Scanning interferometry with reference signal

Apparatus for correcting scanning errors in an interferometric profilometer with an objective adapted to execute interferometric measurements by illuminating a test surface and a first reference surface with a measurement beam adapted to provide a plurality of measurement interference patterns on a measurement detector as the optical path difference between said test surface and first reference surface is varied with a scanner calibrated to produce a sequence of nominal phase steps within a range of scanning operation, said apparatus comprising:

• (a) a light source adapted to produce a reference beam along an optical path exterior to said objective of the profilometer, said reference beam having a coherence length sufficient to cover said range of scanning operation;
• (b) a reflective surface;
• (c) a second reference surface;
• (d) a reference detector;
• (e) means for directing the reference beam toward the reflective surface and the second reference surface to provide a plurality of reference interference patterns on the reference detector as the optical path difference between the reflective surface and the second reference surface is varied with said scanner between acquisition frames of the reference detector;
• (f) means for performing an irradiance analysis on said plurality of reference interference patterns to calculate phase changes produced between said acquisition frames of the reference detector by said nominal phase steps of the scanner during said scanning operation;
• and
• (g) means for performing interferometric analysis of the measurement interference patterns utilizing said phase changes.

A method of correcting scanning errors in an interferometric profilometer with an objective adapted to execute interferometric measurements by illuminating a test surface and a first reference surface aligned in a first optical path with a measurement beam adapted to provide a plurality of measurement interference patterns on a measurement detector as the optical path difference between said test surface and first reference surface is varied with a scanner calibrated to produce a sequence of nominal phase steps within a range of scanning operation, the method comprising the following steps:

• (a) providing a reference beam of coherence length sufficient to cover said range of scanning operation;
• (b) directing the reference beam toward a reflective surface and a second reference surface aligned in a second optical path adapted to provide a plurality of reference interference patterns on a reference detector as the optical path difference between said reflective surface and second reference surface is varied with said scanner, said second optical path being exterior to said objective of the profilometer:
• (c) performing an irradiance analysis on said plurality of reference interference patterns to calculate phase changes produced by said nominal phase steps of the scanner during said scanning operation;
• and
• (d) performing interferometry analysis of the measurement interference patterns utilizing said phase changes calculated in step (c);
• wherein said test surface and reflective surface are separate optical elements of the profilometer.