Defects and Failure Modes in PZT Films for a MEMS Microengine
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Defects And Failure Modes In PZT Films For A MEMS Microengine D.F. Bahr, B.T. Crozier, C.D. Richards and R.F. Richards Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920 ABSTRACT Piezoelectric films for a MEMS microengine have been deposited using solution deposition routines onto platinized silicon wafers. These films are used as membranes above a bulk micromachined cavity. A dynamic bulge tester and interferometer were used to characterize the deformation of the films when pressurized. The mechanical strain at failure, as well as the fatigue behavior, have been characterized. Membranes between 300 and 500 nm thick have been shown to sustain mechanical fatigue damage over approximately 10 million cycles at strains of 30% of the monotonic failure strain. Defects in the films due to growth and thermal stresses during processing, and their role in membrane failure, are identified. Crack growth is demonstrated in these films by compliance measurements during fatigue testing, and interfacial failure is identified between the PZT and Pt layers. INTRODUCTION With the increase in variety of microelectromechanical systems (MEMS), it has become increasingly important to understand the mechanical properties of the materials used in devices which are exposed to significant stresses and harsh environments. A device developed for MEMS power generation consists of a ceramic piezoelectric membrane, PbZrxTi1-xO3 (PZT), which caps a cavity filled with a two phase fluid. This cavity acts as a Carnot cycle engine when heated, resulting in cyclic flexing of the PZT membrane when heat is applied and removed from the cavity. Since the operation of the proposed device requires cycling at rates approaching 1 kHz at strains which may approach 0.5 %, the mechanical strength and reliability of this membrane generator is a concern for device performance. Researchers using silicon based technology (primarily polycrystalline or single crystal silicon) for MEMS have examined the strength and fracture behavior of silicon structures [1]. While some studies have focused on applying external loading mechanisms to thin films, others have centered on creating on-chip loading structures to electrostatically test the strength of a component. It has been noted that there is a wide range of data reported for both the tensile fracture strength as well as the elastic modulus of polycrystalline thin films [2]. These properties can vary by a factor of 4 for different films tested in methods ranging from tensile tests to beam bending tests. Fatigue data for polycrystalline silicon thin films [3] shows that when some MEMS structures are cycled at stresses greater than about three quarters of the stress required for failure, failure can occur around 10 billion cycles. While there exists mechanical test data on silicon for MEMS [1-4], little data exists on the mechanical properties of thin film PZT. There exist some data on hardness and adhesion measured via nanoindentation [5,6], but these measurements do not probe the prope
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