Ion Optics has developed a thin silicon membrane MEMS device that replaces the thermal source, IR filter, IR detector and mechanical chopper in conventional non-dispersive infrared gas sensors. The key enabling technology is a 2-D photonic crystal. The center wavelength and bandwidth of emitted radiation from the photonic crystal depends upon the pattern etched into the surface. Previously we reported designs based on hexagonal arrangements of holes about 2 microns diameter. New results for more intricate designs with deliberate photonic crystal "defects" will be presented. Experimental results will be compared to 3-D electromagnetic models. The 2-D photonic crystal structure consists of an array of air rods produced by self-aligned etching into a thin (100nm) conductor on top of a dielectric membrane. We describe fabrication routes via conventional silicon microlithography and novel approaches including nano-imprinting and transfer molding. We present spectral emission and absorption measurements which relate optical intensity to details of photonic crystal design and fabrication.
A compact, low-cost, multi-wavelength NDIR sensor was designed to measure G-type CW agents at ppm-levels. This 4-color sensor can distinguish between the different agents (sarin, soman, tabun) and is more sensitive than a single wavelength sensor. The design of the sensor and test results with simulants R-12 (dichlorodifluoromethane) and sulfur hexafluoride is presented. These test results support a lower detection limit of 3 ppmv for a 1 sec integration time. Modifications of the sensor design which will enable us to achieve <1 ppmv sensitivity are discussed.
Light-weight, low-power consumption, low-cost IR sources are required for combat ID (IFF, identify friend or foe), trail markers, pallet markers, etc. They must be visible with conventional viewers at 200 meters in the 3-5 micron or 8-12 micron bands and emit no visible or near infrared radiation. Ion Optics has tested a prototype MEMS IR source that can meet all of these requirements. It uses a hermetically sealed filament with a photonic crystal-enhanced (PCETM) coating that efficiently generates narrowband IR light. The photonic crystal surface structure limits emission to (tunable) predetermined bands (3-5 and 8-12 microns specifically). These devices generate 10mW of IR light in the 3-5 micron band with "wall-plug" efficiency of 10%, 2 orders of magnitude more efficient than conventional IR LED's. This high efficiency enables overnight battery operation. Using traditional 3-5 micron MWIR cameras, we measured visibility ranges of 200 meters. Current research and development on wafer-level packaging of the MEMS device promises to increase device yield, improve reliability, reduce package size and reduce total cost.
Sensors of trace gases are of enormous importance to diverse fields such as environmental protection, household safety, homeland security, bio-hazardous material identification, meteorology and industrial environments. The gases of interest include CO for home environments, CO2 for industrial and environment applications and toxic effluents such as SO2, CH4, NO for various manufacturing environments. We propose a new class of IR gas sensors, where the enabling technology is a spectrally tuned metallo-dielectric photonic crystal. Building both the emitting and sensing capabilities on to a single discrete element, Ion Optics’ infrared sensorchip brings together a new sensor paradigm to vital commercial applications. Our design exploits Si-based suspended micro-bridge structures fabricated using conventional photolithographic processes. Spectral tuning, control of bandwidth and direction of emission were accomplished by specially designed metallo-dielectric photonic crystal surfaces.
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.
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.
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 Planck's 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.
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.