ABSTRACT
The article is devoted to the development of an EUV microscope using a wavelength of 13.84â nm. Due to the use of a mirror lens with a large numerical aperture, NA = 0.27, and a short depth of focus, it has been possible to carry out z-tomography of bio-samples for the first time with this type of microscope. A 3D image was reconstructed, and a pixel resolution of 140â nm was obtained. A new simple algorithm for the 3D reconstruction of absorption images from z-tomography data has been proposed that takes into account lens aberrations and a point spread function. The algorithm reduces the inverse absorption task to the corresponding well-studied task of fluorescence microscopy, with an error of 10% for cells up to 10 µm thick.
ABSTRACT
A reflective modification of the Schmidt-Cassegrain system was built and tested. Ultraviolet (UV) and soft x-ray applications are discussed. The system consists of a planoid mirror with an aspheric profile and prime concave and secondary convex spherical mirrors. Spherical aberration in a wide field of view and astigmatism are compensated by the aspheric profile of the planoid. The main parameters of the scheme are as follows: an entrance aperture of 180 mm, a focal ratio F/3.2, an angular resolution better than 3'' (corresponding to a pixel size of a back-side illuminated CCD), a field of view of ±1.5° (2ω=3°) and a flat image field with a diameter of 30.4 mm. Due to the absence of chromatic aberrations and wide field of view, the scheme is of considerable interest for hyperspectral instruments. In particular, the operating range of the instruments can be expanded into vacuum UV and UV regions.
ABSTRACT
Problems in the application of a null lens for surface shape measurements of aspherical mirrors are discussed using the example of manufacturing an aspherical concave mirror for the beyond extreme ultraviolet nanolithographer. A method for allowing measurement of the surface shape of a sample under study and the aberration of a null lens simultaneously, and for evaluating measurement accuracy, is described. Using this method, we made a mirror with an aspheric surface of the 6th order (i.e., the maximum deviation from the best-fit sphere is 6.6 µm) with the parameters of the deviations from the designed surface PV=5.3 nm and RMS=0.8 nm. An approximation of the surface shape was carried out using Zernike polynomials {Z(n)(m)(r,φ),m+n≤36}. The physical limitations of this technique are analyzed. It is shown that for aspheric measurements to an Angstrom accuracy, one needs to have a null lens with errors of less than 1 nm. For accurate measurements, it is necessary to establish compliance with the coordinates on the sample and on the interferogram.
ABSTRACT
The use of point diffraction interferometry is reported for measuring minutes, on the order of 0.01 arcsec angular movements. The algorithm for determining the angular displacement by the dynamics of the interference pattern is described. We also demonstrate results for applying this method to the study of the linearity and hysteresis of the angular shift of the platform, controlled by piezo actuators, which are designed for angular adjustment of the mirror of a solar extreme-ultraviolet telescope.
ABSTRACT
Circular Zernike polynomials are often used for approximation and analysis of optical surfaces. In this paper, we analyse their lateral resolving capacity, illustrating the effects of a lack of approximation by a finite set of polynomials and answering the following questions: What is the minimum number of polynomials that is necessary to describe a local deformation of a certain size? What is the relationship between the number of approximating polynomials and the spatial spectrum of the approximation? What is the connection between the mean-square error of approximation and the number of polynomials? The main results of this work are the formulas for calculating the error of fitting the relief and the connection between the width of the spatial spectrum and the order of approximation.
ABSTRACT
The possibilities of applying the point diffraction interferometry (PDI) method for the detection of the middle spatial frequency roughness of superpolished optical surfaces are analyzed. The point source used in the experiment is based on a single mode optical fiber with the subwavelength exit aperture size, which is about 0.25 µm. In a numerical aperture of 0.01 the reference wave root-mean-square deformation is less than 0.005 nm. It is theoretically shown that the possible diffraction-limited lateral resolution of PDI while measuring a spherical substrate of 100 mm curvature radius is about 8 µm. The experiment demonstrated the possibility of obtaining roughness spectra in the range 0.001-0.05 µm(-1). The surface map obtained by PDI, and the roughness spectra obtained by both the PDI and atomic-force microscopy methods are shown.
ABSTRACT
A new type of a reference spherical wave source (SWS) based on a single mode optical fiber with a narrowed down up to the submicrometer size exit aperture is proposed. It is intended for the precision point diffraction interferometers as a source of a reference wave. Systematic experimental errors which influence the measurement accuracy of the quality of the wave fronts generated by the SWSs are considered. Experimental data on wave front deformations are given. The combined root-mean-square (rms) wave deformation for a couple of the SWSs measured in a numerical aperture of NA=0.27 reaches the value of rms=0.36 nm that corresponds to rms=0.25 nm of a single SWS or about lambda2500 for the red He-Ne laser.
