Borosilicate Glass Compositions

The invention relates to borosilicate glass compositions which are especially useful as the binder component for thick film resistor compositions.

Thick film materials are mixtures of metals, metallic oxides, glass and/or ceramic powders dispersed in an organic medium. These materials, which are applied to nonconductive substrates to form conductive, resistive or insulating films, are used in a wide variety of electronic and light electrical components.

The properties of such thick film compositions depend on the specific constituents of the compositions. Most of such thick film compositions contain three major components. A conductive phase determines the electrical properties and influences the mechanical properties of the final film. A binder, usually a glass and/or crystalline oxide, holds the thick film together and bonds it to a substrate, and an organic medium (vehicle) acts as a dispersing medium and influences the application characteristcs of the composition and particularly its rheology.

High stability and low process sensitivity are critical requirements for thick film resistors in microcircuit applications. In particular, it is necessary that resistivity (R.sub.av) of a resistor be stable over a wide range of temperature conditions. Thus, the thermal coefficient of resistance (TCR) is a critical variable in any thick film resistor. Because thick film resistor compositions are comprised of a functional (conductive) phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.

Glass is most frequently used as the inorganic binder for thick film resistors. Frequently used glass frits have been borosilicate frits, such as barium, calcium or other alkaline earth borosilicate frits. The preparation of such glass frits is well known and consists, for example, in melting together the constituents of the glass in the form of their oxides and pouring such molten composition into water to form the frit. The batch ingredients may, of course, be any compound that will yield the desired oxides under the usual conditions of frit production. For example, boric oxide will be obtained from boric acid; silicon dioxide will be produced from flint; barium oxide will be produced from barium carbonate; etc. The glass is preferably milled in a ball mill with water to reduce the particle size of the frit and to obtain a frit of substantially uniform size.

Despite the wide variety of glasses which have been used as a binder for thick film resistor materials, it has heretofore been difficult to manufacture thick film resistors over a wide range of resistance values using tin oxide pyrochlore-based resistor materials. In particular, there is a significant need for inorganic binder materials with which such pyrochlore-based materials can be formulated to make high end resistors, i.e., those having resistance values in the range of 50 k.OMEGA./.quadrature. to 100M .OMEGA./.quadrature..

The invention is, therefore, directed in its primary aspect to a glass composition which is substantially free of Bi, Cd and Pb comprised by mole %:

A. 50-85% of a material selected from the group consisting of 25-55% B.sub.2 O.sub.3, 15-40% SiO.sub.2, 0-10% ZrO.sub.2 and 0-5% SnO.sub.2 and mixtures thereof, and

B. 50-15% of a material selected from the group consisting of 15-30% BaO, 1-10% CoO and 0-12% alkaline earth metal oxides having atomic numbers 12-38, characterized further in that

(1) the mole ratio B.sub.2 O.sub.3 /SiO2.gtoreq.0.8; and

(2) .SIGMA. (B.sub.2 O.sub.3 +SiO.sub.2).gtoreq.50.

In a secondary aspect, the invention is directed to a screen-printable thick film resistor composition comprising a dispersion in organic medium of an admixture of finely divided particles of tin oxide pyrochlore-based electrically conductive material and the above-described glass composition.

In a still further aspect, the invention is directed to a resistor comprising a patterned thin layer of the above-described dispersion, which has been dried and fired in a nonoxidizing atmosphere to effect volatilization of the organic medium and liquid phase sintering of the inorganic binder.

In this Summary of the Invention as well as in the Detailed Description of the Invention which follows, all percentages are on a molar basis relative to the total glass composition unless otherwise indicated.

The glass composition of the invention is comprised basically of glass-forming materials and glass-modifying materials. By the term "glass-forming material" is meant a material which upon melting and cooling will form a glass by itself without the addition of other materials. The term "glass-forming material" also includes "conditional" glass-forming materials which become part of the glass network. By conditional glass forming material is meant material which upon melting and cooling will form a glass only if other metal oxides are present. Despite the fact that the materials in Group A are characterized as "glass formers", this does not mean that they are necessarily functioning as glass formers in these glass compositions. The term refers only to their general characterization in the art as being capable of glass formation under certain circumstances. Suitable glass-forming materials and conditional glass formers are B.sub.2 O.sub.3, SiO.sub.2, SnO.sub.2 and ZrO.sub.2. Of these, only B.sub.2 O.sub.3 and SiO.sub.2 are essential to all compositions of the invention.

The boron-containing glass former must constitute at least 25% of the glass composition in order that the viscosity of the glass be suitable for application in thick film resistor systems. However, it must not exceed 55% and preferably not more than 50% lest the humidity stability of the resistor in which it is used be too low.

Similarly, the silica component must be at least 15% of the glass in order that the glass has satisfactory durability and an appropriate coefficient of expansion for the particular ceramic substrate on which thick film resistor paste is used. Nevertheless, the amount of SiO.sub.2 should not exceed 40%. When higher amounts of SiO.sub.2 are used, the softening point of the glass may be elevated excessively and the TCR of the resultant resistor becomes too low.

Other glass-forming materials are not essential for all compositions of the invention; however, SnO.sub.2 and ZrO.sub.2 can be used in addition to B.sub.2 O.sub.3 and SiO.sub.2 to adjust the physical properties of the glass for particular applications. For example, SnO.sub.2 is added to lower the resistance of the resistor systems in which they are used. The amount of SnO.sub.2 used for this purpose is, however, limited by the comparatively low solubility of SnO.sub.2 in glass systems of the type described here. Similarly, while small concentrations of ZrO.sub.2 may be quite desirable to adjust TCR and improve durability, the amount of ZrO.sub.2 which can be used is likewise limited because of its low solubility in this system.

The amount of ZrO.sub.2 should not exceed 10% in order to avoid crystallization, 1-7% ZrO.sub.2 being preferred for most applications. Similarly, the amount of SnO.sub.2 should not exceed 5%, 1-4% being preferred.

In addition to the above-described limitations on the amount of each glass former or conditional glass former which can be used, it is essential that the glass-forming materials constitute 50-85% of the glass formulation. Also, the total amount of B.sub.2 O.sub.3 and SiO.sub.2 must constitute at least 50% of the glass composition and 60-70% is preferred. In addition, the mole ratio B.sub.2 O.sub.3 /SiO.sub.2 must be at least 0.8.

Suitable glass-modifying materials for use in the invention are CoO and the oxides of alkaline earth metals having atomic numbers of 12-56. Of these modifiers, both CoO and BaO are essential components of the invention. In particular, the compositions must contain 10-30% BaO and preferably 15-30% BaO. At least 10% BaO is needed to obtain a suitably low softening point but if more than 30% is used, the coefficient of expansion of the glass is affected adversely as is the stability of the glass as well. The other essential glass modifier is CoO, without which the compositions would not be capable of raising the resistance of tin oxide pyrochlore-based resistance materials while at the same time keeping TCR and CV within acceptable limits. In addition to the above-described essential glass modifiers, the compositions of the invention may contain up to 12% of oxides of alkaline earth metals having atomic numbers 12-38, i.e., Mg, Ca, Sr and mixtures thereof. It is preferred that they contain 3-10% of such alkaline earth metal oxides which, when substituted for part of the BaO, tend to raise the coefficient of expansion less than BaO.

Equally important as the concentration of the individual glass modifiers is the total concentration of all glass modifiers which must be within the range of 15-50% and preferably 25-35%.

It will be recognized that the glass compositions of the invention must not contain any materials which are reducible under the nonoxidizing conditions encountered in the use of these glasses in thick film resistor compositions. For this reason, reducible oxides such as Bi.sub.2 O.sub.3, CdO and PbO must not be present in the composition in any significant amount; that is, they may be present in only very small amounts since the reduced materials therefrom adversely effect the durability of the glass. In addition, the exclusion of these compounds also eliminates any toxicity which might arise from the presence in the glass of oxides of Pb, Bi and Cd.

It is particularly preferred that the binder be comprised of 95-99.9% by weight of the above-described bismuth-, cadmium- and lead-free glass and 5-0.1% wt. of a metal fluoride selected from the group consisting of CaF.sub.2, BaF.sub.2, MgF.sub.2, SrF.sub.2, NaF, LiF, KF and NiF.sub.2. The use of such metal fluorides with the frit produces a decrease in resistance of the resistors made therefrom.

The glasses of this invention are prepared with conventional glassmaking 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 were premixed by shaking in a polyethylene jar with plastic balls and then melted in a platinum crucible at the desired temperature. The melt was heated at a peak temperature of 1100.degree.-1400.degree. C. for a period of 1-1/2 hours and then poured into cold water. The maximum temperature of the water during quenching was kept low by increasing the water-to-melt volumetric ratio. After separation from water, the crude frit 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 3-15 hours in alumina containers using alumina balls. Residual 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, excess solvent is removed by decantation and the frit powder is dried in an oven at 110.degree..+-.10.degree. C. The dried powder is then screened through a 325-mesh screen to remove any large particles.

When the glass frits of the invention are used as the binder component of thick film resistor compositions, an important property of the frit is to aid the liquid phase sintering of the inorganic crystalline particulate materials and form noncrystalline (amorphous) or crystalline materials by devitrification during the heating-cooling cycle (firing cycle) in the preparation of such thick film resistors. This devitrification process can yield either a single crystalline phase having the same composition as the precursor noncrystalline (glassy) material or multiple crystalline phases with different compositions from that of the precursor glassy material.

The glasses of the invention may advantageously be admixed with the NiO-containing glasses of copending U.S. patent application Ser. No. 562,966, filed concurrently herewith, for use in such thick film resistor compositions.

The glass compositions of the invention have been found to be particularly effective in the SnO/SnO.sub.2 /Ta.sub.2 O.sub.5 /Nb.sub.2 O.sub.5 pyrochlore-based resistors described in applicant's U.S. patent application Ser. No. 460,572, filed Jan. 24, 1983, which is incorporated herein by reference. The conductive phase of resistors prepared therefrom are comprised of the above-referred pyrochlore-based material or the oxide precursors thereof and SnO.sub.2. These components are all sintered during the firing step to form the resistor element. As used herein, the term "conductive phase" refers to those solid components which are electrically conductive (e.g., the pyrochlore and the SnO.sub.2) or which become electrically conductive upon firing (e.g., the oxide precursors of the pyrochlore--SnO, SnO.sub.2 and Ta.sub.2 O.sub.5 or Nb.sub.2 O.sub.5.

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 a ceramic or other substrate. 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 wetability 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 inert liquids can be used as organic medium. The organic medium for most thick film compositions is typically a solution of resin in a solvent and, frequently, a solvent solution containing both resin and thixotropic agent. 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.

The most widely used solvents for thick film applications are terpenes such as alpha- or beta-terpineol or mixtures thereof with other solvents such as kerosene, 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 requirements for each application.

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.

The ratio of organic medium to solids in the 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 complementary by weight 60-90% solids and 40-10% organic medium. Such dispersions are usually of semifluid consistency and are referred to commonly as "pastes".

The pastes are conveniently prepared on a three-roll mill. The viscosity of the pastes is typically within the following ranges 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.

In the preparation of the composition of the present invention, the particulate inorganic solids are mixed with the organic medium and dispersed with suitable equipment, such as a three-roll mill or a muller, to form a suspension, resulting in a composition for which the viscosity will be in the range of about 100-150 Pa.s at a shear rate of 4 sec.sup.-1.

In the examples which follow, the formulation was carried out in the following manner.

The ingredients of the paste, minus about 5% wt. of the estimated organic components which will be required, are weighed together in a container. The components are then vigorously mixed to form a uniform blend; then the blend is passed through dispersing equipment such as a three-roll mill to achieve a good dispersion of particles. A Hegman gauge is used to determine the state of dispersion of the particles in the paste. This instrument consists of a channel in a block of steel that is 25 .mu.m deep (1 mil) on one end and ramps up to zero depth at the other end. A blade is used to draw down paste along the length of the channel. Scratches will appear in the channel where the agglomerates' diameter is greater than the channel depth. A satisfactory dispersion will give a fourth scratch point of 10-18 .mu.m typically. The point at which half of the channel is uncovered with a well dispersed paste is between 3 and 8 .mu.m typically. Fourth scratch measurement of 20 .mu.m and "half-channel" measurements of 10 .mu.m indicate a poorly dispersed suspension.

The remaining 5% of the organic components of the paste is then added and the resin content of the paste is adjusted for proper screen-printing rheology.

The composition is then applied to a substrate such as alumina, usually by the process of screen printing, to a wet thickness of about 30-80 microns, preferably 35-70 microns and most preferably 40-50 microns. The electrode compositions of this invention can be printed onto the substrates either by using an automatic printer or a hand printer in the conventional manner. Preferably, automatic screen printing techniques are employed using a 200- to 325-mesh screen. The printed pattern is then dried at below 200.degree. C., e.g., about 150.degree. C., for about 5-15 minutes before firing. Firing to effect sintering of both the inorganic binder and the finely divided particles of metal is preferably done in a controlled atmosphere belt conveyor 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.-950.degree. C. lasting about 5-15 minutes, followed by a controlled cooldown cycle to prevent over sintering, unwanted chemical reactions at intermediate temperatures or substrate fracture which can occur from too rapid cooldown. By controlled atmosphere is meant a nonoxidizing atmosphere such as nitrogen, argon or mixtures thereof. 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. In some instances, total cycle times as short as 30 minutes can be used.

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 ten 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 900.degree. C., dwell at 900.degree. C. for 9 to 10 minutes and cooled at a rate of 30.degree. C. per minute to ambient temperature.

Substrates prepared as described above 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 temperature of the chamber is then cooled to -55.degree. C. and allowed to equilibrate and the cold resistance measured and recorded.

The hot and cold temperature coefficients of resistance (TCR) are calculated as follows: ##EQU1##

The values of R.sub.25.degree. C. and Hot and Cold 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.sub.av).times.100 (%)

A tantalum-tin pyrochlore composition corresponding to the formula

was prepared by ball milling 71.42 g of SnO, 117.16 g of Ta.sub.2 O.sub.5 and 11.42 g of SnO.sub.2 using water as a dispersing medium. Upon completion of thorough mixing, the admixtures were dried and placed in alumina crucibles and heated in a furnace containing a nonoxidizing (N.sub.2) atmosphere. The mixtures were then heated for 24 hours at 872.degree. C..+-.10.degree. C. to form the pyrochlore. The preparation and use of such pyrochlores is described in applicant's copending U.S. patent application Ser. No. 460,572, filed Jan. 24, 1983.

The above-described pyrochlore was then divided into 50 and 100 g quantities, each of which was combined with 450 and 400 g of respectively purified SnO.sub.2 and ball milled for one hour using distilled water as a liquid milling medium. Upon completion of ball mill mixing, the mixtures of pyrochlore and SnO.sub.2 were placed in a nitrogen furnace and fired for 25 hours at 875.degree. C. The conductive phase material prepared in this manner was then used to formulate a thick film resistor paste having the composition given above. In the following examples, the 50/450 and 100/400 conductive phases are referred to as Conductive A and Conductive B respectively.

The physical properties of 3 glasses of the invention are given in Table 1 and the electrical properties of the fired resistors made therefrom are given in Table 2 which follows.

In the examples which follow, the glass frit compositions listed in Table 1 below were made by the above described procedure. All were single phase glasses with no secondary phases present.

Using the above-described CoO-containing glasses, a series of eight resistors was prepared. The compositions of the thick film compositions and the properties of the resistors prepared therefrom are given in Table 2, which follows:

In the foregoing examples, the thick film compositions of Examples 1-3 were mixed by mulling, while those of Examples 4-8 were mixed on a three-roll mill. By comparison of Examples 2 and 4, which are essentially identical in composition, it can be seen that the mixing had a significant effect on their respective resistance values. Though both compositions had excellent properties, the mulled sample (Example 2) had almost 50% lower resistivity than the roll-milled sample (Example 4). Aside from this surprising effect of different mixing methods, the foregoing data show that resistors having a wide range of resistance can be made from the glasses of the invention, all of which gave moderately positive HTCR values.

Using the above-described Glasses IV and V, a series of five resistors was prepared. The compositions of the thick film compositions and the properties of the resistors prepared therefrom are given in Table 3, which follows:

Examples 1-3 (Table 2) demonstrate the effects of different glass compositions on electrical properties. In addition, Example 1 shows that a high loading of conductive phase need not result in low resistance, unlike similar cobalt oxide-free glasses which produce lower resistance when compounded in the same way. For example, compare Example 1 (Table 2) with Examples 9 and 10 (Table 3). Examples 2 and 3 show the combined effect of CoO and ZrO.sub.2 to produce both high resistance and positive HTCR. Example 3 shows that HTCR becomes even more positive as the level of ZrO.sub.2 is increased while keeping a constant amount of CoO.

Comparison of Example 2 with Examples 11-13 (Table 3) shows the effect of ZrO.sub.2 -glass. A ZrO.sub.2 -free glass such as Glass V produces a lower resistance and lower HTCR than Glass II which contains CoO and ZrO.sub.2.

Examples 5-7 (Table 2) show the effect of glass concentration and, in particular, show that at a high conductive phase loading, both high resistance and positive HTCR are obtained. This is, of course, desirable in that high loadings of conductive phase assures quite good power handling capability. Of particular interest is the fact that resistors of 50 M.OMEGA./.quadrature. and higher having positive HTCR can be made. This has not heretofore been possible.

Examples 9 and 10 and 11-13 show clearly the effect of reducing the amount of conductive phase, namely that as the concentration of conductive phase is lowered, the resistance is raised. Also comparison of Examples 4 and 8 shows that the use of the conductive phase containing less pyrochlore (B) gave lower resistance but more positive HTCR.