Patent ID: 7892909

Claim:
A method for forming a semiconductor structure, the method comprising: providing a semiconductor substrate; forming a gate dielectric layer on the semiconductor substrate; forming a first silicon-containing layer on the gate dielectric layer, wherein the first silicon-containing layer is substantially free from p-type and n-type impurities; forming a second silicon-containing layer over the first silicon-containing layer, wherein the second silicon-containing layer comprises an impurity; etching the second silicon-containing layer in a region of the semiconductor substrate until the first-silicon containing layer is reached; forming a third silicon-containing layer over the first silicon-containing layer, second silicon-containing layer, and gate dielectric layer; removing the third silicon-containing layer formed over the second silicon-containing layer; and performing an annealing to diffuse the impurity in the second and third silicon-containing layers into the first silicon-containing layer.