Influence of Structure and Chemistry on Piezoelectric Properties of Pzt in a Mems Power Generation Application
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INFLUENCE OF STRUCTURE AND CHEMISTRY ON PIEZOELECTRIC PROPERTIES OF PZT IN A MEMS POWER GENERATION APPLICATION L.M.R. Eakins, D.E. Eakins, C.D. Richards, M.G. Norton, R.F. Richards, and D.F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman WA 99164-2920 ABSTRACT PZT films between 1 and 3 µm thick were grown using solution deposition techniques to study the effects of crystal structure, orientation, chemistry and PZT/PZT crystallization interfaces on the piezoelectric output of these films. By varying the chemistry of the film from Zr-rich to Ti-rich the film orientation increased towards {h00}. PZT with 60 wt% Ti exhibited tetragonality and produced greater electrical output at a given strain than the rhombohedral films with concentrations less than 50 wt% Ti. Multiple steps of solution deposition left identifiable PZT/PZT interfaces within the film. TEM, FESEM, and Auger spectroscopy were used to characterize these interfaces, which form upon crystallization of the amorphous PZT film. Internal PZT interfaces are associated with both structural defects (voids) as well as chemical variations such as Pb deficiencies. INTRODUCTION Lead zirconate titanate (PZT), a piezoelectric oxide, has gained much attention for uses in MEMS applications. One such application transforms mechanical energy into electrical energy by flexing a PZT membrane. PZT can be deposited via solution deposition and subsequent heat treatments. The texture of the PZT has been found to contribute to the electrical properties of the film [1-4]. Texture can be controlled by heat treatments [5,6], PZT thickness [5], chemistry [1,7], growth interface chemistry (including bottom electrode and internal PZT interfaces), and deposition technique of the bottom electrode [8,9]. Therefore, understanding crystal growth and preferred orientation for a given system is important to device optimization. Another serious concern in solution deposited PZT are chemical gradients and grain morphology. In addition to formation of harmful intermetallics and the non-piezoelectric pyrochlore phase, Pb diffusion and loss may also govern orientation. For example, migration of Pb to the nucleating interface can govern the orientation of the PZT. (100) PZT texture nucleates on the PbO (a (001) textured layer compound), while (111) PZT nucleates on Pt5-7Pb (a (111) textured transient intermetallic phase) [5-9]. Also, since nucleation of PZT grains is expected to occur at the interface on which it is deposited, it is feasible to expect that structure and chemistry also play a role in the morphology. Heterogeneous nucleation at the interface leads to columnar grains, which have been shown to exhibit better electrical fatigue and polarization than do equiaxed grains [9,10]. Thus, controlling interfacial chemistry may also be a method of controlling structure and orientation and therefore electrical properties. The current study aims to elucidate the contribution of various processing parameters to orientation and crystal structure and thereby p
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