Nitric acid method for fabrication of gate oxides in TFT

Abstract We have developed low temperature formation methods of SiO 2 layers which are applicable to gate oxide layers in thin film transistors (TFT) by use of nitric acid (HNO 3 ). Thick (>10 nm) SiO 2 layers with good thickness uniformity (i.e., ±4%) can be formed on 32 cm × 40 cm substrates by the two-step nitric acid oxidation method in which initial and subsequent oxidation is performed using 40 and 68 wt% (azeotropic mixture) HNO 3 aqueous solutions, respectively. The nitric acid oxidation of polycrystalline Si (poly-Si) thin films greatly decreases the height of ridge structure present on the poly-Si surfaces. When poly-Si thin films on 32 cm × 40 cm glass substrates are oxidized at azeotropic point (i.e., 68 wt% HNO 3 aqueous solutions at 121 °C), ultrathin (i.e., 1.1 nm) SiO 2 layers with a good thickness uniformity (±0.05 nm) are formed on the poly-Si surfaces. When SiO 2 /Si structure fabricated using plasma-enhanced chemical vapor deposition is immersed in 68 wt% HNO 3 , oxide fixed charge density is greatly decreased, and interface states are eliminated. The fixed charge density is further decreased by heat treatments at 200 °C, and consequently, capacitance–voltage characteristics which are as good as those of thermal SiO 2 /Si structure are achieved.