To achieve the next generation displays, it is becoming increasingly important to develop backplane technology with superior characteristics such as high mobility, high stability, and high transparency. Among the several candidates for suitable backplane in high definition display, amorphous In-Ga-Zn-O (a-IGZO) oxide semiconductor TFTs have attracted much interest due to its high mobility, optical transparency, and large area uniformity. However, due to the intrinsic problem of a-IGZO TFTs caused by oxygen vacancy and hydrogen, it is hard to control of threshold voltage (V_th) and stability under various stress conditions. One of them, hydrogen is the main factor closely related to reliability. According to previous studies, hydrogen acts as positive roles by defect passivation and also play negative roles in creating new defects in a-IGZO active layer. However, incorporation and diffusion of hydrogen into the active layer is an unavoidable issue during the TFTs fabrication, and it is important to control it so that hydrogen plays a positive role. So, in this study, we conducted experiments to verify the effect of hydrogen on the reliability by modifying the Al₂O₃ gate insulator layer (GI) deposition process using atomic layer deposition (ALD) method.

In order to verify the relationship between the hydrogen and electrical properties of a-IGZO TFTs, we fabricated top gate bottom contact (TGBC) structures and applied different GI deposition temperature (T_dep) for controlling the amount of hydrogen. [1] Al₂O₃ GI deposited using trimethylaluminum (TMA) and H₂O precursor. Each T_dep are 200, 250, 270 and 300 degree, respectively. As a result of Al₂O₃ single thin film analysis using secondary ion mass spectroscopy (SIMS) method, it was confirmed that the Al₂O₃ thin film deposited at high T_dep has a relatively small amount of hydrogen than the Al₂O₃ thin film deposited at low T_dep. And then, TFT devices a, b, c, and d were fabricated using Al₂O₃ GI with different T_dep of 200, 250, 270 and 300 degrees, respectively. As a result, there was no significant difference between the devices. All devices had subthreshold swing (SS), value of 0.178 ~ 0.225 V, turn on voltage (V_on) of -0.16 ~ -0.44V, hysteresis of 0.13 ~ 0.27 V and field effect mobility (μ_FE) of 9.6 ~ 10.55 cm²/Vs. This trend was similar to positive bias temperature stress (PBTS) and negative bias temperature stress (NBTS) reliability. However, the reliability of negative bias illumination stress (NBIS) was significantly different for each device. Under the NBIS condition, the V_th shift of each TFT was -4.36 V for a TFT, -4.36 V for b TFT, -3.72V for c TFT, -2.48 V for d TFT. These results indicate that the NBIS characteristic is improved as the T_dep of Al₂O₃ GI increases and proves that reliability varies with the amount of hydrogen. This suggests that as hydrogen increases, more hydrogen-induced defects are formed at the interface between GI and a-IGZO active layer, which causes trapping of positive charges. This phenomenon can be explained by non-bridging oxygen hole center (NBOHC) method, which is one of the positive charge trapping models, and the mechanism for the role of hydrogen in the a-IGZO TFTs can be identified. [2]

Base on these experimental results, we will propose the way to optimize condition of GI deposition process for high stability and high performance in a-IGZO TFTs.

[1] S.J. Yun, K.-H. Lee, J. Skarp, H.-R. Kim and K.-S. Nam, J. Vac. Sci. Technol. A, 15(6) (1997)

[2] M. Tsubuku, R. Watanabe, N. Ishihara, H. Kishida, M. Takahashi, S. Yamazaki, Y. Kanzaki, H. Matsukizono, S. Mori, T. Matsue, SID 2013 DIGEST. 169 (2013)