In a process of forming MISFETs that have gate insulating films that are
mutually different in thickness on the same substrate, the formation of
an undesirable natural oxide film at the interface between the
semiconductor substrate and the gate insulating film is suppressed. A
gate insulating film of MISFETs constituting an internal circuit is
comprised of a silicon oxynitride film. Another gate insulating film of
MISFETs constituting an I/O circuit is comprised of a laminated silicon
oxynitride film and a high dielectric film. A process of forming the two
types of gate insulating films on the substrate is continuously carried
out in a treatment apparatus of a multi-chamber system. Accordingly, the
substrate will not be exposed to air. Therefore, it is possible to
suppress the inclusion of undesirable foreign matter and the formation of
a natural oxide film at the interface between the substrate and the gate
insulating films.