The principle of operation of an electron microscope. Limitations of the electron microscope. Electron microscope: episode I
ELECTRON MICROSCOPE- a high-voltage, vacuum device in which an enlarged image of an object is obtained using a stream of electrons. Designed for research and photography of objects at high magnifications. Electron microscopes have high resolution. Electron microscopes are widely used in science, technology, biology and medicine.
According to the principle of operation, translucent (transmission), scanning, (raster) and combined electron microscopes are distinguished. The latter can work in translucent, scanning or in two modes simultaneously.
Domestic industry began to produce transmission electron microscopes in the late 40s of the 20th century. The need to create an electron microscope was caused by the low resolution of light microscopes. To increase the resolution, a shorter-wavelength radiation source was required. The solution of the problem became possible only with the use of an electron beam as an illuminator. The wavelength of the flow of electrons accelerated in electric field with a potential difference of 50,000 V is 0.005 nm. At present, a resolution of 0.01 nm for gold films has been achieved with a transmission electron microscope.
Scheme of a transmission type electron microscope: 1 - electron gun; 2 - condenser lenses; 3 - lens; 4 - projection lenses; 5 - tube with viewing windows through which you can observe the image; 6 - high voltage cable; 7 - vacuum system; 8 - control panel; 9 - stand; 10 - high-voltage power supply; 11 - power supply of electromagnetic lenses.
The schematic diagram of a transmission electron microscope is not much different from the scheme light microscope(cm.). The path of the rays and the main structural elements of both microscopes are similar. Despite the wide variety of electronic microscopes produced, they are all built according to the same scheme. The main structural element of a transmission electron microscope is the microscope column, which consists of an electron source ( electron gun), a set of electromagnetic lenses, an object stage with an object holder, a luminescent screen and a photorecording device (see diagram). All structural elements of the microscope column are hermetically assembled. A system of vacuum pumps in the column creates a deep vacuum for the unimpeded passage of electrons and protection of the sample from destruction.
The flow of electrons is formed in the microscope gun, built on the principle of a three-electrode lamp (cathode, anode, control electrode). As a result of thermal emission from a heated V-shaped tungsten cathode, electrons are released, which are accelerated to high energies in an electric field with a potential difference from several tens to several hundreds of kilovolts. Through the hole in the anode, the flow of electrons rushes into the gap of the electromagnetic lenses.
Along with tungsten thermionic cathodes in the electron microscope, rod and field emission cathodes are used, which provide significantly greater density beam of electrons. However, their operation requires a vacuum of at least 10 ^ -7 mm Hg. Art., which creates additional design and operational difficulties.
Another main structural element of the microscope column is an electromagnetic lens, which is a coil with a large number of turns of a thin copper wire, placed in a shell of soft iron. When passing through the lens winding electric current an electromagnetic field is formed in it, the lines of force of which are concentrated in the internal annular rupture of the shell. To enhance the magnetic field, a pole tip is placed in the discontinuity region, which makes it possible to obtain a powerful, symmetrical field at a minimum current in the lens winding. The disadvantage of electromagnetic lenses is various aberrations that affect the resolution of the microscope. Highest value has astigmatism caused by the asymmetry of the magnetic field of the lens. To eliminate it, mechanical and electrical stigmatators are used.
The task of dual condenser lenses, like the condenser of a light microscope, is to change the illumination of an object by changing the electron flux density. The diaphragm of a condenser lens with a diameter of 40-80 μm selects the central, most homogeneous part of the electron beam. The objective lens is the shortest focal length lens with a powerful magnetic field. Its task is to focus and initially increase the angle of motion of electrons that have passed through the object. The resolution of the microscope largely depends on the quality of manufacture and the uniformity of the material of the pole tip of the objective lens. In the intermediate and projection lenses, there is a further increase in the angle of electron motion.
Special requirements are imposed on the quality of the object stage and object holder, since they must not only move and tilt the sample in given directions at high magnification, but also, if necessary, subject it to stretching, heating, or cooling.
A rather complex electronic-mechanical device is the photo-recording part of the microscope, which allows automatic exposure, replacement of the captured photographic material, and recording of the necessary microscopy modes on it.
Unlike a light microscope, the object of study in a transmission electron microscope is mounted on thin grids made of non-magnetic material (copper, palladium, platinum, gold). A film-substrate made of collodion, formvar or carbon several tens of nanometers thick is attached to the grids, then the material is applied, which is subjected to microscopic examination. The interaction of incident electrons with sample atoms leads to a change in the direction of their motion, deflection by small angles, reflection or complete absorption. In the formation of an image on a luminescent screen or photographic material, only those electrons that were deflected by the sample substance at insignificant angles and were able to pass through the aperture diaphragm of the objective lens take part. The image contrast depends on the presence of heavy atoms in the sample, which strongly affect the direction of electron motion. To enhance the contrast of biological objects built mainly from light elements, various methods contrasting (see Electron microscopy).
In a transmission electron microscope, it is possible to obtain a dark-field image of a sample when it is illuminated by an inclined electron beam. In this case, the electrons scattered by the sample pass through the aperture diaphragm. Dark field microscopy enhances image contrast with high resolution of sample details. The transmission electron microscope also provides for the mode of microdiffraction of minimal crystals. The transition from bright-field to dark-field regime and microdiffraction does not require significant changes in the microscope scheme.
In a scanning electron microscope, the electron flow is formed by a high-voltage gun. With the help of double condenser lenses, a thin beam of electrons (electron probe) is obtained. By means of deflecting coils, the electron probe is deployed on the surface of the sample, causing radiation. The scanning system in a scanning electron microscope resembles the system by which a television image is obtained. The interaction of an electron beam with a sample leads to the appearance of scattered electrons, which have lost part of their energy when interacting with sample atoms. To build a three-dimensional image in a scanning electron microscope, electrons are collected by a special detector, amplified and fed to a sweep generator. The number of reflected and secondary electrons at each individual point depends on the relief and chemical composition of the sample, and the brightness and contrast of the image of the object on the kinescope change accordingly. The resolution of the scanning electron microscope reaches 3 nm, the magnification is 300,000. The deep vacuum in the column of the scanning electron microscope provides for the obligatory dehydration of biological samples with organic solvents or their lyophilization from a frozen state.
A combined electron microscope can be created on the basis of a transmission or scanning electron microscope. Using a combined electron microscope, you can simultaneously study the sample in transmission and scanning modes. In a combined electron microscope, as well as in a scanning one, an opportunity is provided for X-ray diffraction, energy-dispersive analysis of the chemical composition of an object's substance, as well as for optical-structural machine analysis of images.
To increase the efficiency of using all types of electron microscopes, systems have been created that allow converting an electron microscopic image into digital form with subsequent processing of this information on a computer.
Bibliography: Stoyanova I. G. and Anasknn I. F. Physical foundations methods of transmission electron microscopy, M., 1972; Suvorov A. L. Microscopy in science and technology, M., 1981; Finean J. Biological ultrastructures, trans. from English, M., 1970; Schimmel G. Technique of electron microscopy, trans. with German. M., 1972. See also bibliogr. to Art. Electron microscopy.
To obtain an image in an electron microscope, special magnetic lenses are used that control the movement of electrons in the instrument column using a magnetic field.
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Subtitles
The history of the development of the electron microscope
In 1931, R. Rudenberg received a patent for a transmission electron microscope, and in 1932 M. Knoll and E. Ruska built the first prototype of a modern instrument. This work by E. Ruska was awarded in 1986 Nobel Prize in physics, which was awarded to him and the inventors of the scanning probe microscope Gerd Carl Binnig and Heinrich Rohrer. The use of the transmission electron microscope for scientific research began in the late 1930s, and at the same time, the first commercial instrument built by Siemens appeared.
In the late 1930s - early 1940s, the first scanning electron microscopes appeared, which form an image of an object by sequentially moving an electron probe of a small cross section over the object. The widespread use of these devices in scientific research began in the 1960s when they achieved significant technical sophistication.
A significant leap (in the 1970s) in development was the use of Schottky cathodes and cathodes with cold field emission instead of thermionic cathodes, but their use requires a much larger vacuum.
In the late 1990s and early 2000s, computerization and the use of CCD detectors made digital imaging much easier.
In the last decade, modern advanced transmission electron microscopes have used correctors for spherical and chromatic aberrations, which introduce major distortions into the resulting image. However, their use can significantly complicate the use of the device.
Types of devices
Transmission electron microscopy
The transmission electron microscope uses a high-energy electron beam to form an image. The electron beam is created by means of a cathode (tungsten, LaB 6 , Schottky or cold field emission). The resulting electron beam is usually accelerated to 80-200 keV (various voltages from 20 kV to 1 MV are used), focused by a system of magnetic lenses (sometimes electrostatic lenses), passes through the sample so that some of the electrons are scattered on the sample, and some are not. Thus, the electron beam passed through the sample carries information about the structure of the sample. Next, the beam passes through a system of magnifying lenses and forms an image on a luminescent screen (usually made of zinc sulfide), a photographic plate, or a CCD camera.
TEM resolution is limited mainly by spherical aberration. Some modern TEMs have spherical aberration correctors.
The main disadvantages of TEM are the need for a very thin sample (on the order of 100 nm) and the instability (decomposition) of the samples under the beam.
Transmission scanning (scanning) electron microscopy (SEM)
One of the types of transmission electron microscopy (TEM), however, there are instruments that operate exclusively in the TEM mode. An electron beam is passed through a relatively thin sample, but, unlike conventional transmission electron microscopy, the electron beam is focused to a point that moves across the sample along the raster.
Raster (scanning) electron microscopy
It is based on the television principle of sweeping a thin electron beam over the sample surface.
Coloring
In their most common configurations, electron microscopes produce images with a separate value of brightness per pixel, with results typically shown in shades of gray. However, often these images are then colorized using software, or simply by manual editing with a graphical editor. This is usually done for aesthetic effect or to refine the structure, and usually does not add information about the pattern.
In some configurations, more information about the properties of the sample can be collected per pixel by using multiple detectors. In SEM, attributes of the topography and topography of a material can be captured using a pair of electronic reflectance detectors and such attributes can be superimposed into a single color image, with different primary colors assigned to each attribute. By analogy, combinations of the reflected and secondary electronic signal can be assigned different colors and superimposed on a single color micrograph, simultaneously showing the properties of the sample.
Some types of detectors used in SEM have analytical capabilities and can provide multiple data items per pixel. Examples are Energy Dispersive X-ray Spectroscopy detectors used in elemental analysis, and Cathodoluminescence microscope systems that analyze the intensity and spectrum of electron-stimulated luminescence in (for example) geological samples. In SEM systems, the use of these detectors is common to color code the signals and overlay them into a single color image so that differences in the distribution of different sample components can be clearly seen and compared. Additionally, the secondary electronic imaging standard can be combined with one or more compositional channels so that the structure and composition of the sample can be compared. Such images can be made while maintaining the complete integrity of the original signal, which does not change in any way.
Flaws
Electron microscopes are expensive to manufacture and maintain, but the overall and operating cost of a confocal optical microscope is comparable to basic electron microscopes. Microscopes aimed at achieving high resolutions must be placed in stable buildings (sometimes underground) and without external electromagnetic fields. Samples should generally be considered in a vacuum, as the molecules that make up the air will scatter electrons. One exception is the SEM environment, which allows hydrated samples to be viewed in low pressure (up to 2.7 kPa) and/or humid environments. Scanning electron microscopes operating in the usual high vacuum mode typically image a conductive sample; Therefore, non-conductive materials require a conductive coating (gold/palladium, carbon alloy, osmium, etc.). The low voltage mode of modern microscopes makes it possible to observe non-conductive, uncoated samples. Non-conductive materials can also be depicted by varying pressure (or environment) scanning electron microscope.
Applications
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Semiconductors and storage
Biology and biological sciences
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Scientific research
Industry
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Electron microscopes appeared in the 1930s and came into widespread use in the 1950s.
The figure shows a modern transmission (translucent) electron microscope, and the figure shows the path of the electron beam in this microscope. In a transmission electron microscope, electrons pass through the sample before an image is formed. Such an electron microscope was constructed first.
Electron microscope upside down compared to the light microscope. Radiation is applied to the sample from above, and the image is formed from below. The principle of operation of an electron microscope is essentially the same as that of a light microscope. The electron beam is directed by condenser lenses onto the sample, and the resulting image is then magnified by other lenses.
The table summarizes some of the similarities and differences between light and electron microscopes. At the top of the column of an electron microscope is a source of electrons - a tungsten filament, similar to that found in an ordinary light bulb. A high voltage (for example, 50,000 V) is applied to it, and the filament emits a stream of electrons. Electromagnets focus the electron beam.
A deep vacuum is created inside the column. This is necessary in order to minimize the scattering electrons due to collision with air particles. Only very thin sections or particles can be used for studying in an electron microscope, since the electron beam is almost completely absorbed by larger objects. The relatively denser parts of the object absorb electrons and therefore appear darker in the formed image. Heavy metals such as lead and uranium are used to stain the sample to increase contrast.
Electrons invisible to the human eye, so they are directed to the fluorescent, which reproduces the visible (black and white) image. To take a photograph, the screen is removed and electrons are directed directly onto the film. A photograph taken with an electron microscope is called an electron micrograph.
The advantage of the electron microscope:
1) high resolution (0.5nm in practice)
Disadvantages of an electron microscope:
1) the material prepared for the study must be dead, since in the process of observation it is in a vacuum;
2) it is difficult to be sure that the object reproduces a living cell in all its details, since fixation and staining of the material under study can change or damage its structure;
3) the electron microscope itself and its maintenance are expensive;
4) preparation of material for work with a microscope takes a lot of time and requires highly qualified personnel;
5) the studied samples are gradually destroyed under the action of the electron beam. Therefore, if required detailed study sample, it is necessary to photograph it.
Moscow Institute of Electronic Technology
Electron Microscopy Laboratory S.V. Sedov
The principle of operation of a modern scanning electron microscope and its use for the study of microelectronic objects
The purpose of the work: acquaintance with the methods of studying materials and microelectronic structures using a scanning electron microscope.
Duration of work: 4 hours.
Devices and accessories: scanning electron microscope Philips-
SEM-515, samples of microelectronic structures.
The device and principle of operation of a scanning electron microscope
1. Introduction
Scanning electron microscopy is the study of an object by irradiation with a finely focused electron beam, which is deployed in a raster over the surface of the sample. As a result of the interaction of a focused electron beam with the sample surface, secondary electrons, reflected electrons, characteristic X-ray radiation, Auger electrons, and photons of various energies are produced. They are produced in certain volumes - generation regions inside the sample and can be used to measure many of its characteristics, such as surface topography, chemical composition, electrical properties, etc.
The main reason for the widespread use of scanning electron microscopes is the high resolution in the study of massive objects, reaching 1.0 nm (10 Å). Another important feature of images obtained in a scanning electron microscope is their three-dimensionality, due to the large depth of field of the device. The convenience of using a scanning microscope in micro- and nanotechnology is explained by the relative simplicity of sample preparation and the efficiency of research, which makes it possible to use it for interoperational control of technological parameters without significant loss of time. An image in a scanning microscope is formed in the form of a television signal, which greatly simplifies its input into a computer and further software processing of the research results.
The development of microtechnologies and the emergence of nanotechnologies, where the dimensions of elements are significantly smaller than the wavelength of visible light, make scanning electron microscopy practically the only non-destructive method of visual control in the production of solid-state electronics and micromechanics.
2. Interaction of an electron beam with a sample
When an electron beam interacts with a solid target, a large number of different kinds of signals arise. The source of these signals are radiation regions, the dimensions of which depend on the beam energy and the atomic number of the bombarded target. The size of this area, when using a certain type of signal, determines the resolution of the microscope. On fig. 1 shows the excitation regions in the sample for different signals.
Total energy distribution of electrons emitted by the sample
shown in Fig.2. It was obtained at the energy of the incident beam E 0 = 180 eV, the number of electrons emitted by the target J s (E) is plotted along the ordinate axis, and the energy E of these electrons is plotted along the abscissa axis. Note that the type of dependence
shown in Fig. 2 is also valid for beams with an energy of 5 – 50 keV used in scanning electron microscopes.
G 
The group I consists of elastically reflected electrons with an energy close to the energy of the primary beam. They arise during elastic scattering at large angles. With an increase in the atomic number Z, elastic scattering increases and the fraction of reflected electrons increases. The energy distribution of reflected electrons for some elements is shown in Fig.3.
Scattering angle 135 0 
, W=E/E 0 is the normalized energy, d/dW is the number of reflected electrons per incident electron and per unit energy interval. It can be seen from the figure that as the atomic number increases, not only does the number of reflected electrons increase, but their energy also becomes closer to the energy of the primary beam. This leads to the appearance of a contrast in atomic number and makes it possible to study the phase composition of the object.
Group II includes electrons that have been subjected to multiple inelastic scattering and radiated to the surface after passing through a more or less thick layer of the target material, having lost a certain part of their initial energy.
E 
Group III electrons are secondary electrons with low energy (less than 50 eV), which are formed when the outer shells of target atoms are excited by the primary beam of weakly bound electrons. The topography of the sample surface and local electric and magnetic fields have the main influence on the number of secondary electrons. The number of emerging secondary electrons depends on the angle of incidence of the primary beam (Fig. 4). Let R 0 be the maximum depth of exit of secondary electrons. If the sample is tilted, then the path length within the distance R 0 from the surface increases: R = R 0 sec
Consequently, the number of collisions at which secondary electrons are born also increases. Therefore, a slight change in the angle of incidence leads to a noticeable change in the brightness of the output signal. Due to the fact that the generation of secondary electrons occurs mainly in the near-surface region of the sample (Fig. 1), the resolution of the image in secondary electrons is close to the size of the primary electron beam.
Characteristic X-ray radiation arises as a result of the interaction of incident electrons with electrons from the inner K, L, or M shells of sample atoms. The characteristic radiation spectrum carries information about chemical composition object. Numerous methods of composition microanalysis are based on this. Most modern scanning electron microscopes are equipped with energy dispersive spectrometers for qualitative and quantitative microanalysis, as well as for creating sample surface maps in the characteristic X-ray emission of certain elements.
3 Scanning electron microscope device.
We are starting to publish a blog of an entrepreneur, a specialist in the field information technologies and part-time amateur designer Alexei Bragin, which tells about an unusual experience - for a year now, the author of the blog has been busy restoring complex scientific equipment - a scanning electron microscope - practically at home. Read about what engineering, technical and scientific challenges Alexey had to face and how he coped with them.
Once a friend called me and said: I found an interesting thing, I need to bring it to you, however, it weighs half a ton. So I got a column from a JEOL JSM-50A scanning electron microscope in my garage. She was decommissioned from some research institute a long time ago and taken to scrap metal. The electronics were lost, but the electron-optical column, together with the vacuum part, was saved.
Since the main part of the equipment was preserved, the question arose: is it possible to save the entire microscope, that is, to restore and bring it into working condition? And right in the garage, with your own hands, with the help of only basic engineering and technical knowledge and improvised means? True, I had never before dealt with such scientific equipment, not to mention being able to use it, and had no idea how it works. But it's interesting not just to put the old piece of iron into working condition - it's interesting to figure everything out on your own and check whether it is possible, using the scientific method, to master completely new areas. So I began to restore the electron microscope in the garage.
In this blog, I will tell you about what I have already managed to do and what remains to be done. Along the way, I will introduce you to the principles of operation of electron microscopes and their main components, as well as talk about the many technical obstacles that had to be overcome in the course of work. So let's get started.
In order to restore the microscope I had at least to the state of “drawing with an electron beam on a luminescent screen”, the following was necessary:
Let's start in order. Today I will talk about the principles of operation of electron microscopes. They are of two types:
Transmission electron microscope
TEM is very similar to a conventional optical microscope, only the sample under study is irradiated not with light (photons), but with electrons. The wavelength of an electron beam is much smaller than that of a photon beam, so much higher resolution can be obtained.
The electron beam is focused and controlled by electromagnetic or electrostatic lenses. They even have the same distortions ( chromatic aberration) as optical lenses, although nature physical interaction here is completely different. By the way, it also adds new distortions (caused by the twisting of electrons in the lens along the axis of the electron beam, which does not happen with photons in an optical microscope).
TEM has disadvantages: the samples to be studied must be very thin, thinner than 1 micron, which is not always convenient, especially when working at home. For example, to see your hair through the light, it must be cut along at least 50 layers. This is due to the fact that the penetrating power of an electron beam is much worse than a photon one. In addition, TEM, with rare exceptions, is quite cumbersome. This apparatus, shown below, does not seem to be that big (although it is taller than a human being and has a solid cast-iron frame), but it also comes with a power supply unit the size of a large cabinet - in total, almost a whole room is needed.
But the resolution of TEM is the highest. With it (if you try hard) you can see individual atoms substances.
University of Calgary
This resolution is especially useful for pathogen identification. viral disease. All virus analytics of the 20th century was built on the basis of TEM, and only with the advent of cheaper methods for diagnosing popular viruses (for example, polymerase chain reaction, or PCR), the routine use of TEMs for this purpose has ceased.
For example, here's what the H1N1 flu looks like "through the light":
University of Calgary
Scanning electron microscope
SEM is mainly used for examining the surface of samples with very high resolution(an increase of a million times, versus 2 thousand for optical microscopes). And this is much more useful in the household :)
For example, this is how a single bristle of a new toothbrush looks like:
The same should happen in the electron-optical column of the microscope, only here the sample is irradiated, and not the screen phosphor, and the image is formed on the basis of information from sensors that record secondary electrons, elastically reflected electrons, and so on. It is this type of electron microscope that will be discussed in this blog.
Both the kinescope of the TV and the electron-optical column of the microscope work only under vacuum. But I will talk about this in detail in the next issue.
(To be continued)
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