Method for the preparation of discrete substrate plates of semiconductor silicon wafer

A method for the preparation of discrete substrate plates of semiconductor silicon wafer deeply diffusion-doped on one surface which comprises the steps of:

• (a) subjecting a single crystal wafer of silicon semiconductor, of which the crystallographic orientation of the surface plane is either along the (111) plane or along the (100) plane of the single crystal silicon, after a simultaneous lapping treatment of both surfaces to a first doping treatment with a dopant by heating the silicon wafer in an atmosphere containing the dopant element so as to deposit the dopant on both of the surfaces of the silicon wafer;
• (b) subjecting the silicon wafer after the first doping treatment with the dopant to a second doping treatment of the dopant by heating the silicon wafer at a temperature in the range from 1250.degree. to 1310.degree. C. in an atmosphere of a gaseous mixture consisting of from 99.5% to 90% by volume of argon or helium and from 0.5% to 10% by volume of oxygen for a length of time in the range from 20 hours to 450 hours so as to cause diffusion of the dopant to the core portions of the silicon wafer from the respective surfaces leaving an undoped layer in-between;
• (c) slicing the silicon wafer after step (b) on an internal-blade slicing machine with an annular blade along the center plane of the undoped layer into two silicon wafers each having a thickness smaller than one half of the thickness of the starting silicon wafer and each having a laminar structure consisting of a doped layer and an undoped layer;
• and
• (d) lapping the surface of the undoped layer of each of the silicon wafers obtained in step (c) into a mirror-polished surface, the thickness of the single crystal silicon in step (a) and the thickness of the annular blade in step (c) being correlated by the inequalities
• 2(x.sub.j +x.sub.i)+t.sub.c +75.ltoreq.t.sub.2 .ltoreq.2(x.sub.j +x.sub.i)+t.sub.c +300,
• in which t.sub.2 is the thickness of the single crystal wafer in .mu.m, t.sub.c is the thickness of the annular blade in .mu.m, x.sub.j is the thickness in .mu.m of each of the doped layers formed in step (a), and x.sub.i is the thickness in .mu.m of the undoped layer in each of the discrete substrates after step (d).