Thin films on cantilevers

Abstract

The main goal of the work compiled in this thesis is to investigate thin films for integration in micro electromechanical systems (MEMS). The miniaturization of MEMS actuators and sensors without compromising their performance requires thin films of different active materials with specific properties which differ from those in bulk. The Young's modulus of thin films can be reliably determined from a resonance frequency shift of microcantilevers on which the films are deposited. A high accuracy can be obtained a) by taking the difference in the resonance frequency before and after the deposition of the thin films, b) by taking into account the effective undercut length and thickness of the individual cantilevers, c) by measuring many cantilevers, and d) by applying a rigorous error analysis. To match this high accuracy, a precise value is needed for the effective Young's modulus of the cantilevers, taking into account the anisotropic Young's modulus of silicon and clamping of the cantilever at its base. This value of the effective Young's modulus must therefore be determined by 3-D FE simulations. Compositional dependence of epitaxially grown PZT, deposited by pulsed laser deposition, shows excellent piezoelectric and mechanical properties. Higher Young's modulus results in a higher electromechanical coupling coefficient in comparison to bulk PZT ceramic in clamped conditions. Residual stress can be accurately determined from cantilever curvature and this technique is to be preferred over X-ray diffraction, which gives unlikely high values. We applied the Young's modulus technique to GeTeSb based phase change thin films, because the Young's modulus of the film on the cantilever can be changed by heating, without any change in its mass. Indeed we observed a strong increase in the Young's modulus and residual stress at crystallization temperature. We observed a strong dependence of the residual stress on the annealing history before the crystallization temperature, which should be taken into consideration when designing the structures using GST thin films. The results reported in this thesis facilitate the choice of the correct composition of the materials in thin film form, based for instance on an optimization of figures of merit specific for the intended application.