Method for preparing cap layer of embedded epitaxial silicon-germanium layer

The present invention provides a method for preparing the cap layer of an embedded epitaxial silicon-germanium layer. The method comprises a step of providing a semiconductor substrate with the formation of an epitaxial silicon germanium layer whose two sides are provided with shallow trench isolation structures, a step of forming a monocrystalline silicon layer on the epitaxial silicon germanium layer to cover the epitaxial silicon germanium layer, a step of forming a polycrystalline silicon layer on the monocrystalline silicon layer and forming the cap layer by the monocrystalline silicon layer and the polycrystalline silicon layer together, and a step of carrying out annealing process and nickel silicification process on the cap layer sequentially. According to the method, the problem that a cap layer which completely covers an epitaxial silicon germanium layer can not be formed on the embedded epitaxial silicon-germanium layer in the prior art is solved, the reaction of subsequent nickel silicification process and the silicon germanium of the epitaxial silicon germanium layer is prevented, and a brought stress problem is improved.