PREPARATION AND PROPERTIES OF THICK FILM RESISTORS CONTAINING CADMIUM GLASSES AND CADMIUM OXIDE

The electrical properties of resistors comprising finely divided CdO and cadmium glass, were investigated. The change in a temperature coefficient of resistance from about −500 ppm/℃ to


INTRODUCTION
Thick film resistors are usually produced by using noble metals.We have tried to prepare the resistors either without or with only small percentages of noble materials.In accordance with a model of a conductance mechanism in a ceramic type of resistor, we have proposed using a strongly degenerated semiconductor, cadmium oxide, as the conductive material.The problem of finding suitable glasses was solved by using cadmium glasses.

ELECTRICAL PROPERTIES OF RESISTORS 2.1 Preparation of Thick Film Resistors
Compositions used for the preparation of resistors contained, as a solid ingredient, a mixture consisting essentially of 50% to 100% by weight of finely divided cadmium glass frit, 0 to 50% CdO powder, 0 to 6% further additions and also a liquid vehicle.
The glasses from the system Na20-BaO-CdO-PbO-B2 O3 2 were selected.The glasses used were melted in an electrical furnace in platinum-rhodium crusibles at the temperature of 1300 C. Molten glasses were fritted into distilled water.Glass frit was ground in an agate planet mill into grain sizes smaller than 5/zm.The glasses marked EV 6, and EV 12 contain CdO amounting to 26 mole % and 35 mole %, respectively.The EV 21 glass differs from EV 12 glass, especially in zero content of Na: O.The resistor compositions were applied by a 29 stencil screen printing method onto alumina sub- strates which were then fired in a furnace with a suitable temperature schedule at peak temperatures ranging from 600 C to 760 C.

Resistivity of Resistors
An interesting dependence of resistivity of resistors on temperature was observed in a firing cycle, showing a structure at temperatures of 300C, 440C, 530C and a fiat maximum at 7000C when heated (Figure 1). to be associated with a presence of further crystalline phases (except CdO) in the layer as the following analyses demonstrated.
Measurements o f resistance changes taken at the temperature of 120C after 1000 hours showed only a slight variation (--0.2%) in resistor resistance between firing temperatures of 680 and 760C, as shown by curve 1 in Figure 2. Curve 80% EV 6 glass + 20% CdO; curve 2-80% EV 12 glass + 15.4% CdO + 4.6% Bi Ru O The resistivity vs. firing temperature plot exhibits an outstanding maximum at 700C for resistors containing a high percentage of EV 6 glass (about 80% or more) as shown in Figure 3.For resistors containing a smaller amount of EV 6 glass (about 70%) (curve 2) or containing EV 12 glass as well as EV 21 glass, the maximum does not appear; the resistivity having a much lower dependence on peak firing temperature, especially in the range from 690C to 760C.Firing temperature, C FIGURE 3 Sheet resistance as a function of firing tem- perature (1) 80% EV 6 + 20% CdO; (2) 70% EV 6 + 30% CdO.
By using various kinds of glasses and a suitable peak firing temperature (8 min at peak), it is possible to change the resistivity from about kohm/sq, to Mohm/sq.using resistors of about 10 pm thickness.The resistors containing under 60% glass are not usually available owing to their lower stability.

Temperature Coefficient of Resistance
The temperature dependence of resistivity below 200C is linear (Figure 1) with a constant tempera- ture coefficient of resistance (TCR) of about --500 ppm/ C, disregarding the kind of glass.This value was reduced by adding further compounds to the resistor composition.
The addition of CdC12 reduced the TCR only to -200 ppm/C.Better results were achieved by the addition of Bi2 Ru2 Or. Figure 4a (curve 1) shows a strong change in the TCR of resistors fired at 730C with variations in the amount of the above mentioned compound.The resistor composition contained 80% by weight of EV 12 glass and 20% by weight of a powder mixture of solid CdO and Bi2 Ru2 Or.The minimum of the TCR (about -50 ppm/C) was reached at 4.6% by weight of the compound.These resistors are sufficiently stable at 120 C as shown by curve 2 in Figure 2.
The TCR can be controlled very well from -500 ppm/C up to +70 ppm/C by the addition of Pb2 Ru2 06 to the resistor composition com- prising EV 21 glass as shown in Figure 4 (dashed lines).The zero value of the TCR was achieved at 3.8% by weight of this compound.
The further reduction in the TCR can be reached by using a higher peak firing temperature as illustrated in Figure 4b for both compounds in two amounts.

ANALYSIS OF SPECIAL CADMIUM GLASSES WITH CADMIUM OXIDE
The above given results of measurements of electrical properties of thick film resistors can be explained by the formation and presence of crystalline phases in thermally treated cadmium glasses with additions as following analyses showed.The thermal treatment temperatures of samples were chosen according to the distribution of the peaks along the curve of differential thermal analysis of the basic EV 6 glass as shown in Figure 5.The main D.T.A. peaks coincide well with the "peaks" observed in the temperature dependence of resistivity presented in Figure 1.Specimens were investigated for crystallinity using an X-ray powder diffraction technique.
The ascertained crystalline phases are listed in Table I.
Crystalline phases marked "A", "B" and "C" have not been identified yet.
The values d of the most intensive X-ray dif- fraction interplanar spacings are shown in Table II.
Scanning electron micrographs of the thick film cadmium resistors are shown in Figure 6.
The influence of the temperature treatment on the formation or destruction/dissolving of the crystalline phases is evident from Figure 6.

TABLE
Crystalline phases in the thermally treated EV 6 glass and in the mixture of EV 6 glass with CdO addition.
examinations carried out proved that the most stabilized resistor films could be obtained from a mixture of cadmium glasses and CdO by a proper thermal treatment of the films at the temperatures of 700C to 730C, at which, in most cases, only a fine crystalline CdO phase was determined.All other crystalline phases, such as BaO, "A", "B", "C", were found to influence unfavourably the electric stability of thick film resistors which were prepared.
The analyses of the cadmium glasses with addition of CdO and ruthenium compounds will be published in future.

FIGURE 2
FIGURE 2 Resistance change at 120C after 1000 hours.

FIGURE 4
FIGURE 4 Dependence of TCR (a) on addition of Bi Ru O or Pb Ru 20 (dashed lines); (b) on firing temperature.

FIGURE 5
FIGURE 5 Differential thermal analysis of EV 6 glass.