Optically stimulated diffusion in ultra-shallow junction formation

Formation of pn junctions in advanced Si-based transistors employs rapid thermal annealing after ion-implantation in order to increase the activation of dopants. There has long been suspicion that the strong lamp illumination required for this procedure may nonthermally influence the diffusion of dopants. Identification of such effects is difficult in conventional RTA geometries because lamps provide both heating and photo stimulation, and because the interpretation of conventional dopant diffusion experiments is impeded by complex dopant-defect interactions. We have circumvented these problems with a new experimental design in which heating and illumination can be decoupled. Data interpretation has been simplified by examining the motion of isotopically labelled ³⁰Si tracer in an epitaxial ²⁸Si matrix using SIMS depth profiling, and more recently with dopants such as arsenic in standard ion-implanted Si. Results for self-diffusion show that for n-type Si, self-diffusion rates are increased nonthermally by more than an order of magnitude for modest illumination intensities of roughly 1 W/cm². Diffusion rates for silicon self-diffusion are shown to change by factors of up to 25 in response to optical fluxes on the order of 1 W/cm². Results depend on doping type; the rates of both interstitial formation and migration are affected in the case of n-type material. There is no comparable effect for p-type material, however. Significant effects appear for boron as well in standard implantation experiments.