Grazing incidence interferometer

A grazing incidence interferometer comprising:

• an interferometer main body having
• a light source configured to generate coherent original light,
• a light beam divider configured to divide the original light coming from the light source into measurement light and reference light,
• an illuminator configured to apply the measurement light obliquely to a measurement subject surface of a measurement subject,
• a light beam combining module configured to combine the measurement light reflected from the measurement subject surface with the reference light, and
• a photodetector configure to detect a combined light beam produced by the light beam combining module;
• a stage configured to hold the measurement subject;
• a moving mechanism mounted on the stage, and configured to support the interferometer main body and to be capable of moving the interferometer main body along the measurement subject;
• and
• an auxiliary reflector fixed to the stage and disposed on an extension of an axis of movement of the interferometer main body by the moving mechanism,
• wherein the interferometer main body has
• an auxiliary light beam separator configured to separate auxiliary light from the original light coming from the light source and to apply the separated auxiliary light to the auxiliary reflector, and
• an auxiliary photodetector configured to detect the auxiliary light reflected by the auxiliary reflector, wherein the auxiliary photodetector comprises an auxiliary light beam combining module configured to combine the auxiliary light coming from the auxiliary reflector and the original light coming from the light source and to supply a combined light beam to the light beam divider, and an auxiliary mask disposed on an optical path of the auxiliary light that goes from the auxiliary light beam separator to the auxiliary light beam combining module past the auxiliary reflector and configured to transmit only part, located in an auxiliary light region, of the auxiliary light,
• one of the light beam divider, the illuminator, and the light beam combining module has a measurement mask disposed in an optical path of the measurement light and configured to transmit only part, located in a prescribed measurement light region, of the measurement light, and
• the auxiliary light region of the auxiliary mask and the measurement light region of the measurement mask are set so as not to overlap with each other.