Low Pressure Chemical Vapor Deposition (LPCVD) nitride is a high-temperature deposition process performed in a low-pressure hot-wall furnace. Wafers are loaded vertically into the furnace tube, where dichlorosilane (SiH₂Cl₂) and ammonia (NH₃) react at approximately 750 to 850 °C to deposit silicon nitride. The low pressure and elevated temperature drive a uniform, surface-controlled reaction across the entire load. Because the reaction is thermally driven rather than line-of-sight, LPCVD nitride is highly conformal. It coats every exposed surface, follows underlying topography faithfully, and deposits on both sides of the wafer and the wafer edge. The resulting film is dense, near-stoichiometric Si₃N₄ that contains very little hydrogen and forms an exceptionally effective barrier against moisture, oxygen, and mobile-ion contamination. Standard stoichiometric LPCVD nitride carries high tensile stress, which limits how thick the film can be deposited before cracking. By adjusting the gas ratio toward a silicon-rich composition, the tensile stress can be reduced substantially, producing a low-stress nitride suited to thick films and suspended membranes. Rogue Valley Microdevices offers all three film types from a single stable nitride process. Stoichiometric LPCVD silicon nitride is supplied with a tensile film stress at or above 800 MPa and a refractive index of 2.00 ± 0.05 at 632.8 nm. Low-stress LPCVD nitride is supplied with a tensile film stress below 250 MPa and a refractive index of 2.20 ± 0.05 at 632.8 nm. Super-low-stress LPCVD nitride is supplied with a tensile film stress below 100 MPa and a refractive index of 2.30 ± 0.05 at 632.8 nm. Targeted film stress between these values is available by tuning the gas ratio to match a specific application. Typical Applications: Oxidation (LOCOS) masking; Diffusion and moisture barrier; Etch stop and hard mask; MEMS structural and sacrificial layers; Low-stress suspended membranes; Electrical isolation. Thickness at RVM: 100Å to 4µm, depending on film class.
