Towards Integrated Mid-Infrared Gas Sensors

Abstract

Optical gas sensors play an increasingly important role in many applications. Sensing techniques based on mid-infrared absorption spectroscopy offer excellent stability, selectivity and sensitivity, for numerous possibilities expected for sensors integrated into mobile and wearable devices. Here we review recent progress towards the miniaturization and integration of optical gas sensors, with a focus on low-cost and low-power consumption devices.

Keywords: CMOS; MEMS; gas sensors; integrated sensors; mid-infrared.

Figure 1

Mid-infrared absorption spectra of selected molecules with their relative intensities. H${}_2$O: water; CO${}_2$: carbon dioxide; CO: carbon monoxide; NO: nitric oxide; NO${}_2$; nitrogen dioxide; CH${}_4$: methane; O${}_3$: oxygen; NH${}_3$: ammonia. Source: HITRAN [10].

Figure 2

Optical gas sensor based on the Beer–Lambert’s law. (a) No signal detected when the emitter is off. (b) The detected signal is at a maximum when the emitter is on and no gas is present, and is decreasing (c) with the gas concentration, c, when the gas is present.

Figure 3

Optical gas sensors topologies. Most commonly used sensors rely on gas cells formed between face-to-face configured emitters and detectors. (a) Light can be filtered prior interaction with the gas. An external reference detector can be used to compensate for changes in the emitted light [53]. (b) Dual detector configuration, with [48,58] or without [54] filters. (c) The gas cell can be reduced by increasing the light-gas interaction, e.g., by using photonic crystals [55], optical cavities [56], multi-pass cells [57], or gas enrichment layers [58]. (d) Photoacoustic cell. Acoustic waves created by light-gas interaction are detected by a microphone. It can be resonant [67] or non-resonant [68]. (e) Open cell configuration, using either dual optical detection [64] or microphones sealed with target gases [69]. (f) Cell with emitter and detector in planar configuration. Multiple optical detectors with filters in the range of interest can be used [59], or microphones sealed with target gases [44]. (g) Waveguide sensors based on evanescent field interaction [60,61,62] require optical coupling with both emitters and detectors. (h) Dual photoacoustic cell with reference microphone [70,71,72].

Figure 4

(a) Complementary metal-oxide semiconductor (CMOS) integrated plasmonic microhotplate with drive and temperature control. (b) Mid-infrared spectra of carbon nanotubes (CNT)-coated and uncoated devices [94].

Figure 5

(a) Differential filter-free detection, and (b) spectral response. Adapted from ref. [54].

Figure 6

Integrated optical gas sensor concept. Planar integration of emitters and detectors within the same micro-electro-mechanical systems (MEMS) chip, facing an optimized multi-reflection elliptic mirror (gas cell). The sensor signal can be enhanced with an on-chip amplifier and have the first analogue processing level done on-chip with more complex signal processing done externally through the use of an application-specific integrated circuit (ASIC). The MEMS and ASIC chips are attached via face-to-face flip-chip bonding.