How can you be sure that the measurements from a scanning electron microscope (SEM) are reliable? For SEMs, this fundamental metrological question is usually answered by chip-based, 1D and 2D standards. These can be used to calibrate the vertical and lateral axes and to determine the orthogonality between the lateral axes. However, thanks to special measurement techniques, SEMs can also generate height information from measurement objects. In her PhD research at the Physikalisch-Technische Bundesanstalt, in collaboration with Point electronic GmbH, Celina Hellmich developed reproducible and scalable wafer-based manufacturing processes for 3D micro-standards. These combine the characteristics of 1D and 2D standards and also enable the determination of orthogonality errors between the lateral and vertical axes. 3D micro-standards consist of pyramid structures with at least one height level and reference marks on all height levels. The layout of the reference marks makes it possible to precisely identify the rotation and displacement of measurement objects. This crucial difference from 1D and 2D standards now opens up new possibilities in optical metrology.
Celina Hellmich: “My supervisor, Professor Stefanie Kroker, inspired me to explore more in-depth fields of application beyond the manufacturing aspects. In discussions with my colleagues at the PTB, Dr Matthias Schumann and Dr Christoph Weichert, who had developed a new 2D self-calibration method, the idea arose to apply this method to 3D standards. With the help of Dr Rainer Köning, the calibration was transferred to measurements using a SEM via measurements of traceable marker positions on the PTB’s nanometre comparator. A systematic difference between the markers on the upper and lower levels was observed on both measuring instruments. This enabled us to demonstrate that the error field of the measuring instruments under investigation varies with vertical position. The combination of the 3D micro-normals with the new error separation method thus provides a completely new perspective on distortions in microscope images. It is therefore essential to align measurement objects perpendicular to the measurement axis of the microscopes, even when observing pure 2D grids.”
The potential impact of these findings on work with SEMs and other microscopes is enormous, and this is why Hellmich’s findings were awarded the Paper of the Year prize. Depending on how the error fields vary with height in different analytical contexts, the calibration of such instruments will need to be completely rethought. This, in turn, opens up significant opportunities for a future with even more accurate microscopes.
Publication
Hellmich, C., Köning, R., Schumann, M., & Weichert, C. (2025). Self-calibration of microscopes based on 3D micro standards. Surface Topography: Metrology and Properties, 13(3), 035007. DOI: 10.1088/2051-672X/adf7c3
