Effect of electrode size and silicon residue on piezoelectric thin-film membrane actuators

Piezoelectric micro-electromechanical systems (MEMS) often adopt a membrane structure to facilitate sensing or actuation. Design parameters, such as membrane size, thickness of the piezoelectric thin film, and electrode types, have been studied to maximize actuation, sensitivity, or coupling coefficient. This paper is to demonstrate numerically and experimentally that the size of silicon residue and its relative size to the top electrode are two critical yet unrecognized parameters in maximizing the actuation displacement of PZT thin-film membrane actuators. To study effects of the silicon residue, we have developed a finite element model using ANSYS. The model consists of five components: a square passive silicon membrane, a silicon substrate, a PZT thin film, a square top electrode, and a silicon residue region. In particular, the silicon residue has a circular inner diameter and a square outer perimeter with a trapezoidal cross section. Predictions of the finite element model lead to several major results. First, when the silicon residue is present, there exists an optimal size of the top electrode maximizing the actuator displacement. Second, the optimal electrode size is roughly 50–60% of the inner diameters of the silicon residue. The displacement of the membrane actuator declines significantly as the electrode overlaps with the silicon residue. Third, the maximal actuator displacement decreases as the inner diameter of the silicon residue decreases. Aside from the finite element analysis, a mechanics-of-material model is also developed to predict the electrode size that maximizes the actuator displacement. To verify the simulation results, eight PZT thin-film membrane actuators with progressive electrode sizes are fabricated. These actuators all have a square membrane of 800 μm × 800 μm with the inner diameter of the silicon residue controlled between 500 and 750 μm. A laser Doppler vibrometer is used to measure the actuator displacements. The experimental measurements confirm that there exists an optimal size of the top electrode maximizing the actuator displacement.