Design And Fabrication Of Silicon Based Micro Thermal Conductivity Detectors

With the rapid development of petroleum,chemical industry,natural gas and other fields,the requirements for on-line detection of gas are more and more strict.Traditional thermal conductivity detectors are difficult to meet the detection requirements.Recently,with the mature of MEMS（Micro-Electronic Mechanical System）technology,thermal conductivity detectors with the millimeter scale have been fabricated on silicon wafes by using photolithography,etching,film deposition,bonding and other micro processing technologies.The as-fabricated micro thermal conductivity detectors（μTCDs）not only exhibit the characteristics of good generality as the traditional thermal conductivity detectors,but also lead to significant reduction of the device’s volume and analysis time.And consequently,theseμTCDs exhibit the advantages of good portability and high sensitivity.However,their manufacturing processes are complex and expensive,thus limiting the development of micro thermal conductivity detectors.So,in this thesis,we pay attention to the fabrication of micro thermal conductivity detectors.The main results are shown as follows:（1）The processes for the fabrication of thermistor were investigated.Results reveal that combination of electron beam evaporation and double layer negative adhesive stripping process can lead to the yield of complete metal graphics.Moreover,TCR of Pt thermistor increases after having been treated at a high temperature of 350^oC.But the resistances of Pt will decrease.Compared with that,the resistances of Ni decrease more significantly.（2）The factors influencing wet etching and dry etching were investigated.Results show that the optimal conditions for wet etch of silicon include that KOH concentration is about 30wt%,and the temperature is 80-85 ^oC;The same etching gas in dry etching will etch Si、Si O₂ and Si N_x at different rates.Consequently,thermal conductivity cells and Si N_x micro-bridges have been fabricated by wet etching and dry etching,respectively.The characterization results show that the surfaces of the thermal conductivity cells fabricated by wet etching are smoother.Moreover,the patterns of the micro-bridges fabricated by wet etching are related to the crystal orientations and mask windows,Si N_x-based micro-bridges are easy to be excessively corroded by KOH solutions,thus weakening the mechanical strength.Compared with that,the patterns of the micro-bridges fabricated by dry etching are completely consistent with those of the masks,and the perpendicularity of the side wall reaches 89±1。.However,silicon generally remains on the back of the Si N_x-based micro-bridges,thus affecting the performance of the micro-bridges.（3）Based on the results of the structure design and manufacturing process for the micro thermal conductivity detectors,and the simulation optimization,the as-designed micro-thermal conductivity detectors have been successfully fabricated.The performance of the fabricated micro-thermal conductivity detectors were tested and analyzed.Results show that the as-fabricated detectors can be applied in detecting503 ppm of n-pentane with the gas analysis test time of about 2 minutes,meeting the requirements of on-line test and analysis of natural gases.In the thesis,we have explored the fabrication method of micro thermal conductivity detectors,promoting the basis study for the future.