Transparent amorphous oxide semiconductors (TAOS) represented by IGZO have excellent characteristics such as high mobility, and a low off current compared with amorphous Si (a-Si) semiconductor. Furthermore, TAOS can be deposit by standard sputtering systems. Large area fabrication at a lower cost than low temperature polysilicon semiconductor (LTPS) is also possible. Therefore, there is a possibility to manufacture a display with high definition / low power consumption at low cost. However, many studies have shown the insufficient electrical reliability of IGZO TFTs. Although application to flexible displays is expected, in the conventional fabrication of IGZO TFTs, post annealing processes at a high temperature of 300°C is necessary. Therefore, low temperature formation of IGZO TFT is required.

The inductively coupled plasma (ICP) sputtering system under development is characterized by its capacity to deposit high density films and high target usage efficiency. Figure 1 shows the schematic of the ICP sputtering system. In this system, the antenna is installed in the chamber, and a high-frequency generator is connected to the antenna. ICP is generated in the chamber by applying high-frequency to the antennas. In this device, since the magnetic field is not arranged on the target, ICP diffuses uniformly into the chamber. In order to perform sputtering, a negative bias voltage is applied to the target, ions are pulled from the ICP, and the target material is deposited on the substrate. The target material was uniformly consumed, and usage efficiency showed a very high value of 85%. In addition, since antennas and targets are connected to independent power supplies, independent control is possible. By increasing the high-frequency power applied to the antennas and controlling the plasma density, deposition is possible without lowering deposition rate even at low target voltage. We considered that independent control of RF power and target voltage is important because both may be related to film density. The density of the IGZO films deposited by changing the RF power and the target voltage was obtained using X-ray reflectivity. The results are shown in figures 2 (a) and (b). The relationship between RF power and film density could not be confirmed. When the target voltage was increased, the film density decreased. Therefore, performing deposition at a lower target voltage yielded a high-density IGZO film. Under all conditions, the IGZO films exhibited a film density of more than 6.0 g/cm³. In this study, the bottom-gate top-contact TFTs were fabricated by low-temperature process using high-density two-layer structure IGZO film deposited by ICP sputtering system, and the transfer characteristics and reliability were examined.

Bottom-gate top-contact TFTs structure is shown in figure 3. The two-layer structure IGZO (In:Ga:Zn = 1:1:1) films were deposited by ICP sputtering system with different O₂/Ar gas ratio condition at room temperature. The TFTs were subjected to post annealing at 150°C or 200°C or 250°C for 2 h under the O₂/N₂ mixed gas atmosphere. The passivation of the photosensitive polysilsesquioxane was coated using a straightforward solution process for samples annealed at 250°C, and baked at 250°C. The TFTs were subjected to a positive gate bias stress test (PBS) by applying a gate voltage (V_gs) of 20 V, for a total stress time of 10000 s.

Figure 4 show transfer characteristics of each post annealing temperature. When no annealing is performed, the TFT shows switching behavior but the threshold voltage (V_th) became negative. When post annealing was carried out, V_th became positive. Transfer characteristics at annealing temperature of 150°C to 250°C exhibited similar values. The electric field effect mobility of V_ds = 0.5 V obtained from the transfer characteristic at post annealing temperature of 150°C was 7.9 to 8.8 cm²/Vs. The results of PBS are shown in figure 5. Threshold voltage shift (ΔV_th) was used as an index of reliability. ΔV_th of post annealing temperature of 150°C, 200°C and 250°C were 0.73 V, 0.75 V, 0.66 V, respectively. A sample with post annealing temperature at 250°C coated with polysilsesquioxane passivation has a ΔV_th = 0.09 V.

The reason why TFTs showing comparatively high-mobility can be fabricated even through a low-temperature process is considered to be the deposition of high-density films with low vacancy after deposition by the ICP sputtering system. In addition, it was found that the TFT with the annealing temperature of 150°C has the same reliability as the TFT with the annealing temperature of 250°C. We are set to demonstrate TFTs showing high-reliability at low-temperature process by using passivation that can be coated even at low temperatures.