Nitric acid method for fabrication of gate oxides in TFT

We have developed low temperature formation methods of SiO2 layers which are applicable to gate oxide layers in thin film transistors (TFT) by use of nitric acid (HNO3). Thick (>10 nm) SiO2 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) HNO3 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% HNO3 aqueous solutions at 121 °C), ultrathin (i.e., 1.1 nm) SiO2 layers with a good thickness uniformity (±0.05 nm) are formed on the poly-Si surfaces. When SiO2/Si structure fabricated using plasma-enhanced chemical vapor deposition is immersed in 68 wt% HNO3, 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 SiO2/Si structure are achieved.