Multilayer ceramic capacitors consist of a plurality of interleaved and staggered layers of an electrically conductive film of metal (termed "electrode"), formed by the deposition (usually by screen printing or variations thereof) of a thick film paste (termed an "electrode composition") and electrically insulating layers of a ceramic oxide (termed "dielectric"), formed by laying a cast dielectric tape or by casting a dielectric slurry over the dried electrode. Such capacitors are well-known in the art. U.S. Pat. No. 2,389,420, for example, describes the structure, manufacture and properties of monolithic multilayer ceramic capacitors.
The electrode composition is usually a dispersion of finely divided precious metal powders such as palladium, silver, gold, or platinum or their mixtures in a vehicle which is usually solely organic in nature. Dispersions of non-precious metals such as copper and nickel have also been shown to have utility in electrode compositions. The vehicle is usually composed of a mixture of a polymeric resin which imparts viscosity to the composition and appropriate solvents for processing compatibility, particularly with respect to drying. Other organic additives are usually made to the vehicle to control paste rheology. Typical electrode composition metal concentrations range from 40 to 70% by weight, with the remainder being vehicle. Electrode compositions are deposited, usually by screen printing techniques, on dried dielectric layers, then dried to remove solvents and leave a mixture of metal powders and resin from the vehicle.
The dielectric layer is usually composed of finely-divided oxide powders dispersed in a resin. Barium titanate (BaTiO.sub.3) and other oxides such as neodymium titanate (Nd.sub.2 Ti.sub.2 O.sub.7) and magnesium titanate (MgTiO.sub.3) are used. Additions are usually made to these oxides to control various electrical characteristics, particularly to maximize dielectric constant while controlling the temperature dependence of dielectric constant and insulation resistance, among other properties. The resin is present in the dielectric layers to facilitate handling and printing of electrodes on the layers.
Multilayer ceramic capacitors are manufactured by building up an interleaved configuration of electrode and dielectric layers, dicing individual parts out of the build-up then subjecting the parts to a slow burnout and then high temperature firing. Burnout is done to remove the organic resin in the electrode and dielectric layers to avoid rapid outgassing and rupture of the parts. Firing is done to a peak temperature (the "dielectric maturation temperature") both to densify the dielectric for maximum dielectric constant and physical strength, and to react the chemical constituents of the dielectric so that other desired electrical characteristics are achieved. During the firing step, the powder grains in the electrode layers also sinter and densify so as to produce a continuous, highly electrically-conductive metal film.
A principal problem in multilayer ceramic capacitor manufacture originates from the firing of the electrode and dielectric films simultaneously. During firing, a common physical defect termed a "delamination" is formed. Delaminations are defined as separations of the electrode and dielectric layers which cause a void in what is meant to be a monolithic solid body. Delaminations are detrimental to finished capacitor performance since they can cause electrodes of opposite polarity to contact and form an electrical short. Delaminations can also trap liquid solutions used in subsequent processing. These solutions can leave electrical charge carriers in the delamination voids and reduce the insulation resistance of the finished part. For high parts yields, and maximum reliability of the finished parts, delaminations must be closely controlled, and preferably eliminated entirely.
The dielectric maturation temperature determines the metal powders used in the electrode. The metals are chosen so as to have melting points above the dielectric maturation temperature to avoid melting the electrode during firing and the formation of discontinuous fired metal films. For example, dielectrics which mature at 1350.degree. C. require use of electrode compositions which contain only palladium, gold, or platinum powders or their mixtures, due to the high melting point of these metals and their alloys. Lower firing dielectrics, such as those which fire at 1100.degree. C., typically permit use of electrode compositions which contain mixtures of palladium and silver in the weight ratio of 30%:70%. Dielectrics which can be fired in non-oxidizing atmospheres permit use of non-precious metal electrodes.
Dielectrics are usually designed to have the highest dielectric constant and other optimized electrical properties, while firing at the lowest possible maturation temperature so as to use inexpensive metals such as silver. To maximize dielectric constant, particularly in low (approximately 1100.degree. C.) firing dielectric systems, a flux composed of a glass with softening points in the range of 500.degree. to 700.degree. C. is usually added to promote densification of the dielectric. Low melting fluxes which are crystalline in nature can also be used.
During firing of capacitors made with low-firing dielectrics, the electrode film begins to density first. This is due to the presence of large amounts of silver in the film. Silver has a melting point of about 962.degree. C. and begins to sinter and densify at about 600.degree. C. Dielectrics which mature at around 1100.degree. C. typically begin to sinter and shrink at about 800.degree. to 1000.degree. C. The mismatch between the sintering shrinkage temperatures of the electrode and dielectric is a basic cause of delaminations.
In low fire dielectrics which contain a flux additive, these stresses due to differential shrinkage can be partially alleviated by flow of the molten flux additive. In addition, it is known in the art that addition of metal oxides to the electrode composition can move the sintering shrinkage of the electrode metal powders to higher temperatures to reduce mismatch with the dielectric and relieve the stresses formed due to differential electrode/dielectric shrinkage. Barium titanate, for example, is added as a sintering inhibitor since it will not adversely effect the electrical properties of the dielectric should some be incorporated into the dielectric layers during firing.
The key to the effectiveness of the sintering inhibitor additive is that it must be insoluble in the metals of the electrodes, and can be well-dispersed throughout the electrode paste composition so as to block interparticle contact of the electrode metal grains and keep them from sintering. In general, it is desired to add as little oxide inhibitor as possible to avoid increasing the electrical resistance of the fired electrode, yet ensure sintering shrinkage compatibility with the dielectric. But even with optimum additions of oxide sintering inhibitor, sintering shrinkage matching with the dielectric cannot be fully realized: the mechanism of stress relief due to flow of the flux additive cannot, in some cases, alleviate the stresses, and a single delamination or multiple delaminations will form.
This problem is exacerbated in the case of high firing dielectrics where flux additives are not present and high firing temperatures and long peak temperature soak times must be used to densify the dielectric. In this case also, additives such as barium titanate are used to avoid delaminations.
Furthermore, as dielectric technology advances, dielectrics which are intended to be matured at temperatures such as 1100.degree. C. are being invented which contain fine-grained starting materials which require no flux additives to achieve maximum densification. U.S. Pat. No. 4,640,905, for example, describes such a material. Without these flux additives, the mechanism for alleviating sintering shrinkage mismatch is absent, and delaminations are likely to occur, even with the addition of conventional oxide sintering inhibitors. A more effective means of alleviating the mismatch of the sintering shrinkage of the electrode with the dielectric is needed.
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