Nitrogen implementation to minimize device variation

Abstract

A rapid thermal nitridation (RTN) process produces a nitrogen concentration gradient in an oxynitride layer to compensate for transistor threshold voltage effects from a thickness gradient in the oxynitride layer. The nitrogen concentration gradient is selected to allow greater dopant penetration through thicker gate dielectrics in PMOS transistors formed using the oxynitride layer. Any increases in threshold voltage due to thicker gate dielectrics are counteracted by corresponding decreases in threshold voltage due to dopant penetration, allowing consistent threshold voltage values to be maintained for all PMOS transistors on a single wafer. The nitrogen concentration gradient can be introduced by regulating the flow of nitrous oxide during RTN processing to cause an accumulation of atomic oxygen to develop within the process chamber. The atomic oxygen forms a concentration distribution that increases in the direction of nitrous oxide flow, and therefore removes incorporated nitrogen from the oxynitride layer in corresponding proportions.