Equilibrium Shape of CoSi 2 in Silicon
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EQUILIBRIUM SHAPE OF CoSi 2 IN SILICON D.P. ADAMS, S.M. YALISOVE, AND D.J. EAGLESHAM* Department of Materials Science and Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, MI, 48109-2136. * AT&T Bell Laboratories, 600 Mountain Ave., Murray Hill, N.J., 07974 ABSTRACT The energetics associated with the formation of CoSi2 have been investigated by using the equilibrium shape of isolated precipitates. Two types of precipitates were studied: CoSi2 equilibrated at the Si surface and CoSi2 equilibrated within the Si lattice. CoSi 2 precipitates form by facetting along {11111 and 11001 planes in Si. Using a reverse Wulff approach, a ratio of interfacial free energies of y{ 100}1/ y{ 111) has been measured to be 1.43 + 0.07. INTRODUCTION CoSi 2 thin films are currently used as contacts in very large scale integration and are candidates for future three dimensional integration [1]. The self-aligned-silicide (salicide) process, involving the deposition of thin Co films followed by annealing for brief amounts of time at high temperatures, is used to form CoSi 2 contact layers on Si substrates. It is difficult, however, to apply this process to the fabrication of submicron feature size devices, because silicide agglomeration produces discontinuity within the thin layer [2]. Therefore, a great deal of work has focused on increasing the stability of silicide layers [3-5]. Although a number of different mechanisms have been proposed to explain agglomeration, a precise understanding is still lacking [6-8]. Interest in using CoSi 2 for applications such as vertical integration, has lead to the development of other novel growth techniques, such as the template technique and allotaxy (9-10]. The template technique has proven successful in producing high quality, single crystal CoSi2 films on different substrates, but the quality of heterostructures has been limited by the inability to grow high quality Si back onto CoSi2 [11]. Successful construction of heterostructures for device applications is often complicated by differences in interfacial (surface) free energies. Any attempt to modify the energetics, or possibly the kinetics, of silicide heteroepitaxy requires an understanding of the interfacial energetics associated with growth. In this study, we have investigated the energetics associated with the formation of CoSi 2 in Si. Relative ratios of interfacial free energies were extracted by studying the equilibrium shape of small CoSi2 precipitates. Buried precipitates (i.e. CoSi2 equilibrated within the Si lattice) and surface precipitates (silicide equlibrated at the Si surface) were both used to interpret the equilibrated shape. Studies of small volumes of silicide isolates the energetics of formation by reducing the role of long range diffusion in shape formation. In general, the equilibrium shape
Mat. Res. Soc. Symp. Proc. Vol. 311. ©1993 Materials Research Society
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of a crystalline volume can be predicted if the interfacial energies of that particular material is known. Using a Wulff approach, a common o
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