Method for fabricating semiconductor device

1. A method for fabricating semiconductor device, comprising:

forming a trench in a substrate;

forming a first oxide layer in the trench, wherein the first oxide layer is a single-layered structure made of silicon oxide;

forming a silicon layer on and directly contacting the first oxide layer without filling the trench completely, wherein a thickness of the silicon layer is less than a thickness of the first oxide layer;

performing an oxidation process to transform the silicon layer into a second oxide layer for filling the trench completely, wherein a maximum width of the second oxide layer in the trench is greater than a maximum width of the silicon layer in the trench; and

planarizing the second oxide layer and the first oxide layer to form a shallow trench isolation (STI).

2. The method of claim 1 , further comprising forming the silicon layer on the first oxide layer without filling the trench completely.

3. The method of claim 1 , further comprising transforming the silicon layer into the second oxide layer to fill the trench completely.

4. The method of claim 1 , further comprising performing an atomic layer deposition (ALD) process to form the first oxide layer.

5. The method of claim 1 , wherein the oxidation process comprises an in-situ steam generation (ISSG) process.

6. The method of claim 1 , wherein a thickness of the silicon layer is less than a thickness of the first oxide layer.

7. The method of claim 1 , wherein a thickness of the second oxide layer is greater than a thickness of the silicon layer.

8. The method of claim 1 , wherein the silicon layer comprises an amorphous silicon layer.

9. The method of claim 1 , wherein the first oxide layer and the second oxide layer comprise silicon oxide.