Low Power and Fast Response Carbon Dioxide Gas Sensors

Tu Xiang Zheng

Carbon dioxide can cause negative health affects to humans including drowsiness and at high enough concentrations suffocation. It has been recommended that the maximum time averaged exposure to the atmosphere containing 5,000 ppm carbon dioxide is not over eight hours. As such it is highly desirable to be able to measure carbon dioxide in order to control indoor air quality and in environmental monitoring.

The present author provides a MEMS carbon dioxide gas sensor as shown in the above figure. The sensor is based on a silicon wafer and fabricated utilizing CMOS technologies. Since the sensor is required to be operated at an elevated temperature a thermal insulating pad is formed in the silicon wafer which is used to support the sensor body. Both a resistor for heating and a thermopile for temperature sensing are formed on the thermal insulating pad. Then depositing an electrical insulating layer and laying a tin dioxide layer is formed thereon. By employing such device structure with good thermal insulation to the silicon wafer, the sensor presents a series of advantages such as miniaturized size, low power consumption, and fast response.

Reducing the size of the sensor is the most effective way to reduce overall power consumption. The size of the heater of the sensor can be reduced as small as 60 µm x 800 µm. This affords field operation on a single 9V battery for an acceptable time. In addition, shortening the thermal response time enables the sensor to be operated for a very brief period during measurement. This pulse heating provides a further reduction in the power consumption. All these capabilities make the sensors ideal for the applications ranging from low power wireless to cell phones and wearable and conformal sensing systems.

The thermopile of the sensor is not able to measure the absolute temperature, but generate an output voltage proportional to the temperature difference between the heater and the silicon wafer. This is suitable for the sensor to adapt a pulse width modulation (PWM) based temperature controlled circuit. It is a key to stabilize the temperature of the sensor since the response of the sensor increases and reaches the maximum at a certain temperature, and then decreases rapidly with increasing the temperature. In the modulation circuit a microcontroller is programmed to generate different modulating 8 bit digital signal which is converted to analog signal using digital to analog converter (DAC). The analog signal is then used to control a PWM based driver circuit which drives the heater of the metal oxide gas sensor.

The pulse width modulation (PWM) based temperature controlled circuit has been reported to have three advantages:

• A cyclic temperature variation can give a unique response for each gas as rate of reaction of the different analytic gases are different at different temperature;
• Low temperature may lead to the accumulation of incompletely oxidized contaminant, which may get removed during cyclic oscillating voltage; and
• Thermal cycling can lead to improvements in sensitivity because for each gas there is a heater voltage for which it shows maximum conductance-temperature characteristics.