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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 179–186

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

nvestigation of ultrasonic copper wire wedge bonding onu/Ni plated Cu substrates at ambient temperature

anhong Tiana,∗, Chunqing Wanga, Ivan Lumb, M. Mayerb, J.P. Jungc, Y. Zhoua,b

State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, ChinaMicrojoining Lab, Center for Advanced Materials Joining, University of Waterloo, Waterloo, Canada N2L 3G1Microjoining Lab, University of Seoul, Seoul, Republic of Korea

r t i c l e i n f o

rticle history:

eceived 24 December 2006

eceived in revised form

December 2007

ccepted 23 December 2007

eywords:

opper wire

a b s t r a c t

Copper wire is attracting more and more attention in wire bonding technology due to its

advantages in comparison with gold or aluminum wire. This paper presents an achievement

of ultrasonic wedge bonding with 25 �m copper wire on Au/Ni plated Cu substrate at ambi-

ent temperature. A detailed investigation from the aspects of process optimization, bonding

mechanism, interdiffusion, ultrasonic effects on microstructure and microhardness of the

bonding materials were performed. The results show that it is possible to produce strong

copper wire wedge bonds at room temperature, and the thinning of the Au layer was found

directly below the center of the bonding tool with the bonding power increasing. Interdif-

fusion between copper wire and Au metallization during the wedge bonding at ambient

ltrasonic wedge bonding

esign of experiment (DOE)

ear action

ltrasonic softening

ecrystallization

temperature was assumed negligible. The wedge bonding was achieved by wear action

induced by ultrasonic vibration. The ultrasonic power did contribute to enhancing defor-

mation of the copper wire due to ultrasonic softening effect which was then followed by the

strain hardening of the copper wedge bond, and the dynamic recovery or recrystallization

of the copper wire caused by ultrasonic vibration during wedge bonding was also found.

sal are required in order to facilitate bonding. Deformation

. Introduction

opper wire bonding is an alternative chip interconnectionechnology with promising cost savings compared to goldire bonding and better electrical performance compared

o aluminum wire (Harman, 1997). There are lots of studiesn thermosonic gold or copper ball bonding and ultrasonicluminum wedge bonding (Ho et al., 2003; Harman andlbers, 1977; Krzanowski et al., 1990; Takahashi et al., 1996;angenecker, 1966; Lum et al., 2005, 2006; Murali et al., 2003;i et al., 2006). However, there is a lack of understanding on

he ultrasonic copper wire wedge bonding process. Ultrasonicedge bonding utilizes a normal bond force simultaneouslyith ultrasonic energy to form the first and second bonds at

∗ Corresponding author. Tel.: +86 451 86418359; fax: +86 451 86416186.E-mail addresses: tianyh@hit.edu.cn, yanhtian@yahoo.com (Y. Tian

924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.12.134

© 2008 Elsevier B.V. All rights reserved.

ambient temperatures and is a preferred method in intercon-necting power devices.

Ultrasonic wire bonding is generally accepted to be a solidstate joining process which is supported by various evidencessuch as bonds made at liquid nitrogen temperatures (Harmanand Albers, 1977) and studies of the bond interface with trans-mission electron microscopy (Krzanowski et al., 1990). A majorrequirement to form a metallurgical bond is a relatively con-taminant free surface. Without occurrence of melting in thewire-bonding process other methods of contaminant disper-

).

is the main mechanism responsible for the contaminant dis-persal required for bond formation in thermocompression(using heat and pressure only) wire bonding. The deforma-

n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 179–186

Fig. 2 – Three types of the first bond outcomes during

180 j o u r n a l o f m a t e r i a l s p r o c e s s i

tion mechanism observed in thermocompression bonding issimilarly observed in pressure welding, in which applied pres-sure and the subsequent deformation breaks up the oxidelayer (Takahashi et al., 1996). In ultrasonic wire bonding, ultra-sonic energy is used in addition to pressure. When a metal isirradiated with ultrasonic energy, the yield stress decreases,and this is known as the reversible ultrasonic softening effect(Langenecker, 1966). Recently, Lum et al. proposed the tran-sition from micro-slip to gross slide with ultrasonic powerincreasing based on the micro-slip theory to elucidate the balland wedge bonding mechanism (Lum et al., 2005, 2006). Muraliet al., 2003 reported their un-annealed copper wire after ballbonding showed the lowest hardness in the HAZ zone, whichwas caused by the recrystallization and grain growth fromthe FAB formation process. They concluded that the highesthardness in the Cu ball bond came from the strain hardeninginduced by ultrasonic power. Li et al., 2006 found the atomicdiffusion between Au ball bond and Al pad at a high level ultra-sonic frequency (1.5 MHz) when ultrasonic power is 1.75 W andbonding temperature 200 ◦C, and the thickness of atomic dif-fusion layer is about 500 nm.

This paper will present a detailed investigation on theultrasonic wedge bonding of copper wire at ambient tem-perature, and many aspects of the copper wire wedgebonding including process optimization, bonding mechanism,microstructure, and microhardness of the copper wedge bondand interdiffusion of the bonding materials were studied.

2. Experimental procedure

Copper wire bonding was performed at room temperature onAu/Ni plated Cu substrate with the 3 �m thickness of Au, 7 �mNi and 23 �m Cu. The 25 �m copper wire used is provided byMK Electron Co. Ltd. with 99.99% purity. The bonding machineused was a semi-automatically 4523A Digital K&S wedge bon-der with a frequency of 65 KHz. Fig. 1 illustrates the Cu wirewedge bonding process.

Bond growth and joint strength are related with process-ing parameters and ultrasonic conditions such as ultrasonicvibration accuracy and speed, ultrasonic power, bonding force,bonding time, tail breaking force, surface state, etc. These

parameters should be optimized to get good bond quality atambient temperature. Design of experiment (DOE) was appliedin this study, and a 20-run central composite design basedon response surface methodology was used. There are three

Fig. 1 – Schematic drawing of the Cu wire wedge bondingprocess.

wedge bonding (a) lift-off (b) sticking, and (c) the first bondcut caused by excessive deformation.

factors in the DOE, ultrasonic power, bonding force, and bond-ing time. For each factor, three levels were chosen. There are20 runs in total, and for each set of parametric conditions20 bonds were made. After wedge bonding, pull testing wasperformed on DAGE 4000 to get the pull force of the bond.Pull force was the response for this design. The experimen-tal design and data analysis were done on MiniTAB statisticalsoftware.

The cross-section samples were prepared and chemicallyetched. An etch solution (containing 2 g Na2Cr2O7 + 4 ml satu-rated NaCl solution + 10 ml concentrated H2SO4 + 100 ml H2O)was swabbed onto the cross-section surface for 12 s usinga cotton ball to reveal the microstructure. The cross-sectionsamples were observed using SEM, and energy dispersive X-ray (EDX) was used to study the chemical composition. Vickersmicrohardness testing was conducted on the cross-section ofthe copper wedge bonds at various locations. The methodemploys an indentation measurement by using a 136◦ dia-mond pyramid indenter and 5 gf load and was applied for15 s.

3. Results and discussion

3.1. Effects of process parameters on the pull force ofwedge bonds

During the wedge bonding of the copper wire, three typesof bonding outcomes were obtained: lift-off caused by weakbonding, sticking, and wedge bond cut caused by excessivedeformation, as illustrated in Fig. 2. Lifted off bonds wouldoccur because the frictional force acting at the wire/wire feedhole during the following looping step would be greater thanthe strength of the wedge bond and during the following loop-ing step would lift the bond off the substrate. On the otherhand, if the bond was sufficiently strong, the wedge bondwould stick on the substrate during the following looping step.

The completed wire bond (sticking bond in Fig. 2b) wouldsubsequently be pull tested with a DAGE 4000 pull tester, andthen pull force and failure modes are determined. Three com-mon failure modes in this study are:

1) Interfacial break (bond lifting off from the surface of themetallization).

2) Neck break (wire break at the neck of the bond).

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 179–186 181

Fig. 3 – Plot of pull test results illustrating the differentrd

3

abwtpb

ftialmptmbp

vi

Ff

Table 1 – Parameter levels in the central compositedesign

Variables Level of variables

−� −1 0 +1 +�

and bonding time were determined. Table 1 shows the param-eter levels for the central composite design. In this paper,pull force of the wedge bonds was used as a response orquality characteristic, that is also the output we want to mea-

Table 2 – Details of the ultrasonic power, bonding time,bonding force, bonding responses (pull force), andstandard deviation (StdEv) for each run of DOE for thefirst bonds

Run no. Power Time Force Pull StdEv

egions of failure modes and resulting pull force withifferent ultrasonic powers.

) Bond break (when the bond was deformed excessively).

The failure mode may give insight to the wire bondabilitynd the bond strength. Wire breaks at the neck position of theond are the preferred mode in this experiment because a highire load with a wire break indicates good bonding between

he wire and the Au/Ni/Cu metallization. Bond breaks are notreferred because the bonds deformed excessively under highonding power and force.

Fig. 3 shows the pull test results of the first wedge bondsormed by the different ultrasonic powers when the bondingime and force was fixed (30 ms/40 gf, 1 gf = 9.8 mN). Fig. 3 alsollustrates the different regions of failure modes. With 65 mWnd less ultrasonic bonding power (labeled lift-off region),ift-off bonds would occur since the amount of bonding was

inimal. Bond sticking would occur with ultrasonic bondingower higher than 65 mW. With increased bonding power upo 390 mW, the percentage of interfacial break decreased and

ore neck breaks occurred. It can be seen that the wedgeonds with the neck break failure modes yields the highest

ull forces during pulling test.

Fig. 4 shows pull force of the first wedge bonds formed byarious bonding forces when the ultrasonic power and bond-ng time was fixed (260 mW/30 ms). Here, a pull force of larger

ig. 4 – Pull force of wedge bonds formed by various bondorces.

Power (mW) 150 195 260 325 370Time (ms) 14 20 30 40 46Force (gf) 32 35 40 45 48

than 20 g is chosen as a suitable condition for the bondingforce process window. This condition is fulfilled in the win-dow between 35 and 50 g. Below these bonding forces, contactpressure is insufficient, resulting in a loose contact. At otherextreme, excessive bonding forces are detrimental to the inter-facial motion of wire and pad. Consequently, the resultingbond has a lower pull force.

3.2. DOE process optimization of the wedge bonds

Design of experiment is a quick and cost-effective methodto understand and optimize any manufacturing processes(Raymond and Douglas, 2002). It is a direct replacement of‘one variable-at-a-time’ approach of experimentation, whereexperimenters vary only one variable at a time, keeping allother variables in the experiment fixed. In order to optimizethe process parameters of both the first bond and second bond,a 20-run central composite design was used in this study.Based on the aforementioned experiment results, three lev-els of the factors including ultrasonic power, bonding force,

(mW) (ms) (gf) force (gf)

1 325 20 35 22.8 2.1042 260 30 40 22.3 1.9053 325 40 35 22.0 2.364 325 40 45 19.4 1.1725 260 30 40 20.6 1.9936 195 40 35 20.9 1.7437 195 20 45 18.6 2.278 195 20 35 18.9 2.3759 325 20 45 19.0 2.14

10 195 40 45 20.6 1.95411 260 30 40 20.7 0.94812 260 30 40 22.2 0.96413 260 30 40 21.6 1.19914 150 30 40 20.9 1.97315 260 14 40 19.1 2.18116 260 30 32 21.1 2.51017 260 46 40 19.0 1.44318 370 30 40 19.6 1.46819 260 30 48 19.6 1.00720 260 30 40 21.1 2.876

182 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 179–186

Table 3 – Details of the ultrasonic power, bonding time,bonding force, bonding responses (pull force), andstandard deviation (StdEv) for each run of DOE for thesecond bonds

Run no. Power(mW)

Time(ms)

Force(gf)

Pull force(gf)

StdEv

1 325 20 35 16.1 3.752 260 30 40 18.0 3.443 325 40 35 15.4 4.464 325 40 45 20.2 3.025 260 30 40 19.8 2.146 195 40 35 10.0 3.057 195 20 45 10.1 3.4958 195 20 35 5.9 2.4249 325 20 45 20.4 1.62

10 195 40 45 9.1 3.1711 260 30 40 19.2 3.4112 260 30 40 19.5 3.6613 260 30 40 16.4 2.4714 150 30 40 7.4 3.9515 260 14 40 9.8 3.5516 260 30 32 13.2 6.0317 260 46 40 17.2 5.7118 370 30 40 20.5 2.8619 260 30 48 17.9 2.77

Fig. 5 – Contour plots of pull force vs. bonding power andbonding force when bonding time is 30 ms. (a) First bond

20 260 30 40 19.8 1.32

sure during the experiment. Tables 2 and 3 show details ofthe ultrasonic power, bonding time, bonding force, bondingresponses (pull force), and standard deviation (StdEv) for eachrun for both the first and second bonds. It can be foundthat for the copper wire used in this study, the pull forcesof 20 gf or greater can be obtained for both the first and sec-ond bonds. The higher pull force and lower standard deviationof the first bonds can be achieved compared with the secondbonds.

Fig. 5 shows the contour plots of the pull force whenthe bonding time is 30 ms. It can be found that the high-est pull force of the first bond was achieved with highpower and low force. However, for the second bond, for thehighest pull force, both high power and high force wererequired. This might be because of the tail formation withthe wire clamp which closes and pulls the wire to breakit at the heel of the second bond that requires more forceand power. However, there is no pulling force of the clampon the first bond, as shown in Fig. 1. According to the con-tour plots, the optimized ultrasonic power, bonding force,and bonding time for both the first bond and the secondbond could be achieved, which are 260 mW/35 gf/30 ms and325 mW/40 gf/30 ms, respectively.

Fig. 6(a) shows the wedge bonds obtained with the opti-mized processing parameters when the bonding time is 30 mswith first bonds at a power of 260 mW and bonding force of35 gf, and the second bonds at a power of 325 mW and bond-ing force of 40 gf. Fig. 6(b) shows the first bonds obtained withpower 370 mW, force 40 gf, and time 30 ms. It could be found

that with the increase of ultrasonic power, the deformationof the wedge bond increased, and excessive deformation ofthe first bonds occurred when a higher ultrasonic power wasapplied.

and (b) second bond.

3.3. Microstructure and hardness of the wedge bond

The possibility of using Cu wires bonded to Au/Ni plated Cusubstrate has led to interest in the reliability of this metallur-gical system. Fig. 7 shows the cross-sections of second bondswhen the bonding force is 35 gf, bonding time is 30 ms andultrasonic power is 260 and 370 mW, respectively. With thebonding power increasing, the thickness of the gold layer atthe center of the Au/Ni metallization decreased, as shown inFig. 7b, this will be discussed in detail at the following section.It seems that the ultrasonic power contributed to increased

deformation of the copper wire because higher ultrasonicpower made the wire softer due to ultrasonic softening. Fig. 8shows two lines scan from the same cross-sections of Fig. 7aand b, which indicates that there was no obvious interdif-

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 179–186 183

Fig. 6 – Micrographs of the wedge bonds when the bondingtime is 30 ms (a) the wedge bonds obtained with theoptimized processing parameters, the first bonds (260 mW,35 gf); the second bonds (325 mW, 40 gf); (b) the first bondsw(

fhwm

Fig. 8 – Results of line scan from the same cross-sections ofFig. 5: (a) line scan from line 1 of Fig. 5a and (b) line scan

Fu

ith high ultrasonic power shows excessive deformation370 mW, 35 gf).

usion between Cu and Au at these two bonding interfaces,owever, a little amount of Au elements inside the Cu wireas found, which might be caused by the wear action andechanical mixing.

ig. 7 – Cross-sections of second bonds with different bonding pltrasonic power increasing. (a) 260 mW, 35 gf, 30 ms and (b) 370

from line 2 of Fig. 5b.

Fig. 9 shows fractured surfaces after shear testing. Bothof the fracture surfaces showed dimples which indicateddesirable ductile bonded joints. The composition of the frac-ture surface was Au and Cu, which showed that the fractureoccurred along the bonded interface between the Cu and Au

layer. For 325 mW bonds, top layer of Au/Ni metallizationlifted off somewhere and the Cu substrate was exposed dur-ing shearing, which could be found from EDX result of point A.Point B and point C show good bonding and mixing of the ele-

ower showing thinning of the Au layer occurred withmW, 35 gf, 30 ms.

184 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 179–186

er sh

Fig. 9 – Fractured surface of the wedge bond aft

ments between wire and metallization, as shown in Fig. 9b. Forthe bonds with increased power of 370 mW, a small white par-ticle was found at the fractured surface, as shown in Fig. 9c andd. The EDX result of point D shows that the particle consistsof Cu and Au.

There are three intermetallic phases (Cu3Au, CuAu, CuAu3)in the binary Cu–Au phase diagram when the temperature isbeyond 200 ◦C. According to the literature, Cu moves rapidlythrough the Au film by boundary diffusion at temperatures of100–300 ◦C within approximately 1 h, and the diffusion coef-ficient for Cu in Au is D = 1.64 × 10−20 cm2/s at 200 ◦C (Halland Morabito, 1978). This diffusion is faster than that of Auin Cu because the Cu atom is smaller than the Au atom. Itwas found from some ultrasonic wire bonding investigationsthat temperature rise at the bond interface was between 80and 300 ◦C (Ho, 2004). According to the Fick’s diffusion law,the thermal interdiffusion distance can be obtained from fol-lowing equation: X2 = Dt, where t is the interdiffusion time,which is assumed as the bonding time 50 ms here. As a resultof this, the thermal diffusion distance is not more than 0.3 A◦.It was proposed that the interdiffusion between Al wire andNi metallization was enhanced by ultrasonic vibration, andatomic diffusion between Au bond and Al metallization at ahigh level ultrasonic frequency (1.5 MHz) and bonding tem-perature 200 ◦C was also found (Li et al., 2006). In this paper,the wedge bonding of the Cu wire was achieved at ambienttemperature, and the interdiffusion between Cu and Au was

not found from Fig. 8. Therefore it is concluded that the Cudiffusion into the Au during the wedge bonding at ambienttemperature was negligible, and the formation of IMCs is notexpected.

ear test (a) and (b) 325 mw; (c) and (d) 370 mw.

The above results confirmed that the mixing of the Cu andAu at the interface region and the achievement of the wedgebonding was caused by wearing action. In ultrasonic wedgebonding, the amplitude of the bonding tip oscillation is propor-tional to the applied ultrasonic power and the relative motionexperienced when the wire is sliding, will lead to wear of mate-rial (or contaminant) according to an equation developed forcontacting surfaces in relative motion (Peterson and Winer,1980):

t = dH

K

1PV

(1)

where t is the time required, d is the depth of material worn,P is the mean or nominal pressure, H is the hardness of thematerial, K is the wear coefficient constant, and V is the slidingvelocity. This wear of material is termed fretting when smallamplitude oscillations are involved. A sufficient removal of thecontaminant layer is required for bonding to occur betweenthe underlying metal surfaces. Bonding at room temperatureis related to a wear mechanism induced by the ultrasonicvibrations. During bonding, the ultrasonic stick–slip frictioncauses friction power to be delivered at the interface. Thispower is partly transformed to mechanical wear. The wear-ing action breaks up contaminant and oxide layers allowingfor areas of fresh metal of the opposing bonding partners tocontact and bond to each other.

From Fig. 9(a) and (c), the sheared fracture surface (foot-print) showed the typical ellipse shape of wedge bonding. Itis the wear caused by the relative motion that will allow inti-mate metal–metal contact and promotes subsequent bonding.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 179–186 185

Fig. 10 – Microstructures of bonded joints when power is 325 mW, bonding force 35 gf, and time 50 ms (a) copper wireshowing elongated grains in the middle (b) magnified picture showing the small equiaxed grains.

F r is 2(

Ttt

ditiopi

uawwwgiieorvmsacf

ig. 11 – Microhardness test on the wedge bond when poweb) the second bond.

hese regimes of relative motion combined with the stress dis-ribution at the bonding interface caused by the bonding toolhat account for the bonded footprint morphology.

The rate and uniformity of the wear at the interfaceepends on the stress field amplitudes and uniformity at the

nterface, respectively. Because the stress is lowest right athe periphery of the contact, and is highest at the bondingnterface directly below the tool. The thinning of the Au layerbserved in Fig. 7b can be explained by the larger interfacialeak stresses right below the wedge tool where the thinning

s observed when the higher ultrasonic power is used.Fig. 10 gives the microstructure of the bonded joint where

ltrasonic power, bonding force, and time are 325 mW, 35 gfnd 50 ms, respectively. The microstructures of the copperire in the left upper part of Fig. 10a are elongated grains,hich were produced during the drawing process of the wireshen manufacturing. After wedge bonding, small equiaxed

rains were found at the bonding interface region, as shownn Fig. 10b. As we discussed above, the temperature rising dur-ng room temperature wedge bonding process was not highnough to cause any microstructure change and even recoveryf copper wire, thus it can be concluded that the recovery orecrystallization found in this study was due to the ultrasonicibration. The horizontal ultrasonic vibration and the nor-al force of the wedge induced compressive stress and shear

tress at the interfacial region, which resulted in the slippingnd refining the grains the wedge bond. The slipping of theopper wire and refining grains provided a foundation of theurther plastic deformation. The results were consistent with

60 mW, time 50 ms, and force 35 gf. (a) The first bond and

the others studies in which the dynamic recovery or recrys-tallization of the aluminum wire was found during ultrasonicwedge bonding (Geissler et al., 2006; Krzanowski, 1990).

Micrographs of microhardness test results are given inFig. 11. The test was performed along the centerline of thecopper wire. Five points for the wire part and the bondedjoint part were tested. It could be found that the hardnessat the deformed bonded joint part is a little higher than thewire part, which was probably caused by the strain harden-ing of the wedge bond. It was believed that the ultrasonicenergy enhances the plastic deformation of the bonding wiredue to the fact that the dislocations inside the wire absorbsthe acoustic energy selectively, and the dislocations wereactivated from their anchoring locations, which makes thedeformation of the bonding wire easier. After the acousticenergy was removed, the new defects made the bonding wireharder.

4. Conclusions

(1) Copper wire bonding on an Au/Ni plated Cu substrateat ambient temperature was achieved, and the bondingparameters for both first bond and second bonds wereoptimized by design of experiment. To get strong pull force

of the wedge bond, higher power and force were requiredfor the second bond than for the first bond.

(2) Cross-section analysis showed a continuous connectionbetween the Cu wire and Au metallization when the

n g t

rExperiments, 2nd edition. John Wiley & Sons, Inc., New York.

Takahashi, Y., Shibamoto, S., Inoue, K., 1996. Numerical analysis

186 j o u r n a l o f m a t e r i a l s p r o c e s s i

appropriate bonding parameters were chosen. With theincrease of the ultrasonic power, the thickness of the goldlayer at the center of the metallization was found. This isbecause the interfacial peak stresses occurred right belowthe wedge tool where the thinning is observed.

(3) The interdiffusion between Cu wire and Au layer at thebonding interface during the wedge bonding at ambienttemperature was assumed negligible. The achievement ofcopper wedge bonding was proposed to be the wear actionand mechanical mixing induced by the ultrasonic vibra-tion, and the ultrasonic power contributes to increasingdeformation of the copper wire due to ultrasonic soft-ening which was then followed by the strain hardeningof the copper wedge bond, and the dynamic recovery orrecrystallization of the copper wire caused by ultrasonicvibration during wedge bonding was also found.

(4) Since the Au metallization layer has a good bondability,room temperature wedge bonding of the copper wire isfeasible. However, if the bare Cu or Al substrates are used,it becomes very diffucult. In the future work, differentkinds of the metallization layers should be investigated,and oxidation protection, coated copper wire, as well asthe copper wire with different elongation should also bestudied further.

Acknowledgments

The research work was supported by National Natural ScienceFoundation of China under grant No. 50705021/E052104, MKElectron Ltd. of South Korea and the Development Programfor Outstanding Young Teachers in HIT (Project No.: HIT. 2006:01504489).

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