Ivo Konvalina

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.

Imaging with STEM Detector, Experiments vs. Simulation

Although there are a great many Scanning Electron Microscopes (SEMs) and Scanning Transmission Electron Microscopes (STEMs) currently in existence, obtaining quantitative information from the electron signal is proving difficult to establish [1]. In order to create good quantitative procedures, a comparison between experimental and theoretical signals from well defined samples needs to be carried out. Since the theoretical understanding of electron transport at low energies remains relatively poor, studies which make use of the higher energy scattered primary electrons would be a good place to start such a comparison. Hence, one could compare the signal obtained from a backscattered electron detector in an SEM with what one might expect from a Monte Carlo (MC) simulation. Another approach would be to consider the transmitted electrons (TEs) through thin films of elemental materials in a STEM. It is the latter approach that we will concentrate upon in this report. This is because calibrating the STEM detector using the primary beam is possible using the transmitted beam when no sample is present. However this approach is not possible for backscattered measurements. The aim of this work is to find out the angular and energy distribution of the TEs that are very important for the understanding of image formation in STEM.

Practical Use of Scanning Low Energy Electron Microscope (SLEEM)

The high negative bias of a sample in a scanning electron microscope constitutes the cathode lens (CL), with a strong electric field just above the sample surface [1] offers a tool for controlling the landing energy of electrons down to units or even fractions of electronvolts. 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.

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.