Beam Deceleration is a relatively simple method to reduce electron beam energy and improve imaging parameters such as resolution and contrast. The scanning electron microscope (SEM) uses a sharply focused electron beam to probe the specimen surface. The energy of the electrons forming such a probe is determined by the electrical potential of the electron source, referred to as accelerating voltage or high voltage (HV). No matter how many times the electrons are accelerated or decelerated inside the column, they leave the column with an energy corresponding to the high voltage. The high voltage is usually controllable within a range of 200 V to 30 kV for most commercially available SEMs, allowing the operator to select the electron beam energy suitable for the application. Imaging with very low electron beam energy has great importance, which is illustrated by SEM instrumentation development over the last few decades [1–2]. Low voltage microscopy is a topic discussed at most microscopy-related conferences these days, but generally, it is approached with an immersion lens and field emission gun (FEG) SEM system because of the better beam current densities. However, beam deceleration is also a means to bring low kV improvement to SEMs with thermionic electron sources.

High resolution low-kV STEM imaging is getting more and more attention in the materials research and semiconductor industry, as well as in life sciences research [1]. This has been driven by the need to work with thinner TEM lamella, and with samples containing low-Z and beam sensitive materials. Decreasing electron energies is often favorable from a radiation damage point of view, moreover it improves scattering contrast. In this contribution we focus on STEM imaging extended by diffraction analysis, by integration of pixelated detectors into the FIB/SEM platform.There are two main types of STEM instrumentationdedicated S/TEM systems (which can typically perform in both STEM and TEM modes) and general purpose FIB/SEM systems with a STEM option. The dedicated S/TEM instruments offer higher ultimate imaging performance, usually can go to higher accelerating voltages, might use correctors and advanced analytical techniques such as EELS. In contrast, FIB/SEM platforms offer high system versatility, combining sample preparation with imaging and analysis. However, the STEM options have historically been limited to ~6Å or more, limiting their usefulness on the most challenging samples. Significant improvements in this field was achieved by introducing the Helios G4 platform which brings 3Å STEM resolution, as well in-situ site specific ultrathin lamellas with damage below 1 nm and maximum cut fidelity [2].Resolution of a 30 kV field emission SEM is mainly limited by spherical aberration of the objective lens. The easiest way how to reduce the spherical aberration is to shorten the focal length of the objective lens. In SEMs with conventional lenses the minimum focal length is usually more than 10 mm, in instruments with immersion magnetic (single pole) lenses more than 4 mm. On the other hand, in TEMs where the objective lens utilizes two pole pieces with small gap in between (an in-lens configuration), the focal length can be several times shorter and consequently the coefficients of axial aberrations smaller. The presented accomplishment of STEM performance improvement in a SEM is to equip the system with a second (lower) pole piece in such a way that the system is in-situ configurable and can be operated both in a traditional single-pole SEM (or FIB/SEM) mode and in a high resolution in-lens STEM mode. In order to have the system user-configurable the lower pole piece with integrated multi-segment solid state detector is mounted on a retractable mechanism, which also assures precise mechanical alignment to avoid unwanted aberrations. Improvements in resolution at 30 kV was demonstrated by lattice planes visualizations on several materials such as carbon nanotubes (0.34 nm), silicon (0.31 nm) or tungsten disulfide (0.27 nm) as shown in Figure 1c.To augment the STEM imaging it is desirable to view the diffraction pattern from the sample (as would typically be the case in a conventional S/TEM), which can provide information about sample crystallography, as well as lamella orientation. To investigate this, a pixelated d...

Detection of signal electrons belongs amongst the key parameters of the Scanning Electron Microscope (SEM). The traditional approach is the ETD detector for secondary electrons (SE) and a below-the-lens detector for backscattered electrons (BSE). State-of-the-art SEMs can be equipped with up to three inlens detectors capable of collecting both the SE and BSE signal. Moreover, the possibility to sort electrons according to their energies and/or emission angles is becoming common. Such a selective detection is usually done by influencing the SE or BSE trajectories by electric or magnetic fields. This paper introduces compound-lens-controlled energy selective detection of BSE on a new FEI SEM.The compound final lens combines the magnetic final lens inside the pole piece (ML1), the immersion magnetic lens (ML2) and the electrostatic lens (EL1) formed by the potential at the T1 detector (Fig. 1a). Main function of the compound final lens is to focus the primary electron beam to the sample. However, independent control of these three lenses enables us to use ML1 and EL1 to focus the primary beam and the ML2 to affect the trajectories of signal electrons. The ML2 behaves as a chromatic sensitive lens which focuses the high-loss BSE into the aperture of the annular T1 detector. The low-loss BSE are less sensitive to the ML2 magnetic field and reach the T1 detector (Fig. 1b). SE also pass through the aperture in the T1 and are collected further in the column by the T2 and T3 detectors.The main goal of the energy selective BSE detection is the enhancement of the compositional contrast. The BSE signal typically contains both the high-and the low-loss fractions. The BSE coefficient (η) is little dependent on the atomic number (Z) for the high-loss BSE, while it is strongly dependent in the case of the low-loss BSE. Therefore, in order to enhance the compositional contrast, low-loss BSE need to be selectively detected. This is exactly the mechanism behind the contrast enhancement with the compound lens. Besides the BSE filtration, the magnetic field of the ML2 decreases the diameter of the primary beam and thus improves the resolution. This enables us to acquire compositional contrast images in high resolution, such as the Pd nano-particles on the CeO2 matrix in Fig. 2a [2]. Another important improvement is the collection efficiency of the T1 BSE detector. Thanks to its design and position, the T1 detector covers a large BSE emission angle and provides high signal to noise ratio even at low probe currents. Naturally, BSE filtering cuts off a significant part of the T1 signal. However, the T1 detector keeps providing low noise images at probe currents down to 25 pA and less. The possibility to work at low probe currents together with the energy selective BSE detection enables charge-free imaging of non-conductive samples. The high-loss BSE carrying the charge information are filtered out so they don't contribute to the T1 detector image. Collection of low-loss BSE at probe current of 25pA produces charge free images of insul...

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The scanning electron microscope (SEM) offers a rich array of different electron-beam induced signals with which to create the final micrograph. So in parallel with efforts to improve the beam resolution [1], it is also important to find ways to make the best use of these induced signals to bring out the desired information about the sample. As an example of this effort, this paper will describe the use of energy filtering of the secondary electron (SE) signal using an immersion lens SEM. Applications of this technique include visualizing dopant contrast [2] and improving the sensitivity of passive voltage contrast, Figure 1, as well as the potential for increasing the contrast between key materials of interest by exploiting the different SE energy distributions from different materials (see for example, [3] for experimental measurements of SE spectra).To better understand the complete imaging process Monte-Carlo simulations have been undertaken that include a 3D modeling of the SEM column, including the electromagnetic fields, so that the trajectories of the emitted SE and back-scattered electrons (BSE) from the beam-sample interaction can be accurately followed back into the SEM detection system [4]. Once such a model is in place virtual experiments can be carried to determine the optimum beam and detector conditions for a particular sample type. Such simulations can investigate the signal based on particular SE energies or trajectory angles from the sample, and also to simulate the actual image expected.This modeling has been applied to the energy filtering of the SE signal. Low-pass, high-pass and band-pass filtering can be carried out depending upon the beam, sample and detector conditions. In each case, the magnetic field of the immersion lens SEM combined with an in-lens detector play the crucial role in enabling the filtering. Figure 2 shows modeled results for a low pass filter where with in-lens detector parameter M=-2V the electrons up to ~5 eV are transmitted, while a value of M=-16V allows a wider range (~16 eV). The modeling showed that by adjustment of M it was possible to go from zero to full transmission of the SE range, with Fig.2 just being representative examples. Figure 1 shows experimental results for a high pass filter on a voltage contrast sampleby adjusting the starting point of the range from 1 eV to 2 eV it was possible to change which of the contacts appear bright or dark in the image.