Amorphous (non-crystalline) oxide semiconductors used in a TFT have a high carrier mobility and a large optical bandgap as compared to generalized amorphous silicon (a-Si), and can be deposited at low temperature. Thus, the amorphous oxide semiconductors are expected to be applied to next-generation display devices required for large size, high resolution, and high-speed driving, as well as resin substrates with a low heat resistance, and the like. In formation of the above oxide semiconductor (film), a sputtering method is preferably used which involves a sputtering target made of the same material as the film. The thin film formed by the sputtering method has excellent in-plane uniformity of the composition or thickness in the direction of the film surface (in the in-plane direction) as compared to thin films formed by ion plating, vacuum evaporation coating, and electron beam evaporation. The sputtering method has an advantage that can form the thin film of the same composition as that of the sputtering target. The sputtering target is normally formed by mixing, sintering, and mechanically processing oxide powders.
The compositions of the oxide semiconductor used in the display device include, for example, In-contained amorphous oxide semiconductors, such as “In—Ga—Zn—O, In—Zn—O, or In—Sn—O (ITO)”. However, the oxide semiconductor composition contains “In” as a rare metal, which might lead to the increase in material cost during mass production processes. For this reason, another oxide semiconductor appropriate for the mass production is proposed which can reduce the material cost without using the expensive element “In”. The oxide semiconductor is a ZTO-based amorphous oxide semiconductor formed by adding “Sn” to “Zn”. Patent literatures 1 to 4 disclose sputtering targets useful for manufacturing a ZTO-based oxide semiconductor film.
Patent literature 1 proposes a method for suppressing the occurrence of abnormal discharge or cracking during sputtering by controlling the composition of an oxide sintered body not to contain a tin-oxide phase by burning for a long time. Patent literature 2 proposes a method for suppressing the abnormal discharge during sputtering by performing two-stage processes, namely, a temporary powder burning process at a low temperature of 900 to 1300° C. and a main powder burning process to increase the density of the ZTO-based sintered body. Patent literature 3 proposes a method for improving the conductivity of an oxide sintered body and increasing the density thereof by containing a spinel AB2O4 compound in the sintered body. Patent literature 4 proposes a method for obtaining a dense ZTO-based sintered body by performing two-stage processes of a temporary power burning process performed at a low temperature of 900 to 1100° C. and a main powder burning process.
Patent literature 5 proposes a ZTO-based sputtering target having a low In content, as a sputtering target for formation of a transparent conductive film which has a low specific resistance and a high relative density even after the decrease in In content of the ITO. In general, when the In content of the ITO is decreased, the relative density of the sputtering target becomes lower, but the specific resistance of the bulk is increased. However, in the technique disclosed in Patent literature 5, a bixbyite structure represented by In2O3, and a spinel structure represented by Zn2SnO4 coexist in the target, which achieves the sputtering target having a high density and a small specific resistance, and which can suppress the abnormal discharge in the sputtering.