Design And Fabrication Of Polyimide Capacitive Humidity Sensors By CMOS Process

Humidity sensors are not only widely used in people's daily life, but also in all areas of the national economy such as industry, argriculture, business, meteorology, etc., and even in the areas of military and aerospace as well. Microelectronics manufacturing technology has improved with the development of science and technology so much in recent years that the performance of humidity sensors fabricated by CMOS compatible process has improved dramatically in cost, accuracy and other respects. Furthermore, the capacitive humidity sensors by CMOS process also has the merits of easy processing, simple technology and high yield which earn them wide attention and studies.The structure of a sensor has directly impact on its performance, the sandwiched-type capacitive humidity sensor is adopted in this design by taking principle of work and process of fabrication into consideration. After analyzing the structure, humidity-sensitive materials as well as the humidity-sensing mechanism on the performance of the humidity sensor, and performing numerical simulation of the structure of upper electrode of the sandwiched-typed capacitive humidity sensor, humidity sensors with upper electrode in grid or strip pattern were designed. Fabrication process of a polyimide capacitive humidity sensor by CMOS compatible process was demonstrated, the imidiation conditions of stable insoluble polyimide insulating thin film was given, the conditions of soluble polyimide was also studied, and a optimization of former process was also carried out. Test results showed that, the polyimide capacitive humidity sensors fabricated exhibited good performance in capacitance vs. relative humidity (%RH), humidity sensibility, hysteresis and response time for the sensors with upper electrode width of 2μm, which show fast response speed, high sensibility and small humidity hysteresis. With comparation to humidity sensor with strip upper electrode, the grid-type upper electrode humidity sensor has the advantage of reproducibility, high yield as well as humidity sensibility, while the strip-type upper electrode humidity sensor has the advantage of fast response time. The process of fabrication is simple and easy to control and is proning to mass production.The design requirements of the capacitive humidity sensor has been achieved.