102 research outputs found
Wavelength-multiplexed Multi-mode EUV Reflection Ptychography based on Automatic-Differentiation
Ptychographic extreme ultraviolet (EUV) diffractive imaging has emerged as a
promising candidate for the next-generation metrology solutions in the
semiconductor industry, as it can image wafer samples in reflection geometry at
the nanoscale. This technique has surged attention recently, owing to the
significant progress in high-harmonic generation (HHG) EUV sources and
advancements in both hardware and software for computation.
In this study, a novel algorithm is introduced and tested, which enables
wavelength-multiplexed reconstruction that enhances the measurement throughput
and introduces data diversity, allowing the accurate characterisation of sample
structures. To tackle the inherent instabilities of the HHG source, a modal
approach was adopted, which represents the cross-density function of the
illumination by a series of mutually incoherent and independent spatial modes.
The proposed algorithm was implemented on a mainstream machine learning
platform, which leverages automatic differentiation to manage the drastic
growth in model complexity and expedites the computation using GPU
acceleration. By optimising over 200 million parameters, we demonstrate the
algorithm's capacity to accommodate experimental uncertainties and achieve a
resolution approaching the diffraction limit in reflection geometry. The
reconstruction of wafer samples with 20-nm heigh patterned gold structures on a
silicon substrate highlights our ability to handle complex physical
interrelations involving a multitude of parameters. These results establish
ptychography as an efficient and accurate metrology tool
On-chip interrogator based on Fourier Transform spectroscopy
In this paper, the design and the characterization of a novel interrogator
based on integrated Fourier transform (FT) spectroscopy is presented. To the
best of our knowledge, this is the first integrated FT spectrometer used for
the interrogation of photonic sensors. It consists of a planar spatial
heterodyne spectrometer, which is implemented using an array of Mach-Zehnder
interferometers (MZIs) with different optical path differences. Each MZI
employs a 33 multi-mode interferometer, allowing the retrieval of the
complex Fourier coefficients. We derive a system of non-linear equations whose
solution, which is obtained numerically from Newton's method, gives the
modulation of the sensor's resonances as a function of time. By taking one of
the sensors as a reference, to which no external excitation is applied and its
temperature is kept constant, about 92 of the thermal induced phase drift
of the integrated MZIs has been compensated. The minimum modulation amplitude
that is obtained experimentally is 400 fm, which is more than two orders of
magnitude smaller than the FT spectrometer resolution.Comment: 15 pages, 6 figure
Optical power transmission in a polygon mirror-based swept source optical coherence tomography system
Swept Source Optical Coherence Tomography (SS-OCT) relies on the rapid tuning of a broadband light source to produce narrow laser linewidths. Imaging speed is governed by the sweeping frequency of the source and the axial resolution is given by the total bandwidth generated. Mechanical, free space methods, employing rotating polygonal mirrors with a pair of telescopically arranged lenses, can achieve tuning speeds in excess of 100 kHz. Their success relies upon maximising the light throughput of the swept spectrum by reducing the effects of aberration and vignetting caused by the lens design and the geometrical properties of the polygon respectively. However, these properties impose constrictions on the spectral filter’s design and care must be taken when building the filter to avoid unnecessarily limiting the performance of the system. This paper presents some of the initial stages of a much larger study into the optimisation of such systems.
Theoretical work has been confirmed by experimental observations and compared with ideal simulations for a spectral filter consisting of a dispersive element, a double lens telescope, arranged in a Littman configuration, and a 72-facet, off-axis polygon mirror with end reflector. A non-linear relationship between the linewidth’s location on the telescope in time with the rotation of the polygon was observed and a first approximation for the tuned wavelength with respect to the polygon rotation angle was found. These observations, coupled with ongoing research, will lead to a complete description of polygon based scanners and how their performance can be optimised in future design
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