Design and fabrication of poly-Si/SiO₂ Fabry–Perot filter for nondispersive IR CO₂ sensors

Abstract

The relationship between the transmittance and FWHM of a Fabry-Perot filter for a nondispersive carbon dioxide (CO2) sensor was investigated as a function of the number of distributed Bragg reflector (DBR) pairs consisting poly-Si and SiO¬2 thin films. Given the significant prior research on the fabrication of high-performance Fabry-Perot filters, this study is focused on the relationship between the transmittance and FWHM that can be achieved by controlling the reflectance of the DBR pairs. Each layer of the filter was simulated adequately as the poly-Si and SiO2-based DBR pairs, and poly-Si and SiO2 were deposited on the soda–lime substrate by radio frequency sputtering and low-pressure chemical vapor deposition based on the simulation results. The fabricated filter showed a transmittance of 43.7% and FWHM of 125 nm at 4.26 µm. The NDIR CO2 sensor with Fabry-Perot filter showed enhanced selectivity to CH4 and CO compared with normalized CO2 response.