Expanding Capabilities of Low-kV STEM Imaging and Transmission Electron Diffraction in FIB/SEM Systems

Abstract

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...

Tilt-free EBSD

“…Tomáš Vystavěl 1 , Pavel Stejskal 1 , Marek Unčovský 1 and Chris Stephens 2 1. Thermo Fisher Scientific, Brno, Czech Republic.…”

“…Recent developments in electron detection technology, particularly in pixelated semiconductor detectors, enable the acquisition of electron diffraction patterns in various geometries. Diffraction patterns acquired on thin samples in transmission configuration were presented in [1]. On top of that acquisition of electron backscattered diffraction patterns above the sample is discussed here.…”

Powder Nano-Beam Diffraction in Scanning Electron Microscope: Fast and Simple Method for Analysis of Nanoparticle Crystal Structure

“…The 4D-STEM technique was introduced in the field of TEM and dedicated high-energy STEM microscopy [ 11 ], and, despite being quite new, it has already found several applications, such the mapping of electrostatic fields in 2D semiconductors [ 12 , 13 ], electron ptychographic diffractive imaging [ 11 , 14 ], mapping of structure of amorphous thin films by means of the pair distribution function analysis [ 15 ], or mapping of orientation of organic nanocrystal grains [ 16 ]. In the field of SEM microscopy, the pixelated detectors based on fast DED cameras has been commercialized very recently [ 17 ]. As a result, the 4D-STEM techniques in SEM microscopes are less common, just a few examples were found in the available literature, and almost all methods have been based on traditional CCD or CMOS detectors [ 7 , 18 , 19 , 20 , 21 ].…”

High Resolution Powder Electron Diffraction in Scanning Electron Microscopy

“…In the field of SEM, the 4D-STEM techniques are much less common. The fast pixelated DED detectors for SEM were commercialized quite recently [10]. Consequently, just a few examples of 4D-STEM in SEM can be found in the literature, and most of the studies were based on traditional CCD and CMOS detectors [11][12][13][14].…”