Thermal Losses and Temperature Measurement in SOI MEMS Heater
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Thermal Losses and Temperature Measurement in SOI MEMS Heater Nicholas Moeldersa, Irina Puscasua Mark P. McNeala, Martin U. Prallea, Lisa Lasta, William Hoa, Anton C. Greenwalda, James T. Dalya, Edward A. Johnsona, Thomas Georgeb and Daniel S. Choib a Ion Optics, Inc., 411 Waverley Oaks Road, Waltham, MA 02452 b Jet Propulsion Laboratories, Pasadena, CA. ABSTRACT A sensor chip has been designed and tested that uses a MEMS strip heater as both source and detector of infrared radiation. An optical cavity reflects infrared radiation back onto the source filament. Changes in reflected light intensity modify heater temperature, and the measured signal is a change in resistance. The effects of processing on electrical and thermal isolation were characterized and used to evaluate device performance. Thermally isolated, uniformly heated emitters are achieved using a backside release etch process. The fully released devices demonstrated superior electric to thermal-optical conversion, with the requisite narrow band emission for CO2 detection. Using these sensor-chips, CO2 detection was demonstrated, with projected sensitivities ≤0.1%. INTRODUCTION Sensing carbon dioxide (CO2) gas is of profound importance because of its prevalence as both a physiological and industrial byproduct. CO2 gas sensor applications include respiration monitoring, combustion by-product monitoring, indoor air quality, environmental monitoring. There are several traditional methods for gas and chemical detection. Electrochemical or catalytic sensors require little power and are small, sensitive, and inexpensive but they are subject to interference and false alarms due to other chemical species, they require contact with the environment and they need to be replaced every two years. Non-dispersive infrared (NDIR) spectroscopic gas sensors are based on the fact that many gases have unique infrared absorption signatures in the 2-14 µm region. This enables conclusive identification and quantification of chemicals in liquid and gas phase mixtures. NDIR gas sensors are accurate with a fast response time. Because these sensors do not require direct contact with the environment, they are more reliable. Typically, NDIR gas sensors consist of several discrete components, making them more complex and more expensive than electrochemical sensors. This work seeks to exploit MEMS-based technologies to integrate the functionality of NDIR instruments onto a chip (Fig.1a). This is accomplished by using a thermally isolated, narrow band emitter in combination with reflective optics. The emitter projects a light beam through an optical cell to a reflector, which sends the light back to the emitter (Fig.1b). In the absence of any gas absorption in the optical cell, the filament rapidly reaches thermal equilibrium. The introduction of absorbing gas in the cell reduces the reflected optical power returning to the filament, causing the filament to equilibrate at a lower temperature. The change in thermal equilibrium is detected as a resistance change. By controlling the filame
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