J. Eur. Opt. Society-Rapid Publ. 21, 19( 2025) 197 reconstruction results are especially sensitive to misalignment along the optical axis. Therefore, the accurate alignment of the specimen along the optical axis is crucial for a highly accurate form reconstruction.
In this work, we propose a specimen alignment procedure that uses the so-called Cat’ s Eye reference position as an alignment reference and intermediate step for aligning the specimen into the measurement position. The Cat’ s Eye position is a fixed position, independent from the measured specimen, where the focus of the interferometer’ s optics and the apex of the measured specimen coincide. For aligning a specimen into this position, we show a threestep process: First, a pre-alignment of the specimen perpendicular to the optical axis, second, a rough alignment along the optical axis, and third, a fine alignment using the height values reconstructed from the interferogram. The last step aligns the specimen along the optical axis but can also be enhanced by a fine alignment perpendicular to the optical axis. From the reference position, the specimen can be transferred into the measurement position with the relative distance being tracked by a distance-measuring interferometer. This allows for a accurate positioning in relation to a fixed datum.
In a first step, the resulting camera images for different alignment steps, and the influence of different positioning steps and their sensitivities are investigated using simulations. From these insights, the Cat’ sEyeadjustment procedure is developed and is then tested with the measurement setup and different specimens. The axial positioning shows a good short-term repeatability with a standard deviationintherangeofbetween15 nm and 30 nm. The fine-adjustment perpendicular to the optical axis depends on the chosen specimen but usually has only a small impact on the axial positioning accuracy. The overall impact of the positioning into the Cat’ s Eye on the final form reconstruction result can be preliminary estimated to be around < 5 nm RMS, but further investigations, including the transition to the measurement position have to be done.
Future work will focus on the positioning of specimens into the measurement position, which includes both the lateral position and the specimen tilt. After fine adjusting the lateral alignment in measurement position, the Cat’ s Eye reference will be determined again in order to further reduce possible errors due to lateral misalignment. Then, the specimen is moved to the final measurement position and the surface form is reconstructed from the data acquired. This process will then be repeated several times, which would give insight into the overall repeatability of the measurement process. In addition, the absolute positioning accuracy of the specimen will be investigated: In a first step, well-known, calibrated reference specimens will be measured and the absolute form will be compared to reference values with well-known uncertainty. In a second step, the absolute position of the Cat’ s Eye reference will be investigated utilizing an external white light interferometer to measure the distance between the top surface of a transparent test specimen and the last objective lens surface of the TWI. This would further enhance the matching between the digital twin of the TWI and the physical instrument and would therefore be a leap forward in understanding and improving the measurement uncertainty of the TWI.
Funding
This research has been supported by the Deutsche Forschungsgemeinschaft( grant no. 496703792).
Conflicts of interest
The authors declare that they have no competing interests to report.
Data availability statement
Data associated with this article cannot be disclosed due to legal reason.
Author contribution statement
Conceptualization, G. S. and I. F.; Methodology, G. S. and I. F.; Software, D. E. and G. S.; Validation, G. S. and I. F.; Formal Analysis, G. S.; Investigation, D. E. and G. S.; Resources, I. F.; Data Curation, D. E. andG. S.; Writing – Original Draft Preparation, G. S.; Writing – Review & Editing, I. F.; Visualization, G. S.; Supervision, I. F.; Project Administration, I. F.; Funding Acquisition, I. F.
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