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This paper presents a fully packaged CMOS interdigital capacitive humidity sensor with polysilicon heaters. The sensor was fabricated with the industrial standard CMOS process and the MEMS postprocessing step. Polyimide was used as the sensing material for its good linearity. To enlarge the depth of the recesses between the interdigital electrodes, the polysilicon heater was placed under the Al electrode and also the passivation between the interdigital electrodes was partly etched. As a result, thicker polyimide can be deposited between the Al electrodes and thus the sensitivity of the sensor can be improved. The etching resistant passivation between Al electrodes and polyimide was used to ensure the reliability of the sensor. To put it into practical use, the sensor was fully packaged, and by using the polysilicon heaters, both the hysteresis and the recovery time of the sensor could be greatly improved. The packaged humidity sensor was measured to show a good linearity and repeatability over the whole testing range.