Abstract

Coherent diffractive imaging (CDI) is a lensless microscopy technique where a sample is imaged by recording the far field intensity diffraction pattern and subsequently retrieving the phase by iterative computer algorithms. It nowadays enables imaging with few-nanometers resolution by employing X-ray radiation usually provided either by synchrotron or free-electron laser facilities [1]. During the recent years, table-top coherent XUV sources using high-order harmonic generation (HHG) showed tremendous progress and offer unique opportunities previously accessible only at large scale facilities. Table-top XUV sources can nowadays provide photon flux of up to 10¹³ photons/sec/eV combined with excellent coherence and femtosecond pulse durations, which makes them highly attractive for high-resolution imaging [2]. In this contribution, we present CDI experiments performed with a 68.6 eV XUV source, driven by a high-average power femtosecond fiber laser system similar to that reported in [3]. It delivers pulses with 33 fs duration, 0.66 mJ pulse energy and 30 kHz repetition rate at 1μm wavelength which are subsequently focused onto an argon gas jet for HHG. A high photon flux of 1.4 × 10¹⁰ photons/sec was generated at 68.6 eV and focused onto the CDI sample by a pair of multilayer mirrors. We demonstrated the recording of diffraction from transmission samples with a fringe visibility of > 90% and a high numerical aperture (NA=0.7). Using both real and Fourier space criteria, a half-pitch resolution of less than 15 nm was achieved which is the highest resolution from any table-top XUV or soft X-ray microscope [3]. Due to the high-photon flux of the source, such high-NA diffraction patterns are recorded within few-minutes of acquisition time. To extend the applicability of CDI for extended samples (ptychography) and 3D reconstructions (tomography), hundreds of diffraction patterns have to be recorded. This requires much shorter acquisition times for a single diffraction pattern. To this end, we are able to reach resolutions of 23 nm with 3 seconds (Fig. 1) and 35 nm with only 1 second of acquisition time. This allows e.g. ~ 20 nm resolution ptychography on 100 µm × 100 µm large samples and 3D tomography with measurement times of only few tens of minutes. We will present our latest results in ptychographic and tomographic CDI driven by 68.6 eV HHG-based source. In addition, we will discuss the possibilities of increasing the photon energy for imaging in the silicon–window at ~90 eV which is of great interest for lithographic mask inspection. The rising average power of driving lasers [4] for HHG will enable improved resolution and further reduced integration times in the near future. Furthermore, the extension of available photon energies by using different driving laser wavelengths as well as different HHG gas targets will allow for addressing several absorption edges for material and chemical-state sensitive imaging.

© 2017 IEEE