The Development and Fabrication of Silicon Carbide Based Metal Oxide Semiconductor Gas Sensors for Automotive Applications

Abstract

The detection of gases in hazardous environments has been a subject of great interest due to the number of potential applications. Vehicle and volcano emission monitoring, space exploration and nuclear waste management could all make use of sensitive and selective gas sensors. Traditional silicon based electronic systems have poor resilience in harsh environments and struggle to operate at temperatures in excess of 150◦C. Silicon carbide (SiC) is an alternative material from which temperature-stable sensors can be fabricated. The wide band gap, strong silicon to carbon bonding, and low intrinsic carrier concentration make it a suitable candidate for these devices. SiC based metal oxide semiconductor capacitors have been successfully deployed as hydrogen, oxygen, methane and hydrogen sulfide sensors in the past, however there are still issues around selectivity, as cross-analyte sensitivity occurs, leading to the need for sensor arrays.
In order to ensure compliance with the continuously iterated European legislation on diesel engine emissions, a selective, temperature-stable, real-time sensor could be employed to monitor levels of nitrogen oxide (NOx) emissions. In this study, metal oxide semiconductor (MOS) capacitors were fabricated, comprising a material with a high-dielectric constant (high-κ), and a noble metal layer as the catalytic gate material. Previously, devices based on hafnium oxide (HfO2) and titanium dioxide (TiO2) have shown sensitivity to hydrogen and oxygen at high temperatures, therefore, in this work, the fabrication process for these devices was built upon to target NOx.
Tungsten oxide (WO3) has previously shown sensitivity towards NOx and was incorporated as the dielectric material of the device, with a study carried out to find the optimum tungsten deposition and oxidation conditions. It was concluded that DC sputtering of 50 nm of tungsten metal and subsequent oxidation in air at 500◦C for 10 minutes produces a uniform film of WO3, suitable for use as the dielectric layer of a capacitor.
Platinum was chosen as the gate metal due to its ability to catalytically decompose gases. It has been shown in the past, that a porous gate metal layer can encourage interaction of gases such as ammonia and NOx. Therefore, a study was carried out to optimise the platinum deposition conditions in order to incorporate porosity into the film. Alteration of the sputter power and argon pressure was carried out and it was found that sputtering at 50 W with an argon pressure of 140 mTorr incorporated a suitable level of porosity into the metal.
Devices comprising HfO2, TiO2, ZrO2 and WO3 as the dielectric layer were fabricated, and their capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics were extracted. Sensor behaviour in the presence of H2, O2 and NO was examined. It was found that device response varies depending on the identity of the dielectric material. ZrO2 devices demonstrate a temperature dependant response, showing no sensitivity at room temperature, but displaying a visible shift in characteristics at 573 K. We have also shown for the first time that a Pt/WO3/SiO2/4H-SiC device demonstrates sensitivity to NO at 573K.
A Pt/TiO2/SiO2/4H-SiC device was exposed to different organic solvents and the C-V characteristics were monitored, in order to determine more information about the mechanism of response. A significant relationship between magnitude of response, and solvent dipole moment was discovered, leading to the hypothesis that traps in the dielectric layer drive device response. In conclusion, this work has built upon previous demonstration of SiC based MOS capacitor gas sensors comprising HfO2 and TiO2 as dielectric materials. We have demonstrated the use of a Pt/ZrO2/SiO2/4H-SiC device as an oxygen sensor and a Pt/WO3/SiO2/4H-SiC device as a nitric oxide (NO) sensor, for the first time. Combining these devices into a sensor array would lead to a system capable of selectively targeting specific analyte gases in high temperature environments, such as a diesel exhaust engine.