Highly efficient formation process for functional silicon oxide layers at low temperatures (≤ 120 °C) using very high-frequency plasma under atmospheric pressure

SiO2-like films are deposited with high rates at substrate temperatures in the range of <= 120 degrees C by means of the developed atmospheric-pressure (AP) plasma-enhanced chemical vapor deposition (PECVD) technology. The AP plasma is excited by using a novel parallel-plate-type electrode system under supplying a 150 MHz very high frequency (VHF) electric power. As the process gases, oxygen (O-2) and hexamethyldisiloxane (HMDSO) diluted with helium (He) are used. The electrode system is capable of forming one-dimensional flow of the process gases without any turbulence both inside and outside the plasma zone, which achieves the deposition conditions with no contamination of substrate surface with dust particles. The microstructure and surface roughness of the films are discussed by controlling O-2 flow rate (O-2/HMDSO source ratio) and VHF power density (P-VHF), aiming at the use of the films as the gate insulators of low-temperature thin film transistors (TFTs). Irrespective of the substrate materials (Si wafers or polyethylene naphthalate (PEN) sheets), the films deposited with an appropriate O-2 flow rate condition show smooth surface and cross sectional morphologies with no particles being incorporated during the deposition, which lead to the sufficiently good quality to be used as the gate insulator of low-temperature Si TFTs. The capability of forming functional thin films on temperature sensitive substrates will also lead to various applications of AP-VHF plasma as an efficient coating tool for polymer substrates.