Precision of diamond turning sinusoidal structures as measurement standards used to assess topography fidelity

Abstract

In optical surface metrology, it is crucial to assess the fidelity of the topography measuring signals. One parameter to quantify this is the \textsl{small-scale fidelity limit} $T_\mathrm{FIL}$ defined in ISO 25178-600:2019. To determine this parameter, sinusoidal structures are generated, where the wavelengths are modulated according to a discrete chirp series.

The objects are produced by means of ultra-precision diamond face turning. Planar areas and regions with slopes below $4^\circ$ could be produced with form deviations of $\lesssim 10 \, \mathrm{n m}$.
An initial estimate of the cutting tool's nose radius resulted in a
deviation that caused the ridges of the structures to be too narrow by approximately $150 \, \mathrm{n m}$, while the trenches were too wide.
At the bottom of narrow trenches, deviations are observed in the form of elevations with heights of about $20$ to $100 \, \mathrm{n m}$.

The measurement standard investigated in this study has also been used to characterise optical instruments in a round-robin test within the European project TracOptic, which requires precise knowledge of the geometry of all structures. The geometry of the topography, cosine structures superimposed with form deviations, was measured using the Met.\ LR-AFM metrological long-range atomic force microscope of the German National Metrology Institute.