Method of manufacturing semiconductor device, and recording medium

A method of manufacturing a semiconductor device, comprising:

• (a) loading a substrate including a base and a first film containing silicon and formed on the base into a process container;
• (b) converting a modifying gas containing helium into plasma to generate reactive species of helium;
• and
• (c) supplying the modifying gas containing the reactive species of helium to a surface of the substrate to respectively modify the first film and an interface layer of the base constituting an interface with the first film,
• wherein in (c), a total thickness of the first film after modification and the interface layer whose composition has been changed by modification is adjusted by a ratio (B/A) of a flow rate B of an oxidant or a nitriding agent supplied into the process container to a flow rate A of a helium gas supplied into the process container.