Electrical Resistance Compositions and Methods of Making the Same

The present invention concerns electrical resistance elements and, in particular, compositions for making electrical resistance elements and methods of making the same.

Electrical resistance elements formed from certain compositions are particularly useful in producing microminiature circuitry for the electronics industry wherein electronic elements (or pastes) are screen printed onto substances.

U.S. Pat. No. 3,304,199 describes an electrical resistance element composed of a mixture of RuO.sub.2 or IrO.sub.2 and lead borosilicate glass. The mixture is combined with a vehicle, e.g., organic screening agent, such as ethyl cellulose dissolved in acetone-toluene. The resultant mixture containing the vehicle is applied onto a nonconductive substrate and then air fired.

U.S. Pat. No. 3,324,049 describes a cermet resistance material comprising 40 to 99 weight percent of a lead borosilicate glass, 0.5 to 20 weight percent of a noble metal such as Ag, Au, Pd, Pt, Rh, Ir, Os or Ru and 0.5 to 40 weight percent MnO.sub.2 or CuO. The resultant resistance material is then fired in air.

U.S. Pat. No. 3,655,440 concerns a resistance composition including RuO.sub.2, IrO.sub.2 or PdO, a lead borosilicate glass vitreous binder and an electrically nonconductive crystal growth controlling agent, e.g., alumina comprising submicron inert particles. Such resistance composition is air fired at 975.degree. C. to 1025.degree. C. for 45 minutes to 1 hour.

U.S. Pat. No. 3,682,840 concerns electrical resistor compositions containing lead ruthenate and mixtures thereof with RuO.sub.2, in conjunction with lead borosilicate binders.

U.S. Pat. No. 4,065,743 concerns a vitreous enamel resistor containing a glass frit and conductive particles. Such conductive particles include tin oxide and tantalum oxide.

U.S. Pat. No. 4,101,708 is directed to printable compositions of finely divided powder in an inert liquid vehicle for producing film resistors adherent to a dielectric substrate, such compositions including RuO.sub.2, glass containing PbO, Nb.sub.2 O.sub.5, CaF.sub.2 and an inert vehicle.

German Patentschrift 21 15 814 concerns a resistance paste for air firing on a ceramic. Such resistance paste includes BaRuO.sub.3, SrRuO.sub.3 and CaRuO.sub.3 in a lead borosilicate glass.

Resistor compositions have been made using Ag-Pd and/or PdO, RuO.sub.2, IrO.sub.2, and the so-called "du Pont" pyrochlores. The pyrochlore structures are complex oxides with the general formula A.sub.2 B.sub.2 O.sub.6-7 where the large cation A is in eightfold coordination and the smaller B cation is octahedrally coordinated. Their success is largely based on their stability in variable atmospheres (reducing) and their ability for handling multisubstitution of elements to alter electrical properties. Examples of pyrochlores specifically used in these compositions and discussed in U.S. Pat. Nos. 3,553,109; 3,560,410 and 3,583,931 (all of these patents involve lead borosilicate binders) include Bi.sub.2 Ru.sub.2 O.sub.7 and Pb.sub.2 Ru.sub.2 O.sub.7-x where O<x<1.

The resistivities of various precious metal oxides (including primarily pyrochlores and some perovskites) were tabulated by Bube, K., Proceedings of Inter. Microel. Symp., Oct. 30-Nov. 1, 1972, Washington, D.C., ISHM, as follows:

The perovskite crystal structure was described in Goldsmith, U. M., Skrifter Norske Videnskaps--Akad., Oslo, I: Mat. Nuturv.Kl. 2:8 (1926). In the perovskite composition of ABO.sub.3 the A cation is in twelve-fold coordination with oxygen and the smaller B cation is in octahedral coordination. This perovskite structure is one of high lattice energy and is generally a very stable structure.

Resistance compositions have been applied in screen printing techniques requiring firing in an oxidizing (air) atmosphere which necessitated the use of expensive noble metals such as Au, Ag, Pt and Pd. Less expensive copper as a base metal could not be employed since copper easily oxidizes. Accordingly, there is a need for a stable copper compatible resistance composition that could be fired in non-oxidizing atmospheres, e.g., nitrogen.

Typical previously employed resistance compositions utilized lead borosilicate glass binders. After firing in air, resistance compositions including, for example, strontium ruthenate in a lead borosilicate binder, the strontium would decompose to strontium oxide, which dissolves into the binder, and ruthenium oxide. In the present invention when, for example, strontium ruthenate in a strontium borosilicate binder is fired in nitrogen, there is no decomposition of the conductive component, i.e., the strontium ruthenate remains unchanged.

One object of the present invention is to provide stable copper compatible resistance compositions that can be fired in non-oxidizing atmospheres.

Another object of the present invention is to provide a thick film resistor system which exhibits property reproducibility and reduced processing sensitivity.

The present invention concerns a composition for making electrical resistance elements composed of an electrically conductive component and a binder component.

The conductive component includes a precious metal oxide of the formula A'.sub.1-x A".sub.x B'.sub.1-y B".sub.y O.sub.3, wherein when A' is Sr, A" is one or more of Ba, La, Y, Ca and Na, and when A' is Ba, A" is one or more of Sr, La, Y, Ca and Na; B' is Ru; B" is one or more of Ti, Cd, Zr, V and Co; O<x<0.2; and 0<y<0.2.

The binder component includes:

between 40 weight percent and 75 weight percent C', wherein C' is SrO when A' is Sr, C' is BaO when A' is Ba and C' is SrO+BaO when A' is Sr and A" is Ba and when A' is Ba and A" is Sr;

between 20 weight percent and 35 weight percent B.sub.2 O.sub.3,

between 2 weight percent and 15 weight percent SiO.sub.2, and

between 0.5 weight percent and 6.5 weight percent ZnO.

The present invention also concerns a method of preparing a composition for making an electrical resistor. Such method includes combining a conductive component of the formula A'.sub.1-x A".sub.x B'.sub.1-y B".sub.y O.sub.3 wherein when A' is Sr, A" is one or more of Ba, La, Y, Ca and Na, and when A' is Ba, A" is one or more of Sr, La, Y, Ca and Na; B' is Ru; B" is one or more of Ti, Cd, Zr, V and Co; O<x<0.2; O<y<0.2; a binder having 40 to 75 weight percent C' (C' as defined hereinabove), 20 to 35 weight percent B.sub.2 O.sub.3, 2 to 15 weight percent SiO.sub.2, and 0.5 to 6.5 weight percent ZnO, and an organic vehicle to form a paste.

The binder component can also include between 0.1 and 2.5 weight percent Al.sub.2 O.sub.3.

The binder component can further include between 0.1 weight percent and 1.5 weight percent each of one or more of Bi.sub.2 O.sub.3, CuO, MgO, or Nb.sub.2 O.sub.5.

The binder component can also further include between 0.1 weight percent and 1.5 weight percent TiO.sub.2 or NaF. The binder component may also further include between 5 weight percent and 15 weight percent CaO.

The compositon for making electrical resistance elements of the present invention includes a conductive metal oxide perovskite component and a glass binder component.

The conductive component is represented by the formula A'.sub.1-x A".sub.x B'.sub.1-y B".sub.y O.sub.3 wherein when A' is Sr; A" is one or more of Ba, La, Y, Ca, and Na, and when A' is Ba, A" is one or more of sr, La, Y, Ca and Na; B' is Ru; B" is one or more of Ti, Cd, Zr, V and Co; O<x<0.2; and O<y<0.2. Preferred combinations of B'.sub.1-y B".sub.y include Ru.sub.0.8 Ti.sub.0.2 and Ru.sub.0.9 Ti.sub.0.1. Preferred conductive components include SrRu.sub.0.8 Ti.sub.0.2 O.sub.3, SrRuO.sub.3 and SrRu.sub.0.9 Ti.sub.0.1 O.sub.3. Combinations of these components may also be used, such as SrRuO.sub.3 +SrRu.sub.0.8 Ti.sub.0.2 O.sub.3 or SrRuO.sub.3 +SrRu.sub.0.9 Ti.sub.0.1 O.sub.3. Other non-limiting examples of conductive components include SrRu.sub.0.95 Cd.sub.0.05 O.sub.3, Sr.sub.0.90 Na.sub.0.10 RuO.sub.3, Sr.sub.0.90 Y.sub.0.10 RuO.sub.3, Sr.sub.0.80 Na.sub.0.10 La.sub.0.10 RuO.sub.3 and SrRu.sub.0.8 Ti.sub.0.2 O.sub.3 /SrRuO.sub.3, SrRu.sub.0.8 Zr.sub.0.2 O.sub.3, SrRu.sub.0.9 Zr.sub.0.1 O.sub.3, SrRu.sub.0.75 V.sub.0.25 O.sub.3 and SrRu.sub.0.8 Co.sub.0.2 O.sub.3.

The formula A'.sub.1-x A".sub.x B'.sub.1-y B".sub.y O.sub.3 can be altered by partial substitutions of A, B or A and B (A is A'+A"; B is B'+B"), such as described above and by using other substitutions. Non-limiting examples of substitutions (based on ionic radii and valency) on the A or B sites are as follows:

The binder component of the present invention has as its major constituents C', i.e., SrO or BaO or SrO+BaO; B.sub.2 O.sub.3 ; SiO.sub.2 ; and ZnO in the following amounts:

Additionally, the binder component may have included therein one or more of the following constituents:

Non-limiting examples of preferred binder component formulations include the following:

Examples of other non-limiting examples of binder formulations include the following:

The weight percent loading of binder component/conductive component can vary from 25 wt.% to 75 wt.% binder/75 wt.% to 25 wt.% conductive component, i.e., wt.% binder can be, for example, 30 wt.%, 35 wt.%, 40 wt.%, 50 wt.%, 60 wt.%, 65 wt.% and 70 wt.%.

The binder component and conductive component are mixed together with a suitable "organic vehicle". An organic vehicle is a medium which volatilizes at a fairly low temperature (approximately 400.degree. C.-500.degree. C.), without causing reduction of other paste components. An organic vehicle acts as a transfer medium for screen printing. An organic vehicle for use in the present invention is preferably a resin, e.g., an acrylic ester resin, preferably an isobutyl methacrylate, and a solvent, e.g., an alcohol, preferably tri-decyl alcohol ("TDA"). The resin can be any polymer which depolymerizes at or below 400.degree. C. in nitrogen. Other solvents that can be employed are terpineol or "TEXANOL" of Eastman Kodak. The solvent for utilization in the present invention can be any solvent which dissolves the respective resin and which exhibits a suitable vapor pressure consistent with subsequent milling and screen printing. In a preferred embodiment, the organic vehicle is 10 to 30 weight percent isobutyl methacrylate and 90 to 70 weight percent TDA.

The binder component, conductive component and organic vehicle are mixed, screen printed on Cu termination on a suitable substrate, e.g., 96% Al.sub.2 O.sub.3 and then fired in a nitrogen atmosphere at a high temperature, e.g., 900.degree. C., for a suitable period of time, e.g., 7 minutes.

In preparing compositions for making electrical resistance elements according to the present invention, the conductive component, binder component and organic vehicle are combined to form a paste. The paste is then milled to the required fineness for screen printing techniques.

Without wishing to be bound by any particular theory of operability, it is believed that the binder component (glass matrix) of the present invention prevents decomposition of the conductive component during firing, i.e., the crystal structure (physical) and chemical composition of the conductive component remains stable and unchanged during firing.

Binders were synthesized utilizing reagent grade raw materials, each in the oxide form with the exception of strontium, barium and copper compounds which were in the carbonate form. When the composition was formulated, the individual components were weighed and homogenized for one (1) hour in a V-blender (which is a dry blending operation). After the blending was complete, the homogenized powders were poured into kyanite crucibles in which they would be subsequently melted. The binders were preheated for one (1) hour at 600.degree. C. and then transferred to another furnace where they were melted typically in range of 1100.degree. C. to 1300.degree. C. for 1 to 1.5 hours. The molten material was removed from the furnace at the melting temperature and poured (fritted) into stainless steel buckets filled with deionized water. As the molten stream made contact with the water, solidification and disintegration into glass chunks (size dictated by thermal stresses) occurred. The deionized water was decanted and the glass was placed in a ceramic jar mill with alumina grinding cylinders and an isopropyl alcohol medium. The glasses were ball milled for 24 hours and then wet-sieved through a 200 mesh screen. After drying in a room temperature convection explosion-proof oven, the powders were ready for characterization and incorporation into resistor pastes. The powders ranged in particle size from 1 to 2 .mu.m. Binders prepared as described in the foregoing procedure are those previously identified as Formulations I, II and III. The softening points for Formulations I, II and III were found to be, respectively, 625.degree. C., 635.degree. C. and 660.degree. C. Other binder formulations prepared according to Example 1 include the following:

Conductive components were prepared by formulating the respective compound (e.g., SrRuO.sub.3), calculating the equimolar amounts of, for example, SrCO.sub.3 and RuO.sub.2 which must be weighted in order to ensure stoichiometry, and finally weighing the individual components. Correction factors for Ru metal content, water content, and other volatile components lost on ignition at 600.degree. C. are also incorporated into the calculation. A similar correction factor for loss on ignition was incorporated into calculations for the weights of other components, if necessary. The RuO.sub.2 had a surface area greater than 70 m.sup.2 /g, while the other constituents were less than 5 m.sup.2 /g. The weighed raw materials were ball milled for two (2) hours in ceramic jar mills with alumina grinding media and deionized water, thus, creating a wet milling process. After 2 hours, the homogenized slurry was poured into stainless steel trays and dried for 24 hours at 80.degree. C. The dried blend was passed through an 80 mesh screen prior to calcination.

The meshed powders were calcined in high purity alumina crucibles (99.8% purity) with the cycle being precisely microprocessor-controlled. The heat-up and cool-down rates were not per se critical, but were generally 500.degree. C./hour. The hold times at the respective temperatures (from 800.degree. C. to 1200.degree. C. depending on the compound) varied from one (1) hour to two (2) hours. When the calcination was complete, the powders were milled in a Sweeco-vibratory mill for two (2) hours. This is a high energy milling procedure which utilized alumina grinding media and an isopropyl alcohol medium. The perovskites were wet sieved (200 mesh) at the end of the cycle, dried at room temperature in a convection oven (explosion proof), and prepared for characterization and incorporation into resistor pastes.

Conductive components prepared according to the foregoing procedure included the following:

The binders as prepared in accordance with Example 1 hereinabove were combined with conductive components prepared in accordance with Example 3 hereinabove, along with an organic vehicle. The organic vehicle utilized was "ACRYLOID" B67 a resin (an isobutyl methacrylate) produced by Rohm & Haas of Philadelphia, Pa., and tri-decyl alcohol ("TDA") in a 30/70 wt.% ratio.

The respective binders, conductive components, and organic vehicle were weighted to make the desired paste blends. The solids content (binder plus conductive phase) was maintained at 70 wt.% of the total paste weight. The pastes were three-roll milled to a fineness of grind of <10 .mu.m. Resistor test patterns were screen printed with the following print thicknesses: wet, 29-32 .mu.m; fired, 10-13 .mu.m. The pastes were then printed through either a 325 mesh screen with 0.6 mil-emulsion or a 280 mesh screen with a 0.5 mil-emulsion. The wet prints were dried at 150.degree. C. for 5-10 minutes prior to firing.

The firing profile was dependent on the binder constituent. For example, pastes containing Formulation I were fired at 850.degree. C., while Formulations II and III were fired at 900.degree. C. The 850.degree. C. profile length was 58 minutes from 100.degree. C. to 100.degree. C., i.e., from furnace entrance to furnace exit. The heating rate was 45.degree. C./minute, the cooling rate was 60.degree. C./minute, and the dwell time at peak temperature was 10 minutes. The 900.degree. C. profile had a duration of 55 minutes from 100.degree. C. to 100.degree. C., a heating rate of 50.degree. C./minute, and a cooling rate of 60.degree. C./minute. The time at peak temperature was varied from 5 to 14 minutes.

Various combinations of the aforementioned binder formulations and conductive components to form resistor elements and their resultant properties after firing in nitrogen are given hereinbelow in Tables I and II. In Table II, nitrogen firing in 850.degree. C. was utilized. BASIC ELECTRICAL PROPERTIES OF RESISTORS FIRED IN NITROGEN WT % LOADING .rho.s (CV) HTCR LASER TRIM 85.degree. C./85% RH 150.degree. C. Calc. Firing BINDER/COND. (.OMEGA./.quadrature.) CTCR .DELTA.TCR VCR DRIFT (24 hr) % .DELTA.R % .DELTA.R Conds. Temp. Noise BINDER//COMP. COMP. (ppm/.degree.C.) (ppm/.degree.C.) (ppm/.degree.C.) (ppm/V/inch) % .DELTA.R (120 hr) (120 hr) (.degree.C./hr) (.degree.C.) (dB) FORMULATION I//COMPOSITION II 500 (6) +400 +390 10 1000/2 850 -4 60//40 FORMULATION I//COMPOSITION II/ 5k (8) +290 +280 10 -5 0.4 .2 .5 1000/2 850 +5 COMPOSITION I 1200/2 60//30//10 FORMULATION I//COMPOSITION II/ 20K (7) +140 +125 15 -20 0.5 .2 .5 1000/2 850 +13.7 COMPOSITION I 1200/2 60//25/15 FORMULATION I//COMPOSITION I 500k (12) -120 -200 80 1200/2 850 60//40 FORMULATION I//COMPOSITION II 120 1000/2 850 -14.4 30//70 FORMULATION I//COMPOSTION II 900k (35) +250 +290 40 1000/2 850 70//30 FORMULATION III//COMPOSITION III 2k (8) +650 +750 100 800/1 900 60//40 FORMULATION III//COMPOSITION II 3.5k (12) +485 +415 70 -8 1.00 1.29 .12 1000/2 900 60//40 FORMULATION III//COMPOSITION II 900 (8) +750 +675 75 .15 .57 .20 1000/2 900 -4.5 50//50 FORMULATION III//COMPOSITION II 95 (2) +900 +650 250 1000/2 900 30//70 FORMULATIO N III//COMPOSITION I 45k (4) -275 -550 275 1200/2 900 +11.3 70//30 FORMULATION III//COMPOSITION I 40k (4) 1200/2 900 -14.7 60//40 FORMULATION III//COMPOSITION I 20k (9) -300 -560 260 1200/2 900 +9.0 50//50 FORMULATION III//COMPOSITION I 11k (4) -240 -490 250 1200/2 900 +8.2 40//60 FORMULATION III//COMPOSITION I 400 (5) +180 + 380 200 1200/2 900 -12.5 30//70 FORMULATION III//COMPOSITION I 62k (2) -230 -440 210 1200/2 900 +6.4 60//40 FORMULATION III//COMPOSITION I 20k (3) -185 -360 175 1200/2 900 -15.5 40//60 FORMULATION III//COMPOSITIO N IV 70k (11) +70 +50 20 1200/2 900 70//30 FORMULATION III//COMPOSITION IV 37k (6) +60 +30 30 1200/1 900 6.8 60//40 FORMULATION III//COMPOSITION IV 19k (3) +70 +50 20 1200/1 900 5.4 50//50 FORMULATION III//COMPOSITION IV 2.6k 1200/1 900 -1.8 40//60 FORMULATION III//COMPOSITION IV 147 (2) +600 +630 30 1200/1 900 30//70 FORMULATION III//COMPOSITION II 120 (4) +750 +875 125 1200/2 900 35//65 FORMULATION III//COMPOSITI ON II 2.5k (10) +350 +400 50 1000/2 900 40//60 FORMULATION III//COMPO SITION II 20k +190 +250 60 1000/2 900 +9 50//50 FORMULATION III//COMP OSITION II 100k (10) +80 +90 10 -20 1000/2 900 60//40 FORMULATION III//COMPOSITION V 320 (1) +615 +540 75 1200/2 900 30//70 GLOSSARY .rho.s = sheet resistance HTCR = hot temperature coefficient of resistance CTCR = cold temperature coefficeint of resistance TCR = temperature coefficient of resistance .DELTA.TCR = the absolute difference of HTCR and CTCR VCR = voltage coefficient of resistance (CV) = coefficient of variation expressed as percent

It will be appreciated that the instant specification and claims are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.