Electrically Conductive Pt-Zr-B and Pt-Si Thin Films for Use in High Temperature Harsh Environments
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Electrically Conductive Pt-Zr-B and Pt-Si Thin Films for Use in High Temperature Harsh Environments
R.J. Lad, D.M. Stewart, R.T. Fryer, J.C. Sell, D.J. Frankel, G.P. Bernhardt, R.W. Meulenberg Department of Physics & Astronomy and Laboratory for Surface Science & Technology, University of Maine, Orono, ME 04469-5708, U.S.A.
ABSTRACT Stable, electrically conductive, thin film materials are key components for high temperature sensors operating in harsh environments. In this work, nanocomposite Pt-Zr-B and Pt-Si thin film materials were grown to a nominal thickness of 200 nm on both r-cut sapphire (α-Al2O3) substrates using e-beam evaporation, and their structure, morphology, and chemical composition was characterized following thermal treatments in an air laboratory furnace up to 1300oC. In the Pt-Zr-B system, oxidation of a nanolaminate architecture consisting of ZrB2 and pure Pt layers leads to boron oxide evaporation and the formation of Pt grains decorated by tetragonal-ZrO2 nanocrystallites at high temperature. Electrical conductivity measurements with a 4-point probe show that this nanocomposite film structure can maintain a film conductivity > 1x106 S/m up to 1300oC, depending on the Pt/ZrB2 layer thickness ratio. In the Pt-Si system, film compositions were varied to yield either nanocrystalline Pt3Si, Pt2Si, or PtSi phases depending on the Pt-Si ratio, or an amorphous phase at high Si content. Above 1000oC in air, Pt-oxide and Si-oxide phases form and coexist with the Pt-Si phases, and some Pt-Si film conductivities remain as high as 1x106 S/m after annealing at 1000oC for 6 hours. It was found that a 100 nm thick amorphous alumina capping layer grown by atomic layer deposition (ALD) aids in limiting film oxidation, but film stress leads to regions of delamination. INTRODUCTION There is a critical need to develop electrically conductive thin film materials that remain very stable and nonreactive at temperatures above 1000oC for use in wireless sensors, actuators, and other thin film based electronic devices operating in harsh environments. At these high temperatures, thermodynamic rather than kinetic factors become dominant and often film deterioration occurs by mechanisms including recrystallization, agglomeration, oxidation, and chemical interdiffusion. These thin film degradation phenomena that occur in harsh operating environments are often the cause of sensor degradation over time and the lack of long-term accuracy and reliability. The motivation for the present work is to develop stable thin film electrode materials for use in wireless surface acoustic wave (SAW) sensor technology being applied to high temperature harsh environments in the aeronautic, power, space, and high temperature manufacturing industries [1-4]. The ultimate goal is to apply these thin films to wireless high temperature sensor devices attached to high temperature machinery and other
complex equipment in order to monitor conditions and processes for improved efficiency of industrial processes, energy savings, and reduced dependen
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