Nicholas Moelders

Development of optical MEMS CO2 sensors

Inexpensive optical MEMS gas and chemical sensors offer chip-level solutions to environmental monitoring, industrial health and safety, indoor air quality, and automobile exhaust emissions monitoring. Previously, Ion Optics, Inc. reported on a new design concept exploiting Si-based suspended micro-bridge structures. The devices are fabricated using conventional CMOS compatible processes. The use of photonic bandgap (PBG) crystals enables narrow band IR emission for high chemical selectivity and sensitivity. Spectral tuning was accomplished by controlling symmetry and lattice spacing of the PBG structures. IR spectroscopic studies were used to characterize transmission, absorption and emission spectra in the 2 to 20 micrometers wavelength range. Prototype designs explored suspension architectures and filament geometries. Device characterization studies measured drive and emission power, temperature uniformity, and black body detectivity. Gas detection was achieved using non-dispersive infrared (NDIR) spectroscopic techniques, whereby target gas species were determined from comparison to referenced spectra. A sensor system employing the emitter/detector sensor-chip with gas cell and reflective optics is demonstrated and CO2 gas sensitivity limits are reported.

Photonic crystals for narrow-band infrared emission

MEMS silicon (Si) micro-bridge elements, with photonic band gap (PBG) modified surfaces are exploited for narrow-band spectral tuning in the infrared wavelength regime. Thermally isolated, uniformly heated single crystal Si micro-heaters would otherwise provide gray-body emission, in accordance with Plancks distribution function. The introduction of an artificial dielectric periodicity in the Si, with a surface, vapor-deposited gold (Au) metal film, governs the photonic frequency spectrum of permitted propagation, which then couples with surface plasmon states at the metal surface. Narrow band spectral tuning was accomplished through control of symmetry and lattice spacing of the PBG patterns. Transfer matrix calculations were used to model the frequency dependence of reflectance for several lattice spacings. Theoretical predictions that showed narrow-band reflectance at relevant wavelengths for gas sensing and detection were then compared to measured reflectance spectra from processed devices. Narrow band infrared emission was confirmed on both conductively heated and electrically driven devices.

Modeling Combined Thermal, Electrical, Optical and Mechanical Response for MEMS Spectroscopic Gas Sensor Based On Photonic Crystals

A new type of gas sensor was developed that combines the principles of bolometric infrared detectors with photonic crystals. 1,2 This paper describes a quantitative model used to optimize the materials, geometry, and electrical properties of this suspended membrane MEMS device. Fundamentally the model is concerned with the thermal response of the device using temperature dependent thermal conductivity, specific heat, and electrical resistance to calculate conduction, convection, and radiation losses for a negative temperature coefficient of resistance material. Variations in the electrical drive circuit, dc and ac response, low and high frequency sinusoidal and random noise, along with an exacting calculation of expected signal were used to improve design. The model follows the time evolution of the system. We show how look-up tables with scaling (derived from exact, off-line finite element models for thermal conduction, spectral emission, etc.) provided sufficiently accurate estimates with rapid calculation to enable running the model on a standard PC type computer. The simulations matched the experimental results, accurately predicted the unstable operating regimes, and maximized the signal to noise ratio for the device.

MEMS-based sensor system for environmental monitoring

A new IR-based sensor technology is introduced for environmental monitoring of industrial pollutants (CO2, CO, NOx, etc.). The design concept exploits Si-based, thermally isolated suspended bridge structures. These devices, which function as both IR emitter and detector, are fabricated using MEMS-based processing methods. Photonic bandgap (PBG) modified surfaces enable narrow band IR emission for high chemical selectivity and sensitivity. Spectral tuning is accomplished by controlling symmetry and lattice spacing of the PBG structures. IR spectroscopic studies were used to characterize transmission, absorption and emission spectra in the 2 to 20 micrometers wavelength range. Device characterization studies measured drive and emission power, temperature uniformity, and black body detectivity. Gas detection was achieved using non-dispersive infrared (NDIR) spectroscopic techniques, whereby target gas species and concentrations were determined from comparison to referenced spectra. A sensor system employing the emitter/detector sensor-chip with gas cell and reflective optics is demonstrated and CO2 gas sensitivity limits are reported. A multi-channel microsensor-array is proposed for multigas (e.g., CO2, CO, and NOx, etc.) detection.

Development of optical MEMS carbon dioxide sensors

A NDIR-based sensor-chip using MEMS Si micro-bridge elements, with integrated PBG structure for wavelength tuning is discussed. The effects of processing on device performance, especially device release, were investigated. Thermal and electrical device characterization was used to quantify loss mechanisms. Thermally isolated, uniformly heated emitters were ultimately achieved using a backside release etch fabrication process. The fully released devices demonstrated superior electric to thermal (optical) conversion, with the requisite narrow band emission for CO2 detection. Using the MEMS sensor-chips, 20% CO2 detection was demonstrated, with projected sensitivities of ~3% CO2.

Thermal Losses and Temperature Measurement in SOI MEMS Heater

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 CO 2 detection. Using these sensor-chips, CO 2 detection was demonstrated, with projected sensitivities ≤0.1%.