Nondestructive characterization of the properties of thin diaphragms

The stress-strain curve and residual stresses in thin diaphragms used in microelectronic applications are determined by means of a nondestructive approach. The method is based on the measurement of the deflection of the diaphragm near its center when it is subjected to an externally applied normal pressure and inversion of the deflection data to determine the quantities of interest. The measurements are carried out by means of a Twyman-Green laser interferometer. The relationship between the applied pressure and the deformation profile of the diaphragm is derived based on membrane mechanics. It is shown that if the diaphragm deflection in the neighborhood of its center can be measured accurately, then the strain and stress in this region can be determined through data inversion based on this relationship. It follows that, given the deformation profiles of a diaphragm for a range of pressurer, the biaxial modulus and the residual stress in the diaphragm can be extracted. By utilizing this local bulge testing method, many complex diaphragm shapes can be analyzed, without resorting to complicated numerical modeling. The method is applied to a nitride membrane with initial tensile stress and also to a silicon composite membrane with initial compressive stress, with reasonable results.