Stefan Risse

Freeform manufacturing of a microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo

We report what is to our knowledge the first approach to diamond turn microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo. In recent years ultraprecision machining has been employed to manufacture accurate optical components with 3D structure for beam shaping, imaging and nonimaging applications. As a result, geometries that are difficult or impossible to manufacture using lithographic techniques might be fabricated using small diamond tools with well defined cutting edges. These 3D structures show no rotational symmetry, but rather high frequency asymmetric features thus can be treated as freeform geometries. To transfer the 3D surface data with the high frequency freeform features into a numerical control code for machining, the commonly piecewise differentiable surfaces are represented as a cloud of individual points. Based on this numeric data, the tool radius correction is calculated to account for the cutting-edge geometry. Discontinuities of the cutting tool locations due to abrupt slope changes on the substrate surface are bridged using cubic spline interpolation.When superimposed with the trajectory of the rotationally symmetric substrate the complete microoptical geometry in 3D space is established. Details of the fabrication process and performance evaluation are described.

Development of a low cost high precision three-layer 3D artificial compound eye.

Artificial compound eyes are typically designed on planar substrates due to the limits of current imaging devices and available manufacturing processes. In this study, a high precision, low cost, three-layer 3D artificial compound eye consisting of a 3D microlens array, a freeform lens array, and a field lens array was constructed to mimic an apposition compound eye on a curved substrate. The freeform microlens array was manufactured on a curved substrate to alter incident light beams and steer their respective images onto a flat image plane. The optical design was performed using ZEMAX. The optical simulation shows that the artificial compound eye can form multiple images with aberrations below 11 μm; adequate for many imaging applications. Both the freeform lens array and the field lens array were manufactured using microinjection molding process to reduce cost. Aluminum mold inserts were diamond machined by the slow tool servo method. The performance of the compound eye was tested using a home-built optical setup. The images captured demonstrate that the proposed structures can successfully steer images from a curved surface onto a planar photoreceptor. Experimental results show that the compound eye in this research has a field of view of 87°. In addition, images formed by multiple channels were found to be evenly distributed on the flat photoreceptor. Additionally, overlapping views of the adjacent channels allow higher resolution images to be re-constructed from multiple 3D images taken simultaneously.

Development, fabrication, and testing of an anamorphic imaging snap-together freeform telescope

The fabrication chain for the development of an afocal all aluminum telescope using four anamorphic aspherical mirrors is described. The optical and mechanical design are intended to achieve an enhanced system integration with reduced alignment effort by arranging two optical surfaces monolithically on common mirror bodies. Freeform machining is carried out by a hybrid fabrication approach combining diamond turning and diamond milling in the same machine setup. A direct figure correction of diamond turned aluminum mirrors by magnetorheological finishing is presented, resulting in high-precision athermal mirror modules with excellent figure properties. The interferometric system test highlights the diffraction limited telescope performance and the feasibility of the chosen approaches for freeform machining and mechanical integration.

Electrostatic chucks for lithography applications

Abstract High precision electrostatic chucks with diameters up to 12 inches are being developed at IOF for electron/ion-beam lithography applications. For optimal performance, selection of the appropriate chuck dielectric is crucial. We have tested various materials, including sapphire, quartz and glass-ceramics with respect to chucking force under vacuum conditions. Differences in electrostatic force of more than an order of magnitude were observed and are attributed to Coulomb and Johnsen–Rahbek behaviour. For the former, reasonable agreement with theoretical calculations was obtained when taking the corresponding dielectric constants and a finite gap between wafer and support into account. Time constants for chucking and dechucking were determined for the latter.

Fabrication of high precision metallic freeform mirrors with magnetorheological finishing (MRF)

The fabrication of complex shaped metal mirrors for optical imaging is a classical application area of diamond machining techniques. Aspherical and freeform shaped optical components up to several 100 mm in diameter can be manufactured with high precision in an acceptable amount of time. However, applications are naturally limited to the infrared spectral region due to scatter losses for shorter wavelengths as a result of the remaining periodic diamond turning structure. Achieving diffraction limited performance in the visible spectrum demands for the application of additional polishing steps. Magnetorheological Finishing (MRF) is a powerful tool to improve figure and finish of complex shaped optics at the same time in a single processing step. The application of MRF as a figuring tool for precise metal mirrors is a nontrivial task since the technology was primarily developed for figuring and finishing a variety of other optical materials, such as glasses or glass ceramics. In the presented work, MRF is used as a figuring tool for diamond turned aluminum lightweight mirrors with electroless nickel plating. It is applied as a direct follow-up process after diamond machining of the mirrors. A high precision measurement setup, composed of an interferometer and an advanced Computer Generated Hologram with additional alignment features, allows for precise metrology of the freeform shaped optics in short measuring cycles. Shape deviations less than 150 nm PV / 20 nm rms are achieved reliably for freeform mirrors with apertures of more than 300 mm. Characterization of removable and induced spatial frequencies is carried out by investigating the Power Spectral Density.

Structured Mo/Si multilayers for IR-suppression in laser-produced EUV light sources

Laser produced plasma sources are considered attractive for high-volume extreme-ultraviolet (EUV) lithography because of their high power at the target wavelength 13.5 nm. However, besides the required EUV light, a large amount of infrared (IR) light from the CO2 drive laser is scattered and reflected from the plasma as well as from the EUV mirrors in the optical system. Since these mirrors typically consist of molybdenum and silicon, the reflectance at IR wavelengths is even higher than in the EUV, which leads to high energy loads in the optical system. One option to reduce this is to structure the EUV multilayer, in particular the collector mirror, with an IR grating that has a high IR-suppression in the zeroth order. In this paper, the characterization of such an optical element is reported, including the IR-diffraction efficiency, the EUV performance (reflectance and scattering), and the relevant surface roughness. The measurement results are directly linked to the individual manufacturing steps.

Diamond milling or turning for the fabrication of micro lens arrays: comparing different diamond machining technologies

Diamond-micro milling and ultra-precision free-form turning technologies for fabricating micro lens arrays (MLA) with a large number of lenslets are explained in detail and compared. Besides the programming of the toolpath, correction loops and cutting parameters are presented. Both technologies are compared regarding achievable form deviation, roughness and economic factors like machining time. The paper offers a guideline for ultra-precision machining of micro lens array master molds on planar substrates and curved surfaces.

Ultra-precisely manufactured mirror assemblies with well-defined reference structures

Aspherical surfaces for imaging or spectroscopy are a centerpiece of high-performance optics. Due to the high alignment sensitivity of aspheric surfaces, reference elements and interfaces with a tight geometrical relation to the mirror are as important as the high quality of the optical surface itself. The developed manufacturing method, which accounts for the shape and also for the position of the mirror surfaces, allows controlling and precisely correcting not only the form, but also the alignment of reference marks, interfaces or even other mirrors in the sub-assembly using diamond turning. For Korsch or TMA telescopes it is also possible to diamond turn whole sub-assemblies containing two or more mirrors with a relative position error as low as the machine precision. Reference elements allow the correction of the shape and position of mirrors as well as the position of interfaces for system integration. The presented method opens up a novel manufacturing strategy to enhance the relative positioning accuracy of optic assemblies by one order of magnitude.

Novel TMA telescope based on ultra precise metal mirrors

Modern telescopes for space applications use complex optical elements like aspheres or freeforms. For the multispectral pushbroom scanner for spaceborne Earth remote sensing the Jena-Optonik GmbH has developed a Jena-Spaceborne- Scanner JSS product line. The optic of JSS-56 imager is realised by a Three-Mirror-Anastigmat (TMA) telescope designed in aluminium [1]. For brilliant pictures, mirrors with high shape accuracy and very smooth surfaces are required. The combination of precise diamond turning and post polishing techniques enables the classical infrared application for the visible and ultra-violet range. A wide variety of complex mirror shapes are feasible. A special new solution for lightweight design was applied. Ultra precise metal mirrors with aspherical surface are developed at the Fraunhofer IOF from design to system integration. This paper summarizes technologies and results for design, fabrication and surface finish of ultra lightweight aspherical metal mirrors for novel TMA telescopes.

A novel athermal approach for high-performance cryogenic metal optics

This paper describes a new athermal approach for high performance metal optics, particularly with regard to extreme environmental conditions as they usually may occur in terrestrial as well as in space applications. Whereas for mid infrared applications diamond turned aluminium is the preferred mirror substrate, it is insufficient for the visual range. For applications at near infrared wavelengths (0.8 μm - 2.4 μm) as well as at on cryogenic temperatures (-200°C) requirements exist, which are only partially met for diamond turned substrates. In this context athermal concepts such as optical surfaces with high shape accuracy and small surface micro-roughness without diffraction effect and marginal loss of stray light, are of enormous interest. The novel, patented material combination matches the Coefficient of Thermal Expansion (CTE) of an aluminium alloy with high silicon content (AlSi, Si ≥ 40 %) as mirror substrate with the CTE of the electroless nickel plating (NiP). Besides the harmonization of the CTE (~ 13 * 10-6 K-1), considerable advantages are achieved due to the high specific stiffness of these materials. Hence, this alloy also fulfils an additional requirement: it is ideal for the manufacturing of very stable light weight metal mirrors. To achieve minimal form deviations occurring due to the bimetallic effect, a detailed knowledge of the thermal expansion behavior of both, the substrate and the NiP layer is essential. The paper describes the reduction of the bimetallic bending by the use of expansion controlled aluminium-silicon alloys and NiP as a polishing layer. The acquisition of CTE-measurement data, the finite elements simulations of light weight mirrors as well as planned interferometrical experiments under cryogenic conditions are pointed out. The use of the new athermal approach is described exemplary.