CONDUCTION MECHANISMS IN THICK FILM RESISTORS
Abstract
The work reported here has focused on the influence of substrate dissolution during the high temperature firing on the electrical properties of ruthenium dioxide-lead borosilicate thick film resistors. A resistor 25 (mu)m thick that is fired for 10 minutes at 840(DEGREES)C will dissolve 4 (mu)m of the substrate surface. The presence of the alumina in the glass phase retards the growth of the resistor microstructure or chains of conducting particles by increasing the glass viscosity and slightly decreasing the solubility of RuO(,2). These physical changes brought about measurable changes in the electrical properties. Four glasses were made: a standard glass with 63 w/o PbO:25 w/o B(,2)O(,3):12 w/o SiO(,2); 4 w/o 614 AlSiMag substrate and 96 w/o standard glass; 6 w/o substrate and 94 w/o standard glass; 10 w/o substrate and 90 w/o standard glass. These glasses were powdered and mixed with 5 w/o RuO(,2) powder then blended with screening agents. Four terminal resistors were fired onto 614 AlSiMag substrates under various conditions of time and temperature. The sheet resistance, hot (+ 125(DEGREES)C) and cold (-55(DEGREES)C) temperature coefficients of resistance, normalized resistance versus temperature (-55 to +125(DEGREES)C), and noise index were measured. As predicted, resistors with increasing amounts of substrate in the glass phase demonstrated slower development of electrical properties. It took about 14 minutes at 800(DEGREES)C for a 10 w/o substrate resistor to reach its minimum sheet resistance compared to only 8 minutes for the standard glass resistor. The hot and cold TCR's increased from large negative values (< -400 ppm/(DEGREES)C) for short firing times or low firing temperatures to near-zero values at longer firing times or higher firing temperatures. The noise index, in general, decreased with increasing firing time or temperature from about 40 dB down to 5 dB. With increasing weight percent substrate, the noise index increased. A second group of experiments was performed to determine conduction mechanisms in thin films of the standard and 10 w/o substrate glasses. The glass was rf sputtered onto gold-coated, oxidized silicon wafers, then counter-electrodes of gold were evaporated on these films. Both ac and dc measurements were made. The parallel capacitance was a slowly decreasing function of frequency from 10('2) to 10('5) Hz. The conductivity increased linearly with frequency while the dissipation factor often showed a broad maximum implying the presence of an ionic impurity relaxation. The dielectric constants were found to be 11.5 and 11.2 for the standard and 10 w/o substrate glasses, respectively. The dielectric breakdown voltages were 2.6 x 10('6) V/cm for the standard glass and 5.5 x 10('6) V/cm for the 10 w/o substrate glass. The dc measurements indicate the conduction mechanism in the glass films is Schottky emission with an activation energy of approximately 0.38 eV. Using the normalized resistance versus temperature data for the thick film resistors, a unit model consisting of sintered particles of RuO(,2) and non-sintered contacts with RuO(,2) particles separated by a thin layer of the glass phase has been proposed. The temperature dependence of the theoretical unit has been derived and related to the experimental data by a curve-fitting process.
Degree
Ph.D.
Subject Area
Materials science
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