Transmission Electron Microscopy ("TEM") allows morphological, structural and chemical analysis of solid samples at the atomic scale. 

A lot of informations can be obtained by TEM such as the thickness of layers in complex stacks, the morphology of materials in section, their structure (amorphous or organized), the size of grains for polycrystalline samples, their crystalline orientation, the nature of crystalline defects...

Electron microscopy is similar in principle to optical microscopy. However, the wavelength associated with the electron beam being much smaller than that of a light beam, the lateral resolution in electron microscopy is significantly improved. There are, nevertheless, constraints related to the use of electrons: a high vacuum in the microscope column is essential, as well as the use of ultra-thin samples (thickness lower than 100 nm) in order to be as transparent as possible to electrons. BIOPHY RESEARCH has different preparation techniques, including ultramicrotomy and FIB (Focused Ion Beam).

Scanning Transmission Electron Microscopy (STEM) is based on the same principle as TEM, except that the electron beam is reduced to a sub-nanometer fine brush and scans the sample with deflection coils.

In this mode, the detection of electrons is done at high angles with a HAADF (High Angle Annular Dark Field) detector, providing an image whose contrast is a function of the atomic number (region of high Z in clear on the STEM-HAADF image and vice versa).

 

The electrons are produced by an electron gun located at the top of the column, then accelerated to a stabilized voltage (typically between 80 kV and 300 kV) in order to increase their speed considerably. The emitted electrons then pass through a set of electromagnetic lenses associated with diaphragms, called the condenser system, allowing to modify the illumination mode of the sample (parallel or convergent beam). The role of the objective lens is to focus the electrons on the sample, its characteristics playing a determining role on the resolution offered by the instrument.

The sample is then the seat of various physical phenomena resulting from the interaction of electrons with matter: the TEM and STEM are interested in those transmitted and scattered. 

A diaphragm placed at the exit of the sample, called objective or contrast diaphragm, allows to select the transmitted beam (bright field) or a diffracted beam in a particular direction (dark field). A set of electromagnetic lenses, constituting the projection system, then transfers the image of the sample (or the diffraction pattern) to the observation screen, which emits light in the yellow-green range under the impact of electrons. The digital image acquisition is performed by a CCD camera placed under the observation screen.

By interacting with the sample, some of the electrons lose energy. This loss of energy can be exploited to perform chemical analyses, by energy loss spectrometry (EELS) for example. As for the X-rays resulting from the de-excitation of the atoms of the sample, they can be analyzed by X-ray spectrometry via an EDX detector.

TEM or STEM coupled with a chemical analysis allows for example to access the nature of the layers or the diffusion of an element in a layer and at the interfaces.

MORPHOLOGICAL ANALYSIS

Visualization of the different regions of a sample according to the local electron density  

Based on the scattering contrast, the intensity of the scattered electrons depends on the local electron density of the sample. A dark area on the bright field image (BF) corresponds to a region of the sample that scatters electrons more strongly (high Z).

STRUCTURAL ANALYSIS

Characterization of the crystallinity of a material, respect of Bragg's law

Based on the diffraction contrast, the intensity of the diffracted electrons is related to the orientation of the atomic planes and to the crystal defects (dislocations, inclusions...).

ELEMENTAL COMPOSITION EDX/EDS

Chemical analysis at the nanoscale, imaging or profiling (STEM mode)

• EDX (Energy Dispersive X-ray spectroscopy)
• EELS (Electron Energy Loss Spectroscopy)

TEM STEM EDX EELS technical specifications

• Source: FEG Schottky hot cathode (80 kV, 200 kV, 300 kV)
• Resolution: <0.1 nm thanks to the presence of a geometric aberration corrector
• Lateral resolution : TEM mode : 0.1 nm (300 kV), STEM mode : 0.15 nm 
• Voltage : from 80 to 300 kV
• Magnification : from x45 to x1 250 000
• Liquid nitrogen cooled Si(Li) diode EDX detector (EDAX) with drift correction
• STEM High Annular Dark Field Detector
• EELS module (GIF Tridiem integrating a 2K x 2K camera)

TEM STEM EDX EELS strenghts

• Obtaining morphological, structural and chemical information at the nanoscale
• Chemical imaging with nanometric resolution in STEM mode with ring darkfield detector (HAADF)
• Point chemical analysis in STEM mode for the determination of elemental composition at the nanoscale: 
  • by X-ray detection (EDX)
  • by electron energy loss spectroscopy (EELS)