Ultra-shallow p⁺-junction formation in silicon by excimer laser doping - a heat and mass transfer perspective

A new technique is developed to fabricate the ultra-shallow p⁺-junctions with the depth from 30 nm to 400 nm. The ultra-shallow p⁺-junction is successfully made by the excimer laser doping of crystalline silicon with a solid spin-on-glass (SOG) dopant. High boron concentration of 10²⁰ atoms/cc and the `box-like' junction profile are achieved through the nanosecond pulsed laser heating, melting, and boron mass diffusion in the 100 nm thin silicon layer close to the surface. The key mechanism determining the `box-like' junction shape is found to be the melt-solid interface limited diffusion. 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 (SIMS) versus the laser fluence and the pulse number. Heat and mass transfer are studied at the nanosecond time scale and the nanometer length scale. 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.