MEMS Optical (Infrared) Gas Sensor

Product Overview

Infrared gas sensors exploit the fact that most gases have unique infrared signatures in the 2-14 micron wavelength region. Because each gas has a unique infrared absorption line, infrared gas sensors provide conclusive identification and measurement of the target gas with little interference from other gases. IR gas sensors are also highly accurate, responsive, and reliable. Since the sensor element does not touch the gas -- only infrared light touches the gas -- they are not poisoned by contact with the environment. Conventional infrared gas sensors are big, complicated, and expensive since they are actually a cabinet full of several discrete components which are usually hand-selected and hand-assembled. Ion Optics' patented optical technology platform allows us to build all the optical components -- emitter, filter, detector -- onto a single silicon chip. Since this chip is designed according to standard microelectronics design practices, we can build the chip in standard semiconductor foundries

Infrared Gas Sensor Basics

To the right is a schematic of a conventional infrared gas sensor. (This one is a nondispersive infrared, or NDIR, gas sensor.)

Just as optical fibers carry different conversations at specific optical wavelengths, optical gas sensors depend on measuring the transmission of light at a different wavelength for each gas. The particular wavelength identifies the gas and the amount of light absorbed by the gas determines the gas concentration. This is the reason that optical measurements are accurate and do not suffer from cross-sensitivity and false alarm problems.

Optical measurements are the most accurate and most reliable method for gas analysis. Until now, optical instruments have been big, complex, and expensive. Ion Optics’ unique patented optical technology platform can shrink a high quality optical sensor onto a tiny silicon chip. The key breakthrough is the ability to control optical wavelengths in a flat, two-dimensional structure which is built through conventional silicon processing. This tunable wavelength capability is the heart of Ion Optics’ optical technology platform.

How the IR Gas SensorChipTM Works

The SensorChip is a wavelength-tuned, MEMS-based micro-bridge element. Using photonic bandgap (PBG) technology, the micro-bridge emits and absorbs efficiently in a narrow waveband centered on the signature wavelength of the target gas. Ion Optics tunes the infrared wavelength (like an LED) during production using standard, stable semiconductor manufacturing techniques.

Wavelength Tuning

The key innovation is MEMS-based micro-bridge elements that are wavelength-tuned emitters and wavelength-tuned detectors. Using photonic bandgap (PBG) technology, the micro-bridge element emits AND absorbs efficiently in a narrow waveband centered on the signature wavelength of the target gas.

Ion Optics has made photonic bandgap structures that achieve 10% FWHM (Full Width Half Max) performance. This works reliably for identifying particular gases and it is comparable to the performance of the thin film interference filters used in conventional NDIR units.

We have achieved better than 90% emissivity for these surfaces (in-band) so that the SensorChip makes efficient use of electrical drive power. The yellow curve on this graph shows the emitted power spectrum from a test structure which was tuned for emission at 4 micron wavelength. The red curve shows the absorption of carbon dioxide gas at a wavelength nearby. By making the emission/detection band narrow, we can make the useful (in-band) light large compared to the total thermal budget of the silicon micro-bridge. The gray curve shows emission from an ideal blackbody to illustrate the amount of drive power required to produce this level of in-band signal with a conventional infrared instrument.