Abstract
In this paper, we present a fully integrated Non-dispersive Infrared (NDIR) CO2 sensor implemented on a silicon chip. The sensor is based on an integrating cylinder with access waveguides. A mid-IR LED is used as the optical source, and two mid-IR photodiodes are used as detectors. The fully integrated sensor is formed by wafer bonding of two silicon substrates. The fabricated sensor was evaluated by performing a CO2 concentration measurement, showing a limit of detection of ∼750 ppm. The cross-sensitivity of the sensor to water vapor was studied both experimentally and numerically. No notable water interference was observed in the experimental characterizations. Numerical simulations showed that the transmission change induced by water vapor absorption is much smaller than the detection limit of the sensor. A qualitative analysis on the long term stability of the sensor revealed that the long term stability of the sensor is subject to the temperature fluctuations in the laboratory. The use of relatively cheap LED and photodiodes bare chips, together with the wafer-level fabrication process of the sensor provides the potential for a low cost, highly miniaturized NDIR CO2 sensor.
Highlights
CO2 gas sensing is of great importance in both academia and industry
In our previous work [21], we have demonstrated an on-chip Non-dispersive Infrared (NDIR) CO2 sensor based on an integrating cylinder
We present a fully integrated on-chip NDIR CO2 sensor based on such an integrating cylinder, with the optical source and detector integrated on-chip
Summary
CO2 gas sensing is of great importance in both academia and industry. A low-cost, miniaturized CO2 sensor is desired in many applications such as greenhouse farming, airquality monitoring, and industrial process control [1,2,3]. Electrochemical sensors measure the gas concentration by measuring a change in electrical properties such as resistance, capacitance, or electric potential induced by the adsorption of the gas [12,13] They are advantageous because of their easy fabrication, low cost and high sensitivity to a wide range of compounds. NDIR CO2 sensors rely on the very high absorption coefficient of CO2 in the mid-infrared wavelength range around 4.26 μm Due to this high absorption coefficient of CO2 , the requirement on the interaction length is relaxed and an optical path length of a few centimeters is sufficient to detect small changes in CO2 concentration. The relatively low detection limit, fast response time, and the small footprint of our previously demonstrated CO2 sensor give the potential for a low-cost, highly miniaturized and sensitive CO2 sensor. A qualitative analysis of the long-term stability of the sensor showed that the Sensors 2021, 21, 5347 response of the fully integrated sensor is temperature dependent, due to the temperature dependence of the LED/photodiodes characteristics
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