The Effects of Preheating in Millisecond Anneals on Dopant Activation in Silicon

MOS transistor scaling faces major challenges from the rapid rise in the parasitic resistance as the source/drain (s/d) contact area is reduced. Millisecond annealing (MSA) can provide efficient dopant activation at s/d regions and reduce contact resistivity, but process temperatures are increasingly limited by device integration requirements. Optimization of annealing conditions requires understanding of how different stages of the heating profile affect the process outcome. This paper uses the effective process time concept, where experimentally measured temperature-time data are weighted according to Arrhenius kinetics, to explore the effects of multi-stage MSA recipes on dopant activation. The approach was applied to self-amorphized Si wafers implanted with high doses of P or As at low energies. For heavy As doping, slow cooling cycles strongly affected the electrical activation, whereas for P the hottest stage of the anneal was most significant. The effect of the preheating stage was isolated by using heating cycles where the preheat conditions varied while the slow cooling stages were very similar. Significant differences in dopant activation were found for both MSA processes with different preheat stages and also for low temperature spike anneals with different ramp up characteristics. The results suggest that the final dopant activation depends on more than just peak temperatures and cooling cycles, and that optimization of preheating provides new opportunities for improved activation.