Rapid thermal annealing of high-melting-point films on low-melting-point substrates

Rapid thermal annealing which involves fast heating and cooling rates, is used to activate dopants in thin-film structures yet minimize the dopant diffusion that occurs with excessive thermal exposure. Although the proper resulting electrical properties are the main concern, the structural behavior must also be considered. At the elevated annealing temperature, the heterostructure may be susceptible to both relaxation and yielding. However, the relative effect of these deformations is a function of the material properties, ramp-rate, annealing conditions, and wafer geometry, In particular, for a high-melting-point film on a lower-melting-point substrate, the substrate will experience the inelastic effects prior to the film. More specifically, because germanium has a significantly lower melting point than silicon, previously developed processing technology for silicon cannot be applied directly to germanium processing. A numerical model has been developed to account for the thermo-mechanical effects associated with rapid thermal annealing of relaxing materials. Numerical parametric studies have been conducted for rapid thermal annealing of a thin polysilicon film on a (111) germanium substrate in order to determine the optimum processing window. Results reveal that lower annealing temperatures that still fall within the RTA regime will minimize or even eliminate the plastic damage that could occur during thermal processing.