Investigation of stability and reliability of tin oxide thin-film for integrated micro-machined gas sensor devices

Abstract

The stability and reliability of SnO2/Pt, SnO2–Cu/Pt and SnO2/Pt/SnO2/Pt sensitive thin-films used for CO gas sensor device are discussed in this paper. The stability and reliability are very important for the tin dioxide-based gas sensor devices when these devices are to be integrated with standard CMOS circuitry. The drift in output voltage of tin dioxide sensing thin-film device must be minimized. The stability in the output of the tin dioxide thin-film sensing resistor is very essential to implement reliable integrated sensor device, because a small drift in baseline led to large change in the biasing current in the subsequent signal processing circuit. Here, we define the baseline as the output voltage across the two-terminal sensor in the absence of the signal. Another important parameter is the sensitivity of the sensor. The sensitivity of the device should be repeatable over large number of operation cycles. The drift in baseline and sensitivity of devices which are fabricated using SnO2/Pt, SnO2–Cu/Pt and SnO2/Pt/SnO2/Pt sensitive thin-films as gas sensitive material were studied. The sensor device based on SnO2–Cu/Pt thin-film shows good sensing characteristics. It was observed that the tin dioxide thin-film cracks during large number of operation cycles. The annealing of these thin-film at higher temperature or longer time induced the cracks in the sensing thin-film. These cracks are responsible for the drift in baseline and sensitivity of the thin-film. The cracks change the resistivity of the sensitive thin-film, which directly impact the device performance and reliability of the device. Our result shows, that SnO2–Cu/Pt thin-film is more stable as compared to SnO2/Pt and SnO2/Pt/SnO2/Pt thin-film. The change in surface structure during the operation of device was investigated using SEM technique. The accelerated thermal tests on the devices were performed to estimate the lifetime and evaluate the stability of the sensor device.