Errors in phase-measurement interferometry with high numerical apertures

If phase-measurement interferometry (PMI) is applied for a high numerical aperture (NA), specific errors can occur. The resulting phase no longer varies linearly with the height difference in the test surface. Thus, for height measurements, a constant correction factor or NA scaling factor, which depends only on the NA and the illumination of its stop, does not exist here. This is shown for several pupil illuminations (intensity approximately cos(2) alpha, intensity approximately cos alpha, intensity constant; alpha is the incident angle). For example (NA = 0.9, pupil illumination intensity is constant, wedge angle approximately 0, PMI phase increments = pi/2 for normal incidence), the correction factor can vary from 1.20 (height difference approximately 0) to 1.37 (height difference approximately lambda;/4), i.e., by 17%.