Scanning Electron Microscopy (SEM) is a technique capable of producing high resolution images of the surface of a sample. SEM is used in many fields; from biology to materials science to microelectronics... and on all types of samples, even insulators can be observed after plating. The analysis takes place in a controlled inert atmosphere or under low voltage (close to kV).

SEM is generally used to study the 3D morphology with nanometric resolution of a surface or object. Chemical and elemental composition can also be obtained by X-ray microanalysis.

Scanning electron microscopy is similar in principle to optical microscopy. However, as the wavelength associated with the electron beam is much shorter than that of a light beam, the lateral resolution in electron microscopy is significantly improved. There are, nevertheless, constraints linked to the use of electrons: the presence of a high vacuum in the microscope column is essential.

The principle of this technique is therefore based on the use of an incident electron beam of a few tens of kilovolts scanning the surface of the sample, which then re-emits a whole spectrum of particles and radiation: secondary electrons, backscattered electrons, Auger electrons and X-rays.

The electron beam is produced in an "electron gun" and then directed through a set of electromagnetic lenses and scanning coils forming the SEM column. 

The detection of the different particles or radiation emitted provides information on the sample: its morphology, topography, crystalline structure, elemental chemical composition (qualitative and semi-quantitative analysis)...

Secondary electrons
During the collision between the primary electrons of the beam and the atoms of the sample, a primary electron can give up part of its energy to an electron in the surface layers (< a few nm) of the sample, causing its ejection. The latter, called secondary electrons, are representative of the topography of the sample.

Backscattered electrons
Backscattered electrons result from the quasi-elastic scattering of electrons from the primary beam with the nuclei of atoms on the surface of the sample. They therefore have an energy close to the energy of the primary beam and an escape depth of the order of 10 to 200 nm. The detection of backscattered electrons provides an image whose contrast is related to the chemical composition of the sample. 

Auger electrons
Auger electrons are electrons from a deep layer of the target atom that are ejected during the collision with the primary beam. Their energy is characteristic of the atom that emitted them and thus provides information on the chemical composition of the sample surface.

X-ray microanalysis
The emission of X-rays, whose wavelength is characteristic of the target atoms, provides information on the chemical nature of the atom.

 

TOPOGRAPHIC IMAGING

Topographic visualisation of the sample
By detecting secondary electrons.

CHEMICAL IMAGING

Acquisition of images with visualization of the chemical composition
It provides images whose contrast is a function of the atomic number by detecting backscattered electrons (compositional contrast).

MICROANALYSIS X

POINT

Elemental analysis (typically from carbon) of a point in the sample
X-rays from a volume of the order of µm3 (depending on the acceleration voltage and the nature of the sample) are detected at a point in the sample. Elements below about 0.2 mass % will not be detected.

PROFILE

Elemental analysis along a line drawn on the sample
An elemental analysis is performed pixel by pixel along a selected line.

CARTOGRAPHY

Imaging the distribution of one or more elements on the surface
Still using X-rays, an elemental analysis is carried out pixel by pixel on a given surface of the sample.

MICROSTRUCTURAL ANALYSIS BY EBSD

Information in particular on the orientation, size and shape of grains
Detection of electrons diffracted by the atomic planes of the sample.

HEATING AND/OR TRACTION MODE

Use of a Peltier and traction heating plate
This module allows the study of the surface morphology of a sample subjected to different temperatures or to a ramp and/or increasing stretch.

SEM technical specifications 

• Source: Tungsten or Schottky FEG
• Detected signals: secondary and backscattered electrons, X-rays 
• Elements detected: from Boron
• Detection limits: 0.1 to 1 atomic percent
• Lateral resolution: nano
• Imaging/mapping: Yes
• Multiple SEMs and detectors:
  • Various secondary and backscattered electron detectors
  • STEM detectors [BF (analysis of electrons not scattered or scattered at very small angles => visibility on mass density variation), DF (analysis of electrons scattered at small angles => diffraction and scattering effect by light elements visible), HADF (electrons inelastically scattered at large angles => incoherent scattering does not depend on the crystal structure of the material. The image contrast is proportional to the thickness of the sample and the atomic number)
  • EDX detector
  • EBSD detector
• VP mode (variable pressure)
• Heated stage (up to 1000°C) with or without traction
• Cryogenic stage (cooled by Peltier effect)
• Large chamber for analysis of large samples
• Xe or Ga FIB cross section analysis

SEM strenghts

• High resolution imaging from the very large to the very small (micron field of view)
• Rapid identification of elements present by EDX
• Excellent depth of field (~100x that of optical microscopy)
• Partial vacuum or variable pressure mode allows imaging of insulating and/or hydrated samples