Excimer laser induced heating, melting, and mass diffusion in crystal silicon in nanosecond and nanometer scale

Heat and mass transfer at the nanosecond time scale and the nanometer length scale in pulsed laser fabrication of ultra-shallow p+-junction is studied in this work. A technique is developed to fabricate the ultra-shallow p+-junctions with pulsed laser doping of crystalline silicon with a solid spin-on-glass (SOG) dopant, through the nanosecond pulsed laser heating, melting, and boron mass diffusion in the 100 nm thin silicon layer close to the surface. High boron concentration of 10²⁰ atoms/cc and the 'box-like' junction profile are achieved. The key mechanism determining the 'box-like' junction shape is found to be the melt-solid interface limited diffusion. The ultra-shallow p+-junctions with the depth from 30 nm to 400 nm are successfully made by the excimer laser. The optimal laser fluence condition for SOG doping is found about 0.6 - 0.8 J/cm² by studying the ultra-shallow p+-junction boron profiles measured by the secondary ion mass spectroscopy versus the laser fluence and the pulse number. The 1D numerical analysis agrees reasonably with the experiment, within the available physical picture. Possible mechanisms such as boron diffusivity dependence on the dopant concentration in the molten silicon are proposed.