Comparison of the MOS capacitor hydrogen sensors with different SiO_2 film thicknesses and a Ni-gate film in a 4% hydrogen-nitrogen mixture

摘要

In this study a MOS capacitive-type hydrogen gas sensor with the Ni/SiO_2/Si structure has been fabricated. The influence of SiO_2 film thickness on the sensor response speed, response (R%), and Flat-Band-Voltage (V_(FB)) has been investigated at 140℃ and 100 kHz frequency. Devices were fabricated on (0.22 Ω cm) <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 SiO_2 film thickness (28 nm, 40 nm, 46 nm and 53 nm) and results are compared. The V_(FB) for films with 28 nm and 53 nm thicknesses was measured in pure N_2 and 4% H_2-N_2 mixtures. In the former case, results were obtained at 1V and 2 V and in the later case the V_(FB) shift was 0.3 V and 0.05 V. 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 (OV) for a 28 nm SiO_2 film thickness is 87.5%. The response decreases with the increase of SiO_2 film thickness. The shortest H_2 response/recovery time observed is in the Ni/SiO_2/Si sensor with 28 nm SiO_2 film thickness. Experimental results demonstrate that the sensor is highly sensitive to SiO_2 film thickness, which can be used for response, response/recovery time and V_(FB) studies of MOS capacitive gas sensors and low-cost hydrogen detectors with 4% hydrogen concentration responses.