PURPOSE: To assure remarkable heat-conductivity of AlN ceramics substrate with purity of 95% by a method wherein the separated part of AlN needle type polycrystal is limited to 1/3 or less of substrate thickness.

CONSTITUTION: The purity of AlN ceramics is recommended to exceed 95% in terms of the thermal conductivity while adding 0.1W5wt% of Y2O3 as sintering assistant. AlN powder is formed into a substrate 2 by means of sintering it in N2 at 1,600W1,850°C as it is sealed up in a fire resistant vessel after removing grease. At this time, AlN needle type polycrystal 1 with Al-Si-O- N composition formed on the side only shall not exceed 1/3 of substrate thickness (t) while if they are formed on both sides, the total thickness shall be also limited to 1/3 of the same. In such a constitution, the heat-conductivity of AlN ceramics substrate may not be deteriorated at all.