Effects of residual stresses on lead–zirconate–titanate (PZT) thin-film membrane microactuators

A common design of piezoelectric microactuators adopts a membrane structure that consists of a base silicon structure, a layer of bottom electrode, a layer of piezoelectric thin film, and a layer of top electrode. In particular, the piezoelectric thin film is often made of lead–zirconate–titanate (PZT) for its high piezoelectric constants. When driven electrically, the PZT thin film extends or contracts flexing the membrane generating an out-of-plane displacement. For PZT thin-film microactuators, residual stresses are unavoidable from fabrication and can significantly reduce the actuation displacement. In this paper, the authors present a fourfold approach to address the issue of residual stresses. First, we demonstrate experimentally that two PZT thin-film actuators may present substantially different displacement and natural frequency even though dimensions of the two actuators are similar. Through a series of finite element analyses, we conclude that only residual stresses can produce such a significant frequency shift reducing the displacement of a PZT thin-film microactuator. Second, we measure the warping of the PZT thin-film actuator via interferometry to detect residual stresses. A simple calculation using shell theories indicates that the warping only causes a tiny shift in natural frequencies. Therefore, most of the degradation of actuator performance results from the nature that residual stresses are in-plane rather than the warping caused by the residual stresses. Third, we develop a vibration analysis to determine when residual stresses could significantly reduce actuator displacement. Fourth, we demonstrate two simple ways to mitigate the negative effects of residual stresses: poling at an elevated temperature and applying a DC bias voltage.