Low temperature formation of SiO 2 thin films by nitric acid oxidation of Si (NAOS) and application to thin film transistor (TFT)

Abstract

Nitric acid oxidation of Si (NAOS) can form 1.3–20 nm SiO 2 films on Si(1 0 0) and poly-crystalline silicon (poly-Si) surfaces at low temperatures below 120 °C. The 1.3 nm SiO 2 layer formed by immersion in 68 wt% HNO 3 possesses a low leakage current density and it can be made lower by post-metallization annealing (PMA) in hydrogen than that for thermal oxide with the same thickness. A thicker SiO 2 layer can be formed by immersion in 40 wt% HNO 3 followed by 68 wt% HNO 3, and its leakage current density becomes nearly the same after PMA as that of thermal oxide. The low leakage current density results from elimination of the interface states by hydrogen and the high atomic density of the NAOS–SiO 2 layer as estimated from the vibrational frequencies of transverse and longitudinal optical phonons of the Si–O–Si asymmetric stretching mode. The thickness of the NAOS–SiO 2 layer is uniform even on the rough poly-Si surfaces. The dispersion of the thickness of the NAOS–SiO 2 layer is also found to be less than ±5% on the 32 cm × 40 cm poly-Si substrates.