This invention is an oxynitride film forming method including: a reaction chamber
heating step of heating a reaction chamber to a predetermined temperature, the
reaction chamber containing an object to be processed; a gas heating step of heating
a process gas to a temperature not lower than a reaction temperature at which an
oxynitride film can be formed, the process gas consisting of dinitrogen oxide gas;
and a film forming step of forming an oxynitride film on the object to be processed
by supplying the heated process gas into the heated processing chamber. The temperature
to which the reaction chamber is heated in the reaction chamber heating step is
set at a temperature below a temperature at which the process gas undergoes a reaction.
