A sensor is a device which detect various information such as pressure, temperature, humidity, acceleration, and bio signals from a measurement object and converts them into electrical signals. It is a device that acquires information input to electronic devices in an external environment. Sensor technology is a key element of the Internet of Things (IoT) that acts as a human sensory organ, and has been miniaturized and complex with the development of micro-electro- mechanical systems (MEMS) technology, and has recently developed into intelligent sensors that include data processing and communication functions. Among these, humidity sensors are actively used in various fields such as automobiles, mobile devices, medical devices, environmental devices, and industrial devices in addition to household appliances used in everyday life. In order to be competitive in these markets, it is essential to have small and high-precision characteristics at low prices.
Most humidity sensors are mainly resistive-type and stacked-type. The resistance-type humidity sensor is simple to manufacture by screen printing, but it has limitations in its application field due to problems such as a narrow humidity sensing area and reliability such as changes in characteristics at high temperatures and high humidity. The stacked-type capacitive humidity sensor developed to overcome the problems of the resistive-type humidity sensor has a complicated process such as having to porous the upper electrode metal and separately forming the upper electrode for smooth moisture absorption and desorption, and so it is difficult to miniaturize due to the structure of lead pin.
In this study, a planar-type humidity sensor has been designed to improve the problems of the stacked structure with interdigitated electrode pattern and humidity sensor which has excellent linearity and responsive character at low cost has been proposed by applying thermoset polyimide as a moisture absorbent.
The proposed humidity sensor used a polyimide capable of chemical wet etching as a moisture membrane, and has been designed and fabricated to be process compatible when developing an integrated temperature-humidity complex sensor and performed measurement of special feature and analysis. First, the sensor structure was optimized through the theoretical consideration of factors affecting the capacity value of the capacitive humidity sensor, such as the number, the thickness and spacing of electrodes, and the thickness of the polyimide absorbent film. Based on this, the mask design of the interdigitated electrode capacitive humidity sensor was constructed, and the basic characteristics were analyzed after fabrication using MEMS technology on a silicon substrate.
The fabrication process applied in this thesis has the advantage of being compatible with CMOS processes, so that the readout integrated circuits (ROIC) attached to the back-end can be designed together as a single process. Taking these advantages and considering mass production in the future, the chip size was maintained at 1.54 mm ? 1.54 mm, and the sum of the electrode line width and the inter-electrode spacing was 6 μm, and sensors with various electrode numbers were fabricated on a 4 inch silicon wafer. As a follow-up process for measurement, the lead pin was attached to the PCB pad and packaged to measure and analyze the properties.
The designed and fabricated capacitive humidity sensor shows an average capacitance values are distributed in the range of 24±1 in the range of 20 to 95% relative humidity. In addition, it was confirmed that the linear characteristics were within ±2, the hysteresis characteristic were within ±3, and the response time were within 5 sec.