Effects of AlN film thickness and operating frequency on sensing output of CO₂ pyroelectric-based non-dispersive infrared gas sensor

CMOS compatible AlN-based pyroelectric detectors of different thicknesses (500nm and 1μm respectively) are examined by measuring their responses to different concentrations of CO₂ gas in an NDIR gas sensing system. We note up to ~55% improvement in signal when AlN thickness reduces from 1μm to 500nm. In addition, we design our AlN-based pyroelectric detectors similar to 2 capacitors connected in series and 4 capacitors connected in series while keeping the total pyroelectric sensing layer constant. CO₂ sensing responses of both designs with AlN thicknesses of 500nm and 1μm are measured in the NDIR gas sensing system and the 2-capacitor design with 500nm thick AlN in general gives a more sensitive response. The LOD for CO₂ sensing when using this detector is extracted based on Beer-Lambert law and we obtain an LOD of ~53ppm. As pyroelectric detectors are known to operate at lower frequencies (~tens of hertz), we also operate the detectors at different modulating frequencies (7Hz, 11Hz and 17.4Hz) and observe their effects on the gas sensing signal. Comparing the 4 different detectors in NDIR gas sensing, the AlN-based pyroelectric detector with 500nm thick AlN and of 2-capacitor design presents the best performance in CO₂ gas sensing. This work shows effects of AlN film thickness change and variation in operating frequency on pyroelectric-based NDIR CO₂ gas sensing. The results will provide more understanding on characteristics of AlN-based pyroelectric detectors and their behaviours in NDIR gas sensing.