Sammendrag
Wireless sensors incorporated in RFID systems are important in several industrial, consumer and logistics applications. By extending RFID tags to sensing applications, the products become smarter. Application areas for these smart tags include; health care (verification of the environmental conditions during transport or in storage of e.g. diapers, bandages, etc.), food monitoring (food quality during transport, storage and sales) and construction industry (e.g. building material).In this paper, a small, very low power and low cost humidity sensor tailor made for passive RFID applications is presented. The sensor consists of a glass chip substrate with a sub-micron interdigitated gold electrode structure covered with a humidity sensitive polyimide layer. The humidity absorbed by the sensing layer is measured capacitively. Finite element modeling and analytic calculations were used to determine the design of the interdigitated electrodes and the optimal thickness of the polyimide layer. A read-out electronics circuit was designed and used to evaluate the sensor. Sensors were fabricated and calibrations have been made to verify their function. The sensor response was close to linear from below 20 to above 90 %RH and its response time was proven to be at least as short as that of the climate chamber, namely 0.1 %RH/s. The concept can easily be adapted to measure a range of other parameters such as temperature or the presence of certain substances.