Issues in the Flexible Integration of Sputter-Deposited PZT Thin Films with Polysilicon and Ti/Pt Electrode Layers for U
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Issues In The Flexible Integration Of Sputter-Deposited PZT Thin Films With Polysilicon And Ti/Pt Electrode Layers For Use As Sensors And Actuators In Microelectromechanical Systems (MEMS) C.F. Knollenberg, T.D. Sands, A.S. Nickles1, R.M. White2. Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. 1 Capacitor Division, Applied Materials, San Jose, CA. 2 Department of Electrical Engineering & Computer Sciences, University of California, Berkeley, CA 94720. ABSTRACT Sputter-deposited piezoelectric lead zirconate titanate (PZT) thin films with Ti/Pt and polysilicon electrode layers are being investigated for use in Microelectromechanical Systems (MEMS). Existing research shows the nucleation of the perovskite phase of the PZT is linked to the lattice spacing of the underlying Pt electrode and/or seed layers, and is key in obtaining PZT layers with good piezoelectric/ferroelectric properties. Our research with piezoelectric PZT films on Ti/Pt electrode layers aims at employing these films to generate and receive acoustic waves in flexural plate wave devices (FPWs). Our experiments indicate the formation of a random polycrystalline perovskite phase is linked to the emergence of oriented Pt grains within the dominant -oriented crystal structure during rapid thermal annealing in an oxygen environment. Pt films annealed in nitrogen, in contrast, retained their preferential orientation without the formation of Pt grains. PZT films deposited on these electrodes and annealed in nitrogen were strongly oriented in the direction, but exhibited lossy ferroelectric behavior and were prone to delamination. We are also investigating the feasibility of using doped polysilicon electrode layers with PZT thin films. The multiple layers used with the Pt electrode (Pt, Ti, and SiO2 adhesion layer) have significant interactions with one another, and replacing these layers with a single electrode layer should alleviate these complications. A low-temperature PZT deposition process (300°C) and short annealing cycles (30 sec.), coupled with a TiO2 barrier/seed layer should prevent interdiffusion and reactions between the polysilicon and PZT layers. Our experiments show that PZT films deposited and annealed on doped polysilicon layers develop a random polycrystalline perovskite phase, but are subject to tensile cracking. The use of polysilicon as an electrode layer should also facilitate the integration of piezoelectric PZT layers with polysilicon surface micromachined structures using SiGe sacrificial layers. INTRODUCTION While much research on thin-film PZT focuses on its ferroelectric properties for use in memory applications, there is a parallel interest in using the piezoelectric properties of PZT for sensing and actuation in MEMS [1]. In particular, the large piezoelectric coefficients of PZT may substantially improve the performance of acoustic devices like the flexural plate wave device (FPW), which use the piezoelectric effect to generate and receive acoustic signals [2]. EE5.35.1
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