Cemented doublets and triplets can not be used for objectives working at wavelengths of 248 nm and shorter, because the optical cement can not withstand the high photon energies. It will be shown that high NA deep UV objectives can be designed and built successfully with the help of air spaced doublets. Assuring Strehl ratios above 95% enforces very tight tolerances. For example the distance error of the lens vertex to its mount has to be less than 1 μm. This calls for a new manufacturing precision never realized before in series production. We show how a white light Mirau interferometer can be used to measure lens vertex positions with an accuracy of 200 nm. We also demonstrate how the fine-tuning process can be optimized by using a "simulated star test", where the point-spread function is calculated in real time with a FFT-algorithm from the optical path difference data, acquired by a Twyman-Green interferometer.
To realize the required precision, today various measurement techniques and production processes are used. Picking up the subgroups on different machining tools and measurement systems will loosen the accuracy. Here, we present the concept and the layout of a new manufacturing tool where we implemented the different measurement techniques needed in one CNC machining center. This tool is able to 1) adjust automatically the optical axis of the subgroups related to the machining axis better than 0.5 μm with the help of the stick-slip effect where a mechanical impulse is transferred by an electromagnetically driven hammer, 2) measure the lens vertex relative to the shoulder of the mount with an accuracy of 250 nm and 3) do all steps which are necessary to process the lens mount within the accuracies described above.
Thin-film coatings in modern optical systems as wideband AR-coatings may have >10 layers and an optical thickness of several λ. Such complex thin-films may introduce pronounced changes in transmission phases with varying angles of incidence, polarization and/or wavelength. "Polarization ray tracing" as utilized by current optical design programs models a "ray" as a "localized plane wave" hitting the air/thin-film/glass system and the transmission properties in phase and amplitude for the p- and s-components are taken into account. However, this only approximates the thin film as a pure phase object of vanishing thickness on a flat surface. Any "ray" crossing a layer of finite thickness will undergo lateral displacement and on a surface of notable curvature, this displacement will further change the direction of the refracted "ray". Both effects might become important in high NA, deep UV microscope objectives based on an air-spaced design that involves a large number of highly curved air/glass interfaces, large angles of incidence and tight tolerances. This paper shows how the equivalent lateral ray displacement and bending can be calculated from the film/glass properties and the surface curvature and how it can be incorporated into a polarization ray-tracing program. It also addresses other problems encountered in polarization ray tracing of thin films, as proper conversion from phase shifts to optical path length and how to easily "unwrap" the thin-film induced phase.
The precise positioning of the individual optical elements is essential for attaining diffraction limited performance in high-numerical-aperture (high-NA) microscope objectives. Tolerances are in the micron range or lower for high-end objectives, e.g. for broad-band scanning confocal applications, metrology objectives in general, and especially for deep ultraviolet (DUV) applications. The ever increasing demands on imaging performance ask for the continuous development and improvement of specialized measurement equipment for the production line. Our award-winning 150x/0.90-DUV-AT-infinity/0 objective for wafer inspection and metrology at 248nm employs air
spacings in its doublets because of the instability of optical cements against DUV radiation. This comes however at the cost of a higher number of surfaces and even higher precision demands on their geometry, orientation and positioning. We present several tools enabling us to meet these requirements. A Fourier transform fringe
analysis scheme is adapted to high-NA Fizeau interferometry for surface characterization. A white light Mirau interferometer for dimensional measurements on lens groups with sub-μm resolution enables us to keep surface distance errors lower than 2 μm. Residual aberrations of the objective are compensated for by translating special correction elements under observation of the wave-front using a DUV-Twyman-Green interferometer, which also
incorporates a 903nm branch for the parfocal adjustment of the infrared (IR) autofocus feature of the objective. To adjust the shifting element for the elimination of on-axis coma, we compute an artificial (real-time) star test from the interferogram, allowing interactive manipulations of the element while monitoring their influence on the point spread function (PSF).
Cemented doublets and triplets, which are the principle parts in high quality, high numerical aperture (NA) objectives, can not be used for objectives working at wavelengths of 248 nm and shorter, because the optical cement can not withstand the high photon energies. We will show that high NA deep UV objectives can be designed and built successfully with the help of air spaced doublets. Assuring Strehl ratios above 95% enforces very tight tolerances. For example the distance error of the lens vertex to its mount has to be <1 μm. This calls for a new manufacturing precision never realized before in series production. We show how a white light Mirau interferometer can be used to measure lens vertex positions with an accuracy of ±200 nm. We also demonstrate how the fine-tuning process can be optimized by using a "simulated star test," where the point-spread function is calculated in real time with a FFT-algorithm from the optical path difference data, acquired by a Twyman-Green interferometer.
The quality of high performance microscope objectives is usually verified by interferometric measurement of the wave front. However, a non-interferometric method might be preferable, when appropriate light sources of sufficient coherence length or an interferometric setup are difficult to realize. Under these circumstances, the complex pupil function of an optical system can also be determined from its intensity point spread function (PSF), i.e. from the image of a sub-resolution point object. We present a system that can determine the pupil function of high-NA microscope objectives from defocused images of an artificial point source. An extended version of the "Misell" algorithm is used, which utilizes 4 or more PSF images to overcome the Fourier phase ambiguity and which generally converges rapidly to the correct pupil function both in phase and amplitude. The algorithm can also compensate small errors in the x-, y- and z-positions of the images, which might be caused by vibrations, thermal drift or the limited accuracy of the z-drive.
We discuss the requirements on design and production regarding geometric and chromatic aberrations for objectives used in 4Pi confocal microscopy. We show that even the selection of a category 1/1 glass will not automatically assure that these requirements are met, due to residual variations in the Abbe number v within the manufacturer's tolerances. Consequently, the optical design has to take into consideration the possibility of balancing chromatic aberrations by varying selected air spacings in the final assembly of each individual objective. We also demonstrate, that for analyzing the influence of aberrations on the intensity distribution along the optical axis, a scalar diffraction theory is still applicable and very useful.
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