Recent Development of Using Optical Methods to Measure the Mechanical Properties of Thin Films

The mechanical properties of micron and nanometer scale thin films have become an important issues in modern microelectronics. Here, two recent development of using optical methods to measure the mechanical properties of thin films were introduced. First, a paddle-like cantilever beam test structure with nanometer scale metal films on top was design and developed. Film strain and stresses of different thicknesses were measured through the beam deflection obtained by using a four-step phase-shifting process with a Michelson interferometer. Second, we introduce the XRD measurements of the bulge tested thin film. We annealed thin Ag films and tracked the texture transformation in-situ using synchrotron x-ray diffraction while independently varying the stress in the film using a bulge test apparatus. The bulge height was measured as a function of pressure using a simple Fabry-Perot optical interferometer, using the bulge as the fully reflective surface, and an optically flat half-silvered mirror as a reference surface. A CCD camera was used to record interference fringe motion as the pressure was increased. The bulge height was obtained by counting the number of fringes that passed a given point. A laser light source with a 532 nm wavelength gave a height resolution of 266 nm.