Boron Diffusion from a solid source to form a BJT base: a case study

Boron is the most commonly used p-type dopant for silicon, principally because it is the only column III element that can be masked by SiO₂. The first step, called the predeposition, is an open tube diffusion process that involves the gaseous transfer of a compound containing the dopant to the Si wafer. The gas may be supplied in several different ways, but in almost all cases the final chemical reaction when it reaches the silicon is:

Boron, in the form of B₂O₃, diffuses from the source to the wafer. The surface concentration during such constant source diffusion is usually limited by the solid solubility of the dopant in silicon. The boron predeposition transfers boron from a solid source to the wafer. The borosilicate glass (BSG) is in contact with the silicon surface. A concentration gradient is formed, and since this process occurs at high temperature, diffusion will occur. If the boron source used is a solid source of boron nitride wafers, Si wafers are stacked next to oxidized wafers as shown. At the pre-deposition temperature, concentration gradient induced diffusion is established between the source wafers and the silicon wafers.