Solid--solid vacuum diffusion processes in silicon

Two diffusion techniques are described making use of doped silicon wafers as the diffusion source. The processes are carried out either in an evacuated capsule or inside a high vacuum furnace. Using the capsule technique, boron and phosphorus layers in silicon have been produced, with sheet resistances ranging from approximately 20–4000 ohms/□, and junction depths between approximately 0.3 and 15 µm. Good uniformity and reproducibility of results were obtained, with relative variations below 3% across one wafer, from wafer to wafer, and from run to run. In a series of boron diffusions, the high vacuum technique yielded results comparable to the capsule technique. It offers more experimental flexibility but requires masking layers other than for the production of planar devices.