Absolute surface metrology with a phase-shifting interferometer for incommensurate transverse spatial shifts

Area of Science:

• Optical metrology
• Interferometry
• Surface characterization

Background:

• Phase-shifting interferometry (PSI) measures surface height relative to a reference optic.
• Reference optics (flats, spheres) have inherent surface errors (e.g., tens of nanometers P-V).
• These errors limit the accuracy of traditional PSI measurements.

Purpose of the Study:

• To detail the implementation and practical utility of a novel absolute optical metrology scheme.
• To address the limitations of reference optic errors in PSI.
• To demonstrate a method for accurate absolute surface mapping.

Main Methods:

• The scheme extracts absolute phase differences by differencing phase maps from PSI with slight transverse spatial shifts of the optic under test.
• Absolute phase differences are used in standard wavefront reconstruction to yield the absolute surface map.
• The technique is shown to retain single-pixel spatial resolution even with coarser transverse shifts than the pixel size.

Main Results:

• The novel scheme effectively removes the influence of the reference surface errors.
• Absolute phase differences are accurately obtained, enabling precise surface height mapping.
• The method maintains high spatial resolution irrespective of shift magnitude relative to pixel size.

Conclusions:

• The proposed absolute optical metrology scheme offers a significant advancement in optical metrology.
• It provides an accurate and simple method for removing reference surface effects in PSI.
• The technique has practical considerations for implementation and broader applications in optical testing.