Miniaturized two-chamber photoacoustic CO₂ sensor with a wafer-bonded MEMS (micro-electro-mechanical systems) detector

<p>The measurement of air quality, particularly the concentration of carbon dioxide (CO<span class="inline-formula">₂</span>), has gained significant interest due to increased public health awareness. Elevated concentrations of CO<span class="inline-formula">₂</span> in the air have been linked to negative effects on cognitive functions and directly correlate with the quality of indoor air. This work presents the prototype of a miniaturized sensor module designed for sensing CO<span class="inline-formula">₂</span> concentrations in the air. The sensor module utilizes the two-chamber photoacoustic concept and features a compact design combining an absorption cell with a wafer-bonded detector based on silicon MEMS (micro-electro-mechanical systems) microphone technology. The CO<span class="inline-formula">₂</span>-filled detector chip uses the photoacoustic effect and is able to detect pressure oscillations within an enclosed volume generated by the absorption of modulated infrared (IR) light by CO<span class="inline-formula">₂</span> molecules. The complete sensor module measures only <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">9</mn><mo>×</mo><mn mathvariant="normal">13</mn><mo>×</mo><mn mathvariant="normal">7.8</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="4ef0f2c465de24f9712df844104d42f9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jsss-13-219-2024-ie00001.svg" width="58pt" height="10pt" src="jsss-13-219-2024-ie00001.png"/></svg:svg></span></span> mm<span class="inline-formula">³</span> and the used detector chip is the smallest detector for two-chamber photoacoustic sensors reported to date. Experimental characterization showed that the prototype achieves a detection limit of 81 ppm CO<span class="inline-formula">₂</span> and exhibits a response time <span class="inline-formula"><i>τ</i>₆₃</span> of 53 s. The compact size and performance characteristics make the proposed sensor module suitable for applications in indoor air quality monitoring.</p>