PZT and Electrode Enhancements of Mems Based Micro Heat Engine for Power Generation
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V5.19.1
PZT AND ELECTRODE ENHANCEMENTS OF MEMS BASED MICRO HEAT ENGINE FOR POWER GENERATION A.L. Olson, L.M. Eakins, B.W. Olson, D.F. Bahr, C.D. Richards, R.F.Richards Mechanical and Materials Engineering, Washington State University, Pullman, WA ABSTRACT The P3 Micro Heat Engine relies on a thin film PZT based transducer to convert mechanical energy into usable electrical power. In an effort to increase process yield for these devices, PZT adhesion, cracking, and thickness effects were studied. Also, to increase power generation, novel PZT etching patterns were used to improve the strain at failure. AFM and AES were used on sputtered Ti/Pt bottom electrodes to compare roughness, grain size, and diffusion for annealing temperatures between 550 and 700 °C. For an optimized bottom electrode, process yield for various sized top electrodes were then studied for PZT thickness between 0.54 and 1.62 µm. Analytic strain calculations in pressurized square membranes, showing high strain at half the lateral distance from each corner of the membrane, were then used as a qualitative model for reducing stress concentrations. Two PZT etching geometries on 2.3 µm thick Si/SiO2 membranes, with 1.5-3.5 mm side-lengths, were examined and one was used to increase the strain at failure by at least 40%. Integrating improvements in process yield and strain at failure, single PZT based MEMS devices capable of generating power of up to 1 mW and in excess of 2 volts have been demonstrated operating at frequencies between 300 and 1,100 Hz. INTRODUCTION The P3 Micro Heat Engine is a small mm-scale power generation device relying on twophase fluid expansion to deflect a µm-scale thin-film piezoelectric membrane [1]. In this case, lead zirconate titanate (PZT) is used. Two main sections comprise the heat engine, a heat source and a generator (the piezoelectric transducer). Figure 1 shows the cross section of such a device. Heat is supplied to the engine through the source causing pressure to develop in the engine cavity from evaporation. This pressure deflects the generator and the resulting strain in the piezoelectric film causes a voltage to develop across the top and bottom electrode, effectively a d31 mode of the piezoelectric effect. Voltage cycles are then used to drive a charging circuit that generates power.
Figure 1. The P3 Micro Heat Engine and generator (piezoelectric transducer)
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PZT adhesion, cracking, and electrical defects due to the bottom electrode structure degrade the performance of piezoelectric devices and process yield. For this reason the bottom electrode must have good adhesion to the silica substrate as well as good surface morphology and chemistry to support PZT with large residual tensile stress. Pt is commonly used as the bottom electrode for its temperature stabil
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