Substrate processing apparatus, method of processing substrate, method of manufacturing semiconductor device, and gas rectifier

A method of processing a substrate, comprising:

• accommodating a substrate in a process container including a port that is provided in the process container in which the substrate is accommodated and heat-treated, the port providing an optical path that penetrates an interior of the process container and an exterior of the process container;
• transmitting light through an optical window mounted to the port while maintaining the airtightness of the port;
• guiding a purge gas from a purge gas supplier disposed inside the process container to the optical window;
• and
• guiding the purge gas from the purge gas supplier to the optical window by a partition.

A gas rectifier comprising:

• a plate that is arranged inside a process container in which a substrate is accommodated and heat-treated, to face a port that provides an optical path penetrating an interior of the process container and an exterior of the process container, and to partially cover the port;
• and
• a partition that is provided in substantially parallel to the optical path,
• wherein the plate and the partition are provided between a purge gas supplier disposed adjacent to the port in the process container and an optical window mounted to the port.