Method of making ceramic substrate

DETAILED DESCRIPTION The present invention relates to multi-layer ceramic packaging of hybrid micro-electronic devices, including those for use in implantable medical devices.
The present invention permits size reduction and design simplification in such packaging by eliminating the need for electrolytic or electroless plating, and by eliminating or substantially eliminating the shrinkage variation typically associated with surface metallization techniques.
The present invention also provides a ceramic substrate where the number and topology of electrical interconnections or interconnects is simplified and reduced, and is particularly well adapted to flip chip applications because it permits surface metallization shrinkage variation to be eliminated or substantially eliminated.
Post-firing printing and component placement are aided in the present invention by the external vias not "posting," or having protrusions extending above the top surface of the ceramic substrate after the high-temperature or subsequent firing steps have been completed.
In a ceramic substrate of the present invention, external vias most preferably comprise ruthenium and ceramic and are disposed between internal metallization layers formed most preferably of tungsten or molybdenum, and external surface thin or thick film layers.
In a preferred method of making a ceramic substrate of the present invention, the substrate is fired in a high-temperature firing step at high temperatures in a reducing atmosphere, a thick film external surface layer is applied onto the top or bottom surfaces of the substrate, the substrate is fired in a low-temperature firing step in an inert atmosphere at low temperatures, and finally the substrate is fired at low temperatures in an air atmosphere.
Other objects, features and advantages of the present invention will become apparent upon referring to the appended drawings, detailed description of the preferred embodiments and the claims.