Investigation of IGZO Thin Film Transistors with Copper Electrode Under Varied Channel Lengths and Different Gate Insulator Annealing Temperatures

IGZO TFTs incorporating copper (Cu) electrodes undergo hydrogen treatment to prevent the formation of copper oxide on Cu. However, this treatment introduces hydrogen into IGZO, increasing donor states, boosting majority carriers, and resulting in a negative threshold voltage (Vth) shift, thereby posing reliability concerns. This study explores the characteristics of IGZO TFTs with varying channel lengths and gate insulator (GI) annealing temperatures. Devices subjected to GI annealing at 350°C exhibit a negative $\mathrm{v}_{\mathrm{t}\mathrm{h}}$ and higher mobility compared to those annealed at 280°C. In addition, due to the hydrogen plasma treatment from interlayer dielectric, hydrogen lateral diffusion will cause the difference of $\mathrm{V}_{\mathrm{t}\mathrm{h}}$ in short channel and long channel devices. The capacitance-voltage diagram also shows the C-V earlier turn-on in short channel devices. After positive bias temperature stress, the results indicate devices under GI annealing at 350°C will display a smaller $V_{\text {th }}$ shift. Plausible physical models are proposed subsequently.

I. Introduction

Nowadays, to meet the emerging development of novel displays, high frequency operation or frame rate become paramount. The integration of copper (Cu) electrodes into large-scale and automotive displays has become prevalent, primarily aimed at reducing resistive-capacitive delays owing to low resistance of Cu [1]. However, Cu electrode is easy to be oxidized and becomes copper oxide. Therefore, hydrogen treatment is applied to make copper oxide reduction and becomes Cu electrode [2]. In traditional liquid crystal displays or active matrix organic light-emitting diodes, thin film transistors (TFTs) assume a pivotal role in the driving circuit [3]. In addition, metal oxide is an excellent material in TFTs due to the advantages of IGZO, such as high mobility, ultra-low off-state leakage, and great uniformity [4]. However, IGZO TFTs with copper electrodes remain an urgent reliability issue. Hydrogen's interaction with IGZO leads to the formation of interstitial hydrogen or hydrogen vacancies, acting as shallow donor states that contribute to a negative threshold voltage and high mobility. By analyzing the experimental results, plausible physical models are proposed to elucidate the diffusion of hydrogen from both the gate insulator (GI) and the interlayer dielectric (ILD).