Zirconium diboride thin films for use in high temperature sensors and MEMS devices

David M. Stewart; George P. Bernhardt; Robert J. Lad

doi:10.1117/12.2266561

30 May 2017 Zirconium diboride thin films for use in high temperature sensors and MEMS devices

David M. Stewart, George P. Bernhardt, Robert J. Lad

Proceedings Volume 10246, Smart Sensors, Actuators, and MEMS VIII; 102460Q (2017) https://doi.org/10.1117/12.2266561
Event: SPIE Microtechnologies, 2017, Barcelona, Spain

Abstract

Sensors and MEMS devices operating in high temperature environments require stable thin films with high electrical conductivity for use as electrodes, bond pads, and other components. Metal films are unreliable because of thermodynamically driven morphological instability and agglomeration over long times. Zirconium diboride (ZrB₂) is an ultra-high temperature conducting ceramic with a melting point of 3245°C, with low atomic diffusion rates compared to other materials. To evaluate ZrB₂ as a high temperature film, 200 nm thick ZrB₂ films were synthesized on r-sapphire substrates using e-beam co-evaporation of elemental Zr and B sources. Film stability was characterized after post-deposition thermal treatments from 600-1000°C in both reducing (vacuum) and oxidizing (air) environments. ZrB₂ films deposited at room temperature are amorphous, but have short-range order characteristic of ZrB₂ bonding. ZrB₂ films grown at 600°C are polycrystalline with preferred <0001< texture, whereas at 850°C grains with preferred <10-10< and <10-11< texture become dominant. Negligible grain growth or morphology changes occur after annealing at 850°C for 55 hours in vacuum, and film electrical conductivity remains <105 S/m. Annealing in air, however, leads to ZrB₂ film decomposition into ZrO₂ and B₂O₃ phases, the latter of which is volatile. X-ray diffraction indicates that a 50 nm thick hexagonal boron nitride (h-BN) capping layer grown on top of ZrB₂ via magnetron sputtering hinders oxidation, but the ZrB₂ eventually transforms to ZrO₂. These results indicate that ZrB₂ films are attractive for potential use in sensors and MEMS devices in high temperature reducing environments, and for short times in oxidizing environments when covered with a h-BN capping layer.

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David M. Stewart, George P. Bernhardt, and Robert J. Lad "Zirconium diboride thin films for use in high temperature sensors and MEMS devices", Proc. SPIE 10246, Smart Sensors, Actuators, and MEMS VIII, 102460Q (30 May 2017); https://doi.org/10.1117/12.2266561

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KEYWORDS

Microelectromechanical systems

Thin film devices

Thin films

Environmental sensing

Temperature sensors

Annealing

Sensors

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