Method for producing thin film transistor

1. A method for producing a thin film transistor including an oxide semiconductor layer, the method comprising:

depositing an oxide semiconductor film above a substrate by a sputtering method; and

forming the oxide semiconductor layer into a predetermined shape by processing the oxide semiconductor film,

wherein, in the depositing of the oxide semiconductor film, a first oxide semiconductor film is deposited by using a first power density, and a second oxide semiconductor film is deposited on the first oxide semiconductor film by using a second power density different from the first power density,

the first oxide semiconductor film has a thickness of 10 nm or more, and

the second oxide semiconductor film has a thickness of 5 nm or less.

2. The method for producing a thin film transistor according to claim 1 ,

wherein the second power density is greater than the first power density.

3. The method for producing a thin film transistor according to claim 2 ,

wherein the second power density is 110% or more of the first power density.

4. The method for producing a thin film transistor according to claim 1 , comprising:

forming a gate electrode above the substrate;

forming a gate insulation film on the gate electrode;

forming the oxide semiconductor film on the gate insulation film in the depositing of the oxide semiconductor film;

forming the oxide semiconductor layer into a predetermined shape by processing the oxide semiconductor film in the forming of the oxide semiconductor layer;

forming an insulation layer on the oxide semiconductor layer so as to expose a portion of the oxide semiconductor layer; and

forming a source electrode and a drain electrode on the insulation layer so as to be connected to the exposed portion of the oxide semiconductor layer.

5. The method for producing a thin film transistor according to claim 1 ,

wherein the oxide semiconductor film includes a transparent amorphous oxide semiconductor.

6. The method for producing a thin film transistor according to claim 5 ,

wherein the transparent amorphous oxide semiconductor is InGaZnO.

7. The method for producing a thin film transistor according to claim 1 ,

wherein the depositing of the oxide semiconductor film includes, in order:

depositing the first oxide semiconductor film using the first power density;

temporarily stopping a plasma discharge;

switching the first power density to the second power density;

restarting the plasma discharge; and

depositing the second oxide semiconductor film using the second power density.

8. The method for producing a thin film transistor according to claim 7 ,

wherein the first oxide semiconductor film and the second oxide semiconductor film are deposited in a same chamber, and the plasma discharge is temporarily stopped without breaking a vacuum in the same chamber.

9. The method for producing a thin film transistor according to claim 1 ,

wherein, in the depositing of the oxide semiconductor film, the first power density is changed gradually to the second power density in an analog manner.

10. The method for producing a thin film transistor according to claim 1 ,

wherein, in the depositing of the oxide semiconductor film, the first power density is switched to the second power density in a digital manner.

11. The method for producing a thin film transistor according to claim 1 , further comprising:

forming an insulating layer above the substrate to cover the oxide semiconductor layer, the insulating layer including a three-layer structure of a silicon oxide film, an aluminum oxide film, and a silicon oxide film.