to achieve optimum film quality; this effect is attributed to thermally activated surface diffusion of adatoms on the growing film. Note that a-Si and nc-Si films are deposited at much lower temperature than other polycrystalline thin films for solar cells such as CdTe or CIGS.
A PECVD system usually consists of several major parts: (1) a gas delivery system (gas cylinders, pressure regulators, mass flow controllers, and various gas valves to direct gas flows); (2) a deposition chamber (electrodes, substrate mounts, substrate heaters, and the RF power feed through) capable of high vacuum; (3) a pumping system usually consisting of 3 pumps: a turbomolecular pump and mechanical backing pump for establishing high-purity vacuum, and a separate process pump for removing the gases and other by-products only used during deposition; (4) a pressure control system (capacitance manometer, ionization gauges, thermocouple gauges, and/or throttle valve to monitor and control the chamber pressure); (5) high-frequency power supply (RF or VHF supply with impedance matching network); and (6) an exhaust system for the process gases (typically either a chemical scrubber to neutralize the gases or a “burn box” to pyrolyze them). In multichamber systems there is a transfer system to move substrates inside the vacuum system between various deposition chambers through appropriate gate valves. Large-volume manufacturing systems typically do not have turbomolecular pumps due to their cost and maintenance.