ALUMINUM NITRIDE SUBSTRATE, ALUMINUM NITRIDE CIRCUIT BOARD, SEMICONDUCTOR DEVICE, AND METHOD FOR

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Provided is an aluminum nitride substrate having excellent insulating characteristics, excellent heat dissipating characteristics and a high strength. An aluminum nitride circuit board and a method for manufacturing an aluminum nitride substrate are also provided. The aluminum nitride substrate is composed of a polycrystal, which is provided with a plurality of aluminum nitride crystal grains and composite oxide crystal grains which exist on the grain boundaries of the aluminum nitride crystal grains and include a rare-earth element and aluminum, and the aluminum nitride substrate has the aluminum nitride as the main component. The maximum diameter of the aluminum nitride crystal grain is 10 μm or less, and the maximum diameter of the composite oxide crystal grain is smaller than the maximum diameter of the aluminum nitride crystal grain. In a viewing field of 100 μm×100 μm wherein the surface of the aluminum nitride substrate is observed, 40 or more composite oxide crystal grains of 1 μm or more exist, the deflective strength of the substrate in the unpolished state after baking is 400 MPa or more, and the volume resistivity of the substrate is 1012 Ωm or more.