Many MEMS structures perform best at a specific film stress rather than at the extremes of the stoichiometric or low-stress recipes. Because film stress in LPCVD nitride is governed by the silicon-to-nitrogen ratio, Rogue Valley Microdevices can tune the gas ratio within its single stable nitride process to deliver a targeted film stress between the low-stress and stoichiometric values. The same adjustment shifts the refractive index, which moves predictably from about 2.00 for a stoichiometric film toward about 2.20 as the film becomes more silicon-rich. A targeted-stress film is valuable when a suspended membrane, cantilever, or other released structure must hold a particular curvature, flatness, or resonant frequency that a standard recipe does not provide. Rogue Valley Microdevices develops targeted-stress nitride to a customer specification and can include a stress monitor wafer for films thicker than 2,000Å to verify the delivered stress. Targeted Stress Specifications: Composition: Tuned silicon-to-nitrogen ratio; Film stress: Custom target between the low-stress and stoichiometric values; Refractive index: Tracks composition, about 2.00 to 2.20 at 632.8 nm; Wafer coverage: Both surfaces (furnace process); Process basis: Single stable nitride process, gas ratio set to the target; Verification: Stress monitor available for films thicker than 2,000Å; Typical applications: Membranes and released structures needing a specific stress.
