Two-dimensional phosphorus diffusion for soft drains in silicon MOS transistors

Phosphorus has a considerably less steep concentration profile than arsenic. Therefore phosphorus is considered as an alternative dopand for soft drain concepts in future MOS devices. In-diffusion of phosphorus starting from a high surface concentration generatesexcess point defects which diffuse into the depth of the crystal and lead to a tail in the phosphorus concentration profile by considerably enhancing the phosphorus diffusion in this region. It is also well known that the interface between silicon and a non growing oxide acts as a sink for excess point defects. Since source/drain areas of MOS transistors are surrounded by gate and isolation oxides, the question arises how the resulting excess point defect distribution may influence the lateral and vertical diffusion profile of phosphorus and hence the channel length and the junction depth of the source/drain region in a MOS device. We extended the one-dimensional Fair-Tsai model of phosphorus diffusion into two dimensions and incorporated that the interface between silicon and a gate oxide acts as a sink for excess point defects and modifies their distribution. The appropriate code was implemented in the two-dimensional process simulation program LADIS. Based on this extended model two-dimensional simulations of phosphorus drains have been performed and compared to experimental results and to results from other numerical models. It turns out that the presence of the gate oxide reduces the tail in the phosphorus concentration profile, considerably in lateral direction and less pronounced in vertical direction. Limitations of the model will be discussed in detail.