Researching surface modification to develop high-performing materials is one of the main objectives in surface characterization today. Characterization, when used in materials science, refers to the broad and general process by which a material’s structure and properties are probed and measured. With the time there has been advancement in the characterization techniques and it is continuously evolving and getting better with time.
Microscopy is also a technique of surface characterization that uses photons, electrons, ions or physical cantilever probes to gather data about sample’s surface, sample’s composition and many more on a range of length scales. There are various types of microscopy techniques that can be used for different applications and those are as follows
- Scanning Electron Microscope (SEM)
- Transmission Electron Microscope (TEM)
- Optical Microscope
- Field Ion Microscope (FIM)
- Atomic Force Microscope (AFM)
- X-Ray diffraction Topography
- Scanning Probe Microscope
- Scanning Tunnelling Microscope (STM)
We will be focusing on details of Scanning Electron Microscope in this Blog like working principle of SEM, applications of SEM and more. In the coming blogs we will mainly focus on TEM, AFM as they are widely being used as a characterization technique.
Scanning Electron Microscopy
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. Finally, the information obtained in the detectors is transformed to give rise to a high definition image, with a resolution of 0.4 to 20nm. Since the wavelength of electrons is much smaller than that of light, the resolution of SEMs is superior to that of a light microscope.
Working of SEM
Main component of SEM Equipment is as follows
- Electron Gun
- Anode
- Condenser Lens
- Specimen Stage
- Different types of Detector
- Vacuum System
- Computer to Display the image and controlling panel
Electrons are produced at the top of the column, accelerated down and passed through a combination of lenses and apertures to produce a focused beam of electrons which hits the surface of the sample. The sample is mounted on a stage in the chamber area and, unless the microscope is designed to operate at low vacuums, both the column and the chamber are evacuated by a combination of pumps. The level of the vacuum will depend on the design of the microscope. The position of the electron beam on the sample is controlled by scan coils situated above the objective lens. These coils allow the beam to be scanned over the surface of the sample. This beam rastering or scanning, as the name of the microscope suggests, enables information about a defined area on the sample to be collected. As a result of the electron-sample interaction, a number of signals are produced. These signals are then detected by appropriate detectors.
When electron beam hits the sample, some of the electron is absorbed by the sample, some of the electrons are scattered by the sample and some of the electrons ejects electron from the sample after collision with the sample. Collision between the electron and the sample is elastic.
- Secondary Electrons
- Schematic of Scanning Electron Microscope
- Backscattered Electrons
- Characteristic X-Rays
- Continuous X-Rays
The scanning electron microscope (SEM) produces images by scanning the sample with a high-energy beam of electrons. As the electrons interact with the sample, they produce secondary electrons, backscattered electrons, and characteristic X-rays. These signals are collected by one or more detectors to form images which are then displayed on the computer screen. When the electron beam hits the surface of the sample, it penetrates the sample to a depth of a few microns, depending on the accelerating voltage and the density of the sample. Many signals, like secondary electrons and X-rays, are produced as a result of this interaction inside the sample.
Controlling of the path of electron
In a similar way to optical microscopes, lenses are used to control the path of the electrons. Because electrons cannot pass through glass, the lenses that are used are electromagnetic. They simply consist of coils of wires inside metal pole pieces. When current passes through the coils, a magnetic field is generated. As electrons are very sensitive to magnetic fields, their path inside the microscope column can be controlled by these electromagnetic lenses simply by adjusting the current that is applied to them.
Generally, two types of electromagnetic lenses are used: The condenser lens is the first lens that electrons meet as they travel towards the sample. This lens converge the beam before the electron beam cone opens again and is converged once more by the objective lens before hitting the sample. The condenser lens defines the size of the electron beam (which defines the resolution), while the main role of the objective lens is to focus the beam onto the sample.
The SEM’s lens system also contains scanning coils, which are used to raster the beam onto the sample. In many cases, apertures are combined with the lenses to control the size of the beam.
Back Scattered Electron and Secondary Electron
The interaction of electrons within a sample can generate many different types of electrons, photons, or irradiations. In the case of an SEM, the two types of electrons used for imaging are backscattered (BSE) and secondary electrons (SE). BSEs belong to the primary electron beam and are reflected back after elastic interactions between the beam and the sample. By contrast, secondary electrons originate from the atoms of the sample; they are a result of inelastic interactions between the electron beam and the sample. Because BSEs come from deeper regions of the sample whereas SEs originates from surface regions, the two carry different types of information. BSE images show high sensitivity to differences in atomic number; the higher the atomic number, the brighter the material appears in the image. SE imaging can provide more detailed surface information.
X-Rays
In many microscopes, detection of X-rays generated from the electron-matter interaction is also widely used to perform elemental analysis of the sample. Every material produces X-rays that have a specific energy; X-rays are the material’s fingerprint. By detecting the energies of X-rays that come out of a sample with an unknown composition, it’s possible to identify all the different elements that the sample contains.
Difference between Optical Microscope and SEM
| Quality | Optical Microscope | SEM |
| Magnification | Optical microscopes can have from 4x to about 1000x | SEM can range from 10x to more than 3,000,000x. |
| Field Depth | In the case of optical microscopes, they range from 0.19 microns to 15 microns. | In SEMs, this range is broader, ranging from 0.4 microns to 4 mm. |
| Resolution | Optical microscopes can reach a spatial resolution of about 0.2 microns. | SEMs can reach up to 0.4 nm with some models and lenses. |
Application of SEM
- Product Design Failure Analysis
- Characterization of surface texturing
- Analysis of surface detects and quality control
- Morphological and Structural Study
- Study of Contaminants