Miniaturized CO₂ Gas Sensor Using 20% ScAlN-Based Pyroelectric Detector

Abstract

NDIR CO 2 gas sensors using a 10-cm-long gas channel and CMOS-compatible 12% doped ScAlN pyroelectric detector have previously demonstrated detection limits down to 25 ppm and fast response time of ∼2 s. Here, we increase the doping concentration of Sc to 20% in our ScAlN-based pyroelectric detector and miniaturize the gas channel by ∼65× volume with length reduction from 10 to 4 cm and diameter reduction from 5 to 1 mm. The CMOS-compatible 20% ScAlN-based pyroelectric detectors are fabricated over 8-in. wafers, allowing cost reduction leveraging on semiconductor manufacturing. Cross-sectional TEM images show the presence of abnormally oriented grains in the 20% ScAlN sensing layer in the pyroelectric detector stack. Optically, the absorption spectrum of the pyroelectric detector stack across the mid-infrared wavelength region shows ∼50% absorption at the CO 2 absorption wavelength of 4.26 μm. The pyroelectric coefficient of these 20% ScAlN with abnormally oriented grains shows, in general, a higher value compared to that for 12% ScAlN. While keeping the temperature variation constant at 2 °C, we note that the pyroelectric coefficient seems to increase with background temperature. CO 2 gas responses are measured for 20% ScAlN-based pyroelectric detectors in both 10-cm-long and 4-cm-long gas channels, respectively. The results show that for the miniaturized CO 2 gas sensor, we are able to measure the gas response from 5000 ppm down to 100 ppm of CO 2 gas concentration with CO 2 gas response time of ∼5 s, sufficient for practical applications as the average outdoor CO 2 level is ∼400 ppm. The selectivity of this miniaturized CO 2 gas sensor is also tested by mixing CO 2 with nitrogen and 49% sulfur hexafluoride, respectively. The results show high selectivity to CO 2 with nitrogen and 49% sulfur hexafluoride each causing a minimum ∼0.39% and ∼0.36% signal voltage change, respectively. These results bring promise to compact and miniature low cost CO 2 gas sensors based on pyroelectric detectors, which could possibly be integrated with consumer electronics for real-time air quality monitoring.

CMOS AlN and ScAlN Pyroelectric Detectors with Optical Enhancement for Detection of CO2 and CH4 Gases

“…The pyroelectric coefficients obtained from our in‐house AlN and 12%‐doped ScAlN are comparable with those reported in literature. [ 15,39,42,48,56,57 ] It is also worthwhile to note that the pyroelectric coefficients calculated for AlN and 12% ScAlN are in general independent of background temperatures, agreeing with what Kurz et al. [ 48 ] have reported.…”

“…The pyroelectric coefficients obtained from our in-house AlN and 12%-doped ScAlN are comparable with those reported in literature. [15,39,42,48,56,57] It is also worthwhile to note that the pyroelectric coefficients calculated for AlN and 12% ScAlN are in general independent of background temperatures, agreeing with what Kurz et al [48] have reported. This is because the AlN and 12%-doped ScAlN films deposited in this work are columnar and do not contain abnormally oriented grains (AOGs) that usually resulted in bigger grains in the film.…”

“…Rather, span and 𝜅 change when the gas tested changes [60] or when the gas channel change. [15] The CO 2 gas concentration is increased further to test CO 2 gas sensing at percentage concentrations. CO 2 gases from 10% to ≈100% (99.995%) are input into the gas channel.…”

Portable TDLAS Sensor for Online Monitoring of CO2 and H2O Using a Miniaturized Multi-Pass Cell

“…Optics-based methods have several advantages and are widely used in trace gas monitoring. Non-dispersive infrared (NDIR) has been increasingly applied to atmospheric CO 2 and H 2 O monitoring [ 5 , 6 , 7 ]. However, the application of NDIR is limited because it uses a broadband light source and therefore has poor selectivity.…”

Low-power contactless button system based on MEMS ScAlN pyroelectric detector

“…They are compact and can function at room temperature without the need for liquid nitrogen (N2) cooling, unlike photon detectors which usually require cooling. 4 Their thermal sensing capability is widely used for a number of applications including gas sensing, [5][6][7] temperature sensing, [8][9][10] ultraviolet/ infrared (UV/ IR) light detection, [11][12][13][14][15] thermal imaging [16][17] etc. When considering using pyroelectric detectors in contactless button system, they could potentially replace the traditional way contactless button works.…”