mikroskopia polowa elektron w fem field emission microscopy n.
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Mikroskopia polowa elektronów FEM – Field emission Microscopy

Mikroskopia polowa elektronów FEM – Field emission Microscopy

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Mikroskopia polowa elektronów FEM – Field emission Microscopy

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  1. Mikroskopia polowa elektronów FEM – Field emission Microscopy • Zasada działania • Historia • Przykładowe wyniki

  2. FEM – zasada działania

  3. FEM – zasada działania In its simplest form, FEM consit of a shap needle emitter and a fluorecent screen as shown in Fig. 1. By appling negative field to the emitter, electrons are emitted from the surface of the emitter to the direction of the screen. The image contrast appears due to the difference in current densities of electron, which originated from the difference in work functions and electric field on the emitter surface (Fowler-Nordheim relation).

  4. FEM - historia The field emission microscope was invented by Erwin Mueller in 1936. This instrument approached, for the first time, to view a surface on a scale of atomic dimensions and yet simultaneously allowed one to follow rapid changes at the surface. Erwin Wilhelm Müller was born in Berlin on June 13, 1911, He died on May 17, 1977.

  5. FEM Przykład 1 FEM of crystalline W tip

  6. FEM – obrazy cd Field emission Micrographs of a clean tungsten single crystal surface and a surface with a grain boundary

  7. FEM Przykład 2 Analysis of fiber probes of scanning nearfield optical microscope by field emission microscopy; Ultramicroscopy 89 (2001) 83–87 Illustrating the Field Emission Microscope used to image the metal-coated fiber tip SNOM probes.

  8. FEM Przykład 2- wynik Field emission images of the two di.erent metal-coated SNOM .ber probes with the nominal aperture for the light transmission of 200 nm. Tip potential was equal to 2.2 kV for the .rst tip (a) and to 3.0 kV for the second (b) Coating material: aluminum with the small admixture (10–15%) of chromium.

  9. FEM – applications Field emission has been extensively used in the characterization of surface structures and electronic properties. This technique has given a wealth of information and understanding of surfaces, metal and gas interface systems before the advent of many other techniques for surface analysis. Adsorption of atoms in the submonolayer, monolayer amounts of adatoms, desorption and surface diffusion measurements can be carried out using field emission techniques. The analysis of field emission current fluctuation can yield quantitative information regarding the surface phenomena occurring on the emitter      When the metal under study is the cathode for field emission, its surface condition particularly its surface cleanliness can be judged from the emission pattern. At this point, it becomes clear that role of Ultra High Vacuum is extremely prominent in field emission experiments, as it has an extreme surface sensitivity to change the emission pattern even in the submonolayer regime.  A base pressure less than 10-10  mbar needs to be maintained during the experiments.  Also since, field emission occurs from all single crystal facets, field emission microscopy is best suited to study these facets (crystal planes) and to compare the results under identical conditions in single experiments. These features make the relevance of results of field emission microscopy studies important and unique among other standard surface analytical tools.

  10. Jonowa mikroskopia polowa FIM – Field Ion Microscope • Zasada działania • Historia • Przykładowe wyniki • Tendencje rozwoju

  11. FIM - Zasada działania 1

  12. FIM - Zasada działania 2 Do ostrza próbki K (krzywizna powierzchni rzędu 100 A) przykłada się potencjał dodatni (około 10 kV). Atomy helu w polu elektrycznym ostrza ulegają polaryzacji i zderzają się z ostrzem. Po kilkukrotnym odbiciu - znajdując się w strefie ‘S’ ulegają jonizacji poprzez tunelowanie elektronu do metalu. Zjonizowany dodatni jon zdąża radialnie do katody (ekran fluorescencyjny P) . Na ekranie tworzy się obraz ‘stereograficzny” rzutów atomów z zewnętrznych ścianek monokryształu Mikroskop polowo-jonowy ; Problemy (511) 1989;

  13. FIM - Zasada działania 3 The specimen is in the form of a sharp tip. A positive potential is applied to the tip such that a very large electric field is present at the tip. The ambient gas surrounding the tip is usually Helium or Neon at a pressure of 1-3 x 10 to the minus 3 millibar. The gas atoms move towards the tip and strike it. The gas atoms may strike the surface many times, before an electron from the gas atom tunnels into the metal tip leaving the gas atom positively ionised. The gas atom is then accelerated away from the tip where it strikes a fluorescent screen. The net effect of many gas atoms is to create a pattern on the flourescent screen showing spots of light which correspond to individual atoms on the tip surface. The technique was invented by Erwin Müller in 1951.

  14. FIM – zasada ...4

  15. FIM – Historia The idea of FIM really comes from Prof. E.W. Muller. It all began in 1951 when Muller published his FIM paper, describing the invention of FIM and its improved resolution compared to FEM. However, the atomically resolved  images came into existance only in 1955.

  16. FIM - Układ pomiarowy - schemat Schematics of FIM

  17. FIM - Układ pomiarowy This is a modification of the field emission microscope to study the electron emission from selected crystallographic planes. The anode screen on which the field emission pattern is observed is provided with a hole of 2mm diameter with a Faraday cup collector behind it. With magnetic deflection of the pattern one can bring the single crystal planes of interest to coincide with the hole. The electrons thus falling on the collector are necessarily the ones coming from the probed area. Since the tip to screen distance is about 4-5 cm and the magnification is roughly 106, the probed area on the surface will be few nanometers, which is well within the size of most of the crystal planes observed in the field emission pattern

  18. FIM – obrazy...

  19. FIM – obrazy 1 Field Ion Micrograph of a Field Evaporated Tungsten Tip FIM of crystalline Ir tip

  20. FIM – obrazy 2 – co to jest? Mikroskop polowo-jonowy ; Problemy (511) 1989; a) Rzut stereograficzny monokryształu na płaszczyznę (110) gęstego upakowania w sieci bcc b) Obraz ekranu FIM dla półkolistej próbki monokrysztłu wlframu o strukturze bcc. Scianki typu (110) są ciemne, gdyż ich gęsto ułożone atomy nie produkują wystarczającego pradu jonowego. (wg. E.W. Mulera)

  21. FIM – obrazy 3 Mikroskop polowo-jonowy ; Problemy (511) 1989; Obraz jonowy igły ze stopu Ir-0,1 Zr. Widoczna jest pętla dyslokacyjna wychodząca w zakreślonych kółkach (1972, Phylosophical Magazine)

  22. FIM – zastosowania Mikroskop polowo-jonowy ; Problemy (511) 1989; • badanie fizykochemii powierzchni • adsorpcja • dyfuzja • korozja • kataliza • wzrost warstw krystalicznych (epitaksja)

  23. FEM – FIM Przykłady FEM and FIM images of a clean Ni surface. Both images were obtained from the identical surface of a Ni tip. (Curtsy of K. Hono, NRIM)

  24. Atom probe FIM Połączenie MIF z analizatorem masy jonu (spektrograf masowy) • Zasada działania • Przykłady

  25. Atom probe FIM - historia The first all-metal atom-probe field ion microscope as though it were a weapon, Panitz (at the gunner's position) “fires” a friendly shot at Müller, who is holding the instrument's time-of-flight mass spectrometer tube in this 1969 photo

  26. Atom probe - zasada działania Atom probe tomography (APT) also requires a needle shaped specimen and exploits the effect of a high electric field. When the electric field reaches a critical value, the atoms of the surface of the specimen are ionised and desorpted in a combination called field evaporation. The ions generated are subsequently accelerated away from the specimen surface by the highly divergent elecric field surrounding the specimen, giving also rise to a highly magnified projection of the ions. APT can be considered as being a point projection microscope that enables the observation of single atoms ionised and desorbed from the surface of the specimen via the application of a very intense electric field. Coupled to a time-of-flight mass spectrometer and using a position sensitive detector (PSD), the atom probe becomes a nano-analytical microscope capable of mapping the distribution of the atoms in three dimensions

  27. Atom probe - schemat Mikroskop polowo-jonowy ; Problemy (511) 1989; Nanosekundowy impuls o napięciu U desorbuje atom (odpowiadający zadanej plamce) i nadaje mu energię kinetyczną. Mierzy się czas dojścia atomu do detektora (mikrosek)

  28. Atom probe - Przykład Mikroskop polowo-jonowy ; Problemy (511) 1989;


  30. 3D FIM Early analytical instruments, so called atom probes, were equipped with a single time-of-flight detector to perform a one-dimensional analysis along a specimen cylinder of a few nanometer in diameter. Only some 104 atoms were counted during a typical measurement, so that the statistical significance of the data always was very limited. This situation has recently changed dramatically by the development of two-dimensional detectors, which allow to register a much larger specimen volume and to improve the lateral resolution at the same time. By a reconstruction of the tomographic data, the 3D-distribution of the atoms inside a volume of typically 15 x15x100nm3 is determined in an accuracy that was not achievable before. • Postęp w detekcji jonów

  31. Konstrukcja 3D FIM układuz Gottingen Institut f¨ur Materialphysik, G¨ottingen 20.08.01 background pressure measurement: 5 ¢ 10¡8 Pa; specimen storage: 5 ¢ 10¡7 Pa minimum specimen temperature: ¼ 25K base voltage: 0 : : : 20 kV pulse voltage: 500 : : : 5 kV pulse rate: 2000 Hz data rate: 50 000 . . . 200 000 atoms/h Figure 1: Layout of the new 3D atomprobe at the ’Institut f¨ur Materialphysik’ in G¨ottingen a) top view of the complete vacuum system,

  32. Konstrukcja 3D FIM układu -2 Field evaporation is triggered by negative pulses applied to an extraction electrode in front of the specimen to improve the mass resolution. The geometry of the electrode follows a published design [5]. Using a tip to electrode distance of 4mm and an inner electrode diameter of 4 mm, the requirements to the accuracy of tip positioning are only moderate. After aligning the specimens in standardized holders outside the vacuum system, any mechanical adjustment inside the chamber becomes obsolete, which makes measurements very easy. b) geometry of the specimen stage (inner diameter of the conical electrode bore: 4 mm, tip to electrode distance: 4mm)

  33. Atom probe...konstrukcja The ORNL atom probe includes a three-dimensional atom probe (3DAP)

  34. Atom probe - przykłady In addition to mapping the atoms in an indium arsenide nanowire capped with a gold catalyst (right), modern atom probes can quickly reveal compositional variations in the interior of ultrathin slices (left) made on either side of the interface. Indium is green; arsenic, purple; gold, yellow.

  35. Atom probe – przykłady 2 Visualisation of data obtained from an atom probe, each point represents a reconstructed atom position from detected evaporated ions