Patent ID: 7338555

Claim:
A process for producing a highly crystalline aluminum nitride multi-layered substrate by nitriding a single-crystal α-alumina substrate in the presence of carbon, nitrogen and carbon monoxide to form a crystalline aluminum oxynitride layer and a highly crystalline aluminum nitride film as an outermost layer directly on the crystalline aluminum oxynitride layer, said highly crystalline aluminum nitride film as an outermost layer having a film surface smoothness in terms of Ra and RMS of 100 nm or less, wherein nitriding is carried out under reaction conditions which satisfy the following conditions a to d: a: the equilibrium partial pressure of Al 2 O(g) in the reaction formula (1) is 5×10 −4 bar or less: Al 2 O 3 ( s )+2C( s )=Al 2 O( g )+2CO( g ) (1) b: the equilibrium partial pressure of Al 2 O(g) in the reaction formula (2) is 5×10 −6 bar or less: 2AlN( s )+CO( g )=Al 2 O( g )+C( s )+N 2 ( g ) (2) c: the reaction formula (3) proceeds to the right side: 3 32 - x ⁢ alon ⁢ ⁢ ( AlNsat . ) ⁢ ( s ) + 3 ⁢ C ⁡ ( s ) + ⁢ N 2 ⁡ ( g ) = 64 + x 32 - x ⁢ AlN ⁡ ( s ) + 3 ⁢ CO ⁢ ⁢ ( g ) ( 3 ) wherein alon (AlNsat.) means aluminum oxynitride which is saturated aluminum nitride, and d: the reaction temperature is 1,630° C. (1,903K) or higher and lower than 1,700° C. (1,973K).