Super Poly-Si And Transistor Formed by Nickel-Induced-Lateral-Crystallization
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Super Poly-Si And Transistor Formed by Nickel-Induced-Lateral-Crystallization C.F. Cheng, T.C. Leung, M.C. Poon Dept. of Electrical and Electronic Engineering, Hong Kong University of Science & Technology, Sai Kung, Hong Kong ABSTRACT Nickel-Induced-Lateral-Crystallization (NILC) was carried out on a 6000 Å amorphous Si (a-Si) layer. 6000 Å NILC poly-Si was formed and the upper 5000 Å Si layer was then removed. When compared to the conventional 1000 Å NILC poly-Si, the bottom 1000 Å NILC poly-Si layer was found to have relatively larger grains, less grain boundaries and better transistor mobility. The new high quality (super) NILC poly-Si layer can potentially provide greater contributions for novel device and circuit applications.
INTRODUCTION Metal-Induced-Lateral-Crystallization (MILC) has been studied intensively as a powerful method to form high quality and high growth rate poly-silicon (poly-Si) from amorphous Si (a-Si) at temperatures (around 500-600oC) lower than the conventional solid-phase crystallization (SPC) [1]. Ni has been found to be one of the best metal agents for the MILC process [2], [3] and Nickel-Induced-Lateral-Crystallization (NILC) of a-Si has attracted much attention. Recently we have reported that by applying a post-NILC high temperature annealing (~600-900oC), the grain size of the post-NILC poly-Si film can be much enhanced to around 10 to 100 µm [4], [5]. The film has good grain quality and performance of thin-filmtransistors (TFT) fabricated on the film can be highly comparable (>40%) to that of siliconon-insulator (SOI) TFTs. Such results show that NILC has high potential in many applications such as low cost, large area and even low temperature novel devices and circuit fabrication. This paper further reports the new results in NILC poly-Si and TFT obtained from the optimization of the NILC parameters. It was found that when NILC process was carried out on a 6000 Å a-Si layer, the quality of the bottom 1000 Å NILC poly-Si layer was significantly better than the normal 1000 Å NILC Si layer and the performance of the TFTs was also improved.
EXPERIMENTAL DETAILS The TFT fabrication was started by growing 7000 Å of thermal oxide onto silicon wafers by 1000 °C wet oxidation. Low Pressure Chemical Vapor Deposition (LPCVD) was used to deposit 6000 Å and 1000 Å amorphous Si (a-Si) on the sample and controlled wafers respectively. After that, 4000 Å of Low Temperature Oxide (LTO) was deposited. LTO was patterned and etched to form windows for Ni evaporation. 50 Å of Ni was then evaporated onto the wafers by using an electron-beam evaporator. The wafers were put into a 550 °C N2 furnace for 40 hours NILC first annealing. Ni diffused laterally into the a-Si layer (Figure 1). The a-Si was then crystallized to form NILC poly-Si and the NILC region grew along the direction of the Ni diffusion. After the first A6.4.1
annealing, piranha (H2SO4:H2O2 = 10:1) and BOE solution were used to remove the unreacted Ni and LTO layers respectively. A high temperature annealing (900 °C, 1 hour) was then per
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