Measurement apparatus

A measurement apparatus which measures a distance between a reference surface and a test surface, the apparatus comprising:

• a light source unit including a plurality of light sources each corresponding to one of a plurality of wavelength scanning ranges which are separate from each other and each continuously scans a wavelength of generated light in the corresponding wavelength scanning range of the plurality of wavelength scanning ranges;
• an interferometer unit which splits light emitted by each of the plurality of light sources into reference light and test light, and detects, as an interference signal, an interference fringe formed by the reference light reflected by the reference surface and the test light reflected by the test surface;
• and
• a processor which determines a slope of a phase of the interference signal with respect to wave number of the light based on the interference signal detected by the interferometer unit for each of the plurality of wavelength scanning ranges, and determines the distance from the slope of the phase,
• wherein the light source unit simultaneously scan the plurality of wavelength scanning ranges, and the interferometer unit simultaneously detects the interference fringes each formed by the light from the corresponding light source of the plurality of light sources,
• wherein the processor determines, based on a first interference signal detected by the interferometer unit for a first wavelength scanning range that is one of the plurality of wavelength scanning ranges, a first slope of a phase, that is a slope of a phase of the first interference signal with respect to wave number within the first wavelength scanning range, and a fractional component of a first phase that is a phase of the first interference signal at an arbitrary wave number falling within the first wavelength scanning range,
• determines, based on a second interference signal detected by the interferometer unit for a second wavelength scanning range that is another one of the plurality of wavelength scanning ranges, a fractional component of a second phase that is a phase of the second interference signal at an arbitrary wave number falling within the second wavelength scanning range,
• determines, based on the first slope of the phase, the fractional component of the first phase, and the fractional component of the second phase, a first interference-order-difference that is a difference between order of interference of the first phase and order of interference of the second phase, and
• determines, as an updated slope of a phase for determining the distance, based on the first interference-order-difference, the fractional component of the first phase, and the fractional component of the second phase, a second slope of a phase, that is a slope of a phase of an interference signal, containing the first interference signal and the second interference signal, with respect to wave number.