ARTICLE IN PRESS

Journal of Crystal Growth 312 (2010) 1267–1270

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Real-time intrinsic stress generation during Volmer–Weber growth of Co byelectrochemical deposition

Tian Zhi Luo, Lian Guo, Robert C. Cammarata n

Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA

a r t i c l e i n f o

Available online 21 December 2009

Keywords:

A1. Atomic force microscopy

A1. Stresses

A2. Electrochemical growth

B1. Metals

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.jcrysgro.2009.12.014

esponding author. Tel.: +1 410 516 5462.

ail address: rcc@jhu.edu (R.C. Cammarata).

a b s t r a c t

Real time in situ stress measurements were performed during constant current electrochemical

deposition of Co thin films on amorphous substrates. A three-stage compression-tension–compression-

stress evolution was displayed during the growth similar to behavior that has been observed during

growth of films produced by physical vapor deposition. The different stages of stress generation were

correlated with morphological changes of the thin films that showed that deposition occurred by the

Volmer–Weber island growth mode. Potential transients also displayed a strong correlation with the

morphological changes. A bimodal island size distribution was generated that was associated with

features in the stress and potential evolution.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Over the past three decades, the intrinsic stress evolutionduring thin film growth by physical vapor deposition (PVD) hasbeen extensively investigated [1–21]. A compression-tension–compression variation in the stress has often been observed forboth crystalline and amorphous films [13] that grow by theVolmer–Weber island growth mode. This growth is characterizedby the following stages: island formation, island coalescence, andcontinuous film growth. When the deposition has been haltedduring various stages of growth, a significant tensile relaxation ofthe stress has been seen that was generally fully recoverablewhen deposition was resumed. It has been suggested thatadatom–substrate interactions during the island formation stageis responsible for that part of the initial compressive stress that isrelaxed when deposition is interrupted [19], and that a compres-sive residual (permanent) stress can result owing to capillaryeffects that act on the isolated islands when they first becomefully adhered to the substrate [3,8] When island impingementoccurs, there is generally a concomitant tensile stress jump thathas been associated with elastic strains formed during theformation of the grain boundaries [1,6,12]. After the film becomecontinuous, it has been proposed that processes involving thediffusion of adatoms into the grain boundaries [16] contribute tothe generation of the later stage compressive stress which can berelieved by the adatoms diffusing out of the grain boundarieswhen deposition is halted.

ll rights reserved.

The PVD studies for intrinsic stress evolution just discussedwere generally conducted using amorphous oxidized silicon oramorphous silica substrates and were for growth under high orultrahigh vacuum conditions at a constant deposition rate. Therehas recently been interest in investigating the stress evolutionduring electrochemical deposition of metals under constantoverpotential conditions on crystalline substrates [22–25]. Acompression-tension–compression-stress evolution has beenreported for Cu electrodeposition on crystalline Au substrates[24]. It was suggested that this stress behavior was associatedwith the Volmer–Weber growth but no detailed microstructuralcharacterization was provided.

In the present study, stress evolution and voltage transientswere measured in real time during constant current electro-chemical deposition of Co thin films grown on amorphous NiTi. Byusing constant current conditions (and therefore constantdeposition rates) as well as amorphous substrates, complicationsassociated with possible epitaxial stresses generated by film-substrate lattice matching and possible formation of an under-potential deposition layer could be avoided and a more directcomparison with the behavior of previous described systemsproduced by PVD could be made.

2. Experimental approach

Co electrochemical deposition was conducted using a CoSO4

0.01–0.05 mol/l solution (pH 5.0–6.5) in three electrode modewith platinum acting as the counter electrode and Ag/AgCl actingas the reference electrode. The deposition current, which variedfrom 0.001 to 0.1 mA, was controlled by an EG&G potentiostat/

ARTICLE IN PRESS

T. Zhi Luo et al. / Journal of Crystal Growth 312 (2010) 1267–12701268

galvanostat model 263 A. The film was deposited onto amorphousNiTi that had an average surface roughness of about 2.5 nm andhad been deposited by magnetron sputtering onto a sputtered Cu/Cr overlayer that was on a 150-mm-thick glass slide. The Cr layerprovided strong adhesion and the Cu layer ensured goodconductivity within the plane of NiTi surface so as to avoidnonuniform current distribution during electrodeposition. Thethickness of the Cr, Cu and NiTi layers was about 1, 50, and500 nm, respectively. The deflection of the substrate duringdeposition was continuously monitored using a wafer curvaturesystem and used to calculate the stress-thickness product usingStoney’s equation [26]. The details of the wafer curvatureapparatus are given elsewhere [27]. The morphology of the filmswas characterized ex situ at different times by atomic forcemicroscopy (AFM).

3. Results and discussion

The stress-thickness evolution and the voltage transient duringelectrochemical deposition are shown in Fig. 1a and the derivativeof stress-thickness product (related to the instantaneous stress)as a function of time is given in Fig. 1b. The stress-thicknessbehavior displayed a compression-tension–compression variationqualitatively similar to that often observed during the

Fig. 1. (a) Stress evolution and potential transient; (b) Derivative of the stress-

thickness signal with respect to time.

Volmer–Weber growth of metallic films by PVD [13]. Thevoltage transient displayed four distinct stages, labeled inFig. 1a as OA, AB, BC, and CD. These stages were correlated withfeatures observed during the stress evolution. Stages OA and ABwere associated with the initial compressive stress generationand the tensile stress jump, respectively. Stage BC was associatedwith the stress-thickness evolving from the tensile peak to thesteady-state compressive stress stage CD.

These results can be understood in the following manner. Theinitial increase of the overpotential can be attributed mainly tothe charging of the electric double layer and the adatomcapacitance as well as the establishment of an increasingpopulation of nucleating islands. The overpotential is thedifference between the voltage in Fig. 1a and the equilibriumpotential of (Co/Co2 +) [28]. After completion of this charging,nucleation and growth of islands occurred resulting in a furtherincrease of the overpotential. According to the classical electro-crystallization theory of Scharifker–Hills [29], there is a diffusionzone near the solid–liquid interface for each island. Before islandcoalescence the overpotential continually increased. If islandgrowth is assumed to be diffusion controlled, the total depositioncurrent is initially proportional to N(dr/dt)3, where N is the totalnumber of nuclei and r is the average size of the diffusion zones.With further deposition and island growth, the diffusion zonesstart to overlap, decreasing the effective value of (dr/dt)3. Thus,new nuclei are created accompanied by an increase in theoverpotential in order to maintain the constant current duringisland coalescence. This process continues until the film becomescontinuous, and there is no longer any place on the substratesurface for further nucleation. This would explain the significantincrease of potential associated with the AB stress evolution stage.The overpotential did not change significantly at the later stage ofcontinuous growth CD.

The ex situ film morphology of films after different depositiontimes as characterized by AFM are shown in Fig. 2. The solutionused for these films was 0.05 mol/l CoSO4 and the deposition ratewas 0.7 A/s. All the images have a size of 4�4mm2. The image attime t=40 s shows isolated islands with an average diameter ofabout 200 nm and height of 40 nm, and corresponds to the OAstage in Fig. 1a. The image at t=100 was taken when the stressunderwent the sharp jump from compression to tension (stage ABin Fig. 1a). It can be seen that a large number of new small islandswere formed during this transition. As discussed above, if islandgrowth is assumed to be diffusion controlled and the diffusionzone model is applicable, the newly created islands can beassociated with the sharp increase of the potential during stageAB (see Fig. 1). The image for t=200 s shows a film as it was justbecoming continuous. It can be seen from Fig. 1 that the stressapproached its tensile maximum at this point. Thus, the AFMimages show that the Co films deposited by the Volmer–Webergrowth mode and that the rapid increase of potential wascorrelated with a sudden burst of new islands.

As is shown in Fig. 1b, the derivative of the stress-thicknessproduct displayed two distinctive peaks in the compression-tension transition regime. It is seen in Fig. 2 that there is abimodal distribution of islands after t=100 s. It is suggested thatthe two peaks were associated with two kinds of coalescenceevents. The first involved coalescence between large and smallislands and the other resulted from later coalescence among thesmall islands. A similar rapid increase of tensile stress due tocoalescence during electrodeposition has been observed duringconstrained island coalescence [25].

When the electrodeposition of Co was interrupted, stressrelaxations were observed that were of opposite sign to that of thevalue of the average film stress when the growth was halted. Thisis in contrast to PVD during which the stress relaxations are

ARTICLE IN PRESS

Fig. 2. Atomic force microscopy characterization of film morphology for a deposition rate of 0.7 A/s at different deposition times: t=40, 80, 100, 120, 160, and 200 s. All

images have a size of 4�4 mm2. The scale marker indicates one nanometer.

T. Zhi Luo et al. / Journal of Crystal Growth 312 (2010) 1267–1270 1269

generally always tensile. One reason for this difference may bethat the relaxations presented here are accompanied by a largechange from an overpotential to open-circuit potential that drivessurface morphological changes. Details of the relaxation behaviorduring Co electrodepositon and will be presented elsewhere [27].

4. Summary

The intrinsic stress and potential evolution were measured inreal time during constant current electrochemical deposition ofCo on amorphous substrates. AFM characterization showed thatthe films were deposited by a Volmer–Weber island growthmode. The stress evolution displayed a compression-tension–compression variation, similar to that observed for films grown byphysical vapor deposition. Different stages of the stress andpotential curves were shown to be correlated with the Volmer–Weber growth stages of island formation, island coalescence, andcontinuous film growth. A bimodal island size distribution was

generated that could be associated with details of the stress andpotential evolution.

Acknowledgments

The authors gratefully acknowledge valuable discussions withProfessor P.C. Searson. Support for this work from the NationalScience Foundation, award number DMR 0706178, is also grate-fully acknowledged.

.

References

[1] R.W. Hoffman, Thin Solid Films 34 (1976) 185.[2] R. Abermann, R. Kramer, J. Maser, Thin Solid Films 52 (1978) 215.[3] M. Laugier, Vaccum 31 (1981) 155.[4] R. Abermann, R. Koch, Thin Solid Films 142 (1986) 65.[5] R. Koch, J. Phys.: Condens. Matter 6 (1994) 9519.[6] W.D. Nix, B.M. Clemens, J. Mater. Res. 14 (1999) 3467.[7] C.V. Thompson, Annu. Rev. Mater. Sci. 30 (2000) 159.

ARTICLE IN PRESS

T. Zhi Luo et al. / Journal of Crystal Growth 312 (2010) 1267–12701270

[8] R.C. Cammarata, T.M. Trimble, D.J. Srolovitz, J. Mater. Res. 15 (2000) 2468.[9] S.C. Seel, C.V. Thompson, S.J. Hearne, J.A. Floro, J. Appl. Phys. 88 (2000) 7079.

[10] J.A. Floro, S.J. Hearne, J.A. Hunter, P. Kotula, E. Chason, S.C. Seel, C.V.Thompson, J. Appl. Phys. 89 (2001) 4886.

[11] S.G. Mayr, K. Samwer, Phys. Rev. Lett. 87 (2001) 036105.[12] L.B. Freund, E. Chason, J. Appl. Phys. 89 (2001) 4866.[13] J.A. Floro, E. Chason, R.C. Cammarata, D.J. Srolovitz, MRS Bull. 27 (1) (2002) 19.[14] C. Friesen, C.V. Thompson, Phys. Rev. Lett. 93 (2004) 056104.[15] E. Chason, B.W. Sheldon, L.B. Freund, J.A. Floro, S.J. Hearne, Phys. Rev. Lett. 88

(2002) 156103.[16] A. Rajamani, B.W. Sheldon, E. Chason, A.F. Bower, Appl. Phys. Lett. 81 (2002)

1204.[17] J.A. Floro, P.G. Kotula, S.C. Seel, D.J. Srolovitz, Phys. Rev. Lett. 91 (2003)

096101.

[18] S.C. Seel, C.V. Thompson, J. Appl. Phys. 93 (2003) 9038.[19] C. Friesen, C.V. Thompson, Phys. Rev. Lett. 93 (2004) 056104.[20] C. Friesen, S.C. Seel, C.V. Thompson, J. Appl. Phys. 95 (2004) 1011.[21] C.-W. Pao, D.J. Srolovitz, Phys. Rev. Lett. 96 (2006) 186103.[22] W. Haiss, R.J. Nichols, J.K. Sass, Surf. Sci. 388 (1997) 141.[23] S.J. Hearne, J.A. Floro, J. Appl. Phys. 97 (2005) 14901.[24] O.E. Kongstein, U. Bertocci, G.R. Stafford, J. Electrochem. Soc. 152 (2005) C116.[25] A. Bhandari, B.W. Sheldon, S.J. Hearne, J. Appl. Phys. 101 (2007) 33528.[26] G.G. Stoney, Proc. Roy. Soc. 82 (1909) 172.[27] T.Z. Luo and R.C. Cammarata, to be published.[28] A. Milchev, M.I. Montenegro, J. Electroanal. Chem. 333 (1992) 93.[29] B. Scharifker, G. Hills, Electrochim. Acta 28 (1983) 879.