A material measure for optical surface measurement

Optical areal surface topography metrology tools are often used for noncontact and fast measurements of precision surfaces such as optical mirrors of X-ray or lithography devices. However, using such tools often raises a fundamental question in optical measurements, ‘Are we getting the right answer?’

To answer this challenging question, PTB, in collaboration with Zeiss-SMT, has developed a novel material measure to characterize the 2D-ITF of areal surface topography measuring tools within the scope of the 3D-Stack EMPIR project. Among other things, this new standard features circular structural patterns. Such rotationally symmetric patterns are advantageous for characterizing ITFs along different angular directions, i.e., for investigating the angular anisotropy of measuring instruments. Three different kinds of patterns have been implemented in the design: circular step patterns, circular chirp patterns whose spacing continuously changes, and circular discrete grating patterns. They are designed to represent three kinds of spatial signals for ITF characterization in a complementary way. The patterns have radii ranging from 30 μm to 300 μm and wavelengths from 0.1 μm to 150 μm. They may be combined to meet measurement requirements for various instruments that may have very different bandwidth characteristics and fields of view. The design of this material measure is therefore highly flexible and suited to various applications.

Besides the material measure, software has also been developed for the calibration process and the subsequent data evaluation. This combination allows the 2D-ITF of optical surface measuring devices to be characterized conveniently in just a few minutes. The application of the newly developed method at PTB’s industrial partners shows the advantages of its high repeatability and robustness, along with its excellent flexibility and ease of use.