Resistor Compositions

The invention relates to thick film resistor compositions and especially those which are fireable in low oxygen-containing atmospheres.

Screen printable resistor compositions compatible with nitrogen (or low oxygen partial pressure) fireable conductors are relatively new in the art of thick film technology.

Thick film resistor composites generally comprise a mixture of electrically conductive material finely dispersed in an insulative glassy phase matrix. Resistor composites are then terminated to a conductive film to permit the resultant resistor to be connected to an appropriate electrical circuit.

The conductive materials are usually sintered particles of noble metals. They have excellent electrical characteristics; however, they are expensive. Therefore, it would be desirable to develop circuits containing inexpensive conductive materials and compatible resistors having a range of stable resistance values.

In general, nonnoble metal conductive phases such as Cu, Ni, Al, etc. are prone to oxidation. During the thick film processing, they continue to oxidize and increase the resistance value. However, they are relatively stable if the processing can be carried out at low oxygen partial pressure or "inert" atmosphere. As used herein, low oxygen partial pressure is defined as the oxygen partial pressure that is lower than the eqilibrium oxygen partial pressure of the system consisting of the metal conductive phase and its oxide at the firing temperature. Therefore, developed of compatible resistor functional phases which are capable of withstanding firing in a low oxygen partial pressure without degradation of properties is the prime objective in this technology. The phases must be thermodynamically stable after the processing of the resistor film and noninteractive to the nonprecious metal terminations when they are cofired in an "inert" or low oxygen partial pressure atmosphere. The major stability factor is the temperature coefficient of resistance (TCR). The materials are considered stable when their resistance values do not change appreciably when the resistor components are subjected to temperature changes.

In its primary aspect, the invention is directed to a thick film resistor composition for firing in a low oxygen-containing atmosphere comprising finely divided particles of (a) a semiconductive material consisting essentially of a refractory metal carbide, oxycarbide or mixture thereof; and (b) a nonreducing glass having a softening point below that of the semiconductive material, dispersed in (c) organic medium.

In a second aspect, the invention is directed to a resistor element comprising a printed layer of the above-described composition which has been fired in a low oxygen-containing atmosphere to effect volatilization of the organic medium and liquid phase sintering of the glass.

Huang et al. in U.S. Pat. No. 3,394,087 discloses resistor composition comprising a mixture of 50-95% wt. vitreous glass frit and 50-5% wt. of a mixture of refractory metal nitride and refractory metal particles. Disclosed are nitrides of Ti, Zr, Hf, Va, Nb, Ta, Cr, Mo and W. The refractory metals include Ti, Zr, Hf, Va, Nb, Ta, Cr, Mo and W. U.S. Pat. No. 3,503,801 Huang et al. disclose a resistor composition comprising a vitreous glass frit and fine particles of Group IV, V or VI metal borides such as CrB.sub.2, ZrB.sub.2, MoBr.sub.2, TaB.sub.2 and TiB.sub.2. In U.S. Pat. No. 4,039,997 to Huang et al. a resistor composition is disclosed comprising 25-90 wt. % borosilicate glass and 75-10 wt. % of a metal silicide. Disclosed metal silicides are WSi.sub.2, MoSi.sub.2, VaSi.sub.2, TiSi.sub.2. ZrSi.sub.2, CaSi.sub.2 and TaSi.sub.2. Boonstra et al. in U.S. Pat. No. 4,107,387 disclose a resistor composition comprising a metal rhodate (Pb.sub.3 Rh.sub.7 O.sub.15 or Sr.sub.3 RhO.sub.15), glass binder and a metal oxide TCR driver. The metal oxide corresponds to the formula Pb.sub.2 M.sub.2 O.sub.6-7, wherein M is Ru, Os or Ir. Hodge in U.S. Pat. No. 4,137,519 discloses a resistor composition comprising a mixture of finely divided particles of glass frit and W.sub.2 C.sub.3 and WO.sub.3 with or without W metal. Shapiro et al. in U.S. Pat. No. 4,168,344 disclose resistor compositions comprising a mixture of finely divided particles of glass frit and 20-60% wt. Ni, Fi and Co in the respective proportions of 12-75/5-60/5-70% vol. Upon firing, the metals form an alloy dispersed in the glass. Again, in U.S. Pat. No. 4,205,298, Shapiro et al. disclose resistor compositions comprising a mixture of vitreous glass frit having fine particles of Ta.sub.2 N dispersed therein. Optionally the composition may also contain fine particles of B, Ta, Si, ZrO.sub.2 and MgZrO.sub.3. Merz et al. in U.S. Pat. No. 4,209,764 disclose a resistor composition comprising a mixture of finely divided particles of vitreous glass frit, Ta metal and up to 50% wt. Ti, B, Ta.sub.2 O.sub.5, TiO.sub.2, BaO.sub.2, ZrO.sub.2, WO.sub.3, Ta.sub.2 N, MoSi.sub.2 or MgSiO.sub.3. In U.S. Pat. No. 4,215,020, to Wahlers et al. a resistor composition is disclosed comprising a mixture of finely divided particles of SnO.sub.2, a primary additive of oxides of Mn, Ni, Co or Zn and a secondary additive of oxides of Ta, Nb, W or Ni. The Kamigaito et al. patent, U.S. Pat. No. 4,384,989, is directed to a conductive ceramic composition comprising BaTiO.sub.3, a doping element such as Sb, Ta or Bi and an additive such as SiN, TiN, ZrN or SiC, to lower the resistivity of the composition. Japanese patent application No. 58-36481 to Hattori et al. is directed to a resistor composition comprising Ni.sub.x Si.sub.y or Ta.sub.x Si.sub.y and any glass frit (" . . . there is no specification regarding its composition or method of preparation.").

The compositions of the invention are directed to heterogeneous thick film compositions which are suitable for forming microcircuit resistor components which are to undergo firing in a low oxygen-containing atmosphere. As mentioned above, the low oxygen atmosphere firing is necessitated by the tendency of base metal conductive materials to be oxidized upon firing in air. The resistor compositions of the invention therefore contain the following three basic components: (1) one or more semiconductive materials; (2) one or more metallic conductive materials or precursors thereof; and (3) an insulative glass binder, all of which are dispersed in (4) an organic medium.

The resistance values of the composition are adjusted by changing the relative proportions of the semiconductive, conductive and insulative phases present in the system. Supplemental inorganic materials may be added to adjust the temperature coefficient of resistance. After printing over alumina or similar ceramic substrates and firing in low oxygen partial pressure atmosphere, the resistor films provide a wide range of resistance values and low temperature coefficient of resistance depending on the ratio of the functional phases.

The semiconductive materials which may be used in the compositions of the invention are refractory metal carbides (MeC.sub.x), oxycarbides (MeC.sub.y-x O.sub.x, where y=1-3 and x<1.) or mixtures thereof. In particular, suitable refractory metals are Si, Al, Zr, Hf, Ta, W and Mo. Of the refractory metals, Si is preferred because silicon carbide is widely available in commercial quantities.

Silicon carbide is a semiconductor with a large band gap of nearly 3 ev for hexagonal structure and 2.2 ev for the cubic modification. Details are given in Proc. Int. Conf. Semiconductor Phys., Prague, 1960, 432, Academic Press, Inc. 1961 and Proc. Conf. Silicon Carbide, Boston, 1959, 366, Pergamon Press, 1960. Small amounts of impurities, which are always present in the commercial sample, reduce the band gap. For example, if aluminum is the impurities, the SiC is a p-type conducting with an acceptor level lying about 0.30 ev above the valance band; and if nitrogen is the impurity, then the compound is n-type with a donor level lying about 0.08 ev below the conduction band. Details are given in J. Phys. Chem. Solids 24, 1963, 109 by H. J. Van Daal, W. F. Knippenberg and J. D. Wasscher.

Refractory metal carbides, in general, have a range of solid solubility, resulting in nonstiochiometric compositions with vacant lattice sites (e.g., Ta, Ti, Mo, W, etc.). The range of the solubility, structures, and phase compositions are summarized in Aerojet-General Corporation Report on "Ternary Phase Equilibria in Transition Metal-Boron-Carbon-Silicon System" dated Apr. 1, 1965. Carbides are interstitial compounds and are structurally different from their corresponding oxides. They always contain impurities such as nitrides, oxides and free carbon.

Industrial scale manufacture of SiC by the Acheson Process is described in various handbooks of chemical technology. The process involves heating a mixture of silica and carbon in accordance with a preselected temperature-time cycle. The major reactions that takes place upon heating the mixture are as follows:

Also, there is evidence in the literature of the formation of SiO, which further reduces to Si. It is considered that .alpha.-SiC is an impurity-stabilized form of silicon carbide (R. C. Ellis; Proc. Conf. Silicon Carbide, Boston, 1959, 124, Pergamon Press, 1960).

Fine powders of carbides and metal-doped carbides such as WC-6% Co were prepared by reduction-carburization of metal oxide gels using dry methane gas at 800.degree.-900.degree. C. The amorphous powder thus obtained can be crystallized by heating in an oxygen-free atmosphere at a higher temperature to obtain substantially pure carbides. Alternatively, by heating the amorphous powder in a low oxygen partial pressure atmosphere, oxycarbides are produced. Details were described at the 79th Annual meeting of the American Ceramic Society--Apr. 23-28, 1977, an abstract of which is given in M. Hoch and K. M. Nair, Bulletin American Ceramic Soc., 56, 1977, p. 289. Oxycarbides are also produced by heating a mixture of metal carbide with the corresponding metal oxide in a controlled oxygen atmosphere.

The third major component present in the invention is one or more of insulative phases. The glass frit can be of any composition which has a melting temperature below that of the semiconductive and/or conductive phases and which contains nonreducible inorganic ions or inorganic ions reducible in a controlled manner. Preferred compositions are alumino borosilicate glass containing Ca.sup.2+, Ti.sup.4+, Zr.sup.4+ ; alumino borosilicate glass containing Ca.sup.2+, Zn.sup.2+, Ba.sup.2+, Zr.sup.4+, Na.sup.+ ; borosilicate glass containing Bi.sup.3+, and Pb.sup.2+ ; alumino borosilicate glass containing Ba.sup.2+, Ca.sup.2+, Zr.sup.4+, Mg.sup.2+, Ti.sup.4+ ; and lead germanate glass, etc. Mixtures of these glasses can also be used.

During the firing of the thick film in a reducing atmosphere, inorganic ions reduce to metals and disperse throughout the system and become a conductive functional phase. Examples for such a system are glasses containing metal oxides such as ZnO, SnO, SnO.sub.2, etc. These inorganic oxides are nonreducible thermodynamically in the nitrogen atmosphere. However, when the "border line" oxides are buried or surrounded by carbon or organics, the local reducing atmosphere developed during firing is far below the oxygen partial pressure of the system. The reduced metal is either evaporated and redeposited or finely dispersed within the system. Since these fine metal powders are very active, they interact with or diffuse into other oxides and form metal rich phases.

The glasses are prepared by conventional glass making techniques, by mixing the desired components in the desired proportions and heating the mixture to form a melt. As is well known in the art, heating is conducted to a peak temperature and for a time such that the melt becomes entirely liquid and homogeneous. In the present work the components are premixed by shaking in a polyethylene jar with plastic balls and then melted in a crucible at up to 1200.degree. C., depending on the composition of the glass. The melt is heated at a peak temperature for a period of 1-3 hours. The melt is then poured into cold water. The maximum temperature of the water during quenching is kept as low as possible by increasing the volume of water to melt ratio. The crude frit after separation from water is freed from residual water by drying in air or by displacing the water by rinsing with methanol. The crude frit is then ball milled for 3-5 hours in porcelain containers using alumina balls. The slurry is dried and Y-milled for another 24-48 hours depending on the desired particle size and particle size distribution in polyethylene lined metal jars using alumina cylinders. Alumina picked up by the materials, if any, is not within the observable limit as measured by X-ray diffraction analysis.

After discharging the milled frit slurry from the mill, the excess solvent is removed by decantation and the frit powder is then screened through a 325 mesh screen at the end of each milling process to remove any large particles.

The major properties of the frit are: it aids the liquid phase sintering of the inorganic crystalline particulate matters; some inorganic ions present in the frit reduce to conductive metal particles during the firing at the reduced oxygen partial pressure; and part of the glass frit form the insensitive functional phase of the resistor.

Because the semiconductive resistor materials generally have quite high resistivities and/or highly negative HTCR (Hot Temperature Coefficient of Resistance) values, it will normally be preferred to include a conductive material in the composition. Addition of the conductive materials increases conductivity; that is, lowers resistivity and in some instances may change the HTCR value as well. However, when lower HTCR values are needed, various TCR drivers may be used. Preferred conductive materials for use in the invention are RuO.sub.2, Ru, Cu, Ni, and Ni.sub.3 B. Other compounds which are precursors of the metals under low oxygen containing firing conditions can also be used. Alloys of the metals are useful as well.

The above-described inorganic particles are mixed with an inert liquid medium (vehicle) by mechanical mixing (e.g., on a roll mill) to form a pastelike composition having suitable consistency and rheology for screen printing. The latter is printed as a "thick film" on conventional ceramic substrates in the conventional manner.

The main purpose of the organic medium is to serve as a vehicle for dispersion of the finely divided solids of the composition in such form that it can readily be applied to ceramic or other substrates. Thus, the organic medium must first of all be one in which the solids are dispersible with an adequate degree of stability. Secondly, the rheological properties of the organic medium must be such that they lend good application properties to the dispersion.

Most thick film compositions are applied to a substrate by means of screen printing. Therefore, they must have appropriate viscosity so that they can be passed through the screen readily. In addition, they should be thixotropic in order that they set up rapidly after being screened, thereby giving good resolution. While the rheological properties are of primary importance, the organic medium is preferably formulated also to give appropriate wettability of the solids and the substrate, good drying rate, dried film strength sufficient to withstand rough handling, and good firing properties. Satisfactory appearance of the fired composition is also important.

In view of all these criteria, a wide variety of liquids can be used as organic medium. The organic medium for most thick film compositions is typically a solution of resin in a solvent frequently also containing thixotropic agents and wetting agents. The solvent usually boils within the range of 130.degree.-350.degree. C.

By far, the most frequently used resin for this purpose is ethyl cellulose. However, resins such as ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins, polymethacrylates of lower alcohols, and monobutyl ether of ethylene glycol monoacetate can also be used.

Suitable solvents include kerosene, mineral spirits, dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene glycol, and high-boiling alcohols and alcohol esters. Various combinations of these and other solvents are formulated to obtain the desired viscosity and volatility.

Among the thixotropic agents which are commonly used are hydrogenated castor oil and derivatives thereof and ethyl cellulose. It is, of course, not always necessary to incorporate a thixotropic agent since the solvent/resin properties coupled with the shear thinning inherent in any suspension may alone be suitable in this regard. Suitable wetting agents include phosphate esters and soya lecithin.

The ratio of organic medium to solids in the paste dispersions can vary considerably and depends upon the manner in which the dispersion is to be applied and the kind of organic medium used. Normally, to achieve good coverage, the dispersions will contain complementally by weight 40-90% solids and 60-10% organic medium.

The pastes are conveniently prepared on a three-roll mill. The viscosity of the pastes is typically 20-150 Pa.s when measured at room temperature on Brookfield viscometers at low, moderate and high shear rates. The amount and type of organic medium (vehicle) utilized is determined mainly by the final desired formulation viscosity and print thickness.

The resistor material of the invention can be made by thoroughly mixing together the glass frit, conductive phases and semiconductive phases in the appropriate proportions. The mixing is preferably carried out by either ball milling or ball milling followed by Y-milling the ingredients in water (or an organic liquid medium) and drying the slurry at 120.degree. C. overnight. In certain cases, the mixing is followed by calcination of the material at a higher temperature, preferably at up to 500.degree. C., depending on the composition of the mixture. The calcined materials are then milled to 0.5-2.mu. or less average particle size. Such a heat treatment can be carried out either with a mixture of conductive and semiconductive phases and then mixed with appropriate amount of glass or semiconductive and insulative phases and then mixed with conductive phases or with a mixture of all functional phases. Heat treatment of the phases generally improves the control of TCR. The selection of calcination temperature depends on the melting temperature of the particular glass frit used.

To terminate the resistor composition onto a substrate, the termination material is applied first to the surface of a substrate. The substrate is generally a body of sintered ceramic material such as glass, porcelain, steatite, barium titanate, alumina or the like. A substrate of Alsimag.RTM. alumina is preferred. The termination material is then dried to remove the organic vehicle and fired in a conventional furnace or a conveyor belt furnace in an inert atmosphere, preferably N.sub.2 atmosphere. The maximum firing temperature depends on the softening point of the glass frit used in the termination composition. Usually this temperature varies between 750.degree. C. to 1200.degree. C. When the material cooled to room temperature, there is formed a composite of glass having particles of conductive metals, such as Cu, Ni, embedded in and dispersed throughout the glass layer.

To make a resistor with the material of the present invention, the resistance material is applied in a uniform-drying thickness of 20-25.mu. on the surface of the ceramic body which has been fired with the termination as described earlier. Compositions can be printed either by using an automatic printer or a hand printer in the conventional manner. Preferably the automatic screen printed techniques are employed using a 200-325 mesh screen. The printed pattern is then dried at below 200.degree. C., e.g. to about 150.degree. C. for about 5-15 minutes before firing. Firing to effect sintering of the materials and to form a composite film is preferably done in a belt furnace with a temperature profile that will allow burnout of the organic matter at about 300.degree.-600.degree. C., a period of maximum temperature of about 800.degree.-1000.degree. C. lasting about 5-30 minutes, followed by a controlled cooldown cycle to prevent unwanted chemical reactions at intermediate temperatures or substrate fracture of stress development within the film which can occur from too rapid cooldown. The overall firing procedure will preferably extend over a period of about 1 hour with 20-25 minutes to reach the firing temperature, about 10 minutes at the firing temperature, and about 20-25 minutes in cooldown. The furnace atmosphere is kept low in oxygen partial pressure by providing a continuous flow of N.sub.2 gas through the furnace muffle. A positive pressure of gas must be maintained throughout to avoid atmospheric air flow into the furnace and thus an increase of oxygen partial pressure. As a normal practice, the furnace is kept at 800.degree. C. and N.sub.2 or similar inert gas flow is always maintained. The above-described pretermination of the resistor system can be replaced by post termination, if necessary. In the case of post termination, the resistors are printed and fired before terminating.

In the Examples below, hot temperature coefficient of resistance (HTCR) is measured in the following manner:

Samples to be tested for Temperature Coefficient of Resistance (TCR) are prepared as follows:

A pattern of the resistor formulation to be tested is screen printed upon each of ten coded Alsimag 614 1.times.1" ceramic substrates and allowed to equilibrate at room temperature and then dried at 150.degree. C. The mean thickness of each set of dried films before firing must be 22-28 microns as measured by a Brush Surfanalyzer. The dried and printed substrate is then fired for about 60 minutes using a cycle of heating at 35.degree. C. per minute to 850.degree. C., dwell at 850.degree. C. for 9 to 10 minutes and cooled at a rate of 30.degree. C. per minute to ambient temperature.

The test substrates are mounted on terminal posts within a controlled temperature chamber and electrically connected to a digital ohm-meter. The temperature in the chamber is adjusted to 25.degree. C. and allowed to equilibrate, after which the resistance of each substrate is measured and recorded.

The temperature of the chamber is then raised to 125.degree. C. and allowed to equilibrate, after which the resistance of the substrate is again measured and recorded.

The hot temperature coefficient of resistance (TCR) is calculated as follows: ##EQU1##

The values of R.sub.25.degree. C. and Hot TCR are averaged and R.sub.25.degree. C. values are normalized to 25 microns dry printed thickness and resistivity is reported as ohms per square at 25 microns dry print thickness. Normalization of the multiple test values is calculated with the following relationship: ##EQU2##

The coefficient of variance (CV) is a function of the average and individual resistances for the resistors tested and is represented by the relationship .sigma./R.sub.av, wherein ##EQU3## R.sub.i =measured resistance of individual sample. R.sub.av =calculated average resistance of all samples (.SIGMA..sub.i R.sub.i /n)

n=number of samples

CV=(.sigma./R).times.100(%)

The invention will be better understood by reference to the following examples in which all compositions are given in percentages by weight unless otherwise noted.

In the Examples which follow, the following glass composition was used:

Using the formulation and testing procedures described above, a series of three resistor compositions was prepared in which various concentrations of SiC, a semiconductor, were used as the conductive phase in combination with Glass A. Furthermore, in Example 4, a small amount of AlOOH, a TCR driver, was substituted for part of the SiC as in the composition of Example 1. The composition of the formulations and the electrical properties of the resistors prepared therefrom are given in Table 2 below. The resistor data show that as SiC is used to replace glass, the very high resistance values are lowered only slightly and that the quite highly negative HTCR values become even more highly negative. In addition, it can be seen that the AlOOH functioned as a positive TCR driver in that the HTCR of Example 4 was considerably less negative than that of Example 1.

Again using the formulation and testing procedures described above, a series of three additional resistor compositions was prepared in which an organosilane ester was used to replace a progressively greater amount of the semiconductor. The organosilane ester readily decomposes during firing to form (SiO.sub.4).sup.4- tetrahedra which reacts with components of the glass binder.

The compositions of the formulations and the electrical properties of the resistors prepared therefrom are given in Table 3 below. These data show the inclusion of the silicon ester to replace part of the SiC resulted in slightly lower HTCR values, but the composition still had high resistance values.

A further series of three resistor compositions was formulated in which Ni.sub.3 B, a conductor, was added to the semiconductive SiC. The formulation also contained a small but constant amount of Al.sub.2 O.sub.3. The composition of the formulation and the electrical properties of the resistors prepared therefrom are given in Table 4 below.

Because Ni.sub.3 B is a conductor and SiC is only semiconductive, one would expect that the replacement of SiC with Ni.sub.3 B would result in significant lowering of the resistance values of the composition. However, quite surprisingly, this did not happen, for the resistance values of the composition were only slightly changed. The values of HTCR were little changed as well.