Sputter-induced pits on {100} nickel surfaces
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INTRODUCTION
ENERGETIC particles impinging on the surface of a material cause the ejection of near-surface atoms, a process known as sputtering. For several years, electron microscopists have sputtered or "ion-milled" hard, nonconducting materials, thereby making them thin enough to transmit electrons and suitable for observation in the microscope. More recently, engineers and scientistst~,2~have recognized sputtering in the broader context of micromachining. Sputtering proves beneficial in both contexts because it yields nearly atomically smooth surfaces and also because it can be controlled so as to remove depths of material accurate to within a nanometer. This precise sputter removal of material is applied during Auger depth profile analysis, for example, where monolayers of the sample being analyzed are sputtered away using low energy ions and the composition of the target is measured as a function of depth. Though surface smoothing by sputtering, described previously, may prevail initially during the sputtering of homogeneous materials, it may be replaced by roughening at higher fluences or in inhomogeneous materials. Surface roughening is commonly observed at incident particle fluences beyond ~ 1019/cm2, even in apparently homogeneous metals of high p u r i t y . I3] Such sputter-roughened surfaces are sought after in some instances, for example, as a means of creating or altering catalytic activity, for increasing biocompatibility,~4,5]or for altering optical properties: 6] Though the processes governing surface smoothing vs roughening during sputtering are not fully appreciated at present, they are
J.K. STEELE, formerly Visiting Scientist, Metallurgy Department, University of Connecticut, is a Research Scientist at Metals Research Laboratories, Olin Corporation, New Haven, CT 06511. D.I. POTTER, Professor, is with the Metallurgy Department and Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136. Manuscript submitted June 8, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
important. In using sputtering to alter surfaces, the experimenter clearly seeks either smoothing, as in ion milling for microscopy or in micromachining, or roughening, as in the examples just cited. Early work by Ahmed e t aLtT] showed that temperature plays a critical role in determining whether roughening or smoothing occurs as a result of sputtering. These researchers used A1+ as well as Ni+ ions to sputter polycrystalline substrates. The AI§ ions produced near-surface layers of NiA1 intermetallic compounds, and further investigationtS] revealed the phases present and their stability during annealing. This simple, two chemical element investigation provides a prototype for appreciating what may be accomplished when more complex implanted layers are created by implantation with several elements. Of more direct relevance to the present work, Ahmed e t al. tTl report that electropolished nickel surfaces remain smooth when sputtered at temperatures below 300 ~ and when sputtered at temperatures above 700 ~ S
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