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Image by PTB
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The method of separating the wavefronts of the reference surface and
the test sample surface in the plane of the reference surface � for
which a patent has been applied for � uses a new generation of
commercially available beam splitters, also called "on-axis beam
splitters", which cause a separation of the polarisation directions of
the incident light along the optical axis. In comparison with common
Fizeau interferometers, this has various advantages. On the one hand,
through the generation of polarised light, the measurement can be
traced back to a pure two-beam interference. The analysis of the
signal is thus significantly facilitated and improved. Classic Fizeau
interferometers, however, are based on the analysis of the
multiple-beam interference.
On the other hand, by varying the direction of polarisation, a maximum
contrast can be set, independent of the reflectivity of the test
sample. In contrast to this, in conventional Fizeau interferometers,
different reference surfaces with adapted reflectivity must be held
available in case the reflectivity of the test samples varies strongly.
In order to increase the accuracy of analysis of common Fizeau
interferometers, a variable phase is generated by varying mechanically
the distance between the test sample and the reference surface (phase-shifter
interferometry). Another advantage of the new method is that such a
phase shifting becomes possible through the use of electro-optical
components and thus without using mobile parts. Thanks to the
increased measuring dynamics achieved in this way, it is, for example,
possible to carry out topographical measurements in environments which
are subjected to vibrational strain.
The new method allows a facilitated and improved analysis of the
measurement data, requires only one calibrated reference surface
instead of several, and opens up new possibilities of application in
the field of dynamic interferometry. It is particularly suited for
measurements on structured or unstructured surfaces with different
reflectivities � especially in the optical and semiconductor industry.
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