Miniaturized Mixed‑Gas Detector Using a Mid‑IR Linear Variable Optical Filter and MEMS Thermopile Array

Abstract

This study details the design, fabrication, and testing of a compact mid‑infrared (MIR) linear variable optical filter (LVOF) and an array of MEMS‑based uncooled thermopile detectors for simultaneous measurement of CH₄, CO₂, and CO. The LVOF, a tapered‑cavity Fabry‑Pérot filter, converts the broadband MIR spectrum into a series of narrow band‑pass channels with linearly spaced peak wavelengths. Multi‑layer dielectric Bragg reflectors on both sides of the cavity and an integrated antireflection/out‑of‑band rejection layer enhance transmittance to 70 % with a mean full‑width at half maximum (FWHM) of 400 nm across 2.3–5.0 µm. The thermopile array, fabricated by stacking multiple p‑/n‑poly‑Si thermocouples, delivers a responsivity of 146 µV/°C at room temperature. Experiments demonstrate linear voltage responses to gas concentrations ranging from 50 to 3000 ppm, achieving sensitivities of –0.090 µV/ppm for CH₄, –0.096 µV/ppm for CO₂, and –0.123 µV/ppm for CO. The integrated system accurately identifies and quantifies each component in mixed‑gas samples, validating its potential for low‑cost, high‑throughput environmental and industrial monitoring.

Introduction

Real‑time monitoring of multiple gases is critical for safety, environmental protection, and industrial process control. Conventional chemoresistive sensors provide only qualitative data and require frequent calibration, whereas large‑scale Fourier‑transform infrared (FTIR) spectrometers are bulky and expensive. Miniaturized non‑dispersive infrared (NDIR) systems based on MEMS‑integrated optical filters offer a compelling alternative, combining high sensitivity with low power consumption. This work presents a fully integrated NDIR sensor that couples a custom MIR LVOF with a MEMS thermopile array, enabling simultaneous detection of CH₄, CO₂, and CO in a single, lightweight device.

Design and Experimental Methods

Design and Fabrication of the LVOF

The LVOF is a Fabry‑Pérot resonator featuring a tapered SiO₂ cavity sandwiched between two Si/SiO₂ Bragg reflectors. The cavity thickness varies linearly from 843 to 1908 nm, producing 12 discrete transmission peaks spanning 2.55–4.80 µm. A Ge/SiO multi‑layer antireflection coating on the backside of the Si substrate suppresses short‑wavelength transmission orders and enhances overall transmittance (average 95 % between 2.5–5.0 µm). The filter was fabricated using a gray‑scale lithography process to achieve the tapered profile, followed by dry etching and dielectric deposition steps. Figure 1 shows the fabricated LVOF and its package.

Design and Fabrication of the Thermopile Array

Each thermopile chip measures 1.1 × 1.1 mm with an active area of 0.35 × 0.35 mm. Multiple p‑/n‑poly‑Si thermocouple pairs are wired in series, forming a compact structure that generates a measurable Seebeck voltage without external power or chopper. The fabrication sequence includes thermal oxidation, poly‑Si deposition, ion implantation for p/n doping, metal contact formation, and backside silicon etching to form the suspended membrane. The resulting devices exhibit a responsivity of 146 µV/°C at 100 °C blackbody temperature under room‑temperature conditions.

Integrated Miniaturized Mixed‑Gas Detector

The detector assembly consists of an MIR LED source, a collimator, a gas cell, and the LVOF‑based spectrometer. The LED emits broadband light that is spatially filtered by the LVOF into 12 narrow channels. A linear array of thermopile chips positioned beneath the filter converts the optical power from each channel into electrical signals. The entire system operates over 2.3–5.0 µm with a linear wavelength‑to‑position relation of ~156 nm/mm. Data acquisition is performed by sequentially reading each thermopile output and applying calibration curves to recover individual gas concentrations.

Results and Discussion

Spectral measurements of the LVOF confirmed the designed linear progression of peak wavelengths and a mean FWHM of 400 nm with peak transmittance close to 70 %. The thermopile array showed low optical transmission (<1 %) across the MIR band, ensuring efficient thermal conversion. Calibration experiments using standard CH₄, CO₂, and CO gases produced linear voltage responses with R² values exceeding 0.96, confirming high measurement accuracy. Sensitivities were found to be –0.090 µV/ppm for CH₄, –0.096 µV/ppm for CO₂, and –0.123 µV/ppm for CO, enabling detection over a 50–3000 ppm range. Mixed‑gas tests (CH₄ 800 ppm, CO₂ 500 ppm, CO 800 ppm) validated the sensor’s ability to resolve each component simultaneously.

Conclusion

The developed MIR LVOF and MEMS thermopile array form a robust, low‑cost, and portable mixed‑gas sensor capable of accurately quantifying CH₄, CO₂, and CO. The design offers high transmittance, narrow bandwidth, and strong thermal responsivity, making it suitable for environmental monitoring, industrial safety, and process control applications.

Availability of Data and Materials

All data generated during this study are included in the published article.