Scanning Electron Microscopy: Principle and Application | Soil Mineralogy

In this article we will discuss about:- 1. Principle of Scanning Electron Microscopy 2. SEM Equipment 3. Working Mechanism 4. Specimen Preparation 5. Image Treatment and Analysis 6. Advantages and Applications.

Principle of Scanning Electron Microscopy:

SEM equipment is similar to a television. The principle of SEM is to use a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens, such as secondary electrons, backscattered electrons, and X-rays. These signals are collected by detectors to form images of the sample displayed on a cathode ray tube screen.

These signals are discussed as follows:

1. Secondary Electron Emission:

When the primary electron beam strikes the sample surface, it causes the ion­ization of specimen atoms and emission of loosely bound electrons that are known as secondary electrons. Secondary electrons emitted from a sample surface form three-dimensional images and are most valuable for showing morphology and topography of samples with magnification up to 100000× with a minimum resolu­tion of 10 nm. Secondary electrons that are prevented from reaching the detector will generate shadows or will be darker in contrast than those regions that have an unobstructed electron path to the detector.

2. Backscattered Electrons:

Some of the electrons undergo elastic collision with the specimen atomic nucleus and bounce back and backscattered toward their source with energy greater than 50 eV and these are called backscat­tered electrons (BSEs). Elements with higher atomic numbers have more positive charges on the nucleus, and as a result, more electrons are backscattered, providing atomic number contrast in the SEM images.

3. X-Rays:

Another class of signals produced by the interaction of the primary electron beam with the specimen is characteristic X-rays. The analysis of characteristic X-rays to provide chemical information is the most widely used microanalytical technique in the SEM.

The signals that derive from electron sample interactions also reveal information about the chemical composition and crystalline structure of materials making up the sample. The image produced by SEM is much like an aerial photograph – edges are bright and recesses are dark.

SEM Equipment:

Following are the important components of an SEM equipment:

1. Electron Gun:

The electron gun produces the electrons and accelerates them to an energy level of 0.1-30 keV. The first SEM systems used tungsten “hairpin” or lanthanum hexaboride (LaB₆) cathodes. The diameter of elec­tron beam produced by hairpin tungsten gun is too large to form a high-resolution image.

Modern SEMs use field emission electron guns (FEG), which provide enhanced current and lower energy dispersion. In field emission guns, the electrons are extracted from a very sharply pointed tungsten tip by an extremely high electric field. FEG sources offer brightness up to 1000 times greater than tungsten emitters, but they are much more expensive. Field emitters must operate under ultra-high vacuum (lower than 10^–9 torr or 1.33 × 10^–7 pascals) to stabilize the electron emission and to prevent contamination.

There are three types of FEGs used in the SEM systems – cold field emission (CFE) sources that operate at room temperature, thermal field emission (TFE) sources that operate in elevated temperature and Schottky emission (SE) sources. The performance of CFE and SE sources is superior to that of TFE sources in terms of brightness, source size, and lifetime. SE source is preferred to CFE source because of its higher stability and easier operation.

2. Condenser Lens:

Electromagnetic lenses and apertures are used to focus and define the electron beam and to form a small focused electron spot (of the order of 1 – 100 nm) on the specimen.

3. Object Lens:

The object lens controls the final focus of the electron beam by changing the magnetic field strength. The cross-over image is finally de-magnified to about 10 nm beam spot.

4. Vacuum System:

An ultra-high vacuum system is indispensable for SEMs in order to avoid the scattering on the electron beam and the contamination of the electron guns and other components. Typical vacuum systems con­sist of chamber that holds vacuum, pumps to produce vacuum, valves to control vacuum, and gauges to monitor vacuum. Generally, a mechanical pump and a diffusion pump are utilized to pump down the chamber from atmospheric pressure.

In general, a sufficiently good vacuum for a SEM is produced by either an oil diffusion pump or a turbo-molecular pump (the current standard for most SEMS), in each case backed by a mechanical pre-vacuum pump. The sample must be stable in a vacuum of the order of 0.001 – 0.01 m Torr (0.133 – 1.33 m Pa). Low-vacuum SEMs are to be used for samples likely to outgas at low pressures.

5. Signal Detection System:

Detectors for backscattered electrons and secondary electrons are usually either a scintillation detector or a solid-state detector. In the scintillator case, electrons strike a fluorescent screen that emits light that is amplified and converted into an electrical signal by a photomultiplier tube.

The solid-state detector works by amplifying the minute signal produced by the incoming electrons in a semiconductor device. A third type of detector monitors the net current absorbed by the specimen (beam current less second­ary and backscattered electron emission) or the current induced in a semiconductor junction by the incoming beam electron.

6. Display and Data Output System:

Most modern SEMs have migrated from photographic film to digital media for image recording and storage.

Working Mechanism of SEM Equipment:

The working mechanism of SEM equipment can be summarized as follows:

1. The electron gun produces a stream of monochromatic electrons and the condenser lens is used to form the beam and to limit the amount of current in the beam. It works in conjunction with the condenser aperture to eliminate the high-angle electrons from the beam.

2. The second condenser lens forms the electrons into a thin, coherent beam. A user selectable object aperture fur­ther eliminates high-angle electrons from the beam.

3. The electron beam is scanned in a rectangular raster over the specimen and a set of coils are used to scan the beam in a grid fashion, dwelling on points for a period of time, usually for a few microseconds, as determined by the scan speed. The object lens focuses the scanning beam onto the part of the specimen desired.

4. When the beam strikes the sample (and dwells for a few microseconds), interactions occur inside the sample and are detected with various instruments.

5. Before the beam moves to its next dwell point, these instruments count the number of e-interactions and display a pixel on a cathode ray tube (CRT). The intensity of the pixel is determined by the number of electron-sample interactions (the more the reactions the brighter the pixel). The intensities of various signals created by interac­tions between the beam electrons and the specimen are measured and stored in computer memory.

Specimen Preparation for SEM:

Samples for conventional SEM generally have to be clean, dry, vacuum-compatible, and, ideally,, electrically con­ductive. The vacuum system in SEM necessitates rendering the sample into the solid state, devoid of fluids, which otherwise degas in high vacuum and contaminate the microscope. If the specimen contains any volatile compo­nents such as water, they must be removed by a drying process (or in some circumstances it can be frozen solid) before they can be used in a high-vacuum system.

Non-conducting specimens will accumulate charge under electron bombardment and may need to be coated with a conducting material, such as iridium, carbon, gold, or other metal or alloy. Iridium gives a fine-grained coat­ing and is easily applied in a sputter coater. It gives a good yield of secondary electrons, and consequently, a good quality image of the surface. Carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high-resolution electron imaging applications. An electrically insulating sample can be examined without a conductive coating in an instrument capable of “low vacuum” operation.

There is no restriction on specimen size other than that set by the size of the specimen chamber. The various SEM models in a range differ in the size of their specimen chambers, allowing various sizes of specimens to be introduced and manipulated.

The simplest models accept specimens of a few centimeters in diameter and can move them 50 mm in X and Y directions. Larger models can accommodate samples up to 300 mm in diameter. Most models also allow samples to be tilted to high angles and rotated through 360°.

Image Treatment and Analysis in SEM:

Because the image in a SEM is completely electronically produced, it can be subjected to sophisticated analysis and manipulation using modern digital techniques. This includes contrast enhancement, inversion (black becomes white, etc.), filtering, mixing of images from various detectors, subtraction of the image from one detector from that produced by a different detector, color coding, and image analysis. The application of these techniques must be guided by the primary goal of extracting the best possible information from the specimen.

The emission of secondary electrons depends on the electron density of the specimen atoms. Hence, the number of secondary electrons increases as the atomic number of the specimen increases. The production of BSEs also increases with the atomic number of the specimen. Therefore, the contrast of secondary electron signal and BSE signal can give information about the specimen composition. However, BSE signal produces better contrast concerning composition variation of the specimen.

Advantages and Applications of Scanning Electron Microscopy:

The SEM has a magnification range of 20× to 1,50,000× that enables a spatial resolution of 50 – 100 nm. Clay particles and fracture surfaces through soil masses may be viewed directly. Minimal sample preparation is required. Data acquisition is rapid, each image can be generated in less than 5 min.

The combination of higher magnification, larger depth of field, greater resolution, and compositional and crystallographic information makes the SEM one of the most widely used instruments in academic/national lab research areas and industry for the study of clays and several other materials. Modern scanning electron microscopes generate data in digital formats, which are portable. Typical applications include the study of the topography and morphology as well as mineralogical identification and composition.