Trace chemical sensor based on photoacoustic spectroscopy

Piezoelectric MEMS device fabricated from a three-wafer process greatly decreases the overall size of sensing devices


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Photoacoustic spectroscopy (PAS) is a chemical detection method based on several simple principles: 1) the absorption of light by an analyte molecule; 2) the subsequent generation of an acoustical wave in a cell generated by the molecular relaxation, and 3) the detection of the acoustic wave by a pressure sensing device (a microphone).

Improvements to PAS detection devices have been steady but there is still a shortcoming related to the large size of the cell and ancillary equipment.

As an improvement, Army Research Lab scientists have developed an integrated PAS cell using MEMS manufacturing techniques.

The device incorporates a three-layer sandwich structure with the inner layer patterned to form a resonant cavity and a microphone acoustically coupled to that cavity. The microphone is a piezoelectric thin film membrane formed on one of the outer layers. The resonant cavity is an open-tube structure which receives light from a source. The inner layer is additionally patterned to include buffer cavities on either side of the resonant cavity.

Piezoelectric sensing is extremely attractive for miniature components because it is a high-efficiency conversion of mechanical to electrical energy and a passive sensing technique, i.e. does not require an electrical power source. The photoacoustic cell makes PAS, as a handheld technique, viable for numerous commercial and military chemical-sensing requirements.

The device is applicable to all types of gas sensing and monitoring, including industrial waste gas monitoring, chemical detection on the battlefield, etc. Although the main use of photoacoustics is in trace gas sensing, the technology can be used to detect absorption events in any phase gas, liquid or solid while avoiding effects from scattering phenomenon. This could make the technology viable for other ancillary markets. Additionally, there are several uses for the piezoelectric microphone itself that include passive acoustic detection for the military and physiological sensors for biological diagnostics.

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