CA1240213A - Metal oxide-coated copper powder - Google Patents

Abstract

TITLE
METAL-OXIDE COATED COPPER POWDER
ABSTRACT
Particles of Cu-containing metal having a particle size of 0.5-20 µm in largest dimension bearing a thin, substantially continuous coating of at least one metal oxide having a free energy of formation more negative than -98 kcal/mole and method of making them. The metal oxide-coated particles have improved sintering and shrinking properties which closely match those of green ceramic tape.

Description

~402~ 3 TITLE
PETAL OX I DE- Keyword COPPER POWDER
The invention relate to metal oxide-coated ; copper metal powder which are especially suitable 6 fur conductor in multi layer Background of the Invention The term "hybrid microcircuit" refer to the interconnection and packaging of discrete electronic devices in a thick film or thin film network. In the past, the interconnection have been made by building a circuit or a number of circuit on a ceramic 6ub6trate. Recently multi layer of this type have been made by printing alternating layer of copper thick film conductor material and dielectric material in a desired configuration on a rigid ceramic 6ub6trate such a alumina. Each of the dielectric layer it fired in a non oxidizing atmosphere to effect den6ification of the dielectric material without oxidizing the copper conductive material before the next layer it applied. eke the 6ub6trate prevent lateral shrinkage, the finished multi layer structure remain flat.
wherefore, the thermal coefficient of expansion TOE
of the thick film conductor and dielectric material need only approximate the TOE of the substrate in order to obtain relatively flat multi layer tractor.
Though the use of thick film put it technically adequate, equally 6ati6factory multi layer tractor can be obtained much more economically by the use of the "green tape" method.
This prows involve the use of a tape fabricated from 92-94~ wt. purity Aye ceramic powder and a flexible polymeric binder, one or more layer of which are metallized wit a patterned EL-0181 35 conductive layer, including punched via, tacked and ....... ....

. .

1240~3 then laminated with heat and pussier. After lamination, the multilane assemblage it coiffured in a reducing atmosphere to produce the completed multi layer. A many a forty such alternating layers are used to form high density interconnection for use in various electronic hardware such a computer logic module.
Since Aye it refractory. the conductor metal mutt be correspondingly refractory. In the pat, W, My and Mom have been used a the conductive layer material for metallic traces and via interconnect. Particularly widely used for multi layer ha been the system A12O3/Mo-Mn. Despite its advantage, the use of the Amman ~y6tem ha several disadvantages. For example, the conductivity of Mom it too low for modern high speed data prosing equipment. Allah, Al2o3/Mo-Mn require firing temperatures on the order of 1500~C in a wet Ho atmosphere to achieve proper deification of the AYE Furthermore, AYE ha a temperature coefficient of expansion TOE which it twice that of Six Therefore, large integrated circuit chips often crack when they are bonded to the multi layer substrate because of the mismatch in TOE between AYE and Six To overcome these problem, Heron et at. in 4,234,367 and Camaro et at. in US. 4,3~1,324 have proposed the use of green tape in which the Aye it replaced by a glue ceramic hazing a low crystallization temperature and Mooney it replaced by Cut as the conductive layer material.
Suitable glues are decal Ed to be ~-spodumene (Lucia) and cordierite ~2MgO-2A1203-5SiO2)- Both the 6podumene and cordierite winter below 1000C.

Using the above-refe~red material in the green tape pus, the multiple layer are coiffured 3-5 hour at 775 10C in a Tom atmosphere to burn out the polymeric binder, after which the ~2/~2 atmosphere it replaced by No and fifing it completed at 930-970C to achieve den6ification of the glue ceramic material. Because of the high wintering temperature. the structure shrink about 15% and glue ceramic material it crystallized into cordierite ceramic.
in the above-de6cribed 6y6tem. the fine copper powder begin to winter and shrink wren it reaches 400C in the firing cycle. worry the glass ceramic material Dow not stinter until it reaches 730C. Because of this difference in the wintering and shrinking characteri6tic6 of the two materials, the multi layer tend to incur warping and bowing.
or this Ryan, it has been difficult to produce cordierite/copper multi layer tractor with the required degree of flatten. Thus, it would be very desirable to have a copper-based conductive material which would not incur sistering until it reached about 780C and which would Allah have predictable shrinkage characteristics approaching those of green glue ceramic tape, i.e. about 15% shrinkage occurring between 700 and 970C.
A still further problem with the prior ail copper powder ha been the unpredictability of the copper powders from batch to batch. This it illustrated in Wolf, J. Ed., Powder Metallurgy, Am.
Sock for Metal, Cleveland, OH (1942). In Chapter 31 (page 323-331) of this publication, J. E. Drop.
Jr. presents several sistering curves for copper powder under various sistering conditions which are so diverse in shape that it would be extremely I

' .

,, ~021~

difficult if not impossible to predict the 6i~terîng characteristic of mixture of such material under other sistering conditions or in blends with other copper material. Thus the availability of copper material hazing not only improved but Allah predictable shrinkage characteri6tic6 it an important goal.
Brief Duration of the Invention The invention it therefore directed in it primary apt to particle of Cu-containing metal having wintering and shrinkage characteri6tic6 which more closely match those of green ceramic tape, bearing a thin, substantially continuous coating of at least one metal oxide having a free energy of formation more negative than -98 calmly and having a particle size of 0.5-20 em in largest dimension.
In a secondary aspect, the invention it directed to a method for changing the shrinkage characteri6tic6 and raising the sistering temperature of finely divided Cu-containing metal particle comprising the sequential Taipei of:
(a) dispersing the Cu-containing metal particle in a 601ution comprising I an organometallic compound of a metal, the oxide of which ha a free energy of formation more negative than -98 calmly dissolved in (2) a volatile organic solvent, (b) while maintaining the dispersion by agitation, removing the solvent by evaporation, thus forming a coating of the organometallic compound on the Cu-containing metal particles, and (c) treating the organometallic compound-coated Cu-containing metal particles in a reducing atmosphere at an elevated temperature and for a time sufficient to effect reduction of any copper oxide on ,, ....

toe copper-containing metal particles and decomposition of the organometallic compound to the corresponding metal oxide.
In a third aspect, the invention it directed to printable thick film put comprising a di6per6ion of the above-de6cribed metal oxide-coated Cu-containing metal particle in organic medium.
Brief Description of the Drown The drawing keynote of inure 1-4.
Faker 1-3 are graphical repre~entation6 of the effect of temperature upon the shrinkage of metal oxide-coated and uncoated copper particle, and Figure 4 is a blending curve for coated copper particle.
Prior Art Copper-containing thick film conductor compassion are, of course, well known in the art.
They have, in fact, been formulated in a number of different ways for a wide variety of applications.
Pro example, US. 2,993,815 to Tryout it directed to a method of forming a copper conductive layer for printed circuit on a refractory substrate by screen printing a layer of 5-50 parts by weight copper or copper oxide and 1 part by weight of reduction-resi6tant glue fruit diapered in an organic medium. The conductive layer it formed by firing the applied pate in two stage at 500-1050C. In the first firing stage, the glass it partially wintered in an oxidizing atmosphere and, in the second stage, the glue it completely wintered in a reducing atmosphere.
Boron et at., US. 3,988,647, decal a conductor co~po6ition comprising Cut particle which have been treated to remove oxide from the surface 35 dispersed in a 601ventle66 polymeric binder. This patent it concerned only with oxide-free cut powders and toe conductive composition of this reference contains quite high polymer concentrations.
The crier patent, US 4,07Z,771 it directed to a conductor composition comprising Cut particle, which have been preoxidized to form a surface layer of Cut, and glue fruit diapered in 15-25%, wit, organic tedium. The oxide coaling of the Cut particles constitutes 1-5~ by wt. of the total old (Cut oxide and glass). The patent is silent with respect to the use of oxide other than Cut.
Sputa and Slutsky's US. Patent 4,521,329 is directed to thick film copper conductor compositions for use with Rubbed writer in which the copper particle are coated with a thin layer of copper oxide to promote intern of the copper particles at very low firing temperature in a non oxidizing atmosphere and to obtain good 6ub6trate adhesion and solder ability.
Sweets US. Patent 4,514,321 is directed to thick film copper conductor compositions for use with resistors or dielectrics, which are less susceptible to oxidation during firing in non oxidizing conditions, comprising a conductive metal which is at least 28~ copper, inorganic binder and a small amount of a noncuprous metal such as W, Mow Rye and alloys and mixtures thereof.
In addition, unexamined Japanese Patent Application 57-71115 (published May 5, 1982) doxology an electrode pate comprising a di6per6ion of noble petal powder and organometallic powder diapered in an organic medium. When firing the pate in air, the organometallic compound forts very 1~4(~3 wine particles ox corresponding metal oxides which are dispersed among the metal powder particles and are alleged to suppress growth of the metal particles.
DETAILED DESCRIPTION OF THE INVENTION
A wide variety of organometallic compounds can be used in the invention to coat the copper-containing particles so long as the oxide(s) of the metallic moiety thereof has a free energy of formation per gram atom of oxygen more negative than -98 calmly. Therefore, the oxides of Six Tip Cue, Or, Al, Be, H, So, La, My, Cay V, To and mixtures thereof are all suitable.
The composition of the organometallic compound it not, strictly speaking, critical except to the extent that the compound must (1) be completely 601uble in organic solvent, (2) contain an oxygen-metal linkage, and (3) be readily pyrolyzable in a non oxidizing or reducing atmosphere.
The role of the organometallic compound can be characterized in a generalized way as follows:

(1) Morocco Moe) XROH

(2) Moe) Moo Suitable organometallic compound includes hydrocarbyl mutilates of the formula Morocco and metal chelates such as those having the following structural formula: X -Y
ROW it O OR
YUCCA
wherein is a functional group containing oxygen or nitrogen and Y is C2 3 alkyd.

I.

~2~02~1l3 Titanium compounds of these two classes are quite readily available in commercial quantities and include such compounds as the following:
Tet~aisopcopyl titan ate Tet~a-n-butyl titan ate Tetrakis (2-ethylhexyl) titan ate Lactic acid titanium chelates Titanium acetylacetonate Triethanolamine titanium chelates Titanium ethyl acetoacetonate chelates Though the titanium orqanometallic compounds are most available, nevertheless similar and analogous compounds exist for the other metals which are suitable for the invention. Among those are aluminum diisopropoxide acetate ester and tetraethoxy Solon. Many others will be apparent to those skilled in the art of organometallic materials.
The organometallic compound, which is a precursor of the desired metal oxide, is deposited on the copper powder by dispersing the copper particles in a 601ution of the organometallic compound in an an hydrous volatile organic solvent. Suitable alcoholic solvents include methanol, ethanol, i60propanol and methyl ethyl kitten. Ethylene chloride can Allah be used. By evaporating the solvent from the dispersion in a flash evaporator, the copper particles become coated with a thin layer of the organometallic compound. By increasing the concentration of organometallic compound in the coating 601ution, the coating thickness can be increased accordingly. Consequently, the oxide coating it correspondingly changed.
The resultant oxide layer it in most instances on the order of AYE in thickness and substantially continuous in nature. It is not, I, . .

~:4~3 however, Nasser that the coating be wholly continuous. Nevertheless, the extent of the coating mutt be such a to preclude any significant degree of metal-to-metal contact among toe oxide-coated 5 particles. Though thicker coating tend to give more nearly complete particle coverage, the thicken of the oxide layer should not exceed abut 10~0 A in order to avoid substrate adhesion problems. Such layer are at most only about 100 atomic layer in thicken and therefore do not significantly change the diameter of the copper particle.
With respect to particle configuration, both particle size and particle shape are very important.
To attain ~uitablè wintering properties, it is e66ential that the copper-containing particles fall within the range of 0.5-20 em and that the average particle size be 1-5 em. When the copper-containing particle are lest than about 0.5 em, the surface area of the particles become too high, which necessitates the use of exce66ive amounts of organic medium to obtain suitable printing viscosities.
Furthermore, welling and blistering of the printed Cut layer occurs when it is filed because it it very difficult to burn out high amounts of organic medium. On the other hand, when the size of the copper-containing particle is more than about 20 em, the particle do not winter adequately at low firing temperature and therefore the conductor layer is porous, doe not adhere adequately and ha low conductivity. Such coarse particle Allah exhibit poor printing characteri6tic6. For these tame reason, it it also Nasser that, within these broad limit of acceptable particle size, the average particle size should be 1-5 em. An average copper-containing particle size of 2-4 em it . .

preferred to attain even better properties in the fired film.
It it alto important that the copper-containing particle used in the invention have a 6ub~tantial degree of furriest. That it, they mutt have a surface area-to-weight ratio of below about 1.0 m go A ratio of ORB m go or lower is preferred and 0.2-0.5 mug it an especially preferred practical minimum range. The minimum possible 6urface-to-weight ratio for 2 em size particles would be about 0.30 m go for perfect pharaoh. For 4 em size spherical particle, the minimum pueblo surface area-to-weight ratio would be about 0.15 m go However, a a practical matter, such perfect pharaoh are not obtainable.
An important advantage of the invention it it use in making available Cu-containing particle which will have predictable shrinkage characteri6tic6 in practical applications. A it shown by Examples 11-21, selected shrinkage characteristics can be achieved by blending oxide-coated particles having different shrinkages. Thus, in Moe instances, it will become desirable to blend oxide-coated particle having higher shrinkage, e.g., 20S, with oxide-coated particle having lower shrinkage, e.g., 13%, to obtain predictably intermediate values. Pro this Ryan, it may be desirable to adjust upward the degree of shrinkage of Cu-containing particle made in accordance with the invention. Quite 6urpri6ingly, it has been found that the shrinkage of particle treated in accordance with the invention can be increased by mildly milling the particle.
This effect a limited amount of particle flattening without substantially changing the surface area-to-weight ratio of the particle. This ,' :

Q~3 I operation can be carried out during the oxide coating ¦ step step i or afterward. It it, however, ¦ preferred to carry out this additional operation during the oxide coating step. it should be 5 recognized, however, that this mild milling step Dow I not produce flake particle but merely more elliptical and truncated particle. Bead milling it frequently used for this purpose and it preferred.
Even though the foregoing deacon ha ; 10 been prevented with empha6i~ on the use of copper alone, it will be recognized that the invention is also useful for copper alloy having similarly high electrical conductivity. Included among 6uoh alloys are Quaked, Quizzer, Quote and Quicker. Suitable alloy of this type are disclosed in Dumper, Metal for Conductive and Re6i6tive Functions, Hayden Book Co., ha., New York, NY, 1970, pp. 21-27.
In the Example which follow, metal oxide-coated Cut particlefi were made by the following procedure:
(1) the organometallic compound it dissolved in an exce66 of an alcoholic solvent (methanol). The solution volume it chosen to be about three times the volume of copper powder to be added;
(2) copper powder is added to the 601ution of step (1) and the mixture it agitated to disperse the copper powder:

(3) the di6per6ion of step (2) it continue agitated and heated to 40C in a rotary evaporator to remove the alcoholic vent and leave a coating of organometallic compound on the surface of tube Cut particle;
I the coated Cut particle from step (3) are heated to 220-250C and treated with a reducing 35 atmosphere (e.g. 4% H2/96% No forming gay) ~:4t3~

- '12 for a time sufficient to decompose the organ metallic compound and reduce 6ub6tantially all of the metal oxide formed therefrom (reducible oxide below 0.1% byway Cut). Any high boiling decomp~ition product from the organometallic are alto volatilized in this step.
Shrinkage way determined by prying 3 gram of Cut powder at 3000 pi (211 kg~cm ) in a 0.5 inch (1.27 cm) die. Copper pellet thusly formed were then heated for one hour in an No atmosphere to the indicated elevated temperature. Shrinkage was calculated a the percent change in pellet diameter from measurement made before and after wintering.
Example 1-10 Vying the above-de6cribed procedure, a err of ten metal oxide-coated copper powder way prepared and the 6interinq and shrinkage characteri~Otic6 of each were observed and compared with uncoated copper particle. The beginning wintering temperature of each of the powder way raised from 350C to at least 600C examples 7 and 9). Moreover, beginning wintering temperature of 780C were obtained with Example 4-6 and 10, while Example 2 coated powder did not begin to winter until they were heated above 900C. With the exception of Example 6 all of the oxide-coated Cut powder exhibited total shrinkage value about the tame a the uncoated copper particle. The composition and properties of the Jnember6 of this err are given in Table 1 which follow:

Table 1 EFFECT OF OXIDE Coating I SISTERING OF COPPER POWDERS
Example Organometallic Compound Metal No. composition _ g/lOOqCu Composition Wit 1 (1) 0.67 To 0.0g4 2 (2) 1.07 Al 0.104 3 (3) loo So 0.135

4 (4) 2.00 To 0.154 j 5 (4) 2.00 To 0.154 6 (4)2.00 To 0.154 7 (1) 0.42 To 0.059 8 (1) 0.83 To 0.118 9 (1) 0.67 To 0.094

(5) 0.95 To 0.094 Control - None - None Sistering Temperature Example Metal Oxide (C) Shrinkage No. Composition % Wt. Brain End (I _ 1 Shea 0.15780 980 14 2 Aye >900 - _ 3 Sue 0.24600 900 15 4 Shea 0.267B0 BYWAY
Shea 0.267B0 950 15

6 Shea 0.26780 B50 20

7 Shea 0.10600 900 12

8 Shea 700 BB0 12

9 Shea 0.15600 920 15 Shea 0.157B0 980 14 Control - Nina 850 14 (1) Tetra-n-butyl titan ate (2) Aluminum diisopropoxide acetate ester I Tetraethoxy 6ilane (4) Tetraoctylene glycol titan ate (5) Acetylacetonate chelates of To Example Vying the above-de~cribed uncoated copper powder, two batches of titanium dioxide-coated copper ....

., I,, Pi powder were prepared and the shrinkage characteristic were observed and compared with like uncoated copper particle. The data in Figure 1 show that the extent of toe shrinkage upon heating it 5 substantially delayed upon heating to 60~-7~C when the copper particle are coated with metal oxide.
Moreover. the extent of shrinkage it related directly and linearly to the wintering temperature. The linearity of the shrinkage it, of course, quite important with regard to the predictability of the shrinkage performance of these material.
Example 14-16 For compari60n, the shrinkage characteristic of a series of three uncoated copper powders was observed in which the average particle size was lower than in Examples 11-14 and the surface area was higher than those of the invention (1.0 m I). Figure 2 owe that the variation of shrinkage with temperature was quite diversely curvilinear and therefore unpredictable a a practical matter. Thus, unlike the material of Example 11-13, the use of these material would be very difficult when it it desired to obtain a preselected shrinkage behavior.
Exam~le6 17-21 To illustrate the important predictability of the oxide-coated copper particle of the invention, a series of five compassion was prepared in which quantities of two TiO2-coated copper powder having 6hrinkage6 of 20% (Example 17) and 13%
30 (Example 21) at 1000C were blended and the shrinkage of the blends way measured up to 1000C. These data are given in Figure 3 and show that the shrinkage curve for each of the mixture was approximately linear at least up to 950C. Moreover, when the 35 shrinkage at 1000C it plotted a a function of the ., .

124~ 3 weight % of Example 17 and Example 21 coated copper materials in the mixtures, the resultant blending curve was almost exactly linear figure 4).

I

Claims (8)

16

1. Particles of cu-containing metal having sintering and shrinkage characteristics which more closely match those of green ceramic tape, bearing a substantially continuous coating of at least one metal oxide having a free energy of formation more negative than -98 kcal/mole and having a particle size of 0.5-20 µm in largest dimension and a surface area less than 1.0 m2/g.

2. The coated particles of claim 1 on which the coating is an oxide of a metal selected from the group consisting of Si, Ti, Ce, Zr, Al, Ba, Sr, La, Mg, Ca, V, Ta and mixtures thereof.

3. The coated particles of claim 1 in which the Cu-containing metal is selected from the group consisting of Cu, Cu/Cd, Cu/Zr, Cu/Ti, Cu/Cr and mixtures thereof.

4. The coated particles of claim 1 in which the Cu-containing metal is CU.

5. A method for more closely matching the shrinkage of green ceramic tape and raising the sintering temperature of finely divided Cu-containing metal particles comprising the sequential steps of:
(a) dispersing the Cu metal particles in a solution comprising (1) an organometallic compound of a metal, the oxide of which has a free energy of formation more negative than -98 kcal/mole dissolved in (2) a volatile organic solvent.
(b) while maintaining the dispersion by agitation, removing the solvent by evaporation, thus forming a coating of the organometallic compound on the Cu-containing metal particles, and (c) treating the organometallic compound-coated Cu metal particles in a reducing atmosphere at an elevated temperature and for a time sufficient to effect reduction of any copper oxide on the copper metal particles and decomposition of the organometallic compound to the corresponding metal oxide.

6. A method for increasing the shrinkage of copper powders comprising the method of claim 5 in which the oxide-coated Cu-containing metal particles are mildly milled to effect flattening of the particles without substantially changing the surface area-to-weight ratio of the particles.

7. The method of claim 6 in which milling of the particles is carried out during steps (a) and (b).

8. A printable thick film conductive composition comprising a dispersion of the coated particles of claim 1 in an organic medium.