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


Event details

Date 27.01.2022 14:0015:00  
Speaker Dr. Florent Houdellier - CEMES CNRS
Location Online
Category Conferences - Seminars

The electron microscope (Transmission EM or Scanning EM), like other machines, is made up of several critical components such as the electron source, lenses, and detectors. Each of these elements has a significant impact on the number of applications accessible and the quality of the data captured. For example, the precise design of the objective lens magnetic circuit will substantially influence the instrument's resolution as well as the useable area available around the sample to interact with it, whether in traditional TEM or Scanning EM (STEM or SEM). The influence of geometrical aberrations of the lens will "naturally" rise when this essential space is increased, resulting in a worsening of the instrument resolution. For many years, this well-known intrinsic feature of magnetic electron lenses severely hampered TEM capabilities, but it was solved in the early twenty-first century with the advent of multipole lenses-based aberration correctors. [1, 2]. 
The introduction of the Cs corrector had a favorable side effect of reviving instrumental innovations and stimulating a new period of inventiveness in the electron optics field. [3, 4].
If the objective lens is the instrument's "heart," the electron source is its "brain," isolated in a highly sensitive environment (high voltage and vacuum) and having a significant impact on all of the machine's outcomes. The electron source based on electron cold field emission from a metallic nanotip is the brightest of a large family of electron sources [5]. 
When high beam coherence is required in TEM or high probe current with minimal energy spread is required in STEM and electron spectroscopic techniques, the brightness is an appropriate figure of merit. The brightness of the simplest CFE source assembly, i.e. a fine tip and a large round anode, is optimal due to its short virtual source size in the nm range. In an electron microscope, however, this virtual source size must be imaged properly and with enough flexibility to generate either a coherent collimated beam on the sample in TEM or a narrow-focused probe in STEM/SEM.
Electrostatic gun lenses are usually employed before the accelerating tube, followed by magnetic condenser/objective lenses. However, geometrical aberrations of these elements will gradually deteriorate the optimum beam brightness, provided by the first virtual cross, over in each consecutive image plane. In this well-known mechanism shown by a progressive distortion of the emittance figure in each related conjugate plane, the electrostatic optic design, composed of the extraction anode and the gun lens assembly above the accelerator, is critical. Many source design options have been offered to improve the tip area, extractor, or gun lens assembly to provide an optimization of the final emittance.
We started addressing these concerns a few years ago alongside Aurélien Masseboeuf and Marc Monthioux in CEMES, by replacing the tip material from the original monocrystalline W <310> oriented nanotip to a new form of carbon nanocone to boost the holography capabilities of our FE-TEM [6,7]. This first instrumental achievement allowed us to increase the brightness and beam current stability of an old 200keV CFE source. Together with Arnaud Arbouet, we recently succeeded in generating brilliant and ultrashort electron pulses by mixing a traditional W nanotip cathode with a femtosecond laser, allowing us to implement coherent techniques in an ultrafast TEM, such as electron holography. [8, 9]. 
The strength of these two original developments, as usually in CFE technology, is heavily influenced by the qualities of the initial pre-accelerator electrostatic optic. 
During my presentation, I will quickly summarize these developments and discuss our ongoing research efforts to improve the emittance characteristic of a CFE source and overcome the limitations of our previous designs.

[1] M Haider, S Uhlemann, E Schwan and B Kabius, Development of a spherical corrected 200 kV TEM. Proc. Dreiländertagung, Regensburg, Germany. 1997, Optik 106, 7
[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

About the speaker:

2006 : Thesis at CEMES and Favard Prize of the French Society of Microscopies
2007 : Joining the CNRS, head of the TEM department at CEMES
2009 : Recovery of the first HF2000: towards instrumentation
2018-2022 : Director of the joint laboratory HC-IUMi between Hitachi High Technologies and CNRS
2019 : Ernst Ruska Prize of the German Society of Microscopies