The present invention generally relates to metallic features in bulk ceramic materials and, more particularly, to a method of controlling the densification behavior of the metallic feature in the ceramic material by incorporating in the metallic feature an amount of carbonaceous material which inhibits the sintering of the metallic feature.
While the present invention has applicability to, in general, metallic features in ceramic materials, it has particular application to metallic screened lines or vias in ceramic substrates for electronic applications. Therefore, for the sake of clarity only, the remainder of the discussion of the invention will be concentrated on metallic vias in ceramic substrates.
The use of metal-filled vias and/or screened lines in ceramic substrates and sintering processes for producing them are well known in the semiconductor packaging art as taught, for example, in Herron et al. U.S. Pat. No. 4,234,367, the disclosure of which is incorporated by reference herein. Recently, more interest has been focused on the associated problems of the disparity in shrinkage rates between the metal and ceramic as well as the onset of via "opens", particularly as via diameters are reduced below 100 microns in high circuit density applications. A discussion of such problems is given in Herron et al. U.S. Pat. No. 4,776,978, the disclosure of which is incorporated by reference herein.
As set forth in the cited U.S. Pat. No. 4,776,978, metal particles, such as copper, in the via paste undergo sintering with attendant shrinkage of the thick film pattern (also consisting of the paste) during the initial phase of the sintering cycle whereas the ceramic and glass particles (of the ceramic substrate containing the vias) undergo sintering during the intermediate and final phases of the sintering cycle along with their characteristic shrinkage. One method of delaying the onset of sintering of the metal particles until at least the intermediate phase of the sintering cycle is to intersperse the metal particles in the thick film with a high melting point material such as aluminum oxide.
Although the foregoing generalized considerations have been known in the art for some time and have provided the basis for techniques for overcoming previous shrinkage and related problems, more refined and detailed approaches are required to meet the needs of metal-filled vias and/or screened lines in ceramic substrates with increasing circuit densities and the concomitant via diameters in the range of about 85 to 100 microns. It is also desirable to provide a metallic-based paste mixture which can be adapted for use with the next generation of ceramic packages which may exhibit reduced shrinkage or no shrinkage with sintering wherein the fired metallic conductive features would maintain acceptable electrical conductivity to meet product specifications.
The following references illustrate previous techniques attempting to overcome shrinkage and other problems.
Herron et al. U.S. Pat. No. 4,671,928 teaches the coating of copper particles with an organic material. The coated copper particles are incorporated in a paste and cofired with the ceramic bulk material. The coating on the copper particles is pyrolyzed to a carbonaceous residue during the pyrolysis stage of the sintering cycle. The carbonaceous residue retards the sintering of the copper particles so that they do not begin to densify until the binder burnoff stage of the sintering cycle when an oxidizing ambient is introduced which removes the carbonaceous residue. Once the carbonaceous residue is removed, the copper particles can easily densify. In fact, the copper particles will have densified before the ceramic material sinters.
Siuta U.S. Pat. No. 4,594,181 teaches the dispersal of copper particles in a solution of an organometallic compound in an anhydrous volatile organic solvent towards obtaining a better shrinkage match of copper to ceramic substrates during sintering.
Beil U.S. Pat. No. 4,020,206 discloses a thick film paste for vias which consists of gold particles, glass binder and refractory particles. Among the refractory particles are metallic carbides such as silicon carbide. The purpose of the refractory particles is that since they do not melt at the sintering temperature of the gold paste, they serve to reduce the volumetric shrinkage of the vias to less than 5%
Reed U.S. Pat. No. 4,964,948 discloses generally a conductive ink consisting of silver flakes, carbon black and fumed silica.
A purpose of the present invention is to have a method of sintering a composite structure comprising metallic features in a ceramic material wherein the metallic features have reduced shrinkage during the sintering of the ceramic material.
Another purpose of the present invention is to have a method of sintering a composite structure comprising metallic features in a porous ceramic material wherein the metallic features have reduced shrinkage (or no shrinkage) during the sintering of the low shrinkage (or no shrinkage) porous ceramic material, and where metallic shrinkage can subsequently be promoted post stabilization of the porous ceramic material for optimization of electrical conductivity of the metallic component.
These and other purposes of the present invention will become more apparent after considering the following description of the invention.