INTRODUCTION OF SEM
The scanning electron microscope is a powerful microscopic tool that utilizes electrons to form a magnified image of specimen under study. It is a powerful magnification tool that produces high-resolution, three-dimensional images which provide information on the topography, morphology and composition of the sample/specimen. The SEM was developed by Dr. Charles Oatlev.
The Scanning Electron Microscope, which is employed in various fields like medical, biological, metals, semiconductors and ceramics, is broadening its application frontier. SEM is considered as one of the most powerful tools getting used at R&D institutes and quality control inspection sites all over the world. The primary step of a scientific evaluation is just too thoroughly and in depth thorough analysis to observe the form of the material. For this purpose, we have a hand magnifying glass or an optical microscope. But, as long as light is used, we can’t see anything smaller than the wavelength of light and thus observing a structure right down to several nm is extremely difficult.
The Scanning Microscope (hereinafter “SEM”) enables a transparent observation of very small surface structures, which isnt possible with an optical microscope. The complex interaction between the accelerated electrons and therefore the specimen leads to various physical products like elastically scattered electrons, secondary electrons, X-rays, etc. SEM utilizes electrons to point out an enlarged image of a specimen. The Scanning Electron Microscope (SEM) introduced here utilizes a beam whose wavelength is shorter than that of light and thus observing a structure down to several nm in scale becomes possible.
Principles of SEM
The principle on a SEM is that the same as that of the basic principles as light microscopes, but a focused beam of high-energy electrons is employed rather than photons.
The electrons carry significant kinetic energy. When the incident electrons strike the sample surface, the energy from these electrons is dissipated as a spread of signals. The signals are generated as results of interactions that happen between electron and therefore the sample. These signals include secondary electrons, backscattered electrons (BSE), and diffracted backscattered electrons (EBSD), X-Rays, light (cathodoluminescence) and heat.
The production of SEM images utilize the secondary and back scattered, whereas the crystalline structures, orientation of minerals and micro-fabrics are determined by the diffracted backscattered electrons . The X-Rays are used for elemental analysis of the sample.
INSTRUMENTATION OF SCANNING ELECTRON MICROSCOPE
- Electron Source: There are 3 common sorts of electrons sources:
- Tungsten (W) electron filament –This is basic type of electron source. It produces electrons when heated resistively.
- Solid state crystal (Lanthanum hexaboride (LaB6) or Cerium hexaboride (CeB6)) – This source may be a thermionic emission. It is foremost common high-brightness source and offers about 5-10 times the brightness as that of the tungsten filament. Also, the lifetime of the electron source is far longer lifetime than its tungsten filament counterpart.
- Field emission gun (FEG)- because the name suggests, this source uses field electron emission for production of the electron beam. The tiny tip radius of the FEG provides for improved emission and focusing ability.
The electrons generated by these sources are accelerated to a voltage range of 1-40 kV and then further focussed into a narrow beam will be used for the aim of image formation and analysis of an equivalent.
A series of condenser lenses are present which are used to focus the beam because it passes through the microscopic column. The narrowness of the beam determines what’s going to be the dimensions of the spot when it’ll be contacting the surface. These lenses are tubes, wrapped in coil and mentioned as solenoids.
3. Scanning Coil
The purpose of the scanning coils is to deflect the focused electron beam within the X and Y axes in order that it scans during a raster fashion over the sample surface.
4. Sample Chamber
Sample chamber consists of a vaccum chamber where the samples are mounted and placed. It can also include additional devices to help in sample imaging, like translation stage, tilt and rotation devices, temperature stages, optical cameras etc.
5. SEM Detectors
The electrons from the sample are collected by the detectors. There are various sorts of detectors utilized in SEM.
Type of detector
Secondary electron detector
- Low energy electrons ejected from the orbitals of the sample atoms
- Electrons are accelerated towards a scintillator, which successively produces a current. The present is directed towards a photomultiplier and therefore the amplified signal is read on the monitor
- Everhart-Thornley detector is an example
Backscattered electron detector
- High energy electrons are produced as results of elastic scattering interactions with specific atoms within the sample
- The electrons are reflected back by sample atoms.
- These detectors are often scintillators or semiconductors.
Diffracted backscatter electrons
- Determine crystallographic structures of sample
- Determine orientation of minerals and micro-fabrics.
- Provide information on element and mineral
Working of a SEM
- Electron guns placed at the top of the column generate high energy electrons. These are accelerated down and allowed to pass through a combination of electromagnetic lenses. The lenses help to produce a focused electron beam.
2. The beam moves across a vertical path through the microscope, within the presence of vacuum.
3. The sample chamber area is additionally evacuated by a mixture of pumps. The sample is placed inside this vacuum chamber.
4. The scanning coils are adjusted to allow the electron beam to be focused on the sample surface. Beam scattering enables information for the sample to be collected on a defined area on which the beam has been focused.
5. The operator can adjust the beam through a computer to control magnification and surface area to be scanned.
6. Interaction between the incident electrons and the sample surface leads to the release of a number of energetic electrons from the sample surface.
7. The interaction leads to specific scattering of electrons (e.g., backscattered electrons, secondary electrons etc.) which can provide information on size, shape, texture and composition of the sample.
8. The electrons are collected by detectors and converted into a signal.
9. The signals are sent to a detector to produce a final black and white 3-dimensional image.
Advantages of SEM
- Three-dimensional imaging and topographical, morphological and compositional information obtained of the sample
- User-friendly, fast and easy to operate
- Recent technology advancement allows digital data generation
- Detection, identification and analysis of surface fractures, microstructures, surface contaminations, crystalline structures etc.
- It provides higher resolution as well as larger area to be focused at one time as compared to traditional microscopes.
- Increased control on the degree of high magnification provided by SEM.
Disadvantages of SEM ־
- Expensive, large in size, requires proper housing and maintenance
- Special trained operators are required to be for operation, data analysis and interpretation
- Artefacts generated during sample preparation can be addressed only with help of experienced researchers.
- Limited to solid, inorganic samples
- Size of the samples have to be small enough to fit inside the vacuum chamber
- Samples must be stable in vacuum chamber and bear the vacuum pressure
- There is a mild risk of exposure to radiation, with electrons that have scattered from beneath the sample surface.
- Non-conducting sample to be observed under conventional SEM need to be specifically coated with electrically conducting material.
- It doesn’t provide information on living samples. It is only applicable for fixated samples, which are not living.
Applications of SEM
- Essential research tool in life science, biology, medical and forensic science, metallurgy and many more
- Wider field of applications, including industrial and technological applications
- Deciphering spatial variations in chemical compositions ־ Measurement of very small features and objects down to 50 nm in size
- Back scattered electron images (BSE) are often used for rapid discrimination of phases in multiphase samples.
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