J. Eur. Opt. Society-Rapid Publ. 21, 27( 2025) 293
up further designs. The design goal set for axial defocus between the measurement wavelengths could be achieved. Additionally, a long working distance of 6 mm had been realized.
The realized chromatic differential probe demonstrated axial resolution of 3 nm and a repeatability of 82.8 nm( k = 1). Height measurements on a reference target show well agreement to an AFM measurement. Regarding lateral resolution the probe with an average NA = 0.24 can provide contrast down to grating pitches of p nom = 4lm. Due to diffraction effects of the nearly rectangular structure, these show no correct height measurement anymore. The smallest grating pitch that could show a plateau for height evaluation was p nom = 20lm. Therefore, the application of the developed probe for profilometry on discontinuous substrates is limited. However, considering direct laser writing, continuous substrates are to be expected, where the diffraction effects become negligible.
It could be observed, that specimen induced diffraction in general is amplified by the axially separated measurement beams. Unfortunately, this meant that the supposedly more robust normalized focus error signal turned out to be instable on typical specimen. It is proposed to derive a new optimal defocus distance for chromatic differential confocal microscopes in the future to reduce this susceptibility to diffraction effects. Additionally, the design based on two independently fluctuating light sources, here two laserdiodes, reduces the stability of the differential curve and therefore is a major driver for the axial uncertainty, albeit the employed lock-in-filter. In future, these fluctuations might be compensated by additional observation principles or the reduction to a single light source.
The angular measurements for azimuthal and polar angle were realized by a camera observing the deflection of the nearly collimated reflected beam of the objective lens. Accounting for possible defocused positions of the tool head, the angular resolutions reach below 0.5 ° on a range from �66 °. Given typical slants of supposedly flat substrates, this might be sufficient angular resolution for the approximate perpendicular adjustment on free-form substrates. Further improvement of angular range and resolution is expected by the introduction of another focusing lens that can reduce the spot size on the camera and the influence of interference pattern. A design rule for the optimum spot size should be found in the near future.
All in all, the realized integrated tool head already achieves more than sufficient resolution for following an arbitrary substrate surface with the exposure beam focus. It is considered ready for direct laser writing on free-form substrates and will therefore be integrated into the control structure and trajectory planner of the NPMM-5D in an upcoming development step.
Funding
This research was supported by the Deutsche Forschungsgemeinschaft( DFG) in the scope of the Research Training Group“ Tipand laser-based 3D-Nanofabrication in extended macroscopic working areas”( GRK2182) aswellasthetransfer-project“ Multiachs-Nanofabrikationsmaschine( MNFM)” at Technische Universität Ilmenau.
Conflicts of interest The authors declare no conflict of interest.
Data availability statement
Measurement and simulation data is available upon reasonable request. Parts of the here shown data is only available in proprietary formats.
Author contribution statement
Johannes Belkner: Conceptualization, Methodology, Investigation, Formal analysis, Validation, Visualization, Writing – original draft, Johannes Leineweber: Conceptualization, Methodology, Investigation, Validation, Georg Hein: Investigation, Validation, Visualization, Data curation, Jaqueline Stauffenberg: Investigation, Formal analysis, Writing – review & editing, Alexander Barth: Conceptualization, Investigation, Thomas Kissinger: Conceptualization, Writing – review & editing, Eberhard Manske: Conceptualization, Methodology, Supervision, Funding acquisition, Writing – review & editing, Thomas Fröhlich: Methodology, Supervision, Funding acquisition, Writing – review & editing.
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