(A) study on oxidation of silicon by inductively coupled oxygen plasma and its application to polycrystalline silicon thin film transistors

Abstract

In polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) for active-matrix liquid crystal display (AMLCD), it is crucial to obtain high-quality $SiO_2$ film and Si/$SiO_2$ interface at low temperature for gate dielectric as the quality of channel poly-Si films improves. The interface quality of the oxide formed by oxidation method is better than that of the oxide by deposition method. Plasma oxidation has been extensively investigated as a means of low-temperature oxidation of silicon. But plasma oxides suffer from the poor-oxide quality due to the oxide damage by the bombardment of energetic ions and metallic contamination. Inductively coupled plasma (ICP) isa high density plasma, of which ion density is high because the sheath voltage of the plasma is small; the energy loss per ion lost is small. This small sheath voltage results in reduced oxide-damage and metallic contamination. The ICP source with planar configuration has a simple structure and low-aspectratio (height/diameter) so that it is suitable to low-temperature plasma oxidation process for poly-Si TFT, which requires a large-size glass substrate. We investigated the characteristics of the ICP with oxygen gas and the structural properties of $SiO_2$ films grown in the ICP with oxygen gas, and explained the kinetics of oxide growth in the ICP with oxygen gas. Furthermore, we applied ICP oxide to the poly-Si TFTs and analyzed the effect of the ICP oxidation on the properties of poly-Si films and TFTs. Silicon wafers were oxidized in the ICP oxidation system with oxygen gas at rf powers from 1.0 kW to 3.0 kW, temperatures from 350℃ to 450℃, and oxygen pressures from 5 mtorr to 200 mtorr. Maximum $5×10 ^{11}cm}^{-3}$ of ion density ($n_i$) was obtained in the ICP with oxygen gas at 10 mtorr and 3.0 kW, which is the maximum power of our system. This value is much higher than that of the conventional rf plasma. The electron temperature ($T_e$) and plasma potential ($V_p$) increased as the oxygen...