WO1992000924A1

WO1992000924A1 - Tellurite glass compositions

Info

Publication number: WO1992000924A1
Application number: PCT/GB1991/001123
Authority: WO
Inventors: John Francis Clifford; Simon Edward Pettitt
Original assignee: Cookson Group Plc
Priority date: 1990-07-09
Filing date: 1991-07-09
Publication date: 1992-01-23
Also published as: WO1992000925A1; GB9015072D0

Abstract

A low melting glass composition comprises 50 to 85 mol % TeO2, 0.1 to 30 mol % of an oxide of silver, from 5 to 30 mol % of an oxide of lead and/or zinc, and optionally up to 44.9 % of one or more other oxides, the glass composition having a dilatometric softening temperature, Ts, typically of 380 °C or below.

Description

TELLURITE GLASS COMPOSITIONS

The present invention relates to novel glass compositions and, in particular, to novel low temperature glasses which are useful as sealing glasses or solder glasses, and in electronic paste formulations.

The term "solder glass" or "sealing glass" is a term which is used to describe glasses which form an adhesive bond between two glasses, or between a combination of glass, metal, or a ceramic material. It is well known in the art that commercially available solder glasses capable of sealing ceramic and electrical component parts, such as television tube and semiconductor devices, are practical in the 400° to 500°C range. These solder glasses are generally based upon the lead oxide-boron oxide binary system. The lead oxide-boron oxide eutectic comprises 13% by weight of boron oxide and 87% by weight of lead oxide and represents the most fluid glass in that binary system. It is the starting point from which most commercial solder glasses have been derived. Whilst these glasses, with the addition of certain fillers, have been very successful, there is a need in the art for a solder glass which will seal at temperatures below the lower limit of the range of use for the lead oxide-boron oxide based glasses.

This is typically in the region of 380°C for conventional lead oxide-boron oxide based glasses, although it may be less for such glasses containing thallium or fluorine. Thallium is highly toxic, whilst the reduced durability and ionic mobility encountered in fluorine containing glasses is not acceptable in many electronics applications.

A second useful feature of these glasses is their high thermal expansion which can be modified to be comparable with high expansion materials including copper, silver and aluminium.

Until now no truly satisfactory solder/sealing glasses were available for use with such high expansion metals and alloys.

In order for a glass to qualify as a solder glass it has to meet certain physical criteria. These criteria will depend to a great extent upon the properties of the materials which the solder glass is to unite. Thus, in general, a solder glass should possess the following characteristics: i) a low viscosity in the melting phase; ii) a thermal expansion which will match the thermal expansion of the workpiece; iii) a sealing temperature which is below the lowest annealing and strain points of the materials to which it is to be sealed, iv) good physical and chemical stability; and v) a good adhesion to the workpiece.

Various attempts have been made in the prior art to produce solder glasses which meet the above criteria and which will seal at temperature of the order of 300° to 350°C. Thus, W087/05006 describes a solder glass for low temperature applications which is based upon the addition of bismuth oxide, zinc oxide and phosphorus pentoxide to the lead oxide-vanadium oxide eutectic.

US-A-3408212 describes the effect of adding large quantities of lead fluoride to lead oxide-vanadium oxide mixtures. A narrow glass forming region was found to exist in the centre of the lead oxide-lead fluoride-vanadium pentoxide ternary diagram, with improved glass life stability. US-A-3837866 describes the addition of arsenic oxides to both the lead oxide-vanadium pentoxide and caesium oxide-vanadium pentoxide eutectics to prevent early recrystallization. However, the addition of arsenic oxide tends to increase the viscosity of the resulting glasses. US-A-4186023 describes lead borate and lead zinc borate glasses containing from 0.1 to 10% by weight of cuprous oxide and a non-volatile metal fluoride, the mol ratio of cuprous oxide to the fluoride content of the metal fluoride being in the range of from 1:0.25 to 1:10.

We have now developed a series of solder glass compositions some of which have dilatometric softening temperatures of below 350°C, which are capable of wetting and bonding to a wide range of glasses, metals and ceramics including typical electronic substrates such as alumina, metallised alumina and silicon, and which are not derived from the lead oxide-boron oxide eutectic or the lead oxide- vanadium pentoxide eutectic mixtures. Accordingly, the present invention provides a low melting glass composition which comprises in mol percent calculated on an oxide basis i) from 50 to 85% of Te0₂, or an appropriate amount of a precursor for Te0₂, ii) from 0.1 to 30% of an oxide of silver, or an appropriate amount of a precursor therefor, iii) from 5 to 30% of an oxide of lead and/or an oxide of zinc, or an appropriate amount of a precursor for the said oxide, and iv) optionally from 0.1 to 44.9% of one or more oxides of Mg, Ti, Ta, Mo, B, W, Tl, V, Li, Na, K, Rb, Cs, Ca, Sr, Zr, Hf, Si, Ge, Al, Ga, In, P, Sn, Sb, Bi, La or a rare earth metal, or an appropriate amount of a precursor for one or more of the chosen oxides, provided that when V is included in the composition it is present in an amount of less than 5 mol %, and the glass composition having a dilatometric softening temperature, Ts, typically of 380°C or below.

The low melting glass composition of the present invention thus contain Te0₂ as the major component in an amount of from 50-85 mol %, preferably 60 to 80 mol %, more preferably 65-75 mol %.

The low melting glass composition of the present invention may be a three oxide system, the first oxide comprising Te0₂, with the second and third oxides comprising components (ii) and (iii) as defined above. In such a system the second oxide, component (ii) , will generally be present in an amount of at least 1.0 mol %, preferably 10 to 25 mol %, whilst the third oxide, component (iii) will generally be present in an amount of from 10 to 20 mol %.

The low melting glass composition may be a four or more component system, in which the additional components are as defined above for component (iv) . Component (iv) is preferably an oxide of Mo, W, Mg, Tl, B or V, most preferably M0O₃ or W0₃.

The preferred low melting glass compositions of the present invention have a dilatometric softening point, Ts, in the range of from 210 to 380°C, preferably 350°C or below and more preferably

300°C or below and linear thermal coefficients of expansion in the range of from 150 to 245 X 10~⁷, although the latter may on occasion be advantageously lowered by the use of a filler as described below. _A composition having a thermal coefficient of expansion of greater than 190 x 10^-7 is preferred for sealing to high expansion.

The low melting glass compositions of the present invention may additionally contain at least one solid halide of low volatility, such as lithium or sodium fluoride, in an amount of ≤ 5% by weight.

A further and surprising attribute of many of the glasses of the present invention is their good water and chemical durability. It is the combination of this property with either or both the high expansion and low melting temperature that makes the glasses of this invention such good sealing/bonding materials.

The various oxides used in the preparation of the compositions of the present invention are usually in the form of fine powders. Precursors of these oxides can also be useful, providing that they decompose to form the required oxides at a temperature below the melting temperature of the glass. Suitable precursors are the nitrites, nitrates, carbonates, metal organic salts, for example citrates, acetates, etc. and telluric acid.

The invention also includes within its scope a mixture of the above-described glass compositions of the invention with from 1 to 50% by weight, based on the mixture, of an inert refractory filler material having a thermal coefficient of expansion below that of the glass composition. The filler should be insoluble or only slightly soluble in the glass composition. The filler material is preferably used in an amount of from 5 to 30% by weight, based on the mixture, but the amount will depend upon the thermal expansion of the substrate or parts which the composition is intended to join, and on the specific gravity of the materials, a larger amount of filler resulting in a greater decrease in the thermal coefficient of expansion of the composition. The filler materials are used in order to modify the thermal coefficient of expansion of the glass composition, without effecting significantly the bonding temperature, of the glass. The filler material may thus be added to the glass composition of the invention as a means of controlling the overall thermal expansion and contraction of the resulting solder glass mixture. Increased amounts of a low thermal expansion filler will correspondingly decrease the linear expansion of the glass composite. Suitable filler materials include amorphous Si0₂, zircon, aluminium titanate, corderite, Nb θ5, a θ5 and lithium aluminium silicates e.g. jS-spodumene.

It will be understood by those skilled in the art that mechanically strong adhesive bonds are formed when the thermal coefficients of expansion of the materials being sealed are closely matched and similar to that of the solder glass mixture. It will likewise be appreciated that close matching of the thermal expansion of the solder glass to the substrate to which it is to bond is critical to maintain minimal or zero stress in the seal or bond interface. This maintains the strength and integrity of the seal or bond under conditions of thermal cycling and thermal shock.

The fillers are mixed with the glass composition in amounts in the range of from 1 to 50% by weight based on the mixture. The mixtures may be prepared, for example, by ball milling in a conventional manner to produce a finely divided, uniformly mixed material. The low melting glass compositions of the present invention, optionally in admixture with a filler as described above, may be applied to the substrate surfaces which are to be bonded together either as a molten glass, or as a shaped preform, or the powdered glass may be admixed with an organic vehicle to form a glass paste which is used to coat the substrate. The substrate is then heated initially to a temperature at which the organic vehicle will "burn off" and then at a temperature sufficient to melt the glass and form the seal. Generally, the composite is heated to a temperature in the range of from 300° to 450°C to melt the glass and form the bond. The organic vehicle may be any synthetic organic solvent which preferably boils or decomposes at a temperature below the softening point of the glass composition. In addition to their use as straightforward solder, sealing or bonding glasses, the glasses of this invention may also successfully be employed in passivating, dielectric, resistor, conducting, die attach or similar electronic paste formulations in which the glass acts wholely or in part as the adhesive bond.

As already described, the invention includes within its scope the use of a filler to modify mechanical properties such as thermal expansion. It is also intended to provide, where appropriate, for the use of fillers to modify electrical properties such as conductivity, resistivity and dielectric constant by the inclusion of; high and low resistivity metals, semiconducting oxides, nitrides, borides and carbides and dielectrics such as barium titanate or other insulating oxide materials.

A metal flake or powder filler, such as silver, gold, copper, aluminium or a solder alloy may be admixed with the low melting glass composition of the present invention in an amount of from 25% to 95% by weight based on the total dry weight of the composition, preferably 60 to 95% by weight based on the total dry weight of the composition. The metal-glass mixture may be formulated into a paste by admixture with an organic vehicle. The organic vehicle will generally be used in an amount such that the total solids content of the metal-filled glass paste is in the range of from 70 to 90% solids. The metal-glass mixture is particularly suitable for electronic applications, such as "thick film" conducting pastes and die attach applications in bonding semiconductor devices to a ceramic substrate, such as alumina. The solder glasses and pastes of the present invention may be coated onto metal-glass or ceramic substrates at any chosen thickness but usually at thicknesses in the range of from about 0.5 to 500 micrometres. When the solder glass is used as a paste, either with or without a filler and either with or without a metal flake or powder incorporated therein, the paste will generally be applied to the substrate surface in a conventional manner, for example by brush coating, screen printing, stencilling or stamping. For die attach applications, the paste is typically syringe dispensed. The die is attached by placing it in the centre of the wet paste and setting it by the application of pressure so that the paste flows up the side of the die and leaves a thin film beneath the die. The structure is then heat treated to "burn off" the organic vehicle and the temperature then raised to melt the low melting glass composition.

The present invention will be further described with reference to the following non-limiting Examples.

Examples 1 to 8

A range of three oxide tellurite based glasses were made of the composition Te0₂:X:Y, investigating the tellurite rich glass region. The compositions which were made are given, in mol %, as points 1 to 6 on Figure 1 of the accompanying drawings which is a phase diagram of the compositions. The oxides X and Y which were incorporated into the tellurite glass systems were selected from the following:

Ag 0 = A ZnO = Z

PbO = P

Glasses were prepared from mixtures of two of these oxides with Te0₂ (abbreviation T) in the following ratios:

Composition TXY1 = 85:7.5:7.5 mol %

Composition TXY2 = 80:15:5 mol %

Composition TXY3 =^■*■ 80:5:15 mol %

Composition TXY4 - 70:25:5 mol %

Composition TXY5 - 70:15:15 mol % Composition TXY6 » 70:5:25 mol %

The mixtures were all fused at a temperature of 775°C for 10 minutes in a platinum crucible. The glasses were then poured into graphite moulds maintained at room temperature to produce small bars. The bar was then cut to fit a dilatometer and measurements made of the softening temeperature, Ts, and the coefficient of thermal expansion, TCE. In some cases, exact cutting of the glass bar to fit the dilatometer was not possible as the glass bar shattered. The fragments allowed the measurement of Ts but not the measurement of TCE. Table 1 below gives the results. All of the glass compositions are denoted by the abbreviation of the oxides present, plus a number representing the molar compositions relating to Figure 1 and as defined above.

Examples 9 to 17

An investigation into four oxide tellurite based glasses was made by adding various fourth oxides in amounts of 1, 5, 7.5 or 10 mol % to various of certain of the three oxide glass systems previously described. Glasses were prepared from mixtures of the four oxides by fusing at a temperature of 775°C for 10 minutes in a platinum crucible. The abbreviations for the three oxide system were as described above, the fourth oxide addition being represented by one of the following letters indicating which oxide was added:

Mo0₃ = M

W0₃ = W

ZnO = Z and the number 1 or 5 etcetera indicating whether it was added in an amount of 1, 5, 7.5 or 10 mol %. The results are given in Table 2 below:

Table 2

Example 18

A four oxide tellurite based glass was made from the following components by fusing at a temperature of 775°C for 10 minutes in a platinum crucible:

Two four oxide tellurite based glasses were made from the following components by fusing at a temperature of 775°C for 10 minutes in a platinum crucible:

A test was carried out to show that a tellurite glass could have a use in hermetically sealing high expansion metal joins. A tellurite glass was selected to fuse to an Al/Mg alloy. The glass selected for bonding purposes had a slightly lower TCE value than that of the metal to be bonded so that the glass was put into compression on sealing. The glass was ground to sub 38 micrometre particle size, suspended in heptane and spread thinly and evenly onto the flat metal surface. Another piece of metal was placed over the top of the glass to create a "sandwich" which was heated to 300° to 450°C to bond the two metal pieces together. A manual inspection indicated that a good bond had been obtained. The results are given in Table 4 below:

Table 4

Metal Metal TCE Glass Glass TCE Results X 10^-7oC X 10"⁷

Al/Mg 225 TZA5 224 The glass Alloy bonded quite well. The two metal surfaces wer not easily separated by hand.

Examples 22 to 27

The use of various fillers to lower the thermal coefficient of expansion, TCE, was demonstrated using a Te0₂:Pb^O:Ag₂o composition having the oxide ratio 80:5:15 mol % to which 7.5 mol % of a fourth oxide, ZnO was added. To this base glass composition a range of fillers were added at 23% by weight. The glass and filler were ground and sieved together. The sub 38 micrometre powder mix was pressed at 200 psi into discs and these were fused at 300°C for 20 minutes. The discs were then cut to fit a dilatometer for measurement of the Ts and TCE. The results are given in Table 6 below:

Table 6

Example Filler 23 wt % Ts TCE Number xlO^-7

22 162 23 160

24 143

25 122 26 214 27

182

Base glass None 235 215

Claims

CLAIMS :

1. A low melting glass composition which comprises in mol percent calculated on an oxide basis i) from 50 to 85% of Te0₂, or an appropriate amount of a precursor for Te0₂, ii) from 0.1 to 30% of an oxide of silver, or an appropriate amount of a precursor therefor, iii) from 5 to 30% of an oxide of lead and/or an oxide of zinc, or an appropriate amount of a precursor for the said oxide, and iv) optionally from 0.1 to 44.9% of one or more oxides of Mg, Ti, Ta, Mo, B, W, Tl,

V, Li, Na, K, Rb, Cs, Ca, Sr, Zr, Hf, Si, Ge, Al, Ga, In, P, Sn, Sb, Bi, La or a rare earth metal, or an appropriate amount of a precursor for one or more of the chosen oxides, provided that when V is included in the composition it is present in an amount of less than 5 mol %, and the glass composition having a dilatometric softening temperature, Ts, typically of 380°C or below.

2. A low melting glass composition as claimed in claim 1 which is a mixture of three oxides wherein the silver oxide, component (ii) , is present in an amount of at least 1.0 mol %.

3. A low melting glass composition as claimed in claim 1 which is a four oxide component glass.

4. A low melting glass composition as claimed in claim 1 which is a five oxide or more component glass.

5. A low melting point glass composition as claimed in any one of the preceding claims wherein component (i) is present in an amount of from 65 to 75 mol %.

6. A low melting point glass composition as claimed in any one of the preceding claims wherein component (ii) is present in an amount of from 10 to 25 mol %.

7. A low melting point glass composition as claimed in any one of the preceding claims wherein component (iii) is present in an amount of from 10 to 20 mol %.

8. A low melting glass composition as claimed in any one of the preceding claims wherein component

(iv) is an oxide of:

Mo, W, Mg, Tl, B or V.

9. A low melting point glass as claimed in claim 8 wherein component (iv) is:

Mo0₃ or WO₃

10. A low melting point glass composition as claimed in any one of the preceding claims which has a dilatometric softening temperature, Ts of 350°C or below.

11. A low melting point glass composition as claimed in any one of the preceding claims which has a dilatometric softening temperature, Ts, of 300°C or below.

12. A low melting point glass composition as claimed in any one of the preceding claims which has a thermal coefficient of expansion of greater than 150 X 10"⁷.

13. A low melting point glass composition as claimed in any one of the preceding claims which has a thermal coefficient of expansion of greater than 190 X 10~⁷.

14. A low melting point glass composition as claimed in any one of the preceding claims which includes at least one halide of low volatility therein in an amount of ≤ 5% by weight.

15. A low melting glass composition as claimed in any one of the preceding claims which is admixed with from 1 to 50% by weight, based on the mixture, of one or more filler materials.

16. A low melting point glass composition as claimed in Claim 15 wherein the filler is added to alter the electrical properties of the composition.

17. A low melting glass composition as claimed in claim 15 or claim 16 wherein the filler is zircon, aluminium titanate, corderite, Nb θ5, Ta θ5 or a lithium aluminium silicate.

18. A low melting glass composition as claimed in any one of claims 15 to 17 wherein the filler is incorporated into the mixture in an amount of from 5 to 30% by weight, based on the mixture, of the filler material.

19. A low melting glass composition as claimed in any one of the preceding claims in powder form which is admixed with an organic vehicle.

20. An electronic paste which includes therein a low melting glass composition as claimed in any one of the preceding claims and in which the glass acts wholly or in part as the bonding agent.

21. An electronic paste as claimed in claim 20 which is a passivating, dielectric, resistor, conducting or die attach electronic paste.

22. A metal-filled glass composition which comprise : a) from 25 to 95% by weight of a metal flake or powder filler, b) from 5 to 75% by weight of a low melting glass composition as claimed in any one of claims 1 to 14, the percentages being by weight based on the total weight of the composition.

23. A metal-filled composition as claimed in claim 22 which additionally includes an organic vehicle in an amount such that the metal-filled solder glass paste so formed has a solids content in the range of from 60 to 95% solids.

24. A metal-filled composition as claimed in claim 22 or claim 23 wherein the metal flake or powder filler is silver, gold, copper or aluminium.

25. An adhesive bond between a combination of a glass, metal or a ceramic which is formed by a low melting point glass composition as claimed in any one of claims 1 to 14.

26. A structure which includes therein at least one adhesive bond as claimed in claim 25.

27. A film or coating on a substrate which is formed from a low melting point glass composition as claimed in any one of claims 1 to 14.

28. A method of bonding two substrate surfaces which are the same or different and which are selected from glass, metal or ceramic materials, which method comprises using as the bond forming material a low melting point glass composition as claimed in any one of claims 1 to 14.

29. A method as claimed in claim 28 wherein the glass composition is applied to the surfaces to be bonded together as a molten glass, a shaped preform or as a powdered glass in admixture with an organic vehicle.

30. A method as claimed in claim 29 wherein the composite so formed is heated to a temperature in the range of from 300° to 450°C to melt the glass and form the bond.