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J5.37.1

RESIDUAL STRESS CONTROL TO OPTIMIZE PZT MEMS PERFORMANCE M.S. Kennedy, D.F. Bahr, C.D. Richards, and R.F. Richards Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920 ABSTRACT Flexing piezoelectric membranes can be used to convert mechanical energy to electrical energy. The overall deflection of individual membranes is impacted by the residual stress in the system. Membranes comprised of silicon dioxide, Ti/Pt, lead- zirconate- titanate (PZT), and TiW/Au layers deposited on a micromachined boron doped silicon wafer were examined for both morphology and residual stress. By characterizing the membrane residual stress induced during processing with x-ray diffraction, wafer curvature, and bulge testing and identifying methods to reduce stress, the membrane performance and reliability can be optimized. For Zr:Ti ratios of 52:48, the residual stress in the PZT was 350 MPa tensile, with an overall effective stress in the composite membrane of 150 MPa. A reduction of stress was accomplished by changing the PZT chemistry to 40:60 Zr:Ti in the PZT to obtain a stress in the PZT of 160 MPa tensile and an overall effective membrane stress of 100 MPa. The crystallization of the 52:48 PZT film at 700 ºC causes a 28% reduction in the thickness of the film. INTRODUCTION A new MEMS development is the P3 microengine [1]. This power generation device utilizes a composite flexing membrane to generate power by converting mechanical energy to electrical energy. The membrane is made by depositing layers of silicon dioxide, Ti/Pt, PbZrxTi1-xO3 (PZT), and TiW/Au onto a micromachined boron doped silicon wafer. To obtain the highest electrical output over a given pressure range and the highest coupling coefficient, ideally no residual stress should be within the membrane when fabrication is complete. Residual stresses within the membrane generator can not only increase the pressure needed to obtain the desired electrical output of the generator, but can also lead to material failures like cracking, buckling, and hillock formation [2]. Since residual stress is inherent in metallic and inorganic films regardless of the deposition technique [2], concentration is centered on controlling and lowering of residual stresses found in the membrane generator. Researchers working on similar composite membrane structures have shown that the stress the individual layers is created during the initial deposition and subsequently altered during later processing steps. For example, the residual stress in silicon diaphragm having a boron doped layer becomes compressive since the intrinsic tensile stress in the p+ layer is reduced during thermal drive-in process. The local compressive stress at the edges of the diaphragm is created when the cavity is etched on the back side of the wafer [3].In addition to the silicon support layer, changes are also seen within the PZT. Some researchers have shown that the PZT became less tensile as crystallization into the perovskite structure forms [4]. PZT films on Pt electrodes ha