J. Eur. Opt. Society-Rapid Publ. 21, 27( 2025) 287
Figure 12. Schematic of designed optical measurement and exposure tool.
shapes. To find regions of interest on these, a wide-field observation should be integrated. The already for angular measurements integrated camera is employed for this task. A problem is, that a wide-field illumination is missing. Therefore, wide-field imaging is performed in a rough way via axially defocusing the substrate with the positioning stage. Through the third focusing lens, this results in a rough, yet useful, image of the specimen on the camera.
The schematic overview of the realized total system is shown in Figure 12. Besides the already discussed optical beam-path, the different colored wavelengths are combined into the common fiber via a wavelength division multiplexer. Since their wavelength are very close to each other, the measurement lasers are combined via 2 2 couplers. This also applies to the wavelength-specific photodetectors. They are spectrally distinctive via notch filters for the two employed wavelength. Normal photodiodes are employed for intensity detection. Current signals are amplified via circuits following the design of [ 58 ]. The signals are read by a lock-in filter greatly enhancing the SNR of the signals via demodulation with a reference oscillation [ 59 ]. This reference oscillation was given to the employed laserdiode drivers, modulating the driving current. To reduce crosstalk, both laserdiodes are modulated at different frequencies, f C1 = 150 kHz and f C2 = 200 kHz, respectively. The created amplitude modulation is demodulated with the dual-phase scheme as indicated by Figure 12. This eliminates sensitivity to phase fluctuations in the analog signals. The demodulated two signals were calculated to their difference D and sum R. These signals are then transferred with an output sample rate of f S = 10 kHz to the digital signal processor of the NPMM-5D. There, these two are calculated to the normalized FES, which is used for focus control on the z-axis. The control loops run at f cyl = 3kHz.
The exposure laserdiode is monitored for power control via its own photodetector. A exposure dose controller directly modulates the laserdiode according to the scan speed of the spot on the substrate. A lock-in-filter is not implemented in order to achieve highest power stability for the exposure. The rotational axis will be controlled by the calculated polar and azimuth angle. Currently this evaluation has only been implemented offline. An online control loop is added in the near future.
4 Results
To demonstrate the integrated functions, several tests had been performed. For the larger part, the primary goals regarding the chromatic differential confocal measurement in conjunction with the exposure beam are confirmed. First, this measurement probe is characterized and, second, it is used for illustrative measurements. For another part, the integrated camera-based angular measurement is characterized.
4.1
Characterization of the differential confocal system
At first, it shall be investigated whether the design goals for the optical beam-path are fulfilled. Therefore a plane optical mirror is axially scanned through the focal planes. The z-axis is measured by the highly accurate interferometer. The individual confocal responses are shown in Figure 13. Visibly, the blue exposure laser has its focal plane between the two red measurement lasers. This confirms, that primary design goal( I) has been accomplished. The actual emission wavelength of both laserdiodes was determined to be slightly deviating to k C1 = 638 nm and k C2 = 658 nm, respectively. The axial defocii of the 638 nm-laserdiode and the 658 nm-laserdiode are z D1 = 8.7lm andz D2 = �5.1 lm, respectively. Both values are close to the results from the ray-tracing optimization of z D1 6 lm andz D2 �4lm, but exceed them slightly. This may be caused by variation of the delivered lenses or the axial adjustment of the beam-path. Especially, the axial adjustment of the first negative SF11 lens has been identified as most sensitive. A larger distance towards the fiber facet will increase the axial separation of the response signals.
This eventually will come with the cost of widened spotsizes on the specimen. To estimate the optimality of the adjustment, the full-width at half maximum of the response