New method for an accurate determination of residual strain in polycrystalline silicon films by analysing resonant frequencies of micromachined beams

The residual strain of polycrystalline silicon (poly-Si) film is evaluated using the strain-sensitive nature of the resonant frequency of a doubly-supported beam. By introducing the length dependence curve of the resonant frequency, the effect of the residual strain is discussed. Both measurements and finite-element calculations are performed to accurately determine the residual strain in two types of phosphorus-doped poly-Si films. From the finite-element calculations including nonlinear buckling, the post-buckling frequency is more sensitive to compressive residual strain than the frequency before buckling. By also considering these post-buckling frequencies, a more accurate estimation of the residual strain can be obtained.

A test chip consisting of 97 poly-Si beams of different lengths was fabricated for residual-strain evaluation. By fitting the calculations with measured resonant frequencies, the residual strains of the films are determined to be -48±5×10-6 and -400±100×10-6 for two types of films. The difference in the residual strain is explained by the heat treatment steps of phosphorus-doped poly-Si films. This method is extremely suitable for monitoring small fluctuations in the film residual strain between different process runs.