Electron beam broadening in electron‐transparent samples at low electron energies

Abstract

Summary Scanning transmission electron microscopy (STEM) at low primary electron energies has received increasing attention in recent years because knock‐on damage can be avoided and high contrast for weakly scattering materials is obtained. However, the broadening of the electron beam in the sample is pronounced at low electron energies, which degrades resolution and limits the maximum specimen thickness. In this work, we have studied electron beam broadening in materials with atomic numbers Z between 10 and 32 (MgO, Si, SrTiO3, Ge) and thicknesses up to 900 nm. Beam broadening is directly measured using a multisegmented STEM detector installed in a scanning electron microscope at electron energies between 15 and 30 keV. For experimental reasons, the electron beam diameter is defined to contain only 68% of the total intensity instead of the commonly used 90% of the total beam intensity. The measured beam diameters can be well described with calculated ones based on a recently published model by Gauvin and Rudinsky. Using the concept of anomalous diffusion the Hurst exponent H is introduced that varies between 0.5 and 1 for different scattering regimes depending on t/Λel with the specimen thickness t and the elastic mean free path length Λel. The calculations also depend on the fraction of the beam intensity that defines the electron beam diameter. A Hurst exponent H of 1 is characteristic for the ballistic scattering regime with t/Λel → 0 and can be excluded for the experimental conditions of our study with 6 ≦ t/Λel ≦ 30. We deduced H = 0.75 from measured beam diameters which is larger than H = 0.5 that is expected under diffusion conditions. The deviation towards larger H values can be rationalised by our definition of electron diameter that contains only 68% of the total beam intensity and requires therefore larger sample thicknesses before the diffusion regime is reached. Our results clearly deviate from previous analytical approaches to describe beam broadening (Goldstein et al., Reed, Williams et al., Kohl and Reimer). Measured beam diameters are compared with simulated ones, which are obtained by solving the electron transport equation. This approach is advantageous compared to the commonly used Monte Carlo simulations because it is an exact solution of the electron transport equation and requires less computer time. Simulated beam diameter agree well with the experimental data and yield H = 0.80. Lay Description In scanning transmission electron microscopy (STEM), a focused electron beam is scanned over an electron‐transparent sample and an image is formed by detecting the intensity of the transmitted electrons by a STEM detector. STEM resolution is ultimately limited by the electron beam diameter and can be better than 0.1 nm for the best microscopes. However, the electron‐beam diameter increases with increasing specimen thickness because electrons are scattered by the interaction of the specimen material and electrons. Electron scattering leads to a change of the electron propagation direction and redu...

Beam broadening of keV electrons in matter calculated by numerical solution of the electron transport equation

“…R denotes the percentage of the total beam intensity that defines the beam radius. Hugenschmidt et al [3] experimentally verified Eq. ( 2) by scanning transmission electron microscopy for different materials and electron energies.…”

High-Energy Electron Scattering in Thick Samples Evaluated by Bright-Field Transmission Electron Microscopy, Energy-Filtering Transmission Electron Microscopy, and Electron Tomography

“…The topic discussed here attracted attention since the early days of electron microscopy (Gouldsmit & Saunderson, 1940; Cosslett & Thomas, 1964 a , 1964 b ; Wittry & Kyser, 1967; Everhart & Hoff, 1971; Kanaya & Okayama, 1972; Fitting, 1974; Matsukawa et al, 1974) and are actively investigated at present (Egerton, 2007; Lukiyanova et al, 2009; Gauvin & Rudinsky, 2016; de Jonge et al, 2018; Drees et al, 2018; Hugenschmidt et al, 2019). Early researchers were interested in the energy deposition and range of electrons in materials, often in the context of signal generation volume.…”

Imaging of polymer:fullerene bulk-heterojunctions in a scanning electron microscope: methodology aspects and nanomorphology by correlative SEM and STEM

“…Despite of these advantages, low-energy STEM (≤ 30 keV) in a scanning electron microscope (also denoted as STEM-in-SEM) has not been extensively exploited up to now and only few methodological studies were published [9][10][11]. More recent work comprises, e.g., nanoparticle characterization [12,13], dislocations analysis and manipulation [14][15][16][17], composition quantification [18], beam broadening [19,20], transmission electron microscopy (TEM) specimen thickness determination [21] as well as reduced delocalization and negligible Cherenkov losses in electron energy loss spectroscopy [22]. Low-energy STEM is also well suited to study weakly scattering and beam-sensitive (knock-on damage) materials such as bulk-heterojunction (BHJ) absorber layers of organic solar cells [23].…”