in the present invention , a ceramic powder consisting of at least 95 %, preferably 96 - 99 %, of aluminum nitride powder and not more than 5 %, preferably 1 - 4 %, of a sintering aid is combined with a binder such as paraffin and allyl type resin and optionally an organic solvent . the resultant mixture is molded in the form of plates not more than 2 mm , particularly 0 . 3 - 1 mm , in thickness by the press molding method , the slip casting method , the doctor blade method or some other proper molding method known to the art . examples of the aforementioned sintering aid include oxides of rare earth element such as yttrium oxide , oxides of aluminum , magnesium , calcium strontium , and barium . not more than 20 such molded plates of the mixture are superposed on a ceramic support base , with a ceramic powder of an average particle diameter of 1 to 50 μm interposed between the support base and the lowest molded plate and between the individual molded plates . examples of the aforementioned ceramic powder include boron nitride powder , aluminum nitride powder , and alumina powder . among other ceramic powders cited above , the boron nitride powder proves particularly desirable . for the ceramic powder to fulfil its purpose advantageously , the grain size thereof is required to fall in the range of 1 to 50 μm , preferably 1 to 20 μm . this ceramic powder can be applied to the surface of the support base and on the surface of the molded plates of mixture by suspending the ceramic powder in an organic solvent such as acetone , alcohol , etc ., and directly applying the resultant suspension on the surface by means of a spray , a brush , or a roll by dipping the surfaces in the suspention , or by simply sprinkling the supension on the surfaces . the deposition of the ceramic powder on the molded plates may be effected on both sides or on only one side . the layer ceramic powder deposited on the surface of the molded plates is desired to have a thicknbess of not less than 1 μm . the plurality of molded plates of mixture are placed in an atmosphere formed of one more non - oxidative gases such as n and ar or a vacuum atmosphere and burned at a temperature in the range of 1 , 600 ° to 1 , 850 ° c ., preferably 1 , 700 ° to 1 , 800 ° c ., for a period of one to three hours , under appication of no pressure or under a load of not more than 400 kg / cm 2 . the sintered aluminum nitride plates consequently obtained experience virtually no occurrence of cracks or fractures because the superposed plates of mixture are effectively prevented from mutual fusion by the intervening ceramic powder during the course of the firing . then , the sintered plates of aluminum nitride obtained as described above are given a corrective treatment for the elimination of warp and undulation by the following method . five to ten such sintered plates of aluminum nitride are superposed on a support base , with another support base placed as a weight on the highest of the superposed plates . optionally a plurality of sets of superposed plates are piled up , capped with a weight . the superposed sintered plates are placed in an atmosphere formed of one or more non - oxidative gases such as n 2 and ar or in a vacuum atmosphere and heated therein for a period of 10 minutes to three hours under application of no pressure or under a load of not more than 400 kg / cm 2 . for the purpose of preventing the sintered plates from mutual fusion and keeping grains from further growth , this heating is desired to be carried out at a temperature 50 to 100 centigrade degrees lower than the temperature at which the molded plates of alumina nitride were burned . specifically , this temperature is desired to fall in the range of 1 , 500 ° to 1 , 800 ° c ., for example . the ceramic powder to be used for separating the sintered plates is the same as that which has been used for separating the molded plates . further , the method by which the ceramic powder is interposed between the opposed surfaces is the same as that which has been used for the molded plates of aluminum nitride . for the ceramic powder to fulfil its purpose advantageously , the required to be not less than 1 μm . the sintered aluminum nitride plates are honed with alundum particles 100 to 1 , 000 mesh in grain size or ground with diamond particles 100 to 600 mesh in grain size to remove the ceramic powder adhered on the surface until the surface roughness is decreased to below 10 μm ( in trerms of maximum height , rmax ). the sintered aluminum nitride plates which have been obtained by the method described above optionally have their surface oxidized to be coated with a stable alumina layer 1 to 10 μm in thickness so as to facilitate adhesion thereto of a conductor foil or conductor film . this alumina layer can be formed by heating the surface in the air or some other oxidative atmosphere at a temperature in the range of 1 , 000 ° to 1 , 400 ° c . for a period of 0 . 5 to 10 hours . the surface roughness which the sintered aluminum nitride plates are required to possess varies with the nature of the use for which the plates are intended . properly , the surface roughness is not more than 5 μm where the plates are substrates for thick layers using au , ag / pd , cu , glass , and resistor ; not more than 2 μm where the plates are substrates for thin layers using cu , tl , ag , and au ; not more than 6 μm where the plates are substrates to which copper patterns are directly joined ; and not more than 10 μm where the plates are substrates for structures . in the case of substrates for thickn layers , the surface roughness is desired to be not leas than 2 μm because excessively high smoothness results in loss of adhesive strength of layers . now , the present inventioin will be described below with reference to working examples . a mixture obtained by combining aluminum nitride powder containing 3 % by weight of yttrium oxide with 7 % by weight of acryl type resin as a binder and further adding thereto an organic solvent was molded by the doctor blade method to produce a thin plate 1 mm in thickness . rectangular pieces 80 × 40 mm were punched out of the thin sheet and were degreased in a nitrogen atmosphere at amount 700 ° c . for one hour . separately , 10 g of boron nitride powder having an average particle diameter of 3 μm was suspended in 100 cc of acetone . with a brush , the resultant suspension was applied to the surface of a support base made of aluminum nitride and the surface of the aforementioned rectangular pieces of aluminum nitride to form thereon a layer of boron nitride powder 15 μm in thickness . then , on the support base on which the boron nitride powder had been applied as described above , ten of the rectangular pieces of aluminum nitride ceramic mixture were superposed with the same boron nitride powder interposed between the superposed rectangular pieces . the superposed rectangular pieces were placed in a container of alumina and burned under atmospheric pressure of n 2 gas at 1 , 800 ° c . for two hours . the sintered rectangular pieces consequently obtained were not fused with the support base or with one another and the boron nitride powder could be easily removed from the sintered rectangular pieces . the sintered rectangular pieces possessed a good surface shape showing no discernible sign of crack or warp . a mixture obtained by combining aluminum nitride powder containing 3 % by weight of yttrium oxide with 7 % by weight of acryl type resin as a binder was processed by following the procedure of example 1 , to give rise to rectangular pieces measuring 80 mm × 40 mm × 0 . 8 mm . these rectangular pieces were similarly degreased . then , 20 such rectangular pieces were superposed in the same manner as in example 1 and burned in an atmosphere of nitrogen gas at 1 , 800 ° c . for two hours . part of the sintered rectangular pieces were found to be warped by a maximum size of about 2 mm in the direction of length . the sintered rectangular pieces which were warped were collected . separately , 10 g of boron nitride powder about 400 mesh in grain size was suspended in 100 cc of acetone . the suspension was applied with a brush on the surface of a support base made of aluminum nitride and on the surface of the sintered rectangular pieces of aluminum nitride to form a layer of boron nitride powder about 1 - 10 μm in thickness . then , on the support base of ceramic on which the boron powder had been applied , 10 sintered rectangular pieces of aluminum nitride were superposed each in a total of five sets , with boron nitride powder interposed similarly between the opposed surfaces and support bases placed one each on the sets of superposed sintered rectangular pieces . the superposed rectangular pieces on the support base ( about 0 . 5 kg ) were placed in a container made of alumina and heated in nitrogen gas at about 1 , 700 ° c . for one hour to correct deformation . the sintered rectangular pieces after this heating had warp mended to flatness of not more than 80 . sub . μ m . the corrected sintered pieces were not fused with the support base or with one another . the boron nitride powder could be easily removed from the sintered pieces . a mixture obtained by combining aluminum nitride powder having particle diameters of 1 to 2 μm and containing 3 % of yttrium oxide with 7 % of acryl type resin as a binder and adding thereto an organic solvent was molded in the form of a plate , degreased in nitrogen gas at about 700 ° c . for one hours in the same manner as in example 1 , fired under atmospheric pressure , subjected to honing to remove the ceramic powder from the surface and then heated in the air at about 1 , 200 ° c . for one hour to give rise to a flat alumina nitride type ceramic substrate having a stabilized alumina layer about 8 μm in thickness formed on the surface thereof and possessing an alumina surface roughness of 5 μm ( rmax ). a thin copper sheet 0 . 3 mm in thickness was placed on the substrate and joined thereto at 1065 °- 1080 ° c . under application of heat to test the substrate for tight adhesiveness with the thin copper sheet , and electrical properties . consequently , the adhesive strength to the cooper sheet was found to be about 2 . 5 kg / mm 2 , and the electric resistance to be about 2 . 0 ω - cm . a substrate was produced by following the procedure of example 1 , using aluminum nitride powder of particle diameters of 2 . 5 to 4 . sup . μ m and 3 % of yttrium oxide . the produced substrate had a surface roughness of 13 . sub . μ m ( rmax ). this substrate was honed with abrasive abour 600 mesh in grain size to reduced the surface roughness to 8 . sub . μ m ( rmax ). when a thin copper sheet was joined to this substrate by following the procedure of example 3 , the results were similarly satisfactory . when a thin copper sheet was joined similarly to example 3 to the ( oxidized ) substrate of a surface roughness of 13 μm ( rmax ) obtained in example 4 , it was not joined uniformly throughout the entire surface area . the copper sheet partly peeled from the substrate .