A cutting tool (1) formed of a silicon nitride-based sintered body (2) including a matrix phase (3), a hard phase (4), and a grain boundary phase (10) in which a glass phase (11) and a crystal phase (12) exist. The sintered body (2) contains yttrium in an amount of 5.0 wt % to 15.0 wt % in terms of an oxide, and contains titanium nitride as the hard phase (4) in an amount of 5.0 wt % to 25.0 wt %. In an X-ray diffraction peak, a halo pattern appears at 2 ranging from 25 to 35 in an internal region of the sintered body (2). A ratio B/A of a maximum peak intensity B to a maximum peak intensity A satisfies 0.11B/A0.40 . . . Expression (1) in a surface region of the sintered body (2), and satisfies 0.00B/A0.10 . . . Expression (2) in the internal region of the sintered body (2).

A preparation method for a copper plate-covered silicon nitride ceramic substrate is provided. The structure of the copper plate-covered silicon nitride ceramic substrate includes a silicon nitride ceramic substrate, copper sheets disposed on the upper and lower sides of the silicon nitride ceramic substrate and soldering layers disposed between the copper sheets and the silicon nitride ceramic substrate; the composition of the silicon nitride ceramic substrate comprises a silicon nitride phase (more than or equal to 95 wt %); and a grain boundary phase (containing at least three elements (Y, Mg and O) and less than or equal to 5 wt %, and the content of a crystalline phase in the grain boundary phase is more than or equal to 40 vol %); and the sintering aids are Y.sub.2O.sub.3 and MgO. The two-step sintering process comprises: in a nitrogen atmosphere, performing low-temperature heat treatment and high-temperature heat treatment in sequence.

A silicon nitride-type sintered body has a total content of at least one metal M selected from the group consisting of Mg. Ca and Y of from 0.2 to 8.0% by mass, an Al content of from 4.0 to 12.0% by mass, an O content of from 4.0 to 12.0% by mass, and a fracture toughness value of from 5.0 to 10.0 MPa.Math.m.sup.1/2.

An electrostatic chuck is provided. Implemented according to an embodiment of the present invention is an electrostatic chuck comprising: a silicon nitride sintered body; a surface modification layer covering at least a portion of the external surface of the silicon nitride sintered body and having corrosion resistance and plasma resistance; and an electrostatic electrode laid inside the silicon nitride sintered body. Therefore, the electrostatic chuck includes a ceramic sintered body of silicon nitride, and thus has excellent plasma resistance, chemical resistance, and thermal shock resistance while exhibiting an equivalent or similar level of heat dissipation performance compared to ceramic sintered bodies of aluminum nitride that have been conventionally widely used, so that the electrostatic chuck can be widely used in semiconductor processes.