Disclosed is a method for manufacturing a semiconductor device provided with a sidewall having a high quality and an excellent shape. The sidewall on a gate electrode side wall is formed using a carbon-containing silicon nitride oxide film. The film can be formed by a CVD method using, as starting materials, BTBAS and oxygen where a BTBAS flow rate/oxygen flow rate ratio is appropriately set and a low film formation temperature is set, for example, at about 530° C. When forming the sidewall using this film, improvement in HF resistance and reduction in fringe capacitance can be realized due to contribution of nitrogen atoms and carbon atoms. Further, when forming this film under low temperature conditions, unnecessary diffusion of impurities introduced into a semiconductor substrate can be suppressed. Thus, transistor characteristics are enhanced and stabilized, so that high performance and high quality in the semiconductor device can be realized.