Stability of Thin Film Amorphous Metal Alloys
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Kruislaan 407,
1098 SJ
Cornell University, Ithaca,
ABSTRACT Thermal stability of self supporting amorphous films is reviewed in terms of a simple kinetic model. A semi-empirical relationship between the crystallization temperature and the heat of hole formation is applicable only for systems with negative heat of mixing. Interaction of amorphous diffusion barriers with Si substrates and Al overlayers is also reviewed for near noble-refractory alloys. If their crystallization temperature is higher than their reaction temperature, there is no correlation between the two parameters. The reaction takes place when the annealing temperature reaches a value at which compounds can be formed of both constituents of the alloy. Finally, comparison is made to reactions of crystalline alloys with Si. 1. INTRODUCTION Knowledge of the thermal and irradiation stability of amorphous metal alloys adds to our fundamental understanding of the criteria responsible for amorphous phase formation. Mcreover, the stability of amorphous metals is of crucial importance to any possible application of these unique materials. Thin film amorphous metal alloys have become subject of investigation recently because amorphous films may be applied in widely different areas, such as diffusion barriers in multilevel metallization schemes for integrated circuits; or as protective coatings to improve corrosion, friction or wear properties of metal tools; or as magnetic recording material in information storage techniques. Thin film amorphous metal alloys are frequently the product of modern methods in materials modification with ion, laser or electron beams. Once these metastable films are formed their thermal stability and reactivity become key issues for evaluations. The outline of this paper is to first review the crystallization temperature of self supporting amorphous films and of thin amorphous films in contact with inert substrates as Si0 2 . Then recent experiments on reaction of amorphous metal alloys with Si and with Al will be discussed. These amorphous metal alloys consist of near noble - and refractory metals and two cases will be distinguished: 1) the crystallization temperature of the amorphous alloy lies above the temperature at which reaction with Si and with Al starts, 2) the crystallization temperature of the binary alloy is below its reaction temperature with Si. 2. THERMAL STABILITY It has been proposed [1] that the crystallization of amorphous alloys may be kinetically limited: their stability varies with the energy required to produce and move point defects. Accordingly, high enthalpies of formation and migration suggest a hindrance to crystallization. In this model the crystallization temperature Tc, of amorphous alloys is a key parameter. In recent work, Buschow [2] has observed that there is no correlation between Tc and
Mat. Res. Soc. Symp. Proc. Vol. 54. ' 1986 Materials Research Society
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the heat of alloy formation, instead Tc was found to vary linearly with the energy, AHv, required to produce a hole the size of the
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