Light and electron microscopes. electron microscopy

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

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

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

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

The reflective electron microscope uses elastically scattered electrons.

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

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

Technological archeology)
Some electron microscopes restore, other firmware spacecraft, others are engaged in reverse engineering of circuitry of microcircuits under a microscope. I suspect that the occupation is terribly exciting.
And, by the way, I remembered a wonderful post about industrial archeology.

Spoiler

There are two types of corporate memory: people and documentation. People remember how things work and know why. Sometimes they record this information somewhere and keep their records somewhere. It's called "documentation". Corporate amnesia works the same way: people leave, and documentation disappears, rots, or is simply forgotten.

I spent several decades working for a large petrochemical company. In the early 1980s, we designed and built a plant that converts hydrocarbons into other hydrocarbons. Over the next 30 years, the corporate memory of this plant has waned. Yes, the plant is still running and making money for the firm; maintenance is being done, and wise people know what they need to twitch and kick to keep the plant running.

But the company has completely forgotten how this plant works.

This happened due to several factors:

The downturn in the petrochemical industry in the 1980s and 1990s caused us to stop hiring new people. In the late 1990s, our group consisted of guys under the age of 35 or over 55 - with very rare exceptions.
We slowly switched to designing with the help of computer systems.
Due to corporate reorganizations, we had to physically move the entire office from place to place.
A corporate merger a few years later completely dissolved our firm into a larger one, causing a massive reshuffling of departments and personnel.
Industrial archeology

In the early 2000s, I and several of my colleagues retired.

In the late 2000s, the company remembered the plant and thought it would be nice to do something with it. Say, increase production. For example, you can find a bottleneck in manufacturing process and improve it - the technology has not stood still for these 30 years - and, perhaps, attach another workshop.

And then the company from all over the place is imprinted in brick wall. How was this plant built? Why was it built this way and not otherwise? How exactly does it work? Why is vat A needed, why are workshops B and C connected by a pipeline, why does the pipeline have a diameter of G, and not D?

Corporate amnesia in action. Giant machines built by aliens with their alien technology champ like clockwork, spitting out heaps of polymers. The company has a rough idea of ​​how to maintain these machines, but has no idea what amazing magic is going on inside, and no one has the slightest idea how they were created. In general, the people are not even sure what exactly to look for, and do not know from which side this tangle should be unraveled.

We are looking for guys who were already working in the company during the construction of this plant. Now they occupy high positions and sit in separate, air-conditioned offices. They are given the task of finding documentation on the said plant. It's no longer corporate memory, it's more like industrial archaeology. No one knows what kind of documentation on this plant exists, whether it exists at all, and if so, in what form it is stored, in what formats, what it includes and where it is located physically. The plant was designed project team that no longer exists, in a company that has since been taken over, in an office that has been shut down using pre-computer age methods that no longer apply.

The guys remember their childhood with obligatory swarming in the mud, roll up the sleeves of expensive jackets and get to work.

a device for observing and photographing a multiply (up to 10 6 times) enlarged image of objects, in which, instead of light rays, beams are used accelerated to high energies (30-100 keV or more) in deep vacuum conditions. Physical foundations corpuscular-ray optical instruments were founded in 1834 (almost a hundred years before the appearance of the electron microscope) by W.R., who established analogies between light rays in optically inhomogeneous media and particle trajectories in force fields. The expediency of creating an electron microscope became apparent after the advancement in 1924 about, and the technical prerequisites were created by the German physicist X. Bush, who investigated axisymmetric focusing fields and developed a magnetic electron lens (1926). In 1928, the German scientists M. Knoll and E. Ruska set about creating the first magnetic transmission electron microscope (TEM), and three years later they obtained an image of an object formed by beams. In subsequent years (M. von Ardenne, 1938; V.K., 1942), the first scanning electron microscope (SEM) was built, operating on the principle of scanning (deployment), i.e., sequential from point to point moving a thin electron beam ( probe) on the object. By the mid 1960s. SEMs have reached a high technical perfection, and since that time their use in scientific research. TEMs have the highest (PC), surpassing in this parameter light microscopes several thousand times. T. n. the resolution limit, which characterizes the device to display separately small, as close as possible, details of the object, for TEM is 2-3 . At favorable conditions you can photograph individual heavy atoms. When photographing periodic structures, such as atomic lattices of crystals, it is possible to realize a resolution of less than 1 . Such high resolutions are achieved due to the extremely short length (see ). Optimum aperture [see. in electron (and ion) optics] can be reduced (affecting the PC electron microscope) with a sufficiently small diffraction error. effective methods correction in the Electron microscope (see ) was not found. Therefore, in TEM, magnetic (EL), which have smaller , completely replaced electrostatic EL. PEMs for various purposes are produced. They can be divided into 3 groups: high resolution electron microscope, simplified TEM and high accelerating electron microscope.

high resolution TEM(2-3 Å) - like, multi-purpose devices. By using additional devices and attachments in them, you can tilt the object at different large angles to the optical axis, heat, cool, deform it, carry out research methods, etc. Accelerating electrons reaches 100-125 kV, is regulated in steps and is highly stable: in 1-3 minutes it changes by no more than 1-2 millionths of the original. An image of a typical TEM of the described type is shown in rice. one. In its optical system (column), with the help of a special vacuum system a vacuum is created (up to 10 -6 mm Hg). The scheme of the TEM optical system is shown in rice. 2. The beam, which is served by a heated cathode, (is formed in and then twice focused by the first and second condensers, creating an electronic "spot" of small sizes on the object (when adjusting the spot, it can vary from 1 to 20 microns). After that, the part is scattered through the object and delayed by the diaphragm. Unscattered electrons pass through the diaphragm opening and are focused into the objective intermediate lens.The first magnified image is formed here.Successive lenses create the second, third, etc. images.The last projection lens forms an image on a fluorescent screen that glows under the influence of electrons.Magnification Electron microscope is equal to the magnifications of all lenses.The degree and nature of electron scattering are not the same at different points of the object, since the thickness and chemical composition of the object change from point to point.Accordingly, the number of electrons delayed by the aperture diaphragm after passing through various points of the object changes, and, consequently, current density but on the image, which is converted to on the screen. Under the screen is a store with photographic plates. When photographing, the screen is removed and the electrons act on the photoemulsion layer. The image is focused by a smooth change in the current that excites the lens. The currents of other lenses are adjusted to change the magnification Electron microscope

Rice. 3. Superhigh-voltage electron microscope (SVEM): 1 - a tank into which electrically insulating gas (SF6) is pumped up to a pressure of 3-5 atm; 2 - electron gun; 3 - accelerating tube; 4 - high-voltage source capacitors; 5 - block of condenser lenses; 6 - lens; 7, 8, 9 - projection lenses; 10 - light microscope; 11 - control panel.

Scanning Electron Microscope (SEM) with an incandescent cathode are designed to study massive objects with a resolution of 70 to 200 Å. The accelerating power in the SEM can be adjusted from 1 to 30-50 sq.

The device of a scanning electron microscope is shown in rice. four. Using 2 or 3 ELs, a narrow electron probe is focused on the sample. Magnetic deflectors deploy the probe over a given area on the object. When the probe interacts with an object, there are several types ( rice. 5) - secondary and reflected electrons; electrons that have passed through the object (if it is thin); x-ray and characteristic; radiation, etc.

Rice. 5. Scheme for registering information about the object obtained in the SEM. 1 - primary electron beam; 2 - detector of secondary electrons; 3 - X-ray detector; 4 - detector of reflected electrons; 5 - detector light radiation; 6 - detector of passed electrons; 7 - a device for measuring the electrical potential induced on the object; 8 - device for measuring the current of electrons passing through the object; 9 - device for measuring the current of electrons absorbed in the object.

Any of these radiations can be registered by an appropriate collector containing a sensor that converts into electrical radiation, which, after amplification, are fed to (CRT) and modulate its beam. The CRT beam is scanned with the scanning of the electron probe in the SEM, and an enlarged image of the object is observed on the CRT screen. The increase is equal to the ratio of the frame height on the CRT screen to the width of the scanned object. Photograph the image directly from the CRT screen. The main advantage of SEM is the high information content of the device, due to the ability to observe the image using various sensors. SEM can be used to investigate chemical composition by object, p-n-junctions, produce and much more. The sample is usually examined without pre-training. SEM finds application in technological processes(chip defects, etc.). High for SEM PC is realized when imaging using secondary . It is determined by the diameter of the zone from which these electrons are emitted. The zone size, in turn, depends on the probe diameter, properties of the object, primary beam electrons, etc. At a large penetration depth of primary electrons, secondary processes developing in all directions increase the zone diameter and PC decreases. The secondary electron detector consists of a (PMT) and an electron-photon converter, the main element of which is with two - an extractor in the form of a grid, which is under a positive potential (up to several hundred V), and an accelerating one; the latter imparts to the captured secondary electrons the energy necessary for . Approximately 10 kV is applied to the accelerating electrode; usually it is an aluminum coating on the scintillator. The number of scintillator flashes is proportional to the number of secondary ones knocked out at a given point in the object. After amplification in the PMT and into the signal, the CRT beam modulates. The magnitude of the signal depends on the sample, the presence of local electric and magnetic microfields, and the value of , which in turn depends on the chemical composition of the sample at a given point. Reflected electrons are recorded by a semiconductor (silicon). The contrast of the image is due to the dependence on the angle of incidence of the primary beam and the atomic number . The resolution of the image obtained "in reflected electrons" is lower than that obtained using secondary ones (sometimes by an order of magnitude). Due to the straightness of the flight of electrons to the collector, information about individual sections from which there is no direct path to the collector is lost (shadows appear). The characteristic is allocated either by an X-ray crystal or energy-dispersive sensor - a semiconductor detector (usually made of pure silicon doped with lithium). In the first case, X-ray quanta, after being reflected by the spectrometer crystal, are recorded by a gas, and in the second, the signal taken from a semiconductor is amplified by a low-noise (which is cooled with liquid nitrogen to reduce noise) and a subsequent amplification system. The signal from the crystal modulates the CRT beam, and a picture of one or another appears on the screen. chemical element by object. The SEM also produces a local x-ray. The energy dispersive detector registers all elements from Na to U with high sensitivity. Crystal spectrometer using a set of crystals with different interplanar (see) overlaps from Be to U. Significant disadvantage SEM - long duration of the process of "removal" of information in the study of objects. A relatively high PC can be obtained using an electron probe of a sufficiently small diameter. But at the same time, the probe decreases, as a result of which the influence of , which reduces the ratio of the useful signal to noise, increases sharply. So that the signal-to-noise ratio does not fall below a given level, it is necessary to slow down the scans to accumulate at each point of the object enough a large number primary (and corresponding secondary). As a result, PC is implemented only at low sweep rates. Sometimes one frame is formed within 10-15 minutes.

Rice. 6. circuit diagram transmission scanning electron microscope (SEM): 1 - field emission cathode; 2 - intermediate anode; 3 - anode; 4 - deflecting system for beam alignment; 5 - aperture of the "illuminator"; 6, 8 - deflecting systems for scanning the electronic probe; 7 - magnetic long-focus lens; 9 - aperture diaphragm; 10 - magnetic lens; 11 - object; 12, 14 - deflecting systems; 13 - ring collector of scattered electrons; 15 - collector of unscattered electrons (removed when working with the spectrometer); 16 - magnetic spectrometer in which electron beams are rotated magnetic field by 90° ; 17 - deflecting system for selecting electrons with various energy losses; 18 - spectrometer slit; 19 - collector; VE - flux of secondary electrons hn - X-ray radiation.

SEM with field emission gun have a high for SEM PC (up to 30 Å ). In a field emission gun (as in ) a cathode is used in the form of a tip, at the top of which there is a strong force that pulls electrons out of the cathode (see). The electronic brightness of a gun with a field emission cathode is 10 3 -10 4 times higher than that of a gun with an incandescent cathode. Correspondingly, the electron probe current increases. Therefore, in a SEM with a field emission gun, fast sweeps are performed, and the probe is reduced to increase PC. However, the field emission cathode operates stably only at ultrahigh vacuum (10 -9 -10 -11 mmHg), and this complicates the design of such SEMs and operation on them.

Transmission Scanning Electron Microscope (SEM) have the same high PC as TEM. These devices use field emission guns, which provide enough in a probe with a diameter of up to 2-3 Å. On the rice. 6 a schematic representation of the SEM is shown. Two reduce the diameter of the probe. Below the object are located - central and ring. Unscattered electrons fall on the first one, and after amplification of the corresponding signals, the so-called “electrons” appear on the CRT screen. brightfield image. Scattered electrons are collected on the ring detector, creating the so-called. darkfield image. In PREM, one can study thicker objects than in TEM, since an increase in the number of inelastically scattered objects with thickness does not affect the resolution (there is no optics in PREM after the object). With the help of energy, the electrons that have passed through the object are separated into elastically and inelastically scattered beams. Each beam hits its own detector, and the corresponding image is observed on the CRT, containing Additional information about a scattering object. High resolution in STEM is achieved with slow sweeps, because in a probe with a diameter of only 2–3 Å, the current is too low.

Mixed type electron microscope. The combination in one instrument of the principles of imaging with a fixed beam (as in TEM) and scanning of a thin probe over an object made it possible to realize the advantages of TEM, SEM, and STEM in such an electron microscope. At present, all TEMs provide for the possibility of observing objects in a raster mode (with the help of condenser lenses and creating a reduced image, which is scanned over the object by deflecting systems). In addition to the image formed by a fixed beam, one obtains bitmaps on CRT screens using transmitted and secondary electrons, characteristic, etc. Optical system Such a TEM, located after the object, makes it possible to work in modes that are not feasible in other devices. For example, you can simultaneously observe on the CRT screen and the image of the same object on the device screen.

emission E. m. create an image of an object in electrons, which the object itself emits when heated, by a primary beam, and when a strong electric field pulling electrons out of an object. These devices usually have a narrow purpose.

SLR Electron microscope serve mainly to visualize the electrostatic "potential relief" and magnetic microfields on the object. The main optical element of the device is, and one of the objects is the object itself, which is located under a small negative potential relative to the cathode of the gun. The electron beam is directed to the mirror and reflected by the field in the immediate vicinity of the object. The mirror forms an image "in reflected beams" on the screen. Microfields near the surface of the object redistribute the electrons of the reflected beams, creating a visualization of these microfields on the image.

Development prospects Electron microscope Increasing PC in images of non-periodic objects to 1 Å and more will make it possible to register not only heavy, but also light atoms and visualize at the atomic level. To create an electron microscope with a similar resolution, the accelerating power is increased. Ser. physical”, vol. 34, 1970; Hawks, P., i, trans. from English, M., 1974; Derkach V. P., Kiyashko G. F., Kukharchuk M. S., Electron probe devices, K., 1974; Stoyanova I. G., Anaskin I. F., Physical foundations of methods of transmission electron microscopy, M., 1972; Oatley C. W., The scanning electron microscope, Camb., 1972; Grivet P., Electron optics, 2 ed., Oxf., 1972.

The history 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 modern appliance. 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 Karl 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 mass use of these devices in scientific research began in the 1960s, when they reached significant technical perfection.

A significant leap (in the 70s) 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 90s and early 2000s, computerization and the use of CCD detectors greatly increased stability and (relatively) ease of use.

AT last decade modern advanced transmission electron microscopes use correctors for spherical and chromatic aberrations (which introduce the main distortion in the resulting image), but their use sometimes significantly complicates the use of the device.

Types of electron microscopes

Transmission electron microscopy

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The original view of the electron microscope. 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 up to +200 keV (various voltages from 20 keV to 1 meV are used), focused by a system of electrostatic lenses, passes through the sample so that part of it is scattered on the sample, and part is 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. aaaaa

Transmission scanning (scanning) electron microscopy (SEM)

Main article: Transmission scanning electron microscope

One of the types of transmission electron microscopy (TEM), however, there are instruments that operate exclusively in the TEM mode. The 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.

Low voltage electron microscopy

Fields of application of electron microscopes

The main world manufacturers of electron microscopes

see also

Notes

Links

• Top 15 Electron Microscope Images of 2011 The images on the recommended site are randomly colored, and are of artistic rather than scientific value (electron microscopes produce black and white images rather than color).

Wikimedia Foundation. 2010 .

How does an electron microscope work? What is its difference from an optical microscope, is there any analogy between them?

The operation of an electron microscope is based on the property of inhomogeneous electric and magnetic fields, which have rotational symmetry, to exert a focusing effect on electron beams. Thus, the role of lenses in an electron microscope is played by a set of suitably calculated electric and magnetic fields; the corresponding devices that create these fields are called "electronic lenses".

Depending on the type of electronic lenses electron microscopes are divided into magnetic, electrostatic and combined.

What type of objects can be examined with an electron microscope?

Just as in the case of an optical microscope, objects, firstly, can be "self-luminous", i.e., serve as a source of electrons. This is, for example, an incandescent cathode or an illuminated photoelectron cathode. Secondly, objects that are "transparent" for electrons with a certain speed can be used. In other words, when operating in transmission, the objects must be thin enough and the electrons fast enough to pass through the objects and enter the electronic lens system. In addition, by using reflected electron beams, the surfaces of massive objects (mainly metals and metallized samples) can be studied. This method of observation is similar to the methods of reflective optical microscopy.

By the nature of the study of objects, electron microscopes are divided into transmission, reflection, emission, raster, shadow and mirror.

The most common at present are electromagnetic microscopes of the transmission type, in which the image is created by electrons passing through the object of observation. It consists of the following main components: an illumination system, an object camera, a focusing system, and a final image registration unit consisting of a camera and a fluorescent screen. All these nodes are connected to each other, forming the so-called microscope column, inside which pressure is maintained. The lighting system usually consists of a three-electrode electron gun (cathode, focusing electrode, anode) and a condenser lens (we are talking about electronic lenses). It forms a beam of fast electrons of the desired cross section and intensity and directs it to the object under study located in the object chamber. The electron beam passing through the object enters the focusing (projection) system, which consists of an objective lens and one or more projection lenses.