Wavelength-scale ptychographic coherent diffractive imaging using a high-order harmonic source

Getnet K Tadesse,Thomas Pertsch,Maxim Tschernajew,Matthias Zilk,Wilhelm Eschen,Frederik Tuitje,Robert Klas,Jens Limpert,Michael Steinert,Michael Zürch,Vittoria Schuster,Jan Rothhardt,Christian Spielmann

doi:10.1038/s41598-019-38501-1

Abstract

Ptychography enables coherent diffractive imaging (CDI) of extended samples by raster scanning across the illuminating XUV/X-ray beam, thereby generalizing the unique advantages of CDI techniques. Table-top realizations of this method are urgently needed for many applications in sciences and industry. Previously, it was only possible to image features much larger than the illuminating wavelength with table-top ptychography although knife-edge tests suggested sub-wavelength resolution. However, most real-world imaging applications require resolving of the smallest and closely-spaced features of a sample in an extended field of view. In this work, resolving features as small as 2.5 λ (45 nm) using a table-top ptychography setup is demonstrated by employing a high-order harmonic XUV source with record-high photon flux. For the first time, a Rayleigh-type criterion is used as a direct and unambiguous resolution metric for high-resolution table-top setup. This reliably qualifies this imaging system for real-world applications e.g. in biological sciences, material sciences, imaging integrated circuits and semiconductor mask inspection.

Highlights

Coherent diffractive imaging and related techniques are variants of X-ray microscopy that have become increasingly important for photon energies where focusing optics are limited in terms of quality and achievable spot size[1,2]
An high-order harmonic generation (HHG) source driven by a high average power fiber laser is used and a single harmonic line at 68.6 eV is selected and focused using spherical multilayer XUV mirrors
By imaging a Siemens-star test pattern we resolved the smallest features ever obtained by any table-top ptychography setup

Summary

Introduction

Coherent diffractive imaging and related techniques are variants of X-ray microscopy that have become increasingly important for photon energies where focusing optics are limited in terms of quality and achievable spot size[1,2]. Complete nanoscale 3D images of integrated circuits[8] and whole unstained cells[9] have become possible and attracted huge attention in science and technology, bridging the gap between visible light and electron microscopy Until recently, these ptychography experiments have been conducted at synchrotron facilities, which are not accessible. The smallest contacts and fins in integrated circuits[8], smallest inclusions in functional nanomaterials or the features of cellular sub structures[9] measure only a few ten nanometer in size Resolving such features is more challenging for table-top sources, compared to synchrotrons delivering similar flux, due to the longer wavelengths (>10 nm) www.nature.com/scientificreports/. A Rayleigh-type criterion is used as a direct and reliable resolution metric in a transmission geometry that uses a Siemens star test pattern

Methods

Results

Conclusion