Compatibility studies of zinc oxide varistor materials with integrated ceramic packaging technology.

摘要

Recent developments in electronics have merged thick film and multilayer ceramic manufacturing techniques to form a new breed of "integrated ceramic" substrates. The goal of this study was to incorporate multilayer varistors in these devices. This included determining the compatibility of the materials, characterizing the varistor-substrate interface, and demonstrating an operational varistor cofired in the substrate. The varistor composition consisted of 98.5 mol% ZnO, 0.5 mol% CoO, and 1 mol% Bi{dollar}sb2{dollar}O{dollar}sb3{dollar}. The substrate used was the DuPont Green Tape substrate.; The sintering study revealed that simple varistor compositions with fine particle sizes will densify at lower temperatures than compositions with coarser particles. This promotes grain growth at lower temperatures, reducing breakdown voltages. Calcining these materials either reduces densification and grain growth, increasing breakdown voltages, or forms aggregates which act as large single grains, reducing breakdown voltages. Samples with cobalt calcined into the ZnO exhibited dramatically reduced nonlinearities, suggesting that cobalt should be left at the grain boundary as electron traps or as hole formation sites. The sintering of coarser calcined materials at lower temperatures is improved if fine materials are added to the composition.; The interfacial study showed that the varistor material forms a continuous bond to the substrate material. The interfacial region consists of three zones: (1) a layer of zinc silicate (willemite) bonding to the ZnO surface, (2) a reaction zone containing ZnAl{dollar}sb2{dollar}O{dollar}sb4{dollar} (gahnite) crystals and (Ca,K)AlSi{dollar}sb3{dollar}O{dollar}sb8{dollar} (feldspar) crystals, bonded by a Bi-Si rich amorphous phase, and (3) a zone where bismuth-rich liquid infiltrates the substrate. Liquid bismuth from the varistor attacks the glass first, but eventually dissolves all of the substrate's components. Electrodes at the interface do not prevent these interactions.; The study of substrate-varistor composites indicated that buried devices could be fabricated between 850 and 950{dollar}spcirc{dollar}C. Excess thickness shrinkage and reduced lateral shrinkage were observed in the composite laminates, caused by early densification of the interface. This constrains the lateral dimensions of the composite and forces the increased thickness shrinkage. The buried varistor contained large amounts of porosity, reducing grain growth and increasing breakdown voltage. Insulation resistances were low in samples containing internal electrodes, suggesting localized grain growth. Nonlinearity coefficients of approximately 15 were observed for the buried devices.