(53d) New Mechanism for 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 photostimulation. We have circumvented these problems with an experimental design in which heating and illumination can be decoupled. The present work describes the evidence for a photostimulated diffusion mechanism based on electrostatic coupling between interface and Si bulk.
Photostimulated effects on diffusion of boron were studied in ion implanted crystalline silicon samples. Low intensity illumination (2 W/cm2) was used for nonthermal photostimulation in conjunction with resistive heating of the substrate. The samples were annealed at different temperatures and dopant diffusion and activation data was compared between experiments with and without illumination. The results showed that light interacts with the charged defects at Si-SiO2 interface and modulates the electrical field arising from near-surface band bending. The effects of this modulation exhibited profound effect on diffusion profile evolution near the surface and in the bulk. Simulations using continuum equations for the reaction and diffusion of defects was employed to elucidate underlying physical mechanism of this effect.