Fracture Behavior of Thin Film PZT on Silicon Mems and Membranes
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FRACTURE BEHAVIOR OF THIN FILM PZT ON SILICON MEMS AND MEMBRANES A.L. Olson, J.L. Skinner, D.F. Bahr, C.D. Richards, R.F. Richards Mechanical and Materials Engineering, Washington State University, Pullman, WA ABSTRACT This research focuses on identifying the fracture mechanism in thin film silicon membranes and PZT on silicon composite structures under static and dynamic loading conditions. Square silicon membranes with a 3mm side-length and thickness between 1.5 and 3.0µm, with and without 1.5µm of PZT, were pressurized to failure while laser interferometry was used to determine the maximum strain at failure. The strain at fracture of silicon membranes, initiating from the sharp corner radius inside the micromachined cavity, was improved from 0.37% to 0.8% by an isotropic etch of the sharp corner. Fracture of PZT on silicon membranes, tested at the mechanical resonant frequency suggested that fracture initiates in the blanket PZT layers under reversed bending. Etching the PZT from high strain regions along the membrane surface improved the strain at failure of the composite device by 40%. INTRODUCTION Thin film membranes of silicon and piezoelectric composite layers are used in a variety of microelectromechanical systems (MEMS). These devices can actuate, resonate, sense, or transduce depending on whether their principle function is to generate or collect energy. For an application of generating power from relatively large mechanical strains where more than 100 psi on 3mm X 3µm thick silicon membranes can be applied, the high strength properties of silicon are desirable. Silicon has distinct advantages for batch manufacturing using existing technologies, and piezoelectric materials deposited upon a silicon membrane serve to couple mechanical and electrical signals, in many cases Pb(ZrxTi1-x)O3 (PZT). For applications where MEMS devices are operated at high strains, fracture becomes increasingly important. The current research focuses on square membranes of silicon, and PZT on silicon, and the failure of these devices via fracture. Previous research has shown that fracture of single crystal silicon initiates in regions of highest tensile stress for bending test specimens [1], and may be related to surface roughness and the size of test specimens. Other researchers have shown the strength of micromachined silicon cantilevers to be about 3.1 times stronger from the front than the back (front and back have the same meaning for cantilevers as the square membrane cross-section shown in Figure 1), and about 1.8 times stronger for fractures occurring along (110) than (111) surfaces [2]. Stress concentrations are important to the strength of MEMS devices and are related to the notch sensitivity and theoretical stress concentration for a given geometry [3]. Figure 1 shows the cross-section of a device consisting of 1.3µm of silicon and 1.1µm of PZT between top and bottom electrodes. A feature of this membrane is the relatively sharp corner inside the membrane cavity that is inherent to anisotropic etched (100) sili
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