when molded plates of aluminum nitride obtained by molding a ceramic powder formed preponderantly of aluminum nitride powder are superposed on a support base with a ceramic powder interposed between the support base and the superposed plates and between the superposed plates and are fired in a non - oxidative atmosphere , the sintered plates consequently produced are prevented from sustaining crack or fracture . when warped and undulated sintered plates of aluminum nitride are superposed on a support base with a ceramic powder interposed between the support base and the superposed plates and between the superposed plates and are heated in a non - oxidative atmosphere , the heated plates are relieved of warp and undulation . the sintered plates of aluminum nitride are enabled to acquire improved adhesiveness to a metal film or foil when they are subjected to a heat treatment adapted to reduce the surface roughness to or below 10 μm .