Analysis of Micromachined PZT Membranes for MEMS Power J.H. Cho * , M. J. Anderson ** , R.F. Richards * , D.F. Bahr * and C.D. Richards * * Washington State University Pullman, School of Mechanical and Materials Engineering WA, USA, cill@wsu.edu ** University of Idaho, Department of Mechanical Engineering Moscow, ID, USA, anderson@uidaho.edu ABSTRACT The performance of a piezoelectric generator in terms of the quality factor Q, the electromechanical coupling coefficient k 2 , and the efficiency were examined. The effect of design parameters such as membrane size, piezoelectric thickness, silicon thickness, and top electrode area are explored. The results show that both k 2 and Q are sensitive to PZT thickness and electrode size. Keywords: pzt membrane, mems power, electromechanical coupling coefficient, quality factor, efficiency. 1 INTRODUCTION The need for miniaturized power sources for MEMS and microelectronics devices is widely recognized. Micro- scale concepts to generate electrical power include devices which use the stored energy in fuels to those which harvest energy from the environment. Piezoelectric materials are increasingly employed in both types of devices to convert mechanical to electrical energy [1–5]. The use of piezoelectric materials yields significant advantages for micro power systems. Since piezoelectric materials convert mechanical energy into electrical energy via strain in the piezoelectric material, they lend themselves to devices that operate by bending or flexing which brings significant design advantages. Recent work by our team at Washington State University has been directed at the design of a micro heat engine in which thermal power is converted to mechanical power through the use of a novel thermodynamic cycle. Mechanical power is then converted into electrical power through the use of a thin-film piezoelectric membrane generator [1]. In this paper we examine the performance of the piezoelectric generator in terms of the quality factor Q, the electromechanical coupling coefficient k 2 , and the efficiency. The effect of design parameters such as membrane size, piezoelectric thickness, and silicon thickness are explored. 2 DEVICE FABRICATION The structure of the thin-film piezoelectric membrane generator is a two-dimensional sandwich structure similar to that used for ultrasonic transducers [6]. Figure 1 shows a cross section of the membrane generator, which consists of a silicon membrane, a bottom platinum electrode, a thin- film of the piezoelectric ceramic PZT (Lead Zirconate Titanate) and a top gold electrode. The substrate for the PZT membrane generator is a (100) silicon wafer. Boron is doped into the one side of the silicon for an etch stop, and then an oxide layer is grown and patterned to provide a mask for anisotropic etching in ethylene diamine pyrochatechol (EDP). A 12nm layer of titanium and a 175nm thick layer of platinum are sputtered using DC magnetron sputtering. PZT is spun onto the platinum films in a sol-gel process [7]. A top electrode consisting of a 5nm TiW adhesion layer of gold and 300nm of gold is deposited by DC magnetron sputtering. Photolithography is then used to etch the top electrode and PZT around the top electrode. A completed membrane generator is shown in Fig. 2. 3 PIEZOELECTRIC MEMBRANE CHARACTERISTICS A piezoelectric membrane generator produces power when it is strained in response to an applied pressure. In Figure 1: The structure of the PZT membrane generator. Figure 2: Piezoelectric membrane generator. NSTI-Nanotech 2004, www.nsti.org, ISBN 0-9728422-7-6 Vol. 1, 2004 382