Method of manufacturing semiconductor device and semiconductor device

1. A method of manufacturing a semiconductor device comprising the steps of:

successively forming a gate insulating film and a gate electrode on a semiconductor substrate;

forming a silicon nitride layer of a predetermined thickness by a low-pressure chemical vapor deposition method on a portion that covers the gate insulating film and the side portions of the gate electrode wherein the silicon nitride layer physically directly contacting sidewalls of the gate insulating film and the gate electrode;

exposing the silicon nitride layer to nitrogen under a decompressed atmosphere; and

forming a silicon nitride film that consists of a plurality of laminated silicon nitride layers by repeating the step of forming the silicon nitride layer and the step of exposing the silicon nitride layer to nitrogen.

2. The method of manufacturing a semiconductor device according to claim 1 , wherein the silicon nitride film is formed so as to be connected to the end portions of the gate insulating film that are exposed between the semiconductor substrate and the gate electrode.