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Material property measurements of micromechanical polysilicon beams

Fabrication- and measurement-induced stresses in surface micromachined structures are investigated by wafer-level probing of electrostatically actuated polysilicon test structures fabricated by the MUMPs process of MCNC. The test structures are based on M-Test, an electrostatic pull-in approach for monitoring process uniformity and reproducibility, and, when used in conjunction with suitable geometric data, for measuring material properties. The sensitivity of the pull-in technique reveals that the simple step of placing the die on a vacuum probe station can significantly affect the measured results. The presence of strain gradients in the polysilicon and compliant structural supports for the beams makes the modeling more complex than for ideal geometries, but with appropriate adjustments to the models, and with knowledge of the strain gradient obtained from cantilever tip deflection as a function of beam length, the technique enables a measurement of the elastic modulus and the fabrication-induced residual stress.