Thermal oxidation is a growth process performed in a high-temperature diffusion furnace. Silicon wafers are loaded into a horizontal or vertical furnace where carefully controlled flows of dry oxygen (O₂) or water vapor (H₂O) react directly with exposed silicon at approximately 900 to 1200 °C. As the oxidizing species reach the surface, they chemically convert the substrate into silicon dioxide. Diffusion furnace. Wafers stand vertically in a heated quartz tube while dry oxygen or water vapor reacts with the exposed silicon, converting the wafer surface itself into SiO₂ on every exposed face. Unlike a deposition process, thermal oxidation consumes silicon from the wafer. Approximately 44 percent of the finished oxide thickness grows below the original silicon surface while 56 percent grows above it. Because the film is formed directly from the silicon crystal lattice, the resulting oxide is dense, stoichiometric, contains essentially no bonded hydrogen, and forms an exceptionally clean silicon-to-oxide interface with very low interface trap density, high dielectric strength, and excellent long-term reliability. Two oxidation chemistries are used. Dry oxidation reacts silicon with oxygen, producing slower growth rates but the highest quality, thinner films with superior electrical properties. Wet oxidation uses water vapor to increase the growth rate significantly and is commonly selected for thick oxides used in electrical isolation, MEMS sacrificial layers, and DRIE masking. Because oxidation occurs wherever silicon is exposed, both sides of the wafer and the exposed wafer edge oxidize simultaneously unless a masking or protection method is applied. Grown everywhere silicon is exposed. Because oxidation converts the silicon itself, the film forms on the front, back, and edge of the wafer simultaneously unless a masking or protection method is applied. Thermal oxidation follows predictable kinetics. Thin oxides initially grow rapidly because oxidizing species reach the silicon interface easily. As the oxide thickens, oxygen or water vapor must diffuse through the existing film to reach the interface, which steadily reduces the growth rate. Dry oxidation produces the highest quality thin oxides, while wet oxidation provides the faster growth needed for thick isolation and MEMS applications. Thickness at RVM: 500Å to 10µm Typical process sequence: Early fabrication Typical applications: Gate oxide, field oxide, DRIE hard mask, sacrificial oxide, waveguide undercladding Typical Applications - Gate oxide - Field oxide - Electrical isolation - DRIE hard mask - MEMS sacrificial layers - Optical waveguide undercladding Choose thermal oxide when interface quality, dielectric strength, thick high-quality oxide, DRIE masking, sacrificial oxide, or optical undercladding are the primary requirements.
