Nitric acid method for fabrication of gate oxides in TFT

摘要

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 ℃), 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 ℃, and consequently, capacitance-voltage characteristics which are as good as those of thermal SiO_2/Si structure are achieved.