Dopant diffusion-retarding barrier region formed within polysilicon gate layer

Abstract

A diffusion-retarding barrier region is incorporated into the gate electrode to reduce the downward diffusion of dopant toward the gate dielectric. The barrier region is a nitrogen-containing diffusion retarding barrier region formed between two separately formed layers of polysilicon. The upper layer of polysilicon is doped more heavily than the lower layer of polysilicon, and the barrier region serves to keep most of the dopant within the upper layer of polysilicon, and yet may allow some of the dopant to diffuse into the lower layer of polysilicon. The barrier region may be formed, for example, by annealing the first polysilicon layer in an nitrogen-containing ambient to form a nitrided layer at the top surface of the first polysilicon layer. The barrier region may alternatively be formed by depositing a nitrogen-containing layer, such as a silicon nitride or titanium nitride layer, on the top surface of the first polysilicon layer. The thickness of the nitrogen-containing layer is preferably approximately 5-15 Å thick. Any nitrogen residing at the top of the gate dielectric may be kept to a concentration less than approximately 2%. The present invention is particularly well suited to thin gate dielectrics, such as a those having a thickness of approximately 25-60 Å, when using a p-type dopant, such as boron.