Evidence for nonthermal illumination-enhanced diffusion in implanted silicon

Summary form only given. Formation of pn junctions in advanced Si-based transistors employs rapid thermal annealing (RTA) of ion-implanted regions in order to increase the activation of dopants. The need for ever-shallower junctions with low sheet resistances has driven a shift to "spike" RTA with exceptionally high heating rates near 400°C/s. There has been suspicion that the strong lamp illumination required for this procedure may nonthermally influence the diffusion of dopants. Possible mechanisms include (a) photoenhanced formation of bulk vacancies and interstitials that mediate diffusion, and (b) photostimulated changes in the hopping mobility of existing interstitials. Both mechanisms involve changes in the average electrical charge state of the relevant species. 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 and the presence of built-in electric fields in typical test structures. In this work, we present direct, unambiguous experimental evidence for nonthermally enhanced surface diffusion Group III, IV and V elements on Si at illumination intensities as low as 1 W/cm², showing how activation energies can change by more than 0.5 eV, and pre-exponential factors by four orders of magnitude. We then discuss how similar mechanisms are likely to apply in the case of bulk transient enhanced diffusion of dopants.