Composite electronic substrate of alumina uniformly needled through with aluminum nitride
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates generally to the field of substrates for microcircuits and more directly to a ceramic composite substrate consisting of an aluminum oxide (Al 2  O 3 ) matrix containing electrically non-conductive, thermally conductive needles of aluminum nitride (AlN) uniformly distributed in the substrate in a geometry that allows for improving the heat dissipating abilities of the substrate. The methods for removal of heat from semiconductor devices whether discrete components, integrated circuits or microcircuits, are numerous. Examples of sundry methods are shown in U.S. Pat. Nos. 4,106,188; 4,069,498; 4,024,570; 3,969,754; 3,735,485; 3,723,176; 2,982,892; 2,967,984; 2,857,560; and 2,758,261. 
     In the present invention it is necessary to provide an electrically non-conductive substrate for a micropackage assembly having anisotropic thermal properties. That is, the substrate has improved thermal conductivity in the short transverse direction as is required for heat dissipation from the micropackage assembly and yet maintains good dielectric strength across the surface of the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 of the drawing is a diagrammatic representation of an embodiment of the invention; and, 
     FIG. 1a is an enlargement of a portion of FIG. 1. 
    
    
     DESCRIPTION 
     The figures of the drawing show a pictorial type view of the basic aluminum oxide (Al 2  O 3  O) substrate which contains aluminum nitride needles uniformly distributed such that their orientation is predominantly along the direction parallel to the short transverse direction of the substrate. The base material or matrix of the substrate is preferably aluminum oxide although other suitable dielectric substrate materials may be used. Aluminum oxide, which is intended for use in micropackage assembly, has excellent dielectric and mechanical strength properties but is somewhat limited in thermal conductivity. An improved anisotropic thermal conductivity in the short transverse direction which increases the suitability of the substrate for heat dissipation from the attached micropackage assembly, is provided by the higher thermal conductivity resulting from incorporation of needles of aluminum nitride which is also a dielectric. These dielectric, high thermal conductivity needles of AlN incorporated into the Al 2  O 3  matrix are preferentially aligned parallel to the short transverse dimension of the substrate. The AlN needles as a result of their high thermal conductivity, then serve as solid state heat pipes to dissipate heat away from the microcircuitry during operation thereby providing the desired anisotropic thermal conductivity of the substrate. 
     In a preferred method of preparing the composite substrate a slurry of aluminum oxide powder with necessary solvents, plasticisers and binders is first mixed, and the aluminum nitride needles are then added. The slurry is poured onto a moving sheet in the usual manner of a ceramic tape casting operation to form substrate material which may be about 0.025&#34; thick. The needles of aluminum nitride are uniformly distributed throughout the bulk of the substrate before drying and the concentration of aluminum nitride may be from a few percent up to about 50 volume percent of the total without having adjacent needles in contact with each other in the finished substrate. In an important step of the invention an electric field is applied to align or orient the AlN needles parallel to the short transverse dimension of the substrate while still in the slurry state and while the solvents are being evaporated. As a result of the application of the electric field the aluminum nitride needles are aligned through the thickness of the substrate so that the substrate has anisotropic thermal properties, wherein the needles serve as solid state heat pipes to dissipate heat away from the microcircuitry. Following drying the substrate is fired to eliminate the remaining organic materials and to sinter the composite ceramic substrate to good density.