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 3$\times$3 multi-mode interferometer, allowing the retrieval of the complex Fourier coefficients. We derive a system of non-linear equations whose solution, which is obtained numerically from Newton's method, gives the modulation of the sensor's resonances as a function of time. By taking one of the sensors as a reference, to which no external excitation is applied and its temperature is kept constant, about 92$\%$ 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