A Photosensitive Ployimide Suspended Membrane For Micro-machined Pirani Vacuum Gauge

Pirani vacuum gauge is known as a pressure sensor.The working principle is to use the pressure to change the thermal effect around the resistor,and then reflects the pressure around the heating resistor by the electrical signal.The development of micro-machined Pirani vacuum gauge technology is ameliozated by the rapid development of MEMS technology.In this technology,the key point is the suspended plate which can be fabricated by some thermal isolated film.Generally,the suspension plate of the Micro-machined Pirani vacuum gauge is made of SiO₂,Si_xN_y,or a multi-layer film.However,the specialized deposition equipment is required.Furthermore,it is difficult to eliminate the influence of the residual stress of the film.Thus,the cost is high and the processing is complex.As a material,photosensitive polyimide?PSPI?presents as an excellent candidate of suspended membrane and is widely used in MEMS sensor production.It shows a number of advantages in micro suspended plate which can play a role as a mechanical support layer such as stable chemical and physical properties,excellent thermal stability,and potential low cost.As a MEMS sensor,the structure design of the micro-machined Pirani vacuum gauge consist of four layers in this thesis,including silicon substrate,aluminum film,suspended membrane heating resistor.In order to improve the performance of the Pirani vacuum gauge,we innovatively used the photosensitive polyimide to produce the suspended membrane for the Micro-machined Pirani vacuum gauge.According to its structural characteristics,we develop the details of fabrication on process.The process consists of 27steps,including the spin coating,lithography,metal deposition,ICP etching.We emphasized here the fabrication process of PSPI spin coating and ICP.These two technologies improve significantly the throughput of Pirani vacuum gauge.Finally,a Micro-machined Pirani vacuum gauge based on PSPI suspended membrane was successfully fabricated.In this thesis,the circuit used to test the relationship between the vacuum gauge resistance and the pressure is the"four-line method",which could effectively deduct the line resistance.We finally measured the relationship between the resistance R and the pressure P for this Pirani vacuum gauge using the vacuum system to provide the pressure environment around the heating resistor.The results show an excellent resolution of 1 Pa in the range of 1 to 10 Pa,10 Pa in the range of 10 to 100 Pa,100 Pa in the range of 100 to500 Pa,resepectively.In this experiment,the production of Pirani vacuum gauge with the photosensitive polyimide as a suspended plate was successfully fabricated.Photosensitive polyimide impressed us by its shaping,excellent thermal stability,and strong mechanical properties.It can be extend to all similar micro-mechanical device structures in which the suspension plate is needed.