Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device comprising a first insulating film formed on a semiconductor substrate, a charge storage layer formed on the first insulating film, a second insulating film formed on the charge storage layer, and a control electrode formed on the second insulating film,

• forming the second insulating film comprising:
• forming a lower insulating film containing oxygen and a metal element;
• forming an upper insulating film on the lower insulating film using film forming gas containing at least chlorine;
• and
• thermally treating the lower insulating film and the upper insulating film in an atmosphere containing oxidizing gas, so that a chlorine concentration in the lower insulating film is reduced, the thermal treating being performed under a condition that the oxidizing gas does not completely permeate the lower insulating film.