Numerical Study of the Structural Parameter Effects on the Dynamic Characteristics of a Polyimide Film Micro-Capacitive Humidity Sensor

The parallel-plate capacitive humidity sensor with a polyimide (PI) film is a promising one in improving response time. Its grid upper electrode is for moisture into and out the PI film, whose permittivity changes linearly with the moisture absorbed. It is impractical to carry out a comprehensive experimental research on the response time of humidity sensors. As a result, several scholars have made some trial studies by numerical method. However, the results were still imperfect, and especially, the porosity effect of PI film on the response time was not considered. By introducing the relationship between the effective diffusivity of vapor and the film porosity, a new model was established. The model also consists of the capacitance formulation of parallel-plate capacitor, Fick's law, Looyenga's empirical equation, and the relationship between the fraction volume of water in the film and environmental humidity. One unit of the sensor was selected as the computational domain. The numerical model and method were validated based on the experimental results. Parameter effects on response time of the sensor were studied thoroughly. The results indicate that: the method is practicable to study the micro-humidity sensors even though the model cannot account for the chemical effect and agglomeration of water molecules; the effects of diffusivity and film porosity, bar width of the upper electrodes, and film thickness on the dynamic characteristics of the humidity sensor are remarkable; and the humidity range and the environment temperature have small effect on the response time.