Determination of stress in thin films using micro-machined buckled membranes

In this work, optical profilometry and finite-element simulations are applied on buckled micromachined membranes for the stress analysis of ion-beam-sputtered Ta2O5 and SiO2 thin films. Layers with different thicknesses are grown on silicon substrates, and then several membranes with different geometries are manufactured with standard microsystem technologies; due to a high level of films' compressive stress, buckled membranes are obtained. Thermally grown silica membranes are also produced for comparison. The residual stress values are determined by comparing the measured and simulated deflections of the membranes. The average stress state of Ta2O5 thin films is found to be - 209 MPa. The SiO2 thin films are in a higher compressive stress state whose average value is - 576 MPa. For comparison, the average stress in thermal SiO2 thin layers grown at 1130 °C is found equal to - 321 MPa, in good agreement with the literature.