Internal Oxidation and Mechanical Properties of Pt-IrO 2 Thin Films
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INTERNAL OXIDATION AND MECHANICAL PROPERTIES OF Pt-IrO2 THIN FILMS Richard R. Chromik, Thirumalesh Bannuru and Richard P. Vinci Department of Materials Science and Engineering, Lehigh University 5 East Packer Avenue, Bethlehem, PA 18015 ABSTRACT Pt-IrO2 films, approximately 200 nm thick, were fabricated by co-sputter deposition of Pt and Ir in an Ar-O2 mixture followed by annealing at 700°C in O2 for 4 hours. X-ray photoelectron spectroscopy and x-ray diffraction measurements indicate the presence of IrO2 throughout the thickness of the films. After a thermal cycle in vacuum to 700°C, the room temperature residual stress is significantly lower in the internally oxidized films than in pure Pt films of similar thickness subjected to identical cycling. Initial analysis of the behavior of the films during thermal cycling indicates that the primary cause for the difference in residual stress level is a decrease in the thermoelastic slope associated with the introduction of IrO2. INTRODUCTION The use of Pt as an electrode material for certain sensors and actuators typically springs from a need for a robust oxidation and corrosion resistant metal. In this regard, Pt is an excellent choice. However, Pt can easily develop room temperature residual stresses of greater than 1 GPa after the thermal cycling to which many devices are subjected, much higher stress than is typical for most other common electrode materials. The high residual stress can cause undesirable curvature in micromechanical devices, and can cause long-term reliability problems such as void formation and poor adhesion [1]. Even if Pt film deposition is controlled in such a way that initial residual stress is minimized, thermal expansion-driven high temperature plasticity, followed by nearly elastic behavior during much of the cooling process, determines the final room temperature stress state. This indicates two possible methods for control of room temperature residual stress: a reduction of high temperature plasticity, and a decrease of the thermoelastic modulus M∆α, where M is the biaxial elastic modulus of the film and ∆α is the difference in linear coefficient of thermal expansion between the film and substrate. Both goals could potentially be achieved by appropriate choice of an alloy addition. In bulk materials, solid solution and oxide dispersion alloying have both been effective at inhibiting rapid relaxation at high temperatures [2]. In recent studies on alloy thin films, Pt-Ru solid solution alloying was found to increase low temperature strength, but had little effect on high temperature relaxation [3]. Pt is often oxide dispersion strengthened in bulk form with ZrO2, but this is achieved in ways that are not compatible with thin film deposition. Although bulk Pt is not a prime candidate for internal oxidation [4], the small length scales and unique fabrication processes associated with thin films raise the prospect that successful internal oxidation of 200 nm thick Pt films can be achieved. The aim of the current study was to explore the
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