Source/drain metal-dependent oxygen scavenging from the viewpoint of the decoupling between source/drain resistance and threshold voltage in InGaZnO thin-film transistors

Abstract

Reducing source-drain resistance (RSD) in oxide semiconductor thin-film transistors (TFTs) mainly impacts the performance and reliability of devices and circuits. Oxygen scavenging (OS) is commonly used during process integration to reduce RSD, including contact resistance between source-drain (S/D) metal and oxide semiconductors. Meanwhile, the lower RSD, the better, but the threshold voltage (VT) should be optimized depending on the application. Therefore, it is essential to decouple RSD and VT when applying OS. In this study, the OS effect depending on the S/D metal of amorphous InGaZnO (a-IGZO) TFT was investigated from the perspective of decoupling RSD and VT. As a result of comparing Cu, Ti, and Al as S/D metals, when Al, which has a high metal–oxygen (M−O) bond strength, was used as the source-drain metal, VT and RSD decreased and on-current (Ion) increased compared to when Ti was used. A comprehensive analysis of TFT's electrical characteristics (VT, mobility, Ion), X-ray photoelectron spectroscopy (XPS), subgap density of states (DoS), and lateral distribution of thermal equilibrium carrier concentration (n0(y)) indicates that the occurrence and diffusion of oxygen vacancy (VO) due to OS in the S/D region cause an increase in subgap DoS and gate-to-S/D overlap length (LOV) and a decrease in VT due to an increase in donor concentration in the center of the channel (NCH) due to a change in n0(y) profile. While RSD changes before and after post-annealing are ×1.087 (Cu), ×0.606 (Ti), and ×0.283 (Al), NCH changes are ×0.985 (Cu), ×1.267 (Al), and ×1.183 (Ti). The ΔVT’s before and after post-annealing are + 0.119 V (Cu), −0.206 V (Al), and − 0.045 V (Ti), while ΔIon’s are ×0.724 (Cu), ×1.222 (Al), and ×1.193 (Ti). Therefore, it is found that Ti is more advantageous than Al in terms of decoupling RSD and VT. Our result becomes more critical in employing oxide semiconductor TFTs as the back end of line (BEOL) devices because the phenomenon of VT being affected in improving RSD using OS can become more severe in high-temperature processes. Furthermore, our result suggests that when selecting S/D metals and annealing conditions for OS, it is necessary to fully consider not only the RSD reduction but also the degree to which RSD and VT can be decoupled. © 2025 The Authors