Nucleation of a new phase from the interaction of two adjacent phases: Some silicides
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Nucleation of a new phase from the interaction of two adjacent phases: Some silicides F.M. d'Heurle IBM Research Center, P. O. Box. 218, Yorktown Heights, New York 10598 and Institutetfor Mikrovdgsteknik, Kungliga Tekniska Hogskolan S-100 44 Stockholm, Sweden (Received 12 March 1987; accepted 11 September 1987)
The reactions of metal layers with their silicon substrates resulting in the formation of various silicides are considered generally not only as phenomena common to all diffusion couples where new phases are formed, but also as typical of all transitions from two to three phases. The conditions under which such transitions will display the same characteristics as encountered in the usual one-to-two phase transitions (condensation, crystallization, boiling) are analyzed by comparison to the classical theory of nucleation. Because of the lack of knowledge about the exact values of the relevant parameters, the discussion is carried out mostly in descriptive thermodynamic terms. Although nucleation effects are analyzed in general terms, the main focus of attention is a class of reactions where nucleation dominates the formation of a new phase; a salient feature of these reactions is the absence of any equilibrium temperature, although the nucleation temperatures are relatively well defined within narrow limits. Nucleation effects are correlated to such material characteristics as the stability of the nucleated phases, and to such kinetic characteristics as the sequence of phase formation. The modification of the energy levels of the different phases brought about by stress, ion bombardment, or the replacement of usual phases by metastable ones, are considered with respect to their effect on nucleation processes. The nearly total absence of literature references to nucleation in metalmetal diffusion couples is discussed with respect to some specific aspects of the metal-silicon reactions.
I. INTRODUCTION Nucleation is generally understood to refer to the initial process of creating a new phase, for example, liquid water, from another phase, water vapor, as a result of a temperature excursion that carries the system through some critical temperature (in the case at hand, at normal atmospheric pressure, 100 °C). The processes involved in such transformations have been the object of careful observations for a long time, at least two centuries; a very good historical perspective on this subject is found at the beginning of Ref. 1. The special case of metal vapors condensing on substrates and forming thin solid films is considered in all texts dealing with thinfilm preparation.2'3 Solidification and the precipitation of a new phase, such as CuAl2, from a supersaturated solution (of copper in aluminum) have an immense technological importance that motivates the attention being paid to nucleation phenomena in all metallurgical treatises.4"6 (Detailed discussions are found in Refs. 7 15.) In all cases, nucleation phenomena are controlled by the necessity of creating an interface between the nucleated phase and the orig
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