[Paper Review] Multimode Objective Lens for Momentum Microscopy and XPEEM: Experiments

O. Tkach, Sotirios Fragkos+38 more|arXiv (Cornell University)|Jan 18, 2024

Advanced Electron Microscopy Techniques and ApplicationsBiochemistry, Genetics and Molecular Biology3 citations

TL;DR

This paper presents a novel multimode objective lens for XPEEM and momentum microscopy that uses a concentric ring electrode to shape the electric field in the gap between sample and extractor, reducing field strength by up to an order of magnitude and mitigating field emission. The lens enables zero-field and repeller modes, achieving 250 nm raw resolution (improvable to <100 nm) with reduced space-charge effects and larger fields of view compared to conventional cathode lenses.

ABSTRACT

A new type of objective lens has recently been proposed for use in X-ray photoemission electron microscopes (XPEEMs) and momentum microscopes. Adding a ring electrode concentric with the extractor allows the field in the gap between the sample and the extractor to be shaped. Forming a lens field in this gap reduces the field strength at the sample by up to an order of magnitude. This mitigates the risk of field emission, particularly for cleaved samples with sharp edges. A retarding field can redirect all slow electrons, thus eliminating the primary contribution to the space-charge interaction. Here we present the first experimental investigation of the new lens, examining its performance at photon energies ranging from the extreme ultraviolet produced by a high-harmonic generation (HHG)-based source to soft and hard X-rays at two synchrotron facilities. The gap lens in a region without electrodes enables large working distances up to 23 mm. Reduced aberrations allow for larger fields of view in both k-space and real-space imaging, with resolutions comparable to those of conventional cathode lenses. However, field strengths are an order of magnitude smaller. The zero-field mode enables the study of 3D structured objects and is therefore beneficial for small cleaved samples as well as for operando devices involving top electrodes. The repeller mode reduces space-charge effects, but results in a smaller k-field diameter. This reduction ranges from 10% at hard X-ray energies to 50% in the XUV range. The usable energy interval is also reduced by a factor of two. In time-of-flight XPEEM mode the raw data show a resolution of 250 nm, which can be improved to better than 100 nm through data processing.

Motivation & Objective

• To address field emission and space-charge effects in XPEEM and momentum microscopy, especially for cleaved or sensitive samples.
• To develop a lens design that enables operation in multiple modes—zero-field and repeller—without compromising resolution or field of view.
• To enable high-resolution imaging of 3D-structured samples and operando devices with top electrodes.
• To extend the usable energy range and improve performance across extreme ultraviolet, soft, and hard X-ray regimes.
• To validate the lens experimentally at synchrotron facilities using high-harmonic generation and beamline sources.

Proposed method

• A concentric ring electrode is added around the extractor to shape the electric field in the gap between the sample and the lens, enabling multimode operation.
• The lens operates in two primary modes: zero-field mode, where the field is minimized at the sample surface, and repeller mode, where a retarding field redirects slow electrons.
• The gap between the sample and extractor is field-free in the zero-field mode, allowing working distances up to 23 mm.
• Field shaping reduces field strength at the sample by up to 10×, significantly lowering the risk of field emission.
• The lens design maintains high resolution by minimizing aberrations, enabling large fields of view in both k-space and real space.
• Time-of-flight XPEEM mode is employed to assess resolution, with data processing used to enhance image quality.

Experimental results

Research questions

• RQ1 Can a multimode objective lens reduce field emission in XPEEM and momentum microscopy while preserving high resolution?
• RQ2 How does the inclusion of a ring electrode affect field distribution and electron trajectory control in the lens gap?
• RQ3 To what extent does the repeller mode reduce space-charge effects, and what are the trade-offs in energy range and k-space diameter?
• RQ4 Can the lens support imaging of 3D-structured samples and operando devices with top electrodes?
• RQ5 What is the achievable resolution in time-of-flight XPEEM mode, and how much can it be improved through data processing?

Key findings

• The lens reduces field strength at the sample by up to an order of magnitude, significantly mitigating field emission risks, especially for cleaved samples with sharp edges.
• The zero-field mode enables imaging of 3D-structured samples and operando devices with top electrodes, supporting large working distances up to 23 mm.
• The repeller mode reduces space-charge effects by redirecting slow electrons, but decreases the usable k-field diameter by 10% at hard X-ray energies and up to 50% in the XUV range.
• The usable energy interval is reduced by a factor of two in the repeller mode due to the constraints of electron energy selection.
• Raw resolution in time-of-flight XPEEM mode reaches 250 nm, with data processing improving it to better than 100 nm.
• Aberrations are reduced compared to conventional cathode lenses, enabling larger fields of view in both k-space and real-space imaging while maintaining comparable resolution.