Micromechanical characterization and initial deflection minimization for piezoelectric multi-layer cantilever microactuators

Abstract

This thesis investigates a method to minimize the initial deflection of a multi-layer piezoelectric microactuator without loosing its piezoelectric deflection performance required for light modulating micromirror devices. The multi-layer piezoelectric actuator, composed of PZT, silicon nitride and platinum layers, deflects or buckles due to the gradient of residual stress. From a structural analysis, the deflection of the multi-layer microactuator has been found as a function of the residual stress and the rigidity of each film layer. The residual stress and Young``s modulus of each layer are measured from a blister test. Based on the structural analysis and measured mechanical properties, we have modified the residual stress and the thickness of thin film layers to reduce the initial residual stress deflection without decreasing its piezoelectric deflection performance. The modified designs, fabricated by surface-micromachining process, achieved the 77% reduction of the initial deflection compared with that of the conventional designs, while maintaining the equivalent piezoelectric deflection performance. The present method based on the measured micromechanical material properties is applicable to the design and refinement of multi-layer MEMS devices with controlled strength and residual stress.