Simpler surface analysis
Researchers at Germany’s national metrology institute PTB (Physikalisch-Technische Bundesanstalt), based in Braunschweig, have developed a faster and easier method for analysing surfaces with different reflectivities.
Fizeau interferometry creates an interference between the surface of a test sample and a reference surface, which is recorded and analysed by an imaging optics system. The contrast and shape of the interference signals depend on the reflectivity of the test samples, and can be time-consuming to analyse using traditional methods.
‘The new concept [involves modifying] the reference surface of a Fizeau interferometer to act like an on-axis polarisation beam splitter,’ explains Dr Harald Bosse, Head of the Length and Angle Metrology department at PTB. This causes separation of the polarisation directions along the optical axis.
‘The interferometer can then be configured as a polarisation coding instrument, which allows it to adapt to the different reflectivities of the surfaces being tested, and to obtain an optimum contrast in each measurement case,’ he adds. This means surfaces such as partially metallised materials (photomasks) or those with larger changes of reflectivity (uncoated glass or highly reflective metallised materials) can be analysed using one calibrated reference surface, instead of several. There is also no need to change hardware or use attenuation plates.
By generating polarised light, the measurement can also be traced back to a pure two-beam interference, which is quicker and easier to analyse.
The PTB is looking for partners to commercialise the new method. The organisation is also working on a prototype interferometer system for its laboratory.