Design and optimization of an integrated MEMS gas chamber with high transmissivity

Non-Dispersive InfraRed(NDIR)gas sensor is widely used for gas detection in collieries and the gas chemical industry,etc.The performance of the NDIR gas sensor depends on the volume,optical length and transmittance of the gas chamber.However,the existing gas sensor products have problems of large volume,high cost and incapable of integration,which need to develop towards the miniaturized sensor.This paper first presents the theoretical background of the NDIR gas sensor and the novel structure of a fully integrated infrared gas sensor and its micro-machined gas chamber structure.Then,the light structure and the gas flow of the gas chamber are optimized on Tracepro software and Ansys workbench,respectively,and the technological process for preparing the Micro-Electro-Mechanical System(MEMS)gas chamber is designed.Finally,we produce a gas chamber with a small volume and good transmissivity,which would be the most important part of producing the miniaturized NDIR gas sensor.

Transient thermal response of micro-thermal conductivity detector (μTCD) for the identification of gas mixtures:An ultra-fast and low power method

Micro-thermal conductivity detector(μTCD)gas sensors work by detecting changes in the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems.Conventional TCDs use steady-state resistance(i.e.,temperature)measurements of a micro-heater.In this work,we developed a new measurement method and hardware configuration based on the processing of the transient response of a low thermal mass TCD to an electric current step.The method was implemented for a 100-μm-long and 1-μm-thick micro-fabricated bridge that consisted of doped polysilicon conductive film passivated with a 200-nm silicon nitride layer.Transient resistance variations of theμTCD in response to a square current pulse were studied in multiple mixtures of dilute gases in nitrogen.Simulations and experimental results are presented and compared for the time resolved and steady-state regime of the sensor response.Thermal analysis and simulation show that the sensor response is exponential in the transient state,that the time constant of this exponential variation was a linear function of the thermal conductivity of the gas ambient,and that the sensor was able to quantify the mixture composition.The level of detection in nitrogen was estimated to be from 25 ppm for helium to 178 ppm for carbon dioxide.With this novel approach,the sensor requires approximately 3.6 nJ for a single measurement and needs only 300μs of sampling time.This is less than the energy and time required for steady-state DC measurements.

Application research on the sensitivity of porous silicon

Applications based on sensitive property of porous silicon （PSi） were researched. As a kind of porous material, the feasibility of PSi as a getter material was studied. Five groups of samples with different parameters were prepared. The gas-sensing property of PSi was studied by the test system and suitable parameters of PSi were also discussed. Meanwhile a novel structure of humidity sensor, using porous silicon as humidity-sensitive material, based on MEMS process has been successfully designed. The humidity-sensing properties were studied by a test system. Because of the polysilicon layer deposited upon the PSi layer, the humidity sensor can realize a quick dehumidification by itself. To extend service life and reduce the effect of the environment, a passivation layer （Si3N4） was also deposited on the surface of electrodes. The result indicated the novel humidity sensor presented high sensitivity （1.1 pF/RH%）, low hysteresis, low temperature coefficient （0.5%RH/℃） and high stability.