Semiconductor device

Example 1

[0180]In this example, a substrate in which a tungsten film was formed over an oxide semiconductor film was prepared as a sample, and a cross section of the sample before and after bake treatment was observed. The cross-sectional observation of the sample will be described with reference to FIGS. 10A and 10B.

[0181]First, samples for performing cross-sectional observation were manufactured.

[0182]A 100-nm-thick oxide semiconductor film was formed over a glass substrate by a sputtering method using an In—Ga—Zn—O-based metal oxide target (In2O3:Ga2O3:ZnO=1:1:1 [molar ratio]) under a mixed atmosphere of argon and oxygen (argon:oxygen=30 sccm:15 sccm) at room temperature under the following conditions: the distance between the substrate and the target was 60 mm, the pressure was 0.4 Pa, and the direct current (DC) power was 5 kW.

[0183]Then, a 150-nm-thick tungsten film was formed over the oxide semiconductor film by a sputtering method using a tungsten target.

[0184]Through the above

Example 2

[0190]In this example, results of manufacturing a transistor in which tungsten is used for source and drain electrodes and comparing the transistor characteristics before and after a light bias test will be described with reference to FIG. 11.

[0191]First, a manufacturing method of the transistor used in this example will be described below.

[0192]First, a 100-nm-thick silicon nitride film and a 150-nm-thick silicon oxynitride film were formed successively by a plasma CVD method over a glass substrate to form a base film, and then a 100-nm-thick tungsten film was formed as a gate electrode over the silicon oxynitride film by a sputtering method. The tungsten film was etched selectively, thereby forming the gate electrode.

[0193]Then, a 30-nm-thick silicon oxynitride film was formed as a gate insulating film over the gate electrode by a plasma CVD method.

[0194]Next, a 15-nm-thick oxide semiconductor film was formed over the gate insulating film by a sputtering method using an In—G

Example 3

[0208]In this example, results of calculating, in a stacked-layer structure of an oxide semiconductor layer and an electrode (source electrode or drain electrode) shown in FIG. 12C, an energy change between before and after transfer of oxygen from the oxide semiconductor layer to the electrode will be described.

[0209]Specifically, in the stacked-layer structure, an energy change between before and after an oxygen vacancy was generated in the oxide semiconductor layer and oxygen was inserted between lattices in the electrode was calculated. By comparison of the energy before and after oxygen was extracted from the oxide semiconductor layer to be inserted between lattices in the electrode, the stability after oxygen has transferred was evaluated.

[0210]As a material of the oxide semiconductor layer, an In—Ga—Zn—O-based oxide semiconductor (hereinafter referred to as IGZO) was used. As a material of the electrode, titanium (Ti), molybdenum (Mo), tungsten (W), and platinum (Pt) wer