In recent years, with downsizing and rapid popularization of electronic apparatuses represented by cellular phones, an increase in mounting density of the electronic parts used for these apparatuses and improvement in their performance are required. Especially, demands for thickness reduction, an increase in the number of layers and uniformization of each layer are placed on multilayer ceramic electronic parts that are used as passive elements, in order to meet the above requirements. In addition, development of the manufacturing method that can meet these requirements is also-demanded.
So called metal-ceramic composite sintering is an example of conventional manufacturing method used for manufacturing the aforementioned multilayer ceramic electronic parts such as multilayer ceramic capacitors having electrodes formed in the interior thereof, which can meet the aforementioned requirements. Here, the metal-ceramic composite sintering technology will be described briefly. In this technology, a plurality of electrodes are simultaneously formed on a so-called ceramic green sheet using an electrically conductive paste composed of a metal powder and an organic binder material.
Subsequently, a plurality of simple ceramic green sheets and ceramic green sheets on which electrodes have been formed etc. are laminated to form a ceramic laminated member. The electrodes will constitute internal electrodes of multilayer ceramic electronic parts when they are finished. In addition, the ceramic laminated member is pressed in its thickness direction so that the green sheets will be brought into close contact with each other. The laminated member brought into close contact is cut and separated to a certain size, and then subjected to sintering. On the outer surface of the sintered member thus obtained, external electrodes are formed fitly. Thus, a multilayer ceramic electronic part is obtained.
In recent years, further downsizing and thickness reduction of the aforementioned multilayer ceramic electronic parts have been required, and it is necessary to reduce the thickness of dielectric layers made of a ceramic or the like sandwiched between internal electrodes. Therefore, it is required to perform the above-described process while further reducing the thickness of ceramic green sheets that constitute the ceramic laminated member. In view of these requirements, the thickness of the thinnest ceramic green sheet presently used is as small as approximately 2 to 3 μm.
However, electrodes printed on the ceramic green sheets have a thickness of approximately 1.5 to 2.0 μm. Accordingly, when a ceramic laminated member is formed, the thickness of the portion where the internal electrode is laminated becomes very large as compared to the thickness of the portion where the internal electrode is not present. Consequently, unevenness is likely to be produced. In addition, such unevenness can cause lamination displacement upon laminating a green sheet.
As described above, the laminated member is pressed in the thickness direction after it is formed, and preferably, the above-mentioned unevenness will be nearly eliminated by the pressing process. However, a pressing pressure of, for example, as large as 1 ton/cm2 is required to achieve pressure contact of the green sheets and internal electrodes, and improvements in this process are required. In addition, since the applied pressure is localized only in the portions on which internal electrodes are present, there is a risk that insufficiency of pressing in the other areas or deformation after pressing may occur. Furthermore, such localization of applied pressure may sometimes lead to interlaminate separation called delamination in the laminated member after sintering.
Such deformations of the laminated member result in problems such as, for example in the case of the multilayer ceramic capacitor, variations in the electric capacity. Such problems contribute to generation of or increase in variations of electric characteristics of compact, highly-integrated, multilayer ceramic electronic parts such as multilayer ceramic inductors, LC composite parts and EMC related parts.
The technology disclosed in Japanese Patent Application Laid-Open No. 9-115766 was devised with a view to prevent generation of the above-mentioned unevenness. Specifically, in forming electrodes on a green sheet by screen printing, an electrically conductive paste composed of an organic binder having water-repellent properties and a metal powder is used. In addition, after electrodes are formed, water-based ceramic slurry is applied on them. The slurry is repelled from the electrode by virtue of the aforementioned water-repellent properties, so that a ceramic sheet that fills the unevenness is formed around the electrode patterns.
In this technology, however, a gap is formed between the side surface of the internal electrode and the water-based slurry, and there is a risk that displacement in lamination may be caused by the gap. Moreover, adhesion strength of the interface between the surface of the water-repellent electrode and the ceramic green sheet pressed against it may possibly be weaker than the adhesion strength between a conventional electrode and a sheet. Furthermore, it is considered that in the region surrounded by water repellent electrodes, the thickness of the water-based slurry is likely to be increased due to its surface tension. This may lead to a decrease in the degree of accuracy of lamination.
Japanese Patent Application Laid-Open No. 4-215414 discloses a technology in which recesses are formed at the positions on a ceramic green sheet at which electrodes are formed, and the recesses are filled with electrically conductive paste by screen printing to reduce unevenness caused by thickness of electrodes on a ceramic sheet. However, in the case where the green sheet is thin, the green sheet is deformed by the projections of the base member on which the green sheet is formed. Consequently, in the green sheet produced, projections are formed at the positions corresponding to the recesses on the surface of the green sheet opposite to the surface on which the recesses are formed.
Accordingly, when screen printing for filling the recesses with electrically conductive paste is performed, there is a risk that the recesses may deform to cause problems concerning accuracy of the shape of the recesses and their positions etc. Therefore, it is practically impossible to match the accuracy of recess formation and accuracy of electrode formation by screen printing, and there is a risk that unevenness or gaps may be created on the sheet due to the difference between these position accuracies or positional displacement in forming them. In addition, the presence of projections will promote generation of formation density difference when laminated layers are pressed as described in connection with the Japanese Patent Application Laid-Open No. 4-215414. Accordingly, although generation of unevenness is suppressed to some extent, it is considered that the above-mentioned variations in electric characteristics still occur.