The integration of microspheres within the instruments of optical metrology and mask-less lithography could already show a significant enhancement of their lateral resolution. Exposing complex large structures exploiting this high resolution requires the lateral movement of the microsphere over the substrate. Challenging remains the accurate lateral and axial positioning of the microsphere ensuring the constant exposure conditions at every point. Preserving the advantage of optical instruments to not actually contact the specimen, the microsphere must be kept at a nanometer-close, yet constant distance from the surface. Here, we introduce the, to our best knowledge, novel approach to combine the principle of the differential confocal microscope with a scanning microsphere. This produces a differential signal towards the surface allowing a nanometer-sensitive and fast control of the axial position of the microsphere above the substrate. In preliminary experiments we show the repeatable pick-up of microspheres and their precise lateral scanning using a nanopositioning and nanomeasuring machine as well as axial depth responses and differential signals from the realized microsphere assisted differential confocal probe.
Direct laser writing is a popular method for mask-less lithography that already achieved commercial grade. However, it is still challenging to realize homogeneously exposed structures on 3D-shaped substrates. A common source of the variation in exposure is a changing distance of the substrate surface with the photoresist towards the plane of the narrowest waist of the exposure beam. To tackle this issue, we propose a differential confocal probe that employs a spatially modulated pinhole. This phase-modulated lock-in principle enables highly resolved depth sensing without ambiguity about the direction of the deviation from the focal plane. However, the modulation contrast must be high enough to achieve this, which is why the measuring beam passes the pinhole twice. This probe is integrated into the nanopositioning and nanomeasuring machine via a position-based controller. We demonstrate the capability to follow 10◦ inclined substrates.
Confocal microscopy is a working horse of optical profilometry since decades. It is a pointwise measurement method, where the whole sample must be scanned in all three dimensions. The high lateral resolution thereby outstrips its lowered scanning speed compared to widefield based principles. Furthermore, for a single 3D surface, even single-digit nanometre depth-resolution has been shown. However, albeit such high axial resolution, the accuracy may suffer from sample or optics induced wavefront distortions that differ from point to point. The acquired signal then experiences a shift that leads to a wrong acquired depth. Here we model this error through a low NA scalar model. We further present a method to compensate this error significantly by enhancing the principle of differential confocal microscopy. Theoretical results show the possibility for ideal compensation of the error caused by such in-stationary aberrations in confocal depth measurements.
Metrological stages such as the nano-positioning and nano-measurement machine (NPMM) can position single-digit nanometer accurately on centimeter working volumes. However, their measurement system requires a feedback to the arbitrary shaped specimen by another probe. The differential confocal microscopy (DCM) offers the possibility to have a sensitivity down to that single-digit nanometers but suffers from noise and aberration. Recently the principle of the LockIn filtering could be successfully adapted in DCM and therefore achieved a high SNR. Contrary to the there employed acoustically driven tunable GRIN lens (TAG lens) at the objective, we demonstrate a microelectromechanical system (MEMS), an AFM cantilever, as an ultrafast oscillating pinhole in front of the detector. Its first resonance at 96kHz makes it very competitive regarding acquisition speed, but the low oscillation amplitude lowers contrast. By principle inheriting the possibility to compensate a change in reflectivity, we present another advancement for the evaluation of the resulting differential signal to make it robust against sample induced systematic depth errors, e.g. a tilt-angle. This could be advantageous for DCM with static beam-paths, as well. Potentially, the highest improvement can be achieved in conjunction with the NPMM’s highly accurate measurement interferometers, because the residual error for the depth of a specimen under the influence of varying aberration is kept below 20nm.
By combining classic differential interference contrast (DIC) with the chromatic confocal principle, we show that phaseshifting calibration can be avoided in DIC by using spectral information induced by the investigated sample. The created spectral fringe can be further used to unwrap the phase. This unwrapping is limited by the spectral resolution of the spectrometer. Therefore, the depth-difference around a single measurement point can be determined instantaneously. To reconstruct the depth profile, the integration of a depth-gradient is necessary. By combining the depth information of the chromatic confocal carrier signal with the differential depth information of the carried DIC signal, the accumulation of measurement uncertainty can be reduced. To our best knowledge, the proposed chromatic confocal differential interference contrast (CCDIC) is a novel profile reconstruction principle. To verify the feasibility of the CCDIC, a prototype probe with an adjustable shear and phase has been developed. Preliminary experiments achieve sub-micrometer depth resolution. A current challenge requiring further work is the stable unwrapping of the phase-difference by spectral frequencies.
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