The present invention relates to a process for making a ceramic green body of a type which may be fired into a thin ceramic sheet or tape for use in a multilayer capacitor.
A multilayer capacitor is a stack of a number of ceramic capacitor sheets connected in parallel to produce a capacitor of high total capacitance. These capacitors may be very small, and typically comprise between 40 and 50 ceramic sheets, each about 20 .mu.m thick. The entire capacitor may be only about 1 mm thick. These devices find many uses in microcircuitry.
However, there is a need for even thinner ceramic bodies, within the range of about 3 to about 10 .mu.m in thickness. This is due to the fact that equivalent capacitance could be achieved using only half as many of these thinner sheets, since capacitance is inversely proportional to the thickness of the ceramic sheet. The reduction in the number of sheets would then result in a savings in material costs, because the electrode layers interspersed between the ceramic layers are made of expensive noble metals, and with fewer, thinner sheets, fewer electrode layers would be required. In the past it has been very difficult to make sheets of this reduced thickness because current commercial methods of producing the green (unfired) tapes involve processing of solids, including ceramic powders.
These ceramic powders, which are dielectric, are dispersed into an organic solvent such as a methyl ethyl ketone/ethanol mixture. The powders are often barium titanate admixed with other compounds such as strontium titanate, lead titanate, calcium zirconate, lead oxide, borates and silicates. Ball-milling is generally necessary to maximize dispersion in the solvent, and often requires several hours at a minimum. The dispersed powders are then mixed with polymeric organic binders, plasticizers and surfactants to form a slip, which is tape-cast onto a nonporous substrate and dried in an oven to form a flexible "green tape."
To produce a capacitor, this green tape or green body is screen-printed with a noble metal electrode ink. Forty or fifty layers, typically, of the printed tape are then laminated and, after dicing into chips or sheets, fired to burn off the organic binder. The inorganic chips are sintered at high temperatures, typically between about 1000.degree. C. and about 1400.degree. C., to densify the sheets and improve their strength and conductivity. The result is a mechanically and electrically acceptable device.
A problem very commonly faced in this process, however, that tends to militate against reducing the thickness of the sheets or tapes beyond thicknesses currently being produced, is the fact that ceramic powders are usually agglomerated when received and remain so to some extent even after lengthy ball-milling prior to and after dispersion in a solvent. This agglomeration makes it difficult to produce sheets of only about 3 .mu.m to about 10 .mu.m in thickness that are of uniform quality, since the size and shape of the initial ceramic particles is a critical factor in producing a good quality final product. It also tends to produce sheets that exhibit less than optimum loading levels, and the reduced density increases the degree of shrinkage of the tape during firing. Ideally, the particles should be uniformly sized and equiaxially shaped, and should also demonstrate high purity.
Therefore, because of the problems associated with the ceramic powders and their general unsuitability to producing the thinner ceramic sheets now sought, it would be desirable to have a ceramic green body prepared from a slip formulation which does not generally require ball-milling or other milling steps at any point and for which powder agglomeration does not present a significant problem, which exhibits good ceramic loading levels, and which therefore does not generally require processing of dry ceramic powders, with the accompanying quality problems and thickness limitations that such processing entails. The present disclosure describes such an invention and involves an in situ finely dispersed slurry that may be used to produce a ceramic green body.