Study On A Miniature MEMS-based NDIR Spectrometer And Its Application

Infrared spectrometer is an instrument which makes use of the absorption characteristics of different wavelengths of infrared radiation to analyze the molecular structure and chemical composition.According to the different spectroscopic devices,it can be divided into dispersion type and interference type.At present,large and expensive desktop interferometric Fourier Transform infrared(FTIR)spectrometers have been widely used in the laboratory.Miniaturization and low-cost are the main trends in the development of infrared spectrometer.Faster,more reliable and cheaper portable infrared spectrometers are likely to be used in family market in the future to meet the urgent needs of people for real-time detection for living environment gases,food and beverage ingredients and pesticide residues.In this paper,the fabrication techniques of miniaturized Non-Dispersive infrared(NDIR)spectrometer were studied based on Micro-Electro-Mechanical System(MEMS)technology.MEMS infrared light source,MEMS infrared thermopile detector and linear variable optical filter(Linear Variable Optical Filter,LVOF)were designed and prepared.Based on the above MEMS devices,we have assembled a small infrared spectrometer to realize the verification of CO/CO2/CH4 mixed gas detection.The main contents of the paper include the following parts:(1)Silicon-based MEMS infrared light source was designed and fabricated.In the aspect of design,the preparation processes based on SOI wafer were proposed.The electro-thermal-optical-mechanical properties of microstructure were studied by designing two kinds of light-emitting thin film structures(bridge and closed suspended film structure).Through the finite element simulation and experimental comparison using COMSOLTM multi-physical field,it is determined that the bridge structure design is the best design for optimizing the performance of light source.In the aspect of preparation,the back absorption and self-heating effect of radiation light were realized by heavy doping-boron in Si layer.The low-resistance polycrystalline silicon film was prepared by LPCVD,ion implantation and wet etching technology.The suspension structure was realized using the DRIE deep ion reaction etching technique.The bridge structure has a higher heat generation temperature,a larger heat generation area,and can withstand greater stress than the conventional suspension film structure at the same driving voltage.The surface temperature of the light source can reach 600?(at 12V working voltage),and the modulation frequency can reach 24 Hz(at 70%modulation depth).(2)MEMS infrared thermopile detector was designed and fabricated.In the aspect of design,SiO2/Si3N4 composite thin film layer was used as support layer and infrared absorption layer material to realize thermal stress compensation and reduce thermal stress of thin film.The thermocouples were made of two kinds of polysilicon thermocouples with different doping of n-type and p-type polysilicon,and the thermocouples were connected by aluminum.Several thermocouples of different lengths were stacked on rectangular thin films in series to improve the output of thermal potential.In the aspect of fabrication,the support and thermal insulation thin films were grown using MEMS processes uch as LPCVD,PECVD on the substrate.The doping of polysilicon was realized by ion implantation.The back cavity structure was released using dry etching process for achieving the infrared suspended thin film.The responsivity is 146 ?V·?-1 at room temperature.(3)Design and preparation of the mid-infrared linear variable optical filter(LVOF).Based on the tapered Fabry-Perot(F-P)cavity,a full-media F-P type filter was designed which can transform the mid-infrared continuous spectrum into an infinite number of narrow pass-band filters with discrete peak wavelength with linear changes.The Bragg reflectors on both sides of the tapered cavity and the anti-reflection film combined with the out-of-band suppression function were designed using high(Si and Ge),low(SiO and SiO2)refractive index dielectric layers.The optical transmitance sprctrum of the LVOF was simulated by the multi-layer medium film transmission matrix theory and the MATLAB software.In the device preparation,the tapered cavity of SiO2 was prepared by the technique of gradient gray-scale exposure,thermal reflux and dryetching,and the multi-layer dielectric film was prepared by the process of thermal evaporation.After the test,the working wavelength of the LVOF was 2.3?5 ?m,the half-height width of the narrow band-pass filter channel was not more than 400 nm,and the transmittance was not lower than 70%.Multi-beam optical filtering with one piece of filter is achieved compared to other single-wavelength splitting filters.(4)The MEMS infrared light source,LVOF and MEMS thermopile detector were integrated into a miniaturized MEMS non-dispersive infrared spectrometer to identify and quantify the components of CO/CO2/CH4 mixed gases.The sensitivity of infrared spectrometer was-0.090 ?V/ppm for CH4,-0.096 ?V/ppm for CO2 and-0.123 ?V/ppm for CO,respectively.After analyzing the characteristics of miniaturization and portability of infrared spectrometers,the above-mentioned devices are integrated into a multi-channel composite small MEMS non-dispersive infrared spectrometer.The compact structure and mature manufacturing process make the designed spectrometer portable.High performance,moderate cost,and detection accuracy can be adapted to different applications.Using simulation and simulation techniques,the influence of the film design of the linear gradient filter on the spectral performance of the infrared light in the mid-range is deeply analyzed,and the gradient transmittance and the band resistance of the reflector are determined.It provides the basis for future application of LVOF.