The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The SEM’s job is to use an electron beam to trace over the object, creating an exact replica of the original object on a monitor. As the electron beam traces over the object, it interacts with the surface of the object, dislodging secondary electrons from the surface of the specimen in a unique pattern.
A secondary electron detector attracts those scattered electrons and, depending on the number of electrons that reach the detector, registers different levels of brightness on a screen. The scanning electron microscope has many advantages over traditional microscopes.
The SEM has a large depth of field, which allows more of a specimen to be in focus at one time, producing strikingly clear images. The SEM also has much higher resolution, so closely spaced specimens can be magnified at much higher levels. Because the SEM uses electromagnets rather than lenses, the researcher has much more control over the degree of magnification.
The first SEM image was obtained by Max Knoll, who in 1935 obtained an image of silicon steel showing electron channelling contrast. Subsequently M. vonArdenne (1938) constructed a scanning transmission electron microscope by adding scan coils to a transmission electron microscope. The SEM was further developed by Professor Sir Charles Oatley and Gary Stewart in1965. The first SEM used to examine the surface of a solid specimen was described by Zworykin et al. (1942), who was working in the RCA Laboratories in the United States.
Principles of Scanning Electron Microscopy
Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images), backscattered electrons (BSE), diffracted backscattered electrons (EBSD that are used to determine crystal structures and orientations of minerals), photons (characteristic X-rays that are used for elemental analysis and continuum X-rays), visible light (cathodoluminescence–CL), and heat.
Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples. X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete shells of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength. SEM analysis is considered to be “non-destructive”; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly.
The SEM can produce very high-resolution images of a sample surface that can be magnified up to 300,000 times the size of the object, revealing details about 1 to 5 nm in size. Due to the way these images are created, SEM micrographs have a very large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample but SEMs cannot produce colour images.
COMPONENTS OF A SCANNING ELECTRON MICROSCOPE
Electron gun produces steady stream of electrons necessary for SEMs to operate. Electron guns are typically one of the two types: Thermionic guns, which apply thermal energy to a filament (usually made of tungsten) to detach electrons away from the gun and toward the specimen. Field emission guns, on the other hand, create a strong electrical field to pull electrons away from the atoms they’re associated. The anode, which is positive with respect to the filament, forms powerful attractive forces for electrons. This causes electrons to accelerate toward the anode.
When a SEM is used, the column must always be at a vacuum. There are many reasons for this. If the sample is in a gas filled environment, an electron beam cannot be generated or maintained because of a high instability in the beam. Gases could react with the electron source, causing it to burn out, or cause electrons in the beam to ionize, which produces random discharges and leads to instability in the beam. The transmission of the beam through the electron optic column would also be hindered by the presence of other molecules.
Lenses: SEMs use lenses to produce clear and detailed images but the lenses work differently and they are made of magnets capable of bending the path of electrons. By doing so, the lenses focus and control the electron beam, ensuring that the electrons end up precisely where they are needed.
Sample chamber: The sample chamber of an SEM is where specimen is placed in a vacuum. Because the specimen must be kept extremely still for the microscope to produce clear images, the sample chamber must be very sturdy and insulated from vibration. In fact, SEMs are so sensitive to vibrations that they’re often installed on the ground floor of a building. They also manipulate the specimen, placing it at different angles and moving it so that researchers don’t have to constantly remount the object to take different images.
Detectors: These devices detect the various ways that the electron beam interacts with the sample object. For instance, Everhart-Thornley detectors register secondary electrons, which are electrons dislodged from the outer surface of a specimen. These detectors are capable of producing the most detailed images of an object’s surface. Other detectors, such as backscattered electrondetectors and X-ray detectors, can tell researchers about the composition of a substance.
Vacuum chamber: SEMs require a vacuum to operate. Without a vacuum, the electron beam generated by the electron gun would encounter constant interference from air particles. Not only would these particles block the path of the electron beam, they would also be knocked out of the air and onto the specimen, which would distort the surface of the specimen.
Scanning coils create a magnetic field using fluctuating voltage, to manipulate the electron beam. The scanning coils are able to move the beam precisely back and forth over a defined section of an object. If a researcher wants to increase the magnification of an image, he or she simply sets the electron beam to scan a smaller area of the sample.
Sample preparation can be minimal or elaborate for SEM analysis, depending on the nature of the samples and the data required. Minimal preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. Carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of “low vacuum” operation.
The sputtercoater uses argon gas and a small electric field. The sample is placed in a small chamber which is at vacuum. Argon gas is then introduced and an electric field is used to cause an electron to be removed from the argon atoms to make the atoms ions with a positive charge. The Argon ions are then attracted to a negatively charged piece of gold foil. The Argon ions act like sand in a sandblaster, knocking gold atoms from the surface of the foil. These gold atoms now settle onto the surface of the sample, producing a gold coating.
Conductive materials in current use for specimen coating include gold, gold/palladium alloy, platinum, osmium, iridium, tungsten, chromium and graphite. Coating prevents the accumulation of static electric charge on the specimen during electron irradiation.
The electron beam, which typically has an energy ranging from a few hundred eV to 40 keV, is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface.
The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by elastic scattering, emission of secondary electrons by inelastic scattering and the emission of electromagnetic radiation, each of which can be detected by specialized detectors. The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current.
The raster scanning of the CRT display is synchronised with that of the beam on the specimen in the microscope, and the resulting image is therefore a distribution map of the intensity of the signal being emitted from the scanned area of the specimen.
Unlike optical and transmission electron microscopes, image magnification in the SEM is not a function of the power of the objective lens. SEMs may have condenser and objective lenses, but their function is to focus the beam to a spot, and not to image the specimen. In an SEM, magnification results from the ratio of the dimensions of the raster on the specimen and the raster on the display device. Magnification is therefore controlled by the current supplied to the x,y scanning coils, and not by objective lens power.
A scanning device near the bottom of the vacuum chamber controls the movement of the electron beam across the specimen, row by row. As the electron beam sweeps the surface, it excites electrons present in the atomic structure of molecules, causing some of them to escape from the surface. These escaping electrons, known as deflected secondary electrons, have specific energies that can be measured. As they are released from each area of the sample, they are collected and counted by a detector that sends their amplified signals. The various electronic energies produced are analyzed by computer software, and the resulting image is displayed on a computer monitor.