Electron Microscope: Episode I

How does an electron microscope work? What is its difference from an optical microscope, is there any analogy between them?

The operation of an electron microscope is based on the property of inhomogeneous electric and magnetic fields, which have rotational symmetry, to have a focusing effect on electron beams. Thus, the role of lenses in an electron microscope is played by a set of appropriately calculated electric and magnetic fields; the corresponding devices that create these fields are called “electronic lenses”.

Depending on the type of electronic lenses electron microscopes are divided into magnetic, electrostatic and combined.

What type of objects can be examined using an electron microscope?

Just as in the case of an optical microscope, objects, firstly, can be “self-luminous,” that is, serve as a source of electrons. This is, for example, a heated cathode or an illuminated photoelectron cathode. Secondly, objects can be used that are “transparent” to electrons having a certain speed. In other words, when working in transmission, the objects must be thin enough and the electrons fast enough so that they pass through the objects and enter the electron lens system. In addition, by using reflected electron beams, the surfaces of massive objects (mainly metals and metallized samples) can be studied. This observation method is similar to reflective optical microscopy methods.

According to the nature of the study of objects, electron microscopes are divided into transmission, reflection, emission, raster, shadow and mirror.

The most common at present are transmission-type electromagnetic microscopes, in which the image is created by electrons passing through the object of observation. It consists of the following main components: a lighting system, an object camera, a focusing system and a final image recording unit, consisting of a camera and a fluorescent screen. All these nodes are connected to each other, forming a so-called microscope column, inside which pressure is maintained. The lighting system usually consists of a three-electrode electron gun (cathode, focusing electrode, anode) and a condenser lens (we are talking about electron lenses). It forms a beam of fast electrons of the required cross-section and intensity and directs it to the object under study located in the object chamber. A beam of electrons passing through an object enters a focusing (projection) system consisting of an objective lens and one or more projection lenses.

To study nanoobjects, the resolution of optical microscopes ( even using ultraviolet) is clearly not enough. In this regard, in the 1930s. The idea arose to use electrons instead of light, the wavelength of which, as we know from quantum physics, is hundreds of times shorter than that of photons.

As you know, our vision is based on the formation of an image of an object on the retina of the eye by light waves reflected from this object. If light passes through an optical system before entering the eye microscope, we see an enlarged image. In this case, the path of light rays is skillfully controlled by the lenses that make up the lens and eyepiece of the device.

But how can one obtain an image of an object, and with a much higher resolution, using not light radiation, but a flow of electrons? In other words, how is it possible to see objects using particles rather than waves?

The answer is very simple. It is known that the trajectory and speed of electrons are significantly influenced by external electromagnetic fields, with the help of which the movement of electrons can be effectively controlled.

The science of the movement of electrons in electromagnetic fields and the calculation of devices that form the necessary fields is called electron 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. Therefore, in an electron microscope, devices for focusing and scattering an electron beam are called “ electronic lenses”.

Electronic lens. The coils of wires carrying current focus the electron beam in the same way that a glass lens focuses a light beam.

The magnetic field of the coil acts as a converging or diverging lens. To concentrate the magnetic field, the coil is covered with a magnetic " armor» made of a special nickel-cobalt alloy, leaving only a narrow gap in the inner part. The magnetic field created in this way can be 10–100 thousand times stronger than the Earth’s magnetic field!

Unfortunately, our eyes cannot directly perceive electron beams. Therefore they are used for “ drawing” images on fluorescent screens (which glow when hit by electrons). By the way, the same principle underlies the operation of monitors and oscilloscopes.

There are a large number of different types of electron microscopes, among which the most popular is the scanning electron microscope (SEM). We will get its simplified diagram if we place the object under study inside the cathode ray tube of an ordinary TV between the screen and the source of electrons.

In such microscope a thin beam of electrons (beam diameter about 10 nm) runs around (as if scanning) the sample along horizontal lines, point by point, and synchronously transmits the signal to the kinescope. The whole process is similar to the operation of a TV during the scanning process. The source of electrons is a metal (usually tungsten), from which electrons are emitted when heated as a result of thermionic emission.

Scheme of operation of a scanning electron microscope

Thermionic emission– release of electrons from the surface of conductors. The number of electrons released is small at T=300K and increases exponentially with increasing temperature.

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 it. In some cases, secondary electrons are emitted, X-ray radiation is induced, etc. All these processes are recorded by special detectors and in a converted form are displayed on the screen, creating an enlarged picture of the object being studied.

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. Because the wavelength of an electron is orders of magnitude smaller than that of a photon, in modern SEMs this magnification can reach 10 million15, corresponding to a resolution of a few nanometers, which makes it possible to visualize individual atoms.

Main disadvantage electron microscopy– the need to work in complete vacuum, because the presence of any gas inside the microscope chamber can lead to ionization of its atoms and significantly distort the results. In addition, electrons have a destructive effect on biological objects, which makes them inapplicable for research in many areas of biotechnology.

History of creation electron microscope is a remarkable example of an achievement based on an interdisciplinary approach, when independently developing fields of science and technology came together to create a new powerful tool for scientific research.

The pinnacle of classical physics was the theory of the electromagnetic field, which explained the propagation of light, electricity and magnetism as the propagation of electromagnetic waves. Wave optics explained the phenomenon of diffraction, the mechanism of image formation, and the play of factors that determine resolution in a light microscope. Success quantum physics we owe the discovery of the electron with its specific particle-wave properties. These separate and seemingly independent development paths led to the creation of electron optics, one of the most important inventions of which was the electron microscope in the 1930s.

But the scientists did not rest on this either. The wavelength of an electron accelerated by an electric field is several nanometers. This is not bad if we want to see a molecule or even an atomic lattice. But how to look inside an atom? What is a chemical bond like? What does the process of a single chemical reaction look like? For this purpose, today scientists in different countries are developing neutron microscopes.

Neutrons are usually found in atomic nuclei along with protons and have almost 2000 times more mass than an electron. Those who have not forgotten de Broglie’s formula from the quantum chapter will immediately realize that the wavelength of a neutron is the same amount of times shorter, that is, it is picometers, thousandths of a nanometer! Then the atom will appear to researchers not as a blurry speck, but in all its glory.

Neutron microscope has many advantages - in particular, neutrons map hydrogen atoms well and easily penetrate thick layers of samples. However, it is also very difficult to build: neutrons do not have an electrical charge, so they easily ignore magnetic and electric fields and strive to elude sensors. In addition, it is not so easy to expel large, clumsy neutrons from atoms. Therefore, today the first prototypes of a neutron microscope are still very far from perfect.

ELECTRON MICROSCOPE- a device for observing and photographing a multiply (up to 10 6 times) enlarged image of an object, in which instead of light rays, accelerated to high energies (30-1000 keV or more) in deep conditions are used. Phys. fundamentals of corpuscular-beam optics. devices were founded in 1827, 1834-35 (almost a hundred years before the advent of electron microscopy) by W. R. Hamilton, who established the existence of an analogy between the passage of light rays in optically inhomogeneous media and the trajectories of particles in force fields . The feasibility of creating E. m. became obvious after the hypothesis about de Broglie waves was put forward in 1924, and technical. the prerequisites were created by H. Busch, who in 1926 investigated the focusing properties of axisymmetric fields and developed a magnetic field. electronic lens. In 1928, M. Knoll and E. Ruska began creating the first magnet. Transmission electron microscopy (TEM) and three years later obtained an image of the object formed by electron beams. In subsequent years, the first raster electron microscopy (SEM) were built, operating on the principle of scanning, that is, the sequential movement of a thin electron beam (probe) across an object from point to point. K ser. 1960s SEMs have reached high technology. perfection, and from that time on their widespread use in science began. research. PEMs have the highest resolution, surpassing light ones in this parameter microscopes in several thousand times. The resolution limit, which characterizes the ability of the device to separately image two maximally closely spaced details of an object, for TEM is 0.15-0.3 HM, i.e. it reaches a level that allows observing atomic and the molecular structure of the objects under study. Such high resolutions are achieved due to the extremely short wavelength of the electrons. Lenses of E. m. have aberrations, effective methods of correction for which have not been found, unlike a light microscope (see. Electronic and ion optics).Therefore, in TEM mag. electronic lenses(EL), in which aberrations are an order of magnitude smaller, have completely replaced electrostatic ones. Optimal aperture (see. Diaphragm in elec tronic and ion optics) it is possible to reduce the spherical. lens aberration affecting

on the resolution of E.M. The TEMs in use can be divided into three groups: high-resolution E.M., simplified TEMs, and unique ultra-high-resolution E.M.

High resolution TEM(0.15-0.3 nm) - universal multi-purpose devices. They are used to observe images of objects in a light and dark field, to study their electronographic structure. method (see Electronography), carrying out local quantities. using an energy spectrometer. losses of electrons and x-ray crystals. and semiconductor and obtaining spectroscopic. images of objects using a filter that filters out electrons with energies outside the specified energy. window. Energy losses of electrons passed through the filter and forming an image are caused by the presence of a single chemical in the object. element. Therefore, the contrast of areas in which this element is present increases. By moving the window along the energy. the spectrum receives the distributions of different elements contained in an object. The filter is also used as a monochromator to increase the resolution of electron microscopy when studying objects of large thickness, which increase the electron energy spread and (as a consequence) chromatic aberration.

With the help of additional devices and attachments, the object studied in TEM can be tilted in different planes at large angles to the optical lens. axis, heat, cool, deform. The electron-accelerating voltage in high-resolution emitters is 100–400 kV; it is regulated in steps and is highly stable: within 1–3 minutes, its value is not allowed to change by more than (1–2)·10 -6 from the initial value. The thickness of the object, which can be “illuminated” by an electron beam, depends on the accelerating voltage. In 100-kilovolt electromagnetic waves, objects with a thickness of 1 to several meters are studied. tens of nm.

A TEM of the described type is shown schematically in Fig. 1. In its electron-optical a deep vacuum is created in the system (column) using a vacuum system (pressure up to ~10 -5 Pa). Electro-optical circuit. The TEM system is shown in Fig. 2. A beam of electrons, the source of which is a thermionic cathode, is formed in electron gun and a high-voltage accelerator and then is focused twice by the first and second condensers, creating a small electronic “spot” on the object (when adjusted, the diameter of the spot can vary from 1 to 20 microns). After passing through the object, some of the electrons are scattered and delayed by the aperture diaphragm. Unscattered electrons pass through the aperture and are focused by the lens into the object plane of the intermediate electron lens. Here the first enlarged image is formed. Subsequent lenses create a second, third, etc. image. The last - projection - lens forms an image on a cathodoluminescent screen, which glows under the influence of electrons. The degree and nature of electron scattering are not the same at different points of the object, because thickness, structure and chemical. the composition of an object varies from point to point. Accordingly, the number of electrons passing through the aperture diaphragm changes, and, consequently, the current density in the image. An amplitude contrast arises, which is converted into light contrast on the screen. In the case of thin objects, it prevails phase contrast, caused by a change in phases scattered in the object and interfering in the image plane. Under the emulsion screen there is a magazine with photographic plates; when photographing, the screen is removed and the electrons act on the photoemulsion layer. The image is focused by the objective lens using a smooth current adjustment that changes its magnetic field. field. The currents of other electronic lenses regulate the magnification of the emitter, which is equal to the product of the magnifications of all lenses. At high magnifications, the brightness of the screen becomes insufficient and the image is observed using a brightness amplifier. To analyze an image, analog-to-digital conversion of the information contained in it and processing on a computer are performed. The image, enhanced and processed according to a given program, is displayed on a computer screen and, if necessary, entered into a storage device.

Rice. 1. Transmission electron microscope (PEM): 1 -electron gun with accelerator; 2-condenweed lenses; 3 -objective lens; 4 - projection lenses; 5 -light microscope, additionally zoomedreading the image observed on the screen; b-thatbeads with observation windows through which you can observegive an image; 7 - high voltage cable; 8 - vacuum system; 9 - Remote Control; 10 -stand; 11 - high-voltage power supply device; 12 - lens power supply.

Rice. 2. Electron-optical scheme of TEM: 1 -cathode; 2 - focusing cylinder; 3 -accelerator; 4 -pervy (short throw) condenser creating reduced image of the electron source; 5 - a second (long-focus) condenser, which transfers a reduced image of the source electrons per object; 6 -an object; 7 -aperture dialens aperture; 8 - lens; 9 , 10, 11 -system projection lenses; 12 - cathodoluminescent screen.

Simplified FEM intended for scientific studies that do not require high resolution. They are also used for pre-treatment. viewing objects, routine work and for educational purposes. These devices are simple in design (one condenser, 2-3 electronic lenses to magnify the image of an object), have a lower (60-100 kV) accelerating voltage and lower stability of high voltage and lens currents. Their resolution is 0.5-0.7 nm.

Ultra-high voltage E. m . (SVEM) - devices with accelerating voltage from 1 to 3.5 MB - are large-sized structures with a height of 5 to 15 m. They are equipped with special equipment. premises or construct separate buildings that are an integral part of the SVEM complex. The first SVEMs were intended for studying objects of large (1-10 microns) thickness, which preserved the properties of a massive solid body. Due to the strong influence of chromatic aberrations, the resolution of such E. m. decreases. However, compared with 100-kilovolt electron microscopes, the resolution of images of thick objects in ultraviolet electron microscopy is 10–20 times higher. Since the energy of electrons in an SVEM is greater, their wavelength is shorter than in a high-resolution TEM. Therefore, after solving complex technical problems (this took more than a decade) and the implementation of high vibration resistance, reliable vibration isolation and sufficient mechanical and electric Stability on the UVEM was achieved at the highest resolution (0.13-0.17 nm) for translucent electron microscopy, which made it possible to photograph images of atomic structures. However, spherical Lens aberration and defocus distort images taken at extreme resolution and interfere with obtaining reliable information. This information barrier is overcome with the help of focal series of images, which are obtained by dif. defocusing the lens. In parallel, for the same defocusing, a computer simulation of the atomic structure being studied is carried out. Comparison of focal series with series of model images helps to decipher micrographs of atomic structures taken with ultraviolet electron microscopy with extreme resolution. In Fig. Figure 3 shows a diagram of a SVEM located in a special building. Basic The components of the device are combined into a single complex using a platform, the edges of which are suspended from the ceiling on four chains and shock-absorbing springs. On top of the platform there are two tanks filled with electrical insulating gas under a pressure of 3-5 atm. A high-voltage generator is placed in one of them, and an electrostatic generator in the other. electron accelerator with electron gun. Both tanks are connected by a pipe, through which high voltage from the generator is transmitted to the accelerator. The electron-optical unit is adjacent to the tank with the accelerator from below. a column located in the lower part of the building, protected by a ceiling from x-rays. radiation generated in the accelerator. All of the listed nodes form a rigid structure that has physical properties. pendulum with a large (up to 7 s) period of its own. , which are damped by liquid dampers. The pendulum suspension system provides effective isolation of the SVEM from the outside. vibrations The device is controlled from a remote control located near the column. The design of lenses, columns and other components of the device is similar to the corresponding FEM devices and differs from them in larger dimensions and weight.

Rice. 3. Ultra-high voltage electron microscope (SVEM): 1-vibration-isolating platform; 2-chain, on which the platform hangs; 3 - shock-absorbing springs; 4-tanks containing the generatorhigh voltage and electron accelerator with electronnoah gun; 5-electron-optical column; 6- ceiling dividing the SVEM building into the upper and lower halls and protecting personnel working lower hall, from x-ray radiation; 7 - remote control microscope control.

Raster E. m. (SEM) with a thermionic gun - the most common type of device in electron microscopy. They use tungsten and hexaboride-lanthanum thermal cathodes. The resolution of the SEM depends on the electronic brightness of the gun and in devices of the class under consideration is 5-10 nm. The accelerating voltage is adjustable from 1 to 30-50 kV. The SEM device is shown in Fig. 4. Using two or three electron lenses, a narrow electron probe is focused onto the surface of the sample. Magn. deflection coils deploy the probe over a given area of ​​the object. When electrons of the probe interact with an object, several types of radiation arise (Fig. 5): secondary and reflected electrons; Auger electrons; x-ray bremsstrahlung and characteristic radiation (see Characteristic spectrum); light radiation, etc. Any of the radiation, currents of electrons passing through the object (if it is thin) and absorbed in the object, as well as the voltage induced on the object, can be recorded by appropriate detectors that convert these radiations, currents and voltages into electricity. signals, which, after amplification, are fed to a cathode ray tube (CRT) and modulate its beam. The scanning of the CRT beam is carried out synchronously with the 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 frame size on the CRT screen to the corresponding size on the scanned surface of the object. The image is photographed directly from the CRT screen. Basic The advantage of SEM is the high information content of the device, due to the ability to observe images using various signals. detectors. Using SEM, you can study the microrelief, the distribution of chemicals. composition for the object, p-n-transitions, produce x-rays. spectral analysis, etc. SEMs are widely used in technology. processes (monitoring in electronic lithographic technologies, checking and identifying defects in microcircuits, metrology of micro-products, etc.).

Rice. 4. Scheme of a scanning electron microscope (REM): 1 -electron gun insulator; 2 -V-imagethermal cathode; 3 -focusing electrode; 4 - anode; 5 - condenser lenses; 6 -diaphragm; 7 - two-tier deflection system; 8 -lens; 9 -lens aperture diaphragm; 10 -an object; 11 -secondary electron detector; 12 -crystallic spectrometer; 13 -proportional counter; 14 - pre-amplifier; 15 - amplification block; 16, 17 - equipment for registration x-ray radiation; 18 - amplification block; 19 - magnification control unit; 20, 21 - blocks burnzontal and vertical scans; 22, 23 -elekthrone ray tubes.

Rice. 5. Scheme for registering information about an object, obtained in SEM; 1-primary electron beam; 2-secondary electron detector; 3-rent detectorgen radiation; 4-reflected electron detectorronov; 5-Auger electron detector; 6-detector lightcommercial radiation; 7 - detector of transmitted electrodesnew; 8 - circuit for recording the current passing through electron object; 9-circuit for current recording electrons absorbed in the object; 10-scheme for reregistration of electrical energy induced at the object potential.

The high resolution of the SEM is achieved by image formation using secondary electrons. It is inversely related to the diameter of the zone from which these electrons are emitted. The size of the zone depends on the diameter of the probe, the properties of the object, the speed of the electrons of the primary beam, etc. With a large penetration depth of primary electrons, secondary processes developing in all directions increase the diameter of the zone and the resolution decreases. The secondary electron detector consists of photomultiplier tube(PMT) and electron-photon converter, main. the element of which is a scintillator. The number of scintillator flashes is proportional to the number of secondary electrons ejected at a given point of the object. After amplification in the PMT and video amplifier, the signal is modulated by the CRT beam. The magnitude of the signal depends on the topography of the sample, the presence of local electric currents. and mag. microfields, coefficient values. secondary electron emission, which, in turn, depends on the chemical. composition of the sample at a given point.

The reflected electrons are captured by a semiconductor detector with p - n-transition. The contrast of the image is determined by the dependence of the coefficient. reflection from the angle of incidence of the primary beam at a given point of the object and from at. substance numbers. The resolution of the image obtained in “reflected electrons” is lower than that obtained using secondary electrons (sometimes by an order of magnitude). Due to the straightness of the flight of electrons, information about the department. areas of the object from which there is no direct path to the detector are lost (shadows appear). To eliminate loss of information, as well as to form an image of the relief of the sample, the cut is not affected by its elemental composition and, conversely, to form a picture of the distribution of chemicals. elements in an object, which is not affected by its topography, the SEM uses a detector system consisting of several. detectors placed around the object, the signals of which are subtracted from one another or summed, and the resulting signal, after amplification, is fed to the CRT modulator.

X-ray characteristic radiation is recorded crystalline. (wave-dispersive) or semiconductor (energy-dispersive) spectrometers, which complement each other. In the first case, x-ray. radiation, after reflection by the spectrometer crystal, enters the gas proportional counter, and in the second - x-ray. The quanta excite signals in a semiconductor cooled (to reduce noise) detector made of lithium-doped silicon or germanium. After amplification, the spectrometer signals can be fed to a CRT modulator and a picture of the distribution of a particular chemical will appear on its screen. element along the surface of the object.

On a SEM equipped with X-ray. spectrometers produce local quantities. analysis: the number of pulses excited by x-rays is recorded. quanta from the area where the electronic probe was stopped. Crystallic. spectrometer using a set of analyzer crystals with different. interplanar distances (see Bragg-Wulf condition)discriminates with a high spectrum. characteristic resolution spectrum by wavelength, covering the range of elements from Be to U. The semiconductor spectrometer discriminates x-rays. quanta by their energies and simultaneously registers all elements from B (or C) to U. Its spectral resolution is lower than that of crystalline ones. spectrometer, but higher sensitivity. There are other advantages: fast delivery of information, simple design, high performance characteristics.

Raster Auger-E. m. (ROEM) devices, in which, when scanning an electron probe, Auger electrons are detected from an object depth of no more than 0.1-2 nm. At this depth, the exit zone of Auger electrons does not increase (unlike secondary emission electrons) and the resolution of the device depends only on the diameter of the probe. The device operates at ultra-high vacuum (10 -7 -10 -8 Pa). Its accelerating voltage is approx. 10 kV. In Fig. 6 shows the ROEM device. The electron gun consists of a hexaboride-lanthanum or tungsten thermal cathode operating in the Schottky mode, and a three-electrode electrostatic one. lenses. The electron probe is focused by this lens and magnet. a lens in which the object is located in the focal plane. Auger electrons are collected using a cylindrical. a mirror energy analyzer, the internal electrode of which covers the lens body, and the external electrode is adjacent to the object. Using an analyzer that discriminates Auger electrons by energy, the chemical distribution is studied. elements in the surface layer of an object with submicron resolution. To study deep layers, the device is equipped with an ion gun, which is used to remove the upper layers of an object using the method of ion-beam etching.

Rice. b. Scheme of a scanning Auger electron microscope(ROEM): 1 - ion pump; 2- cathode; 3 - three-electrode electrostatic lens; 4-channel detector; 5-aperture lens; 6-bunk deflection system for scanning the electronic probe; 7-lens; 8- outer cylindrical electrode mirror analyzer; 9-object.

SEM with field emission gun have high resolution (up to 2-3 nm). A field emission gun uses a tip-shaped cathode, at the top of which a strong electric shock occurs. field that removes electrons from the cathode ( auto-electronic emissions). The electron brightness of a gun with a field emission cathode is 10 3 -10 4 times higher than the brightness of a gun with a thermionic cathode. Accordingly, the electron probe current increases. Therefore, in an SEM with a field emission gun, fast scanning is carried out along with slow scanning, and the diameter of the probe is reduced to increase resolution. However, the field emission cathode operates stably only in ultra-high vacuum (10 -7 -10 -9 Pa), which complicates the design and operation of such SEMs.

Translucent raster E. m. (STEM) have the same high resolution as TEM. These devices use field emission guns operating under ultra-high vacuum conditions (up to 10 -8 Pa), providing sufficient current in a small-diameter probe (0.2-0.3 nm). The diameter of the probe is reduced by two magnets. lenses (Fig. 7). Below the object there are detectors - central and ring. The first one receives unscattered electrons, and after converting and amplifying the corresponding signals, a bright-field image appears on the CRT screen. The ring detector collects scattered electrons, creating a dark-field image. In a STEM it is possible to study thicker objects than in a TEM, since the increase in the number of inelastically scattered electrons with thickness does not affect the resolution (after the object there is no electron optics for image formation). Using an energy analyzer, electrons passing through an object are separated into elastically and inelastically scattered beams. Each beam hits its own detector, and corresponding images containing complementary images are observed on the CRT. information about the elemental composition of the object. High resolution in a STEM is achieved with slow scans, since in a probe with a diameter of only 0.2-0.3 nm the current is small. PREMs are equipped with all analytical devices used in electron microscopy. research of objects, and in particular energy spectrometers. electron losses, x-ray spectrometers, complex systems for detecting transmitted, backscattered and secondary electrons, highlighting groups of electrons scattered on various. angles that have different angles energy, etc. The devices are equipped with a computer for complex processing of incoming information.

Rice. 7. Schematic diagram of a translucent rasternew electron microscope (STEM): 1-autoemision cathode; 2-intermediate anode; 3- anode; 4- "illuminator" aperture; 5-magnetic lens; 6-twotiered deflection system for electron scanningnogo probe; 7-magnetic lens; 8 - aperture lens aperture; 9 -object; 10 - deflection system; 11 - ring detector of scattered electrons; 12 - detector of unscattered electrons (removed when operation of a magnetic spectrometer); 13 - magnetic spectrometer; 14-deflection system for selection electrons with various energy losses; 15 - slot spectrometer; 16-detector spectrometer; VE-secondaryny electrons; hv- X-ray radiation.

Emission E. m. create an image of an object with electrons, which are emitted by the object itself when heated, bombarded with a primary beam of electrons, under the influence of electric magnets. radiation and when a strong electric current is applied. field that strips electrons from an object. These devices usually have a narrow purpose (see. Electronic projector).

Mirror E. m. serve as ch. arr. for visualization of electrostatic "potential reliefs" and magnetic microfields on the surface of an object. Basic electron-optical element of the device is electronic mirror, and one of the electrodes is the object itself, which is under slight negative pressure. potential relative to the gun cathode. An electron beam is directed into an electron mirror and reflected by a field in the immediate vicinity of the object's surface. 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 a contrast in the image that visualizes these microfields.

Prospects for the development of E. m. The improvement of electronic meters in order to increase the volume of information obtained, which has been carried out for many years, will continue in the future, and improving the parameters of instruments, and above all increasing the resolution, will remain the main task. Work on the creation of electron-optical devices. systems with small aberrations have not yet led to a real increase in the resolution of emitters. This applies to non-axisymmetric aberration correction systems, cryogenic optics, and lenses with corrective spaces. in the axial region, etc. Searches and research in these directions are underway. Exploration work continues to create electronic holographic images. systems, including those with correction of frequency-contrast characteristics of lenses. Miniaturization of electrostatic lenses and systems using advances in micro- and nanotechnology will also help solve the problem of creating electronic optics with low aberrations.

Lit.: Practical scanning electron microscopy, ed. D. Gouldstein, X. Yakovits, trans. from English, M., 1978; Spence D., Experimental high-resolution electron microscopy, trans. from English, M., 1986; Stoyanov P. A., Electron microscope SVEM-1, "Izvestia of the USSR Academy of Sciences, ser. physics.", 1988, v. 52, no. 7, p. 1429; Hawks P., Kasper E., Fundamentals of electron optics, trans. from English, t. 1-2, M., 1993; Oechsner H., Scanning auger microscopy, Le Vide, les Couches Minces, 1994, t. 50, no. 271, p. 141; McMullan D., Scanning electron microscopy 1928-1965, "Scanning", 1995, t. 17, no. 3, p. 175. P. A. Stoyanov.

ELECTRON MICROSCOPE- a high-voltage, vacuum device in which a magnified image of an object is obtained using a flow of electrons. Designed for research and photographing objects at high magnifications. Electron microscopes have high resolution. Electron microscopes are widely used in science, technology, biology and medicine.

Based on the principle of operation, transmission (transmission), scanning, (raster) and combined electron microscopes are distinguished. The latter can operate in transmission, scanning or in two modes simultaneously.

The domestic industry began producing transmission electron microscopes in the late 40s of the 20th century. The need to create an electron microscope was caused by the low resolution of light microscopes. To increase the resolution, a shorter wavelength radiation source was required. The solution to the problem became possible only with the use of an electron beam as an illuminator. The wavelength of a flow of electrons accelerated in an electric field with a potential difference of 50,000 V is 0.005 nm. Currently, a resolution of 0.01 nm for gold films has been achieved on a transmission electron microscope.

Diagram of a transmission electron microscope: 1 - electron gun; 2 - condenser lenses; 3 - lens; 4 - projection lenses; 5 - tube with viewing windows through which you can observe the image; 6 - high-voltage cable; 7 - vacuum system; 8 - control panel; 9 - stand; 10 - high-voltage power supply device; 11 - power supply for electromagnetic lenses.

The schematic diagram of a transmission electron microscope is not much different from the diagram of a light microscope (see). The beam path and basic design elements of both microscopes are similar. Despite the wide variety of electron microscopes produced, they are all built according to the same scheme. The main design element of a transmission electron microscope is a microscope column, consisting of an electron source (electron gun), a set of electromagnetic lenses, a stage with an object holder, a fluorescent screen and a photorecording device (see diagram). All structural elements of the microscope column are assembled hermetically. A system of vacuum pumps in the column creates a deep vacuum for the unhindered passage of electrons and protects the sample from destruction.

The flow of electrons is generated in a microscope gun, built on the principle of a three-electrode lamp (cathode, anode, control electrode). As a result of thermal emission, electrons are released from a heated V-shaped tungsten cathode, which are accelerated to high energies in an electric field with a potential difference from several tens to several hundred kilovolts. Through a hole in the anode, a stream of electrons rushes into the lumen of the electromagnetic lenses.

Along with tungsten thermionic cathodes, electron microscopes use rod and field emission cathodes, which provide a significantly higher electron beam density. However, for their operation a vacuum of at least 10^-7 mmHg is required. Art., which creates additional design and operational difficulties.

Another main element of the microscope column design is the electromagnetic lens, which is a coil with a large number of turns of thin copper wire, placed in a soft iron shell. When electric current passes through the winding of the lens, an electromagnetic field is formed in it, the lines of force of which are concentrated in the internal annular rupture of the shell. To enhance the magnetic field, a pole piece is placed in the discontinuity area, which makes it possible to obtain a powerful, symmetrical field with minimal current in the lens winding. The disadvantage of electromagnetic lenses is various aberrations that affect the resolution of the microscope. The most important is astigmatism, caused by the asymmetry of the magnetic field of the lens. To eliminate it, mechanical and electrical stigmators are used.

The task of dual condenser lenses, like the condenser of a light microscope, is to change the illumination of an object by changing the electron flux density. The diaphragm of the condenser lens with a diameter of 40-80 microns selects the central, most homogeneous part of the electron mass. The objective lens is the shortest focal length lens with a powerful magnetic field. Its task is to focus and initially increase the angle of motion of electrons passing through an object. The resolving power of the microscope largely depends on the quality of workmanship and the uniformity of the material of the pole piece of the objective lens. In the intermediate and projection lenses, the angle of electron motion further increases.

Special requirements are placed on the quality of manufacturing of the object stage and object holder, since they must not only move and tilt the sample in given directions at high magnification, but also, if necessary, subject it to stretching, heating or cooling.

A rather complex electronic-mechanical device is the photorecording part of the microscope, which allows automatic exposure, replacement of photographic material, and recording of the necessary microscopy modes on it.

Unlike a light microscope, the object of study in a transmission electron microscope is mounted on thin grids made of non-magnetic material (copper, palladium, platinum, gold). A substrate film made of collodion, formvar or carbon with a thickness of several tens of nanometers is attached to the grids, then a material is applied that is subjected to microscopic examination. The interaction of incident electrons with sample atoms leads to a change in the direction of their movement, deflection at small angles, reflection or complete absorption. Only those electrons that were deflected by the sample substance at small angles and were able to pass through the aperture diaphragm of the objective lens take part in the formation of an image on a luminescent screen or photographic material. The image contrast depends on the presence of heavy atoms in the sample, which strongly influence the direction of electron motion. To enhance the contrast of biological objects, constructed mainly from light elements, various contrast methods are used (see Electron microscopy).

A transmission electron microscope provides the ability to obtain a dark-field image of a sample when illuminated by an inclined beam of electrons. In this case, electrons scattered by the sample pass through the aperture diaphragm. Dark-field microscopy increases image contrast while resolving sample details at high resolution. The transmission electron microscope also provides a microdiffraction mode for minimal crystals. The transition from bright-field to dark-field mode and microdiffraction does not require significant changes in the microscope design.

In a scanning electron microscope, a stream of electrons is generated by a high-voltage gun. Using dual condenser lenses, a thin beam of electrons (electron probe) is obtained. By means of deflection coils, the electron probe is deployed on the surface of the sample, causing radiation. The scanning system in a scanning electron microscope is similar to the system that produces television images. The interaction of the electron beam with the sample leads to the appearance of scattered electrons that have lost some of their energy when interacting with the atoms of the sample. To construct a three-dimensional image in a scanning electron microscope, electrons are collected by a special detector, amplified and fed to a scanning generator. The number of reflected and secondary electrons at each individual point depends on the relief and chemical composition of the sample; the brightness and contrast of the image of the object on the kinescope changes accordingly. The resolution of a scanning electron microscope reaches 3 nm, magnification - 300,000. The deep vacuum in the column of a scanning electron microscope requires the mandatory dehydration of biological samples using organic solvents or their lyophilization from a frozen state.

A combined electron microscope can be created on the basis of a transmission or scanning electron microscope. Using a combined electron microscope, you can simultaneously study a sample in transmission and scanning modes. In a combined electron microscope, as in a scanning microscope, the possibility is provided for X-ray diffraction and energy dispersive analysis of the chemical composition of an object’s substance, as well as for optical-structural machine analysis of images.

To increase the efficiency of using all types of electron microscopes, systems have been created that allow converting an electron microscopic image into digital form with subsequent processing of this information on a computer. Optical-structural machine analysis allows for statistical analysis of the image directly from the microscope, bypassing the traditional “negative-print” method.

Bibliography: Stoyanova I. G. and Anaskin I. F. Physical foundations of transmission electron microscopy methods, M., 1972; Suvorov A. L. Microscopy in science and technology, M., 1981; Finean J. Biological ultrastructures, trans. from English, M., 1970; Schimmel G. Technique of electron microscopy, trans. with him.. M., 1972. See also bibliogr. to Art. Electron microscopy.

Electron microscopes appeared in the 1930s and came into widespread use in the 1950s.

The picture shows a modern transmission (transparent) electron microscope, and the figure shows the path of the electron beam in this microscope. In a transmission electron microscope, electrons pass through the sample before an image is formed. Such an electron microscope was the first to be constructed.

Electron microscope turned upside down compared to a light microscope. Radiation is applied to the sample from above, and an image is formed at the bottom. The operating principle of an electron microscope is essentially the same as that of a light microscope. The electron beam is directed by condenser lenses onto the sample, and the resulting image is then magnified using other lenses.

The table summarizes some of the similarities and differences between light and electron microscopes. At the top of the electron microscope column there is a source of electrons - a tungsten filament, similar to that found in a regular light bulb. A high voltage (eg 50,000 V) is applied to it and the filament emits a stream of electrons. Electromagnets focus the electron beam.

A deep vacuum is created inside the column. This is necessary in order to minimize dispersion electrons due to their collision with air particles. Only very thin sections or particles can be used for examination in an electron microscope, since the electron beam is almost completely absorbed by larger objects. Parts of the object that are relatively denser absorb electrons and therefore appear darker in the resulting image. Heavy metals such as lead and uranium are used to stain the sample to increase contrast.

Electrons invisible to the human eye, so they are directed to a fluorescent one, which reproduces a visible (black and white) image. To take a photograph, the screen is removed and electrons are directed directly onto the film. A photograph taken in an electron microscope is called an electron microphotography.

Advantage of electron microscope:
1) high resolution (0.5 nm in practice)

Disadvantages of the electron microscope:
1) the material prepared for research must be dead, since during the observation process it is in a vacuum;
2) it is difficult to be sure that the object reproduces a living cell in all its details, since fixation and staining of the material under study can change or damage its structure;
3) the electron microscope itself and its maintenance are expensive;
4) preparing material for working with a microscope is time-consuming and requires highly qualified personnel;
5) the samples under study are gradually destroyed under the action of an electron beam. Therefore, if a detailed study of a sample is required, it is necessary to photograph it.