SINGLE-MATERIAL MEMS USING POLYCYRSTALLINE DIAMOND
By
Zongliang Cao

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Electrical and Computer Engineering
2011

ABSTRACT
SINGLE-MATERIAL MEMS USING POLYCYRSTALLINE DIAMOND
By
Zongliang Cao
Diamond, due to its unique mechanical, thermal, chemical and electrical properties, is an
excellent material for Microelectromechanical Systems (MEMS). Furthermore, due to its large
band gap of 5.5 eV, diamond offers the possibility of making MEMS structures out of a single
material by varying the doping level to achieve the semiconducting, metallic and insulating
(undoped) properties needed in a typical MEMS structure. Such single-material MEMS (SMM)
can alleviate problems of complicated multilayer devices, such as thermal mismatch, adhesion,
inter-layer diffusion, contact resistance.
Since polycrystalline diamond (poly-C) is inexpensive and retains many of the unique
properties of single-crystal diamond, SMM technology based on poly-C has been developed in
this research work. Moreover, poly-C can be layered to perform a number of functions, whereas
a complex stack of materials would otherwise be required. Consequently, due to poly-C’s high
etching selectivity to most other materials, the SMM fabrication process developed in the current
work can reduce the number of fabrication masks by a factor of 1.5 - 2 as compared to a
conventional MEMS process.
The development of SMM technology faces three major challenges; (a) production of highly
conducting and highly insulating films, (b) the development of dry etching techniques for
multilayer poly-C structures without any damage to the sacrificial layers necessary to produce
multilayer structures and (c) the surface micromachining technology of building multilayer
SMM structures containing insulating, semiconducting and highly-conducting layers. All of

these challenges and other associated poly-C micromachining technologies are discussed in
depth. A number of complex poly-C SMM structures were fabricated using Si or SiO2 as a
sacrificial layer to address the initial SMM issues. Additionally, the surface micromachining
processes of SMM devices and SMM thin film packaging have been developed.
In this research, the design, fabrication and testing of novel poly-C RFMEMS resonators with
piezoresistive detection, a potential application for SMM technology, are developed and
presented for the first time. These resonator uses undoped poly-C with a resistivity above 10

9

Ω·cm as an insulating layer as well as a structural layer. Lightly boron-doped poly-C with a
resistivity of 9 Ω·cm is used as a semiconducting material with piezoresistive properties. Highly
-3

doped poly-C with a resistivity on the order of 10

Ω·cm is used as interconnect to reduce the

total parasitic resistance in signal transmission. SMM system integration has been achieved for a
RFMEMS application with piezoelectric actuation and piezoresistive detection.

ACKNOWLEDGEMENTS

The author would like to express his deep appreciation to my thesis advisor and mentor, Dr.
Dean M. Aslam, for his encouragement, guidance and support throughout this research.
Additional thanks are extended to Dr. Donnie K. Reinhard, Dr. Terence Brown and Dr. Brian F.
Feeny for their valuable discussions and academic advices.
The author would like to acknowledge all the members of Dr. Aslam’s research group,
particularly Dr. Hoyin Chan, Sean Hatch, Mike Varney and Sedat Gur for their assistance and
helpful discussions. The author is also thankful to staffs and friends of the WIMS center at
University of Michigan for their great help and friendship, particularly Jay Yoong Cho and
Seowyuen Yee.
Last but not least, the author wishes to thank my whole family for their patience,
understanding and support during this study. Greatest thanks to my wife Leting Zhao for her
sacrifice.
This work was supported primarily by the Engineering Research Centers Program of the
National Science Foundation under Award Number EEC-9986866.

iv

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................ vii 
LIST OF FIGURES ......................................................................................................... viii 
Chapter 1  Research Motivation and Goals ........................................................................ 1 
1.1  Introduction .............................................................................................................. 1 
1.2  Objective of This Work ............................................................................................ 3 
1.3  Dissertation Organization ......................................................................................... 7 
Chapter 2  Diamond MEMS Technologies ........................................................................ 8 
2.1  Introduction .............................................................................................................. 8 
2.2  Forms of Diamond.................................................................................................... 8 
2.3  Unexpected Electrical Conductivity of Undoped Poly-C ...................................... 15 
2.4  Patterning of Poly-C ............................................................................................... 16 
2.5  Diamond Piezoresistive Sensors ............................................................................ 18 
2.6  Diamond RFMEMS ............................................................................................... 22 
2.7  BioMEMS .............................................................................................................. 26 
2.7.1  Microfluidic channel ....................................................................................... 26 
2.7.2  Neural Probes .................................................................................................. 27 
2.8  Diamond Packaging ............................................................................................... 29 
Chapter 3  Experimental Design and Setup ...................................................................... 31 
3.1  Introduction ............................................................................................................ 31 
3.2  Design of MEMS-Based Characterization Structures ............................................ 31 
3.2.1  Electrical resistivity characterization of doped poly-C ................................... 31 
3.2.2  Contact resistivity characterization ................................................................. 33 
3.2.3  Electrical resistivity characterization of undoped poly-C ............................... 37 
3.2.4  Residual stress characterization ...................................................................... 38 
3.3  Measurement of Gauge Factor by Beam Bending Method .................................... 40 
3.4  Piezoresistive Resonator Testing............................................................................ 41 
3.4.1  Piezoelectric actuation setup ........................................................................... 41 
3.4.2  Driver circuit for piezoelectric sheet............................................................... 43 
3.4.3  Piezoresistive detection circuit ....................................................................... 46 
Chapter 4  Single-Material MEMS Poly-C Film Technologies ....................................... 49 
4.1  Introduction ............................................................................................................ 49 
4.2  Achievement of Highly Insulating Poly-C Thin Film ............................................ 49 
4.2.1  Poly-C growth with the presence of O2 .......................................................... 50 
4.2.2  Characterization of electrical resistivity ......................................................... 53 
4.3  Characterization of Semiconducting and Metallic Poly-C ..................................... 56 
4.3.1  Characterization of electrical resistivity ......................................................... 57 
4.3.2  Characterization of contact resistivity ............................................................ 62 

v

4.4  Characterization of Residual Stress ........................................................................ 63 
4.4.1  Substrate curvature measurement ................................................................... 63 
4.4.2  Characterization using micro-mechanical structure........................................ 67 
4.5  Multilayer Single-Material MEMS Fabrication Technologies .............................. 70 
4.5.1  Achievement of conformal poly-C coating..................................................... 70 
4.5.2  Optimization of dry etching of poly-C............................................................ 72 
4.5.3  Surface micromachining of poly-C structures and devices ............................ 80 
4.6  Single-Material MEMS Examples ......................................................................... 87 
4.6.1  SMM capacitive RFMEMS ............................................................................ 88 
4.6.2  SMM integrated field emission devices .......................................................... 90 
4.6.3  SMM pieozoresistive sensors ......................................................................... 92 
Chapter 5  Single-Material MEMS Piezoresistive Resonators for RFMEMS ................. 95 
5.1  Introduction ............................................................................................................ 95 
5.2  Sensor Design ......................................................................................................... 96 
5.3  Sensor Fabrication .................................................................................................. 99 
5.4  Sensor Characterization ........................................................................................ 101 
5.4.1  Electrical conductivity and contact resistivity characterization.................... 101 
5.4.2  Displacement sensitivity and gauge factor characterization ......................... 106 
5.5  Measurement of Frequency Response .................................................................. 107 
Chapter 6  Single-Material MEMS Thin Film Packaging .............................................. 114 
6.1  Introduction .......................................................................................................... 114 
6.2  Process Design and Fabrication ........................................................................... 115 
6.3  Release and Sealing Using Porous Diamond ....................................................... 119 
6.4  Evaluation of Poly-C Hermeticiy ......................................................................... 122 
Chapter 7  System Integration ........................................................................................ 124 
Chapter 8  CONCLUSIONS AND FUTURE RESEARCH .......................................... 127 
8.1  Summary of Contributions ................................................................................... 127 
8.2  Future Research .................................................................................................... 128 
APPENDICES ................................................................................................................ 129 
APPENDIX A ................................................................................................................. 130 
APPENDIX B ................................................................................................................. 135 
APPENDIX C ................................................................................................................. 137 
APPENDIX D ................................................................................................................. 141 
APPENDIX E ................................................................................................................. 142 
APPENDIX F.................................................................................................................. 145 
APPENDIX G ................................................................................................................. 146 
APPENDIX H ................................................................................................................. 147 
APPENDIX I .................................................................................................................. 150 
BIBLIOGRAPHY ........................................................................................................... 156 

vi

LIST OF TABLES

Table 2-1 Comparison of diamond properties with other semiconductors ................................... 10 
Table 2-2 Properties of various forms of diamond [2] [20] [23-27] ............................................. 14 
Table 2-3 The reported room temperature quality factor Q’s at resonant frequencies f0’s of
flexural resonators......................................................................................................................... 24 
Table 3-1 Design values of micro strain gauge ............................................................................ 39 
Table 4-1 The processing parameters in various steps used in this study .................................... 51 
Table 4-2 The measured resistivities of samples A-D .................................................................. 54 
Table 4-3 Electrical resistivity, doping concentration for various TMB ratios ............................ 58 
Table 4-4 Residual stresses for CH4/H2 of 0.5%, 1.0% and 2.0% ............................................... 66 
Table 4-5 Three-step etching conditions of poly-C ...................................................................... 77 
Table 5-1 Analytical, simulation results of k and εPZR ................................................................ 97 
Table A-1 Gauge factors of poly-C reported by different groups .............................................. 133 
Table H-1 Values of x, y, f(x,y), g(x,y), k and εPZR. .................................................................. 149 

vii

LIST OF FIGURES

Figure 1-1 Concept diagram of SMM. For interpretation of the references to color in this and all
other figures, the reader is referred to the electronic version of this dissertation. .......................... 3 
Figure 1-2 Overview of single-material MEMS using polycrystalline diamond. .......................... 6 
Figure 2-1 SEM images of (a) microcrystalline diamond (MCD) [3], (b) nanocrystalline diamond
(NCD) [17], (c) ultrananocrystalline diamond (UNCD) [17] and (d) diamond-like carbon (DLC)
[18]. ................................................................................................................................................. 9 
Figure 2-2 Raman spectrum of (a) microcrystalline diamond (MCD) [21], (b) nanocrystalline
diamond (NCD) [17], (c) ultrananocrystalline diamond (UNCD) [17], (d) diamond-like carbon
(DLC) including a-C and ta-C [137]. ............................................................................................ 13 
Figure 2-3 Comparison of the highest gauge factors achieved in previous work at room
temperature between poly-C and other metals and semiconductors. [88] .................................... 20 
Figure 2-4 Gauge factors of poly-C at room temperature in literatures [89-104] [106] [180]. .... 20 
Figure 2-5 (a) Cantilever test structure to characterize the gauge factor [99], (b) a pressure sensor
using p-type poly-C piezoresistor [109], (c) diamond acceleration sensor (left) and diamond
pressure sensor (right) [99], (d) position sensor made of p-type poly-C in cochlear implant probe
[180]. ............................................................................................................................................. 21 
Figure 2-6 (a) Schematic cross-setion of diamond microwave relay (b) SEM-micrographs of
microwave relay structures (beam length: 2500 μm); left: coplanar line pattern deposited on the
substrate and oriented orthogonally to the cantilever: right: complanar line deposited onto the
beam surface and oriented along the cantilever. [142] [143]........................................................ 25 
Figure 2-7 SEM of microfluidic channels with 4 μm tunnels. [146] ............................................ 26 
Figure 2-8 Poly-C probe used for electrochemically detecting neurotransmitters. [157]............. 28 
Figure 2-9 Completed sealed poly-C packaging. [158] ................................................................ 29 
Figure 3-1 Cross-sectional view of micro-fabricated 4-point probe structure. ............................. 32 
Figure 3-2 Top view of micro-fabricated Van der Pauw structure. .............................................. 33 
Figure 3-3 Schematic diagram of micro-fabricated Kelvin bridge structure used to measure the
contact resistivity between doped poly-C (p poly-C) and metal with or without highly doped
interlayer (p+ poly-C). .................................................................................................................. 34 

viii

Figure 3-4 Schematic diagram of micro-fabricated structure of linear Transmission line model
(TLM) pattern used to characterize the contact resistivity of doped poly-C ................................ 36 
Figure 3-5 A MEMS design used to characterize the electrical resistivity of undoped poly-C ... 37 
Figure 3-6 Micro strain gauge used to measure the internal stress in poly-C thin film. Test beam,
slope beam and indicator beam are released. ................................................................................ 38 
Figure 3-7 Schematic diagram of the measurement setup to characterize the gauge factor (GF) 40 
Figure 3-8 Schematic diagram of testing sample mounted on the piezoelectric actuator ............ 41 
Figure 3-9 Schematic diagram of actuation of piezoelectric actuator .......................................... 42 
Figure 3-10 Schematic diagram of the driving circuit .................................................................. 43 
Figure 3-11 Characterization of the driving circuit by oscilloscope............................................. 45 
Figure 3-12 Schematic diagram of piezoresistive detection circuit .............................................. 46 
Figure 3-13 The frequency response of the detection circuit ....................................................... 47 
Figure 4-1 Characteristics of the poly-C film seeded by 5% DPLW spin method. (a) front surface
of the film; (b) side view of the film; (c) AFM image of the film; (d) Raman spectrum of the film.
....................................................................................................................................................... 50 
Figure 4-2 Growth rate of poly-C with the presence of O2 versus ratio of O2 to CH4. ............... 53 
Figure 4-3 I-V plot of a 0.68-μm-thick undoped poly-C grown with O2 present based on high
ultra-high nucleation density......................................................................................................... 55 
Figure 4-4 (a) SEM images of the fabricated structure with four metal lines in 4-point-probe
configuration and (b) measurement result. ................................................................................... 55 
Figure 4-5 (a) Optical image and concept diagram of the sample for resistivity measurement (b)
resistivity of doped poly-C as a function of TMB ratio. ............................................................... 57 
Figure 4-6 Electrical resistances (kΩ) of a 600-800 nm thick boron-doped poly-C layer versus
reciprocal temperature (K). ........................................................................................................... 60 
Figure 4-7 (a) Kelvin Bridge Structure used to measure the contact resistivity between p poly-C
(piezoresistive layer) and metal with or without p+ poly-C interlayer; (b) Plot of contact
resistivity as a function of the resistivity of piezoresistive layer with the inset of I-V plot by
Semiconductor Analyzer indicating excellent ohmic contact. ...................................................... 61 

ix

Figure 4-8 (a) Raw data of curvature measurement, (b) quadratic fit of smoothed curvature data,
and (c) curvature variation with x position along the scan length. ............................................... 66 
Figure 4-9 Reading from micro strain gauge indicating a tensile stress of 26-36 MPa................ 67 
Figure 4-10 Stress gradients characterization using cantilever beam arrays. Beams are made of (a)
undoped poly-C grown without O2, (b) undoped poly-C grown with 2000 ppm O2 and (c) highly
doped poly-C with 1 ppm TMB. ................................................................................................... 69 
Figure 4-11 (a) PECVD SiO2 was patterned to form a high step as shown by the optical image on
the right side. (b) Low seeding density results in a discontinuous film along the line of
intersection as shown by SEM images on the right side. (c) 5% DPLW spun at 2000 rpm results
in a continuous film with the absence of powder aggregation. (d) Powder aggregation when
powder concentration in DPLW is too high or spin rate is too low. (e) Film still has pinholes
after 15 hours growth caused by the relatively low seeding density when 5% DPLW is spun at
3000 rpm. ...................................................................................................................................... 71 
Figure 4-12 Perspective view of remaining diamond columns after 2-μm-thick poly-C film on
sample A was etched in O2 plasma; Inset is the close-up view of diamond columns. (b)
Perspective view of remaining poly-C after 3.5-μm-thick poly-C film was etched in O2 plasma;
Inset is the perspective view of the remaining porous poly-C layer which was broken
intentionally on sample B’. (c) Holes were created in Si by CF4 etching through the pores of
remaining porous poly-C on sample B’’. Inset is the close-up view of holes underneath the
porous poly-C layer. (d) EDS analysis of the etched surface on sample A and B indicating the
existence of Al. ............................................................................................................................. 74 
Figure 4-13 Etch rate of poly-C and Al at various pressures in (a) O2 plasma and (b) CF4 plasma
....................................................................................................................................................... 76 
Figure 4-14 Schematic diagram of 3-step etching of poly-C ........................................................ 77 
Figure 4-15 (a) The SEM image of smooth etched surface using multi-step recipes in Table I; (b)
a dot of 5 μm diameter was designed to study the etching of side wall; (c) smooth side wall is
achieved when poly-C was etched in O2 plasma and H2 plasma at 10 mTorr. ............................ 80 
Figure 4-16 Process of fabricating released structures with one poly-C layer ............................. 81 
Figure 4-17 Left: Photoresist 9260 patterned on top of rough poly-C surface using optimized
photolithography parameters; Right: an Al ribbon was patterned successfully via lift-off step. . 82 
Figure 4-18 Released cantilevers using the surface micromachining techniques......................... 83 
Figure 4-19 SEM images of various SMM structures made of poly-C. ....................................... 84 

x

Figure 4-20 Inspection of (a) lateral gap and (b) vertical gap using fabricated poly-C structures.
....................................................................................................................................................... 85 
Figure 4-21 poly-C beams with widths varying from 200 nm – 2 μm inspected by (a) SEM and
(b) optical microscopy. ................................................................................................................. 87 
Figure 4-22 SEM images of fabricated capacitive SMM RFMEMS and the fabrication process
flow. .............................................................................................................................................. 88 
Figure 4-23 (a) The concept diagram and SEM picture of fabricated IFED; (b) diamond needles
created in O2 plasma by ICP. ........................................................................................................ 91 
Figure 4-24 Field emission measurement result with vacuum as insulator. ................................. 92 
Figure 4-25 SEM images of SMM piezoresistive sensors. ........................................................... 92 
Figure 4-26 Cross-sections of the fabrication sequence. .............................................................. 94 
Figure 4-27 Relative change of resistance vs. cantilever tip deflection ....................................... 94 
Figure 5-1 Schematic diagram of piezoresistive resonator. .......................................................... 96 
Figure 5-2 (a) stress contour of piezoresistive resonator, (b) tip deflection and (c) average strain
in piezoresistors vs. applied force at the tip. ................................................................................. 98 
Figure 5-3 Fabrication process flow of SMM piezoresistive resonator........................................ 99 
Figure 5-4 SEM images of fabricated piezoresistive resonator .................................................. 101 
Figure 5-5 SEM images of fabricated Van Der Pauw structure used to measure the electrical
resistivity of doped poly-C.......................................................................................................... 102 
Figure 5-6 Electrical resistivity of piezoresistive layer versus temperature. .............................. 103 
Figure 5-7 (a) SEM image of transmission line model used to characterize the contact resistance
(b) The total resistance between two neighboring metal pads as a function of spacing between
pads is plotted for four different contact lengths. ....................................................................... 104 
Figure 5-8 ΔR/R vs. (a) cantilever tip deflection and (b) microstrain in piezoresistive sensor .. 106 
Figure 5-9 Concept diagram of measurement setup for piezoresistive detection and piezoelectric
actuation. ..................................................................................................................................... 108 
Figure 5-10 Piezoresistive detection and piezoelectric actuation boards with wire-bonded sample
shown at the bottom left.............................................................................................................. 109 

xi

Figure 5-11 The measured frequency response in air for resonator with length l = 430 μm when
power supplied to Wheastone bridge is (a) 24 V and (b) 0 V. .................................................... 110 
Figure 5-12 (a) experimental and (b) simulation results of frequency response in air for resonator
with beam length l = 630 μm; (c) experimental and (d) simulation results with beam length l =
430 μm. ....................................................................................................................................... 113 
Figure 6-1 SMM package concept diagram and the SEM picture of the structures ................... 116 
Figure 6-2 General fabrication process flow of SMM thin film package ................................... 118 
Figure 6-3 (a) Creation of porous diamond by RIE of diamond (b) Release the structure by using
HF to penetrate the porous diamond ........................................................................................... 119 
Figure 6-4 Poly-C package having porous diamond as release holes before being soaked in HF
..................................................................................................................................................... 120 
Figure 6-5 Poly-C package having porous diamond as release holes after being soaked in HF for
(a) 10 minutes and (b) 30 minutes .............................................................................................. 121 
Figure 6-6 Porous diamond grows into continuous poly-C film in MPCVD ............................. 122 
Figure 6-7 Sample chips with poly-C layer of (a) 2.62 μm (b) 2.92 μm after 2 days soak in HF.
..................................................................................................................................................... 123 
Figure 7-1 Design of system integration ..................................................................................... 124 
Figure 7-2 Frequency response in air for a resonator with beam length of 430 μm measured using
the system integration setup in Figure 7-1. ................................................................................. 124 
Figure A-1 Simplified Schematic of valence bands split-off model (a) no stress, (b) under tensile
stress, and (c) under compressive stress. .................................................................................... 130 
Figure B-1 (a) Schematic diagram of temperature control system and (b) sample mounting on the
heater. .......................................................................................................................................... 135 
Figure C-1 Schematic diagram of seeding process (a) Si has hydrophobic surface after removing
the native oxide by diluted hydrofluoric acid (HF). Separate water drops can be observed after
taking the wafer out from deionized (DI) water. (b) Super-hydrophilic surface is achieved after
O2 plasma treatment. A thin water layer forms on the surface after taking the wafer out from DI
water. (c) Several drops of DPLW are put on silicon wafer immediately after O2 plasma
treatment. (d) Wafer is spun. Uniform and high seeding density is achieved across the wafer. 137 
9

-2

Figure C-2 For the case of using 0.5% DPLW, (a) seeding density of 10 cm was achieved and
(b) 8 hours are needed to form continuous poly-C thin film. For the case of using 5% DPLW, (c)

xii

11

-2

seeding density above 10 cm was achieved and (d) less than 20 minutes are needed to form
continuous film. .......................................................................................................................... 138 
Figure D-1 Measurement setup to characterize the electrical resistivity of undoped poly-C films.
..................................................................................................................................................... 141 
Figure F-1 Labview program used to measure the change of the resistance of piezoresistive
sensor as function of cantilever tip deflection. ........................................................................... 145 
Figure G-1 Labview program used to measure frequency response........................................... 146 
Figure G-2 The block diagram of the programming ................................................................... 146 
Figure H-1 Top and cross-sectional view of piezoresistive resonator ........................................ 148 
Figure H-2 (a) f(x,y) and (b) g(x,y) as a function of x and y ...................................................... 149 

xiii

Chapter 1
Research Motivation and Goals
1.1 Introduction
The development of methods for the production of miniaturized mechanical components and
devices in Si is a natural outgrowth of the Si surface machining methods that have been
developed for the production of microcircuits. However, Si has its material limitations [1]. In
particular, it has relatively low values for its Young’s modulus and its fracture toughness; against
both itself and many other materials, it has a comparatively large coefficient of friction (COF)
and high rate of wear, such that Si-based MEMS exhibit high energy loss due to high friction. In
addition, Si-based MEMS devices that involve extensive rolling or sliding motion typically wear
out in a very short time [2] [3] [4]. The friction problem can be exacerbated by capillary
adhesion in humid environments [5] [6] or by low operation temperature and heat dissipation due
to the small energy band gap [3]. Clearly, Si is not an appropriate material for some MEMS
devices, even for room temperature applications. Si MEMS components coated with Si3N4 or
SiC exhibit better behavior than pure Si in room-temperature MEMS applications, but they also
have materials and fabrication limitations [7] [8].
Researchers continue to seek new materials for use in MEMS. Metals, semiconductors and
polymers are being studied for their suitability. Among these candidate materials, diamond is
believed to be amongst the most promising materials for MEMS applications. Usually
polycrystalline diamond (poly-C) is used for MEMS applications instead of single crystal
diamond (SCD) because it not only retains many of the properties of single crystal diamond but

1

is also inexpensive and can be deposited on a wide range of substrates with reasonable
deposition rates and good uniformity, making it suitable for very large-scale production. Two
distinct forms of poly-C are microcrystalline diamond (MCD) and ultrananocrystalline diamond
(UNCD), grown in H rich and H poor environments, and having grain sizes in the ranges of 0.510 μm and 3-5 nm, respectively [2]. The MCD with grain sizes in the nanometer range, which
increase with film thickness is called nanocrystalline diamond (NCD) [9].
Diamond possesses a number of excellent properties that Si and other materials lack. These
include generally reduced COF and increased resistance to wear, both particularly useful for
microsystems containing components with sliding interfaces. Poly-C also has a very high
Young’s modulus, high tensile and fracture strength, and is suitable for high frequency MEMS
devices. It is also chemically inert and stable in air up to 600 ˚C, making it an attractive material
for operation at above 300 ˚C, where conventional Si devices cannot operate [3]. It has excellent
thermal conductivity (on the contrary UNCD has a low thermal conductivity which makes it an
ideal thermal insulation layer) and a low coefficient of thermal expansion (CTE) which make it
useful in developing lab-on-chip devices, packages or substrates with better heat dissipation. It
can also be used as bimorph thermal actuators with large displacements. Diamond is
biocompatible, mechanically strong and tribologically effective, and as such is an excellent
coating material for implantable medical devices. Diamond-based micro-mechanical devices can
be integrated with potential diamond electronics on the same substrate for high temperature
applications as well. All in all, these properties show the potential for a great deal of novel
MEMS applications using diamond.

2

1.2 Objective of This Work

Figure 1-1 Concept diagram of SMM. For interpretation of the references to color in this and all
other figures, the reader is referred to the electronic version of this dissertation.

The focus of this work is on developing the technology to build single-material MEMS (SMM)
using poly-C and demonstrate SMM concept. The concept diagram of SMM is shown in Figure
1-1. It consists of diamond MEMS and circuits encapsulated inside a diamond package.
Everything will be built on a diamond substrate. Metals are only used for long interconnects or
bonding pads. The development of the technology required for diamond MEMS devices,
interconnects and thin film packaging is reported in this work.
An overview of the work is illustrated in Figure 1-2. The study of SMM poly-C technology
starts from the fundamental research of characterization and improvements of basic poly-C
deposition techniques to achieve highly insulating and conducting poly-C thin films.
Fundamental research also includes the achievement of poly-C conformal coating and dry

3

etching of poly-C. Then enabling technologies have been developed to build free-standing SMM
devices and SMM thin film packaging with multiple poly-C layers. SMM system integration was
achieved for a RFMEMS application with piezoelectric actuation and piezoresistive detection.
This work involves following specific goals:
Fundamental studies to characterize MEMS quality poly-C thin film
 Highly insulating undoped poly-C growth and characterization;
 Characterization of highly conducting and semiconducting poly-C thin film;
 Achievement of conformal poly-C coating;
 Optimization of dry etching of multilayer poly-C structures;
Enabling technologies
 Fabrication process development for SMM structures in micrometer & nanometer scales;
 Development of SMM examples: non-tested structures and functional structures;
SMM System integration
 SMM system components: study the piezoelectric actuation and piezoresistive detection
for RFMEMS resonators;
 Measure the resonant frequency and quality factor Q of piezoresistive resonators;
Successful accomplishment of these goals leads to the following unique contributions:


Achievement of highly insulating poly-C in a reactor also used for doped poly-C growth.



Optimization of dry etching of multilayer poly-C structures without causing damage to
the underlying layer



Development of multilayer SMM structures using poly-C



First fabrication of SMM thin film packaging with rapid release and sealing using porous
diamond in the release-hole region.

4



First fabrication and testing of SMM piezoresistive RFMEMS resonators.



First achievement of SMM system integration.

5

Figure 1-2 Overview of single-material MEMS using polycrystalline diamond.

6

1.3 Dissertation Organization
Chapter 2 presents an overview of current diamond MEMS technologies. In chapter 3, some
characterization methodologies and experimental setups are described. In chapter 4, important
technologies needed for building multilayer SMM structures are demonstrated. Typical surface
micromaching steps in poly-C microfabrication are provided, leading to the successful
fabrication of multilayer SMM devices reported at the end of chapter 4. In chapter 5, SMM
piezoresistive resonators for RFMEMS applications are persented as a typical SMM example.
The design, fabrication and testing of the device were performed. The frequency response was
achieved using piezoelectric actuation and piezoresistive detection. The RFMEMS resonator
needs high-vacuum packaging to improve its performance. Chapter 6 discusses the fabrication
technology for a SMM thin film packaging process. SMM system integration is explored in
Chapter 7 to measure the frequency response of piezoresistive resonator using MSP430
microcontroller. Chapter 8 presents conclusions of this study and considers possible future
directions.

7

Chapter 2
Diamond MEMS Technologies
2.1 Introduction
This chapter presents an overview of current diamond MEMS technologies. First, different
forms of diamond thin films, typically used in MEMS, and their growth mechanism and
properties are briefly reviewed in section 2.2. The cause of the low electrical resistivity of
undoped poly-C is discussed in section 2.3, along with the role of O2 in increasing its resistivity.
Contemporary patterning methods for poly-C films, and their challenges, are reviewed in section
2.4. Diamond piezoresistive sensors, perhaps being the most widely used diamond sensors, are
then discussed in section 2.5. At the end of this chapter, state-of-the-art diamond RFMEMS,
BioMEMS and thin film packaging results are briefly reviewed.

2.2 Forms of Diamond
There are several types of diamonds for different applications, including natural diamond,
synthetic industrial diamond, chemical vapor deposition (CVD) polycrystalline diamond (poly-C)
and diamond-like carbon (DLC). The properties of natural diamond are summarized and
compared with other semiconductor materials in Table 2-1. Synthetic industrial diamond is
produced by the High Pressure High Temperature (HPHT) synthesis process. Both natural
diamond and HPHT diamond are single-crystalline (SCD). Considering the high cost and
difficulty of processing, these two types of diamonds are very limited in MEMS applications.

8

However, CVD poly-crystalline diamond (poly-C) can still provide remarkable quality for
MEMS application with relatively low fabrication cost.

Figure 2-1 SEM images of (a) microcrystalline diamond (MCD) [3], (b) nanocrystalline diamond
(NCD) [17], (c) ultrananocrystalline diamond (UNCD) [17] and (d) diamond-like carbon (DLC)
[18].

9

Table 2-1 Comparison of diamond properties with other semiconductors
Properties

Si

GaAs

SiC

Natural Diamond

Density (g/cm )

2.329

5.317

3.216

3.52

Melting point (˚C)

1412

1240

2540

3827

8.5

7

24.8

100

Young’s modulus (GPa)

130-180

85

700

1050-1200

Poisson’s ratio

0.22-0.24

0.31-0.32

Lattice constant (Å)

5.43

5.65

4.36

3.57

Band Gap (eV)

1.12

1.42

3.0

5.45

1450

8500

400

1800-2000

Hole (cm /V·s)

2

500

400

50

1600-2100

Dielectric constant

11.7

10.9

10

5.7

0.37-0.5

0.6

2-3

4-20

3

Hardness (GPa)

2

Carrier mobility Electron (cm /V·s)

6

Breakdown voltage (x10 V/cm)

1x10

Intrinsic resistivity (Ω·cm)
Thermal conductivity (W/cm·K)
-6

Thermal expansion coef. (x10 /˚C)

3

1x10

0.1-0.21

8

1x10

16

1.5

0.5

5

20

2.6

5.9

4.7

1.1

10

Three types of poly-C thin films have been synthesized and systematically studied, which
exhibit different microstructure, surface morphology, and properties. Standard poly-C thin film
deposition methods, based on microwave plasma chemical vapor deposition (MPCVD), hot
filament chemical vapor deposition (HFCVD) or any other CVD methods, involve hydrogen-rich
chemistry (CH4/H2=0.1-4%) [10] [11] [12], which results in microcrystalline diamond (MCD)
(typically 0.5-10 μm grains) with columnar structures as shown in Figure 2-1 (a) [3] and
nanocrystalline diamond (NCD) (typically 50-100 nm grains) in Figure 2-1 (b) [17]. NCD is
usually defined as a small grain size poly-C with a very high nucleation density to allow very
thin coalesced films as demonstrated by Philip et al. [13] and Sekaric et al. [14], and not the
cauliflower type grown with high CH4 levels or under conditions designed to reduce atomic
hydrogen and thus increase re-nucleation [15] [16]. The MCD and NCD film surface coarsens
with thickness. Correspondingly, they exhibit a rough, highly faceted morphology with root
mean square (RMS) roughness typically ~10% of the film thickness. In contrast to the growth
process for MCD and NCD films, ultrananocrystalline diamond (UNCD) films growth uses an
argon-rich chemistry (typically Ar (99%)/CH4 (1%)) with no hydrogen added [2] [4]. As a result,
UNCD is a fundamentally different material. Grains of UNCD (typically 2-5 nm) are much
smaller than those of MCD and NCD films as shown in Figure 2-1 (c) [17]. Unlike other poly-C,
UNCD can be grown to any thickness with no appreciable increase in roughness due to the high
re-nucleation rate resulting from its plasma chemistry [17].
In addition to poly-C, DLC is also exploited in MEMS. DLC is a metastable, amorphous
carbon material which may contain microcrystalline phase of diamond. The SEM image of DLC
2

3

is shown in Figure 2-1 (d) [18]. It contains a mixture of sp and sp carbon phases. DLC films in

11

general are very smooth and can be deposited at low temperatures. Various methods used in
deposition have led to a wide range of DLC films. It can be deposited by plasma-enhanced CVD
(PECVD), ion beam deposition, sputtering from a graphite target or laser ablation of graphite [3].
Based on different source gases and/or deposition methods, DLC can be either hydrogen free (aC or ta-C) or hydrogenated (a-C:H or ta-C:H). Usually, both a-C and a-C:H represent a structure
2

3

with a high sp /sp ratio. However, using high plasma density sputtering and plasma-enhanced
3

deposition procedures films can be grown possessing a high fraction of sp bonding. In order to
distinguish them from sp

2

DLC, tetrahedral amorphous carbon (ta-C) was suggested by
3

McKenzie [ 19 ] for high sp amorphous carbon and ta-C:H was referred to hydrogenated
tetrahedral amorphous carbon by Weiler [20].

12

Figure 2-2 Raman spectrum of (a) microcrystalline diamond (MCD) [21], (b) nanocrystalline
diamond (NCD) [17], (c) ultrananocrystalline diamond (UNCD) [17], (d) diamond-like carbon
(DLC) including a-C and ta-C [137].

13

Table 2-2 Properties of various forms of diamond [2] [20] [23-27]
Density
-3

(g/cm )

Young’s
Hardness

3.52

3

H

Gap

Surface

(%)

(at%)

(eV)

roughness

100

<0.1

5.45

modulus
(Gpa)
(GPa)

SCD

sp

100

1050-1200

MCD (grain size

400 nm – 1
3.52

70-100

800-1200

~100

<1

5.45

~0.5-10 μm)

μm

NCD (grain size 5030-75

800-1020

88-98

>50

916-980

<1

2-4.7

50-100 nm

100 nm)
5.4-

95-

UNCD (grain size
3.50

5.65

98

~2-5 nm)

20-40 nm

<1

ta-C

0.41.9-2.0

2-5

(evaporated)

0.7
5-100 nm

757
0.5ta-C (MSIB)

3.0

30-130

90±5

<9
1.5

DLC

ta-C:H

2.9

61

75

22-28

30a-C:H (hard)

1.6-2.2

10-20

0.810-40

60

1.7

300
a-C:H

1-30 nm
50-

0.9-1.6

40-65

<5
80

(soft)

14

1.6-4

Raman spectroscopy is one of the principle characterization tools to investigate diamond film
-1

quality. Good quality diamond shows a high-intensity peak at 1332 cm in the Raman scan with
a very small Full Width at Half Maximum (FWHM) value as shown in Figure 2-2 (a) [21]. The
-1

broad band approximately centered at 1550 cm

is attributed to three bands: the D and G peaks

-1

of polycrystalline graphite at around 1345 cm

-1

and 1560 cm , and a low intensity band

-1

centered approximately at 1470 cm attributed to a tetrahedrally bonded diamond precursor [22].
The Raman spectrum of NCD, UNCD and DLC are shown in Figure 2-2 (b-d) [17] [137],
3

respectively. MCD has the highest quality indicated by its high intensity of sp peak and
3

amorphous carbon has the lowest quality as indicated by the almost invisible sp peak. A
comparison of basic properties of various forms of diamond is given in Table 2-2 [2] [20] [23]
[24] [25] [26] [27].

2.3 Unexpected Electrical Conductivity of Undoped Poly-C
14

The electrical resistivity of natural diamond is approximately 10 -10

16

Ω·cm [28] [29] [30].

Due to its high electrical resistivity, diamond is a good candidate for electrical insulation in
MEMS applications. But after certain surface treatment is done to remove the poly-C surface
conduction as demonstrated previously, undoped poly-C films still show a resistivity of a few
orders of magnitude lower than natural diamonds. The conduction mechanism in undoped polyC has been extensively studied [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
[44]. The conduction of undoped poly-C can be attributed to the impurities or traps in diamond
and the grain boundary regions. After comparing the quality of diamond grown under different

15

CH4/H2 ratio with SEM and Raman, Kulkarni et al. attributed the low resistivity on the order of
6

10-10 Ω·cm to the grain boundary regions [36] [37]. Lee applied Cu electroplating to diamond
film showing that the grain boundaries within the films were the main conduction path in
undoped poly-C [41]. The conduction mechanism due to impurities had also been investigated by
deriving the activation energy based on the change of the electrical conductivity as a function of
the temperature. Usually, boron corresponds to the activation energy about 0.3-0.4 eV. Nitrogen
corresponds to the activation energy about 1.4-1.6 eV. The electrical conductivity as a function
of inverse temperature in [35] suggests that electrically active boron and nitrogen incorporation
is correlated with the film grown along (111) and (100) directions, respectively. Wang, et al. [40]
also reported activation energy of 1.68 eV which was attributed to Si trapped in a vacancy in the
diamond lattice. It is speculated that the possible source of boron contamination in their films
could be the Si wafer which is heavily doped with boron. Stoner et al. found that hydrogen trap
can also be an origin for conduction of undoped poly-C with activation energies less than 0.6 eV
[44]. The real cause for low resistivity of undoped poly-C still needs further study.

2.4 Patterning of Poly-C
For integrating poly-C devices into Si-based microsystems, patterning of poly-C films using
techniques consistent with silicon processing is required. As wet etching poly-C is impractical,
the commonly used patterning techniques are in-situ patterning, selective dry etching. Laser
beam [45] and synchrotron excited radiation [46] were also reported as an optional way to etch
poly-C.
Selective deposition is achieved by selective nucleation or by masking the areas where growth
is not desirable. SiO2 was successfully used as a masking layer by Masood et al. [47], Roppel et

16

al. [48] and Davidson et al. [49]. Patterned amorphous Si masking was also reported [50].
Hirabayashi et al. first nucleated the Si substrate by ultrasonic treatment, then used a photoresist
mask for etching Si to 60-70 nm using an Ar+ ion beam [51] or reactive ion etching [50]. A
simple selective nucleation technique, which consists of spinning a layer of photoresist premixed with diamond powder and lithographically patterning it, was developed by researchers at
Michigan State University [47].
However, in all of these in-situ techniques, the lateral growth at the pattern edges and, in some
cases, growth on unwanted areas can pose serious problems for small feature sizes (< 10 μm).
Consequently, selective dry etching is a better choice for poly-C patterning. Selective etching of
poly-C with SiO2 or Si3N4 as a mask, was performed at atmospheric pressure, in oxygen
environment at 700

˚C,

in a rapid thermal processor [47]. Dry etching technique using

conventional dry etching tools has become the primary approach for poly-C patterning in recent
years, such as ion beam etching (IBE) [52] [53], reactive ion etching (RIE) [54] [55] [56] [57]
[58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74], electron
cyclotron resonance etching (ECR) [75] [76] [77] [78] [79] [80] [81] and inductively coupled
plasma etching (ICP) [82] [83] [84] [85].
RIE of poly-C has been extensively studied for the realization of micro- and nano-structures.
Generally, O2 is used to etch diamond through the generation of CO and CO2 byproducts [86].
However, diamond column structures (also called needles or whiskers) which are usually created
in pure O2 plasma etching unintentionally [59] [60] [61] [70] [72] [73] [74]. These column
structures are generated simultaneously at the beginning of the etching process along grain
boundaries due to the difference in chemical composition of grains and grain boundaries. The

17

columns on the grains are attributed to the intrinsic defects in the crystals and unintentionally
redeposited micro-masks. The origin of the columns on grains and grain boundaries were also
explained as the difference of crystal planes in [74] for grains have (100) plane and grain
boundaries have (111) plane. These diamond column structures can even be formed without a
metal coating existing on the sample [73] [74].
It was suggested that a small amount of CF4 in O2 can increase the etch rate by avoiding the
formation of diamond column structures [55] [65], however, fluorine (F) in plasma etching
causes damage to Si substrates or other underlying layers such as SiO2 or Si3N4 during the
patterning of poly-C [55]. As a result, a new etching recipe is needed to preserve the underlying
layer after the removal of poly-C, while maintaining a reasonable etch rate and aspect ratio.

2.5 Diamond Piezoresistive Sensors
Piezoresistance is a change in resistivity of a material under the influence of an externally
applied stress or strain. The piezoresistive property of semiconductors such as doped diamond
under uniaxial stress can be explained by the valence-band split-off model which is discussed in
APPENDIX A. The strain will change the concentration of heavy and light holes, so the strain
can change the conductivity of the p-type semiconductor. The piezoresistivity of a material is
normally quantified by the gauge factor (GF). The GF of a material is defined as the fractional
change of resistance (ΔR/R) per unit strain ε which is the figure of merit for piezoresistive
material and expressed by equation (2.1).

GF=(ΔR/R)/ε

(2.1)

18

The piezoresistive effect in silicon MEMS is widely used for pressure, force and acceleration
sensors. The most prominent example is piezoresistive acceleration sensors for airbags in cars.
Commercially available sensors cannot operate at high temperatures because of pn-insulation of
the piezoresistors. Since the first report of piezoresistivity in SCD (single crystalline diamond)
and poly-C by researchers at Michigan State in 1992 [87], there has been enormous interest in
the utilization of p-type poly-C prepared by chemical vapor deposition (CVD) as a material for
sensor applications due to its high GF as shown in Figure 2-3 [88] and its potential application at
high temperatures. A number of studies have focused on the piezoresistive effect of poly-C [89]
[90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [106] [180]. The

GF is usually characterized by measuring the change in resistance of the piezoresistor by
deflecting the structural material which is usually made of a cantilever or diaphragm made of
either Si, stainless steel or undoped poly-C. The reported GFs of poly-C as a function of
electrical resistivity are given in Figure 2-4 with details given in APPENDIX A. GFs are found
to increase with the increase of resistivity and are mainly in the range of 5 to 100 at room
temperature.

19

Figure 2-3 Comparison of the highest gauge factors achieved in previous work at room
temperature between poly-C and other metals and semiconductors. [88]

Figure 2-4 Gauge factors of poly-C at room temperature in literatures [89-104] [106] [180].

20

Figure 2-5 (a) Cantilever test structure to characterize the gauge factor [99], (b) a pressure sensor
using p-type poly-C piezoresistor [109], (c) diamond acceleration sensor (left) and diamond
pressure sensor (right) [99], (d) position sensor made of p-type poly-C in cochlear implant probe
[180].

Figure 2-5 (a) shows a cantilever made of undoped poly-C was used as a structural material to
characterize the GF of p-type poly-C piezoresistor [99]. Among the applications of poly-C
piezoresistive sensors, pressure sensors have been studied the most [105] [106] [107] [108] [109].
Figure 2-5 (b) and (c) show pressure sensors and an accelerometer from the work in [109] and
[99]. A position sensor using poly-C was developed for application in cochlear implant probe
[180] as shown in Figure 2-5 (d). RFMEMS resonators with piezoresistive detection are also
being developed [110] [111]. DLC also has piezoresistive effect. Pressure sensors and force

21

sensors made of DLC have also been reported [112] [113] [114] [115] [116]. Piezoresistive
detection, which is receptive to device scaling without decreasing the detection sensitivity, can
help be improved by (a) adjusting the piezoresistor dimensions and (b) increasing the mobility of
holes by using intra-grain piezoresistor.

2.6 Diamond RFMEMS
Radio frequency micro electro mechanical systems (RFMEMS) can yield on-chip
micromechanical resonators with ultra-high quality factors over 10,000 at GHz frequencies in
both vacuum and air making them excellent candidates for broadband wireless communications
[117]. So far, polycrystalline silicon has been the preferred material for RFMEMS resonators,
having been demonstrated with measured quality factor Q’s above 8400 at a frequency of 50.35
MHz [118] for free-free beam design, 2650 at a frequency of 1.156 GHz [119] for radial disk
design, and 2800 at a frequency of 1.52 GHz for extensional wine-glass design [120]. While
already impressive, such an achievement could be greatly amplified if Q’s >10,000 were
achievable at the same GHz frequencies and in the same tiny sizes. In order to further extend
frequencies, the use of alternative structural materials with higher acoustic velocities ( E  , E
is Young’s modulus and ρ is density) than poly-silicon such as silicon carbide [121] [122] and
diamond, has been explored. Among the currently available set of thin-film materials, single
crystal diamond offers the largest acoustic velocity on the order of 18,076 m/s [123]. This is to
be compared with the 8,024 m/s of single crystal silicon [124] and 11,500 m/s of silicon carbide
[125], which are 2.25X and 1.57X smaller, respectively. Given that resonance frequency is
generally

proportional

to

acoustic

velocity,

diamond

potentially

provides

higher

micromechanical resonator frequencies. In addition, the electrical and mechanical properties of

22

polycrystalline silicon begin to rapidly degrade at temperatures above 350 ˚C, making it
increasingly unsuitable for high temperature applications.
Fabrication and testing of diamond-based RFMEMS resonators have been reported in the
literature. They have been made of SCD [126], MCD [127] [128] [129] [130], NCD [14] [ 131]
[132] [133] [134] [135], UNCD [4] [136] and DLC [137] [138]. Reported quality factor Q’s at
resonant frequencies f0’s of diamond-based RFMEMS resonators are given in Table 2-3. The
dissipation (affecting Q) is mainly due to the relaxation of large number of defects in the bulk or
the surface of the film. Although SCD seems to be the best potential material for high Q
achievement, SCD fabrication technology is not developed as well as other types of diamond
material. As a result, poly-C is the most feasible candidate to achieve higher quality factor.
Sepúlveda et al. [129] reported a quality factor Q for poly-C (grain size ~300 nm) cantilevers in
the range of 4,000-165,000, which includes values higher than the Q’s observed for UNCD
cantilevers with comparable dimensions [138]. They suggested that, as the film percentage
occupied by the nucleation layer containing fine grained diamond is increased, the quality factor

Q of the resonators is reduced. This is consistent with the idea that the higher dissipation in the
UNCD or DLC is mainly due to the presence of a higher proportion of grain boundaries and
defects. Low quality factor Q of NCD resonators in Table 2-3 can be attributed to the fact that as
the resonator dimensions become smaller, dissipation due to surface effects also play a role. This
conclusion is supported by the work of Imboden et al [139].

23

Table 2-3 The reported room temperature Q’s at resonant frequencies f0’s of flexural resonators
Quality factor

Measured

Q

frequency f0

Method

510

~230 KHz

Piezoelectric

SCD

Doubly-clamped beam [140]

6,225

3.022 MHz

Electrostatic

MCD

Combdrive [130]

36,460

27.352 KHz

Electrostatic

MCD

Cantilever [128]

15,260

384.9 KHz

Piezoelectric

MCD

Cantilever [129]

116,000

8-50 KHz

Piezoelectric

MCD

2,400-3,500

6-30 MHz

Piezoelectric

NCD

Mesh membrane [14]

3,000

8-20 MHz

Piezoelectric

NCD

Disk resonator [141]

11,555

1.51 GHz

Electrostatic

NCD

Doubly-clamped beam [133]

~3,000

14-157 MHz

Magnetomotive

NCD

Tuning fork [134]

8,000

37.0 MHz

Laser

NCD

Square paddle (pillared) [134]

1,500

14.77 MHz

Laser

NCD

3,000

13.85 MHz

Laser

NCD

Ring resonator [134]

5,000

40.18 MHz

Laser

NCD

Antenna structure [135]

23,200

630.6 MHz

Magnetomotive

NCD

600-2,400

17-66 MHz

Magnetomotive

NCD

11,460

11 KHz

Piezoelectric

UNCD

5,000-16,000

12-35 KHz

Piezoelectric

UNCD

Resonator design

Cantilever [126]

Doubly-clamped paddle [14]

Square

paddle

Actuation
Mater.

(freely

suspended) [134]

Harp structure [132]
Cantilever [4]
Cantilever [136]

24

(a)

(b)
Figure 2-6 (a) Schematic cross-setion of diamond microwave relay (b) SEM-micrographs of
microwave relay structures (beam length: 2500 μm); left: coplanar line pattern deposited on the
substrate and oriented orthogonally to the cantilever: right: complanar line deposited onto the
beam surface and oriented along the cantilever. [142] [143]

A classical electrostatically driven, cantilever-beam-structure switch made of poly-C is shown
in Figure 2-6 [142] [143]. RFMEMS switches have great potential in RF communications
because of their high isolation and low power consumption. The main part is a freestanding polyC cantilever that can be deflected electrostatically by applying a voltage between the cantilever
gate contact and the substrate gate contact. All essential parts of the switch are made of diamond

25

except the silicon substrate. Thus, heat generated caused by the power loss in the signal contacts
can be effectively dissipated over the cantilever and the substrate. Also, no sticking caused by
the melting of metal contacts or formation of insulating oxide layers on the signal contacts can
occur. An on-state attenuation of ~3 dB and an off-state attenuation of -23 dB at 10 GHz were
reported in [143]. The results indicate that for operation in air a diamond microswitch exhibits a
maximum frequency of operation approximately eight times higher than a silicon microswitch of
identical geometry [144].

2.7 BioMEMS
2.7.1

Microfluidic channel

Figure 2-7 SEM of microfluidic channels with 4 μm tunnels. [146]

Heat dissipation is an increasingly important challenge for the electronics industry.
Miniaturized coolers using a liquid passing through microfluidic channels represent one of the
most advanced technologies used to extract thermal energy from microelectronics circuits such

26

as CPUs. Microfluidic elements are also an essential part of lab-on-a-chip systems to perform
complex chemical and biochemical synthesis, as well as analytical tasks. Ramesham, et al. used
anisotropic chemical etching of silicon and selective growth of diamond to form poly-C
microfluidic channels of 17-μm wide [145]. This resulted in channels with diamond forming the
top half of the structure and silicon the bottom half. Such channels may not be useful in many
microfluidic applications due to the relative chemical reactivity and thermal instability of silicon
compared to diamond. Guillaudeu, et al. developed free-standing all-diamond fluidic channels
using Si as a mold as shown in Figure 2-7 [146]. This process is simpler but only narrow width
can be designed to close the trench in a proper time by additional poly-C growth.

2.7.2

Neural Probes

Many research groups are developing unique processes for creating neural probes. However,
silicon can be an undesirable probe material as, without modification, it has been found to have
poor flexibility, solubility in water, and can induce undesirable glial responses [147]. Many
groups have solved this problem through coating silicon with biocompatible materials. As an
alternative to coating silicon, undoped diamond has been explored as a substrate material due to
its structural properties and high biocompatibility [148]. The optical transparency of the diamond
substrate is important for in vitro experiment because it allows the electrode’s position on the
probe to be easily located under a microscope. Boron-doped poly-C sensors and electrodes were
integrated in Si cochlear microprobes for the first time [149]. Boron-doped poly-C electrodes
have also been shown to be effective for the detection of neurotransmitters [150]. Chan et al.
[151] [152] [153] developed a process for fabricating neural probes using undoped poly-C as the
structural material and boron-doped poly-C as the electrode material. Non-diamond materials are

27

used for interconnects and electrical insulation layers. This probe has been used to measure
electrical neural activity. In addition, the comparatively wide potential window (the reported
values range from 1.4 to 4 V) in an aqueous environment [154 ] [155 ], low double layer
2

capacitance (ranging from 5 to 40 μF/cm ) [156], chemical inertness and stability, and resistance
to fouling of boron doped diamond make it an excellent site material for electrochemically
detecting neurotransmitters in vitro as shown in Figure 2-8 [157].

Figure 2-8 Poly-C probe used for electrochemically detecting neurotransmitters. [157]

28

2.8 Diamond Packaging

Figure 2-9 Completed sealed poly-C packaging. [158]

Although MEMS packaging can take advantage of mature packaging techniques from the
semiconductor IC industry, MEMS packaging is still complicated due to the diversity of
applications. Poly-C has emerged as a novel material for MEMS packaging [158] [159]. The
potential of poly-C as a thin film packaging material was explored in view of its excellent
mechanical strength, electrical properties, chemical stability and thermal management as a heat
sink. A poly-C thin film packaging process was developed to encapsulate poly-C cantilever
resonators [159] integrated with highly doped poly-C interconnects [158], as shown in Figure 2-9.
The efficacy of the encapsulation process was evaluated by measuring the resonator frequency
and quality factor before and after the packaging process, indicating the poly-C packaging can be
integrated into conventional MEMS fabrication without affecting the yields of the device. The

29

fluidic hermeticity was also tested by an HF soak test. The encapsulated cantilevers are suitable
for applications in harsh environments and are able to withstand chemical and mechanical attack
and high temperatures. A technology using an all-diamond package for wireless-integrated
Microsystems with boron-doped diamond as built-in interconnects was also developed [160] and
demonstrated the ability to achieve an outstanding level of heat dissipation.

30

Chapter 3
Experimental Design and Setup
3.1 Introduction
First, the designs of MEMS-based characterization structures are demonstrated. These
structures are widely used in this work. The advantage of using these designs is that it can be
integrated with the designer’s own chip design very conveniently without taking up too much
space. Then the experimental setup and the equations that are used to characterize the gauge
factor (GF) of piezoresisive poly-C film are discussed. Finally the design and characterization of
the circuits that are used to measure the frequency response of piezoresistive RFMEMS
resonators in Chapter 5 are discussed.

3.2 Design of MEMS-Based Characterization Structures
3.2.1

Electrical resistivity characterization of doped poly-C

Four-point probe MEMS structure
The purpose of the 4-point probe is to measure the resistivity of any semiconductor material
to exclude the influence from contact resistivity. A micro-fabricated structure with a four-point
probe configuration is used to characterize the electrical resistivity of doped poly-C in this work
as shown by the schematic diagram in Figure 3-1. Doped poly-C resistors were fabricated on an
insulating layer (thermal SiO2 or undoped poly-C) with a length of 410 μm and a width of 20, 40
and 60 μm first and then metal interconnects (either Cr/Au 50/500 nm or Ti/Au 50/500 nm) were
patterned in a four-point probe measurement setup. A current of 10 μA was applied to the two
31

bottom electrodes, while the voltage drop was measured across the two top electrodes. With 10
μA applied current, the resistive heating is less than 0.35 mW, which leads to a negligible
temperature rise. The thickness of the doped poly-C resistor was measured by a Dektak 6M
surface profilometer. Current is driven through the outer two electrodes while voltage is sensed
on the inner electrodes. The resistivity is calculated based on the current source and sensed
voltage as expressed in equation (3.1).



V wt

I L

(3.1)

where I is the applied current, V is the voltage drop, and L, w and t are the length, width and
thickness of the doped poly-C resistor, respectively.

Figure 3-1 Cross-sectional view of micro-fabricated 4-point probe structure.
Van der Pauw MEMS Structure
A micro-fabricated Van Der Pauw structure is also used to measure the electrical resistivity of
doped poly-C. The top view is shown in Figure 3-2 with the same cross-sectional view as in
2

Figure 3-1. The square membrane is made of doped poly-C with a size of 400x400 μm . First 10
μA of current was applied to two contacts, while the voltage drop was measured across the two
32

remaining metal pads. The same procedure was then repeated with perpendicularly oriented
contacts as well as by switching measurement polarity. When the structure is symmetrical, the
electrical resistivity can be expressed by equation (3.2).



 t Vh I h  Vv I v
ln 2



(3.2)

2

Figure 3-2 Top view of micro-fabricated Van der Pauw structure.

3.2.2

Contact resistivity characterization

Kelvin bridge MEMS structure
A micro-fabricated Kelvin bridge structure is used to characterize the contact resistivity as
shown by Figure 3-3. A given current I flows from the metal layer to the doped poly-C layer
through the contact area. The generated voltage difference V is created between the other two
electrodes. The contact resistivity can be calculated by

33

C  RC  A  V I   A

(3.3)
2

2

where ρc is the contact resistivity in Ω·cm , Rc (Ω) is the contact resistance, and A (cm ) is the
size of the contact area. Several values of contact areas used in this work are 100, 400, 900, 1600,
-8

2

2500 and 10000 (10 cm ).

Figure 3-3 Schematic diagram of micro-fabricated Kelvin bridge structure used to measure the
contact resistivity between doped poly-C (p poly-C) and metal with or without highly doped
interlayer (p+ poly-C).

34

transmission line model MEMS structure
A transmission line model (TLM) was also used to study the contact resistivity as shown in
Figure 3-4. Metal contacts to the mesa of doped poly-C were fabricated on top of the insulating
layer which could be SiO2 or undoped poly-C. The total resistance value RT (obtained by V/I )
can be expressed by [161]

RT  RS

d  2 LT
Z

(3.4)

for rectangular geometry, where RS is the sheet resistance of the semiconductor layer and d is the
spacing. d has values of 6, 10, 15, 25, 30 and 35 μm. Z is the length of the rectangular pads, i.e.
contact length. In this study, the contact length was designed for four series, e.g. 150, 300, 450
and 600 μm, in order to compare the effect of contact length on contact resistivity. The transfer
length LT is the averaged portion of the contact width H (H=200 μm in this study) which current
actually flows and can be expressed by

LT  c RS

(3.5)
2

where ρc is contact resistivity in Ω·cm . Fitting the RT value of different spacing d to the
appropriate formula (equation (3.4)) yields the free parameters RS and LT, and ρc can be
calculated from equation (3.5).

35

Figure 3-4 Schematic diagram of micro-fabricated structure of linear Transmission line model
(TLM) pattern used to characterize the contact resistivity of doped poly-C

36

3.2.3

Electrical resistivity characterization of undoped poly-C

Figure 3-5 A MEMS design used to characterize the electrical resistivity of undoped poly-C

Typically, the four-point probe resistivity arrangement, based on four needle-like contacts
2

2

having a very small contact area (4 x 10 µm ), is not usable for resistivity measurements on
undoped poly-C because the measured current is too low to be measured. In the present study,
using a special design as shown in Figure 3-5, the contact area for each of the four metal contacts
7

2

was increased to 6.58 x 10 µm , and the inter-probe spacing was decreased to 4 µm. The five
orders of magnitude increase in contact area and the small length to width ratio of the resistor
makes it possible to measure the high resistivity of undoped poly-C up to 10

17

Ω·cm when a

Semiconductor Analyzer (Agilent 6514C) is used. In addition, convenient measurement can even
37

be performed only using the inner two electrodes. For example, when the resistivity of a 1-µmthick undoped poly-C film is 10

12

Ω·cm, a resistance of approximately 24 MΩ would be

expected, which is well below the 200 MΩ measurement range of a typical hand-held digital
multimeter (MASTECH MY-65).

3.2.4

Residual stress characterization

Figure 3-6 Micro strain gauge used to measure the internal stress in poly-C thin film. Test beam,
slope beam and indicator beam are released.

A micro strain gauge (from [162]) is used to evaluate the internal film stress as shown in
Figure 3-6. Design values are given in Table 3-1. The design utilizes a lever arm structure to
mechanically amplify strains caused by stress. Residual stress, either tensile or compressive,
generates a small displacement δtb in the “test beam” (beam elongates or contracts). This
displacement in turn generates an angular deflection along the “slope beam”, which then
generates a much larger displacement δv at the tip of the indicator beam that can be read visually
via optical microscope. Via this structure, the strain in the “test beam” is effectively amplified to
38

a larger displacement. The mechanical gain factor is given by: δv/δtb=3Lib/(2Lsb)=37.5. The
residual stress can be represented in equation (3.6).

0 

 tb E

(3.6)

37.5  Ltb

Table 3-1 Design values of micro strain gauge
Value
Symbol

Value

Element

Symbol

Element

(μm)

Ltb
Lsb

Lib
h

(μm)

500

Wtb

20

Length of the test beam

Wsb

Length of the slope
beam
Length of the indicator

Wib

500
beam
Thickness of thin film

~3

39

Width of the test beam

30

Width of the slope
3
beam
Width of the indicator
3
beam

3.3 Measurement of Gauge Factor by Beam Bending Method

Figure 3-7 Schematic diagram of the measurement setup to characterize the gauge factor (GF)

The measurement setup to characterize the GF of doped poly-C is shown in Figure 3-7. An
undoped poly-C cantilever with doped poly-C at the anchor end was micro-fabricated. The tip of
the

cantilever

beam

was

pressed

down

using

a

micromanipulator.

Another

two

micromanipulators were located on the two metal contacts at the two ends of doped poly-C in
order to read the change of the resistance by multi-meter by every certain amount of change of
the tip deflection. A Labview program is made to facilitate the measurement which is discussed
in APPENDIX F.
The GF can be computed using the equation below.

GF 

R R



;

3h 
l 
L  1 
3
2
2L 

(3.7)

40

where ΔR/R is the relative resistance change of doped poly-C resistor, ε is the longitudinal
average strain in doped poly-C, L and h are the length and thickness of undoped poly-C
cantilever, respectively. l1 is the length of doped poly-C resistor and δ is the tip deflection of the
cantilever. In equation (3.7), the thickness of doped poly-C resistor is assumed to be zero and its
effect on the cantilever deformation is also ignored.

3.4 Piezoresistive Resonator Testing
In order to achieve the frequency response of the piezoresistive resonator in the study
presented in Chapter 5, a piezoelectric actuation setup, a driver circuit to actuate the piezoelectric
sheet and a detection circuit to detect the piezoresistive response are needed.

3.4.1 Piezoelectric actuation setup

Figure 3-8 Schematic diagram of testing sample mounted on the piezoelectric actuator

41

The schematic for the testing sample mounted on the piezoelectric actuator is shown in Figure
3-8. A single-layer piezoceramic sheet (PSI-5A4E, from Piezo System, INC.) with a thickness of
0.267 mm is used as piezoelectric actuator. For piezoelectric material, if a voltage is applied
across it, the electrical field that developed would cause it to deform. For the rectangular
piezoelectric actuator used in this study as shown in Figure 3-9, when a voltage is applied
longitudinally, a displacement ΔL occurs and can be closely approximated by:

L    L0   E  d33  L0  V  d33

(3.8)

where ΔL is the change in length (m), ε is the strain, E is the electrical field strength (V/m) and

L0 is the length of actuator (m). The term d33 is the piezoelectric coefficient (m/V), where the
first subscript identifies the axis of the field and the second subscript identifies the axis of
displacement. In order to generate a large enough vibration amplitude, a high voltage has to be
applied.

Figure 3-9 Schematic diagram of actuation of piezoelectric actuator

The use of the spacers underneath the piezoelectric sheet is optional; they are included here to
decrease energy loss due to ultrasonic radiation to the board underneath. Silver paint was used to
bond the spacer to the piezoceramic sheet since the interface between the two sheets requires
electrical conduction. Super glue was used to bond the actuator to the silicon die and also hold
42

the spacers in place on the board. It is important to ground the Si substrate. Since the actuator can
be regarded as a single capacitor (~10 nF), when it is driven by an AC voltage, charge and
discharge of the capacitor occur. This effect can induce unwanted current in signal transmission
lines on the testing sample. With Si substrate grounded, the induced current can be greatly
reduced.

3.4.2

Driver circuit for piezoelectric sheet

Figure 3-10 Schematic diagram of the driving circuit

When the actuator is driven by a periodic voltage source whose frequency is below or above
the resonant frequency of the actuator, it can be modeled by a single capacitor. Therefore, the
impedance presented to the driving source Zload is, to a good approximation, simply:

Z load  1  2π  f  CPA 

(3.9)

43

where f is the frequency of driving source and CPA is the equivalent capacitance of the
piezoelectric actuator. CPA of the piezoelectric sheet used in this work is 10 nF. A high voltage
results in high current due to the relative large capacitance of the actuator. Since the actuator is
driven at high voltage (targeted peak-to-peak VAC is 48 Volts) with relatively high frequency
(10-100 kHz), the driving circuit must allow high speed or high slew rate. However, to combine
the high power and high speed, the choices of suitable amplifiers with cost less than $100 shrink
very fast. A power amplifier PA78 together with its evaluation board EK60 is used in this work
to build the inexpensive driver circuit as shown by the schematic diagram in Figure 3-10. The
circuit can deliver current up to 10 A with a slew rate of 300 V/μs where the current is limited by
the two power MOSFETs at the output and the slew rate is controlled by the power amplifier
PA78. The driving circuit was characterized when the input is a sine wave or square wave as
shown in Figure 3-11. A square wave is used to simulate the high-frequency pulses generated by
the microcontroller. The circuit shows a gain of 17.5-18 dB at 40 kHz. The gain keeps constant
when the driving frequency is less than 100 kHz.

44

Figure 3-11 Characterization of the driving circuit by oscilloscope

45

3.4.3

Piezoresistive detection circuit

Figure 3-12 Schematic diagram of piezoresistive detection circuit

Figure 3-12 illustrates the circuit design and configuration for measuring the frequency
response of a resonator using piezoresistive detection. A Wheastone bridge is used to convert the
change of the resistance in the piezoresistors to a small AC voltage. RPZR in the Wheastone
bridge is the total resistance of a piezoresistive sensor on the testing sample whose resistance
changes when the resonator is vibrating. R1 and R2 are the variable resistors that are used to null
the output voltage from the Wheastone bridge (VA-VB) when resonator is not vibrating. RREF is
the reference resistor that is identical in dimensions and normal resistance to RPZR, but is located
on the substrate to cancel out the voltage due to the RPZR. RREF also provides a first-order
cancellation of thermal and electromagnetic disturbances that may occur to RPZR (assuming

RPZR and RREF experience the same disturbance). The Wheastone bridge is used since this

46

configuration has maximum sensitivity of the output voltage, VA-VB, over the varied resistance.
A constant 24 Vdc voltage, achieved by voltage regulation, is used as a stable and low-noise
input source for the piezoresistive sensor. When RPZR / R1 = RREF / R2, the output voltage from
the Wheastone bridge can be expressed in (3.11)

VA  VB 

 RPZR

R  R2
Vin
 R1    RREF  R2 

(3.10)

where ΔR is the resistance change of the piezoresistor when resonator is vibrating. When RPZR =

RREF = R, equation (3.10) can be simplified as given below

VA  VB 

R
Vin
4R

(3.11)

Since Vin = 24 V, VA-VB ≈ 6ΔR/R.

Figure 3-13 The frequency response of the detection circuit

47

An instrumentation amplifier (IN103) is used to amplify the small AC signal with a gain of 40
dB. Next, two active filters are used to achieve a fourth order Butterworth high pass filter with
corner frequency of 9 kHz and gain of 0 dB. The Butterworth filter type is selected because it
provides the flattest pass-band response as compared to Bessel and Chebyshev filter types. The
low-noise opamp (OPA128) before the output is used to further amplify the signal with a gain of
20 dB. Since the total gain of the circuit after the Wheastone bridge in Figure 3-12 is 60 dB, the
output voltage from the circuit Vout is equal to 6000ΔR/R. The frequency response of the
detection circuit is characterized by plotting Vout as a function of frequency. A flat pass-band
between 10-100 kHz is clearly seen. The actual gain in the pass-band is 58.8 dB.

48

Chapter 4
Single-Material MEMS Poly-C Film Technologies
4.1

Introduction

In order to build single-material MEMS (SMM) structures and devices, the achievement of
MEMS-quality poly-C thin films is essential. Major challenges are addressed in this chapter
including the achievement of highly insulating poly-C thin film, conformal poly-C coating, dry
etching of poly-C and the high-resolution patterning of poly-C film with high surface roughness.
The electrical properties characterization and the residual stress characterization of poly-C thin
films are also discussed in this chapter to complete the study on poly-C film used in SMM.

4.2 Achievement of Highly Insulating Poly-C Thin Film
Insulating poly-C layers are important for SMM devices. When conducting transmission lines
are fabricated on top of undoped poly-C in SMM devices, crosstalk can be a problem depending
upon spacing between the neighboring lines. However, it is a challenge to achieve a high
6

electrical resistivity of undoped poly-C above 10 Ω·cm in a growth system which is also used
for doped poly-C due to unintentional boron doping. Since O2 can suppress the boron
incorporation into poly-C film as discussed in section 2.3, O2 is used in this work to increase the
3

9

resistivity of undoped poly-C from ~10 Ω·cm to above 10 Ω·cm.

49

4.2.1

Poly-C growth with the presence of O2

(a)

(b)

(c)

(d)

Figure 4-1 Characteristics of the poly-C film seeded by 5% DPLW spin method. (a) front surface
of the film; (b) side view of the film; (c) AFM image of the film; (d) Raman spectrum of the
film.

CVD poly-C films (MCD and NCD) require a pretreatment step to generate seeds (or nuclei)
on the substrate before growth begins. Ultra-high seeding density is required for highly
insulating thin film, because a low nucleation density or small variation in seeding density could
50

create a non-covered area in the poly-C thin film, such as pin-holes, leading to current leakage
through the non-covered area. The seeding procedure used to achieve ultra-high seeding density
of approximately 10

11

-2

cm is discussed in APPENDIX A.

Table 4-1 The processing parameters in various steps used in this study
Gas components

Microwave Power Pressure

Time

Step 1 H2

2.3 kW

10 Torr

10 min

Step 2 CH4:H2=3:200

2.3 kW

40 Torr

30 min

Step 3 O2:CH4:H2=0.5:2:200

2.3 kW

40 Torr

The processing parameters for various steps used for insulating poly-C growth in MPCVD are
given in Table 4-1. Etching in H2 plasma is performed first to remove silicon dioxide and also
impurities on the substrate and substrate holder. Relatively low pressure is applied here which
results in a larger plasma size. This step is crucial because it provides a well-defined and
reproducible substrate surface for poly-C growth. Also, a stable temperature is achieved across
the substrate surface, which can help the control of poly-C growth. Next, the sample is exposed
to a rich CH4 in H2 plasma in order to form SiC and a diamond nuclei layer. In this process,
diamond powders are chemically bonded to each other and are bonded to the silicon substrate. It
is also suggested that oxygen should be introduced only after the nucleation stage because the
addition of O2 can suppress diamond nucleation and result in a decrease of nucleation density
2

[163]. Finally, O2 is introduced to improve the diamond quality primarily by preferential sp
51

etching and the suppression of boron or silicon incorporation in the grown film. The CH4 to H2
ratio is also lowered relative to step 2 leading to higher diamond quality.
As-grown poly-C film was characterized by SEM, AFM (atomic force microscope) and
Raman spectroscopy (excitation wavelength of 532 nm, laser spot size of < 2 μm). The poly-C
film was grown on a substrate having a seeding density of 10

11

-2

cm . The film shows well-

faceted grains. The average grain size is about 0.7 μm as observed by SEM imaging, as shown in
Figure 4-1 (a). The thickness is inspected by taking an SEM image of the sample’s cross-section
as shown in the Figure 4-1 (b). As the ratio of O2/CH4 varies from 0-75%, the growth rate is
computed as the sample’s thickness divided by its growth time as shown in Figure 4-2. The
highest growth rate occurs when the O2 to CH4 ratio is 25%, and no poly-C is grown when the
O2 to CH4 ratio is higher than 75%. Measurement of surface roughness can be achieved by AFM
imaging as shown in Figure 4-1 (c). An average arithmetic surface roughness ranges from 30100 nm for a poly-C layer having a thickness of 2-6 μm. Raman spectroscopy is a well-known
method to investigate the diamond film quality. A sharp, narrow and high intensity peak
-1

observed at 1332.9-1333.5 cm
-1

around 1350 cm

-1

and 1560 cm

with an absence of appreciable amorphous carbon signature
indicates good quality of poly-C, as shown in Figure 4-1 (d)
-1

when the film’s thickness ranges from 2-6 μm. The peak shift away from 1332 cm

is most
-1

likely caused by the residual stress. The full width at half-maximum (FWHM) is 7.8-8.4 cm .
The narrow FWHM is comparable to the reported FWHM in [21], also indicating good poly-C
quality.
52

Figure 4-2 Growth rate of poly-C with the presence of O2 versus ratio of O2 to CH4.

4.2.2

Characterization of electrical resistivity

Throughout the study of films grown under different conditions, it was observed that different
processes performed before the undoped poly-C growth can affect the film resistivity. The
measured resistivities of samples A-D grown with/without O2 presence and with different
process histories prior to growth are given in Table 4-2. All measurements were performed after
the hydrogenated layer was removed by O2 plasma etching in a plasma asher as discussed in
APPENDIX C. It should be noted that only n-type Si wafers are used for undoped poly-C growth,
because it was suspected that boron can diffuse out of p-type Si wafers and unintentionally
incorporate into undoped poly-C films [157].

53

Table 4-2 The measured resistivities of samples A-D
Sample Process history of MPCVD chamber Sample growth with O2?
A

B

Doped poly-C growth

Resistivity ρ (Ω·cm)
2

3

4

5

5

6

9

11

No

No

10 – 10

Yes

10 – 10

Yes

Undoped poly-C growth without O2

10 - 10

10 – 10

for over 100 hours
C

D

Doped poly-C growth
Undoped poly-C growth with O2
for over 100 hours

54

Figure 4-3 I-V plot of a 0.68-μm-thick undoped poly-C grown with O2 present based on high
ultra-high nucleation density.

Figure 4-4 (a) SEM images of the fabricated structure with four metal lines in 4-point-probe
configuration and (b) measurement result.

The results in Table 4-2 indicate that undoped poly-C resistivity is relatively low if it is grown
after doped poly-C growth. This is due to the fact that boron can stick on the dome or the
substrate holder, and act as a dopant for the next undoped poly-C sample. It is also observed
55

from Table 4-2 that the addition of O2 during growth can increase the electrical resistivity of
undoped poly-C significantly. So in order to achieve a highly insulating poly-C layer, a sample
must be grown with O2 present after a long period of undoped poly-C growth with O2. A 0.68μm-thick undoped poly-C film based on ultra-high nucleation density is grown under this
condition. The measurement result is shown in Figure 4-3 with the measurement setup discussed
10

in APPENDIX D. This result corresponds to an electrical resistivity of ~2·10

Ω·cm. Such high

resistivity is not necessary for most SMM applications. The undoped poly-C layers used in this
work usually has a relatively lower resistivity. The resistivity is generally characterized using the
on-chip MEMS design discussed in section 3.2.3. The fabricated structure is shown in Figure 4-4
(a) and a typical I-V plot of 3-μm-thick undoped poly-C film is shown in Figure 4-4 (b)
9

corresponding to an electrical resistivity of ~10 Ω·cm.

4.3 Characterization of Semiconducting and Metallic Poly-C
To study the efficacy of poly-C as a semiconductor and as a conductor, in situ doping of polyC was achieved by introducing trimethylboron (B(CH3)3, TMB), diluted in hydrogen
(TMB/H2=0.098%), during the poly-C growth. The doping concentration is controlled by
changing the TMB ratio while keeping the ratio of CH4/H2 constant to make the poly-C
semiconducting or metallic.

56

4.3.1

Characterization of electrical resistivity

(a)

(b)
Figure 4-5 (a) Optical image and concept diagram of the sample for resistivity measurement (b)
resistivity of doped poly-C as a function of TMB ratio.

Doped poly-C resistivity as a function of TMB ratio is performed using a 4-point probe
configuration as demonstrated in section 3.2. Doped poly-C resistors were patterned on thermal
SiO2 with a length of 410 μm and a width of 20, 40 and 60 μm. Then Cr/Au interconnects were
57

patterned in a four-point probe measurement setup as shown in Figure 4-5 (a). Cr/Au is chosen
because it is resistant to hydrofluoric acid (HF), which is commonly used in SMM to remove
sacrificial SiO2 layers. The measurement of doped poly-C resistivity as a function of TMB ratio
(0-1 ppm) is shown in Figure 4-5 (b). 1000 ppm O2 is added to increase the electrical resistivity
by 10-100 times to allow for more resistivity control.

Table 4-3 Electrical resistivity, doping concentration for various TMB ratios
-3

TMB (ppm) Resistivity ρ (Ω·cm) Doping concentration NA (cm )
19

0.010

58.145

2.949x10

0.025

43.608

3.492 x10

0.050

21.823

4.045 x10

0.100

7.016

5.071x10

1.000

0.010

>1.7 x 10

19
19

19
20

The activation energy can be inferred by plotting the change of resistance of doped poly-C
resistors as a function of temperature. The temperature-control setup is demonstrated in
APPENDIX B. The resistance can be read directly from a digital multimeter. The resistance R is
dependent on the activation energy Ea (eV) and absolute temperature T (K) which is rewritten
from section 2.3 as shown below

58

R  C exp  Ea kT 

(4.1)

where Ea is the activation energy, k is the Boltzmann constant and T is temperature in K. Since
lnR = lnC - Ea/kT, a straight line in a plot of lnR vs. 1/T is expected. A linear fit was used to find
the slope, where slope = Ea/kT is the fit parameter. As a result, Ea can be calculated. Then, the
-3

doping concentration NA cm can be derived using equation (4.2) [164].

Ea  0.37  6.7  108 N 1 3 eV
A

(4.2)

59

Figure 4-6 Electrical resistances (kΩ) of a 600-800 nm thick boron-doped poly-C layer versus
reciprocal temperature (K).

60

Figure 4-7 (a) Kelvin Bridge Structure used to measure the contact resistivity between p poly-C
(piezoresistive layer) and metal with or without p+ poly-C interlayer; (b) Plot of contact
resistivity as a function of the resistivity of piezoresistive layer with the inset of I-V plot by
Semiconductor Analyzer indicating excellent ohmic contact.
61

The change of resistance of doped poly-C resistor versus reciprocal temperature was plotted
in Figure 4-6 for five typical TMB ratios. The sample was placed in a box during the
measurement to avoid light-induced photo-current. The activation energy Ea is computed using
the fit parameter of the slope. NA based on the known value of activation energy Ea is then
computed. NA’s for typical TMB ratios were listed in Table 4-3. The resistance of highly doped
poly-C remains nearly constant when the temperature increases, indicating zero activation energy
which corresponds to a doping concentration greater than 1.7 x 10

4.3.2

20

-3

cm .

Characterization of contact resistivity

Kelvin Bridge (KB) structures are used for contact resistance measurements between doped
poly-C and Cr/Au. The detection scheme is demonstrated in section 3.2.2. The fabricated
structure is shown in Figure 4-7 (a). The I-V curve was plotted with a semiconductor analyzer
(where a p+ poly-C interlayer was sandwiched in between the p (lightly-doped) poly-C layer)
indicating a good ohmic contact (even without annealing), as shown by the inset of Figure 4-7
(b). This is probably due to the partial carbide formation by heat treatment during sample
preparation and during the lift-off process [165], or the high roughness of the poly-C film [166].
The contact resistivity between p-type poly-C (piezoresistive layer) and Cr/Au with or without a

p+ poly-C interlayer as a function of the resistivity of p-type poly-C layer is plotted in Figure 4-7
(b). The result, as measured by a Kelvin Bridge structure, shows that the p+ poly-C interlayer
can reduce the contact resistivity to 30% -45% of the original values. This result is very
important for piezoresistive sensor designs.

62

4.4 Characterization of Residual Stress
The study of residual stress minimization is essential, because unacceptably high residual
stress may cause buckling, warping, or other damages in released SMM structures. Residual
stresses in thin films are typically measured by the curvature method, XRD-based methods and
Raman peak-shift methods [ 167 ]. Film stress can also be monitored and characterized by
micromechanical structures. The curvature method and micromechanical structures are used in
the current study.

4.4.1

Substrate curvature measurement

A general uni-axial residual stress field in a thin film can be represented by equation (4.3).

 total   0   1

y
h 2

(4.3)

where y   h 2, h 2 is the coordinate across the thickness h, with an origin chosen at the
film’s midplane. σ0 is the mean stress and σ1 is the stress gradient. The stress gradient is caused
by localized effects including atomic diffusion through h to the film/substrate interface,
interstitial or substitutional defects and atomic peening [168]. σ0 can be calculated based on the
Stoney’s equation as given in equation (4.4):

0 

2
Es d s
6 R(1  s )h

(4.4)

63

where Es, νs and ds are the Young’s modulus, Poisson ratio and thickness of the substrate, and h
is the thickness of the thin film. Values of σ0 are determined through measurement of R (radius
of curvature).
Three poly-C samples grown with CH4 to H2 ratio of 0.5%, 1% and 2%, were used for this
study to evaluate the stress range. All the samples were grown on a 2” circular silicon substrate
with an average thickness of 275 μm. A Dektak stylus profilometer was used to measure the
curvature of the wafer. Since all the samples have a concave top surface (indicating tensile
stress), the measurements were taken on the backside. A scan length of 3 cm was run along the
diameter of sample and 6 points were measured every 10 μm as shown in Figure 4-8 (a). The
sample with CH4 to H2 ratio of 1.0% is given as an example here. The data was smoothed in
Matlab, and subsequently fit to a quadratic as shown in Figure 4-8 (b). Equation (4.5) was then
used to calculate the curvature.

y ''  x 
1
 
R 
2 32
 y'  x  
1  
 



(4.5)

where κ is the curvature, R is the radius of curvatusre, x is the scan direction and y is normal
to the scan direction. Since bending is not perfectly circular, the curvature varies with x as shown
in Figure 4-8 (c). An average curvature was used to compute the residual stress. In this study, the
substrate thickness ds is 275 μm. Young’s modulus Es and Poisson ratio νs are 165 GPa and 0.27
for silicon, respectively. The film thickness h was measured by profilometer after poly-C was
patterned. Finally, the residual stress for these three samples were computed and given in
64

. All stresses are tensile and below 100 MPa. Stresses increase with CH4 concentration (become
more tensile when CH4 concentration is high). These stress values are smaller or similar to the
reported stress values of poly-C films used for MEMS. For example, NCD film has a stress of
180-390 MPa in [169], MCD has a stress approximately 500 MPa in [170] and UNCD has a
stress of 50-100 MPa in [171].

Table 4-4 Residual stresses for CH4/H2 of 0.5%, 1.0% and 2.0%
CH4/H2 Thickness h Curvature κ Curvature Radius R
-1

0.5%

4±0.4 μm

0.6535 m

1.0%

6±0.6 μm

1.3495 m

2.0%

6±0.8 μm

1.5329 m

-1
-1

Stress

Strain

153.0 cm

47±4.7 MPa ~0.00482%

74.1 cm

64±6.4 MPa ~0.00656%

65.2 cm

73±7.3 MPa ~0.00752%

65

Figure 4-8 (a) Raw data of curvature measurement, (b) quadratic fit of smoothed curvature data,
and (c) curvature variation with x position along the scan length.

66

4.4.2

Characterization using micro-mechanical structure

Figure 4-9 Reading from micro strain gauge indicating a tensile stress of 26-36 MPa

A MEMS structure is used to characterize the residual stress. Details of the structural design
are discussed in section 3.2.4. A poly-C film grown on a silicon substrate with a CH4/H2 ratio of
0.5% was used in this study. After poly-C patterning, the structure was released by isotropic
etching of silicon using a Xactix XeF2 tool. The released structure is shown in the optical image
in Figure 4-9. Due to the existence of out-of-plane stress gradients, the tip-position of the
indicator beam cannot be determined under optical microscopy because the microscope’s depth
of field is not sufficient to focus on the tip and the fingers (which are used to read the tip
movement) at the same time. As a result, the tip-position was inspected by SEM. The reading
shows a tensile stress of 26-36 MPa. This result is comparable to the stresses calculated by
curvature characterization. A more accurate reading would require a more sensitive structure,

67

which could be obtained by increasing the ratio of Lib to Lsb. The extremely small residual stress
of the poly-C films in this study guarantees the success of building multilayer SMM structures.
Micro-fabricated cantilever beam arrays were also used to inspect the stress gradient. Poly-C
used in this study includes undoped poly-C grown with or without O2 during growth, and also
highly doped poly-C. The SEM images of the released cantilever beams are shown in Figure
4-10. Undoped poly-C films grown without O2 on top of SiO2 generally have an almost zero
stress gradient as shown in Figure 4-10 (a). Beams made of undoped poly-C films grown with
O2 on top of SiO2 bend upward as shown in Figure 4-10 (b) with σ1 being positive in equation
(4.3). Higher O2 concentrations result in more significant bending. As a result, low O2
concentration, such as 1000 ppm, is usually used for released insulating poly-C structures as long
as the electrical resistivity of the insulating poly-C remains high enough for applications.
Cantilever beams made of highly doped poly-C films grown on top of SiO2 always stick to the
substrate after release with σ1 being positive in equation (4.3). The elucidation of the origins of
stress gradients requires further study. It should be noted that the downward bending in Figure
4-9 may be attributed to the poly-C growth on Si instead of on SiO2.

68

(a)

(b)

(c)
Figure 4-10 Stress gradients characterization using cantilever beam arrays. Beams are made of
(a) undoped poly-C grown without O2, (b) undoped poly-C grown with 2000 ppm O2 and (c)
highly doped poly-C with 1 ppm TMB.
69

4.5 Multilayer Single-Material MEMS Fabrication Technologies
4.5.1

Achievement of conformal poly-C coating

In order to build thin film structures, the film must be continuous everywhere. However,
problems exist with the step coverage of poly-C film grown by MPCVD due to low seeding
density. Step coverage for different spin speeds and various seeding concentrations in DPLW
was studied.
A 6-μm plasma-enhanced-chemical-vapor-deposition (PECVD) SiO2 was patterned to create
a large step height as shown in Figure 4-11 (a). Typical step heights encountered in surface
micromachining are usually less. It has been found that low seeding density will result in
discontinuous film along the line of intersection, even after an extremely long growth time.
Figure 4-11 (b) shows a sample that was grown for 15 hours with a growth rate of ~0.15 μm per
hour; holes still remain in the film, especially along the line of intersection. Even though these
holes can be eventually sealed in another 10-20 hours, the film is too thick for use in many
MEMS applications. Samples with different spin rates and various powder concentrations in
DPLW were studied. The continuity of samples was inspected by SEM after 15 hours of poly-C
growth. In this study, 15 hours of growth time is used because it will result in a thickness of 2.53 μm which is suitable for a structural material with both excellent flexibility and mechanical
strength. In addition, thicker film also brings difficulty to the dry etching. It was found that ultrahigh seeding density has to be used in order to generate continuous film within a proper growth
time. This can be achieved by either using DPLW with high concentration of diamond powders
or spinning DPLW at low spin rate. 5% DPLW spun at 2000 rpm can achieve ultra-high seeding
density of 10

11

-2

cm

without obvious powder aggregation. This condition will also result in
70

continuous film with the presence of step as shown in Figure 4-11 (c) with film thickness of 2.53.0 μm. The larger grain size along the line of intersection compared to other areas results from
the relatively lower seeding density.

Figure 4-11 (a) PECVD SiO2 was patterned to form a high step as shown by the optical image on
the right side. (b) Low seeding density results in a discontinuous film along the line of
intersection as shown by SEM images on the right side. (c) 5% DPLW spun at 2000 rpm results
in a continuous film with the absence of powder aggregation. (d) Powder aggregation when
powder concentration in DPLW is too high or spin rate is too low. (e) Film still has pinholes
after 15 hours growth caused by the relatively low seeding density when 5% DPLW is spun at
3000 rpm.
71

If the powder concentration is higher than 5% or the spin rate is lower than 2000 rpm,
significant powder aggregation will occur. For example, 8.43% DPLW spun at 2000 rpm and 0.5%
DPLW spun at 500 rpm can result in a continuous film in the presence of a step in a rather short
growth time, but the film also shows significant powder aggregation as shown in Figure 4-11 (d).
On the other hand, if the powder concentration is 5% with spin rate higher than 2000 rpm, it may
result in pinholes in poly-C film after 15 hours of growth. For example, Figure 4-11 (e) shows
the film grown for 15 hours with pinholes when 5% DPLW is spun at 3000 rpm. As a result, 5%
DPLW spun at 2000 rpm is an optimized seeding condition which can generate a continuous 2.5
or 3-μm-thick poly-C film over the step with a near absence of powder aggregation on the
sample.

4.5.2

Optimization of dry etching of poly-C

Recipes of RIE of poly-C films of different thicknesses are required to obtain smooth etched
surfaces and a satisfactory etch-rate. For the first time, RIE-related damage to underlying
substrate (specific to RIE of poly-C) was eliminated to optimize the technology for single- and
multi-material MEMS made from poly-C.
As discussed in chapter 2, O2 is generally used to etch diamond through the generation of CO
and CO2 byproducts. However, diamond columns may be formed unintentionally in O2 plasma.
Furthermore, these columns can even be formed without a pre-coated metal layer. Since the etchrate of a poly-C layer is determined by the etching of diamond columns, the existence of these
columns prevents the effective removal of poly-C. It was suggested that a small amount of CF4
in O2 can increase the etch rate by prohibiting the formation of diamond column structures [53].
72

However, fluorine (F) in plasma etching causes damage to Si substrates and other underlying
layers such as SiO2 or Si3N4 during the patterning of poly-C [55]. Such damage is inevitable,
because poly-C growth starts with diamond seeds and, thus, the initial layer (this thickness
depends on seeding density) of poly-C has holes between the growing seeds. The thickness of
11

this porous layer is 200-300 nm when ultra-high nucleation density of 10

-2

cm was used in our

study. When the poly-C etching process reaches this thickness, the etching of the underlying
substrate begins. This becomes a serious problem in poly-C MEMS fabrication, especially with
poly-C films grown with low nucleation density. The roughness of as-grown poly-C is another
factor to be considered during poly-C patterning. If the etch rate of poly-C is assumed to be
uniform all over the sample, the roughness of poly-C would be “transferred” to the Si substrate
or other underlying layers (such as SiO2 or Si3N4) and can even be amplified due to the likely
higher etch rate of the underlying layer as compared to that of poly-C in the same etching
environment.
RIE was carried out using a RIE etching system, Plasmatherm 790. Typical film thickness is
2-4 μm in this study. A 1.5-μm-thick Al layer, patterned through lift-off, was used as a masking
layer during RIE of Poly-C. Al is used as a mask because highest mask selectivity was measured
as compared to Ti, Au and SiO2. The measured selectivity of Al and poly-C is above 100 in O2
plasma at a pressure of 50 mTorr and RF power of 300 W. The selectivity reduces to
approximately 11 when a small amount of CF4 is present in plasma etching. SEM was used to
observe surface morphologies. Energy dispersive spectroscopy (EDS) was used to characterize
the etched surface. Etching depth was estimated from Dektak stylus profilometer measurements
and surface roughness was measured by an Atomic force microscope (AFM).
73

Figure 4-12 Perspective view of remaining diamond columns after 2-μm-thick poly-C film on
sample A was etched in O2 plasma; Inset is the close-up view of diamond columns. (b)
Perspective view of remaining poly-C after 3.5-μm-thick poly-C film was etched in O2 plasma;
Inset is the perspective view of the remaining porous poly-C layer which was broken
intentionally on sample B’. (c) Holes were created in Si by CF4 etching through the pores of
remaining porous poly-C on sample B’’. Inset is the close-up view of holes underneath the
porous poly-C layer. (d) EDS analysis of the etched surface on sample A and B indicating the
existence of Al.
74

Etched surfaces, as shown in Figure 4-12 (a-d) were used to demonstrate the problems
associated with the typically-used etching scheme from the literature, i.e. poly-C etching in O2
plasma with/without CF4 gas. Al masks were used in this study. Sample A with a poly-C
thickness of 2 μm and sample B with a poly-C thickness of 3.5 μm were prepared for this study.
After the removal of poly-C in O2 plasma on sample A, scattered diamond columns were
observed on the etched surface as shown by the SEM image in Figure 4-12 (a). On the other
hand, a thin poly-C layer was left after the poly-C on sample B was etched in O2 plasma as
shown in Figure 4-12 (b). Then, Sample B was cut into two pieces, B’ and B’’. The remaining
poly-C layer on sample B’ was broken intentionally to find the thickness of it. A thickness of 1
μm was determind as shown in the inset of Figure 4-12 (b). Before breaking the remaining polyC layer on sample B’’, CF4 gas was added to O2 plasma to continue etching for a time. Then, the
remaining layer was broken intentionally and holes in Si were observed as shown in Figure 4-12
(c). It is believed that CF4 passed through the pores of the remaining poly-C layer and etched the
underlying Si. Actually, both diamond columns on sample A and the remaining poly-C layer on
sample B were etched very slowly in O2 plasma. It is believed that the generation of diamond
columns and thin poly-C layers is caused by nano-masks, and these nano-masks impede the fast
removal of poly-C underneath. EDS analysis on sample A and B provides the evidence for the
existence of nano-masks as shown in Figure 4-12 (d). It indicates the presence of Al on the
etched surface, revealing that Al particles are redeposited on the etched poly-C surface, probably
in the form of Al2O3 serving as nano-masks. It is also speculated that the density of nano-masks
increases with time. Therefore, separate nano-masks begin connecting with each other after long
75

etching periods and result in a thin porous layer as shown in Figure 4-12 (b). It should be noted
that such a nano-masking layer also exists even when the sample does not have an Al masking
layer. The etching result is almost the same as those shown in Figure 4-12 (a) and (b). The origin
of the Al nano-masking layer is probably from Al emanating from the chamber itself—it has
already been pointed out that materials sputtered from the chamber itself can act as nano-masks
in [68]. CF4 can be added to O2 plasma to avoid the nano-masking. However, the underlying Si
surface can be greatly damaged due to the presence of fluorine (F) as indicated in Figure 4-12 (c).
The same problem occurs to the SiO2 and Si3N4.

Figure 4-13 Etch rate of poly-C and Al at various pressures in (a) O2 plasma and (b) CF4 plasma

76

Table 4-5 Three-step etching conditions of poly-C
Flow Rate

RF Power

Pressure

Etch Rate

Slope

(sccm)

(W)

(mTorr)

(nm/min)

Angle

Condition Gases

Step 1

CF4

5

300

50

~30

55˚

Step 2

O2

50

300

80

~12

~75˚

Step 3

H2

20

300

50

<5

>80˚

Figure 4-14 Schematic diagram of 3-step etching of poly-C

77

Due to the above problems with the typical poly-C etching recipes, a new three-step RIE of
poly-C films is used in this work in order to obtain smooth etched surfaces. Etching parameters
are given in Table 4-5. Pressures were selected to achieve the highest etch rate based on the
measurement results in Figure 4-13. Slope angles as inspected by SEM are also given. The
schematic diagram of 3-step etching is shown in Figure 4-14. In addition to the etched surface,
study of the side wall is also important for certain applications, such as AFM cantilever tip
production. As a result, the influence of side wall slope angle is also considered in Figure 4-14.
Slope angle is defined as shown in Figure 4-14 (a). It should be noted that slope angles of the
side wall in O2 and H2 plasmas is assumed to be 90˚. CF4 plasma is used in the first step to etch
the poly-C film without the formation of diamond columns until a thin continuous poly-C layer
remains as shown in Figure 4-14 (a). The slope angle is 55˚. Since pure O2 plasma effectively
does not attack the commonly used underlying layers (such as Si, SiO2 and Si3N4) as indicated
in Figure 4-12 (a) and (b), it is used next to continue etching the poly-C film accompanied by the
formation of diamond columns as shown in Figure 4-14 (b). It should be noted that the area
between the columns was etched faster than the columns themselves in O2 plasma as indicated
by the increasing height of diamond columns on the side wall in Figure 4-14 (c). Therefore, the
etch rate of the poly-C layer in O2 plasma is limited by the etch rate of the columns. Due to the
slow etch rate of these diamond columns, they are difficult to remove entirely in O2 plasma, but
the diamond columns on the etched surface are much shorter than those on the side wall.
Therefore, they can be effectively removed in H2 plasma as shown in Figure 4-14 (d). An ultrasmooth etched surface is achieved using this 3-step RIE process as shown in Figure 4-15 (a). The
78

root mean square roughness (RRMS) is less than 2 nm as measured by AFM. The use of H2 as the
termination step can also allow the etching time in CF4 plasma to be less strict. If the etching
time of CF4 plasma was not controlled very well, a relatively thick poly-C layer would remain
for O2 plasma etching. It is highly possible that a porous layer as in Figure 4-12 (b) would be
formed instead of the scattered short diamond columns as in Figure 4-12 (a). Such a relatively
thick porous layer will take a very long time to be remove in O2 plasma due to the low etch rate.
Although there is a fast removal rate of poly-C using the 3-step etching scheme, the residual
diamond columns on the side wall at the foot still remain even when diamond columns on other
areas are already removed in H2 plasma as shown in Figure 4-15 (b). This is mainly due to the
equivalent etch rate of diamond columns and the areas between columns on the side wall in H2
plasma. In order to avoid the formation of diamond columns on the side wall, more anisotropic
etching in all plasmas are required. Currently, a smooth side wall is achieved by using only O2
plasma and H2 plasma as shown in Figure 4-15 (c). 10 mTorr is used for O2 plasma etching
because it generates almost 90˚ slope angle. However, the removal of poly-C is not as fast as the
3-step etching scheme in Figure 4-14, especially when a porous poly-C layer remains after O2
plasma etching. As a result, further study of anisotropic etching in CF4 plasma is required to
achieve both a smooth etched surface and side wall.

79

Figure 4-15 (a) The SEM image of smooth etched surface using multi-step recipes in Table I; (b)
a dot of 5 μm diameter was designed to study the etching of side wall; (c) smooth side wall is
achieved when poly-C was etched in O2 plasma and H2 plasma at 10 mTorr.

4.5.3

Surface micromachining of poly-C structures and devices

In order to produce multilayer poly-C structures, a general surface micromachining process
for poly-C film must be developed. The process for fabricating released structures with one polyC layer is shown in Figure 4-16. The process is developed based on traditional Si-based surface
micromachining technology, and includes the following basic steps: (i) photolithography, metal
evaporation and lift-off processes are performed to pattern Al masking layers on top of poly-C;
(ii) The pattern of Al is transferred to poly-C by dry etching; (iii) Al is removed by Al Etchant
Type A, and a hydrofluoric acid (HF) wet etch is used to remove the sacrificial SiO2 layer,
leaving the poly-C film suspended above the substrate.
The smallest feature size of photolithographic patterning is limited by the surface roughness
ranging from 30-100 nm for the 2- to 6-μm-thick poly-C layers that are commonly used in SMM.
This is because radiation reflects off of a rough surface and travels back through the photoresist
interfering with the incoming waves during exposure leading to unwanted exposure in the
80

masked areas. The resolution is also limited by stress-induced wafer-warping. The wafer usually
shows concave shape with the center of wafer lower than the edge after poly-C is grown on top
of the substrate. Therefore, the mask cannot have hard contact with the center of the wafer during
exposure. This also leads to the unwanted exposure in the masked areas. The resolution of the
photolithographic patterning also propagates to poly-C patterning performed afterward. As a
result, photolithographic parameters must be optimized to achieve high-resolution patterning.
These parameters primarily include the type of photoresisit, photoresist spin rate, soft bake
temperature and time, rehydration time, exposure time and development time (refer to the
APPENDIX E for detailed parameters). By using an optimized photolithography recipe and liftoff process, a narrow Al layer can be patterned on a rough poly-C surface successfully as shown
in Figure 4-17. The released cantilever beams are shown in Figure 4-18 based on the surface
micromachining steps in Figure 4-16. A repeated sequence of procedure in Figure 4-16 can be
used to build multilayer SMM structures as shown in Figure 4-19.

Figure 4-16 Process of fabricating released structures with one poly-C layer

81

Figure 4-17 Left: Photoresist 9260 patterned on top of rough poly-C surface using optimized
photolithography parameters; Right: an Al ribbon was patterned successfully via lift-off step.

82

Figure 4-18 Released cantilevers using the surface micromachining techniques.

83

Figure 4-19 SEM images of various SMM structures made of poly-C.

84

(a)

(b)
Figure 4-20 Inspection of (a) lateral gap and (b) vertical gap using fabricated poly-C structures.
85

The primary purpose of fabricating testing structures such as those in Figure 4-18 and Figure
4-19 is to inspect the efficacy of surface micromachining techniques. Beams having a width of 3
μm can be fabricated with photolithography patterning as shown in Figure 4-18. The smallest
lateral gap that can be achieved by photolithography is also examined as shown Figure 4-20 (a),
which shows a 3 μm gap. Smaller features of about 2 μm can be achieved if the stress-induced
wafer-warping is less and the contact between the mask plate and wafer is more sound.
The vertical gap between the suspended poly-C layer and the underlying below it was
inspected and is detailed in Figure 4-20 (b). Flat and suspended beams having a uniform gap
above the underlying layer are ascribed to the relatively small tensile stress of the film. Also,
there appears to be bumps formed on the backside of the suspended beam. This is probably due
to etching of SiO2 through the gaps between diamond seeds at the early stage of poly-C growth.
The size of these bumps can be suppressed by using higher quality SiO2 as well as high seeding
density. Higher quality SiO2 can be achieved by annealing at high temperatures in N2.
In relation to how small features can be fabricated, recently, cantilever arrays having a
minimum width of 400 nm were fabricated as shown in Figure 4-21 with the help of e-beam
writing. This result contradicts the comment in [171] which mentions that such small features
cannot be patterned in microcrystalline diamond (MCD) due to the limitation imposed by the ~1
μm grain size. This shows that the device dimensions can be even smaller than the grain size.
Therefore it is possible to build a single-crystal sensor within one grain of poly-C.

86

Figure 4-21 poly-C beams with widths varying from 200 nm – 2 μm inspected by (a) SEM and
(b) optical microscopy.

4.6 Single-Material MEMS Examples
Based on the technologies demonstrated in this chapter, three single-material MEMS (SMM)
devices were proposed and fabricated successfully. They are SMM capacitive RFMEMS, SMM
integrated field emission devices (IFEDs) and SMM piezoresistive sensors. They have been
proven to be mechanically and electrically functional after preliminary testing, but further testing
is still needed for future study. They are not only all-diamond MEMS but also SMM, because an
undoped insulating poly-C layer and a highly conducting poly-C layer are used in all three
devices. In addition a semiconducting poly-C is also used in the SMM piezoresistive sensor. This
is the first time that the SMM concept is used for RFMEMS, IFEDs and piezoresistive sensors.
87

The advantage of using SMM has been discussed in chapter 1. Taking these three devices as
SMM examples, the efficacy of using the previously demonstrated technologies to build
functional SMM devices are demonstrated.

4.6.1

SMM capacitive RFMEMS

Figure 4-22 SEM images of fabricated capacitive SMM RFMEMS and the fabrication process
flow.

Poly-C is a promising material for RFMEMS resonators, because a resonator made of poly-C
will have a much higher resonant frequency than a silicon resonator having equivalent
dimensions due to its higher acoustic wave velocity. Poly-C cantilever beams have shown the
88

highest quality factor of cantilevers made of any polycrystalline material [172] at a resonant
frequency of 536 KHz. Furthermore, poly-C is more advantageous for high temperature
applications as compared to silicon.
A SMM capacitive RFMEMS was fabricated as shown in Figure 4-22. As it can be seen,
poly-C surface micromachining technology is similar to poly-Si technology in concept. A
released bridge resonator with spacing around 1.0 μm was fabricated. The bottom poly-C layer is
used as the I/O electrodes and interconnects. It consists of an undoped poly-C layer and a highly
doped poly-C layer which are grown in sequence in an MPCVD chamber on an n-type Si wafer
as shown in Figure 4-22 (a). N-doped Si wafer is used instead of p-doped to avoid the outdiffusion of boron from p-type Si when undoped poly-C groµµwth is under way. The undoped
9

poly-C layer serves as an insulation layer. High resistivities over 10 Ω·cm are achieved by
using O2 in the growth environment. Poly-C growth for another 4 hours creates a high resistance
between the highly doped poly-C and the Si substrate. Then 1 ppm TMB is used to grow the p+
-3

(highly doped) poly-C having a resistivity on the order of 10

Ω·cm. After poly-C growth the

poly-C is patterned by RIE. All these techniques have been illustrated in Chapter 4. Then a 1μm-thick sacrificial PECVD SiO2 layer is deposited and annealed in N2 at 750 °C. The SiO2
layer is patterned by RIE to create the anchor for the eventual resonators as shown in Figure 4-22
(b). The SiO2 layer supports the structural poly-C layer during deposition and patterning, and
defines the electrode-to-resonator gap spacing. A 2.3 μm-thick highly doped poly-C layer is then
grown and patterned by RIE, as shown in Figure 4-22 (c). In the final step of the process, the
wafer is dipped into a solution of hydrofluoric acid (HF) which etches away the sacrificial SiO2
89

layer. Since poly-C is chemically inert, even chemically-aggressive HF causes no damage to the
poly-C surface. A supercritical CO2 drying step is not necessary for poly-C beams. Free-standing
poly-C resonators are achieved after careful cleaning, as shown in Figure 4-22 (d).

4.6.2

SMM integrated field emission devices

The fabrication process shown in Figure 4-22 can also be applied to build IFEDs as shown in
Figure 4-23 (a). The built-in top anode is used to permit (a) lower operating voltage, (b) high
accuracy of anode to emitter spacing, and (c) well defined emitter area. This IFED has a welldefined small distance between the tips and the anode with very sharp tips of 200 nm in diameter,
created in O2 plasma by an inductively plasma etching tool (ICP) as shown in Figure 4-23 (b).
The field emission process was confirmed using a needle anode instead of the integrated top
poly-C anode. The measurement was taken at the pressure of ~10

-6

torr, as shown in Figure 4-24.

A high voltage supply (Keithley, model 248) applies a voltage between the tip and the sample,
and the resulting emission current is measured with a digital multimeter (Keithley, model 2010)
as the voltage across a 3.3 kΩ resistor. The F-N curve shows a non-linearity in the high anode
voltage region, which might be due to partial thermionic emission. The field at the onset of
-1

emission is in the range of 0.5 - 0.75 MVcm .

90

Figure 4-23 (a) The concept diagram and SEM picture of fabricated IFED; (b) diamond needles
created in O2 plasma by ICP.

91

Figure 4-24 Field emission measurement result with vacuum as insulator.

4.6.3

SMM pieozoresistive sensors

Figure 4-25 SEM images of SMM piezoresistive sensors.

92

This is the first reported application of piezoresistive sensors based on the SMM concept. The
SEM images of SMM cantilever-based piezoresistive sensors made of poly-C are shown in
Figure 4-25. The fabrication process is shown in Figure 4-26. Undoped poly-C with a resistivity
9

above 10 Ω·cm is used as both an insulating and structural material. Lightly-doped poly-C with
a resistivity of 22 Ω·cm was chosen for the piezoresistor with consideration of both the gauge
-3

factor (GF) and signal to noise ratio (SNR). Highly doped poly-C with a resistivity of 5x10

Ω·cm is used as interconnects and also the interlayer between metal and poly-C resistor for the
purpose of reducing the contact resistance. The SMM device is released by wet etching of silicon
in KOH etchant. Displacement sensitivity S was characterized by pressing down the cantilever
beam at its tip with a micro-manipulator which was explained in section 3.3. The resistance
change was measured for every 1 μm tip deflection and is plotted in Figure 4-27. S = 0.47
mΩ/Ω/μm for the long cantilever (l = 400 μm) and S = 0.66 mΩ/Ω/μm for the short cantilever (l
= 250 μm). Due to the small contact area in this design and a certain level of misalignment, the
contact resistance is about 3.7 MΩ which is comparable with the 5.2 MΩ of the piezoresistor.
Considering the contribution of contact resistance, the actual GF is 20-28. Another piezoresistive
sensing design has been developed for RFMEMS applications with larger contact area to reduce
the affect from the contact resistance. This will be discussed in Chapter 5.

93

Figure 4-26 Cross-sections of the fabrication sequence.

Figure 4-27 Relative change of resistance vs. cantilever tip deflection

94

Chapter 5
Single-Material MEMS Piezoresistive Resonators
for RFMEMS
5.1 Introduction
Usually the MEMS resonators in RFMEMS use capacitive transduction to excite and detect
the mechanical resonance. However these resonators typically exhibit high impedance at
resonance. The high impedance can be even further increased when the resonator is downscaled
in order to increase its resonance frequency. Additionally, approaches aimed at lowering the
impedance always increase manufacturing complexity. The transduction scheme of using
piezoresistive detection and piezoelectric actuation can be implemented using simple processing
and allows for resonators with low effective impedance by (a) adjusting the piezoresistor
dimensions and (b) increasing the mobility of holes by using intra-grain piezoresistors (which
also leads to a piezoresistive gauge factor of over 4000 [97]). Furthermore, this type of
transduction is insensitive to geometric scaling and allows for the realization of miniature high
frequency MEMS resonators and oscillators without performance reduction. It should also be
noted this scheme may also increase the quality factor of the resonator because there is no
damping due to the electrostatic force as in the case of capacitive transduction. In this chapter,
using piezoresistive detection and piezoelectric actuation, the fabrication and testing of
polycrystalline diamond based single-material MEMS (SMM) resonators is reported for the first
time.
95

5.2 Sensor Design

Figure 5-1 Schematic diagram of piezoresistive resonator.

Figure 5-1 shows the fabricated device with two piezoresistors connected in a half-bridge
configuration. The two piezoresistors, connected in series due to the presence of metal layer on
the front portion of the cantilever, are located near the anchor point. Since the resonant frequency
of resonator is proportional to √k, a high spring constant is desired for potential RFMEMS
applications. Considering the fabrication difficulties and the total size of the device, the
reasonable values used in this paper are: wPZR=20 μm, lPZR=20 μm, w=70 μm, l= 430 and 630
μm. In addition, values of t = 3 μm and tPZR = 0.6 μm are also used by taking into account the
sensitivity and also the poly-C etching and growth time. The values of device spring constant k,
96

and the average strain in piezoresistors εPZR are computed and listed in Table 5-1, by assuming

E is 1000 GPa. The calculation details are given in APPENDIX H.
Finite element analysis (FEA) software (Coventorware) was also used to derive the spring
constant of the cantilever and the average strain of the piezoresistive layer when l is 630 μm. The
stress distribution at F=1 μN is shown in Figure 5-2 (a). Then force F, with an increase of 1-μN
per step, is applied at the tip of cantilever. Tip deflection versus force is plotted in Figure 5-2 (b).
The spring constant k is calculated as k=dF/dz where dF is the change in force applied (1 μN)
and dz is the deflection in the cantilever measured at its tip. The results are given in Table II with
analytical results also provided for comparison. The average strain in piezoresistors versus force
is plotted in Figure 5-2 (c). The simulation results are fairly consistent with analytical results as
shown in Table 5-1. The values of k and εPZR are not significantly affected by the undercut.

Table 5-1 Analytical, simulation results of k and εPZR
Analytical Sim. no undercut Sim. 50 μm undercut

k N/m

3.054

3.045

3.028

εPZR μstrain

18.262

15.297

15.196

97

Figure 5-2 (a) stress contour of piezoresistive resonator, (b) tip deflection and (c) average strain
in piezoresistors vs. applied force at the tip.

98

5.3 Sensor Fabrication

Figure 5-3 Fabrication process flow of SMM piezoresistive resonator.

Figure 5-3 shows the main steps of the 3-mask fabrication process. 3-μm-thick undoped poly9

C with resistivity > 10 Ω·cm is used as the structural material for the resonator. A 0.6-μm-thick
boron-doped poly-C film with a resistivity of 9 Ω·cm is used as a piezoresistive layer. 50-nm-3

thick highly doped poly-C with a resistivity of 5×10

Ω·cm is used as an interlayer between the

piezoresistive layer and Ti/Au (50/400 nm) double layer to reduce the contact resistance. 50 nm
Ti, 500 nm Au and 700 nm Al layers were evaporated in sequence and patterned by a lift-off
99

technique using the first mask. Al is used as a masking layer to etch highly doped and
piezoresistive poly-C layers by RIE. The etching condition is 20 sccm CF4 at a pressure of 80
mTorr and a RF power of 300 W. The etching time is monitored by measuring the electrical
conductivity of remaining thin film. Then another 1000 nm Al is evaporated and patterned using
the second mask. Undoped poly-C is then etched in O2 plasma in an ICP tool using coil power of
1000 W, platen power of 100 W, pressure of 10 mTorr thus leaving a very smooth etched surface
and vertical sidewall. After removing the Al, the third mask is used to wet etch the Ti and Au.
Then Ti/Au is used as a mask layer to etch the highly doped poly-C in RIE leaving the
piezoresistive poly-C layer exposed. Since the highly doped poly-C is very thin, only a very
short etching time is needed. As a result, the Au layer is negligibly damaged which ensures the
successful wire bonding later on. Then the cantilever is released by isotropic Si etching in XeF2
gas. The SEM image of a fabricated piezoresistive resonator is shown in Figure 5-4 (a). Two
boron-doped rectangular piezoresistive sensing regions (labeled as PZR) are connected in series
by Ti/Au metal layers and form a piezoresistive sensor as shown in Figure 5-4 (b). This sensor is
located at the end of resonator. The undercut boundary can also be seen clearly. The highly
doped poly-C interlay can be seen clearly in between the Ti/Au and piezoresistive layer to reduce
the contact resistance as shown in Figure 5-4 (c).

100

Figure 5-4 SEM images of fabricated piezoresistive resonator

5.4 Sensor Characterization
5.4.1

Electrical conductivity and contact resistivity characterization

A micro-fabricated Van Der Pauw structure is also used to measure the electrical resistivity of
doped poly-C as shown in Figure 5-5. The explanation of this structure is provided in section
3.2.1. Equation (3.2) is used to calculate the resistivity. The resistivity as a function of
temperature is also measured by using the temperature control system explained in APPENDIX
B. The result is shown in Figure 5-6. It shows a resistivity of 9 Ω·cm at room temperature and
101

decreases to ~1 Ω·cm at 200 ˚C, indicating an activation energy of 0.145 eV and a doping
19

concentration of 3.8·10

-3

cm . Based on this measurement result, the resistance of a

piezoresistive sensor can be calculated based on its actual dimensions inspected under SEM. The
calculated resistance is 976±156 kΩ at room temperature. The error is caused by the rough
estimation of the length and the thickness of the piezoresistive sensor. The actual measured
resistance of the piezoresistive sensor is 1088±26 kΩ based on two-terminal measurement. The
measured result is fairly consistent with the calculated result. The measured resistance decreases
to ~120±4 kΩ at 200 ˚C.

Figure 5-5 SEM images of fabricated Van Der Pauw structure used to measure the electrical
resistivity of doped poly-C.

102

Figure 5-6 Electrical resistivity of piezoresistive layer versus temperature.

103

Figure 5-7 (a) SEM image of transmission line model used to characterize the contact resistance
(b) The total resistance between two neighboring metal pads as a function of spacing between
pads is plotted for four different contact lengths.

The contact resistivity is characterized using a transmission line model which is explained in
section 3.2.2. The fabricated structure is shown in Figure 5-7 (a). The total resistance between
104

two neighboring metal pads as a function of spacing between pads is plotted in Figure 5-7 (b) for
four different contact lengths. Equations (3.4) and (3.5) are used to calculate the contact
-3

resistivity and transfer length. Contact resistivity ρc is = 6.8x10

2

Ω·cm and transfer length LT

is about 3.3 μm. A contact resistance Rc of approximately 10 kΩ is calculated for a single
contact. Since there are four contacts for the pieziresistive sensor, the total contact resistance is
~40 kΩ. Therefore Rc is approximately 3.7% of the total resistance for the piezoresistive sensor
in an actual device. The contact resistance in this design is significantly reduced as compared to
the piezoresistive sensor design discussed in section 4.6.3.

105

5.4.2

Displacement sensitivity and gauge factor characterization

Figure 5-8 ΔR/R vs. (a) cantilever tip deflection and (b) microstrain in piezoresistive sensor

Displacement sensitivity S was characterized by pressing down the cantilever beam at its tip
with a micro-manipulator. The resistance change was measured per 5 μm tip deflection and is
plotted in Figure 5-8 (a). S = 0.1591 mΩ/Ω/μm for the long cantilever (l = 630 μm) and S =
0.3674 mΩ/Ω/μm for the short cantilever (l = 430 μm). GF can be derived as shown in Figure
106

5-8 (b) showing a GF of 9.2±0.5 when resistivity is about 9 Ω·cm. GF is small due to the high
doping concentration which is not necessarily disadvantageous in terms of signal-to-noise ratio.

5.5 Measurement of Frequency Response
The concept diagram of the measurement setup for achieving the frequency response of the
RFMEMS resonators using piezoresistive detection and piezoelectric actuation is shown in
Figure 5-9. Labview is used to control the function generator and provide an AC voltage at a
desired frequency to the driver circuit. The driver circuit is used to drive the piezoelectric
actuator to vibrate the resonator. The details of driver circuit have been given in section 3.4.2.
The mounting of the resonator sample on the piezoelectric actuator is also discussed in section
3.4.1. The piezoresistive sensor located at the anchor area of the resonator is used to detect the
vibration amplitude of the resonator. The response of the piezoresistive sensor is converted to a
small AC voltage by a Wheastone bridge structure. Then the AC voltage is amplified and filtered
by the detection circuit. The details of the detection circuit have been demonstrated in section
3.4.3. The output of the detection circuit is read by a digital multimeter and sent to Labview.
Labview is programmed to sweep the frequency in a desired frequency range. The details of
Labview programming are discussed in APPENDIX G. It is worth mentioning that the output
data sent to Labview at every driving frequency were repeated ten times and averaged in
Labview. This results in that the thermal noise power is reduced by 10 times. Flicker noise
cannot be reduced by averaging the data. However, it is proportional to the reciprocal of
frequency and mostly blocked by the high-pass filter on the board. The actual images of circuits
are shown in Figure 5-10. The detection circuit with a wire-bonded sample is shown at the
bottom left in Figure 5-10.
107

Figure 5-9 Concept diagram of measurement setup for piezoresistive detection and piezoelectric
actuation.

After frequency response data were collected by Labview, Lorentz curving fitting method was
used to generate the most accurate fit for the resonant curve of micromechanical resonators. The
equation for a Lorentz curve is given by [173]:

y

ymax
2


f
1  4Q   1
 f0 

2

(5.1)

where f0 is peak frequency at resonance and Q is the quality factor. f0 and Q can be extracted
from the fit.

108

Figure 5-10 Piezoresistive detection and piezoelectric actuation boards with wire-bonded sample
shown at the bottom left.

The measured frequency response for a resonator with length l = 430 μm, when the power
supply to Wheastone bridge is 24 V, is plotted in Figure 5-12 (a) showing a resonant frequency

f0 of 49.2 kHz and a quality factor Q of 197 in air. When the power supply of Wheastone bridge
is set to be zero, the output does not change with frequency as shown in Figure 5-12 (b). The
comparison of results in Figure 5-12 verifies that the peak observed in Figure 5-12 (a) is the
109

resonant peak of the resonator. The non-zero output in Figure 5-12 (b) is due to the induced
current in the circuit caused by the piezoelectric actuator.

(a)

(b)
Figure 5-11 The measured frequency response in air for resonator with length l = 430 μm when
power supplied to Wheastone bridge is (a) 24 V and (b) 0 V.

110

A more accurate estimation of Q was achieved by taking measurements with a smaller
frequency increment per step. The measured frequency response for resonator with length l =
630 μm is plotted in Figure 5-12 (a) showing f0 of 25.1 kHz and Q of 72 in air. The measured
frequency response for resonator with length l = 430 μm is plotted in Figure 5-12 (c) showing f0
of 49.7 kHz and Q of 142 in air. The variation of Q for the resonators with same dimensions may
be due to the measurement inaccuracy in Figure 5-12 (a). The harmornic analysis feature in
Coventorware was used to simulate the frequency response of the resonator. A harmonic
displacement with vibration amplitude of 100 nm is applied to the anchor of the resonator in
simulation to simulate the vibration of the piezoelectric actuator. The displacement of the
resonator tip is plotted as a function of the frequency to achieve the frequency response.
Simulation results are shown in Figure 5-12 (b) and (d). Q is also preset to be 100 (damping ratio
is 0.005), which is suitable for resonators with designed dimensions in this work [174]. As seen
from Figure 5-12, measurement results match the simulation results very well. In simulation,
Young's modulus of diamond is set to be 1000 GPa. The matching of the results also indicates
that the actual Young's modulus of the poly-C film used in this work is about 1000 GPa, which
agrees well with the previous published results in our group [175].

111

(a)

(b)

112

Figure 5-12 (cont’d)

(c)

(d)
Figure 5-12 (a) experimental and (b) simulation results of frequency response in air for resonator
with beam length l = 630 μm; (c) experimental and (d) simulation results with beam length l =
430 μm.
113

Chapter 6
Single-Material MEMS Thin Film Packaging
6.1 Introduction
Poly-C has emerged as a novel material for MEMS packaging. The unique chemical
properties of poly-C offer protection against corrosive environments; the mechanical strength
offers resistance to package deformation in thin film packaging applications. Poly-C’s electrical
properties make it a good candidate for use as both conducting and insulating materials. The high
thermal conductivity also makes it a good heat sink.
This work reports two designs of a SMM thin film packaging process, each including an
encapsulated SMM device. The first design uses boron-doped poly-C as electrical feedthroughs
which can be embedded into the undoped, electrically insulating poly-C package. Access ports
were opened along the package edge to release the thin film package and the device. Then,
additional poly-C growth was used to seal the access ports. The second design is based on the
concept of using porous diamond to release the structures from the top of the package, thereby
significantly reducing the release and sealing time of the package, without significantly affecting
the device. A preliminary test regarding the package’s fluidic hermeticity was performed to
demonstrate that the poly-C thin film package has good fluidic hermeticity in an acidic
environment. This is the first time that porous diamond created by RIE has been used in alldiamond thin film packaging including poly-C electrical feedthroughs, and an encapsulated
SMM device has been reported. This is also the most complicated poly-C structures fabricated
successfully.
114

6.2 Process Design and Fabrication
The concept diagram and SEM images of SMM thin film packaging using poly-C is shown
in Figure 6-1. It includes poly-C pads, poly-C feedthroughs, poly-C SMM device and poly-C
package on an insulating undoped poly-C layer. The fabrication process is given by Figure 6-2.
The fabrication process started with a thick, undoped insulating poly-C layer grown on a Si
substrate. 0.5-μm thick feedthroughs and contact pads were deposited and patterned using highly
boron-doped poly-C (Figure 6-2 (a)). Then SMM devices made of boron-doped poly-C were
fabricated using two masks (Figure 6-2 (b, c)). The sacrificial PECVD SiO2 for poly-C device is
~1 μm. The fluidic access ports for the removal of sacrificial layer are also created after these
two steps. Next, a second PECVD SiO2 layer was deposited and patterned to create the anchor
region for the packaging cap (Figure 6-2 (d)). A 4-μm-thick undoped poly-C layer was then
grown and patterned to form the thin film packaging cap (Figure 6-2 (e)). The nearly zero stress
in poly-C film and the good adhesion between poly-C and SiO2 guarantee the success of
fabricating such multilayer structures. The sample was released in concentrated HF and sealed in
a MPCVD system for an additional poly-C growth (Figure 6-2 (f) and (g)) with typical sealing
pressures of 40 Torr. Another mask is used to remove the undoped poly-C layer in the area
where the highly-doped-poly-C contacts pads are (Figure 6-2 (h)), which has not been
accomplished yet. Since the released structures consist of poly-C only, the thermal mismatch
stress is minimized. Furthermore, the feedthrough is also made of poly-C leading to perfect
sealing around it.

115

Figure 6-1 SMM package concept diagram and the SEM picture of the structures

The thickness of the second SiO2 sacrificial layer needs to be selected carefully to make sure
the package does not touch the underlying device due to the deflection of the package under the
pressure difference after the package is sealed. The deflection of the poly-C package can be
estimated using the pressure-deflection relationship given as:

p

3.41 th
a2



2.45Eth3
a4

(6.1)

where p is the pressure difference applied on the packaging; t, 2a and h are the thickness, width
and deflection of the packaging respectively; E is the Young’s Modulus of the packaging, which
is assumed to be 1000 GPa. Neglecting the residual stress (i.e. σ = 0) and assuming the pressure
inside the packaging is 0, it is estimated that a 4-μm-thick poly-C packaging that is 650 μm on a
116

side would deflect around 4.8 μm. Therefore 6-μm-thick PECVD SiO2 was used for the second
sacrificial layer between the SMM device and package.

117

Figure 6-2 (cont’d)

Figure 6-2 General fabrication process flow of SMM thin film package

118

The sealing time for the current design is reduced by a factor of 12 by reducing the height of
the fluidic access ports compared to the time in the previous work [158] [159]. The package
sealing is also enhanced because sealing occurs between two poly-C layers instead of between
silicon and a poly-C layer used in [158] [159].

6.3 Release and Sealing Using Porous Diamond

Figure 6-3 (a) Creation of porous diamond by RIE of diamond (b) Release the structure by using
HF to penetrate the porous diamond

The application of porous diamond techniques to the SMM thin film package in the second
design can further reduce the release time as well as the sealing time due to the small pore size.
As a result, the encapsulated SMM device is negligibly affected during the release and sealing
process. It is known that poly-C growth rate is usually very low under low pressure conditions.

119

However, the application of porous diamond can make sealing possible to use ECR diamond
growth at 10 mTorr [176].
The fabrication steps with the application of porous diamond are shown in Figure 6-3. Since
most of the steps are the same as those in Figure 6-2, Figure 6-3 only shows the fabrication steps
of porous diamond creation after the patterning of poly-C thin film packaging and the release of
the structure through porous diamond. Porous diamond is created in the region of the release
holes. It was created with O2 etching in RIE as illustrated in 4.5.1.

Figure 6-4 Poly-C package having porous diamond as release holes before being soaked in HF

120

Figure 6-5 Poly-C package having porous diamond as release holes after being soaked in HF for
10 minutes (top) and 30 minutes (bottom).

Figure 6-4 shows a poly-C package with release holes before HF etching while Figure 6-5
shows the SEM images after soaking in HF for 10 and 30 minutes. Some SiO2 remained after 10
minutes in HF indicating that HF can penetrate through the porous diamond to etch the
underlying SiO2. In contrast, the continuous poly-C layer can protect the etching of underlying
SiO2. The results also indicate the etch rate of SiO2 through the porous diamond is lower than
the normal SiO2 etch rate in HF. The time for the package sealing is reduced approximately by at

121

least a factor of 10 using porous diamond. As seen in Figure 6-6, the holes in porous diamond are
closed by additional poly-C growth.

Figure 6-6 Porous diamond grows into continuous poly-C film in MPCVD

6.4 Evaluation of Poly-C Hermeticity
An experiment was designed to test the hermeticity of a poly-C package in an acidic
environment, as shown in Figure 6-7. The poly-C cap is fabricated, leaving the 6-μm SiO2 inside
the package. The fabricated chip was then soaked in a concentrated hydrofluoric acid (HF)
solution for up to two days at room temperature. Samples were then broken intentionally to see if
the HF had penetrated the poly-C layer and etched the SiO2. Test chips with different thickness
of poly-C package were prepared by breaking the chips at various steps of the testing, showing
that a poly-C thin film with a thickness of 2.6 µm (corresponds to 17 hours growth time) cannot
prevent HF from penetrating it, as shown in Figure 6-7 (a). Thus, the SiO2 sealed inside the polyC package was etched away. However, the poly-C layer with a thickness of 2.9 µm (corresponds
122

to 19 hours growth time) shows good fluidic hermeticity in an acidic environment, as shown by
Figure 6-7 (b). It also indicates that the 4- µm-thick poly-C package that is generally used in this
study can provide excellent hemedicity.

Figure 6-7 Sample chips with poly-C layer of (a) 2.62 μm (b) 2.92 μm after 2 days soak in HF.

123

Chapter 7 System Integration

Figure 7-1 Design of system integration

Figure 7-2 Frequency response in air for a resonator with beam length of 430 μm measured using
the system integration setup in Figure 7-1.

124

MEMS system integration is becoming very complex because of the increased number of
components and requirements of system miniaturization. A schematic diagram for wireless
MEMS system integration is proposed in Figure 7-1. In this work, system integration is built to
achieve the frequency response of piezoresistive RFMEMS resonators. A Texas instruments
MSP430G series microcontroller is used to control the piezoelectric actuator and process the
piezoresistive sensing signal. For the actuation, a timer module of the microcontroller is
programmed to generate 50% duty cycle PWM (pulse-width modulation) signals on one of its
outputs. The PWM signal is then amplified by analog interface (discussed in section 3.4) to drive
the piezoelectric actuator. The piezoresistive sensing signal is amplified, filtered and rectified by
analog interface, and then read by the 10-bit Analog-to-Digital converter module integrated in
the microcontroller. Digital data is saved in its internal flash memory. In order to save more data,
a RAM module can be added to the system to increase the memory size. Instead of using a
wireless interface for communication with computers, C code in IAR Workbench, a developer
environment for the microcontroller, is used to save the digital data in the microcontroller’s flash
memory as a text file. Then a Python script is written to read the file and convert the data in the
text file into analog voltages. With the help of the Python script, a table of voltages versus
frequency can be created to achieve the frequency response of the system. The C code and
Python code are given in APPENDIX I. The frequency response in air of a resonator with beam
length of 430 μm is plotted in Figure 7-2. In order to achieve the resonant frequency f0 and
quality factor Q, Lorentz curving fitting method discussed in section 5.5 was used to generate the
most accurate fit for the resonant curve of micromechanical resonators. It shows f0 of 48.4 kHz
with Q of 122. This result is fairly consistent with the result reported in Chapter 5, indicating that
125

the system integration works as well as the previous measurement setup but with a more compact
setup.

126

Chapter 8
CONCLUSIONS AND FUTURE RESEARCH
8.1 Summary of Contributions
Fundamental research on SMM technology
Three crucial issues are addressed in this work for fabrication of SMM structures and devices.
They are reported and discussed for the first time. First, highly insulating poly-C thin film (<1
μm) with electrical resistivity higher than 10

10

Ω·cm is achieved in a reactor which is also used

to dope poly-C. Second, conformal coating is achieved, which ensures poly-C film is continuous
everywhere. Third, the dry etching of poly-C is optimized to achieve smooth etched surfaces as
well as smooth and vertical side walls after the removal of poly-C. MEMS structures used to
characterize poly-C electrical resistivity, contact resistivity and residual stress are also discussed.
They can be integrated with designer’s own chip design very conveniently without taking too
much space. This is very helpful for future SMM designers.

Enabling technology development for SMM structures and devices
Surface micromachining technology was developed for multilayer SMM structures and
devices. For the first time, SMM Piezoresistive RFMEMS resonators using highly doped poly-C
as interconnects and electrodes, semiconducting poly-C as piezoresistive layer, and undoped
poly-C as insulating layer are fabricated and tested. Two very important characteristics of
micromechanical resonators, resonant frequency and quality factor Q, are extracted from the plot
of measured frequency response. The determination of these two parameters can be used for the
127

mechanical filters in RFMEMS or the chemical sensor in BioMEMS in the future. Thin film
packaging with SMM devices encapsulated is also reported for the first time and presented as the
most complicated SMM structure. It shows excellent resistance to acidic corrosion environment.

8.2

Future Research

1. Develop the technology to smooth poly-C thin film. Film with low surface roughness can
ease the fabrication process. The performance of poly-C nano-devices can be improved with
low surface roughness. For example, the reduction of surface roughness can potentially
increase the quality factor of poly-C nano-resonators in high vacuum. Smooth poly-C film
can be achieved either by polishing the as-grown film or by using UNCD film for SMM
instead of MCD or NCD.
2. Explore highly sensitive SMM piezoresistive resonator using intra-grain piezoresistive
sensors. The gauge factor of doped poly-C is 4000 when the sensor is within a single grain
[97]. The intra-grain sensor can be patterned using e-beam lithography. This sensor can be
applied to chemical and gas analysis.
3. Explore the application of SMM in biosensors. The advantages of diamond in the field of
biosensors are enormous: chemical stability, biocompatibility, easy surface modification,
largest electrochemical potential window, etc.

128

APPENDICES

129

APPENDIX A

Figure A-1 Simplified Schematic of valence bands split-off model (a) no stress, (b) under tensile
stress, and (c) under compressive stress.

The piezoresistive property of semiconductor such as doped diamond under uniaxial stress
can be explained by the valence-bands split-off model, as shown in Figure A-1. The strain will
change the concentration of heavy and light holes, so the strain can change the conductivity of
the p-type semiconductor. The GF for single crystalline doped diamond was derived [177]

130

GF  


C  m

A  EeC
1 A  EeC

 kT  A  EeC  kT
1

C  m1 2  m1 2
l
h
12
h

 m1 2
l

 m
 m

12
l

 m1 2
h

12
l

 m1 2
h




tensile strain

(A.1)

compressive strain

where A and Ee are labeled in Figure A-1. mh and ml are the effective mass of heavy- and light
band hole. The suffix ‘h’ and ‘l’ represent heavy and light holes, respectively. C represents the
relative difference between the effective mass of heavy and light holes, in other words, they also
represent the relative difference between the effective mobility of heavy and light holes. For
diamond, C=-0.89 under tensile strain and C=0.89 under compressive strain. k (J/K) is
Boltzmann constant, T (K) is the absolute temperature. The great difference of the effective mass
between the heavy and light holes in diamond is one of the main factors causing the excellent
piezoresistive effect of diamond [177] as indicated by equation (A.1). Since GF is positive under
tensile, it indicates the resistivity increase under tensile stress. Likewise, the resistivity decreases
under compressive stress due to the negative value of GF.
In general, the polycrystalline materials consist of crystalline grains interconnected by grain
boundaries. Grains are considered as single crystals with a diamond lattice. The grain boundaries
(GBs) are composed of disordered atoms and contain a large number of defects and dangling
bonds which act as trapping states and/or segregation sites [178]. Trapping of holes creates a
potential barrier at the boundary and a depletion region into the grains. The film can be modeled
as grains of length Lg with boundary thickness δ and depletion width wg [179]. The fractional
change in resistivity per unit strain of the polycrystalline materials can be represented as [180]:

131

 1

 



Lg  (2 wd   )  g 1 (2 wd   ) b 1

g 
b 
Lg
Lg

(A.2)

where ρg and ρb are the resistivity of grain and grain boundary. Normally, it is believed that the
piezoresistive effect of the GBs is much lower than the grain crystal. The random orientations of
the GBs also lower the overall piezoresistive effect of grain. This is consistent with the reported
low GF of polycrystalline materials than that of their single crystal counterparts. The effect of
GBs on the gauge factor was studied in [98] and presented as below assuming there is no GF for
GBs:

 1
GF  
1


 GFg


(A.3)

where is α the ratio of GBs resistance to grains resistance along the conduction path when film
has no strain and GFg is the gauge factor of single-crystalline material. This explains why
polycrystalline film with high resistivity results in larger GF.
A more explicit equation of GF for polycrystalline material is given in [177]

GF   M  F  GFg  F  D  C 

1 dEe
kT d 

(A.4)

D is also constant relating to the difference of the effective mass of holes like C. D equals to -1
and 1 under tensile and compressive stress for diamond film, respectively. M and F are two
-6

constants related to the grain-boundary scattering. When the stain is very small (~ 10 ), M is
equal to 1 and 0<F<1. The last term on the right side of equation (A.4) is negative. Therefore,

GF is smaller than GFg, indicating the GF of polycrystalline material is smaller than that of
single crystalline material. Equation (A.4) also means if there is excellent piezoresistive effect in
132

the crystalline materials, the more significant piezoresistive effect will exist in the corresponding
polycrystalline material. As a result, poly-C is a good candidate for the application of
piezoresistive sensor. The reported GFs of poly-C in the past are given in Table A-1. They range
from 5-1000 at room temperature when poly-C resistivity is in the range of 0.01-300 Ω·cm.

Table A-1 Gauge factors of poly-C reported by different groups
Boron

GF

ρ
(Ω·cm)

Concentration

Deposition

Diamond/SiO2

HFCVD

16

4

MPCVD
C

2.5x10

116

18

DC plasma
Si
CVD

5-30

8

HFCVD

[87]

BCl3

[181]

B2O3 powder

[91]

boron powder

[182]

MPCVD

0.2

10

SiO2

Si

6-25

Ref.

-3

undoped poly5.4

Doping

(cm )
1.2x10

6

Substrate

2.2

67

[106]
solid wafer

225

100

Boron compound

300
undoped poly-

1000

300

MPCVD
C
133

B2H6

[93]
[94]

Table A-1 (cont’d)
7-9

0.1

70-75

100

0.1

1000

30-35

10

Si

Si
0.27

MPCVD

B(CH3)3

[96]

boron powder

[97]

undoped
poly-C

0.1

6.7/8.9*
~10

20

undoped
MPCVD

[98]

poly-C

25**
33

[95]

MPCVD
4000**

B(CH3)3 in ethanol

6

690
283*

MPCVD

3.15
SiO2

HFCVD

H3BO3 in methanol

[100]

and acetone

50

2.65

40

28

SiO2

MPCVD

B(CH3)3

[180]

8±0.5

0.01

SiO2

MPCVD

B(CH3)3

[102]

*intergrain piezoresistor
*intragrain piezoresistor

134

APPENDIX B

(a)

(b)
Figure B-1 (a) Schematic diagram of temperature control system and (b) sample mounting on the
heater.

135

A temperature control system was built to measure the resistance change of doped poly-C
resistor as a function of temperature as shown in Figure B-1 (a). It can control the temperature
from room temperature to 200 ˚C with a precision of ±1 ˚C. A rectangular Kapton insulated
2

flexible heater (rate temperature range -200 - 200 ˚C) with a power of 10 Watts per inch was
used. Two DC power supplies (Keithley E3612) with maximum output power of 120 W in total
are used to power the electric heater. By reading the heater temperature via a K-type thermal
couple, the temperature controller (CN7800) controls the mechanical relay to open and close.
When the heater temperature exceeds the setting value, the mechanical relay will be open. It
results in an open circuit and the temperature of the heater will decrease. A solid-state DC relay
is used to isolate the mechanical relay from the heating loop (the left side of the schematic
diagram) because the maximum current that can flow through the mechanical relay is much
lower than the current flowing though the electric heater.
The sample mounting method is shown in Figure B-1 (b). The electrical heater was put on the
wafer chuck of probe station with a quartz plate in between. Quartz plate is used for thermal
isolation due to its low thermal conductivity (1.3 W/(m·k)). Electric heater warps due to the
thermal expansion when temperature increases. As a result, a thin metal sheet was placed in
between the sample and the electric heater. Two clips are used to fix them. Two probes were
used to pick up the resistance which is read by a digital multimeter (Keithley 2010).

136

APPENDIX C

Figure C-1 Schematic diagram of seeding process (a) Si has hydrophobic surface after removing
the native oxide by diluted hydrofluoric acid (HF). Separate water drops can be observed after
taking the wafer out from deionized (DI) water. (b) Super-hydrophilic surface is achieved after
O2 plasma treatment. A thin water layer forms on the surface after taking the wafer out from DI
water. (c) Several drops of DPLW are put on silicon wafer immediately after O2 plasma
treatment. (d) Wafer is spun. Uniform and high seeding density is achieved across the wafer.

It is difficult to create highly insulating poly-C due to the reduction of layer thickness--a low
nucleation density or small variation in seeding density could create a non-covered area in poly137

C thin film, such as pin-holes, leading to current leakage through the non-covered area.
Therefore, in addition to the use of O2 during growth, another study is focused on achieving pinhole free diamond film having a layer thickness less than 1 μm.

(a)

(b)

(c)

(d)
9

-2

Figure C-2 For the case of using 0.5% DPLW, (a) seeding density of 10 cm was achieved and
(b) 8 hours are needed to form continuous poly-C thin film. For the case of using 5% DPLW, (c)
seeding density above 10

11

-2

cm

was achieved and (d) less than 20 minutes are needed to form

continuous film.
138

Undoped poly-C layers are always grown on top of a Si substrate, and are used as the bottom
(insulating) layer for SMM devices in this research work. Si is chosen as the base layer because
the adhesion force between poly-C and Si is generally stronger than that between poly-C and
SiO2 or poly-C and Si3N4 due to the formation of SiC at the interface between poly-C and Si.
Therefore, the study on nucleation density is only focused on undoped poly-C growth on Si
substrates.
The usual seeding procedure is shown in Figure C-1. An O2 plasma treatment in a plasma
asher at a power of 250 W and a pressure of 500 mTorr is used to convert the hydrophobic nature
of the Si surface to super-hydrophilic before DPLW is spun. Such a treatment can allow a more
uniform seeding density compared to the wet chemical treatment using ammonium hydroxide
(NH4OH) and peroxide (H2O2) [183] reported previously by our group. Furthermore, O2 plasma
treatment is more convenient and controllable, and it generally creates a cleaner surface by
removing organic residue.
Seeding density depends on the diamond powder concentration more than the spin rate.
Seeding density was studied for diamond powder concentrations varying from 0.5%-5% when
the spin rate is 2000 rpm. The seeding density was calculated using a scanning electron
microscope (SEM, Hitachi S-4700II). The seeding density, defined by the number of seeding
2

particles per cm , was calculated by a graphical counting method. Figure C-2 (a) shows the
seeding distribution on silicon with 0.5% DPLW (diamond powder concentration is 0.5%) with a
9

-2

seeding density of 10 cm . 8 hours growth time is needed to form a continuous thin film in this
case as indicated by Figure C-2 (b). Figure C-2 (c) shows an ultrahigh seeding density of

139

approximately 10

11

-2

cm , which was achieved using 5% DPLW. This result is two orders of

magnitude higher than that produced with 0.5% DPLW. Only 20 minutes growth time is needed
to form a continuous thin film in this case as indicated by Figure C-2 (d).
Other advantages of ultrahigh seeding density are (i) it ensures continuous poly-C step
coverage and (ii) it protects the underlying layer from being damaged by H2 plasma in a
MPCVD system prior to the formation of pin-hole free poly-C film.

140

APPENDIX D
A convenient way that is also used to characterize the electrical resistivity of undoped poly-C
films grown with or without O2 in the growth environment is shown in Figure D-1. Undoped
poly-C is grown on highly conductive n-type Si chip having areas of 1 cm x 1 cm. A Cr/Au
metal layer was deposited on top of undoped poly-C as a top electrode. Al was deposited on the
bottom of the Si substrate forming an ohmic contact. The chip was then glued to a metal plate
using silver conductive epoxy. An I-V plot can be achieved using the voltage reading in power
supply and the current reading in ampere meter.

Figure D-1 Measurement setup to characterize the electrical resistivity of undoped poly-C films.

141

APPENDIX E
Fabrication process flow of one released poly-C layer with SiO2 as sacrificial layer
I. Prepare Substrates
1) Material: 1.5”x1.5” square n-type Si wafers coated with SiO2 on top; vendor: WRS materials
2) Clean: Acetone for 20 seconds, IPA for 20 seconds and DI water for 1 minute
3) Blow dry
II. Seeding
1) Diluted hydrofluoric acid (HF:DI 1:100) etch for 5 seconds
Comment: to remove the native oxide
2) Blow dry
3) O2 plasma etching for 3 minutes:
Plasma asher: 250 W and 250 mTorr
Comment: oxidize Si surface
4) Spin 5% DPLW at 2000 rpm for 40 seconds.
Comment:
a) If there is a high step height, spinning at 500 rpm for 5 seconds is necessary to form
continuous poly-C film everywhere.
b) Ultrasonic bath must be used before spinning
c) Spinning must be done immediately after O2 plasma etching; otherwise the hydrophilic
property of oxidized Si surface would change.
III. Poly-C growth
1) Pump down MPCVD system for 30 minutes
2) Grow poly-C at ~750 ˚C
Power 2.3 kW, pressure 40 Torr.
Insulating poly-C: O2 : CH4: H2 = 0.5 : 2 : 200; Highly conducting poly-C: TMB 1 ppm
Semiconducting poly-C: TMB 0.01-0.1 ppm.
3) Vent system and load sample after system has been cooled down for 30 minutes
IV. Photolithography
1) Clean: Acetone for 20 seconds, IPA for 20 seconds and DI water for 1 minute
2) Blow dry
3) O2 plasma etching for 1 minute
Plasma asher: 100 W and 250 mTorr
Comment: to remove the surface conducting layer of poly-C
142

4) Prebake on hotplate to dehydrate the sample for 5 minutes at 115 ˚C
5) Spin HMDS at 4000 rpm for 15 seconds
6) Spin AZ9260 at 4000 rpm for 30 seconds
7) Soft bake on hotplate for 2.5 minutes at 115 ˚C
Comment: kimwipe must be used underneath the wafer
8) Rehydration for 15 minutes
Comment: to develop rate will be lower than normal if rehydration time is not sufficient.
9) Exposure
Tool: MA/BA6; hard contact, for 35 seconds
10) Develop
Developer AZ400 : DI = 1 : 4 for 1.5-2 minutes depending on the rehydration time.
11) Blow dry
V. Metal Lift-off
1) Descum in O2 plasma
Plasma asher: 120W 250 mT for 100 seconds
Comment: to remove the photoresist AZ9260 residuals.
2) Al deposition
Tool: Enerjet Evaporator
Comment: Al thickness depends on the thickness of poly-C.
The measured selectivity of Al and poly-C is above 100 in O2 plasma and 11 in CF4 plasma
in Plasmatherm RIE tool.
Selectivity of Al and poly-C is about 10 in O2 plasma in LAM9400 ICP tool.
3) Lift-off
a) Leave in acetone facing down for more than 3 hours
b) Put some acetone in acetone lift-off beaker. Put the wafer facing up.
Provide ultrasonic for 5 minutes; Drain the acetone.
c) Put some IPA in acetone lift-off beaker.
Provide ultrasonic for 5 minutes; Drain the IPA.
d) Squirt DI using the shower. Drain the DI water; repeat three times.
e) Rinse 2 minutes in DI water
f) Blow dry
VI. Dry etching of poly-C
1) Tool: Plasmatherm 790 or LAM9400
Comment: for poly-C film with thickness less than 2 μm, O2 plasma can be used in both
tools to achieve smooth etched surface after poly-C removal.
For poly-C film with thickness more than 2 μm, three-step etching must be used in
Plasmatherm 790. However, O2 plasma can still be used in LAM9400.
For efficient poly-C etching, three-step etching scheme (a sequential etching in CF4, O2 and
143

H2 plasma) should always be used if the requirement for aspect ratio is not critical
2) Al Etching
In Al etching type A.
Comment: Due to the Al2O3 on top, the etch rate of Al is not a constant. Putting the wafer in
the etchant more than 10 minutes usually can entirely remove the Al.
3) Rinse in DI for more than 10 minutes
Comment: Rinsing time must be longer than 10 minutes to ensure no residual remained.
4) Blow dry
VII. Release
1) Immerse the wafer in BHF until all devices are released.
Comment: vertical etch rate of thermal SiO2 is about 10 minutes per μm; vertical etch rate of
PECVD SiO2 is about 3 minutes per μm. Lateral etch rate is 5-6 times faster than the vertical
etch rate. Therefore, the release time can be estimated.
2) Drain BHF and Put DI water; Leave wafer in DI water for 5 minutes; Repeat 3 times
3) Drain DI water and put methanol; leave wafer in methanol for 10 minutes;
4) Put wafer in basket and put basket on hotplate at 115 ˚C. Wait till methanol is totally
evaporated.
Comment: methanol is used because its surface tension force is much less than DI water.

144

APPENDIX F

Figure F-1 Labview program used to measure the change of the resistance of piezoresistive
sensor as function of cantilever tip deflection.

The interface is shown in Figure F-1. It can fulfill the following functions:
1. Compute the mean value and the standard deviation of a set number of read points from
digital multimeter, and then plot these points on the left graph.
2. After clicking on “write to file” button, the mean value of resistance will be saved to the file
whose save path is defined by users. The point will be plotted on the right graph.
3. Every time after the tip moves by a certain of distance, “write to file” should be clicked.
After a number of clicks, the resistance change versus tip deflection is plotted. All data has
been saved.
4. Data can be cleared and the software is ready for next data collection
145

APPENDIX G

Figure G-1 Labview program used to measure frequency response

The interface of the program is shown in Figure G-1. The block diagram of the programming
is shown in Figure G-2.

Figure G-2 The block diagram of the programming

The program can set the low and high frequency range, voltage amplitude, and the number of
frequency steps. It can show the current frequency and voltage reading from the digital
multimeter. The read data from digital multimeter at every frequency point can be averaged in
order to reduce the thermal noise.
146

APPENDIX H
The top view and cross-sectional view of a piezoresistive resonator are shown in Figure H-1
with two short segments connected to the anchor. The cantilever has a spring constant k. f(x,y),
which is a geometric factor, represents the relative change of the spring constant due to the two
short segments as shown in equation (H.1), where k0 is the spring constant of a resonator with
length l, width w and thickness t + tPZR without the connecting segaments. E is the Young’s
modulus of poly-C. x= lPZR/l (0<x<1) and y= 2wPZR/w (0<y<1).

k k0  f ( x , y )

(H.1)

k0  Ew(t  t PZR )3 (4l 3 )

(H.2)





f  x, y   1 1  x3  3x2  3x 1 y  1 





(H.3)

The displacement sensitivity S is defined as the normalized change in the resistance with the
tip deflection of the cantilever δ equal to 1 μm, which is given in equation (H.4) and (H.5).

S

R R



 3t

 GF   PZR  GF   2  g ( x, y) 
 4l


g ( x, y )    2  x  y   f  x , y 



(H.4)

(H.5)

εPZR is the average strain in piezoresistors at δ = 1 μm. It should be noted that highly doped
poly-C and Ti/Au metal layers are not considered in the above and below analytical results.

147

Figure H-1 Top and cross-sectional view of piezoresistive resonator

Since the resonant frequency of resonator is proportional to √k, a high spring constant is
desired for potential RFMEMS applications. f and g have to be maximized to achieve a high
displacement sensitivity and spring constant. The plots of f(x,y) and g(x,y) as a function of x and

y are shown in Figure H-2 (a) and (b), respectively. It should be noted that in Figure H-2 (b), the
minimum y is set to be 0.1, because g(x,y) becomes infinite when y approaches zero.
Considering the fabrication difficulties and the total size of the device, the reasonable values
used in this paper are: wPZR=20 μm, lPZR=20 μm, w=70 μm, l= 430 and 630 μm. In addition,
values of t = 3 μm and tPZR = 0.6 μm are also used by taking into account of the sensitivity and

148

also the poly-C etching and growth time. The values of x, y, f(x,y), g(x,y), k, and εPZR are
computed and listed in Table H-1 by assuming E is 1000 GPa.

(a)

(b)

Figure H-2 (a) f(x,y) and (b) g(x,y) as a function of x and y

Table H-1 Values of x, y, f(x,y), g(x,y), k and εPZR.

l μm

x

y

f

g

k N/m εPZR μstrain

430

0.046 0.57 0.91 10.3

9.337

37.823

630

0.031 0.57 0.94

3.054

18.262

3.3

149

APPENDIX I
//C Code

#include "msp430g2231.h"
#include <stdio.h>
#include <math.h>

void WRT2MEM(unsigned int value, int addr);
void PWM(int freq); //PWM Signal Generator Function.

int main( void )
{
WDTCTL = WDTPW + WDTHOLD;
BCSCTL1 = CALBC1_16MHZ;
DCOCTL = CALDCO_16MHZ;
WDTCTL = WDTPW + WDTHOLD;
P1DIR |= 0x04;
P1SEL |= 0x04;
PWM(50);
_BIS_SR(CPUOFF);
__delay_cycles(90000000);

150

ADC10CTL0 = SREF_0 + ADC10SHT_0 + ADC10ON + ADC10IE;
ADC10CTL1 = INCH_7;
ADC10AE0 |= 0x80;
int addr = 0xF900;

for(int i=1; i<=500; i++){
ADC10CTL0 |= ENC + ADC10SC;
__bis_SR_register(CPUOFF + GIE);
unsigned int temp = ADC10MEM;
WRT2MEM(temp, addr);
addr +=2;
__delay_cycles(8);
}
return 0;
}

void WRT2MEM(unsigned int value, int addr){
FCTL2 = FWKEY + FSSEL0 + FN5 + FN4 + FN3 + FN2 + FN1;
unsigned int *MEM_PTR;

MEM_PTR = (unsigned int *)addr;

FCTL3 = FWKEY;
151

FCTL1 = FWKEY + BLKWRT + WRT;

*MEM_PTR = value;
FCTL1 = FWKEY;
FCTL3 = FWKEY + LOCK;
}

void PWM(int freq){
float step_dif = (1.0/(10.0*1000.0))-(1.0/(freq*1000.0));
float cycle = roundf(step_dif/0.00000004665);
unsigned long int icycle = (int)cycle;
unsigned long int count0 = 2144 - icycle;
unsigned long int count1 = count0/2;
CCR0 = count0;
CCTL1 = OUTMOD_7;
CCR1 = count1;
TACTL = TASSEL_2 + ID_0 + MC_1;
}
#pragma vector=ADC10_VECTOR
__interrupt void ADC10_ISR(void){
__bic_SR_register_on_exit(CPUOFF);
}

152

//Python code

from __future__ import division

def Convert2Volt(value,Vhi,Vlo):
result = float((int(value,16)/int(1023))*(Vhi-Vlo))
return (result)

fname = raw_input("File Name (*.txt) :")
Vhi = float(raw_input("High Converter Voltage:"))
Vlo = float(raw_input("Low Converter Voltage:"))
text = open(fname,'r')
start_addr = text.readline()
datalst=[]
null= 'ff'
data = 0
pickvolt = 0

print "\nStart Address:",start_addr

while True:
line = text.readline()
line = line.split(" ")
153

count = 8
mem = 0
if line[mem] == 'q':
break
while count > 0:
count = count-1
data += 1
if line[mem+1] == 'ff':
break
if mem != 14:
value = line[mem+1] + line[mem]
#print value
voltage = Convert2Volt(value,Vhi,Vlo)
datalst.append(voltage)
print 'Read Voltage %d : %.4f V'%(data,voltage)
mem = mem+2
else:
linelast = line[mem+1].split('\n')
value = linelast[0]+line[mem]
voltage = Convert2Volt(value,Vhi,Vlo)
datalst.append(voltage)
print 'Read Voltage %d : %.4f V'%(data,voltage)

154

for i in datalst:
if i > pickvolt:
pickvolt =i
else:
pass

print '\nPeak voltage: %.4fV\n'%pickvolt
print '***END'

155

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156

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