We develop H2 gas sensors based on CMOS compatible 20% ScAlN-based pyroelectric detectors fabricated in-house. Leveraging on the high thermal conductivity of H2, ScAlN-based pyroelectric detector is used in the H2 sensor for H2 to conduct away thermal energy received by the detector, resulting in a drop in signal received by the detector, thereby leading to different voltage signals measured for different H2 gas concentrations. The higher the H2 gas concentration, the lower the voltage measured as more thermal energy is conducted away from the detector. We successfully demonstrate H2 gas sensing with the signal received by the pyroelectric detector at concentration ranging from 400 ppm to 1% H2 concentration. The gases are cycled at 2-minute intervals between different concentrations of H2, using N2 as the reference gas. Our measurements show H2 sensing down to 400 ppm gas concentration with response time ranging from ~3-7 s. In addition, a linear relationship is also observed between the measured output signal from the H2 gas sensor and the H2 gas concentration flowing across the pyroelectric detector. The results show promise in using CMOS compatible 20% ScAlN-based pyroelectric detectors for development of thermal conductivity H2 gas sensor in H2 leakage sensing to increase confidence towards adoption of H2 as a clean energy as we move towards a sustainable society.
Microelectromechanical system (MEMS)-based thermal emitter is a key component in an optical sensor to provide broadband emission at mid-infrared wavelengths, where a lot of molecules have their unique absorption profile. However, the thermal emission from a MEMS emitter is typically fixed at a specific spatial coordinate. In this work, a MEMS thermal emitter with piezoelectric actuation to realize active tuning is demonstrated. Thermal emission comes from a doped silicon layer acting as a resistive heater. Piezoelectric actuation is enabled by an aluminum nitride layer on a designed cantilever. The devices are fabricated on a complementary metal-oxide semiconductor (CMOS)-compatible process line. The fabricated thermal emitter at the tip of the cantilever generates broadband MIR thermal emission with spectrum peaked around 10 μm wavelength, and piezoelectric actuation with a displacement of more than 20 μm. The work paves the way towards self-adaptable MEMS directional emitter for various applications including chemical/gas sensing.
A demonstration of an on-chip CO2 gas sensor is reported. It is constructed by the integration of a MEMS-based thermal emitter, a scandium-doped aluminum nitride (ScAlN) based pyroelectric detector, and a sensing channel built on Si substrate. The integrated sensor has a small footprint of 13mm × 3mm (L×W), achieved by the replacement of bulky bench-top mid-IR source and detectors with MEMS-based thermal emitter and ScAlN-based pyroelectric detector, with their footprints occupying 3.15 mm × 3 mm and 3.45 mm × 3 mm, respectively. In addition, the performance of the integrated sensor in detecting CO2 of various concentrations in N2 ambient is also studied. The results indicate that the pyroelectric detector responds linearly to the CO2 concentration. The integration of MEMS emitter, thermal pathway substrate, and pyroelectric detector, realized through CMOS compatible process, shows the potential for massdeployment of gas sensors in environmental sensing networks.
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