Transmission electron microscope working principle briefly. Microscopy, electronic translucent. Transmission microscope magnification

Transmission electron microscopy is one of the highest resolution research methods. Wherein transmission electron microscope(TEM) is an analogue of a traditional optical microscope. The analogy lies in the fact that the change in the trajectory of propagation of the flow of optical quanta under the action of a refractive medium (lenses) is similar to the action of magnetic and electric fields on the trajectory of charged particles, in particular electrons. The similarity, in terms of focusing electrons and forming an image of the object under study, turned out to be so close that the electron-optical columns of the first magnetic and electrostatic TEMs were calculated using the dependences of geometric optics.

As focusing lenses in modern TEM (Fig. 15.2), electromagnetic coils enclosed in a magnetic circuit are used, which create focusing magnetostatic fields (Fig. 15.3). The magnetic circuit of the lens performs two functions: it increases the field strength

Rice. 15.2.

• 1 - electron gun; 2 - block of condenser lenses; 3 - an objective lens block with an object holder; 4 - a block of projection lenses; 5 - screens for image visualization; 6- high voltage power supply; 7- vacuum system
• (i.e. enhances its focusing power) and shapes it to produce an image that most closely matches the subject. Unlike glass lenses, the refractive power of a magnetic lens can be easily changed by changing the excitation current in the winding. Due to this, the magnification provided by the microscope can be changed continuously from several hundreds to millions of times.

Rice. 15.3. Scheme of the electromagnetic lens of the electron microscope: I- magnetic core; 2 - magnetic field excitation coil;

3- field that focuses the electron beam

In TEM, samples are “examined” through the light. That is, they are irradiated with an electron beam and the necessary information is obtained in the form of an image formed using electrons that have passed through the sample. Any image consists of areas of a certain size, differing in brightness. These differences in TEM arise due to the fact that electrons, passing through the dense medium of the sample, are scattered in it (partially absorbed, change the direction of motion, and, as a rule, lose part of their energy). Moreover, the angular distribution of electrons that have passed through the sample carries information about the density of the sample, its thickness, elemental composition, and crystallographic characteristics.

Rice. 15.4. Absorption of an electron flow in a thin-film amorphous sample with a region of increased density: a - b- current density distribution j

Rice. 15.5. Electron flux absorption in a thin-film amorphous sample of variable thickness: a - the passage of an electron flow through the sample; b - current density distribution j in the electron flow passing through the sample

Thus, areas containing heavier atoms scatter electrons at large angles and cause their more efficient absorption (Fig. 15.4). Similarly, thicker regions of an amorphous sample deflect and absorb electrons to a greater extent than thinner regions (Fig. 15.5). If the plane of the sample and the plane of the receiver-converter are optically matched with the help of lenses, an enlarged image will appear on the surface of the latter.

If the sample is a crystal or polycrystal, the interaction of the electron beam, which is a plane wave, with the crystal lattice leads to the appearance of a diffraction pattern (Fig. 15.6). The geometry of this picture is described by the Wulf-Bragg equation known from the course of physics and is uniquely related to the crystallographic parameters of the sample. Knowing the energy of the irradiating electrons, these parameters can be set with high accuracy. In order to obtain an enlarged image of such a pattern (diffractogram), it is sufficient to optically match the plane of formation of the diffraction pattern (it is located behind the plane of the sample) and the plane of the receiver-converter.

Rice. 15.6. Electron diffraction patterns obtained from single crystal (I) and polycrystalline (b) samples

To visualize these images, the transmitted electrons are focused on the surface of the receiver-converter using a lens system (objective, intermediate, etc.). In this case, from all the electrons that have passed through the sample, either electrons scattered at large angles or not scattered are selected (less often, electrons scattered at small angles are used to form an image, usually with small-angle diffraction). In the first case, areas characterized by low scattering power look darker in the resulting image (this is the so-called dark-field imaging mode), and in the second case, vice versa (bright-field mode).

The schematic diagram of the TEM is shown in fig. 15.7. The microscope consists of an electron gun and a system of electromagnetic lenses forming a vertically arranged electron-optical column in which a vacuum of 10 -3 h -10~2 Pa is maintained. The illumination system of the microscope includes an electron gun and a two-lens condenser. An electron gun, as a rule, is a thermionic one, it consists of a cathode (a heated filament made of W or LaB 6) that emits electrons, a control electrode (it is supplied with a negative potential relative to the cathode) and an anode in the form of a plate with a hole. A powerful electric field with an accelerating voltage of 100-150 kV is created between the cathode and anode.

It should be noted that there is a small class of so-called ultrahigh voltage microscopes, in which the accelerating voltage can reach several megavolts. With an increase in speed, the wavelength decreases (A. \u003d h/mv-h/(2teU) 0 5) electron. As the wavelength decreases, the resolution of the optical system of any microscope, including TEM, increases. An increase in the accelerating voltage, in addition, leads to an increase in the penetrating power of electrons. At operating voltages of 1000 kV and more, it is possible to study samples up to 5–10 µm thick.

Rice. 15.7.

• 1 - cathode; 2 - anode; 3 - the first condenser; 4 - the second condenser;
• 5 - adjustment corrector; 6 - goniometric table with object holder;
• 7 - aperture diaphragm; 8 - sector diaphragm; 9 - intermediate lens;
• 10 - projection lens; 11 - receiver-converter;
• 12 - aperture of the field of view; 13 - stigmatator of the intermediate lens;
• 14 - stigmatator of an objective lens; 15 - objective lens;
• 16 - the object under study; 17- stigmatator of the second condenser;
• 18 - diaphragm of the second condenser; 19 - diaphragm of the first condenser; 20 - control electrode

However, when studying materials in a high-voltage TEM, it is necessary to take into account the formation of radiation defects such as Frenkel pairs and even complexes of point defects (dislocation loops, vacancy pores) in its structure during long-term exposure to a high-energy electron beam. For example, in aluminum, the threshold energy of mixing an atom from a lattice site for an electron beam is 166 eV. These electron microscopes are effective tool to study the appearance and evolution of radiation defects in crystalline solids.

Passing through the anode hole, the electron beam enters the condensers and the alignment corrector, where the electron beam is finally aimed at the sample under study. In TEM, condenser lenses regulate and control the size and angle of exposure of the sample. Further, using the fields of the objective and projection lenses, an information image is formed on the surface of the receiver-converter.

For microdiffraction studies, the microscope includes a movable selector diaphragm, which in this case replaces the aperture diaphragm. For greater versatility, an additional lens is installed between the objective and intermediate lenses in the TEM. It sharpens the image throughout the magnification range. The main purpose of the lens is to provide a quick transition to the mode of electron diffraction studies.

As a receiver-converter, a luminescent screen can be used, where the electron flow is converted into an optical radiation flow in the phosphor layer. In another design, the receiver-converter includes a sensitive matrix (sectioned microchannel plates, matrix image intensifier tubes, CCD arrays (abbreviated from charge-coupled device)), in which the electron flow is converted into a video signal, and the latter is output on the monitor screen and is used to create a TV picture.

Modern TEM provides resolution down to 0.2 nm. In this regard, the term "high-resolution transmission electron microscopy" appeared. Useful magnification of the final image can reach 1 million times. It is interesting to note that at such a huge magnification, a 1 nm structure detail in the final image is only 1 mm in size.

Since the image is formed from electrons that have passed through the sample, the latter, due to the low penetrating power of electrons, must have a small thickness (usually tenths and hundredths of a micrometer). There is a rule of thumb according to which the thickness of the sample does not exceed the value of the required resolution by more than an order of magnitude (for obtaining ultra-high resolution of 0.2 nm, this rule no longer works). As a result, the sample is prepared in the form of a foil or a film race called replica.

Depending on how the sample is prepared, its examination can be direct, indirect or mixed.

direct method gives the most complete information about the structure of the object. It consists in thinning the initial massive sample to the state of a thin film, which is transparent or translucent to electrons.

Sample thinning is a laborious process, since the use of mechanical devices at the last stage is impossible. Usually, the sample is cut into millimetric plates, which are mechanically polished to a thickness of ~50 µm. The sample is then subjected to precision ion etching or electrolytic polishing.

(two-sided or reverse side from the surface being examined). As a result, it thins to a thickness of ~ 100-1000 A.

If the sample has a complex composition, then it should be taken into account that the erosion rate of various materials during ion sputtering and electropolishing is different. As a result, the resulting layer provides direct information not about the entire initial sample, but only about its extremely thin near-surface layer remaining after etching.

However, this situation is not critical if the sample itself is a fine structure, for example, a grown epitaxial film or a nanodispersed powder.

In some cases, usually related to non-metallic plastic materials such as organics and biological objects, thin films for research are cut off from a massive initial sample using special devices called ultramicrotomes (Fig. 15.8). Ultramicrotome is a miniature guillotine with a precision (usually piezoceramic) drive for moving the sample under the knife. The thickness of the layer cut by the device can be several nanometers.

Rice. 15.8.

In some cases, films are also obtained by physical vacuum deposition on water-soluble substrates (NaCl, KS1).

In studies using transmission (transmission) electron microscopy, one can study the dislocation structure of materials (see, for example, Fig. 2.28), determine the Burgers vectors of dislocations, their type and density. Also, using TEM, it is possible to study accumulations of point defects (including radiation defects), stacking faults (with determination of their formation energy), twin boundaries, grain and subgrain boundaries, segregations of second phases (with identification of their composition), etc.

Sometimes microscopes are equipped with special attachments (for heating or stretching the sample during the study, etc.). For example, when using an attachment that makes it possible to stretch the foil during the study, one observes the evolution of the dislocation structure during deformation.

When studying by the TEM method, it is also possible to carry out microdiffraction analysis. Depending on the composition of the material in the study area, diagrams (electron diffraction patterns) are obtained in the form of dots (samples are single crystals or polycrystals with grains exceeding the study area), solid or consisting of individual reflections. The calculation of these electron diffraction patterns is similar to the calculation of X-ray debyegrams. Microdiffraction analysis can also be used to determine crystal orientations and misorientations of grains and subgrains.

Transmission electron microscopes with a very narrow beam make it possible to conduct a local chemical analysis of the material, including analysis for light elements (boron, carbon, oxygen, nitrogen), based on the energy loss spectrum of electrons that have passed through the object under study.

indirect method is associated with the study of not the material itself, but thin film replicas obtained from its surface. A thin film is formed on the sample, repeating the surface structure of the sample to the smallest detail, and then it is separated using special techniques (Fig. 15.9).

The method is implemented either by vacuum deposition on the sample surface of a film of carbon, quartz, titanium, or other substances, which is then relatively simply separated from the sample, or by oxidizing the surface (for example, copper), obtaining easily detachable oxide films. Even more promising is the use of replicas in the form of polymer or lacquer films deposited in liquid form on the surface of a section.

The indirect method does not require expensive high-voltage microscopes. However, it is significantly inferior to the direct method in informativeness. First, it excludes the possibility of studying the crystallographic characteristics of the sample, as well as evaluating the features of its phase and elemental composition.

Rice. 15.9.

Secondly, the resolution of the resulting image is usually worse. The useful magnification of such images is limited by the accuracy of the replica itself and reaches at best (for carbon replicas) (1-2) 10 5 .

In addition, distortions and artifacts may appear in the process of making the replica itself and separating it from the original sample. All this limits the application of the method. Many problems associated with the study by an indirect method, including fractography, are currently being solved by scanning electron microscopy.

Note that the method of deposition of a thin layer on the surface of a sample is also used in the direct study of thinned objects. In this case, the created film provides an increase in the contrast of the formed image. A well-absorbing electron material (Au, Mo, Cu) is sprayed onto the surface of the sample at an acute angle so that it condenses more on one side of the protrusion than on the other (Fig. 15.10).

Rice. 15.10.

mixed method sometimes used in the study of heterophase alloys. In this case, the main phase (matrix) is studied using replicas (indirect method), while the particles extracted from the matrix into a replica are studied by a direct method, including microdiffraction.

In this method, the replica is cut into small squares before separation, and then the sample is etched according to a regime that ensures the dissolution of the matrix material and the preservation of particles of other phases. Etching is carried out until the complete separation of the replica film from the base.

The mixed method is especially convenient for studying finely dispersed phases in a matrix with their small volume fraction. The absence of a replica of its own structure makes it possible to study the diffraction patterns from particles. With the direct method, such patterns are extremely difficult to identify and separate from the picture for the matrix.

In connection with the development of nanotechnology and especially methods for obtaining ultrafine and nanosized powders (fulleroids, NT, etc.), this method has provided a high interest of researchers in TEM. The ultrafine and nanosized particles to be studied are deposited on a very thin and practically transparent membrane for electron beams, after which they are placed in a TEM column. Thus, one can observe their structure directly - practically in the same way as in a conventional optical microscope, only with incomparably more high resolution.

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Communication, communication, radio electronics and digital devices

Transmission electron microscope design. The use of a transmission electron microscope. Secondly, a significant increase to 1 Å or less in the resolution of electron microscopes, which made them competitive with field-ion microscopes in obtaining direct images of the crystal lattice. Today it is difficult to imagine a biological medical physical metallographic chemical laboratory without an optical microscope: by examining blood droplets and a tissue section, physicians draw up a conclusion about the state of...

Introduction ................................................ ................................................. ............5

1. History reference................................................ ....................................7

2. Transmission electron microscopy....................................................................11

2.1 Construction of a transmission electron microscope.......................11

2.2 Image................................................. ...........................................sixteen

2.3 Permission................................................. ...............................................21

2.4 Sources of electrons................................................... .........................26

2.5 Lighting system................................................ ......................27

2.6 Astigmatism Correction .................................................................. ................28

2.7 Accessories for Conventional High-Resolution Transmission Electron Microscopy .................................................................31

3. Preparation of objects for research and special requirements for them.32

4. Using the Transmission Electron Microscope ..................................33

4.1 Non-biological materials ................................................................ .................34

4.2 Biological preparations .................................................................. ......................37

4.3 High-voltage microscopy .............................................................. .............38

4.4 Radiation damage .................................................................. ..............39

5. Modern types of transmission electron microscopy..... 39

6. Disadvantages and limitations, features of the use of transmission electron microscopy .............................................................. ...............................43

Conclusion................................................. ................................................. .....46

Bibliography................................................ ...............................................48

Introduction

Electron microscopy techniques have gained such popularity that it is currently impossible to imagine a material research laboratory that does not use them. The first successes of electron microscopy should be attributed to the 1930s, when it was used to reveal the structure of a number of organic materials and biological objects. In the study of inorganic materials, especially metal alloys, the position of electron microscopy was strengthened with the advent of microscopes with high voltage (100 kV and higher) and even more thanks to the improvement in the technique of obtaining objects, which made it possible to work directly with the material, and not with replica casts. Strong positions are occupied by electron microscopy in a number of other branches of materials science.

The growing interest in electron microscopy is explained by a number of circumstances. This is, firstly, the expansion of the possibilities of the method due to the appearance of a wide variety of attachments: for research at low (up to -150°C) and high (up to 1200°C) temperatures, observation of deformation directly in a microscope, study of X-ray spectra of microsections (up to 1 μm and less) of objects, obtaining images in scattered electrons, etc. Secondly, a significant increase (up to 1 Å and less) in the resolution of electron microscopes, which made them competitive with field-ion microscopes in obtaining direct images of the crystal lattice. Finally, the opportunity to study in detail diffraction patterns in parallel with microscopic studies up to the observation of such fine details as diffusion scattering of electrons.

The main advantage of using transmission electron microscopes is their high resolution, which is made possible by the short wavelength of the electrons. At an accelerating voltage of 200 kV, the electron wavelength is only 0.025 Å. Transmission electron microscopes use accelerating voltages of up to 3000 kV, with the highest point resolution achievable in state-of-the-art instruments being better than 1 Å. This makes it possible to study the structure of materials at the atomic level.

When using the methods of transmission electron microscopy, the necessary information is obtained by analyzing the results of the scattering of an electron beam as it passes through an object. There are two main types of scattering: a) elastic scattering - the interaction of electrons with the effective potential field of nuclei without energy loss; b) inelastic scattering - the interaction of an electron beam with the electrons of an object, in which there are energy losses, i.e. absorption. The diffraction pattern appears only in elastic scattering.

1. History reference

The history of microscopy is the history of man's continuous quest to penetrate the mysteries of nature. The microscope appeared in the 17th century, and since then science has been rapidly moving forward. Many generations of researchers spent long hours at the microscope, studying visible to the eye world. Today it is difficult to imagine a biological, medical, physical, metallographic, chemical laboratory without an optical microscope: examining blood droplets and a tissue section, physicians draw up a conclusion about the state of human health. Establishing the structure of metal and organic substances made it possible to develop a number of new high-strength metal and polymer materials.

Our century is often called the electronic age. Penetration into the secrets of the atom made it possible to design electronic devices - lamps, cathode-ray tubes, etc. In the early 1920s, physicists had the idea of ​​using an electron beam to form an image of objects. The implementation of this idea gave rise to the electron microscope.

Ample opportunities for obtaining a wide variety of information, including from areas of objects commensurate with an atom, served as an incentive for the improvement of electron microscopes and their use in almost all areas of science and technology as instruments for physical research and technical control.

A modern electron microscope is able to distinguish such small details of the image of a microobject that no other instrument is able to detect. Even more than the size and shape of the image, scientists are interested in the structure of the micro-object; and electron microscopes can tell not only about the structure, but also about the chemical composition, imperfections in the structure of sections of a micro-object with a size of fractions of a micrometer. Due to this, the scope of the electron microscope is constantly expanding and the device itself is becoming more complex.

The first transmission electron microscopes operated with an electron-accelerating voltage of 30–60 kV; the thickness of the studied objects barely reached 1000 Å (1 Å – 10-10 m). At present, electron microscopes with an accelerating voltage of 3 MV have been created, which made it possible to observe objects as thin as a few micrometers. However, the success of electron microscopy was not limited to a quantitative increase in the accelerating voltage. A milestone was the creation of a serial scanning electron microscope (SEM), which immediately gained popularity among physicists, chemists, metallurgists, geologists, physicians, biologists, and even forensic scientists. The most significant features of this device are a large image depth of field, which is several orders of magnitude higher than that of an optical microscope, and the possibility of studying massive samples practically without any special preparation. The evolution of the ideas of physics is inextricably linked with the development of research methods that make it possible to explain the phenomena occurring in the microcosm. In the development of any science that studies real physical bodies, two questions are basic: how does a body behave under certain conditions? Why does it behave in a certain way? The most complete answer to these questions can be obtained if we consider the structure of the body and its behavior in a complex way, that is, from microconnections and microstructure to macrostructure in a macroprocessor. In the 19th century, the imaging theory was finally formulated, and it became obvious to physicists that in order to improve the resolution of a microscope, it is necessary to reduce the wavelength of the radiation that forms the image. At first, this discovery did not lead to practical results. Only thanks to the work of Louis de Broglie (1924), in which the wavelength of a particle was related to its mass and speed, from which it followed that the phenomenon of diffraction should also take place for electrons; and Bush (1926), who showed that electric and magnetic fields act almost like optical lenses, it became possible to talk concretely about electron optics.

In 1927, the American scientists K. Devissoy and L. Germer observed the phenomenon of electron diffraction, and the English physicist D. Thomson and the Soviet physicist P. S. Tartakovskii conducted the first studies of this phenomenon. In the early 1930s, Academician A. A. Lebedev developed the theory of diffraction as applied to an electron diffraction recorder.

Based on these fundamental works, it became possible to create an electron-optical device, and de Broglie suggested that one of his students, L. Szilard, do this. He, in a conversation with the famous physicist D. Tabor, told him about de Broglie's proposal, but Gabor convinced Szilard that any object in the path of the electron beam would burn to the ground and, in addition, living objects could not be prevented from vacuum.

Szilard refused his teacher's offer, but by that time there were no more difficulties in obtaining electrons. Physicists and radio engineers successfully worked with vacuum tubes, in which electrons were obtained due to thermionic emission, or, simply put, by heating the filament (cathode), and the directed movement of electrons to the anode (i.e., the passage of current through the lamp) was formed by applying voltage between anode and cathode. In 1931, A. A. Lebedev proposed an electron diffraction scheme with magnetic focusing of the electron beam, which formed the basis of most of the instruments manufactured in our country and abroad.

In 1931 R. Rudenberg filed a patent application for a transmission electron microscope, and in 1932 M. Knoll and E. Ruska built the first such microscope, using magnetic lenses to focus electrons. Ruska was rewarded for his work by being awarded the Nobel Prize in Physics in 1986.

In 1938, Ruska and B. von Borries built a prototype of an industrial TEM for Siemens-Halske in Germany; this instrument eventually made it possible to achieve a resolution of 100 nm. A few years later, A. Prebus and J. Hiller built the first high-resolution TEM at the University of Toronto (Canada).

The wide possibilities of TEM became apparent almost immediately. Its industrial production was started simultaneously by Siemens-Halske in Germany and RCA Corporation in the USA. In the late 1940s, other companies began to produce such devices.

The SEM in its current form was invented in 1952 by Charles Otley. True, preliminary versions of such a device were built by Knoll in Germany in the 1930s and by Zworykin with employees at the RCA corporation in the 1940s, but only the Otley device could serve as the basis for a number of technical improvements that culminated in the introduction of an industrial version of the SEM into production in the middle 1960s. The circle of consumers of such a rather easy-to-use device with a three-dimensional image and an electronic output signal has expanded with the speed of an explosion. At present, there are a dozen industrial SEM manufacturers on three continents and tens of thousands of such devices used in laboratories around the world. In the 1960s, ultrahigh-voltage microscopes were developed to study thicker samples. , where a device with an accelerating voltage of 3.5 million volts was put into operation in 1970. RTM was invented by G. Binnig and G. Rohrer in Zurich in 1979. This very simple device provides atomic resolution of surfaces. For the creation of the RTM, Binnig and Rohrer (simultaneously with Ruska) received the Nobel Prize in Physics.

The wide development of electron microscopy methods in our country is associated with the names of a number of scientists: N. N. Buynov, L. M. Utevsky, Yu. A. Skakov (transmission microscopy), B. K. Vainshtein (electronography), G. V. Spivak (scanning microscopy), I. B. Borovsky, B. N. Vasichev (X-ray spectroscopy), etc. Thanks to them, electron microscopy has left the walls of research institutes and is increasingly being used in factory laboratories.

2. Transmission electron microscopy

2.1 Construction of a transmission electron microscope

Electron microscope– a device that allows you to get a greatly enlarged image of objects, using electrons to illuminate them. An electron microscope (EM) makes it possible to see details that are too small to be resolved by a light (optical) microscope. The electron microscope is one of the most important instruments for fundamental scientific research into the structure of matter, especially in such fields of science as biology and solid state physics.

The principle of its construction is generally similar to the principle of an optical microscope; there are lighting (electron gun), focusing (lenses) and recording (screen) systems. However, it is very different in details. For example, light propagates freely in air, while electrons are easily scattered when interacting with any substance and, therefore, can only move freely in a vacuum. In other words, the microscope is placed in a vacuum chamber.

1- high voltage cable; 2- electron gun; 3 - cathode; 4- control electrode (modulator);; 5 - anode; 6- first condenser lens; 7- second condenser lens; 8- windings of the electron beam tilting and moving system; 9 – sample chamber; 10- objective lens; 11- aperture diaphragm; 12 - stigmatator ; 13 - intermediate lens; 14-diffraction camera; 15- projection lens; 16- binocular (optical microscope); 17- tube (surveillance camera); 18- fluorescent screens; 19- photo shop (camera with photo plates and interchangeable mechanism)

Let's take a closer look at the components of the microscope. The system of filament and accelerating electrodes is called the electron gun (1). In essence, the gun resembles a triode lamp. The flow of electrons is emitted by a hot tungsten wire (cathode), is collected in a beam and accelerated in the field of two electrodes. The first is the control electrode, which surrounds the cathode, and a bias voltage is applied to it, a small negative potential of several hundred volts relative to the cathode. Due to the presence of such a potential, the electron beam emerging from the gun is focused on the control electrode. The second electrode is the anode (2), a plate with a hole in the center through which the electron beam enters the microscope column. An accelerating voltage, typically up to 100 kV, is applied between the filament (cathode) and the anode. As a rule, it is possible to change the voltage stepwise from 1 to 100 kV.

The task of the gun is to create a stable flow of electrons with a small emitting region of the cathode. The smaller the area emitting electrons, the easier it is to obtain their thin parallel beam. For this, V-shaped or specially sharpened cathodes are used.

Next, lenses are placed in the microscope column. Most modern electron microscopes have four to six lenses. The electron beam leaving the gun is directed through a pair of condenser lenses (5,6) to the object. The condenser lens makes it possible to change the illumination conditions of an object over a wide range. Typically, condenser lenses are electromagnetic coils in which the current-carrying windings are surrounded (with the exception of a narrow channel with a diameter of about 2–4 cm) by a soft iron core (Fig. 2.1.2).

When the current flowing through the coils changes, the focal length of the lens changes, as a result of which the beam expands or contracts, the area of ​​the object illuminated by electrons increases or decreases.

The geometric dimensions of the pole piece are indicated; the dashed line shows the contour appearing in Ampère's law. The dashed line also shows the magnetic flux line, which qualitatively determines the focusing effect of the lens. In r - field strength in the gap away from the optical axis. In practice, the lens windings are water-cooled and the pole piece is removable

To obtain a large magnification, it is necessary to irradiate the object with high-density fluxes. The condenser (lens) usually illuminates an area of ​​the object that is much larger than that of interest to us at a given magnification. This can lead to overheating of the sample and its contamination with the decomposition products of oil vapors. The temperature of the object can be reduced by reducing the irradiated area to approximately 1 µm with the second condenser lens, which focuses the image produced by the first condenser lens. This increases the flow of electrons through the sample area under study, increases the brightness of the image, and the sample is less contaminated.

The sample (object) is usually placed in a special object holder on a thin metal mesh 2–3 mm in diameter. The object holder moves by a system of levers in two mutually perpendicular directions, tilts in different sides, which is especially important when examining a tissue cut or such defects in the crystal lattice as dislocations and inclusions.

In this design, the hole diameter of the upper pole piece, the hole diameter of the lower pole piece and the pole clearance (R 1 , R 2 and S are defined in Fig.2.1.2): 1 – object holder, 2 – sample stage, 3 – sample, 4 – objective diaphragm, 5 – thermistors, 6 – lens winding, 7 – upper pole piece, 8 – cooled rod, 9 - lower pole piece, 10 - stigmator, 11 - channels of the cooling system, 12 - cooled diaphragm

A relatively low pressure, approximately mm Hg, is created in the microscope column using a vacuum pumping system. Art. This takes quite a lot of time. To speed up the preparation of the device for operation, a special device for quick object change is attached to the object chamber. In this case, only a very small amount of air enters the microscope, which is removed by vacuum pumps. Sample change usually takes 5 minutes.

2.2 Image

When an electron beam interacts with a sample, the electrons passing near the atoms of the object's substance are deflected in the direction determined by its properties. This is mainly due to the visible contrast of the image. In addition, electrons can still undergo inelastic scattering associated with a change in their energy and direction, pass through the object without interaction, or be absorbed by the object. When electrons are absorbed by a substance, light or X-ray radiation is produced, or heat is released. If the sample is sufficiently thin, then the fraction of scattered electrons is small. The designs of modern microscopes make it possible to use for image formation all the effects arising from the interaction of an electron beam with an object.

The electrons that have passed through the object enter the objective lens (9) designed to obtain the first magnified image. The objective lens is one of the most important parts of the microscope, "responsible" for the resolving power of the instrument. This is due to the fact that the electrons enter at a relatively large angle of inclination to the axis, and as a result, even slight aberrations significantly worsen the image of the object.

The final enlarged electronic image is made visible by means of a fluorescent screen that glows under the influence of electron bombardment. This image, usually low contrast, is usually viewed through a binocular light microscope. With the same brightness, such a microscope with a magnification of 10 can create an image on the retina that is 10 times larger than when observed with the naked eye. Sometimes a phosphor screen with an image intensifier tube is used to increase the brightness of a weak image. In this case, the final image can be displayed on a conventional television screen, allowing it to be recorded on videotape. Video recording is used to record images that change over time, for example, due to a chemical reaction. Most often, the final image is recorded on photographic film or photographic plate. A photographic plate usually makes it possible to obtain a sharper image than that observed with the naked eye or recorded on videotape, since photographic materials, generally speaking, register electrons more efficiently. In addition, 100 times more signals can be recorded per unit area of ​​photographic film than per unit area of ​​videotape. Thanks to this, the image recorded on the film can be further enlarged by about 10 times without loss of clarity.

Electronic lenses, both magnetic and electrostatic, are imperfect. They have the same defects as the glass lenses of an optical microscope - chromatic, spherical aberration and astigmatism. Chromatic aberration occurs due to inconsistency focal length when focusing electrons with different velocities. These distortions are reduced by stabilizing the electron beam current and the current in the lenses.

Spherical aberration is due to the fact that the peripheral and internal zones of the lens form an image at different focal lengths. The winding of the coil of a magnet, the core of the electromagnet, and the channel in the coil through which the electrons pass cannot be done perfectly. The asymmetry of the magnetic field of the lens leads to a significant curvature of the electron motion trajectory.

If the magnetic field is not symmetrical, then the lens distorts the image (astigmatism). The same can be attributed to electrostatic lenses. The manufacturing process of the electrodes and their alignment must be highly accurate, because the quality of the lenses depends on this.

In most modern electron microscopes, symmetry violations of magnetic and electric fields are eliminated with the help of stigmators. Small electromagnetic coils are placed in the channels of electromagnetic lenses, changing the current flowing through them, they correct the field. Electrostatic lenses are supplemented with electrodes: by selecting the potential, it is possible to compensate for the asymmetry of the main electrostatic field. The stigmators very finely regulate the fields and make it possible to achieve their high symmetry.

There are two more important devices in the lens - the aperture diaphragm and the deflection coils. If deflected (diffracted) rays are involved in the formation of the final image, then the image quality will be poor due to the spherical aberration of the lens. An aperture diaphragm with a hole diameter of 40–50 µm is inserted into the objective lens, which delays rays diffracted at an angle of more than 0.5 degrees. Rays deflected by a small angle produce a bright-field image. If the aperture diaphragm blocks the transmitted beam, then the image is formed by the diffracted beam. In this case, it is obtained in a dark field. However, the dark field method gives a lower quality image than the bright field method, since the image is formed by rays intersecting at an angle to the microscope axis, spherical aberration and astigmatism are more pronounced. Deflecting coils are used to change the slope of the electron beam.

To obtain the final image, you need to increase the first enlarged image of the object. A projection lens is used for this purpose. The overall magnification of the electron microscope should vary over a wide range, from a small corresponding magnification magnifier ( x 10, x 20), in which you can examine not only part of the object, but also see the entire object, up to the maximum magnification, which allows you to make the most of the high resolution of the electron microscope (usually up to X 200000). A two-stage system (lens, projection lens) is no longer enough here. Modern electron microscopes, designed for maximum resolution, must have at least three magnifying lenses - an objective, an intermediate and a projection lens. Such a system guarantees a change in magnification over a wide range (from x 10 to x 200000).

The change in magnification is carried out by adjusting the current of the intermediate lens.

Another factor contributing to obtaining greater magnification is the change in the optical power of the lens. To increase the optical power of the lens, special so-called "pole tips" are inserted into the cylindrical channel of the electromagnetic coil. They are made of soft iron or alloys with high magnetic permeability and allow the magnetic field to be concentrated in a small volume. In some models of microscopes, it is possible to change the pole tips, thus achieving an additional increase in the image of the object.

On the final screen, the researcher sees an enlarged image of the object. Different parts of the object scatter the electrons incident on them differently. After the objective lens (as already mentioned above), only electrons will be focused, which, when passing through the object, are deflected by small angles. These same electrons are focused by the intermediate and projection lenses on the screen for the final image. On the screen, the corresponding details of the object will be light. In the case when electrons are deflected at large angles while passing through sections of the object, they are delayed by the aperture diaphragm located in the objective lens, and the corresponding sections of the image will be dark on the screen.

The image becomes visible on a fluorescent screen (luminous under the action of electrons falling on it). It is photographed either on a photographic plate or on film, which are located a few centimeters below the screen. Although the plate is placed below the screen, due to the fact that electronic lenses have a rather large depth of field and focus, the clarity of the image of the object on the photographic plate does not deteriorate. The change of the plate is through a hermetic hatch. Sometimes photoshops are used (from 12 to 24 plates), which are also installed through lock chambers, which makes it possible to avoid depressurization of the entire microscope.

2.3 Permission

Electron beams have properties similar to those of light beams. In particular, each electron is characterized by a certain wavelength. The resolution of an electron microscope is determined by the effective wavelength of the electrons. The wavelength depends on the speed of the electrons and, consequently, on the accelerating voltage; the greater the accelerating voltage, the greater the speed of the electrons and the shorter the wavelength, and hence the higher the resolution. Such a significant advantage of the electron microscope in resolving power is due to the fact that the wavelength of electrons is much smaller than the wavelength of light. But since electronic lenses do not focus as well as optical ones (the numerical aperture of a good electronic lens is only 0.09, while for a good optical lens this value reaches 0.95), the resolution of an electron microscope is 50 - 100 electron wavelengths. Even with such weak lenses in an electron microscope, a resolution limit of about 0.17 nm can be obtained, which makes it possible to distinguish individual atoms in crystals. To achieve resolution of this order, very careful tuning of the instrument is necessary; in particular, highly stable power supplies are required, and the instrument itself (which may be about 2.5 m high and weigh several tons) and its accessories require vibration-free mounting.

To achieve a dot resolution better than 0.5 nm, it is necessary to keep the instrument in excellent condition and, in addition, to use a microscope that is specifically designed for work related to obtaining high resolution. Objective lens current instability and object stage vibration should be kept to a minimum. The examiner must be sure that there are no remnants of objects left from previous examinations in the pole tip of the objective. Diaphragms must be clean. The microscope should be installed in a place that is satisfactory in terms of vibrations, extraneous magnetic fields, humidity, temperature and dust. The spherical aberration constant should be less than 2mm. However, the most important factors when working with high resolution are the stability of the electrical parameters and the reliability of the microscope. The object contamination rate must be less than 0.1 nm/min, and this is especially important for high resolution dark field work.

Temperature drift should be minimal. In order to minimize contamination and maximize high voltage stability, vacuum is required and should be measured at the end of the pump line. The interior of the microscope, especially the volume of the electron gun chamber, must be scrupulously clean.

Convenient objects for checking the microscope are test objects with small particles of partially graphitized carbon, in which the planes of the crystal lattice are visible. In many laboratories, such a sample is always kept on hand to check the condition of the microscope, and every day, before starting work at high resolution, clear images of the system of planes with an interplanar spacing of 0.34 nm are obtained on this sample using a sample holder without tilt. This practice of testing the instrument is highly recommended. It takes a lot of time and energy to keep a microscope in top condition. Examinations requiring high resolution should not be planned until the condition of the instrument is maintained at an appropriate level, and, more importantly, until the microscopist is not completely confident that the results obtained using high resolution images will justify the investment. time and effort.

Modern electron microscopes are equipped with a number of devices. A very important prefix is ​​to change the inclination of the sample during the observation. Since the image contrast is obtained mainly due to electron diffraction, even small tilts of the sample can significantly affect it. The goniometric device has two mutually perpendicular tilt axes, which lie in the plane of the sample and are adapted for its rotation through 360°. When tilted, the device ensures that the position of the object remains unchanged relative to the axis of the microscope. A goniometric device is also necessary when obtaining stereo images to study the relief of the fracture surface of crystalline samples, the relief of bone tissues, biological molecules, etc.

A stereoscopic pair is obtained by shooting in an electron microscope the same place of an object in two positions, when it is rotated at small angles to the objective axis (usually ±5°).

Interesting information about the change in the structure of objects can be obtained by continuously monitoring the heating of the object. With the help of the attachment, it is possible to study surface oxidation, the process of disordering, phase transformations in multicomponent alloys, thermal transformations of some biological preparations, and to carry out a complete cycle of heat treatment (annealing, hardening, tempering), moreover, with controlled high heating and cooling rates. Initially, devices were developed that were hermetically attached to the chamber of objects. Using a special mechanism, the object was removed from the column, heat-treated, and then placed back into the object chamber. The advantage of the method is the absence of column contamination and the possibility of long-term heat treatment.

Modern electron microscopes have devices for heating the object directly in the column. Part of the object holder is surrounded by a microfurnace. The heating of the tungsten spiral of microfurnaces is carried out by direct current from a small source. The temperature of the object changes when the heater current changes anddetermined from the calibration curve. The device retains a high resolution when heated up to 1100°C, about 30 Å.

Recently, devices have been developed that make it possible to heat an object with the electron beam of the microscope itself. The object is located on a thin tungsten disk. The disk is heated by a defocused electron beam, a small part of which passes through a hole in the disk and creates an image of the object. The temperature of the disk can be varied over a wide range by changing its thickness and the diameter of the electron beam.

There is also a table in the microscope for observing objects in the process of cooling to -140 ° C. Cooling - liquid nitrogen, which is poured into a Dewar vessel,connected to the table with a special cold pipe. In this device, it is convenient to study some biological and organic objects that are destroyed under the influence of an electron beam without cooling.

With the help of an attachment for stretching an object, it is possible to study the movement of defects in metals, the process of initiation and development of a crack in an object. Several types of such devices have been created. In some, mechanical loading is used by moving the grips in which the object is attached, or by moving the pressure rod, while others use heating of bimetallic plates. The sample is glued or clamped to bimetallic plates that move apart when heated. The device allows you to deform the sample by 20% and create a force of 80 g.

The most important attachment of an electron microscope can be considered a microdiffraction device for electron diffraction studies of any particular area of ​​an object of particular interest. Moreover, the microdiffraction pattern on modern microscopes is obtained without reworking the device. The diffraction pattern consists of a series of either rings or spots. If many planes in an object are oriented in a way that is favorable for diffraction, then the image consists of focused spots. If an electron beam hits several grains of a randomly oriented polycrystal at once, diffraction is created by numerous planes, and a pattern of diffraction rings is formed. By the location of the rings or spots, one can determine the structure of the substance (for example, nitride or carbide), its chemical composition, the orientation of the crystallographic planes and the distance between them.

2.4 Electron sources

Four types of electron sources are commonly used: tungsten V-shaped cathodes, tungsten point (point) cathodes, lanthanum hexaboride sources, and field electron sources. This chapter briefly discusses the advantages of each type of electron source for high-resolution transmission electron microscopy and their characteristics. The following basic requirements are imposed on electron sources used in high-resolution electron microscopy:

1. High brightness (current density per unit solid angle). The fulfillment of this requirement is essential for experiments in obtaining high-resolution images with phase contrast, when it is necessary to combine a small illumination aperture with a sufficient current density, which makes it possible to accurately focus the image at high magnification.

2. High efficiency of electron use (the ratio of brightness to the total value of the current of the primary electron beam), which is achieved due to the small size of the source. Reducing the illuminated area of ​​the sample reduces its heating and thermal drift during exposure.

3. Long lifetime in the existing vacuum.

4. Stable emission during long-term (up to a minute) exposure, which is typical in high-resolution microscopy.

An ideal illumination system for a conventional high resolution transmission microscope would be one that allows the operator to independently control the size of the illuminated area of ​​the specimen, the illumination intensity and the beam coherence. Such possibilities are achieved only when working with an autoelectronic source. However, for most laboratories, the use of a tungsten point cathode is the best compromise for both cost and performance for high resolution transmission microscopy. At present, the possibility of using sources from lanthanum hexaboride is also being considered. Also promising is a cathode heated by a laser beam, the brightness of which, as reported, is 3000 times greater than the brightness of a V-shaped cathode with an effective source diameter of about 10 nm. These cathodes operate in moderate vacuum (Torr).

2.5 Lighting system

The system has two condenser lenses C1 (strong lens) and C2 (weak lens). F - cathode; W – Wepelt cylinder; S is an imaginary electron source, S" and S" are its images; SA2 - second condenser diaphragm. distances,are electron-optical parameters, while the distanceseasily measured in the microscope column.

On fig. 2.5.1 shows two condenser lenses included in the illumination system of an electron microscope. It is usually possible to independently change the focal length of these lenses ( and) . The excitation of the first condenser lens is changed using an adjustment knob, sometimes referred to as "spot size". Usually, such an excitation is chosen in which the S, S" planes and the sample surface are conjugate, i.e., so that a focused image of the source is formed on the sample (focused illumination).

For a V-shaped cathode, the source size is approximately 30 µm. To prevent unwanted heating and radiation damage to the sample, it is necessary to form a reduced image of the source on it. Working distanceit must also be large enough to allow the object holder to move when the sample is changed. It is difficult to meet these conflicting requirements when using a single condenser lens - low magnification at long distance- because for this it is necessary that the distance be excessively large. Therefore, a strong first condenser lens is usually used,which serves to reduce the image of the source by 5 - 100 times, and the second weak one following the first one lens with an increase of about 3 provides a large working distance.

2.6 Astigmatism Correction

The adjustment of the stigmatator of the objective lens is very critical to ensure high resolution. Some devices adjust astigmatism in both direction and strength, while others provide for adjusting astigmatism strength in two fixed orthogonal directions. First of all, astigmatism should be roughly corrected with a stigmator until the symmetry of the Fresnel ring is obtained. When working with high resolution, it is necessary to correct astigmatism as accurately as possible, which can be done by imaging the structure of a thin amorphous carbon film at high magnification. A microscope magnification of at least 400,000x and an optical binocular x10 are required to carefully correct for astigmatism in the details of such a 0.3 nm image. Use the focus adjustment knobs and the stigmatator to achieve the minimum contrast that is achieved by using the finest adjustment knobs. When the lens is underfocused by a few tens of nanometers, a uniform granular structure of the carbon film should be visible without anisotropy in any preferred direction. This is a difficult procedure requiring considerable skill. The optical X-ray diffraction pattern is the fastest way to check the correctness of astigmatism correction, and its use is especially important when mastering the astigmatism correction procedure. The following points are important:

1. Eyes must be fully dark-adapted. To do this, spend at least 20 minutes in the dark.

2. The position and cleanliness of the objective iris and cooled iris in the lens field will critically affect the required stigmatator placement. Never touch either aperture after correcting astigmatism before photographing the image. Most importantly, astigmatism does not change over time and can be corrected. Slight contamination of the objective diaphragm does not create interference that cannot be corrected with a stigmator. A dirty diaphragm, which creates field fluctuations, is a more serious interference. Check how dirty the lens iris is by moving it while viewing the image. With small aperture shifts, there should not be a strong deterioration in astigmatism. The cleanliness of the aperture of a cooled diaphragm can be checked at the magnification at which it limits the field of view. The check is made by moving the cooled diaphragm slightly, if possible, observing at low magnification.

3. The astigmatism correction current varies depending on the type of object holder used, the accelerating voltage, and the drive current of the objective lens. The latter is slightly dependent on magnification, possibly due to the magnetic interaction of the lenses.

4. A common cause of severe astigmatism is the presence of a chipped or partially evaporated specimen in the objective pole piece.

5. There is no point in correcting astigmatism until the cooled diaphragm reaches liquid nitrogen temperature and the cold diaphragm reservoir has to be periodically topped up with liquid nitrogen (preferably with a pump). Astigmatism also appears quickly as liquid nitrogen evaporates from the reservoir, causing the diaphragm to move as it heats up. It may take at least half an hour for the diaphragm temperature to stabilize from the start of filling the reservoir.

The sensitivity of high-resolution images to astigmatism can be judged by observing planes of graphitized carbon in a bright field with non-tilted illumination while adjusting the stigmatator. To obtain images of grating planes located in all possible directions, it is necessary to accurately compensate for astigmatism in two directions. It is easier to image the grating planes in one direction, but it does not provide precise astigmatism correction control.

Finally, it is worth reiterating that astigmatism needs to be corrected after each movement of the lens aperture.

2.7 Accessories for Conventional High Resolution Transmission Electron Microscopy

In addition to the microscope itself, there are various auxiliary devices that complement the microscope, which were mentioned earlier. Collectively, they are all covered in this section.

1. A mass spectrometer or partial pressure gauge is an extremely useful addition to an electron microscope. The mass spectrometer gives a complete analysis of the contamination products in the microscope. Some devices have magnets in their designs; such a device should be positioned taking into account the possible influence on the electron microscope image.

2. When working with high resolution, it is useful to use bottled dry nitrogen. The microscope is filled with dry nitrogen whenever internal repairs are needed to reduce the amount of water vapor entering the column.

3. To calibrate the magnification of the device in conditions of a changing length of the focus of the objective lens, it is useful to use a device for measuring the current of the objective lens.

4. In view of the importance of ensuring thermal stability when photographing dark-field images with long exposures, it is advisable to have a pump for pumping liquid nitrogen.

5. To blow off any dust or product residue left after cleaning the microscope gun chamber, it is always a good idea to have a blower with a nozzle.

3. Preparation of objects for research and special requirements for them

Foil is most often prepared in the following way. A round blank 3 mm in diameter and 0.2-0.3 mm thick is cut out from the sample to be studied, which is then thinned by grinding to 0-1-0-15 mm. The final thinning of the plate is carried out by chemical or electrolytic (most common) polishing in a suitable reagent (by chemical composition, temperature). The prepared plate is immersed in the electrolyte as an anode. The cathodes are two metal plates located on both sides of the sample (foil). Electropolishing, at an optimal ratio of current and voltage, is continued until one or more small holes (0.2-0.8 mm in diameter) appear in the central part of the polished plate. At the edges of such holes, the foil sections are the thinnest and can be used for viewing in an electron microscope.

When examining replicas and foils under an electron microscope at high magnifications, the appearance of the microstructure changes significantly. Therefore, in order to correctly decipher the structure, it is necessary to start the study with small magnifications, gradually moving to large ones.

For metal-physical studies, microscopes with an accelerating voltage of 100-200 kV are usually used, which allow electron beams to shine through objects with a thickness of 0.2-0.4 μm (the limiting thickness depends on atomic mass material). With an increase in the accelerating voltage, the penetrating power of electrons increases, which makes it possible to study thicker objects. Electron microscopes UEMV-100, PEM-100 have been widely used. EM-200, etc. Electron microscopes with an accelerating voltage of 500, 1000, 1500 and even 3500 kV are known. Such microscopes make it possible to study objects up to several micrometers thick.

4. Transmission Electron Microscope Application

There is hardly any sector of research in the field of biology and materials science where transmission electron microscopy (TEM) has not been applied; this is due to advances in sample preparation techniques.

All techniques used in electron microscopy are aimed at obtaining an extremely thin sample and providing maximum contrast between it and the substrate that it needs as a support. The basic technique is designed for samples with a thickness of 2–200 nm, supported by thin plastic or carbon films, which are placed on a grid with a cell size of about 0.05 mm. A suitable sample, no matter how it is obtained, is processed in such a way as to increase the intensity of electron scattering on the object under study. If the contrast is high enough, then the observer's eye can easily distinguish details that are at a distance of 0.1 - 0.2 mm from each other. Therefore, in order for the image created by an electron microscope to distinguish details separated on a sample by a distance of 1 nm, a total magnification of the order of 100 - 200 thousand is necessary. The best of microscopes can create an image of a sample on a photographic plate with such a magnification, Too small area shown. Usually a micrograph is taken at a lower magnification and then enlarged photographically. A photographic plate resolves about 10,000 lines over a length of 10 cm. If each line corresponds on the sample to a certain structure with a length of 0.5 nm, then to register such a structure, an increase of at least 20,000 is required, while with the help of TEM, about 1000 lines can be resolved.

4.1 Non-biological materials

The main goal of high-resolution electron microscopy today is to visualize details of the ultrastructure of imperfect crystalline materials. Currently, there are no other methods capable of providing such information at the atomic resolution level or at the elementary cell resolution level. A detailed understanding of the structure of crystal defects determines the progress both in crystal chemistry and in the field of studying the strength of materials. Using an electron beam to control the rate of a chemical reaction in crystals, one can also study the motion of defects during phase transitions almost at the atomic level. High-resolution electron microscopy is also widely used to study the microstructure of very small crystals, from which it is impossible to obtain an x-ray diffraction pattern. In recent years, this method has been widely used to study minerals and ceramic materials.

Studies of minerals by the replica method began several decades ago. Mica and clay minerals were the first to be studied directly by transmission electron microscopy. Among the first mineralogists who used electron microscopy in their research are Ribbe, McConnell and Fleet. The work of McLaren and Fakey (since 1965) and Nissen (since 1967) had a great influence on the development of electron microscopy as applied to mineralogy; their research program was entirely devoted to the electro-microscopic study of minerals. In 1970, work on the study of lunar materials by TEM methods contributed to the emergence of an extraordinary boom in electron microscopy of minerals, in which, along with mineralogists, materials scientists and physicists were involved. The results obtained by them within five years, which had a tremendous impact on modern mineralogy, showed that electron microscopy is a very powerful tool in the hands of a scientist. To date, new data have made a significant contribution to the deciphering of the structure of feldspars and pyroxenes, and in almost every group of minerals, studies using electron microscopy reveal a number of unexpected properties.

Electron microscopy has also been used to determine the age of terrestrial, lunar, and meteorite rocks. In this case, the fact was used that during the radioactive decay of the nucleus, particles are released that penetrate into the surrounding material with high speed and leaving a visible "trace" in the crystal. Such tracks can be seen with an electron microscope, using it in scanning or transmission modes. The density of decay tracks around a radioactive inclusion is proportional to the age of the crystal, and their length is a function of the particle's energy. Long tracks indicating high particle energy have been found in lunar rock; Hutcheon and Price attributed this extraordinarily long track to the decay of element 244. Po , which, due to its short half-life, has disappeared by now, but could still exist 4 billion years ago. Tracks in material taken from the surface of the Moon or from meteorites (Fig. 4.1.1) provide information about the evolution of cosmic radiation and allow conclusions to be drawn about the age and composition of the universe.

The high track density is caused by the presence of energetically heavier nuclei (mainly Fe) in a solar flare before meteorite formation.

TEM is used in materials research to study thin crystals and interfaces between different materials. To obtain a high-resolution image of the interface, the sample is filled with plastic, the sample is cut perpendicular to the interface, and then it is thinned so that the interface is visible on the sharp edge. The crystal lattice strongly scatters electrons in certain directions, giving a diffraction pattern. The image of a crystalline sample is largely determined by this pattern; the contrast is highly dependent on the orientation, thickness, and perfection of the crystal lattice. Changes in the contrast in the image make it possible to study the crystal lattice and its imperfections on the scale of atomic sizes. The information obtained in this way supplements that provided by X-ray analysis of bulk samples, since EM makes it possible to directly see dislocations, stacking faults, and grain boundaries in all details. In addition, electron diffraction patterns can be taken in EM and diffraction patterns from selected areas of the sample can be observed. If the lens diaphragm is adjusted so that only one diffracted and unscattered central beam passes through it, then it is possible to obtain an image of a certain system of crystal planes that gives this diffracted beam. Modern instruments make it possible to resolve grating periods of 0.1 nm. Crystals can also be studied by dark-field imaging, in which the central beam is blocked so that the image is formed by one or more diffracted beams. All these methods have provided important information about the structure of very many materials and have significantly clarified the physics of crystals and their properties. For example, the analysis of TEM images of the crystal lattice of thin small-sized quasicrystals in combination with the analysis of their electron diffraction patterns made it possible in 1985 to discover materials with fifth-order symmetry.

4.2 Biologicals

Electron microscopy is widely used in biological and medical research. Techniques for fixing, embedding, and obtaining thin tissue sections for TEM studies have been developed. These techniques make it possible to study the organization of cells at the macromolecular level. Electron microscopy revealed the components of the cell and details of the structure of membranes, mitochondria, the endoplasmic reticulum, ribosomes, and many other organelles that make up the cell. The sample is first fixed with glutaraldehyde or other fixatives, and then dehydrated and embedded in plastic. Cryofixation methods (fixation at very low - cryogenic - temperatures) allow preserving the structure and composition without the use of chemical fixatives. In addition, cryogenic methods allow imaging of frozen biological samples without dehydration. Using ultramicrotomes with polished diamond or chipped glass blades, tissue sections can be made with a thickness of 30–40 nm. Mounted preparations can be stained with heavy metal compounds (lead, osmium, gold, tungsten, uranium) to enhance the contrast of individual components or structures.

Biological studies have been extended to microorganisms, especially viruses, which are not resolved by light microscopes. TEM made it possible to reveal, for example, the structures of bacteriophages and the location of subunits in the protein coats of viruses. In addition, positive and negative staining methods have been able to reveal the structure with subunits in a number of other important biological microstructures. Nucleic acid contrast enhancement techniques have made it possible to observe single- and double-stranded DNA. These long, linear molecules are spread into a layer of basic protein and applied to a thin film. Then a very thin layer of heavy metal is applied to the sample by vacuum deposition. This layer of heavy metal "shades" the sample, due to which the latter, when observed in TEM, looks as if illuminated from the side from which the metal was deposited. If, however, the sample is rotated during deposition, then the metal accumulates around the particles from all sides evenly (like a snowball).

4.3 High voltage microscopy

Currently, the industry produces high-voltage versions of PEMs with an accelerating voltage of 300 to 400 kV. Such microscopes have a higher penetrating power than low-voltage instruments, and are almost as good as the 1 million volt microscopes that were built in the past. Modern high-voltage microscopes are quite compact and can be installed in an ordinary laboratory room. Their increased penetrating power proves to be a very valuable property in the study of defects in thicker crystals, especially those from which it is impossible to make thin specimens. In biology, their high penetrating power makes it possible to examine whole cells without cutting them. In addition, these microscopes can be used to obtain three-dimensional images of thick objects.

4.4 radiation damage

Because electrons are ionizing radiation, the sample in an EM is constantly exposed to it. Therefore, samples are always exposed to radiation damage. The typical dose of radiation absorbed by a thin sample during a TEM micrograph is roughly equivalent to the energy that would be sufficient to completely evaporate cold water from a 4 m deep pond with a surface area of ​​1 ha. To reduce radiation damage to the sample, it is necessary to use various methods of its preparation: staining, pouring, freezing. In addition, it is possible to register an image at electron doses that are 100–1000 times lower than by the standard method, and then improve it using computer image processing methods.

5. Modern types of transmission electron microscopy

The state-of-the-art transmission electron microscope Titan™ 80 – 300 provides images of nanostructures at the sub-angstrom level. Electron microscope Titan operates in the range of 80 - 300 kV with the ability to correct spherical aberration and monochromaticity. This electron microscope meets stringent requirements for maximum mechanical, thermal and electrical stability, as well as precise alignment of advanced components. Titanium expands the resolution capabilities of spectroscopy in measuring band gaps and electronic properties and allows the user to obtain clear images of interfaces and more fully interpret the obtained data..

The 300-kilovolt high-precision, ultra-high-resolution analytical electron microscope is designed to simultaneously observe the image at the atomic level and accurately analyze the sample. This microscope uses many new developments, including a compact 300 kV electron gun, an illumination system with five lenses.

The use of a built-in ion pump ensures a clean and consistently high vacuum. Dot resolution: 0.17 nm. Accelerating voltage: from 100 to 300 kV. Magnification: from x 50 to x 1,500,000.

A transmission electron microscope equipped with a high-brightness electron gun with a heated field emission cathode with increased emission current stability. Allows you to directly observe the details of the atomic structure and analyze individual atomic layers. The field emission heated cathode electron gun, most suitable for the analysis of nanodomains, provides a probe current of 0.5 nA at a probe diameter of 1 nm and 0.1 nA at 0.4 nm. Resolution per dot: 0.17 nm. Accelerating voltage: 100, 200, 300 kV. Magnification: from x60 to x1,500,000.

The field emission electron gun, which provides an electron beam with high brightness and coherence, plays a key role in obtaining high resolution and in the analysis of nanostructures. The JEM - 2100F is a complex TEM equipped with an advanced electronic control system for various functions.

The main features of this device: 1) High brightness and stability of the electron gun with thermal field emission provides analysis of nanoscale regions at high magnification. 2) The probe diameter less than 0.5 nm allows to reduce the point of analysis to the level of nanometers. 3) New highly stable side loading sample stage provides easy tilting, turning, heating and cooling, programmable settings and more without mechanical drift.

Allows not only to acquire transmission images and diffraction patterns, but also includes a computer control system that can integrate a TEM, a scanning mode imaging device (STEM), an energy dispersive spectrometer (JED - 2300 T) and an electron energy loss spectrometer (EELS ) in any combination.

The high resolution (0.19 nm) is achieved due to the stability of the high voltage and beam current, together with an excellent lens system. The new microscope column frame structure gently reduces the effect of instrument vibration. The new goniometric stage allows sample positioning with nanometer precision. computer system microscope control provides network connection of other users (computers) and information exchange between them.

6. Disadvantages and limitations, features of the use of transmission electron microscopy

To begin with, we note the shortcomings of the transmission electron microscope. Materials require special preparation before direct examination, since it is necessary to make a sample of such a thickness that electrons sufficiently pass through it. The samples under study can only be placed on graphene, a carbon nanomaterial one atom thick, which will provide sufficient throughput. The TEM field of view is limited, which does not allow evaluation of the entire sample surface. In the case of biomaterials, the probability of damage to the sample is high.

Let's look at permission restrictions next. TEM resolution is often limited by spherical and chromatic aberrations. The new generation of correctors already makes it possible to overcome a significant part of spherical aberrations. Software although the correction of spherical aberrations made it possible to obtain an image of a carbon atom in a diamond with a sufficiently high resolution. Previously, this could not be done because the interatomic distance was 0.89 angstroms (89 pikameters. 1 angstrom \u003d 100 pikameters \u003d 10 ~ 10 m). The increase in this case was 50 million times. The ability to determine the arrangement of atoms in materials has made TEM an indispensable tool for nanotechnology, research and development in many areas, including heterogeneous catalysis, as well as in the development of semiconductor devices in electronics and photonics.

Finally, consider the use of transmission electron microscopy. If scanning electron microscopy can explain how the destruction occurred in the studied material of the product, how the mechanical surface of the part responds to thermoplastic action external environment, then transmission electron microscopy can explain why this happens, how this is facilitated by the structural-phase state of the material.

The method of transmission electron microscopy makes it possible to study the internal structure of the studied metals and alloys, in particular:

• determine the type and parameters of the crystal lattice of the matrix and phases;
• define oriented relationships between phase and matrix:
• study the structure of grain boundaries;
• determine the crystallographic orientation of individual grains, subgrains;
• determine misorientation angles between grains, subgrains;
• determine the plane of occurrence of defects in the crystal structure;
• to study the density and distribution of dislocations in the materials of products;
• to study the processes of structural and phase transformations in alloys:
• study the effect of technological factors (rolling, forging, grinding, welding, etc.) on the structure of structural materials.

All of the above tasks are constantly encountered in practical activities researchers of metals and alloys. Chief among them is the taskthe choice of material for structures with specified mechanical properties, such that the finished structure can work stably in the conditions of its further operation. This problem can be solved only by the joint efforts of crystallographers, metallurgists and technologists. The success of its solution depends on: 1) On right choice base metal with desired type crystal lattice. 2) From alloying and thermoplastic processing of metal in order to form a given structure in it - this is the field of metal science. 3) From the development of technological processes for the manufacture of structures - this is the area of ​​\u200b\u200btechnology.

The task of creating an alloy with desired mechanical properties implies the creation of a material with the desired internal structure, since almost all mechanical properties are structure-sensitive. Without exception, all changes in the properties of metals and alloys in deep or surface layers are a response to a change in their internal structure at the macro, micro and submicroscopic levels.

The study of surface microtopography and the internal structure of structural materials is one of the most effective applications of powerful modern and rapidly developing methods of scanning and transmission electron microscopy.

Conclusion

Until relatively recently, mineralogists had two classical tools in their hands - a polarizing microscope and X-ray diffraction equipment. Using an optical microscopewe can study the morphology and optical properties of minerals, study twins and lamellas if they exceed the wavelength of the incident light in size. X-ray diffraction datamake it possible to accurately determine the position of atoms in a unit cell on a scale of 1 – 100 Å. However, such a definition of the crystal structure gives us a certain structure averaged over many thousands of elementary cells; therefore, we assume in advance that all elementary cells are identical.

At the same time, the importance of structural details that characterize minerals on a scale of 100-10,000 Å is becoming increasingly clear. Diffuse reflections on X-ray patterns were interpreted as evidence of the existence of small domains; the asterism observed in the Laue patterns, or the small values ​​of the extinction coefficients during the refinement of the structure, indicated that the crystals are imperfect in their structure and contain various defects. To study inhomogeneities whose sizes are within the indicated limits, the electron microscope is an ideal tool.Such studies are an important source of geological information characterizing the parameters of cooling and formation of minerals and rocks or the conditions of their deformation.

In contrast to X-ray diffraction, which began to be used in mineralogy immediately after its discovery, electron microscopy was initially most developed and used in metallurgy. After the creation of industrial instruments in 1939, it took more than 30 years for the electron microscope to become a common instrument in mineralogy and petrography.

The advantage of electron microscopy is that structures and textures can be depicted in real space with it, and therefore the results are easier to visualize than they can be obtained by calculating diffraction patterns. It is appropriate here to mention the need to exercise some caution. Unlike observations in an optical microscope, the structure cannot be seen directly through an electron microscope. We simply observe the contrast arising, for example, from the strain field around the dislocations, and this contrast is transformed into an image inside the device. Electron microscopy does not replace research conducted by X-ray diffraction methods. On the other hand, there are many examples where electron microscopy data served as a basis for interpreting X-ray data. These two methods complement each other perfectly.

Bibliography

1. V. G. Dyukov, S. A. Nepiiko, and N. N. Sedov Electron microscopy of local potentials./ Academy of Sciences of the Ukrainian SSR. Institute of Physics. - Kyiv: Nauk. Dumka, 1991. - 200 p.

2. Kulakov Yu.A. Electron microscopy. - M.: Knowledge, 1981. – 64 p.

3. Ch. Pool, F. Owens Nanotechnologies: Per. from English / Ed. Yu. I. Golovina. - M.: Technosfera, 2005. - 336 p.

4. Spence J. Experimental high-resolution electron microscopy: TRANS. from English / Ed. V. N. Rozhansky. – M.: Science. Ch. ed. Phys.-Math. Lit., 1986. - 320 p., ill.

5. Thomas G., Gorinzh M. J. Transmission electron microscopy of materials: Per. from English / Ed. B.K. Weinstein - M: Science. Main edition of physical and mathematical literature, 1983 - 320s

6. Electron microscopy in mineralogy: Per. from English / Under the general editorship. G.-R. Wreath. - M.: Mir, 1979. - 485 p., ill.

7. AI Vlasov, KA Elsukov, IA Kosolapov Electron microscopy;

Edited by Honored Scientist of the Russian Federation, Corresponding Member of the Russian Academy of Sciences, Professor V. A. Shakhnov

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He expanded the resolution limit from the wavelength of light to atomic dimensions, or rather to interplanar distances of the order of 0.15 nm. The first attempts to focus an electron beam using electrostatic and electromagnetic lenses were made in the 1920s. The first electron microscope was made by I. Ruska in Berlin in the 30s. Her microscope was translucent and was intended for the study of powders, thin films and sections.

Reflecting electron microscopes appeared after World War II. Almost immediately they were superseded by scanning electron microscopes combined with microanalysis tools.

High-quality preparation of a sample for a transmission electron microscope is a very difficult task. However, methods for such training exist.

There are several methods for sample preparation. With good equipment, a thin film can be prepared from almost any technical material. On the other hand, don't waste time studying a poorly prepared sample.

Let us consider methods for obtaining thin samples from a block material. Methods for the preparation of biological tissues, dispersed particles, as well as the deposition of films from the gas and liquid phases are not considered here. It should be noted that almost any material has features of preparation for an electron microscope.

Mechanical restoration.

The starting point for sample preparation is usually a disk 3 mm in diameter and a few hundred microns thick, cut from a massive piece. This disc can be punched out of metal foil, cut out of ceramic, or machined from a block pattern. In all cases it is necessary to minimize the risk of micro-cracking and maintain a flat sample surface.

The next task is to reduce the thickness of the sheet. This is done by grinding and polishing, as in preparing a sample for an optical microscope. Choice best way grinding is determined by the rigidity (modulus of elasticity), hardness and degree of plasticity of the material. Ductile metals, ceramics and alloys are polished differently.

electrochemical etching.

During machining, as a rule, near-surface damage such as plastic shear or microcracking appears. In the case of a conductive metal, the sample thickness can be reduced by chemical or electrochemical dissolution in an electropolishing solution. However, it should be borne in mind that the processing parameters of thin samples differ significantly from macrosamples, primarily due to the smallness of the processed area. In particular, in the case of thin samples, much higher current densities can be used. The problem of cooling the material due to the occurrence of a chemical reaction is solved by carrying out the reaction in a solvent jet, and the processing of the disk can be two-sided.

Thin films of metals, alloys and other electrically conductive materials are often successfully jet polished. However, the conditions for polishing such materials differ in composition, solution temperature, and current density.

The areas around the neutral hole should be transparent (typically 50-200 nm in diameter). If the areas suitable for examination are too small, this is due to too long etching, which should be stopped immediately after the hole appears. If these areas are too rough, then either the current density is too low, or the contaminated and overheated polishing solution should be changed.

ion etching.

The ion etching (bombardment) method has the following advantages:

(a) Ion etching is a gas-phase process carried out at low pressure, where it is easy to control the degree of surface contamination.

(b) Electrochemical methods are limited to conductive metals, while ion etching is applicable to non-conductive materials as well.

(c) Although ion etching can result in near-surface radiation damage to the material, its extent can be reduced by appropriate selection of process parameters.

(d) Ion etching removes surface oxide layers from previous electropolishing. This does not change the surface composition, since the process is usually carried out at low temperatures, when there is no surface diffusion.

(e) Ion etching makes it possible to process multilayer materials consisting of several layers deposited on a substrate in a plane perpendicular to the layers. Note that standard chemical etching methods do not allow this.

(c) The ion etching method allows processing areas smaller than 1 µm, which is impossible chemical methods. It is very useful for preparing thin films.

Of course, this method also has disadvantages. Etching speed is maximum. if the ion beam is perpendicular to the sample surface, and the atomic weights of the ions and the material being processed are close. However, the ion beam transfers momentum, and at an angle of 90 0 the microdamage of the surface layer is maximum. In addition, due to the danger of chemical interaction of ions with the treated surface, only inert gases (usually argon) are used as a beam.

The etch rate can be increased by increasing the energy of the ions, but at the same time they begin to penetrate the material and create a damaged surface layer. In practice, the ion energy is limited to a few keV when the penetration depth is not too high and the ions can diffuse to the surface without damaging the material.

The etching rate does not exceed 50 µm per hour. As a consequence, prior to ion processing, samples must be mechanically (disc or wedge-shaped) or electrochemically processed to a thickness of 20-50 µm. During ion bombardment, the sample is rotated. in order to guarantee uniform processing, and to increase the etching speed, the initial processing stage is carried out simultaneously on both sides at an angle of 18 0 . After that, the beam angle (and, consequently, the speed of the process) is reduced. The minimum angle that makes it possible to obtain a flat surface and approximately the same film thickness in a sufficiently large area is determined by the geometry of the ion beam. At small angles of incidence, the beam ceases to hit the sample, and the chamber material sprayed in this case is deposited and contaminates the surface of the sample. The minimum angles of incidence of the beam at the final stage of processing are usually equal to 2-6 0 .

As a rule, processing is completed when the first hole appears on the surface of the sample. In modern ion plants you can monitor the area being treated and the process of work. which allows the process to complete correctly.

Spray coating.

Since the electron beam carries an electrical charge, the sample can be charged during operation of the microscope. If the charge on the sample becomes too high (but in many cases this is not the case, since the residual surface conductivity often limits the amount of charge), the sample must be covered with an electrically conductive layer. The best material for this is carbon, which after sputtering has an amorphous structure and has a low atomic number (6).

The cover is created by passing electricity through two contacting carbon rods. The second method consists in sputtering the carbon material by bombarding it with inert gas ions, after which the carbon atoms are deposited on the surface of the sample. "Problem" materials may require coating on both sides. Sometimes thin (5-10 nm) nanometer coatings are barely visible in the image.

replica method.

Instead of preparing a thin sample for a transmission electron microscope, a replica (imprint) of the surface is sometimes made. In principle, this is not required if the surface can be examined with a scanning electron microscope. However, in this case, there may be a number of reasons for preparing replicas, for example:

(a) If the specimen cannot be cut. After cutting the part, it can no longer be used. On the contrary, removing the replica allows you to save the part.

(b) When looking for certain phases on the sample surface. The surface of the replica reflects the morphology of such phases and makes it possible to identify them.

(c) It is often possible to extract one of the components of a multiphase material, for example by chemical etching. This component can be isolated on the replica, while retaining it on the original material. The chemical composition, crystallographic structure and morphology of the selected phase can be studied in isolation from the main material, the properties of which sometimes interfere with the study,

d) Finally, sometimes it is necessary to compare the image of a replica with the original surface in a scanning electron microscope. An example is the study of a material under mechanical fatigue conditions, when the surface changes during the test.

The standard technique is to obtain a negative replica using a plastic polymer. The replica is obtained by using a cured epoxy or solvent-softened polymer film pressed against the surface to be examined before the solvent evaporates. In some cases it is required to remove surface contamination. To do this, before creating the final replica, ultrasound is used or a preliminary “cleaning” replica is made before removing the final replica. In some cases, the object of study may be a "pollutant".

After the polymer replica has solidified, it is separated from the test sample and coated with a heavy metal layer (usually an alloy of gold and palladium) to increase the image contrast. The metal is chosen so that during sputtering the size of its droplets is minimal, and the scattering of electrons is maximal. The metal droplet size is usually on the order of 3 nm. After metal shading, a 100–200 nm thick carbon film is sputtered onto the polymer replica, and then the polymer is dissolved. The carbon film, together with the particles extracted by the polymer from the original surface, as well as the metal layer shading it (reflecting the topography of the original surface), is then rinsed, placed on a thin copper grid and placed in a microscope.

Surface preparation.

The use of multilayer thin-film materials in electronics has led to the need to develop methods for their preparation for examination in a transmission electron microscope.

The preparation of multilayer samples has several stages:

First, the sample is immersed in liquid epoxy, which is then cured and cut perpendicular to the plane of the layers.

The flat specimens are then either machined with a disc or polished to obtain wedge-shaped specimens. In the latter case, the thickness of the removed material and the angle of the wedge are controlled with a micrometer. Polishing has several stages, the last of which uses particles of diamond powder with a diameter of 0.25 microns.

Apply ion etching until the thickness of the area under study is reduced to the desired level. The final processing is carried out with an ion beam at an angle of less than 6 0 .

Literature:

Brandon D, Kaplan W. Microstructure of materials. Methods of research and control // Publisher: Tekhnosfera.2006. 384 p.

Transmission electron microscope with field emission cathode, energy OMEGA filter, Köhler illumination system (patented by Carl Zeiss SMT) – the microscope is designed for high resolution.

Transmission electron microscope Zeiss Libra 200FE

Libra 200 FE is an analytical transmission electron microscope for studying solid and biological samples. Equipped with a high-efficiency field emission emitter and energy OMEGA filter to perform precision measurements with high resolution of the crystal lattice and chemical composition of nano-sized objects. Images obtained in the MRC in the direction of "Nanotechnology".

The main characteristics of the microscope:

Accelerating voltage:

200 kV, 80 kV, 120 kV.

Increase:

• in TEM (TEM) mode 8x - 1,000,000x;
• in the PREM (STEM) mode 2,000x - 5,000,000x;
• in EELS mode 20x - 315x.

Limit resolution:

• in TEM mode 0.12 nm;
• in STEM mode 0.19 nm.

Resolution of the EELS spectrometer: energy 0.7 eV.

• - high resolution electron microscopy (HREM);
• - transmission electron microscopy (TEM);
• - scanning transmission electron microscopy (STEM);
• - TEM with energy filtering;
• - electron diffraction (ED);
• - Converging Beam ED (CBED);
• - analytical electron microscopy (EELS, EDS);
• - Z-contrast;
• - observation of an object in the temperature range from -170 o C to 25 o C.

Areas of use:

• - characterization of the crystal lattice and chemical nature of nanoobjects;
• - local analysis of elemental composition;
• - analysis of the structural perfection of multilayer heterostructures for micro- and optoelectronics;
• - identification of defects in the crystal lattice of semiconductor materials;
• - fine structure of biological objects.

Sample requirements:

The standard sample size in the plane of the TEM holder is 3 mm in diameter. Typical thicknesses for TEM, for example: aluminum alloys, semiconductor materials TEM - 1000 nm; HREM - 50 nm.

Energy Dispersive X-Ray Detector X-Max

The spectrometer type is energy dispersive (EDS).

Detector type - Analytical Silicon Drift Detector (SDD): X-Max;
active area of ​​the crystal - 80 mm 2;
nitrogen-free cooling (Peltier);
motorized slider.

Spectral resolution - 127 eV (Mn), corresponds to ISO 15632:2002;

Sensitivity to concentration - 0.1%.

Image holders for LIBRA 200

Gatan Model 643 Single Axis Analytical Holder

Designed for imaging and analytical applications such as electron diffraction and EDX analysis of TEM samples where two axes of sample tilt are not required.

Main characteristics:

• drift speed 1.5 nm/min
• holder material beryllium
• tilt angle maximum 60ᵒ

Gatan Model 646 Biaxial Analytical Holder

Designed for high resolution imaging, the holder includes design features, optimized for electron diffraction and EDX analysis of crystalline samples.

Main characteristics:

• drift speed 1.5 nm/min
• 0.34 nm resolution at zero tilt angle
• sample size 3 mm diameter, 100 micron thickness
• holder material beryllium
• slope angles α =60ᵒ β = 45ᵒ

Gatan Model 626 Single Axial Cryo Transfer Analytical Holder

The cryo holder is used in applications for low temperature studies of frozen hydrated samples, It can also be used for in-situ studies of phase transitions and the reduction of fouling due to carbon migration, reducing unwanted thermal effects in EELS.

Main characteristics:

• drift speed 1.5 nm/min
• 0.34 nm resolution at zero tilt angle
• sample size 3 mm diameter, 100 micron thickness
• cryogen liquid nitrogen
• holder material copper
• tilt angle maximum 60ᵒ

Model 626 workstation

Gatan Model 636 Biaxial Cryo Analytical Holder

The cryo holder is used in applications for research at low temperatures, in-situ phase transitions and the reduction of contamination due to carbon migration. It can also be used to reduce unwanted thermal effects in EELS and EDX analytical methods.

Main characteristics:

• drift speed 1.5 nm/min
• 0.34 nm resolution at zero tilt angle
• sample size 3 mm diameter, 100 micron thickness
• Max. working temperature 110ᵒС
• min. operating temperature - 170ᵒС
• cryogen liquid nitrogen
• temperature stability ± 1ᵒС
• cooling time 30 minutes to -170ᵒС
• holder material beryllium
• slope angles α =60ᵒ β = 45ᵒ

Gatan Model 652 Heated Biaxial Analytical Holder

The heated sample holder is designed for in situ observation of micro-structural phase changes, nucleation, growth and dissolution during elevated temperatures.

Main characteristics:

• drift rate 0.2 nm/min (at 0 to 500ᵒC)
• 0.34 nm resolution at zero tilt angle
• sample size 3 mm diameter, 500 micron thickness
• Max. operating temperature 1000ᵒС
• min. operating temperature room
• holder material beryllium, copper
• slope angles α =45ᵒ β = 30ᵒ

Used in conjunction with the following devices:

Model 652.09J Water recirculator

Gatan Model 654. Single axis deformation holder

The holder is designed for in situ study of sample tension.

Main characteristics:

• drift speed 1.5 nm/min
• 0.34 nm resolution at zero tilt angle
• sample size 2.5mm X 11.5mm, 500 micron thickness

Used in conjunction with the following device:

Controller Accuroller Model 902

Fischione Model 2040 Dual Axis Tomography Holder

The holder with an additional axis of rotation is designed to obtain a series of images for tomography.

Main characteristics:

• drift speed 1.5 nm/min
• 0.34 nm resolution at zero tilt angle
• sample size 3 mm diameter, 100 micron thickness
• holder material copper
• tilt angle maximum 70ᵒ

The main difficulty in using TEM is the preparation of samples. The thickness of the sample should be no more than a micron. Typically, such samples are made using photolithography and chemical etching. Jet and ion etching are also used. Variants of sample configurations for TEM are shown in Fig.6.

Often, when determining the LSI morphology, special test crystals containing components intended for TEM studies are used. An example of an electron microscopic image of a test crystal is shown in Fig. 7.

Whiskers have formed between the polysilicon layers in the thin oxide layer, which can cause a decrease in the breakdown voltage of the oxide and lead to circuit failure. It is extremely difficult to detect such "weak" points of the circuit by another method.

To study the surface relief of massive samples, replicas (prints) are made from the surface by applying special plastics (or graphite). The replicas are then separated from the sample and a thin layer of metal is applied to enhance the contrast. When studying the surface with the help of replicas, the resolution is (5 - 10) nm, while the object is not destroyed.

The energy of primary electrons in the TEM method is (0.6 - 3.5)·10 5 eV. When studying thin films by TEM at an accelerating voltage of (1 - 2) 10 5 V, the film thickness should not exceed (0.2 - 0.3) μm; ) µm.

TEM resolution is limited by spherical aberration and is (0.1 – 1.0) nm. In the image observation mode, the magnification of the microscope reaches the values ​​(2 - 5)·10 5 .

Main characteristics of TEM

Scanning electron microscopy

Scanning electron microscopy (SEM) is designed to study the surface topography, determine the composition, and detect defects in the crystal lattice. The method is indispensable in studying the causes of IC failures, determining the electrical potential on the surface.

Figure 8 shows the scheme of a scanning electron microscope. The source of electrons is an electron gun with a thermionic cathode, the filament of which is made of tungsten or LaB 6 . Electrons are accelerated by an electric field to energies E 0 = (0.2 - 4) 10 4 eV, that is, smaller than in TEM. The formation of an electron beam and its control is carried out using magnetic lenses and deflecting coils, which make it possible to obtain a beam of small diameter (2–10 nm) and unfold it into a raster on the sample surface.

REMwith thermionic cathode designed to study massive objects with a resolution of 70 to 200 A°. The accelerating voltage in the SEM can be adjusted in the range from 1 kV to 30–50 kV.

The device of such a SEM is shown in Fig.9. Using 2 or 3 magnetic electron lenses (EL), a narrow electron probe is focused onto the sample surface. Magnetic 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. 10) - secondary and reflected electrons; electrons that have passed through the object (if it is thin); x-ray bremsstrahlung and characteristic radiation; light emission, etc.

Any of these radiations can be registered by an appropriate collector containing a sensor that converts the radiation into electrical signals, which, 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 increase is equal to the ratio of the height of the frame on the CRT screen to the width of the scanned surface of the object. Photograph the image directly from the CRT screen. The main advantage of SEM is the high information content of the device, due to the ability to observe the image using signals from various sensors. With the help of SEM, one can study the microrelief, the distribution of the chemical composition over the object, p-n-transitions, perform X-ray diffraction analysis, and much more. The sample is usually examined without prior preparation. SEM also finds application in technological processes (control of microchip defects, etc.).

Fig.9. SEM block diagram

1 – electron gun insulator;

2 - heated V-shaped cathode;

3 - focusing electrode;

5 – block of two condenser lenses;

6 - diaphragm;

7 - two-tier deflecting system;

8 - lens;

9 - diaphragm;

10 - object;

11 – detector of secondary electrons;

12 - crystal spectrometer;

13 - proportional counter;

14 - preliminary amplifier;

15 - gain block:

16, 17 - equipment for recording X-rays;

18 - amplification block;

19 - block for adjusting the increase;

20, 21 - blocks of horizontal and vertical scans;

22, 23 - cathode ray tubes.

High for SEM PC is realized when forming an image using secondary electrons. It is determined by the diameter of the zone from which these electrons are emitted. The size of the zone, in turn, depends on the probe diameter, the properties of the object, the velocity of the primary beam electrons, etc. At a large penetration depth of primary electrons, secondary processes developing in all directions increase the zone diameter and PC decreases. The secondary electron detector consists of a PMT and an electron-photon converter, the main element of which is a scintillator with two electrodes - an extracting one in the form of a grid, which is under a positive potential (up to several hundred volts), and an accelerating one; the latter imparts to the captured secondary electrons the energy necessary to excite the scintillator. A voltage of about 10 kV is applied to the accelerating electrode; it is usually an aluminum coating on the surface of 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 magnetic microfields, and the value of the secondary electron emission coefficient, which in turn depends on the chemical composition of the sample at a given point. The reflected electrons are registered by a semiconductor (silicon) detector. The contrast of the image is due to the dependence of the reflection coefficient on the angle of incidence of the primary beam and the atomic number of the substance. 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 to the collector, information about individual sections from which there is no direct path to the collector is lost (shadows appear).

Characteristic X-ray radiation is emitted either by an X-ray crystal spectrometer or an energy-dispersive sensor - a semiconductor detector (usually made of pure silicon doped with lithium). In the first case, X-ray quanta, after being reflected by the spectrometer crystal, are recorded by a gas proportional counter , and in the second, the signal taken from the semiconductor detector is amplified by a low-noise amplifier (which is cooled with liquid nitrogen to reduce noise) and a subsequent amplification system. The signal from the crystal spectrometer modulates the CRT beam, and a picture of the distribution of one or another chemical element over the surface of the object appears on the screen. RMA is also produced at REM. The energy dispersive detector registers all elements from Na to U with high sensitivity. The crystal spectrometer, using a set of crystals with different interplanar spacings, covers the range from Be to U. A significant drawback of SEM is the long duration of the process of “removing” information when studying objects. A relatively high PC can be obtained using an electron probe of a sufficiently small diameter. But in this case, the probe current strength decreases, as a result of which the influence of the shot effect sharply increases, which reduces the ratio of the useful signal to noise. In order for the signal-to-noise ratio not to fall below a given level, it is necessary to slow down the scanning speed in order to accumulate a sufficiently large number of primary electrons (and the corresponding number of secondary ones) at each point of the object. As a result, high PC is realized only at low sweep rates. Sometimes one frame is formed within 10 - 15 minutes.

SEM with field emission gun have a high for SEM PC (up to 30 Å). A field emission gun (as well as an electron projector) uses a cathode in the form of a point, at the top of which a strong electric field arises, pulling electrons out of the cathode. The electronic brightness of a gun with a field emission cathode is 10 3 - 10 4 times higher than that of a gun with an incandescent cathode. Correspondingly, the electron probe current increases. Therefore, in a SEM with a field emission gun, fast sweeps are performed, and the probe diameter is reduced to increase PC. However, the field emission cathode operates stably only at ultrahigh vacuum (1·10 -9 – 1·10 -11 mmHg), and this complicates the design of such SEMs and operation on them.

Transmission scanning electron microscopes (STEM) have the same high PC as TEM. These devices use field emission guns that provide a sufficiently high current in a probe with a diameter of up to 2 – 3 Å. Figure 11 shows a schematic representation of the SEM. Two magnetic lenses reduce the diameter of the probe. Below the object are detectors - central and ring. Not scattered electrons fall on the first one, and after converting and amplifying the corresponding signals, the so-called bright-field image appears on the CRT screen. Scattered electrons are collected on a ring detector, creating a so-called dark-field image. In PREM, one can study thicker objects than in TEM, since an increase in the number of inelastically scattered electrons with thickness does not affect the resolution (there is no optics in PREM 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 image is observed on the CRT, containing additional information about the scattering properties of the object. High resolution in STEM is achieved with slow sweeps, because in a probe with a diameter of only 2 – 3 Å, the current is too low.

Fig.11. Schematic diagram of a transmission scanning electron microscope (SEM).

1 – field emission cathode;

2 – intermediate anode;

4 – deflecting system for beam alignment;

5 - aperture of the "illuminator";

6, 8 - deflecting systems for sweeping the electron probe;

7 - magnetic long-focus lens;

9 - aperture diaphragm;

10 - magnetic lens;

11 - object;

12, 14 - deflecting systems;

13 – ring collector of scattered electrons;

15 – collector of non-scattered electrons (removed when working with the spectrometer);

16 - magnetic spectrometer, in which electron beams are rotated by a magnetic field by 90°;

17 - deflecting system for selecting electrons with various energy losses;

18 – spectrometer slit;

19 - SE collector - secondary electrons;

h - X-ray radiation.

Mixed type electron microscopes. The combination of the principles of im- aging with a fixed beam (as in TEM) and scanning of a thin probe over an object in one instrument made it possible to realize the advantages of TEM, SEM, and STEM in such an electron microscope. At present, all TEMs provide for the possibility of observing objects in a raster mode (using condenser lenses and an objective that creates a reduced image of the electron source, which is scanned over the object by deflecting systems). In addition to the image formed by a stationary beam, raster images are obtained on CRT screens using transmitted and secondary electrons, characteristic X-ray spectra, etc. The optical system of such a TEM, located after the object, makes it possible to operate in modes that are not feasible in other devices. For example, one can simultaneously observe an electron diffraction pattern on a CRT screen and an image of the same object on the device screen.

Emission electron microscopes they create an image of an object in electrons, which the object itself emits when heated, bombarded by a primary electron beam, illuminated, and when a strong electric field is applied that pulls electrons out of the object. These devices usually have a narrow purpose.

Mirror electron microscopes serve mainly to visualize the electrostatic "potential relief" and magnetic microfields on the surface of an object. The main optical element of the device is an 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 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. Microfields near the surface of the object redistribute the electrons of the reflected beams, creating a contrast in the image that visualizes these microfields.

P

Fig.12. Contours of the penetration regions of primary electrons near the sample surface depending on their energy E 0

When a sample is irradiated, X-ray quanta and secondary and reflected (backscattered) electrons appear. The electrons of the primary beam penetrate deep into the sample and experience collisions. The narrow primary beam is scattered. The contours of the scattering region are shown in Fig.12.

Penetration depth R depends on the electron energy E 0 and matter density ρ. Experiments have shown that the product Rρ is almost constant, and the energy dependence E 0 is described by the following empirical formula

, (2)

where BUT- atomic weight; Z - atomic number. For Si, the values R change within (0.02 - 10) microns when changing E 0 from 1 to 100 keV.

Secondary electrons are divided into two groups. Electrons that have experienced elastic collisions with nuclei have an energy close to the energy of electrons in the primary beam E 0 . These are reflected electrons. The fraction of reflected electrons is small, it is approximately (1 - 2)% of the number of secondary ones and increases with increasing atomic number Z.

Inelastic collisions with atoms cause ionization of atoms and the formation of true secondary electrons with an energy below 50 eV and a distribution maximum near 5 eV (Fig. 13).

Decline in the yield of secondary electrons in the region of high energies E 0 is due to an increase in the penetration depth R with an increase in energy. The fraction of secondary electrons produced in the bulk of the sample and capable of reaching the surface decreases in this case. Metals and semiconductors have secondary emission coefficients of the order of unity, for dielectrics these are values ​​​​in the range (1.5 - 23). For metals, the mean free path of secondary electrons is ~ 1 nm and the maximum exit depth is ~ 5 nm, for dielectrics it is 5 and 50 nm, respectively, which is explained by the interaction of secondary electrons with free carriers in metals. Energy losses in dielectrics are due only to scattering by phonons. Secondary electrons can also arise due to reflected electrons, their share is (20 - 70)% of the total number of secondary electrons. The number of secondary electrons significantly depends on the work function of the material, the influence of the quantity Z expressed to a lesser extent than for reflected electrons. Thus, the yield of secondary electrons from SiO 2 is greater than from Si (the internal work function for SiO 2 is 0.9 eV, and for Si it is 4.15 eV).

The image on the microscope screen is formed by secondary and reflected electrons. The signal goes to the detector and, after amplification, to the cathode ray tube (CRT). The beam sweep in the CRT is synchronized with the sweep of the primary electron beam of the SEM. The intensity of the electron beam in a CRT is modulated by a signal coming from the sample. Therefore, the image of the surface depends on the intensity of the reflected electron beam. Scanning the electron beam makes it possible to observe a certain area of ​​the sample on the CRT screen.

The contrast is determined by the chemical composition of the sample and the surface morphology. With an increase in the atomic number of an element, the electron reflection coefficient increases, so areas containing elements with a large Z, give a greater signal. Thus, the yield of reflected electrons for Au is 10 times higher than for carbon. The image contrast of Al inclusions in Si is about 7%; such inclusions are quite distinguishable. The regions of metallization, oxide, and Si are also easily distinguishable in images formed by secondary electrons.

SEM studies use secondary electrons of different energies (slow and fast reflected). The resulting images carry different information and differ in contrast and resolution. Figure 14 explains the mechanism of formation of contrast in secondary and reflected electrons due to surface morphology.

To provide the necessary image contrast K current must be less than

,

where ε - signal detection efficiency; t f- time of beam scanning of the area under study. With low image contrast, a larger diameter beam must be used to provide current I min.

The spatial resolution of the image depends on the composition, orientation of the surface, the size of the surface area, and the characteristics of the SEM itself. The beam diameter can be reduced by decreasing the beam current and increasing its energy. Electron beam diameter at energy 10 – 30 keV and current I\u003d 1 10 -11 A is (4 - 13) nm.

The main limitation on the spatial resolution is related to the need to ensure sufficient current of the electron beam. The spatial resolution of the SEM is less than 10 nm, and the depth of field (2 – 4) µm with increasing x 10 4 and (0.2 – 0.4) mm with magnification x 10 2 .

The linear resolution of images formed by secondary electrons is equal to the sum of the diameters of the primary electron beam and the blur region in the plane at a depth equal to the mean free path of the electrons. Secondary electrons can also be excited by reflected electrons, which worsens the resolution.