Comments on measuring thin-film stresses using bi-layer micromachined beams

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Abstract
Bi-layer structures are formed by depositing a thin pre-stressed film onto an existing micromachined beam which is made of a base-layer material. The resulting static deformation of the built-up beam is measured through optical profilometry, and the mean residual strain in the thin film is determined by calibrating a strain-deflection model to the measurements. The method is demonstrated with AlCu and diamond-like carbon films that are deposited onto SiO2 cantilevers produced through bulk machining. Critical to the technique is the inclusion of initial and non-ideal deflections of the base-layer cantilever, which have not been reported in previous investigations of the bi-layer method and which can play a significant role in determining the film`s stresses.