Buckling and Delamination of Ti/Cu/Si Thin Film DuringAnnealing

QIJING LIN,1 SHUMING YANG,1,3 WEIXUAN JING,1 CHANGSHENG LI,1

CHENYING WANG,1 ZHUANGDE JIANG,1 and KELY JIANG2

1.—State Key Laboratory of Mechanical Manufacturing Systems Engineering, Xi’an JiaotongUniversity, Xi’an 710049, China. 2.—School of Mechanical Engineering, University of Birming-ham, Edgbaston, Birmingham B15 2TT, UK. 3.—e-mail: shuming.yang@mail.xjtu.edu.cn

In this paper, the formation of buckling and delamination of sandwichedstacking of Ti/Cu/Si thin film are investigated. The crystallization structures,the composition of the Cu/Ti thin films, and the surface morphology aremeasured during annealing. The results show that the solid-phase reactionbetween Cu and Ti occurs at the interface. Buckling is initiated in the thin filmannealed at 600�C. The volume expansion promotes the buckling and furtherproduces microcracks. With increasing volume expansion, there are cavitiesformed in the middle layer when the annealing temperature is up to 700�C.Finally, thin film is delaminated from the substrate.

Key words: Thin Film, interface reaction, buckling, delamination,microcracks

INTRODUCTION

Various kinds of thin film are broadly applied forphotoelectric devices. Adhesion between two heter-ogeneous thin film layers and the substrate signifi-cantly influence the performance of devices. Manyfactors such as morphology,1 plasticity,2 environ-mental effects,3,4 residual stresses,5 and chemistryreaction6 have significant effects on the interfacialadhesion of thin film. Residual stresses includeintrinsic growth stresses produced during deposi-tion together with thermal stresses. Thermalstresses are commonly induced by a thermal mis-match between the thin film and the substrate,including temperature changes during depositionand annealing.7,8 High stress concentrations afterannealing developed along dissimilar material areincentives of failure/crack initiation.9 The crackinitially propagates, possibly along the paths ofleast fracture resistance, and then debonding eventsdriven by film stresses begin to propagate in thethin film.

The internal stress, surface morphology, andcontamination are important for the adhesion of the

thin film structures. The occurrence of interfacereaction and formation of reaction compoundenhance the adhesion of interfaces.10,11 However,some reaction products may make the interfacelayer brittle and result in premature spalling of thethin film.12,13 These reaction products, segregation,or impurities existing in the interface significantlyweaken the adhesion, reducing the fracture resis-tance. This is a straightforward reason for thin filmfailure initiation such as buckling, debond, delami-nation, and microcracks.

Cu and Ti have been widely used for the fabrica-tion of thin films. According to the Cu/Ti structuraldiagram, intermetallic compounds such as Cu3Tican generate at the certain annealed temperature inCu/Ti thin film systems.14,15 In this paper, theinfluence of reaction and reaction products on thefilm buckling and delamination for sandwichedstacking of Ti/Cu/Si after annealing was discussed.

EXPERIMENTS

Sandwich stacked Ti/Cu/Si thin films weredeposited on a double-sided polished Si(100) sub-strate at room temperature using a DC magnetronsputtering system (Explorer 14; USA) in an Aratmosphere with a flow rate of 20 sccm under the(Received December 9, 2013; accepted May 3, 2014)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-014-3238-7� 2014 TMS

condition of a base pressure of 2 9 10�5 Pa. Thepurity grade of the Cu and Ti targets was 99.99%.The sputtering power was 99 W for Cu and 105 Wfor Ti, and the deposition rates were 4 A/s and1.2 A/s, respectively. The deposited Ti/Cu/Si sand-wiched films were annealed for 30 min at tempera-tures of 600�C and 700�C using a rapid thermalannealing (RTA) system (RTP-500; China). This isin an Ar atmosphere with ramping rate at 50�C/sand then furnace-cooling to room temperature. Thethicknesses of the Ti and Cu films are 70 nm and20 nm, respectively.

The crystallization structures of the Cu/Ti thinfilms annealed at different temperatures weremeasured by x-ray diffraction (XRD) (Xpert Pro;Netherlands), with a monochromatic CuKa(k = 1.54 A), in the scan range of 2h from 15� to65� with a step size of 0.06(2h/s). The surface of thefilms was measured using atomic force microscope(AFM) (Innova; USA) in contact mode at a smallscale (<4 lm) and field emission scanning electronmicroscopy (FESEM) (SU-8010; Japan), for whichan energy dispersive spectroscopy (EDS) used forcomposition analysis is attached.

RESULTS AND DISCUSSION

The microstructure and the formation of com-pounds of the Ti/Cu sandwiched thin films annealedat 600�C and 700�C are shown in Fig. 1. Cu(111),Cu(200), and Cu3Ti peaks are observed for the thinfilm annealed after 600�C. The exhibition of Cu3Tiphase indicates the formation of Cu/Ti intermetalliccompounds.16,17 The same results also can be foundin other Cu/Ti thin film systems in our previousworks.14,15 After annealed at 700�C, Cu(111) andCu(200) peaks disappear and only Cu3Ti isobserved. There is no obvious reflection peak of Tibut weak TiSi peaks in both annealed thin films.

The stress (r) developed in the films can be cal-culated from the XRD data employing the relation-ship18,19

r ¼ �Eða0 � aÞ2ma

(1)

where E is the Young’s modulus, m is the Poisson’sratio, a0 is the lattice parameter of bulk material,amd a is the lattice parameter of the measured film.The values of E and m for Cu3Ti are about 120 GPaand 0.27 GPa, respectively. For the Cu3Ti peak, thetensile stress in the film annealed at 600�C is about53.1 GPa, and 41 GPa at 700�C.

Two potential limitations of the sandwichedsample geometry include microstructural andcompositional changes of the thin film stack. Thisis related to the elevated temperature and buck-ling of the thin film.8 The AFM measurementresult of 70 nmTi/20 nmCu/Si thin film annealingat 600�C is shown in Fig. 2(a). It can be seen thatthe surface is smooth and void-free. The grainsare tiny and homogeneously distributed. TheFESEM morphology of the thin film is shown inFig. 2(b). Clear depleted grain boundaries areformed. There are black holes and separatedgrains. The depleted regions appear as isolatedbranched clusters with self-affine behavior, so itcan be described by a fractal model.20,21 Thefractal dimension D was determined usingthe boxcounting method.22,23 The boxes with borderlength d (1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128,1/256, 1/512) were arranged at the center of themass of the patterns, and the number of the pixelsN with gray levels corresponding to the patternsas a function of box size d were counted. The curveof lnN-lnd�1 is shown in Fig. 3. The fitting curveof the data points is a straight line, so the rela-tionship between the number N and the boxlength d is N � (1/d)D, where D = 1.70 that wascalculated from the slope of the fitting straightline.

As shown in Fig. 2(b), some bright particles orclusters distribute in the surface. These are Cu3Tiintermetallic compounds formed by the solid-phase reaction indicated by the XRD pattern. TheEDS was employed to identify the composition ofthe black depleted region (region A) and brightregion (region B). The spectra are shown in Fig. 4.The EDS spectra of region A (Fig. 4a) shows thatthe content of Cu is low (about 1.21 at.%) and thestoichiometry of this region has deviated sharply.The atom rate of Ti to Cu is about 25.8. IThistmuch exceeds the normal rate in as-deposited thinfilm which is about 2.34. This indicates thedepletion of Cu in this region. But in region B(Fig. 4b), the content of Cu is high (about27.26 at.%) and the atom rate of Ti to Cu is about1.3. This rate is lower than the normal rate inas-deposited thin film. The increasing of Cu atoms

Fig. 1. X-ray diffraction patterns of Ti/Cu/Si thin films annealed atvarious temperatures.

Lin, Yang, Jing, Li, Wang, Z. Jiang, and K. Jiang

is induced by the formation of Cu3Ti that leads tothe aggregation of Cu atoms in the local region.

Sputtered films deposited at room temperaturenormally show a compressive stress state whendeposited at room temperature.24 It is attributed tothe shot peening action of the bombarding ions andneutrals on the advancing film surface. However, onannealing, the room temperature stress moves inthe tensile direction due to the densification of theas-deposited films.24,25 The formation of initial filmdebonding and buckling produces an adhesion losswithin an area.26 Therefore, in the Cu-depletedregion (Region A), the top Ti film debond from sub-strate because of the loss of middle Cu layer. Thereaction products of Cu with Ti at the interface mayweaken the film adhesion, and then the initialbuckling forms around the region of the products.

Moreover, volume expansion accompanying thereaction benefits the formation of buckling. Theaggregation of reaction production of Cu3Ti leads tothe stresses in the border of particles increasingsharply. It exceeds the fracture resistance, so theformation and expansion of microcracks are pro-duced. As shown in Fig. 5, microcracks induced by alarge stress in Ti film around and in top of Cu3Tiparticles are observed. This large stress is developedby the volume expansion due to the aggregation ofthe intermetallic compounds Cu3Ti. Specially,buckling and volume expansion together with themicrocracks can be seen in Fig. 5(b).

According to the solid diffusion theory, the diffu-sion of atoms in thin film is a process of thermalactivation. The relationships between diffusioncoefficient Ds and temperature T can be expressedusing the Arrhenius’ law:16,27

Ds ¼ D0e�Ea=kT; (2)

where D0 is the diffusion coefficient at room tem-perature, Ea is the thermal activation energy. In thestacking of Ti/Cu/Si, the thermal activation energyof Cu is lower than that of Ti.28 The melting point ofCu is also lower than that of Ti. Consequently, atthe same annealing temperature, the diffusioncoefficient of Cu is larger than Ti. So, the Cu crys-tals are relatively easy to disassemble and theactivated Cu atoms can pass across the originalinterface and diffuse into shot peening actionTi film.Therefore, the depleted grain boundaries occur inthe Cu film. On the other hand, because the crystalstructure of Cu is face-centered cubic (Fcc) while Tiis close-packed hexagonal (Hcp), the Cu atoms aredifficult to pass through the Ti thin film layer andthe top Ti film can be regarded as a dense barrierlayer of stacking and restrict Cu layer on the Sisubstrate. So, buckling appears in the top Ti film.The diffusion coefficient and the acceleration of the

Fig. 2. (a) AFM topographic images and (b) FESEM morphology of athin film annealed at 600�C.

Fig. 3. The curve of lnN-lnd obtained using the box counting methodto determine the fractal dimension D.

Buckling and Delamination of Ti/Cu/Si Thin Film During Annealing

solid-phase reaction in the interface increase withthe rising of temperature.

The FESEM morphology of the sandwiched thinfilm annealed at 700�C is shown in Fig. 6. Regulardendritic patterns with an open ramified branchedstructure grown in a certain line orientation areobtained. A number of microcracks are noted in thesurface and they all almost always occur and expand

along this line branches (Fig. 6b). The XRD pattern ofthe thin film annealed at 700�C (Fig. 1) has indicatedthat all Cu atoms in the thin film exist in the form ofCu3Ti intermetallic compounds, so the line structurebranches are composed of the aggregation of Cu3Ticlusters. The cross-section of the interface structure ofthe thin film annealed at 700�C is shown in Fig. 7.Obvious film buckling is obtained and the top surface

Fig. 4. EDS spectra of (a) region A and (b) region B as shown in Fig. 2(b).

Lin, Yang, Jing, Li, Wang, Z. Jiang, and K. Jiang

thin film layer has debonded from the substrate. Filmdelamination and lots of cavities are formed. The topthin film layer over these cavities is supported by thebackbones that are formed by the aggregated Cu3Ticlusters in the branches of dendritic pattern. Thecavities also indicate the formation of debonding forthin film from the substrate.

The delamination develops from the initial bucklingof the thin film.3 Similar to film buckling, the propa-gation of the delamination crack is also connected withthe chemical reaction between Cu and Ti at theinterface, and the reaction products crucially deter-mine the weakening of the film adhesion. Normally,volume expansion accompanying with the reactionproduces delamination cracks.4,12,24 If the reactionproducts exhibit a sufficiently high mobility at theinterface and aggregate in the local region, the volumeexpansion increases several times. As shown inFig. 7(a), the height between the thin film layer andthe substrate is about 385 nm, three times larger thanthe total thickness of stacking. Some abnormal vol-ume expansion results in the stress concentration andfracture cracks propagating in the direction of thestress. An extraordinary Cu3Ti cluster on the sub-strate is shown in Fig. 8. Because of the abnormal

height of the cluster, microcracks propagate around itand the top thin film tends to collapse. Compared todensification of thin film, delamination film releasesthe concentration stress at a certain degree. This is thereason that the stress value annealed at 700�C islower than that at 600�C.

CONCLUSION

The sandwiched stacking of Ti/Cu/Si thin films wasfabricated by a DC magnetron sputtering system, andthen the films were annealed at temperatures of600�C and 700�C for 30 min using a RTA system. Thetensile stress in the film for Cu3Ti peak was about53.1 GPa and 41 GPa at annealed temperatures of600�C and 700�C, respectively. The solid-phase reac-tion between Cu and Ti occurred at the interface, andthe aggregation of reaction products in the local regioninduces volume expansion. This produced the con-centration of stress in the thin film and initiated theinitial buckling. The forward promotion of film buck-ling resulted in the formation of microcracks aroundthe clusters aggregated by the reaction productsCu3Ti when the stacking annealed at 600�C. The mi-crocracks initiated and propagated with increasing

Fig. 5. Microcracks formed around Cu3Ti particles. Fig. 6. FESEM surface morphology at annealed temperature of700�C.

Buckling and Delamination of Ti/Cu/Si Thin Film During Annealing

stress concentration. With the annealing temperaturerising and solid-phase reaction further proceeding,cavities formed in the middle layer of the Ti/Cu/Sistacking together with obvious film delamination.

ACKNOWLEDGEMENTS

Theauthorswould like to thankthefinancial supportof National Natural Science Foundation of China (No.51175418), National Natural Science Foundation ofChina Major Research Program on Nanomanufactur-ing (No. 91323303), Program for New Century Excel-lentTalents inUniversity (No.93JXDW02000006), 111Program (No. B12016), Program of Chang JiangScholars and Innovative Research Team in University(No. IRT1033), the Fundamental Research Funds forthe Central Universities (No. 2011jdgz09, No.xjj2011068), and the fund of the State Key Laboratoryof Digital Manufacturing Equipment & Technology(No. DMETKF2012009, Huazhong University of Sci-ence and Technology).

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Fig. 7. Cross-section of the interface structure of a thin film annealedat 700�C.

Fig. 8. The collapse of the top thin film because of the abnormalvolume expansion of the Cu3Ti cluster.

Lin, Yang, Jing, Li, Wang, Z. Jiang, and K. Jiang