Enhanced stability of Ti/Pt micro-heaters using a-SiC:H passivation layers

Due to their chemical inertness, hydrogenated amorphous silicon carbide (a-SiC:H) thin films exhibit excellent properties as passivation layers for micro-sensors and micro-actuators operating in harsh environmental applications. They are typically synthesized with a plasma-enhanced chemical vapor deposition process at moderate temperatures, providing an excellent backend compatibility. To demonstrate their suitability and their signal stability for hot film anemometers in high pressure loaded automotive applications, molybdenum and titanium/platinum micro-heaters without and with a-SiC:H passivation were operated in an oil medium for up to 380 h at nominal thin film temperatures of 60 and 160 °C. As expected, the drift of the electrical resistance increased with rising temperature. In the unpassivated case, the molybdenum metallization exhibited an increased long-term stability when operated as the flow sensitive thin film. A protection of the platinum based thin film element with a 1 μm thick a-SiC:H layer reduced the drift in sensor resistance by more than one order of magnitude compared to a bare hot film probe. As a result of the tensile stress in the metallic thin film and a compressive stress in the a-SiC:H coating, a 30 h pre-aging is recommended in order to stabilize the micro-heater structure. At a high-pressure hydraulic test bench, an a-SiC:H/Pt/Ti test structure on a low temperature co-fired ceramics substrate was exposed to the dynamic pressure fluctuations with a maximum value of 135 MPa (1350 bar) for 1 h. No cracks or delaminations were found in the a-SiC:H surface passivation layer, as determined by optical inspection and scanning electron microscopy.