MEMS Microresonators for High Temperature Sensor Applications

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MEMS Microresonators for High Temperature Sensor Applications Dharanipal Doppalapudi1, Richard Mlcak1, Jeremie LeClair1, Patrick Gwynne1, Jeffrey Bridgham1, Scott Purchase1, Martin Skelton1, Gerald Schultz1, Harry Tuller1,2 1 2

Boston MicroSystems Inc., Woburn, MA 01801

Department of Materials Science and Engineering, MIT, Cambridge, MA 02139

ABSTRACT Microelectromechanical Systems (MEMS) are being extensively investigated as a means of miniaturizing piezoelectric sensors thereby offering higher sensitivity, reduced power consumption, and ability to form compact multi-sensor arrays. Such devices typically employ one or more silicon micromechanical elements (e.g. membranes, cantilever beams and tethered proof masses) driven electromechanically by a polycrystalline piezoelectric film. The use of polycrystalline materials results in inherently less stable and irreproducible device characteristics. For elevated operating temperatures, more robust and refractory materials are also required. In this paper, we describe a MEMS microresonator array capable of operating to temperatures exceeding 600°C enabled by the integration of epitaxially grown piezoelectric AlN films onto single crystal SiC tethered plates. The operation of the microresonators as sensors is illustrated by examining their response to temperature, pressure and chemical analytes. INTRODUCTION Robust temperature, pressure and chemical sensors are required for several harsh environment applications including combustion environments in power generation, exhaust monitoring in automobiles, environmental monitoring and process control. In such applications, the sensors need to operate at temperatures as high as 1000°C and be stable in the presence of corrosive gases over extended periods. In addition to high sensitivity, selectivity and the ability to track multiple gas concentrations simultaneously are highly desired. Many applications further require sensors with rapid response and non-invasive installation enabling real-time process control. Currently, there is no commercial sensor technology that can satisfy all these requirements. Sensors based on MEMS platforms have many attractive features including small size, low power consumption, low detection limit, rapid response and ability to form multi-element arrays [1]. However, conventional MEMS based on silicon and polycrystalline materials are inherently limited to low temperatures. SiC-AlN bimorph resonator based sensors enable extension of the MEMS technology to harsh environments. SiC has excellent mechanical strength (Young’s Modulus of 448 GPa is 2.5 times that of Si), chemical and thermal stability and an exceptionally high thermal conductivity (5 W/cm-°C), making it an ideal material for harsh environment applications. AlN is similarly a refractive material, with excellent piezoelectric properties [2,3] that are preserved at high temperatures above 1000°C [4,5]. AlN and SiC have excellent acoustic match, enabling the fabrication of low-loss resonators. In this paper, we present the app