Low Oxygen Content Fine Spherical Copper Particles and Process for Producing Same by Fluid Energy Milling and High Temperature Processing

This invention is related to application Ser. No. 07/161,535 entitled "Process For Producing Fine Copper Powder With Enhanced Sinterability" and Ser. No. 07/292,788 entitled "Process For Producing Fine Copper Flakes", both assigned to the same assignee as the present application and both filed concurrently herewith.

This invention relates to fine spherical copper powder particles and to the process for producing the particles which involves mechanically reducing the size of an electrolytically produced dendritic copper starting material by fluid energy or jet milling following by high temperature processing to produce fine spherical particles having oxygen contents of less than about 0.5% by weight. More particularly the high temperature process is a plasma process.

Replacement of precious metal powder by copper powder for thick film paste hybrid circuit applications is a current trend in the electronic circuit fabrication industry. Copper offers several economic and performance advantages over precious metals. However, copper thick film paste technology is in its infancy, and many improvements still must be made in the process. One of the improvement is reduced cost. One of the ways by which costs may be reduced is to use a smaller diameter copper powder which allows a thinner circuit trace to be fabricated.

One of the better commercial processes for producing such copper particles is by gas atomization. Only a small percentage of the powder produced by atomization is less than about 15 micrometers in maximum particle size. An even smaller percentage is below about 3 micrometers in size. Therefore, yields are low and metal powder costs are high as a result.

Another process for producing copper powders for thick film electronic circuit applications is by chemical or electrical precipitation. While these powders are made with a high yield under 10 micrometers in diameter, the particles are not smooth and spherical. This leads to problems with flowability, packing density, and organic vehicle removal.

U.S. Pat. Nos. 3,909,241 and 3,974,245 to Cheney et al. relates to free flowing powders which are produced by feeding agglomerates through a high temperature plasma reactor to cause at least partial melting of the particles and collecting the particles in a cooling chamber containing a protective gaseous atmosphere where the particles are solidified.

U.S. Pat. 4,264,354 to Cheetham relates to producing spherical dental alloy powders by high frequency induction coil heating followed by cooling in a liquid medium.

In European Patent Application W08402864 published Aug. 2, 1984, there is disclosed a process for making ultra-fine powder by directing a stream of molten droplets at a repellent surface whereby the droplets are broken up and repelled and thereafter solidified as described thereinn. While there is a tendency for spherical particles to be formed after rebounding, it is stated that the molten portion may form elliptical shaped or elongated particles with rounded ends.

U.S. Pat. Nos. 4,711,660 and 4,711,661 relate to spherical particles and process for producing same by mechanically reducing the particles size of the material and high temperature processing followed by rapid solidification. The oxygen content of the spherical particles when the material is reduced in size by the preferred attritor milling is typically greater than about 0.8 % by weight. It is desirable that the oxygen content be lower than this value for better sintering and better mechanical properties, etc.

In accordance with one aspect of this invention there is provided a powder material consisting essentially of fine spherical copper particles which are essentially free of elliptical material and essentially free of elongated particles having rounded ends. The particles have a particle size of less than about 20 micrometers in diameter, have an oxygen content of less than about 0.5% by weight, and a carbon content of no greater than the carbon content of the starting material.

In accordance with another aspect of this invention, there is provided a process for producing the above described particles which comprises reducing the size of an electrolytically produced starting dendritic copper powder material by fluid energy milling to produce a finer powder, essentially all of which has a particle size of less than about 20 micrometers in diameter, entraining the finer powder in a carrier gas and passing it through a high temperature zone at a temperature above the melting point of the finer powder, the temperature being from about 5500.degree. C. to about 17,000.degree. C., to melt at least about 50% by weight of the finer powder to form essentially fine spherical particles of the melted portion, and rapidly and directly resolidifying the resulting high temperature treated material while the material is in flight.

FIG. 1 is an SEM photograph at 2000.times. of electrolytically produced dendritic copper powder that is the typical starting material for the process of the present invention.

FIG. 2 is an SEM photograph at 2000.times. of the above described powder after jet milling according to the process of the present invention.

FIG. 3 is an SEM photograph at 2000.times. of the above processed powders which have been plasma processed according to the process of the present invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above described drawings and description of some of the aspects of the invention.

The starting material of this invention is copper metal particles. The material is dendritic copper powder. By dendritic is meant fern-or-tree-like single crystals feathery in appearance and produced by electrolytic deposition. Typically the copper powder is electrolytically produced because it is easier to reduce in size in the fluid energy milling step. The size of the material is of any size or diameter typically less than about 2,000 microns in diameter, measured at the widest part of the particle.

The size of the starting material is first reduced to produce a finer powder material. Essentially all of the material is reduced to a particle size of less than about 20 micrometers in diameter as measured by conventional techniques such as air or liquid settling, laser diffractometry, or Microtrac. With Microtrac, it is assumed that the particles are spherical, and therefore for non-spherical particles, the reported particle sizes are relative values.

The size reduction is accomplished by a group of processes commonly called "jet milling" or "fluid energy milling", including fluidized bed opposed jet milling, the "Coldstream" process in which a stream of gas and the starting material are impinged against a fixed target, etc. All references made herein to "jet milling" or "fluid energy milling" are understood to refer to this group of processes. In the process of the invention, there are no moving parts except for gas compressors to produce the fluid energy stream. Energy is imparted to the particles by the fluid or gas that is, by the velocity of the fluid. All of these processes impart high velocities to the material which is being ground and impact the accelerated particles against each other or against a solid substrate at a sufficient force to shatter or break the particles into smaller fragments.

In U.S. Pat. Nos. 4,711,660 and 4,711,661 relate to particle size reduction followed by high temperature processing and rapid solidification to form spherical particles. These patents stress media/mechanical motion or vibration to reduce particle size. These patents relate to processes in which the size reduction is done in a liquid medium and the material must be dried before subsequent high temperature processing. Both of these steps increase the likelihood for oxidation of the powder. By contrast, according to this invention, the size reduction can be done with the material in the dry state in an inert atmosphere. Only the correct size powder is produced and therefore there is no need for screening or size classification before high temperature processing. Furthermore, the processes of these patents result in powders which have a carbon content exceeding that required in some applications. This is due to the fact that size reduction occurs typically in a liquid organic medium which breaks down or is trapped within the powder particles. This results in an increase in the carbon content of the powder. By contrast, the present invention is carried out with the material in the dry state and the carbon content is therefore not increased. Carbon content affects the resistivity and other performance measurements for copper in electronic applications. When fluid energy milling is used, the oxygen content in the resultant spherical powder particles is less than about 0.5% by weight, and the carbon content is essentially no greater than that of the starting material. Also the process operates at a higher efficiency than prior art methods of gas or water atomization or the processes of U.S. Pat. Nos. 4,711,660 and 4,711,661 because only the correct size powder is discharged from the jet milling apparatus to convert it to spherical particles by high temperature processing. The prior art methods of mechanical size reduction are batch processes. Therefore all material undergoes high temperature processing, even if a portion of it is not the correct size. Thus, more material must undergo the high temperature processing to yield a given amount of product, and more post-high temperature treatment classifying is necessary to yield the desired final size distribution. The process of this invention yields a more uniform size reduced material for subsequent high temperature processing than does prior art processing. This is so because the fluid energy milling is a continuous process. The oversize powder is recycled to the fluid energy milling process while the correct size material which is finer than the starting material is discharged from the mill for subsequent high temperature process. This is important because melting efficiency (the weight ratio of melted particles to total particles) is increased when the material that is subjected to the high temperature process is more uniform in size.

The preferred jet mill to accomplish size reduction is the fluidized bed opposed jet mill invented by Alpine. The mill is comprised of a cylindrical grinding chamber with an Alpine classifier mounted at the top. Compressed air, nitrogen, or inert gases is introduced into the mill through three or more horizontally oriented nozzles circumferentially spaced around the lower portion of the grinding chamber. Material is introduced into the chamber by a feeder at the bottom of the chamber or through a tube entering the grinding chamber above the gas jets. Because of the gas flowing into the mill, the material which is being size reduced forms a fluidized bed at the bottom of the grinding chamber. Gas leaves the nozzles at supersonic velocities and accelerates the material to be reduced in size. Particles of material are entrained in each gas jet and impact near the center of the grinding chamber with particles entrained in the other gas jets. Particles fracture and therefore, size reduction occurs at this stage of the process. The mixture of size reduced and unground material travels upwards through the grinding chamber to the air classifer, which is a finned wheel (similar in appearance to a "squirrel cage" blower) rotating at a high speed (>5,000 rpm). The wheel rejects particles above a certain size (which is adjustable) and returns these unground or partially ground particles to the fluidized bed of the grinding chamber. The oversize material rejected by the classifier wheel is reentrained in the gas jets for further grinding. Fine particles of the desired size pass through the classifier wheel, where they are collected by conventional means, such as gas cyclones or filters. New starting material is fed into the mill at a rate equal to the rate at which fine size reduced powder leaves the mill.

If a metal or metal alloy powder is size reduced by the above described jet mill with nitrogen or an inert gas as the grinding/atmosphere gas, the oxygen content of the size reduced powder is only slightly greater than the starting oxygen content. No matter which gas is used for milling, contamination of the material other than by oxygen during size reduction is minimal, even compared to other jet milling processes, because the material impacts and fractures against itself. Wear of the jet milling apparatus, which implies contamination of the material which is being size reduced, is minimal. The above described eqipment offers many advantages over conventional tumbling or stirred ball mills for the size reduction of metal powders. In conventional mills, milling is usually conducted in an organic solvent, which leads to carbon contamination. This does not happen in the process of the present invention. Also, the size reduced material must be dried before conversion to essentially spherical particles, and oxidation is nearly unavoidable.

The reduced size material is then entrained in a carrier gas such as argon and passed through a high temperature zone at a temperature above the melting point of the finer powder for a sufficient time to melt at least about 50% by weight of the finer powder and form essentially fine particles of the melted portion. Some additional particles can be partially melted or melted on the surface and these can be spherical particles in addition to the melted portion. The preferred high temperature zone is a plasma.

Details of the principles and operation of plasma reactors are well known. The plasma has a high temperature zone, but in cross section the temperature can vary typically from about 5500.degree. C. to about 17,000.degree. C. The outer edges are at low temperatures and the inner part is at a higher temperature. The retention time depends upon where the particles entrained in the carrier gas are injected into the nozzle of the plasma gun. Thus, if the particles are injected into the outer edge, the retention time must be longer, and if they are injected into the inner portion, the retention time is shorter. The residence time in the plasma flame can be controlled by choosing the point at which the particles are injected into the plasma. Residence time in the plasma is a function of the physical properties of the plasma gas and the powder material itself for a given set of plasma operating conditions and powder particles. Larger particles are more easily injected into the plasma while smaller particles tend to remain at the outer edge of the plasma jet or are deflected away from the plasma jet.

After the material passes through the plasma, it cools, and is rapidly and directly solidified. Generally the major weight portion of the material is converted to spherical particles. Generally greater than about 75% and most typically greater than about 85% of the material is converted to spherical particles by the high temperature treatment. Nearly 100% conversion to spherical particles can be attained. The particles are less than about 20 micrometers in diameter. The major portion of the spherical particles are less than about 10 micrometers in diameter. The particle size of the plasma treated particles is largely dependent of the size of the material obtained in the size reduction step. Most typically greater than about 99% of the particles are less than about 15 micrometers.

More preferred particle sizes are less than about 7 micrometers in diameter and most preferably less than about 3 micrometers in diameter, and it is preferred that the particles be greater than about 0.1 micrometer in diameter.

After cooling and subsequent resolidification, the resulting high temperature treated material can be classified to remove the major spheroidized particle portion from the essentially non-spheroidized minor portion of particles and to obtain the desired particle size distribution. The classification can be done by standard techniques such as screening or air classification.

The unmelted minor portion can then be reprocessed according to the invention to convert it to fine spherical particles.

The powder materials of this invention are essentially smooth-surfaced spherical particles which are essentially free of elliptical shaped material and essentially free of elongated particles having rounded ends. These characteristics can be present in the particles made by the process described in European Patent Application W08402864 as previously mentioned.

Furthermore, the levels of chemical contamination (carbon, oxygen, etc.) in the final product of this invention are much lower than those found in the spherical particles made by prior art high temperature processes. The oxygen levels in the particles produced by the process of the present invention are typically less than about 0.5% by weight and more typically less than about 0.3% by weight with levels as low as about 0.15% by weight can be achieved.

Spherical particles have an advantage over non-spherical particles in thick film electronic circuit applications. The lower surface area of spherical particles as opposed to non-spherical particles of comparable size, and the flowability of spherical particles makes spherical particles easier to mix with binders and easier to remove binders from.

To more fully illustrate this invention, the following non-limiting example is presented.

Dendritic morphology copper powder produced by electrolytic deposition is reduced in size by opposed jet milling using compressed air at a jet inlet pressure of from about 82 to about 85 pounds per square inch. The reduced size powder is then plasma melted using argon as the plasma gas and 10.5 kilowatts (300 amperes, 35 volts) of input powder the plasma torch. Scanning electron microscope pictures at a magnification of 2000.times. are shown in the Figures: the starting dendritic copper powder in FIG. 1, the powder reduced in size by jet milling in FIG. 2, and the plasma melted powder before classification in FIG. 3. Note in FIG. 3 there are submicron particles clinging to the fine spherical copper particles. These can be easily removed by air classification. The particle size as measured by Microtrac and the analyses of the powder at various stages in the process are shown in the Table below.

Note that the carbon and oxygen contents are essentially the same before and after plasma melting.

While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.