Technique for determining the Poisson's ratio of thin films

摘要

The theory and experimental approach for a new technique used to determine the Poisson's ratio of thin films are presented. The method involves taking the ratio of curvatures of cantilever beams and plates micromachined out of the film of interest. Curvature is induced by a through-thickness variation in residual stress, or by depositing a thin film under residual stress onto the beams and plates. This approach is made practical by the fact that the two curvatures air, the only required experimental parameters, and small calibration errors cancel when the ratio is taken. To confirm the accuracy of the technique, it was tested on a 2.5 (mu)m thick film of single crystal silicon. Micromachined beams 1 mm long by 100 (mu) wide and plates 700 (mu)m by 700 (mu)m were coated with 35 nm of gold and the curvatures were measured with a scanning optical profilometer. For the orientation tested ((100) film normal, (011) beam axis, (0(bar 1)1) contraction direction) silicon's Poisson's ratio is 0.064, and the measured result was 0.066 (+-) 0.043. The uncertainty in this technique is due primarily to variation in the measured curvatures, and should range from (+-) 0.02 to 0.04 with proper measurement technique.