Carbon Dioxide Sensing Using Amine-Functionalized Capacitive Microsensors As a Function of Sensing Film Temperature

Abstract

Introduction This paper systematically investigates the sensing performance of amine-functionalized, microfabricated capacitive carbon dioxide (CO2) sensors as a function of sensor temperature and sensing film thickness. The interdigitated capacitor structure was heated directly by an integrated resistive heater. Using thin sensing films, sensor sensitivities around 1% per 1,000ppm CO2 are obtained at response times of <2 minutes. Background Monitoring of carbon dioxide, a greenhouse gas, using miniaturized sensors is of significant interest for indoor air quality and health monitoring applications [1, 2]. However, CO2 molecule are chemically inert and, thus, pose a challenge for solid-state gas detectors. As a result, optical sensing using nondispersive infrared (NDIR) sensors has become the standard for CO2 sensing. However, NDIR sensors are generally large and costly compared to solid-state sensors. To address this challenge, amino-functionalized sensing films in combination with a variety of transducers have been investigated for CO2 sensing, but exhibit small sensitivities at room temperature [3, 4]. This work systematically investigates the CO2 detection capabilities of amino-functionalized, microfabricated capacitive sensors as a function of the sensor temperature and sensing film thickness. Method Each silicon die contains four 1mm2 interdigitated electrode structures as capacitive sensors, surrounded each by a resistive heater and temperature sensor. The simple 2-mask fabrication process starts with growing a 1µm thermal oxide on top of the silicon wafer for electrical insulation, followed by deposition and patterning a 200nm aluminum film to define the interdigitated electrodes, the resistive heater, and a resistive temperature sensor. The wafer was then passivated using a 20nm thick SiO2 film deposited by plasma-enhanced atomic layer deposition and contact pads were patterned and etched. Figure 1 shows a picture of the sensor die with 4 sensors and a schematic of one sensor with heater and temperature sensor around it. The sensing film was prepared by mixing 3-amino-propyltrimethoxysilane and propyltrimethoxysilane with a volume-ratio of 70:30. The mixture was spray-coated on the sensor and cured at 120˚C for 16 hours in a vacuum oven. The capacitive sensors were mounted in a dual-in-line (DIL) packages and wire bonded. To increase the thermal resistance to the DIL package the sensor chips were mounted on 500µm Kapton tape at its four corners as shown in Figure 2a. The packaged sensors exhibit a heating efficiency of approximately 0.2˚C/mW (Figure 2b). Results and Conclusions An Environics 2040 gas mixing system was used to expose the packaged sensors to defined concentrations of CO2 gas. A commercial NDIR sensor was mounted in line with the capacitive gas sensor and used as a reference sensor. Figure 3 shows a typical response of a capacitive sensor coated with a 500nm thick sensing film when exposed to different CO2 concentrations. This specific measurement was performed at 85˚C die temperature and shows a decrease in capacitance with increasing CO2 concentration. Similar measurements were performed using sensors with three different film thicknesses (100nm, 200nm and 500nm) at die temperatures ranging from 50-120˚C. For the capacitor coated with a 100nm sensing film, Figure 4 shows the capacitance change measured at 3700ppm CO2 concentration as well as the t90 response time as a function of the sensor temperature. A maximum capacitance change was observed at 85˚C and exhibited an average response time t90=11minutes. Faster response times were found at higher temperatures at the expense of sensor sensitivity. Figure 5 compares the capacitance change at 3,700ppm CO2 as a function of temperature for different film thicknesses. While large sensor sensitivities can be obtained for thicker films, the response times increase. For example, for a 500nm thick sensing film, t90≈26minutes with a signal change of ≈17%/1,000ppm.