The second stage of the diffusion process, called the drive-in, has as its objective the redistribution of the dopant deeper into the silicon, and is referred to as a constant source diffusion. For this process we first remove our silicon wafer from the pre-deposition furnace and strip off the SiO2 - B2O3 glass using HF. The HF actually attacks the SiO2, not the B2O3, so care must be used in the pre-deposition step to prevent the deposition of excessive amounts of B2O3. This is done mainly by assuring no H2O is present in the furnace, which would greatly increase the rate of transfer of B2O3 to the silicon. Also note the thin SiBx phase on the wafer is hydrophilic, so the wafer will not de-wet after etching. The actual drive-in is performed at very high temperatures (~1100°C). The drive-in is usually initiated in an oxidizing environment; the initial growth of SiO2 prevents the out-diffusion of the boron during the rest of the process.
The boron concentration near the surface after predeposition is too high and the junction depth is too shallow to act as a good base. After the BSG is removed from the surface of the wafer, a 'sourceless' diffusion (drive) will lower the surface concentration and simultaneously drive the dopant deeper into the wafer. The distribution can be approximated as a Gaussian profile:
