A silicon nitride film is first formed on a semiconductor substrate and
serves as a polishing stopper film. Then, the silicon nitride film and the
semiconductor substrate are etched in a predetermined region to form an
isolating trench which partitions an active region. Next, a silicon
dioxide film is deposited on the semiconductor substrate so that the
isolating trench is filled with the silicon dioxide film. Next,
first-stage chemical mechanical polishing (CMP) is performed with a
SiO.sub.2 -contained slurry which can efficiently polish the surface of
the silicon dioxide film regardless of level difference. Finally,
second-stage CMP is performed with a CeO.sub.2 -contained slurry ensuring
a large polishing selectivity ratio of the silicon dioxide with regard to
silicon nitride films.
