Characterization of Bond Wires YOUJIE...

Micro Assembly for Radio Frequency Electronics Characterization of Bond Wires

YOUJIE CHEN

KTH ROYAL INSTITUTE OF TECHNOLOGY E L E C T R I C A L E N G I N E E R I N G A N D C O M P U T E R S C I E N C E

IL246X DEGREE PROJECT IN ELECTRICAL ENGINEERING SECOND CYCLE 30 CREDITS STOCKHOLM, SWEDEN 2019

Micro Assembly for Radio Frequency Electronics Characterization of Bond Wires

Youjie Chen

2019-07-18

Master’s Thesis

Examiner Gunnar Malm Academic adviser Saul Rodriguez Industrial adviser Francesc Purroy Martin

KTH Royal Institute of Technology School of Electrical Engineering and Computer Science (EECS) Department of Electronics SE-100 44 Stockholm, Sweden

Abstract | i

Abstract

Due to the increasing number of components involved in Radio Frequency design, integration and packaging become an important topic of developing power-efficient and cost-effective solutions. Furthermore, interconnections are a key factor in such a topic because they are heavily used in Radio Frequency engineering, especially in the Fifth Generation. Among the interconnections, bond wires are one of the most commonly used.

In micro assembly design, it is crucial to understand and model the behavior of each component, including interconnections. Radio Frequency engineers usually use the bond wire models in the software directly without questioning if the model actually has the same behavior as the fabricated one. Therefore, how to accurately model and characterize the bond wires becomes valuable, and furthermore, how the physical dimensions affect the transmission performance. This Master’s thesis project aims to solve this problem by building simple models for single bond wire and double bond wires with coupling, and verifying them by electromagnetic simulation and measurement.

The project has built bond wire models in Matlab and in electromagnetic simulators NI AWR and ANSYS HFSS. The actual test structures are also fabricated using the bonding machine, and measured by vector network analyzer. A sufficient amount of data has been collected from these sources and then analyzed. The proposed analytical model of bond wires is valid after comparing its results with those from simulation and measurement. In addition, the effect of the loop height and separation distance on the transmission performance is studied and has a well verified conclusion.

This thesis work will be helpful to Radio Frequency engineers, who use bond wires in the micro assembly of their design. They would be able to characterize the bond wires more accurately and adjust the physical dimensions in order to achieve the desired performance.

Keywords

Bond wire interconnection, Radio frequency, Analytical model, Electromagnetic simulation, Electromagnetic measurement

Sammanfattning | iii

Sammanfattning

På grund av det ökande antalet komponenter i radiofrekvensdesign, integration och förpackning blivit ett viktigt ämne för att utveckla energieffektiva och kostnadseffektiva lösningar. Sammankopplingar är en nyckelfaktor i ett sådant ämne, eftersom de är starkt används i radiofrekvensteknik. Bland dem, bondtrådar är en av de vanligaste.

Det är viktigt att förstå och modellera beteendet hos varje komponent. Därför hur att noggrant modellera och karakterisera bondtrådarna blir ett värdefullt problem, och dessutom, hur de fysiska dimensionerna påverkar överföringsprestanda.

Projektet har byggt bondtrådsmodeller i Matlab och i elektromagnetiska simulatorer NI AWR och ANSYS HFSS. De faktiska teststrukturerna tillverkas också med hjälp av bindningsmaskinen och mäts av vektornätverksanalysatorn. Den föreslagna analysmodellen för bindningstrådar är giltig efter att ha jämfört dess resultat med dem från simulering och mätning. Dessutom studeras effekten av slinghöjden och separationsavståndet på transmissionens prestanda och har en väl verifierad slutsats.

Nyckelord

Bindningstråd sammankoppling, Radiofrekvens, Analytisk modell, Elektromagnetisk simulering, Elektromagnetisk mätning

Acknowledgments | v

Acknowledgments

I would like to thank my examiner, Gunnar Malm, and supervisor at KTH, Saul Rodriguez, for their advice and constant supervision of this project. Thanks to my supervisor at Huawei Technologies Sweden AB, Francesc Purroy Martin, who has been giving me valuable insights and help throughout this journey. In addition, I would like to express my gratitude to my manager Dr. Shi Cheng, and my colleagues at the company, Igor Blednov, Peng Li, Qingzhao Du, and Tomasz Kaczkowski for welcoming me to the team and offering all the help I need along the way.

I would like to thank my family and friends for all the love and moral support that they always give me. This thesis project could not have been completed without the assistance and guidance that I received from all these people, to whom I am deeply grateful.

Stockholm, July 2019 Youjie

Table of contents | vii

Table of contents

Abstract ........................................................................................................ i Keywords ..................................................................................................................... i

Sammanfattning......................................................................................... iii Nyckelord ................................................................................................................... iii

Acknowledgments ...................................................................................... v Table of contents ...................................................................................... vii List of Figures ............................................................................................ ix List of Tables.............................................................................................. xi List of acronyms and abbreviations ...................................................... xiii 1 Introduction .......................................................................................... 1

1.1 Background .................................................................................................... 1 1.2 Problem .......................................................................................................... 1 1.3 Purpose .......................................................................................................... 1 1.4 Goals .............................................................................................................. 2 1.5 Research Methodology .................................................................................. 2 1.6 Delimitations .................................................................................................. 2 1.7 Structure of the thesis ................................................................................... 2

2 Background .......................................................................................... 5 2.1 Transmission Line Theory ............................................................................. 5 2.2 Bond Wire ....................................................................................................... 6

2.2.1 Bonding methods .............................................................................. 6 2.2.2 Relation with transmission line theory ................................................ 7

2.3 Microwave Network Analysis ........................................................................ 7 2.3.1 Impedance and Admittance Matrices ................................................. 8 2.3.2 Scattering Matrices ............................................................................ 9

2.4 Related work................................................................................................. 10 2.4.1 Transmission performance analysis of single and double bond

wires ............................................................................................... 10 2.4.2 Comprehensive performance analysis of single bond wire in

simulation ........................................................................................ 11 2.4.3 Modeling and minimizing the self-inductance of bond wire ............... 12 2.4.4 Analysis of capacitance compensation structure for bond wire ......... 14 2.4.5 Electromagnetic simulation of bond wires and comparison with

measurements ................................................................................ 16 2.4.6 Novel approach to model the packaged RF power transistors .......... 17

2.5 Summary ...................................................................................................... 18 3 Methodologies and Methods ............................................................. 21

3.1 Research Process ........................................................................................ 21 3.2 Research Paradigm ...................................................................................... 21 3.3 Data Collection ............................................................................................. 21

3.3.1 Sample Size .................................................................................... 21 3.4 Experimental design/Planned Measurements ............................................ 22

viii | Table of contents

3.4.1 Test environment/test bed/model ..................................................... 22 3.4.2 Hardware/Software to be used ........................................................ 22

3.5 Assessing reliability and validity of the data collected .............................. 22 3.5.1 Reliability......................................................................................... 22 3.5.2 Validity ............................................................................................ 23

3.6 Planned Data Analysis ................................................................................. 23 3.6.1 Data Analysis Technique ................................................................. 23 3.6.2 Software Tools ................................................................................ 23

4 Experimental and Modeling Approach ............................................. 25 4.1 Analytical modeling ..................................................................................... 25

4.1.1 Shape model ................................................................................... 25 4.1.2 Electrical characteristics .................................................................. 26 4.1.3 Network analysis ............................................................................. 27

4.2 Simulation .................................................................................................... 28 4.2.1 Circuit modeling and simulation ....................................................... 28 4.2.2 3D modeling and simulation............................................................. 30

4.3 Fabrication and measurement ..................................................................... 31 4.3.1 Bond wire prototyping ...................................................................... 32 4.3.2 Thru-Reflect-Line (TRL) network analyzer calibration ....................... 33 4.3.3 Measurement .................................................................................. 35

5 Results and Analysis ......................................................................... 37 5.1 Major results................................................................................................. 37

5.1.1 Analytical model results ................................................................... 37 5.1.2 Simulation results ............................................................................ 39 5.1.3 Fabrication and measurement results .............................................. 42

5.2 Reliability Analysis ...................................................................................... 48 5.2.1 Single bond wire .............................................................................. 48 5.2.2 Double bond wires ........................................................................... 48

5.3 Validity Analysis........................................................................................... 49 5.4 Discussion .................................................................................................... 49

5.4.1 Analytical model .............................................................................. 49 5.4.2 Fabrication and measurement ......................................................... 49 5.4.3 Loop height ..................................................................................... 50 5.4.4 Separation distance ......................................................................... 50

6 Conclusions and Future work ........................................................... 51 6.1 Conclusions ................................................................................................. 51 6.2 Limitations .................................................................................................... 51 6.3 Future work .................................................................................................. 51 6.4 Reflections ................................................................................................... 52

References ................................................................................................ 53

List of Figures | ix

List of Figures

Figure 2.1.1: Equivalent electric circuit of a transmission line [6] .................................... 5 Figure 2.2.1: Bond wires on chip [8] .............................................................................. 6 Figure 2.2.2: Ball-wedge bond wire [9] .......................................................................... 7 Figure 2.2.3: Wedge-wedge bond wire [10] ................................................................... 7 Figure 2.3.1: Two-port network...................................................................................... 8 Figure 2.4.1: Simulation results of single bond wires with different height [8] ............... 10 Figure 2.4.2: Fabricated single bond wire structure [8] ................................................. 11 Figure 2.4.3: Equivalent circuit for bond wire interconnection [11] ................................ 11 Figure 2.4.4: Transmission performance of the fabricated single bond wire [11] ........... 12 Figure 2.4.5: Bond wire model of Gaussian distribution function [12] ........................... 13 Figure 2.4.6: Measurement of return loss of bond wires with radius 12.5 µm and

62.5 µm [12] ........................................................................................... 13 Figure 2.4.7: Measurement of insertion loss of bond wires with radius 12.5 µm and

62.5 µm [12] ........................................................................................... 14 Figure 2.4.8: Structure of the interconnection [13] ....................................................... 14 Figure 2.4.9: Capacitance compensation model for bond wire [13] .............................. 14 Figure 2.4.10: 3D capacitive compensation structure [13] .............................................. 15 Figure 2.4.11: Simulated return loss of capacitive compensated and uncompensated

structure [13] .......................................................................................... 15 Figure 2.4.12: SEM picture of the bond wire [14] ........................................................... 16 Figure 2.4.13: Geometry and corresponding lumped element model of the bond wire

[14] ......................................................................................................... 16 Figure 2.4.14: SEM picture of the inside of a power transistor [15] ................................. 17 Figure 2.4.15: Extracted model for electromagnetic simulation of resistance and

inductance in FastHenry [15] .................................................................. 18 Figure 3.1.1: Research Process .................................................................................. 21 Figure 4.1.1: Rayleigh distributions [17] ....................................................................... 26 Figure 4.1.2: Shape model .......................................................................................... 26 Figure 4.1.3: Equivalent p circuit ................................................................................. 28 Figure 4.1.4: Cascaded p circuits ................................................................................ 28 Figure 4.2.1: Single bond wire model in AWR.............................................................. 29 Figure 4.2.2: Double bond wires model in AWR .......................................................... 29 Figure 4.2.3: Equivalent circuit for single bond wire ..................................................... 29 Figure 4.2.4: Equivalent circuit for two bond wires with coupling .................................. 30 Figure 4.2.5: Overall test structure in HFSS ................................................................ 31 Figure 4.2.6: Close-up view of test structure in HFSS .................................................. 31 Figure 4.3.1: Empty test board .................................................................................... 32 Figure 4.3.2: Detailed structure of the bond wire in top view ........................................ 33 Figure 4.3.3: VNA measurement of two-port device [6] ................................................ 34 Figure 4.3.4: Board for calibration ............................................................................... 34 Figure 5.1.1: Comparison of analytical model and Rayleigh distribution function .......... 37 Figure 5.1.2: Matlab results of single lossless bond wires with different loop heights ... 38 Figure 5.1.3: S-parameters of single bond wires in AWR simulation and Matlab .......... 39 Figure 5.1.4: Return loss of single bond wires in AWR simulation and Matlab .............. 40 Figure 5.1.5: Error between single bond wire model in AWR simulation and Matlab ..... 40 Figure 5.1.6: S-parameters of single bond wire in HFSS simulation ............................. 41 Figure 5.1.7: Return loss of single bond wire in HFSS simulation ................................ 42 Figure 5.1.8: SEM picture of single bond wire with gap = 1 mm, and loop = 1 .............. 43 Figure 5.1.9: Errors among single bond wires of loop = 1 ............................................ 44

x | List of Figures

Figure 5.1.10: Errors among single bond wires of loop = 10 .......................................... 44 Figure 5.1.11: S-parameters of fabricated single bond wires with different loop

heights ................................................................................................... 45 Figure 5.1.12: Return loss of fabricated single bond wires with different loop heights ..... 45 Figure 5.1.13: S-parameters of fabricated double bond wires with different

separation .............................................................................................. 46 Figure 5.1.14: Return loss of fabricated double bond wires with different separation ...... 47 Figure 5.2.1: Error between two measurements on a single bond wire......................... 48 Figure 5.2.2: Error between two measurements of double bond wires ......................... 49

List of Tables | xi

List of Tables

Table 2.5.1: Technical methods of related works ........................................................ 18 Table 4.3.1: Test board structure ............................................................................... 32 Table 5.1.1: Bond wire parameters in Matlab ............................................................. 38 Table 5.1.2: Optimized values for single bond wire ..................................................... 41 Table 5.1.3: Coupling coefficient of double bond wires with different separation ......... 42 Table 5.1.4: Single bond wires with 1 mm gap ........................................................... 43 Table 5.1.5: Optimized electrical characteristics for single bond wires ........................ 46 Table 5.1.6: Optimized coupling coefficients of measured double bond wires with

different separation ................................................................................. 47

List of acronyms and abbreviations | xiii

List of acronyms and abbreviations

2D Two Dimensional 3D Three Dimensional 5G Fifth Generation AAU Active Antenna Unit DUT Device under Test EM Electromagnetic FEM Front End Module FDTD Finite Difference Time Domain HMIC Hybrid Microwave Integrated Circuit IC Integrated Circuit LNA Low Noise Amplifier MIMO Multiple-Input and Multiple-Output PA Power Amplifier PCB Printed Circuit Board RF Radio Frequency SEM Scanning Electron Microscope S-parameter Scattering Parameter TRL Thru-Reflect-Line VNA Vector Network Analyzer

Introduction | 1

1 Introduction

Bond wire is one of the most commonly used interconnections in integrated circuit (IC) fabrication and packaging [1]. This is because it is a reliable, cost-effective and flexible technology. One of the common usages of bond wire is to connect the chip and its packaging. In the Fifth Generation (5G) technology, the increasing data transmission rate and the number of components in the device make the transmission performance of the interconnections more and more crucial. This gives rise to studies into the electrical behavior of the bond wires, such as how to accurately model and characterize the bond wires, and how the physical dimensions affect the transmission performance.

1.1 Background

Phased-arrays are one of the most fundamental blocks in the 5G multiple-input and multiple-output (MIMO) active Antenna units (AAUs) [2]. Each sub-array is fed by a Front-End Module (FEM), which typically consists of a Power Amplifier (PA), a switch and a Low Noise Amplifier (LNA) [3]. Because of the wide frequency range in 5G, there are a large number of components in Radio Frequency (RF) design. Therefore, integration and packaging become an important aspect of developing power-efficient and cost-effective solutions. This project focuses on the Power Amplifier integration.

1.2 Problem

For sub 6 GHz frequency, the Power Amplifier integration design is implemented by using a hybrid microwave integrated circuit (HMIC) [3]. There are a lot of components and interconnections used in the HMIC. Bond wire is one of the most frequently used interconnections in RF design, and its behavior can affect the performance of the system significantly [1]. In order to understand the behavior of the system better, all the components used in HMIC Micro Assembly should be able to be correctly characterized and modeled in terms of basic electromagnetic components. For bond wire, its transmission performance under different frequency and with different physical parameters is of interest. The problem in this project is how to accurately characterize bond wires, and how the physical dimensions influence the transmission performance.

1.3 Purpose

This project is meant to research the characterization of bond wires, and to gain insights into how parameterizing the physical dimensions would impact its transmission performance in a certain frequency range.

This thesis will be beneficial to engineers, who need to use bond wires in their designs and want to get a better sense of the characterization of the bond wires. Once this thesis is successfully completed, the engineers can use the model proposed in this project for bond wires, and the results and conclusions of this thesis will help them understand better how the model in the software actually behaves in real hardware, and choose the most suitable shape of bond wire for their specific design requirements.

As this thesis is based on the bigger goal of developing and implementing power-efficient and cost-effective solutions by looking into the integration and packaging, it is meant to be environmentally friendly and sustainable.

2 | Introduction

1.4 Goals

The goal of this project is to accurately characterize both single bond wire and double bond wires with coupling, and to draw conclusion about how the physical dimensions influence the transmission performance. This is divided into the following four sub-goals:

1. Build mathematical models for single bond wire and for double bond wires with coupling 2. Design and simulate test structures in simulation 3. Build test structures on bond wire prototype machine, and perform Vector Network Analyzer

(VNA) measurement 4. Investigate the effect of different loop height and separation distance on the transmission

performance Deliverables: simple models for single bond wire and for double bond wires with coupling

Results: 1. The transmission performance of the analytical models should be consistent with

simulation and measurement results 2. Conclusion about the effect of loop height and separation distance on the transmission

performance

1.5 Research Methodology

This project is a quantitative research, because it is meant to prove that the proposed model is consistent with the real bond wire by experiments and measurements. Due to the “experimental and testing character” of this project, the philosophical assumption is positivism [4]. As this project also includes studying the relationships between the physical dimension parameters and the electromagnetic performance, it needs to employ an experimental research method.

1.6 Delimitations

Due to the limited resource of physical wires, this project would only use one specific kind of wire. This means that this project would not be able to study the effect of other factors, such as different material or diameter of the wire. The measurement device Vector Network Analyzer only gives accurate calibration results under 15 GHz. Therefore, the frequency domain will be from 1 GHz to 15 GHz in theoretical calculation, simulation environment and the physical measurement. For simplicity reasons, only ball-wedge bond wires will be used, and this project would not study the impact of different bonding method.

1.7 Structure of the thesis

• Chapter 2 introduces relevant background information about transmission line theory, bond wires and network analysis. It also contains a selection of valuable related work that other researchers have done in this area.

• Chapter 3 presents the methodology and method that are used in this degree project in order to solve the problem.

• Chapter 4 shows how the research is designed and implemented in detail, which includes mathematical modeling, simulation, fabrication and measurement.

• Chapter 5 presents the data and results that are obtained from these three sources, and the reliability and validity analysis of the data. In addition, it discusses briefly about the

Introduction | 3

3

analytical model and fabrication, and draws a conclusion about the effect of loop height and separation distance.

• Chapter 6 is a final summary and conclusion of the work, including advantages, shortcomings and limitations. It also suggests what can and should be done in the future based on the results of this thesis project.

Background | 5

2 Background

This chapter provides some basic background information about transmission line theory and bond wire to help readers understand the subject this project deals with. It also introduces how to perform network analysis, which is applied in this project. Several related works on modeling and characterizing bond wires are also discussed in this chapter.

2.1 Transmission Line Theory

Transmission lines are commonly used for transmitting electrical power and information in electrical engineering [5]. As a transmission line can have a considerable length compared to wavelengths, it can be treated as a distributed parameter network [6]. From the perspective of electric circuit theory, a two-wire transmission line can be represented as a cascade of N segments of the structure shown in Figure 2.1.1.

For a segment of lengthΔ𝑧, it is modeled as an equivalent circuit consisting of series resistance, series inductance, shunt conductance and shunt capacitance. R is the series resistance of the wire per unit length; L is the series inductance of the wire per unit length; G is the shunt conductance between two conductors per unit length; C is the shunt capacitance between two conductors per unit length. Due to the nature of R and G, they both indicate loss in the circuit [6]. The values of these parameters are dependent of the geometric dimensions and material of the transmission line.

Figure 2.1.1: Equivalent electric circuit of a transmission line [6]

When analyzing this circuit, Kirchhoff’s law can be applied here and will get [6]

𝑣(𝑧, 𝑡) − 𝑅Δ𝑧𝑖(𝑧, 𝑡) − 𝐿Δ𝑧𝜕𝑖(𝑧, 𝑡)𝜕𝑡 − 𝑣(𝑧 + Δ𝑧, 𝑡) = 0

𝑖(𝑧, 𝑡) − 𝐺Δ𝑧𝑣(𝑧 + Δ𝑧, 𝑡) − 𝐶Δ𝑧𝜕𝑣(𝑧, 𝑡)𝜕𝑡 − 𝑖(𝑧 + Δ𝑧, 𝑡) = 0

These can be derived to

𝑑4𝑉(𝑧)𝑑𝑧4 − 𝛾4𝑉(𝑧) = 0

𝑑4𝐼(𝑧)𝑑𝑧4 − 𝛾4𝐼(𝑧) = 0

where

6 | Background

𝛾 = 𝛼 + 𝑗𝛽 = ;(𝑅 + 𝑗𝜔𝐿)(𝐺 + 𝑗𝜔𝐶)

is the complex propagation constant, 𝛼 is the attenuation constant and 𝛽 is propagation constant [6].

The solutions to these traveling wave equations are [6]

𝑉(𝑧) = 𝑉=>𝑒@AB + 𝑉=@𝑒AB

𝐼(𝑧) = 𝐼=>𝑒@AB + 𝐼=@𝑒AB

The characteristic impedance of the transmission line is defined as

𝑍= =𝑉=>

𝐼=>=−𝑉=@

𝐼=@=𝑅 + 𝑗𝜔𝐿

𝛾 = D𝑅 + 𝑗𝜔𝐿𝐺 + 𝑗𝜔𝐶

In the case of lossless transmission line, 𝑅 = 𝐺 = 0 and the corresponding propagation constant

is 𝛾 = 𝛼 + 𝑗𝛽 = 𝑗𝜔√𝐿𝐶. The characteristic impedance will be 𝑍= = FGH .

2.2 Bond Wire

Bond wires are typically used to form interconnections between a chip and its package, and there are also other bond wires like crossovers and printed circuit board (PCB) bonds [3]. It is a flexible and cost-effective type of interconnection technique, and it had been used for more than 90% interconnections in semiconductor packages by 2013 [7]. Figure 2.2.1 is a picture showing that a number of bond wires connect the chip in the middle with its packaging on the surrounding.

Figure 2.2.1: Bond wires on chip [8]

2.2.1 Bonding methods

There are two bonding techniques: ball bonding and wedge bonding. Ball bonding method begins with forming a free air ball at the tip of the wire by creating a spark, and then applying force and ultrasonic energy on the ball against the bond pad until a ball bond is formed. While for wedge bonding, it uses the foot of the wedge to deform the wire and applies force and ultrasonic energy at the same time until the bond is made.

Background | 7

7

There are two types of bond wires in terms of techniques: ball-wedge wire and wedge-wedge wire [7]. In Figure 2.2.2 is a ball-wedge gold bond wire. The first bond is a ball bonding made on the bond pad of the die, and the second one is a wedge bonding on the lead frame of the packaging. This is the type of bond wires that this project studies.

Figure 2.2.2: Ball-wedge bond wire [9]

In Figure 2.2.3 is a wedge-wedge bond wire. Both of the bonds are formed by wedge bonding

method.

Figure 2.2.3: Wedge-wedge bond wire [10]

2.2.2 Relation with transmission line theory

A bond wire can be interpreted by the transmission line theory with some variations. Applying the model of Figure 2.1.1 to the case of single bond wire, R is the resistance of the wire per unit length; L is the inductance of the wire per unit length; C is the capacitance between the wire and the ground per unit length. G should be the shunt conductance between two conductors, and it is not applicable in this case, which simply means G=0.

Among all the electrical parameters, inductance is the one that makes most difference in the transmission performance. Hence, it is one of the evaluation metrics in this project.

2.3 Microwave Network Analysis

There are mainly two ways to perform analysis on a microwave problem, which are (1) circuit or network analysis and (2) field analysis using Maxwell’s equations [5]. Circuit analysis is a quite intuitive and basic method to solve microwave problems, but it gives limited information and it

8 | Background

could be too simplified for certain problems. Field analysis, on the other hand, provides comprehensive solutions, but it is very complicated, difficult and time-consuming to apply. Most of the electromagnetic simulation software would use field analysis to get a full picture of the electromagnetic fields around the test structure.

Network analysis is more suitable for analytical modeling in the cases when only the voltage and current at the terminals are needed. In order to perform network analysis, the values of the basic components, for example, the characteristic impedance, of the circuit need to be derived first. Then, connect all the components properly and apply electric circuit theory or transmission line theory to assess the behavior of the system [6].

2.3.1 Impedance and Admittance Matrices

Impedance and admittance matrices show the relationship of total voltages and currents at all terminals, which becomes a description of the whole network [6]. Here, consider a simple two-port network as in Figure 2.3.1. 𝑡I and 𝑡4 are the terminal plane defined for the first and second port, respectively. (𝑉I>, 𝐼I>) and (𝑉4>, 𝐼4>) are the equivalent voltages and currents for the incident waves to the port 1 and port 2, respectively. (𝑉I@, 𝐼I@) and (𝑉4@, 𝐼4@) are those for the reflected waves.

Figure 2.3.1: Two-port network

The impedance matrix [Z] relates the voltages with currents by

J𝑉I𝑉4K = J

𝑍II𝑍4I

𝑍I4𝑍44

K J𝐼I𝐼4K

and the corresponding admittance matrix [Y] will be

J𝐼I𝐼4K = J

𝑌II𝑌4I

𝑌I4𝑌44K J𝑉I𝑉4K

The relationship between impedance and admittance matrices is

[𝑌] = [𝑍]@I

The element Zij and Yij can be found by [6]

𝑍OP =𝑉O𝐼PQRST=UVWXYP

𝑌OP =𝐼O𝑉PQZST=UVWXYP

Background | 9

9

2.3.2 Scattering Matrices

Different from the impedance and admittance matrix, the scattering matrix indicates the relationship of the voltages of the incident and reflected waves on all ports [6]. It can be obtained by calculations, or measurement performed on a Vector Network Analyzer.

In the same case as Figure 2.3.1, 𝑉[> and 𝑉[@ are the amplitude of the voltage of the wave incident and reflected at port 𝑛. The scattering matrix [𝑆] can be expressed as

J𝑉I@

𝑉4@K = J𝑆II 𝑆I4

𝑆4I 𝑆44K ^𝑉I>

𝑉4>_

The element 𝑆OP can be directly found by [6]

𝑆OP =𝑉O@

𝑉P>QZS`T=UVWXYP

which means that 𝑆OP can be calculated by driving port 𝑗 with 𝑉P> and measuring 𝑉O@ at port 𝑖. As a result, 𝑆OO is the voltage reflection coefficient (G) at port 𝑖 when other ports are all terminated with matched load, and the return loss at port 𝑖 will be 𝑅𝐿 = −20 log(|Γ|) dB = −20 log(|𝑆OO|) dB . Similarly, 𝑆OP is the transmission coefficient (Τ = 1 + Γ) from port 𝑗 to port 𝑖 when other ports are all terminated with matched load, and the insertion loss will be 𝐼𝐿 = −20 log(|Τ|) dB =−20 logkl𝑆OPlm dB.

Another way to calculate the scattering matrix is from the impedance matrix or admittance matrix, which is easier to manage in some cases [6]

[𝑆] = ([𝑍] + 𝑍=[𝑈])@I([𝑍] − 𝑍=[𝑈])

The scattering parameters are one of the metrics used in this project to evaluate the transmission performance of the bond wires, including the return loss and insertion loss.

2.3.2.1 Reciprocal network and lossless network

A reciprocal network is one that the transmission of signal does not depend on the direction of propagation, or in other words, the input and output ports can exchange and the transmission performance would still be the same [6]. The scattering matrix of such network is symmetric, which means

𝑆I4 = 𝑆4I

For a lossless network, no real power is actually delivered to the network, which indicates that the incident power is equal to the reflected power. The scattering matrix of a lossless network is unitary, which can be written as [6]

[𝑆]oooo ∙ [𝑆]q = [𝑈]or[𝑆] ∙ [𝑆]> = [𝑈]

where [𝑆]oooo is the conjugate matrix of [𝑆], [𝑆]q is the transpose matrix, [𝑆]> is the conjugate transpose and [𝑈] is the identity matrix. This equation can be expressed as

s𝑆XO ∙ 𝑆XtoooouT4

XTI

= 1,for𝑖 = 𝑗

and

s𝑆XO ∙ 𝑆XwoooouT4

XTI

= 0,for𝑖 ≠ 𝑗

10 | Background

If more explicitly,

|𝑆II|4 + |𝑆4I|4 = 1

|𝑆44|4 + |𝑆I4|4 = 1

𝑆II ∙ 𝑆I4oooo + 𝑆4I ∙ 𝑆44oooo = 0

𝑆I4 ∙ 𝑆IIoooo + 𝑆44 ∙ 𝑆4Ioooo = 0

These two conditions can be used to check if the scattering matrix obtained from the analytical model is fundamentally correct.

2.4 Related work

There have been a considerable number of studies regarding how to model and characterize bond wires and also how the physical parameters affect the transmission performance in the last two decades. They all have shown satisfying results, and are quite helpful for the development of this project. This section will introduce six pieces of work of this topic.

2.4.1 Transmission performance analysis of single and double bond wires

This work [8] studies the relationship between the height and the transmission performance of single bond wire, and the crosstalk or coupling between double bond wires. It approximates the shape of the bond wire as an arc of a circle with a radius of 0.5 mm, and then simulates the model in electromagnetic analysis software CST MWS 2015 to get its scattering parameters in the frequency range from 0 Hz to 43.5 GHz [8]. After comparing the performance of different loop heights ranging from 0.4 mm to 0.8 mm in simulation as in Figure 2.4.1, it concludes that a lower loop height gets a better return loss and a better performance.

Figure 2.4.1: Simulation results of single bond wires with different height [8]

It designs and fabricates the test structure of single bond wire on PCB shown in Figure 2.4.2 and

performs VNA measurement. The simulated and measured results agree with each other. This work

Background | 11

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also fabricated the double bond wires with the pitch of 8 mm to observe the crosstalk between them. Compared to simulation results, the crosstalk is quite severe in the frequency range of 0 Hz to 10 GHz, and the measured results are more consistent with simulation in higher frequency.

Figure 2.4.2: Fabricated single bond wire structure [8]

2.4.2 Comprehensive performance analysis of single bond wire in simulation

The paper [11] performs a comprehensive electrical performance analysis of bond wire interconnection in the frequency up to 170 GHz. The bond wire is simplified as an arc of a circle. It proposes an electrical model for the entire bonding, including the series inductance L of the bond wire, and the shunt capacitance C for the bond pad and ball, as shown in Figure 2.4.3. The work then derives the transfer function of the two-port network [11]

It studies the effect of the inductance and capacitance in the transfer function of the bond wire, and finds out that there is a trade-off between them in terms of achieving a better performance for the system. Note that it does not derive the values of the elements according to the shape profile of bond wire.

Figure 2.4.3: Equivalent circuit for bond wire interconnection [11]

12 | Background

3D simulation software is also to study the impact of different gap distances, pad areas and loop

heights on the transmission behavior. The simulation results demonstrate that the electrical performance of the single bond wire can be improved by a shorter bond wire length and a bonding pad of proper size. This paper also emphasizes the process-related limitations and variations of the loop height, ball size and bonding pad distance, which is of great value when it comes to realistic analysis. It fabricates bond wire test structures on the PCB, and measures the distance between two bonding positions and scattering parameters (S-parameters). It performs simulation of the bond wire, and compares it with the measured result, shown in Figure 2.4.4.

Figure 2.4.4: Transmission performance of the fabricated single bond wire [11]

The results can be considered consistent up to around 130 GHz, and after that the parasitic is beyond the scope of simulation. It is also indicated in this paper that the bond wire profile in the simulation is not a perfect match for the fabricated one.

2.4.3 Modeling and minimizing the self-inductance of bond wire

In the work [12], it presents an analytical calculation for the partial self-inductance of single bond wire. It proposes a novel way to calculate the partial self-inductance, including internal and external ones, which uses the bonding geometric parameters as variables in the functions. These parameters include the loop height, distance between two bonding bumps and the thickness of metallization. The shape of the bond wire is approximated to follow the Gaussian distribution function in Figure 2.4.5, because the paper [12] considers this as a realistic and accurate representation of the bond wire profile. It uses Matlab to obtain the values of all the elements in the analytical model and validates the model by performing simulation in Ansys Q3D.

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Figure 2.4.5: Bond wire model of Gaussian distribution function [12]

This paper [12] also looks into possible ways to minimize the partial self-inductance by

examining the analytical model, and discovers that increasing the radius can reduce the partial self-inductance. This is also verified in simulation, and it takes a further step by quantifying the impact of the radius through fabrication and measurement. The results in the following two figures are consistent with that from analytical modeling and simulation.

Figure 2.4.6: Measurement of return loss of bond wires with radius 12.5 µm and 62.5 µm [12]

14 | Background

Figure 2.4.7: Measurement of insertion loss of bond wires with radius 12.5 µm and 62.5 µm [12]

2.4.4 Analysis of capacitance compensation structure for bond wire

In this work [13], a capacitance compensation structure for bond wire in multiple-chip module is studied. It approximates the bond wire structure as in Figure 2.4.8, and proposes an equivalent circuit for bond wire as shown in Figure 2.4.9. In the equivalent circuit, L is the series inductance, R is the series resistance, and C is the parallel capacitance between the bond pad and the ground. It analytically derives the expressions of all these three elements in terms of the geometric parameters of the bond wire.

Figure 2.4.8: Structure of the interconnection [13]

Figure 2.4.9: Capacitance compensation model for bond wire [13]

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It investigates the relationship between the geometric parameters and the behavior of the bond wire using the analytical model. The parameters are the diameter, loop height and the number of bond wires in parallel, and the key to improve the performance is to decrease the inductance. Then, it builds the entire capacitive compensation structure in the 3D electromagnetic simulator ANSYS HFSS, shown in Figure 2.4.10. There are two pads added on the edge of the microstrips, and two striplines underneath them.

Figure 2.4.10: 3D capacitive compensation structure [13]

This capacitance compensated structure is compared with the uncompensated one, whose

results are in Figure 2.4.11. The conclusion is that the compensated structure can minimize the parasitic effects and improve the return loss [13].

Figure 2.4.11: Simulated return loss of capacitive compensated and uncompensated structure [13]

16 | Background

2.4.5 Electromagnetic simulation of bond wires and comparison with measurements

The work [14] looks into different techniques to perform analysis on bond wires in wide band frequency domain. It utilizes a scanning electron microscope (SEM) to take close and clear pictures of the long bond wires (shown in Figure 2.4.12), discretizes them by an edge detection algorithm, and then generates suitable orthogonal grids for electromagnetic simulation. This method gives a very accurate and realistic profile of the bond wire.

Figure 2.4.12: SEM picture of the bond wire [14]

A numerical technique, full wave finite difference time domain (FDTD) simulation, is performed to obtain the function of the scattering parameters in the frequency domain under 20 GHz. In order to extract the lumped elements, an analytical model in Figure 2.4.13 is considered, where the capacitance is the one between the bond wire and the ground plane. It uses the Monte Carlo technique and FastHenry to calculate the inductance, and the finite element method for the capacitance.

Figure 2.4.13: Geometry and corresponding lumped element model of the bond wire [14]

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These results from FDTD, Monte Carlo and FastHenry are all compared with measurements. They are further analyzed by dividing the results into high frequency range and low frequency range. The computational effort of different techniques is discussed as well. [14] gives a summary of the advantages and disadvantages of all the techniques it uses.

2.4.6 Novel approach to model the packaged RF power transistors

This work [15] uses three-dimensional electromagnetic simulator and SPICE to model the packaged radio frequency power transistors. The key problem is the model accuracy of the bond wires and package. To solve this problem, [15] first calculates the inductance and resistance of the systems of bond wires, including building an equivalent model for the package. In order to get a realistic and accurate profile of the bond wires inside the power transistor, the novel approach used here is to take scanning electron microscope micrographs, as in Figure 2.4.14, and run software in Java to extract the three-dimensional geometries of the bond wires.

Figure 2.4.14: SEM picture of the inside of a power transistor [15]

The 3D electromagnetic simulator FastHenry is used for the simulation of inductance and resistance, with the input of the geometrical extraction from the Java program as in Figure 2.4.15. FastCap is used to estimate the capacitance of the package, and the capacitance model is simplified.

18 | Background

Figure 2.4.15: Extracted model for electromagnetic simulation of resistance and inductance in FastHenry [15]

In the result part, [15] discusses different options and factors that have different impacts on the model accuracy, such as simple model, frequency-dependent model, skin effect, mutual inductance and package capacitance.

2.5 Summary

All the works mentioned in this chapter are a great value to the research design planning of this project. Even though there are already many studies regarding bond wires, they have different techniques and research focuses. Table 2.5.1 is a summary of the technical methods that related works use in terms of how they approximate the shape of the bond wire, if they build a mathematical model and what elements they include in it, if they simulate the model and if they perform fabrication and measurement on the test structure.

Table 2.5.1: Technical methods of related works

The work [8] studies the relationship of the loop height and the transmission performance in simulation, and the crosstalk between two bond wires in measurement. [11] uses analytical modeling and simulation to conclude the effects of gap distances, pad areas and loop heights. However, it does not extract the inductance or capacitance of the bond wire, and measurement is used to see how it differs from simulation results at different frequency. [12] puts emphasis on the analytical calculation of inductance, and investigates its dependency on gap distance and loop

2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 Shape model Arc Arc Gaussian

distribution - Extraction

from SEM Extraction from SEM

Mathematical model

No Yes Yes Yes Yes Yes

Elements - L, C L L, C, R L, C, R L, R Simulation Yes Yes Yes Yes Yes Yes Fabrication

and measurement

Yes Yes Yes No Yes Yes

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height in calculation and simulation, and metallization thickness in measurement. [13] compares the capacitance compensated structure and the uncompensated in simulation, and also studies the impact of different diameter, loop height and number of bond wires in mathematical modeling. But it doesn’t perform lumped element extraction. Unlike all the papers mentioned previously, [14] and [15] focus more on the techniques and methods rather than simply studying the behavior of bond wires. They all use the scanning electron microscope to extract the realistic geometrical profile of the bond wire, and perform electromagnetic simulation, inductance and capacitance extraction using different methods.

As shown previously, a few studies have investigated the relationship between certain physical dimension parameters and the transmission performance of bond wires. Almost all of them model the bond wire in terms of a simple equivalent electrical circuit, which usually consists of one inductor and/or two capacitors and/or one resistor. They, more or less, use the means of mathematical modeling, simulation, fabrication and measurement to verify their results.

In light of these, this project would try to build a more accurate analytical model of the bond wires and apply all three techniques to verify the accuracy of the proposed model.

Methodologies and Methods | 21

3 Methodologies and Methods

This chapter introduces the research methods used in this project. It includes the research process, research paradigm, data collection techniques, experimental design, how to evaluate the reliability and validity of the data collected, and data analysis.

3.1 Research Process

This project follows the steps in Figure 3.1.1 to conduct the research.

Figure 3.1.1: Research Process

3.2 Research Paradigm

The research paradigm or philosophical assumption for this project is positivism. It assumes that the reality is objective and does not depend on the researcher or instrument [4]. This project is quantitative because of its purpose to prove that a certain model fits the behavior of bond wire. The behavior is assumed to follow the fundamental electromagnetic theory and transmission line theory, and they are objective given any circumstances.

3.3 Data Collection

This project uses experiments to collect data of the scattering parameters of the network. The data would include the return loss, insertion loss and the angles, and it would come from three different sources: calculation, simulation and measurement. With the scattering parameters, the corresponding values for inductance, capacitance and resistance can be obtained as well. In the scope of the project, no human participant is involved, which means that it is not subject to privacy or human right issues. No violation of ethics is present in the research process.

3.3.1 Sample Size

In mathematical calculation, at least three samples of different loop height are taken. In simulation, three samples are taken to verify the results from calculation, and three samples of different separation are needed as well. Because the calculation and simulation are always theoretically correct and certain, it is not necessary to take multiple samples of the same test structure.

22 | Methodologies and Methods

For measurement, two different loop height and three different separation are needed. Each of these cases should have three test structures fabricated, and each test structures should be measured for at least twice, and three times if necessary.

3.4 Experimental design/Planned Measurements

This project would combine mathematical modeling, simulation, fabrication, measurement and analysis to solve the problem. This process involves a lot of different technologies. This section introduces all the technologies and conditions needed for the experimental design.

3.4.1 Test environment/test bed/model

All the tests are performed in a professional laboratory, and should not have strong electromagnetic interference in the surrounding. The test bed involves the floating table, VNA, WinCal XE 4.6 and the test board. The test model is fabricated with the bond wires on an original test board.

3.4.2 Hardware/Software to be used

For hardware, there are in total twenty empty test boards available to build test structures and perform calibration. The bond wire prototype machine MP iBond 5000 is used to fabricate test structures, and vector network analyzer N5242A by Agilent Technologies to perform S-parameter measurements. The three-pin probes from Cascade Microtech are used for connection with the test structure. The regular optical microscope is necessary to observe and measure the dimensions of the bond wires. The scanning electron microscope is used to take micrographs of the fabricated bond wires.

There are mainly four kinds of software used in this project. Matlab is used to build mathematical model. National Instruments AWR software is used to perform 2D simulation of the bond wire models and equivalent circuits. Its optimization functionality can extract the values of electrical elements in the equivalent circuit. This project also uses a 3D electromagnetic simulator ANSYS HFSS to simulate the entire test structures and test environment. WinCal XE 4.6 by Cascade Microtech is connected with the VNA and passes the commands to the VNA in order to perform calibration and measurement. The measurement results from the VNA is then transmitted back to the software WinCal XE 4.6.

3.5 Assessing reliability and validity of the data collected

As a part of quality assurance, it is important to assess whether every test is measuring the right content and whether it is measuring accurately, which means reliability and validity respectively. This section talks about the fundamentals of such assessment.

3.5.1 Reliability

Reliability is the consistency of the results. This will be guaranteed by performing multiple measurements on the same test structure. Test it twice at the beginning, and if the two results agree with each other, then the result is reliable; if not, it is necessary to do a third one, and rule out the problematic one.

Methodologies and Methods | 23

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3.5.2 Validity

Validity can be achieved by comparing the measurement results with the simulation or calculation results. If the measurement shows the same behavior as the simulation, then it is valid; if not, the testing procedures, simulation and analytical model need to be examined again and find out the possible source of error.

3.6 Planned Data Analysis

The collected data will be analyzed in order to draw conclusions about the model, simulation and fabrication. This section shows the data analysis method and the corresponding tools used in this project. The evaluation metrics in this project are the scattering parameters and the inductance of the bond wires.

3.6.1 Data Analysis Technique

In order to check if two sets of scattering parameters are close enough, the average L2 norm would be calculated and evaluated. This weighted difference is the “average squared magnitude of the difference between each element of the S-parameter matrix” [16]

Average L2 norm error =∑ ∑ kl𝑆OP{ − 𝑆OP|lm

4uPTI

uOTI

𝑁4

where N is the number of ports of the network, and the error will be calculated in the unit of dB.

In order to compare simpler numeric data, such as coupling coefficient, the percent error is calculated according to

Percenterror =|Experimental − Expected|

Expected × 100%

3.6.2 Software Tools

NI AWR is used to show the graphs of scattering parameters obtained from measurement, perform optimization to get element values, and to calculate and display the average L2 norm.

Experimental and Modeling Approach | 25

4 Experimental and Modeling Approach

This chapter presents how the research is designed and implemented. After studying the related works, a comprehensive research is designed in accordance of the goals of this project, which includes building an analytical model in Matlab, performing electromagnetic simulation in NI AWR and ANSYS HFSS, and hardware fabrication and measurement.

A mathematical model is helpful to understand the fundamentals of the bond wire, and it needs to be validated in simulation. In this case, two simulators are employed, with AWR for two-dimensional and HFSS for three-dimensional simulation. The model is also compared with the measurement to see if they have the same behavior. This kind of comprehensive design would give reliable and trustable results regarding the purpose of this project.

With the techniques mentioned above, the project would investigate how accurate the model is by comparing the results from mathematical modeling with those from simulation and measurement. Furthermore, it would look into how the loop height affects the transmission performance of the single bond wire, and the separation distance influences that of the double bond wire with coupling.

4.1 Analytical modeling

This section explains how to build a mathematical model of the bond wire in Matlab. It is based on the lump-element model of the transmission line. Both single bond wire and double bond wires with coupling are considered.

One single bond wire can be considered as a cascade of N segments of wires. Each segment can be treated as a transmission line, and has the equivalent circuit as shown in Figure 2.1.1. In lossless situation, R would be considered neglectable.

4.1.1 Shape model

In Section 2.4, two works use the arc of a circle to model the shape of the bond wire, [12] use the Gaussian distribution function, and [14] and [15] extract the discretized shape from the photo taken by SEM. [14] and [15] have the most realistic and accurate profile of the wire, but it is also difficult and complicated to obtain. A compromising solution is to use a certain mathematical function to model the shape of the bond wire. By observation, the shape of the wire can be better approximated as a Rayleigh distribution function than a Gaussian distribution function [17]:

𝑓(𝑥; 𝜎) =𝑥𝜎4𝑒@

��4�� ,𝑥 ≥ 0

The probability density function for Rayleigh distribution has the shape shown in Figure 4.1.1.

26 | Experimental and Modeling Approach

Figure 4.1.1: Rayleigh distributions [17]

In order to build a quantifiable model, divide the gap evenly into N sections along the x axis, and take the height of the middle point as the equivalent height for this section of wire. The length of each section is estimated as the distance between two middle points. The case with 𝑁 = 10 is illustrated in Figure 4.1.2. The dashed line is the bond wire outline following a Rayleigh distribution, and the red solid line is the approximated segmented line. Each section will be considered as parallel to the ground when evaluating their electrical characteristics.

Figure 4.1.2: Shape model

4.1.2 Electrical characteristics

In the equivalent circuit, there are series inductance, series resistance and parallel capacitance. Among these three elements, the inductance is the one that can influence the transmission performance the most.

4.1.2.1 Inductance

The inductance of each section normally consists of the self-inductance of the round wire and the mutual inductance between the wire and the ground. Here, the mutual inductance is small enough to be negligible. The self-inductance can be calculated by [18]

𝐿��U = 2 �𝑙 log ^𝑙 + √𝑙4 + 𝑟4

𝑟_ − ;𝑙4 + 𝑟4 +

𝑙4 + 𝑟

�


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where 𝑟 is the radius, and 𝑙 is the length of the wire.

In case of two parallel bond wires, the inductive coupling between them needs to be considered as well. The mutual inductance can be found analytically by [18]

𝑀I = 2�𝑙 log ^𝑙 + √𝑙4 + 𝑑4

𝑑_ − ;𝑙4 + 𝑑4 + 𝑑�

where 𝑑 is the distance between two bond wires. Another way to get the mutual inductance is through the coupling coefficient

𝑀4 = 𝑘;𝐿I ∙ 𝐿4

in which 𝐿I and 𝐿4 are the inductance of the two conductors, and 𝑘 is the coupling coefficient between these two inductances. In the ideal case that the two bond wires are identical, 𝐿I should be equal to 𝐿4.

The total inductance of the double bond wires with coupling will be

𝐿�V�� =𝐿I ∙ 𝐿4 −𝑀4

𝐿I + 𝐿4 − 2𝑀

4.1.2.2 Capacitance

As mentioned previously, each section of the bond wire is considered as a straight line or a very thin metal plate parallel to and over the ground. The surface facing the ground would be in the shape of a rectangle, and its area can be calculated as 2𝑟𝑙 . The capacitance between this section of bond wire and the ground plane can be found by [19]

C = 𝜀=𝐴ℎ ≈ 𝜀=

2𝑟𝑙ℎ

where 𝜀= is the electric constant and ℎ is the equivalent height from the ground plane.

4.1.2.3 Resistance

The resistance of a conductor can be calculated by [19]

R = 𝜌𝑙𝐴

in which 𝜌 is the resistivity of the material of the bond wire, which is gold in this case. The resistance of the bond wires in this case is extremely small.

4.1.3 Network analysis

As introduced in Section 2.1, the characteristic impedance of each segment of transmission line can be found by

𝑍= = D𝑅 + 𝑗𝜔𝐿𝐺 + 𝑗𝜔𝐶

The Y matrix for each section is [5]

[𝑌] = −𝑗𝑍=∙ J 𝑐𝑜𝑡(𝜃) −𝑐𝑠𝑐(𝜃)−𝑐𝑠𝑐(𝜃) cot(𝜃) K

where 𝜃 = 𝛽𝑙.


Each section can be converted to a 𝜋 equivalent circuit, shown in Figure 4.1.3, with each element represented by the admittance parameters

Figure 4.1.3: Equivalent p circuit

where 𝑧I = 𝑧ª =I

«¬¬>«¬� , and 𝑧4 = − I

«¬� .

As the entire bond wire is a cascade of all N segments, it also means that it is a cascade of all N equivalent 𝜋 circuits as Figure 4.1.4. In order to obtain the scattering parameters of the entire bond wire, the easier way is to find the impedance parameters first. The impedance matrix can be found by definition, and then transform it into the scattering matrix using the formula mentioned in Section 2.3.2.

Figure 4.1.4: Cascaded p circuits

The whole analytical model has been done in Matlab, and this gives a theoretically ideal results of the transmission performance of bond wires.

4.2 Simulation

One way to verify the mathematical model is to perform electromagnetic simulation. This project would use two simulators to investigate the behavior of the bond wires: NI AWR and ANSYS HFSS. NI AWR is a tool for RF and microwave circuit design and simulation, and ANSYS HFSS is used to perform 3D electromagnetic simulation which takes the whole test environment around the test structure into account.

4.2.1 Circuit modeling and simulation

In NI AWR, there are existing bond wire models that can be used directly as a component in a design. Figure 4.2.3 is the model for single bond wire, and Figure 4.2.4 for double bond wires. Since the bond wire studied in this thesis project is quite short, it cannot achieve very accurate results if it is divided into too many segments in simulation. In fact, the attempt to use more segments in the model failed due to the warning message in AWR. Therefore, a simpler three-segment bond wire component is used in AWR simulation. This requires approximating the ten-segment shape used in


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Matlab to a three-segment one. The general rule for such approximation is maintaining the similar shape profile and total length.

The ports used in this project all have the impedance of 𝑍 = 50Ω.

Figure 4.2.1: Single bond wire model in AWR

Figure 4.2.2: Double bond wires model in AWR

The bond wire model with the approximated physical dimensions is then compared to the equivalent circuit with element values calculated in Matlab. For equivalent circuits, the single bond wire is modeled by an inductor, a resistor and two capacitors in Figure 4.2.3. The resistance would simply be zero in the lossless case.

Figure 4.2.3: Equivalent circuit for single bond wire

For double bond wires with coupling, the circuit in Figure 4.2.4 is used. This fundamentally consists of two of the circuits from Figure 4.2.3 in parallel, and coupling between the inductors as well.


Figure 4.2.4: Equivalent circuit for two bond wires with coupling

In NI AWR, the optimization functionality is used to fit the bond wire model to the equivalent circuit by minimizing the average L2 norm difference between the scattering parameters of the model and the circuit. The optimization process can obtain the optimized values for all the elements in the circuit, which are inductance, capacitance and resistance in the single bond wire, and coupling coefficient in the double bond wires. The corresponding values for the elements in the circuit can then be compared with the ones from Matlab calculations.

4.2.2 3D modeling and simulation

In ANSYS HFSS, the whole testing environment with the test model can be constructed and simulated, and this should give very accurate results as well. As presented in Figure 4.2.5, the box on the outside is the environment of air or vacuum and also the radiation boundary for electromagnetic simulation; the plane inside is the copper ground plane, where the bond wire is built on.


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Figure 4.2.5: Overall test structure in HFSS

From a closer view of the test structure in Figure 4.2.6, on the edges of the single bond wire are

the lumped ports for excitation. All these components are required in order to build a complete test structure and achieve better results in the 3D simulator. The bond wire model used in HFSS should also be a three-segment model which has the same parameters and shape as the one used in AWR.

Figure 4.2.6: Close-up view of test structure in HFSS

4.3 Fabrication and measurement

The last section of the design and implementation is to build the test structures by using the bond wire prototype machine, and to perform scattering parameter measurement on them via Vector Network Analyzer. The fabrication is done on a two layered board with microstrips on the top, shown in Figure 4.3.1. The detailed information of the board from the top to the bottom is listed in Table 4.3.1.


Figure 4.3.1: Empty test board

Table 4.3.1: Test board structure

Material Thickness (mm)

Microstrip Copper 0.035

Substrate Ro4350B 0.254

Ground Copper 0.035

The wires for fabrication are golden wires with the diameter of 0.038 mm, and there are three sets of different gaps: 1 mm, 1.5 mm and 2 mm. The width of the microstrip is 0.55 mm, and the length is 5 mm. The structure at the end of the microstrip is to connect the three-pin ground-signal-ground probe for measurement.

4.3.1 Bond wire prototyping

In this project, MP iBond 5000 bonding machine is used to perform bond wire prototyping. There is only one type of bond wire used in this thesis project, which is the ball-wedge wire. For this project, the bonding should be done with high consistency. In order to eliminate as many error sources as possible, it is necessary to make sure that all the bond wires have similar or same shape and distance between two bonding points. Referring to Figure 4.3.2, the distance between two bonding points of one bond wire would be

𝐷 = 𝑟°�� + 𝑑I + 𝐷±�² + 𝑑4

in which 𝑟°�� is the radius of the bonding ball, 𝐷±�² is the width of the gap, and 𝑑I and 𝑑4 are the distances between the bonding and the edge of the microstrip.


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Figure 4.3.2: Detailed structure of the bond wire in top view

The two important parameters on the bonding machine are loop and step. The loop parameter is how much the bonding head rises after forming the first bonding. The important note is that this loop is not the actually loop height of the bond wire. The step parameter is the distance it moves from the first bonding point to the second one. As all the parameters on the machine are relative, it is difficult to map the parameters in the machine to absolute values accurately. The distance can be measured under a regular microscope, but the height is difficult to measure and derive.

For this machine, the loop height that it can reach is very limited, and the maximum height is around 0.5 mm. In order to study the effect of different loop height, bond wires are built with the same step but significantly different loop. However, if the bond wires are built with different but close loop parameters, the difference is too subtle to notice.

When building two bond wires with coupling, the bonding balls should not touch each other, and also the distance between them is constrained by the width of the microstrip. This distance is the distance between the center of the ball of the first bond wire and the one of the second bond wire, which is 𝐷��² in Figure 4.3.2. As the width of the microstrip is 0.55 mm and the diameter of the ball is about 0.1 mm, the distance range can be 0.1 mm ~ 0.45 mm in theory. But in fabrication, it is more realistic to leave some margins. Note that the industry design standard of such distance is 0.13 mm.

4.3.2 Thru-Reflect-Line (TRL) network analyzer calibration

Before measurement, the Thru-Reflect-Line calibration technique needs to be performed. Because the primary reference plane of the VNA is somewhere within the analyzer, the calibration is necessary to characterize the possible errors caused by all the cables and probes that are connected to the actual device under test (DUT), as shown in Figure 4.3.3 [6].


Figure 4.3.3: VNA measurement of two-port device [6]

The idea is to replace the DUT with something that is already known. For Thru, it connects port 1 directly with port 2; for Reflect, it normally uses short or open circuit; for Line, it connects port 1 and port 2 with a matched transmission line [6]. In Figure 4.3.4 is the test board for calibration. The layer structure and all the materials are the same as the test boards for fabrication. The first three structures are all for Line connections, but they have different lengths. (1) has the length of 15 mm, (2) is 9 mm, and (3) is 4 mm. (4) is the open structure for Reflected connection, and (5) is the short structure. Both (6) and (7) are Thru connections.

Figure 4.3.4: Board for calibration

Assume the system is symmetric, the errors of each side would be the same, except the direction of propagation. The calibration is able to capture the scattering parameters of the error boxes, and it can use [𝑆] and the measured data ³{

´

H´|´µ´¶to calculate the scattering parameter of the actually

device under test ³{·

H·|·µ·¶.

As the measured data can be represented as [6]

J𝐴¸

𝐶¸𝐵¸

𝐷¸K = [𝑆] ∙ ^

𝐴º

𝐶º𝐵º

𝐷º_ ∙ [𝑆],


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the error-corrected scatter matrix of the device under test can be found by

^𝐴º

𝐶º𝐵º

𝐷º_ = [𝑆]@I ∙ J

𝐴¸

𝐶¸𝐵¸

𝐷¸K ∙ [𝑆]@I

Theoretically, calibration needs to be done before every measurement if they are done on different days. This is because the Earth is rotating all the time, therefore the magnetic field of the Earth is always changing. From a practical point of view, before measuring new test structures, measure something that has already been measured before. And if the results agree with each other, it is acceptable to start new measurements; but if not, it means the change of the magnetic field is noticeable to affect the results, and calibration needs to be done again.

4.3.3 Measurement

After the calibration is done, measurement can be performed via the vector network analyzer, and the calibrated results would be of the device under test instead of the whole system. For each test structure, at least two measurements need to be done in order to ensure reliability. The parameter that needs to be measured is the scattering parameters of the two-port network.

To further study the relationship between the physical dimension and the electromagnetic performance of the bond wires, it would be necessary to measure single bond wires with different loop heights and double bond wires with different separations. These measurements will then be compared with each other and the simulation results.

Results and Analysis | 37

5 Results and Analysis

This chapter presents the results from analytical modeling, simulation and measurement, and how the reliability and validity analysis is performed. Based on the results, it discusses how the loop height and separation distance affect the transmission performance. How well the proposed analytical model and the fabrication perform is also discussed.

5.1 Major results

This section shows the major results from the mathematical modeling, simulation and measurement. It also includes studies of the impact of different loop height for single bond wire and separation distance for double bond wires with coupling.

5.1.1 Analytical model results

As mentioned in Section 4.1, the shape of the analytical model is based on the Rayleigh distribution function. In the following graph is a comparison of the ten-segment model and the original Rayleigh distribution function in Matlab. The model follows very closely with the original function.

Figure 5.1.1: Comparison of analytical model and Rayleigh distribution function

The model built in Matlab is purely based on the basic electrical elements, and the results are obtained by applying the circuit or network analysis. This should give the theoretical transmission performance of the bond wires. For this model, it also compares three lossless single bond wires with different maximum loop heights. The details of these three sets of bond wires obtained in Matlab are in Table 5.1.1.

38 | Results and Analysis

Table 5.1.1: Bond wire parameters in Matlab

Index Maximum loop height (mm)

Inductance (nH) Capacitance (fF)

1 0.4 0.807 1.347

2 0.5 0.927 1.376

3 0.6 1.056 1.406

From the results, it can be derived that the higher the loop, the larger the inductance. The following four graphs are the magnitude and angle of the scattering parameters 𝑆II and 𝑆I4. When the loop height is higher, the return loss is larger and the insertion loss is smaller, which also indicates a larger inductance.

(a) (b)

(c) (d)

Figure 5.1.2: Matlab results of single lossless bond wires with different loop heights


39

5.1.2 Simulation results

The bond wire models are also built in simulation environment, and the results from Matlab and simulation are compared in order to see if the analytical model gives the correct outcomes. Therefore, the models in simulation should have the similar physical dimensions as in Matlab. NI AWR has the optimization function, which can be used to fit the model to the equivalent circuit and get the optimized value for the electrical elements in the circuit. The 3D electromagnetic simulator HFSS is also used to simulate the bond wire test structures, and the results are compared with those from AWR. In addition, double bond wires with different separation distances are also simulated in AWR and studied to derive a conclusion of the its effect on the transmission performance.

5.1.2.1 Single bond wire

In this section, the results from AWR simulation will be compared to those from Matlab modeling. It also uses the optimization functionality within AWR to get the values of all the electrical elements for both the lossless and lossy cases. 3D electromagnetic simulation in HFSS is also performed to see if the they agree with each other.

5.1.2.1.1 AWR simulation results

In NI AWR, the three-segment bond wire model is used and its shape is approximated as a simplified version of the one in Matlab. The model can be simulated directly and get the result. Figure 5.1.3 is a Smith chart of the scattering parameters of a single bond wire that resembles #1 in Table 5.1.1, and Figure 5.1.5 is the return loss in dB.

Figure 5.1.3: S-parameters of single bond wires in AWR simulation and Matlab


Figure 5.1.4: Return loss of single bond wires in AWR simulation and Matlab

These results can be compared to the lossless model from Matlab, and the maximum error

between them as shown in Figure 5.1.5 is around -50 dB at 15 GHz. The result is fairly acceptable despite certain error, which verifies the model in Matlab. A likely source of error is the fact that the shape models in Matlab and AWR are slightly different.

Figure 5.1.5: Error between single bond wire model in AWR simulation and Matlab

Fit this model to the equivalent circuit of both the lossless and lossy single bond wire models

(Figure 4.2.3), and optimize the error under -60 dB for all frequencies. The optimized values for all the elements are obtained and shown in Table 5.1.2.


41

Table 5.1.2: Optimized values for single bond wire

Model Inductance (nH) Capacitance (fF) Resistance (Ohm)

Lossless 0.723 9.390 0

Lossy 0.755 7.577 5.867e-5

5.1.2.1.2 HFSS simulation results

The same test structure is also built and simulated in HFSS. Figure 5.1.6 is the scattering parameters of the single bond wire as in Section 5.1.2.1.1, and Figure 5.1.7 is the return loss. These two graphs are almost identical with those from the AWR simulation. Therefore, from a practical point of view, either of these two simulators is sufficient for the purpose of this project.

Figure 5.1.6: S-parameters of single bond wire in HFSS simulation


Figure 5.1.7: Return loss of single bond wire in HFSS simulation

5.1.2.2 Double bond wires with coupling

For double bond wires with coupling, how the separation distance between two bond wires influences the behavior of the coupling effect is of interest. This part uses the bond wire models of the same shape as in Section 5.1.2.1, but with different separation distances, which are 0.12 mm, o.2 mm and 0.26 mm, respectively. In the equivalent circuit of lossy double bond wires with coupling as in Figure 4.2.4, the capacitance, inductance and resistance are set as the same values in the lossy case obtained in Table 5.1.2. Because the lossy scenario should be the more realistic result in theory. In this case, the coupling coefficient is the variable that the optimization process changes. Use the same optimization method mentioned in Section 5.1.3.1, and the coupling coefficient for each case can be derived, shown in Table 5.1.3.

Table 5.1.3: Coupling coefficient of double bond wires with different separation

Index Separation (mm) Coupling coefficient

1 0.12 0.436

2 0.20 0.279

3 0.26 0.219

From the table above, it can be deducted that a smaller separation distance results in a larger coupling coefficient, which leads to a larger mutual inductance. This is to be expected, because the influence of the magnetic field becomes stronger when it is closer, and the inductive coupling effect gets more significant as well.

5.1.3 Fabrication and measurement results

For the purpose of fully investigating the effect of loop height and separation distance, corresponding test structures are built and measured. This section presents the key results obtained from measurement, both for single and double bond wires. The raw data of scattering parameters from the VNA is imported to NI AWR for display and analysis.


43

Figure 5.1.8 is a picture of a fabricated single bond wire with 1 mm gap and loop parameter 1 of the prototyping machine taken by a scanning electron microscope. The distance between the edges of the copper microstrips is actually 1.125 mm, which is larger than the alleged 1 mm.

Figure 5.1.8: SEM picture of single bond wire with gap = 1 mm, and loop = 1

5.1.3.1 Single bond wire

On a test board of a fixed gap, three single bond wires are built with the same parameters on the prototyping machine. This is meant to check the repeatability of the prototyping process. For all the cases shown in Table 5.1.4, the gap is 1 mm. The first three are built with the maximum loop parameter 10 of the bonding machine, and the last three with the loop parameter 1. Table 5.1.4 also shows the distance between the center of the bonding ball and the center of the wedge. This distance has been explicitly demonstrated in Figure 4.3.2. Since this distance is measured under a regular optical microscope, the accuracy is actually very limited. The distance between two bonding points should be kept as small as possible. But for ball bonding, the diameter of the ball is already close to 0.1 mm, therefore the minimum distance between the bonding ball and the edge of the copper microstrip would be 0.05 mm. The distance between the two bonding points is at least 0.05+1.125=1.175 mm. Table 5.1.4: Single bond wires with 1 mm gap

Index Loop parameter Distance between two bonding points (mm)

1.1 1 1.27

1.2 1 1.26

1.3 1 1.26

2.1 10 1.26

2.2 10 1.26

2.3 10 1.26


Comparing the first three cases, the errors among them shown in Figure 5.1.9 are all less than -60 dB. This means that they have almost the identical transmission performance, even though their physical dimensions are slightly different. The last three cases also exhibit the same performance. These comparisons show that the fabrication of bond wires is quite consistent and repeatable in terms of electrical performance.

Figure 5.1.9: Errors among single bond wires of loop = 1

Figure 5.1.10: Errors among single bond wires of loop = 10


45

5.1.3.1.1 Single bond wires with different loop heights

Figure 5.1.12 and Figure 5.1.12 show the comparison of the measured scattering parameters of single bond wires 1.1 and 2.1, which have the same gap but different loop heights. These two graphs look very similar with those from Section 5.1.1.

Figure 5.1.11: S-parameters of fabricated single bond wires with different loop heights

Figure 5.1.12: Return loss of fabricated single bond wires with different loop heights

The values of the equivalent inductance, capacitance and resistance can be found in Table 5.1.5. The inductance of 2.1 is larger than 1.1, which agrees with the conclusion obtained from the mathematical model that a larger loop height leads to a greater inductance.


Table 5.1.5: Optimized electrical characteristics for single bond wires

Index Inductance (nH) Capacitance (fF) Resistance (Ohm)

1.1 1.193 35.47 2.27e-3

2.1 1.261 44.2 4.71e-3

5.1.3.2 Double bond wires with coupling

This section presents the measurement results of fabricated double bond wires. In this case, double bond wires are built on the test board with 1 mm gap. They all have the same parameters in the prototyping machine (loop parameter = 1), which means that they should have very close shape profiles.

5.1.3.2.1 Double bond wires with different separation

There are three sets of bond wires built with different separation, as simulated in Section 5.1.2.2. The scattering parameters and return loss of the fabricated double bond wires are shown in Figure 5.1.14 and Figure 5.1.14, respectively. From these pictures, it is clear that double bond wire #1 has the largest overall inductance and #3 has the smallest, which means that #1 has the greatest coupling coefficient and #3 has the smallest. This is consistent with the conclusion drawn from Section 5.1.2.2.

Figure 5.1.13: S-parameters of fabricated double bond wires with different separation


47

Figure 5.1.14: Return loss of fabricated double bond wires with different separation

In order to get the optimized coupling coefficients, the measurement data needs to be fit into the equivalent circuit for the double bond wire as in Figure 4.2.4. The values for the electrical elements in the circuit can be taken from those of the single bond wire 1.1 in Table 5.1.5. They are the optimized values from fitting as well.

In the table below, it shows the separation distances, the corresponding optimized coupling coefficients, the simulated ones from Table 5.1.3 in Section 5.1.2.2, and the percent error between them. This error is calculated as

Percenterror =|Optimized − Simulated|

Simulated × 100%

Table 5.1.6: Optimized coupling coefficients of measured double bond wires with different separation

Index Separation (mm)

Optimized coupling coefficient

Simulated coupling coefficient

Percent error

1 0.12 0.476 0.436 9.17%

2 0.20 0.345 0.279 23.66%

3 0.26 0.325 0.219 48.40%

Compared to the results from simulation, the coupling coefficient for double bond wire #1 is quite close to theory, but for #2 and #3, there are significantly noticeable errors between them. One possible source of this error is the fabrication process. Because for double bond wires, fabrication is much more complicated and a lot of factors need to be considered. It is very likely that double bond wires #2 and #3 have some fabrication problems or they are not close to parallel. Another possible reason is that the values used in the equivalent circuit are from the previous fitting, and these values may not be a good match for #2 or #3.


5.2 Reliability Analysis

As discussed in Section 3.5.1, the reliability of the measurement results will be examined by taking multiple tests on the same test structure, and this will be able to identify and eliminate any possible problematic result. This section shows some measurement results of both single and double bond wires as examples.

5.2.1 Single bond wire

In this case, both measurements are taken from a single bond wire, which is built with loop parameter = 1 over a 1 mm gap. Figure 5.2.1 shows the error between these two measurement results, which is less than -140 dB. The error is extremely small, and it means that both measurements are reliable.

Figure 5.2.1: Error between two measurements on a single bond wire

5.2.2 Double bond wires

In this section, the measurements are performed on double bond wires, which are built with loop parameter = 1 and o.12 mm separation on a 1 mm gap. The error is well below -130 dB, which indicates that the measurements are reliable.


49

Figure 5.2.2: Error between two measurements of double bond wires

5.3 Validity Analysis

In order to examine the validity, the measurement results are compared with analytical modeling results and simulation results. This has already been examined in Section 5.1.3, where the results are verified by the conclusions from analytical modeling and simulation.

5.4 Discussion

In this section discusses some of the key points from the implementation and results. The research techniques, including how well this analytical model, fabrication and measurement of single and double bond wires perform, are all under discussion. In addition, it sums the conclusions of how the loop height and separation distance impact the transmission performance of bond wires.

5.4.1 Analytical model

The model built in Matlab is purely analytical, and is based on the physical dimensions of the bond wires. It represents the most ideal case of how the bond wire should behave. From the comparison of the mathematical model and simulation results, it concludes that the analytical model is accurate, despite a slight difference between them. Such difference could come from how the shape of the bond wire is approximated.

5.4.2 Fabrication and measurement

The repeatability, reliability and validity of the measurement are all examined in this chapter. Even with the same parameters on the prototype machine, the bond wires may still have different shapes. However, given the results in Section 5.1.3.1, the repeatability of the single bond wire fabrication is actually quite well. The measurement is very reliable in most cases. Even if it is not, the wrongful measurement result can be detected and eliminated by taking multiple measurements. The validity is ensured by referring to the mathematical or simulation results.


For double bond wire fabrication, the situation is more complicated. There are a couple of factors that need to be considered. First, the shape of the two bond wires should be identical, or very similar. Second, they need to be parallel, which means the positions of the bonding points need to be parallel and have the same distance apart. Third, the separation between two bond wires should be carefully chosen. The bonding balls should not overlap.

5.4.3 Loop height

The relationship between the loop height of the bond wire and the behavior has been discussed in both analytical modeling and measurement. In analytical modeling, it computes the inductance and scattering parameters of single bond wires with three different and known loop heights, and the results are conclusive. In fabrication and measurement, it compares the inductances of two single bond wires with different loop heights. The observation of the measurement is consistent with the conclusion from analytical modeling, which is that when other conditions are identical, a greater loop height leads to a larger inductance.

5.4.4 Separation distance

How the distance between the two parallel bond wires affects the coupling effect, and therefore the performance of the system, is already examined in simulation and also measurement. The simulation gives the optimized coupling coefficients of three cases with different separation distances, and concludes that a shorter distance causes a larger coupling coefficient and a greater mutual inductance. Afterwards, this is verified by the measurements.

Conclusions and Future work | 51

6 Conclusions and Future work

This chapter concludes the whole thesis project, including evaluation of the results, positive effects and shortcomings. It also discusses the limitations during the research process, and possible future work that can and should be done on the basis of this project. In addition, it reflects on different aspects of the project.

6.1 Conclusions

From the results gained in Chapter 5, it can be concluded that the analytical model and simulation model are quite accurate after comparing the results from Matlab, AWR, HFSS and measurement. It also concludes about the effect of loop height and separation distance, which is that a greater loop height results in a larger inductance, and a shorter separation distance leads to a larger coupling efficient and a larger total inductance. Given the results, the goals of this project are fully reached.

This project is comprehensive in terms of research techniques, because it employs modeling, simulation, fabrication and measurement. This improves the credibility and validity of the results. The results obtained from analytical modeling, simulation and measurement all give the consistent conclusions, which indicates that the research methodology is reasonable and the design and implementation is proper for the purpose of this project.

One drawback during the process is that the realistic profile of the bond wires cannot be properly captured and simulated accordingly. The author only had access to the SEM near the end of the research process, and there was not enough time or tool to process the SEM images, extract the geometric model and perform a more realistic simulation.

6.2 Limitations

When considering factors that can affect the transmission behavior of the bond wires, there are quite a few. But due to the limited resource, this project only considers two of them, which are loop height and separation distance.

In the fabrication part, the available tool is the prototyping bonding machine. As mentioned previously, even if built with the same parameters, the bond wires are very likely to look different from each other. This can be improved to some extent by a lot of practice, but the problem cannot be eliminated with this kind of machine.

Because of limited resource, it cannot measure the physical profile of the fabricated bond wires. The regular optical microscope can only measure the length on a two-dimensional plane. The scanning electron microscope is able to take pictures of the bond wires from a three-dimensional angle, but cannot measure the loop height directly because it cannot tilt the platform by 90 degrees. However, it is possible to use image processing algorithm to get the discretized profile as shown in the related work. This project is not able to do that due to lack of time.

6.3 Future work

If equipped with adequate devices and time, a detailed examination of the physical profile of the bond wires should take place. For example, with the pictures taken from SEM, the discretized structure and shape can be extracted by applying a proper edge detection algorithm. This realistic shape can be applied to the analytical and simulation models, and the results can be compared to see if they have the same performance.

52 | Conclusions and Future work

For double bond wires, there are more variations in fabrication using the current prototyping machine, such as the two bond wires are not exactly identical, and they are not strictly parallel to each other. A realistic analysis should build the unideal test structure in simulation according to the results from SEM.

Another device that can capture the detailed geometry of the bond wires is an advanced digital microscope. It is different from the optical microscope and the scanning electron microscope. But it is able to use image processing techniques to get the 3D profile from a 2D image.

One way to avoid such problem is to use an industrial bonding machine, which is able to give the exact shape as designed and has extremely high repeatability. However, such equipment may be very difficult to gain access to.

6.4 Reflections

This section discusses the ethical, environmental and social aspects of this thesis project.

• As discussed in Section 1.3, there is no expected ethical matter involved in this project, because no human subject participates. This remains true throughout the research process. No human privacy or personal data is handled for the purpose of this project.

• The basis of this project is the attempt to develop and implement power-efficient and cost-effective solutions regarding integration and packaging in 5G technology, whose nature serves an environmentally friendly agenda.

• The project poses no harm to any living beings, and rather it is valuable and beneficial to a certain group of human beings and the development of this topic. The results of this project are worthy of studying and referring in the future.

References | 53

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