Investigation of Amorphous IGZO TFT Employing Ti/Cu Source/Drain and SiNx Passivation
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Investigation of Amorphous IGZO TFT Employing Ti/Cu Source/Drain and SiNx Passivation Young Wook Lee1, Sung-Hwan Choi1, Jeong-Soo Lee1, Jang-Yeon Kwon2 and Min-Koo Han1 1
School of Electrical Engineering, Seoul National University, Gwanak-gu, Seoul 151-742, Korea 2 Department of Material Science and Engineering, Seoul National University, Gwanak-gu, Seoul 151-742, Korea ABSTRACT We successfully fabricated a-IGZO TFTs employing a Ti/Cu source/drain (S/D) and SiNx passivation in order to reduce the line-resistance, as compared to most oxide TFTs that use Mo (or TCO) and SiO2 for their S/D and passivation, respectively. Although passivated with SiNx, the TFT exhibits good transfer characteristics without a negative shift. However, the TFT employing a Mo S/D exhibited conductor-like characteristics when passivated with SiNx. Our investigation suggests that the IGZO oxygen vacancies found in the Ti/Cu S/D are controlled, resulting in low concentrations, and so prevent the SiNx-passivated TFT from having a negative shift. INTRODUCTION Amorphous Indium-Gallium-Zinc-Oxide Thin Film Transistors (a-IGZO TFT) have attracted considerable attention because they exhibit excellent electrical properties, such as a high field-effect mobility and a large on-current along with a small leakage current [1], [2] Its low sub-threshold swing enables fast on/off switching; the uniformity of a-IGZO TFTs is also good [3]. TFT-LCDs and AMOLEDs employing a-IGZO TFTs suggest that an oxide semiconductor is a good candidate for switching devices in displays [3-6]. High resolution and large display sizes require a low data-line resistivity along with high mobility TFTs. When display devices increase in size, resolution, and refresh rate, the required short charging time makes it difficult to charge the pixel electrodes up to the necessary data voltage. High mobility TFTs can overcome this charging problem, however, signal distortions due to the RC-delay of the data-line are still a liability. The RC-delay, therefore, needs to be decreased in order to avoid image distortion. Since the capacitance is usually fixed by the data-line width, the resistivity of the data-line needs to be reduced in order to decrease the RC-delay. Al is widely used for the data-line in displays. Cu, which is less resistive than Al by about 30%, is being considered for the reduction of the data-line resistance [7]. Because the data line is formed simultaneously with the source/drain (S/D) electrodes, employing Cu for a TFT S/D is very important. Oxide semiconductor TFTs are vulnerable to moisture and oxygen[8]. For that reason, they need a passivation layer, such as SiO2, Al2O3 or PMMA, in order to block ambient gases [8-10]. Among them, SiO2 is widely used because it can be deposited using the conventional PECVD methods and it exhibits superior stability to the environment[8]. However, SiNx passivation, which is widely used in a-Si TFTs, is difficult to apply to an oxide semiconductor TFT. Because oxide semiconductors are easily damaged by hydrogen plasma, SiNx passivation
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