A scanning electron microscope (SEM) makes use of a focused and high energy electron beam to generate high definition magnified two-dimensional images of the sample. For this purpose, the electron beam is directed on a selected portion of the surface of the solid sample. The interaction between the electrons of the beam and the sample results in the production of various signals. These signals are recorded and processed further to produce images in a digital format. It can be used to reveal information about the internal structure of the sample, the external texture of the specimen, the chemical composition of substances, and the orientation and arrangement of the elements that make up the sample. The scanning electron microscope was first made by a German researcher and applied physicist and inventor Manfred von Ardenne in 1937. The magnification of a scanning electron microscope typically ranges from 20X to approximately 30,000X. The spatial resolution range of a scanning electron microscope is from 50 to 100 nm.
Working of the Scanning Electron Microscope
The working of a scanning electron microscope typically depends on the detection of the reflected electrons after they strike the surface of a specimen. The main element of a scanning electron microscope is an electron source. Generally, the heated tungsten filament is used as the electron source in most scanning electron microscopes. Here, the heat tends to supply more energy to the electrons, thereby directing them in a particular direction and producing a single focused electron beam. An anode or a positively charged electrode plate is present between the electron source and the condenser. The main purpose of the anode is to deflect the electrons away and align them in a thin and single straight line. This is because the electrons contain a negative charge on them, while the anode plate is positively charged. A scan coil and an objective lens are present below the condenser. The electron beam generated by the source passes through the condenser, scan coil, and objective lens. When the electrons contained by the electron beam hit the sample, they reflect and get scattered in all directions randomly. This is known as electron escape, which helps the user to establish a relationship between the number of scattered and retained electrons. The signal that is produced as a result of the electron-sample interaction and electron escape gets detected by the detector. The detector is further connected to a sensor. The sample generally consists of bumps and valleys. When the electrons hit the bump area of the sample, a greater number of electrons tend to escape, and when the electrons hit the valleys, comparatively few electrons manage to reflect and escape. This difference in the escape of the electrons helps to develop an image of the sample at the micro-level.
Applications of the Scanning Electron Microscope
A scanning electron microscope is used as an analysis tool in a number of fields including biology, pharmaceuticals, manufacturing industries, physics laboratories, and many more. Some of the prime uses of the scanning electron microscope are listed as follows:
1. Scanning electron microscope is widely used in energy-Dispersive X-ray Spectroscopy for spot chemical analysis.
2. It is prominently employed in biology laboratories to study the internal structures of microorganisms at the cellular level.
3. A scanning electron microscope has multiple applications in industries. For instance, it can be used to study the surface of solid objects and analyse the distribution of atoms in various elements.
4. Cosmetologists make use of a scanning electron microscope to analyse fine details of tiny cosmetic components.
5. Manufacturing industries employ scanning electron microscopes to look for the contaminants and impurities in manufactured items.
6. Quality control departments of various industries use scanning electron microscopes to determine the purity of a particular substance. For instance, pharmaceutical industries use them for good-bad testing of drugs, medicines, and other products.
7. Scanning electron microscope is also used in qualitative chemical analysis of elements by proving a clear and magnified image of the crystalline structures.
8. A scanning electron microscope is quite advantageous in nanotechnology and other related fields. It can provide precise measurements and detailed images of objects that are over 50nm in size.
9. It can be used to distinguish different phases of a multiphase sample.
10. Some of the scanning electron microscopes are equipped with diffracted backscattered electron detectors, which helps to examine and determine the microfabric and crystallographic orientation of substances.
11. A scanning electron microscope is typically used to produce high definition images of objects that can display spatial variation of chemical compounds.
12. A scanning electron microscope is generally preferred when it is required to perform the analysis of selective point locations on the sample.
13. It is generally used in the medicine domain to observe the bacterial interaction with the skin and body organs. This helps the doctors determine the nature of the bacterial disease and to find the cure.
Advantages of the Scanning Electron Microscope
A scanning electron microscope is quite advantageous over the other microscopes. Some of these advantages are listed below:
1. Scanning electron microscopes are user friendly and easy to use.
2. They can produce and generate results in digital format.
3. Scanning electron microscopes are able to provide quick results, i.e., data can be obtained within a few minutes.
4. A scanning electron microscope requires minimum sample preparation.
5. The resolution of scanning electron microscopes is significantly high.
Disadvantages of the Scanning Electron Microscope
There are certain limitations and disadvantages of a scanning electron microscope. Some of them are given below:
1. Scanning electron microscopes are comparatively expensive.
2. Some microscopes must fulfil certain special conditions before their use. For instance, the room must be free of vibrations and electromagnetic radiation.
3. Scanning electron microscopes have a bulky structure.
4. A consistent voltage level must be maintained for the proper operation of a scanning electron microscope. This may require additional electronic circuitry or voltage regulators to fix the voltage magnitude to a constant value.
5. A cooling system should be attached with such microscopes.
6. The sample should be small enough to fit within the chamber of the microscope. The horizontal dimensions of the sample should not exceed 10 cm, while the vertical dimension is much more constrained and must be less than 40 mm.
7. The sample to be examined with the help of a scanning electron microscope needs to be solid in nature. Wet samples are unsuitable and are required to be decrepitated first.
8. A scanning electron microscope can not be used for light materials such as hydrogen, helium, lithium, etc.
9. To study samples that are insulators with the help of a scanning electron microscope, an electrically conductive coating is required to be applied on their surface. However, it can be neglected if the device is able to work in low vacuum mode.
10. Scanning of living samples with the help of a scanning electron microscope is not possible.