A monolithic integrated 4 × 4 tin oxide gas sensor array with on-chip multiplexing and differential readout circuits

This paper presents a monolithic 4 × 4 tin oxide gas sensor array together with on-chip multiplexing and differential read-out circuitry. A robust fabrication process focusing on the integration of the CMOS circuitry and the microelectromechanical systems (MEMS) structure is first described. The gas sensor uses a surface micro-machined micro-hotplate based structure. Platinum is selected as the material of choice for both the micro heater and the sensor’s electrodes, for its good thermal stability and compatibility with the sensing film. Different post-treatments are used to modify the characteristics of the tin oxide gas sensors hence improving the sensor’s selectivity of the overall sensor array. Furthermore, in contrast to the conventional voltage divider read-out technique, a novel differential read-out circuit (DRC) for tin oxide gas sensors is proposed. The DRC applies a constant current to drive the sensor and uses a unit-gain single stage amplifier (inverter) to generate a fully differential output, directly from the voltage drop across the sensor. The output of the DRC is simply proportional to the difference between the voltage on the two electrodes of the sensor but not to the transistor parameters such as mobility and threshold voltage, neither to the supply voltage. The monolithic sensor array and its pre-processing circuitry have been implemented in our in-house 5 μm process. Experimental results showed good linearity at the output of the DRC for a wide range of sensor resistance variation (over 20 MΩ). The fabricated micro-hotplate sensor array was tested for four target gases. Results show good sensitivity and interesting thermal characteristic leading to only 15.5 mW power consumption for 300 °C operating temperature.