Tetraethylorthosilicate (TEOS, chemical formula Si(OC2H5)4) is an organosilicon compound widely used as a silicon dioxide precursor in semiconductor CVD processes. TEOS is a liquid at room temperature (boiling point 168°C) that is delivered to the CVD chamber as a vapor by heating a liquid source and using a carrier gas or through direct liquid injection vaporization. Upon thermal decomposition or plasma-assisted dissociation, TEOS reacts with oxygen or ozone to deposit high-quality SiO2 films. The primary advantage of TEOS over silane (SiH4)-based oxide deposition is superior step coverage and conformality. The TEOS molecule is relatively large and has lower sticking coefficient on the growing surface, allowing it to migrate along surfaces before reacting, resulting in more uniform coverage over topographic features. In LPCVD at 680-720°C, TEOS thermally decomposes to produce dense, high-quality oxide films with properties approaching thermal oxide — low hydrogen content, low wet etch rate ratio (WERR close to 1.0), and excellent dielectric properties. LPCVD TEOS oxide is widely used for spacer deposition, hard masks, and conformal liner applications. In PECVD at 350-400°C, TEOS combined with O2 plasma produces oxide with better conformality than SiH4-based PECVD oxide, though film quality is lower than thermal LPCVD TEOS due to incomplete precursor decomposition, resulting in carbon and hydrogen incorporation. SACVD at 400-480°C uses ozone (O3) as the co-reactant with TEOS, where ozone's high reactivity enables highly conformal deposition at moderate temperatures with excellent gap-fill capability. The O3/TEOS ratio, ozone concentration, deposition temperature, and chamber pressure are critical parameters controlling film quality, deposition rate, and conformality. TEOS handling requires attention to safety — it is a flammable liquid that hydrolyzes in moisture to produce ethanol and silica, and decomposition at high temperature can generate toxic byproducts. Storage and delivery systems use nitrogen-purged, temperature-controlled bubblers or pressurized liquid delivery systems with mass flow controllers for precise dose control.