Electron microscope An electron microscope is a device that allows you to obtain images of objects with a maximum magnification of up to 10 6 times, thanks to the use of luminous flux beam of electrons. The resolution of an electron microscope is 1000÷10000 times greater than the resolution of a light microscope and for the best modern instruments can be several angstroms (10 -7 m).
The appearance of the electron microscope became possible after a number of physical discoveries late XIX beginning of the 20th century. This is the discovery of the electron in 1897 (J. Thomson) and the experimental discovery in 1926 of the wave properties of the electron (K. Davisson, L. Germer), confirming the hypothesis put forward in 1924 by de Broglie about the wave-particle duality of all types of matter. In 1926, the German physicist H. Busch created a magnetic lens that allowed focusing electron beams, which served as a prerequisite for the creation of the first electron microscope in the 1930s. 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 device. This work by E. Ruski was awarded the Nobel Prize in Physics in 1986, which was awarded to him and the inventors of the scanning probe microscope, Gerd Karl Binnig and Heinrich Rohrer. In 1938, Ruska and B. von Borries built a prototype of an industrial transmission electron microscope 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). In the late 1930s and early 1940s, the first scanning electron microscopes (SEMs) appeared, forming an image of an object by sequentially moving a small cross-section electron probe across the object. Massive use of these devices in scientific research began in the 1960s, when they achieved significant technical excellence. 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 and his colleagues at the RCA Corporation in the 1960s, but only Otley’s device was able to serve as the basis for a number of technical improvements, culminating in the introduction of an industrial version of the SEM into production in the mid-1960s. x years.
There are two main types of electron microscopes. transmission electron microscopeIn the 1930s, a conventional transmission electron microscope (OPEM) was invented, a raster (scanning) electron microscope in the 1950s - a raster (scanning) electron microscope (SEM)
Transmission electron microscope from an ultrathin object Transmission electron microscope (TEM) is a setup in which an image from an ultrathin object (about 0.1 µm thick) is formed as a result of the interaction of an electron beam with the sample substance, followed by magnification with magnetic lenses (objective) and recording on a fluorescent screen. A transmission electron microscope is in many ways similar to a light microscope, but it uses a beam of electrons rather than light to illuminate samples. It contains an electronic illuminator, a series of condenser lenses, an objective lens, and a projection system that matches the eyepiece but projects the actual image onto a fluorescent screen or photographic plate. The electron source is usually a heated tungsten or lanthanum hexaboride cathode. The cathode is electrically isolated from the rest of the device, and the electrons are accelerated strongly electric field. To create such a field, the cathode is maintained at a potential of the order of B relative to other electrodes, which focus electrons into a narrow beam. This part of the device is called an electronic spotlight. one billionth of the atmosphere. Since electrons are strongly scattered by matter, there must be a vacuum in the microscope column where the electrons move. Here the pressure is maintained not exceeding one billionth of atmospheric pressure.
The magnetic field created by the turns of the coil carrying the current acts as a converging lens, the focal length of which can be changed by changing the current. The coils of wire carrying current focus the beam of electrons in the same way that a glass lens focuses a beam of light. An electronic image is formed by electric and magnetic fields in much the same way as a light image is formed by optical lenses. The principle of operation of a magnetic lens is illustrated by the following diagram.
CONVENTIONAL TRANSMISSION ELECTRON MICROSCOPE (OPEM). 1 – source of electrons; 2 – accelerating system; 3 – diaphragm; 4 – condenser lens; 5 – sample; 6 – objective lens; 7 – diaphragm; 8 – projection lens; 9 – screen or film; 10 – enlarged image. The electrons are accelerated and then focused by magnetic lenses. The magnified image created by electrons that pass through the lens diaphragm is converted into a visible image by a fluorescent screen or recorded on a photographic plate. A series of condenser lenses (only the last one is shown) focuses the electron beam onto the sample. Typically, the former creates a non-magnified image of the electron source, while the latter controls the size of the illuminated area on the sample. The aperture of the last condenser lens determines the beam width in the object plane. Sample The sample is placed in the magnetic field of an object lens with high optical power - the most important lens of the OPEM, which determines the maximum possible resolution of the device. Aberrations in an objective lens are limited by its aperture, just as they are in a camera or light microscope. An object lens produces a magnified image of an object (usually about 100 magnification); the additional magnification introduced by the intermediate and projection lenses ranges from slightly less than 10 to slightly more. Thus, the magnification that can be obtained in modern OPEMs ranges from less than 1000 to ~ (At a million times magnification, a grapefruit grows to the size of the Earth) . The object under study is usually placed on a very fine mesh placed in a special holder. The holder can be mechanical or electrically smoothly move up and down and left and right.
The final enlarged electronic image is converted into a visible image by a fluorescent screen that glows under electron bombardment. This image, usually of low contrast, is typically viewed through a binocular light microscope. At 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, to increase the brightness of a weak image, a phosphor screen with an electron-optical converter is used. In this case, the final image can be displayed on a regular television screen. A photographic plate usually produces a clearer image than that observed with the naked eye or recorded on videotape, since photographic materials, generally speaking, record electrons more efficiently. Resolution.Resolution. Electron beams have properties similar to those of light beams. In particular, each electron is characterized by a specific wavelength. The resolution of an EM is determined by the effective wavelength of the electrons. The wavelength depends on the speed of the electrons, and therefore on the accelerating voltage; The higher the accelerating voltage, the higher the speed of the electrons and the shorter the wavelength, which means the higher the resolution. Such a significant advantage of EM in resolution is explained by the fact that the wavelength of electrons is much shorter than the wavelength of light. But since electron lenses do not focus as well as optical lenses (the numerical aperture of a good electron lens is only 0.09, while a good optical lens has a NA of 0.95), the resolution of EM is 50–100 electron wavelengths. Even with such weak lenses, an electron microscope can achieve a resolution limit of ~0.17 nm, which makes it possible to distinguish individual atoms in crystals. To achieve a resolution of this order requires very careful adjustment of the instrument; in particular, highly stable power supplies are required, and the device itself (which can be ~2.5 m high and weigh several tons) and its additional equipment require vibration-free installation. In OPEM you can get an increase of up to 1 million. The limit of spatial (x, y) resolution is ~0.17 nm.
Raster electron microscopy Scanning electron microscope (SEM) is a device based on the principle of interaction of an electron beam with matter, designed to obtain an image of the surface of an object with high spatial resolution (several nanometers), as well as information about the composition, structure and some other properties near-surface layers. The spatial resolution of a scanning electron microscope depends on the transverse size of the electron beam, which in turn depends on the electron-optical system that focuses the beam. Currently modern models SEMs are produced by a number of companies around the world, including: Carl Zeiss NTS GmbH Germany FEI Company USA (merged with Philips Electron Optics) FOCUS GmbH Germany Hitachi Japan JEOL Japan (Japan Electron Optics Laboratory) Tescan Czech Republic
1 – source of electrons; 2 – accelerating system; 3 – magnetic lens; 4 – deflection coils; 5 – sample; 6 – reflected electron detector; 7 – ring detector; 8 – analyzer In an SEM, electron lenses are used to focus an electron beam (electron probe) into a very small spot. It is possible to adjust the SEM so that the diameter of the spot in it does not exceed 0.2 nm, but, as a rule, it is a few or tens of nanometers. This spot continuously runs around a certain area of the sample, similar to a beam running around the screen of a television tube. Electrical signal, which occurs when an object is bombarded by beam electrons, is used to form an image on the screen of a television kinescope or cathode ray tube (CRT), the scanning of which is synchronized with the electron beam deflection system (Fig.). Magnification in this case is understood as the ratio of the size of the image on the screen to the size of the area covered by the beam on the sample. This increase is from 10 to 10 million electron column Electron lenses (usually spherical magnetic) and deflection coils form a system called an electron column. However, the SEM method is characterized by a number of limitations and disadvantages, which are especially pronounced in the submicron and nanometer measurement ranges: insufficiently high spatial resolution; the difficulty of obtaining three-dimensional images of the surface, due primarily to the fact that the height of the relief in the SEM is determined by the efficiency of elastic and inelastic electron scattering and depends on the depth of penetration of primary electrons into the surface layer; the need to apply an additional current-collecting layer to poorly conductive surfaces to prevent effects associated with charge accumulation; carrying out measurements only in vacuum conditions; the possibility of damaging the surface under study with a high-energy focused electron beam.
Due to the very narrow electron beam, SEMs have a very large depth of field (mm), which is two orders of magnitude higher than that of an optical microscope and allows one to obtain clear micrographs with a characteristic three-dimensional effect for objects with complex topography. This SEM property is extremely useful for understanding the surface structure of a sample. A micrograph of pollen demonstrates the capabilities of SEM.
Scanning probe microscopes Scanning probe microscopes (SPM Scanning Probe Microscope) are a class of microscopes for measuring the characteristics of an object using various types probes. The imaging process is based on scanning the surface with a probe. In general, SPMs make it possible to obtain a three-dimensional image of a surface (topography) with high resolution. The main types of scanning probe microscopes: Scanning tunneling microscope Scanning tunneling microscope (STM scanning tunneling microscope) or scanning tunneling microscope (RTM) - a tunnel current between the probe and the sample is used to obtain an image, which allows obtaining information about the topography and electrical properties sample. Scanning atomic force microscope Scanning atomic force microscope (AFM) - records the various forces between the probe and the sample. Allows you to obtain the surface topography and its mechanical properties. Scanning near-field optical microscope Scanning near-field optical microscope (SNOM) - uses the near-field effect to obtain an image.
A distinctive feature of SPM is the presence of: a probe, a system for moving the probe relative to the sample along 2nd (X-Y) or 3rd (X-Y-Z) coordinates, a recording system. At a small distance between the surface and the sample, the action of interaction forces (repulsion, attraction, and other forces) and the manifestation of various effects (for example, electron tunneling) can be recorded using modern means registration. For registration, various types of sensors are used, the sensitivity of which makes it possible to detect small disturbances. The operation of a scanning probe microscope is based on the interaction of the sample surface with a probe (cantilever - English beam, needle or optical probe). Cantilevers are divided into hard and soft along the length of the beam, and this is characterized by the resonant frequency of cantilever oscillations. The process of scanning a surface with a microprobe can occur both in the atmosphere or a predetermined gas, and in a vacuum, and even through a liquid film. Cantilever in a scanning electron microscope (magnification 1000X) coordinates,
The recording system records the value of a function that depends on the probe-sample distance. To obtain a full raster image, use various devices scans along the X and Y axes (for example, piezo tubes, plane-parallel scanners). Surface scanning can occur in two ways: scanning with a cantilever and scanning with a substrate. If in the first case the cantilever moves along the surface under study, then in the second case the substrate itself moves relative to the stationary cantilever. feedback To maintain the scanning mode, - the cantilever must be close to the surface, - depending on the mode, - whether it is a constant force mode, or a constant height mode, there is a system that could maintain such a mode during the scanning process. For this purpose in electronic circuit The microscope includes a special feedback system, which is connected to the system for deflecting the cantilever from its original position. The main technical difficulties when creating a scanning probe microscope: The end of the probe must have dimensions comparable to the objects under study. Providing mechanical (including thermal and vibration) stability at a level better than 0.1 angstrom. Detectors must reliably detect small disturbances of the recorded parameter. Creation precision system scans. Ensuring smooth approach of the probe to the surface.
Scanning tunneling microscope (STM, English STM scanning tunneling microscope) or raster tunneling microscope (RTM) The scanning tunneling microscope in its modern form was invented (the principles of this class of devices were laid down earlier by other researchers) by Gerd Karl Binnig and Heinrich Rohrer in 1981. For this invention they were awarded the Nobel Prize in Physics for 1986, which was shared between them and the inventor of the transmission electron microscope, E. Ruska. In STM, a sharp metal needle is brought to a sample at a distance of several angstroms. When a small potential is applied to the needle relative to the sample, a tunneling current occurs. The magnitude of this current depends exponentially on the sample-needle distance. Typical pA values at distances of about 1 A. This microscope uses a small diameter metal tip to provide electrons. An electric field is created in the gap between the tip and the sample surface. The number of electrons pulled by the field from the tip per unit time (tunneling current) depends on the distance between the tip and the surface of the sample (in practice, this distance is less than 1 nm). As the tip moves along the surface, the current is modulated. This allows you to obtain an image related to the surface topography of the sample. If the tip ends in a single atom, then an image of the surface can be formed by passing atom by atom.
RTM can only work under the condition that the distance from the tip to the surface is constant, and the tip can be moved with precision down to atomic dimensions. The high resolution of STM along the normal to the surface (~0.01 nm) and in the horizontal direction (~0.1 nm), which is realized both in vacuum and with dielectric media in the tunnel gap, opens up broad prospects for increasing the accuracy of measurements of linear dimensions in nanometer range. Platinum-iridium needle of a scanning tunneling microscope close-up.
Scanning atomic force microscope Scanning atomic force microscope (AFM) Surface atomic force microscopy (AFM), proposed in 1986, is based on the effect of force interaction between closely spaced solids. Unlike STM, the AFM method is suitable for carrying out measurements on both conducting and non-conducting surfaces, not only in vacuum, but also in air and liquid media. The most important element An AFM is a microprobe (cantilever), at the end of which there is a dielectric tip with a radius of curvature R, to which the surface of the sample under study is brought to a distance of d0.1÷10 nm using a three-coordinate manipulator. The tip of the cantilever is usually mounted on a spring made in the form of a bracket with low mechanical rigidity. As a result of interatomic (intermolecular) interaction between the sample and the tip of the cantilever, the bracket is deflected. The AFM resolution along the surface normal is comparable to the corresponding STM resolution, and the resolution in the horizontal direction (longitudinal resolution) depends on the distance d and the radius of curvature of the tip R. Numerical calculation shows that at R = 0.5 nm and d = 0.4 nm the longitudinal resolution is ~1 nm. It must be emphasized that the AFM probe is the tip of a needle, which makes it possible to obtain information about the profile of a surface relief element having nanometer dimensions, but the height (depth) of such an element should not exceed 100 nm, and the neighboring element should be located no closer than at a distance of 100 nm. If certain AFM-specific conditions are met, it is possible to restore the element profile without loss of information. However, these conditions are practically impossible to implement experimentally.
View Spatial resolution (x,y) Z-coordinate resolution Field size Magnification Optical microscopy 200 nm-0.4 -0.2 mm x Confocal microscope 200 nm 1 nm White light interferometry 200 nm 0.1 nm 0.05 to x Holographic microscopy 200 nm 0.1 nm 0.05 to x Transmission electron microscope 0.2 nm- to Scanning electron microscope (SEM) 0.4 nm 0.1 nm 0.1-500 µm along z - ~1-10 mm to x Scanning probe microscopes 0.1 nm 0.05 nm ~150 x 150 µm in z - 
We are starting to publish a blog of an entrepreneur, a specialist in the field information technologies and part-time amateur designer Alexey Bragin, who talks 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 dealt with them.
A friend called me one day and said: I found an interesting thing, I need to bring it to you, however, it weighs half a ton. This is how a column from a JEOL JSM-50A scanning electron microscope appeared in my garage. It was written off long ago from some research institute and taken to scrap metal. The electronics were lost, but the electron-optical column, along 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, restore it and bring it into working condition? And right in the garage, with my own hands, using only basic engineering knowledge and available tools? True, I had never dealt with such scientific equipment before, let alone knew how to use it, and had no idea how it worked. But it’s interesting not just to put an old piece of hardware into working order - it’s interesting to figure it out on your own and check if it’s possible using scientific method, explore completely new areas. So I started restoring an 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, and also tell you about the many technical obstacles that had to be overcome along the way. So let's get started.
To restore the microscope I had in my possession to at least the “we draw with an electron beam on a fluorescent screen” state, the following was necessary:
Let's start in order. Today I will talk about the principles of operation of electron microscopes. They come in 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 the electron beam is much shorter than the photon beam, so significantly greater resolution can be obtained.
The electron beam is focused and controlled using electromagnetic or electrostatic lenses. They even have the same distortions (chromatic aberrations) as optical lenses, although the nature of the physical interaction 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 under study 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, you need to cut it lengthwise into at least 50 layers. This is due to the fact that the penetrating power of the electron beam is much worse than the photon beam. In addition, FEMs, with rare exceptions, are quite cumbersome. This device, pictured below, does not seem to be that big (although it is taller than human height and has a solid cast-iron frame), but it also comes with a power supply the size of a large cabinet - in total, almost an entire room is needed.
But TEM has the highest resolution. With its help (if you try hard) you can see individual atoms of a substance.
University of Calgary
This resolution can be especially useful for identifying the causative agent of a viral disease. All virus analytics of the twentieth century were built on the basis of FEM, 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, this is what the H1N1 flu looks like “in the light”:
University of Calgary
Scanning electron microscope
SEM is used mainly to examine the surface of samples with very high resolution (a million-fold magnification, versus 2 thousand for optical microscopes). And this is much more useful in the household :)
For example, this is what an individual bristle on a new toothbrush looks like:
The same thing should happen in the electron-optical column of a microscope, only here the sample is irradiated, not the phosphor of the screen, and the image is formed based on information from sensors that record secondary electrons, elastically reflected electrons, etc. This is the type of electron microscope that will be discussed in this blog.
Both the TV picture tube and the electron-optical column of the microscope operate only under vacuum. But I will talk about this in detail in the next issue.
(To be continued)
ELECTRON MICROSCOPE- a high-voltage, vacuum device in which a magnified image of an object is obtained using a flow of electrons. Designed for research and photographing objects at high magnifications. Electron microscopes have high resolution. Electron microscopes are widely used in science, technology, biology and medicine.
Based on the principle of operation, transmission (transmission), scanning, (raster) and combined electron microscopes are distinguished. The latter can operate in transmission, scanning or in two modes simultaneously.
The domestic industry began producing transmission electron microscopes in the late 40s of the 20th century. The need to create an electron microscope was caused by the low resolution of light microscopes. To increase the resolution, a shorter wavelength radiation source was required. The solution to the problem became possible only with the use of an electron beam as an illuminator. Wavelength of a stream of electrons accelerated at electric field with a potential difference of 50,000 V, is 0.005 nm. Currently, a resolution of 0.01 nm for gold films has been achieved on a transmission electron microscope.
Diagram of a transmission electron microscope: 1 - electron gun; 2 - condenser lenses; 3 - lens; 4 - projection lenses; 5 - tube with observation windows, through which the image can be observed; 6 - high-voltage cable; 7 - vacuum system; 8 - control panel; 9 - stand; 10 - high-voltage power supply device; 11 - power supply for electromagnetic lenses.
The schematic diagram of a transmission electron microscope is not much different from the diagram of a light microscope (see). The beam path and basic design elements of both microscopes are similar. Despite the wide variety of electron microscopes produced, they are all built according to the same scheme. The main design element of a transmission electron microscope is a microscope column, consisting of an electron source (electron gun), a set of electromagnetic lenses, a stage with an object holder, a fluorescent screen and a photorecording device (see diagram). All structural elements of the microscope column are assembled hermetically. System vacuum pumps A deep vacuum is created in the column to allow electrons to pass unhindered and protect the sample from destruction.
The flow of electrons is generated in a microscope gun, built on the principle of a three-electrode lamp (cathode, anode, control electrode). As a result of thermal emission, electrons are released from a heated V-shaped tungsten cathode, which are accelerated to high energies in an electric field with a potential difference from several tens to several hundred kilovolts. Through a hole in the anode, a stream of electrons rushes into the lumen of the electromagnetic lenses.
Along with tungsten thermionic cathodes, electron microscopes use rod and field emission cathodes, which provide a significantly higher electron beam density. However, for their operation a vacuum of at least 10^-7 mmHg is required. Art., which creates additional design and operational difficulties.
Another main element of the microscope column design is an electromagnetic lens, which is a coil with a large number turns of 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 piece is placed in the discontinuity area, which makes it possible to obtain a powerful, symmetrical field with minimal current in the lens winding. The disadvantage of electromagnetic lenses is various aberrations that affect the resolution of the microscope. Highest value has astigmatism caused by the asymmetry of the magnetic field of the lens. To eliminate it, mechanical and electrical stigmators are used.
The task of dual condenser lenses, like the condenser of a light microscope, is to change the illumination of an object by changing the electron flux density. The diaphragm of the condenser lens with a diameter of 40-80 microns selects the central, most homogeneous part a bunch of electrons. Objective lens - the shortest focal length lens with powerful magnetic field. Its task is to focus and initially increase the angle of motion of electrons passing through an object. The resolving power of the microscope largely depends on the quality of workmanship and the uniformity of the material of the pole piece of the objective lens. In the intermediate and projection lenses, the angle of electron motion further increases.
Special requirements are placed on the quality of manufacturing of the object stage and object holder, since they must not only move and tilt the sample in given directions at high magnification, but also, if necessary, subject it to stretching, heating or cooling.
A rather complex electronic-mechanical device is the photorecording part of the microscope, which allows automatic exposure, replacement of photographic material, and recording of the necessary microscopy modes on it.
Unlike a light microscope, the object of study in a transmission electron microscope is mounted on thin grids made of non-magnetic material (copper, palladium, platinum, gold). A substrate film made of collodion, formvar or carbon with a thickness of several tens of nanometers is attached to the grids, then a material is applied that is subjected to microscopic examination. The interaction of incident electrons with atoms of the sample leads to a change in the direction of their movement, deflection at small angles, reflection or complete absorption. Only those electrons that were deflected by the sample substance at small angles and were able to pass through the aperture diaphragm of the objective lens take part in the formation of an image on a luminescent screen or photographic material. The image contrast depends on the presence of heavy atoms in the sample, which strongly influence the direction of electron motion. To enhance the contrast of biological objects, built mainly from light elements, they use various methods contrasting (see Electron microscopy).
A transmission electron microscope provides the ability to obtain a dark-field image of a sample when illuminated by an inclined beam of electrons. In this case, electrons scattered by the sample pass through the aperture diaphragm. Dark-field microscopy increases image contrast while resolving sample details at high resolution. The transmission electron microscope also provides a microdiffraction mode for minimal crystals. The transition from bright-field to dark-field mode and microdiffraction does not require significant changes in the microscope design.
In a scanning electron microscope, a stream of electrons is generated by a high-voltage gun. Using dual condenser lenses, a thin beam of electrons (electron probe) is obtained. By means of deflection coils, the electron probe is deployed on the surface of the sample, causing radiation. The scanning system in a scanning electron microscope is similar to the system that produces television images. The interaction of the electron beam with the sample leads to the appearance of scattered electrons that have lost some of their energy when interacting with the atoms of the sample. To construct a three-dimensional image in a scanning electron microscope, electrons are collected by a special detector, amplified and fed to a scanning generator. The number of reflected and secondary electrons at each individual point depends on the relief and chemical composition of the sample; the brightness and contrast of the image of the object on the kinescope changes accordingly. The resolution of a scanning electron microscope reaches 3 nm, the magnification is 300,000. The deep vacuum in the column of a scanning electron microscope requires the mandatory dehydration of biological samples using organic solvents or their lyophilization from a frozen state.
A combined electron microscope can be created on the basis of a transmission or scanning electron microscope. Using a combined electron microscope, you can simultaneously study a sample in transmission and scanning modes. In a combined electron microscope, as in a scanning microscope, the possibility is provided for X-ray diffraction and energy dispersive analysis of the chemical composition of an object’s substance, as well as for optical-structural machine analysis of images.
To increase the efficiency of using all types of electron microscopes, systems have been created that make it possible to convert an electron microscopic image into digital form with subsequent processing of this information on a computer. Optical-structural machine analysis allows for statistical analysis of the image directly from the microscope, bypassing traditional method"negative print".
Bibliography: Stoyanova I. G. and Anaskin I. F. Physical foundations of transmission electron microscopy methods, M., 1972; Suvorov A. L. Microscopy in science and technology, M., 1981; Finean J. Biological ultrastructures, trans. from English, M., 1970; Schimmel G. Technique of electron microscopy, trans. with him.. M., 1972. See also bibliogr. to Art. Electron microscopy.
Moscow Institute of Electronic Technology
Electron Microscopy Laboratory S.V. Sedov
The operating principle of a modern scanning electron microscope and its use for studying microelectronic objects
Purpose of the work: familiarization with methods for studying materials and microelectronic structures using a scanning electron microscope.
Operating time: 4 hours.
Devices and accessories: Philips scanning electron microscope-
SEM-515, samples of microelectronic structures.
Design 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 into 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 appear. They are born in certain volumes - generation areas 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 when studying 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 allows it to be used for interoperational monitoring of technological parameters without significant loss of time. The 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 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 visual inspection technique in the production of solid-state electronics and micromechanics products.
2. Interaction of the electron beam with the sample
When an electron beam interacts with a solid target, a large number of different types of signals arise. The source of these signals are radiation regions, the sizes 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. In Fig. Figure 1 shows the excitation regions in the sample for different signals.
Complete energy distribution of electrons emitted by the sample
shown in Fig. 2. It was obtained at an incident beam energy 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 preserved for beams with an energy of 5–50 keV used in scanning electron microscopes.
G 
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. As the atomic number Z increases, 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 - normalized energy, d/dW - 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 allows one to study the phase composition of the object.
Group II includes electrons that have undergone multiple inelastic scattering and are emitted to the surface after passing through a more or less thick layer of target material, losing 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 a primary beam of weakly bound electrons. The main influence on the number of secondary electrons is exerted by the topography of the sample surface and local electric and magnetic fields. 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 release 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 in which secondary electrons are produced 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 image resolution in secondary electrons is close to the size of the primary electron beam.
Characteristic X-ray radiation results from the interaction of incident electrons with electrons from the inner K, L, or M shells of the sample atoms. The spectrum of characteristic radiation carries information about the chemical composition of the object. Numerous methods for microanalysis of composition 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 maps of the sample surface in the characteristic X-ray radiation of certain elements.
3 Scanning electron microscope design.
A transmission electron microscope is a device for obtaining magnified images of microscopic objects, which uses electron beams. Electron microscopes have greater resolution than optical microscopes, and they can also be used to obtain additional information regarding the material and structure of an object.
The first electron microscope was built in 1931 by German engineers Ernst Ruska and Max Barrel. Ernst Ruska received for this discovery Nobel Prize in physics in 1986. He shared it with the inventors of the tunneling microscope because the Nobel Committee felt that the inventors of the electron microscope had been unfairly forgotten.
An electron microscope uses focused beams of electrons to produce images, which bombard the surface of the object under study. The image can be observed different ways– in rays that passed through the object, in reflected rays, registering secondary electrons or x-rays. Focusing an electron beam using special electron lenses.
Electron microscopes can magnify images 2 million times. The high resolution of electron microscopes is achieved due to the short wavelength of the electron. While the wavelength of visible light ranges from 400 to 800 nm, the wavelength of an electron accelerated at a potential of 150 V is 0.1 nm. Thus, electron microscopes can practically view objects the size of an atom, although this is difficult to achieve practically.
Schematic structure of an electron microscope The structure of an electron microscope can be considered using the example of a device operating in transmission. A monochromatic beam of electrons is formed in an electron gun. Its characteristics are improved by a condenser system consisting of a condenser diaphragm and electronic lenses. Depending on the type of lens, magnetic or electrostatic, a distinction is made between magnetic and electrostatic microscopes. Subsequently, the beam hits the object, scattering on it. The scattered beam passes through the aperture and enters the objective lens, which is designed to stretch the image. A stretched beam of electrons causes the phosphor to glow on the screen. Modern microscopes use several levels of magnification.
The aperture diaphragm of the electron microscope lens is very small, amounting to hundredths of a millimeter.
If a beam of electrons from an object is directed directly onto the screen, then the object will look dark on it, and formations will form around it. light background. This image is called Svitlopolnym. If, however, it is not the fundamental beam that enters the aperture of the objective lens, but a scattered beam, then a dark-field Images. The dark-field image is more contrasty than the light-field image, but its resolution is lower.
There are many different types and designs of electron microscopes. The main ones are:
Transmission electron microscope is a device in which an electron beam shines through an object.
A scanning electron microscope makes it possible to study separate areas object.
A scanning electron microscope uses secondary electrons knocked out by an electron beam to examine the surface of an object.
A reflector electron microscope uses elastically scattered electrons.
An electron microscope can also be equipped with a system for detecting X-rays, which are emitted by highly excited atoms of matter when they collide with high-energy electrons. When an electron is knocked out of the inner electron shell, characteristic X-ray radiation is formed, by studying which it is possible to establish chemical composition material.
Studying the spectrum of inelastic-scattered electrons allows one to obtain information about the characteristic electronic excitations in the material of the object under study.
Electron microscopes are widely used in physics, materials science, and biology.
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