A method for manufacturing an SOI substrate superior in film thickness
uniformity and resistivity uniformity in a substrate surface of a silicon
layer having a film thickness reduced by an etch-back method is provided.
After B ions is implanted into a front surface of a single-crystal Si
substrate 10 to form a high-concentration boron added p layer 11 having a
depth L in the outermost front surface, the single-crystal Si substrate
10 is appressed against a quartz substrate 20 to be bonded at a room
temperature. Chemical etching is performed with respect to the
single-crystal Si substrate 10 from a back surface thereof to set its
thickness to L or below. A heat treatment is carried out with respect to
an SOI substrate in a hydrogen containing atmosphere to outwardly diffuse
B from the high-concentration boron added p layer 11, thereby acquiring a
boron added p layer 12 having a desired resistance value. During this
heat treatment, B in an Si crystal is diffused to the outside of the
crystal in a state where it is coupled with hydrogen in the atmosphere,
and a B concentration in the high-concentration boron added p layer 11 is
reduced. In regard to a heat treatment temperature at this time, in view
of a softening point of the insulative substrate, an upper limit of the
heat treatment temperature is set to 1250.degree. C., and 700.degree. C.
is selected as a lower limit of the temperature at which B can be
diffused.