A conventional phase shifting interferometer is capable of measuring opaque surfaces with sub-nanometer precision.
However, it cannot be used to measure an object with multiple parallel reflective surfaces such as a transparent plate, a
glass disk, or an Extreme Ultraviolet Lithography (EUVL) mask blank. This is because the plane parallel reflective
surfaces generate multiple interferograms that are superimposed in the recording plane of the interferometer. Although
every individual interferogram is associated with phase information that is related to the height or thickness, the
conventional interferometer is not able to differentiate one surface from another. To measure these surfaces, we have
developed a method that integrates a Fizeau interferometer with a tunable light source and a weighted least-square
technique. The tunable light source controls the wavelength during the data acquisition process, producing phase shift
speeds that are proportional to the optical path difference (OPD). The weighted least-square signal processing
technique separates each surface from the others in an optimal manner. Thus the desired information, such as the front
surface height, back surface height, and relative optical thickness of a plane-parallel transparent glass plate are extracted
without multi-surface fringe print-through artifacts. In this paper we will present the method and demonstrate its
performance. The demonstrated surface height accuracy for EUVL mask blank substrates is 5 nm and the RMS
repeatability is <0.01 nm.