Patent ID: 8791566

Claim:
An aluminum nitride substrate suitable for a circuit board, comprising: a polycrystal, wherein the polycrystal further comprises: a plurality of aluminum nitride grains; and a plurality of complex oxide grains including a rare earth element and aluminum at grain boundaries of the aluminum nitride grains, wherein the plurality of aluminum nitride grains have a maximum aluminum nitride grain size of 10 μm or less, the plurality of complex oxide grains have a maximum oxide grain size smaller than the maximum aluminum nitride grain size, a number of complex oxide grains in the plurality of complex oxide grains of grain size of 1 μm or more in an area of 100 μm×100 μm of a surface of the aluminum nitride substrate observed is 40 to 70, an average grain size of the complex oxide grains is from 2 to 3 μm, a content of the rare earth element with respect to the aluminum nitride substrate is from 3% by mass to 6% by mass in terms of a rare earth oxide equivalent amount, the aluminum nitride substrate has a relative density of 99% or more, the aluminum nitride substrate has a bending strength of 400 MPa to 500 MPa in an unpolished state after firing, the aluminum nitride substrate has a volume resistivity of 10 12 Ωm to 10 15 Ωm, and the aluminum nitride substrate has a thermal conductivity of 160 W/m·K to 190 W/m·K; wherein the aluminum nitride substrate has the bending strength of 450 MPa to 550 MPa when the aluminum nitride substrate is polished to a surface roughness Ra of up to 1 μm after firing.