The Production of Palladium Powders for Electronic Applications
The Production of Palladium Powders for Electronic Applications
Reaction Conditions Determine Surface Character
Recent investigations into metal powder fabrication by chemical precipitation techniques have shown that a wide variety of powder morphologies can be obtained by making subtle changes in the reaction parameters. Palladium powders produced using these techniques find wide application in the microelectronics industry, particularly as metallisations for multilayer ceramic capacitors or in thick film interconnection systems.
The microelectronics industry requires metal powders for incorporation into thick film inks or pastes, and the technology of thick film materials and their application to microelectronics has been extensively reviewed (1–6). The major uses of such inks occur during the manufacture of hybrid integrated circuits where they are screen-printed and then fired to form the conducting layers of the interconnection system or to form the internal electrodes of multilayer ceramic capacitors (MLCCs).
The properties of the metallic components of thick film inks intended for the internal electrodes of MLCCs are extremely important as compatibility is required between the metal powder and the organic medium of an ink, and between the ink itself and the surrounding dielectric material of the MLCC. The most widely used metals for this application are palladium or palladium-silver composites.
Historically, metal powders prepared by the chemical reduction of simple metal salts were used in the manufacture of MLCCs. However, such powders were often extremely fine with particle diameters of 0.001 to 0.1 μ m, they exhibited a high surface area (>10m2/g) and as a result were often catalytically active or even pyrophoric. Consequently they were of only limited use when combined with an organic medium to form screen printable thick film inks. They suffered from three very serious disadvantages, in particular:
[i] High surface area powders require excess organic medium for adequate wetting of the powder during paste fabrication, and this can have a detrimental effect on the final rheology. The high oxide content associated with such powders can promote excessive gelling of the ink and screen printing may therefore become difficult.
[ii] During the firing stages, the fine material can interact catalytically with the organic medium of the ink creating hot spots inside the assemblies and leading to rapid vapour release, blistering and delamination of the capacitors as they are formed.
[iii] During the latter stages of firing, the metal powder sinters and shrinks. The shrinkage characteristics of very fine metal powders are usually excessive and unlikely to match that of the surrounding dielectric material. Consequently, tiny “islands” of metal are formed which result in discontinuous metal patterns of poor conductivity. Shrinkage incompatibility can also lead to delamination.
Thus it was considered necessary to produce relatively non-active metal powders which would be fine enough to be used in thick film pastes but of sufficiently low surface area to inhibit catalytic activity. On firing, such materials should also possess shrinkage compatible with the surrounding dielectric in order to form smooth, high density metallisations. Therefore there was a requirement for a range of powder types of varying morphology, surface area and tap density, depending upon the organic medium chosen to make the ink and the dielectric selected for the MLCC.
Recent investigations at the Johnson Matthey Technology Centre have been concerned with the fabrication of suitable metal powders for such applications. The method adopted was derived from electroless plating techniques, with metal salts and complexes in an aqueous medium being precipitated to the metal by the action of a chemical reducing agent.
Studies reported in the literature (7, 8, 9) have led to the general observation that precipitation is a combination of a number of stages, namely: nucleation, where solid phase particles are formed out of a liquid phase; growth, where solid material grows around the original nuclei; and agglomeration where the particles collide and combine. A supersaturated solution is, of course, one which contains more solute than that indicated by the equilibrium solubility or saturation point, and is a prerequisite for the growth mechanism to take place; while nucleation, which may be promoted by the presence of impurity substrates, requires even higher levels of saturation. However, when the precipitation of metals by the chemical reduction of metal ions from aqueous solution is under consideration, concepts such as saturation, supersaturation, nucleation and growth become less certain in view of the vanishingly small solubility of metals in water, and there appears to be very little published in the literature about this form of precipitation.
The investigations made at these laboratories have shown that although the basic reaction remained the same, which in the case of palladium is Pd2+ + 2e→Pd°, subtle variations in the reaction parameters can have a remarkable effect on the physical characteristics of the metal powder formed. For this purpose important variables include the concentration of the reactant or the reducing agent, the form of the metal salt or complex, the choice of the reducing agent, the reaction temperature, the mode and degree of agitation of the solution, the solution viscosity and the presence of additives or polyelectrolyte surfactants
Surface Morphology of the Powders
The results of this work are illustrated by the scanning electron micrographs which show a variety of palladium powders, all at similar magnifications. A very fine, high surface area (>10m2/g) material which, for the reasons outlined earlier, is of little use in MLCC applications, is shown in Figure 1. Figure 2 illustrates a nodular powder having a lower surface area (1 to 2m2/g); however, the surface irregularity and wide particle size distribution promotes uneven sintering during the firing stages and this leads to poorly defined conducting layers. Figure 3 shows a dendritic material which displays surprisingly good sintering characteristics which are considered to be due to the formation of a mat-type structure during printing. This same factor, however, can result in screen blockage which makes the powder of little direct use. A powder composed of spherical particles is shown in Figure 4; again the high level of agglomeration would result in poor asfired properties. Figures 5 and 6 illustrate powders consisting of spherical particles but here there is less agglomeration. This is the type of powder most useful for MLCC fabrication, the smoother surfaced material possessing superior sintering properties.
Subtle alterations to precipitation parameters can also be used to control particle size and agglomeration, as shown in Figures 6, 7 and 8. Consequently surface area, tap density, Scott Density and Fischer Sub Sieve Size can also be regulated. The powder illustrated in Figure 8 has a particularly low particle size (≍0.1 μ m diameter) that facilitates printing of a buried electrode to a minimal thickness, but which retains a coherent structure. The use of this type of powder is expected to increase in the near future as it offers economic advantages.
The assessment of a metal powder for a particular application requires further characterisation in addition to measurements of surface area, tap density and particle size distribution. The determination of oxide layers or the presence of surface contaminants which may interact harmfully with any dielectric material requires the use of X-ray Photo-electron Spectroscopy. Transmission Electron Microscopy and X-ray Diffraction are used for the determination of fundamental crystallite size, which incidentally provides information on the particle growth mechanism. The presence of bulk impurities which can drastically alter MLCC performance is determined by X-ray Fluorescence Spectroscopy, in addition to the traditional methods of chemical analysis. Finally, Simultaneous Thermal Analysis and Dilatometry are used to examine sintering and shrinkage behaviour.
The investigations carried out at the Johnson Matthey Technology Centre have involved mainly palladium and silver-palladium, reflecting the importance of the platinum group metals in the manufacture of MLCC electrodes. This work has provided a better understanding of the fundamental aspects of precipitation of these metal powders, which can also be utilised for other applications such as improved MLCC end terminations and hybrid interconnect metallisations. The electronics industry requires and demands high purity powders of advanced formulation for existing and predicted future applications; it is knowledge gained from investigations such as these recorded here that will enable the constantly changing needs of this important industry to be satisfied.
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