Investigation on different passivation technologies for thin film sensors on ceramic substrates

A proper passivation technology for thin film sensor elements, exposed to the harsh environmental conditions of an automotive injection system, is a critical factor for a long-term stable and reliable operation of the device. For thermal mass flow sensors, diamond or diamond-like coatings promise excellent thermal material properties with a low influence on the sensor characteristics. With an arc discharge plasma jet CVD technology, however, no closed surface characteristics for the passivation layer could be obtained on LTC (Low Temperature Cofired) ceramics, which serve as the substrate for hot film anemometers. By using a plasma-enhanced CVD process for the deposition of amorphous silicon carbide (a-SiC:H), the complete glass-ceramic substrate is covered with a 1 μm thick thin film, yielding a closed coating of the surface. The a-SiC:H layer can be patterned with a NF₃/O₂ gas mixture by a plasma assisted dry etch technology. After exposing the coated thin film sensors to the dynamic operation of a hydraulic test bench up to fuel pressures of 135 MPa, no failure of the passivation layer by delamination or cracking occurred.