Chemical Vapor Deposition (CVD) is any process in which chemical vapors are delivered to a substrate (usually heated) where the vapors react chemically to form a layer.
A related process is physical vapor deposition (PVD) in which the depositing material is delivered by a physical means such as sputtering or evaporation.
Hybrid processes, such as reactive sputtering, are in use which involve both physical and chemical effects. Initially silicon semiconductor devices were fabricated with just single crystal silicon, thermally-grown silicon oxide, physically deposited aluminum, and vapor transported dopants. Where a separate single crystal layer was required (e.g. bipolar devices), an epitaxial silicon layer was deposited by CVD. When the need developed to create two layers of metal interconnect (a cross-over), a deposited insulator layer was required. Both CVD and PVD were investigated, and over time, CVD silicon oxide won that competition for quality and cost. As a result, CVD became a widespread process in semiconductor fabrication.
CVD requires volatile precursors that can be delivered to the substrate by vapor transport.
Initially, silicon was delivered as silane or chlorosilanes. Later, volatile organic compounds were developed to deliver silicon and other metals, thus giving rise to metal-organic-CVD (MOCVD) technology.
Variations on the basic CVD concept include plasma-enhanced CVD (PECVD, CVD stimulated by plasma), photo-CVD (CVD stimulated by light energy, usually at ultraviolet wavelengths), and LPCVD (CVD at low pressures where the mobility of the vapors permits wafers to be arrayed parallel to each other for efficient processing).