Effect of Grain Boundary Misorientation on Electromigration ......Effect of Grain Boundary...

Effect of Grain Boundary Misorientation on Electromigrationin Lead-Free Solder Joints

AMANEH TASOOJI,1,4 LETICIA LARA,2,5 and KYUOH LEE3,6

1.—Arizona State University, PO Box 876106 Tempe, AZ 85287-6106, USA. 2.—Honeywell Cor-poration, Phoenix, AZ, USA. 3.—Intel Corporation, Chandler, AZ, USA. 4.—e-mail: [email protected]. 5.—e-mail: [email protected]. 6.—e-mail: [email protected]

Reduction in microelectronic interconnect size gives rise to solder bumpsconsisting of few grains, approaching a single- or bicrystal grain morphologyin C4 bumps. Single grain anisotropy, individual grain orientation, presence ofeasy diffusion paths along grain boundaries, and the increased current densityin these small solder bumps aggravate electromigration. This reduces thereliability of the entire microelectronic system. This paper focuses on elec-tromigration behavior in Pb-free solder, specifically the Sn-0.7 wt.%Cu alloy.We discuss the effects of texture, grain orientation, and grain boundary mis-orientation angle on electromigration (EM) and intermetallic compound for-mation in EM-tested C4 bumps. The detailed electron backscatter diffraction(EBSD) analysis used in this study reveals the greater influence of grainboundary misorientation on solder bump electromigration compared with theeffect associated with individual grain orientation.

Key words: Electromigration, lead-free solder, grain orientation effect, grainboundary misorientation effect, intermetallic compound,C4 bump

INTRODUCTION

The continued form-factor reduction and minia-turization of flip chip controlled collapse chip con-nection (C4) solder bumps and the transition tolead-free solders in support of health, safety, andenvironmental protection have introduced manychallenges in terms of reliability issues in micro-electronics packaging systems. Maintaining theelectromigration (EM) resistance of solder inter-connects is critical to ensure that product integrityand performance are preserved during these tran-sitions. Many investigators have reported thatelectromigration, which involves mass transport ofatoms driven by the combined forces of electric fieldand charge carriers, is influenced by materialchemistry, structure/anisotropy, and microstruc-ture. In addition to grain orientation effects, theincrease in current density caused by the reductionin interconnect size further aggravates the effects of

electromigration. The preferred techniques forstudying how these factors impact EM behaviorinclude orientation imaging microscopy and elec-tron backscatter diffraction (EBSD).1 Industry hasfocused on solder alloy material development andmicrostructure control to overcome this increasedEM susceptibility.

Flip Chip Microelectronics Package Design

Flip chip technology and C4 interconnect designare commonly used in microelectronics systems. C4solder joints provide the signal path between solid-state devices and adjacent levels of electronicspackaging. Schematics of C4 interconnect designconsisting of solder bumps on wettable copper metallayers and the current pathways in a C4 device areshown in Fig. 1.

Lead-Free Solder Issues

Shifting to lead-free solder systems involves newsolder material development, including chemistry

(Received April 15, 2014; accepted July 5, 2014;published online August 5, 2014)

Journal of ELECTRONIC MATERIALS, Vol. 43, No. 12, 2014

DOI: 10.1007/s11664-014-3321-0� 2014 The Minerals, Metals & Materials Society

4386

modification to compositions containing mixtures oftin, silver, copper, etc. Some of the concerns intransitioning from lead-rich to lead-free soldersinclude:

� Increased melting temperature and higher reflowtemperature requirements

� Higher intermetallic compound (IMC) formationat the solder joint, due to the increased dissolu-tion rate and solubility of Cu in solder associatedwith this higher reflow temperature

� Degraded electrical performance due to reducedconductivity with enlargement of IMC

� Reduction in mechanical integrity due to brittleIMC at solder joints

� Higher thermal loading developed during manu-facturing with higher reflow temperature

� Greater coefficient of thermal expansion (CTE)mismatch stress between the substrate and thesilicon die

� Increased surface and interfacial energies, whichcan influence wetting characteristics and increasethe contact angle between solder and copper layer

� Reduced EM resistance associated with increasedcurrent density and current crowding

� Microstructural differences and the presence ofdistributed IMC precipitate phases (e.g., Sn5Cu6

and SnAg3 in Sn-Ag-Cu solder alloy), impactingmaterial behavior

Tin-Based Solder and IMC Crystal Structure

The primary component in most lead-free solders isSn, which has a body-centered tetragonal (BCT) lat-tice structure at temperatures above 13.2�C. Themost and least close-packed directions/planes in tincrystals are [001]/(100) and [110]/(001), respectively,and the active slip system is reported to be(100)[001].2,3 The crystal structure of the Sn5Cu6

IMC which forms in Cu-Sn solder joints is hexagonalclose-packed (HCP), potentially resulting in IMCgrains with basal plane growth front alignment.

Electromigration

Electromigration associated with mass transportdue to applied current is of critical concern inmicroelectronics systems. EM in solder bumps leads

to the formation of voids, hillocks [IMCs formingdue to limited solubility of migrating under bumpmetallurgy (UBM) atoms in solder material], and/orIMC. Such changes affect the mechanical, electrical,and thermal properties of the joint, and can result inpremature failure of the solder connection. Voidsgenerally form at the cathode side, whereas hillocks/IMC atoms accumulate at the anode side. Thesevoid and IMC regions also introduce an impedancemismatch in the interconnects, increasing theresistance and impacting signal integrity and elec-trical performance.3 Lastly, increased levels of IMCformed through the EM process can lead to localembrittlement and premature mechanical failure.4

The reduced current-carrying ability of the solderdue to the formation of IMC and voids not onlyincreases the current density but also introducescurrent stressing/crowding at the contact areabetween the solder and UBM. These factors furtheraggravate electromigration.5

Mechanism of Electromigration and ControllingFactors

Electromigration is described as the mass trans-port of atoms driven by the combined forces ofelectric field and charge carriers. The drifting elec-trons collide with atoms, causing one of the atoms toexchange position with a neighboring vacancy dur-ing current stressing. The flow of energetic electronscan lead to increased atom migration. This effectfurthers the mobility of Cu atoms from UBM into Snsolder, leading to an increased level of IMC growthat the anode side.

Many studies6–8 have shown an increased level ofIMC growth due to electromigration, over andbeyond what is expected from thermally diffusedatoms. The augmented electromigration is due tothe increased movement of atoms when subjected toa current density exceeding a threshold level. Afterstressing for an extended time, atoms in intercon-nects accumulate on the anode side, resulting inUBM dissolution on the cathode side that can leadto open failure with time.

The main conditions for electromigration include:(i) sufficient energy for atoms to overcome the bar-rier for diffusion, with grain boundary migrationdominating at low temperature due to its reducedenergy requirements, and (ii) availability of siteswhich are energetically and geometrically favorable‘‘sinks’’ for the atom to move to. Vacancies, disloca-tion cores, grain boundaries, and voids are potentialsinks or favorable sites for the accumulation ofatoms and IMC formation. Current crowding, Jouleheating, and solder grain orientation also influencethe rate of electromigration.1,5,9,10

Many researchers have investigated the effect ofgrain orientation on electromigration. Kinney et al.1

studied the influence of crystal orientation onintermetallic growth in lead-free single-crystalsolder samples subjected to uniaxial current. The

Fig. 1. Schematic of a flip chip package and current path throughsolder interconnects and trace layers.

Effect of Grain Boundary Misorientation on Electromigration in Lead-Free Solder Joints 4387

EM tests in that study were performed at currentdensities of 10,000 A/cm2 and 11,500 A/cm2, held at100�C for 24 h, 48 h, 96 h, and 120 h. Kinney et al.1

used EBSD orientation maps constructed per ste-reographic projection 2 (SP2) in evaluating thegrain orientation effect. In SP2 the grain orienta-tion is quantified according to the angle that itsc-axis [001] makes with the electron flow direction(the direction perpendicular to the solder/Cu joint).The authors concluded that grains oriented withtheir c-axis [001] closely aligned to the electron flowpath experience significantly higher IMC growthcompared with grains with their c-axis perpendic-ular to the electron flow. They further showed thatthe IMC growth front was nearly planar, exhibitinga constant orientation along the Cu/Sn interface.Other investigators have also reported similarbehavior.11

ELECTROMIGRATION IN FIRST-LEVELINTERCONNECTS (FLIP CHIP C4 BUMPS)

In the study presented here, we demonstrate theeffect of grain orientation on EM behavior of lead-free solder in actual C4 bumps. The applicability ofthe results from simple test configurations reportedby other investigators to more complex configura-tions such as C4 bumps was also evaluated.

Experimental Procedures

The lead-free (Sn-0.7Cu) C4 solder bumps evalu-ated in this study were sampled from first-levelinterconnects (FLIs) in flip chip ball grid array(FCBGA) packages supplied by Intel Corporation.The FLI C4 bumps were encapsulated in a nonme-tallic underfill layer (a commonly used practicein microelectronic packages). The samples wereEM-tested using accelerated life tests (ALTs) at165�C and 800 mA/bump. The test current densitywas estimated to be �24,880 A/cm2. The currentdensity was calculated using the bump pad diame-ter measured on pre-EM-tested end-of-line (EOL)sample bumps. Three sets of bumps from threepackages, totaling 104 bumps (27, 41, and 36 bumpsfrom package numbers 31, 36, and 39, respectively),were evaluated in this study. Package numbers 31,36, and 39 were tested for 227 h, 222 h, and 215 h,respectively.

The current direction during ALT was the samein all cases. Figure 2 demonstrates the current andelectron flow directions with respect to the cathode(substrate side, shown at the bottom of the solderbumps) and the anode (die side, shown at the topside of the solder bump).

Materials Characterization

The C4 bumps were characterized using orienta-tion imaging microscopy, scanning electron micros-copy (SEM), and EBSD analysis. Optical imageswere used to select representative solder samples

for further EBSD analysis. The selection was basedon the severity of EM-induced degradation, noted bythe extent of IMC formation; 23 solders from threepackages were selected for detailed EBSD analysis.

The EBSD sample preparation included brief pol-ishing with 1200 grit SiC paper to improve sampleflatness. Additional polishing steps were done using1-lm alumina followed by 0.3-lm alumina. The finalpolish was done with 0.02-lm colloidal silica. Finally,a carbon coating of approximately 15 A and silverpaint were applied to improve conductivity andreduce the charging effect.

Optical Analysis

We evaluated IMC growth using optical micros-copy and assessed the degree of electromigration inthese three samples. Images were captured for theentire row of solders at 5009 magnification.Figure 3 shows an optical photomicrograph ofsample 31. Individual solders were then selected forEBSD and texture analyses. C4 bumps (and theirlocation in the FLI solder array) selected for theEBSD analysis are shown in Fig. 3. Note that insome cases IMCs are observed in the cathode side inour two-dimensional (2D) picture/cross-section ofthe representative three-dimensional (3D) solderbump. These cathode-side IMCs could be an exten-sion of the IMC starting on the anode away from theplane of cross-section.

Scanning Electron Microscopy and ElectronBackscatter Diffraction Analysis

EBSD maps of representative bumps were col-lected to quantify the grain orientation and its effecton electromigration. High-quality orientation ima-ges were developed for the selected bumps byindexing Kikuchi bands and poles. Reliability of theEBSD analysis was based on confidence index (CI)in excess of 0.7. (CI is a metric with values varyingbetween 0 for low reliability and 1 for highreliability).

Pre- and postpolished images of the solder bumpswere evaluated to confirm that the characteristicsobserved in the original EM-tested bumps weremaintained during the EBSD sample preparationsteps. Comparison of the optical photomicrographwith the SEM secondary electron (SE) image, shownin Fig. 4 for solder 31-2, validated the traceability ofthe features to the original optical photomicro-graphs.

To differentiate the solder material from theintermetallic layers and the copper traces, an ele-mental map was also obtained (Fig. 4c).

Orientation image maps (OIM) were developedusing the EBSD data, in accordance with the SP2approach used in Ref. 1. Recall that in SP2 the grainorientation is identified based on the angle that its[001] c-axis makes with the electron flow direction.Using this approach, grains oriented with theirc-axis parallel to the electron flow (an angle of 0�)

Tasooji, Lara, and Lee4388

are represented by blue color, while grains with [001]perpendicular to the electron flow (an angle of 90�)are red. The EBSD map of solder 31-2 constructed

using this SP2 approach is shown in Fig. 5. Itshould be noted that, in SP2, any and all grains withtheir c-axis perpendicular to the electron/current

Fig. 2. Example solder bump (31-1) showing the electron flow direction in the solder. Electron flow direction is from the cathode side (bottom ofsolder joint) to anode side (top of solder joint). Diffusion of atoms occurs along the same direction as the electron flow, creating voids in thecathode side and IMC at the anode side.

Fig. 3. Optical images for solder array in sample 31.

Fig. 4. Optical image (a), SE SEM image (b), and elemental map (c) of solder bump 31-2.


flow (c-axis residing in the plane of the solder joint)appear as red. In other words, all [uw0]-orientedgrains appear red in SP2 (Fig. 5).

RESULTS

OIM data for sample 31 are presented in Fig. 6.Figure 7 shows representative grain mapping datagenerated for HCP-structured IMC layers inselected solders. Key observations from the EBSDresults are highlighted below. It should be notedthat the reported results are based on data fromtwo-dimensional cross-sections of the samples.

1. No significant IMC growth was noted within anysingle grain (e.g., no IMC region surrounded bysame-colored grain);

2. The observed IMCs in all cases resided at grainboundaries, neighboring grains with differentorientations on two sides;

3. Low-angle grain boundaries (adjacent grainswith similar orientations) did not show anyIMC (e.g., solders 31-7 and 31-10);

4. Solders 31-15, 36-10, and 39-26 were the onlysolders with blue grains (grain orientation mostsusceptible to EM). Only 3 out of 23 soldersstudied via EBSD contained [001] grain orienta-tion along the current flow direction. However,green grains were more abundant;

5. The majority of IMC grains had preferred orien-tation with the basal plane (close-packed plane ofHCP structure) perpendicular to the electronflow;

6. IMC-rich regions were noted on both the die(anode) and package (cathode) sides. Since IMCis expected to form on the anode side, it istheorized that the IMCs in the package/cathodeside are the extension of IMC formed in the die/anode side away from the plane of view. Future3D tomography is recommended to validate thishypothesis.

DISCUSSION

One of the objectives of this study is to quantifythe reported ‘‘preferred’’ grain orientation effect on

EM in actual C4 bumps. OIM of 23 EM-tested C4solder bumps in this study did not show any sig-nificant preferential IMC growth in individualgrains to validate ‘‘preferred’’ grain orientationpromoting EM and IMC formation during ALT (seepoint 1 in ‘‘Results’’ section).

The critical finding of this study is the effect ofgrain boundary misorientation angle on electromi-gration in C4 bumps. A direct correlation betweenthe severity of EM and grain boundary misorienta-tion angle was noted (see point 2 in ‘‘Results’’section).

The number of grains associated with the mostEM-susceptible orientation (blue grains) was verylimited. The authors propose further studies toinvestigate: (i) low probability for nucleation/growthof such grains at reflow, and (ii) consumption ofsuch grains during the EM tests.

Detailed discussion on the observed behaviors isprovided in the following sections.

Effect of Crystal Packing on Electromigrationin Single-Crystal Solders

Using planar density calculations, we can explainthe observed orientation effect on EM in single-crystal solder. Table I shows that, for single crystalsand in the absence of defects (such as grain bound-ary and dislocation), a grain oriented with its least-packed plane (001) aligned with the IMC growthfront will have a more open structure for latticediffusion and migrating atoms, hence facilitatingEM and IMC growth. Interestingly, this high EMgrain orientation is that represented by the ‘‘blue’’designation, hence explaining the reported ‘‘pre-ferred’’ orientation effects discussed previously.

Effect of Grain Misorientation on IMCFormation and Electromigration

This study revealed the importance of grainboundary for electromigration. In this section, theeffect of grain boundary misorientation angle on theextent of IMC growth is quantified. The misorien-tation angle data were determined via EBSD analysis

Fig. 5. SP2 map reflecting the [001] orientation of the noted grains w.r.t. the current flow direction (current crowding not considered).


Fig. 6. SP2 maps for selected solder bumps in sample 31.

Fig. 7. Representative solders showing HCP crystal orientation w.r.t. basal plane [0001]. Most IMC grains have their close-packed basal planesperpendicular to the electron flow direction.

Table I. Correlation between crystal planar density and reported EM in single-crystal solder

Orientation Color Planar Density (atoms/nm2) Electromigration

(001) Blue 2.9 High(011) and (101) Green 5.2 Medium–high(110) Red 7.6 Low(100) and (010) Red 9.7 Low


(see Fig. 8, showing grain boundary misorientationangles in sample 31-2).

The IMC fraction in bulk solder was measuredusing OIM software on the elemental maps. Thesedata were then plotted against the correspondingmisorientation angles. The data clearly show thatthe large misorientation angles (15� to 53�) displaythe most IMC formation, while the solders with low-angle grain boundaries exhibit minimal IMC growth(Fig. 9). This behavior is expected, as the increase inmisorientation angle provides a more open site foratom migration and a more suitable sink for IMCduring EM testing.

The scatter in the data in Fig. 9 is believed to beassociated with the fact that %IMC values werebased on measurement from 2D cross-sections ofsolders. IMC data using 3D tomography are rec-ommended for improved accuracy. As mentionedabove, the IMC data denoted ‘‘cathode side’’ arebelieved to be associated with extension of 3D IMCoriginating on the ‘‘anode side’’ away from the cross-sectioned plane of view.

Our data clearly reveal that the majority of theIMC growth evolved from grain boundaries, and thegrain boundary diffusion effect dominates the EMprocess and atom migration. This effect of misori-entation angle on diffusion rate is similar to thebehavior shown previously for other materials(Fig. 10). A large misorientation angle betweenadjacent grains promotes a higher rate of diffusionalong the boundary. As the misorientation drops tozero, the diffusion coefficient in the boundary dropsto that of lattice diffusion (Fig. 10a).12 The misori-entation angle’s effect on diffusivity is due to thefact that the grain boundary surface energy (cG) isinfluenced by this angle (Fig. 10b).13 The EM trendnoted in Fig. 9 correlates well with the trend of themisorientation angle’s effect on the grain boundarysurface energy and the diffusion rate, shown inFig. 10a and b, respectively.

Solder diffusivity data were evaluated for furtheranalysis of these observed behaviors. Table II sum-marizes the reported data.1,11,14 Diffusivity valuesreported by Dyson et al.11 and Kinney et al.1 cor-respond to lattice diffusion since the solder samplesused in their studies were primarily single crystals,and the IMC growth was observed within the grain.The effect of grain orientation on lattice diffusion isreflected in the Dlattice column in this table. These

Fig. 8. Misorientation angles measured between different grains insolder 31-2.

Fig. 9. Effect of grain boundary misorientation on IMC. Note that %IMC data are based on 2D cross-sections; IMC data using 3D tomography arerecommended for improved accuracy. The IMC data denoted ‘‘cathode side’’ are believed to be associated with extension of 3D IMC originatingon the anode side away from the cross-sectioned plane of view.


data show a factor of 659 to 5009 higher latticediffusion along the c-axis compared with that per-pendicular to the c-axis.1,11 The large scatter in thereported lattice diffusivity ratio data could be asso-ciated with differences in the secondary orientationw.r.t. the current trajectory in the two studies (alldirections in the {001} plane being perpendicular tothe c-axis).

The grain boundary diffusivity data reported byLi and Basaran14 are shown in the Dgb column ofTable II. A significantly higher diffusivity along thegrain boundary (six orders of magnitude) comparedwith the lattice diffusivity is reported in Ref. 11.

Based on these data,1,11,14 one would expect theGB diffusion to be at least three orders of magnitudehigher than lattice diffusion along any directionwithin the grain. Therefore, for multigrained solder,GB diffusion will be the dominant mechanism forEM. This could explain why we did not observe theindividual grain orientation effect as observed byKinney et al. in their single-crystal solder study.

Low Occurrence of EM-Susceptible Grainsin C4 Array Bumps

The scarcity of the most EM-susceptible (blue)grains in this study was puzzling. The authors pro-pose further studies and EBSD analysis of EOLsamples to investigate the possibility of completeconsumption of such grains during the EM tests and/or potential preferential grain growth resulting uponsolidification from reflow (i.e., lower probability of

blue grain nucleation and growth). The statisticaldistribution of the blue grains in EOL and EM-testedsolders should provide valuable insight in validatingthese hypotheses. Such study will test the grainconsumption hypothesis and also shed light on po-tential preferential stress-assisted grain nucleationand growth occurring during solder solidification.Variation in CTE-induced thermal stresses devel-oped during reflow at different C4 solder locations[with varying distance from neutral points (DNP)]may impact grain nucleation in these solders.

CONCLUSIONS

Electromigration has proven to be a critical reli-ability issue in Pb-free solder joints. The high cur-rent densities in miniaturized solder bumps, thetemperatures to which the bumps are exposed dur-ing processing and application of microelectronicsystems, solder composition, microstructure, andgrain orientation greatly impact EM in theseinterconnects. The effect of microstructure and ori-entation on electromigration in C4 bumps was themain focus in this study, and the current density,temperature, and solder composition were similarfor all samples tested.

This study showed a significant effect of grainboundary misorientation angle on electromigrationin C4 bumps. For the ALT duration of this study,minimal/no EM was observed on any single soldergrain, and the ‘‘preferred’’ orientation effectreported by others was not evident in C4 bumps.Grain orientations in C4 bumps varied significantly,ranging from the [001] direction of the grain ori-ented along the current flow (blue grains) to thatperpendicular to the current flow (reflected as redgrains). A statistically lower number of grains withc-axis parallel to the e� flow, associated with thehighest lattice diffusivity, was present in the 23 C4bumps studied via EBSD compared with other grainorientations.

The amount of IMC on the cross-sectioned surfaceof EM-tested solders was determined. ExtensiveIMC formation was noted on some of the EM-tested

Fig. 10. Effect of grain boundary misorientation angle on (a) diffusion of atoms along grain boundaries (normalized to lattice diffusion), and (b) ongrain boundary energy (Cu at 1338 K, normalized to free surface energy cS).

Table II. Diffusivity ratios (using data fromRefs. 1, 11, 14) comparing lattice and grainboundary diffusivities

Study Dlattice Dgb

Dyson et al.11 DkcD?c¼ 500 –

Kinney et al.1DkcD?c¼ 65 –

Li and Basaran14 –Dgb

Dl> 106


C4 solder bumps (in excess of 40% IMC layers con-suming the solder in samples 31-25 and 39-22).

EBSD analysis was used in determining the grainorientations, as well as grain boundary misorien-tation angles. High-angle grain boundaries werenoted to be the most dominant path for diffusionand IMC formation in C4 bumps. TransgranularIMC and the lattice orientation effect reported byothers1 under idealized single-crystal testing werenot observed during the course of this study. Thegrain orientation effect reported by other investi-gators and the observations from this study wereexplained using lattice and grain boundary diffu-sivity data. The observed trend for the effect of grainboundary misorientation on EM and IMC formationwas explained through the grain boundary surfaceenergy and diffusion data found in literature.

Even though an individual grain orientationeffect on EM was not evident in this study, we doexpect the anisotropic properties in EM to act forsingle-grained BCT tin solder, in the absence ofgrain boundary.

FUTURE WORK

The extent of IMC formation and the severity ofEM in C4 bumps should be further quantified using3D x-ray tomography and/or serial sectioning inconjunction with EBSD analysis. This approach willprovide valuable data on the IMC distribution andthe misorientation angles involved throughout thesolder volume. Such analysis should be helpful inexplaining the cathode-side IMCs noted on the 2Dcross-sectional views in this study. In addition,EBSD analysis of EOL samples is recommended toobtain statistical data on the number of blue grainsin untested conditions. This will provide informa-tion on whether blue grains are consumed duringEM testing and/or whether package processinginfluences bump grain orientation.

Electromigration testing of controlled bicrystalsolder samples, with various degrees of misorien-tation angles, is recommended in developing a moreaccurate analytical model.

Solder alloy development and processing methodsto control the grain boundary diffusivity and grain

orientation should be further investigated. Additionof alloying elements to reduce grain boundary dif-fusion, and developing a solidification process toachieve preferred single-crystal solder orientation(and/or limiting secondary grain formation to low-angle boundaries), are potential improvementmethods in managing electromigration.

ACKNOWLEDGEMENTS

The authors would like to recognize Intel Corpo-ration’s support for this research and their dedica-tion to higher education. Special thanks go to RonWitt from EBSD Analytical for his assistance withour EBSD analysis. We acknowledge the use offacilities within the Center for Solid State Science atArizona State University. Ms. Leticia Lara isgrateful to her academic advisor Prof. AmanehTasooji, and to her industry mentor Dr. Kyuoh Leefor their dedicated support and guidance during herstudies at ASU.

REFERENCES

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3. T. Bieler and T.-K. Lee, Encyclopedia of Materials: Scienceand Technology (2010) pp. 1–12.

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5. G.A. Rinne, Microelectron. Reliab. 43, 1975–1980 (2003).6. S.-H. Chae, B. Chao, X. Zhang, J. Im, and P.S. Ho, Electronic

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7. B. Chao, S.-H. Chae, X. Zhang, K.-H. Lu, J. Im, and P. Ho,Acta Mater. 55, 2805–2814 (2007).

8. C.C. Lee, P.J. Wang, and J.S. Kim, Electronic Componentsand Technology Conference (2007).

9. K.N. Chian, C.C. Lee, C.C. Lee, and K.M. Chen, Appl. Phys.88, 072102 (2006).

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