Comparison of the MOS capacitor hydrogen sensors with different SiO2 film thicknesses and a Ni-gate film in a 4% hydrogen–nitrogen mixture

Abstract

In this study a MOS capacitive-type hydrogen gas sensor with the Ni/SiO2/Si structure has been fabricated. The influence of SiO2 film thickness on the sensor response speed, response (R%), and Flat-Band-Voltage (VFB) has been investigated at 140°C and 100kHz frequency. Devices were fabricated on (0.22Ωcm) 〈400〉 n-type Si and oxide layer has been characterized using Scanning electron microscopy (SEM) and Atomic force microscopy (AFM). Four sensors are reported at different SiO2 film thickness (28nm, 40nm, 46nm and 53nm) and results are compared. The VFB for films with 28nm and 53nm thicknesses was measured in pure N2 and 4% H2–N2 mixtures. In the former case, results were obtained at 1V and 2V and in the later case the VFB shift was 0.3V and 0.05V. Using MOS C–V measurement under the Bias Thermal Stress (BTS) technique, the trapped charges were measured. Results indicate an increase in trapped charge which is due to an increase in the oxide film thickness. The highest response observed at the bias voltage of (0V) for a 28nm SiO2 film thickness is 87.5%. The response decreases with the increase of SiO2 film thickness. The shortest H2 response/recovery time observed is in the Ni/SiO2/Si sensor with 28nm SiO2 film thickness. Experimental results demonstrate that the sensor is highly sensitive to SiO2 film thickness, which can be used for response, response/recovery time and VFB studies of MOS capacitive gas sensors and low-cost hydrogen detectors with 4% hydrogen concentration responses.