Identification Of Intermetallic Compound At Cu Wire – Al Bond Pad Interface By Using X-Ray Diffraction.
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
IDENTIFICATION OF INTERMETALLIC COMPOUND AT
CU WIRE – AL BOND PAD INTERFACE BY USING X-RAY
DIFFRACTION
This report submitted in accordance with requirement of the Universiti Teknikal
Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering
(Engineering Materials) (Hons.)
by
NOR RAFIKA BINTI NOR HAMID
B050910115
900115-05-5110
FACULTY OF MANUFACTURING ENGINEERING
2013
ABSTRAK
Kajian ini akan memberi perhatian dalam pengenalpastian struktur kristal pada ikatan
dawai kuprum (Cu) dan aluminium (Al) dalam skala nanometer dengan menggunakan
Teknik pembelauan sinar-X. Terdapat beberapa kajian tentang pengenalan Cu-Al IMC
telah dikaji dengan menggunakan pelbagai jenis teknik seperti Mikroskop Pengimbas
Elektron (SEM) dan Mikroskop Transmisi Elektron (TEM). Secara dasarnya, untuk
mengenal pasti fasa Cu-Al IMC yang dalam skala nanometer, kebiasaannya TEM
merupakan teknik yang digunakan kerana ia mempunyai resolusi yang tinggi yang
membolehkan untuk mendapatkan maklumat secara lebih tepat untuk mikrostruktur
bahan. Walaubagaimanapun, terdapat beberapa batasan dalam penggunaan TEM ini,
oleh itu teknik XRD telah digunakan untuk menggantikan TEM untuk membuat analisa
dalam pengenalpastian fasa Cu-Al IMC. Terdapat beberapa cabaran dalam penggunaan
teknik XRD ini. Oleh itu, kajian ini memberi tumpuan dalam penyediaan dua kaedah
sampel seperti kaedah pembetukan serbuk dan kaedah proses punaran. Antara dua
kaedah ini, satu kaedah akan dipilih berdasarkan keberkesanan kedua-dua kaedah
tersebut yang akan bakal dikenalpasti pada akhir kajian uji kaji ini.
i
ABSTRACT
This study will be focused on the identification of the crystalline structure at the Copper
(Cu) wire - Aluminum (Al) bond pad interface by using X-ray Diffraction (XRD)
analysis. There are a few studies about the identification of the Cu-Al IMC have been
studied by using other techniques such as Scanning Electron Microscope (SEM) and
Transmission Electron Microscope (TEM). Basically, in order to identify the Cu-Al IMC
phase, TEM is used to be the selective technique, since it has high resolution that can
reveal ultrafine details of material microstructure. Nevertheless, due to the some
limitations, XRD technique being the selective technique to replace TEM in order to
make the analysis to identify Cu-Al IMC phase at the bonding interface. However, there
are some challenges while using this technique. Hence, this study will be focused on the
two sample preparation methods which are powder method and etching process. From
these two methods, one method would be the selective method by identifying the
effectiveness for the both techniques at the end of the experiment.
ii
DEDICATION
Dedicated to my beloved parents, siblings, friends, lecturer and supervisor
iii
ACKNOWLEDGEMENT
I would like to express my deepest gratitude to my main supervisor Dr. Mohd Warikh
bin Abd. Rashid, and also to my co-supervisor Dr. T.Joseph Sahaya Anand lecturer in
Faculty of Manufacturing Engineering (Engineering Materials), University Teknikal
Malaysia Melaka (UTeM), for their guidance, assistance and concern throughout my
research project.
I would also like to extend my sincere thanks to the Mr. Chua Kok Yau who provided
me with the extensive discussion and information around my works and interesting
exploration in operations.
Special thanks to my friends for giving me supports and helps especially in periods of
uncertainties and difficulties. My special thanks are also due to my classmates, Nor
Diyanna bt Norazemi and Clement Khoo for their critical suggestions and motivating me
throughout in this research.
I am also particularly grateful to the Infineon Company in Melaka for providing me the
place to do the research and also to the Engineering Materials Department Laboratory
for the facilities.
Finally, I would like to express my gratitude with highly appreciation and dedication to
my parents and family for their concerns and encouragements. Without them, I will not
be here to complete my study in UTeM.
iv
TABLE OF CONTENTS
Abstrak
i
Abstract
ii
Dedication
iii
Acknowledgement
iv
Table of Contents
v
List of Tables
viii
List of Figures
ix
List Abbreviations, Symbols and Nomenclatures
xi
CHAPTER 1: INTRODUCTION
1.1
Background
1
1.2
Problem Statement
3
1.3
Objectives
5
1.4
Scopes of Study
5
1.5
Report Organization
6
CHAPTER 2: LITERATURE REVIEW
2.1
Wire Bonding
7
2.2
Wire Bonding Techniques
8
2.2.1 Thermosonic Bonding Technique
2.3
2.4
8
2.2.2 Ultrasonic Bonding Technique
11
2.2.3 Thermocompression Bonding Technique
14
Wire Materials
15
2.3.1 Gold (Au) Wire
15
2.3.2 Copper (Cu) Wire
16
Intermetallic Compound (IMC)
16
v
2.5
2.4.1 Nucleation of IMC
17
2.4.2 IMC Behavior between Cu Bond and Au Bond
20
2.4.3 Mechanism of Nucleation of Intermetallics
22
Failure Mode of Wire Bonding
22
2.5.1 Cratering on Wire Bonding
22
2.5.2 Kirkendall Effect
24
2.6
Analysis Technique by XRD Analysis
25
2.7
Summary of the Literature Review
29
CHAPTER 3: METHODOLOGY
3.1
Chapter Overview
30
3.2
Materials
32
3.3
Sample Preparation Method
32
3.4
3.3.1 Powder Method
32
3.3.2 Etching Process
34
Sample Analysis
36
3.4.1 XRD Analysis
36
3.4.2 SEM/EDX Analysis
37
CHAPTER 4: RESULTS & DISCUSSION
4.1
Chapter Overview
38
4.2
Powder Sample
38
4.2.1 Optical Inspection Analysis
38
4.2.2 XRD Analysis
40
4.2.3 SEM/EDX Analysis
41
Etched Sample
44
4.3.1 Optical Inspection Analysis
44
4.3.2 XRD Analysis
48
4.3.3 SEM/EDX Analysis
49
4.3
vi
CHAPTER 5: CONCLUSION & RECOMMENDATIONS
5.1
Conclusion
53
5.2
Recommendations
54
REFERENCES
55
APPENDICES
A
Gantt Chart for PSM I
59
B
Gantt Chart for PSM II
60
vii
LIST OF TABLES
2.1
Misfit and vacancy-solute binding energy of elemental pairs
24
4.1
Atomic composition for the powder sample of Cu-Al IMC
42
4.2
Table of the lifted ball against immersion duration time
47
4.3
Atomic composition for the etched Sample 2 (1400s) of Cu-Al IMC
50
4.4
Atomic composition for the etched Sample 6 (2200s) of Cu-Al IMC
50
viii
LIST OF FIGURES
2.1
Ball bonding formation
9
2.2
Wedge bonding formation
9
2.3
A ball wedge interconnection between two bond pads
10
2.4
Schematic procedures of ball-wedge bonding
10
2.5
Capillary tool for ball-wedge bonding
11
2.6a
Au wire for ultrasonic bonding; High frequency bond
12
2.6b
Au wire for ultrasonic bonding; Low frequency bond
12
2.7
Wedge tool for ultrasonic technique
13
2.8
Schematic procedures for wedge-wedge bonding
14
2.9
IMC coverage and growth of the intermetallics on high temperature
18
storage
2.10
Relationship between bonded IMC coverage and probability of ball lift
18
2.11
Cu-Al intermetallics have lower resistivity than Au-Al intermetallics
20
2.12
A cross section of a Cu wire bonded to Al pad shows cratering occurs
22
under the bond. The wire is slightly lifted and cratering could be seen after
chemical etching
2.13
Diffractogram of sample in powder form that consists of Cu wires and Si
25
chip
Diffractogram of selected 2θ ranges with slow scan time
26
2.17a Diffractogram of etched powder sample with faster scan
27
2.17b Comparison of diffractogram of unetched and etched powder samples
27
3.1
General overview of the process flow for the study
31
3.2
The process flow for the mechanical removal process for the powder
33
2.14
sample of the Cu-Al IMC
3.3
Optical Microscope
33
ix
3.4
The process flow for the etched sample of Cu-Al IMC
35
3.5
The process flow for the etching optimization process
35
3.6
XRD machine
36
3.7
SEM machine
37
4.1
The collected of the powder samples for Cu-Al IMC
39
4.2
Close view of the Cu-Al IMC parts
40
4.3
Diffractogram for the powder samples of Cu-Al IMC
40
4.4
IMC formation at the ball bond area with a different stress area which (1)
41
low stress area, (2) and (3) high stress area
4.5
Stress distribution under a ball bond during Thermosonic process
42
4.6a
EDX analysis for; point 1(low stress area)
43
4.6b
EDX analysis for; point 2 (high stress area)
43
4.6c
EDX analysis for; point 3 (high stress area)
43
4.7a
Condition of the sample; before the etching process
44
4.7b
Condition of the sample; after the etching process
44
4.8
The condition of the sample at different etching duration
46
4.9
Percentage of the lifted ball vs time
47
4.10
Diffractogram of the etched samples for Sample 2 (1400s) and Sample 6
48
(2200s)
4.11a Ball bond area for; Sample 2 (1400s)
49
4.11b Ball bond area for; Sample 6 (2200s)
49
4.12a EDX analysis for Sample 2;point 1 (high stress area)
51
4.12b EDX analysis for Sample 2;point 2 (high stress area)
51
4.12c EDX analysis for Sample 2;point 3 (high stress area)
51
4.13a EDX analysis for Sample 6;point 1 (high stress area)
52
4.13b EDX analysis for Sample 6;point 2 (high stress area)
52
4.13c EDX analysis for Sample 6;point 3 (low stress area)
52
x
LIST OF ABBREVIATIONS, SYMBOLS AND
NOMENCLATURE
Ag
-
Silver
Al
-
Aluminum
Au
-
Gold
Cu
-
Copper
EDX
-
Energy Dispersive X-ray
EFO
-
Electronic Flame Off
FAB
-
Free Air Ball
HF
-
Hydrofluoric Acid
HNO3
-
Nitric Acid
HTS
-
High Temperature Storage
IC
-
Integrated Circuit
IMC
-
Intermetallic Compound
kHz
-
Kilohertz
KOH
-
Potassium Hydroxide
N
-
Newton
nm
-
Nanometer
OM
-
Optical Microscope
SEM
-
Scanning Electron Microscope
Si
-
Silicon
SiO2
-
Silicon Oxide
TAB
-
Tape-Automated Bonding
TEM
-
Transmission Electron Microscope
%
-
Percentage
°
-
Degree
°C
-
Degree C
xi
CHAPTER 1
INTRODUCTION
1.1
Background
There are three main interconnection electrical interconnection methods in intergrated
circuit (IC) packages such as flip-chip, tape-automated bonding (TAB), and wire
bonding for microelectronic packaging (Murali et al., 2003). Wire bonding technique is
the technique that is widely used in industry for making the electrical connection
between the chips and the lead frames compared to the other two techniques (Xu et al.,
2011). Basically, in wire bonding technique, there are several techniques that have been
used in order to make a bonding. The techniques are thermocompression, ultrasonic and
thermosonic bonding technique.
Generally in industry, the technique that is widely used is thermosonic bonding
technique since it is the fastest process compared to the other two processes. Other than
that, since the process is fast, then it will produce high production in a short time. In this
technique, thermal and ultrasonic energy are needed in order to make a good
interconnection between the two materials. Gold (Au) wire is the preferred interconnect
bond wire material to bond with the Aluminum (Al) metalized bond pad by the
thermosonic technique (Srikanth et al., 2004). The general overview of this technique is
started by forming the Free Air Ball (FAB), and the ball is then pressed onto the bond
1
pad metallization thus formed a first bond or known as ball bond formation. After that,
the wire will make a looping then formed the second bond (wedge bond).
For bonding wire material, Au wire is the most preferred bonding wire material because
of its properties which do not oxidize during the process and easily to deform which
capable to form a good ball bond. Normally, Au wire is connected to Al bond pad
metallization then produced a new phase which is known as intermetallic compound
(IMC). However, this bonding system can cause the bond degradation and failure
because of the excessive formation of Au-Al IMC and voids (Xu et al., 2011). In order
to prevent the failure, thus a Copper (Cu) wire has become a selected wire bond material
instead of Au wire since it has many advantages over Au wire. For example, it has high
electrical, mechanical and thermal properties and low in terms of cost over Au wire.
Other than that, the formation of IMC and void growth is much lower which can prevent
from the failure (Xu et al., 2011). For bond pad metallization material, the Al bond pad
metallization is the most preferred and suitable in wire bonding industry for decades due
to the inexpensive and easily wire bondable.
The hardness of the Al bond pad metallization is well matched to the Au wire but less
suited for Cu wire because the hardness of the FAB formation of Cu wire is higher than
Al bond pad metallization. Thus it can damage the bond pad metallization and lead to
the failure (Clauberg et al., 2010). IMC is an interface which is made up of two or more
metallic elements. Basically, IMC will produce a new phase with a new crystal structure,
composition and properties (Askeland and Fulay, 2010). The common IMC is occurred
between Au wire and Al bond pad metallization because the perfect match between
these two materials but the performance of the bond will be degraded because of the
excessive growth of the IMC. Cu-Al IMC layer is also can be formed but need some
enhancement for the Cu wire in order to make a better match between these two
materials. This is because, Cu wire is oxidized during the process and will make FAB
formation become hard and can damage the Al bond pad metallization. Moreover, if the
thickness of the IMC becomes increase, it will be produced a high electrical resistance
which lead to high heat generation when the current is flow (Hang et al., 2008). Apart
2
from that, the bonding strength will be increased when the IMCs are formed in the right
and proper amount during the wire bonding process, but it will be degraded when the
excessive growth of the IMCs are detected (Xu et al., 2009). The IMC formation and
growth are basically based on the parameters such as bond force, heat and ultrasonic
energy (Chen et al., 2010).
The presence of the IMC formation can be checked by using Optical Microscopy (OM)
and Scanning Electron Microscope (SEM). The IMC thickness which is affected by
thermal annealing temperatures and times also can be measured by using these two
techniques (Kim et al., 2003). The study of the effect of thermal annealing or baking of
the Cu-Al IMC has been widely investigated. Basically, these studies relied upon OM
and SEM with Energy Dispersive X-ray spectrometry (EDX). However, all the
techniques as mention above cannot directly examine the nanostructure and
crystallography of the IMC formation (Xu et al., 2011). The detail information is not
provided by using those techniques since they are not able to do so, hence the IMC
phases of Cu-Al bonds are not clearly identified. In order to get the details of the Cu-Al
IMC phase, the Transmission Electron Microscopy (TEM) is used since it is able to
analyze the Cu-Al bond interface on the nanoscale and lead to identification of the its
crystallographic structure (Xu et al., 2009).
1.2
Problem Statement
The thermosonic Au wire bonded to the Al bond pad was widely used due to the
oxidation free of the Au wire material. However, the excessive formation of the Au-Al
IMC and voids can lead to the degradation of the bonding and lead to the failure as well
(Xu et al., 2011). In the semiconductor industry, the reliability of a manufactured
product is very important and that is why the failure is needed to be avoided (Yu et al.,
2010). Apart from using Au wire as wire material, the Cu wire also can be used.
Nowadays, the usage of the Cu wire is widely used in industry due to the better
properties over the Au wire. One of the important thing is the cost of the Cu wire is
3
much cheaper compared to the Au wire. Other than that, Cu wire will provide better
electrical, mechanical and thermal properties (Xu et al., 2011). The growing speed of the
Cu-Al IMC is much lower than Au-Al IMC, thus formation of Cu-Al IMC and void
growth is much lower over the Au wire. Other than that, the low growing speed of
Cu-Al IMC can also lead to a lower electrical resistance and less heat generation hence
produced a better reliability and better device performance as well (Zhong, 2011).
The IMC phase at the bonding interface which is in nanometer (nm) scale is affecting
the performance or reliability of the microchip, thus detail research is necessary. The
IMC growth in a Thermosonic Cu-Al bonding interface is observed to be thin (a few
tenth to hundredth nanometers). In this context, one technique has been commonly used
which is TEM with EDX and electron diffraction functionality (Xu et al., 2011). TEM
technique has been commonly used to obtain the detail information about the IMC
morphology and the crystallographic. However, this technique is highly localized and
not able to represent the global structure of a bonding interface.
X-ray Diffraction (XRD) technique is a relatively cost effective technique and a
promising tool to identify the phase of a specimen. Besides, this technique has been
widely used for characterizing thin film and bulk specimen. Other than that, detail
information of crystallography and phase identification can be achieved. Nevertheless,
there are some challenges while using this technique in order to analyse IMC phase at
bonding interface of semiconductor sample. However, a direct measurement of IMC
formed at the bonding interface is restricted due to the position of IMC which
sandwiches between Cu ball bond and Silicon (Si) substrate. This leads to low intensity
of IMC in the diffractogram. Hence, this study is to evaluate two sample preparation
methods for the XRD measurement in order to reveal the minor peaks for the Cu-Al
IMC phases.
4
1.3
Objectives
The objectives for this study that need to be achieved are:
I.
To prepare the sample preparation method for XRD analyis by powder method
and etching process for Cu-Al IMC samples.
II.
To identify the capability to reveal the minor peak between these two sample
preparation methods by using XRD analysis for detecting IMC growth at the
bonding interface.
1.4
Scopes of Study
1.
Study for the sample preparation method for the powder method.
2.
Study on sample preparation for the etching process.
3.
Study about the IMC phase at the bonding interface for Cu-Al IMC phases.
4.
Study about the XRD analysis for identifying the Cu-Al IMC formation at
bonding interface.
5
1.5
Report Organization
This report will be divided into five major chapters:
I.
Chapter 1: Introduction
This chapter will explain about the background, problem statement, objectives,
and scopes of the study.
II.
Chapter 2: Literature review
This chapter will explain with more details which related to the study by
referring journals, books or website as sources of the information.
III.
Chapter 3: Methodology
This chapter describes the steps on how this study is conducted by making a flow
chart of the study. It also explains the procedures of the study that is useful and
be as a guideline in order to make the project run smoothly and properly.
IV.
Chapter 4: Results and Discussion
This chapter shows the results of the study and discussing the results with more
details.
V.
Chapter 5: Conclusion
This chapter is the summary of the study. Other than that, some
recommendations or suggestions are needed in order to make the future
reference.
6
CHAPTER 2
LITERATURE REVIEW
2.1
Wire Bonding
In microelectronic products, current is supplied by the microelectronic packaging
towards the IC chips and also distributing the signals between the microelectronic
devices. (Zhong et al., 2006). Generally, there are three main electrical interconnection
techniques in IC packages that involved in microelectronic packaging such as flip-chip,
Tape Automated Bonding (TAB), and wire bonding (Murali et al., 2003). Among these
three techniques, wire bonding is said to be the prime technique and widely used in the
industry which make a connection between the chips and the lead frames (Hang et al.,
2007). Basically, wire bonding is the technique which provides an electrical
interconnection between IC chips and lead frames in microelectronics (Xu et al., 2009).
Usually Au and Cu wire are used for wire bonding process. The usage of the Au wire is
widely used because it does not oxidize during the wire bonding process and make a
better contact to the Al bond pad metallization (Zhong et al., 2006). Nevertheless, the
cost of Au wire is expensive compared to the Cu wire. Thus, the Cu wire is being
selected as the replacement for the Au wire because of its superior properties and low in
terms of the cost.
In the semiconductor industry, there are three types of bonding techniques for wire
bonding. The first bonding technique was Thermocompression bonding technique which
7
introduced by Bell Laboratories in 1957. This technique was used until the Ultrasonic
and Thermosonic bonding techniques were introduced in 1970. In 1963, the first
commercial bonding machine was introduced by Kulicke and Soffa Industries for
semiconductor industry. Thermosonic Au wire bonding process towards the Al bond pad
metallization is the technique that has been widely used in the industry. The energy that
is needed in order to make a bonding for this technique is thermal and also ultrasonic
energy (Shah et al., 2008). However, many studies of Au-Al IMC phase reported that the
bonding strength will be degraded due to the excessive growth of IMC and void
formation. Hence, in order to produce a better interconnection reliability, many
researchers are focusing on Cu wire for making a replacement for the Au wire (Hang et
al., 2007).
Recently the bonding machines used in the industry are operated automatically come
with the equipment is also advanced software installed to control the operation of the
machine. Most of the wired bonds, 90% are bonded with the Thermosonic bonding
technique (ball-wedge bonding) and the rest of the 10% is done with the Ultrasonic
bonding technique (wedge-wedge bonding) (Hasnida et al, 2008).
2.2
Wire Bonding Techniques
2.2.1
Thermosonic Bonding Technique
Thermosonic bonding technique or also be known as ball-wedge bonding technique, has
a higher speed for a bonding formation and also provide a good electrical and
mechanical properties of the IMC phase. Basically, there are two types of wire bonds
formation that will be formed in this particular technique. The first wire formation is
known as ball bonding formation as shown in Figure 2.1. Meanwhile the second wire
formation is known as wedge or stitch bonding formation as shown in Figure 2.2. There
are some factors that are desirable to be considered in order to form a good or strength
bonding formation. For example like heat energy, ultrasonic energy and pressure (force)
(Singh and Mamat, 2011).
8
Figure 2.1: Ball bonding formation (Murali et al., 2003).
Figure 2.2: Wedge bonding formation (Breach and Wulff, 2009).
From Figure 2.4 shows the schematic procedures of ball-wedge bonding. Ball-wedge
bonding is usually applied on the Au wires. In ball-wedge bonding, the wire is passed
through a hollow capillary and fed until a small tip of the wire is extending beneath the
capillary. An Electronic Flame Off (EFO) system was introduced which placed under
the capillary and a spark is used to melts the tip and forms a small ball. Then, the ball is
pressed down onto the bond pad by a specific force with ultrasonic vibration converted
from sinusoidal electrical signal by the transducer (Chen et al., 2011).
9
IDENTIFICATION OF INTERMETALLIC COMPOUND AT
CU WIRE – AL BOND PAD INTERFACE BY USING X-RAY
DIFFRACTION
This report submitted in accordance with requirement of the Universiti Teknikal
Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering
(Engineering Materials) (Hons.)
by
NOR RAFIKA BINTI NOR HAMID
B050910115
900115-05-5110
FACULTY OF MANUFACTURING ENGINEERING
2013
ABSTRAK
Kajian ini akan memberi perhatian dalam pengenalpastian struktur kristal pada ikatan
dawai kuprum (Cu) dan aluminium (Al) dalam skala nanometer dengan menggunakan
Teknik pembelauan sinar-X. Terdapat beberapa kajian tentang pengenalan Cu-Al IMC
telah dikaji dengan menggunakan pelbagai jenis teknik seperti Mikroskop Pengimbas
Elektron (SEM) dan Mikroskop Transmisi Elektron (TEM). Secara dasarnya, untuk
mengenal pasti fasa Cu-Al IMC yang dalam skala nanometer, kebiasaannya TEM
merupakan teknik yang digunakan kerana ia mempunyai resolusi yang tinggi yang
membolehkan untuk mendapatkan maklumat secara lebih tepat untuk mikrostruktur
bahan. Walaubagaimanapun, terdapat beberapa batasan dalam penggunaan TEM ini,
oleh itu teknik XRD telah digunakan untuk menggantikan TEM untuk membuat analisa
dalam pengenalpastian fasa Cu-Al IMC. Terdapat beberapa cabaran dalam penggunaan
teknik XRD ini. Oleh itu, kajian ini memberi tumpuan dalam penyediaan dua kaedah
sampel seperti kaedah pembetukan serbuk dan kaedah proses punaran. Antara dua
kaedah ini, satu kaedah akan dipilih berdasarkan keberkesanan kedua-dua kaedah
tersebut yang akan bakal dikenalpasti pada akhir kajian uji kaji ini.
i
ABSTRACT
This study will be focused on the identification of the crystalline structure at the Copper
(Cu) wire - Aluminum (Al) bond pad interface by using X-ray Diffraction (XRD)
analysis. There are a few studies about the identification of the Cu-Al IMC have been
studied by using other techniques such as Scanning Electron Microscope (SEM) and
Transmission Electron Microscope (TEM). Basically, in order to identify the Cu-Al IMC
phase, TEM is used to be the selective technique, since it has high resolution that can
reveal ultrafine details of material microstructure. Nevertheless, due to the some
limitations, XRD technique being the selective technique to replace TEM in order to
make the analysis to identify Cu-Al IMC phase at the bonding interface. However, there
are some challenges while using this technique. Hence, this study will be focused on the
two sample preparation methods which are powder method and etching process. From
these two methods, one method would be the selective method by identifying the
effectiveness for the both techniques at the end of the experiment.
ii
DEDICATION
Dedicated to my beloved parents, siblings, friends, lecturer and supervisor
iii
ACKNOWLEDGEMENT
I would like to express my deepest gratitude to my main supervisor Dr. Mohd Warikh
bin Abd. Rashid, and also to my co-supervisor Dr. T.Joseph Sahaya Anand lecturer in
Faculty of Manufacturing Engineering (Engineering Materials), University Teknikal
Malaysia Melaka (UTeM), for their guidance, assistance and concern throughout my
research project.
I would also like to extend my sincere thanks to the Mr. Chua Kok Yau who provided
me with the extensive discussion and information around my works and interesting
exploration in operations.
Special thanks to my friends for giving me supports and helps especially in periods of
uncertainties and difficulties. My special thanks are also due to my classmates, Nor
Diyanna bt Norazemi and Clement Khoo for their critical suggestions and motivating me
throughout in this research.
I am also particularly grateful to the Infineon Company in Melaka for providing me the
place to do the research and also to the Engineering Materials Department Laboratory
for the facilities.
Finally, I would like to express my gratitude with highly appreciation and dedication to
my parents and family for their concerns and encouragements. Without them, I will not
be here to complete my study in UTeM.
iv
TABLE OF CONTENTS
Abstrak
i
Abstract
ii
Dedication
iii
Acknowledgement
iv
Table of Contents
v
List of Tables
viii
List of Figures
ix
List Abbreviations, Symbols and Nomenclatures
xi
CHAPTER 1: INTRODUCTION
1.1
Background
1
1.2
Problem Statement
3
1.3
Objectives
5
1.4
Scopes of Study
5
1.5
Report Organization
6
CHAPTER 2: LITERATURE REVIEW
2.1
Wire Bonding
7
2.2
Wire Bonding Techniques
8
2.2.1 Thermosonic Bonding Technique
2.3
2.4
8
2.2.2 Ultrasonic Bonding Technique
11
2.2.3 Thermocompression Bonding Technique
14
Wire Materials
15
2.3.1 Gold (Au) Wire
15
2.3.2 Copper (Cu) Wire
16
Intermetallic Compound (IMC)
16
v
2.5
2.4.1 Nucleation of IMC
17
2.4.2 IMC Behavior between Cu Bond and Au Bond
20
2.4.3 Mechanism of Nucleation of Intermetallics
22
Failure Mode of Wire Bonding
22
2.5.1 Cratering on Wire Bonding
22
2.5.2 Kirkendall Effect
24
2.6
Analysis Technique by XRD Analysis
25
2.7
Summary of the Literature Review
29
CHAPTER 3: METHODOLOGY
3.1
Chapter Overview
30
3.2
Materials
32
3.3
Sample Preparation Method
32
3.4
3.3.1 Powder Method
32
3.3.2 Etching Process
34
Sample Analysis
36
3.4.1 XRD Analysis
36
3.4.2 SEM/EDX Analysis
37
CHAPTER 4: RESULTS & DISCUSSION
4.1
Chapter Overview
38
4.2
Powder Sample
38
4.2.1 Optical Inspection Analysis
38
4.2.2 XRD Analysis
40
4.2.3 SEM/EDX Analysis
41
Etched Sample
44
4.3.1 Optical Inspection Analysis
44
4.3.2 XRD Analysis
48
4.3.3 SEM/EDX Analysis
49
4.3
vi
CHAPTER 5: CONCLUSION & RECOMMENDATIONS
5.1
Conclusion
53
5.2
Recommendations
54
REFERENCES
55
APPENDICES
A
Gantt Chart for PSM I
59
B
Gantt Chart for PSM II
60
vii
LIST OF TABLES
2.1
Misfit and vacancy-solute binding energy of elemental pairs
24
4.1
Atomic composition for the powder sample of Cu-Al IMC
42
4.2
Table of the lifted ball against immersion duration time
47
4.3
Atomic composition for the etched Sample 2 (1400s) of Cu-Al IMC
50
4.4
Atomic composition for the etched Sample 6 (2200s) of Cu-Al IMC
50
viii
LIST OF FIGURES
2.1
Ball bonding formation
9
2.2
Wedge bonding formation
9
2.3
A ball wedge interconnection between two bond pads
10
2.4
Schematic procedures of ball-wedge bonding
10
2.5
Capillary tool for ball-wedge bonding
11
2.6a
Au wire for ultrasonic bonding; High frequency bond
12
2.6b
Au wire for ultrasonic bonding; Low frequency bond
12
2.7
Wedge tool for ultrasonic technique
13
2.8
Schematic procedures for wedge-wedge bonding
14
2.9
IMC coverage and growth of the intermetallics on high temperature
18
storage
2.10
Relationship between bonded IMC coverage and probability of ball lift
18
2.11
Cu-Al intermetallics have lower resistivity than Au-Al intermetallics
20
2.12
A cross section of a Cu wire bonded to Al pad shows cratering occurs
22
under the bond. The wire is slightly lifted and cratering could be seen after
chemical etching
2.13
Diffractogram of sample in powder form that consists of Cu wires and Si
25
chip
Diffractogram of selected 2θ ranges with slow scan time
26
2.17a Diffractogram of etched powder sample with faster scan
27
2.17b Comparison of diffractogram of unetched and etched powder samples
27
3.1
General overview of the process flow for the study
31
3.2
The process flow for the mechanical removal process for the powder
33
2.14
sample of the Cu-Al IMC
3.3
Optical Microscope
33
ix
3.4
The process flow for the etched sample of Cu-Al IMC
35
3.5
The process flow for the etching optimization process
35
3.6
XRD machine
36
3.7
SEM machine
37
4.1
The collected of the powder samples for Cu-Al IMC
39
4.2
Close view of the Cu-Al IMC parts
40
4.3
Diffractogram for the powder samples of Cu-Al IMC
40
4.4
IMC formation at the ball bond area with a different stress area which (1)
41
low stress area, (2) and (3) high stress area
4.5
Stress distribution under a ball bond during Thermosonic process
42
4.6a
EDX analysis for; point 1(low stress area)
43
4.6b
EDX analysis for; point 2 (high stress area)
43
4.6c
EDX analysis for; point 3 (high stress area)
43
4.7a
Condition of the sample; before the etching process
44
4.7b
Condition of the sample; after the etching process
44
4.8
The condition of the sample at different etching duration
46
4.9
Percentage of the lifted ball vs time
47
4.10
Diffractogram of the etched samples for Sample 2 (1400s) and Sample 6
48
(2200s)
4.11a Ball bond area for; Sample 2 (1400s)
49
4.11b Ball bond area for; Sample 6 (2200s)
49
4.12a EDX analysis for Sample 2;point 1 (high stress area)
51
4.12b EDX analysis for Sample 2;point 2 (high stress area)
51
4.12c EDX analysis for Sample 2;point 3 (high stress area)
51
4.13a EDX analysis for Sample 6;point 1 (high stress area)
52
4.13b EDX analysis for Sample 6;point 2 (high stress area)
52
4.13c EDX analysis for Sample 6;point 3 (low stress area)
52
x
LIST OF ABBREVIATIONS, SYMBOLS AND
NOMENCLATURE
Ag
-
Silver
Al
-
Aluminum
Au
-
Gold
Cu
-
Copper
EDX
-
Energy Dispersive X-ray
EFO
-
Electronic Flame Off
FAB
-
Free Air Ball
HF
-
Hydrofluoric Acid
HNO3
-
Nitric Acid
HTS
-
High Temperature Storage
IC
-
Integrated Circuit
IMC
-
Intermetallic Compound
kHz
-
Kilohertz
KOH
-
Potassium Hydroxide
N
-
Newton
nm
-
Nanometer
OM
-
Optical Microscope
SEM
-
Scanning Electron Microscope
Si
-
Silicon
SiO2
-
Silicon Oxide
TAB
-
Tape-Automated Bonding
TEM
-
Transmission Electron Microscope
%
-
Percentage
°
-
Degree
°C
-
Degree C
xi
CHAPTER 1
INTRODUCTION
1.1
Background
There are three main interconnection electrical interconnection methods in intergrated
circuit (IC) packages such as flip-chip, tape-automated bonding (TAB), and wire
bonding for microelectronic packaging (Murali et al., 2003). Wire bonding technique is
the technique that is widely used in industry for making the electrical connection
between the chips and the lead frames compared to the other two techniques (Xu et al.,
2011). Basically, in wire bonding technique, there are several techniques that have been
used in order to make a bonding. The techniques are thermocompression, ultrasonic and
thermosonic bonding technique.
Generally in industry, the technique that is widely used is thermosonic bonding
technique since it is the fastest process compared to the other two processes. Other than
that, since the process is fast, then it will produce high production in a short time. In this
technique, thermal and ultrasonic energy are needed in order to make a good
interconnection between the two materials. Gold (Au) wire is the preferred interconnect
bond wire material to bond with the Aluminum (Al) metalized bond pad by the
thermosonic technique (Srikanth et al., 2004). The general overview of this technique is
started by forming the Free Air Ball (FAB), and the ball is then pressed onto the bond
1
pad metallization thus formed a first bond or known as ball bond formation. After that,
the wire will make a looping then formed the second bond (wedge bond).
For bonding wire material, Au wire is the most preferred bonding wire material because
of its properties which do not oxidize during the process and easily to deform which
capable to form a good ball bond. Normally, Au wire is connected to Al bond pad
metallization then produced a new phase which is known as intermetallic compound
(IMC). However, this bonding system can cause the bond degradation and failure
because of the excessive formation of Au-Al IMC and voids (Xu et al., 2011). In order
to prevent the failure, thus a Copper (Cu) wire has become a selected wire bond material
instead of Au wire since it has many advantages over Au wire. For example, it has high
electrical, mechanical and thermal properties and low in terms of cost over Au wire.
Other than that, the formation of IMC and void growth is much lower which can prevent
from the failure (Xu et al., 2011). For bond pad metallization material, the Al bond pad
metallization is the most preferred and suitable in wire bonding industry for decades due
to the inexpensive and easily wire bondable.
The hardness of the Al bond pad metallization is well matched to the Au wire but less
suited for Cu wire because the hardness of the FAB formation of Cu wire is higher than
Al bond pad metallization. Thus it can damage the bond pad metallization and lead to
the failure (Clauberg et al., 2010). IMC is an interface which is made up of two or more
metallic elements. Basically, IMC will produce a new phase with a new crystal structure,
composition and properties (Askeland and Fulay, 2010). The common IMC is occurred
between Au wire and Al bond pad metallization because the perfect match between
these two materials but the performance of the bond will be degraded because of the
excessive growth of the IMC. Cu-Al IMC layer is also can be formed but need some
enhancement for the Cu wire in order to make a better match between these two
materials. This is because, Cu wire is oxidized during the process and will make FAB
formation become hard and can damage the Al bond pad metallization. Moreover, if the
thickness of the IMC becomes increase, it will be produced a high electrical resistance
which lead to high heat generation when the current is flow (Hang et al., 2008). Apart
2
from that, the bonding strength will be increased when the IMCs are formed in the right
and proper amount during the wire bonding process, but it will be degraded when the
excessive growth of the IMCs are detected (Xu et al., 2009). The IMC formation and
growth are basically based on the parameters such as bond force, heat and ultrasonic
energy (Chen et al., 2010).
The presence of the IMC formation can be checked by using Optical Microscopy (OM)
and Scanning Electron Microscope (SEM). The IMC thickness which is affected by
thermal annealing temperatures and times also can be measured by using these two
techniques (Kim et al., 2003). The study of the effect of thermal annealing or baking of
the Cu-Al IMC has been widely investigated. Basically, these studies relied upon OM
and SEM with Energy Dispersive X-ray spectrometry (EDX). However, all the
techniques as mention above cannot directly examine the nanostructure and
crystallography of the IMC formation (Xu et al., 2011). The detail information is not
provided by using those techniques since they are not able to do so, hence the IMC
phases of Cu-Al bonds are not clearly identified. In order to get the details of the Cu-Al
IMC phase, the Transmission Electron Microscopy (TEM) is used since it is able to
analyze the Cu-Al bond interface on the nanoscale and lead to identification of the its
crystallographic structure (Xu et al., 2009).
1.2
Problem Statement
The thermosonic Au wire bonded to the Al bond pad was widely used due to the
oxidation free of the Au wire material. However, the excessive formation of the Au-Al
IMC and voids can lead to the degradation of the bonding and lead to the failure as well
(Xu et al., 2011). In the semiconductor industry, the reliability of a manufactured
product is very important and that is why the failure is needed to be avoided (Yu et al.,
2010). Apart from using Au wire as wire material, the Cu wire also can be used.
Nowadays, the usage of the Cu wire is widely used in industry due to the better
properties over the Au wire. One of the important thing is the cost of the Cu wire is
3
much cheaper compared to the Au wire. Other than that, Cu wire will provide better
electrical, mechanical and thermal properties (Xu et al., 2011). The growing speed of the
Cu-Al IMC is much lower than Au-Al IMC, thus formation of Cu-Al IMC and void
growth is much lower over the Au wire. Other than that, the low growing speed of
Cu-Al IMC can also lead to a lower electrical resistance and less heat generation hence
produced a better reliability and better device performance as well (Zhong, 2011).
The IMC phase at the bonding interface which is in nanometer (nm) scale is affecting
the performance or reliability of the microchip, thus detail research is necessary. The
IMC growth in a Thermosonic Cu-Al bonding interface is observed to be thin (a few
tenth to hundredth nanometers). In this context, one technique has been commonly used
which is TEM with EDX and electron diffraction functionality (Xu et al., 2011). TEM
technique has been commonly used to obtain the detail information about the IMC
morphology and the crystallographic. However, this technique is highly localized and
not able to represent the global structure of a bonding interface.
X-ray Diffraction (XRD) technique is a relatively cost effective technique and a
promising tool to identify the phase of a specimen. Besides, this technique has been
widely used for characterizing thin film and bulk specimen. Other than that, detail
information of crystallography and phase identification can be achieved. Nevertheless,
there are some challenges while using this technique in order to analyse IMC phase at
bonding interface of semiconductor sample. However, a direct measurement of IMC
formed at the bonding interface is restricted due to the position of IMC which
sandwiches between Cu ball bond and Silicon (Si) substrate. This leads to low intensity
of IMC in the diffractogram. Hence, this study is to evaluate two sample preparation
methods for the XRD measurement in order to reveal the minor peaks for the Cu-Al
IMC phases.
4
1.3
Objectives
The objectives for this study that need to be achieved are:
I.
To prepare the sample preparation method for XRD analyis by powder method
and etching process for Cu-Al IMC samples.
II.
To identify the capability to reveal the minor peak between these two sample
preparation methods by using XRD analysis for detecting IMC growth at the
bonding interface.
1.4
Scopes of Study
1.
Study for the sample preparation method for the powder method.
2.
Study on sample preparation for the etching process.
3.
Study about the IMC phase at the bonding interface for Cu-Al IMC phases.
4.
Study about the XRD analysis for identifying the Cu-Al IMC formation at
bonding interface.
5
1.5
Report Organization
This report will be divided into five major chapters:
I.
Chapter 1: Introduction
This chapter will explain about the background, problem statement, objectives,
and scopes of the study.
II.
Chapter 2: Literature review
This chapter will explain with more details which related to the study by
referring journals, books or website as sources of the information.
III.
Chapter 3: Methodology
This chapter describes the steps on how this study is conducted by making a flow
chart of the study. It also explains the procedures of the study that is useful and
be as a guideline in order to make the project run smoothly and properly.
IV.
Chapter 4: Results and Discussion
This chapter shows the results of the study and discussing the results with more
details.
V.
Chapter 5: Conclusion
This chapter is the summary of the study. Other than that, some
recommendations or suggestions are needed in order to make the future
reference.
6
CHAPTER 2
LITERATURE REVIEW
2.1
Wire Bonding
In microelectronic products, current is supplied by the microelectronic packaging
towards the IC chips and also distributing the signals between the microelectronic
devices. (Zhong et al., 2006). Generally, there are three main electrical interconnection
techniques in IC packages that involved in microelectronic packaging such as flip-chip,
Tape Automated Bonding (TAB), and wire bonding (Murali et al., 2003). Among these
three techniques, wire bonding is said to be the prime technique and widely used in the
industry which make a connection between the chips and the lead frames (Hang et al.,
2007). Basically, wire bonding is the technique which provides an electrical
interconnection between IC chips and lead frames in microelectronics (Xu et al., 2009).
Usually Au and Cu wire are used for wire bonding process. The usage of the Au wire is
widely used because it does not oxidize during the wire bonding process and make a
better contact to the Al bond pad metallization (Zhong et al., 2006). Nevertheless, the
cost of Au wire is expensive compared to the Cu wire. Thus, the Cu wire is being
selected as the replacement for the Au wire because of its superior properties and low in
terms of the cost.
In the semiconductor industry, there are three types of bonding techniques for wire
bonding. The first bonding technique was Thermocompression bonding technique which
7
introduced by Bell Laboratories in 1957. This technique was used until the Ultrasonic
and Thermosonic bonding techniques were introduced in 1970. In 1963, the first
commercial bonding machine was introduced by Kulicke and Soffa Industries for
semiconductor industry. Thermosonic Au wire bonding process towards the Al bond pad
metallization is the technique that has been widely used in the industry. The energy that
is needed in order to make a bonding for this technique is thermal and also ultrasonic
energy (Shah et al., 2008). However, many studies of Au-Al IMC phase reported that the
bonding strength will be degraded due to the excessive growth of IMC and void
formation. Hence, in order to produce a better interconnection reliability, many
researchers are focusing on Cu wire for making a replacement for the Au wire (Hang et
al., 2007).
Recently the bonding machines used in the industry are operated automatically come
with the equipment is also advanced software installed to control the operation of the
machine. Most of the wired bonds, 90% are bonded with the Thermosonic bonding
technique (ball-wedge bonding) and the rest of the 10% is done with the Ultrasonic
bonding technique (wedge-wedge bonding) (Hasnida et al, 2008).
2.2
Wire Bonding Techniques
2.2.1
Thermosonic Bonding Technique
Thermosonic bonding technique or also be known as ball-wedge bonding technique, has
a higher speed for a bonding formation and also provide a good electrical and
mechanical properties of the IMC phase. Basically, there are two types of wire bonds
formation that will be formed in this particular technique. The first wire formation is
known as ball bonding formation as shown in Figure 2.1. Meanwhile the second wire
formation is known as wedge or stitch bonding formation as shown in Figure 2.2. There
are some factors that are desirable to be considered in order to form a good or strength
bonding formation. For example like heat energy, ultrasonic energy and pressure (force)
(Singh and Mamat, 2011).
8
Figure 2.1: Ball bonding formation (Murali et al., 2003).
Figure 2.2: Wedge bonding formation (Breach and Wulff, 2009).
From Figure 2.4 shows the schematic procedures of ball-wedge bonding. Ball-wedge
bonding is usually applied on the Au wires. In ball-wedge bonding, the wire is passed
through a hollow capillary and fed until a small tip of the wire is extending beneath the
capillary. An Electronic Flame Off (EFO) system was introduced which placed under
the capillary and a spark is used to melts the tip and forms a small ball. Then, the ball is
pressed down onto the bond pad by a specific force with ultrasonic vibration converted
from sinusoidal electrical signal by the transducer (Chen et al., 2011).
9