Pt/TiO 2 Growth Templates for Enhanced PZT films and MEMS Devices
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Pt/TiO2 Growth Templates for Enhanced PZT films and MEMS Devices Daniel M. Potrepka1, Glen R. Fox2, Luz M. Sanchez1,3, and Ronald G. Polcawich1 1
U.S. Army Research Laboratory, Adelphi, Maryland 20783, U.S.A.
2
Fox Materials Consulting LLC, Colorado Springs, CO 80908, U.S.A.
3
Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, U.S.A.
ABSTRACT The crystallographic texture of lead zirconate titanate (PZT) thin films strongly influences the piezoelectric properties used in MEMS applications. For PZT films poled to saturation, the piezoelectric response is sequentially greater for random, {111}, and {001} texture. Textured growth can be achieved by relying on crystal growth habit and can also be initiated by the use of a seed layer that provides a heteroepitaxial template. Template choice and the process used to form it determine the structural quality and ultimately influence performance and reliability of MEMS PZT devices such as switches, filters, and actuators. This study focuses on how {111}-textured PZT is generated by a combination of crystal habit and templating mechanisms that occur in the PZT/bottom-electrode stack. The sequence begins with {0001}textured Ti deposited on thermally grown SiO2 on a Si wafer. The Ti is converted to {100}textured TiO2 (rutile) through thermal oxidation. Then {111}-textured Pt can be grown to act as a template for {111}-textured PZT. The Ti and Pt are deposited by DC magnetron sputtering. The TiO2 and Pt film textures and structure were optimized by variation of sputtering deposition times, temperatures and power levels, and post-deposition anneal conditions. The relationship between Ti, TiO2, and Pt texture and their impact on PZT growth will be presented. INTRODUCTION A wide variety of the physical properties of materials, such as ferroelectricity, ferromagnetism, piezoelectricity, conductivity, and dielectric permittivity depend upon material anisotropy and are therefore strongly affected by chrystallographic texture [1]. With the appropriate choice of thin film texture, device operating efficiency and reliability can be strongly affected. Therefore texture is a critical factor for thin film process control and is fundamental to device reproducibility. This report on deposition and characterization of TiO2 and Pt thin film electrodes leverages the electrode developments reported for FRAM (ferroelectric random access memory) and applies the technology to PZT-MEMS device fabrication. Because the Pt electrode crystallographic texture acts as a template for ferroelectric PZT film growth, the remanant polarization properties of the PZT can be degraded by uncontrolled Pt bottom electrode texture [2]. In addition, factors such as bottom or top electrode film density, thickness and grain size and Pt top electrode etch damage and thermal processing influence PZT ferroelectric performance. Academic publications rarely describe Pt texture and density control, but a patented process has been reported for a bottom electrode consisting
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