A scanning electron microscope (SEM) is a type of electron microscope that generates images by scanning the surface of a specimen with a concentrated beam of electrons. Electrons interact with atoms in the sample, generating signals that carry information about the sample’s surface topography and composition. The electron beam is scanned in a raster scan pattern, and the beam’s position is coupled with the detected signal’s intensity to create an image. Using a secondary electron detector, the most prevalent SEM mode detects secondary electrons generated by atoms stimulated by the electron beam (Everhart–Thornley detector). The amount of secondary electrons that can be detected, and consequently the signal intensity, is dependent on specimen topography, among other factors. Certain SEMs have resolutions greater than 1 nanometer.
Specimens are viewed in a normal SEM under high vacuum, in low vacuum or wet conditions in a variable pressure or environmental SEM, and at a wide range of cryogenic or higher temperatures with specialised instruments.
The scanning electron microscope (SEM) was developed in the 1930s and 1940s by a number of researchers, including Max Knoll, Ernst Ruska, and James Hillier.
Max Knoll was a German physicist who developed the concept of the electron microscope in the 1930s. He designed a prototype electron microscope that used a beam of electrons to produce images of the surface of a sample, but the device was not practical for use as a microscope.
Ernst Ruska was a German physicist who worked with Knoll and developed a more practical electron microscope in the 1930s. Ruska’s design used a series of electromagnetic lenses to focus the beam of electrons onto the sample, and it was the first electron microscope to produce images of a sample that were visible to the naked eye.
James Hillier was an American physicist who worked with Ruska and developed the first practical scanning electron microscope (SEM) in the 1940s. Hillier’s design used a focused beam of electrons to scan the surface of the sample, producing a high-resolution image of the surface. Hillier’s SEM was the first instrument to be called a “scanning electron microscope.”
Overall, the development of the SEM was the result of the work of several researchers who made important contributions to the field of electron microscopy.
Scanning Electron Microscope Definition
A scanning electron microscope (SEM) is a type of microscope that uses a focused beam of electrons to produce high-resolution images of the surface of a sample. SEMs are used in a variety of fields, including materials science, biology, and geology, to examine the surface structure and composition of a wide range of materials.
In an SEM, a beam of electrons is generated by a high-voltage electron gun and focused onto the surface of the sample using electromagnetic lenses. The electrons interact with the atoms in the sample, causing the emission of secondary electrons and other types of electromagnetic radiation, such as X-rays. These emitted particles are detected by detectors, and the data is used to create a high-resolution image of the surface of the sample.
SEMs have a number of advantages over other types of microscopes, including the ability to produce high-resolution images at a wide range of magnifications, the ability to image samples in three dimensions using techniques such as backscattered electron imaging, and the ability to analyze the elemental composition of the sample using techniques such as energy dispersive X-ray spectroscopy (EDS).
Overall, SEMs are powerful tools for studying the surface structure and composition of a wide range of materials at the microscopic level.
Characteristics of Scanning Electron Microscopy
- High-resolution imaging (1-2 nm), high-speed acquisition (30-60 s) (30-60 s).
- Live observation of the specimen with 5-6 orders of magnification (10x to 500,000x) (10x to 500,000x).
- Vacuum compatibility necessary. Vacuum chamber accommodates. specimens up to 4 inches in diameter.
- Versatility: various modes of operation possible.
- Readily available cross-sectional measurements.
Principle of Scanning Electron Microscope
In contrast to the Transmission Electron Microscope, which employs electrons that are transmitted, the Scanning Electron Microscope uses electrons that are emitted. Utilizing kinetic energy, the scanning electron microscope generates data based on the interaction between electrons. These electrons, which are secondary electrons, backscattered electrons, and diffracted backscattered electrons, are utilised to observe crystalline elements and photons. To create a picture, secondary and backscattered electrons are utilised. The secondary electrons released by the specimen are primarily responsible for detecting the specimen’s morphology and topography, whilst the backscattered electrons reveal differences in the specimen’s elemental makeup.
Fundamental Principles of Scanning Electron Microscopy (SEM)
Significant amounts of kinetic energy are carried by accelerated electrons in a SEM; this energy is released as a variety of signals created by electron-sample interactions when the incident electrons decelerate in the solid sample. These signals consist of secondary electrons (which produce SEM images), backscattered electrons (BSE), diffracted backscattered electrons (EBSD, used to determine crystal structures and orientations of minerals), photons (characteristic X-rays used for elemental analysis and continuum X-rays), visible light (cathodoluminescence—CL), and heat. Secondary electrons and backscattered electrons are frequently employed for imaging samples: secondary electrons are most useful for displaying morphology and topography on samples, whilst backscattered electrons are most useful for displaying compositional contrasts in multiphase samples (i.e. for rapid phase discrimination). Inelastic collisions of incoming electrons with electrons in discrete ortitals (shells) of atoms in the sample generate X-rays. As the electrons return to their lower energy states, they emit X-rays with a constant wavelength (that is related to the difference in energy levels of electrons in different shells for a given element). Thus, X-rays are produced for each element in a mineral “stimulated” by an electron beam. SEM examination is called “non-destructive” because x-rays generated by electron interactions do not result in sample volume loss, allowing repeated study of the same materials.
Parts of Scanning Electron Microscope
Scanning electron microscope contain these following parts;
- Electron Source: A electron source help in the production of electron beams in SEM. There are present different types of electron sources are used in SEM such as;
- A Thermionic filament: It is a tungsten filament. When heated, it emits electron beams.
- A Field emission gun (FEG): It creates a strong electrical field, which pulls electrons away from their atoms.
- A Cerium Hexaboride cathode (CeB6): It provides ten-time brightness as compared to other electron sources.
- Electromagnetic Lens: It focuses the electron beams on the specimen from the source.
- Vacuum chamber: It prevents the intersection between an electron beam and air particles.
- Sample chamber and stage: It holds the specimen inside the vacuum.
- Computer: It controls the magnification power and the surface to be scanned.
- Secondary electron detectors: It detects the secondary electrons.
- BackScattered Electron (BSE) detector: It detects the backscattered electrons.
- Power Supplier: It Supply Power to the SEM.
Sample Preparation for Scanning Electron Microscope
1. Primary Fixation:
- This is done with ALDEHYDES (PROTEINS).
- This step will help to stabilize the ultrastructure of the specimen with the crosslinking the Proteins by glutaraldehyde and formaldehyde.
2. Secondary Fixation:
- This is done with the OSMIUM TETROXIDE (LIPIDS).
- This step prevents the Blipid membranes extraction during dehydration.
- It also increases sample conductivity and minimizes image distortions
- This is done by incubation of fixed specimen in solvents such as ETHANOL OR ACETONE.
- To remove the water from the specimen without shrinking it the Solvent concentration is increased gradually.
- In this method, the dehydration solvents are replaced with the Hexamethyldisilazane (HMDS) or liquid CO2 to prevent the artifacts and micro-ripping of the surface.
5. Mounting on a Stub:
- This is done with a sticky carbon dis, which mount the specimen on a metal stub.
- This step helps to increase the conductivity of the specimen.
- To increase further conductivity Silver-containing glue can be added to it.
6. Sputter Coating With Conductive Material:
- In this method, the specimen is coated with a conductive material to prevent the charge buildup on specimen surface.
What is causing the sample degradation?
Depending on the acceleration voltage, electrons in the electron beam can interact with electrons within the sample’s atoms. If a valence electron — an electron that can participate in the creation of a chemical bond — is removed from an atom, it will leave behind an electron hole. This vacancy must be filled within 100 femtoseconds (the usual period of an atomic vibration) or the bond will break.
This is not a problem in conductive materials because the electron hole is filled within 1 millisecond (fs). However, it can take many microseconds for non-conductive materials to fill the electron hole, potentially breaking the link and chemically changing the material’s structure.
How does the Scanning Electron Microscope (SEM) work?
- First of all, an electron source or electron gun located at the top of the SEM column is heated with high-voltage.
- As a result, it will release electron beams.
- Electron beams are now accelerated down the column and onto a series of electromagnetic lenses.
- These lenses and tubes are also called solenoids, because they are wrapped in a coil.
- These coils create fluctuations in the voltage, which results in increasing/decreasing the speed of electrons. Thus, how they create focused electron beams.
- This electron beam is focused onto a specimen.
- A computer is attached to the SEM, which controls the magnification power of the microscope and as well as determines the surface area to be scanned.
- The electron beams and atoms of the sample are combined, the rate is determined by the acceleration rate of incident electrons, which carry significant amounts of kinetic energy before focusing on the sample.
- When the incident electrons come in contact with the sample, energetic electrons are released from the surface of the sample. The scatter patterns made by the interaction yields information on the size, shape, texture, and composition of the sample.
- Different types of electrons are emitted from the sample after the interaction between electron beams and sample.
- An electron detector is placed over the sample, called BackScattered Electron (BSE) detector, which will detect backscattered electrons.
- The secondary electrons are detected using a Secondary Electron (SE) detector, which is placed at the side of the electron chamber. It will provide more detailed surface information.
Scanning process and image formation
- An electron beam is thermionically emitted from an electron cannon with a tungsten filament cathode in a standard SEM.
- Tungsten is typically employed in thermionic electron cannons due to its high melting point and low vapour pressure, allowing it to be electrically heated for electron emission, and its low cost.
- Other types of electron emitters include lanthanum hexaboride (LaB 6) cathodes, which can be used in a standard tungsten filament SEM if the vacuum system is upgraded, and field emission guns (FEG), which can be of the cold-cathode type with tungsten single crystal emitters or the thermally assisted Schottky type with tungsten single crystal emitters coated in zirconium oxide.
- The electron beam, which normally possesses an energy between 0.2 keV and 40 keV, is focussed by one or two condenser lenses to a point with a diameter between 0.4 nm and 5 nm.
- The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, often in the last lens, which deflect the beam in the x and y axes such that it scans over a rectangular portion of the sample surface in a raster pattern.
- When the primary electron beam interacts with the material, the electrons lose energy by repeated random scattering and absorption throughout the interaction volume, which extends from less than 100 nm to nearly 5 m beneath the surface.
- The size of the interaction volume is determined by the electron’s landing energy, the specimen’s atomic number, and its density.
- The energy exchange between the electron beam and the sample causes the reflection of high-energy electrons by elastic scattering, the emission of secondary electrons by inelastic scattering, and the emission of electromagnetic radiation, all of which can be detected by specialised detectors.
- The beam current absorbed by the specimen can also be monitored and utilised to generate pictures of the specimen current distribution.
- Diverse types of electronic amplifiers are used to amplify the signals, which are shown on a computer monitor as fluctuations in brightness (or, for vintage models, on a cathode-ray tube).
- Each pixel of computer video memory is synchronised with the position of the microscope’s beam on the specimen, and the resulting image is a distribution map of the strength of the signal emitted from the scanned area of the specimen.
- The majority of current microscopes capture images digitally, whereas older microscopes captured images on film.
- In a SEM, magnification can be adjusted across a range of approximately six orders of magnitude, from around 10 to 3,000,000 times. In contrast to optical and transmission electron microscopes, picture magnification in a SEM is independent of the objective lens’s power.
- Condenser and objective lenses may be present in SEMs, but their purpose is to focus the beam to a pinpoint, not to create an image of the specimen.
- In theory, if the electron gun can generate a beam with a sufficiently tiny diameter, a SEM may function totally without a condenser or objective lenses, although it would likely be less adaptable and have a lower resolution.
- Similar to scanning probe microscopy, magnification in a SEM is determined by the ratio between the raster on the display device and the raster on the specimen.
- Assuming the display screen has a fixed size, increasing magnification is achieved by decreasing the raster size on the specimen, and vice versa.
- Therefore, magnification is determined by the current given to the x, y scanning coils or the voltage supplied to the x, y deflector plates, and not by the objective lens power.
How secondary electrons are detected?
- The most common imaging technique gathers low-energy (50 eV) secondary electrons released from conduction or valence bands of specimen atoms by inelastic scattering interactions with beam electrons. Because of their low energy, these electrons originate from a few nanometers beneath the sample’s surface.
- A form of collector-scintillator-photomultiplier device, the Everhart–Thornley detector detects the electrons. The secondary electrons are initially gathered by attracting them into a grid electrically biassed at about +400 V, and then accelerated toward a phosphor or scintillator positively biassed at about +2,000 V.
- The accelerated secondary electrons are now powerful enough to cause the scintillator to emit flashes of light (cathodoluminescence), which are conducted to a photomultiplier outside the SEM column through a light pipe and a window in the specimen chamber’s wall.
- The amplified electrical signal emitted by the photomultiplier is presented as a two-dimensional intensity distribution that can be viewed and photographed on an analogue video display, or converted from analogue to digital and shown and stored as a digital image.
- This method utilises a main beam that is raster-scanned. Quantity of secondary electrons reaching the detector determines signal brightness. If the beam penetrates the sample perpendicular to the surface, the activated zone is homogeneous about the beam’s axis, and a specific amount of electrons “escape” from the sample.
- As the angle of incidence increases, the interaction volume grows and the “escape” distance of one side of the beam decreases, leading to an increase in the number of secondary electrons emitted from the sample.
- Consequently, steep surfaces and edges are typically brighter than flat surfaces, resulting in images with a distinct, three-dimensional aspect. It is feasible to obtain an image resolution of less than 0.5 nm using the signal of secondary electrons.
How backscattered electrons are detected?
- Backscattered electrons (BSE) are high-energy electrons that originate from the electron beam and are reflected or backscattered out of the specimen interaction volume due to elastic scattering interactions with specimen atoms.
- Since heavy atoms (high atomic number) backscatter electrons more strongly than light elements (low atomic number) and hence appear brighter in the image, BSEs are used to detect contrast between regions with different chemical compositions.
- The Everhart–Thornley detector, which is typically positioned on one side of the specimen, is ineffective for the detection of backscattered electrons because few such electrons are emitted in the solid angle subtended by the detector and the positively biassed detection grid has little ability to attract the higher energy BSE.
- Dedicated backscattered electron detectors are placed above the sample in a “doughnut” configuration, concentric with the electron beam, to maximise the solid angle of collection.
- BSE detectors are typically made of either scintillator or semiconductor material. When all sections of the detector are utilised to capture electrons symmetrically around the beam, atomic number contrast is produced.
- Strong topographic contrast is achieved, however, by collecting back-scattered electrons from one side above the specimen with an asymmetrical, directional BSE detector; the resulting contrast appears as illumination of the topography from that side.
- Semiconductor detectors can be fabricated with radial segments that can be turned on or off to adjust the sort of contrast produced and its directionality.
- Backscattered electrons can also be used to create an electron backscatter diffraction (EBSD) image, which can be utilised to determine the crystallographic structure of the material.
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Resolution of the SEM
- However, unlike a CCD array or film, the detector in a SEM does not continually create an image. The resolution is not constrained by the diffraction limit, the fineness of lenses or mirrors, or the resolution of the detector array, as it would be in an optical system.
- Focusing optics can be on the huge and crude side, and the SE detector, which measures current, is about the size of a human fist.
- Instead, the wavelength of the electrons and the electron-optical system that produces the scanning beam determine the size of the electron spot, and hence the spatial resolution of the SEM.
- The volume of specimen material that interacts with the electron beam is another factor limiting the resolution. In contrast to a transmission electron microscope, a scanning electron microscope (SEM) lacks the resolution necessary to provide detailed images of individual atoms due to the size of the spot and the interaction volume (TEM).
- However, the SEM has a number of advantages that make up for this, including the ability to image a large region of the specimen, the capability to image bulk materials (as opposed to only thin films or foils), and the availability of a wide range of analytical modes for determining the specimen’s composition and properties.
- The resolution can range from below 1 nm to above 20 nm, depending on the type of instrument used. Using a secondary electron detector, the highest resolution conventional (30 kV) SEM in the world as of 2009 achieved a point resolution of 0.4 nm.
Application of Scanning Electron Microscope
Scanning electron microscopes (SEMs) are advanced microscopes that use a focused beam of electrons to create high-resolution images of the surface of a sample. They are widely used in a variety of fields, including materials science, biology, and engineering, to study the structure, composition, and properties of a wide range of samples.
Some common applications of SEMs include:
- Microscopy: SEMs can be used to study the surface features of small samples at very high magnifications, typically in the range of 100x to 1,000,000x. This makes them useful for studying the structure and composition of materials at the micro- or nanoscale.
- Surface analysis: SEMs can be used to identify the elemental composition of a sample by measuring the energy of the electrons that are emitted from the sample as they are bombarded by the electron beam. This technique, called energy dispersive x-ray spectroscopy (EDS), allows researchers to determine the chemical makeup of a sample at a high level of accuracy.
- Materials Science: In materials science, SEMs are used for study, quality assurance, and problem diagnosis. Modern materials science relies extensively on SEMs for research and examination into nanotubes and nanofibers, high temperature superconductors, mesoporous structures, and alloy strength. Because of SEMs, the entire field of material science has advanced, from aerospace and chemistry to electronics and energy usage.
- Nanowires For Gas Sensing: By refining and extending current fabrication methods, scientists are investigating the potential of nanowires as gas sensors. Characterizing nanowires and learning how they behave as gas sensors are two areas where electron microscopy plays a crucial role.
- Semiconductor Inspection: Accurate topographical information is essential for semiconductor reliability. SEMs’ ability to provide high-resolution, three-dimensional images allows for a rapid, precise analysis of the semiconductor’s composition. In reality, SEMs are one of three crucial quality control tools utilised in virtually every step of the wafer production process. Quality control inspectors’ eyestrain can be alleviated by using larger monitors (19 inches or more) during daily, repetitive inspections.
- Microchip Assembly: There is a growing reliance on scanning electron microscopes (SEMs) in the microchip industry for evaluating the efficacy of novel manufacturing and assembly processes. The high resolution, three-dimensional capacity of SEMs is important to microchip design and production in an age of shrinking sizes and materials, as well as the promise of sophisticated self-assembling polymers. SEMs will remain crucial in the development of low-cost, low-power chipsets for non-traditional computers and networked devices as the Internet of Things (IoT) grows increasingly pervasive in the daily lives of consumers and manufacturers.
- Forensic Investigations: SEMs are frequently used in criminal and other forensic investigations to help unearth evidence and generate forensic insight. Applications include: gunshot residue analysis, jewellery inspection, bullet mark comparison, handwriting analysis, print analysis, currency inspection. investigation of traffic accidents using study of paint particles, fibres, and light bulbs. It is possible to draw conclusions, trace material origins, and add to a body of evidence in forensic sciences thanks to SEMs because of their capacity to investigate a wide variety of materials at high and low magnification without sacrificing depth of focus. Automated gun shot residue analysis is a specialty of the desktop Phenom GSR instrument.
- Surface imaging: SEMs can be used to create detailed images of the surface of a sample, revealing features such as cracks, pits, and other defects. These images can be used to study the surface properties of materials and identify potential problems or defects.
- 3D imaging: By using a technique called serial sectioning, SEMs can be used to create 3D images of the internal structure of a sample. This allows researchers to study the internal structure of materials and identify features such as pores, defects, and other microstructural features.
- Biological Sciences: SEMs have a wide range of applications in the biological sciences, from studying bacteria and viruses to examining insects and animal tissue. Applications include: quantifying how species are being impacted by climate change. discovery of novel and dangerous bacterial strains, Analysis of Vaccines, Finding new species through genetic research.
- Soil And Rock Sampling: In geology, taking samples and analysing them with a scanning electron microscope can reveal information on the morphology and weathering processes of the samples. Compositional variations can be seen with backscattered electron imaging, and elemental composition can be obtained with microanalysis. Example of acceptable applications: localization of prehistoric artefacts and toolkits, Evaluation of soil for agricultural use and date of archaeological sites, Soil chemistry, poisons, and other such factors can be used as forensic evidence.
- Medical Science: SEMs have numerous applications in medicine, including the examination of blood and tissue samples to ascertain disease origins and the evaluation of therapy efficacy (while contributing to the design of new treatments). Usage examples: Detecting Pathogens and Illnesses, vaccine and drug trials, testing samples over the course of a patient’s life, vs comparing tissue samples between individuals in a control and test group.
- Art: The practicality of several SEM uses is debatable. SEM micrographs have been incorporated into digital paintings. High-resolution 3D photos captured from a variety of materials create a wide variety of landscapes, including both unfamiliar and recognisable subjects.
- Materials characterization: SEMs can be used to study the microstructure and properties of materials, such as their strength, toughness, and fatigue resistance. This information is useful for optimizing the design and performance of materials in a variety of applications.
Advantages of Scanning Electron Microscope
- It provides a 3D and topographical image of the specimen with great detail.
- Require less time.
- Require minimal preparation action of the sample.
- Modern SEMs produces portable digital data.
- SEM is easy to operate with proper training.
Disadvantages of Scanning Electron Microscope
- SEMs are a costly item.
- It is large in size, that is why required a room to operate SEM.
- The room should be free of electric, magnetic field, and vibration.
- Required a steady voltage.
- Required cool water.
- Required proper training to operate SEM.
- It is limited to solid, inorganic samples small enough to fit inside the vacuum chamber.
- It also carried a risk of radiation exposure.
- Samples must be solid and they must fit into the microscope chamber. Maximum size in horizontal dimensions is usually on the order of 10 cm, vertical dimensions are generally much more limited and rarely exceed 40 mm. For most instruments samples must be stable in a vacuum on the order of 10^-5 – 10^-6 torr. Samples likely to outgas at low pressures (rocks saturated with hydrocarbons, “wet” samples such as coal, organic materials or swelling clays, and samples likely to decrepitate at low pressure) are unsuitable for examination in conventional SEM’s. However, “low vacuum” and “environmental” SEMs also exist, and many of these types of samples can be successfully examined in these specialized instruments.
- EDS detectors on SEM’s cannot detect very light elements (H, He, and Li), and many instruments cannot detect elements with atomic numbers less than 11 (Na).
- Most SEMs use a solid state x-ray detector (EDS), and while these detectors are very fast and easy to utilize, they have relatively poor energy resolution and sensitivity to elements present in low abundances when compared to wavelength dispersive x-ray detectors (WDS) on most electron probe microanalyzers (EPMA).
- An electrically conductive coating must be applied to electrically insulating samples for study in conventional SEM’s, unless the instrument is capable of operation in a low vacuum mode.
Scanning Electron Microscope (SEM) Images