Research On The Rapid Response Structure Of MEMS Humidity Sensor

Humidity sensors are widely applied in many fields such as meteorology, agriculture, industry, medicine and so on. The most popular humidity sensors are capacitive humidity sensors, resistive humidity sensors, piezoresistive humidity sensors and so on. Because of low power-consumptions and fabrication costs, capacitive humidity sensors have been the most available one. Researchers always endeavor to improve the response properties of humidity sensors. This research presents the design and fabrication of a rapid response sandwich capacitive humidity sensor and a flip-chip capacitive humidity sensor. The main contents of this research are:(1) A rapid response sandwich capacitive humidity sensor is presented. In this study, the fabrication process of a porous electrode was optimized. Because of the porous structure of the electrode, water molecules can be captured by the humidity sensing layer more easily. Polyimide was chosen as the sensing material due to its compatibility with semiconductor processes and good humidity sensing properties. A dielectric layer between the bottom electrode and sensing material was employed to ensure the reliability of the sensor. The layouts of the sensor were designed with L-edit software.(2) The fabricated process flow of the rapid response sandwich capacitive humidity sensor was developed in this research. Porous electrode was fabricated by a glancing angle deposition (GLAD) process with a special wafer clamp and an evaporation-equipment. The porous electrode was patterned by a lift-off process. Two sandwich humidity sensors with porous electrode layers deposited by different glancing angles were fabricated in this study. Results of measurement show good linearity, sensitivity and hysteresis of the rapid response sandwich capacitive humidity sensor. The respond time is about 1 second.(3) A capacitive humidity sensor with a flip-chip fabrication process is presented. With the flip-chip technology, the top and bottom electrode are connected by several bumps. There is a narrow gap between the polyimide layer and the bottom electrode, so that water molecules can be more available to the polyimide, so the sensor can achieve a good response property. The layouts of the chip were designed with L-edit software and the PCB substrates were designed with Altium designer software.(4) The fabricated process flow of the flip-chip capacitive humidity sensor was developed in this research. Gold ball bumps was fabricated on the PCB bonding block with gold wire ball bonding process. The top electrode and the bottom electrode of sensor were bonded with a flip-chip bond process. Measurement results show good response properties of the flip-chip capacitive humidity sensor, the response time is about 3 seconds.This paper presents the design and fabrication of a rapid response sandwich capacitive humidity sensor and a flip-chip capacitive humidity sensor. This research may provide useful experiences for the study of these two types of sensors.