microscope, electron transmission abbr., PEM (English) abbr., TEM) — a variety is a high-vacuum high-voltage device in which an image from an ultrathin object (thickness of the order of 500 nm or less) is formed as a result of the interaction of an electron beam with the sample substance when passing through it.
Description
The principle of operation of a transmission electron microscope is almost the same as that of an optical microscope, only the first one uses magnetic lenses instead of glass lenses and electrons instead of photons. The electron beam emitted by the electron gun is focused with a condenser lens into a small spot ∼2–3 μm in diameter on the sample and, after passing through the sample, is focused with an objective lens to obtain a projection of an enlarged image on a special sample screen or detector. A very important element of the microscope is the aperture diaphragm located in the rear focal plane of the objective lens. It determines the contrast of the image and the resolution of the microscope. The formation of image contrast in TEM can be explained as follows. When passing through the sample, the electron beam loses part of its intensity to scattering. This part is larger for thicker sections or for sections with heavier atoms. If the aperture stop effectively cuts off scattered electrons, then thick areas and areas with heavy atoms will appear as darker. A smaller aperture increases contrast but results in loss of resolution. In crystals, elastic scattering of electrons leads to the appearance of a diffraction contrast.
The authors
- Veresov Alexander Genrikhovich
- Saranin Alexander Alexandrovich
Source
- Handbook of microscopy for nanotechnology, Ed. by Nan Yao, Zhong Lin Wang. - Boston: Kluwer Academic Publishers, 2005. - 731 p.
Description
For TEM studies, samples with a thickness of less than 500 nm (often less than 100–200 nm) are usually used. The thicker the sample, the greater should be the accelerating voltage of the electron beam. The resolution of TEM is tens of nanometers, however, there are modifications of the TEM method for which the resolution can reach 0.2 nm, and even 0.05 nm when using special spherical aberration correctors. These varieties are often considered as an independent research method - high resolution transmission electron microscopy (HREM, HRTEM).
An electron microscope with the use of additional detectors makes it possible to implement various methods of microanalysis of samples - X-ray spectral microanalysis, etc.
The authors
- Zotov Andrey Vadimovich
- Saranin Alexander Alexandrovich
Source
- Terminology for nanoscale measurement and instrumentation, PAS133:2007. - BSI (British standard), 2007.
Transmission microscope magnification
In transmission electron microscopy, TEM (Transmission electron microscopy, TEM) electrons are accelerated to 100 keV or higher (up to 1 MeV), focused onto a thin sample (less than 200 nm thick) using a condenser lens system, and pass through the sample either deflected or undeflected. The main advantages of TEM are its high magnification, ranging from 50 to 10 6 , and its ability to acquire both an image and a diffraction pattern from the same sample.
The scattering undergone by electrons during their passage through the sample determines the type of information received. Elastic scattering occurs without energy loss and makes it possible to observe diffraction patterns. Inelastic collisions between primary electrons and electrons of such sample inhomogeneities as grain boundaries, dislocations, particles of the second phase, defects, density variations, etc., lead to complex absorption and scattering processes, which lead to spatial variations in the intensity of transmitted electrons. In TEM, it is possible to switch from the sample imaging mode to the diffraction pattern registration mode by changing the field strength of the electromagnetic lenses.
The high magnification or resolution of all transmission electron microscopes is the result of the small effective electron wavelength X, which is given by the de Broglie relation:
Where m and q are the mass and charge of the electron, h is Planck's constant, and V is the accelerating potential difference. For example, electrons with an energy of 100 keV have a wavelength of 0.37 nm and are able to effectively penetrate a layer of silicon ˜0.6 μm thick.
Transmission microscope resolution
The greater the accelerating voltage of a transmission electron microscope, the higher its lateral spatial resolution. The theoretical limit of microscope resolution is proportional to λ 3/4 . Transmission electron microscopes with high accelerating voltage (eg 400 kV) have a theoretical resolution limit of less than 0.2 nm. High-voltage transmission electron microscopes have added benefit- greater penetration depth of electrons, since high-energy electrons interact with matter more weakly than low-energy electrons. Therefore, high-voltage transmission electron microscopes can work with thicker samples. One of the disadvantages of TEM is the limited depth resolution. Information about the scattering of electrons in TEM images comes from a 3D sample but is projected onto a 2D detector. Therefore, information about the structure obtained along the direction of the electron beam overlaps on the image plane. Although the main problem of the TEM method is sample preparation, it is not so relevant for nanomaterials.
Limited area diffraction (SAD) offers a unique opportunity to determine the crystal structure of individual nanomaterials, such as nanocrystals and nanorods, and the crystal structure of individual sample parts. When observing diffraction from a limited area, the condenser lenses are defocused to create a parallel beam incident on the sample, and an aperture is used to limit the volume involved in the diffraction. Diffraction patterns from a limited region are often used to determine the type of Bravais gratings and the lattice parameters of crystalline materials in an algorithm similar to that used in XRD. Despite the fact that TEM is not capable of distinguishing atoms, electron scattering is extremely sensitive to the target material, and chemical elemental analysis has developed different kinds spectroscopy. These include energy dispersive X-ray spectroscopy (EDAX) and characteristic electron energy loss spectroscopy (EELS).
Transmission electron microscope and nanotechnology
In nanotechnology, TEM is used not only to diagnose the structure and chemical analysis but also for other tasks. Among them is the determination of the melting points of nanocrystals, when an electron beam is used to heat the nanocrystals, and the melting point is determined by the disappearance of the electron diffraction pattern. Another example is the measurement of mechanical and electrical parameters of individual nanowires and nanotubes. The method makes it possible to obtain an unambiguous correlation between the structure and properties of nanowires.
Guozhong Cao Ying Wang, Nanostructures and nanomaterials: synthesis, properties and applications - M .: Scientific world, 2012
Introduction
1. Historical background
2. Transmission electron microscopy
2.1 Electron sources
2.2 Lighting system
2.3 Astigmatism correction
2.4 Auxiliary equipment for OPEM
3. Application of the transmission electron microscope
3.1 Non-biological materials
3.2 Biologics
3.3 High voltage microscopy
3.4 Radiation damage
4. Modern types of TEM
Conclusion
Bibliography
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 of the technique for obtaining objects, which made it possible to work directly with the material, and not with replica casts. It is the so-called transmission electron microscopy that owes its appearance and constant development to the theory of dislocations, the mechanism of plastic deformation of materials. 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 studies at low (up to -150°C) and high (up to 1200°C) temperatures, observation of deformation directly in a microscope, studies 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.
Scanning electron microscopy, which has concentrated all the achievements of transmission electron microscopy, is also moving wider and wider.
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 the world that is not visible to the eye. 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 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 experts. The most significant features of this device are a large depth of field of the image, 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 for electrons (as well as for light sols) the phenomenon of diffraction must take place; 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 investigations of this phenomenon. In the early 1930s, Academician A. A. Lebedev developed the theory of diffraction as applied to an electron diffraction recorder.
On the basis of 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 ashes 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 towards 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. This instrument was the forerunner of modern OPEM. (Ruska was rewarded for his work by winning the 1986 Nobel Prize in Physics.)
In 1938, Ruska and B. von Borries built a prototype of an industrial OPEM for Siemens-Halske in Germany; this instrument eventually made it possible to achieve a resolution of 100 nm. A few years later, A. Prebus and J. Hiller built the first high-resolution OPEM at the University of Toronto (Canada).
The wide possibilities of OPEM became apparent almost immediately. His industrial production It was launched 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. Currently, 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. Buinov, 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
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.
Let's get acquainted with the design of a modern transmission electron microscope.
Figure 1 - Section showing the main components of a transmission electron microscope
1 – electron gun; 2 -anode; 3 – coil for gun alignment; 4 – gun valve; 5 – 1st condenser lens; 6 – 2nd condenser lens; 7 – coil for beam tilting; 8 – condenser 2 diaphragms; 9 – objective lens; 10 – sample block; 11 – diffractive diaphragm; 12 – diffractive lens; 13 – intermediate lens; 14 – 1st projection lens; 15 – 2nd projection lens;
16 – binocular (magnification 12); 17 – vacuum block of the column; 18 – camera for 35mm roll film; 19 – focus screen; 20 – record chamber; 21 – main screen; 22 – ion sorption pump.
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.
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, or the so-called "Wenelt cylinder", 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 coming out of the gun is focused on the Wehnelt cylinder. 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 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).
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.
electron microscope correction astigmatism
Figure 2 - Simplified diagram of a magnetic electronic lens
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 is moved by a system of levers in two mutually perpendicular directions, tilted in different directions, which is especially important when examining a tissue section or such crystal lattice defects as dislocations and inclusions.
Figure 3 - Configuration of the pole tip of the high-resolution objective of the Siemens-102 electron microscope.
In this successful industrial design, the hole diameter of the upper pole piece 2R 1 =9 mm, the hole diameter of the lower pole piece 2R 2 =3 mm and the interpole gap S=5 mm (R 1 , R 2 and S are defined in Fig. 2): 1 – object holder 2 – sample table, 3 - sample, 4 – objective diaphragm, 5 – thermistors, 6 – winding lens, 7 – upper pole piece, 8 – cooled rod, 9 – lower pole piece, 10 – stigmatator, 11 - channels of the cooling system, 12 – cooled diaphragm
In a microscope column using vacuum system pumping creates a relatively low pressure, approximately 10 -5 mm Hg. 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.
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.
Figure 4 - Formation of the first intermediate image by an objective lens and the effect of aberration.
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. Asymmetry magnetic field lens leads to a significant curvature of the electron trajectory.
Work in the modes of microscopy and diffraction. The shaded areas mark the course of the equivalent beams in both modes.
If the magnetic field is not symmetrical, then the lens distorts the image (astigmatism). The same can be attributed to electrostatic lenses. The process of manufacturing electrodes and their alignment should be in high degree accurate, because the quality of the lenses depends on it.
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.
Figure 5 - The path of rays in a transmission type electron microscope
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 magnification corresponding to the magnification of a magnifying glass (10, 20), at which one can examine not only part of the object, but also see the entire object, to the maximum magnification, which allows one to make full use of the high resolution of the electron microscope ( usually up to 200,000). 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 10 to 200,000).
The change in magnification is carried out by adjusting the current of the intermediate lens.
Another factor contributing to obtaining higher 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. Change of the plate - through a sealed 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.
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 resolution
The breaking power is explained by 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, 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 each 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 sure 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 attachment for changing the inclination of the sample during the observation (goniometric device). 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, to carry out a complete cycle of heat treatment (annealing, hardening, tempering), and 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 and is determined 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 is with 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 a 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 manner 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.
2.1 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 using electrons (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 under existing vacuum.
4. Stable emission with 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 sample, 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 is reportedly 3000 times higher than the brightness of a V-shaped cathode with an effective source diameter of about 10 nm. These cathodes operate in moderate vacuum (10 -4 Torr).
2.2. Lighting system
Sample
Figure 6 - Illumination system of a modern electron microscope
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 U 1 , U 2 , V 1 ,V 2 are electron-optical parameters, while the distances D 1 , D 2 , D 3 easily measured in the microscope column. .
On fig. Figure 6 shows two condenser lenses included in the electron microscope illumination system. It is usually possible to independently change the focal length of these lenses (C1 and C2) . 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. The working distance D 3 must also be large enough to allow the object holder to move when changing the sample. When using a single condenser lens, it is difficult to meet these conflicting requirements - low magnification at a large distance D 3 - as this requires that the distance D 1 be excessively large. Therefore, a strong first condenser lens C1 is usually used, which serves to reduce the image of the source by a factor of 5–100, and the second weak lens C2 following the first one with a magnification of about 3 provides a large working distance,
2.3 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 and stigma knobs 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 fully adapt to the dark. To do this, spend at least 20 minutes in the dark.
2. The position and cleanliness of the objective iris and the cooled iris in the field of the lens will critically affect the required stigmatator setting. 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 the temperature of liquid nitrogen and until the cooled 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.
2.4 Accessories for conventional transmission electron microscopy high resolution
In addition to the microscope itself, there are various auxiliary devices, complementing the microscope, which were mentioned earlier in this book. 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 . APPLICATIONS OF A TRANSMISSION ELECTRON MICROSCOPE
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, whichever way it is obtained, is processed so as to increase the intensity of electron scattering on the object under study.) If the contrast is high enough, then the observer's eye can distinguish details that are at a distance of 0.1 - 0.2 mm without strain 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, but 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 using TEM, about 1000 lines can be resolved.
3.1 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. AT last years this method is 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 around whitlockite inclusions in lunar rock; Hutcheon and Price attributed this unusually long track to the decay of element 244 Rho, 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. 7) provide information on 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. Noteworthy is the tabular structure due to the decomposition of solid solutions.
Figure 7 - Dark-field TEM picture of a pyroxene grain from the Pesiano meteorite
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.
3.2 Biologicals
Electron microscopy is widely used in biological and medical research. Techniques for fixing, pouring, and obtaining thin tissue sections for research in OPEM 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 "shadows" the sample, due to which the latter, when observed in the OPEM, looks like it is 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).
3.3 High voltage microscopy
Currently, the industry produces high-voltage versions of OPEM 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.
3.4 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 the recording of a microphotograph in OPEM approximately corresponds to the energy that would be sufficient to completely evaporate cold water from a pond 4 m deep with a surface area of 1 ha. To reduce radiation damage to the sample, it is necessary to use various methods 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.
4 . MODERN TYPES OF TEM
Transmission electron microscope Titan 80 – 300 with atomic resolution
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 resolving capabilities of spectroscopy in measuring band gaps and electronic properties and allows the user to obtain clear images of interfaces and to interpret the data in the most complete way.
JEOL JEM-3010
300 kV transmission electron microscope
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: 100 to 300 kV
Increase: 50 to 1,500,000
JEOL JEM - 3000FasTEM
300 kV field emission transmission electron microscope
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.
Dot resolution: 0.17 nm
Accelerating voltage: 100, 200, 300 kV
Magnification: from x60 to x1,500,000
JEOL JEM-2100F
200 kV field emission transmission electron microscope
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:
· The high brightness and stability of the thermal field emission electron gun enables analysis of nanoscale regions at high magnification.
· Probe diameter less than 0.5 nm allows to reduce the point of analysis to the level of nanometers.
· New, highly stable, side-loading sample stage provides easy tilting, turning, heating and cooling, programmable settings, and more without mechanical drift.
JEOL JEM-2100 LaB6
200 kV analytical transmission electron microscope
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 at 200 kV on the LaB 6 cathode) is achieved due to high beam voltage and current stability, 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.
CONCLUSION
Until relatively recently, mineralogists had two classical tools in their hands - a polarizing microscope and X-ray diffraction equipment. With the help of an optical microscope, we 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 data make 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 heterogeneities whose sizes are within the specified limits, an ideal tool is an electron microscope. 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 it can depict structures and textures in real space, 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 Dyukov V.G., Nepiiko S.A., Sedov N.N. 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., Goring 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.
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. In the presence of good equipment 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. The choice of the optimal grinding method 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.
At machining, as a rule, near-surface damages such as plastic shear or microcracking appear. 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 with 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 units, it is possible to monitor the treated area 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 coated 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. Chemical composition, the 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.
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