HIGH FUNCTIONAL DENSITY THROUGH RIGID-FLEX

By

Vincens Gjokaj

A DISSERTATION

Submitted to

Michigan State University

in partial fulfillment of the requirements

for the degree of

Electrical Engineering – Doctor of Philosophy

2021

ABSTRACT

HIGH FUNCTIONAL DENSITY THROUGH RIGID-FLEX

By

Vincens Gjokaj

In the last decade Additive Manufacturing (AM), often referred to as 3D printing, has garnered

signficant interest in systems packaging. It has proven as an effective way of producing Radio

Frequency (RF) to millimeter wave (mmWave) components and modules. AM enables flexible

and rapid realization of structures with arbitrary shapes and complexity. AM is considered as one

of the most important emerging material processing technologies that will drive the future designs

of high functional density modules and systems. This thesis investigates the us of AM for the

design and fabrication of microwave and mmWave circuits and systems.

Different AM technologies are used to design and demonstrate a range of high frequency pas-

sive and active components. Stereolithography printing is utilized to create passive components in

the X-band (10-18 GHz) and K-band (18-27 GHz) frequency bands. These passive component de-

signs are then combined with active RF circuits to design transmit and receive modules. Aerosol jet

printing (AJP) is used to create an entirely printed mmWave components that avoids the use of tra-

ditional photolithography fabrication methods. All the designs are compared to their counterparts

fabricated using conventional techniques. It is demonstrated that self-packaged RF components

3D form can readily be produced using AM leading to high functional density systems working

well into the W-band frequency band.

Copyright by
VINCENS GJOKAJ
2021

For my parents and siblings, Pellumb, Gjyljana, Miklovan, and Desantila.

iv

ACKNOWLEDGMENTS

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

erated by Honeywell Federal Manufacturing & Technologies, LLC under contract number DE-

NA0002839.

v

TABLE OF CONTENTS

LIST OF FIGURES .

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CHAPTER 1

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1.1.1

INTRODUCTION .

System-On-Chip (SOC)
System-On-Package (SOP)

1.1 Traditional Wireless systems design and packaging . . . . . . . . . . . . . . . .

System .
1.1.1.1 Multi-Chip-Modules (MCM)
1.1.1.2
1.1.1.3

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . .
1.1.2 Transmission lines
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.3 Antenna fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . .

1
1
2
3
4
5
6
7
7
8
Fused Deposition Modeling (FDM)
Stereolithography (SLA)
9
Polyjet Printing/ Material Jetting (MJ) . . . . . . . . . . . . . . . . . . . . 10
Inkjet/Aerosol Jet Printing (AJP) . . . . . . . . . . . . . . . . . . . . . . . 11
1.3 Limitations of Combining Standard Technology and Additive Manufacturing . . . 13
1.4 Rigid-Flex System On Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.5 Dissertation Outline .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.2 Additive Manufacturing of Wireless Components

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

1.2.1
1.2.2
1.2.3
1.2.4

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. .

Introduction .

2.1
.
2.2 Design and Simulation .
.

CHAPTER 2 HIGH FREQUENCY APPLICATIONS OF POLYJET PRINTING . . . . . 18
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.1 Metallization .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.2 Reduced Height Waveguide Transmission Line . . . . . . . . . . . . . . . 20
2.2.3 Reduced Height Waveguide Resonator Resonator . . . . . . . . . . . . . . 21
2.2.4 Reduced Height Waveguide Power Divider
. . . . . . . . . . . . . . . . . 24
2.2.5 Reduced Height Waveguide Leaky-Wave Antenna . . . . . . . . . . . . . . 24
2.2.6 Reduced Height Waveguide Vivaldi Antenna
. . . . . . . . . . . . . . . . 25
2.3 Fabrication and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.1 Reduced Waveguide Transmission line . . . . . . . . . . . . . . . . . . . . 28
. . . . . . . . . . . . . . . . . . . 29
2.3.2 Reduced Height Waveguide Resonator
2.3.3 Reduced Height Waveguide Power Divider
. . . . . . . . . . . . . . . . . 29
2.3.4 Reduced Height waveguide Leaky-wave antenna
. . . . . . . . . . . . . . 30
2.3.5 Reduced Height waveguide Vivaldi antenna . . . . . . . . . . . . . . . . . 30
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.4 Summary .

. .

. .

.

.

.

Introduction .

CHAPTER 3 RIGID-FLEX COUPLED ANTENNA . . . . . . . . . . . . . . . . . . . . 32
3.1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2 Apperature Coupled Patch Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . 33
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3 Rigid-Flex Patch array .
3.4 Apperature Coupled Vivaldi Antenna in Large structure . . . . . . . . . . . . . .
. 35

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vi

3.5 Characterization of Conductive Film Coatings . . . . . . . . . . . . . . . . . . . . 37
. . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.6 Antenna Design and Measurement
. 40
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
.

3.6.1 Vivaldi Antenna Design . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.7 Antenna on Structure .
.
3.8 Summary . .

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4.1 Rigid-Flex Coax Structure Simulation and Fabrication . . . . . . . . . . . . . . .

CHAPTER 4 RIGID-FLEX COAX TRANSMISSION LINE . . . . . . . . . . . . . . . . 44
. 44
4.1.1 Rigid-Flex Coax Transmission Lines . . . . . . . . . . . . . . . . . . . . . 44
4.1.2 Rigid-Flex Coax Resonator . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.1.3 Rigid-Flex Coax Transmission line With Substrate steps
. . . . . . . . . . 48
4.1.4 Rigid-Flex Hemispherical Coax Resonator . . . . . . . . . . . . . . . . . . 48
4.1.4.1 Rigid-Flex Resonator error analysis . . . . . . . . . . . . . . . . 50
4.2 Rigid-Flex Coax Antenna Feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.1 Rigid-Flex Coax Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.2 Rigid-Flex Coax Antenna Array . . . . . . . . . . . . . . . . . . . . . . . 54
4.3 Rigid-Flex Vivaldi Array with Printed Resistor . . . . . . . . . . . . . . . . . . . . 58
4.4 Summery .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

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CHAPTER 5 RIGID-FLEX SOA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
. 64
. 65
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.1 System Design and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Package Design and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Measurement
5.4 Summery .

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6.1 Aerosol Jet Printing .

CHAPTER 6 RIGID-FLEX PRINTED VIA SOA . . . . . . . . . . . . . . . . . . . . . . 69
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.1.1 Circuit Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.1.2 VIA Fabrication .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.2 Transmitter Circuit Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2.1 Vivaldi Design and Measurement . . . . . . . . . . . . . . . . . . . . . . . 74
SOA Fabrication and Measurement . . . . . . . . . . . . . . . . . . . . . . 75
6.2.2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

6.3 Summary .

. .

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CHAPTER 7 CONCLUSIONS .
.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.1 Conclusion .
7.2 Additive Manufacturing Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 81

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APPENDIX .

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BIBLIOGRAPHY .

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

vii

LIST OF FIGURES

Figure 1.1:

Three major parts of a system . . . . . . . . . . . . . . . . . . . . . . . . .

.

Figure 1.2:

Time-line of packaging advances . . . . . . . . . . . . . . . . . . . . . . . .

Figure 1.3:

Drawing of MCM .

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

Figure 1.4:

Drawing of SOC .

Figure 1.5:

Drawing of SOP .

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.

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

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

Figure 1.6:

Drawing of the FDM process . . . . . . . . . . . . . . . . . . . . . . . . .

.

2

3

4

5

6

9

Figure 1.7:

Drawing of the SLA .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 1.8:

Drawing of the MJ .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 1.9:

Drawing of Inkjet and AJP . . . . . . . . . . . . . . . . . . . . . . . . . .

. 13

Figure 1.10: SoA Mock Up .

.

.

.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 2.1:

Designed inset for the RHW structure. λ is wavelength of the center frequency. 21

Figure 2.2:

Designed RHW structure. (A) 40 mm length (B) 60 mm length. . . . . . . .

. 22

Figure 2.3:

RHW Through simulated results.

. . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 2.4:

Designed RHW Resonator.

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

. 23

Figure 2.5:

RHW Resonator simulated and measured results.

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

. 23

Figure 2.6:

Designed power divider. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 24

Figure 2.7:

RHW power divider simulated and measured results.

. . . . . . . . . . . . . 25

Figure 2.8:

Designed RHW leaky-wave antenna.

. . . . . . . . . . . . . . . . . . . . . . 25

Figure 2.9:

RHW leaky-wave antenna simulated and measured S11results.

. . . . . . . . 26

Figure 2.10: RHW leaky-wave antenna simulated and measured gain results. . . . . . . . . 26

Figure 2.11: Designed Vivaldi antenna with dimensions. . . . . . . . . . . . . . . . . . . . 27

viii

Figure 2.12: Damascene process.

.

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

. 27

Figure 2.13: RHW Through before (left) and after (right) assembly. . . . . . . . . . . . .

. 28

Figure 2.14: RHW Through measured results.

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

. 28

Figure 2.15: RHW Resonator before (left) and after right blanket metalization. . . . . . . . 29

Figure 2.16: RHW power divider before (left) and after assembly (right). . . . . . . . . . . 30

Figure 2.17: Fabricated RHW leaky wave antenna prior to assembly (left) and after as-

sembly (right). . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 2.18: Angled (left) and side (right) view of fabricated Vivaldi antenna.

. . . . . .

. 31

Figure 2.19: Measured and simulated return loss (left) and gain (right). . . . . . . . . . .

. 31

Figure 3.1:

Simulated and measured return loss (left) and gain (right) shown for the
patch antenna.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

.

.

.

Figure 3.2:

The schematic design (left) of the antenna and a the fabricated patch (right)
are shown. The dashed box denotes slot size. . . . . . . . . . . . . . . . . . . 34

Figure 3.3:

Designed Patch (left) and fabricated patch (right).

. . . . . . . . . . . . . . . 34

Figure 3.4:

The Rigid-Flex patch array design (left) and power divider dimensions (right). 36

Figure 3.5:

The measured and simulated return losses for planer and bent arrays.

. . . . . 36

Figure 3.6: Measured and simulated array radiation pattern at 15.2 GHz for flat (left)

and bent (right).

.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Figure 3.7:

Fabricated Rigid-Flex patch array (left) and after bending (right) around a
+ 90o angle is shown.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Figure 3.8:

Painted silver and electroplated copper silver paint.

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

. 39

Figure 3.9:

Designed (left) and fabricated (right) printed Vivaldi with coupled feed.

. .

. 40

Figure 3.10: Planner Vivaldi returnloss (left) and gain (right).

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

. 41

Figure 3.11:

(a) Planner Vivaldi printed (b) Vivaldi antenna coated in Silver paint (c)
Vivaldi antenna after electroplating copper.
(d) Fully fabricated Vivaldi
Antenna.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

. .

. .

.

.

ix

Figure 3.12: Automobile bumper design with Vivaldi antennas embedded in corners

(Bottom). Fully fabricated with metalized Vivaldis (top).

. . . . . . . . . . . 42

Figure 3.13: Measured transmit and reflected signal of the Vivaldi antennas on the bumper.

43

Figure 4.1:

Flex-Rigid coax design (left) and fabricated (right).

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

. 45

Figure 4.2:

Simulated vs Measured insertion loss of printed Rigid-Flex Coax. . . . . . . . 46

Figure 4.3:

Rigid-Flex T-line resonator structure, exploded view (left) and fabricated
(right).

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

. .

. .

.

.

.

Figure 4.4:

Resonator simulated and measured results.

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

. 47

Figure 4.5:

Exploded view of the Rigid-Flex coax design with varying trenches (left)
and fabricated (right).

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Figure 4.6:

Simulated and measured return loss of the Rigid-Flex coax with varying depths. 49

Figure 4.7:

Close up view of the slot in the ground plane (left) and blown out view (right). 50

Figure 4.8:

Printed and meatallized resonator pieces (left) and fully assembled (right).

.

. 50

Figure 4.9:

Simulated and Measured results for the Hemispherical resonator.

. . . . . .

. 51

Figure 4.10: Fabricated Slot dimensions.

. . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Figure 4.11: Measured and simulation results after correction for slot dimensions.

. . . .

. 52

Figure 4.12: Designed front-to-back cavity (Left) and Cavity S-parameters (Right).

. . . . 53

Figure 4.13: Designed Vivaldi (left) Fabricated Vivaldi (Right). . . . . . . . . . . . . . . . 54

Figure 4.14: Vivaldi simulated and measured S-parameters (left) Vivaldi simulated and

measured gain (Right).

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Figure 4.15: Wilkinson Power divider design (left) and fabricated (right). . . . . . . . . .

. 55

Figure 4.16: Wilkinson Power divider simulated and measured S-Parameters.

. . . . . .

. 55

Figure 4.17: Vivaldi array design (left) and fabricated (right).

. . . . . . . . . . . . . . . . 56

Figure 4.18: Vivaldi array S-parameters results (left) and gain (right). . . . . . . . . . . .

. 57

Figure 4.19: Vivaldi vertical array design (left) and fabricated (right). . . . . . . . . . . .

. 57

x

Figure 4.20: Vertical Vivaldi array S-parameters results (left) and gain (right).

. . . . . . . 58

Figure 4.21: Measured results of the Wilkinson power divider. PR- Printed Resistor,

SM- Surface Mount. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Figure 4.22:

(a) 1x2 Vivaldi array with surface mount resistor. (b) 1x2 Vivaldi array
with printed resistor. .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Figure 4.23:

(a) Simulated and Measured S11 . (b) Simulated and Measured gain results.
PR- Printed Resistor, SM- Surface Mount.

. . . . . . . . . . . . . . . . . . . 62

Figure 5.1:

(a) System schematic of receive circuit. (b) Fabricated circuit. . . . . . . . . . 65

Figure 5.2:

(a) Rigid-Flex Vivaldi antenna. (b) Modified Rigid-Flex Vivaldi for SoP
receive circuit. (c) Vivaldi with circuit embedded in the cavity. (d) Final
package with cap covering the cavity. . . . . . . . . . . . . . . . . . . . . .

. 66

Figure 5.3:

(a) Horn antenna with receive circuit. (B) 3D printed SoP receive module.

. . 67

Figure 5.4:

Receive power difference between the active horn and Vivaldi antenna (nor-
malized to the horn). .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Figure 5.5:

(a) Measured radiation pattern at 12 GHz. (b) Measured radiation pattern at
13 GHz. (c) Measured radiation pattern at 14 GHz. (d) Measured radiation
pattern at 15 GHz.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Figure 6.1:

An ultra compact receive front-end. . . . . . . . . . . . . . . . . . . . . . . . 70

Figure 6.2:

AJP printed transmitter circuit.

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

. 72

Figure 6.3:

VIA close ups. . .

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

. 74

Figure 6.4:

3D Printed Vivaldi Design (left) 3D Printed Vivaldi Fabricated(right). . . . .

. 75

Figure 6.5:

Simulated and Measured S11 (left) and Gain (right).

. . . . . . . . . . . . . . 76

Figure 6.6:

Designed SOA (left) Fabricated SOA (right). . . . . . . . . . . . . . . . . . . 77

Figure 6.7: Measurement Set-Up.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Figure 6.8:

Spectrum Analyzer results.

. . . . . . . . . . . . . . . . . . . . . . . . . . . 77

xi

CHAPTER 1

INTRODUCTION

1.1 Traditional Wireless systems design and packaging

In the area of electromagnetics, a major breakthrough happened in 1873 when James Clerk

Maxwell published his book Treatise on Electricity and Magnetism which theoretically explained

how these phenomenon work. Following this work, German physicist Heinrich Rudolf Hertz was

able to prove Maxwells ideas in experimentation. The first science experiment with millimeter

waves occurred in 1897, conducted by Jagadish Chandra Bose. The first public demonstration

of a functional radio system was done by Guglielmo Marconi in England. Marconi transmitted a

telegraph message across the English Channel in 1899, and sequentially transmitted a long-distance

message from England to Canada in 1901. The greatest surge in radio technology was in the 1920s

when radio broadcasts began happening each day and in the 1930s when radios became a common

home appliance. in 1957 the Soviets launched the first satellite Sputnik into orbit. With the advent

of the first transistor at Bell Labs in 1947, and the first integrated circuit (IC) patented by Jack

Kilby in 1959 [1]. This marked a major path forward for radio systems. They were able to become

smaller and less bulky which paved the way in the 1970s for Global Positioning System (GPS)

and cellular networks to become commercially available. This was followed by wireless-fidelity

networks in the 1990s and in the 2010s with the development of the 5th-generation (5G) networks.

A major use for wireless systems is for radar systems. This technology stretches across many

platforms in modern technology. This was mainly utilized for military purposes when it first began

being studied but now spreads to commercial aviation and is being incorporated into automobiles

for driver assistance. The first notable radar patented was in 1903 in Germany. It was a spark-gap

transmitter that was connected to an array of dipoles [2]. It had a basic function of noting when a

mateallic body was approaching. This technology functioned on the principle of a transmit (Tx)

module and a receive module (Rx). This would set the groundwork for future radar systems to be

1

Figure 1.1: Three major parts of a system.

developed. The Tx and Rx modules can be broken down into three major parts as can be seen in

Fig. 1.1. These parts of the module have been studied and developed over the years to achieve

a high efficiency module. While the modules have been optimized for performance they still are

expensive to produce and use. Recently, additive manufacturing (AM) has been studied to create

some of these aspects of the modules to reduce the cost and make them compact. No real break

through has been made to create a module that combines the standard components with the AM

components to create a high efficiency module at a low cost.

1.1.1 System

The major development of radio technology can be mainly attributed to the development of pack-

aging methods. In traditional manufacturing, remarkable changes are happening to wireless tech-

nology through miniaturization, integration, and innovation in both integrated circuits (IC) and

packaging methods in order to meet on going challenges of greater functionality, higher perfor-

mance, smaller size, lower cost, and faster time to market. Printed circuit boards (PCBs) are the

essential parts to assemble modern electronic circuits. The PCB design and development process

has a direct impact on the system cost and time to market. On average, companies report that

PCB represents 31% of the overall product cost[3]. Industry uses various methods in modern day

technology packaging to achieve the highest performance in a small lightweight package that is

2

Figure 1.2: Time-line of packaging advances.

extremely cost effective to produce. The most commonly used packaging methods are System-

On-A-Package (SOP), System-On-A-Chip (SOC), System-In-A-Package (SIP), and Multi-Chip-

Modules (MCM). The time-line of these advances can be seen in Fig. 1.2. While high functional

density has been achieved with the packaging of individual components, no solution to combing

these systems to antennas without the use of bulky connectors or enlarging the circuit board or

silicon die to fit the antenna.

1.1.1.1 Multi-Chip-Modules (MCM)

For the active analog/RF and digital/DSP functions, individual ICs can be interconnected using

flip-chip bonding to eliminate bondwires (and their associated parasitic effects) and allow more

compact MCM floor planning. An additional benefit of this approach is that complete system

redesign is not necessarily needed to change one part of the subsystem/module; individual func-

tional component dies could be upgraded as long as their interconnection scheme and footprint

were kept the same to remain compatible with the MCM layout. The incorporation of micro-

electromechanical systems (MEMS), smart materials, smart antennas, and etc., with mixed-signal

ICs in such an environment is now increasingly referred to as “mixed-technology” or “heteroge-

neous” integration [4]. The multilayer ceramic MCM package offers an attractive alternative to

the conventional plastic QFP package in terms of significant size reduction, thickness, and per-

formance at high frequencies for the mobile communication market [5]. A strong alternative to

complete single-chip SOC integration is some advanced form of an MCM solution, which inte-

grates antenna, filters, resonators, baluns, and other RF components. These can be integrated in

either ceramic, such as low-temperature cofired ceramic (LTCC), or organic, such as multilayer

3

Figure 1.3: Drawing of MCM.

organic (MLO). The basic philosophy for MCM is to fabricate devices and circuits in the optimum

available technology for their particular function (e.g., digital circuits in submicrometer CMOS,

analog components in BiCMOS, RF components in GaAs or InP) and then integrate them into a

system or subsystem using MCM technology. High-Q passive structures could potentially be em-

bedded directly into the MCM structure. Since MCM utilizes the optimum substrates for each the

devices packaged it overcomes a lot of the issues that SOC faces. A drawing of MCM can be seen

in Fig. 1.3.

1.1.1.2 System-On-Chip (SOC)

Integration of complete systems of RF analog & digital functions in a single chip is typically

referred to as System-On-A-Chip (SOC). But this method is facing many obstacles due to the

requirements for high capacity memory devices, duplexer and filter banks, MEMS, and High Q

Devices[3]. Another major limitation of SOC is the extremely high initial cost of producing the

system. Developing and fabricating ICs is one of the highest expenses a manufacturer of electronic

components can have. When combining many functions into one Chip, the SOC movement is

ambushed by the cost of additional mask layers needed to marry digital logic with memory and

analog function on one specific and optimum substrate [4]. Also if any changes are wanted to be

made to the system the chip has to be completely re-fabricated from the masks and this makes

it extremely expensive. SOC offers many benefits, out of all packaging methods SOC offers the

smallest size & footprint, it also offers an increased sensor density[6]. Because of this SOC are

most commonly used today for powering smart phones and tablets because they offer the small

4

Figure 1.4: Drawing of SOC.

size needed for all the components. Typical smart phone applications for SOC package the Central

Processing Unit (CPU), Graphical Processing Unit (GPU), Random Access Memory (RAM), Read

Only Memory (ROM), and even the Modem for the wireless connectivity of the smart phone (LTE,

Bluetooth, Cellular) into one packaged IC chip. A drawing of SOC can be seen in Fig. 1.4. A

downside of utilizing SOC is that its impossible to connect wires to it; which this limits the package

to be utilized only on printed circuit boards (PCBs). Also the SOC packaging method is really hurt

by the cost added by additional mask layers needed to marry digitial logic with memory and analog

functions on one specific substrate [4]. Another downside of SoC is the inability to add antennas

that have high gain and large bandwidths because of the size of the antenna. This increases the

size of the chip which increases the production cost as will as decreasing the performance of the

antenna due to utilizing silicon as the substrate.

1.1.1.3 System-On-Package (SOP)

Recently, a new concept called SOP is introduced in which the package, and not the board, is the

system. As such, SOP is beginning to address the shortcomings of both SOC and SIP[4]. The

system-on-a-package (SOP) concept is considered as the solution of future communication mod-

ules, which would need more functionality, better performance, low cost, and more integrity[7].

The concept of SOP can be thought of a conceptual paradigm in which the package, and not the

bulky board acts as the system and the package provides all the system functions in one single

module, not as an assemblage of discrete components to be connected together, but as a continu-

ous merging of various integrated thin film technologies to comprise a system solution in a small

package[8]. SOP aims to utilize the best of on-chip SOC integration and package integration to

5

Figure 1.5: Drawing of SOP.

achieve the highest system performance at the lowest cost. Similar to a wafer-to-IC concept, the

SOP packages are fabricated on panels using low cost processes used in high density organic pack-

ages and diced to smaller sizes. This leads to tremendous size and cost reduction, functionality,

performance, and reliability. SOP overcomes both the computing limitations and integration lim-

itations of SOC, SIP, MCM and traditional system packaging. The limitation of this technology

is the interconnect delay is becoming a serious bottleneck for the new generation of high speed

digital systems [8]. As well as one of the major issues for developing SOP is integrating an an-

tenna with a module efficiently. Fabricating an antenna directly on the package has the advantages

of reducing interconnect loss and size of an entire module. SOP goes one step beyond multichip

module (MCM) by enhancing overall performances and adding more functionalities[7]. A drawing

of SOP can be seen in Fig. 1.4.

1.1.2 Transmission lines

Transmission line is one of the most basic RF passive components. A transmission lines sole

purpose is to carry a signal from one point to another. The three forms of transmission lines

are microstrip, coaxial, and waveguide based. Hollow metallic waveguide based transmission

lines have been utilized since they offer low loss characteristics. As new dielectric materials have

been developed with low-loss, low tan-δ, low loss microstrip and coaxial transmission lines are

6

commonly employed in the design of high frequency circuits.

1.1.3 Antenna fabrication

There are three main forms of antennas that are commonly employed. The most basic form of

antenna is the wire antenna. This is known as the Monopole and Dipole antenna. Sometimes they

are referred to as a Whip antenna and most notably found on automobiles. These are low gain

antennas that have a broad beam width. These antennas are very basic antennas that are designed

to be quarter wavelength long. Whip antennas offer a very narrow operating frequency band.

The second form of antenna is the the planar or microstrip patch antenna. These substrate

based antennas are ground plane back antennas that offer higher gain at the expense of a smaller

beamwidth, which is needed in many applications. These offer the benefit of being easily incorpo-

rated with planar circuits. Microstip antennas are dependent on substrate properties and it becomes

increasingly expensive more expensive as the frequency increases the substrates need to be lower

loss. These antennas allow for various designs to meet specific criteria such as operating frequency

range, gain, directivity and polarization.

The third form is traveling wave antennas. The most common are Vivaldi, leaky-wave, and

Yagi-Uda antennas. These offer the benefit of being very high gain, high directivity, and large

bandwidth. The downside of these antennas are the size is large and typically linearly polarized.

The other difficulty of these antennas are the feeding network is more complex. They are not

ground plane back antennas which limits a direct microstrip feed without multilayer fabrication.

Traveling wave antennas are also larger when compared to their counterparts.

1.2 Additive Manufacturing of Wireless Components

AM is a form of manufacturing in which a Computer Aided Design (CAD) model is captured

and then subsequently fabricated in a layer-by-layer manner – eliminating tooling and enabling

complex geometries not possible with traditional manufacturing [9]. It is revolutionizing how crit-

ical parts get manufactured, fast prototyping, reduced waste, allows custom designs, easier design

7

adjustments, and parts can be printed as a single unit. This contrasts with traditional manufac-

turing, which cuts, drills, and grinds away unwanted excess from a solid piece of material, often

metal. The term AM is a term that encompasses several different methods and technologies of

AM. In 2015 the ISO/ASTM 529000 [10] standard was created in an effort to standardize the

terminology and classify each method of AM. In total there are seven different categories of AM

processes have been established and are used to identify which process of AM is used. Though

in general all of the processes follow the same basic steps of generating individual physical layers

and combining them together [11]. This section will introduce and describe each of the classifi-

cations with an example of a typical 3D printer that utilizes that method followed by a section on

the process of blanket metallization used to metallize all the plastic AM. components. The cost of

3-D printers and the materials used in them are continuing to decrease as the technology gains in

popularity and utility which makes it a promising technology for the RF world especially for small

scale operations and prototyping.

1.2.1 Fused Deposition Modeling (FDM)

The most basic form of AM is FDM printing sometimes referred to as Fused Filament Fabrication

(FFF). This process uses a spool of plastic filament that is fed through a printer head which is

heated and the filament is pushed through the head which causes it to melt. The printer head is

moved to specific points laying down the melted filament in layers on a build plate, the filament

then cools down on the build-plate and solidifies into a solid object. For layers that are floating or

not directly touching a layer below it; supports are also printed to support the layer in the position

it is in as can seen in 1.6. This process is repeated layer by layer until the object is fully created.

This method of 3D printing is very affordable, a standard FDM printer costs a few hundred dollars

which has made it extremely popular among hobbyist. Typical FDM printers are the Makerbot and

Flashforge.

The surface of these AM structures has a certain roughness level due to laying separate layers

of the melted filament on top of each other. When looking at the root-mean-square (RMS) of

8

Figure 1.6: Drawing of the FDM process.

the surface roughness it is dependent on a few conditions, the printing tip diameter, the nozzle

temperature, ambient temperature, and it is also dependent on which material is being printed[12].

For a 200 µm print nozzle it can be expected to have a RMS surface roughness of about 100 µm.

Due to this relatively high surface roughness it makes FDM not an ideal choice of AM to use above

X-band frequency range (8-12 GHz).

1.2.2 Stereolithography (SLA)

Stereolithography referred to as (SLA) is the oldest form of AM. The first patent was filed for

in 1986 and the company 3D systems commercialized the technology [13]. SLA printers use

mirrors in the X and Y-axis, and the mirrors aim a sold-state laser to cure a reservoir of resin

selectively.The resin is a photo-polymer which is cured by light. The build platform moves up out

of the reservoir and layer by layer the object is built. A drawing of this process can be seen in

Fig. 1.7. The most common type of commercially available SLA printer is the Formlabs which

offers SLA printers with accuracy down to 25 µm and various material choices from flexible, high

temperature, and standard [14]. The high temperature materials offered with SLA printing offer an
ability to maintain structure and form at over 200oc.

SLA is one of the most accurate AM for large structures [11]. SLA provides a superior and

smoother surface finish but with photo-curable materials that suffer from poor mechanical strength

or durability and degrade or discolor with prolonged UV exposure [15]. SLA produces structures

9

Figure 1.7: Drawing of the SLA.

with a RMS surface roughness of 25 µm which makes it an good choice to use on structures with

feature as small as 100 µm [16]. This low surface roughness allows SLA to be used to create

structures up to the W-band frequency Range (75-110 GHz).

1.2.3 Polyjet Printing/ Material Jetting (MJ)

Polyjet printing which is the commercial name for material jetting works very similarly to a stan-

dard home inkjet printer. Just as a inkjet printer prints a single layer of ink- Material Jetting prints

multiple layers, moving the build platform down a layer thickness between each layer until the

object is created. This process can be seen in a sketch in Fig. 1.8. The print-head jets hundreds

of droplets of a photopolymer, which is then followed by a ultraviolet (UV) light to crosslink the

droplets and create a solid object. MJ printing produces solid objects at a faster pace than other

forms of AM. Also, an added benefit of MJ method is that it allows for different materials to be

printed in the same object. The MJ method does require objects to be printed with a support ma-

terial, typically a wax that is printed simultaneously during the printing of the object. With the

latest advances in MJ the support material can be removed from the printed structure with relative

ease in the post processing state using standard solvents like acetone and isopropyl alcohol (IPA).

MJ is one of the only types of AM technologies to offer printing with multi-material and full color

printing. MJ offers the best surface finish of AM, MJ produces injection mold-like prototypes. MJ

offers print resolution of 16 µm and surface roughness down to 10 µm RMS. A standard MJ 3D

10

Figure 1.8: Drawing of the MJ.

printer is stratsys connex objet 350.

With MJ offering such a superior surface finish, it has been used for various RF components

from microstrip circuits, waveguides, antennas, and dielectric lenses [17]. Recently techniques

have been developed that utilize MJ to create microstrip structures and be able to selectively met-

alize using a damascene-like method [18]. While this method does not produce fine features to

the same level as the industry standard of photolithography, it does allow creating fine featured

microstrip structures without the use of chemicals, masks, and the need for precise alignment. Ar-

ticles have been published that utilize MJ to print side-coupled filters and utilized the damascene-

like method [18] to create the side-coupled filter structure. Horn antennas have been shown to

work up into the W-Band frequency range utilizing MJ but dielectric probes have been fabricated

using MJ up into the THz frequency range (300 GHz and above) [19]. From a RF standpoint the

main disadvantage of MJ is typically the printed polymers have high dielectric losses (Tan-δ=0.04)

which why it has been mainly used to make guided wave electronics that utilize air as the substrate

and the AM components acts as a structure for the metal layer deposited on top of the plastic.

1.2.4

Inkjet/Aerosol Jet Printing (AJP)

Aerosol Jet Printing (AJP) and inkjet printing are very similar in nature, the principles of operation

are, an ink which can be either dielectric or conductive is sent to the print head and printed and the

object is created layer by layer. There are 3 major differences between these technologies. AJP

11

offers a better print resolution almost 4 times better when compared to inkjet printing; AJP has a

5 µm print resolution where inkjet typically offers about 20 µm. The second main difference is the

method of which AJP with using a sheath gas to columnize the ink where inkjet just does droplets

as shown in Fig. 1.9; this allows AJP to have a greater variance in ink viscosity. Typically for AJP

the ink can range from 0.5 to 2000 cP where inkjet is typically limited below 20 cP [20]. The third

main difference is that AJP can have a larger variance in distance between the print head and the

substrate; typically the distance for AJP can vary between 1 and 5 mm where for inkjet printing

that distance is fixed typically around 1 mm. This allows AJP to print on non-flat objects where in

some instances it may be impossible to do utilizing Inkjet. A standard AJP printer is the Optomec

5XJ.

The typical use for AJP and is to print fine featured conductive traces. Out of all the methods of

AM; AJP offers the highest print resolution which makes it impractical to use for large 3D objects.

AJP has been used to make very small microstrip lines or high frequency antennas, THz filters,

and interconnects between substrates and bare-die IC components using conductive inks [20]. The

conductive inks have been shown in recent publications to achieve conductivity as high as 70% of

bulk silver (Ag) [21]. With this high conductivity , a recent publication showed AJP interconnects

having a loss of 0.49 dB/mm for the entire structure including the trapezoidal transition. This

same publication showed an AJP printed CPW on Rogers Ultralam 3850HT (LCP) exhibited 0.38

dB/mm at 110 GHz loss that is by far better than other AM techniques. AJP having such an

extremely high resolution, it was recently used to create band-pass and band-stop filter at 250

GHz and 550 GHz [22]. This makes AJP highly attractive for fine featured microelectronics that

otherwise would need to be fabricated in cleanroom (microfabrication facilities) using various

harsh chemicals.

12

Figure 1.9: Drawing of Inkjet and AJP.

1.3 Limitations of Combining Standard Technology and Additive Manufac-

turing

Additive manufacturing has presented a new method in the evolution of RF passive component

manufacturing. By providing a low cost means of rapidly producing prototype components nec-

essary during design stages, as well as a method for fabricating components with minimal waste,

these passive components can be easily designed and tested through out the design stages. While

additive manufacturing has recently proven as an effective manufacturing method, and the stan-

dard manufacturing methods described have been utilized for a few years now, there has been very

limited work that combines these different technologies together.

Each of the additive manufacturing methods offers distinct advantages over one an other. The

limitations of these technologies are faced when trying to combining them with standard technolo-

gies. MJ and FDM printing offer a good solution to printing large structures but it becomes to

incorporate the printed pieces with systems. The ability to easily fabricate large structures with MJ

and FDM, limits this technology to incorporate it with standard and widely used active component

packaging methods. With the high loss tangent material these printed components have to use

external coupling methods like coaxial or waveguides to feed the signal. This greatly limits the

ability to further increase the density.

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

13

als while achieving good print resolution. Fine featured structures in the several millimeters can

be fabricated. Aerosol jet printing has been used to fabricate of a wide range of electrical cir-

cuits, including transmission lines operating in W-band [23], filters [24], and antennas [25]. Other

passive components have been shown as well inductors, capacitors [26], and resistors [27]. Un-

like the other forms of printing AJP offers a great method to fabricate fine structures and directly

combine the standard forms of system integration. While this offers the best method to attaching

standard packages to printed components, the disadvantage is the limitation in creating large thick

structures. The thick structures will be needed for the traveling wave antennas.

In recent publications these various techniques have shown very effective. There have been no

real work at combing additive manufacturing with standard packages to create a system. There

needs to be a technique that combines various forms of additive manufacturing with standard com-

ponent packages to create a system that maximizes the functional density of the entire system. This

will minimize the cost and waste of manufacturing as well as allowing for the next generation of

RF components through 100 GHz and beyond.

1.4 Rigid-Flex System On Antenna

The polyjet printing of these AM has been utilized to create antennas up into the V-band with

high repeatability. AM can create large structures with resolutions up to 16 µm in a fast process

with surface roughness below 100 µm Ref. [28]. AM techniques present a new manufacturing

process in the evolution of RF component manufacturing by utilizing the Z-axis. By providing

a low cost means of rapidly producing prototype components necessary during design stages, as

well as a low cost method for fabricating components that can be integrated directly into modern

systems. Some processes to package electronics utilizing additive manufacturing techniques have

already been demonstrated. Packaged electronics have been embedded within printed objects,

connected by wires or by traces formed from post-processing of the printed substrate [29]. By

embedding devices in an object during the print process, blanket metallization has been utilized to

create a resonator and antenna that can be tuned up to 7 GHz [30].

14

The objective of this thesis, is to investigate and demonstrate a fundamentally new approach

to manufacture electronic systems. Here we exploit AM to realize high functional density systems

by integrating planner circuits directly into a AM antenna to create a System on Antenna (SoA)

as shown in Fig. 1.10. The process includes integrating active and passive elements, directly in a

AM Vivaldi antenna. This allows the creation of a novel module that is fully enclosed inside of the

unused portion of the antenna. This creates the smallest feed line for the system which inevitably

leads to a higher signal-to-noise ratio (SNR). This thesis paves the way for AM components to

be taken from a novelty of passive components to being utilized as a fundamental aspect of the

entire system. This allows for the printed component to be utilized as the structure and foundation

for the system to be created with in. The utilization of Aerosol printing further increases the

functional density by allowing fine conductive traces to be created as well as finer VIAs to ground.

This work creates a small form factor module that as almost entirely fabricated through A.M. This

methodology allows for cost effective modules to be created which avoids the use of power splitters

as well as the need for high power amplifiers due to the high efficiency of the individual modules.

This work shows how a front end system can go from four antennas being fed from one power

source to four individual modules being fed individually without the use of an inefficient high

power amplifier. This achieves higher power transmission and better energy efficiency with out the

need for a bulky heat dissipation metal plate. Each of these individual modules can be tuned to

operate at the same frequency point through a simple phase lock loop (PLL) module which has the

benefits of being very easy to implement and operates at a very low power level.

1.5 Dissertation Outline

The main objective of this work is to develop the methodology of combining RF circuits cre-

ated on flexible substrates with AM components to create entirely packaged systems from the

microwave frequency range into the W-band spectrum and pave the way for future work into the

500 GHz range. This work is organized as follows:

In chapter 2, this work will dive into better understanding of polyjet printing and understand-

15

Figure 1.10: SoA Mock Up.

ing this technology. Components like transmission line, resonators, and antennas are studied to

understand performance. Reduced height waveguides RHW are utilized to avoid using AM mate-

rial as the substrate. Complex geometries are shown that can be fabricated with relative ease. The

antennas presented have a efficiency above 80%.

In chapter 3, a combination of a flexible substrate and AM structures is presented. Beginning

with Transmission lines, performance is studied through 67 GHz. Then This feed is utilized to

create various antennas and antenna arrays that can be reconfigured at the 10-30 GHz frequency

range. This chapter is then finished with a Vivaldi antenna directly embedded into a scaled down

version of a automotive bumper.

In chapter 4, a combination of flexible LCP and AM is used to create a coax-like transmission

line. This transmission line is utilized to create various RF components. This expands from very

low-loss transmission lines, high Q-factor resonator, antennas, and antenna array. This approach

shows a very effective way of creating small form factor RF components.

In chapter 5, this methodology is used to build a entirely 10-15 GHz packaged system inside

of a AM antenna to create a receive SoA. This circuit that is fabricated through standard pho-

tolithography will be embedded inside of the unused space inside of the AM antenna. This system

is compared to a standard gain horn antenna with the same circuit attached with edge mount con-

nectors. Performance of this SoA is based on the received power compared to a standard gain horn

an antenna with the same connectorized circuit..

16

in chapter 6, a transmitter SoA is built. With the high accuracy of the Aerosol Printer to put

down conductive traces, the circuit traces for this system are printed directly onto the flexible

substrate. The benefit of using the aerosol jet printer for the traces, it also allows to create VIAs

to ground directly when printing the traces. This creates a high functional density SoA at 15 GHz

that is within 1 dB of theoretical power transfer calculations.

17

CHAPTER 2

HIGH FREQUENCY APPLICATIONS OF POLYJET PRINTING

2.1 Introduction

Traditionally, fabrication of components utilizes various processes such as micromachining,

and other lithographic techniques [31]. These processes require cleanroom facilities, multiple

pieces of dedicated equipment, as well as highly skilled technicians to fabricate these components.

The processes described above utilize various chemicals to layout a mask of a desired component

pattern and then chemically etch the metal layer to only leave the desired pattern. While this

method has been developed and become very good at fabricating components this method is very

expensive and has a lot of material waste.

With advances in additive manufacturing (AM), prototype structures with precise dimensions

and tolerances can readily be fabricated in a short turn around time. Now structures that would

have been difficult, if not impossible, to produce using conventional micromachining techniques

can readily be produced [32]. Recently, 3D printing has become a hot bed for research in the

electromagnetic community and circuits operating well into the terahertz (THz) frequency range

have been demonstrated [33]. The use of cleanroom facilities can be minimized to reduce overall

labor and material costs. Due to these benefits and reduced material waste 3D printing has attracted

significant interest in the design and fabrication of RF circuits and components [34].

Additive manufacturing (AM) has been studied as a potential replacement to traditional meth-

ods in the fabrication of RF components. Advantages of AM include fast prototyping, low material

loss, and low hazardous waste generation. With AM, a single tabletop system is sufficient to fab-

ricate a wide range of components, rather than requiring multiple machines, each dedicated to a

specific fabrication task. Recently, a range of 3D printed RF components have been demonstrated

using polyjet printing (MJ). This includes various structures like waveguide [35], coaxial cables

[36] and antennas [34]. MJ has also been utilized to create components up in the THz [37], this

18

work utilized MJ to create molds for the components.

The MJ process operates similar to the standard home printer. It jets droplets of photopolymer

which after being exposed to a UV light crosslinks to become a solid. This process is repeated

until the desired 3D geometry is completed. For complex geometries that have holes or overhangs,

the 3D printer also jets a support material which is gel like. This material is sacrificial which is

utilized to create a solid structure when pared with the photoplymer. Once the entire structure is

printed the support material can be either mechanically removed or dissolved by solvents.

The goal of this chapter is to study the performance of AM components. In contrast to work

presented in [34] & [36]; There have been several papers that have noted the dielectric properties

of the printed material, a dielectric constant of 2.9 and a dielectric loss-tangent of 0.04 can be

expected. To over come this realitivily high loss of the print material, this chapter will focus

on Reduced Height Wave guides (RHW) which will utilize the print material as a support for

copper and will allow air to be the dielectric. This work will focus on various RHW structures.

RHW structures are investigated to test various complex geometries. These structures are AM then

blanket metalized through sputter deposition and followed by a Damascene like process to achieve

a desired metal pattern. This investigation will primarily focus on the 1- 30 GHz frequency range.

2.2 Design and Simulation

Several components have been fabricated to study the performance of the MJ components in the

1-30 GHz range. The components are printed using a commercially available Objet Connex 350

polyjet printer. VeroWhitePlus dielectric material, a UV-cured polymer, was used in the fabrication

as it has the lowest loss among the different materials compatible with this printer. The printer has

a 16 µm print resolution and can provide a smooth surface finish with surface roughness of <5

µm. After printing, the structures are metalized using sputter deposition with a 0.1 µm layer of Ti

followed by 0.5 µm of Cu. The Ti layer is used to promote adhesion between Cu and the dielectric

material. This is followed by electroplating of 6 µm of copper. The surface area around the copper

was raised by 200 µm to use the Damascene-like process of mechanically polishing away excess

19

copper [38]. Several constraints are considered in the design of each component to be fabricated.

Due to the limitations of AM, no features smaller than 200 µm will be considered for fabrication.

2.2.1 Metallization

Most forms of AM the printing is limited to plastics (FDM, MJ, SLA) which requires blanket

metalization of the AM object to be used for waveguide, microstrip, or antenna structures. One

one of the most common methods for blanket metalization is Sputter Deposition, in which the

objects are placed inside of a vacuum chamber and then a seed layer of Titanium (Ti) is coated

(500 nm) around the structure, Ti acts as an adhesion layer for the the next layers of metal, Ti is

typically followed by a layer of copper (Cu) which is coated on about 1 µm thick [38]. But other

metals can be sputter coated aswell; the choice of metal is chosen based of desired conductivity

and particular application. To avoid metallic losses caused by the skin-depth affect [39], which is

defined by Egn. 2.1, the Cu coated pieces can either be electroplated or electrolysis with additional

metal until the desired metal thickness is achieved. The main difference between electrolysis and

electroplating is that electrolysis is the use of electrical current to drive a chemical reaction whereas

electroplating is the use of electrical current to plate one metal on another metal. Sputter deposition

is the most common method of blanket metallization due to its ability to produce a smooth metal

layer on top of the AM object which helps with RF performance.

δ =

1√
π f µσ

(2.1)

2.2.2 Reduced Height Waveguide Transmission Line

Two waveguide based transition are designed to operate in the T E10 mode. The height of the
waveguide was set to 1.52 mm such that a standard edge launch SMA connector can be used, see

Fig. 2.1. This waveguide does not have the normal 50 Ω impedance, thus a transition section from

microstrip to this waveguide is necessary so that standard edge launch SMA connectors can be

20

Figure 2.1: Designed inset for the RHW structure. λ is wavelength of the center frequency.

used. The inset of the 50 Ω microstrip line was set to be λ
waveguide to the 50 Ω microstrip line was optimized to achieve low insertion loss.

4 at 20 GHz and the metal tapering from

fc10 =

1
√
µε
2a

(2.2)

Two waveguides having the same transition but different lengths (40 and 60 mm) were designed

and fabricated. Waveguide loss can be determined with higher accuracy from the difference in the

insertion loss measurements of these waveguides. This approach removes any losses associated

with the transition section. The two RHW structures are shown in Fig. 2.2. At 20 GHz the 40 mm

and 60 mm waveguides have a simulated insertion loss of 0.0975 dB and 0.128 dB, respectively.

The difference in insertion loss between the two structures was 0.0053 dB which is effectively

the loss of a 20 mm long section. It also shows that the loss of these individual waveguides is

dominated by the transition sections and can be further optimized to reduce loss. The simulated

results can be seen in Fig. 2.3.

2.2.3 Reduced Height Waveguide Resonator Resonator

Typically when describing filters and resonators, the Q-factor is an important measure. Higher

Q-factors denote a better performance and this is inversely proportional to the loss in the resonator.

Standard microstrip resonators have a Q-factor between 10-100 whereas waveguide based T-line

resonators have Q-factors > 1000. For this RHW resonator the 60 mm RHW through from above

21

Figure 2.2: Designed RHW structure. (A) 40 mm length (B) 60 mm length.

Figure 2.3: RHW Through simulated results.

22

Figure 2.4: Designed RHW Resonator.

Figure 2.5: RHW Resonator simulated and measured results.

was modified with a λ

4 stub added to the structure. The RHW resonator is shown in Fig. 2.4. The
frequency of operation was chosen to be 14.75 GHz as it is a common communication band. This

resonator had a simulated insertion loss of 48.34 dB at 14.75 GHz, a simulated Q-factor of > 1500

was achieved.

Q =

fc

∆3dBBandwidth

23

(2.3)

Figure 2.6: Designed power divider.

2.2.4 Reduced Height Waveguide Power Divider

In modern RF applications, power dividers and power splitters are a key component in the design

of many circuits. Microstrip based power dividers (e.g., Wilkinson) have higher loss compared to

metal waveguide based designs. Waveguide based power dividers offer low loss across the entire

band of operation. The designed RHW power divider is shown in Fig. 2.6. As can be seen,

the RHW inset was used in this section. A back-to-back structure was used to allow for easier

measurement. The power divider was optimized to have a low insertion loss in the X-band. The

simulated results can be seen in Fig. 2.7. The power divider exhibits low loss, with a maximum

insertion loss of 3.85 dB. The fabrication follows the same process described above.

2.2.5 Reduced Height Waveguide Leaky-Wave Antenna

A leaky-wave antenna is a waveguide based antenna that radiates along the cut slit. A half-width

microstrip leaky wave antenna (HWLWA) has been commonly studied and is attractive due to its

compact size [40]. It uses a half-width microstrip transmission line, with one edge grounded using

either a via wall or copper tape. Recently, a 3D printed leaky wave antenna was demonstrated [41].

This work showed that leaky wave antennas can readily be prototyped using 3D printing; however,

the efficiency was low due to the high loss of the 3D printed dielectric material used. In this work,

a RHW structure which avoids the use of solid dielectric substrates is used to design the antenna.

24

Figure 2.7: RHW power divider simulated and measured results.

Figure 2.8: Designed RHW leaky-wave antenna.

The RHW-based leaky wave antenna structure is shown in Fig. 2.8. It is designed to operate at

10 GHz where the top width of the waveguide is made to be equal to one quarter of a wavelength.

The simulated return loss for the antenna is shown in Fig. 2.9. It has a simulated gain of 9.42 dBi

at 10 GHz, as shown in Fig. 2.10, which is about a 2 dB improvement in gain over the work that

has been demonstrated using a solid 3D printed substrate [41].

2.2.6 Reduced Height Waveguide Vivaldi Antenna

The Vivaldi antenna is an ultra-wide band antenna that offers high gain due to the high directivity.

Unlike previous work that showed 3D printed Vivaldi antennas [42] this work does not require a

coaxial cable to feed the antenna. The Vivaldi antennas smallest spacing between the two flares

dictates the highest frequency the Vivaldi antenna will operate at and the largest spacing dictates

the lowest frequency which the antenna will operate. To achieve constant beam-width the flare rate

25

Figure 2.9: RHW leaky-wave antenna simulated and measured S11results.

Figure 2.10: RHW leaky-wave antenna simulated and measured gain results.

change should be proportional to the wavelength. The design of the rate of separation can be seen

in Fig 2.11. One of the design issues that was encountered when using a waveguide was that both

legs of the Vivaldi antenna would be at separate elevations, but due to the enhanced capabilities

of 3D printing it was feasible to design the Vivaldi antenna where the legs would start at two

elevations and be able to meet at one elevation which would reduce cross-polarization as shown in

Fig 2.11.

26

Figure 2.11: Designed Vivaldi antenna with dimensions.

Figure 2.12: Damascene process.

2.3 Fabrication and Measurement

These sections are printed on a Stratsys Connex350 3D printed using plastic material. Print

resolution as high as 16 µm and surface roughness <6 µm can be achieved using this printer. The

components come out of the printer with a relativity smooth layer which can make it difficult for

proper adhesion between the plastic and the metal. To promote better adhesion some mechanical

polishing was done after printing to prepare the surface for metalization.

For metalization, the printed peaces are metalized utilizing a Denton desktop pro sputtering

system, first 60 nm of titanium (Ti) followed by 1 µm of copper are sputter deposited. Using this

Cu as a seed layer, a total of 6 µm of copper is electroplated. To pattern the copper, a Damascene-

like process was used, as described in [38]. This process can be seen in Fig. 2.12

The performance of the components for return loss and insertion loss is measured utilizing

an Agilent N52227A PNA. The far field components of the antennas are measured utilizing a
SATIMO Starlab® near field measurement system which extrapolates fair field data by utilizing

calibration horn antennas.

27

Figure 2.13: RHW Through before (left) and after (right) assembly.

Figure 2.14: RHW Through measured results.

2.3.1 Reduced Waveguide Transmission line

The reduced waveguide transmission line were printed in two half sections and then blanket metal-

ized, The two printed pieces followed the damascene process to process the desired pattern. After

patterning, the two sections are assembled using a "Lego-like" process. During this assembly, an

18 GHz SMA connector is attached between the two pieces. The pieces and SMA connector are

held together with conductive silver epoxy. Waveguides after assembly are shown in Fig. 2.13.

The measured results for these two RHW structures are shown in Fig. 2.14. These structures have

about 2 dB greater loss than the simulated results. However, the difference in transmitted signal is

small, which indicates low loss characteristics of these waveguides. The measured higher loss is

largely due to the transition section.

28

Figure 2.15: RHW Resonator before (left) and after right blanket metalization.

2.3.2 Reduced Height Waveguide Resonator

The RHW resonator followed the same fabrication process discussed above, the 3D printed pieces

after blanket metalization are shown in Fig. 2.15 and the fully assembled resonator is shown in

Fig. 2.15. The measured results for the RHW resonator can be seen in Fig. 2.5. As shown, there

is good correlation between the simulation and measured results. The resonator has a measured

Q-factor of 245. surface roughness of the print material and the dimensional accuracy of the printer

is believed to be the main cause of Q-factor reduction when compared to simulation. In simulation

perfect dimensional accuracy and no surface roughness was simulated.

2.3.3 Reduced Height Waveguide Power Divider

The AM RHW power divider shell is shown in Fig. 2.16, and fully assembled RHW power divider

with SMA connectors is shown in Fig. 2.16. The measured results can be seen in Fig. 2.7.

The measured results deviated slightly from simulation, as a maximum insertion loss of 5.64 dB

(including power divider loss) was measured, the higher loss was believed to be mainly due to the

loss added by the conductive epoxy as it has poor conductivity at higher frequencies as well as the

dimensional accuracy of the printer.

29

Figure 2.16: RHW power divider before (left) and after assembly (right).

Figure 2.17: Fabricated RHW leaky wave antenna prior to assembly (left) and after assembly
(right).

2.3.4 Reduced Height waveguide Leaky-wave antenna

The fabrication process of the RHW leaky-wave antenna was done similarly as described above,

the 3D printed pieces after blanket metalization are shown in Fig. 2.17 and the fully assembled

antenna is shown in Fig. 2.17. The measured results in comparison to simulation can be seen in

Fig. 2.9. There is good correlation between simulation and measured results. The RHW leaky

wave antenna has a measured gain of 8 dBi as shown in Fig. 2.10. Again, the simulation and

measured results match closely. The difference is largely due to the losses associated with the

connectors and conductive epoxy which are missing from the simulation results.

2.3.5 Reduced Height waveguide Vivaldi antenna

Similar to the other components the Vivaldi antenna was fabricated in two pieces for uniform

metalization, Both pieces of the antenna are joined together using a Lego-like assembly, using

30

Figure 2.18: Angled (left) and side (right) view of fabricated Vivaldi antenna.

Figure 2.19: Measured and simulated return loss (left) and gain (right).

conductive silver epoxy (MG Chemicals 8331S). Between the two half sections is an SMA edge-

mount connector. Fig 2.18 also shows the picture of the assembled antenna from its side. A slow

tapper in the flare section can be seen in Fig 2.18. The simulated reflection coefficient as well as

maximum gain are shown in Fig 2.19. Simulation results show that the entire substrate integrated

waveguide Vivaldi antenna has high return loss and high gain across the entire bandwidth. A

maximum gain of 10.2 dB is shown a return loss entirely above 10 dB across the entire bandwidth.

2.4 Summary

In this chapter, a few components were studied. Unlike previous publications, a reduced height

waveguide was utilized that allowed for the usage of a standard SMA connector for connections

instead of coaxial cables or waveguides. This reduced height waveguide allowed for the usage of

air as the substrate which gave a low loss design. The transmission line showed 0.13 dB/mm as

well as a wideband Vivaldi antenna which showed over 10 dBi gain.

31

CHAPTER 3

RIGID-FLEX COUPLED ANTENNA

3.1 Introduction

Typical printable dielectrics currently available have εr = 2.9, tan-δ = 0.04 [41]. Such high

dielectric losses lead to antennas and other components with inefficiencies. To overcome the loss

of the printed materials, air substrate designs can be implemented using 3D printing [43, 44].

However, due to material limitations, 3D printed polymer structures are rigid, brittle and do not

lend themselves for printing of free standing thin film form. Thus, flexible circuits are difficult to

fabricate using current print technology.

On the other end, photolithography is the main technique of the semiconductor and electronics

industry to fabricate circuits with fine line resolution [45]. Photolithography is a subtractive fabri-

cation method that utilizes a mask to transfer a pattern. Photolithography is the highest resolution

standard fabrication method with feature sizes as small as 10s of nm. Using electron beam lithog-

raphy, features smaller then 5nm can be achieved [45]. Photolithography is the clear choice to

fabricate planar structures since it offers extensive material options with a wide range of electrical

characteristics. These material choices offer the benefits of lower losses and flexibility. However,

thick or high aspect ratio structures are difficult to produce using this technique.

A.M. has been gaining interest from the automotive industry as a possible manufacturing tech-

nique to manufacturing of various automotive body parts. Interest has gained for direct printing of

fascias (bumper) on the assembly line. The automotive market is very price competitive in nature

and thus new and innovative methods of integrating antennas are needed. Here we present a novel

technique to make antennas an integral part of the 3D printed or molded plastic automotive parts.

This chapter combines the 3D printed rigid structures with flex circuits to fabricate novel RF

components that would be difficult to fabricate otherwise. This technique will couple transmission

lines directly into antennas. An aperture-coupled 2x2 element patch antenna array designed to

32

operate at 15.2 GHz is presented. Patch elements with air substrate are fabricated using 3D printing

and the slots and power divider circuit are fabricated using conventional lithography on a flex

substrate (Rogers 3850, LCP). As well as a couple Vivaldi antenna printed directly into a scaled

down automotive fascia (Bumper) that operates in the V-band (30-67 GHz).

3.2 Apperature Coupled Patch Antenna

An aperture-coupled antenna consists of two dielectric and three metal layers. The middle

metal layer is used as the common ground plane to the microstrip transmission line and the patch

element. The microstrip power divider is fabricated on the bottom metal layer and the patch an-

tenna elements are fabricated on the top layer. The coupling between the transmission line and the

patch resonant cavity occurs through a slot in the ground plane [46].

The structure was designed following the design procedure in [47]. Here, the microstrip and

the slots were fabricated onto a flexible LCP film. The patch was fabricated onto a 3D printed

plastic structure. With 3D printing, the spacing between the slot and the patch can be customized

unlike traditional board methods where it is limited by the available substrate thicknesses. Also, an

air substrate can be designed using 3D printing. For this work, an air spacing of 250 µm between

the slot and patch was used.

All simulations were carried out using ANSYS HFSS. A return loss of 19 dB at 15.2 GHz was

simulated as can be seen in Fig. 3.1. The radiation pattern at 15.2 GHz is shown in Fig. 3.1 and a

maximum gain of 8.4 dBi is achieved. The fabricated patch can be seen in Fig. 3.2 and the entire

antenna structure is shown in Fig. 3.3. The antenna had a measured return loss of 18 dB (see Fig.

3.1). The measured radiation pattern is shown in Fig. 3.1, and a gain of 7.5 dBi was measured.

This slight deviation between simulation and measurement was mainly due to the loss contributed

by the SMA connector and slight misalignment of the patch. The return loss and radiation pattern

matches closely with the simulation results. To fabricate the patch, a table like structure is 3D

printed, metalized and patterned, see Fig. 3.2. The 3D printed patch structure was attached to the

LCP using an adhesive paste at the outer edges of the structure.

33

Figure 3.1: Simulated and measured return loss (left) and gain (right) shown for the patch antenna.

Figure 3.2: The schematic design (left) of the antenna and a the fabricated patch (right) are shown.
The dashed box denotes slot size.

Figure 3.3: Designed Patch (left) and fabricated patch (right).

34

3.3 Rigid-Flex Patch array

To enhance gain, a 2x2 antenna array was designed and fabricated. Similar to the single ele-

ment, a flexible LCP substrate was combined with the 3D printed rigid structure that supports the
patch array. The pitch between each patch was λ

2 at 15.2 GHz, and for the power splitter a sim-
ple quarter-wave step impedance transformer was used. In simulation, a return loss of 31 dB was

achieved as is shown in Fig.3.5. The array also had a simulated gain of 13.4 dBi at 15.2 GHz (see

Fig. 3.6). The fabrication of this array was similar to the single element patch, and the fabricated

structure is shown in Fig. 3.7 while the layout of the power splitter is shown in Fig. 3.4. Here,

a manual alignment of the patch to the slots was carried out, and a slight misalignment led to the

difference in the return loss between simulated and measured results as can be seen in Fig. 3.5.

A gain of 10.8 dBi was measured as seen in Fig. 3.6 which is slightly lower than simulated; this

was mainly due to the misalignment of patch antennas to the slots and the losses contributed by

the connector.

One of the major benefits of this hybrid technology is the ability to fabricate antennas that can

be mounted on curved or bent surfaces. As demonstrated in Fig. 3.7, the patch array was bent

around the corner of a foam material. Fig. 3.5 shows the simulated and measured return loss, and

shows good agreement. The simulated radiation pattern is shown in Fig. 3.6; for which a gain of

7.4 dBi was achieved. The drop in gain, as compared to its flat counterpart, is due to the decrease
in the array factor. A half power beamwidth of 200o was obtained in simulation. For the bent array

a gain of 7.2 dBi was measured as shown in Fig. 3.6. These results correlate well with simulations.

3.4 Apperature Coupled Vivaldi Antenna in Large structure

The number of sensors used in an automobile is growing at a rapid rate over the past decade.

Sensors include radars, cameras, Lidars, ultrasonic parking aid, etc. There are many antennas op-

erating in different frequency bands used in a typical automobile. This includes antennas for radios

(analog, digital, satellite), GPS, cell phones, radars, Bluetooth, etc. With rapid advances in wireless

35

Figure 3.4: The Rigid-Flex patch array design (left) and power divider dimensions (right).

Figure 3.5: The measured and simulated return losses for planer and bent arrays.

communication, 5G for example, the number of antennas in a car is expected to grow significantly

over the next decade. Mounting of sensors and integration of antennas is labor-intensive, and in

most instances, integration of antennas is an afterthought.

Allowing the packaging of RF components that are fabricated and patterned directly onto large

printed structures will lead to substantial cost savings and allow for further customization of elec-

tronics within automobiles. For the integration of electronic components, esp. antennas, pattern-

ing of conductive traces with fine resolution on large area non-planar surfaces is a major challenge.

Conventionally, conductive ink (silver) is patterned using aerosol printing [22], inkjet printing [48],

and screen or stencil [49]. However, these techniques are slow and difficult to implement for print-

ing on large non-planar surfaces. Here we propose the use of two techniques to pattern conductive

36

Figure 3.6: Measured and simulated array radiation pattern at 15.2 GHz for flat (left) and bent
(right).

Figure 3.7: Fabricated Rigid-Flex patch array (left) and after bending (right) around a + 90o angle
is shown.

layer: (1) insitu printed stencil layer and (2) damascene-like patterning.

This method has huge benefits to be utilized in industries where additive manufacturing is

being studied for component manufacturing. This section studies the fabrication of a scaled-down

car bumper manufacturing with Vivaldi antennas directly printed with the bumper. The V-band

(30-67 Ghz) frequency range chosen as this can be used for parking and driving sensors as well as

vehicle-to-vehicle communication.

3.5 Characterization of Conductive Film Coatings

Off the shelf (OTS) aerosol paint from MG Chemicals is used here. Conductive paints are

available in both liquid and aerosol spray form. To demonstrate this concept, the use of an aerosol

37

spray of silver particles is used as it has a very high conductivity, and provides a smoother surface

finish. Traditionally, conductive spray paints have been used for RF and electromagnetic interfer-

ence (EMI) protection or FCC compliance on plastics and mobile electronics packaging. Due to

the relatively high conductivity of the silver paint, the material coating on plastics is well suited

for direct electroplating of copper. Other spray paints were also tried, however, the conductivity

achieved was low and had a rough surface finish. The aerosol paint is used as a method to avoid

the use of vacuum chambers since it becomes difficult as the size of components increase. This

becomes impractical as the size of components increase so will the size of the vacuum chamber

which will lead to longer metalization times.

The conductivity of the as-deposited silver ink and after plating was measured. A four-point

probe and source measurement process was used with 10 mm square conductive structures to

measure the DC conductivity. For the measurements, conductive structures were fabricated on

glass substrates using 3 mil polyimide stencils purchased from OshStencils to pattern the structures

for four-point probing and thickness measurements. Fig. 3.8 shows the patterned structures for

four-point probe measurements. The DC measurement was done using a Lucas Labs 4-Probe

conductivity probe and the Keithley source measurement unit. It is difficult to achieve controlled

thickness using spray coating; therefore, several samples were made in order to determine the

average conductivity of the paint.

For the measurements, after deposition, the coating was air-dried 10 minutes for handling be-
fore baking at 65oC for 10 minutes to remove any moisture before measurement. Thickness was

measured using an AEP Technology NanoMap-500LS advanced stylus contact surface profiler to

determine step height and average RMS surface roughness of the films on glass.

The provided datasheet lists the conductivity as 1.5 %, while our measurements show the con-

ductivity to be 1.3% of bulk silver. This conductivity is low for the design of RF circuits. Thus,

to further increase the conductivity of these samples they were electroplated with an additional 5

µm of copper. Leads were attached to the patterned silver structures for copper electroplating. The

same process for drying and measurement was then taken on the copper-plated structures to de-

38

Figure 3.8: Painted silver and electroplated copper silver paint.

termine a before and after conductivity value for this process. This yielded an effective 20% bulk

conductivity of copper. Several thicknesses of the paint were studied to see how it might affect

performance.

Fabrication of the Vivaldi antenna on 3D printed plastic used the same process as discussed

above. Patterning of the conductive layer was done using a shadow mask. Once dried and plated,

the Vivaldi antenna was measured and compared to a traditional fabricated Vivaldi using DC sput-

tered copper. Details of design, fabrication, and measurement are discussed below.

3.6 Antenna Design and Measurement

A process called rigid-flex is utilized in this paper to demonstrate a Vivaldi antenna fabricated

on a 3D printed part using silver ink. This process bridges the gap between standard PCB tech-

nologies (planar) and probing of 3D printed structures. This was recently presented and it showed

promising results [50]. Here, instead of a coax to feed the 3D printed Vivaldi, the feed network
is fabricated on Panasonic Felios R-F705 LCP (Liquid Crystal Polymer, εr=3 tanδ=0.004) and is
capacitively coupled to the Vivaldi structure. This feed is commonly utilized in planner Vivaldi

antennas designs. Here, instead of Vivaldi structure on a planar substrate, the Vivaldi structure is

3D printed and is coupled to the feed network fabricated on the LCP substrate (flexible substrate).

39

Figure 3.9: Designed (left) and fabricated (right) printed Vivaldi with coupled feed.

3.6.1 Vivaldi Antenna Design

Numerous papers have demonstrated 3D printed Vivaldi antennas [51, 52, 53]. The 3D Vivaldi

antenna has the benefit of thickness that can be utilized to achieve good impedance matching and

to enhance gain. This section starts a new design to a printed Vivaldi antenna. The new design is

shown in Fig. 3.9. Here, 100 µm LCP is stacked below the 2.2mm thick VeroWhite Plus polymer

structure to provide the feed network. The LCP has the microstrip coupler which couples to the

Vivaldi which is embedded into the VeroWhite layer. The Vivaldi structure is depressed 0.8 mm

into the VeroWhite layer. The design was optimized for this stack up. The simulated return loss is

shown in Fig. 3.10. The return loss is greater than 10 dBi over a frequency range of 40 - 67 GHz.

The simulated gain is shown in Fig. 3.10, and gain as high as 10 dBi can be attained.

The Vivaldi structure was printed on Stratsys Connex 350 SLA printer which has a 16 µm

print resolution in the z-direction. To metalized the printed Vivaldi structure, a stencil is utilized

which aligns with the Vivaldi depression formed in the 3D printed material. Fig. 3.11 shows the

fabrication and assembly of the Vivaldi antenna. The metalization of this Vivaldi followed the same

process as described above with one crucial difference. This method utilized a condutive aerolsol

paint to initiate the seed layer for the electroplate process. The process started with a shadow mask

(or stencil) on top of the Vivaldi. Approximately 10 µm of silver paint is spray-coated and the

stencil is then peeled off. Any over-spray on the edge is removed using the damascene-like process
discussed earlier. The paint is then allowed to dry at 65oC for 10 minutes. It is then electroplated

with an additional 5 µm of copper in a sodium persulfate solution. The entire fabricated Vivaldi is

shown in Fig. 3.9.

40

Figure 3.10: Planner Vivaldi returnloss (left) and gain (right).

Figure 3.11: (a) Planner Vivaldi printed (b) Vivaldi antenna coated in Silver paint (c) Vivaldi
antenna after electroplating copper. (d) Fully fabricated Vivaldi Antenna.

The antenna was measured using Agilent N5221A PNA with 1.85 mm southwest edge mount

connectors added to the transmission line on the LCP substrate. The measured results are shown

in Fig. 3.10. There is good correlation between the simulation and measured results. The slight

difference can be attributed to any misalignment between the Vivaldi structure and the coupler

LCP, alignment was done manually.

3.7 Antenna on Structure

The main aim behind this paper is to demonstrate antennas on large 3D printed (or molded)

structures. To demonstrate the applicability of the proposed process, the above antenna design was

incorporated into edges of the scaled-down bumper as shown in Fig. 3.12. Note, here two antennas

are incorporated into the structure. The Vivaldi structures are bent to match the curvature of the

bumper. Stratsys Connex 350 SLA printer (16 µm resolution) was again used in the printing of this

new design. It took approximately 12 hours to print the bumper model, with print resolution set at

41

Figure 3.12: Automobile bumper design with Vivaldi antennas embedded in corners (Bottom).
Fully fabricated with metalized Vivaldis (top).

32 µm. The print resolution can be reduced to increase print speed. The whole structure, as shown

in Fig. 3.12, was printed in one piece.

The antenna structure was metalized as described earlier, and the coupled fabricated on LCP

substrate was utilized. The bumper measured results are shown in Fig. 3.13. The two antennas have

similar performance and show good return loss. The difference in performance can be attributed

to the alignment tolerance of the coupler to the Vivaldi structure. Also, shown in the graph is the

measured isolation between the antenna elements which is greater than 30dB over the measured

frequency range. In other words, the Vivaldi antennas are highly directive and this is beneficial

for distance ranging, parking aid, and anti-collision. To demonstrate the antenna in transmit and

receive mode, a copper sheet was placed approximately 15 cm in front of the antennas and the

insertion loss between the antennas was measured. This measured transmit and reflection results

are shown in Fig. 3.13. This demonstrates that antenna designs can directly be incorporated in the

plastic structures of the car, and can be used in radar and parking aid applications. The antenna

was mounted on the exterior of the bumper to show the method being studied. The antenna can be

mounted on the interior of the bumper with no noticeable degradation to the antenna. This would

allow for a slick undisturbed look to the bumper which is desired in the next generation automotive

42

Figure 3.13: Measured transmit and reflected signal of the Vivaldi antennas on the bumper.

design.

3.8 Summary

This chapter covers creating two versions of coupled antennas. The patch coupled antenna

showed an effective method to achieve high gain antennas as well as achieving a 200 degree 3dB

beamwidth. The Vivaldi antenna which was directly printed with a scaled down bumper showed

an effective way that the automotive industry can start incorporating antennas and sensors directly

into printed bumpers. This method studied using conductive aerosol paints and shadow masks to

create the desired pattern. This method proved effective in the V-band frequency range and allows

for a method of metalization which can be directly used in the assembly line.

43

CHAPTER 4

RIGID-FLEX COAX TRANSMISSION LINE

While it has A.M. has been proven to be efficient at rapid component prototyping, which this leads

to significantly reducing component production time and cost. As seen in the precious chapter,

the fabrication of components that would be difficult to fabrication could be easily create through

A.M. With the ability to deposit material in a wide range of geometries it has been used for wide

verities of RF geometries [34, 54, 55]. This allowed for optimized RF performance geometries to

be created.

This chapter demonstrates, for the first time a new hybrid structure that is coaxial like. This

structure will be referred to as rigid-flex as it will contain both flexible substrate material and rigid

AM structures. This flex-rigid structure is similar to higher cost air dielectric coax cables with

being low loss, but has far greater system integration capabilities. This unique structure bridges the

gap between standard PCB technologies and 3D printed structures. For the first time a combination

of LCP and 3D printing are combined to create new coaxial like structures. In this paper rigid-flex

coax, rigid-flex coax filter, flex UWB power dividers, and rigid-flex antenna feeds for both single

and multiple element arrays are shown.

4.1 Rigid-Flex Coax Structure Simulation and Fabrication

Recently, a 3D printed Coaxial transmission line was demonstrated having low loss (≈ 0.1
dB/mm at 20 GHz) [36]. The advantage of such a structure is that it allows for fabrication of

3D geometries while avoiding the use of conventional planar substrates. However, in [36], the

fabrication required complex assembly which does not make it practical for most applications..

4.1.1 Rigid-Flex Coax Transmission Lines

Current A.M. materials are typically lossy and are not designed for RF applications. For this

reason another material was chosen that could be used as a substrate. The material chosen was 7

44

Figure 4.1: Flex-Rigid coax design (left) and fabricated (right).

mil Liquid Crystal Polymer (LCP)(180 µm) Rogers ULTRALAM 3850HT (εr = 2.9, tanδ = .001)

that has very low loss and is flexible [56]. The flex-rigid coax design can be seen in Fig 4.1. The

rigid-flex coaxial structure unlike traditional coaxial cables, only requires a half circle for the air

gap. This is beneficial as it shrinks the overall size of the structure. The radius R, which is the

radius of the interior coax shell, as shown in Fig. 4.1 is calculated using the formula shown in 4.1.

For this structure air was used as the dielectric so εr is 1. This formula was initially described for

a shielded microstrip in [57] with a rectangular opening and was modified for a circular opening.

The fully fabricated Rigid-Flex coax can be seen in Fig. 4.1.

Looking at the S-parameters, the simulation results for this coaxial transmission line are shown

in Fig. 4.2. This structure has an insertion loss less than 2 dB over the 10 - 40 GHz frequency

range. At 40 GHz, the insertion loss is 0.024 dB/mm, which is significantly low compared to

results reported in literature for microfabricated coax structures. The measured S-parameters of

this transmission line which includes the loss of two connectors and the extended 50 Ω line. At 40

GHz this Rigid-Flex coax showed a measured insertion loss of 0.04 dB/mm.

R ≥

λ
20∗√
εr

(4.1)

4.1.2 Rigid-Flex Coax Resonator

The design of high-selectivity, narrow-band resonators with tailored transmission zeros presents

important challenges [58]. The main advantage of using coaxial lines for filters is that they typ-

ically have higher quality (Q) factors resulting in lower losses. The difficulty in manufacturing

45

Figure 4.2: Simulated vs Measured insertion loss of printed Rigid-Flex Coax.

coaxial filters can be substantially decreased by using this rigid-flex technology which uses AM to

augment fabrication.

This filter design followed the same approach for the design as the flex-rigid coax described

above and it was fabricated in 3 parts and assembled using conductive epoxy. The designed filter

can be seen in Fig. 4.3. The performance of the resonator is judged by the increase in Q fac-

tor(which was evaluated as in [59]). which compares favorably to traditional microstrip T-line

resonator Q factors of 10-20 and demonstrates the possibilities for increased performance by com-

bining these fabrication methods.

The simulated results for the flex-rigid resonator are shown in Fig. 4.4. The fabricated resonator

can be seen in Fig. 4.3. Similar to that shown in Fig. 4.3, the 50 Ω feed line was extended by a

microstrip 50 Ω line, like the coax line, to allow for edge mount SMA connectors to be attached.

The measured results are plotted in Fig. 4.4; a Q factor of 113 was measured using the center

frequency over a 3 dB bandwidth as described in [60]. When compared to the other work on AM

46

Figure 4.3: Rigid-Flex T-line resonator structure, exploded view (left) and fabricated (right).

Figure 4.4: Resonator simulated and measured results.

resonators [61], this work showed a slightly increased Q, but with a significant reduction in size

and weight. Other work had a measured weight of 60 grams [61] where this work had a weight of

5.1 grams (an order of magnitude improvement).

47

4.1.3 Rigid-Flex Coax Transmission line With Substrate steps

To demonstrate the advantage of the proposed rigid-flex design, the structure above is further

optimized varying geometry along the transmission line is first demonstrated, Fig. 4.5. This design

while having various line widths and trench depths is a simple continuous 50 Ω line. The top

portion of this coax shell was a half cylinder with a radius of 1.3 mm and the bottom section is flat.

As shown in Fig. 4.1(c), 2 trench depths were used for demonstration (0.3 mm and 0.6 mm deep).
A slope of 45◦ between the trench transition was used. Similarly, a 45◦ transition angle was used
for the center conductor, see Fig. 4.5.

The simulation results for this coaxial transmission line are shown in Fig. 4.6. This structure

has an insertion loss less than 1 dB over the 10 - 40 GHz frequency range. At 40 GHz, the

insertion loss is 0.013 dB/mm, which is significantly low compared to results reported in literature

for microfabricated coax structures. The fabricated Rigid-Flex coax can be seen in Fig. 4.5,

the 50 Ω was fabricated on LCP using standard photolitography process, and the coaxial shells

was 3D printed and blanket metalized using the same process described in [41]. To allow for a

standard SMA connector to be attached, a standard microstrip 50 Ω line was extended out of the

structure. The three pieces (top, bottom and LCP layer) were snapped together using a Lego-

like self assembly process. Silver epoxy was used to permanently join the pieces together. Fig.

4.5 shows the measured S-parameters of this transmission line which includes the loss of two

connectors and the extended 50 Ω line. Below 20 GHz, the simulation and measured results

match closely. Overall, it shows that low-loss rigid-flex coax-like transmission line can readily be

designed using the proposed process.

4.1.4 Rigid-Flex Hemispherical Coax Resonator

Some of the best benefits of A.M. is the ability to produce spherical geometries with ease. Here, a

hemisphere design was chosen over a rectangular structure for the fact that spheres will have less

parasitic capacitance since there are no corners. The inner diameter of the sphere was designed to

be 3 mm to achieve a resonance frequency near 44 GHz. To be able to easily couple to a planar

48

Figure 4.5: Exploded view of the Rigid-Flex coax design with varying trenches (left) and fabricated
(right).

Figure 4.6: Simulated and measured return loss of the Rigid-Flex coax with varying depths.

transmission line, a semi-hemispherical design was chosen, at the expense of slight degradation in

performance over a fully spherical design.

For the RF signal to couple into the air-filled semi-hemisphere, a small slot was created in the

ground plane of the transmission line which directly covers the cavity. With a slot in the ground

plane to the resonator, a higher Q can be achieved by optimizing both the structure simultaneously.

The optimized slot can be seen in Fig. 4.7, and Fig. 4.7 shows the assembly of the pieces. The

fully fabricated resonator can be seen in Fig. 4.8.

The simulated and measured results are shown in Fig. 4.9. The simulation results for this

assembled structure shows a return loss of 28 dB at 44.04 GHz with a 4.15 MHz 3dB bandwidth.

Using Eq. 1, this yields a Q-factor of 10600. The fabricated resonator has a resonance frequency

49

Figure 4.7: Close up view of the slot in the ground plane (left) and blown out view (right).

Figure 4.8: Printed and meatallized resonator pieces (left) and fully assembled (right).

at 44.23 GHz with 5 MHz 3 dB bandwidth. Utilizing Eq. (1), the Q-factor was calculated to be

approximately 8800.

4.1.4.1 Rigid-Flex Resonator error analysis

The resonant frequency of the fabricated structure is 220 MHz higher than the simulation results. In

the simulations, it was assumed that the semi-hemisphere has perfect dimensions (radius), whereas

the fabricated structure is slightly elongated. Also, the feed line which was fabricated through

photolithography was off by 15 µm which showed no major difference in simulations. The biggest

difference, between simulated vs fabricated, is noted in the slot dimensions. Fig. 4.10 shows the

50

Figure 4.9: Simulated and Measured results for the Hemispherical resonator.

Figure 4.10: Fabricated Slot dimensions.

measured dimensions of the slot, in comparison to the simulated structure, Fig. 4.7, a considerable

difference can be noted. The difference in the slot dimension is approximately 225 µm in one

direction and 250 µm in the other direction. This dimensional inaccuracy is due to the Connex 350

printer adding a wax-like support material which slightly changed the structure dimensions during

print. Also, as specified in the datasheet, the printer has a dimensional accuracy of 100 µm [62]

in the x- and y-direction. Fig. 4.11 compares the simulated S21 against measured results after the

as-fabricated slot dimensions are included in the simulations. A good match between the measured

and simulation results is achieved.

51

Figure 4.11: Measured and simulation results after correction for slot dimensions.

4.2 Rigid-Flex Coax Antenna Feed

Vivaldi antennas have been fabricated using AM with good results, but that work required the

antenna to be fed using an expensive semi-rigid coax cable [63]. Using this method degraded the

quality of the fabricated antenna due to the semi-rigid coax cable adding a series resistance which

contributed to achieving only about 9 dBi gain and 60% radiation efficiency (the semi-rigid coax

cable also increases the cost of the structure). In the previous chapter a A.M. Vivaldi antenna used

substrate integrated waveguides (SIW) to feed the Vivaldi which further increased the radiation

efficiency up to 81% . But this method really limits the ability to create an array of antennas that

are wideband.

4.2.1 Rigid-Flex Coax Antenna

The Vivaldi antenna presented here will be similar to the one published in [63]. This antenna will

be designed to work between 10 GHz and 25 GHz. The antenna feed network was first optimized

in a back to back fashion as shown in Fig. 4.12; the cavity in the back of the structure was set to
have a radius of λ

4 for the center frequency which was 3.75 mm. On top of the cavity, plates were

52

Figure 4.12: Designed front-to-back cavity (Left) and Cavity S-parameters (Right).

added which would help with impedance matching and reducing the radiated signal. The width of
the gap of the structure was set to be smaller than λ

4 of the highest frequency. The structure was
optimized to have the lowest insertion loss between both ports. The structure had a a max insertion

loss of 3.8 dB, as can be seen in Fig. 4.12.

With this optimized configuration half of the structure was removed and then the Vivaldi legs

were attached as shown in Fig. 4.13 and the table of dimensions. To optimize the Vivaldi antenna

further the rate at which the Vivaldi legs flared up was changed until a return loss of > 10 for the

entire bandwidth was achieved, as shown in Fig. 4.14. The antenna simulated a gain of greater
than 9 dBi for the entire bandwidth with a maximum gain of ≈ 13 dBi. The fabricated antenna
after blanket metalization and assembly can be seen in Fig. 4.13. The Vivaldi measured return

loss can be seen in Fig. 4.14; as can be seen there is good correlation between simulated and
measured results. The 50 Ω line attached to the Vivaldi was attached with MG Chemicals® 8331

conductive epoxy. The measured gain is shown in Fig. 4.14.

this measurement showed good

results when compared to simulation. The antenna was measured up to 18 GHz as that is the

maximum frequency the SATIMO can measure. The effects of the conductive epoxy is apparent;

as we saw about 1 dB of difference which is expected as it is not designed for RF applications. The

simulation

53

Figure 4.13: Designed Vivaldi (left) Fabricated Vivaldi (Right).

Figure 4.14: Vivaldi simulated and measured S-parameters (left) Vivaldi simulated and measured
gain (Right).

4.2.2 Rigid-Flex Coax Antenna Array

This chapter presents a major advantage that the coax structure uses a standard substrate material;

with this advantage a power divider can be made fairly easy and can be used for various circuits

incorporated with the flex-rigid coax. The power divider being chosen to be studied is a Wilkinson

divider because of its ability to work for UWB applications [64] and its low insertion loss. The

center frequency was chosen to be 20 GHz which allows the Wilkinson power divider to operate

between 10-30 GHz[60]. The designed Wilkinson power divider can be seen in Fig. 4.15. The

100 Ω resistor was modeled as a 0603 resistor since it was easily accessible to get physical 0603

resistors that operate up to 57 GHz. The fabricated Wilkinson divider can be seen in Fig. 4.15. It

54

Figure 4.15: Wilkinson Power divider design (left) and fabricated (right).

Figure 4.16: Wilkinson Power divider simulated and measured S-Parameters.

was fabricated using standard photolithography and the resistor was hand soldered to the board.

As seen in Fig. 4.21, the power divider had about 1 dB of insertion loss up to 23 GHz. These

results include the loss of the two SMA connectors.

This part of the chapter combines the UWB Vivaldi and Wilkinson power divider described

55

Figure 4.17: Vivaldi array design (left) and fabricated (right).

above to create a 1x2 Vivaldi array. The designed 1x2 Vivaldi array can be seen in Fig. 4.17. For
the array spacing, due to the geometry, it was impossible to get λ

2 spacing between each antenna
element; so the spacing was minimized as much as possible to about 56 mm. As can be seen in Fig.

4.18 the array had a return loss larger than 10 dB for the entire desired bandwidth. The simulated

realized gain for the structure was larger than 12 dBi for the entire bandwidth as is shown in Fig.

4.20. Since further optimization was not possible in this orientation the array was fabricated. The

fabricated array can be seen in Fig. 4.17, The antennas were 3D printed and blanket metalized up

to 6 µm of copper, and the power divider was fabricated using standard photolithography process.

The elements were attached to the power divider in a similar way as the 50 Ω line in the Vivaldi

antenna described above. The measured return loss in comparison to simulation can be seen in

Fig. 4.18. These measured results show very good correlation to simulation results. The fabricated

array did maintain a return loss > 10 dB. Between 16-18 GHz a resonance that does not correlate

to simulation was seen and this was due to the 0603 resistor being a 3D dimensional component

with larger parasitic effects than simulated. The measured gain can be seen in Fig. 4.18. Between

10-17 GHz the array had a measured gain > 9 dBi.

The main disadvantage of using Vivaldi antennas is the physical size of the antennas that limits

56

Figure 4.18: Vivaldi array S-parameters results (left) and gain (right).

Figure 4.19: Vivaldi vertical array design (left) and fabricated (right).

the λ
be λ

2 spacing of the antennas in array applications. Shown above the array spacing was set to
2 for 3 GHz due to the Vivaldi leg size. The benefit of this flex-rigid coaxial structure being
used for the feed lines for the antenna, is that the Vivaldi antennas can be in vertical orientation to
allow for better λ
shown in Fig. 4.18 a λ

2 spacing. This can be seen in Fig. 4.19, this allowed for better array spacing, as
2 for 10 GHz was used. The returnloss as shown in Fig. 4.20 showed a good
improvement. The simulated gain shown in Fig. 4.20; shows a good improvement across the the

entire bandwidth. A maximum gain of 16.25 dBi was achieved. The fabricated Vivaldi array was

simply bent around Styrofoam that was 15 mm thick as shown in Fig. 4.19. Styrofoam was used

as it has characteristics similar to air, so no loading effects should be seen by the antennas and the

spacing will be maintained at 15 mm. The measured return loss can be seen in Fig. 4.20; an overall

1.3 dB increase in return loss was seen when compared to the horizontal case. The measured gain

can be seen in Fig. 4.20.

57

Figure 4.20: Vertical Vivaldi array S-parameters results (left) and gain (right).

4.3 Rigid-Flex Vivaldi Array with Printed Resistor

With surface mount components, there are physical and electrical limitations that could po-

tentially be improved by using additive manufacturing (AM). Previous work has been done with

static (DC) circuits to fabricate passive components such as resistors, capacitors, and inductors us-

ing injet printing techniques [65, 66, 67, 68]. However, to the best of our knowledge, resistors for

microwave applications. By fabricating the passive elements directly onto a circuit there is a poten-

tial for reducing undesired parasitics. There are many materials available for AM that can be used

individually or in combination to fabricate novel passive components. A common material used

often in the fabrication of flexible circuits and solar cells for electrode connection is conductive

polymers. For the purpose of replacing a chip-resistor with an aerosol printed thin-film resistor,

we chose to use of PEDOT:PSS (Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)). PE-

DOT:PSS is a conjugated polymer [69] whose conductivity is low compared to copper and can

readily be used in the design of a resistor. The geometry of the printed resistor can be used to

achieve desired resistance values.

In order to replace the 100 Ω chip-resistor with a printed thin-film conductive polymer resistor,

the same footprint was used. By using the same footprint for the resistor, a one-to-one comparison

58

can be made between the commercially available and the customized aerosol-jet printed resistor.

Using an Optomec 5X aerosol-jet printer with ultrasonic atomizer, to deposit thin-film conductive

polymers onto a liquid crystal polymer (LCP) substrate with direct contact to etched copper pads

and traces. Since PEDOT:PSS is a low viscosity material, it can easily be aerosolized and de-

posited. The area taken up by the chip-resistor was used as the pattern area of the printed thin-film

resistor, with feature sizes as small as 40µm. By adjusting the amount of material deposited and

successive layers of PEDOT:PSS, the resistance of the film can be precisely controlled. Aerosol jet

printing allows for successive layering of material and overcomes many geometrical challenges. If

a printed component such as a thin-film resistor is not within a margin of error, then additional ge-

ometric structures can be added to increase or reduce the resistivity. This is a major improvement

over the current processes of laser trimmed chip-resistors.

An identical Wilkinson power divider was fabricated using the same process as discussed above

and instead of utilizing a high frequency surface mount resistor the resistor was printed utilizing

PEDOT:PSS and was printed in layers until the 100 Ω resistance was achieve. The measured

results for the Wilkinson power divider with the printed resistor compared to the Wilkinson power

divider that utilizes a standard surface-mount resistor are shown in Fig. 4.21. As can be seen, the

results for both Wilkinson power dividers matches closely, and shows that the printed resistor can

provide similar performance to a commercially available high frequency resistor.

The Wilkinson power divider described is combined with the 3D printed Vivaldi to create a

1x2 element antenna array. The Wilkinson power dividers from above will have their connectors

removed and used with the printed Vivaldi structure. The array S11 results are shown in Fig. 4.23a.
There is good correlation between simulation and measured results for the arrays. As shown in

Fig. 4.23a, the array that utilized a 0603 surface mount resistor had a resonance between 16-19

GHz; through simulation it was found that the resonance was caused from the parasitic inductance

and capacitance of the 0603 package and soldering to the board. The PEDOT:PSS based resistor

did not show any resonance over the measured frequency band. Fig. 4.23b shows the gain of the

array. The array had a simulated gain of 12 dBi at 10 GHz and a max gain of 15 dBi at 18 GHz.

59

Figure 4.21: Measured results of the Wilkinson power divider. PR- Printed Resistor, SM- Surface
Mount.

The measured gain for the surface mount resistor array was approximately 10 dBi at 10 GHz with

a maximum gain of approximately 11.75 dBi at 16 GHz; however, the gain begins to drop where

the resonance occurred. For the printed resistor array the gain started at approximately 10 dBi and

with a maximum gain of 12.5 dBi at 17 GHz.

4.4 Summery

This chapter showed the an effective way of creating coaxial like transmission line. This ap-

proach utilized standard substrates to create the fine features through photolithography and additive

manufacturing to create the larger features. This approach created the lowest loss transmission line

in recent publications. This work achieved a resonator with a Q-factor over 10000. This transmis-

sion line design also allowed us to create a Vivaldi antenna and for the first time in publication a

UWB Vivaldi antenna array. This approach shows an effective and an easy way to create compact

transmission lines, resonators, and antennas in a small form factor.

60

(a)

(b)

Figure 4.22: (a) 1x2 Vivaldi array with surface mount resistor. (b) 1x2 Vivaldi array with printed
resistor.

This also showed for the first time utilizes PEDOT:PSS as a high frequency resistor for the

Wilkinson power divider and shows measured results up to 25 GHz. This chapter showed an

effective way of creating RF resistors through additive manufacturing.

61

(a)

(b)

Figure 4.23: (a) Simulated and Measured S11 . (b) Simulated and Measured gain results. PR-
Printed Resistor, SM- Surface Mount.

62

CHAPTER 5

RIGID-FLEX SOA

Microwave and mm-wave systems are important for many automotive, communications, and mil-

itary applications.

In the electronic front-end of radar systems, the transmit and receive (T/R)

modules are of critical importance [70]. T/R modules play a critical role in determining the overall

cost and performance of radar systems. The T/R module needs to be compact and light-weight, and

have low-loss and low-distortion characteristics [71]. On the receiver end, it is of utmost impor-

tance to achieve high signal-to-noise ratio. Apart from using low-noise components, it is critical

to also reduce integration losses. One approach to meet this goal is to integrate the receive module

in close proximity to the antenna element.

Many system design approaches are commonly employed in the design of compact T/R mod-

ules, including System-on-Chip (SoC), System-in-Package (SiP) and System-on-Package (SoP).

SoC offers the promise for the most compact, light-weight system that can be mass-produced.

This has been and continues to be the road map for the wireless RF industry. SoC seeks to in-

tegrate numerous system functions on one silicon (or other semiconductor) platform horizontally,

typically a chip [4]. SoC is a very attractive solution to achieve high levels of integration in a

small package. Some of the challenges associated with the SoC concept are long design times due

to integration complexities, high wafer fabrication costs, and incompatibility with heterogeneous

integration [4]. Another challenge faced is degraded antenna performance due to lossy silicon sub-

strates when designed in CMOS and SiGE-BiCMOS, which is the optimal technology for modern

SoC integration [72, 73, 74].

This chapter shows, building upon the concept of SoP, a compact front-end module that utilizes

a section of the antenna element to embed the receive circuit elements. The antenna and active

circuits can be co-designed and combined to achieve small form factor while enhancing system

performance. This co-design between the antenna and the circuit can help solve the major issue

faced with developing SoP, which is how to efficiently integrate an antenna, including active feed

63

structures [75]. A receive module is shown that is compatible with system designs for unmanned

aerial vehicles, automotive anti-collision, and other constrained installations that will need to be

compact and lightweight [76]. In the future, RF modules will be embedded in wings, bumpers or

other sections of a platform. 3D printing is becoming the key enabling technology for these higher

levels of integration and conformality.

The Rigid-Flex fabrication technique demonstrated in the previous chapter is adopted here
to design a complete receive module. This technique utilizes Rogers® 3850HT (Liquid Crystal

Polymer, LCP) flex and 3D printed rigid structures to fabricate antennas and antenna arrays. In

this chapter, the fabrication technique is applied more broadly to include active elements as integral

parts of the antenna structure, and a compact receive module is demonstrated.

5.1 System Design and Fabrication

Figure 5.1a shows the schematic diagram of the receive module design chosen for demonstra-

tion. It is comprised of an amplifier and a mixer integrated circuit. The integrated mixer down-

converts the RF to an IF band. This was done as a practical concern, since in a real application the

IF frequency will be used to for the signal processing rather than the RF signal itself. The amplifier
used in this design was a Mini Circuits® AVA-24+ surface mounted amplifier, which has 12.3 dB
of gain[77]. The mixer used was a surface mount Mini Circuits® SIM-153+ ceramic mixer [78].

The receive module housing the active and passive components (resistors and capacitors) was
fabricated on Rogers® 3850HT (LCP). The circuit was patterned using a standard positive pho-

tolithography process. The vias to the ground for the mixer and the amplifier were formed using
a high speed 200 µm diameter drill bit. The vias were filled by MG Chemicals® 8331 conductive
epoxy to complete the electrical connections. The conductive epoxy was cured at 60oc for one

hour at atmospheric pressure. The surface mount components were reflow-soldered using hot air.

Fig. 5.1b shows the fabricated circuit. To verify the circuit was operating as intended, it was first

tested on a spectrum analyzer using a signal generator as the RF-input.

64

(a)

(b)

Figure 5.1: (a) System schematic of receive circuit. (b) Fabricated circuit.

5.2 Package Design and Fabrication

Previously, as shown in Fig. 5.2, a 3D printed Vivaldi antenna fed by a Rigid-Flex transmission

line was developed [79] and is the basis for this design. The 3D printed Vivaldi antenna had a gain

greater then 9 dBi over 10 - 25 GHz with a maximum gain of 13 dBi at 25 GHz. For the Vivaldi

antenna, the inside volume of the legs is important for maintaining the radiation characteristics but

the rest of the Vivaldi leg volume is electrically unused space. Here, the exterior of the Vivaldi leg

was used as a cavity to package the receive system circuit. The cavity was slightly larger than the

Vivaldi leg in order to accommodate the surface-mounted mixer, which is shown in Fig. 5.2.

To fabricate the Vivaldi antenna, a similar approach as described in [79] was followed. To

create the SoP, we removed the RF input connector and excess substrate material from the receive

circuit and placed it into the cavity as shown in Fig. 5.2. The ground plane of the receive circuit was
adhered to the Vivaldi antenna package using MG Chemicals® 8331 conductive epoxy. Following

that, the 3D printed cavity cover was placed onto the antenna as shown in Fig. 5.2 to complete the

SoP. This package also has the added benefit of providing isolation for the receive circuit.

5.3 Measurement

To measure the performance of this 3D printed SoP, a duplicate receive circuit was fabricated

and attached to a Cobham H-1498 horn antenna with a SMA male to male through as shown in Fig.

65

Figure 5.2: (a) Rigid-Flex Vivaldi antenna. (b) Modified Rigid-Flex Vivaldi for SoP receive circuit.
(c) Vivaldi with circuit embedded in the cavity. (d) Final package with cap covering the cavity.

5.3a. This horn antenna with receive circuit is compared directly to the 3D printed SoP Vivaldi

with identical measurement conditions. The measurement conditions for both setups consist of a

Cobham H-1498 transmitting horn antenna driven by an HP 83592A signal generator as the RF

input. A HP 83731A signal generator was the LO drive for the receive circuit. The IF power from

the receive circuit was measured using an HP 8562A spectrum analyzer. Both front-end modules

were measured inside of an anechoic chamber.

The transmitter was set to 10 dBm for each of the frequency points. The LO drive was 11 GHz

with 7 dBm output power and held constant throughout the measurement. As can be seen in Fig.

5.4, the 3D printed SoP front-end module at 12 GHz collects 1 dB more power than the reference

horn and linearly increases to approximately 5 dB more at 15 GHz. This directly corresponds to the

Vivaldi antenna gain, which starts at 9 dBi and increases to 12 dBi with increasing frequency over

this range whereas the Cobham H-1498 horn maintains 11 dBi gain across the band [35]. Also,

the loss from the SMA male-to-male adapter and the SMA connector used to connect the horn to

the receive circuit contributes to the lower received power. As can be seen in Figs. 5.5a-5.5d the

radiation characteristics of the carrier frequency match closely to the theoretical end-fire radiation

66

(a)

(b)

Figure 5.3: (a) Horn antenna with receive circuit. (B) 3D printed SoP receive module.

Figure 5.4: Receive power difference between the active horn and Vivaldi antenna (normalized to
the horn).

pattern for both the horn and 3D printed Vivaldi antennas.

5.4 Summery

A demonstration of a hybrid additive manufacturing approach for the fabrication of high functional-

density packages has been presented. Building on previous work with passive-structures, the hy-

brid Rigid-Flex transmission lines, an active receive circuit embedded in a 3D printed Vivaldi

antenna has been measured. When compared directly to a standard rigid horn antenna the 3D

printed Vivaldi antenna showed 5 dB improvement on receive. This work demonstrates a low-cost,

light-weight, and high-density package with improved performance through 3D printing.

67

(a)

(b)

(c)

Figure 5.5: (a) Measured radiation pattern at 12 GHz. (b) Measured radiation pattern at 13 GHz.
(c) Measured radiation pattern at 14 GHz. (d) Measured radiation pattern at 15 GHz.

(d)

68

CHAPTER 6

RIGID-FLEX PRINTED VIA SOA

This chapter covers new approach to packaging of RF front-ends u sing a combination of polyjet

printing and aerosol jet printing (AJP) techniques. The transmit system demonstrated consists of

a Vivaldi antenna having a pocket for packaging as an integral part of the design and a transmit

circuit fabricated on LCP using AJP that is placed in this pocket. The transmit circuit consists

of a VCO, PA and the bias network. The radiated power was measured using a horn antenna

and a spectrum analyzer. The effective power loss in the circuit is approximately 0.5 dB. The pro-

posed process avoids the use of conventional photolithography or machining techniques. This work

shows that high functional density complex RF systems can readily be fabricated using additive

manufacturing.

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

and systems. Aerosol jet printing offers a ability to put down a wide range of conductive materials.

With the large standoff distance of aerosol jet printing, complete metallization of out-of-plane

features can be achieved, enabling complete circuit fabrication without the use of clean room

facilities and chemicals. By reducing the time and cost associated with prototyping, designs can

quickly be iterated to converge on an optimal solution.

Building upon the work in the previous chapter, the aim of this chapter is to demonstrate a

truly compact and light weight transmit system fabricated using Additive Manufacturing . Fig. 6.1

highlights the aim of this paper where the antenna and the transmit (Tx) module are combined as

a single platform.

AJP has shown promising results for the fabrication of passive components working over a

wide frequency range, DC to THZ applications [69, 80, 81, 82, 83]. AJP shows a method that

can be utilized as a cost-efficient technology for various applications like internet of things (IOT),

radio frequency identification (RFID), and wireless sensors. Recent publications have been looking

into the fabrication of VIAs in circuits with AJP. VIAs are one of the most critical factors for the

69

Figure 6.1: An ultra compact receive front-end.

realization of highly integrated systems, packages, and multilayered structures[9]. This paper

studies and shows the results of VIAs created with AJP in holes created utilizing a standard rotary

tool.The issue faced in [41] was the VIAs were drilled in the substrate and manually filled with

a conductive epoxy which really increased the dimensional size of the circuit so much so that the

Vivaldi had to be slightly increased to fit the circuit. This work utilizes a combination of SLA and

AJP to create a SOA transmitter all inside a small form factor Vivaldi antenna. A recent publication

showed results of VIAs created with AJP in holes that were created with femtosecond lasers [84]

and this work shows similar results. This works studies utilizing these AJP VIAs to create a high

functional density circuit which is utilized to create a system. The circuit is fabricated on standard

liquid crystal polymer dielectric (LCP, Rogers Ultralam 3850HT) with AJP completely cutting

out the need to utilize photolithography to create the circuit for the components. Utilizing AJP

for fabrication of the circuit is far superior in creating circuits when compared to other additive

manufacturing methods. This AJP circuit is then housed inside of printed antenna to create the

packaged system.

This chapter shows a VCO and an amplifier inside of the Vivaldi antenna to create a transmitter
which is within ± 1 dB of the theoretical calculation. For this entire fabrication process no standard

70

fabrication processes were utilized.

6.1 Aerosol Jet Printing

With Additive Manufacturing (AM) as the main goal for this design, an Optomec 5x Aerosol

Jet Printer (AJP) was used to pattern and fabricate the conductive traces for the PCB. Using an

AJP process expands the abilities for small flexible circuits requiring design rules that traditional

photolithography fabrication may not easy to produce. With a contactless fabrication process,

planar and nonplanar substrates may be used, as well as the elimination of chemicals traditionally

used for the patterning of copper.

6.1.1 Circuit Fabrication

The conductive ink deposited by the printer is Clariant Prelect TPS 50G2 50 wt% loaded silver

(Ag) nanoparticle ink. This ink has good RF characteristics with thermal sintering below 200C,

where the substrate begins to warp and lose its structural properties. The fine features of the

AJP allow for precise resolution down to 10um, and the control over thickness of the deposited

material. This design included 10um thickness (z axis) of Ag traces and pads, with 40um feature

size (x,y axis) using the Fine Feature setup on the Optomec 5x. Connections to the surface mount

components and leads for DC connection were made using Epotek H2OE-FC conductive epoxy

with a low temperature cure of 150C on a hotplate. This conductive epoxy works well with the

printed Ag ink on both pads and ground planes. The printed circuit can be seen in Fig. 6.2 before

being populated with components.

6.1.2 VIA Fabrication

Vias are commonly used in electronics for connections between physical layers to connect elec-

trical pathways of two or more layers. For multi-layer PCBs (printed circuit boards), Vias can

be used to connect signal paths, bias voltages, and ground planes between various layers for easy

routing around or under components such as ICs (integrated circuits). For single-layer or two-sided

71

Figure 6.2: AJP printed transmitter circuit.

PCBs, the most common use of VIAs are to make connections to ground without routing traces

on both sides. This technique is very common, where a simple through-hole VIA is used to con-

nect components to ground through the substrate. A few fabrication methods include laser drilling,

photolithographic masking and chemical etching, and mechanical drilling. If fine precision of laser

drilling is not required, and chemical processes need to be avoided, mechanical drilling is the sim-

plest and least costly choice. Mechanical drilling is also material-agnostic, as it does not require

particular chemicals or specific wavelengths and power to penetrate through various material types.

While there are many different techniques for fabricating VIAs, the authors chose to use me-

chanical drilling due to the low cost and availability of tools for the process. Two types of VIAs

were created with this method: through-hole and blind VIAs. Mechanical drilling allows for ease

of use in prototyping and allows for a material-agnostic process usable on most rigid and flexi-

ble laminated substrates. This method works well for LCP, leaving smooth tapered VIA wells.

Blind VIAs were created with various diameters ranging from 100 µm to 600 µm using a high-

speed drill press and CNC drills. The drill was used to create a well in the LCP that bottoms out

at the copper cladding on the backside of the substrate. This type of well is best suited for the

fabrication of plugged or plated VIAs, as there is a large conductive area at the bottom to build

upon. Through-hole VIAs were created using the same technique as the blind VIAs, except the

drilled well continued through the copper cladding at the bottom. By drilling through the cladding

by a small amount, a tapered hole was formed allowing for metallization of the VIA walls to be

formed.Plating of VIAs requires chemicals as well as leaving behind a lot of unused waste. Ad-

72

ditive Manufacturing avoids the unnecessary waste caused by chemical plating solutions which

use a large volume of material for plating of small areas in VIAs. Selective VIA plating is also

time consuming and difficult, so AM was chosen as a method to create plug-like VIAs in the LCP

substrate. By printing in the tapered VIA well using conductive silver ink, a conductive path was

formed from the bottom copper to the LCP sidewalls and upward onto the surface of the LCP

where pads of conductive silver were printed previously. Once the ink was dried and sintered into

a solid conductor, a plugged VIA had been created.

With the use of a Optomec 5x AJP (aerosol jet printer), many traditional methods can be

supplemented or supplaced by depositing aerosolized particles layer at a time in selective areas

and geometries. Many features can be created with the use of AJP, including ones which may

have been previously impractical. Because AJP does not require contact with the substrate or

conductors like plating processes do, conductive or dielectric materials can be used as fill agents

for VIAs easily. The tapered well created by mechanical drilling is perfect to print on from the

vertical position of the AJP without any special setup to print on a vertical surface. A simple spiral

pattern starting at the bottom or top surface was used to connect the silver pads at the LCP surface

to the copper cladding on the backside of the substrate. Some close up of the VIAs can be seen in

Fig. 6.3. High vertical aspect rations were realized by decreasing the speed of the printer tool path.

Applying 2-3 layers as needed, 100-200 µm deep wells were plugged on both test coupons as well

as the final SOA circuit board. Test coupons were created in order to measure contact resistance

between the ground plane and the printed VIAs and pads on 4 mil and 9 mil LCP laminates. The

measurement of the VIAs was done with a 4-point conductivity probe. The measured resistance

for both 4 mil and 9 mil LCP was less then 0.01 Ω.

6.2 Transmitter Circuit Package

This portion covers the design of a 3D printed ultra wide band (UWB) Vivaldi antenna which

is blanket metalized and fed directly with a simple 50 Ω transmission line on LCP. This technique

of Rigid-Flex yields great performance with out the need of utilizing coaxial feeds or complex

73

Figure 6.3: VIA close ups.

complex coupling feeds. This process of design and fabrication was described in great detail in the

previous chapter.

6.2.1 Vivaldi Design and Measurement

The same process is utilized in this paper but the length of the antenna is shortened which yields

to a smaller gain of the antenna. The designed antenna can be seen in Fig. 6.4. The total length

of this antenna is approximately 40 mm and the total with is 16.5 mm. The legs of the Vivaldi

are 8 mm in width with a gap of 500 µm between the legs. With the Vivaldi antenna being rather

short for the frequency range, impedance matching can difficult, an advantage of 3D printing the

antenna the thickness of the antenna can be changed to the get good impedance matching. For this

antenna to get a good impedance match (return loss > 10 dB) the Vivaldi leg was designed to be
4 mm. The 50 Ω transmission line is modeled for 7 mil (≈150 µm) LCP where the ground plane
is connected to one of the Vivaldi legs. The center conductor of the 50 Ω line continues to the

opposite Vivaldi leg without a ground plane beneath it. The simulated returnloss can be seen in

Fig. 6.5. The simulation was done in Ansys HFSS (version 18.2). The antenna has a S11 below -10
dB for the frequency range 12-17 GHz. The antenna showed a simulated gain between 9 - 11 dBi

for the frequency range as can be seen in Fig. 6.5. With these results the antenna was fabricated.

The fabricated Vivaldi antenna can be seen in Fig. 6.4b. The antenna was blanket metalized

with 0.50 µm of Titanium (Ti) followed by 1 µm of Copper (Cu). The layer of Ti acts an adhesion

74

Figure 6.4: 3D Printed Vivaldi Design (left) 3D Printed Vivaldi Fabricated(right).

layer between the copper and 3D printed plastic material. The copper layer was thickened to a

total Cu thickness of 6 µm with electroplating. The antenna was fabricated in two pieces to ensure

proper Cu coating in all the crevasses and the two pieces where combined together using a MG

Chemicals 8331 conductive epoxy. The Rigid-Flex feed was fabricated on 7 Mil LCP utilizing

standard photolithography to. The antenna was fed with a 3.5 mm edge-mount connector. The

measured S11 of the antenna can be seen in Fig. 6.5. The measurement Slight discrepancies can be
seen, This was due to the longer 50 Ω to account for the edge-mount connector causing a standing

wave. The gain of the antenna is as can be seen in Fig. 6.5 matches simulation within 1 dB.

6.2.2 SOA Fabrication and Measurement

The outside portions of the Vivaldi leg is essentially unused space. A cavity was created in the

Vivaldi leg and the transmitter circuit was set inside of it as can be seen in Fig. 6.6. Following

the same fabrication steps as above a new 3D printed Vivaldi was created with the cavity for the

circuit. The transmitter circuit was placed inside of it and the ground plane was epoxied to the

antenna with the MG chemicals epoxy. This created a DC ground for the entire antenna and the

75

Figure 6.5: Simulated and Measured S11 (left) and Gain (right).

Vivaldi leg with the cavity became a RF ground. The RF-out of the amplifier is connected to the

opposite Vivaldi leg with the Rigid-Flex feed to create the transmitter circuit.

Looking at the Fig. 6.7, with the VCO outputting 5 dBm and we can account for 3 dB of loss

between the VCO and amplifier due to the loss contributed by the silver printed transmission line,

the amplifier amplifies the signal by 11.8 dB into a 8.8 dB antenna. Calculating the free space

loss Egn. 6.1 for a distance of 10 cm gives approximately 30 dB of attenuation. That signal

goes into a 10 dB standard horn antenna which is connected to the spectrum analyzer through a

SMA coaxial cable which measures 1.7 dB of attenuation. The theoretical received power at the

spectrum analyzer is calculated to be -0.01 dBm, as can be seen in Fig. 6.8, for the measurement

set up shown in Fig. 6.7 this SOA is within 0.7 dB of theoretical value. This small derivation from

the theoretical can be easily accredited to contact resistance between the VCO and AMP to the

circuit due to the conductive epoxy.

Lair = 20Log10(

4πd f

c

)

(6.1)

6.3 Summary

This chapter shows a new approach to the packaging of RF front-end is demonstrated using

a combination of polyjet printing and aerosol jet printing (AJP) techniques. The transmit system

76

Figure 6.6: Designed SOA (left) Fabricated SOA (right).

Figure 6.7: Measurement Set-Up.

Figure 6.8: Spectrum Analyzer results.

77

demonstrated consists of a Vivaldi antenna having a pocket for packaging as an integral part of the

design and a transmit circuit fabricated on LCP using AJP that is placed in this pocket. The transmit

circuit consists of a VCO, PA and the bias network. The radiated power was measured using a horn

antenna and a spectrum analyzer. The effective power loss in the circuit is approximately 0.5 dB.

The proposed process avoids the use of conventional photolithography or machining techniques.

This work shows that high functional density complex RF systems can readily be fabricated using

additive manufacturing.

78

CHAPTER 7

CONCLUSIONS

7.1 Conclusion

In this dissertation, various processes have been shown and developed to combine additive

manufacturing with subtractive manufacturing techniques to create high frequency Rf electronics

that can be created into a small package which can be capable of operating in the 5G/mm-wave

frequency range. This technique began by leveraging photolithography to create fine featured

circuits and 3D printing to create the packages to create a System-On-Antenna package. This

was further developed to utilize aerosol jet printing to remove the need for photolithography, this

reduced the time and cost to create the fine featured circuits which removed the need for specialized

equipment and facilities which are typically used to create high frequency systems.

In chapter 2, Polyjet printing was demonstrated to create various passive circuits. This work

demonstrated an alternative method to subtractive methods and A.M. methods to create passive

components. This showed how Reduced Wave Guide components can be could be made to avoid

the use of the printed material as a substrate and allowed for the use of standard edge mount

connectors. This chapter demonstrated transmission lines, power divider, a resonator, and various

antennas up to 30 GHz. This work demonstrated waveguide like performance in a compact design.

In chapter 3, A.M. components were combined with a flexible substrate LCP to create two

antenna versions. The first was a slot coupled patch antenna where the patch was mounted above

the ground plane with an A.M. table that supported the patch with air as the substrate. This showed

both a single patch and a patch array. High gain was able to be achieved for both the single

patch aswell as the 2x2 array. The flexible substrate allowed the array to be bent around a 90

degree bend which yielded a wide 3 dB beamwidth. A scaled down automotive bumper was

also demonstrated which had Vivaldi antennas directly embedded within the structure. This work

showed how the use of vacuum chambers can be eliminated with the use of aerosol conductive

79

paints which can be incorporated in the assembly for ease of manufacturing. This bumper with

embedded antennas operated in the V-band. This demonstrated a promising technique to directly

embedding components can be achieved in large structures.

In chapter 4, This built upon the method described in chapter 3. This work utilized a com-

bination of flexible substrates and rigid printed components to fabricate a Rigid-Flex coax-like

transmission line. The center conductor of the Rigid-Flex coax was fabricated on the flexible

substrate through photolithography and the rigid shell was 3D printed. The Rigid-Flex coax trans-

mission line showed very low loss characteristics. This technique was then utilized to create a

high Q-factor resonator as well as a Vivaldi antenna. This technique can be fed with a microstrip

50 Ω line which then allows for the use of a Wilkinson UWB power divider, this allowed for the

creation of an A.M. UWB antenna array. This technique allows for a method of achieving 3D

printed antenna arrays without the need for coax cables or waveguide structures. This chapter also

demonstrated combing aerosol jet printing with the Rigid-Flex Vivaldi array to 3D print the RF

resistor for the power divider. This chapter showed a method of achieving high Q resonators as

well as 3D printed antenna arrays. With the combination of Aerosol jet printing this technique

showed how RF resistors can be printed and fine tuned to achieve the desired resistance.

In chapter 5, a demonstration of a hybrid additive manufacturing approach for the fabrication

of high functional-density package was demonstrated. Building on previous chapter work with

the hybrid Rigid-Flex transmission lines and Vivaldi, an active receive circuit embedded in a 3D

printed Vivaldi antenna was shown. This work allowed for the unused portion of the Vivaldi

antenna to be the package for the active circuit. This technique allows to create high-functional

density System-On-Antennas that can deliver better performance then traditional methods that

utilize connectors and other components to connect directly to horn antennas. This is a promising

method to further decreasing the size of the feed between the circuit and the antenna.

in chapter 6, this chapter combines aerosol jet printing and polyjet printing to create a high-

functional density package. This technique printed the circuit which has been previously created

through photolithography. Using the aerosol jet printer allowed for the creation of the circuit as

80

well as the VIAs to ground to further increase the density of the circuit. The antenna package was

printed with the polyjet printer. This technique allows for an extremely cost effective method of

producing Systems-On-Antennas with the ability to go into the THz range since the fed size is

further decreased.

This work showed the advantages of utilizing two forms of additive manufacturing and com-

mercially available low loss substrates to create high functional density systems. This technique

allows for systems to be created without the need for sterile enivroments and wasteful and harmful

chemicals. This work shows methods for very fats turn around time at a low cost.

7.2 Additive Manufacturing Limitations

Post printing cleaning of the polyjet printed components poses a significant challenge for met-

allization. The support material has to be either chemical removed with solvents or mechanically

removed, the challenge of this is with any remaining support material the metal comes off. This

poses a challenge of needing multiple attempts to metalize if the object is not entirely cleaned.

This can be minimized with support material removal optimization.

The metalized layers are when exposed to the environment begin oxidizing over time. This

degrades the performance of the components and system. While traditional methods of plating

additional metals like gold can help mitigate the issue of oxidation. The issue faced with this is

the needing of extra steps to achieve the additional layer. This requires equipment and additional

personal. Another method that can be utilized is the use of dielectrics to encapsulate the metal but

the dielectric loading effect will need to be taken into account in the design.

The printed metal out of the aerosol jet printer presents a few challenges, it needs to be sintered

at temperatures that are not compatible with printed dielectrics. This ink also has relativity low

adhesion to polymers which has issues with soldering components or perform rework. The printed

metal also has low conductivity limits its ability to preform in high power applications. This can

cause the metal to self-heat which can lead to metal degradation and substrate destruction.

81

APPENDIX

82

A.1 Future Work

Additive manufacturing has a promising future for creating passive components, with advances

in metal printing, components can come off the print bed ready to be used without the need of

metallization. With direct metal printing allows for avoidance of utilizing vacuum chambers or

chemicals to metalize the structure which will further speed up the process of fabrication and

allows for stronger metal.

With the variety of printable polymer materials with the aerosol jet printed, low-loss substrate

materials can be deposited, this provides the added benefit of not needing to utilize commercially

available substrates. Using these materials, the System-On-Antenna can be fabricated directly.

This reduces the cost and time necessary to fabricate package. This would allow for an entirely

printed System-On-Antenna with virtually no material waste.

The fine print resolution of the aerosol jet printer can be utilized to create the package directly

inside of the polyjet printed antennas. Bare die components can be placed directly inside the cavity

of the printed antennas and low-loss materials can be printed to create a specific substrate pattern.

Following this the aerosol printer can be utilized to create connections between the die bonding

pads and the antennas to create System-On-Antennas well into the THz frequency range. Reducing

the number of connections between devices allows for an increase in functional density.

83

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