The Design Of A Capacitive Relative Humidity Sensor With Cmos Process

This paper reports a novel capacitive humidity sensor integrated on a polysilicon heater. The sensor was fabricated with the industrial standard CMOS process to achieve a cost-effective solution for accuracy and reliability. Humidity sensors are widely used in many measurement and control application, including process control, meteorology, agriculture and medical equipment. Humidity sensors can be categorized into capacitive, resistive, mechanical, oscillating and thermo elemental type based on the sensing principle used.The capacitive humidity sensor is integrated on a polysilicon heater in which the response time of desorption is faster than that in previously reported devices, and it has long-term reliability by using passivation between the electrodes and sensing materials. The layer of passivation with flexible selection of the electrode is located between the electrode and the sensing material in order to improve the reliability of the RH sensors. The novel humidity microsensor has been fabricated with the 3 micron CMOS technology. The humidity sensor device consists of comb electrodes, a comb polysilicon heater and the polyimide as the moisture sensing material. The novel structure consists of two interdigitated combs containing 21 aluminum fingers and 60 polysilicon fingers which act as the heater. The fingers of electrodes are 3 micron wide and 400 micron long .The size of the entire structure is 508 micron×450 micron.we have measured the sensitivity, hysteresis, temperature coefficient, stability, response speed and the speed of desorption by the heater The sensitivity of the RH sensor is 6fF/%RH at 2.5micron thickness (spun at 2000rpm) and 5fF/%RH at 1micron thickness (spun at 4000rpm), The humidity sensor was measured to show a fairly linear dependence on the relative humidity in the range of 20%~70%. Under normal operation condition (at 27 degree Celsius, 1 atm) the hysteresis of a 1 micron-thick sensor is record when the relative humidity of chamber goes up from 20% to85%, and then goes down to 20% with a step of 5% RH. The maximum hysteresis is 2% at 80%RH.We also measured the temperature coefficient of the sensor when the temperature of the chamber rises 20℃ to 60℃ and drops from 60℃ to 20℃. At 75%RH the temperature coefficient was 0.14%RH/℃.To obtain the high-reliability, it is necessary to cover a layer of passivation on the Al electrode. No measurable drift was observed after aging the sensor in 90% RH ,27℃ for 7 days. Using a novel way to measuring the response time, we obtained an accurate measurement within about 10 seconds. Finally we get the speed of desorption with the heater in the 85% RH at 25 ℃.The period spended about 51 seconds and is three times faster than that no heating heater. And the heater can be also used to eliminate the volatile impurity.Consequently, the novel humidity sensor provides the high reliability, the better hysteresis, sensitivity and linearity with the standard CMOS process.