Recent optical measurements for the mechanical properties of thin films

Abstract

With advances in the technology behind micro-systems, the mechanical properties of sub-micron and nano-scale thin films have become an item of particular interest. We demonstrate two recent development of using optical methods to measure the mechanical properties of thin films. First, a paddle-like cantilever beam test structure with nano-scale metal films deposited on its top was successfully fabricated and calibrated using standard CMOS processes. Paddle cantilever beam deflection was obtained using a four-step phase-shifting process with a Michelson interferometer. Film strain was determined using a simple force equilibrium equation. Residual stresses were measured and observed at different thicknesses. High tensile stress forms during the early deposition stage for thin film due to grain coalescence, and a decrease in stress with an increase in film thickness. With thicknesses greater than 150 nm of Cu film, lattice relaxation associated with the surface mobility of metallic atoms changes residual stress from tension to compression. Second, it summaries the XRD measurements of the bulge tested thin film. We annealed thin films and tracked the texture transformation using 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 optical interferometer, using the bulge as the fully reflective surface, and an optically flat halfsilvered mirror as a reference surface. A CCD camera was used to record interference fringe motion as the pressure was increased. Results show that applied stresses have no links on the kinetics of the texture transformation in films.