Investigation of wide-range stylus profilometry

Stylus profilometry plays an important part in characterising topography of substrates and films for x-ray optics, through such commercial instruments as the Rank Taylor Hobson Talystep and Form Talysurf. The Talystep, further developed as the Nanosurf by Lindsey and co-workers at the NPL, has 50pm vertical resolution and is well suited to the measurement of large flat substrates; however, its vertical range of 10 microns limits use on curved substrates. The Form Talysurf avoids this range limitation by use of an interferometric transducer but its vertical resolution of 10nm is inadequate for measurement of x-ray optical surfaces. We report an experimental investigation of the factors limiting the resolution of the Form Talysurf. It is shown that the limit can be reduced without modification to the interferometric transducer, and limits to accuracy from differential non-linearity of the transducer and the performance of the cantilever stylus arm are considered.