(566f) Optically Stimulated Diffusion In Ultrashallow 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 photostimulation, 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.
Photostimulated effects were studied using the diffusion of isotopically labelled 30Si tracer in an epitaxial 28Si matrix. 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. 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. A physical model based on photostimulated changes in interstitial charge state explains much of the behavior. A non-equilibrium steady-state model was formulated to describe the electronic occupation of defect levels under photo-stimulation. The model is based on Shockley-Read statistics for an arbitrary distribution of energy levels.
Photostimulated diffusion of dopants, however, exhibited more complicated features. Depending on annealing temperature and time, boron diffusion in silicon could be either enhanced or inhibited. Dopant activation was similarly affected. Simulations using continuum equations for the reaction and diffusion of defects were used to determine whether illumination affects cluster dynamics or steady state boron diffusion.