In chemical vapour deposition (CVD) a solid is deposited from a chemically reactive vapour onto a solid surface or ‘substrate’. The application of CVD is particularly important in the formation of thin films of semiconductors, insulators and metals. For example, a thin film of the compound semi-conductor gallium arsenide GaAs can be deposited as a layer on a substrate by the reaction between Ga(C2H5)3 and AsH3. This enables the construction of very large scale integrated (VLSI) circuits which contain up to one megabyte of memory on a silicon wafer chip about 1 cm in area and comprise millions or transistors and capacitors. New processes to form diamond coatings at moderate pressures and temperatures are also based on the use of CVD. A more mundane example is the production of a ‘gold’ finish on fashion accessories and domestic furniture by depositing TiN on a metallic substrate.