Improving electron microscope capabilities through the design of new cold field emission electron source

Session

PST.3 - New Instrumentation

Authors

Dr Florent HOUDELLIER (1)

Affiliations

1. CEMES-CNRS

Keywords

Electron source, lenses aberrations, cold field emission

Abstract text

Like all machines the electron microscope (Transmission EM or Scanning EM) is composed by various key elements such as the electron source, the lenses and the detectors. Each of these components has a strong influence in the range of applications available and the quality of the recorded results. For instance, either in classical TEM or in Scanning EM (STEM or SEM), the precise design of the objective lens magnetic circuit will strongly influence the resolution of the instrument as well as the useful space available around the sample to interact with it. Increasing this vital space will “automatically” increase the influence of geometrical aberrations of the lens leading to a deterioration of the instrument resolution. This well-known intrinsic property of magnetic electron lenses has strongly limited the TEM capabilities for many years, but at the early of the 21^st century it has been overcome with the development of multipole lenses-based aberration corrector [1, 2]. 

A positive side effect in the advent of Cs corrector was somehow a revival of instrumental developments and the stimulation of a new era of creativity in the electron optics community [3, 4].

If the objective lens may be considered as the “heart” of the instrument, the electron source can be seen as its “brain”, isolated in a very sensitive environment (high voltage and vacuum) and deeply affecting all the outcome of the machine. Among the wide family of electron source, the one based on electron cold field emission from a metallic nanotip is the brightest one [5]. The brightness is indeed an appropriate figure of merit when high beam coherence is needed in TEM or high probe current with minimum energy spread in STEM and electron spectroscopic techniques. With its small virtual source size in the nm range, the brightness of the simplest CFE source assembly, i.e. a fine tip and a large round anode, is optimum. However, in an electron microscope this virtual source size needs to be properly imaged with sufficient flexibility to be able to form either, a coherent collimated beam on the sample in TEM, or a fine focused probe in STEM/SEM. To do so, electrostatic gun lens is in general used before the accelerating tube and magnetic condenser/objective lenses after. But, progressively, in each successive image plane, geometrical aberrations of these elements will deteriorate the optimum beam brightness originally contained in the first virtual cross over. The influence of the electrostatic optic design, composed by the extracting anode and the gun lens assembly above the accelerator, is crucial in this well-known mechanism revealed by a progressive distortion of the emittance figure in each associated image planes. Many design concepts of source have been proposed to provide an optimization of the final emittance either by improving the tip area, the extractor or the gun lens assembly. 

Few years ago, together with Aurélien Masseboeuf and Marc Monthioux in CEMES, we have begun to tackle these questions to improve the holography capabilities of our FE-TEM, by replacing the tip material from the original monocrystalline W <310> oriented nanotip to a new type of carbon nanocone [6,7]. This first instrumental achievement allowed us to improve the brightness of an old 200keV CFE source as well as its beam current stability.  Recently, together with Arnaud Arbouet, we have succeeded to generate bright and ultrashort electrons pulses by coupling a classical W nanotip cathode with a femtosecond laser leading to the implementation of coherent techniques in an ultrafast TEM such as electron holography [8, 9]. 

However, as always in CFE technology, the strength of these two originals developments remains strongly altered by the properties of the original pre-accelerator electrostatic optic. 

During my presentation I will rapidly introduce these results and present our current development direction to optimize the emittance characteristic of a CFE source and overcome the properties limitation of our prior designs.  

References

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[2] O.L Krivanek , P.D Nellist,N Dellby,M.F Murfitt and Z Szilagyi Towards sub-0.5 Å electron beams. 2003 Ultramicroscopy 96, 229–237

[3] N Shibata, Y Kohno, A Nakamura et al. Atomic resolution electron microscopy in a magnetic field free environment. 2019 Nat Commun 10, 2308

[4] F Börrnert, F Kern, F Harder, T Riedel, H Müller, B Büchner, A Lubk, The Dresden in-situ (S)TEM special with a continuous-flow liquid-helium cryostat,Ultramicroscopy. 2019, 203, 12-20,

[5] A.V Crewe, D.N Eggenberger, J Wall and L.M Welter An electron gun using a field emission source. 1968, Rev. Sci. Instr. 39:4.

[6] F Houdellier, A Masseboeuf, M Monthioux, M.J. Hÿtch. New carbon cone nanotip for use in a highly coherent cold field emission electron microscope. Carbon. 2012, 50 (5), 2037-2044

[7] S Mamishin, Y Kubo, R Cours, M Monthioux, F Houdellier. 200 keV cold field emission source using carbon cone nanotip: Application to scanning transmission electron microscopy. Ultramicroscopy. 2017, 182, 303 - 307

[8] F. Houdellier, G.M. Caruso, Sébastien J. Weber, M. Kociak, A. Arbouet. Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source. Ultramicroscopy. 2018, 186, 128 - 138.

[9] F. Houdellier, G.M. Caruso, S. Weber, M.J. Hÿtch, C. Gatel, A. Arbouet Optimization of off-axis electron holography performed with femtosecond electron pulses. Ultramicroscopy, 2019, 202, 26-32