En

FSa

b

c

a

ARR2AA

KENGA

1

eld(abCpid

sdie

e

h1

Journal of Materials Science & Technology 34 (2018) 1885–1890

Contents lists available at ScienceDirect

Journal of Materials Science & Technology

j o ur nal homepage: www.jm s t .org

lectrodeposition and growth mechanism of preferentially orientatedanotwinned Cu on silicon wafer substrate

u-Long Suna,b, Li-Yin Gaoa,b, Zhi-Quan Liua,b,c,∗, Hao Zhangc, Tohru Sugaharac,hijo Nagaoc, Katsuaki Suganumac

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, ChinaUniversity of Chinese Academy of Sciences, Beijing 100049, ChinaInstitute of Scientific and Industrial Research, Osaka University, Osaka 5670047, Japan

r t i c l e i n f o

rticle history:eceived 9 November 2017eceived in revised form0 December 2017ccepted 22 December 2017vailable online 31 January 2018

a b s t r a c t

Homogeneous columnar Cu film with fully embedded nanotwins was successfully fabricated on Ti/Cuseed layer on silicon wafer. The nanotwins with thickness of tens of nanometers are generally parallelto the silicon surface, showing a strong (111) preferred orientation. The acid concentration was foundto be important in influencing the formation of nanoscale twins. By adjusting the acid concentration,the nanotwins can be induced from the top columnar grain to middle columnar grain and reach thebottom equiaxed grain, and a microstructural transformation model was given. A theory focusing on the

eywords:lectrodepositionanotwinned Curowth mechanismcid adsorption

cathode overpotential was proposed to reveal the effect of acid concentration on the growth mechanismof nanoscale twins. An appropriate adsorption proportion of hydrogen on cathode (acid concentration17 ml L−1) could increase the overpotential which supplies adequate nucleation energy for nanoscaletwins formation.

© 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science &Technology.

. Introduction

Cu has been widely used as an interconnect material in micro-lectronic industries [1–5]. At ambient temperature, it exhibitsow electric resistivity (1.75 �W cm) and superior thermal con-uctivity (401 W m−1 K−1) compared with other materials, like Al2.83 �� cm, 217 W m−1 K−1). Thus, Cu has been adopted in manyspects in microelectronic packaging industries, such as wiringonding material, under-bump-metallization (UBM) material, etc.u UBM is commonly utilized in electronic products like com-uters and mobile phones, to form stable electric and mechanical

nterconnection, which is usually produced by direct-current (DC)eposition method.

At present, with packaging density soaring and interconnectize declining continually, there are increasing temperature gra-

ient, stress intensity and electric current density applied to Cu

nterconnect, which requests Cu interconnect to own better prop-rties like good resistance to electromigration and we have done

∗ Corresponding author at: Institute of Metal Research, Chinese Academy of Sci-nces, Shenyang 110016, China.

E-mail address: zqliu@imr.ac.cn (Z.-Q. Liu).

ttps://doi.org/10.1016/j.jmst.2018.01.016005-0302/© 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of

some research previously on how to improve the interfacial relia-bility [6–11]. Here, the reliability performance of Cu interconnectis thought to have a close relationship with its microstructure, tex-ture and crystalline orientation. Thus, modifying the metallurgicalstructure of Cu interconnect to meet the steadily rising demand ofhigh reliability performance is an increasingly serious issue for themicroelectronic packaging industries.

Nanotwinned Cu owns ultra-high strength and superior con-ductivity, winning great attraction of researchers [12–17]. Up tonow, the mechanical property and deformation mechanism of nan-otwinned material have been studied extensively [18–20], while itsreliability performance as interconnect material is very few. Chenet al. used electro beam evaporation to fabricate nanotwinned Cuand found that by introducing nanometer-scale twin defects into Cugrains, the grain-boundary structure and atomic-diffusion behavioralong the boundary were changed [21]. The electromigration-induced atomic diffusion in the twin-modified grain boundarieswas slowed down by one magnitude. It was also reported thatwhen using columnar nanotwinned Cu as UBM material to reflow

with solder, unidirectional intermetallic compound (IMC, Cu6Sn5)growth along (0001) direction was found [12,22]. It implies that

Materials Science & Technology.

1886 F.-L. Sun et al. / Journal of Materials Science & Technology 34 (2018) 1885–1890

F film e4 ark-fie d lines

nl

mrapclietcodne

patcnTaoeia

2

G

ig. 1. (a) Cross-sectional SEM image of preferentially orientated nanotwinned Cu00 nm-thick Cu seed layer (substrate) and plated nanotwinned Cu (NT-Cu), (c) dlectron diffraction pattern in which diffraction spots of matrix are denoted by soli

anotwinned Cu would be a promising candidate to solve the prob-ems derived from the soaring packaging density.

For realistic application, it is necessary to find out the growthechanism of nanoscale twins. Xu et al. [23] and Liao et al. [24] fab-

icated Cu film with twins using pulsed electrodeposition methodnd made an attempt to illuminate the twin formation by first-rinciple calculation method. The cyclical charge during on-timeould produce a high tensile stress, and stress relaxation in off-timeeads to twin growth. Probably this could account for the conditionn pulsed current plating because the forward electricity was highnough to generate high stress in film. Mahajan [25] regarded thathe stacking faults derived from high arrival rates of Cu atoms onathode could be the reason for twin formation. But the explanationnly illustrated the overall growth condition, like physical vaporeposition (PVD), chemical vapor deposition (CVD) and deposition,o detailed information was included quantitatively to clarify thelectrodeposition process.

In this work, we report the detailed direct-current electrode-osition process of preferentially orientated nanotwinned Cu. Thecid concentration was found to greatly influence the nanoscalewin growth in plated Cu film. With a reasonable value of acidoncentration, columnar grained Cu with preferentially orientatedanoscale twins could be fabricated on Ti/Cu seed layer on Si wafer.he mechanism of twin boundary formation being dependent oncid concentration was proposed to get a better understandingn the microstructure change. From the perspective of interfacialnergy, the formation sequence of three kinds of interface, includ-ng straight columnar grain boundaries, coherent twin boundariesnd curved equiaxed grain boundaries, could be classified.

. Experimental

A direct-current power supply (KEITHLEY 2200-20-5 PRO-RAMMABLE POWER SUPPLY) was used to electrodeposit

lectrodeposited with 30 mA cm−2 and 3 ml L−1 acid, (b) XRD pattern of wafer witheld TEM image of nanoscale twins (GD: growth direction) and (d) corresponding

and those of twins are denoted by dashed lines.

columnar-grained nanotwinned Cu film on Ti/Cu seed layer of sin-gle crystal Si wafer in [100] orientation. The thickness of Ti and Cuseed layers are 100 nm and 400 nm respectively. They were sequen-tially sputtered on the silicon wafer. The exposed area of waferfor Cu plating was 1 cm × 1 cm. Before each experiment, the wafersurface was acidic rinsed to remove the contamination on cath-ode surface, exposing the fresh metal for deposition. The anodewas a highly purified (99.995 wt%) electrolytic Cu sheet. The arearatio of anode to cathode was set to be 10 so as to improve thethrowing power of cathodic electricity. The basic plating bath wasmade up of 0.8 mol L−1 Cu cations and different amount of sulfuricacid (3 ml L−1 to 31 ml L−1), while 40 ppm NaCl and gelatin wereadded in the electrolyte in order to control the Cu electrodeposi-tion process and form preferential crystalline orientation. The highconcentration of sulfuric acid combining with appropriate amountof gelatin additive used in the bath was not reported elsewhere.The plating experiment was conducted at room temperature witha current density of 30 mA cm−2 and lasted for 2 h every time. Thewhole plating process was conducted in a 500 ml PP (Polypropy-lene) tank, which has a good resistance to acidic solution. For eachexperiment, a fresh bath was prepared to make sure the consis-tency of each plating process. A rotating magnetic bar was utilizedto agitate the electrolyte (300 rpm), maintaining the uniform con-centration of ingredients in the bath, which is quite different fromthe high convection condition reported in Ref. [12,22]. High stir-ring rate of the bath would generate bubbles adsorbed on cathodesurface to disturb Cu atom deposition, and produce a mixed andnonuniform twin structure. Yet our bath with low stirring rate couldguarantee the uniformity of growth twinning structure repeatedly.

The crystalline orientation of electrodeposited film was identi-

fied using a high-resolution X-ray diffractometer (XRD, Bruker D8Advance X-ray Diffractometer). After mechanically polishing of thecross section of Cu film, precision ion etching (Leica RES101) wasused to display the microstructure. Observation of cross-sectional

cience & Technology 34 (2018) 1885–1890 1887

stita

3

f3FogAthta7istttpetsostndTtstp

dmrfiwpeaiiadtgioCtsbfctd

Fig. 2. Cross-sectional bright-field TEM images of plated Cu from (a) bottom part

F.-L. Sun et al. / Journal of Materials S

tructure of electroplated Cu film was conducted in a scanning elec-ron microscope (SEM, FEI Quanta 600) with backscattered electronmaging. Finer characterization of Cu film was performed using aransmission electron microscope (TEM, JEOL JEM-2100) with anccelerating voltage of 200 kV.

. Results and discussion

The preferentially orientated nanotwinned Cu was successfullyabricated on Ti/Cu seed layer on Si wafer in a current density of0 mA cm−2 with 3 ml L−1 sulfuric acid in the bath as shown in Fig. 1.ig. 1(a) shows the cross-sectional metallurgical structure of nan-twinned Cu film deposited on wafer. At the early stage of filmrowth as marked in area A, the grain was too small to figure out.fterwards, it transformed into columnar shape but without much

win lamella as marked in area B, whose average grain size in theorizontal direction was about 6.8 �m, being significantly largerhan that near the substrate side. At the last stage of film growths marked in area C, the lateral crystalline size increased to about.9 �m wide, and densely packed parallel twin lamella appeared

n each grain. It could be seen that nanotwinned Cu film has beenuccessfully fabricated on Ti/Cu seed layer on Si wafer, althoughhe high density of twins only formed on the film surface ratherhan in the whole cross section. XRD pattern in Fig. 1(b) showshat the plated columnar nanotwinned Cu film has a strong (111)referred crystalline orientation. This could be explained consid-ring that coherent parallel twin boundaries were primarily (111)ype in faced-centered cubic Cu, and the surface of columnar grainshould also be (111) orientated planes. There is a difference in termsf XRD peak intensity between nanotwinned Cu and sputtered Cueed layer, which probably is due to the fact that in magnetic sput-ering Cu atoms are more prone to deposit and transport to theormal lattice sites and locating on (111) planes, the most highlyense packed planes, own the least system energy. The dark-fieldEM image in Fig. 1(c) confirms that nanoscale twins formed inhe columnar grain. By tilting the zone axis to <110> direction ashown in Fig. 1(d), the twin boundaries will be edge-on and thewinning (111) plane could be observed, verifying the formation ofreferentially orientated nanoscale Cu twins.

In order to characterize the detailed microstructure evolutionuring the film growth, finer TEM analysis was conducted on thearked A, B and C positions in Fig. 1(a), which are shown in Fig. 2,

espectively. At the Ti/Cu seed layer side in Fig. 2(a), the depositedlm consisted small equiaxed grains with size of around 100 nm,hich is similar with that of Cu seed layer. The grain boundary of thelated Cu film originated from that of the Cu seed layer, which couldxplain the similar preferred (111) orientation between the filmnd the substrate. With the film thickness growing continually, thenfluence of Ti/Cu seed layer on the plated microstructure becamencreasingly weak, while the plating bath conditions became toffect the film growth. The cross-sectional TEM images in the mid-le of Cu foil (Fig. 2(b)) and at the surface side (Fig. 2(c)) showshe same microstructural transformation from columnar twin-freerain to columnar nanotwinned grain, as what has been observedn SEM. It was reported by You et al. [26,27] that using Ti sheetr amorphous Ni-P film as cathode, the microstructure of platedu near the substrate might exhibit differently in comparison withhat in the middle of the film. Liu et al. [28] observed that the tran-ition layer consisting nanoscale grains without twins might existetween plated Cu film and the Ti/Cu seed layer. The results derived

rom our experimental observation and literatures seemed to beonsistent with the classic film growth theory [29]. It was supposedhat in case of homogeneous epitaxy, strain energy will accumulateuring film growth and when the film thickness reached to a cer-

near substrate, (b) middle part of film and (c) top part near surface. The label at theupper left corner corresponds to those marked in Fig. 1(a).

tain value, it could be released by forming lattice defects, like a highdensity of twin lamellas.

According to the above theory, it would be difficult for nanoscaletwin lamella to occur from the Ti/Cu seed layer directly. However,in order to facilitate its application as an interconnect material, it isessential to promote nanoscale twins to grow from the seed layercontinuously. For instance, the thickness of direct-current platedCu UBM film could range from 5 to 50 �m, thus the uniform struc-ture along the growth direction should be emphasized to ensureits reliability performance equally [22]. Interestingly, some smallcolumnar crystals (grain size = 344 nm) with nanoscale twins (twinthickness = 2–60 nm) were observed within the bottom film nearthe Ti/Cu seed layer side. It is expected that columnar nanotwinnedCu grain with homogeneous microstructure in the whole cross sec-tion of film could grow from Ti/Cu seed layer. So the key factorinfluencing nanoscale twin formation was investigated further.

The sulfuric acid combining with gelatin was found playing acritical role in the growth of preferentially orientated nanotwinnedCu from the Ti/Cu seed layer. Fig. 3 shows the cross-sectional SEM

1888 F.-L. Sun et al. / Journal of Materials Science & Technology 34 (2018) 1885–1890

F 3 ml Lt

iatb(eigfbtsaCfispotta

ppspi[alotttnti

ig. 3. Microstructures of Cu film plated at different concentration of H2SO4: (a)

hickness from Cu film in Fig. 3(c).

mages for Cu films deposited at various concentration of sulfuriccid in a current density of 30 mA cm−2. After the acid concen-ration rose from 3 ml L−1 to 10 ml L−1, parallel nanoscale twinsegan to form in the columnar Cu grains as shown in Fig. 3(a) andb). Afterwards, with increasing acid percentage in the bath, thequiaxed grain zone at the Ti/Cu seed layer side became to van-sh, and the regularity of nanotwinned Cu grain was enhanced. Therowth direction of columnar grains is vertical to the wafer sur-ace, and nanotwinned Cu grain inclined to be formed from theottom part. With 17 ml L−1 of acid concentration, densely packedwin lamellas in parallel with the substrate were formed from theubstrate side as shown in Fig. 3(c). Further addition of sulfuriccid in the electrolyte could decrease the regularity of columnaru grains, and lead to the formation of equiaxed grains within thelm as shown in Fig. 3(d) and (e), although nanoscale twins couldtill be seen in the columnar Cu grains. The cross sectional sam-le of plated Cu film with the acid concentration of 17 ml L−1 wasbserved in TEM to identify the twin density. Measurement of twinhickness along [110] zone axis is shown in Fig. 3(f), in which thewin thickness ranges from several nanometer to 140 nm with anverage of 22 nm by Gaussian distribution fitting.

The mechanism of nanotwin formation is still not fully known,artly due to the difficulty of direct observation of growing twins inlated Cu film. Xu et al. [23] and Jin et al. [30] focused on the internaltress to explain the formation of this nanostructured material. Theroduction and relaxation of high internal stress during pulsed plat-

ng was regarded as the main reason for nanoscale twin formation23]. But Jin et al. [30] supposed that continuous deposition of Cutoms at lower direct current density could also result in high stressevel, leading to high density of twins. If the formation possibilityf nanoscale twin is in proportion to internal stress, adding addi-ive that could produce high film stress should lead to nanoscalewin formation, which is not the case as reported [31]. The rup-ure of plated film happened when high stress existed inside, but

anoscale twins were not found in these films [30]. Thus, nanoscalewin formation could not be interpreted from the perspective ofnternal stress solely.

−1; (b) 10 ml L−1; (c) 17 ml L−1; (d) 24 ml L−1; (e) 31 ml L−1; (f) histogram of twin

On the other hand, Winand [32,33] introduced a diagram ofmain types of electrodeposits according to the film microstruc-ture as a function of two main parameters: current density andinhibition concentration. By enhancing the parameter value, thedeposits could transform from basis-oriented reproduction (BR)type to field-oriented texture (FT) type, which was quite similarwith the microstructure evolution in our case. An example using BR,FT, etc. structure to describe the microstructure effect of additivesin Cu electrodeposition could be seen in Kelly’s work [34]. Here,hydrogen ions and gelatin were supposed to change the microstruc-ture type by adjusting the cathodic overpotential. Hydrogen ionswere regarded as weak inhibitors and gelatin additive was a strongone by Winand [32,33], and adsorption of hydrogen ions and gelatinmolecules on cathode could raise the cathodic overpotential, whichwas the driven force for nucleation and growth of deposits. Yetthese inhibitors function differently in deposition. According toour investigation, gelation is essential in our solution to get nan-otwin structure, which should be combined with adequate amountof sulfuric acid. It is believed that gelation can enhance the effect ofacid to increase the cathodic overpotential for nanotwin formation.And the gently changing overpotential with respect to sulfuric acidconcentration could supply varying level of energy to form varioustypes of electrodeposits. As the cathodic overpotential value rosewith enhancing the acid concentration from 3 ml L−1 to 31 ml L−1,the microstructure of plated Cu film evolved from mixture ofcolumnar twin-free grain (in the middle of film) and equiaxed grain(near the Ti/Cu substrate), to columnar nanotwinned Cu grain (inthe whole cross section of Cu film), and finally to mixed structure ofcolumnar nanotwinned Cu and equiaxed grain(both are randomlydistributed in film), which is summarized and illustrated in Fig. 4.

For columnar Cu grain without twins, the grain boundary wasstraight and its energy was comparably lower than that of equiaxedgrain boundary. Thus, when the acid concentration was compara-tively low (3 ml L−1) and the cathodic overpotential was not so high,

the microstructure was in columnar grain shape and twin lamellawas only observed at the top surface side where the Cu seed layerdid not dominate the structure. On the other hand, the equiaxed

F.-L. Sun et al. / Journal of Materials Science

Fo

g

otmsissTsttfehafcnobdcntw

t(Sotntffio(a

4

mTund

[[[

[[[[[[[[

[

[

[

[[

ig. 4. Schematic illustration of microstructural change of plated Cu vs overpotentialr acid concentration.

rain at the Ti/Cu seed layer side was derived from strong influence

f Cu seed layer. Because the incoherent grain boundary energy wasen times higher than the coherent twin fault energy, twin lamella

ay form ahead of equiaxed grain boundaries when increasing theulfuric acid amount in the plating bath. When acid concentrationncreased from 10 ml L−1 to 17 ml L−1, equiaxed grains at the sub-trate side disappeared gradually, meaning that the influence of Cueed layer on nucleation and growth of Cu deposits was suppressed.he adsorption of sulfuric acid (17 ml L−1) on cathode surface wastrong enough so that adequate amount of energy was suppliedo form a high density of twin boundary in columnar grains fromhe substrate side (Fig. 3(c)). It was supposed that the film wouldorm twin boundary to reduce the total energy of system until thenergy supplied during electrodeposition was sufficient to formigh-angel grain boundary. In other words, it means that as acidmount was high enough, incoherent equiaxed grain boundary mayorm instead of coherent twin boundary. Therefore, when the acidoncentration was beyond 17 ml L−1, a mixed structure of columnaranotwinned Cu grain and equiaxed grain formed. The percentagef equiaxed grain increased with acid concentration, which coulde seen when comparing the cross-sectional microstructures ofeposits in Fig. 3(d) (24 ml L−1) and Fig. 3(e) (31 ml L−1). Thus, itould be derived from the results that the percentage of colum-ar grained Cu with nanoscale twins increased initially, reachedhe maximum value at 17 ml L−1 of acid and decreased afterwardsith respect to the sulfuric acid amount in the electrolyte.

With a low convection condition, combining the effect of gela-ion and sulfuric acid, we successfully fabricated homogeneous111)-orientated columnar nanotwinned Cu on Ti/Cu seed layer ofi wafer using direct-current deposition method. The uniform nan-twinned Cu structure across the whole film from the bottom to theop was firstly reported. In each columnar grain, (111)-orientatedanoscale twins grow from the substrate directly and no transi-ion layer is found near the substrate, which is completely differentrom those reported in Ref. [12,22,23,35]. Such a homogeneous Culm with fully embedded nanoscale twins formed preferentiallyn Si wafer, can be used as UBM [36] or filler of through silicon viaTSV) [37] in electronic packaging, which will be further studiednd reported later.

. Conclusion

We reported here a detailed direct-current electrodepositionethod to fabricate preferentially orientated nanotwinned Cu on

i/Cu seed layer on silicon wafer. Generally the deposit was madep of equiaxed crystals in small size (grain size about 100 nm)ear the bottom substrate, columnar twin-free grains in the mid-le (lateral size about 6.8 �m) and columnar nanotwinned grains

[[[[

& Technology 34 (2018) 1885–1890 1889

near the top surface (lateral size about 7.9 �m). A preferred (111)orientation was observed in the columnar Cu film, with whichthe nanotwins are parallel to the substrate surface with an aver-age twin thickness of 22 nm. Gelation is essential to get nanotwinstructure, which should be combined with adequate amount ofsulfuric acid. It is believed that gelation can enhance the effectof acid to increase the cathodic overpotential for nanotwin for-mation. The sulfuric acid was found playing an important rolein (111)-orientated coherent nanotwin formation. By switchingthe acid proportion in the plating solution, the growth mode ofdeposit changes from columnar twin-free grain to columnar nan-otwinned grain with the mixture of equiaxed grain. With an acidconcentration of 17 ml L−1, homogenous columnar Cu film withfully embedded nanotwins was successfully fabricated from thebottom substrate to the top surface. It was supposed that cathodicoverpotential localized in each (111)-orientated grain could beimproved gently by hydrogen adsorption from acid on the cathodesurface. These overpotential changes can provide different nucle-ation energies for the formation of different boundaries, includinglow angle columnar grain boundaries, parallel coherent nanotwinboundaries and high angle incoherent equiaxed grain boundaries.In total, sulfuric acid combining with gelatin was the main reasonof forming (111)-orientated columnar-grained twinning structure.This work sheds light on the growth mechanism of electrodepositednanoscale twins, which will facilitate its practical application inelectronic packaging.

Acknowledgements

This work was financially supported by the National Natural Sci-ence Foundation of China (No. 51401218) and the Osaka UniversityVisiting Scholar Program (No. J135104902).

References

[1] C.K. Hu, J.M.E. Harper, Mater. Chem. Phys. 52 (1998) 5–16.[2] R. Rosenberg, D.C. Edelstein, C.K. Hu, K.P. Rodbell, Annu. Rev. Mater. Sci. 30

(2000) 229–262.[3] C.V. Thompson, J.R. Lloyd, MRS Bull. 18 (2013) 19–25.[4] P.S. Ho, T. Kwok, Rep. Prog. Phys. 52 (1989) 301.[5] D.C. Edelstein, G.A. Sai-Halasz, Y.J. Mii, IBM J. Res. Dev. 39 (1995) 383–401.[6] H. Zhang, Q.S. Zhu, Z.Q. Liu, L. Zhang, H. Guo, C.M. Lai, J. Mater. Sci. Technol. 30

(2014) 928–933.[7] X.Y. Pang, Z.Q. Liu, S.Q. Wang, J.K. Shang, J. Mater. Sci. Technol. 26 (2010)

1057–1062.[8] L.Y. Gao, H. Zhang, C.F. Li, J. Guo, Z.Q. Liu, J. Mater. Sci. Technol. 34 (2018)

1305–1314.[9] H. Zhang, D. Wu, L. Zhang, Z.Z. Duan, C.M. Lai, Z.Q. Liu, Acta Metall. 48 (2012)

1273–1280.10] L.Y. Gao, C.F. Li, P. Wan, Z.Q. Liu, J. Mater. Sci. 28 (2017) 8537–8545.11] L.Y. Gao, Z.Q. Liu, C.F. Li, J. Electron. Mater. 46 (2017) 5338–5348.12] H.Y. Hsiao, C.M. Liu, H.W. Lin, T.C. Liu, C.L. Lu, Y.S. Huang, C. Chen, K.N. Tu,

Science 336 (2012) 1007–1010.13] L. Lu, Y.F. Shen, X.H. Chen, L.H. Qian, K. Lu, Science 304 (2004) 422–426.14] L. Lu, X. Chen, X. Huang, K. Lu, Science 323 (2009) 607–610.15] K. Lu, L. Lu, S. Suresh, Science 324 (2009) 349–352.16] L. Lu, J. Mater, Sci. Technol. 24 (2008) 473–482.17] J. Guo, K. Wang, L. Lu, J. Mater, Sci. Technol. 22 (2006) 789–792.18] S.S. Cai, X.W. Li, N.R. Tao, J. Mater. Sci. Technol. 34 (2018) 1364–1370.19] G.Z. Liu, N.R. Tao, K. Lu, J. Mater, Sci. Technol. 26 (2010) 289–292.20] B. Fu, L. Fu, S. Liu, H.R. Wang, W. Wang, A. Shan, J. Mater. Sci. Technol. 34

(2018) 695–699.21] K.C. Chen, W.W. Wu, C.N. Liao, L.J. Chen, K.N. Tu, Science 321 (2008)

1066–1069.22] T.C. Liu, C.M. Liu, H.Y. Hsiao, J.L. Lu, Y.S. Huang, C. Chen, Cryst. Growth Des. 12

(2012) 5012–5016.23] D. Xu, V. Sriram, V. Ozolins, J.M. Yang, K.N. Tu, G.R. Stafford, C. Beauchamp, J.

Appl. Phys. 105 (2009) 023521.24] C.N. Liao, Y.C. Lu, D. Xu, J. Electrochem. Soc. 160 (2013) D207–D211.25] S. Mahajan, Scr. Mater. 68 (2013) 95–99.

26] Z.S. You, L. Lu, K. Lu, Acta Mater. 59 (2011) 6927–6937.27] Z. You, X. Li, L. Gui, Q. Lu, T. Zhu, H. Gao, L. Lu, Acta Mater. 61 (2013) 217–227.28] C.M. Liu, H.W. Lin, C.L. Lu, C. Chen, Sci. Rep. 4 (2014) 6123.29] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation and

Surface Evolution, Cambridge University Press, Cambridge, 2004.

1 cience

[[[[[[

[

890 F.-L. Sun et al. / Journal of Materials S

30] S. Jin, Q. Pan, L.S. Lu, Acta Metall. Sin. 49 (2013) 635–640 (in Chinese).

31] S. Shao, Z. Fan, R. Fan, Laser Optoelectr. Prog. 42 (2005) 22–27.32] R. Winand, Electrochim. Acta 39 (1994) 1091–1105.33] R. Winand, Hydrometallurgy 29 (1992) 567–598.34] J.J. Kelly, C.Y. Tian, A.C. West, J. Electrochem. Soc. 146 (1999) 2540–2545.35] S. Seo, S. Jin, G. Wang, B. Yoo, J. Electrochem. Soc. 161 (2014) D425–D428.

[

& Technology 34 (2018) 1885–1890

36] Z.Q. Liu, F.L., Sun, C.F. Li, A Novel Structure of Unidirectional Growth of Cu

Pillar Bump and its Manufacturing Merhod, China Patent, 201410709245.1,2014.

37] Z.Q. Liu, F.L., Sun, C.F. Li, An Manufacturing Method of Electrodeposition ofUnidirectional Nanotwinned Cu in Through Silicon Vias and its Application,China Patent, 201510249314.X, 2015.