Microstructural glass modifications in as-fired and high-voltage-surged RuO 2 -based thick film resistors
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Juji Ishigame and Shyu Sekihara Sumitomo Metal Mining Company, Electronics Division, Ome, Tokyo 198, Japan (Received 28 February 1991; accepted 28 March 1991)
Electrical conduction in thick film resistors has been studied, and microstructures, especially around conducting RuO 2 phases in a lead-borosilicate glass matrix in as-fired and high-voltage-surged thick film resistors, have been observed in detail using transmission electron microscopy. Lattice images of as-fired thick film resistors have suggested the presence of subtle structural modifications in the very thin area across the RuO 2 /glass interface, whereas in the glass matrix very small dot-like contrasts on the order of 1 nm were occasionally observed and were interpreted as being small crystallites or Ru clusters. Heavy electrical loadings of thick film resistors were found to induce the local formation of plate-like crystals in glass, which were identified by electron diffraction to be a slightly modified anorthite. The significance of these observations in terms of the conduction network and the degradation due to the electrical overloading of thick film resistors are discussed.
I. INTRODUCTION Thick film resistors (TFRs) are used in single and networked chip resistor units and as an essential component in hybrid ICs. Fine powders consisting of glass frits and conducting phases such as RuO 2 and Pb2Ru206_7 are mixed with organic vehicles, screen-printed on an alumina substrate, and fired in air at temperatures between 1073 K and 1173 K to form a resistor component. TFR is a complex system, and in order to control electrical properties such as resistivity, temperature coefficient (TCR), etc., numerous studies have been conducted to clarify the mechanism of the electrical conduction in TFR1"6. Variation of resistivity measured as a function of the volume fraction of the conductive phase1'7 indicates that the conduction network would not be formed completely via adjoining conductive particles, since an addition of merely a 2 to 8% conductive phase could give rise to a dramatic decrease of glass resistivity by a factor of more than 10 10 . A direct observation using transmission electron microscopy (TEM) showed8 that most of the conductive particles exist as individual entities surrounded by the glass phase. Involvement of the glass phase as part of the conduction path appears indispensable. Prudenziati6 determined that the charge transfer through an insulating glass can be done by electron hopping between localized low levels,9 electron tunneling between conductive particles with the aid of localized resonant centers,4 or electron transport in narrow energy bands.10 However, no definite conclusions are given, due partly to a lack of information on J. Mater. Res., Vol. 6, No. 8, Aug 1991
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very local microstructures in TFR. Thus, this work was intended to supply detailed TEM observations of RuO 2 based TFRs, especially around conducting phases, to understand better the electrical transport in the c
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