Transistor having recess channel and fabricating method thereof

1. A method for fabricating a transistor, comprising:

forming a trench in a substrate;

forming a polysilicon layer over the substrate and filled in the trench;

etching the polysilicon layer in a manner that a portion of the polysilicon layer having a greater thickness over an edge portion of a bottom surface of the trench than over a middle portion of the bottom surface of the trench remains in the trench;

oxidizing the entire remaining portion of the polysilicon layer to form an insulation layer over the bottom surface of the trench and forming a gate insulation layer over inner sidewalls of the trench to a smaller thickness than the insulation layer at substantially the same time; and

forming a gate electrode over the insulation layer and filled in the trench.

2. The method of claim 1 , wherein forming the polysilicon layer comprises forming an undoped polysilicon layer which is not doped with impurity ions.

3. The method of claim 1 , wherein forming the polysilicon layer comprises forming the polysilicon layer in a manner that a depression is generated over a middle portion of the trench.

4. The method of claim 1 , wherein etching the polysilicon layer in the manner that the portion of the polysilicon layer remains in the trench comprises performing an etch-back process.

5. The method of claim 4 , wherein the etch-back process comprises performing the etch-back process at a pressure ranging from approximately 0.4 Pa to approximately 0.6 Pa at a temperature ranging from approximately 500° C. to approximately 800° C. using sulfur hexafluoride (SF 6 ) gas.

6. The method of claim 1 , wherein oxidizing the remaining portion of the polysilicon layer to form the insulation layer and forming the gate insulation layer over the inner sidewalls of the trench comprise using nitrogen (N 2 ) gas and oxygen ( 0 2 ) gas mixed at a ratio of approximately 20:4 (SLPM) at a temperature ranging from approximately 1,000° C. to approximately 1,200° C.

7. The method of claim 1 , further comprising, after forming the trench:

oxidizing inner surfaces of the trench to form a first sacrificial oxide layer;

removing the first sacrificial oxide layer; and

forming a second sacrificial oxide layer.

8. The method of claim 7 , further comprising, after etching the polysilicon layer in the manner that the portion of the polysilicon layer remains in the trench, removing the second sacrificial oxide layer.

9. The method of claim 1 , wherein the insulation layer has a thickness ranging from approximately 2,000° C. to approximately 3,000° C.

10. The method of claim 1 , wherein the substrate comprises:

a silicon substrate; and

an epitaxial layer formed over the silicon substrate.