Origin of Lower Film Density and Larger Defect Density in Amorphous In–Ga–Zn–O Deposited at High Total Pressure

Amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) are employed in current flat-panel displays. It is known that deposition conditions and post-deposition thermal annealing affect structure and electrical properties of a-IGZO thin films. It was previously reported that total pressure during sputter-deposition deteriorates subthreshold swing, defect density and operation characteristics of a-IGZO TFTs. Here, we provide comprehensive results on effects of total pressure on film density, chemical composition, and TFT characteristics. Rutherford backscattering measurements detected a small amount of argon incorporated in all of the a-IGZO films. We found that increasing the total pressure deteriorated TFT characteristics; i.e., saturation mobility was dropped from 10 to 4 ${\hbox{cm}}^{2}/({\hbox{V}}\cdot {\hbox{s}})$ , subthreshold swing was increased from 0.2 to 0.5 V/dec, and threshold voltage was positive-shifted from 2 to 15 V. It is related to increased oxygen concentration and decreased weight density of the a-IGZO films. Thermal desorption spectra showed that amounts of weakly-bonded oxygen increased as the total pressure increased, which is considered to be related to the TFT deterioration.