Projecting elongate stub walls are provided on the planar surfaces of a substrate at positions where bonding of the substrate to a clamping lid or base is to be carried out. On firing of the substrate, the surfaces thereof are mechanically processed but since the stub walls protrude from the substrate, the grinding and polishing tools make contact with the surfaces of these stub walls, rather than with the entire substrate surface. As a result, the area of the substrate to be processed is minimised and problems with dishing and erosion are alleviated. This allows the clamping lid, or frame to be bonded, using conventional conductive adhesive processes, avoiding the cracking and stress problems associated with non-uniformity of the surface of the ceramic substrates.