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The development of more complex integrated circuits (IC's) with larger chip sizes and greater power generation IC's, emitter-coupled logic (ECL) circuits, very large-scale integration (VLSI), and laser diodes requires new packaging schemes with improved thermal management. From the limited number of some ten nonmetallic inorganic solids known from the literature to have thermal conductivities greater than 100 W/m°K, aluminum nitride (AIN) is the most attractive substrate material. By means of a suitable power technological process, an AIN ceramic has been produced with a thermal conductivity five times higher than that of alumina, which is about one half of the theoretical value estimated for defect-free single crystal AIN. Another advantage of this newly developed material is its low coefficient of thermal expansion which, in the temperature range of interest (20-200°C) closely matches that of silicon. It also has a high electrical resistivity and a moderate dielectric loss. These and some other physical, mechanical, and chemical properties are shown and partly discussed in relation to other ceramics in use or of potential interest as substrate materials.