Patent ID: 7605407

Claim:
A semiconductor device comprising: a semiconductor substrate; a gate stack on the semiconductor substrate; and a stressor adjacent the gate stack and having at least a portion in the semiconductor substrate, wherein the stressor comprises an element selected from the group consisting essentially of germanium and carbon and having a lattice constant different from a lattice constant of the semiconductor substrate, and wherein the stressor comprises: a lower portion comprising the element, wherein the element in the lower portion has a first atomic percentage; and a higher portion on the lower portion, wherein the element in the higher portion has a second atomic percentage substantially greater than the first atomic percentage, wherein an interface between the lower portion and the higher portion of the stressor comprises a first portion and a second portion, and wherein the first portion is between a bottom of the higher portion and a top of the lower portion of the stressor, and the second portion is between sidewalls of the higher and the lower portions of the stressor.