Abstract
A masking structure using a sandwich of silicon dioxide and silicon nitride layers has been investigated for the masking of deep gallium and aluminum diffusions in silicon. It has been shown that a thermal oxide layer is necessary between the nitride layer and the substrate silicon in order to prevent reactions between the nitride and the silicon, and also to prevent the occurrence of cracks in the nitride during heat cycling. This thermal oxide must have a thickness of between 1000 and 2000Å. It has also been shown that a covering oxide layer is necessary over the nitride to prevent the reaction of gallium with the nitride film. Using the above masking structure, it has been shown that it is possible to mask gallium diffusions of more than 275 μm and aluminum diffusions of more than 450 μm in silicon. The lateral diffusion at the edge of diffusion windows has been demonstrated to be about 125% of the junction depth for gallium diffusions and less than 100% of the junction depth for aluminum diffusions. These measurements allow the conclusion that the silicon dioxide‐silicon nitride sandwich structure is an excellent mask for performing deep planar p‐type diffusions in silicon.