ADDITIVE MANUFACTURING FOR RF ELECTRONICS PACKAGING

By

Christopher Ryan Oakley

A DISSERTATION

Submitted to

Michigan State University

in partial fulllment of the requirements

for the degree of

Electrical Engineering  Doctor of Philosophy

2020

ABSTRACT

ADDITIVE MANUFACTURING FOR RF ELECTRONICS PACKAGING

By

Christopher Ryan Oakley

From the development of the rst wireless communications systems, there has been growing

demand for ever smaller, lighter weight, lower cost devices. Electronics packaging techniques

have evolved with this demand, with a variety of process to create compact, high functional

density systems. Recent developments in additive manufacturing technologies have enabled

the application of low cost, rapid fabrication techniques to the development of radio frequency

(RF) electronics. Using available direct-write technologies such as inkjet and aerosol jet

printing, a wide range of electronic components, from RF devices to sensors and antennas,

can be combined to form functional systems quickly and aordably.

The purpose of this thesis is to investigate the application of a variety of additive manu-

facturing process to the packaging of radio frequency electronics operating into the mm-wave

(30 GHz to 300 GHz) frequency range and beyond. Applications of aerosol jet printing for

fabrication of passive circuit components operating in the THz frequency regime have been

demonstrated, as well as limitations of this process, and potential improvements. A process

for rapid prototyping of RF circuits operating in the X-band (8 GHz to 12 GHz), com-

bining commercially available materials and packaged components, was developed. Wide-

bandwidth printed interconnections to devices have been demonstrated, enabling the pack-

aging of bare integrated circuits with very low loss, and low cost. Finally, a self-packaging

process combining multiple additive manufacturing techniques is demonstrated for fabrica-

tion of a Ku-band (12 GHz - 18 GHz) transmitter. These packaging techniques pave the way

for low cost fabrication of circuits and systems, while minimizing unwanted parasitic eects,

enabling ecient operation beyond 100 GHz.

Copyright by
CHRISTOPHER RYAN OAKLEY
2020

For my parents, Robert and Patricia.

iv

ACKNOWLEDGEMENTS

This work is funded by the Department of Energy's Kansas City National Security Campus,

operated by Honeywell Federal Manufacturing & Technologies, LLC under contract number

DE-NA0002839.

v

TABLE OF CONTENTS

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 Electronics Packaging Processes . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Additive Manufacturing Overview . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Additive Manufacturing for Electronics . . . . . . . . . . . . . . . . . . . . .
1.4 Electronics Packaging by Additive Manufacturing . . . . . . . . . . . . . . .
1.5 Dissertation Outline
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ix

1
1
4
7
10
12

CHAPTER 2 HIGH FREQUENCY APPLICATIONS OF AEROSOL JET PRINTING 14
14
16
17
19
23
25
27
30
34
35

2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Design and Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Printed Silver Characterization . . . . . . . . . . . . . . . . . . . . .
2.2.2 Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Polarizers
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Fabrication and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 Polarizers
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Electroless Plating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 3 RAPID PROTOTYPING OF RF OSCILLATOR WITH AEROSOL

JET PRINTING . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Passive Circuit Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Microstrip Transmission Line Simulation and Fabrication . . . . . . .
3.2.2 Microstrip Resonator Simulation and Fabrication . . . . . . . . . . .
3.3 X-Band Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Design and Simulation . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Measurement
3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 4 HYBRID ADDITIVE AND SUBTRACTIVE PROCESSES FOR

ELECTRONICS PACKAGING . . . . . . . . . . . . . . . . . . . . .
4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Design and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Measurement
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Rework of printed lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5 Diode eects of Ag on GaAs . . . . . . . . . . . . . . . . . . . . . . . . . . .

37
37
38
38
39
40
42
42
43
44

46
46
48
53
57
59

vi

4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

60

CHAPTER 5 ADDITIVELY MANUFACTURED SELF-PACKAGED KU-BAND

TRANSMITTER . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Design and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Measurement
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 6 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Technical Limitations of Additive Manufacturing . . . . . . . . . . . . . . . .
6.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67
67
68
72
77

80
80
81
82

84

vii

LIST OF TABLES

Table 2.1: 250 GHz band-stop lter dimensions.

. . . . . . . . . . . . . . . . . . . .

Table 2.2: 550 GHz band-stop lter dimensions.

. . . . . . . . . . . . . . . . . . . .

Table 2.3: 250 GHz band-pass lter dimensions.

. . . . . . . . . . . . . . . . . . . .

Table 2.4: Electroless copper plated 550 GHz band-stop lter dimensions.

. . . . . .

29

30

32

36

viii

LIST OF FIGURES

Figure 1.1: Typical packaged integrated circuit with bonding wires. . . . . . . . . .

Figure 1.2: Chip-on-board style construction with bonding wire. . . . . . . . . . . .

Figure 1.3:

Flip-chip style construction with solder balls. . . . . . . . . . . . . . . .

Figure 1.4:

Example of MCM construction incorporating both SoP and SiP processes.

Figure 1.5:

Example of chip-rst construction.

. . . . . . . . . . . . . . . . . . . .

Figure 1.6:

FDM printed piece with embedded metal [1]. . . . . . . . . . . . . . . .

Figure 1.7:

SLA printed part metalized using damascene-like process from [2]. . . .

Figure 1.8: Aerosol jet printed ll level sensor on bucket [3]. . . . . . . . . . . . . .

Figure 1.9: Aerosol jet printed chip-to-board connection over dielectric ramp [4]. . .

Figure 1.10: Additively manufactured packaging concept.

. . . . . . . . . . . . . . .

Figure 2.1: Aerosol jet system components. Left: ultrasonic atomizer. Right:

print head with nozzle.

. . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 2.2:

Pattern for printed silver conductivity measurement.

. . . . . . . . . .

1

2

3

4

4

7

8

10

11

12

16

18

Figure 2.3: Measured silver cross-sectional area of printed metal conductivity mea-

surement structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19

Figure 2.4:

Schematic representations of band-stop and band-pass lter unit cell.
A: lter line width, B: lter line length, C: lter unit cell width and
length.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 2.5:

Simulated transmission coecient of 250 GHz band-stop lters.

. . . .

Figure 2.6:

Simulated transmission coecient of 550 GHz band-stop lters.

. . . .

Figure 2.7:

Simulated transmission coecient of 550 GHz band-stop lters of dif-
ferent line widths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 2.8:

Simulated transmission coecient of 250 GHz band-pass lters.

. . . .

20

21

21

22

22

ix

Figure 2.9:

Schematic representations of polarizer unit cell. D: polarizer line
width, E: line separation, F: unit cell width and length. . . . . . . . . .

Figure 2.10: Simulated polarizer extinction ratios.

. . . . . . . . . . . . . . . . . . .

Figure 2.11: THz frequency domain measurement setup. . . . . . . . . . . . . . . . .

Figure 2.12: Unit cell of fabricated 250 GHz band-stop lters. Left: Copper metal

with lithographic process. Right: Aerosol jet printed.

. . . . . . . . . .

Figure 2.13: Measured transmission coecient of 250 GHz band-stop lters. . . . . .

Figure 2.14: Fabricated 550 GHz band-stop lter unit cells. Left: Copper metal

with lithographic process. Right: Aerosol jet printed.

. . . . . . . . . .

Figure 2.15: Measured transmission coecient of 550 GHz band-stop lters. . . . . .

Figure 2.16: Unit cell of fabricated 250 GHz band-pass lters. Left: Copper metal

with lithographic process. Right: Aerosol jet printed.

. . . . . . . . . .

Figure 2.17: Measured transmission coecient of 250 GHz band-pass lters. . . . . .

Figure 2.18: Unit cell of fabricated 40 µm polarizer grid. Left: Copper metal with

lithograph process. Right: Aerosol jet printed.

. . . . . . . . . . . . . .

Figure 2.19: Unit cell of fabricated 10 µm aerosol jet printed polarizer grid.

. . . . .

Figure 2.20: Simulated and measured polarizer rejection ratios. . . . . . . . . . . . .

Figure 2.21: Unit cell of 550 GHz band-stop lter after electroless copper plating. . .

24

25

27

28

28

29

30

31

31

33

33

34

35

Figure 2.22: Measured transmission coecient of 550 GHz band-stop lter, before

and after electroless copper plating.

. . . . . . . . . . . . . . . . . . . .

36

Figure 3.1:

Simulated vs measured insertion loss printed transmission line. Inset:
17 mm transmission line with GSG pads. . . . . . . . . . . . . . . . . .

Figure 3.2:

Simulated vs measured insertion loss of printed microstrip resonator.

.

Figure 3.3: A photograph of the printed microstrip resonator.

. . . . . . . . . . . .

Figure 3.4: A photograph of the printed oscillator circuit.

. . . . . . . . . . . . . .

39

41

41

43

Figure 3.5: The measured power spectrum of the printed oscillator fundamental

output frequency.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45

x

Figure 4.1:

Substrate preparation process. . . . . . . . . . . . . . . . . . . . . . . .

50

Figure 4.2:

Prolometer measurement of trenches in LCP substrate. Inset shows
cross-section representation of a lled trench. . . . . . . . . . . . . . . .

Figure 4.3:

Simulated insertion loss of printed interconnection over trench. . . . . .

Figure 4.4:

Fabricated interconnection over trench.

. . . . . . . . . . . . . . . . . .

Figure 4.5:

Insertion loss of interconnection over each trench, as calculated by
Equation 4.1.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 4.6:

Fabricated interconnection to 0 dB attenuator. . . . . . . . . . . . . . .

Figure 4.7:

Per-interconnection loss measured through 0 dB attenuator, as calcu-
lated by Equation 4.2.

. . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 4.8:

Fabricated connections to amplier MMIC. . . . . . . . . . . . . . . . .

Figure 4.9: Gain performance of amplier MMIC with printed connections.

. . . .

Figure 4.10: Amplier circuit input return loss. . . . . . . . . . . . . . . . . . . . . .

Figure 4.11: Amplier circuit output return loss. . . . . . . . . . . . . . . . . . . . .

51

52

54

55

56

57

58

59

60

61

Figure 4.12: Measured transmission coecient of 4.5 mm long transmission line

printed over a trench, before and after printing additional silver.

. . . .

62

Figure 4.13:

Image of die with cracking at Ag-GaAs interface before repair (left)
and after repair (right). . . . . . . . . . . . . . . . . . . . . . . . . . . .

63

Figure 4.14: Measured transmission coecient of attenuator circuit with cracked

interface before and after repair. . . . . . . . . . . . . . . . . . . . . . .

64

Figure 4.15: Measured insertion loss of interconnection to die after repair as calcu-

lated using equation 4.2, compared to one with no cracking.

. . . . . .

Figure 4.16: Measured I-V curve of Ag-GaAs interface on attenuator.

. . . . . . . .

Figure 4.17: Schematic representation of Ag-GaAs diode.

. . . . . . . . . . . . . . .

Figure 5.1:

Surface prole plot of PMMA before and after thermal leveling.

. . . .

Figure 5.2:

Surface prole plot of a transition from an attenuator to the surround-
ing epoxy resin, before and after PMMA deposition. . . . . . . . . . . .

65

66

66

72

73

xi

Figure 5.3:

Stackup of completed active circuit. . . . . . . . . . . . . . . . . . . . .

74

Figure 5.4: Aerosol jet printed interconnection, silver over PMMA to amplier RF

output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75

Figure 5.5:

Fabrication process for the printed antenna and active circuit components. 76

Figure 5.6: Top: schematic representation of the active circuit. Bottom: assem-

bled antenna with integrated active electronics.

. . . . . . . . . . . . .

Figure 5.7: Received power spectrum.

. . . . . . . . . . . . . . . . . . . . . . . . .

77

78

Figure 5.8: Anticipated received power spectrum, as measured from output of

active devices, scaled by Equation 5.1.

. . . . . . . . . . . . . . . . . .

79

xii

CHAPTER 1

INTRODUCTION

1.1 Electronics Packaging Processes

From the invention of the rst transistor at Bell Labs in 1947, and the rst integrated

circuit (IC) patented by Jack Kilby in 1959 [5], electronics have transitioned from large

pieces of heavy, specialized equipment, to being integral parts of our daily lives in the form

of computers, televisions, and cellular phones, to small items such as watches, banking and

identication cards.

Electronics packaging techniques have evolved alongside the circuits they contain. These

packages provide essential functions such as physical protection of the device, as well as

thermally conductive pathways for heating and cooling. Basic integrated circuit packages

are formed from a metal, plastic or ceramic body with leads connecting power and digital

or analog input and output between the IC and the board to which it is mounted. An

example of this package is shown in Figure 1.1. While providing a convenient method

to protect ICs during transportation, as well as handling during assembly, the long leads

passing through the package body, as well as the bonding wires connecting these leads to

the die, introduce signicant parasitic inductance, capacitance and resistance, limiting high-

frequency performance of the circuit [6].

Chip-on-board (CoB) assembly provides an alternate process to connect an IC to a larger

Figure 1.1: Typical packaged integrated circuit with bonding wires.

1

Figure 1.2: Chip-on-board style construction with bonding wire.

circuit board, eliminating some of the undesirable characteristic of a larger package. In this

process, as illustrated in Figure 1.2, individual die are attached directly to a circuit board,

either at the surface or within a recess formed in the board, and are subsequently connected

to metal traces by bonding wires. Though these bond wires still introduce some unwanted

parasitic eects, they are greatly reduced compared to their packaged counterparts. These

eects can be further reduced by using multiple bond wires in parallel, or with wide ribbons

of wire [7, 8].

With interest in radio frequency electronics moving from the microwave regime to mil-

limeter and sub-millimeter ranges, further reductions in unwanted parasitic eects due to

packaging are required. The ip-chip assembly process eliminates bonding wire entirely, in-

stead utilizing a solder bump or micro-bump, to form connections between individual pads

on an IC die and the board to which it is attached. Figure 1.3 shows an example of ip-chip

style connections. In addition to a reduction in unwanted parasitic interactions, this process

also allows for increased circuit density due to the small outline of typical devices used.

Though eects such as detuning of sensitive components on the die can occur as a result

of interactions with adjacent metal and dielectric materials, this process provides low-loss,

impedance matched connections operating beyond 60 GHz [9, 10, 11]. Fan-out wafer-level

packaging further improves on this process, fanning out the on-chip connections with a re-

distribution layer (RDL), eliminating the need for chip under-ll materials [12] typically

required to provide additional mechanical support to the IC.

Reducing or eliminating losses associated with connections to an individual device is ben-

2

Figure 1.3: Flip-chip style construction with solder balls.

ecial; however, rarely are these devices used individually. In the digital domain, micropro-

cessors and microcontrollers are commonly combined with external memories and interfaces

for signal conditioning and sampling. In radio systems, ampliers, lters, mixers, oscillators

and other components are combined to create functional transmitter and receiver chains.

While each of these pieces can be attached to a circuit board individually, high connection

density may make the PCB fabrication process prohibitively expensive. Excessively long

connections between critical components, such as between microprocessors and high speed

memory, can suer from dispersion eects and dielectric losses of the substrate, as well as

resistive losses incurred in the metal trace of the circuit board. By combining these compo-

nents into multi-chip modules (MCMs), critical connections between devices can be formed

with well-controlled impedances, as well as time or phase delay, while providing external

connections for less critical signals and power. For high power applications, substrate choice

becomes important as some materials, such as aluminum nitride, are better able to conduct

heat than others [13]. Cooling channels can also be fabricated throughout the substrate,

allowing for integration of higher power devices with ecient cooling mechanisms [14].

System-on-Package (SoP) and System-in-Package (SiP) processes build on the MCM

concept, taking advantage of unused space to increase functional density. SoP processes take

advantage of space available within the substrate, incorporating passive components such as

resistors, inductors and capacitors [15, 16], as well as antenna networks [17, 11, 18] which

may otherwise be impractical to implement on a die. SiP further reduces interconnection

3

Figure 1.4: Example of MCM construction incorporating both SoP and SiP processes.

Figure 1.5: Example of chip-rst construction.

length between devices by stacking die vertically, with direct connections between bonding

pads of devices [19]. Figure 1.4 shows an example of a multi-chip module incorporating both

SoP and SiP processes.

Chip-rst fabrication processes, in which a device is embedded within the substrate, while

additional dielectric and metal layers are deposited and patterned, can further shorten critical

path lengths between devices. This process was rst patented by the Phillips Corporation

in the 1960s, embedding a die in a exible substrate with metal interconnects contacting

bonding pads on the device directly [20]. General Electric created a High Density Overlay

process by patterning a suitable carrier in which devices are attached and encapsulated in an

epoxy material, with dielectric layers and metal interconnections are subsequently deposited

[21]. As layers are added, additional devices can be included, allowing for greater levels of

integration than by including devices in only a single plane [22]. Figure 1.5 shows an example

of a chip-rst stackup.

1.2 Additive Manufacturing Overview

Additive manufacturing, the processes by which material is deposited layer-by-layer to

form three-dimensional objects, has made its way from the laboratory to a wide range of uses

4

in manufacturing today. Where traditional subtractive manufacturing technologies such as

lathes and mills have been the mainstay of fabricating intricate objects, additive processes

are now being leveraged to fabricate products for a wide range of industries, from automotive

and aircraft components [23], to prosthetic limbs [24], tissue regeneration [25], medication

delivery [26] and beyond. By directly depositing the build material, rather than removing

excess material from an existing object, intricate features can be fabricated which would be

otherwise dicult, if not impossible, to achieve with traditional machining processes, while

minimizing material waste.

As interest in additive manufacturing has grown over the past decades, the number of

additive processes has also increased, each with their own unique set of advantages and

considerations. The materials available for these processes is also widely varied, from ex-

truded plastics, to photopolymers, ceramics and metals. Stereolithographic printing (SLA)

has been in use as far back as 1982, rst reported by Hideo Kodama [27]. In this process,

a build platform is submerged in a photopolymer resin, while a laser draws a pattern over

this surface. The build platform moves to prepare for deposition of the next layer, and the

process is repeated until the print is complete. While this process can be time consuming for

particularly large objects, ner print resolution is achievable than with other additive pro-

cesses, typically less than 50 µm, enabling the fabrication of structures with small features

such as microuidic systems [28]. Digital light projection (DLP) processes can be applied to

improve the standard SLA process, imaging complete layers in a single exposure rather than

drawing each layer with a single laser point, greatly improving printing speeds.

While SLA printing processes can produce high quality parts, long print times and the

high cost typically associated with these systems have created a need for lower cost printing

technologies. Fused deposition modeling (FDM), also known as free form fabrication (FFF),

provides a simple, low cost method to fabricate mechanical structures. In this method, a

thermoplastic is passed through a heated nozzle, partially melting the material, extruding

and depositing it on a build platform. Common materials for this printing process include

5

Nylon, acrylonitrile butadiene styrene (ABS), and polycarbonate, among others. Despite

the low print resolution, typically greater than 200 µm, due to their low cost and rapid

fabrication time, printers using the FDM process have found extensive use both among

casual home users, as well as in laboratory settings, such as reconstruction of dental castings

[29], where models can be digitally stored for printing at a later time, when the physical

object is needed.

Similar to SLA printing, polyjet printing processes utilize photopolymers to create ob-

jects. Rather than employing a resin tank in which the build platform and object are

submerged, polyjet printing selectively deposits the polymers which are cured when exposed

to a UV light source, reducing the amount of material required to initiate the build process.

This selective deposition enables the printing of multiple photopolymers simultaneously, al-

lowing for fabrication of a wider range of structures which would otherwise need to be created

separately and subsequently assembled. By depositing soft or soluble materials, removable

structures can be created to facilitate the printing of overhangs or other unsupported fea-

tures which would be dicult to achieve using the SLA printing technique. Using the polyjet

process, objects such as compliant mechanisms have been fabricated [30], as well as shape

changing structures [31].

Powder-based printing processes function similarly to that of SLA printing, using pow-

dered material rather than a liquid resin. In binder jetting, a binder material is deposited

on the powder bed, additional powder is deposited for the next layer, and the process is re-

peated to completion. Parts are then sintered in a post-processing step, removing the binder

material and fusing the powder [32]. Selective laser sintering (SLS) eliminates the need for

binder materials, by directly heating the powder material with a laser. This process has

been used for printing a wide range of materials including metals, ceramics and polymers

[33]. These processes have been used to fabricate both wire antennas [34], as well as horn

antennas [35].

6

Figure 1.6: FDM printed piece with embedded metal [1].

1.3 Additive Manufacturing for Electronics

With the continuing trend towards additive manufacturing supplementing and/or replac-

ing traditional subtractive processes, attention has turned toward applying these techniques

to the fabrication of electronic devices and systems. Several simple, low cost approaches to

creating electrical connections between components have been developed over recent decades.

Embedding metal traces, by placing metal strips in the object during printing, or by embed-

ding wires in the printed object after printing is completed [1, 36], provide simple methods

to create conductive paths between electrical components. By modifying parts to include

recesses for metallic traces, either by machining printed pieces in a post-processing step,

or modifying the structure to be printed, conductive metallic inks can be applied to form

interconnections throughout the nal printed object [36].

As operating frequencies increase for both analog and digital systems, new techniques for

metallization of printed components become necessary to achieve the features required for

high-frequency operation. Waveguide structures, which require smooth metallic surfaces to

achieve minimal transmission losses, have been fabricated using SLA printing and subsequent

metallization. Metal layers are formed by the deposition of a metallic seed layer by way of

sputter coating, evaporative coating, or electroless deposition. Additional metal can then be

7

Figure 1.7: SLA printed part metalized using damascene-like process from [2].

deposited over this seed layer by using an electrolytic plating processes to obtain a suciently

thick conductive layer [37, 38, 39]. This blanket metallization process provides good large

area coverage, but is insucient for realization of ne features without additional processing.

By modifying the structure of the object to be printed, elevating areas in which copper metal

is unwanted, a damascene-like process can be employed to remove excess metal by mechanical

polishing [40]. By taking advantage of the SLA printing processes ability to deposit multiple

materials during printing, ner features can be realized without the need to modify the

structure of the printed object. Sacricial material can be printed over areas in which metal

is unwanted, enabling the use of a lift-o process to remove unwanted metal which has been

blanket deposited over the printed part [40, 2].

While FDM and SLA processes can be combined with blanket metalization for fabrication

of electrical components by employing both lift-o and damascene-like processes, the feature

sizes which can be achieved by these methods are limited by printer resolution. With interest

in wireless systems moving from the microwave frequency range to mm-wave and beyond,

the resolution of these printing technologies poses a signicant challenge in achieving desired

operation characteristics. Inkjet and aerosol jet printing technologies have been proven to

overcome some of these limitations, enabling deposition of smaller features, as well as the

ability to deposit metallic inks along with a variety of other materials, while sacricing

8

printing speed and build area.

Inkjet printing has been used widely for printing both dielectric materials, as well as

inks containing metal nanoparticles. However, the materials available for this method have

been limited to low viscosities, typically in the range of 10 to 30 cP. Though inks can be

formulated to comply with this limitation, this typically increases the time required to print

a part, as loading factors of materials may be low, requiring additional passes to deposit the

required amount of material. Inkjet printing has been used in the fabrication of a wide range

of objects, from ceramic dental crowns [41], to polymer transistor circuits [42]. Complete

inkjet printing of polymer and metal materials have been demonstrated in the creation of

inductors and capacitors [43, 44], antennas [45], and lters [46].

Inkjet printing has also

been used to deposit structures operating in the sub-millimeter wave frequency range, such

as polymer-based beam splitters [47], and metamaterials printed with silver nanoparticle ink

[48].

Aerosol jet printing has recently proven to also provide a convenient means to deposit

materials while achieving good print resolution. Where inkjet printable materials are limited

to somewhat low viscosities, aerosol jet printing can utilize materials with viscosities up to

1000 cP. Inkjet and aerosol jet printing are both capable of depositing thin layers of materials,

typically in the range of several hundred nanometers, as well as narrow features down to 10

µm or less. However, aerosol jet printing can also be congured to deposit individual features

as wide as several millimeters, providing an increase in printing speed when large areas of

metalization are required, at the sacrice of ne resolution. Aerosol jet printing has been

used to fabricate of a wide range of electrical circuits, including transmission lines operating

beyond 100 GHz [49], coupled line lters [50], as well as antennas [51], inductors, capacitors

and resistors [52]. Unlike other printing technologies such as inkjet printing, aerosol jet

printing has a large stand-o distance between the work surface and the print head, enabling

printing on non-planar surfaces [3]. Figure 1.8 shows an example of a circuit printed on a

tank with the aerosol jet printing process.

9

Figure 1.8: Aerosol jet printed ll level sensor on bucket [3].

1.4 Electronics Packaging by Additive Manufacturing

Additive manufacturing techniques present a new step in the evolution of electronics

packaging. By providing a low cost means of rapidly producing prototype circuits necessary

during design stages, as well as a low cost method for fabricating devices, electronic systems

will continue to be integrated in our daily activities. While fabrication process such as

wire bonding have provided easy means of connecting integrated circuits to boards and

carriers, [53] demonstrates that bonding to printed metal traces can be dicult to achieve.

Processes to directly connect printed metal traces to device bonding pads must be developed

to facilitate further advancements in additive packaging processes. A benet to fabricating

these interconnections by direct-write methods, parasitic eects of the connections can be

greatly reduced, enabling operation into millimeter wave and sub-millimeter wave frequency

ranges.

Some processes to package electronics utilizing additive manufacturing techniques have

already been demonstrated. Packaged electronics have been embedded within printed ob-

jects, connected by wires or by traces formed from post-processing of the printed substrate

[36]. By embedding devices in an object during the print process, blanket metallization has

been employed to create tunable resonators and antennas operating through 7 GHz [54].

With their ner feature resolution and wider range of printable materials, inkjet and

10

Figure 1.9: Aerosol jet printed chip-to-board connection over dielectric ramp [4].

aerosol jet printing can be used to package bare integrated circuits. Aerosol jet printing of

chip-to-chip connections have been demonstrated in [55]. In this process, devices are attached

to a carrier substrate and encapsulated in an epoxy material. The epoxy is subsequently

patterned by laser ablation, followed by aerosol jet printing of silver connections between the

bonding pads of the device. Though eective, this process does require a degree of planarity

of the individual die, which may be dicult to ensure. Aerosol jet printing has also been used

for fabrication of chip-to-board interconnections. With a large stand-o distance of 3 mm

or more between the print head and work piece, substrates can be manipulated and rotated

to expose surfaces for printing which would be dicult to perform using processes such as

inkjet printing. Due to the thin layers of material typically deposited by aerosol jet printing

( < 1 µm), and the signicantly greater device thicknesses typically encountered (> 100 µm),

dielectric ramps are necessary to provide a surface over which metallic inks can be deposited.

By depositing these llets, electrical contact can be formed with devices, enabling a chip-on-

board style construction using additive processes [56]. These bond wire replacements have

been shown to perform well, reducing losses associated with such connections compared

to typical bonding wires [4, 57]. Figure 1.9 shows an aerosol jet printed connection over

dielectric ramp demonstrated in [4].

Figure 1.10 illustrates a possible additively manufactured package. Printed dielectric

11

Figure 1.10: Additively manufactured packaging concept.

materials can enable localized fan-out of high-density interconnections, a process which tra-

ditionally requires a complex printed circuit board with multiple layers to achieve. Printed

dielectrics can be tailored for localized deposition of materials with varied dielectric con-

stants, enabling antennas and passive components to exist within the same substrate needed

for component protection, heat dissipation, and signal and power distribution.

1.5 Dissertation Outline

The objective of this work is to develop processes for packaging electronics operating from

the microwave frequency range into the terahertz spectrum utilizing additive manufacturing

techniques. This work is organized as follows:

In chapter 2, high frequency limitations of the aerosol jet printing process are investigated.

Band-pass and band-stop lters are designed, fabricated and measured, operating from 200

GHz through 550 GHz. Polarizing screens are demonstrated from 100 GHz to beyond 700

GHz. Impacts of line dimensions of aerosol jet printed silver traces are demonstrated. Eects

of electroless plating processes on printed lter performance is investigated. The performance

of each of these lters and polarizers are measured using a commercial frequency-domain

terahertz system.

In chapter 3, a xed-frequency oscillator is designed and fabricated using aerosol jet

printing. Designed for operation at 10.1 GHz, performance of aerosol jet printed distributed

microwave circuit elements is easily measured. Processes for via fabrication using both

12

conventional mechanical drilling, as well as reactive ion etching, and their compatibility

with aerosol jet printed metal traces, are demonstrated.

Chapter 4 demonstrates aerosol jet printed interconnections to bare-die integrated cir-

cuits embedded within a commercially available substrate. By embedding devices within a

substrate, undesirable parasitic eects due to interconnection length are minimized. Pro-

cesses for embedding devices within a substrate are demonstrated. The ability to iterate the

printing process to repair failed interconnections is also demonstrated. Finally, undesirable

eects of printing metal interconnections over a bare semiconductor material are investigated.

Chapter 5 combines two additive manufacturing processes, aerosol jet printing and se-

lective laser annealing, in the fabrication of a completely packaged system-on-antenna. By

combining additive processes, a complete system can be developed and deployed quickly and

at low cost, using the antenna body as the protective package for the electronics contained

within. This process utilizes SLA printing for the fabrication of a passive Vivaldi antenna,

with aerosol jet printing of connections to devices embedded using processes demonstrated

in chapter 4. An additional polymer ink is developed and characterized to improve the

transition from the substrate to the bonding pads of the integrated circuits.

13

HIGH FREQUENCY APPLICATIONS OF AEROSOL JET PRINTING

CHAPTER 2

2.1

Introduction

Over the last two decades, interest has grown in the development of systems operating

in the frequency range of 100 GHz to 10 THz, commonly referred to as the terahertz (THz)

spectrum. Systems operating in the THz frequency range have a wide array of applications,

including medical imaging [58], biology [59], substance detection and imaging for security

[60, 61], among many more.

Traditionally, fabrication of components operating in the THz spectrum employ pro-

cesses such as micromachining, and other lithographic techniques [62], as well as the use

of dedicated machinery such as that used to fabricate polarizing screens [63], [64]. These

processes typically require cleanroom facilities, multiple pieces of dedicated equipment, as

well as highly skilled technicians to fabricate intricate THz components. While subtractive

processes such as laser machining have been demonstrated in the fabrication of polarizing

screens [65], such a process is dicult to adopt in the fabrication of free-standing or layered

structures.

Additive manufacturing (AM) has been touted as a replacement to traditional methods

in the fabrication of THz components. Advantages of AM include fast prototyping, low

material loss and low hazardous waste generation. With AM, a single tabletop system is

sucient to fabricate a wide range of components, rather than requiring multiple machines,

each dedicated to a specic fabrication task. Recently, a range of 3D printed THz com-

ponents have been demonstrated using AM technologies. This includes the use of FDM

to fabricate focusing grating couplers [66], focusing lenses for near-eld microscopy [67], as

well as gradient-refractive-index lenses [68]. SLA processes have been used to fabricate rib-

bon waveguides, power splitters and micro-lens arrays [69, 70]. SLA has also been used to

14

produce molds for the injection molding of THz components using high-density polyethy-

lene [71], allowing for the fabrication of THz components using low-loss dielectrics that are

dicult to print directly using other AM techniques.

Inkjet printing has also been used as another low cost method to print thin lm structures,

especially conductive inks on planar substrates, such as polarizer screens [72, 73]. High res-

olution structures having 5 µm resolution have been demonstrated using this approach [74].

Similarly, laser printing has been demonstrated in the fabrication of metamaterial structures

in the THz frequency spectrum [75]. Electrohydrodynamic jet printing can produce smaller

features, and has been shown as a potential method to fabricate THz metamaterials [76], as

well as sensors for detection of biological materials [77]. While these techniques work well for

depositing ne features, they are dicult to implement for printing structures on non-planar

surfaces.

With a much greater stando distance compared to inkjet printing technologies, aerosol

jet printing is capable of depositing materials with great accuracy, enabling printing on a

much greater variety of substrates. Aerosol jet printing has been successfully used to fabricate

planar circuits operating at 160 GHz [78], as well as the fabrication of multilayer passive

circuitry [79]. More recently, aerosol jet printing has been demonstrated for fabrication of

terahertz passive components [80, 81].

Figure 2.1 depicts the critical components in the aerosol jet printing process. A silver

nanoparticle ink, as an example, is atomized by an ultrasonic transducer, and is carried

to the print head by means of a nitrogen gas stream.

In the print head, the aerosolized

nanoparticle stream is combined with a focusing sheath gas, creating a well formed column

of ink which is then deposited on the substrate material of choice. While inkjet printing

typically requires inks with low viscosity (10 - 30 cP), the aerosol jet process can utilize inks

with viscosity ranging from 1 - 1000 cP, enabling the deposition of a wider range of materials.

However, due to the aerosolization process, particle size of materials dispersed in these inks

is typically limited to less than 200 nm. Adhesion of inks to substrate materials introduces

15

Figure 2.1: Aerosol jet system components. Left: ultrasonic atomizer. Right: print head
with nozzle.

additional constraints, limiting the material choice for both inks and substrates.

The goal of this chapter is to investigate the performance of aerosol jet printed compo-

nents compared to lithographic fabrication for use in the THz frequency regime. Jahn et al.

recently demonstrated aerosol jet printed resonant structures in [81]. In contrast, the struc-

tures presented in this chapter demonstrate the applicability of aerosol jet printing to larger,

more complex designs, and these are compared to their counterpart fabricated using conven-

tional photolithography. Three lter designs are used for this study, including a band-pass

lter designed to operate at 250 GHz, and two band-stop lters, one operating at 250 GHz

and the other at 550 GHz. In addition, polarizing screens having ne line resolution with

long continuous lines are investigated. Finally, reduction in loss characteristics of printed

structures by means of an electroless copper plating system using aerosol printed silver ink

as a seed layer is demonstrated. These structures demonstrate printing of components re-

quiring maximum surface coverage with metal (band-pass lters), minimal surface coverage

(band-stop) and long ne line structures (polarizers).

2.2 Design and Simulation

Several components have been fabricated to evaluate high-frequency performance of

aerosol jet printed structures on exible organic materials. Each of the components has been

fabricated on a liquid crystal polymer (LCP) substrate, Rogers Ultralam 3850HT. While

16

other materials such as quartz are traditionally used in the fabrication of THz components

due to their excellent dielectric properties, this cannot be exed to conform to other surfaces.

Additionally, the particular ink used in this work does not adhere well to quartz after sinter-

ing, resulting in poor performance. LCP has been selected for its low loss characteristics in

the frequency range of interest [82], its compatibility with the silver nanoparticle ink to be

printed, and its thermal stability at the temperatures required for sintering of the silver ink.

To reduce the impact of substrate loss on transmission characteristics, the thinnest available

substrate, 25.4 µm, is used here for all the designs. Several constraints are considered in the

design of each component to be fabricated. Due to the limitations of low cost masks avail-

able for photolithography, no features smaller than 40 µm will be considered for performance

comparison between aerosol jet printed structures and lithographically fabricated structures.

2.2.1 Printed Silver Characterization

Printed silver nanoparticle ink requires a post-print processing step to yield a metal layer

with acceptable electrical conductivity. Thermally sintering printed silver is a simple process,

typically requiring little more than a heat source. By varying both time and temperature, a

range of electrical conductivity can be easily achieved [83]. The range of suitable substrate

materials is further reduced due to the need for this additional thermal process. While the
chosen LCP substrate material has a melting temperature beyond 300 â—¦C, due to dierences
in coecient of thermal expansion between the substrate and the printed metal, substrate
deformation has been observed beyond 200 â—¦C. As a result, a post-print sintering process is
performed at 180 â—¦C for 3 hours.

Figure 2.2 shows the layout of a structure used for measuring conductivity of sintered

printed silver metal. Four pads measuring 0.5 mm x 0.5 mm allow for contact by a four-

point probe. At the outer two conductors, a current is applied across the metal trace, with

the inner two conductors allowing for measurement of the voltage drop between these pads.

Conductivity of the sintered metal can be calculated as:

17

Figure 2.2: Pattern for printed silver conductivity measurement.

σ =

lI
AV

(2.1)

where σ is the metal conductivity, l is the length of the line between voltage measurement

pads, V is the measured voltage drop, A is the cross sectional area of the line, and I is the test

current injected into the structure. The cross sectional area of the line under measurement

can be measured using a variety of techniques. Figure 2.3 shows the cross-sectional area of

a silver metal trace printed on a glass microscope slide, measured using a NanoMap-500LS

contact surface prolometer. Using the above sintering process, conductivity as high as 1.75

x 107 S/m has been achieved. Due to process variations throughout the duration of a print,

simulations are performed using a lower conductivity of 1.2 x 107 S/m to ensure adequate

performance of the nal fabricated structure.

Performance of both lters and polarizer screens may be degraded due to insucient

metal thickness. Skin depth, dened by Equation 2.2 [84], where f is the frequency of

operation, µ is the magnetic permeability of the material, and σ is the DC conductivity of
the material, is the depth at which the current density in a metal has decreased to e−1, or
approximately 36.8%, of its maximum at the metal surface.

δ =

1√
πf µσ

18

(2.2)

Figure 2.3: Measured silver cross-sectional area of printed metal conductivity measurement
structure.

To minimize lter and polarizer degradation due to insucient metal thickness, metal

layers approximately 5 skin-depths (δ) in thickness as calculated at 100 GHz are used. For

copper metal, which has a conductivity of approximately 5.8 x 107 S/m, this corresponds to

a thickness of 1 µm, and approximately 2 µm of metal for aerosol jet printed silver.

2.2.2 Filters

Three lters have been designed, providing band-pass and band-stop behavior, to demon-

strate applicability of aerosol jet printing for the THz frequency spectrum. Each of these

lters are designed for both fabrication by photolithography of copper metal, and aerosol jet

printed silver metal.

Each of the designed band-stop lters is formed using a cross-like unit cell metal structure.

Band-pass lters are formed by the inverse of this structure, where each cross is formed by an

19

Figure 2.4: Schematic representations of band-stop and band-pass lter unit cell. A: lter
line width, B: lter line length, C: lter unit cell width and length.

opening in a metallic sheet. Simulation of each of these lters is performed by designing each

lter as a unit cell, and applying the Floquet Theorem, approximating a two-dimensional

structure of innite size. Figure 2.4 depicts the unit cell geometry of each lter. The cross

structure used for each of these is necessary, due to the elliptically polarized radiation of the

measurement system which will be used to verify each design. Simulation of each lter is

performed using ANSYS High Frequency Structure Simulator (HFSS).

A band-stop lter has been designed for operation at 250 GHz. This lter is formed by a

metal cross with line widths (A) of 50 µm, line lengths (B) of 433 µm, and a unit cell width

and length (C) of 473 µm. These dimensions are summarized in Table 2.1. Simulation of this

lter, both for copper metal and aerosol jet printed silver metal, can be seen in Figure 2.5.

Simulated results of each of these lters are similar, demonstrating resonance at 250 GHz,

with a transmission coecient of approximately -30 dB for both copper metal and printed

silver.

The geometry of the 250 GHz band-stop lter can be modied to achieve operation at

20

Figure 2.5: Simulated transmission coecient of 250 GHz band-stop lters.

Figure 2.6: Simulated transmission coecient of 550 GHz band-stop lters.

21

Figure 2.7: Simulated transmission coecient of 550 GHz band-stop lters of dierent line
widths.

Figure 2.8: Simulated transmission coecient of 250 GHz band-pass lters.

22

a higher frequency. For operation at 550 GHz, the cross line width is maintained at 50 µm,

while the line length is reduced to 209 µm, and the unit cell length and width are reduced to

249 µm. These dimensions are summarized in Table 2.2. Figure 2.6 shows simulation results

of this lter. While the resonance frequency is not centered on 550 GHz, this lter will be

more sensitive to dimensional variation due to the fabrication process. Figure 2.7 shows
simulation results for line widths varied by ±6 µm, resulting in a shift in resonance between
530 and 550 GHz. The transmission coecient for this conguration both in copper metal

and printed silver metal simulations are similar. Each of these lters results in resonance at

535 GHz with a transmission coecient of -30 dB.

A band-pass lter designed for operation at 250 GHz is formed by the inverse of the

band-stop conguration, where each cross is formed from a void in a solid metal sheet.

This lter is formed with a line width of 50 µm, a line length of 454 µm, and a unit cell

length and width of 494 µm. These dimensions are summarized in Table 2.3. Simulation

results of this lter are shown in Figure 2.8. The pass-band transmission coecient of this

lter conguration is approximately -0.6 dB for copper metal, while silver metal results in a

slightly higher pass-band transmission coecient of -0.9 dB.

2.2.3 Polarizers

Two polarizers have been designed, formed by a wire screen. Design and simulation of these

polarizers is performed in a similar manner to that which has been used for the design of each

of the lters above, by simulating a single unit cell approximating a structure of innite size

in two dimensions. Figure 2.9 shows a schematic representation of this unit cell structure.

Each polarizer is designed with identical line widths (D), and line separations (E). A pair

of polarizers, fabricated from copper metal as well as aerosol jet printed silver, are designed

with line widths and spacings of 40 µm. The unit cell width and length (F) of each of

these polarizers is 400 µm. Performance of each polarizer is characterized by the ability of

the polarizer to extinguish electromagnetic waves which are polarized parallel to the wire

23

Figure 2.9: Schematic representations of polarizer unit cell. D: polarizer line width, E: line
separation, F: unit cell width and length.

orientation. This extinction ratio can be expressed as:

Er = 10 log(

Ptm
Pte

)

(2.3)

where Pte is the power of the incident wave polarized parallel to the wire grid, and Ptm is

the wave polarized perpendicular to the wire grid. Simulation results of these polarizers are

shown in Figure 2.10. The extinction ratio of each of these polarizers is approximately 30

dB at 200 GHz, decreasing with frequency to approximately 20 dB at 700 GHz.

An additional polarizer has been designed for fabrication only by aerosol jet printing.

This polarizer is formed with line widths and spacings of 10 µm, and unit cell lengths and

widths of 100 µm. Figure 2.10 shows simulation results of this polarizer. Simulation results

of this polarizer demonstrate a signicant improvement over the simulated results of the 40

µm polarizers, with an extinction ratio of approximately 45 dB at 200 GHz, to approximately

32 dB at 700 GHz.

24

Figure 2.10: Simulated polarizer extinction ratios.

2.3 Fabrication and Measurement

Fabrication of each structure begins with preparation of the substrate material to be

used. The LCP material as provided has a 9 µm thick copper metal laminate on both the

front and back side. This copper metal is chemically removed in a sodium persulfate bath,

and subsequently cleaned in deionized water to remove residue. After drying, this bare LCP

substrate can be used without any further processing for aerosol jet printing.

For structures which are to be fabricated with a lithographic process, a copper metal

layer 1 µm thick is deposited on a single side of each substrate piece to be used, by means

of a Denton Desk Top Pro sputtering system. A tape-peel adhesion test was performed to

verify the deposited copper metal would remain attached to the LCP material without the

need for additional processing of the substrate. A photoresist material is applied by spin

coating, which is then patterned by UV exposure. Each structure is etched in a sodium

persulfate bath, and subsequently rinsed in deionized water and dried.

Each of the substrates to be patterned by aerosol jet printing is axed to a ceramic

plate to facilitate handling throughout both the printing and sintering processes. Clariant

Prelect TPS 50 G2 silver nanoparticle ink has been selected to fabricate each of the aerosol

25

jet printed lters. This ink consists of silver nanoparticles which are dispersed in ethylene

glycol. The ultrasonic atomization process requires a low viscosity ink, typically less than

10 cP. As provided by the manufacturer, the ink used has a viscosity of approximately 30

cP. To reduce the viscosity to an acceptable range, 1 mL of nanoparticle ink is diluted into

3 mL of deionized water.

Each aerosol jet printed lter has been fabricated using a print nozzle 200 µm in diameter,

an atomizer gas ow rate of 22 SCCM, and a focusing sheath gas ow rate of 90 SCCM.

The print stage velocity is limited to 1.5 mm/s, to allow for individual printed layers of

approximately 1 µm thickness to be deposited per pass. 2 metal layers are printed, to yield

a nal metal thickness of approximately 2 µm.

Performance of each fabricated structure has been measured on an Emcore PB-7200

frequency-domain THz system. This system is capable of measuring transmittance over a

frequency range of 100 GHz to approximately 1.7 THz. This system has frequency measure-

ment resolution of 100 MHz, and amplitude error of approximately 1 dB. The THz radiation

is generated by an optically excited photo-conductive switch, and focused through a silicon

lens. An o-axis parabolic mirror directs this well collimated beam to the sample to be

measured. Due to the spiral shape of the antenna coupled to the photo-conductive switch,

the radiation generated by this system is elliptically polarized. In each measurement, only

the normal angle of incidence is considered, with each sample xed to the receiver head unit

to ensure proper orientation.

In an eort to reduce the size of each lter and to ensure

normal incidence of the THz beam, a window of 9 mm x 9 mm was cut from a piece of thick

stock copper-clad circuit board material. This window is placed over the receiving head.

Figure 2.11 illustrates this measurement conguration. The transmission coecient of each

measured lter is calculated by:

T Cf ilter = 10 log(

Pf ilter

Pbackground

)

(2.4)

where Pf ilter is the measured power transmitted through the lter, and Pbackground is the

26

Figure 2.11: THz frequency domain measurement setup.

measured transmitted power with no lter in the signal path.

2.3.1 Filters

Figure 2.12 shows an image of a unit cell of the nal fabricated band-stop lters designed for

operation at 250 GHz. The copper metal structure can be seen on the left, with the aerosol

jet printed structure on the right. Measured dimensions of the fabricated copper metal lter

show a line width (A) of 40 µm, and line length (B) of 430 µm. Measured dimensions of

the aerosol jet printed lter show a line width (A) of 30 µm, and line length (B) of 430 µm.

These dimensions are summarized in Table 2.1.

Figure 2.13 shows measured results of each of the 250 GHz band-stop lters, as well as

simulation results utilizing measured dimensions of each fabricated lter. The lter fabri-

27

Figure 2.12: Unit cell of fabricated 250 GHz band-stop lters. Left: Copper metal with
lithographic process. Right: Aerosol jet printed.

Figure 2.13: Measured transmission coecient of 250 GHz band-stop lters.

cated from copper metal demonstrates resonance at 250 GHz, with a transmission coecient

of -30 dB at resonance. The aerosol jet printed lter results in resonance of approximately

225 GHz, with a transmission coecient of -27 dB. As can be observed, the shift in reso-

nance of the aerosol jet printed lter is due to each arm of the cross being both shorter and

narrower than the design goal.

28

Filter

250 GHz Band-Stop
Copper

Silver

Line Width (A) (um)

Line Length (B) (um)

Unit Cell (C) (um)

Designed
Fabricated
Designed
Fabricated
Designed
Fabricated

50
40
433
430
473
480

50
30
430
430
473
480

Table 2.1: 250 GHz band-stop lter dimensions.

Figure 2.14: Fabricated 550 GHz band-stop lter unit cells. Left: Copper metal with litho-
graphic process. Right: Aerosol jet printed.

Figure 2.14 shows an image of a unit cell of the nal fabricated lters designed for

operation at 550 GHz, with simulation results utilizing dimensions of the fabricated lters.

The measured line width of the copper metal lter is 45 µm, line length is 198 µm. The lter

fabricated from aerosol jet printed silver has a line width of 50 µm, line length of 205 µm.

Measured performance of these lters can be seen in Figure 2.15. Each of these lters has

resonance close to the target frequency of 550 GHz.

Figure 2.16 shows an image of a unit cell of each of the fabricated 250 GHz band-pass

lters. Final measured dimensions of the copper metal lter include a line width of 70 µm

and line length of 471 µm. Dimensions of the fabricated silver metal lter include a line

width of 50 µm, and line length of 490 µm. Measured performance of each of these lters,

as well as simulation results using dimensions of each fabricated lter, can be seen in Figure

29

Figure 2.15: Measured transmission coecient of 550 GHz band-stop lters.

Filter

550 GHz Band-Stop
Copper

Silver

Line Width (A) (um)

Line Length (B) (um)

Unit Cell (C) (um)

Designed
Fabricated
Designed
Fabricated
Designed
Fabricated

50
45
209
198
249
250

50
50
209
205
249
250

Table 2.2: 550 GHz band-stop lter dimensions.

2.17. While these oer similar performance, their resonance is slightly lower than the desired

frequency of 250 GHz. The measured transmission coecients of each lter are within the

amplitude error of the measurement system.

2.3.2 Polarizers

Aerosol jet printed polarizers have been fabricated utilizing dierent parameters to achieve

dierent line dimensions. Aerosol jet printed polarizers designed with line widths and spac-

ings of 40 µm utilize the same parameters chosen to fabricate the lters. To fabricate

30

Figure 2.16: Unit cell of fabricated 250 GHz band-pass lters. Left: Copper metal with
lithographic process. Right: Aerosol jet printed.

Figure 2.17: Measured transmission coecient of 250 GHz band-pass lters.

31

Filter

250 GHz Band-Pass
Copper

Silver

Line Width (A) (um)

Line Length (B) (um)

Unit Cell (C) (um)

Designed
Fabricated
Designed
Fabricated
Designed
Fabricated

50
70
454
471
494
500

50
50
454
490
494
500

Table 2.3: 250 GHz band-pass lter dimensions.

polarizers with line widths and spacings of 10 µm, a nozzle with a 100 µm diameter is uti-

lized, with an atomizer gas ow rate of 20 SCCM, and a focusing sheath gas ow rate of 35

SCCM.

Measurement of polarizers requires a linearly polarized source; however, the measure-

ment system used to characterize these structures generates an elliptically polarized wave.

To achieve linear polarization, a polarizer with a high extinction ratio must be placed be-

tween the source and the component under measurement.

Initially, a pair of aerosol jet

printed polarizers with 10 µm line widths and spacings has been measured to evaluate their

performance. All subsequent measurements have been performed with one of these high ex-

tinction ratio polarizers on the measurement source, to ensure adequate linear polarization

of the wave incident on the samples under test.

Figure 2.18 shows both copper metal and aerosol jet printed polarizers designed for 40

µm line widths and spacings. Measured dimensions of each of these polarizers show a nal

line width of 30 µm, resulting in an extinction ratio approximately 5 dB less than the

design goal. This deviation in line width also impacts high frequency performance, as the

wider gap is electrically larger at higher frequencies. Figure 2.19 shows the nal aerosol jet

printed polarizer with line widths of 10 µm. The nal measured line width of this polarizer is

approximately 9 µm. This deviation in line width does not signicantly impact the measured

extinction ratio of this polarizer over the frequency range of interest. Final measured results,

compared to simulated results of each polarizer utilizing these measured line dimensions, can

32

Figure 2.18: Unit cell of fabricated 40 µm polarizer grid. Left: Copper metal with lithograph
process. Right: Aerosol jet printed.

Figure 2.19: Unit cell of fabricated 10 µm aerosol jet printed polarizer grid.

33

Figure 2.20: Simulated and measured polarizer rejection ratios.

be seen in Figure 2.20. Both the copper and aerosol jet printed polarizers designed for 40 µm

line widths show almost identical performance over the measured frequency range, while the

aerosol jet printed polarizer designed for 10 µm line widths shows a signicant improvement

in extinction ratio of approximately 12 dB compared to the 40 µm line width design.

2.4 Electroless Plating

While aerosol jet printing is capable of depositing features as small as 10 µm reliably,

thick metal layers can be dicult to achieve while maintaining this ne line resolution. One

possible method to reduce the time required to fabricate structures with suciently thick

metal layers while maintaining ne line resolution is to combine aerosol jet printing with a

metal plating process. Electroless plating of metal provides a means to deposit additional

metal layers over a printed seed metal layer, without any design modication.

To demonstrate the feasibility of this method, a band-stop lter designed for operation

34

Figure 2.21: Unit cell of 550 GHz band-stop lter after electroless copper plating.

at 550 GHz has been aerosol jet printed in a single layer, at a print velocity of 2 mm/s,

yielding a metal layer thickness of approximately 600 nm. Measured performance of this

structure before plating can be seen in Figure 2.22. Next, this lter has been submerged

in an electroless copper plating solution from Caswell Plating for 30 minutes, depositing an

additional 2.5 µm of copper metal over the existing printed silver seed layer. A closeup view

of nal plated structure can be seen in Figure 2.21. As can be seen in Figure 2.22, while

the unmodied structure results in a lter with poor performance, after plating performance

improves drastically, with attenuation comparable to that of a lter printed with a much

thicker metal layer. This process can be utilized to thicken the metal layer, and improve

conductivity while reducing aerosol print time. Dimensions of the lter before and after

plating are listed in Table 2.4.

2.5 Summary

Aerosol jet printing can provide a wide range of exibility in fabrication of THz compo-

nents. A number of materials can be deposited in layers from several hundred nanometers

35

Figure 2.22: Measured transmission coecient of 550 GHz band-stop lter, before and after
electroless copper plating.

Filter

550 GHz Band-Stop
Before

After

Line Width (A) (um)

Line Length (B) (um)

Unit Cell (C) (um)

Designed
Fabricated
Designed
Fabricated
Designed
Fabricated

50
31
209
199
249
249

50
34
209
203
249
249

Table 2.4: Electroless copper plated 550 GHz band-stop lter dimensions.

thick, to tens of microns. Aerosol jet printed silver metal has been shown to provide adequate

performance in the fabrication of band-pass and band-stop lters, as well as polarization

screens for operation to at least 700 GHz. Additionally, fabrication time can be signicantly

reduced by combining aerosol jet printing techniques with electroless plating of other metals

to achieve the desired nal metal thickness. The combination of methods presented, as well

as the exibility of aerosol jet printing, can provide a path for fabrication of a wide range of

components operating in the THz frequency spectrum.

36

CHAPTER 3

RAPID PROTOTYPING OF RF OSCILLATOR WITH AEROSOL JET

PRINTING

3.1

Introduction

Rapid component prototyping signicantly reduces product development time and cost,

as well as enables the fabrication of structures which would otherwise not be easily created

through traditional subtractive manufacturing techniques. Recently, there has been focus

on using additive manufacturing for development of radio frequency components such as

antennas [85, 86, 45], passive circuit elements [87, 79, 88], as well as integration of active

and passive elements [89].

With their ability to deposit a wide range of conductive and polymer materials, inkejt and

aerosol jet printing technologies present a new technique for rapid production of RF circuits

and systems. With the large stando distance of aerosol jet printing, complete metallization

of out-of-plane features can be achieved, enabling complete circuit fabrication without the

need for additional processes typically seen in traditional circuit board fabrication. By

reducing the time and cost associated with prototyping and small production quantities,

designs can quickly be iterated to converge on an optimal solution.

In this chapter, two aerosol jet printed passive circuit elements are presented, as well as a

printed oscillator circuit. The passive circuit elements, a transmission line and a microstrip

resonator, form the fundamental building blocks necessary for the realization of the nal

oscillator circuit. Complete metallization of the oscillator circuit, including metallization

of drilled via walls, will be accomplished through aerosol jet printing. This method of

fabrication will allow for quick turn-around prototype fabrication on commercially available

substrate material using packaged, o the shelf components.

37

3.2 Passive Circuit Elements

Two passive circuit elements have been simulated, printed and measured. These are a 50

Ω microstrip transmission line, and a microstrip resonator. The performance of these struc-

tures have been rst designed and simulated using Ansys HFSS full-wave electromagnetic

simulation software.

3.2.1 Microstrip Transmission Line Simulation and Fabrication

A microstrip transmission line has been designed for a characteristic impedance of 50 Ω.

This line forms one of the fundamental building blocks necessary for the fabrication of the

desired oscillator, as well as serve to verify metal layer conductivity and surface roughness

models for design and simulation of other printed structures.

The 50 Ω microstrip transmission line (MSTL) is 231 µm wide, and 17.4mm in length.

Reported conductivities of sintered silver nanoparticle metal layers can range from 106 S/m

to 107 S/m, as reported in [87, 45, 90]. A silver conductivity of 107 S/m, as well as metal

layer thickness of 2 µm with 0.5 nm of surface roughness are used to simulate the transmission

line.

This MSTL was fabricated by printing four layers of silver nanoparticle ink. The line

was printed using a deposition nozzle with a 200 µm diameter, and a stando height of

approximately 3.5 mm and a printing stage velocity of 1 mm/s, resulting in a metal layer

approximately 2 µm thick. Ground pads have been printed on either side of the transmission

line, to facilitate probing of the transmission line with Ground-Signal-Ground (GSG) probes.

The nal printed transmission line can be seen in the inset in Fig. 3.1. The printed metal is
sintered at 200 â—¦C for 1 hour, to achieve a highly conductive layer.

The performance of the printed transmission line was measured on a MPI TS150-THZ

on-wafer probe station utilizing a Keysight N5227A PNA Microwave Network Analyzer, over

a frequency range of 1 GHz to 40 GHz. Measured insertion loss at 10 GHz was approxi-

38

mately .037 dB/mm, while the simulated insertion loss at this frequency was approximately

0.043 dB/mm. At 40 GHz, measured insertion loss is approximately .12 dB/mm. Similarly

sized transmission lines at this frequency fabricated from copper have shown reported in-

sertion loss of approximately .06 dB/mm [91]. Fig. 3.1 compares measured vs simulated

insertion loss of this transmission line. The top surface of the nal printed transmission line

has approximately 0.5 µm of surface roughness, as measured by a NanoMap-500LS Surface

Prolometer.

Figure 3.1: Simulated vs measured insertion loss printed transmission line. Inset: 17 mm
transmission line with GSG pads.

3.2.2 Microstrip Resonator Simulation and Fabrication

A microstrip resonator has been designed for use as a resonant structure for an X-band xed-

frequency oscillator. This resonant structure is designed to have a band-stop characteristic,

39

which is necessary for proper operation of the nal desired oscillator.

The structure is capacitively coupled to a MSTL. This is accomplished by printing a line

which is 200 µm wide and approximately 6 mm long, separated from the transmission line

by a 60 µm gap. The rest of the resonator is formed by a 200 µm wide line, 1.82 mm long,

connected to a half-circle 2.42 mm in diameter, which forms a parallel-plate capacitor with

the ground plane. The structure of the nal printed resonator can be seen in Fig. 3.3.

Simulation was performed both by approximating the metal as a perfectly conducting

layer, as well as a 2 µm thick layer with a conductivity of 107 S/m and surface roughness of

0.5 µm, as was measured for the nal printed transmission line. The resonant frequency of

the structure using a perfectly conducting metal layer was approximately 10.24 GHz, with

an insertion loss of 20.78 dB. However, when simulated using parameters for printed silver,

the resonant frequency shifted to 9.32 GHz, with an insertion loss of 3.73 dB.

The resonator was then printed with four metal layers, providing a nal metal thickness

of approximately 2 µm. The measured resonant frequency of this structure is 10.3 GHz,

with an insertion loss of 4.3 dB. This measured resonance corresponds to the same structure

simulated utilizing a perfectly conducting metal layer, however the insertion loss corresponds

to simulation with a nite conductivity and 0.5 µm of surface roughness.

3.3 X-Band Oscillator

Using the simulated and measured transmission line and resonator results, an oscillator

has been designed and fabricated for operation in the X-band frequency range. This is a

xed-frequency oscillator, fabricated on 4 mil (approximately 102 µm) thick Rogers Ultralam

3850HT LCP substrate material. All metal layers have been printed using the aerosol jet

printing method.

40

Figure 3.2: Simulated vs measured insertion loss of printed microstrip resonator.

Figure 3.3: A photograph of the printed microstrip resonator.

41

3.3.1 Design and Simulation

The X-band oscillator design utilizes and Avago Technologies ATF-36163 pHEMT low noise

transistor. S-parameters of the device have been simulated using a device model provided

by Avago Technologies, for Keysight's Advanced Design System (ADS). The device is being

operated in an unstable condition by means of adding a small inductance of a few nH

between the source pins and ground. A load reection coecient of ΓL = 0.1 + j0.2 has

been selected to provide sucient instability for oscillation. A microstrip matching network

has been designed to transform a 50 Ω load impedance to this desired value. An additional

impedance matching network has been designed to transform the source impedance presented

by the resonator to match the input impedance of the transistor.

Time-domain simulation of this circuit results in a fundamental resonance at 10.1 GHz,

with an output power of approximately -3 dBm. The power of the second harmonic at 20.2

GHz is approximately 12 dB lower than the fundamental frequency of oscillation.

3.3.2 Fabrication

Using the microstrip line dimensions found through simulation, this oscillator was fabricated

and measured. Holes for vias between the top metal layer and the ground plane are rst

mechanically drilled using a drill bit 300 µm in diameter. Following this step, the existing

17 µm copper foil is chemically removed using a sodium persulfate solution. After the

substrate has been prepared, the top metal layer and ground plane are printed using the

above print parameters. Thermal sintering is performed after printing each complete metal
layer, performed at 200 â—¦C for 1 hour.

Critical to the operation of this circuit are conductive vias, which are necessary both

for terminating the oscillator, as well as for providing conductive paths for the DC biasing

network. The vias necessary for this circuit were formed by rotating the sample stage by
45â—¦ around one axis, exposing the walls of the drilled holes directly to the print head.
A rectangular pattern was printed along the walls of each via, resulting in a conductive

42

Figure 3.4: A photograph of the printed oscillator circuit.

path between the ground plane and the microstrip lines printed on the top layer of the

substrate. After the nal sintering step is performed for the printed vias, each of the necessary

components (resistors, DC bias line, and transistor) were attached using solder paste (Kester

ESP265), and hot air reow soldering. The nal fabricated circuit is shown in Figure 3.4.

3.3.3 Measurement

This circuit was measured using an HP 8562A spectrum analyzer, and a 425 µm pitch ground-

signal (GS) probe. An external bias tee was used to block DC voltage from the RF input of

the spectrum analyzer. At a bias point of Vgs = 2.5 V, the transistor drew approximately 20

mA of current. Output of the oscillator was measured to be 2.65 dBm at a resonant frequency

of 8.513 GHz, with approximately 1 dB of loss across the GS probe tip and coaxial cable

used. Fig. 3.5 shows the measured fundamental frequency of the printed oscillator. This

measurement was performed with a resolution bandwidth of 10 kHz. The power in the second

harmonic of the oscillator was found to be 22 dB lower than at the fundamental frequency

at 17.03 GHz. The DC-RF conversion eciency is approximately 3.68%. Initial phase noise

measurements at 1 kHz oset from the carrier frequency is approximately -37 dBc/Hz. This

43

can likely be reduced through design modications, as well as improvement in via fabrication

techniques.

The measured frequency of oscillation of the fabricated circuit is lower than the frequency

predicted through simulation. A possible cause of this is due to poor metallization of the via

walls, which is likely a result of poor hole drilling quality. As a result of mechanically drilling

the soft LCP material, tearing occurs throughout the via, as well as at the entry and exit of

the LCP, resulting in poor metallic connection between the top and bottom layers of metal

to the metal deposited on the via walls. To simulate the impact of poor quality metallization

of the vias, the simulation has been modied to include an inductance of approximately 60

nH between the terminating 50 Ω resistor and ground, and 20 nH between the transistor

gate bias line and ground. This has resulted in a signicant deviation in the fundamental

frequency of oscillation, shifting down from 10.1 GHz to 8.48 GHz. Additionally, the power

of the second harmonic of the oscillator has decreased, to approximately 20 dB lower than

at the fundamental frequency, which agrees well with the measured results.

3.4 Summary

In this chapter, aerosol jet printing is utilized in the rapid fabrication of an oscillator

circuit. Two passive circuit elements have been presented: a 50 Ω transmission line, and

a band-stop microstrip resonator. Transmission line measurements have supported printed

metal conductivity and surface roughness approximations necessary for the accurate model-

ing and simulation of other printed microstrip structures.

An oscillator circuit has been presented, in which all metal layers have been printed

using aerosol jet technology, including metallized vias. Measurement of this oscillator, and

subsequent simulation, has demonstrated that via metallization quality can have a drastic

impact on the circuit performance. While this process will require improvement in the

future, this demonstrates the capability of aerosol jet printing systems to completely realize

microwave and millimeter-wave circuits.

44

Figure 3.5: The measured power spectrum of the printed oscillator fundamental output
frequency.

45

CHAPTER 4

HYBRID ADDITIVE AND SUBTRACTIVE PROCESSES FOR

ELECTRONICS PACKAGING

4.1

Introduction

As operating frequencies continue to increase into the millimeter-wave range and beyond,

unwanted parasitic eects of component packaging, such as added capacitance and induc-

tance, becomes an obstacle which must be addressed. Conventionally, to overcome some of

these limitations, unpackaged devices are directly wire- or ribbon-bonded to transmission

lines and other circuit board traces necessary for proper device operation [7, 8]. While an

improvement over typical packaged components, these bonding wires and ribbons present

unwanted inductance and resistance, and are dicult to implement due to the special han-

dling required to achieve good metal bonding. Furthermore, these techniques do not allow

for high density integration of RF systems.

With the growing body of work over recent decades in applying AM techniques for fabrica-

tion of electronic circuits, interest has turned towards applying these processes for fabrication

of high functional density systems operating in the microwave and millimeter-wave frequency

range.

Over the last few years, there has been growing interest in the use of additive manufactur-

ing (AM) for the fabrication of high functional density systems operating in the microwave

and millimeter-wave frequency range. Towards this goal, many passive components such as

transmission lines [92, 93], waveguides [37], antennas [45, 94], as well as capacitors, induc-

tors and resistors [44, 95] have been demonstrated using AM. Direct printing of transmission

lines on substrates through AM has also been explored. However, when combining active

circuit elements with circuits fabricated by methods such as inkjet or aerosol jet printing,

the forces necessary to attach a bond wire to these printed metals can present great diculty

46

in achieving a quality bond [53]. To avoid direct bonding of chips on printed traces, there

has been work focusing on the incorporation of active components with thick-lm additively

manufactured structures and circuits [96, 97].

By embedding active devices within the substrate of choice, interconnection lengths be-

tween bonding pads for both RF and DC lines can be reduced, signicantly reducing un-

wanted parasitic inductance due to bonding wires. Chip-rst processes have been developed,

where devices are initially attached to an appropriate carrier, with dielectric materials sub-

sequently deposited by lamination [98, 99, 21] or spin-coating [100, 18]. These dielectrics

are then processed by means of laser ablation, or photolithography in the case of photo-

sensitive dielectrics. Metal layers are deposited and processed by traditional lithographic

processes. Chip-last approaches have also been developed, in which metal and dielectric lay-

ers are deposited and processed, with the active device attached at the end of the fabrication

process [101], [102]. These processes provide device connections with very low insertion loss

[17, 103, 104], and can be used to process many devices simultaneously. However, these

processes require good planarization of devices, either by means of varied pocket depths in

substrates, or by planarization of the deposited material.

Direct-write technologies of conductive lms present a method by which chip-to-chip and

chip-to-board connections can be fabricated through selective deposition of dielectric and

metal materials, eliminating the requirement of planarization of devices to be encapsulated.

Chip-to-chip connections for high-speed dierential signaling [55], and chip-to-board connec-

tions [56], have been shown to reduce insertion loss over equivalent wire bonds [4], facilitating

operation up through 110 GHz [105]. Dielectric ramp structures can also be reduced and/or

eliminated by placing components in a pocket fabricated within the substrate, further re-

ducing the unwanted parasitic eects of each interconnection [57]. While these preliminary

results demonstrate a reduction in losses associated with connections between transmission

lines, such performance has not been demonstrated with connection to a monolithic mi-

crowave integrated circuit (MMIC). A chip-rst method, in which devices are attached to a

47

ground plane with substrate material and supporting circuitry subsequently deposited, has

been shown to provide very low-loss performance through 67 GHz [106]. Among the many

AM technologies, aerosol printing is attractive for such applications due to its ability to print

ne resolution structures on planar and non-planar surfaces from a large stando distance,

enabling the integration of devices with varying thickness without the need for pre- or post-

planarization. However, printing of thick structures (>100 µm) with good resolution using

this approach is still a challenge due to slow printing speeds. By printing metal interconnec-

tions, bond wire height can be virtually eliminated, and a transmission line with a controlled

impedance can be brought to the edge of the device, improving circuit performance and

robustness.

This chapter demonstrates a hybrid process whereby a thick substrate having deep cav-

ities to accommodate passive and active elements is fabricated using a conventional litho-

graphic method, and interconnections and supporting circuitry are fully aerosol jet printed.

To the best of our knowledge, this is the rst demonstration of an aerosol jet printed inter-

connection to an embedded MMIC and a package. Printed interconnections to both passive

and active devices, formed by depositing a constant width line to the edge of the device,

is demonstrated operating through 60 GHz. The ability to iteratively deposit metal will

be demonstrated, both to achieve desired circuit performance and to repair defects created

during the fabrication process. Additionally, compatibility challenges of printed metals over

semiconductor materials will be demonstrated and discussed.

4.2 Design and Fabrication

Utilizing a hybrid subtractive and additive manufacturing process, low-loss commercially

available substrate materials can be used to quickly fabricate microwave and millimeter wave

circuits. A liquid crystal polymer (LCP) substrate, Panasonic RF705T, is used here due to

its low-loss characteristics over the frequency range of interest, with a dielectric constant (
r)

of 3.1, and a loss tangent (tan-δ) of approximately 0.004. A substrate thickness of 100 µm

48

is used, equal to the thickness of each device to be embedded, eliminating the requirement

of any additional llets to be formed and minimizing losses associated with the transitions.

Substrate preparation consists of a two-step etching process, beginning with the pattern-

ing of the existing copper cladding by means of a standard photolithographic process. This

patterned copper forms a hard mask, capable of withstanding the etchant solution utilized

to selectively etch LCP. The exposed LCP material is etched using a solution consisting of 40

w.t. % potassium hydroxide, 30 w.t. % ethanolamine, and 30 w.t. % water [107]. This solu-
tion is heated to 85◦ C, providing an etch rate of approximately 2.5 µm per minute. After the
exposed LCP has been etched, the substrate is rinsed in deionized water and blown dry using

nitrogen gas. Each of the cavities are lled with an acetone-soluble resist material to preserve

the RF ground plane, and the remaining exposed copper is chemically stripped away. After

etching, through-substrate features, including all pockets as well as vias to ground, have a
tapered wall of approximately 56â—¦. This non-vertical feature provides an ideal surface on
which metal can be deposited without the need for additional manipulation of the substrate

to expose vertical surfaces. This tapered wall also provides a good transition between the

surrounding LCP, and the epoxy used to ll the pocket around each device, resulting in a

void-free surface onto which silver metal can be deposited.

After the substrate has been etched, devices are attached to the exposed ground plane

using a silver epoxy, Ablestik 84-1LMISR4. After the epoxy has been cured, the remaining

spaces around each die are lled with a UV curable epoxy, Formlabs High Temp resin, which
has a heat deection temperature Tg of 238â—¦ C at 0.45 MPa , enabling it to withstand the
high temperature required to sinter the printed silver nanoparticle ink.

After curing this epoxy, silver ink structures are aerosol jet deposited to form transmission

lines, interconnections to each device, and biasing lines for each active device to be measured.

Diluted Clariant Prelect TPS 50 G2 silver nanoparticle ink is atomized using the ultrasonic

atomization process, and is carried to the print head with a gas ow rate of 23 SCCM. In the

print head, the ink stream is combined with a focusing sheath gas at a ow rate of 50 SCCM.

49

Figure 4.1: Substrate preparation process.

This column of ink is directed towards the substrate through a print nozzle with a 150 µm

diameter, providing a line width of approximately 25 µm, with 1.5 µm thickness. Two layers
are deposited, providing a nal metal thickness of 3 µm. This ink is sintered at 180 ◦C for 3
hours. Figure 4.1 outlines the complete substrate preparation and metal deposition process.

A series of trenches have been fabricated to demonstrate the ability to form low-loss

interconnections over the epoxy material to be deposited around each die. These trenches

represent the anticipated gap between each die and the surrounding LCP material. While

50

Figure 4.2: Prolometer measurement of trenches in LCP substrate.
section representation of a lled trench.

Inset shows cross-

the particular dielectric properties of this material have not been characterized, many printed

materials have been shown to have a dielectric constant of approximately 3 [108] with a loss

tangent ranging from 0.01 to 0.04. This material is assumed to have a dielectric constant

of 3, and a loss tangent of 0.04. Due to lateral etching of the LCP during the wet-etching

process as well as some over-etching to ensure all existing LCP has been eliminated from the

cavity, the nal widths of these trenches are signicantly wider than the 200 µm gap which

will surround each device, at approximately 650 µm.

Figure 4.2 shows a two-dimensional prole of each trench after lling with epoxy, mea-

sured using a NanoMap-500LS contact surface prolometer. As can be seen, each trench is

slightly under lled, resulting in an average drop of approximately 25 µm from the surface

of the LCP. Simulation of a 650 µm wide trench having a 265 µm wide transmission line

51

Figure 4.3: Simulated insertion loss of printed interconnection over trench.

passing over top is shown in Figure 4.3. Three congurations are considered: a fully lled

trench, a trench exhibiting a concave meniscus 30 µm deep, and an under lled trench with

the epoxy surface 30 µm below the surface of the surrounding LCP substrate. As can be

seen, insertion loss of the fully lled conguration is low, remaining below 0.25 dB through

60 GHz. Insertion loss of a constant width line printed over a trench with a 30 µm concave

meniscus increases to a maximum of 0.36 dB, and to a maximum of 0.55 dB for the under

lled case. These losses are attributable to the changing transmission line impedance over

this under lled section, and can be minimized by shaping the interconnection to maintain

a 50 Ω impedance across the surface.

Two bare-die devices are used to demonstrate the ability to fabricate a low-loss wide-

bandwidth connection by aerosol jet printing to devices embedded in the LCP substrate.

A 0 dB attenuator capable of operating from DC to 40 GHz, Mini-Circuits KAT-0-DG+,

52

is used to evaluate the performance of each printed interconnection without introducing

excessive additional losses. Finally, a low-noise amplier (LNA), Quorvo TGA2512, is used

to demonstrate the compatibility of this process with active components. An LNA is used

in this application to reduce any potential unknown eects due to power dissipation in the

printed silver transmission lines and interconnections. The maximum RF power considered
here is −3 dBm, or 500 µW.

Each circuit is fabricated by printing a transmission line 255 µm wide to within 100 µm

of the edge of each device, where the line width is reduced to slightly less than the width of

each bond pad on the chip, and printed to completely cover each pad. To facilitate probing of

this circuit, a coplanar waveguide (CPW) with a conductor width of 203 µm, and gap width

of 60 µm, is used as a probe pad. Each ground strip of this CPW is connected to RF ground

by pockets patterned into either side of the structure. This CPW line is transitioned to the

microstrip transmission line over a length of 100 µm.

Images of each of these fabricated

circuits are shown in Figures 4.4, 4.6, and 4.8.

4.3 Measurement

Due to overspray which occurs during the fabrication process, the nal dimensions for

each line are slightly larger than the design goal. Each transmission line has been designed

with a goal of 255 µm wide line, where the nal fabricated width is approximately 265 µm.

Each of the CPW pads used to facilitate probing of the circuits have a nal conductor width

of approximately 220 µm, with a gap of 40 µm. The deviation in nal line dimensions does

not signicantly impact circuit performance, due to signicantly reduced metal thickness at

the edge of each printed feature.

Performance of each device has been measured on an MPI TS150-THZ probe station,

and Keysight N5227A PNA Microwave Network Analyzer, using GSG probes with 250 µm

pitch. An image of the nal fabricated interconnection over a trench is shown in Figure 4.4.

Insertion loss of each of these connections over a trench can be seen in Figure 4.5. This

53

Figure 4.4: Fabricated interconnection over trench.

insertion loss is calculated as:

ILT rench = ST hru

21 − SM eas

21

(4.1)

where SM eas

21

is the total measured insertion loss of each transmission line with its associated

interconnection with a total length of 4.3 mm, and ST hru

21

is the measured insertion loss of a

3.9 mm transmission line over LCP only. As is shown, the measured performance of each of

these connections agrees well with simulation, with an average insertion loss of 0.15 dB at

40 GHz. At higher frequencies, the characteristic impedance of the interconnection changes

due to the underlled trench, resulting in an increase in eective loss. By tailoring the line

dimensions to maintain a constant impedance throughout the transition region can greatly

reduce this loss at high frequencies.

In Figure 4.6, the nal fabricated circuit connecting a 0 dB attenuator is shown. The

distance from the edge of the die the edge of the surrounding LCP is approximately 450

µm, shorter than the 650 µm distance considered for the individual interconnection over a

trench. Insertion loss of each interconnection is shown in Figure 4.7. This insertion loss is

calculated as:

(cid:16)

1
2

ILAtten =

ST hru
21 + SDev

21 − SM eas

21

(cid:17)

(4.2)

54

Figure 4.5: Insertion loss of interconnection over each trench, as calculated by Equation 4.1.

where SM eas

21

and ST hru

21

are the total measured insertion loss and the measured insertion loss

of a zero-length through line respectively, and SDev

21

is the device insertion loss as measured

directly through a representative device with no external connections. As can be seen in

Figure 4.7, each connection introduces approximately 0.15 dB of insertion loss at 23 GHz,

and less than 0.2 dB at 40 GHz. The loss of each interconnection remains low throughout

the measured frequency range, and can be further improved by reducing the gap surround

each device, as well as by tailoring the line dimensions over the epoxy ller to maintain a

constant impedance to the device edge.

Figure 4.8 shows one of the fabricated circuits connecting to an LNA. The RF input line

can be seen in the lower left, with RF output in the lower right, and the DC biasing line

connecting to the top of the die. The biasing line has been printed over a 100 pF capacitor, as

specied by the device manufacturer. Additional leads have been attached to the board for

55

Figure 4.6: Fabricated interconnection to 0 dB attenuator.

connection to an external power supply for biasing of each amplier. A ground lead, and a

positive bias lead for each amplier, are attached to the board by means of a conductive silver

epoxy, MG Chemicals 8331S. Due to an oset of approximately 100 µm from their intended

positions, each transmission line has a slight bend, allowing for a symmetric transition from

the width of the transmission line to the width of the RF bonding pads.

Figure 4.9 shows the measured gain performance of two LNA circuits compared to the

amplier data sheet. A DC bias of 5 V is applied, drawing approximately 93 mA of current.

Both fabricated circuits demonstrate similar gain, which is slightly less than the manufacturer

provided data, due to several ohms of series resistance in the biasing circuit as a result of the

various epoxies used for attaching leads and the die itself. RF input return loss and output

return loss match well with the amplier data sheet, as can be seen in Figures 4.10 and 4.11.

56

Figure 4.7: Per-interconnection loss measured through 0 dB attenuator, as calculated by
Equation 4.2.

4.4 Rework of printed lines

One unique advantage of utilizing an additive process to deposit metal structures, either

complete circuits or interconnections between devices and boards with existing metal traces,

is the ability to iterate the deposition process to achieve desired performance characteristics.

Figure 4.12 demonstrates the insertion loss of a 4.5 mm long transmission line printed over

a trench, measured with dierent metal thicknesses.

Initially, a very thin layer of silver

metal was deposited, less than 1 µm thick. Measured performance of this line is rather poor

due to high resistance, with an insertion loss of nearly 4.5 dB at 60 GHz. An additional

3 µm of silver was then deposited over the top of this line, and remeasured. Performance

of this thickened line improved greatly, with total insertion loss at 60 GHz reduced to less

than 1.75 dB. This ability to iteratively deposit material can also be applied to deposition of

57

Figure 4.8: Fabricated connections to amplier MMIC.

impedance matching circuits, as well as deposition of other materials, such as for fabrication

of printed resistors or capacitors.

Another case in which this iterative deposition process can be useful is in the repair

of interconnections which have failed due to thermal stress. The left image seen in Figure

4.13 shows an attenuator connection which has cracked due to thermal stress. This crack is

approximately 2 µm wide, at the interface between the GaAs die and the surrounding epoxy

material, resulting in an open circuit. An additional 3 µm of silver metal was deposited on
top of the existing printed traces, and cured for 3 hours at 180 â—¦C on a hot plate, with a
heating and cooling temperature gradient of approximately 5 â—¦C per minute. The repaired
connection is shown in the right image of Figure 4.13. Measurement of this circuit after

repair shows insertion loss through the device matches well to the device which did not

58

Figure 4.9: Gain performance of amplier MMIC with printed connections.

exhibit any cracking, as can be seen in Figure 4.15.

4.5 Diode eects of Ag on GaAs

One possible unwanted eect which can be introduced through direct writing of inter-

connections to bonding pads is the unintentional formation of a Schottky diode due to a

metal-semiconductor contact where silver is printed over an area lacking any passivation.

Ag/GaAs diodes have been demonstrated by [109]. In this instance, the 0 dB attenuators

considered are formed on a GaAs substrate, with an exposed area of GaAs at the edge

of the die. By printing silver directly over this area of exposed semiconductor, a diode is

unintentionally formed. Figure 4.16 shows the measured current vs applied voltage of the

attenuator circuit shown in Figure 4.6. A schematic representation of this conguration is

shown in Figure 4.17. This unintentional diode has the possibility of introducing unwanted

59

Figure 4.10: Amplier circuit input return loss.

harmonic content in applications where voltages can exceed the diode turn-on voltage. How-

ever, this eect can be reduced or eliminated by the careful deposition of an insulating

material over any exposed semiconductor material where metal is to be printed.

4.6 Summary

As operating frequencies continue to increase, unwanted parasitic eects of component

packaging need to be mitigated and/or eliminated. By embedding devices in the substrate, as

demonstrated here using a hybrid process, controlled impedance lines can be extended to the

edge of the device, greatly reducing the losses associated with traditional wire bonding, pro-

viding a low cost path for device integration operating into the mm-wave frequency range and

beyond. Interconnections have been demonstrated operating through 60 GHz, with insertion

loss well below 0.5 dB, which can be further reduced by leveraging AM processes to tailor

60

Figure 4.11: Amplier circuit output return loss.

circuit geometry in real-time to achieve desired circuit performance. Compatibility with

bare-die devices has been demonstrated operating through 40 GHz with per-interconnection

insertion loss below 0.2 dB, as well as compatibility with active RF MMICs. With the ex-

ibility of additive processes, requirements of device positioning can be relaxed and adjusted

for in real-time. This per-interconnection loss compares well to that demonstrated in [106].

While there exists a possibility of the unintentional creation of a diode within the signal

path by depositing metal directly over a semiconductor material, this eect can be reduced

or eliminated entirely by depositing an insulating material (e.g., polyimide) between the

conductive trace and the semiconducting substrate.

61

Figure 4.12: Measured transmission coecient of 4.5 mm long transmission line printed over
a trench, before and after printing additional silver.

62

Figure 4.13: Image of die with cracking at Ag-GaAs interface before repair (left) and after
repair (right).

63

Figure 4.14: Measured transmission coecient of attenuator circuit with cracked interface
before and after repair.

64

Figure 4.15: Measured insertion loss of interconnection to die after repair as calculated using
equation 4.2, compared to one with no cracking.

65

Figure 4.16: Measured I-V curve of Ag-GaAs interface on attenuator.

Figure 4.17: Schematic representation of Ag-GaAs diode.

66

CHAPTER 5

ADDITIVELY MANUFACTURED SELF-PACKAGED KU-BAND

TRANSMITTER

5.1

Introduction

In this chapter, we address a key problem facing increased demand for wireless electronic

systems. This demand ranges from applications such as automotive radar systems and

vehicle-to-everything (V2X) communications, to consumer electronics, military and space-

based platforms. As the operating frequencies of these systems continues to increase, from

microwave to mm-wave and beyond, losses associated with electronics packaging, as well

as long interconnection distances between active components and their associated antennas,

introduces a bottleneck in system performance. By integrating transmit and receive (T/R)

electronics with their respective antenna, these losses can be minimized, improving system

eciency and performance, while achieving a small overall size and reduced weight. By

employing additive manufacturing processes, high levels of integration can be achieved while

minimizing system losses associated with packaging, reducing production time and costs, as

well as allowing for greater exibility in component selection.

Many electronics packaging techniques have been developed which provide varying de-

grees of integration and functional density, such as System-on-Chip (SoC), System-on-

Package (SoP) and System-in-Package (SiP). SoC provides a high level of system integration,

incorporating analog, digital and RF electronics on a single die [110]. By including antennas

on the same die, the shortest possible distance between the T/R electronics and the radiating

element can be achieved [111]. However, below mm-Wave frequencies, the area required for

such an antenna can make this approach cost-prohibitive. Performance of on-chip antennas

can also be greatly degraded due to the substrates on which they are fabricated [112]. While

SoC designs provide a high level of integration, their components are xed at the design

67

stage, increasing development cost and time.

SiP and SoP technologies provide a lower cost alternative to SoC development while

providing high functional density. By leveraging existing integrated circuits, non-recurring

engineering costs are reduced, as well as the time required for development and testing.

Through both SiP and SoP processes oer a reduction in size, at mm-wave frequencies and

beyond, the chip-to-chip and chip-to-board interconnections traditionally fabricated by bond-

wires presents a signicant limitation to system performance. Connections between active

circuits and their respective antennas may require expensive connectors and cables, increasing

system complexity, cost and weight, while contributing additional losses to transmitted and

received signals.

This chapter demonstrates a compact system-on-antenna utilizing a light-weight, low

cost antenna with integrated transmit electronics operating in the Ku frequency band. By

employing a hybrid additive and subtractive fabrication process, COTS substrate materi-

als with good RF performance can be combined with aerosol jet printed interconnections,

ensuring desired system performance, as well as providing the smallest footprint possible

compared to traditionally packaged components. Combining a rigid 3D printed antenna

with a exible substrate, as demonstrated in [113, 114], the active devices can be readily

integrated within the volume of the antenna, providing the shortest possible interconnection

between the transmit electronics and the antenna feed location. By eliminating the need for

additional RF connectors and cables, the mass of the transmitter can be minimized, while

maximizing eciency by eliminating cable and connector insertion losses.

5.2 Design and Fabrication

Design and fabrication of the antenna and active electronics can be performed in parallel.

A Vivaldi antenna is designed and measured to ensure proper radiator performance over the

required frequency band. The active electronics are designed to achieve desired system

performance, and arranged to t within a cavity formed inside the legs of the antenna. The

68

active electronics are then assembled and combined with the antenna to form a completed

system. The substrate of choice for this work is Panasonic Felios RF705T liquid crystal

polymer (LCP), due to its excellent RF characteristics over the frequency range considered,

as well as its compatibility with a variety of fabrication techniques.

The circuit demonstrated in this work combines a commercially available voltage con-

trolled oscillator (VCO) with a medium power amplier. The VCO used is a United Mono-

lithic Semiconductors CHV2270, oering a tunable output frequency of approximately 12

GHz to 13.5 GHz. Measured output power of this device varies over the tunable frequency
range, from approximately −1 dBm around 12.1 GHz, to nearly 4 dBm at 13.5 GHz. This
output is passed through an Analog Devices HMC451 medium power amplier, providing

approximately 20 dB of gain, with a measured saturated output power of approximate 21

dBm. A 0 dB attenuator, Mini-Circuits KAT-0-DG+, is placed before the RF input to

the amplier, providing exibility for designs requiring attenuation of the RF signal be-

fore amplication without the need for additional alterations to the substrate. A schematic

representation of this circuit can be seen in the top of Figure 5.6.

Fabrication of the antenna begins by printing the body and covers of the antenna on

an Objet Connex350 polyjet printer. After removal of support material and cleaning, a 60

nm thick layer of titanium metal is deposited on the surface of the antenna to aid in the

adhesion of subsequent metal layers. A 500 nm thick layer of copper metal is then sputter

deposited over the titanium layer, providing a seed layer for plating. An additional 6 µm of

copper metal is electroplated on top of the seed layer, providing a nal metal thickness of

approximately 6.5 µm.

The active circuits are fabricated on a 100 µm thick LCP substrate, providing a substrate

surface level with the surfaces of both the VCO and amplier. This material is prepared

by removing existing copper cladding material from a 75 µm thick sheet of LCP material.

This is then laminated to a 0.5 mm thick copper shim with a 25 µm thick bonding sheet,

Panasonic R-BM17, providing a dielectric thickness of approximately 100 µm. The copper

69

shim serves as an RF and DC ground plane, as well as providing a degree of heatsinking for

the active components. Pockets are then etched from this laminated LCP stack with the use

of a laser cutter utilizing a 40 watt CO2 laser. After etching of the pockets for the active

devices, the substrate is cleaned in a KOH solution described in Chapter 4.

After substrate preparation, active devices are placed within their respective cavities.

Active components, as well as bypass capacitors, are attached to the exposed copper ground
plane with Epo-Tek H20E silver epoxy and cured at 150 â—¦C for 1 hour. The remaining areas
within each pocket are then lled with an ultraviolet light curable, high temperature epoxy,

Formlabs High Temp resin. This epoxy is deposited by syringe and allowed to cure under

UV light exposure for 5 hours.

As was shown in chapter 4, a Schottky diode was unintentionally formed when silver metal

was deposited over the exposed semiconductor material of an IC. This diode may introduce

additional harmonic content in a transmitted or received signal, as well as potentially drawing

additional power, reducing system eciency. By depositing an additional dielectric material

over the exposed semiconductor, this eect can be minimized or eliminated.

A poly(methyl methacrylate) (PMMA) ink has been created to provide an insulating

layer between each bare die IC used and the silver nanoparticle ink to be printed. This ink

is prepared by combining 45 w.t.% dimethyl sulfoxide (DMSO) and 45 w.t.% toluene, into
which 10 w.t.% PMMA powder is added. This solution is heated to approximately 65 â—¦C and
stirred for 6 hours until all solid material has dissolved. This ink is then aerosol jet printed

using a pneumatic atomization process. A nozzle with a 300 µm diameter is used, with an

atomizer gas ow rate of 555 SCCM, exhaust gas ow rate of 525 SCCM, and sheath gas

ow rate of 80 SCCM, with a print stage velocity of 1 mm/s, yielding lines approximately 50

µm wide and 4 µm thick. During printing, the printer stage is heated to approximately 60
â—¦C to remove some of the solvent, allowing multiple layers to be printed without signicant
deformation to previously deposited layers. After printing, each piece is dried on a hotplate
at 120 â—¦C for 30 minutes to completely remove any remaining solvent.

70

One advantage to using a thermoplastic polymer as a printed dielectric layer is that it can

be melted to form a smooth surface. After initial printing of the PMMA ink, the ink surface

exhibits signicant roughness due to the printing process. After printing, this PMMA layer

can be melted at a temperature greater than the melting temperature of the PMMA, and

below the melting temperature of the LCP substrate. After drying of the PMMA material
is complete, it is subsequently placed under vacuum at 180 â—¦C for 12 hours. This vacuum
environment removes any additional gas which may be held within the printed layer, as well

as allowing the material to melt and self-level. Figure 5.1 shows a surface prole of a section

of printed PMMA, before and after this nal thermal leveling process.

By printing PMMA around each device, transitions from the die surface to the sur-

rounding epoxy material can be improved, removing sharp features present from possible

under-lling of the pocket surrounding each device. Figure 5.2 shows the transition from the

surface of a 0 dB attenuator to the surrounding epoxy material, before and after deposition

of a PMMA layer. As is shown, the PMMA layer remains thin, while providing a smooth

transition from the exposed bonding pad, to the die surface, and nally to the surrounding

pocket.

After printing and thermally treating the PMMA layer, silver metal is deposited to form

transmission lines, device interconnections, and DC biasing lines. The silver ink used here is

diluted Clariant Prelect TPS 50 G2, and aerosolized using an ultrasonic atomization process.

The ink is printed using a print nozzle diameter of 150 µm, an atomizer carrier gas ow rate

of 23 SCCM, and sheath gas ow rate of 50 SCCM, resulting in lines approximately 2 µm

thick and 20 µm wide. Three layers of silver metal are printed to achieve a nal metal layer
thickness of 6 µm, and cured at 180 ◦C for 3 hours. Figure 5.3 shows the nal cross-sectional
stackup of the active components. Figure 5.4 shows an optical image of an interconnection

formed by printed silver over printed PMMA to the input of a 0 dB attenuator.

With printing of the PMMA and silver metal layers complete, a 4 conductor ribbon cable

connector is attached using the same die attach epoxy and curing schedule. This ribbon

71

Figure 5.1: Surface prole plot of PMMA before and after thermal leveling.

cable provides DC power for the VCO and amplier. The nal assembled active components

are mounted within the leg of the Vivaldi antenna. The copper sheet is attached to the
antenna leg with MG Chemicals 8331 silver epoxy, and thermally cured at 65 â—¦C for 3 hours.
The antenna feed line is attached to a recess in the opposing leg using the same silver epoxy.

The assembled antenna can be seen in Figure 5.6. Figure 5.5 summarizes the complete

fabrication process.

5.3 Measurement

The received power spectrum was measured using an HP 8562A spectrum analyzer con-

nected to a Cobham Sensor Systems H-1498 horn antenna by a 2 meter long coaxial cable.

This antenna has a measured gain of 10.4 dBi at 13 GHz. Cable losses have been measured

at 2.2 dB over the 2 meter length at 13 GHz. The transmitting antenna is placed at a

72

Figure 5.2: Surface prole plot of a transition from an attenuator to the surrounding epoxy
resin, before and after PMMA deposition.

distance of 2 meters from the receiving antenna. Expected received power Pr is calculated

from [84] as:

Pr = Pt − Lt + Gt − L0 + Gr − Lr (dBm)

(5.1)

where Pt is the transmit power, Lt is the feed line loss, Gt is the transmitter antenna gain,

L0 is the free space path loss, Gr is the receiver antenna gain, and Lr is the loss in the

receiver cable. Transmitter feed line loss Lt is approximately 0.2 dB, transmitter gain Gt at

12 GHz has been measured to be 8.9 dBi. Free space path loss is calculated from [84] as:

where d is the distance from the transmitter to the receiver, and λ is the wavelength in free

L0 = 20log(

4Ï€d

λ

)

(5.2)

73

Figure 5.3: Stackup of completed active circuit.

space. At a distance of 2 meters, L0 is approximately 60 dB. With a transmit power level Pt
of 18.4 dBm, this corresponds to an expected received power level of −24.9 dBm. Received
power, shown in Figure 5.7, is approximately -34 dBm, 10.6 dB less than that predicted by

Equation 5.1. Figure 5.8 shows the expected received power spectrum. The spectrum shown

in this picture has been measured from the output of the medium power amplier, and scaled

by Equation 5.1. This reduction in received power is currently being investigated, however

it is most likely due to a poor electrical connection between the 0.5 mm copper sheet on

which the active devices are mounted, and the copper metal of the antenna, resulting in a

74

Figure 5.4: Aerosol jet printed interconnection, silver over PMMA to amplier RF output.

75

Figure 5.5: Fabrication process for the printed antenna and active circuit components.

76

Figure 5.6: Top: schematic representation of the active circuit. Bottom: assembled antenna
with integrated active electronics.

high inductance in the RF ground connection. Antenna modications are currently being

developed to improve the assembly process. Measurement results are pending, and will be

published upon completion.

5.4 Summary

This chapter demonstrates a compact system-on-antenna capable of operating in the Ku

band. By utilizing additive manufacturing techniques, active electronic components can be

77

Figure 5.7: Received power spectrum.

easily integrated with antennas, providing a high degree of functionality with a minimum

system footprint. Elimination of expensive and heavy connectors, as well as bonding wires

typically associated with packaged electronics allows for low-loss operation well into the mm-

wave frequency range, enabling a wide range of systems to be developed at low cost with

short development times.

78

Figure 5.8: Anticipated received power spectrum, as measured from output of active devices,
scaled by Equation 5.1.

79

CHAPTER 6

CONCLUSIONS

6.1 Conclusions

In this dissertation, a number of processes have been developed combining additive and

subtractive manufacturing techniques to package RF electronics capable of operating into

the mm-wave frequency range and beyond. By leveraging AM practices, fabrication time

and cost can be reduced, while simultaneously eliminating the need for highly specialized

equipment and facilities normally used to fabricate and package high frequency circuits and

systems.

In chapter 2, aerosol jet printing was demonstrated as a viable alternative to traditional

lithographic processes for fabrication of passive circuit elements capable of operating beyond

200 GHz. Polarizing screens, consisting of long parallel lines with narrow line widths and

separations were demonstrated, taking advantage of the ne feature sizes which can be

achieved with the aerosol jet printing process. The combination of printing a thin seed layer,

onto which additional metal can be plated with an electroless plating solution was shown,

reducing printing time and cost.

In chapter 3, aerosol jet printed circuits were combined with packaged components to form

a functional high frequency oscillator circuit. Distributed circuit elements were designed,

simulated and fabricated, forming the building blocks necessary for the complete circuit. By

leveraging the long stand-o distance of the print nozzle from the substrate surface, complete

metallization of the circuit, including vias connecting the top metal layer to the underlying

ground plane, were demonstrated, as well as limitations due to via wall quality as a result of

mechanical drilling operations. By combining aerosol jet printing with commercially available

components, circuit designs can be rapidly iterated, reducing time between initial system

concept and demonstration of a functional prototype.

80

Chapter 4 demonstrated a hybrid additive and subtractive process for packaging of bare

die integrated circuits. By utilizing commercially available substrates, materials can be cho-

sen with low-loss characteristics over the required frequency range, greatly improving system

performance over the more lossy materials currently capable of being printed. Replacing

bonding wires with printed interconnections provides much greater exibility and control of

the interconnection characteristics, allowing for low loss operation beyond 60 GHz, paving

the way for system-level integration of circuits operating beyond mm-wave frequencies.

In chapter 5, multiple AM techniques were combined to package electronics operating

from 12 GHz to 13.5 GHz. Using a polyjet printing process, a low cost, low-weight Vivaldi

antenna can be readily combined with active electronics, using the antenna body as the

system package. Printing additional polymer material, additional parasitic eects of printed

interconnections can be reduced or eliminated, while providing a high quality transition from

each die to its surrounding dielectric.

This work demonstrates the advantages of using additive manufacturing for RF circuit

prototyping and device packaging. Combined with subtractive processes, commercially avail-

able materials and components can be brought together to create a functional system rapidly,

without the need for complex machinery, sterile environments or highly skilled technicians.

6.2 Technical Limitations of Additive Manufacturing

Material compatibility poses a signicant challenge for packaging electronic components.

Many metal nanoparticle inks require thermal sintering at temperatures incompatible with

most printable polymers. Additionally, dierent coecients of thermal expansion can easily

result in failures of interconnections, or complete assemblies. By tailoring materials to achieve

desirable properties, these unwanted eects can potentially be minimized, greatly increasing

the range of applications possible with additive manufacturing techniques.

Printed materials are subject to environmental stresses. When left untreated, printed

silver metal oxidizes over time, degrading system performance. Traditional methods, such

81

as gold plating of conductors, have proven to be eective in mitigation of these factors.

However, these processes require additional personnel skilled in safe and accurate operation

of the systems necessary to carry out these tasks. By taking advantage of existing AM

processes, encapsulation of printed metals using thin dielectric materials may present one

method to combat these limitations.

The low conductivity of most printed metals can present a limitation to high power appli-

cations. Resistive losses encountered in these metals can lead to signicant self-heating of the

printed metal traces, ultimately resulting in destruction of the substrate onto which it has

been printed. Self-heating of printed metals may also speed degradation due to interaction

with atmospheric conditions. Dielectric and encapsulation materials must be compatible

with the temperatures that will be encountered during normal operating conditions.

6.3 Future Work

Additive manufacturing techniques can be combined to achieve a wide range of goals.

Taking advantage of metal and ceramic printing processes, a wide range of structures can

be fabricated capable of withstanding high temperatures experienced with high power elec-

tronics. Combined with inkjet and aerosol jet printing, high power, high frequency circuits

can be realized without the need for additional exotic materials and processes.

With the growing variety of printable polymer materials, low-loss substrate materials can

be deposited with both inkjet and aerosol jet technologies, providing an additional avenue for

fabrication of low-loss circuits operating well into the mm-wave frequency range. Using these

materials, a chip-rst packaging process can be developed using AM processes, reducing the

cost and time necessary to fabricate systems operating at these high frequencies. By com-

bining materials, functional substrates can be fabricated, incorporating lters, capacitors,

resistors, inductors, as well as microuidic channels for thermal cooling.

AM processes can be used to package both individual components, as well as complete

systems. Chip-scale and wafer-level packaging processes can be replaced or augmented by

82

leveraging AM techniques, printing multi-layered fan-out circuitry directly to the bonding

pads of a device, reducing the number of intermediate connections between devices and

enabling an increase in functional density.

The ne print resolution of inkjet and aerosol jet printing can be combined with existing

packaged components to fabricate complete systems rapidly and at low cost. Many digital

and mixed-signal integrated circuits have a large number of connections, with ne pitch

between each signal pad, typically requiring a multi-layer circuit board fabrication process to

fan out these connections, potentially increasing development cost when these high density

connections are localized to a few individual devices. By depositing metal and dielectric

materials, these high density connections can be fanned out locally, reducing fabrication

costs. When combined with RF integrated circuits, complete systems can be designed,

fabricated and deployed quickly and inexpensively.

83

BIBLIOGRAPHY

84

BIBLIOGRAPHY

[1]

C. Kim, D. Espalin, M. Liang, H. Xin, A. Cuaron, I. Varela, E. Macdonald, and R. B.
Wicker. 3D Printed Electronics With High Performance, Multi-Layered Electrical
Interconnect. IEEE Access, 5:2528625294, 2017.

[2] K. Y. Park, M. I. M. Ghazali, N. Wiwatcharagoses, and P. Chahal. Thick 3D Printed
RF Components: Transmission Lines and Bandpass Filters. In 2018 IEEE 68th Elec-
tronic Components and Technology Conference (ECTC), pages 21862191, May 2018.

[3]

J. A. Paulsen, M. Renn, K. Christenson, and R. Plourde. Printing conformal electronics
on 3D structures with Aerosol Jet technology.
In 2012 Future of Instrumentation
International Workshop (FIIW) Proceedings, pages 14. IEEE, Oct 2012.

[4] K. Lomakin, M. Sippel, G. Gold, J. Ringel, D. Weiÿ, K. Helmreich, M. Ankenbrand,
and J. Franke. Substituting bond wires by additively manufactured interconnections.
In 2018 11th German Microwave Conference (GeMiC), pages 367370, March 2018.

[5]

[6]

[7]

[8]

[9]

J. S. Kilby. Miniaturized electronic circuits, February 06 1959. US Patent 3,138,743.

T. S. Horng, S. M. Wu, H. H. Huang, C. T. Chiu, and C. P. Hung. Modeling of lead-
frame plastic CSPs for accurate prediction of their low-pass lter eects on RFICs.
IEEE Transactions on Microwave Theory and Techniques, 49(9):15381545, Sep. 2001.

J. Y. Kim, H. Y. Lee, J. H. Lee, and D. P. Chang. Wideband characterization of
multiple bondwires for millimeter-wave applications. In 2000 Asia-Pacic Microwave
Conference. Proceedings (Cat. No.00TH8522), pages 12651268, Dec 2000.

C. C. Wei, C. T. Fan, T. H. Chiang, M. K. Chiu, and S. P. Ru. A comparison study
of high-frequency performance between ball bonding and ribbon bonding. In 2009 4th
International Microsystems, Packaging, Assembly and Circuits Technology Conference,
pages 685688, Oct 2009.

J. Wolf, H Reichl, O Ehrmann, K Buschick, FJ Schmuckle, W Heinrich, M Toepper,
and A Owzar. System integration for high frequency applications. In PROCEEDINGS-
SPIE THE INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING, pages
2936. SPIE INTERNATIONAL SOCIETY FOR OPTICAL, 1997.

[10] W. Heinrich. The ip-chip approach for millimeter wave packaging. IEEE Microwave

magazine, 6(3):3645, 2005.

[11] S. Lee, S. Song, Y. Kim, J. Lee, C. Cheon, K. Seo, and Y. Kwon. A V-Band Beam-
Steering Antenna on a Thin-Film Substrate With a Flip-Chip Interconnection. IEEE
Microwave and Wireless Components Letters, 18(4):287289, April 2008.

85

[12] J. H. Lau, M. Li, Q. M. Li, I. Xu, T. Chen, Z. Li, K. H. Tan, Q. X. Yong, Z. Cheng,
K. S. Wee, R. Beica, C. T. Ko, S. P. Lim, N. Fan, E. Kuah, W. Kai, Y. Cheung,
E. Ng, C. Xi, J. Ran, H. Yang, Y. H. Chen, N. C. Lee, M. Tao, J. Lo, and R. Lee.
Design, Materials, Process, Fabrication, and Reliability of Fan-Out Wafer-Level Pack-
aging. IEEE Transactions on Components, Packaging and Manufacturing Technology,
8(6):9911002, June 2018.

[13] R. A. Fillion. A forecast on the future of hybrid wafer scale integration technol-
IEEE Transactions on Components, Hybrids, and Manufacturing Technology,

ogy.
16(7):615625, Nov 1993.

[14] R. Hahn, A. Kamp, A. Ginolas, M. Schmidt, J. Wolf, V. Glaw, M. Topper, O. Ehrmann,
and H. Reichl. High power multichip modules employing the planar embedding tech-
nique and microchannel water heat sinks. In Thirteenth Annual IEEE. Semiconductor
Thermal Measurement and Management Symposium, pages 4956, Jan 1997.

[15] B. Lee, V. Sundaram, B. Wiedenman, C. K. Yoon, V. Kripesh, M. Iyer, and R. R.
Tummala. Chip-last Embedded Active for System-On-Package (SOP). In 2007 Pro-
ceedings 57th Electronic Components and Technology Conference, pages 292298, May
2007.

[16] V. Sridharan, A. Goyal, S. Sitaraman, N. Kumbhat, N. Sankaran, H. Chan, F. Liu,
D. Dawn, V. Nair, T. Kamgaing, F. Juskey, V. Sundaram, and R. Tummala. Ultra-
miniaturized WLAN RF receiver with chip-last GaAs embedded active. In 2011 IEEE
61st Electronic Components and Technology Conference (ECTC), pages 13711376,
May 2011.

[17] R. Carrillo-Ramirez and R. W. Jackson. A highly integrated millimeter-wave active
IEEE Transactions on Microwave

antenna array using BCB and silicon substrate.
Theory and Techniques, 52(6):16481653, June 2004.

[18] X. Yang, A. Kaur, and P. Chahal. Embedded diodes for microwave and millimeter
wave circuits. In 2014 IEEE 64th Electronic Components and Technology Conference
(ECTC), pages 21442150, May 2014.

[19] R. R. Tummala. Packaging: past, present and future.

In 2005 6th International

Conference on Electronic Packaging Technology, pages 37, Aug 2005.

[20] T. Siebolt Tevelde and A. Schmitz. Semiconductor circuit having active devices em-

bedded in exible sheet, October 10 1968. US Patent 3,579,056.

[21] R. A. Fillion, R. J. Wojnarowski, T. B. Gorcyzca, E. J. Wildi, and H. S. Cole. De-
velopment of a plastic encapsulated multichip technology for high volume, low cost
commercial electronics. IEEE Transactions on Components, Packaging, and Manufac-
turing Technology: Part B, 18(1):5965, Feb 1995.

[22] M. Topper, T. Baumgartner, M. Klein, T. Fritzsch, J. Roeder, M. Lutz, M. von Su-
chodoletz, H. Oppermann, and H. Reichl. Low cost wafer-level 3-D integration without

86

TSV. In 2009 59th Electronic Components and Technology Conference, pages 339344,
May 2009.

[23] T. Duda and L. V. Raghavan. 3D metal printing technology. IFAC-PapersOnLine,

49(29):103110, 2016.

[24] J. Zuniga, D. Katsavelis, J. Peck, J. Stollberg, M. Petrykowski, A. Carson, and C. Fer-
nandez. Cyborg beast: a low-cost 3d-printed prosthetic hand for children with upper-
limb dierences. BMC research notes, 8(1):10, 2015.

[25] A. V. Do, B. Khorsand, S. M. Geary, and A. K. Salem. 3D printing of scaolds for
tissue regeneration applications. Advanced healthcare materials, 4(12):17421762, 2015.

[26] J. Wang, A. Goyanes, S. Gaisford, and A. W. Basit. Stereolithographic (SLA) 3D
printing of oral modied-release dosage forms. International journal of pharmaceutics,
503(1-2):207212, 2016.

[27] T. M. Rankin, N. A. Giovinco, D. J. Cucher, G. Watts, B. Hurwitz, and D. G. Arm-
strong. Three-dimensional printing surgical instruments: are we there yet? Journal of
Surgical Research, 189(2):193197, 2014.

[28] A. Bertsch, S. Heimgartner, P. Cousseau, and P. Renaud. Static micromixers based

on large-scale industrial mixer geometry. Lab on a Chip, 1(1):5660, 2001.

[29] M. Kasparova, L. Grafova, P. Dvorak, T. Dostalova, A. Prochazka, H. Eliasova,
J. Prusa, and S. Kakawand. Possibility of reconstruction of dental plaster cast from
3D digital study models. Biomedical engineering online, 12(1):49, 2013.

[30] A. T. Gaynor, N. A. Meisel, C. B. Williams, and J. K. Guest. Multiple-material
topology optimization of compliant mechanisms created via PolyJet three-dimensional
printing. Journal of Manufacturing Science and Engineering, 136(6):061015, 2014.

[31] S. Tibbits. 4D printing: multi-material shape change. Architectural Design, 84(1):116

121, 2014.

[32] J. A. Gonzalez, J. Mireles, Y. Lin, and R. B. Wicker. Characterization of ceramic com-
ponents fabricated using binder jetting additive manufacturing technology. Ceramics
International, 42(9):1055910564, 2016.

[33] J. P. Kruth, X. Wang, T. Laoui, and L. Froyen. Lasers and materials in selective laser

sintering. Assembly Automation, 2003.

[34] O. S. Kim. Rapid Prototyping of Electrically Small Spherical Wire Antennas. IEEE

Transactions on Antennas and Propagation, 62(7):38393842, July 2014.

[35] C. R. Garcia, R. C. Rumpf, H. H. Tsang, and J. H. Barton. Eects of extreme surface

roughness on 3D printed horn antenna. Electronics letters, 49(12):734736, 2013.

87

[36] D. Espalin, D. W. Muse, E. MacDonald, and R. B. Wicker. 3D Printing multifunc-
tionality: structures with electronics. The International Journal of Advanced Manu-
facturing Technology, 72(5-8):963978, 2014.

[37] E. G. Geterud, P. Bergmark, and J. Yang. Lightweight waveguide and antenna com-
ponents using plating on plastics. In 2013 7th European Conference on Antennas and
Propagation (EuCAP), pages 18121815, April 2013.

[38] M. I. M. Ghazali, K. Y. Park, V. Gjokaj, A. Kaur, and P. Chahal. 3D printed met-
alized plastic waveguides for microwave components. In International Symposium on
Microelectronics, volume 2017, pages 000078000082. International Microelectronics
Assembly and Packaging Society, 2017.

[39] M. I. M. Ghazali, E. Gutierrez, J. C. Myers, A. Kaur, B. Wright, and P. Chahal.
Aordable 3D printed microwave antennas. In 2015 IEEE 65th Electronic Components
and Technology Conference (ECTC), pages 240246, May 2015.

[40] J. A. Byford, M. I. M. Ghazali, S. Karuppuswami, B. L. Wright, and P. Chahal.
Demonstration of RF and Microwave Passive Circuits Through 3-D Printing and Selec-
tive Metalization. IEEE Transactions on Components, Packaging and Manufacturing
Technology, 7(3):463471, March 2017.

[41] J Ebert, E Özkol, A Zeichner, K Uibel, Ö Weiss, U Koops, R Telle, and H Fischer.
Direct inkjet printing of dental prostheses made of zirconia. Journal of dental research,
88(7):673676, 2009.

[42] H Sirringhaus, T Kawase, RH Friend, T Shimoda, M Inbasekaran, W Wu, and
EP Woo. High-resolution inkjet printing of all-polymer transistor circuits. Science,
290(5499):21232126, 2000.

[43] D. Redinger, S. Molesa, Shong Yin, R. Farschi, and V. Subramanian. An ink-jet-
deposited passive component process for RFID. IEEE Transactions on Electron De-
vices, 51(12):19781983, Dec 2004.

[44] B. S. Cook, J. R. Cooper, and M. M. Tentzeris. Multi-Layer RF Capacitors on Flexible
Substrates Utilizing Inkjet Printed Dielectric Polymers. IEEE Microwave and Wireless
Components Letters, 23(7):353355, July 2013.

[45] G. McKerricher, D. Titterington, and A. Shamim. A Fully Inkjet-Printed 3-D
Honeycomb-Inspired Patch Antenna. IEEE Antennas and Wireless Propagation Let-
ters, 15:544547, 2016.

[46] H. Kao, C. Cho, and L. Chang.

Inkjet-Printed Interdigital Coupled Line Filter on
Liquid Crystal Polymer Substrate. IEEE Electron Device Letters, 34(12):15841586,
Dec 2013.

[47] B. S. Y. Ung, B. Weng, R. Shepherd, D. Abbott, and C. Fumeaux. Inkjet printed con-
ductive polymer-based beam-splitters for terahertz applications. Opt. Mater. Express,
3(9):12421249, Sep 2013.

88

[48] M. Walther, A. Ortner, H. Meier, U. Löelmann, P. J. Smith, and J. G. Ko-
rvink. Terahertz metamaterials fabricated by inkjet printing. Applied Physics Letters,
95(25):251107, 2009.

[49] F. Cai, Y. H. Chang, K. Wang, W. T. Khan, S. Pavlidis, and J. Papapolymerou. High
resolution aerosol jet printing of D-band printed transmission lines on exible LCP
substrate.
In 2014 IEEE MTT-S International Microwave Symposium (IMS2014),
pages 13. IEEE, 2014.

[50] M. T. Craton, J. Sorocki, I. Piekarz, S. Gruszczynski, K. Wincza, and J. Papapoly-
merou. Realization of Fully 3D Printed W-Band Bandpass Filters Using Aerosol Jet
Printing Technology. In 2018 48th European Microwave Conference (EuMC), pages
10131016, Sep. 2018.

[51] Y. He, C. Oakley, P. Chahal, J. Albrecht, and J. Papapolymerou. Aerosol Jet printed
24 GHz end-re quasi-Yagi-Uda antenna on a 3-D printed cavity substrate. In 2017 In-
ternational Workshop on Antenna Technology: Small Antennas, Innovative Structures,
and Applications (iWAT), pages 179182, March 2017.

[52] A. A. Gupta, A. Bolduc, S. G. Cloutier, and R. Izquierdo. Aerosol Jet Printing for
printed electronics rapid prototyping. In 2016 IEEE International Symposium on Cir-
cuits and Systems (ISCAS), pages 866869, May 2016.

[53] C. Kaestle, J. Hoerber, F. Oechsner, and J. Franke. Prospects of wire bonding as an
approach for contacting additive manufactured Aerosol Jet printed structures. In 2015
European Microelectronics Packaging Conference (EMPC), pages 16, Sep. 2015.

[54] M. I. M. Ghazali, S. Karuppuswami, S. Mondal, A. Kaur, and P. Chahal. Embed-
ded Actives Using Additive Manufacturing for High-Density RF Circuits and Sys-
tems. IEEE Transactions on Components, Packaging and Manufacturing Technology,
9(8):16431651, Aug 2019.

[55] A. Elmogi, W. Soenen, H. Ramon, X. Yin, J. Missinne, S. Spiga, M. Amann, A. Srini-
vasan, P. De Heyn, J. Van Campenhout, J. Bauwelinck, and G. Van Steenberge.
Aerosol-Jet Printed Interconnects for 2.5 D Electronic and Photonic Integration. Jour-
nal of Lightwave Technology, 36(16):35283533, Aug 2018.

[56] Y. Gu, D. R. Hines, V. Yun, M. Antoniak, and S. Das. Aerosol-Jet Printed Fillets
for Well-Formed Electrical Connections between Dierent Leveled Surfaces. Advanced
Materials Technologies, 2(11):1700178, 2017.

[57] F. X. Röhrl, J. Jakob, W. Bogner, R. Weigel, and S. Zorn. Bare Die Connections
via Aerosol Jet Technology for Millimeter Wave Applications. In 2018 48th European
Microwave Conference (EuMC), pages 10331036, Sep. 2018.

[58] M. Tonouchi. Cutting-edge terahertz technology. Nat Photon, 1(2):97105, Feb 2007.

[59] P. H. Siegel. Terahertz technology in biology and medicine. IEEE Transactions on

Microwave Theory and Techniques, 52(10):24382447, Oct 2004.

89

[60] M. C. Kemp, P. F. Taday, B. E. Cole, J. A. Clu, A. J. Fitzgerald, and W. R. Tribe.

Security applications of terahertz technology. Proc.SPIE, 5070:44  52, 2003.

[61] J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars.
THz imaging and sensing for security applicationsexplosives, weapons and drugs.
Semiconductor Science and Technology, 20(7):S266, 2005.

[62] S. Biber, J. Schur, A. Hofmann, and L. P. Schmidt. Design of new passive THz de-
vices based on micromachining techniques. In The Fifth International Kharkov Sympo-
sium on Physics and Engineering of Microwaves, Millimeter, and Submillimeter Waves
(IEEE Cat. No.04EX828), volume 1, pages 2631 Vol.1, June 2004.

[63] A. E. Costley, K. H. Hursey, G. F. Neill, and J. M. Ward. Free-standing ne-wire grids:
Their manufacture, performance, and use at millimeter and submillimeter wavelengths.
J. Opt. Soc. Am., 67(7):979981, Jul 1977.

[64] P.A.R. Ade, A.E. Costley, C.T. Cunningham, C.L. Mok, G.F. Neill, and T.J. Parker.
Free-standing grids wound from 5 µ m diameter wire for spectroscopy at far-infrared
wavelengths. Infrared Physics, 19(5):599  601, 1979.

[65] N. Born, R. Gente, I. Al-Naib, and M. Koch. Laser beam machined free-standing

terahertz metamaterials. Electronics Letters, 51(13):10121014, 2015.

[66] D. Jahn, M. Weidenbach, J. Lehr, L. Becker, F. Beltrán-Mejía, S. F. Busch, J. C.
Balzer, and M. Koch. 3D Printed Terahertz Focusing Grating Couplers. Journal of
Infrared, Millimeter, and Terahertz Waves, 38(6):708716, Jun 2017.

[67] B. Zhu, S. Vanloocke, J. Stiens, D. De Zutter, A. Elhawil, C. De Tandt, and
R. Vounckx. A novel 3D printed focusing probe in scattering-type scanning near-
eld millimetre amp; Terahertz wave microscope. In Proceedings of the 5th European
Conference on Antennas and Propagation (EUCAP), pages 745748, April 2011.

[68] A. I. Hernandez-Serrano, M. Weidenbach, S. F. Busch, M. Koch, and E. Castro-
Camus. Fabrication of gradient-refractive-index lenses for terahertz applications by
three-dimensional printing. J. Opt. Soc. Am. B, 33(5):928931, May 2016.

[69] A. Kaur, J. C. Myers, M. I. M. Ghazali, J. Byford, and P. Chahal. Aordable tera-
hertz components using 3D printing. In 2015 IEEE 65th Electronic Components and
Technology Conference (ECTC), pages 20712076, May 2015.

[70] K. Y. Park, N. Wiwatcharagoses, and P. Chahal. Wafer-level integration of micro-lens
for THz focal plane array application. In 2013 IEEE 63rd Electronic Components and
Technology Conference, pages 19121919, May 2013.

[71] J. A. Byford, Z. Purtill, and P. Chahal. Fabrication of Terahertz Components Using
In 2016 IEEE 66th Electronic Components and Technology

3D Printed Templates.
Conference (ECTC), pages 817822, May 2016.

90

[72] T. Kondo, T. Nagashima, and M. Hangyo. Fabrication of Wire-Grid-Type Polarizers
for THz Region Using a General-Purpose Color Printer. Japanese Journal of Applied
Physics, 42(4A):L373, 2003.

[73] A. Das, T. M. Schutzius, C. M. Megaridis, S. Subhechha, T. Wang, and L. Liu. Quasi-
optical terahertz polarizers enabled by inkjet printing of carbon nanocomposites. Ap-
plied Physics Letters, 101(24):243108, 2012.

[74] K. Takano, T. Kawabata, C. F. Hsieh, K. Akiyama, F. Miyamaru, Y. Abe, Y. Tokuda,
R. P. Pan, C. L. Pan, and M. Hangyo. Fabrication of Terahertz Planar Metamaterials
Using a Super-Fine Ink-Jet Printer. Applied Physics Express, 3(1):016701, 2010.

[75] H. Kim, J. S. Melinger, A. Khachatrian, N. A. Charipar, R. C. Y. Auyeung, and
A. Piqué. Fabrication of terahertz metamaterials by laser printing. Opt. Lett.,
35(23):40394041, Dec 2010.

[76] H. T. Yudistira, A. P. Tenggara, S. S. Oh, V. Nguyen, M. Choi, C. Choi, and D. Byun.
High-resolution electrohydrodynamic jet printing for the direct fabrication of 3D mul-
tilayer terahertz metamaterial of high refractive index. Journal of Micromechanics and
Microengineering, 25(4):045006, 2015.

[77] A. P. Tenggara, S. J. Park, H. T. Yudistira, Y. H. Ahn, and D. Byun. Fabrication of
terahertz metamaterials using electrohydrodynamic jet printing for sensitive detection
of yeast. Journal of Micromechanics and Microengineering, 27(3):035009, 2017.

[78] F. Cai, S. Pavlidis, J. Papapolymerou, Y. H. Chang, K. Wang, C. Zhang, and B. Wang.
Aerosol jet printing for 3-D multilayer passive microwave circuitry. In 2014 44th Eu-
ropean Microwave Conference, pages 512515, Oct 2014.

[79] F. Cai, Y. h. Chang, K. Wang, W. T. Khan, S. Pavlidis, and J. Papapolymerou. High
resolution aerosol jet printing of D- band printed transmission lines on exible LCP
substrate.
In 2014 IEEE MTT-S International Microwave Symposium (IMS2014),
pages 13, June 2014.

[80] C. Oakley, A. Kaur, J. A. Byford, and P. Chahal. Aerosol-Jet Printed Quasi-Optical
Terahertz Filters. In 2017 IEEE 67th Electronic Components and Technology Confer-
ence (ECTC), pages 248253, May 2017.

[81] D. Jahn, R. Eckstein, L. M. Schneider, N. Born, G. Hernandez-Sosa, J. C. Balzer,
I. Al-Naib, U. Lemmer, and M. Koch. Digital Aerosol Jet Printing for the Fabrication
of Terahertz Metamaterials. Advanced Materials Technologies, 3(2):1700236, 2018.

[82] J. A. Hejase, P. R. Paladhi, and P. Chahal. Terahertz Characterization of Dielectric
Substrates for Component Design and Nondestructive Evaluation of Packages. IEEE
Transactions on Components, Packaging and Manufacturing Technology, 1(11):1685
1694, Nov 2011.

91

[83] M. Abt, A. Roch, J. A. Qayyum, S. Pestotnik, L. Stepien, A. Abu-Ageel, B. Wright,
A. C. Ulusoy, J. Albrecht, L. Harle, J. Papapolymerou, and T. Schuelke. Aerosol-
printed highly conductive Ag transmission lines for exible electronic devices. IEEE
Transactions on Components, Packaging and Manufacturing Technology, 8(10):1838
1844, Oct 2018.

[84] D.M. Pozar. Microwave Engineering. Wiley, 2004.

[85] M. I. M. Ghazali, K. Y. Park, J. A. Byford, J. Papapolymerou, and P. Chahal. 3D
printed metalized-polymer UWB high-gain Vivaldi antennas. In 2016 IEEE MTT-S
International Microwave Symposium (IMS), pages 14, May 2016.

[86] B. S. Cook and A. Shamim. Inkjet Printing of Novel Wideband and High Gain Anten-
nas on Low-Cost Paper Substrate. IEEE Transactions on Antennas and Propagation,
60(9):41484156, Sept 2012.

[87] G. McKerricher, J. Gonzalez, and A. Shamim. All inkjet printed 3D microwave ca-
pacitors and inductors with vias. In Microwave Symposium Digest (IMS), 2013 IEEE
MTT-S International, pages 13, June 2013.

[88] C. Mariotti, M. M. Tentzeris, and L. Roselli. Demonstration and charaterization of
fully 3D-printed RF structures. In Microwave Symposium (MMS), 2015 IEEE 15th
Mediterranean, pages 14, Nov 2015.

[89] F. Cai, A. Ç Ulusoy, and J. Papapolymerou. 2.4 GHz low cost low noise amplier
on exible organic substrate. In 2013 Asia-Pacic Microwave Conference Proceedings
(APMC), pages 270272, Nov 2013.

[90] O. Azucena, J. Kubby, D. Scarbrough, and C. Goldsmith. Inkjet printing of passive mi-
crowave circuitry. In Microwave Symposium Digest, 2008 IEEE MTT-S International,
pages 10751078, June 2008.

[91] D. C. Thompson, O. Tantot, H. Jallageas, G. E. Ponchak, M. M. Tentzeris, and J. Pa-
papolymerou. Characterization of liquid crystal polymer (LCP) material and transmis-
sion lines on LCP substrates from 30 to 110 GHz. IEEE Transactions on Microwave
Theory and Techniques, 52(4):13431352, April 2004.

[92] F. Cai, Y. H. Chang, K. Wang, C. Zhang, B. Wang, and J. Papapolymerou. Low-Loss
3-D Multilayer Transmission Lines and Interconnects Fabricated by Additive Man-
ufacturing Technologies. IEEE Transactions on Microwave Theory and Techniques,
64(10):32083216, Oct 2016.

[93] M. Craton, J. A. Byford, V. Gjokaj, J. Papapolymerou, and P. Chahal. 3D Printed
High Frequency Coaxial Transmission Line Based Circuits. In 2017 IEEE 67th Elec-
tronic Components and Technology Conference (ECTC), pages 10801087, May 2017.

[94] V. Gjokaj, P. Chahal, J. Papapolymerou, and J. D. Albrecht. A novel 3D printed
Vivaldi antenna utilizing a substrate integrated waveguide transition. In 2017 IEEE

92

International Symposium on Antennas and Propagation USNC/URSI National Radio
Science Meeting, pages 12531254, July 2017.

[95] B. J. Kang, C. K. Lee, and J. H. Oh. All-inkjet-printed electrical components and
circuit fabrication on a plastic substrate. Microelectronic Engineering, 97:251254,
2012.

[96] M. I. Mohd Ghazali, S. Karuppuswami, S. Mondal, and P. Chahal. Embedded Active
Elements in 3D Printed Structures for the Design of RF Circuits. In 2018 IEEE 68th
Electronic Components and Technology Conference (ECTC), pages 10621067, May
2018.

[97] C. Oakley, P. Chahal, J. Papapolymerou, and J. D. Albrecht. Fabrication of X-band
Oscillator on LCP Substrate Using Aerosol Printing. In International Symposium on
Microelectronics, volume 2017, pages 000052000055. International Microelectronics
Assembly and Packaging Society, 2017.

[98] R. W. Johnson, J. L. Davidson, R. C. Jaeger, and D. V. Kerns. Silicon hybrid wafer-
scale package technology. IEEE Journal of Solid-State Circuits, 21(5):845851, Oct
1986.

[99] M. Gdula, A. Yerman, V. Krishnamurthy, and R. Fillion. The GE high density overlay
MCM interconnect method solves high power needs of GaAs system design. In 1993
Proceedings Fifth Annual IEEE International Conference on Wafer Scale Integration,
pages 339345, Jan 1993.

[100] M. Topper, K. Buschick, J. Wolf, V. Glaw, R. Hahn, A. Dabek, O. Ehrmann, and
H. Reichl. Embedding technology-a chip-rst approach using bcb. In Proceedings 3rd
International Symposium on Advanced Packaging Materials Processes, Properties and
Interfaces.

[101] W. Cheng, M. A. Beiley, and S. S. Wong. Membrane multichip module technology on
silicon. In Proceedings 1993 IEEE Multi-Chip Module Conference MCMC-93, pages
6973, May 1993.

[102] C. H. J. Poh, C. E. Patterson, S. K. Bhattacharya, S. D. Philips, N. E. Lourenco,
J. D. Cressler, and J. Papapolymerou. Packaging Eects of Multiple X-Band SiGe
LNAs Embedded in an Organic LCP Substrate. IEEE Transactions on Components,
Packaging and Manufacturing Technology, 2(8):13511360, Aug 2012.

[103] F. Geng, X. Ding, and L. Luo. Study on a 3D packaging structure with benzocy-
clobutene as a dielectric layer for radio frequency application. In 2009 International
Conference on Electronic Packaging Technology High Density Packaging, pages 8590,
Aug 2009.

[104] J. Tang, H. Wang, X. Chen, W. Ning, G. Xu, X. Sun, and L. Luo. An mcm package
process for 24 ghz driver amplier using photosensitive bcb. In 2010 11th International
Conference on Electronic Packaging Technology High Density Packaging.

93

[105] J. A. Qayyum, M. Abt, A. Roch, A. C. Ulusoy, and J. Papapolymerou. Ultra wideband
In 2017 12th European

3D interconnects using aerosol jet printing up to 110 GHz.
Microwave Integrated Circuits Conference (EuMIC), pages 372375, Oct 2017.

[106] M. T. Craton, J. D. Albrecht, P. Chahal, and J. Papapolymerou. A Chip-First Ap-
proach to Millimeter-Wave Circuit Packaging. IEEE Microwave and Wireless Compo-
nents Letters, pages 13, 2019.

[107] R. Yang and G. Mao. Liquid crystal polymer for exible circuits, June 11 2002. US

Patent 6,403,211.

[108] P. I. Deenbaugh, R. C. Rumpf, and K. H. Church. Broadband Microwave Frequency
Characterization of 3-D Printed Materials. IEEE Transactions on Components, Pack-
aging and Manufacturing Technology, 3(12):21472155, Dec 2013.

[109] Z. Zhai, L. Ma, Y. Jiang, Q. Xie, F. Zhang, X. Wu, and J. Gao. Giant photosensitivity
of aC: Co/GaAs/Ag pn-metal junctions. Optical Materials Express, 5(11):26672674,
2015.

[110] H. J. Ng, J. Wessel, D. Genschow, R. Wang, Y. Sun, and D. Kissinger. Miniaturized
122 GHz system-on-chip radar sensor with on-chip antennas utilizing a novel antenna
design approach. In 2016 IEEE MTT-S International Microwave Symposium (IMS),
pages 14, May 2016.

[111] S. Pan, F. Caster, P. Heydari, and F. Capolino. A 94-GHz Extremely Thin
Metasurface-Based BiCMOS On-Chip Antenna. IEEE Transactions on Antennas and
Propagation, 62(9):44394451, Sep. 2014.

[112] W. T. Khan, A. Ça§r Ulusoy, G. Dufour, M. Kaynak, B. Tillack, J. D. Cressler,
and J. Papapolymerou. A D-Band Micromachined End-Fire Antenna in 130-nm SiGe
BiCMOS Technology. IEEE Transactions on Antennas and Propagation, 63(6):2449
2459, June 2015.

[113] V. Gjokaj, J. Papapolymerou, J. D. Albrecht, B. Wright, and P. Chahal. A Compact
Receive Module in 3D Printed Vivaldi Antenna. IEEE Transactions on Components,
Packaging and Manufacturing Technology, pages 11, 2019.

[114] V. Gjokaj, J. Papapolymerou, J. D. Albrecht, and P. Chahal. Design and Fabrication
of Additively Manufactured Hybrid Rigid-Flex RF Components. IEEE Transactions
on Components, Packaging and Manufacturing Technology, 9(4):779785, April 2019.

94