Flat band voltage shift and oxide properties after rapid thermal annealing

The flat band voltage (V_fb) shift observed for MOS samples exposed to rapid thermal annealing (RTA) (N₂, 20 s, 1040°C) is examined for (1 0 0), (1 1 0) and (1 1 1) orientation-silicon substrates. Using a mercury gate C-V system, the V_fb shift can be attributed to changes in the electronic properties of the oxide layer and not polysilicon gate effects, as had previously been suggested. In addition, this work indicates that the flat band voltage shift results from a reduction of interface and fixed oxide charge due to the RTA process. The interface and oxide charge densities are related to the density of available bonds for each surface orientation, both before and after an RTA step. Based on these results, we argue that the V_fb shift following RTA is primarily due to a reduction of fixed positive charge in the oxide, and to a lesser degree, to a reduction of negative interface charge. The net effect is that the RTA step reduces the total oxide charge density.