Effects of masking oxide on diffusion into silicon

During semiconductor processing it is common to use check slices to monitor individual process steps. Unfortunately, the sheet resistance values obtained from check slices do not always agree with actual resistor values made on slices diffused at the same time. The effect has been studied for the case of boron deposition from a vapour source and found to be real and due to the masking oxide which surrounds the resistor, but which is not present on the check slice. For a typical base diffusion, the sheet resistance in the masked region is about 30% higher than in the unmasked check slice. A series of experiments has shown that this effect occurs at the deposition stage and is independent of background concentration, surface orientation, oxide thickness and oxide growth conditions. On the other hand, the effect disappears if the boron is ion implanted, if a `spin-on¿ source is used, or if silicon nitride is used as the mask instyead of silicon dioxide. This result has implications not only for resistor design but also for active device modelling, since practical devices are oxide masked, but the profiles are measured on large-area check slices.