HWCVD and Thermal Evaporation Systems
The technique of hot wire chemical vapour deposition (HWCVD) is being increasingly used for the deposition of materials such as hydrogenated amorphous or microcrystalline silicon and its alloys and as diamond films. The main part of a deposition system consists in a vacuum chamber evacuated by a pumping unit to some appropriate vacuum level. After achieving the desired ultimate vacuum, the process gas mixture is introduced via Mass Flow Controllers and the pressure is kept constant by a variable conductance valve. Process gas is then decomposed by the heat generated by a hot filament of tungsten, tantalum or other material into radicals which are then deposited as thin films.
The important figure of merit for any vacuum system, and in particular for a deposition chamber, is the total leak and degassing rate when the reactor is at deposition temperature: the ratio of this parameter to the process gas flow rate will largely determine the achievable purity of films deposited in that reactor. In order to optimize the properties of each layer and therefore the characteristics of the electronic devices, it is desirable to deposit each layer, and in particular the active semiconductor layer, in high purity conditions; it is therefore necessary to minimize all external sources of contamination and in particular any contamination by the wire. The general vacuum and pressure control features of a HWCVD are similar to those of a PECVD reactor.