Optimised Piezoelectric PZT Thin Film Production on 8' Silicon Wafers for Micromechanical Applications

摘要

Piezoelectric materials are of great interest for new MEMS devices for sensors, actuators and energy harvesting. Due to its large electromechanical coupling coefficient Lead Zirconate Titanate (PZT) is one of the most favourable materials for realisation of fast and energy efficient microactuators. Here we report results of process optimisation for high quality thin film PZT deposition using standard magnetron sputter equipment (Oerlikon Balzers) of up to 2μm thickness at a deposition rate of 45nm/min. A reliable industrial production process was developed leading to optimised material quality for micromechanical applications. High quality films have been prepared on standard 8" silicon wafers using an Oerlikon CLN 200 tool with heated substrate holder. Ti/Pt bottom electrode configuration, deposition temperatures and process control have been varied for optimisation of thin film piezoelectric coefficients. Using Electron Probe Micro Analysis (EPMA) and X-ray diffraction (XRD) it was determined that best material composition and crystalline structure was achieved using a wafer chuck temperature of 600°C. Optimised PZT films of various thicknesses show a high dielectric constant ε_r of about 1500. A remarkable high average transversal piezoelectric module e_(31,f) of -17,3C/m~2 was achieved. An effective longitudinal d_(33,f) coefficient of 160pm/V was determined for 2μm thick PZT films. Using the optimised magnetron sputter process maximum values for the transversal piezoelectric module e_(31,f) of-21C/m~2 have been determined. Top electrodes on PZT thin films were realised in Cr/Au. These electrodes were used as hard masks for structuring of the piezoelectric material. Free standing PZT beams were produced by etch removal of a sacrificial layer below the PZT films. Various designs have been realised. Process flow and design examples for new sound transducers in the sound and ultrasound region are presented and discussed.