Interferometer, exposure apparatus, exposure method and interference length measurement method

An interferometer, comprising:

• a light source;
• a beam splitter, which is disposed in the optical path of the light emitted by the light source and which splits the light into a measurement beam and a reference beam;
• a fixed reflector, which is disposed in the optical path of the reference beam;
• a moving mirror, which is disposed in the optical path of the measurement beam and includes at least two mutually orthogonal reflection planes;
• and
• an optical member, which is disposed in the optical path of the measurement beam reflected by the moving mirror and which returns the measurement beam toward the moving mirror in a manner causing the incident measurement beam to be reflected odd number of times before exiting therefrom.

An interference length measurement method, comprising:

• a first step of splitting light emitted by a light source into a measurement beam and a reference beam;
• a second step of causing the reference beam to be reflected by a fixed reflector;
• a third step of causing the measurement beam to be reflected by a moving mirror that includes at least two mutually orthogonal reflection planes;
• a forth step of returning the measurement beam having traveled via the moving mirror by an optical member;
• and
• a fifth step of detecting the reference beam reflected by the fixed reflector and the measurement beam returned by the optical member via the moving mirror,
• wherein, in the forth step, the measurement beam exits from the optical member at the point different from the incident point while the optical member keeping the shifted direction and shifted amount between the incident light rays, incident points of which are shifted from each other, unchanged.