Wavelength-multiplexed Multi-mode EUV Reflection Ptychography based on Automatic-Differentiation

TL;DR

This paper introduces a wavelength-multiplexed EUV ptychography algorithm utilizing automatic differentiation, enabling high-resolution, efficient imaging of complex semiconductor samples in reflection geometry.

Contribution

A novel wavelength-multiplexed ptychography algorithm using automatic differentiation is developed, improving throughput and accuracy in EUV reflection imaging of semiconductor wafers.

Findings

Achieved near-diffraction-limited resolution in reflection geometry.

Successfully reconstructed complex wafer structures with 20-nm features.

Demonstrated the algorithm's robustness against experimental uncertainties.

Abstract

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.