A Low Power MEMS IDE Capacitor with Integrated Microhotplate: Application as Methanol Sensor using a Metal Organic Framework Coating as Affinity Layer
Capacitors made of interdigitated electrodes (IDEs) as a transducer platform for the sensing of volatile organic compounds (VOCs) have advantages due to their lower power operation and fabrication using standard micro fabrication techniques. Integrating a micro electromechanical system (MEMS) such as a microhotplate with IDE capacitor, further allows study of the temperature dependent sensing response of VOCs. In this paper, the design, fabrication, and characterization of a low power MEMS microhotplate with IDE capacitor to study the temperature dependent sensing response to methanol using Zeolitic imidazolate framework (ZIF 8) a class of metal organic framework (MOF) is presented. A Titanium nitride (TiN) microhotplate with aluminum IDEs suspended on a silicon nitride membrane is fabricated and characterized. The power consumption of the ZIF 8 MOF coated device at an operating temperature of 50 C is 4.5 mW and at 200 C it is 26 mW. A calibration methodology for the effects of temperature of the isolation layer between the microhotplate electrodes and the capacitor IDEs is developed. The device coated with ZIF 8 MOF shows a response to methanol in the concentration range of 500 ppm to 7000 ppm. The detection limit of the sensor for methanol vapor at 20 C is 100 ppm. In situ study of sensing properties of ZIF 8 MOF to methanol in the temperature range from 20 C to 50 C using the integrated microhotplate and IDE capacitor is presented. The kinetics of temperature dependent adsorption and desorption of methanol by ZIF 8 MOF are fitted with double exponential models. With the increase in temperature from 20 C to 50 C, the response time for sensing of methanol vapor concentration of 5000 ppm decreases by 28% whereas the recovery time decreases by 70%
Sensors