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Figure 1. CAD model view of the significant parts of the NPMM-5D.
optical design is the integration of an optical surface feedback system that works online. This means that the substrate is assumed to be of unknown shape and cannot be compensated by feed-forwarding its profile to the trajectory planner of the NPMM-5D. Suitable feedback systems therefore must be fast enough for the positioning axes that run at 10 kHz. Optically, such systems must provide a point of measurement that aligns with the POS. It should provide nanometer height resolution to achieve homogeneous exposure. Thereby, the measurement wavelength must be sufficiently far from exposure wavelength to prevent undesired structuring. Furthermore, it must be possible to miniaturize and integrate it within the given space for the tool-head. In agreement with the alignment of the probing point and the POS, a common axis for both systems with a common coupled fiber would be advantageous for easier integration into the small building volume.
Optical surface probing of microscopic specimens is a widely studied field of science. Possible principles can be generally classified into interferometric and non-interferometric systems. A prominent example for the former category is coherence scanning interferometry [ 18 ], confocal microscopy is an example for the latter [ 19 ]. Both require axial scanning to reconstruct the surface profile and therefore are not suitable for the required online feedback demanded by the in this article discussed application.
A further advanced interferometric technique is Fourier domain optical coherence tomography( FD-OCT). FD-OCT can achieve nanometer axial resolution in a single spectral acquisition cycle [ 20 ]. However, complex broadband sources, such as sweeping laser sources, are typically used to realize the necessary spectral resolution, leading to the necessity of tight dispersion control between measurement and reference arm [ 21 ]. Furthermore, interferometric systems are sensitive to all vibrations and other parasitic drifts occurring between the reference and measurement arm. Therefore, placement of the reference close to the substrate is often necessary to maintain stability, which leads to a larger beam-path and incompatibility with the exposure beam.
A non-interferometric approach could be the employment of chromatic confocal microscopy( CCM) [ 22 ]. Due to axial chromatic aberration in the focusing objective,
broadband light is focused onto different axial planes. The substrate’ s surface then would cause an axial confocal signal along the spectrum. For well designed chromatic objectives, this spectral axis would be linear towards the depth of the substrate [ 23 ]. By using peak fitting and reconstruction algorithms, nanometer axial resolution can be achieved in a single acquisition cycle [ 24 ]. Compared to an FD-OCT system, not requiring a reference arm, is expected to provide measurements at comparable axial resolution, but with higher stability. This requires a sufficiently high resolution of the spectral confocal peak, which for typical spectrometers require a broader spectrum. This may lead to longer evaluation times and incompatibility to a common singlemode fiber.
FD-OCT and CCM both require axially distributed probing points to reconstruct the surface position via the knowledge of the whole peak. A less data intensive approach and therefore faster evaluation emerges from the analysis of the axial response signal of a classic confocal microscope. As shown in Figure 2 this ideally resembles a sinc 2-peak for I z [ 25 ]. The abscissa of the diagram u is given in normalized axial optical coordinates [ 26 ]:
u ¼ 8p k z sin2
h max
2
: ð1Þ
Thereby, k is the employed effective wavelength and h max the semi-aperture angle of the objective for NA < 0.5. I z has a maximum signal response, if the surface of a flat substrate is in the focal plane of the objective at the measurement wavelength. Without any additional information, such a peak signal has two additional drawbacks for online feedback. First, the intensity value is ambiguous. A deviation from the focal plane can not immediately be assorted to a direction for a following correction. Second, looking at the sensitivity S z of such a peak signal in Figure 2, at the maximum of the peak, the sensitivity is zero. Small deviations might not be detected with noisy systems. Contrary, S z solves both these issues by its steep slope at the zero-crossing and non-ambiguous range u [ �2, 2 ]. Therefore, it would be advantageous to find an optical probe, that can provide a( pseudo-) derivative of the confocal peak signal.
1.2 Chromatic differential confocal microscopy
Systems that can provide such derivative signals towards the surface are called differential optical probes. Famous representatives of these for surface probing are laser focus sensing( LFS) [ 27 ] and differential confocal microscopy( DCM) [ 28 ]. LFS may be realized with Focault’ s knife edge [ 29 ], astigmatism [ 30 ] or two orthogonal line-gratings [ 31 ]. The latter thereby had been miniaturized and widely applied in optical disk drives and has proven useful for nanometer surface probing. However, it is incompatible with the high-integration approach through a single common single-mode fiber( herein after: common fiber).
In DCM the confocal principle is expanded by the arrangement of source- and / or detection-pinholes.