| 摘要 |
Interferometric apparatus and method for measuring changes in distance to an object are compensated for the presence of undesirable phase shifts in measurement beams that result from their interacting with non-polarization preserving optical elements in at least one interferometer measurement leg. Compensation is provided by phase plates, multi-order phase plates set at a predetermined angle with respect to beam components, coatings on reflecting surfaces, or a segmented phase plate at least part of which is rotated with respect to polarized beam components, and combinations thereof. Compensation is provided in interferometers having measurement legs folded with reflecting surfaces that cause relative phase shifts in propagating polarized beams because of non-normal incidence. Compensation is also provided in upward and downward looking interferometers for measuring altitude and changes in altitude to a surface such as a translating wafer stage of a photolithographic exposure apparatus.
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