| The increasing level of human industrial production has also caused the release of a large amount of toxic volatile organic compound（VOC）gas,which has become a direct threat to human beings.Therefore,the development of high-sensitivity VOC sensors is important for environmental monitoring.The use of integrated circuit CMOS process to prepare optical waveguide devices for VOC detection has the advantages of low cost,large scale fabrication and high sensitivity.This thesis investigates the key scientific problem of light-analyte interaction for fast and sensitive detection of VOCs,and designs and fabricates different structures and sizes of silicon nitride waveguide gas sensors based on CMOS process.The innovative work of this thesis includes the following two points:First,a systematic theoretical and experimental optimization of the light-analyte interaction for waveguide evanescent field sensing was carried out to achieve ultra-sensitive detection of volatile toxic pyridine vapor down to sub ppm(part per billion,10-6concentration)detection limit.Theoretically,the Mach-Zehnder interferometer（MZI）waveguide with high refractive index sensing sensitivity was designed by varying the waveguide size to regulate the group refractive index,effective refractive index,and evanescent field strength;experimentally,the MZI waveguide with silicon nitride was prepared by CMOS processes such as photolithography and etching,and the dipole polycarbonate functional layer was used as the waveguide sensitive cladding.The MZI sensors with two different waveguide widths and different sensitive cladding thicknesses were fabricated and encapsulated for VOC detection by CMOS process such as photolithography.The comparison of the final pyridine concentration detection shows that the nanometer-thick sensitive cladding helps the pyridine molecules penetrate quickly to the sensing interface with the strongest evanescent field,and the pyridine sensing sensitivity of the device was improved from 20 pm/ppm to 63 pm/ppm,the detection limit was as low as 0.476 ppm,and the response time was shortened from45 min to 10 min.Second,for the above sensors in response speed and material stability issues,iterative innovation is carried out to further optimize the sensor performance.Firstly,a cross-linked material with solvent corrosion resistance was adopted as the sensitive layer to solve the disadvantage of poor environmental stability of functional polymer materials,and the cross-linking conditions and optical loss of the material were studied in depth;secondly,in order to further enhance the strong interaction between light and the evanescent field of pyridine gas,the waveguide sensor design was optimized,and a200 nm thick silicon nitride with higher evanescent field strength and a thickness of only about 100 nm was used to prepare the MZI sensor.The MZI sensor was prepared using a 200 nm thick silicon nitride with a higher extinction field strength and a cross-linked material with a thickness of only 100 nm,and achieved a sensitivity of 50pm/ppm for pyridine vapor sensing and a much faster response time of 1 min.At the same time,a systematic analysis was carried out and solutions were proposed for the problem of low interference extinction ratio of MZI sensors.In summary,this thesis thoroughly investigated the scientific issues of light-analyte interaction from the theoretical and experimental aspects,and optimized the silicon nitride MZI pyridine sensor in a systematic way,which has significantly improved from sensitivity to response speed and device stability.The developed pyridine gas sensor has the significant advantages of high sensitivity,fast response speed,small size,simple structure and convenient for large-scale low-cost fabrication,and can be used for portable or real-time detection of pyridine vapor leakage. |
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