Patent ID: 7338723

Claim:
An aluminum nitride substrate comprising: a planar sintered aluminum nitride body containing carbon and including free carbon, the sintered aluminum nitride body comprising a work mounting portion for placing a work thereon and a base layer; and an electrode having a substantially planar configuration buried in the base layer of the sintered aluminum nitride body; wherein an average concentration of free carbon in the work mounting portion is higher than an average concentration of free carbon in the base layer, and wherein the average concentrations of free carbon are 500 ppm or lower; wherein an average concentration of all carbon per unit volume of the work mounting portion is lower than an average concentration of all carbon per unit volume of the base layer; and wherein the average concentration of free carbon is greater than 80 ppm in the work mounting portion, and wherein the average concentration of free carbon is not greater than 70 ppm in both of a region of the base layer between planar segments of the electrode and a region of the base layer below the electrode.