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J. Eur. Opt. Society-Rapid Publ. 21, 19( 2025)
Figure
7. Scheme of the specimen positioning algorithm using the Zernike development of the height values reconstructed from the interference image.
Figure 8. Measurement setup for surface measurements with the tilted-wave interferometer consisting of the interferometer itself( only objective shown), the specimen positioning system( large z-axis and positioning hexapod), the specimen, and the distance measuring interferometer.
hexapod mounted on top, and a DMI below. The DMI measures the distance to the back of the positioning hexapod’ s stage, were a retroreflector( PS975M-A, Thorlabs, Newton, NJ, USA) is mounted. Due to the limited beam displacement acceptance of the DMI, the lateral movement of the hexapod is limited to about ± 0.5 mm until the DMI looses the measurement beam. The specimen is placed in a hydraulic chuck( 05.043.258, GEWEFA Josef C. Pfister GmbH & Co. KG, Burladingen, Germany) in the center of the hexapod stage. The TWI objective has an assumed focal point of f = 48.609 mm, which resembles the distance between the last lens surface of the TWI objective and the Cat’ s Eye reference position. The DMI( RLE10, Renishaw, Wotton-under-Edge, Gloucestershire, England, UK) uses a 90 ° sensor head( RLD10, Renishaw) and an environmental compensation unit( RCU10, Renishaw) with an air temperature sensor.
For the measurement of the repeatability of the adjustment of a specimen into the Cat’ s Eye reference position, the specimen is inserted into the hydraulic chuck in the top stage of the positioning hexapod. The specimen is moved upwards in axial direction towards the estimated reference position until a rectangular interference patch of sufficient size appears on the camera image( compare Fig. 3). From here, a coarse lateral alignment of the specimen is performed using the center of gravity of the interference patch. Since this can lead to movements of the hexapod of more than 0.5 mm, this step is prone to the DMI loosing the reflected beam. The axial translation into the linear region of the Zernike developement of the height values follows, using data from the simulation as a lookup