Microstructure of LaB 6 -base thick film resistors
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The microstmcture of LaB6-base thick film resistors was investigated by cross-sectional transmission electron microscopy. The specimens were prepared by a technique that polished them to a thin wedge, thus avoiding ion-milling and permitting imaging over a distance of tens of microns. The resistor microstmcture contained a finely divided electrically conductive phase of TaB2 and nonconducting crystals of CaT^On, formed during high temperature processing of glass and LaBg ingredients of the thick film ink. Using higher surface area ingredients virtually suppressed the formation of CaT^On crystals, and the microstructure became more uniform. Resistors made with higher surface area intermediates also had better voltage withstanding properties. I. INTRODUCTION Thick film resistors are a common component in hybrid microcircuit technology. They are fabricated from a screen-printable ink that consists of finely divided powders of a glass (e.g., PbO-SiO 2 -Al 2 O 3 ) and a conducting phase such as RuO 2 , mixed with an organic vehicle. The particular ratio of the conducting component to the glass determines the sheet resistivity, which can usually be varied between 10 and 106 ft/D. The resistor itself is formed by printing it through a screen with holes that conform to the desired resistor geometry on an AI2O3 substrate. The printed resistor is dried of solvents at about 100 °C and then fired in a moving beltfurnace to a peak temperature of 800-900 °C, beyond the softening temperature of the glass, which upon cooling binds the conducting particles together in a continuous network. The mechanism for electrical conduction in these complex composite systems has been the subject of numerous studies. 1 ^ From the very small activation energy (~10~ 4 eV) for conduction at low temperature, Pike and Seager1 argue that the conduction mechanism involves some form of tunneling between adjacent primary conductive particles, while the dynamics of the development of conducting networks in thick film resistors has been studied extensively by Professor R. Vest and his students.5'6 However, only a few studies have actually examined the microstructure of thick film resistors. Nordstrom and Hills7 and more recently Adachi et al.8 used transmission electron microscopy to study in plan-view the microstructure of Ru-base thick film resistors. TEM is a powerful tool for examining resistor microstructure, but the conventional sample preparation method, which both groups used, limits the imaged region and is potentially flawed by artifacts. In the conventional preparation method for TEM, the specimen is first mechanically polished to a thickness of about 100 fxva, followed by dimpling, before a final step J. Mater. Res., Vol. 7, No. 8, Aug 1992 http://journals.cambridge.org
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of extensive Ar ion-milling to perforation. In the electron microscope, the region adjacent to the hole is usually thin enough to be electron transparent for imaging. However, the area that can be imaged is usually very small compared to typical thick
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