Investigation Of MEMS-based Polyimide Thin Film Humidity Sensors

Humidity plays an important role in the areas of daily life. In recent years, polymer capacitive humidity sensor attracts more and more attentions due to its wide working range, fast response time, high precision, low temperature coefficient, and long-term stability. In this work, several kinds of the capacitive humidity sensors based on polyimide (PI) thin films was designed and fabricated. The main work is as following:First, the capacitive humidity sensors based on the metal-PI-metal sandwich structure were studied. Firstly, the sandwich structure with the lower electrode of1μ m, the upper electrode of0.4μ m, and the PI film of0.35μ m, was fabricated. The total yield ratio of sensor process is more than80%. In the range of15%RH to95%RH, the maximum linearity error was1.5%RH, the sensitivity0.29PF/%RH, the response time of4s. Meanwhile, capacitor showed good temperature coefficient and long-term stability. The results demonstrated that the capacitor was suitable for batch fabrication and can be used for high altitude meteorological observation.Secondly, the humidity sensor with integrated resistor as heater was investigated. The width of resistor was about0.5mm, the width of space was about1mm. The chip temperature is almost linear with the applied voltage. It took about5minutes to achieve the dynamic thermal equilibrium. A new ground-calibration method was presented which can deduce the real environmental humidity values from the heating sensor data. And it had an error of less than3%RH.Thirdly, the PI film humidity sensor integrating with PTCR as heating element to improve the defrosting problem was investigated. With PTCR as heating element, the temperature of humidity sensor increased as the applied voltage increased, and the maximum fluctuation was within+/-2℃. If the voltage was+24V, the temperature of the sensor reached more than40℃. During the heating and cooling period, the capacitance fluctuation of humidity sensor was only0.5PF, which is smaller than the capacitance due to the humidity variation. The defrosting problem of humidity sensor was solved. It demonstrated that PTCR is a prospective material for low temperature sensor field.Fourthly, the humidity sensor with IDE structure were studied. The theoretical model was proposed and Grapher software was used to simulate the electrical field behavior. It was found that the preferred thickness range of the moisture-sensitive PI layer was10-20μm. MEMS process was used to fabricate IDE capacitors, in which aluminum was used as electrode material. The width of finger and the spacing of the fingers were10μm, respectively. From15%RH to95%RH, the maximum linearity error was less than2.2%RH. The sensitivity reached about0.35PF/%RH. The largest hysteresis error was about6%. Moreover, the IDE structure showed fast response time compared to the sandwich one. Both the absorption and the desorption time was less than1.5s.In this work, we investigated the humidity sensor together with Shanghai Changwang Meteotech Co.,Ltd. The humidity sensor was expected to have good performance for the industrialization and suitable for high-altitude meteorological detecting. We focused on the compony’s need, prepared and tested several kinds of humidy sensors, the testing results showed that our products have good performance and meet specific needs.