SILICON NITRIDE AS A MASK IN PHOSPHORUS DIFFUSION

By means of tracer and activation experiments it is shown that during phosphorus diffusion in silicon nitride diffusion profiles are produced, which are very similar to the profiles in silicon dioxide processed in the same experiment. When oxygen was used as part of the carrier gas as is customary in semiconductor technology, a glass was produced in both cases which consisted of SiO2 and P2O5 and whose identity was proved by i.r. absorption and determination of the refractive index. When the medium does not contain oxygen, part of the nitride is transformed to silicon phosphide which is incorporated in the glass or escapes at higher temperatures. When oxygen is present, the silicon phosphide occurs briefly in an intermediate reaction. In this way the normal, slow oxidation of the nitride is accelerated and phosphorus pentoxide acts as a catalyst. The analogous formation of glass from oxide and nitride is the reason why silicon nitride exhibits a similar masking effect as silicon dioxide in phosphorus diffusion, even though it masks better than silicon dioxide against the diffusion of a number of other elements. © 1969.