SF6 gas sensor is developed to measure SF6 gas at different concentrations mixed with N2 based on mid-IR absorption of SF6 at a wavelength of ~10.6 μm. An optical bandpass filter of ~10.6 μm is put in front of a thermal emitter source to allow light of this wavelength to pass through. A CMOS compatible pyroelectric detector is put on the other end of the gas channel to measure the voltage change due to presence of SF6 gas. Here, we use AlN-based and 12% ScAlN-based pyroelectric detectors respectively. The results show for 100% SF6 gas sensing, 12% ScAlN-based pyroelectric detector gives ~73% higher response compared to when using AlN-based pyroelectric detector. The voltage drop between reference N2 gas and different SF6 gas concentrations is also higher (up to 2x) when using 12% ScAlN-based pyroelectric detector. Based on the measured SF6 gas responses, we try to estimate the lower limit of detection of our gas sensors when using AlN- and ScAlN- based pyroelectric detectors respectively. Response times taken for both detectors to detect SF6 concentrations are measured to be ~6.26 s for AlN-based pyroelectric detector and ~1.99 s for 12% ScAlNbased pyroelectric detector. Finally, both pyroelectric detectors’ electrical responses across different frequencies are measured and their 3-dB frequency cutoffs are extracted to be ~13.5 Hz and ~12.6 Hz for AlN- and 12% ScAlN- based pyroelectric detector respectively. The results provide more understanding on characteristics of pyroelectric detectors in SF6 greenhouse gas sensing based on mid-IR absorption.
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.
We demonstrate a system-level low-power contactless button using MEMS ScAlN-based pyroelectric detector. As pyroelectric detectors can sense instantaneous temperature change, the human finger can act as a thermal source to activate the button. Using our in-house fabricated ScAlN-based pyroelectric detector which does not require any IR source, we package it into a contactless button system designed with electrical read-out circuits and signal processing. This contactless button system could detect the presence of a finger at a center distance measured up to ~4 cm away, ~2 cm radius circle area, suitable for application as contactless elevator button. Our contactless button system using ScAlN-based pyroelectric effect is characterized, tested and compared with a commercial contactless button. The power consumed is measured ~3.5× lower than that of commercial contactless button. The results obtained provide a potential solution towards energy efficient low-power contactless button system.
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.
A thermal emitter fabricated on complementary metal-oxide-semiconductor (CMOS)-compatible facilities is a key component for low-cost mid-infrared gas sensing. While conventional thermal emitters have broad spectrum and wide emission angle, which limit the sensing performance. In this work, a microelectromechanical system (MEMS)-based thermal emitter with photonic crystal has been designed and fabricated using CMOS-compatible technology. The photonic crystal enables the emission wavelength selectivity within mid-infrared regime. By engineering photonic crystal dimension, the emission enhancement wavelength can be matched to the fingerprint wavelength of chemical gas for efficient chemical gas sensing purpose.
We present the optical and electrical properties of AlN-based and 12% doped ScAlN-based pyroelectric detectors fabricated on 8-inch wafers respectively. Both AlN and ScAlN materials are deposited at a temperature of ~200oC, making them potential candidates for CMOS compatible MEMS pyroelectric detectors. FTIR spectroscopy is used to measure the absorption of these pyroelectric detectors over the wavelength range of ~2–14 μm and the results show absorption improvement up to ~75% for ScAlN-based pyroelectric detectors compared to that of AlN-based pyroelectric detectors at the wavelength of 4.26 μm where CO2 gas absorption of IR radiation is anticipated. Higher output current (~3-fold increase) is also observed from ScAlN-based pyroelectric detectors. Other than pyroelectric coefficient that contributes to improved performance for ScAlN-based pyroelectric detectors, we believe that absorptivity of the device also plays a major role in the performance of pyroelectric IR detectors. The results obtained from the study of the electrical and optical properties of AlN-based and ScAlN-based CMOS compatible MEMS pyroelectric detectors will bring forth potential applications of these detectors onto multi-functional integrable and monolithic platforms.
Gas sensors have wide applications including industrial process control, environment monitoring, safety control, etc. The distribution of these sensors enables data generation for the emerging trend of big data and internet of things. In this work, chip-based non-dispersive infrared (NDIR) gas sensors are demonstrated. Silicon substrate-integrated hollow waveguide (Si-iHWG), which is formed through silicon wafer etching and bonding, is used as optical channel and gas cell. A high sensitivity of 50 ppm for CO2 sensing is demonstrated. The Si-iHWG chip-based sensor with compactness, low cost, versatility, and robustness provides a promising platform for miniaturized gas sensing in various application scenarios.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.