Dual beam scanning electron microscopy (FIB) (english version)

Technical Manager

Nicolas STEPHANT

Scientist Officer

Philippe MOREAU

The instruments and the activity described in this page take part in the GIS "CIMEN" in progress. It can be discovered on the website : https://www.gis-cimen.fr/

 

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Equipment

A ZEISS Crossbeam 550L dual-beam (ions and electrons) scanning electron microscope (FIB) installed in late 2019.

• • Site Lombarderie (UFR Sciences)

The instrument is installed on the Science campus in the centre de microcaractérisation (CMC) building dedicated to scanning and transmission electron microscopy and atomic force microscopy (AFM).

The sample can be observed under conventional scanning electron microscopy using the electron beam generating column or milled with an ion beam using the second column of the microscope.

It allows to machine a sample by abrasion to extract a thin section for transmission electron microscopy. An in-situ nanomanipulator is provided to remove the thin section and fix it on a TEM grid. Two organometallic gas injectors allow to weld the sample, to contact it or to etch it by metal deposition (platinum or carbon).

It is also possible to mill a sample layer by layer while acquiring images each time. These images are then processed by software to restore the 3D volume destroyed by the beam and quantify different parameters about the observed phases.

The apparatus is equipped with an Oxford EDS detector for the analysis of chemical elements and an EBSD detector to characterize the crystalline orientations. Both techniques are usable in combination with a 3D acquisition.

A cryofreezing system allows to work at very low temperature on samples which are sensitive under the beam (Quorum). It includes a cold transfer system between the microscope and a glove box under controlled atmosphere.

The microscope is also coupled to a Raman spectrometer (Renishaw).

An uncooled vacuum transfer system is provided between a glove box and the microscope.

The FIB is accessible to external users.

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Extraction of a TEM lamella

To work on a solid material in transmission electron microscopy, it must be thin enough so that the electron beam can pass through it and so that the information transmitted is not compromised by the thickness of the observed area. Before the advent of the FIB, this was a tricky job performed by mechanical or chemical methods or by exposure to an ion beam. These methods are very rough to achieve a correct thickness, and especially to target a very precise area in a block that is observed with the naked eye or by optical instruments. The precision of FIB milling now makes it possible to extract a TEM lamella of a hundred nanometers thick in a solid sample. The location from which it is extracted is located on the sample with electronic imaging, which allows the lamella to be removed from the exact location that we intend to analyze with precision.

In a first step, a thick slide is carved out of the material. It is then welded on a nanomanipulator by organometallic deposition under ion beam (platinum) then moved and welded on a copper grid compatible with the TEM sample holder. The final refinement is then performed with the ion beam.

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Observation of a sample section

In most abrasion operations, the surface of the sample is perpendicular to the ion beam and tilted at 54° to the electron beam. This disposition allows to observe with the electron column the face of an excavation made by the ion beam, as far as the observation is not obstructed by the opposite face. By choosing the shape of the hole to clear the perspective for the electron column (a cavity that can be compared to that of a ramp to access an underground parking), a cross-section in the depth of the sample can be observed. This is done exactly where it is needed and without any prior preparation of the sample, which should have been cut and polished before being introduced into the instrument if it had not been provided with an ion column.

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3D Reconstruction

The abrasion capacity of the ion beam is controlled precisely enough to remove a very thin layer of material (theoretically down to about ten nanometers). If this operation is repeated a lot of times on the same surface, a volume of the sample is explored whose depth is proportional to the number of layers removed. If an electronic image of the surface is taken each time a layer is removed, we obtain a collection of images which, stacked one on top of the other, reflect the internal morphology of the volume that has been milled by successive layers.

It remains to process this collection of images with a software dedicated to 3D reconstruction to obtain an in-volume visualization of the sample. Several image analysis type treatments are available on the software to extract numerical data (for example porosity or volume occupied by a phase) and obtain a graphical representation.

 

3D acquisition on a filtration cell by Hélène Roberge

in the context of the e-BRIDGE project (NExT junior talent program).

 

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Observation and cold milling

Some materials are either degraded by the beam or degraded by the vacuum in the column of the scanning microscopes. These are mainly samples containing water but also so-called "fragile" materials.

The dual beam microscope is equipped to observe these materials at cold temperature with a QUORUM system that allows to freeze a sample in liquid nitrogen before introducing it into a preparation chamber connected to the microscope by an airlock. A nitrogen gas circulation system cooled in a liquid nitrogen dewar allows to maintain the sample at -140° in this one under the protection of a trap cooled at -170° (to trap the residual contaminants). This chamber is provided with knives to fracture the sample then metalize it after a possible sublimation.

The sample is then introduced into the microscope for observation on a holder cooled in the same way as in the preparation chamber and similarly protected by a cold trap.

The freezing prior to all this can be done under vacuum in slashed nitrogen.

This device opens the way to the observation of samples that are not observable otherwise.

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The FIB was funded by the 2015-2020 Plan Etat-Region Contract (CPER).The cost of the microscope (1.5M€) was funded by :

ETAT 18%
FEDER (Europe) 50%
REGION PAYS DE LA LOIRE 14%
NANTES METROPOLE 14%
CNRS 4%