Scanning Electron Microscopy: an overview on application and perspective

Transcription

1 Scanning Electron Microscopy: an overview on application and perspective Elvio Carlino Center for Electron Microscopy - IOM-CNR Laboratorio Nazionale TASC - Trieste, Italy

2 Location of the Center for Electron Microscopy (CME) Elettra Synchrotron Fermi FEL IOM-CME

3 Elvio Carlino - Centro Microscopia Elettronica TEM/STEM laboratory Jeol JEM 2010F FEG UHR TEM/STEM: 200 kv accelerating voltage T. A. Field Emission Source ZrO/W[100] Low Cs UHR pole-piece: (0.47 ±0.01) mm Scherzer resolution: 0.19 nm Minimum probe size: nm STEM B. F./HAADF detectors: resolution in zcontrast = nm Oxford Energy Dispersive x-ray Spectrometer (EDS) (Z 5) 70 pm demonstrated by coherent electron diffraction imaging* *L. De Caro, E. Carlino, G. Caputo, P. D. Cozzoli, C. Giannini Electron diffractive imaging of oxygen atoms in nanocrystals at sub-ångström resolution Nature Nano. 5 (2010) 360

4 Digital processing and simulation laboratory Digital processing of TEM/STEM images: Digital Micrographs Simulations of EDS and EELS spectra Modelling and Simulations of HRTEM results: JEMS; MacTempas; Kristal Kit Phasing algorithm for coherent electron diffraction imaging* Modelling and simulations of HAADF results based on parallel codes** *L. De Caro, E. Carlino, G. Caputo, P. D. Cozzoli, C. Giannini Electron diffractive imaging of oxygen atoms in nanocrystal at sub-angstrom resolution Nature Nano. 5 (2010) 360 **E Carlino et al.: Accurate and Fast Multi-slice Simulations of HAADF Image Contrast by Parallel Computing MSM Springer Proceedings in Physics ISSN V0l 120 pp A. Cullis, P. Midgley Eds. DOI: / _38 Springer Netherlands

5 Diffraction limited resolution R = λ / n sinα

6 Two ideas behind electron microscopy Louis De Broglie [1] postulated on theoretical basis related to the symmetry of the nature that to any particle is associated a wave: λ = h p λ = h [ 2eVm 0 (1 + ev/2m 0 c 2 ) ] - 1/2 E = 200keV => λ = 2.507pm E = 30keV => λ = 6.977pm Hans Bush [2] demonstrated that the magnetic field of a short solenoid acts on electrons in the same way as a convex glass lens acts on light Ernst Ruska [3] made the hypothesis and demonstrated that a microscope using electrons instead of light photon was possible [1] De Broglie L D 1925 Ann. Phys. Fr [2] Busch H 1926 Ann. Phys. Lpz [3] Ruska E and Knoll M 1931 Z. Tech. Phys

7 Microscopy by electrons: ingredients #1 = electrons

8 Microscopy by electrons: ingredients #1 = electrons Field Emission Gun (FEG)

9 Features electron guns

10 Microscopy by electrons: ingredient #2 = lens Busch H 1926 Ann. Phys. Lpz

11 Aberrations and resolutions with electron lenses Electron lenses are extremely poor: if glass lenses were as bad, we should see as well with the naked eye as with a microscope! The demonstration by Otto Scherzer in 1936 that skilful lens design could never eliminate the spherical and chromatic aberrations of rotationally symmetric electron lenses was therefore most unwelcome and the other great electron optician of those years, Walter Glaser, never ceased striving to find a loophole in Scherzer s proof. In the wartime and early post-war years, the first proposals for correcting C s were made and in 1947, in a second milestone paper, Scherzer listed these and other ways of correcting lenses; soon after, Dennis Gabor invented holography for the same purpose. P. W. Hawkes - Phil. Trans. R. Soc A 28 Sept 2009 vol 367 n Nominal image resolution at 20 kv = 1.5nm O. Scherzer, Z. Phys. 101(9 10) (1936)

12 Electron-matter interactions

13 Electron intensity distribution vs energy

14 Image formation in SEM

15 Cross section secondary electrons (SE) Q SE (E SE ) = n c e 4 k 3 F A 3πEρN 0 (E SE E F ) 2 Differential cross section of low energy secondary electrons Q SE is in terms of secondary electrons per unit energy interval per incident electron per (atom/cm 2 ) k F is the magnitude of the wave-vector corresponding to the Fermi energy E F A is the atomic weight, n c is the number of conduction-band electrons ρ is the material density E SE is the secondary electron energy E is the beam energy

16 Cross section back-scattered electrons (BSE) Rutheford differential cross section for elastic scattering vsscattering angle θ for a constant value of the electron energy E: dq(θ ) = e 4 Z 2 16(4πε 0 E) 2 dω [sin 2 (θ / 2) + θ 0 2 / 4)] 2 dω = 2πsinθdθ Solid angle into which the electron of energy E is scattered at an angle θ from its incident direction e is the electronic charge Z is the atomic number of the scattering atom ε 0 is the dielectric constant (θ 0 /2) 2 is the screening parameter

17 Signals generated by primary electrons

18 Interaction volume Secondary X-ray fluorescence Auger electrons 1 nm secondary electrons 5-50 nm Back-scattered electrons 1-2µm Characteristic X-ray 2-5µm bremsstrahlung

19 Interaction volume vs HV&Z Low Z High Z Tilt angle Up: high accelerating voltage Down: low accelerating voltage

20 SE yeld vs thickness

21 BSE coefficient vs Z

22 Spatial distribution of the back-scattered electrons

23 SE & BSE coefficients vs Z

24 SE: surface and depth of field Elvio Carlino - Centro Microscopia Elettronica

25 SE & BSE Topography Composition

26 Elvio Carlino - Centro Microscopia Elettronica Tungsten with Titanium and Titanium Nitride Al Ti W

27 Example EBSD

28 Low energy imaging of insulator

29 Imaging at low energy HT

30 Imaging at low energy HT

31 Nanotubes

32 American Journal of Physics 1976 Physics world 2002

33 STEM in SEM

34 Set up for STEM imaging in SEM

35 S(T)EM image

36 S(T)EM in biology

37 ???

38 Lattice fringes by diffractive imaging in SEM

39 Low voltage ~ 100 ev

40 Summary SEM is a powerful and flexible tool to study inorganic and organic matter at nanometer resolution, and more. In many cases the images reveals the specimen properties in an intuitive way giving access to quantitative morphology, crystal structure, chemistry, etc.. It is worthwhile to underline how electron microscopy also represents a flexible tool that can be tuned to design new experiments to access subtle properties of the electron matter interaction paving the way to the knowledge of new science.

41 Nobody can resist to electron microscopy!