Relationships Between Microstructure and Reliability in Pzt Mems
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RELATIONSHIPS BETWEEN MICROSTRUCTURE AND RELIABILITY IN PZT MEMS B.W. Olson, L.M. Randall, C.D. Richards, R.F. Richards, and D.F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman WA 99164-2920 ABSTRACT Piezoelectric oxide films, such as lead zirconate titanate (PZT), are now being integrated into MEMS applications. Many PZT derived systems are deposited using a sol-gel process, which can be used in a microelectronics processing route using spin coating as the deposition method. An application of interest for PZT films is in power generation, where a flexing membrane is used to transform mechanical to electrical energy. The current study was undertaken to identify the relationships between the processing, microstructure, and mechanical reliability of these films. Films were deposited onto both monolithic and bulk micromachined platinized silicon wafers using standard sol-gel chemistries, with roughness and grain size tracked using electron and scanning probe microscopy. Mechanical properties were evaluated in a dynamic bulge testing apparatus. Grain size variations in the Pt film between 35 and 125 nm are shown to have little effect on grain size of the subsequent PZT film and the adhesion of the PZT to the Pt film. Only the Pt film with 125 nm grains was shown to undergo any significant interfacial fracture. Fatigue tests suggest film lifetime is primarily limited by the number of preexisting flaws in the film from processing. Reducing the microcrack density has been shown to produce films and devices that fail at strains of 1.4% and have mechanical fatigue lifetimes in excess of 100 million cycles at strains simulating the operating conditions.
INTRODUCTION Lead zirconate titanate (Pb(ZrxTi1-x)O3, or PZT) has gained attention for its piezoelectric properties to be used in power generating microelectromechnical systems (MEMS) [1]. A proposed MEMS power generator requires multiple layers of piezoelectric ceramic and metallic thin films that are capable of withstanding significant strain while maximizing electrical output. The mechanical integrity as well as the piezoelectric properties are both critical to the development of a reliable device. In applications of power generation, a low coercive field with high saturation and low remnant polarizations is desired. This combination will permit maximum polarization while minimizing time delays and power loss during switching through the loading cycle. Preferred crystallographic orientation [2-6], grain size [7-11], chemistry [10,12-14], surface roughness [3,15,16], and residual stresses [4,17] in piezoelectric films as well as electrodes [8,9,18,19] have been reported to alter electrical properties of piezoelectric and ferroelectric thin films. It has recently been shown that different materials used for the bottom electrode produce different phase formation, microstructure and ferroelectric properties [8]. Pt bottom electrodes in MEMS devices are attractive for inherent oxidation resistance, good thermal conductivity, and preferential cry
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