Electron microscope in the garage. The principle of operation of the electron microscope

AT modern world The microscope is considered an indispensable optical device. Without it, it is difficult to imagine such areas of human activity as biology, medicine, chemistry, space research, and genetic engineering.


Microscopes are used to study a wide variety of objects and allow the smallest details consider structures that are invisible to the naked eye. To whom does humanity owe the appearance of this useful device? Who invented the microscope and when?

When did the first microscope appear?

The history of the device is rooted in ancient times. The ability of curved surfaces to reflect and refract sunlight was noticed as early as the 3rd century BC by the explorer Euclid. In his works, the scientist found an explanation for the visual increase in objects, but then his discovery did not find practical application.

The earliest information about microscopes dates back to XVIII century. In 1590, the Dutch craftsman Zachary Jansen placed two lenses from glasses in one tube and was able to see objects magnified from 5 to 10 times.


Later, the famous explorer Galileo Galilei invented a telescope and drew attention to interesting feature: if it is greatly pushed apart, then small objects can be significantly enlarged.

Who built the first model of an optical device?

A real scientific and technological breakthrough in the development of the microscope occurred in the 17th century. In 1619, the Dutch inventor Cornelius Drebbel invented a microscope with convex lenses, and at the end of the century, another Dutchman, Christian Huygens, presented his model, in which eyepieces could be adjusted.

A more advanced device was invented by the inventor Anthony Van Leeuwenhoek, who created a device with one large lens. Over the next century and a half, this product gave the highest image quality, so Leeuwenhoek is often called the inventor of the microscope.

Who invented the first compound microscope?

There is an opinion that the optical device was not invented by Leeuwenhoek, but by Robert Hooke, who in 1661 improved Huygens' model by adding an additional lens to it. The resulting type of device became one of the most popular in the scientific community and was widely used until mid-eighteenth centuries.


In the future, many inventors put their hand in the development of the microscope. In 1863, Henry Sorby came up with a polarizing device that made it possible to study, and in the 1870s, Ernst Abbe developed the theory of microscopes and discovered the dimensionless quantity "Abbe number", which contributed to the manufacture of more advanced optical equipment.

Who is the inventor of the electron microscope?

In 1931, scientist Robert Rudenberg patented new device, which could magnify objects using electron beams. The device was called an electron microscope and found wide application in many sciences due to high resolution, thousands of times greater than conventional optics.

A year later, Ernst Ruska created a prototype of a modern electronic device, for which he was awarded the Nobel Prize. Already in the late 1930s, his invention began to be widely used in scientific research. At the same time, Siemens began producing electron microscopes for commercial use.

Who is the author of the nanoscope?

The most innovative type of optical microscope today is the nanoscope, developed in 2006 by a group of scientists led by the German inventor Stefan Hell.


The new device allows not only to overcome the barrier of the Abbe number, but also provides an opportunity to observe objects with dimensions of 10 nanometers or less. In addition, the device provides high-quality three-dimensional images of objects, which was previously inaccessible to conventional microscopes.

A transmission electron microscope is a device for obtaining an enlarged image of microscopic objects, which uses electron beams. Electron microscopes have a higher resolution than optical microscopes and can also be used to obtain additional information regarding the material and structure of an object.
The first electron microscope was built in 1931 by German engineers Ernst Ruska and Max Stem. Ernst Ruska received for this discovery Nobel Prize in physics in 1986. He shared it with the inventors of the tunneling microscope because the Nobel Committee felt that the inventors of the electron microscope had been unfairly forgotten.
In an electron microscope, focused beams of electrons are used to obtain an image, with which the surface of the object under study is bombarded. Image can be observed different ways- in the rays that have passed through the object, in the reflected rays, registering secondary electrons or X-rays. Electron beam focusing using special electronic lenses.
Electron microscopes can magnify an image 2 million times. The high resolution of electron microscopes is achieved due to the short wavelength of the electron. While the wavelength visible light lies in the range from 400 to 800 nm, the wavelength of an electron accelerated in a potential of 150 V is 0.1 nm. Thus, electron microscopes can practically examine objects the size of an atom, although this is difficult to implement in practice.
Schematic structure of an electron microscope The structure of an electron microscope can be considered using the example of a transmission device. A monochromatic electron beam is formed in an electron gun. Its performance is enhanced by a condenser system consisting of a condenser diaphragm and electronic lenses. Depending on the type of lens, magnetic or electrostatic, a distinction is made between magnetic and electrostatic microscopes. Later, the beam hits the object, scattering on it. The scattered beam passes through the aperture and enters the objective lens, which is designed to stretch the image. The stretched electron beam causes the phosphor to glow on the screen. Modern microscopes use several degrees of magnification.
The aperture diaphragm of the objective of an electron microscope is very small, being hundredths of a millimeter.
If a beam of electrons from an object hits the screen directly, then the object will look dark on it, and a light background will form around it. Such an image is called svitlopolnym. If, however, it is not the base beam that enters the aperture of the objective lens, but the scattered one, then darkfield Images. A dark-field image has more contrast than a bright-field image, but its resolution is lower.
There are many various types and designs of electron microscopes. The main ones among them are:

A transmission electron microscope is a device in which an electron beam shines through an object.

Scanning transmission electron microscope allows you to study individual parts of the object.

A scanning electron microscope uses secondary electrons knocked out by an electron beam to study the surface of an object.

The reflective electron microscope uses elastically scattered electrons.

An electron microscope can also be equipped with a system for detecting X-rays, which emit highly excited atoms of matter when colliding with high-energy electrons. When an electron is knocked out of the inner electron shells, characteristic X-ray radiation is formed, by examining which it is possible to establish the chemical composition of the material.
The study of the spectrum of inelastic-scattered electrons makes it possible to obtain information about the characteristic electronic excitations in the material of the object under study.
Electron microscopes are widely used in physics, materials science, and biology.

Yesterday I photographed a white Audi. It turned out a great photo of the audi from the side. It is a pity that the tuning is not visible in the photo.

ELECTRONIC 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 are used, accelerated to high energies (30-1000 keV or more) in deep conditions. Phys. fundamentals of corpuscular-beam optical. devices were laid down in 1827, 1834-35 (almost a hundred years before the advent of electromagnetic materials) 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 expediency of creating E. m. became obvious after the nomination in 1924 of the hypothesis of de Broglie waves, and tehn. prerequisites were created by H. Busch, who in 1926 studied the focusing properties of axisymmetric fields and developed a magnetic field. electronic lens. In 1928, M. Knoll and E. Ruska set about creating the first magn. translucent E. m. (TEM) and three years later received an image of the object, formed by electron beams. In subsequent years, the first raster electron beams (SEMs) were built, operating on the principle of scanning, i.e., moving a thin electron beam (probe) over an object sequentially from point to point. K ser. 1960s REM have reached a high tech. perfection, and from that time began their widespread use in scientific. research. TEMs have the highest resolution, exceeding in this parameter the light microscopes in several thousand times. The resolution limit, which characterizes the ability of the device to display separately two as close as possible details of an object, is 0.15-0.3 HM for TEM, i.e., it reaches a level that allows one to observe the atomic and molecular structure of the studied objects. Such high resolutions are achieved due to the extremely short wavelength of electrons. The lenses of E. m. have aberrations, effective methods correction to-rykh was not found, in contrast to the light microscope (see. Electronic and ion optics). Therefore, in the TEM magn. electronic lenses(EL), for which the aberrations are an order of magnitude smaller, completely replaced the electrostatic ones. Optimum aperture (see. Diaphragm in electronic and ion optics), it is possible to reduce the spherical. lens aberration affecting

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

high resolution TEM(0.15-0.3 nm) - universal devices multi-purpose. They are used to observe the image of objects in a bright and dark field, to study their structure by electro-nographic. method (see Electronography), carrying out local quantities. using an energy spectrometer. loss 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. The energy loss of electrons passed through the filter and forming an image is 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 energetic spectrum receive distribution decomp. the elements contained in the object. The filter is also used as a monochromator to increase the resolution of electromagnetic meters in the study of thick objects, which increase the energy spread of electrons and (as a consequence) chromatic aberration.

With the help of add. devices and attachments, the object studied in TEM can be tilted in different planes at large angles to the optical. axis, heat, cool, deform. The electron-accelerating voltage in high-resolution electromagnetic meters is 100-400 kV, it is regulated stepwise and is highly stable: in 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 "enlightened" by the electron beam, depends on the accelerating voltage. In 100-kilovolt E. m. study objects with a thickness of 1 to several. tens of nm.

Schematically, a TEM of the described type is shown in Fig. 1. In his electron-optical. system (column) with the help of a vacuum system creates a deep vacuum (pressure up to ~ 10 -5 Pa). Scheme of electron-optical. TEM system is shown in fig. 2. An electron beam, the source of which is a thermal cathode, is formed in electron gun and a high-voltage accelerator, and then it is focused twice by the first and second condensers, which create a small-sized electronic "spot" on the object (with adjustment, the spot diameter can vary from 1 to 20 μm). After passing through the object, some of the electrons are scattered and retained by the aperture diaphragm. Unscattered electrons pass through the diaphragm opening and are focused by the objective in 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, since the thickness, structure and chem. the composition of the object varies from point to point. Accordingly, the number of electrons passing through the aperture diaphragm changes, and hence the current density in the image. There is an amplitude contrast, which is converted into light contrast on the screen. In the case of thin objects prevails phase contrast, caused by a change in phases scattered in the object and interfering in the image plane. A magazine with photographic plates is located under the E. M. screen; when photographing, the screen is removed and the electrons act on the photoemulsion layer. The image is focused by an objective lens using a smooth adjustment of the current, which changes its magn. field. The currents of other electronic lenses regulate the increase in E. m., 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 the 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 memory device.

Rice. 1. Transmission type electron microscope (PEM): 1 - electron gun with an accelerator; 2-condenweed lenses; 3 -objective lens; 4 - projection lenses; 5 - light microscope, additionally magnifiedadjusting the image observed on the screen; b-thatbeads with viewing 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; 12 - lens power supply.

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

Simplified TEM designed for scientific studies, in which high resolution is not required. They are also used for pre- 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 the 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.

UHV E. m. (SVEM) - devices with an accelerating voltage of 1 to 3.5 MB - are large structures with a height of 5 to 15 m. Special equipment is equipped for them. premises or build separate buildings that are integral part complex SVEM. The first SVMs were designed to study objects of large (1–10 µm) thickness, with which the properties of a massive solid body. Due to the strong influence of chromatic aberrations, the resolution of such E. m. is reduced. However, compared with 100-kilovolt E. m., the resolution of the image of thick objects in SVEM is 10-20 times higher. Since the energy of electrons in UHEM is greater, their wavelength is shorter than in high-resolution TEM. Therefore, after solving complex technical. problems (it took more than one decade) and the implementation of high vibration resistance, reliable vibration isolation and sufficient mechanical. and electric stability, the highest resolution (0.13-0.17 nm) for translucent electromagnetic meters was achieved, which made it possible to photograph images of atomic structures. However, spherical aberration and defocusing of the lens distort the images obtained with the maximum resolution, and interfere with obtaining reliable information. This informational barrier is overcome with the help of focal series of images, to-rye obtained with decomp. lens defocus. Simultaneously, for the same defocusings, the atomic structure under study is simulated on a computer. Comparison of focal series with series of model images helps to decipher the microphotographs of atomic structures taken with UHEM with the highest resolution. On fig. 3 shows a diagram of the SVEM located in the special. building. Main the components of the device are combined into a single complex using a platform, which is suspended from the ceiling on four chains and shock-absorbing springs. On top of the platform there are two tanks filled with electrically insulating gas at a pressure of 3-5 atm. A high-voltage generator is placed in one of them, and an electrostatic generator is placed in the other. electron accelerator with electron gun. Both tanks are connected by a branch pipe, through which high voltage from the generator is transferred to the accelerator. From the bottom to the tank with the accelerator adjoins the electron-optical. a column located in the lower part of the building, protected from X-ray by a ceiling. radiation generated in the accelerator. All of these nodes form a rigid structure that has the properties of physical. a pendulum with a large (up to 7 s) period of its own. , which are extinguished by liquid dampers. The pendulum suspension system provides effective isolation of the SVEM from the external. vibrations. The device is controlled from a remote control located near the column. The arrangement of lenses, columns, and other units of the device is similar to the corresponding TEM devices and differs from them in large dimensions and weight.

Rice. 3. Ultrahigh voltage electron microscope (SVEM): 1-vibration isolation platform; 2-chains, on which the platform hangs; 3 - shock-absorbing springs; 4-tanks in which the generator is locatedhigh voltage and electron accelerator with electronnoah gun; 5-electron-optical column; 6- ceiling separating the SVEM building into the upper and lower halls and protecting personnel working lower hall, from x-rays; 7 - remote control microscope control.

Raster E. m. (SEM) with a thermionic gun - the most common type of devices in electron microscopy. They use tungsten and hexaboride-lanthanum thermal cathodes. The resolution of the SEM depends on the electron brightness of the gun and in devices of this class 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 sample surface. Magn. deflection coils deploy the probe over a given area on the object. When the probe electrons interact with the 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 radiations, the currents of electrons that have passed 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 the corresponding detectors that convert these radiations, currents and voltages into electric. signals, to-rye, after amplification, are fed to a cathode ray tube (CRT) and modulate its beam. The CRT beam is scanned 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. Photograph the image directly from the CRT screen. Main The advantage of SEM is the high information content of the device, due to the ability to observe images using signals decomp. detectors. Using SEM, you can explore the microrelief, the distribution of chemical. composition for the object, pn-transitions, produce x-rays. spectral analysis etc. SEM are widely used in technol. processes (control in electronic-lithographic technologies, testing and detection of defects in microcircuits, metrology of micro-products, etc.).

Rice. 4. Diagram 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 deflecting system; 8 -lens; 9 - aperture diaphragm of the lens; 10 -an object; 11 -detector of secondary electrons; 12 -crystalpersonal spectrometer; 13 -proportional counter; 14 - preamplifier; 15 - amplification block; 16, 17 - registration equipment x-ray radiation; 18 - amplification unit; 19 - magnification control unit; 20, 21 - burn blocksumbrella and vertical scans; 22, 23 -electhrone ray tubes.

Rice. 5. Scheme of registration of information about the object, received in SEM; 1-primary electron beam; 2-detector of secondary electrons; 3-rent detectorgene radiation; 4-detector of reflected electronsronov; 5-detector of Auger electrons; 6-light detectornew radiation; 7 - detector of passed electronew; 8 - circuit for registering the current passed through electron object; 9-circuit for current registration electrons absorbed in the object; 10-scheme for rehystration of the electrical potential.

The high resolution of the SEM is realized in the formation of an image 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 probe diameter, the properties of the object, the velocity of the primary beam electrons, etc. At a large penetration depth of the primary electrons, secondary processes developing in all directions increase the zone diameter and the resolution decreases. The secondary electron detector consists of photomultiplier(PMT) and electron-photonic converter, osn. an element to-rogo is the scintillator. The number of scintillator flashes is proportional to the number of secondary electrons knocked out at a given point of the object. After amplification in the PMT and in the video amplifier, the signal modulates the CRT beam. The magnitude of the signal depends on the topography of the sample, the presence of local electric. and magn. microfields, the magnitude of the coefficient. secondary electron emission, to-ry, in turn, depends on the chemical. sample composition at a given point.

Reflected electrons are captured by a semiconductor detector with p - n-transition. The contrast of the image is due to the dependence of the coefficient. reflections from the angle of incidence of the primary beam at a given point of the object and from at. substance number. The resolution of the image obtained in "reflected electrons" is lower than that obtained with the help of secondary electrons (sometimes by an order of magnitude). Due to the straightness of the flight of electrons, information about the sep. sections of the object, from direct way to the detector is not, it is lost (shadows appear). To eliminate information loss, as well as to form an image of the relief of the sample, its elemental composition does not affect the swarm and, conversely, to form a picture of the distribution of chemical. elements in the object, which is not affected by its relief, the SEM uses a detector system consisting of several. detectors placed around the object, the signals of which are subtracted from one another or added, and the resulting signal, after amplification, is fed to the CRT modulator.

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

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

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 such a depth, the exit zone of Auger electrons does not increase (in contrast to secondary emission electrons) and the instrument resolution depends only on the probe diameter. The device works at ultrahigh vacuum (10 -7 -10 -8 Pa). Its accelerating voltage is approx. 10 kV. On fig. 6 shows the ROEM device. The electron gun consists of a lanthanum hexaboride or tungsten thermal cathode operating in the Schottky mode and a three-electrode electrostatic. lenses. The electron probe is focused by this lens and the magnet. a lens in the focal plane to-rogo is an object. The collection of Auger electrons is carried out using a cylindrical. a mirror energy analyzer, the inner electrode of which covers the lens body, and the outer electrode adjoins the object. With the help of an analyzer that discriminates Auger electrons by energy, the distribution of chem. elements in the surface layer of the object with submicron resolution. To study the deep layers, the device is equipped with an ion gun, with the help of which the upper layers of the object are removed by 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 aperture; 6-double deflecting system for sweeping the electronic probe; 7-lens; 8- outer electrode cylindrical mirror analyzer; 9-object.

SEM with field emission gun have high resolution (up to 2-3 nm). The field emission gun uses a cathode in the form of a point, at the top of which a strong electric current occurs. field pulling electrons out of the cathode ( field emission). The electronic 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. Correspondingly, the current of the electron probe increases. Therefore, in a SEM with a field emission gun, along with a slow sweep, a fast sweep is carried out, and the probe diameter is reduced to increase the resolution. However, the field emission cathode operates stably only at ultrahigh 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 conditions of ultrahigh vacuum (up to 10 -8 Pa), providing sufficient current in a probe of small diameter (0.2-0.3 nm). The probe diameter is reduced by two magn. lenses (Fig. 7). Below the object are detectors - central and ring. Unscattered electrons fall on the first one, and after conversion and amplification of the corresponding signals, a bright-field image appears on the CRT screen. Scattered electrons are collected on the ring detector, creating a dark-field image. In STEM, one can study thicker objects than in TEM, since the increase in the number of inelastically scattered electrons with thickness does not affect the resolution (there is no electron optics for imaging after the object). Using an energy analyzer, the electrons that have passed through the object are separated into elastically and inelastically scattered beams. Each beam hits its own detector, and the corresponding images containing complements are observed on the CRT. information about the elemental composition of the object. High resolution in STEM is achieved with slow sweeps, because in a probe with a diameter of only 0.2–0.3 nm, the current is small. PREM are equipped with all devices used in electron microscopy for analytical. research objects, and in particular spectrometers energetic-tich. electron loss, x-ray spectrometers, complex systems for detecting transmitted, backscattered and secondary electrons that select groups of electrons scattered on decomp. angles having different energy, etc. The devices are equipped with a computer for the complex processing of incoming information.

Rice. 7. Schematic diagram of a translucent rasterelectron microscope (PREM): 1-auto-emissionion cathode; 2-intermediate anode; 3- anode; four- diaphragm "illuminator"; 5-magnetic lens; 6-twotiered deflection system for electron sweepleg probe; 7-magnetic lens; 8 - aperture lens aperture; 9 - object; 10 - deflecting system; 11 - ring detector of scattered electrons; 12 - detector of unscattered electrons (removed when operation of the magnetic spectrometer); 13 - magnetic spectrometer; 14-deflecting system for selection electrons with various losses energy; 15 - gap spectrometer; 16-spectrometer detector; RE-secondarynew electrons; hv- x-ray radiation.

Emission E. m. create an image of an object with electrons, to-rye emits the object itself when heated, bombarded by a primary electron beam, under the action of an e-mag. radiation and when applying a strong electric. field pulling electrons out of the object. These devices usually have a narrow purpose (see. electronic projector).

Mirror E. m. serve the arr. for visualization of electrostatic. "potential reliefs" and magn. microfields on the surface of the object. Main electron-optical element of the device is electronic mirror, and one of the electrodes is the object itself, which is under a small negative. potential relative to the cathode of the gun. The electron beam is directed to the electron mirror and reflected by the field in the immediate vicinity of the surface of the object. The mirror forms an image "in reflected beams" on the screen: the 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. Improvement of electromagnetic meters with the aim of increasing the amount 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. systems with small aberrations have not yet led to a real increase in the resolution of E. m. 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 areas are underway. Research work on the creation of electronic holographic features continues. systems, including those with correction of the frequency-contrast characteristics of lenses. Miniaturization of electrostatic lenses and systems using the achievements of micro- and nanotechnologies will also contribute to solving the problem of creating electronic optics with small aberrations.

Lit.: Practical scanning electron microscopy, ed. D. Gouldstein, X. Yakovitsa, trans. from English, M., 1978; Spence D., Experimental high-resolution electron microscopy, trans. from English, M., 1986; Stoyanov P. A., Electron microscope SVEM-1, "Proceedings of the Academy of Sciences of the USSR, series of physics", 1988, vol. 52, no. 7, p. 1429; Hawks P., Kasper E., Fundamentals of electronic optics, trans. from English, vol. 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.

Table of contents of the subject "Electron Microscopy. Membrane.":

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

The figure shows a modern transmission (translucent) 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 constructed first.

Electron microscope upside down compared to the light microscope. Radiation is applied to the sample from above, and the image is formed from below. The principle of operation 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 by other lenses.

The table summarizes some of the similarities and differences between light and electron microscopes. At the top of the column of an electron microscope is a source of electrons - a tungsten filament, similar to that found in an ordinary light bulb. A high voltage (for example, 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 the scattering electrons due to collision with air particles. Only very thin sections or particles can be used for studying in an electron microscope, since the electron beam is almost completely absorbed by larger objects. Parts of an object that differ relatively more high density, absorb electrons and therefore appear darker in the formed image. To stain the sample in order to increase the contrast, use heavy metals such as lead and uranium.

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

The advantage of the electron microscope:
1) high resolution (0.5nm in practice)

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

electraboutmicroskaboutP(English - electron microscope) – This is a device for observing and photographing a multiply (up to 1 x 10 6 times) enlarged image of objects, in which, instead of light rays, electron beams accelerated to high energies (30 - 100 keV and more) in deep vacuum are used.

The transmission electron microscope (TEM) has the highest resolution, surpassing light microscopes in this parameter by several thousand times. The so-called resolution limit, which characterizes the ability of the device to display separately small as close as possible details of the object, for TEM is 2 - 3 A°. At favorable conditions you can photograph individual heavy atoms. When photographing periodic structures, such as the atomic planes of crystal lattices, it is possible to realize a resolution of less than 1 A°.

To determine the structure of solids, it is necessary to use radiation with a wavelength λ shorter than the interatomic distances. In an electron microscope, electron waves are used for this purpose.

de Broglie wavelength λ B for an electron moving at a speed V

where p- his momentum h is Planck's constant, m 0 - electron rest mass, V- its speed.

After simple transformations, we obtain that the de Broglie wavelength for an electron moving in an accelerating uniform electric field with a potential difference U, is equal to

. (1)

In expressions for λ B, the relativistic correction is not taken into account, which is significant only at high electron velocities V>1 10 5 V.

The value of λ B is very small, which makes it possible to provide a high resolution of the electron microscope.

For electrons with energies from 1 eV up to 10,000 eV, the de Broglie wavelength lies in the range from ~1 nm to 10 −2 nm, that is, in the wavelength range x-ray radiation. Therefore, the wave properties of electrons should manifest themselves, for example, when they are scattered by the same crystals on which diffraction x-rays. [

Modern microscopes have a resolution of (0.1 - 1) nm at an electron energy of (1 10 4 - 1 10 5) eV, which makes it possible to observe groups of atoms and even individual atoms, point defects, surface relief, etc.

Transmission electron microscopy

The electron-optical system of a transmission electron microscope (TEM) includes: electron gun I and condenser 1, designed to provide the illumination system of the microscope; objective 2, intermediate 3 and projection 4 lenses that display; observation and photographing camera E (Fig. 1).

Fig.1. Ray path in TEM in image observation mode

The source of electrons in the electron gun is a tungsten thermionic cathode. A condenser lens makes it possible to obtain a spot with a diameter of several microns on an object. With the help of a display system, an electron microscopic image of the object is formed on the TEM screen.

In the plane associated with the object, the objective lens forms the first intermediate image of the object. All electrons emanating from one point of the object fall into one point of the conjugate plane. Then, using intermediate and projection lenses, an image is obtained on a fluorescent microscope screen or photographic plate. This image conveys the structural and morphological features of the specimen.

TEM uses magnetic lenses. The lens consists of a winding, a yoke and a pole piece, which concentrates the magnetic field in a small volume and thereby increases the optical power of the lens.

TEMs have the highest resolution (PC), surpassing light microscopes in this parameter by several thousand times. The so-called resolution limit, which characterizes the ability of the device to display separately small, as close as possible, details of an object, for a TEM is 2 - 3 A°. Under favorable conditions, individual heavy atoms can be photographed. When photographing periodic structures, such as the atomic planes of crystal lattices, it is possible to realize a resolution of less than 1 A°. Such high resolutions are achieved due to the extremely short de Broglie wavelength of electrons. Optimum iris can reduce the spherical aberration of the lens, which affects the PC TEM, with a sufficiently small diffraction error. No effective methods for correcting aberrations have been found. Therefore, in TEM, magnetic electron lenses (ELs), which have smaller aberrations, have completely replaced electrostatic ELs. PEMs for various purposes are produced. They can be divided into 3 groups:

simplified TEM,

high resolution TEM,

TEM with increased accelerating voltage.

1. Simplified TEM designed for research that does not require a high PC. They are simpler in design (including 1 condenser and 2–3 lenses to magnify the image of an object), they are distinguished by a lower (usually 60–80 kV) accelerating voltage and its lower stability. PCs of these instruments range from 6 to 15. Other applications are object preview, routine research, teaching purposes. The thickness of an object that can be "enlightened" by an electron beam depends on the accelerating voltage. In a TEM with an accelerating voltage of 100 kV, objects with a thickness from 10 to several thousand A° are studied.

2. High resolution TEM(2 - 3 Å) - as a rule, universal multi-purpose devices (Fig. 2, a). With the help of additional devices and attachments, it is possible to tilt an object in different planes at large angles to the optical axis, heat, cool, deform it, carry out X-ray structural analysis, studies using electron diffraction methods, etc. The voltage accelerating electrons reaches 100–125 kV, it is regulated stepwise and is highly stable: in 1 - 3 minutes it changes by no more than 1 - 2 millionths of the original value. A deep vacuum is created in its optical system (column) (pressure up to 1 10 -6 mm Hg). The scheme of the TEM optical system is shown in Fig. 2, b. A beam of electrons, the source of which is a thermal cathode, is formed in an electron gun and then twice focused by the first and second condensers, which create an electronic “spot” on the object, the spot diameter of which can be varied from 1 to 20 μm. After passing through the object, some of the electrons are scattered and retained by the aperture diaphragm. Unscattered electrons pass through the diaphragm opening and are focused by the objective in the object plane of the intermediate lens. Here the first enlarged image is formed. Subsequent lenses create a second, third, etc. image. The last lens forms an image on a fluorescent screen that glows when exposed to electrons.

Rice. 2 a. TEM: 1 – electron gun; 2 - condenser lenses; 3 - lens; 4 - projection lenses; 5 - light microscope, additionally magnifying the image observed on the screen: 6 - tube with viewing windows through which the image can be observed; 7 - high-voltage cable; 8 - vacuum-smart system; 9 - control panel; 10 - stand; 11 - high-voltage power supply; 12 - lens power supply.

Rice. 2 b. Optical scheme of TEM. 1 - V-shaped cathode made of tungsten wire (heated by current passing through it up to 2800 K); 2 - focusing cylinder; 3 – anode; 4 - the first (short-focus) condenser, which creates a reduced image of the electron source; 5 - the second (long-focus) condenser, which transfers a reduced image of the electron source to the object; 6 - object; 7 - aperture diaphragm; 8 - lens; 9, 10, 11 - system of projection lenses; 12 - cathodoluminescent screen on which the final image is formed.

The magnification of the TEM is equal to the product of the magnifications of all lenses. The degree and character of electron scattering are not the same at different points of the object, since the thickness, density and chemical composition of the object vary from point to point. Accordingly, the number of electrons delayed by the aperture diaphragm after passing through various points of the object changes, and, consequently, the current density in the image, which is converted into light contrast on the screen. Under the screen is a store with photographic plates. When photographing, the screen is removed and the electrons act on the photoemulsion layer. The image is focused by changing the current that excites the magnetic field of the lens. The currents of the other lenses are adjusted to change the magnification of the TEM.

3. FEM with increased accelerating voltage(up to 200 kV) are designed to study thicker objects (2-3 times thicker) than conventional TEMs. Their resolution reaches 3 – 5 Å. These devices differ in the design of the electron gun: to ensure electrical strength and stability, it has two anodes, one of which is supplied with an intermediate potential equal to half the accelerating voltage. The magnetomotive force of the lenses is greater than in a TEM with an accelerating voltage of 100 kV, and the lenses themselves have increased dimensions and weight.

4. Ultrahigh voltage electron microscopes(SVEM) - large-sized devices (Fig. 3) with a height of 5 to 15 m, with an accelerating voltage of 0.50 - 0.65; 1 - 1.5 and 3.5 MV.

They build special rooms for them. SVEM are intended for research of objects with thickness from 1 · to · 10 microns. The electrons are accelerated in an electrostatic accelerator (so-called direct-acting accelerator) located in a tank filled with electrically insulating gas under pressure. A high-voltage stabilized power supply is located in the same or in an additional tank. In the future - the creation of a TEM with a linear accelerator, in which electrons are accelerated to energies of 5 - 10 MeV. When studying thin objects, PC SVEM is lower than that of TEM. In the case of thick objects, the PC SVM is 10–20 times superior to the PC TEM with an accelerating voltage of 100 kV. If the sample is amorphous, then the contrast of the electronic image is determined by the thickness and absorption coefficient of the sample material, which is observed, for example, when studying the surface morphology using plastic or carbon replicas. In crystals, in addition, electron diffraction takes place, which makes it possible to determine the crystal structure.

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Fig.4. Aperture position D with bright field ( a) and dark-field ( b) images: P - transmitted beam; D- diffracted beam; arr - sample; I - electron gun

PEM can implement the following modes of operation:

the image is formed by the transmitted beam P, the diffracted beam D is cut off by the aperture diaphragm D (Fig. 4, a), this is a bright-field image;

aperture diaphragm D transmits diffracted D beam, cutting off the past P, this is a dark-field image (Fig. 4, b);

to obtain a diffraction pattern, the rear focal plane of the objective lens is focused on the microscope screen (Fig. 4). Then a diffraction pattern from the translucent portion of the sample is observed on the screen.

To observe the image in the rear focal plane of the lens, an aperture stop is installed, as a result, the aperture of the rays forming the image is reduced and the resolution is increased. The same diaphragm is used to select the observation mode (see Fig. 2 and 5).

Fig.5. Ray path in TEM in microdiffraction mode D - aperture; And - the source of electrons; arr - sample; E - screen; 1 - condenser, 2 - objective, 3 - intermediate, 4 - projection lens

wave length  at voltages used in TEM, is about 1∙10 –3 nm, that is, much less than the lattice constant of crystals a, so the diffracted beam can propagate only at small angles θ to the passing beam (
). The diffraction pattern from a crystal is a set of individual dots (reflexes). In TEM, in contrast to the electron diffraction pattern, it is possible to obtain a diffraction pattern from a small area of ​​the object using a diaphragm in the plane conjugated with the object. The size of the region may be about (1×1) μm 2 . You can switch from the image observation mode to the diffraction mode by changing the optical power of the intermediate lens.