Development and Testing of a MEMS Piezoelectric Energy Harvester Ryan R. Knight, Changki Mo, and William W. Clark Mechanical Engineering and Materials Science Department, University of Pittsburgh ABSTRACT This paper presents a finite element analysis of an interdigitated ( 33 d ) piezoelectric unimorph cantilever beam for harvesting vibration energy. The key feature that is analyzed is the poling behavior of the piezoelectric material. While simplified models of interdigitated piezoelectric devices assume some uniform and well-defined poling pattern, the finite element modeling shows that not to be the case. In this paper, a “percent poling factor” is developed with which to capture the real losses associated with non-uniform poling. A parametric study is carried out in which electrode patterns, piezoelectric layer dimensions and electrode dimensions are varied to see their effect on this percent poling factor. Optimal parameters are pointed out. Finally, experimental energy harvesting results for a micro-scale interdigitated beam are presented. Keywords: MEMS, Piezoelectric, Interdigitated, Electrode, Energy harvesting, Finite element analysis. 1. INTRODUCTION Piezoelectric materials are incorporated into many of today’s commercial and research applications. Accelerometers, crystal oscillators, sonar, audio transducers, igniters, positioning systems, vibrations absorbers, and energy harvesters are to name a few. The development of thin film PZT (lead zirconate titanate) fabrication, and all of the research dedicated to it has spawned many applications in MEMS. Of those, MEMS piezoelectric harvester structures have been investigated, built, and tested by a great deal of researchers. In Sood 1 , Jeon 2 , and Choi 3 , interdigitated electrode (IDE) mode MEMS cantilevers were developed and tested. They utilized the mode because the high output voltage of the device would be able to overcome the forward bias from the rectifying circuit. Their MEMS structure could produce about 1µW of power. They were the first to develop MEMS scale energy harvesting structures and to present empirical power data for MEMS scale structures. Lee 4 also created MEMS energy harvesting structures, however, no power data was provided. At the micro scale, it is easier to fabricate interdigitated piezoelectric beams than conventional (d 31 ) beams because of the reduced number of layers required. Given the small dimensions, however, it can be difficult to achieve electrode patterns that have ratios of spacing and width to thickness that are commonly found in macro-scale interdigitated beams. This paper considers the effects of electrode patterns and dimensions on the real piezoelectric material poling. The interdigitated beam elicits the 33 d piezoelectric constant. The 33 d constant for PZT is commonly known to be twice as large as the 31 d constant. Therefore, designing a 33 d structure properly could produce more energy and larger tuning range than a 31 d structure. However, depending on the electrode spacing and the piezoelectric layer thickness this assumption might not be valid. In this regard the poling electric field is examined in this study 6 . Following a finite element simulation of a beam’s poling electric field, the non-uniform electric field loss is then presented. A model is produced that simulates various interdigitated electrode geometries and calculates the electric field density in the piezoelectric layer. An optimal electrode width to piezoelectric layer thickness ratio is also found from the finite element analysis, and the planar interdigitated electrode loss or percent poling is analyzed. Finally, an example set of experimental results is presented for a micro-scale cantilever beam. 2. MATHEMATICAL MODEL OF INTERDIGITATED d 33 PIEZOELECTRIC BEAM 33 d 33 d Active and Passive Smart Structures and Integrated Systems 2009, edited by Mehdi Ahmadian, Mehrdad N. Ghasemi-Nejhad, Proc. of SPIE Vol. 7288, 72880A · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.815533 Proc. of SPIE Vol. 7288 72880A-1