Chemical Vapour Deposition

Chemical depositions of thin films (Growth & Synthesis)

CVD is a thermal process because a certain thermal energy is needed to decompose the gas precursors and reassemble them in the material to be deposited. Processing conditions in a CVD should avoid gas-phase reactions (homogenous deposition) and should favor that the deposited layer is assembled in the substrate as a surface process (heterogeneous deposition). This can be achieved at atmospheric pressure (APCVD) by heavily diluting the active gases. When these gaseous precursors are let to react in a rarefied the process is termed Low Pressure CVD (LPCVD). The partial vacuum avoids gas-phase reactions. The low pressure also favors larger diffusivity of the species so that an LPCVD process can decorate cavities and led to conformal coatings when a topography is present. The conformality is also spurred by the process temperature which favors movement of the species along the surface. Pressures are usually in the millitorr-torr range and temperatures range from 400-800C (depending on the energy needed to decompose the gas precursors). PECVD is a particular form of CVD that takes place in vacuum but in which a plasma is used to assist the decomposition of the gas precursors. This usually allows lowering the temperature of the process to a few hundred centigrade degrees, which can be run on less temperature resistant substrates or without setting off temperature triggered unwanted processes.