by Chen, Li

Abstract (Summary)
The potential of low-stress, heavily-nitrogen-doped, (111)-oriented polycrystalline 3C-silicon carbide (poly-SiC) formed by LPCVD as an advanced MEMS material for harsh-environment and demanding applications was studied through the development and characterization of two MEMS devices: pressure sensor and IR emitter. A research prototype of a low-cost, miniature, mass-producible sensor for measurement of high pressure at operating temperatures of 300 deg.C to 600 deg.C, e.g., in-cylinder engine pressure monitoring applications, was developed. This all-SiC capacitive sensor, i.e., a SiC diaphragm on a SiC substrate, takes advantage of the excellent harsh environment material properties of SiC and was fabricated by surface micromachining. The sensor was packaged in a high-temperature ceramic package and characterized under static pressures of up to ~5MPa (700psi) and temperatures of up to 574 deg.C in a custom chamber. An instrumentation amplifier integrated circuit was used to convert capacitance into voltage for measurements up to 300 deg.C; beyond 300 deg.C, the capacitance was measured directly from an array of identical sensor elements using a LCZ meter. After high temperature soaking and several tens of temperature/pressure cycles, packaged sensors continued to show stable operation. The sensor was also packaged in a custom probe and successfully demonstrated dynamic pressure monitoring after being inserted into the cylinder head of a research internal combustion engine. A thermal infrared emitter (blackbody) capable of fast thermal cycling was realized for pulsed operation at high frequency using free-standing poly-SiC micro-bridge elements. High emissivity, high thermal conductivity, low thermal mass and excellent mechanical robustness of poly-SiC enable this development. Poly-SiC's peak emission wavelength falls in the range of short wavelength infrared. Devices were pulsed at frequencies up to 100Hz with modulation depth near 50%. Materials analysis examining the surfaces of pre- and post-heated emitter elements was performed using Auger electron spectroscopy (AES) and showed extremely low oxidation effect up to about 700 deg.C. Poly-SiC micro-hotplate-based IR emitter platforms, including poly-SiC heating and sensing resistors, were used for a reliability study using an accelerated degradation methodology testing. For comparison, platinum (Pt) was used on another set of otherwise similar SiC micro-hotplates for the heating and sensing elements. Results show that Pt will rapidly degrade when operated above ~800 deg.C, while poly-SiC is stable up to ~1100 deg.C. In short, poly-SiC stands out for many higher-temperature applications, thanks to its outstanding material properties and chemical stability.
Bibliographical Information:


School:Case Western Reserve University

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:silicon carbide pressure sensors infra red emitters mems harsh environment high temperature


Date of Publication:01/01/2008

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