J. Eur. Opt. Society-Rapid Publ. 2025, 21, 27 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025017 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
EOSAM 2024 Guest editors: Luca De Stefano and Raffaele Velotta
RESEARCH ARTICLE
An integrated exposure and measurement tool for 5-DOF direct laser writing based on chromatic differential confocal sensing
Johannes Belkner *, Johannes Leineweber, Georg Hein, Jaqueline Stauffenberg, Alexander Barth, Thomas Kissinger, Eberhard Manske, and Thomas Fröhlich
Technische Universität Ilmenau, Department of Mechanical Engineering, Institute of Process Measurement and Sensor Technology, Gustav-Kirchhoff-Straße 1, 98693 Ilmenau, Germany
Received 31 January 2025 / Accepted 1 April 2025
Abstract. Accurate and uniform fabrication of microstructures on highly curved substrates requires exposure with the waist of a focused laser beam at every point. In order to realize this, the exposure beam must be held perpendicular and focused onto the local substrate. Here we present an optical tool for our developed 5-axis nano-positioning and nano-measurement machine based on the chromatic differential confocal microscope. Thereby, we introduce the optical design methodology to realize high axial sensitivity from differential optical feedback via axial chromatic aberration. Additionally, the deflection angle is measured via a camera sensor to provide angular feedback. Overall, our probe attains a nanometer axial sensitivity and arc-minute angular sensitivity in a confined space of 50 80 36 mm 3. Keywords: Differential confocal microscopy, Chromatic optical design, Nano-positioning.
1 Introduction
Flexible prototyping of microstructures via direct laser writing( DLW) or laserablation recently enabled a whole new field for the creation of micro-optics [ 1, 2 ], micro-fluidics [ 3, 4 ], MEMS [ 5, 6 ] and metamaterials [ 7, 8 ]. Thereby, a single laser spot exposes or ablates a small volume of photoresist on a substrate. Using multiphoton absorption in DLW, this even allowed for the creation of extended complex three-dimensional structures [ 9, 10 ].
The fabrication on, and modification of, highly curved substrates such as lenses, mirrors or free-form substrates, however, remains an open challenge. The accurate structuring of such substrates could enhance the manufacturing of hybrid diffractive-refractive optical elements, highly integrated systems such as wearables [ 11, 12 ] or new approaches for classic [ 13 ], optical [ 14 ] and organic circuit design [ 15 ].
In order to achieve highest accuracy at the current point of structuring on the substrate( POS), the optical tool needs to follow local surface slope as indicated by the tilted objective in Figure 1. A current project at our institute is the metrological 5-axis nano-positioning and nano-measuring machine( NPMM-5D) [ 16 ], also shown in Figure 1, that allows to position and rotate around the POS with nanometer-accuracy by following the extended Abbe-principle [ 17 ]. Thereby, all position measurement axes and the tool focus
* Corresponding author: johannes. belkner @ tu-ilmenau. de must intersect in the POS as indicated by the dashed lines in Figure 1. For the rotations, the spherical position measurement is realized by encoders in rotational actuators, but expanded by Fabry-Pérot interferometers that observe the eccentricity towards a reference hemispherical mirror. This limits the available space for tool-heads to 50 80 36 mm 3 and therefore requires innovative miniaturized measurement and structuring systems. Such a miniature tool-head has to provide three major functions. At first, the tool-head shall realize a small structuring spot for nano-fabrication. Here we aim for the exposure of photoresist and therefore the employment of a blue diode laser. Second, the tool-head must provide a feedback signal for the three Cartesian nano-positioning axes to keep the substrate surface within the focal spot of said structuring beam. Third, to adjust the tool-heads structuring axis towards the surface slope, angular feedback signals must be generated for the rotational nano-positioning axes. Finally, as a secondary functional requirement, the integration of a wide-field microscope for manual rough positioning would be desirable.
1.1 Optical feedback systems
As the generation of a diffraction limited structuring laser spot can be realized easily by focusing a collimated beam with a suitable precision asphere. A major challenge in
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