I3EM
Interferometry, In situ and Instrumentation
for Electron Microscopy
The I3EM group, created at the beginning of 2016 during the reorganisation of the laboratory, stems from the desire to create a group focused on methodological and instrumental developments in transmission electron microscopy (TEM) and their use for advanced studies of the properties of nanosystems. The complementarity and diversity of the expertise of the members of I3EM allows original methodological developments in TEM to be carried out in order to solve material problems with optimal sensitivity and spatial or temporal resolution.
Key word: Interferometry, in situ, instrumentation, nanomaterials, electric field, magnetism, deformation field, spectroscopy, field emission gun, cathodoluminescence, ultrafast electron microscopy
Objective :
The control of physico-chemical phenomena on the nanometre scale and over short times is at the origin of the development of nanotechnologies. The small dimensions of nanodevices require analytical methods adapted to these scales, making TEM an essential tool that gives access to local structural, physical and chemical properties. However, the complete characterisation of a nanosystem also requires its study in operation and at an adapted time scale. The development of new methods of analysis by TEM, the optimisation of their sensitivity and access to temporal resolution are therefore major challenges for nanoscience.
The research of the I3EM group is to develop extreme measurements in TEM by carrying out instrumental and methodological developments to carry out complete operando studies on the properties of nanosystems (nanoparticles, nanowires, thin films, nanocomponents).
Research topics
1. Instrumental and Methodological Developments
The i3EM group is involved in a number of instrumental development projects such as the development of new cold field emission electron sources (CFEG) and microscope control.
Contact : Sophie Meuret, Hugo Lourenço Martins (Neo), Florent Houdellier, Arnaud Arbouet (Neo)
– Ultrafast TEM holographyIn collaboration with the NeO group, we have developed an ultrafast field emission TEM. This new microscope was designed to study the dynamics of materials. Its originality comes from the high brilliance of its source (Cold-FEG), which allows holograms to be produced with a pulsed beam containing one to a few electrons per pulse. The stability of the microscope makes it possible to acquire holograms despite the long exposure times associated with the low total intensity of the pulsed beam. Time-resolved cathodoluminescence (TR-CL) and energy-gain electron spectroscopy (PINEEM) studies are also possible.
Series of holograms and corresponding phase images obtained in ultrafast TEM.
Houdellier et al. Ultramicroscopy 186, 128 (2018)
PhD student involved: Giuseppe Caruso (PhD thesis defended in 2019)
Time-resolved cathodoluminescence:
Within the FemtoTEM project in collaboration with the Neo team, we have developed the first time-resolved cathodoluminescence experiment in an ultrafast TEM. We measured the spatial variation of the lifetime of atomic defects (nitrogen gap centre (NV°)) in nano-diamonds with a sub-nanosecond time resolution and a spatial resolution of 12 nm. We are currently extending this study to other materials and are open to new collaborations.
Lifetime map of a diamond cluster. a) TEM image of the diamond cluster. b) Intensity map obtained by summing all channels of the histogram, 5 nm resolution. c) Lifetime value map extracted from the fit for each pixel (12 nm resolution). d) Curve and fit of the two coloured pixels on c). Acquisition time 20 seconds per pixel.
S. Meuret et al, App. Phys. Lett. 119 062106 (2021)
Contact : sophie.meuret@cemes.fr
Contact : Christophe Gatel, Martin Hytch
The microscope automation project seeks to compensate for instabilities in real time. The methodological developments were based on new optical configurations offered by the I2TEM microscope in interferometry.
We developed a dynamic control of the I2TEM microscope to compensate for experimental instabilities and to optimise the acquisition of holograms over several tens of minutes in order to improve the signal/noise ratio. In addition, we have developed a simulation of this microscope to accurately model the electron trajectories and define new optical alignments in order to fully exploit the capabilities of the microscope.
Holograms and phase images obtained after different exposure times with and without the microscope.
Y. Kubo et al. Ultramicroscopy 175, 67 (2017), C. Gatel et al. Appl. Phys. Letters 113, 133102 (2018)
PhD students involved: Loic Grossetête, Julien Dupuy (Thesis defended in 2021)
Contact : Florent Houdellier
Electrostatic gun lenses are used in almost all CFEG (cold field emission gun) illumination systems. The most widely used and the simplest of all triode configurations remains the Butler configuration used in the Hitachi HF CFEG (the most widely used microscope for our instrumental developements). More elaborated electrostatic system has been proposed such as the pentode configuration described by Veneklasen et. al. [1]. Tetrode configurations were also studied and implemented during the high voltage STEM project (called the MEBAHT project) in the ”Laboratoire d’optique électronique du CNRS” in Toulouse, the former name of the CEMES laboratory [2]. Without getting into the details of a comparison between the advantages and disadvantages of different types of electrostatic CFEG, they all have the same drawbacks. On one hand they give comparable brightness and beam current and on the other hand the beam characteristics are limited by aperture Cs and chromatic Cc aberrations of the electrostatic lens (of the order of a few cm).
In 1980 Troyon et. al. developed a 100kV field emission gun using a pre-accelerator magnetic lens [3]. Besides it’s high brightness, which is specific to all types of FEG, it showed superior emission current density compare to electrostatic CFEG (about 7 times larger at maximum brightness). In particular it allowed a 25 times larger beam current compare to electrostatic CFEG due to its low aberration coefficients in the range of Cs = 2 mm and Cc = 1 mm for the aperture and the chromatic contributions respectively. They have even demonstrated that beam current values close to the one generated using thermionic sources can be obtained without sacrificing any brightness (see figure below. Moreover, they showed that pre-accelerated magnetic lens CFEG (MCFEG) allowed the source position to be practically constant in a large range of acceleration voltage, enabling very appreciable flexibility compare to electrostatic system.
Outline of a multistage acceleration CFEG equipped with a pre-accelerating magnetic lens showing the evolution of the source image size from the virtual source size in the emission area to the formation of the focused electron probe in the sample plane. mm, me and M are respectively the linear magnifications of the pre-accelerated magnetic lens, the electrostatic lens and the condenser lens. Csm, Ccm the spherical and chromatic aberrations coefficients of the pre-accelerated magnetic lens lens. Cse, Cce the spherical and chromatic aberrations coefficients of the electrostatic lens. Csc, Ccc the spherical and chromatic aberrations coefficients of the condenser lens (extracted from [4]).
The aim of the MOLENS (MOdular magnetic LENS) project is to develop this clever magnetic solution for the new optic of our ultrafast CFEG, originally developed within the Hitachi HF2000 CFEG, to ultimately improve the ultrafast probe current for a given brightness.
However, the actual Hitachi CFEG design is impracticable in our situation due to the presence of the laser focusing optic preventing the use of standard magnetic lens mechanical configuration. In collaboration with Pierre Abeilhou in CEMES (mechanical workshop) and Jérôme Béard in LNCMI (http://lncmi.cnrs.fr), we have then adapted the mechanical configuration of the magnetic circuit allowing to insert the ultrafast gun assembly while preserving the optical working modes of the magnetic gun lens.
Implementing this Ultrafast MCFEG configuration as a future source of coherent UTEM will be one of the medium-term activities of the overall instrumental research project we would like to manage, along with the development of new computational tools (field computation, paraxial trajectories and aberrations evaluations).
While the goal of MOLENS developments is to design an original aberration-optimized source optics, in the near future we would like to deal with aberration-corrected source optics. These more ambitious concepts will be studied in a longer-term basis in our group.
[1] Veneklasen,L.H.andSiegel,B.M.J.Appl.Phys.43,4989(1972)
[2] Hawkes, P.W. Cold Field Emission and the Scanning Transmission Electron Microscope. Advances in imaging and electron physics. 159 (2009).
[3]Troyon,M.Highcurrentefficiencyfieldemissiongunsystemincorporatingapreacceleratormagneticlens.Itsuse in CTEM. Optik 57, 401 (1980).
[4] Houdellier, F. HDR manuscript : https://hal.archives-ouvertes.fr/tel-03695440v2
Contact : Etienne Snoeck
Development of off-axis electron holography for biology. This project in collaboration with the CBI laboratory. The objective of the project is to successfully image various biological samples, ranging from cells at the micrometer scale to proteins at the nanometer scale. The challenges are numerous, such as the imaging conditions (thick sample) and the fragility of biological samples.
PhD student involved: Elio Karim
2. Nanomagnetism/in-operando
Another part of our research concerns the development of in-situ experiments where the major effort has been focused ion beam (FIB) sample preparation and clean room nanofabrication. In parallel, we have implemented simulation and modelling methods to support the studies, for example: finite element method [3], electron optics [7,11], micromagnetism [8,16] and dynamic diffraction [20].
The magnetic transitions of thin films at temperature have been studied using electron holography, allowing the identification of domain nucleation mechanisms [5,6] (see highlights). Beyond these model systems, the analysis of components in operation (operando) is an integral part of the group’s objectives and has been illustrated with the quantification of the magnetic field radiated by an operating read head [15]. Magnetic patterns in nanowires and nanocrystals have been studied by electron holography on a scale never before achieved [8,10] and also in thin films by electron energy loss spectroscopy (EELS) [12,13,19].
In situ electron holography with I2TEM and studies of components in operation (operando)
In situ electron holography has been used to map the magnetic induction of different nanosystems at the nanometre scale. For example, we studied the temperature-dependent magnetic transition of FeRh and MnAs thin films. The results obtained highlight the influences of interfaces and structural properties on the mechanisms of the magnetic transition. We also quantified, in collaboration with Seagate, the magnetic flux emitted by a hard disk write head during its operation (operando). This first experiment on a component from production lines is being continued with the operando study by TEM of microelectronic components in collaboration with STMicroelectronics within the framework of the ANR IODA that we are coordinating.
Phase images showing the evolution of the magnetic induction as a function of temperature in MnAs thin films.
C. Gatel et al. Nano Letters 17, 2460 (2017), C. Gatel et al. Nature Communications 8, 15703 (2017), J.F. Einsle et al. Nano Research 8, 1241 (2015), I.M. Andersen et al. ACS Nano 14, 2 (2020)
PhD students involved: Mia Andersen (thesis defended in 2020), Anais Fondet
3. NanoElectronics / In-operando
In the continuity of our research on nano-magnetism by electronic holography, we are also working on nano-electric phenomena. For example, in collaboration with ST microelectronics, we are interested in the behaviour of electronic components such as capacitors under electrical excitation. A project is also being developed to observe in-situ under optical excitation optoelectronic components such as light-emitting diodes.
Contact : Christophe Gatel, Martin Hÿtch
Electron holography allows the characterization of electrical properties of microelectronic devices, such as capacitors, at the nanoscale. The so-called in-situ studies would allow to study such systems in operation at the nanoscale. In order to carry out these studies two steps are essential and indispensable: sample preparation and modelling of the results. Our team has developed an expertise in these two areas, which allows us to control all the steps of the experimental study.
C. Gatel et al, Phys. Rev. Lett 129, 137701 (2022) ; M. Brodovoi et al. App Phys. Lett 120 233501 (2022)
Post-doctoral fellow: Leifeing Zang
PhD student: Kilian Bruel, Maria Brodovoi
4. Materials Studies
The group’s papers reflect the use and development of microscopy for the quantification of fields (stress, chemical, electrical, magnetic) in nanoobjects and thin films. Several parameters have been considered in data acquisition and analysis, such as the two-dimensional projection of a three-dimensional configuration, the effect of thinning and amorphisation of materials related to thin film fabrication processes or low signal-to-noise ratio. Numerous studies have been carried out in electron spectroscopy through experiments performed at UMR Castaing or in collaboration with the Laboratoria de Microscopia Avanzada of the Instituto de Nanociencia de Aragon (LMA-INA) in Zaragoza (Spain).
Contact : bénédicte Warot
Solar cells
A recent thesis work focused on Transmission Electron Microscopy (TEM) characterisation protocols of materials that constitute solar cells. Two separate studies were carried out, one on the characterisation of a stack of single crystal silicon (c-Si) passivation layers consisting of amorphous aluminium oxide and amorphous hydrogenated silicon nitride (a-AlOx/a-SiNx:H) and another on the characterisation of an epitaxial layer of boron-doped and highly hydrogenated silicon. These materials are part of a joint project for a tandem solar cell coupling silicon and perovskite technologies.
Low-temperature (< 200°C) silicon epitaxy by RF-PECVD is an alternative to standard boron diffusion or ion implantation, and in addition to a lower thermal budget, it ensures a sharper doping profile. The complexity of this process lies in the fact that many impurities are incorporated during growth. Particular attention is paid to the defects by combining several microscopy techniques (HRTEM, STEM-HAADF) and by using an image processing routine (GPA) that allows the study of the deformation field. For the epitaxial layer before annealing, we have shown how the unconventional PECVD growth process influences its microstructure and gives it a non-uniform deformation field. In the annealed epitaxial layer, no deformation is measured, but nanomacles have been detected and analysed through a geometrical model.
The stacking of a-AlOx/a-SiNx:H passivation layers on c-Si aims to preserve the electronic properties of c-Si. In this stacking, a-AlOx has the distinctive feature of giving both chemical and field effect passivation, which requires further research to be more controlled. A-AlOx is also known to be unstable under electron beam, so a detailed study on damage caused by electron irradiation was carried out. Radiolysis damage was found to be dominant. Thus, several STEM-EELS acquisition parameters such as accelerating voltage, electron dose and scan orientation were taken into account and modified to limit radiolysis. Once the irradiation was under control, a STEM-EELS investigation was conducted using Si and Al L2,3 and O K thresholds on the fine structures. The interface was found to be composed of a-SiOx and non-stoichiometric aluminosilicate with a predominance of Al in tetrahedral coordination in its first layers.
EELS spectra recorded at the Si/AlOx interface showing the existence of a metallic aluminium layer at the interface
Contact : Bénédicte Warot
The correlation between mechanical properties and their evolution and local chemical compositions have been little studied so far.
Composition measurements can be obtained by various techniques, such as SIMS or Castaing probe, but the spatial resolution does not allow differentiating the chemistry of the matrix from that of nanometric precipitates. Tomographic atomic probing and electron microscopy allow spatial resolution to be coupled with chemical quantification. However, each alloy requires the optimisation of the spectra acquisition conditions according to the chemical elements to be studied.
A concrete example of the role of local chemistry concerns the AD730 polycrystalline superalloy developed by the Aubert & Duval group (2009 patent), which is being considered for the manufacture of turbine discs for future turbojet engines for civil applications. This alloy, whose chemical composition and thermomechanical treatments were designed on the basis of thermodynamic calculations and knowledge acquired on similar alloys (Udimet 720Li, René 88DT), has very promising mechanical characteristics at an optimal cost. The recent development of this alloy means that there are few fundamental studies on the origin of the increase in mechanical properties in relation to solutes. A thorough characterisation of the microstructure in relation to the chemistry, of its stability and of the mechanical properties at the end of the thermomechanical ranges and during ageing representative of the cycles encountered during the future missions envisaged, is therefore essential.
The STEM-EELS and STEM-EDX studies that I have developed on this nickel-based alloy AD730 have made it possible to map the chemical compositions of the matrix and the precipitates according to the annealing conditions envisaged.
Pettinari-Sturmel, F.; Vultos, W.; Hantcherli, M.; Warot-Fonrose, B.; Marcelot, C.; Douin, J.; Cormier, J.; Villechaise, P.; Devaux, A., Creep Behavior in the New AD730TM Nickel-Based Disk Superalloy – Influence of Aging Heat Treatment and Local Chemical Fluctuations. Materials Science and Engineering: A 2019, 754, 9–17.
Members
Projects
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