Thomas Sure

Phase shifting Fizeau interferometry of front and back surfaces of optical flats

The interpretation of Fizeau interferograms of optical flats is not straightforward because they are composed of more than two reflections. This results in a confusing fringe pattern. There are three main contributions to the interferogram given by the reflections from the reference surface, the front and the rear surface of the sample. We present a new to the best of our knowledge solution to the problem. We use phase shifting measurements of the wave fields, which are reflected by and transmitted through the sample. This eliminates the need for the suppression of reflections by immersion or other methods. As an illustration of this method, several examples will also be presented.

New tools for high-precision positioning of optical elements in high-NA microscope objectives

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).

Artificial star test by real-time video holography for the adjustment of high-numerical-aperture micro-objectives

The star test is a useful tool for fast visual inspection of the aberrations of micro-objectives during final adjustment. One of its most valuable properties is that it permits instantaneous observation of the effect of adjustments of lens groups, for instance, the shifting element during on-axis coma adjustment. Sometimes, however, it is difficult to perform the star test, e.g., in the ultraviolet region, which represents a field of growing interest driven by applications in semiconductor inspection and metrology. In addition, it is difficult to display the point-spread functions with video cameras because of the high dynamic range needed. We present a simple work-around with which to overcome these problems. If an interferometer is available for quantitative wave-front analysis on the production line, the point-spread function may quite easily be computed from an interferogram of the wave front in the back focal plane of a micro-objective. We describe the achievement and application of such a simulated star test in various spectral regions, together with some of its useful applications, including real-time wave-front manipulation.

Iterative full-bandwidth wavefront reconstruction from a set of low-tilt Fizeau interferograms for high-numerical-aperture surface characterization

Full-bandwidth phase-shifting methods as well as band-limited fringe carrier techniques are both problematic when testing high-NA spherical surfaces in Fizeau interferometers. Phase stepping is usually performed by moving a sample and reference sphere relative to each other along the optical axis. At a high NA the method suffers from phase-shift inhomogeneity across the sample surface. Fringe carrier techniques rely on a minimum fringe frequency and call for an off-axis position of the sample, which in turn introduces condenser aberrations. Distortion of the imaging optics generates further apparent aberrations. We propose to combine both principles. The phase shifts are replaced by a set of very low tilts such that the sample is virtually on axis. Initial wavefront estimates are generated by a fringe carrier method. An adaptive Misell-type algorithm combines the interferometric data and iteratively improves the reconstructed wavefront until full spatial bandwidth is achieved.

Ultra High Performance Microscope Objectives: The State of the Art in Design, Manufacturing and Testing

During the last years, new microscope applications require an increased resolution which enforces the development of new state of the art high NA immersion objectives. With the introduction of the 4Pi confocal fluorescence microscope, the increase of the numerical aperture from NA=1.4 to NA=1.46 makes sense, although the gain of lateral resolution is quite small. On the other hand, for inspection and metrology in the semiconductor industry the continuously decreasing structures need the highest possible resolution, which can be achieved with high NA water immersion objectives working in the DUV wavelength range. Building this kind of objectives requires special measuring and testing technologies and a manufacturing precision which has never been realized before in series production.

Automatically high precision manufacturing technology for micro-optic subgroups

To realize the image quality of high end objectives, e. g. high NA microscope objectives working in the DUV spectral region the subgroups have to be manufactured with a mechanical precision which is difficult to achieve cost effectively. For high end microscope objectives the accuracy of the diameter of the lens mount must be within 1 µm, the run-out must be met within 1 µm and the distance of the lens vertex relative to the shoulder of the mount must fit within 1 µm. 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 in one CNC machining center.

Requirements for the design and production of high numerical aperture objectives for 4-π confocal microscopy

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 manufacturers 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.