Design And Preparation Process Of Miniaturized Infrared Carbon Dioxide Gas Sensor

Carbon dioxide infrared gas sensor is used broadly in the fields of industrial and agricultural production,medical and health,environmental protection,aerospace,and other fields.With the rapid development of micro electro-mechanical system（MEMS）technology,Internet of things,wearable devices,and intelligent mobile terminals,a new requirement of miniaturization,intelligence,and low power consumption is proposed for carbon dioxide infrared gas sensors.Therefore,a new infrared gas sensor is designed in this dissertation,which has great significance for great-leap forward development of gas sensors.The research of MEMS infrared light,Si-based integrated gas chamber,and signal processing circuit are systematically carried out for infrared gas sensors.The miniaturized infrared CO₂gas sensor is developed through three-dimensional integration technology and successfully applied to subway stations to realize real-time online detection of CO₂in Suzhou and Shanghai.The high stability and small volume of the infrared light source is required according to the characteristic absorption band of CO₂.And three different light structures are designed in this dissertation,including single bridge-type light,bridge-type light arrays,and single plate-type light.It demonstrates that the single bridge-type light has the advantages of small heating areas,high radiation intensity,and low power by the finite element simulation analysis.The light chip is a micro heat plate structure with suspending four-bridge membrane with a tungsten heater,while with Si O₂and Si₃N₄ dielectric thin film as the mechanical support.The dimension of the micro hot plate is 2×2×0.3 mm³,with a heating area of 300μm²,and a suspended area of 600μm².The width of the supporting cantilever beam is 100μm.The chip is fabricated by a4-inch silicon-based MEMS process.The results show that this chip can quickly rise to407℃in only 24 ms,the power consumption is 46 m W,and the working Voltage is 2.85V.The chip has the advantages of fast thermal response,low power,and high integration,which can provide an important technical foundation for CO₂ gas sensors.Due to the disadvantage of large volume and difficult integration for traditional infrared gas sensor gas chamber,a micro-chamber of double stacked silicon wafer is proposed with single hole for light input and double holes for light output based on anisotropic etching of monocrystalline silicon.And a silicon-based integrated gas chamber of box-type inner cavity is studied with high light transmission efficiency and small Volume by Trace Pro and COMSOL.The optical distance of this gas chamber is21mm with the light transmittance of 17.6%.The dimension of light inlet and outlet holes is 1.0×1.0 mm².The integrated gas chamber has a central symmetric structure with five air holes and the diffusion time of air equilibrium is only 5s in gas chamber.Then,the fabrication process of the integrated gas chamber is carried out,including deposited silicon dioxide,mask making,lithography,corrosion,coating,and dicing.A MEMS integrated gas chamber is successfully produced,with small volume（9.5×9.5×0.74 mm³）,long optical path,and high light transmission efficiency.Focusing on the problem of high-precision and miniaturized signal extraction and processing of weak signals in infrared gas sensors,the method for converting the pyroelectric charge signal in the infrared sensitive unit into current signal is studied by means of impedance transformation and charge integration,and the implementation method,which effectively improved the anti-noise ability of current signal.Moreover,the temperature compensation function of the processing circuit based on modulation amplification and analog-to-digital conversion technology is proposed.It can effectively extract the output signal of pyroelectric sensitive unit.Therefore,the integrated circuit layout of signal processing application specific integrated circuit（ASIC）chip is designed and fabricated.The signal processing circuit ASIC chip is developed with 14A/D sampling bits,a sensitivity of 6.15μV,and a sampling rate of 39.9 k Hz.Finally,MEMS infrared light source,integrated gas chamber and signal processing circuit ASIC chip are integrated on a low temperature co-fired ceramic（LTCC）substrate is proposed to prepare the miniaturized infrared gas sensor based on the technologies of high-density LTCC substrate interconnection,wafer bonding integration,and wafer level vacuum packaging.The test results show that the measurement range of the miniaturized infrared gas sensor is 0～60%Vol,the measurement accuracy is 0.09%,the repeatability error is 1.97%,and the volume is 10×10×6 mm³.The infrared gas sensor successfully applied to subway stations in Suzhou and Shanghai.