| AVS 59th Annual International Symposium and Exhibition | |
| Advanced Surface Engineering | Tuesday Sessions |
| Session SE+PS-TuA |
| Session: | Atmospheric Pressure Plasmas |
| Presenter: | H. Kakiuchi, Osaka University, Japan |
| Authors: | H. Kakiuchi, Osaka University, Japan H. Ohmi, Osaka University, Japan T. Yamada, Osaka University, Japan A. Hirano, Osaka University, Japan T. Tsushima, Osaka University, Japan K. Yasutake, Osaka University, Japan |
| Correspondent: | Click to Email |
Hydrogenated amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) prepared at low temperatures are promising thin film materials for use in large-area electronic devices. The goal of our study is to develop a highly efficient deposition process of good-quality a-Si and μc-Si films on polymer substrates using an atmospheric- pressure (AP) plasma technology in which stable reactive plasma excited by a 150-MHz very high-frequency (VHF) power under AP is effectively used.
The experiments were conducted in an AP plasma CVD system that had a parallel-plate-type electrode (2x8 cm2), whose surface was coated by alumina of ~0.1 mm thickness. By supplying a VHF power through an impedance matching unit, AP He/H2/SiH4 plasma was stably confined in the narrow gap region (0.3–0.7 mm) between the electrode and a substrate. Under a constant process pressure of 1x105 Pa, VHF power density (PVHF), H2 and SiH4 flow rates, plasma gap and substrate heating temperature (Tsub) were varied as principal parameters. Si dusty particles formed by gas-phase condensation in the outside of the plasma region were completely removed by sucking the gas flow before their adhering to the substrate surface.
By examining the influence of gas residence time in the plasma on the film growth behavior, it was shown that the source SiH4 gas was immediately decomposed after being introduced into the plasma region and contributed to the film growth. Under the condition of PVHF = 14 W/cm2, H2 and SiH4 flow rates of 500 and 50 SCCM, respectively (H2/SiH4 = 10), and Tsub = 220 °C, the film started to crystallize in only 0.3 msec. Both increasing PVHF and H2/SiH4 ratio caused the decrease in gas residence time necessary for the phase transition of the resultant Si films. On the other hand, an excessively long gas residence time (> 1 msec) led to the formation of highly crystallized μc-Si films even if H2 was not added to the process gas mixture. However, such μc-Si films showed poor electrical properties, which resulted from the sparse film structure without enough passivation of the grain boundaries with amorphous Si tissues. These suggest that the precise control of gas residence time is primarily important for the formation of good-quality a-Si and μc-Si films using AP-VHF plasma, together with the optimization of PVHF and H2/SiH4 ratio.
The a-Si and μc-Si films deposited with high rates (>10 nm/s) in AP He/H2/SiH4 plasma were used as the channel layers of bottom-gate thin film transistors (TFTs). The performance of the TFTs will be presented in the conference.