Method of manufacturing ceramic substrate for fine-line electrical circuitry

In accordance with the invention there is provided a sintered monolithic ceramic substrate for fine-line electrical circuitry, the substrate comprising a base layer containing at least 90% by weight crystalline alumina or beryllia and a thin upper layer containing at least 90% by weight crystalline material the same as that of the base layer but with an average grain size less than that of the crystalline material of the base layer, the upper layer having an irregular interface with the base layer but a planar upper surface with minimal surface irregularities for improved reception of the fine-line circuitry to be applied. Further in accordance with the invention such substrate is made by first forming and firing a ceramic plate of alumina or beryllia ceramic to provide the base layer and then applying and firing a coating of fine grain alumina or beryllia, as the case may be, on the plate to provide the upper layer.