History of the creation of the electron microscope
In 1931, R. Rudenberg received a patent for a transmission electron microscope, and in 1932, M. Knoll and E. Ruska built the first prototype of a modern device. This work by E. Ruska was awarded the Nobel Prize in Physics in 1986, which was awarded to him and the inventors of the scanning probe microscope, Gerd Karl Binnig and Heinrich Rohrer. The use of transmission electron microscopes for scientific research began in the late 1930s, with the first commercial instrument built by Siemens.
In the late 1930s and early 1940s, the first scanning electron microscopes appeared, forming an image of an object by sequentially moving a small cross-section electron probe across the object. The widespread use of these devices in scientific research began in the 1960s, when they achieved significant technical excellence.
A significant leap (in the 70s) in development was the use of Schottky cathodes and cold field emission cathodes instead of thermionic cathodes, but their use requires a much higher vacuum.
In the late 90s and early 2000s, computerization and the use of CCD detectors greatly increased stability and (relative) ease of use.
In the last decade, modern advanced transmission electron microscopes have used correctors for spherical and chromatic aberrations (which introduce the main distortion into the resulting image), but their use sometimes significantly complicates the use of the device.
Types of electron microscopes
Transmission electron microscopy
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Initial view of an electron microscope. A transmission electron microscope uses a high-energy electron beam to form an image. The electron beam is created by means of a cathode (tungsten, LaB 6 , Schottky or cold field emission). The resulting electron beam is usually accelerated to +200 keV (various voltages from 20 keV to 1 meV are used), focused by a system of electrostatic lenses, passes through the sample so that part of it passes through scattering on the sample, and part does not. Thus, the electron beam passing through the sample carries information about the structure of the sample. The beam then passes through a system of magnifying lenses and forms an image on a fluorescent screen (usually made of zinc sulfide), a photographic plate or a CCD camera.
TEM resolution is limited mainly by spherical aberration. Some modern TEMs have spherical aberration correctors.
The main disadvantages of TEM are the need for a very thin sample (about 100 nm) and the instability (decomposition) of samples under the beam.
Transmission raster (scanning) electron microscopy (STEM)
Main article: Transmission scanning electron microscope
One of the types of transmission electron microscopy (TEM), however, there are devices that operate exclusively in the TEM mode. A beam of electrons is passed through a relatively thin sample, but unlike conventional transmission electron microscopy, the electron beam is focused to a point that moves across the sample in a raster.
Raster (scanning) electron microscopy
It is based on the television principle of scanning a thin beam of electrons over the surface of a sample.
Low Voltage Electron Microscopy
Applications of electron microscopes
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Semiconductors and Data Storage
Biology and Life Sciences
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Scientific research
Industry
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The world's main manufacturers of electron microscopes
see also
Notes
Links
- 15 Best Electron Microscope Images of 2011 The images on the recommended site are randomly colored, and have more artistic than scientific value (electron microscopes produce black-and-white images, not color).
Wikimedia Foundation.
2010.
Technological archeology)
Some electron microscopes restore the firmware of spacecraft, others reverse engineer the circuitry of microcircuits under a microscope. I suspect that the activity is terribly exciting.
And, by the way, I remembered the wonderful post about industrial archeology.
Spoiler
There are two types of corporate memory: people and documentation. People remember how things work and know why. Sometimes they write this information down somewhere and store their notes somewhere. This is called "documentation". Corporate amnesia works the same way: people leave, and documentation disappears, rots, or is simply forgotten.
I spent several decades working for a large petrochemical company. In the early 1980s, we designed and built a plant that converts hydrocarbons into other hydrocarbons. Over the next 30 years, corporate memory of the plant faded. Yes, the plant is still operating and bringing money to the company; maintenance is carried out, and highly wise specialists know what they need to pull and where to kick in order for the plant to continue to operate.
But the company has completely forgotten how this plant works.
This happened due to several factors: Decline in petrochemical industry
in the 1980s and 1990s caused us to stop hiring new people. In the late 1990s, our group consisted of guys under the age of 35 or over 55 - with very rare exceptions.
We have slowly switched to designing using computer systems.
Due to corporate reorganizations, we had to physically move our entire office from place to place.
A corporate merger a few years later completely dissolved our firm into a larger one, causing a major departmental overhaul and personnel reshuffling.
Industrial archeology
In the late 2000s, the company remembered the plant and thought it would be nice to do something with it. Let's say, increase production. For example, you can find a bottleneck in the production process and improve it - technology has not stood still these 30 years - and, perhaps, add another workshop.
And then the company rushes into the brick wall. How was this plant built? Why was it built this way and not otherwise? How exactly does it work? Why is vat A needed, why are workshops B and C connected by a pipeline, why does the pipeline have a diameter of D and not D?
Corporate amnesia in action. Giant machines, built by aliens with the help of their alien technology, champ as if wound up, producing heaps of polymers. The company has some idea of how to maintain these machines, but has no idea what kind of amazing magic happens inside, and no one has the slightest idea how they were created. In general, people are not even sure what exactly to look for, and do not know which side to unravel this tangle.
We are looking for guys who were already working in the company during the construction of this plant. Now they occupy high positions and sit in separate, air-conditioned offices. They are given the task of finding documentation for the designated plant. This is no longer corporate memory, it is more like industrial archaeology. Nobody knows what documentation exists for this plant, whether it exists at all, and if so, in what form it is stored, in what formats, what it includes and where it is physically located. The plant was designed project team, which no longer exists, in a company that has since been acquired, in an office that has been closed, using pre-computer age methods that are no longer used.
The guys remember their childhood with the obligatory digging in the dirt, roll up the sleeves of their expensive jackets and get to work.
Electron microscopy is a method for studying structures that are beyond the visibility of a light microscope and have dimensions of less than one micron (from 1 μm to 1-5 Å).
The operation of an electron microscope (Fig.) is based on the use of a directed flow, which acts as a light beam in a light microscope, and the role of lenses is played by magnets (magnetic lenses).
Due to the fact that different areas of the object under study retain electrons in different ways, the electron microscope screen produces a black and white image of the object being studied, magnified tens and hundreds of thousands of times. Transmission electron microscopes are mainly used in biology and medicine.
Electron microscopy arose in the 1930s, when the first images of certain viruses (tobacco mosaic virus and bacteriophages) were obtained. Currently, electron microscopy has found the widest application in virology and virology, leading to the creation of new branches of science. In electron microscopy of biological objects, special preparation methods are used. This is necessary to identify individual components of the objects being studied (cells, bacteria, viruses, etc.), as well as to preserve their structure in high vacuum conditions under an electron beam. Electron microscopy is used to study external shape object, the molecular organization of its surface, using the method of ultra-thin sections, the internal structure of the object is studied.
Electron microscopy in combination with biochemical, cytochemical research methods, immunofluorescence, as well as X-ray diffraction analysis make it possible to judge the composition and function of the structural elements of cells and viruses.
Electron microscope from the 1970s
Electron microscopy is the study of microscopic objects using an electron microscope.
An electron microscope is an electron-optical instrument with a resolution of several angstroms and allows visual study thin structure microscopic structures and even some molecules.
A three-electrode gun, consisting of a cathode, a control electrode and an anode, serves as a source of electrons to create an electron beam that replaces a light beam (Fig. 1).
Rice. 1. Three-electrode gun: 1 - cathode; 2 - control electrode; 3 - electron beam; 4 - anode.
Electromagnetic lenses, used in electron microscopes instead of optical ones, are multilayer solenoids enclosed in shells made of soft magnetic material, which have inside non-magnetic gap (Fig. 2).
Rice. 2. Electromagnetic lens: 1 - pole piece; 2 - brass ring; 3 - winding; 4 - shell.
The electric and magnetic fields created in an electron microscope are axially symmetric. Due to the action of these fields, charged particles (electrons) emerging from one point of the object within small angle, are reassembled in the image plane. The entire electron-optical system is contained in the electron microscope column (Fig. 3).
Rice. 3. Electro-optical system: 1 - control electrode; 2 - diaphragm of the first capacitor; 3 - diaphragm of the second capacitor; 4 - stigmatizer of the second capacitor; 5 - object; 6 - objective lens; 7 - objective lens stigmatizer; 8 - intermediate lens stigmatizer; 9 - projection lens aperture; 10 - cathode; 11 - anode; 12 - first capacitor; 13 - second capacitor; 14 - focus corrector; 15 - object holder table; 16 - lens aperture; 17 - selector diaphragm; 18 - intermediate lens; 19 - projection lens; 20 - screen.
The electron beam created by the electron gun is directed into the field of action of condenser lenses, which allow the density, diameter and aperture of the beam incident on the object under study to be varied within a wide range. A table is installed in the object's chamber, the design of which ensures the movement of the object in mutually perpendicular directions. In this case, you can sequentially inspect an area equal to 4 mm 2 and select the most interesting areas.
Behind the subject's camera is an objective lens, which allows for a sharp image of the subject. It also gives the first enlarged image of the object, and with the help of subsequent, intermediate and projection lenses overall increase can be increased to the maximum. The image of the object appears on a screen that luminesces under the influence of electrons. Behind the screen are photo plates. The stability of the electron gun, as well as the clarity of the image, along with other factors (constancy of high voltage, etc.) largely depend on the depth of vacuum in the column of the electron microscope, therefore the quality of the device is largely determined by the vacuum system (pumps, pumping channels, taps, valves, seals) (Fig. 4). The required vacuum inside the column is achieved thanks to the high efficiency of vacuum pumps.
A mechanical fore-vacuum pump creates a preliminary vacuum in the entire vacuum system, then the oil diffusion pump comes into operation; both pumps are connected in series and provide a high vacuum in the microscope column. The introduction of an oil booster pump into the electron microscope system made it possible to long time turn off the foreline pump.
Rice. 4. Vacuum circuit of an electron microscope: 1 - trap cooled with liquid nitrogen (cooling line); 2 - high-vacuum valve; 3 - diffusion pump; 4 - bypass valve; 5 - small buffer cylinder; 6 - booster pump; 7 - mechanical forevacuum pump of preliminary vacuum; 8 - four-way valve; 9 - large buffer cylinder; 10 - electron microscope column; 11 - air inlet valve into the microscope column.
Electrical diagram The microscope consists of high voltage sources, cathode heating, power supply for electromagnetic lenses, as well as a system that provides alternating mains voltage to the electric motor of the forevacuum pump, the diffusion pump furnace and the lighting of the control panel. The demands placed on the power supply device are very high requirements: for example, for a high-resolution electron microscope, the degree of instability of the high voltage should not exceed 5·10 -6 in 30 seconds.
An intense electron beam is formed as a result of thermal emission. The source of filament for the cathode, which is a V-shaped tungsten filament, is a high-frequency generator. The generated voltage with an oscillation frequency of 100-200 kHz provides a monochromatic electron beam. The electron microscope lenses are powered by a constant, highly stabilized current.
Rice. 5. Electron microscope UEMV-100B for studying living microorganisms.
Devices are produced (Fig. 5) with a guaranteed resolution of 4.5 Å; In individual unique photographs, a resolution of 1.27 Å was obtained, approaching the size of an atom. The useful increase in this case is 200,000.
An electron microscope is a precision instrument that requires special preparation methods. Biological objects have low contrast, so it is necessary to artificially enhance the contrast of the drug. There are several ways to increase the contrast of preparations. By shading the preparation at an angle with platinum, tungsten, carbon, etc., it becomes possible to determine dimensions along all three axes of the spatial coordinate system on electron microscopic photographs. With positive contrast, the drug combines with water-soluble salts of heavy metals (uranyl acetate, lead monoxide, potassium permanganate, etc.). With negative contrast, the specimen is surrounded by a thin layer of amorphous substance high density, impenetrable to electrons (ammonium molybdate, uranyl acetate, phosphotungstic acid, etc.).
Electron microscopy of viruses (viroscopy) has led to significant progress in the study of the ultrafine, submolecular structure of viruses (see). Along with physical, biochemical and genetic research methods, the use of electron microscopy also contributed to the emergence and development of molecular biology. The subject of study of this new branch of biology is the submicroscopic organization and functioning of human, animal, plant, bacterial and mycoplasma cells, as well as the organization of rickettsia and viruses (Fig. 6). Viruses, large molecules of protein and nucleic acids (RNA, DNA), individual fragments cells (for example, the molecular structure of the membrane of bacterial cells) can be examined using an electron microscope after special treatment: metal shading, positive or negative contrast with uranyl acetate or phosphotungstic acid, as well as other compounds (Fig. 7).
Rice. 6. Cynomolgus monkey heart tissue culture cell infected with variola virus (X 12,000): 1 - nucleus; 2 - mitochondria; 3 - cytoplasm; 4 - virus.
Rice. 7. Influenza virus (negative contrast (X450,000): 1 - envelope; 2 - ribonucleoprotein.
Using the negative contrast method, regularly arranged groups of protein molecules—capsomeres—were discovered on the surface of many viruses (Fig. 8).
Rice. 8. Fragment of the surface of the herpes virus capsid. Individual capsomeres are visible (X500,000): 1 - side view; 2 - top view.
Rice. 9. Ultrathin section of the bacterium Salmonella typhimurium (X80,000): 1 - core; 2 - shell; 3 - cytoplasm.
The internal structure of bacteria and viruses, as well as other larger biological objects, can be studied only after dissecting them using an ultratome and preparing the thinnest sections 100-300 Å thick. (Fig. 9). Thanks to improved methods of fixation, embedding and polymerization of biological objects, the use of diamond and glass knives during ultratomization, as well as the use of high-contrast compounds for staining serial sections, it was possible to obtain ultrathin sections of not only large, but also the smallest viruses of humans, animals, plants and bacteria.
ELECTRON MICROSCOPE
a device that allows you to obtain highly magnified images of objects using electrons to illuminate them. An electron microscope (EM) allows you to see details that are too small to be resolved by a light (optical) microscope. EM is one of the most important instruments for fundamental scientific research into the structure of matter, especially in such fields of science as biology and solid state physics. There are three main types of EM. In the 1930s, the conventional transmission electron microscope (CTEM) was invented, in the 1950s, the raster (scanning) electron microscope (SEM), and in the 1980s, the scanning tunneling microscope (RTM). These three types of microscopes complement each other in studying structures and materials of different types.
CONVENTIONAL TRANSMISSION ELECTRON MICROSCOPE
OPEM is in many ways similar to a light microscope see MICROSCOPE, but it uses a beam of electrons rather than light to illuminate samples. It contains an electronic spotlight (see below), a series of condenser lenses, an objective lens, and a projection system that matches the eyepiece but projects the actual image onto a fluorescent screen or photographic plate. The electron source is usually a heated tungsten or lanthanum hexaboride cathode. The cathode is electrically isolated from the rest of the device, and the electrons are accelerated by a strong electric field. To create such a field, the cathode is maintained at a potential of about -100,000 V relative to other electrodes, which focus the electrons into a narrow beam. This part of the device is called an electron spotlight (see ELECTRON GUN). Since electrons are strongly scattered by matter, there must be a vacuum in the microscope column where the electrons move. Here the pressure is maintained not exceeding one billionth of atmospheric pressure.
Electronic optics. An electronic image is formed by electric and magnetic fields in much the same way as a light image is formed by optical lenses.
The principle of operation of a magnetic lens is illustrated by the diagram (Fig. 1). The magnetic field created by the turns of the coil carrying the current acts as a converging lens, the focal length of which can be changed by changing the current. Since the optical power of such a lens, i.e. the ability to focus electrons depends on the magnetic field strength near the axis; to increase it, it is desirable to concentrate the magnetic field in the minimum possible volume. In practice, this is achieved by the fact that the coil is almost completely covered with magnetic “armor” made of a special nickel-cobalt alloy, leaving only a narrow gap in its inner part. The magnetic field created in this way can be 10-100 thousand times stronger than the Earth's magnetic field on the earth's surface. The OPEM diagram is shown in Fig. 2. A series of condenser lenses (only the last one is shown) focuses the electron beam onto the sample. Typically, the former creates an unenlarged image of the electron source, while the latter controls the size of the illuminated area on the sample. The aperture of the last condenser lens determines the beam width in the object plane. The sample is placed in the magnetic field of an objective lens with high optical power - the most important lens of the OPEM, which determines the maximum possible resolution of the device. Aberrations in an objective lens are limited by its aperture, just as they are in a camera or light microscope. An objective lens produces a magnified image of an object (usually about 100 magnification); the additional magnification introduced by the intermediate and projection lenses ranges from slightly less than 10 to slightly more than 1000. Thus, the magnification that can be obtained in modern OPEMs ranges from less than 1000 to 1,000,000 ELECTRON MICROSCOPE. (At a million times magnification grapefruit grows to the size of the Earth.) The object being studied is usually placed on a very fine mesh placed in a special holder. The holder can be mechanical or electrically
smoothly move up and down and left and right. The contrast in OPEM is due to electron scattering as the electron beam passes through the sample. If the sample is thin enough, the fraction of scattered electrons is small. When electrons pass through a sample, some of them are scattered due to collisions with the nuclei of the sample's atoms, others are scattered due to collisions with the electrons of the atoms, and still others pass through without undergoing scattering. The degree of scattering in any region of the sample depends on the thickness of the sample in this region, its density and the average atomic mass (number of protons) at a given point. Electrons leaving the diaphragm with an angular deviation exceeding a certain limit can no longer return to the beam carrying the image, and therefore highly scattering areas of increased density, increased thickness, and locations of heavy atoms appear in the image as dark zones on light background. Such an image is called bright-field because in it the surrounding field is brighter than the object. But it is possible to make sure that the electrical deflection system allows only some of the scattered electrons to pass into the lens diaphragm. Then the sample looks light on dark field. It is often more convenient to view a weakly scattering object in dark-field mode. The final enlarged electronic image is converted into a visible image by a fluorescent screen that glows under electron bombardment. This image, usually of low contrast, is typically viewed through a binocular light microscope. At the same brightness, such a microscope with a magnification of 10 can create an image on the retina that is 10 times larger than when observed with the naked eye. Sometimes, to increase the brightness of a weak image, a phosphor screen with an electron-optical converter is used. In this case, the final image can be displayed on a regular television screen, allowing it to be recorded on videotape. Video recording is used to record images that change over time, for example, due to the occurrence of a chemical reaction. Most often, the final image is recorded on photographic film or a photographic plate. A photographic plate usually produces a clearer image than that observed with the naked eye or recorded on videotape, since photographic materials, generally speaking, record electrons more efficiently. In addition, 100 times more signals can be recorded per unit area of photographic film than per unit area of video tape. Thanks to this, the image recorded on photographic film can be further enlarged by approximately 10 times without loss of clarity.
Permission. Electron beams have properties similar to those of light beams. In particular, each electron is characterized by a specific wavelength. The resolution of an EM is determined by the effective wavelength of the electrons. The wavelength depends on the speed of the electrons, and therefore on the accelerating voltage; The higher the accelerating voltage, the higher the speed of the electrons and the shorter the wavelength, which means the higher the resolution. Such a significant advantage of EM in resolution is explained by the fact that the wavelength of electrons is much shorter than the wavelength of light. But since electron lenses do not focus as well as optical lenses (the numerical aperture of a good electron lens is only 0.09, while for a good optical lens this value reaches 0.95), the resolution of EM is equal to 50-100 electron wavelengths. Even with such weak lenses, an electron microscope can achieve a resolution limit of approx. 0.17 nm, which makes it possible to distinguish individual atoms in crystals. To achieve a resolution of this order requires very careful adjustment of the instrument; in particular, highly stable power supplies are required, and the device itself (which can be approximately 2.5 m high and weigh several tons) and its additional equipment require vibration-free installation.
RASTER ELECTRON MICROSCOPE
The SEM, which has become an essential instrument for scientific research, serves as a good complement to the OPEM. SEMs use electron lenses to focus an electron beam into a very small spot. It is possible to adjust the SEM so that the diameter of the spot in it does not exceed 0.2 nm, but, as a rule, it is a few or tens of nanometers. This spot continuously runs around a certain area of the sample, similar to a beam running around the screen of a television tube. Electrical signal, which occurs when an object is bombarded by beam electrons, is used to form an image on the screen of a television kinescope or cathode ray tube (CRT), the scanning of which is synchronized with the electron beam deflection system (Fig. 3). Magnification in this case is understood as the ratio of the size of the image on the screen to the size of the area covered by the beam on the sample. This increase is between 10 and 10 million.
The interaction of electrons from the focused beam with the atoms of the sample can lead not only to their scattering, which is used to obtain images in OPEM, but also to the excitation of X-rays, the emission of visible light, and the emission of secondary electrons. In addition, since the SEM has only focusing lenses in front of the sample, it allows the examination of “thick” samples.
Reflective SEM. Reflective SEM is designed for studying massive samples. Since the contrast that arises when recording reflected, i.e. backscattered and secondary electrons is mainly related to the angle of incidence of electrons on the sample, the surface structure is revealed in the image. (The intensity of backscattering and the depth at which it occurs depend on the energy of the electrons in the incident beam. The emission of secondary electrons is determined mainly by the surface composition and electrical conductivity of the sample.) Both of these signals carry information about the general characteristics of the sample. Due to the low convergence of the electron beam, observations can be made from much greater depth sharpness than when working with a light microscope, and obtain excellent volumetric micrographs of surfaces with a highly developed relief. By registering the X-ray radiation emitted by a sample, in addition to data on the relief, it is possible to obtain information about the chemical composition of the sample in a surface layer with a depth of 0.001 mm ELECTRON MICROSCOPE. The composition of the material on the surface can also be judged by the measured energy with which certain electrons are emitted. All the difficulties of working with SEM are mainly due to its recording and electronic visualization systems. The device with a full range of detectors, along with all SEM functions, provides for the operating mode of an electron probe microanalyzer.
Scanning transmission electron microscope. A scanning transmission electron microscope (RTEM) is special kind SEM. It is designed for thin samples, the same as those studied in OPEM. The RPEM diagram differs from the diagram in Fig. 3 only in that it does not have detectors located above the sample. Since the image is formed by a traveling beam (rather than a beam illuminating the entire sample area under study), a high-intensity electron source is required so that the image can be recorded in a reasonable time. High-resolution RTEMs use high-brightness field emitters. In such an electron source, a very strong electric field (approx. V/cm) is created near the surface of a tungsten wire of very small diameter sharpened by etching. This field literally pulls billions of electrons out of the wire without any heat. The brightness of such a source is almost 10,000 times greater than the heated tungsten wire source (see above), and the electrons emitted by it can be focused into a beam with a diameter of less than 1 nm. Beams with a diameter close to 0.2 nm have even been obtained. Field electronic sources can only operate under ultra-high vacuum conditions (at pressures below Pa), in which contaminants such as hydrocarbon vapors and water are completely absent, and high-resolution imaging becomes possible. Thanks to such ultrapure conditions, it is possible to study processes and phenomena that are inaccessible to EM with conventional vacuum systems. RPEM studies are carried out on ultra-thin samples. Electrons pass through such samples almost without scattering. Electrons scattered at angles of more than a few degrees without slowing down are recorded when they hit a ring electrode located under the sample (Fig. 3). The signal picked up from this electrode is highly dependent on the atomic number of the atoms in the region through which the electrons pass - heavier atoms scatter more electrons towards the detector than lighter atoms. If the electron beam is focused to a point less than 0.5 nm in diameter, individual atoms can be imaged. In fact, it is possible to distinguish individual atoms with the atomic mass of iron (i.e. 26 or more) in the image obtained in the RTEM. Electrons that have not undergone scattering in the sample, as well as electrons that have slowed down as a result of interaction with the sample, pass into the hole of the ring detector. An energy analyzer located under this detector allows one to separate the former from the latter. By measuring the energy lost by electrons during scattering, important information about the sample can be obtained. Energy losses associated with the excitation of X-ray radiation or the knocking out of secondary electrons from the sample make it possible to judge the chemical properties of the substance in the region through which the electron beam passes.
RASTER TUNNEL MICROSCOPE
The EMs discussed above use magnetic lenses to focus electrons. This section is devoted to EM without lenses. But before moving on to the scanning tunneling microscope (RTM), it will be useful to briefly look at two older types of lensless microscopes that produce a projected shadow image.
Auto-electronic and auto-ion projectors. The field electronic source used in RPEM has been used in shadow projectors since the early 1950s. In a field emission projector, electrons emitted by field emission from a very small diameter tip are accelerated towards a fluorescent screen located a few centimeters from the tip. As a result, a projected image of the surface of the tip and the particles located on this surface appears on the screen with an increase equal to the ratio of the radius of the screen to the radius of the tip (order). Higher resolution is achieved in a field ion projector, in which the image is projected using helium ions (or some other elements), the effective wavelength of which is shorter than that of electrons. This produces images that show the true arrangement of atoms in the crystal lattice of the tip material. Therefore, field ion projectors are used, in particular, to study the crystal structure and its defects in the materials from which such tips can be made.
Scanning tunnel microscope (RTM). This microscope also uses a small diameter metal tip to provide electrons. An electric field is created in the gap between the tip and the sample surface. The number of electrons pulled by the field from the tip per unit time (tunneling current) depends on the distance between the tip and the surface of the sample (in practice, this distance is less than 1 nm). As the tip moves along the surface, the current is modulated. This allows you to obtain an image related to the surface topography of the sample. If the tip ends in a single atom, then an image of the surface can be formed by passing atom by atom. RTM can only work under the condition that the distance from the tip to the surface is constant, and the tip can be moved with precision down to atomic dimensions. Vibrations are suppressed thanks to the rigid design and small size of the microscope (no larger than a fist), as well as the use of multi-layer rubber shock absorbers. High precision is provided by piezoelectric materials, which elongate and contract under the influence of an external electric field. By applying a voltage of the order of 10-5 V, it is possible to change the dimensions of such materials by 0.1 nm or less. This makes it possible, by attaching the tip to an element made of piezoelectric material, to move it in three mutually perpendicular directions with an accuracy of the order of atomic sizes.
ELECTRON MICROSCOPY TECHNIQUE
There is hardly any sector of research in the field of biology and materials science that does not use transmission electron microscopy (TEM); this is ensured by advances in sample preparation techniques. All techniques used in electron microscopy are aimed at obtaining an extremely thin sample and providing maximum contrast between it and the substrate that it needs as a support. The basic technique is designed for samples with a thickness of 2-200 nm, supported by thin plastic or carbon films, which are placed on a grid with a mesh size of approx. 0.05 mm. (A suitable sample, no matter how it is obtained, is processed so as to increase the intensity of electron scattering on the object under study.) If the contrast is high enough, then the observer's eye can easily distinguish details located at a distance of 0.1-0.2 mm from each other. Consequently, in order for details separated by a distance of 1 nm on the sample to be distinguishable in the image created by an electron microscope, a total magnification of the order of 100-200 thousand is necessary. The best microscopes can create an image of a sample on a photographic plate with such a magnification, but at the same time The area shown is too small. Typically a micrograph is taken at lower magnification and then enlarged photographically. The photographic plate resolves approx. at a length of 10 cm. 10,000 lines. If each line on the sample corresponds to a certain structure with a length of 0.5 nm, then to register such a structure a magnification of at least 20,000 is necessary, whereas with the help of SEM and RPEM, in which the image is recorded by an electronic system and displayed on a television screen, only OK. 1000 lines. Thus, when using a television monitor, the minimum required magnification is approximately 10 times greater than when photographing.
Biological drugs. Electron microscopy is widely used in biological and medical research. Methods for fixation, embedding and obtaining thin sections of tissue for examination in OPEM and RPEM, and fixation techniques for examination of volumetric samples in SEM have been developed. These techniques make it possible to study cell organization at the macromolecular level. Electron microscopy revealed the components of the cell and the structural details of the membranes, mitochondria, endoplasmic reticulum, ribosomes and many other organelles that make up the cell. The sample is first fixed with glutaraldehyde or other fixatives and then dehydrated and embedded in plastic. Cryofixation methods (fixation at very low - cryogenic - temperatures) allow you to preserve the structure and composition without the use of chemical fixing substances. In addition, cryogenic methods allow imaging of frozen biological samples without dehydration. Using ultramicrotomes with blades made of polished diamond or chipped glass, tissue sections with a thickness of 30-40 nm can be made. Mounted histological preparations can be stained with compounds of heavy metals (lead, osmium, gold, tungsten, uranium) to enhance the contrast of individual components or structures.
Biological research has been extended to microorganisms, especially viruses, which are not resolved by light microscopes. TEM made it possible to reveal, for example, the structures of bacteriophages and the location of subunits in the protein shells of viruses. In addition, using positive and negative staining methods, it was possible to identify the structure with subunits in a number of other important biological microstructures. Nucleic acid contrast enhancement techniques have made it possible to observe single- and double-stranded DNA. These long, linear molecules are spread into a layer of basic protein and applied to a thin film. The sample is then vacuum sprayed with very thin layer heavy metal. This layer of heavy metal “shades” the sample, due to which the latter, when observed in OPEM or RPEM, appears as if illuminated from the side from which the metal was deposited. If you rotate the sample during deposition, the metal accumulates around the particles on all sides evenly (like a snowball).
Non-biological materials. TEM is used in materials research to study thin crystals and boundaries between different materials. To obtain a high-resolution image of the interface, the sample is filled with plastic, the sample is cut perpendicular to the interface, and then thinned so that the interface is visible at a sharp edge. The crystal lattice scatters electrons strongly in certain directions, producing a diffraction pattern. The image of a crystalline sample is largely determined by this pattern; the contrast is highly dependent on the orientation, thickness and perfection of the crystal lattice. Changes in contrast in the image allow the crystal lattice and its imperfections to be studied on an atomic scale. The information obtained in this case complements that provided by X-ray analysis of bulk samples, since EM makes it possible to directly see dislocations, stacking faults and grain boundaries in all details. In addition, electron diffraction patterns can be taken using EM and diffraction patterns from selected areas of the sample can be observed. If the lens aperture is adjusted so that only one diffracted and unscattered central beam passes through it, then it is possible to obtain an image of a certain system of crystal planes that produces this diffracted beam. Modern instruments allow resolving lattice periods of 0.1 nm. Crystals can also be studied using dark-field imaging, in which the central beam is blocked so that the image is formed by one or more diffracted beams. All these methods provided important information about the structure of many materials and significantly clarified the physics of crystals and their properties. For example, analysis of TEM images of the crystal lattice of thin small-sized quasicrystals in combination with analysis of their electron diffraction patterns made it possible in 1985 to discover materials with fifth-order symmetry.
High voltage microscopy. Currently, the industry produces high-voltage versions of OPEM and RPEM with an accelerating voltage from 300 to 400 kV. Such microscopes have a higher penetrating power than low-voltage devices, and are almost as good in this regard as the 1 million volt microscopes that were built in the past. Modern high-voltage microscopes are quite compact and can be installed in a regular laboratory room. Their increased penetrating power proves to be a very valuable property when studying defects in thicker crystals, especially those from which it is impossible to make thin samples. In biology, their high penetrating ability makes it possible to study whole cells without cutting them. In addition, with the help of such microscopes it is possible to obtain three-dimensional images of thick objects.
Low voltage microscopy. SEMs with accelerating voltages of only a few hundred volts are also available. Even at such low voltages, the electron wavelength is less than 0.1 nm, so the spatial resolution here is also limited by the aberrations of the magnetic lenses. However, because electrons with such low energy penetrate shallowly below the surface of the sample, almost all of the electrons involved in image formation come from a region located very close to the surface, thereby increasing the resolution of surface relief. Using low-voltage SEMs, images have been obtained on solid surfaces of objects smaller than 1 nm.
Radiation damage. Since electrons are ionizing radiation, the sample in the EM is constantly exposed to it. (This exposure produces secondary electrons used in SEM.) Consequently, samples are always subject to radiation damage. The typical dose of radiation absorbed by a thin sample during the recording of a microphotograph in an OPEM approximately corresponds to the energy that would be sufficient to completely evaporate cold water from a pond 4 m deep with a surface area of 1 hectare. To reduce radiation damage to the sample, it is necessary to use various methods its preparation: coloring, pouring, freezing. In addition, it is possible to record an image at electron doses that are 100-1000 times lower than using the standard technique, and then improve it using computer image processing methods.
HISTORICAL REFERENCE
The history of the creation of the electron microscope is a wonderful example of how independently developing fields of science and technology can, by exchanging information received and joining forces, create a new powerful tool for scientific research. The pinnacle of classical physics was the theory electromagnetic field, which explained the propagation of light, the emergence of electric and magnetic fields, and the movement of charged particles in these fields as the propagation of electromagnetic waves. Wave optics made clear the phenomenon of diffraction, the mechanism of image formation, and the play of factors that determine resolution in the light microscope. We owe advances in the field of theoretical and experimental physics to the discovery of the electron with its specific properties. These separate and seemingly independent paths of development led to the foundations of electron optics, one of the most important applications of which was the invention of EM in the 1930s. A direct hint of this possibility can be considered the hypothesis about the wave nature of the electron, put forward in 1924 by Louis de Broglie and experimentally confirmed in 1927 by K. Davisson and L. Germer in the USA and J. Thomson in England. This suggested an analogy that made it possible to construct an EM according to the laws of wave optics. H. Bush discovered that using electric and magnetic fields it is possible to form electronic images. In the first two decades of the 20th century. the necessary technical prerequisites were also created. Industrial laboratories working on the electron beam oscilloscope produced vacuum technology, stable high voltage and current sources, and good electron emitters. In 1931, R. Rudenberg filed a patent application for a transmission electron microscope, and in 1932, M. Knoll and E. Ruska built the first such microscope, using magnetic lenses to focus electrons. This device was the predecessor of the modern OPEM. (Ruska was rewarded for his efforts by winning the Nobel Prize in Physics for 1986.) In 1938, Ruska and B. von Borries built a prototype industrial OPEM for Siemens-Halske in Germany; this instrument eventually made it possible to achieve a resolution of 100 nm. A few years later, A. Prebus and J. Hiller built the first high-resolution OPEM at the University of Toronto (Canada). The wide possibilities of OPEM almost immediately became obvious. Its industrial production was started simultaneously by Siemens-Halske in Germany and the RCA Corporation in the USA. At the end of the 1940s, other companies began to produce such devices. The SEM in its current form was invented in 1952 by Charles Otley. Is it true, preliminary options Such devices were built by Knoll in Germany in the 1930s and by Zworykin and his collaborators at the RCA Corporation in the 1940s, but only Otley’s device was able to serve as the basis for a number of technical improvements, culminating in the introduction of an industrial version of the SEM into production in the mid-1960s . The range of consumers of such a fairly easy-to-use device with a three-dimensional image and an electronic output signal has expanded exponentially. Currently, there are a dozen industrial manufacturers of SEMs on three continents and tens of thousands of such devices used in laboratories around the world. In the 1960s, ultra-high-voltage microscopes were developed to study thicker samples. The leader of this direction of development was G. Dupuy in France , where in 1970 a device with an accelerating voltage of 3.5 million volts was introduced. RTM was invented by G. Binnig and G. Rohrer in 1979 in Zurich. This device, very simple in design, provides atomic resolution of surfaces. on the creation of RTM, Binnig and Rohrer (simultaneously with Ruska) received Nobel Prize in physics.
see also
a device for observing and photographing multiply (up to 10 6 times) enlarged images of objects, in which, instead of light rays, beams are used that are accelerated to high energies (30-100 keV or more) in deep vacuum conditions. Physical basis of corpuscular-beam optical instruments were founded in 1834 (almost a hundred years before the appearance of the electron microscope) by U. R., who established analogies between light rays in optically inhomogeneous media and the trajectories of particles in force fields. The feasibility of creating an electron microscope became obvious after its advancement in 1924, and the technical prerequisites were created by the German physicist H. Busch, who studied focusing axisymmetric fields and developed a magnetic electron lens (1926). In 1928, German scientists M. Knoll and E. Ruska began creating the first magnetic transmission electron microscope (TEM) and three years later obtained an image of an object formed by beams. In subsequent years (M. von Ardenne, 1938; V.K., 1942), the first raster electron microscopes (SEM) were built, operating on the scanning (sweeping) principle, i.e., sequential movement of a thin electron beam from point to point ( probe) by object. By the mid-1960s. SEMs achieved high technical perfection, and from that time on their use in scientific research began. FEMs have the highest (PC), surpassing in this parameter light microscopes several thousand times. T.n. The resolution limit, which characterizes the device to separately display the smallest possible details of an object, is 2-3 for TEM. At favorable conditions individual heavy atoms can be photographed. When photographing periodic structures, such as atomic crystal lattices, it is possible to achieve a resolution of less than 1 . Such high resolutions are achieved thanks to the extremely short length (see). Optimal aperture [see. in electron (and ion) optics] can be reduced (affecting the PC Electron microscope) with a sufficiently small diffraction error. Effective methods no correction was found in the Electron microscope (see). Therefore, in TEMs, magnetic (EL) ones, which have smaller values, have completely replaced electrostatic EL. PEMs are produced for various purposes. They can be divided into 3 groups: High-resolution electron microscope, simplified TEM and high-acceleration electron microscope.
High resolution TEM(2-3 Å) - like, multi-purpose devices. With the help of additional devices and attachments in them, you can tilt the object at different large angles to the optical axis, heat, cool, deform it, carry out research methods, etc. Accelerating electrons reaches 100-125 kV, is adjustable in steps and is highly stable: for In 1-3 minutes it changes by no more than 1-2 ppm from the original one. An image of a typical TEM of the type described is shown in rice. 1. A vacuum is created in its optical system (column) using a special vacuum system (up to 10 -6 mm Hg). The TEM optical system diagram is shown in rice. 2. The beam, which serves as a heated cathode, (is formed in and then twice focused by the first and second condensers, creating a small electronic “spot” on the object (when adjusting the spot, it can vary from 1 to 20 microns). Afterwards, the part is scattered through the object and delayed by the diaphragm. The unscattered electrons pass through the aperture and are focused into the object's intermediate lens. Here, the first magnified image is formed. Subsequent lenses create the second, third, etc. images. The last projection lens forms an image on a fluorescent screen, which glows under the influence of electrons. Magnification Electron microscope. equal to the magnification of all lenses. The degree and nature of electron scattering are not the same at different points of the object, since the thickness and chemical composition of the object changes from point to point. The number of electrons retained by the aperture diaphragm after passing through different points of the object changes accordingly, and, consequently, the number of electrons retained by the aperture diaphragm after passing through different points of the object changes. current density in the image, which is converted into on the screen. Under the screen there is a magazine with photographic plates. When photographing, the screen is removed and electrons act on the emulsion layer. The image is focused by a smooth change in the current exciting the lens. The currents of other lenses are adjusted to change the magnification Electron microscope
Rice. 3. Ultra-high-voltage electron microscope (UHVEM): 1 - tank into which electrical insulating gas (SF6 gas) is pumped to a pressure of 3-5 atm; 2 - electron gun; 3 - accelerating tube; 4 - high-voltage source capacitors; 5 - block of condenser lenses; 6 - lens; 7, 8, 9 - projection lenses; 10 - light microscope; 11 - control panel.
Scanning Electron Microscope (SEM) with an incandescent cathode are designed for studying massive objects with a resolution from 70 to 200 Å. The accelerator in the SEM can be adjusted in the range from 1 to 30-50 kV.
The device of a scanning electron microscope is shown in rice. 4. Using 2 or 3 ELs, a narrow electron probe is focused on the sample. Magnetic deflectors deploy the probe over a given area of the object. When a probe interacts with an object, several types arise ( rice. 5) - secondary and reflected electrons; electrons passing through the object (if it is thin); X-ray and characteristic; radiation, etc.
Rice. 5. Scheme for recording information about an object received in the SEM. 1 - primary electron beam; 2 - secondary electron detector; 3 - X-ray detector; 4 - reflected electron detector; 5 - light radiation detector; 6 - detector of transmitted electrons; 7 - device for measuring induced on an object electric potential; 8 - a device for measuring the current of electrons passing through an object; 9 - a device for measuring the current of electrons absorbed in an object.
Any of these radiations can be recorded by a corresponding collector containing a sensor that converts into electrical radiation, which, after amplification, is fed to the (CRT) and modulates its beam. The scanning of the CRT beam is carried out with scanning of the electron probe in the SEM, and an enlarged image of the object is observed on the CRT screen. The magnification is equal to the ratio of the height of the frame on the CRT screen to the width of the scanned object. The image is photographed directly from the CRT screen. The main advantage of the SEM is the high information content of the device, due to the ability to observe the image using various sensors. Using SEM, you can study the chemical composition of an object, p-n junctions, produce and much more. The sample is usually examined without prior preparation. SEM is also used in technological processes (microcircuit defects, etc.). High for SEM PC is realized when forming images using secondary . It is determined by the diameter of the zone from which these electrons are emitted. The size of the zone, in turn, depends on the diameter of the probe, the properties of the object, the electrons of the primary beam, etc. At a large penetration depth of primary electrons, secondary processes developing in all directions increase the diameter of the zone and PC decreases. The secondary electron detector consists of a photomultiplier and an electron-photon converter, the main element of which is two - an extractor in the form of a grid under a positive potential (up to several hundred V), and an accelerator; the latter provides the captured secondary electrons with the energy necessary for . About 10 kV is applied to the accelerating electrode; It usually consists of an aluminum coating on the scintillator. The number of scintillator flashes is proportional to the number of secondary ones emitted at a given point of the object. After amplification, the PMT and the signal are modulated by the CRT beam. The magnitude of the signal depends on the sample, the presence of local electric and magnetic microfields, the value of , which in turn depends on the chemical composition of the sample at a given point. The reflected electrons are recorded by a semiconductor (silicon) device. The contrast of the image is due to the dependence on the angle of incidence of the primary beam and the atomic number. The resolution of the image obtained “in reflected electrons” is lower than that obtained using secondary ones (sometimes by an order of magnitude). Due to the straightness of electron flight to the collector, information about separate areas, from which there is no direct path to the collector, is lost (shadows appear). The characteristic is isolated either by an X-ray crystalline or energy-dispersive sensor - a semiconductor detector (usually made of pure silicon doped with lithium). In the first case, X-ray quanta, after reflection by the spectrometer crystal, are recorded by a gas spectrometer, and in the second, the signal taken from a semiconductor is amplified by a low-noise one (which is cooled with liquid nitrogen to reduce noise) and a subsequent amplification system. The signal from the crystal modulates the CRT beam, and a picture of a particular chemical element in the object appears on the screen. SEMs also produce local X-rays. The energy-dispersive detector registers all elements from Na to U with high sensitivity. A crystal spectrometer, using a set of crystals with different interplanar values (see), covers from Be to U. A significant disadvantage of the SEM is the long duration of the process of “removing” information when studying objects. A relatively high PC can be obtained using an electron probe of a sufficiently small diameter. But at the same time, the probe decreases, as a result of which the influence sharply increases, reducing the ratio of the useful signal to noise. To ensure that the signal-to-noise ratio does not fall below a given level, it is necessary to slow down the scans to accumulate a sufficiently large number of primary (and corresponding secondary) at each point of the object. As a result, PC is implemented only at low scan rates. Sometimes one frame is formed within 10-15 minutes.
Rice. 6. Schematic diagram transmission scanning electron microscope (STEM): 1 - field emission cathode; 2 - intermediate anode; 3 - anode; 4 - deflection system for beam adjustment; 5 - “illuminator” diaphragm; 6, 8 - deflection systems for scanning the electronic probe; 7 - magnetic long-focus lens; 9 - aperture diaphragm; 10 - magnetic lens; 11 - object; 12, 14 - deflection systems; 13 - ring collector of scattered electrons; 15 - collector of unscattered electrons (removed when working with the spectrometer); 16 - magnetic spectrometer in which electron beams are rotated by a magnetic field by 90°; 17 - deflection system for selecting electrons with various energy losses; 18 - spectrometer slit; 19 - collector; SE - flow of secondary electrons hn - x-ray radiation.
SEM with field emission gun have high PC for SEM (up to 30 Å). In a field emission gun (as in) a tip-shaped cathode is used, at the top of which a strong wave appears, tearing electrons out of the cathode (see). The electron brightness of a gun with a field emission cathode is 10 3 -10 4 times higher than that of a gun with a hot cathode. Accordingly, the electron probe current increases. Therefore, in an SEM with a field emission gun, fast scans are carried out, and the probe is reduced to increase PC. However, the field emission cathode operates stably only in ultra-high vacuum (10 -9 -10 -11 mmHg), and this complicates the design of such SEMs and operation on them.
Transmission scanning electron microscope (STEM) have the same high PC as PEM. These devices use field emission guns, providing enough in a probe with a diameter of up to 2-3 Å. On rice. 6 A schematic representation of a PREM is shown. Two reduce the diameter of the probe. Below the object are located - central and ring. Unscattered electrons fall on the first one, and after amplification of the corresponding signals, the so-called bright field image. Scattered electrons are collected on a ring detector, creating the so-called. dark field image. In the STEM it is possible to study thicker objects than in the TEM, since the increase in the number of inelastically scattered objects with thickness does not affect the resolution (after the object there is no optics in the STEM). With the help of energy, electrons passing through an object are separated into elastically and inelastically scattered beams. Each beam hits its own detector, and a corresponding image is observed on the CRT, containing additional information about the scattering object. High resolution in a STEM is achieved with slow scans, since in a probe with a diameter of only 2-3 Å the current is too small.
Mixed type electron microscope. The combination in one device of the principles of image formation with a stationary beam (as in TEM) and scanning of a thin probe over an object made it possible to realize the advantages of TEM, SEM and STEM in such an electron microscope. Currently, all TEMs provide the ability to observe objects in raster mode (using condenser lenses and creating a reduced image that is scanned over the object by deflection systems). In addition to the image formed by a stationary beam, raster images are obtained on CRT screens using transmitted and secondary electrons, characteristic images, etc. The optical system of such a TEM, located after the object, makes it possible to operate in modes that are not feasible in other devices. For example, you can simultaneously observe on the CRT screen and an image of the same object on the device screen.
Emission E. m. create an image of an object in electrons, which the object itself emits when heated, by a primary beam, and when a strong electric field is applied, which pulls electrons out of the object. These devices usually have a narrow purpose.
Mirror Electron Microscope serve mainly to visualize the electrostatic “potential relief” and magnetic microfields on an object. The main optical element of the device is, and one of them is the object itself, which is under a slight negative potential relative to the gun cathode. The electron beam is directed into the mirror and reflected by the field in the immediate vicinity of the object. The mirror forms an image on the screen “in reflected beams”. Microfields near the surface of the object redistribute the electrons of the reflected beams, creating an image that visualizes these microfields.
Development prospects Electron microscope Increasing PC in images of non-periodic objects to 1 Å or more will make it possible to record not only heavy but also light atoms and visualize at the atomic level. To create an electron microscope with a similar resolution, the accelerating speed is increased. Ser. Physical", vol. 34, 1970; Hawks P., and, trans. from English, M., 1974; Derkach V.P., Kiyashko G.F., Kukharchuk M.S., Electronoprobe devices, K., 1974; Stoyanova I. G., Anaskin I. F., Physical foundations of transmission electron microscopy methods, M., 1972; Oatley S. W., The scanning electron microscope, Camb., 1972; Grivet P., Electron optics, 2 ed., Oxf., 1972.