Pulsed very high-frequency plasma-enhanced chemical vapor deposition of silicon films for low-temperature (120 °C) thin film transistors

Abstract

With the aim of improving silicon (Si) film quality deposited by atmospheric-pressure (AP) plasma-enhanced chemical vapor deposition at a low substrate temperature of 120 °C, we examine the difference between continuous wave and pulsed plasmas for deposition characteristics depending on H2 and SiH4 flow rates and inpur VHF power. A specifically designed parallel-plate-type electrode system is used to generate the AP plasma, which is excited by 150 MHz very high-frequency (VHF) electric power. Hydrogen and monosilane diluted with helium are used as the process gases. The electrode system enables us to form turbulence-free one-dimensional gas flow in the plasma zone, thereby ensuring a deposition process without contamination of the substrate surface with dust particles. The electrical property of the deposited Si layers was evaluated by fabricating bottom-gate thin film transistors (TFTs). The performance of the bottom-gate TFTs revealed that the a-Si TFTs, i.e. those channel layers prepared with a pulse duty cycle of 10%, exhibit reasonably high field effect mobilities of around 1 cm2 · V–1 · s–1, suggesting that the pulse modulation of input VHF power is highly effective in achieving a plasma chemistry suitable for high-quality Si growths at low temperatures.