Influence of film thicknesses on the electrical properties of RuO₂-thick film resistors on aluminum nitride ceramics (AlN)

Aluminum nitride ceramics are very suitable as substrate material for thick film- / hybrid applications in the field of power electronics and microwave technology. Main features are high thermal conductivity, a thermal expansion coefficient close to silicon, good mechanical stability and dielectric properties. Thick film resistor pastes are usually composed of lead- and bismuth oxide free glass frit and ruthenium dioxide as electrical conductive component. In contrast to oxide ceramics like alumina ceramics, thermo dynamical calculations predict the reaction of ruthenium dioxide with AlN. Thus, resistor pastes for AlN are designed to build defined porosity in the films. In dependence of composition of the paste, substrate roughness and -quality varying interactions of the components and deviating film structures can be observed. On basis of different paste compositions resistors were built with varying film thicknesses. Electrical properties like resistance and its temperature coefficient as well as the power dissipation and short term overload voltage were correlated with geometrical data, chemical composition and substrate properties. Furthermore FESEM was used to characterise film properties like porosity and the interface resistor – ceramics. Users of thick film pastes are shown aspects under which conditions high quality resistors are maintained under maximum materials savings.