The quality of a scanning electron micrograph is determined by both the resolution and the contrast: the size of the features that can be resolved and the type of information that the micrograph gives about them. Recent advances in SEM have improved both, for example using magnetic immersion or electrostatic lenses for ultra-high resolution, or by employing in-lens, angle-sensitive detection for tunable contrast. This paper introduces a further improvement in resolution and in contrast using a new FEI SEM equipped with a compound electrostatic-magnetic final lens.
The compound final lens SEM combines a magnetic final lens in the pole piece, a magnetic immersion final lens and an electrostatic lens formed by the potential at the bottom of the column. This new final lens design provides a resolution equal to 1.0 nm at 1 kV acceleration voltage. Figure 1 demonstrates the ultra-high resolution performance by imaging the SBA-15 sample at 500 V landing energy.
The contrast performance is based on the independent in-lens detection of secondary (SE) and backscattered (BSE) electrons. Secondary electrons are further separated and independently detected by in-lens (T2, higher SE energies) and in-column (T3, lowest SE energies) detectors. Separate collection of the lowest energy SEs delivers extremely surface sensitive imaging. Simultaneous detection of high energy SEs adds more freedom for imaging of non-conductive samples – high energy SEs are less sensitive to sample charging while still clearly showing the topography – see the high energy SE image of a hydroxyapatite nano-sheets grown on bioactive glass fiber  in Figure 2.
The backscattered electrons are detected by the T1 detector which is positioned at the very bottom of the pole piece. Thanks to that position it receives a high signal intensity which enables ultra-low beam current BSE imaging. This is important for beam sensitive samples, such as polymers, porous materials or other fragile samples, which require the maximum amount of signal to be acquired in the shortest amount of time with the smallest dose possible. With the T1 detector BSE imaging is possible during TV-rate navigation, so that material contrast is always available. Combined with the new compound final lens, it is possible to further energy filter the backscattered electrons detected on the T1 detector. When high-loss (low energy) BSEs are filtered out, T1 provides strong material contrast images formed by low-loss (high energy) BSEs only – see surface of the Pt activated carbon particle in Figure 3. The acquisition of topographical information is ensured by the SEs simultaneously collected by the T2 and T3 detectors. The T1 detector keeps providing low noise images even at probe currents down to few tens of pA. The low probe current operation together with the energy selective BSE detection enables charge-free material contrast imaging of non-conductive samples.
The SE and BSE filtering combined with excellent low dose performance (low kV and low current) and “smart” scanning strategies allow for high vacuum imaging of uncoated insulators that are otherwise susceptible to charging. The capabilities on these samples are completed with a low vacuum mode with chamber pressure up to 500 Pa, which is essential for analytical measurements where high acceleration voltages and probe currents are required.
In conclusion, the new SEM equipped with the compound final lens and the in-lens and in-column detectors improves both the imaging resolution and contrast performance. It allows researchers to capture the maximum amount of information from conductive as well as insulating samples, with the right detail and with the least amount of compromises.
 Hydroxyapatite sample courtesy of Devin Wu, FEI China
To cite this abstract:Petr Wandrol, Ernst Jan Vesseur; A New Compound Lens Equipped UHR SEM.. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/a-new-compound-lens-equipped-uhr-sem/. Accessed: February 28, 2020
EMC Abstracts - https://emc-proceedings.com/abstract/a-new-compound-lens-equipped-uhr-sem/