Microstructure development and electrical properties of RuO₂-based lead-free thick film resistors

Lead-free thick film resistive compositions suitable for hybrid microelectronics were prepared. The compositions were made with RuO₂ as the conducting phase and bismuthate glasses. This blend of bismuthate glasses constitutes a suitable choice for avoiding negative effects such as devitrification, bleeding out of the glass on alumina substrates, anomalous distribution of conductive grains in the glassy matrix and phase separation observed in other systems. The morphology, microstructure and electrical properties have been studied. X-ray diffraction (XRD), electron scanning microscopy (SEM) and energy dispersion spectroscopy (EDS) show that a defect pyrochlore phase of bismuth titanate formed at about 700 °C in all the compositions studied. Transmission electron microscopy (TEM) analysis of the original RuO₂ powder shows that a single grain is made of many smaller grains of different crystalline orientations. The sheet resistance spans two decades by changing the RuO₂ fraction from about 14-52 wt%. The resistors exhibit good reproducibility and their temperature coefficient of resistance is in the range of ± 300 ppm/°C.