Effective phase control of silicon films during high-rate deposition in atmospheric-pressure very high-frequency plasma: Impacts of gas residence time on the performance of bottom-gate thin film transistors

Hydrogenated amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) films were grown in atmospheric-pressure (AP) He/H2/SiH4 plasma excited by a 150-MHz very high-frequency (VHF) power at a temperature of 220 °C. The variations in thickness and crystallinity of the deposited Si films along the gas flow direction were studied as functions of gas residence time in the plasma, VHF power density and H2 flow rate. Furthermore, the electrical characteristics of bottom-gate thin film transistors (TFTs) were investigated to evaluate the film quality. The results revealed that the chemical reactions both in gas phase and on the growing film surface were significantly enhanced in AP-VHF plasma, promoting phase transition from amorphous to microcrystalline in a time of the order of 0.1 ms. The performance of the TFTs showed that a-Si layers formed in the upstream portion of the plasma zone had reasonably good electrical property (field-effect mobility of approximately 2 cm2/V s) despite very high deposition rates around 20 nm/s. While μc-Si layers deposited in the downstream portion were very defective, which might come from the insufficient passivation of grain boundaries with a-Si tissues due to a too long gas residence time in the plasma. The precise control of gas residence time by adjusting the length of plasma will be effective for the phase control of Si films with desired quality.