How does an electron microscope work 1



A electron microscope is a microscope that can image the inside or the surface of a sample with electrons.

Since fast electrons have a much smaller wavelength than visible light (→ matter waves) and the resolution of a microscope is limited by the wavelength, a significantly higher resolution (currently about 0.1 nm) can be achieved with an electron microscope than with a light microscope ( about 200 nm). While the resolution in optical microscopes actually almost reaches the physical limit set by the light wavelength, in electron microscopes the aberrations of the electron-optical components worsen the usable resolution by about two orders of magnitude compared to the electron wavelength, which is about 0.0037 nm for 100 keV electron energy.

When interpreting the data obtained with electron microscopes, especially images, it must always be taken into account how the signals are generated so as not to draw incorrect conclusions.

construction

The main components of an electron microscope are:

  • The electron gun that generates the free electrons in an electron source and accelerates them in the direction of an anode located in a ring around the beam axis. There is a high voltage between the anode and cathode which, depending on the microscope, varies from a few kV to 3 MV.
  • Electron lenses that can deflect the trajectories of electrons. Magnetic lenses are mostly used, in some cases electrostatic ones in the electron gun. Electron lenses have the same function as glass lenses in a light microscope. While the focal length of glass lenses is fixed, it can be regulated with electron lenses. Therefore, in contrast to a light microscope, an electron microscope does not contain any exchangeable or displaceable lenses (systems) such as the objective or eyepiece of a light microscope. As with the light microscope, diaphragms are also used in addition to lenses.
  • The vacuum system that ensures that the electron source can work more efficiently and that the electrons are not hindered on their way by colliding with gas molecules or suspended particles (dust).
  • The sample holder, which must guarantee a stable position of the sample. In addition, manipulation options are often desired, of which different combinations can be realized depending on the type of specimen holder: displacement, rotation, tilting, heating, cooling, stretching, etc.
  • Detectors that register the electrons themselves or secondary signals

Modes of operation

Electron microscopes can be divided into two basic aspects.

  • The first is the type of imaging:
    • Secondary electron microscopes (SEM) use an electron-optical system of electromagnetic and electrostatic lenses to generate a fine electron beam on the object, which is guided line by line over the rectangular object area to be examined ("rasterized, hence the designation" scanning electron microscope "," SEM "). The image comes here through the synchronous registration of a characteristic signal caused by secondary electrons emitted by the object.
    • Still image microscopes irradiate an object area with a fixed, wide electron beam. The image is generated here by using part of the electrons emanating from the object for image generation by means of an electron-optical system.
  • The second possible classification relates to the geometry of the arrangement.
    • In transmission (“Transmission electron microscope”, “TEM”) is carried out in that the fast beam electrons are used to generate images after passing through the object, with only very small scattering angles being recorded as a rule. Transmission electron microscopes mostly work according to the still image method (TEM), occasionally the raster method is used here (“STEM” from English “scanning transmission electron microscopy / microscope”).
    • For the opposite, the registration of electrons that emerge at very large angles to the incident electron beam, there is no fixed designation, in the following table, which clarifies the given classification, there is no.B.d.A. Backscatter.
  Still image EMRaster EM   
transmission     TEM STEM
Backscatter     Reflection microscope REM (English SEM)

The most common electron microscopes according to the number of devices installed are SEM / SEM, followed by TEM. STEM is even less common, but the STEM mode is often possible as an operating mode in TEM, pure STEM devices ("dedicated STEM") are extremely rare. As far as the author knows, reflection microscopes can only be found as laboratory setups in some institutes, but are not commercially available. Reflection microscopy, i.e. electron-optical imaging of surfaces, is used, for example, for short-term experiments in which the electron beam is only available for very short times; the short period of time would not be sufficient to scan the image field with an electron beam in the same way as with the SEM.

In addition, there is the field electron microscope that works without imaging optics and in which the sample itself forms the cathode from which the electrons emerge.

Scanning electron microscope

At the Scanning electron microscope (SEM) (or English SEM, Scanning electron microscopy / microscope) a thin electron beam is scanned over the usually massive object. Electrons that emerge or backscatter from the object, or other signals, are detected synchronously, the registered current determines the intensity value of the assigned pixel. Most of the time, the data is displayed immediately on monitors so that the image structure can be followed in real time. With old SEMs without a computer connection, a cathode ray tube was controlled directly with the signal intensity, and the image written on the fluorescent screen of this tube was then photographed with a camera with a correspondingly long shutter opening time to save the image.

The most important signals used in the SEM to image the object surface are secondary electrons (SE) and backscattered electrons (BE or BSE from engl. Back scattered electrons). The cathodoluminescence (KL) Signal (or CL from engl. cathodoluminescence) is of minor importance and is only used in special investigations.

The SE are low-energy electrons that are released by primary electron bombardment. This enables a very high resolution. They are accelerated by suction in the direction of the detector and there generate a number of pulses corresponding to their number. A different image is generated depending on the position of the detector in the object chamber. The standard SE detector is attached to the side above the object and provides a very natural, three-dimensional image because the side facing the detector is brighter than the side facing away. A SEM that only worked in this operating mode was previously called a secondary electron microscope. Another SE detector present in modern SEMs is the so-called "Inlens" detector, which is attached in the form of a ring above the object inside the "column". Due to the very short working distance, it enables very high-resolution images (a few nm) with low acceleration voltages of the primary beam (several 100 V).

The BE or BSE are electrons from the primary beam that are elastically reflected on the atomic nuclei hit on the object surface. The energy of the electrons is in the range of the irradiated primary electrons, the image resolution is in the micrometer range depending on the primary energy. The BSE detector is usually placed as a 4-quadrant semiconductor detector directly above the object. Depending on the wiring of the semiconductor crystals, different topographic contrasts are obtained, with deep-lying areas of the object appearing dark. The property that heavy elements reflect the electrons more strongly than light ones is used with the so-called Z-contrast (Z = atomic number of the elements). The brightness of the image area allows conclusions to be drawn about the chemical nature of the object surface.

KL is the luminescence of the object surface triggered by electron bombardment. The KL signal, i.e. the visible light, is led out of the object chamber via special mirrors and light guides, spectrally broken down by means of a monochromator and detected via a photomultiplier or a CCD detector.

Another examination method on the SEM that is currently gaining in importance, but which does not depict the object surface, is that EBSD-Procedure (from Engl. Electron back scatter diffraction). EBSD can be used to determine the crystallographic orientation of crystals on the object surface. This is of great importance, for example, for the characterization of material properties in materials science and geology. For this purpose, the electrons reflected from the crystal surfaces of the object are projected onto a detector screen and the resulting "Kikuchi lines"analyzed with the help of a computer and assigned to crystallographic directions.

The Electron microprobe is a special scanning electron microscope that is optimized to carry out chemical analyzes on surfaces in the µm range. Here comes the wavelength dispersive (WDX) or the energy dispersive (EDX) X-ray analysis method for application.

A ESEM (from Engl. Environmental Scanning Electron Microscope) allows you to work with a relatively high gas pressure (a few dozen mbar) close to the object. This makes it possible to examine moist objects (e.g. living cells or growing crystals).

Transmission electron microscope

At the Transmission electron microscope (TEM) the electrons radiate through the object, which for this purpose must be correspondingly thin. Depending on the atomic number of the atoms that make up the object material, the level of the accelerating voltage and the desired resolution, the sensible object thickness can range from a few nanometers to a few micrometers. The higher the atomic number and the lower the accelerating voltage, the thinner the object must be.

By changing the projective lens system, instead of the intermediate image, the focal plane (focal plane) of the objective lens can also be shown enlarged (see illustration). An electron diffraction image is obtained, which can be used to determine the crystal structure of the object.

The transmission electron microscope can be expediently expanded with various analysis methods; energy-dispersive X-ray analysis (EDA) and electron energy loss spectroscopy (EELS) are particularly widespread. Both methods can be used to determine the concentration and distribution of chemical elements in the sample, the small achievable diameters of the electron beam in principle also allowing the investigation of very small object areas. When using these methods, one often speaks of analytical transmission electron microscopy.

A further development of the electron energy loss spectroscopy method in TEM is the Energy-filtered transmission electron microscopy (EFTEM), in which mostly images of inelastically scattered electrons of certain, characteristic energies are recorded. This means that the distribution of chemical elements in the image field can often be determined very quickly and effectively. Similarly, energy-filtered electron diffraction images can also be recorded.

If the primary electron beam is finely bundled and scanned over the object, the electrons that have passed through are detected and assigned to the respective beam position on the object, this process is referred to as Scanning transmission electron microscopy (STEM from engl. Scanning Transmission Electron Microscope). An electron-optical object imaging does not take place.

Optical aberrations

This is a measurement method for the optical errors (aberrations) of an electron microscope "Zemlin Tableau". In the electron microscope, images of carbon foils are recorded with different beam tilts. The power spectra of these images are arranged in a tableau according to the azimuth of the beam tilt. With the help of this tableau all paraxial aberrations can be measured. The "Zemlin Tableau" thus serves the exact adjustment of the electron microscope and the correction of the optical errors. (F. Zemlin, K. Weiss, P. Schiske, W. Kunath and K.-H. Herrmann, Ultramicroscopy 3 (1978) 49-60)

Object preparation

In the past, non-conductive objects, especially in the scanning electron microscope (SEM), had to be provided with a thin conductive layer to prevent electrostatic charging. This is usually achieved by plasma coating with gold in a sputtering device or vapor deposition with carbon. In the meantime, the use of low-pressure gas in the object space (device type ESEM, here gas molecules transport surface charges away) or the use of very low-energy primary electrons (thanks to sophisticated electron optics; here the total electron current on the sample is set to close to 0) are available, to depict non-conductive objects without a conductive coating.

For transmission electron microscopy, the objects have to be brought to a maximum thickness of 100 nm (in special cases 1 µm) using different methods. The ion etching process, in which the object is eroded by an ion beam, is very common.

disadvantage

The time-consuming preparation of the objects can lead to artifacts - structures that have only arisen through the preparation and have nothing to do with the actual object - which makes it difficult to evaluate the images. In addition, the material properties in the TEM can deviate from those of compact objects due to the disproportionate proportion of areas near the surface in the analyte volume. Another problem is the damage to the objects by the electron beam, for example by heating or pushing away entire atoms after colliding with the fast electrons, but also by injecting foreign atoms from the vacuum into the sample. Due to the vacuum inside the microscope and the associated drying, it is also only possible in a few cases to view living objects.

Another disadvantage is the very high acquisition and maintenance costs for electron microscopes, which often do not allow private companies to operate their own devices. Therefore, electron microscopes are mainly found in research institutes and service companies.

history

The first lens based on magnetic forces was developed by Hans Busch in 1926. As the first electron microscope, a TEM was built by Ernst Ruska and Max Knoll in 1931, although initially no electron-transparent objects, but rather small metal grids were imaged as a test. For this work Ruska received the Nobel Prize in Physics in 1986. He also developed the first commercial electron microscope at Siemens in 1938.

Contrasting biological objects with osmic acid was proposed by Ladislaus Marton in 1934. The first STEM was built in 1937 by Manfred von Ardenne.

While in the early years the elucidation of pathogens (viruses) that were invisible under the light microscope was an important driving force for the development of the electron microscope, interest later expanded to include materials science in particular, after Robert D. Heidenreich succeeded in preparing thin radiolucent metal films in 1949.

In the 1960s, TEM was developed with ever higher acceleration voltage (up to 3 MV, around 1965 in Toulouse, 1970 in Osaka), primarily to be able to penetrate thick objects. Atomic resolution was also achieved for the first time in this decade.

At the end of the 1960s, Albert Crewe introduced the field emitter for STEM and thus made this technology important.

The ESEM was developed in the late 1980s.

Schottky field emitters have been used in TEM since the late 1980s.

FESEM with Schottky field emitters have been in use since the early 1990s.

It is also worth mentioning the increasing use of computers since the 1990s. For example, complicated lens systems can be adjusted automatically by analyzing the recordings of a CCD camera, which significantly relieves the user of the microscope. The use of computers to compensate for aberrations of the electron-optical lenses with magnetic multipole lenses is indispensable, a technique that has become more and more important in recent years in SEM, TEM and STEM.

See also

literature

  • Stanley L. Flegler, John W. Heckman Jr., Karen L. Klomparens: Electron microscopy: basics, methods, applications. Spectrum Academic Publishing House, Heidelberg, Berlin, Oxford 1995, ISBN 3-86025-341-7
  • Ludwig Reimer, Gerhard Pfefferkorn: Scanning electron microscopy. 282 pages - 2nd, ext. Edition. Springer, Berlin 1999, ISBN 3-540-08154-2.
  • David B. Williams and C. Barry Carter: Transmission Electron Microscopy - A Textbook for Material Sciences. 729 pages - Plenum Press, New York, London 1996, ISBN 0-306-45247-2.

Category: surface physics