A number of forms of CVD are in wide use and are frequently referenced in the literature. These processes differ in the means by which chemical reactions are initiated (e.g., activation process) and process conditions.Classified by operating pressure: Atmospheric pressure CVD (APCVD) – CVD process at atmospheric pressure. Low-pressure CVD (LPCVD) – CVD process at sub-atmospheric pressures. Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the wafer. Ultrahigh vacuum CVD (UHVCVD) – CVD process at very low pressure, typically below 10−6 Pa (~10−8 torr). Note that in other fields, a lower division between high and ultra-high vacuum is common, often 10−7 Pa.