Design of cooling system for large-area and high-rate deposition of hydrogenated microcrystalline silicon

Abstract

We have developed a high frequency (40 MHz) plasma-enhanced chemical vapor deposition (PECVD) system for large-area and high-rate deposition of hydrogenated microcrystalline silicon (μc-Si:H). The large-area and high-rate deposition is known to be the key for mass production of silicon thin film solar cells, and the high-power operation made it possible to obtain deposition rates up to 1.8 nm/s. However, it was reported that there were several drawbacks, and temperature was typically concerned by many researchers. The temperature is one of the most important factors in μc-Si:H deposition processes using PECVD for the reason that temperature is in charge of the surface reaction and the growth kinetics. Without cooling, the temperature was observed to increase considerably during deposition of a thick intrinsic layer with thickness of 2 μm. In order to operate PECVD processes with constant temperature, it is required to design an efficient temperature control system. We have developed a cooling system which is designed for a susceptor and a showerhead. Susceptor cooling is performed in a direct cooling mode where a coolant is circulated through a tube inside a susceptor, and appropriate flow rates and temperatures of the coolant were calculated. Showerhead cooling system is designed to make use of an indirect cooling mode where there is efficient thermal radiation heat exchange between a showerhead and chamber wall. The efficiency was substantially improved by anodizing surface of a showerhead. This cooling system allows stable processes of large-area μc-Si:H deposition at high power.