Improving SEM Imaging Performance Using Beam Deceleration (original) (raw)
Related papers
1 Low-energy electron microscopy
2011
Low-energy electron microscopy (LEEM) images a beam of low-energy electrons that have been reflected from a sample. The technique characterizes the sample’s surface in real-space with nanometer-scale lateral resolution. Through a variety of contrast mechanisms, different aspects of the surface can be imaged, including the distribution of different phases and the location of atomic steps. LEEM instrumentation can also acquire electron diffraction patterns from local regions of the surface. The ability to acquire images quickly during temperature changes, while depositing films and exposing materials to reactive gases makes LEEM extremely useful for studying dynamical processes on surfaces. New developments include aberration correction systems for improved spatial resolution and bright spin-polarized electron sources.
Advanced Structural and Chemical Imaging, 2016
Serial block-face scanning electron microscopy (SBEM) is quickly becoming an important imaging tool to explore three-dimensional biological structure across spatial scales. At probe-beam-electron energies of 2.0 keV or lower, the axial resolution should improve, because there is less primary electron penetration into the block face. More specifically, at these lower energies, the interaction volume is much smaller, and therefore, surface detail is more highly resolved. However, the backscattered electron yield for metal contrast agents and the backscattered electron detector sensitivity are both sub-optimal at these lower energies, thus negating the gain in axial resolution. We found that the application of a negative voltage (reversal potential) applied to a modified SBEM stage creates a tunable electric field at the sample. This field can be used to decrease the probe-beam-landing energy and, at the same time, alter the trajectory of the signal to increase the signal collected by ...
Interpretation of secondary electron images obtained using a low vacuum SEM
Ultramicroscopy, 2003
Charging of insulators in a variable pressure environment was investigated in the context of secondary electron (SE) image formation. Sample charging and ionized gas molecules present in a low vacuum specimen chamber can give rise to SE image contrast. ''Charge-induced'' SE contrast reflects lateral variations in the charge state of a sample caused by electron irradiation during and prior to image acquisition. This contrast corresponds to SE emission current alterations produced by sub-surface charge deposited by the electron beam. ''Ion-induced'' contrast results from spatial inhomogeneities in the extent of SE signal inhibition caused by ions in the gaseous environment of a low vacuum scanning electron microscope (SEM). The inhomogeneities are caused by ion focusing onto regions of a sample that correspond to local minima in the magnitude of the surface potential (generated by sub-surface trapped charge), or topographic asperities. The two types of contrast exhibit characteristic dependencies on microscope operating parameters such as scan speed, beam current, gas pressure, detector bias and working distance. These dependencies, explained in terms of the behavior of the gaseous environment and sample charging, can serve as a basis for a correct interpretation of SE images obtained using a low vacuum SEM.
Scanning Electron Microscopy and X-Ray Microanalysis, 2017
Low Beam Energy SEM 11 11.1 What Constitutes "Low" Beam Energy SEM Imaging?-166 11.2 Secondary Electron and Backscattered Electron Signal Characteristics in the Low Beam Energy Range-166 11.3 Selecting the Beam Energy to Control the Spatial Sampling of Imaging Signals-169
Scanning Electron Microscopy with Samples in an Electric Field
Materials, 2012
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.
Simulations and measurements in scanning electron microscopes at low electron energy
Scanning, 2016
The advent of new imaging technologies in Scanning Electron Microscopy (SEM) using low energy (0-2 keV) electrons has brought about new ways to study materials at the nanoscale. It also brings new challenges in terms of understanding electron transport at these energies. In addition, reduction in energy has brought new contrast mechanisms producing images that are sometimes difficult to interpret. This is increasing the push for simulation tools, in particular for low impact energies of electrons. The use of Monte Carlo calculations to simulate the transport of electrons in materials has been undertaken by many authors for several decades. However, inaccuracies associated with the Monte Carlo technique start to grow as the energy is reduced. This is not simply associated with inaccuracies in the knowledge of the scattering cross-sections, but is fundamental to the Monte Carlo technique itself. This is because effects due to the wave nature of the electron and the energy band structu...
Scanning Microscopy 2009, 2009
The secondary electron and backscattered electron coefficients have been measured as a function of primary beam energy for as-inserted and cleaned pure element samples. Clearly, the effect of cleaning samples makes a significant effect on both these key measurements needed for understanding the electron transport measurements in scannng electron microscopy and a number of other technologies. The results from the cleaned samples suggest that the currently accepted theory for secondary electron emission (SEE) of Baroody does not take account of an important physical effect. We propose that the SEE in transition metals is mainly controlled by the inelastic mean free path (IMFP) of the secondary electrons. In combination with current theories on the transport of hot electrons in transition metals, where sensitivity to the density of empty d states is important, the apparent correlation of the work function with SEE can be explained. The effect of errors in the electron elastic scattering cross-section and the electron stopping power on the estimates of backscattered electron coefficient, η, are explored for the case of Cu. It is found that percentage errors in one parameter (e.g. stopping power) cause very similar changes in η as equal but opposite percentage errors in the other parameter (e.g. elastic scattering cross-section).
Scanning transmission electron microscopy imaging dynamics at low accelerating voltages
2011
Motivated by the desire to minimize specimen damage in beam sensitive specimens, there has been a recent push toward using relatively low accelerating voltages (o 100 kV) in scanning transmission electron microscopy. To complement experimental efforts on this front, this paper seeks to explore the variations with accelerating voltage of the imaging dynamics, both of the channelling of the fast electron and of the inelastic interactions. High-angle annular-dark field, electron energy loss spectroscopic imaging and annular bright field imaging are all considered.