Hollow tin/chromium whiskersJing Cheng, Paul T. Vianco, and James C.M. Li Citation: Applied Physics Letters 96, 184102 (2010); doi: 10.1063/1.3419837 View online: http://dx.doi.org/10.1063/1.3419837 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/96/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nucleation and growth of tin whiskers Appl. Phys. Lett. 98, 241910 (2011); 10.1063/1.3597653 Creep and its effect on Sn whisker growth J. Appl. Phys. 106, 094903 (2009); 10.1063/1.3248277 A model of Sn whisker growth by coupled plastic flow and grain boundary diffusion Appl. Phys. Lett. 94, 191904 (2009); 10.1063/1.3136865 Relieving Sn whisker growth driven by oxidation on Cu leadframe by annealing and reflowing treatments J. Appl. Phys. 102, 043521 (2007); 10.1063/1.2770832 Optimization of wear-resistant coating architectures using finite element analysis J. Vac. Sci. Technol. A 21, 332 (2003); 10.1116/1.1531650

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Hollow tin/chromium whiskersJing Cheng,1,a� Paul T. Vianco,2 and James C.M. Li11Materials Science Program, University of Rochester, Rochester, New York 14627, USA2Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

�Received 28 October 2009; accepted 9 January 2010; published online 4 May 2010�

Tin whiskers have been an engineering challenge for over five decades. The mechanism has notbeen agreed upon thus far. This experiment aimed to identify a mechanism by applying compressivestresses to a tin film evaporated on silicon substrate with an adhesion layer of chromium in between.A phenomenon was observed in which hollow whiskers grew inside depleted areas. Using focusedion beam, the hollow whiskers were found to contain both tin and chromium. At the bottom of thedepleted areas, thin tin/tin oxide film remained over the chromium layer. It indicates that tintransport occurred along the interface between tin and chromium layers. © 2010 American Instituteof Physics. �doi:10.1063/1.3419837�

In the microelectronic industry, tin �Sn� whiskers havebeen a reliability concern for over fifty years.1–4 The whis-kers are able to grow from pure Sn coatings, reaching lengthsof up to tens to hundreds of microns within time periods ofmonths to years. Tin whiskers can potentially cause short-circuits between neighboring conductors or contaminate mi-croelectronic devices as foreign object debris. Several satel-lites and switch terminals have been suspected of failing dueto Sn whisker growth.5 The problem has been mitigated byelectroplating Sn coatings having greater than 3 wt % lead�Pb�.6 However in recent years, several environmental regu-lations have forced manufacturers to remove lead from mi-croelectronic finishes. The most economic replacement ispure Sn.

Investigators continue to search for a mechanism for Snwhisker growth. There have been several proposed pro-cesses, including dislocation loops,7,8 helical dislocations,9

recrystallization,10–14 grain boundary diffusion,15,16 and grainboundary fluid flow.17 However, none of these theories haveyet been fully substantiated, experimentally.18

A study is described here, in which whisker growth wasinvestigated from a vapor-deposited 1 �m thick pure Snfilm on a silicon �Si� substrate �0.350 mm thick, n-type,�100� orientation, 25.4 mm in diameter� after deposition of a20 nm chromium �Cr� adhesion layer, by which the effects ofinterface reactions can be eliminated. Since the compressivestress is a driving force for whisker growth, a special fixturewas designed to apply biaxial compressive stresses across thewafer in bending as shown in Fig. 1. The upper surface hadthe Sn coating. A load of 500 g was applied at the center ofthe wafer, which was supported below around the edge tocreate the compressive stresses over the tin film. The entireassembly was annealed for one week at 180 °C in a vacuumoven maintained at 10−4 Torr.

Tin whiskers grew from the film. Nearly all were presentwithin depleted areas as observed in the scanning electronmicroscope �SEM� image in Fig. 2. Moreover, all the whis-kers we examined were hollow. It is well-known that Snwhiskers are single crystals, and there have been few reportsof hollow Sn whiskers.19–21 Such reports have not included amention of depleted zones. The sequence of the appearance

of the whisker and depleted zone is critical toward determin-ing a relevant mechanism. A potential mass transport path isthe interface between the Sn layer and the Cr layer as sug-gested previously in the case of hillock growth.22 Supportingevidence of this interface pathway is provided in the currentstudy.

Shown in Fig. 3 is a whisker showing a “skin” of Sn orSn oxide �approximately 20–60 nm thick� surrounding it;underneath, the Sn film is largely gone. This whisker wasmilled at the base by focused ion beam �FIB� at lower energy�93 pA� for shorter time to avoid the whisker falling down.The whisker appears as hollow at the bottom.

Two other whiskers, which had grown within depletedareas, were cut off by the FIB technique, using a highercurrent of 46 nA. Each whisker was carefully picked up bythe Omni probe and moved to a transmission electron mi-croscopy �TEM� copper grid, then bonded there with a thinlayer of platinum �Pt�. About 1 �m of the top section of thewhisker was milled �93 pA� by the FIB to either fracture theend or remove one-half of the whisker longitudinally so as toreveal the hollow configuration, as shown in Fig. 4.

The hollow whisker shown in Fig. 4�a� was examinedfurther. A TEM diffraction image is shown in Fig. 5�a�, re-vealing that the whisker was polycrystal. Energy dispersivex-ray �EDX� analysis was used to identify the composition of

a�Electronic mail: jicheng@me.rochester.edu.

Dead Weight

Siliconwafer

FIG. 1. �Color online� Sample fixture with a dead weight. The applied loadwas 500 g and was located at the center of the wafer through an indenter; thediameter of the indenter’s tip was 0.5 mm.

APPLIED PHYSICS LETTERS 96, 184102 �2010�

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the whiskers. The data were taken from three areas on thiswhisker, shown in Fig. 5�b�. Area 1 contained Sn peaks; area2 showed some Cr; and area 3 had a combination of Sn/Cr.The elemental map in Fig. 5�c� illustrates the Sn and Crdistribution within the whisker, which indicates that both Snand Cr were transported into the whisker during its develop-ment. The equilibrium phase diagrams indicate negligible

FIG. 2. SEM image showing Sn whiskers of about 10 �m long, and0.5–1 �m in diameter grew inside Sn depleted areas of 50–75 �m2 afterone week incubation.

FIG. 3. SEM image of a Sn whisker which was milled by FIB to show it tobe hollow at the bottom.

(b)(a)

FIG. 4. SEM images of tin whiskers bonded on a TEM copper grid forobservation: �a� a whisker that was broken into two sections by the FIBtechnique to show the hollow interior, and �b� a whisker that was FIB cutlongitudinally to show the walls due to the hollow interior.

(a)

(b1)

(b3)

(b2)

(c)

FIG. 5. �Color online� �a� TEM diffraction pattern of a polycrystal whisker.�b� EDX analysis revealed the existence of Cr and Sn at three locations inthe hollow whisker. �c� Element mapping illustrated the distribution of Crand Sn in the whisker segment.

184102-2 Cheng, Vianco, and Li Appl. Phys. Lett. 96, 184102 �2010�

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solubility of Cr in Sn and about 1.5 wt % Sn soluble in Crfrom room temperature up to �700 °C.23 The EDX analysisshowed a more substantial presence of Cr in the whisker thanwould be expected from the limited equilibrium solubilityvalues. This observation suggests the following three points:�a� the interface has a significant role in the transport processsupporting whisker growth in thin films; �b� the dynamics ofthat transport process are of such a magnitude as to capturethe Cr atoms in the building process of the whisker; and �c�the whisker is a metastable equilibrium structure in order toincorporate such high levels of Cr.

The SEM image in Fig. 6 shows the lateral side view ofthe Sn film. Only vertical grain boundaries are present in theFIB cut. If the intragranular transport processes are not activeduring the formation of either whiskers18 or depleted zones,then the only possible diffusion paths are the Sn/Cr interfaceand the vertical grain boundaries.

A depleted area is shown in Fig. 7. The edges are steepand show partially consumed grains. These features suggest

that Sn grains shrank horizontally to create the depleted area.It appears that the tin atoms diffuse along the grain bound-aries next to the grain along the edge of the depleted areatowards the tin/chromium interface causing the neighboringgrains to shrink horizontally by sliding along the tin/chromium interface. The driving force for this diffusion isthe strain energy stored in the tin film due to the biaxialcompressive stresses. Then the tin atoms diffuse or flowalong the Sn/Cr interface carrying some Cr atoms with theminto the hollow whisker. The hollow whisker and the de-pleted zone probably started at the same time when the tinatoms push out the grain boundaries surrounding a smallgrain to nucleate a hollow polycrystalline whisker. More de-tails will be reported in a subsequent publication.

In summary, Sn whiskers were found to grow inside Sndepleted areas in an evaporated Sn film over Si with a Crlayer in between. Both Sn and Cr were found in the whis-kers, indicating that the Sn/Cr interface was the principalmass transport path that supported whisker growth. The highconcentration of Cr in the Sn whisker indicated that whiskersare highly nonequilibrium structures in terms of chemicalcomposition.

This research was supported by Sandia National Labora-tory through the efforts of P.T.V. Sandia is a multiprogramlaboratory operated by Sandia Corporation, a Lockheed Mar-tin Co., for the United States Department of Energy’s Na-tional Nuclear Security Administration under Contract No.DE-AC04-94AL85000. J.C. also wishes to thank Dr. Mal-colm Thomas of Cornell for his diligent work and invaluablehelp in the FIB fabrication of hollow Sn whiskers. A greatappreciation is for Dr. Brian McIntyre upon his very kindassistance in TEM analysis.

1G. T. T. Sheng and C.F. Hu, J. Appl. Phys. 92, 64 �2002�.2G. T. Galyon, IEEE Trans. Electron. Packag. Manuf. 28, 94 �2005�.3J. W. Osenbach, J. M. DeLucca, B. D. Potteiger, A. Amin, and F. A.Baiocchi, J. Mater. Sci.: Mater. Electron. 18, 283 �2007�.

4K. Zeng and K. N. Tu, Mater. Sci. Eng. R. 38, 55 �2002�.5Satellite News Digest, http://www.sat-index.com/failures/.6S. M. Arnold, Proceedings of IEEE Electronics and Components Confer-ence, 1959 �unpublished�, pp. 75–82.

7J. D. Eshelby, Phys. Rev. 91, 755 �1953�.8F. C. Frank, Philos. Mag. 44, 854 �1953�.9S. Amelinckx, W. Bontinck, W. Dekeyser, and F. Seitz, Philos. Mag. 2,355 �1957�.

10W. C. Ellis, D. F. Gibbons, and R. C. Treuting, in Growth and Perfectionof Crystals, edited by R. H. Doremus, B. W. Roberts, and D. Turnbull�Wiley, New York, 1985�, pp. 102–120.

11T. Kakeshita, K. Shimizu, R. Kawanaka, and T. Hasegawa, J. Mater. Sci.17, 2560 �1982�.

12B. D. Dunn, ESA Scientific and Technical Review 2, 1 �1976�.13I. Boguslavsky and P. Bush, Proceedings of the 2003 APEX Conference,

Anaheim, CA, March 2003 �unpublished�, pp. S12-4-1–S12-4-10.14P. Vianco and J. Rejent, J. Electron. Mater. 38, 1815 �2009�.15K. N. Tu, Acta Metall. 21, 347 �1973�.16K. N. Tu, Phys. Rev. B 49, 2030 �1994�.17K. N. Tu and J. C. M. Li, Mater. Sci. Eng., A 409, 131 �2005�.18R. M. Fisher, L. S. Darken, and K. G. Caroll, Acta Metall. 2, 368 �1954�.19H.-P. Kehrer and H. G. Kadereit, Appl. Phys. Lett. 16, 411 �1970�.20A. Politycki and H.-P. Kehrer, Z. Metallkd. 60, 17 �1969�.21E. E. Thomas, Acta Metall. 4, 94 �1956�.22J. Cheng, S. Chen, P. Vianco, and J. C. M. Li, IEEE Electronic Compo-

nents and Technology Conference, 2008, pp. 472–477 �There is a misprintin Eq. �1�: the number 8 should be 3�.

23M. Venkatraman and J. P. Neumann, J. Phase Equilib. 9, 159 �1988�.

FIG. 6. SEM image showing FIB milling shown a single layer of Sn grainsexisted in the 1 �m thick coating. There are no horizontal grain boundaries.

FIG. 7. SEM image of a Sn-depleted area showing the steep edge of thebordering grains.

184102-3 Cheng, Vianco, and Li Appl. Phys. Lett. 96, 184102 �2010�

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