SIOS Meβtechnik GmbH developed a universal interferometrical profilometer for 3D measurements of freeform optics topography. Due to the measurement principle using a scanning differential interferometer, no expensive and individually shaped reference optics are required. All optic shapes such as plane-,spherical-, and freeform-optics with local slopes up to 7 mrad and sizes up to 100 × 100 mm2 can be measured with sub-nanometer resolution. The capability of the setup has been proven by measurements of highly precise machined silicon mirrors (plane and spherical). A maximum of ± 3 nm peak-valley deviation between two subsequent measurements of a 30 mm × 100 mm plane mirror topography has been achieved, which proves a very good repeatability. Furthermore, measurement results show very good accordance with those from Fizeau interferometer measurements of this precision plane mirror. The maximum deviation was ± 10 nm, which is a hint to a very good accuracy of our measurements. Furthermore, form parameters such as the radii of spherical mirrors can be determined precisely due to the interferometer-based synchronous measurements of the x- and y- positions of the z- topography. A reproducibility of 1.4 × 10-4 of the radius measurements of a 29 m radius mirror was achieved, whereat the mirror was measured on different supports and in different orientations.
In 2019, the Institut für angewandte Photonik (IAP) e. V. in cooperation with Nano Optics Berlin (NOB) GmbH and SIOS Meßtechnik GmbH has made an important progress in the technology for precision soft X-ray optics – the development of three-dimensional (3-D) reflection zone plates (RZPs) with diffractive compensation of slope errors. 2-D mapping of spherical and toroidal grating substrates was used for the metrology of their individual profile. Based on these data, the inscribed grating structure, which corrects the slope error distribution, was computed. The correction algorithm has been implemented as a Python script, and first pilot samples of slope error compensated RZPs are in fabrication process. The 3-D device can replace two or three components in an optical scheme and, therefore, reduce absorption losses by several orders of magnitude. Beyond, the fabrication of customized 3-D Fresnel structures on curved substrates promises considerable improvements for efficiency, resolution and energy range in wavelength dispersive applications. As an example, we present simulations for a compact instrument within (150 – 250) eV. Further development of this approach toward commercial availability will enable the design and construction of compact soft Xray monochromators and spectrometers with unique parameters.
There are many established technologies for precise characterization of the mirror geometries available. The paper presents a high precision measuring setup based on a single beam homodyne laser interferometer. The single beam interferometer is moved by a linear stage between the reference and measuring surfaces and delivers the differences between them. The reference mirror defines an absolute accuracy of the method. This point based method allows a high spatial resolution of the mirror shape and is suitable for measurements of the free form mirror geometries as long as the radius of curvature does not exceed the maximal toleranced tilt.
The measuring results have been obtained for optics with dimensions of up to 50 mm and have been verified both for plan mirrors and for mirrors with radii of curvature in the range between 6 m and 10 m. A repeatability of the measuring results in sub-nanometer range can be shown.
Especially for mirrors with a very big radius of curvature the knowledge of the exact position of the each measuring point on the surface is important for minimizing the errors of the mathematical fitting algorithms. Therefore a triple beam interferometer has been used for measurements of the stage position. The tight synchronization between all interferometer channels of 0.1 ns allows very fast “on-the-fly” scans of the surface.
A three-degree-of-freedom measurement system for the acquisition of the straightness and roll errors of a moving linear
stage is described. The horizontal (Δx) and vertical (Δy) straightness errors are obtained by measuring the lateral displacement
of a triple prism with a laser beam and position sensitive detectors. From two simultaneously performed vertical
straightness measurements the roll angle (Θz) can be calculated. The system consists of a cable-free reflector head
and a detector head. The position sensitive detectors have been calibrated using a precision x,y-stage equipped with two
plane mirror interferometers. Different position sensitive detectors are compared with regard to position sensitivity, linearity,
null-shift stability and sensitivity to the intensity profile of the detected laser beam. In combination with an already
known triple-beam plane mirror interferometer, additional information about the linear position (Δz) and the pitch (Θx)
and yaw (Θy) angle can be obtained from three parallel linear measurements. Thus all six-degree-of-freedom geometric
errors can be measured simultaneously.
Systematic errors of the three-degree-of-freedom measurement due to misalignment of the laser beams and geometric
errors of the triple reflectors are discussed. An approach for correction of those errors caused by the triple reflectors is
shown. The method is based on determination of the reflector geometry and calculation using the additional information
(Δz) acquired by the interferometer. Furthermore the metrological properties of the proposed system for the measurement
of straightness and roll are compared to other measurement principles. Experimental results demonstrate the
measurement capabilities of the system.