Method for fabricating semiconductor device

1. A method for fabricating semiconductor device, comprising:

providing a substrate having at least one fin-shaped structure thereon, wherein the fin-shaped structure comprises a top portion, a bottom portion, and a hard mask thereon;

forming a first shallow trench isolation (STI) around the fin-shaped structure;

using the hard mask to remove the first STI;

forming a spacer on the hard mask and adjacent to the top portion of the fin-shaped structure;

completely removing the first STI followed by removing part of the bottom portion of the fin-shaped structure;

forming an epitaxial layer on the substrate to surround the bottom portion of the fin-shaped structure;

transforming the bottom portion of the fin-shaped structure into the epitaxial layer; and

removing part of the epitaxial layer.

2. The method of claim 1 , further comprising:

removing the spacer after removing part of the epitaxial layer;

depositing an insulating layer on the substrate;

planarizing the insulating layer;

removing the hard mask; and

etching back the insulating layer for forming a second STI.

3. The method of claim 1 , wherein the hard mask and the spacer comprise silicon nitride.

4. The method of claim 1 , further comprising performing a thermal anneal process after forming the epitaxial layer to transform the bottom portion of the fin-shaped structure into the epitaxial layer.

5. The method of claim 1 , wherein the fin-shaped structure comprises silicon.

6. The method of claim 1 , further comprising transforming the entire bottom portion of the fin-shaped structure into the epitaxial layer.

7. The method of claim 1 , further comprising removing all of the epitaxial layer not covered by the top portion of the fin-shaped structure.

8. The method of claim 1 , wherein the epitaxial layer comprises silicon germanium.

9. The method of claim 8 , wherein the semiconductor device comprises a NMOS transistor.