Miloš Hovorka

Scanning Electron Microscopy with Samples in an Electric Field

The high negative bias of a sample in a scanning electron microscope constitutes the “cathode lens” with a strong electric field just above the sample surface. This mode offers a convenient tool for controlling the landing energy of electrons down to units or even fractions of electronvolts with only slight readjustments of the column. Moreover, the field accelerates and collimates the signal electrons to earthed detectors above and below the sample, thereby assuring high collection efficiency and high amplification of the image signal. One important feature is the ability to acquire the complete emission of the backscattered electrons, including those emitted at high angles with respect to the surface normal. The cathode lens aberrations are proportional to the landing energy of electrons so the spot size becomes nearly constant throughout the full energy scale. At low energies and with their complete angular distribution acquired, the backscattered electron images offer enhanced information about crystalline and electronic structures thanks to contrast mechanisms that are otherwise unavailable. Examples from various areas of materials science are presented.

Very low energy electron microscopy of graphene flakes

Commercially available graphene samples are examined by Raman spectroscopy and very low energy scanning transmission electron microscopy. Limited lateral resolution of Raman spectroscopy may produce a Raman spectrum corresponding to a single graphene layer even for flakes that can be identified by very low energy electron microscopy as an aggregate of smaller flakes of various thicknesses. In addition to diagnostics of graphene samples at larger dimensions, their electron transmittance can also be measured at very low energies.

Scanning transmission low-energy electron microscopy

We discuss an extension to the transmission mode of the cathode-lens-equipped scanning electron microscope, enabling operation down to the lowest energies of electrons. Penetration of electrons through free-standing ultrathin films is examined along the full energy scale, and the contribution of the secondary electrons (SEs), released near the bottom surface of the sample, is shown, enhancing the apparent transmissivity of the sample to more than 100%. Provisional filtering off of the SEs, providing the dark-field signal of forward-scattered electrons, was made using an annular 3-D adjustable detector inserted below the sample. Demonstration experiments were performed on the graphene flakes and on a 3-nm-thick carbon film. Electron penetrability at the lowest energies was measured on the graphene sample.

Very low energy scanning electron microscopy in nanotechnology

The group of low energy electron microscopy at ISI AS CR in Brno has developed a methodology for very low energy scanning electron microscopy at high image resolution by means of an immersion electrostatic lens (the cathode lens) inserted between the illumination column of a conventional scanning electron microscope and the sample. In this way the microscope resolution can be preserved down to a landing energy of the electrons one or even fractions of an electronvolt. In the range of less than several tens of electronvolts the image signal generation processes include contrast mechanisms not met at higher energies, which respond to important features of the 3D inner potential of the target and visualise its local crystallinity as well as the electronic structure. The electron wavelength comparable with interatomic distances allows observation of various wave–optical phenomena in imaging. In addition, the cathode lens assembly secures acquisition of electrons backscattered from the sample at large angles with respect to the surface normal, which are abandoned in standard microscopes although they provide enhanced crystallinity information and surface sensitivity even at medium electron energies. The imaging method is described and illustrated with selected application examples.

Strain Mapping by Scanning Low Energy Electron Microscopy

The use of the scanning low energy electron microscopy (SLEEM) has been slowly making its way into the field of materials science, hampered not by limitations in the technique but rather by relative scarcity of these instruments in research institutes and laboratories. This paper reports the results obtained from an investigation of the microstructure of ultra fine-grained (UFG) copper fabricated using equal channel angular pressing (ECAP) method, namely in the as-pressed state and after annealing. SLEEM is very sensitive to the perfection of crystal lattice and using SLEEM, local strain can be effectively imaged.

High-Throughput Large Volume SEM Workflow using Sparse Scanning and In-painting Algorithms Inspired by Compressive Sensing

We are presenting a new extension to our Cell and Tissue/Neurobiology large volume imaging workflow, with the goal of increasing acquisition speed by more than five times. Instead of scanning dense square-grid frames, in the conventional way, our approach is here to explore the use of sparse scanning and inpainting techniques inspired by Compressive Sensing (CS) [1]. Sparse samples are obtained by pseudo-random scan patterns, and reconstruction algorithms are used to recover the dense volume data. The goal is to recover 3D datasets with minimum loss of information. Techniques inspired by CS gained wide attention over the last decade and are now being used in various applications where sensor bandwidth is a limiting factor. They have been recently explored for SEM and STEM applications [2][3]. In the context of nano-scale cell biology volume acquisition, we expect these techniques to ultimately increase the imaging throughput by nearly an order of magnitude. We will discuss additional advantages of this approach, such as the low-dose imaging of sensitive specimens, and the good compatibility with backscatter electron imaging. A key enabler of any sparse scan application to EM is the accurate control of scan locations. It has been shown in [2] and in our own experiments that precise positioning of the beam at the planned sampling locations is essential for a good CS reconstruction. We have developed advanced minimum-path scanning strategies to address this issue. The scanning technique is illustrated in Fig. 1, where the left two images show a conventional raster scan at 300ns dwell visiting a random set of points with the compressive sensing reconstruction obtained from such scan strategy. The right two images of Fig. 1 show an example minimum-path scan pattern and a much improved reconstruction result from images acquired with this second method. In future work we will compare pseudo-random sparse sampling, in combination with a reconstruction algorithm based on CS-inspired in-painting, to conventional grid sampling of the same effective dose, in combination with a de-noising algorithm, also based on CS. CS machine learning algorithms build patch-dictionaries, which are used as the building blocks for data representation [3]. During live acquisition runs, such dictionaries can be used to in-paint with high fidelity, the sparsely sampled datasets (Figure 2). We are implementing the new sparse scanning modules on SEM platforms, which also employ the Multi Energy Deconvolution SEM (MED-SEM) technology and Serial Block Face (SBF) imaging [4]. By incorporating CS, we will have an instrument allowing for both high-resolution isotropic imaging, and the fast acquisition of very large datasets (Figure 3).

Strategies for Collection of Secondary Electrons in the SEM

The standard way of secondary electron (SE) detection in the scanning electron microscope (SEM) is to use the Everhart-Thornley (ET) detector. Only weak electrostatic field attracts low energy SEs. Let us call this system the standard detector. Although the ET detector has been around for more than fifty years, it remains the most frequently used type of detector in SEMs. Modern SEMs have improved their image resolution by so called immersion systems, allowing a strong magnetic field of the objective lens to penetrate into the specimen region. In that case, two ET detectors are usually used: one is located above the objective lens, and the other below it (upper and lower detector). The resulting contrast of the SE images depends on SE energy and on the angular sensitivity of detectors, which is a result of specific distributions of electrostatic and magnetic fields in the specimen region.