Vacuum Sensor Based On Micro-Hotplate And Its Monolithic Integrating Technology

Thermal conductivity vacuum sensors which are widely used in various vacuumapparatuses, if miniaturized, can be extended into the fields of leak hunting of microelectronicpackage, integrated vacuum gauge, portable instruments for gas pressure measurement, etc.Therefore, thermal conductivity vacuum microsensors have become one of the most importantresearch directions in the MEMS fields. In this thesis, thermal conductivity vacuum sensorsbased on micro-hotplate (MHP)are systematically investigated, including design theory,fabricating processes, characteristics measurement and integration with circuit in a single chip,etc.Two types of MHP-based thermal conductivity vacuum sensors with polysilicon orplatinum heaters respectively are designed. According to typical heat transfer theory andrarefied gas dynamics, a heat transfer model for the MHP-based sensor is constructed toanalyze the MHP thermal characteristics, and effects of heating current and the MHP size onthe sensor response. Temperature distribution and thermal deformation of the MHP aresimulated with finite element method. The results show that temperature distribution alongthe MHP supporting beams, heat losses through supporting beams or through gas thermalconduction from the MHP central zone vary with gas pressure. For the two designed sensors,temperature distributions of the MHP central zone are uniform, and the thermal deformationis small.Surface micromachining technology for the two designed sensors is investigated. Theappropriate fabricating processes are designed according to the practical condition. Manypivotal technics including sacrificial layer etching, window etching, step smoothing, andresidual stress control of dielectric thin films, etc are discussed and solved. The MHP-basedthermal conductivity vacuum sensors with polysilicon or platinum heater respectively use0.5μm thick polysilicon film or 0.5μm aluminum thin film as sacrificial layer, which are wetetched with TMAH etchant which doesn't etch exposed aluminum pads or aluminum etchantwhich doesn't etch Pt/Ti layer. The annealing temperature and time have great influence onthe MHP residual stress, and two different annealing processes are applied for the two typesof sensor so that the MHP residual deformation is controlled successfully and output capacityis guaranteed. Measurement technology of the MHP-based thermal conductivity vacuum sensor isstudied. The automatic test system consisting of vacuum system, test circuit, computeradopting and controlling parts is constructed to measure the sensor response characteristics atconstant current, constant voltage, constant temperature respectively. The measurementresults show that for constant current and constant voltage operating mode, the sensor isapparently sensitive in the pressure range of 1～5×10~4pa, while the sensitivity falls down athigher gas pressure due to too low working temperature, and the output voltage amplitude isonly a few hundreds of millivolts from 1Pa to 10~5Pa. For constant temperature mode, thesensor output voltage increases if elevating gas pressure, and higher sensitivity appears in thepressure range of 1～10~5Pa so that the output voltage amplitude is about a few volts.Studies about integrating MHP-based thermal conductivity vacuum sensor withintegrated circuits are also implemented. The fabricating processes of monolithic MHP-basedthermal conductivity vacuum sensor chip are designed according to Post-IC technology. Thedesigned monolithic sensor chip integrates MHP-based thermal conductivity vacuum sensorwith polysilicon heater, CMOS operational amplifier and resistors to make the sensor operateat constant voltage mode. The monolithic sensor chip is fabricated through MPW approach.The test results show that the monolithic sensor chip can measure gas pressure. This researchprovides foundation for deeply studying integrated MHP-based thermal conductivity vacuumsensor.