6. Scanning Electron Microscopy (SEM) Literature: Rasterelektronenmikroskopie L. Reimer und G. Pfefferkorn, Springer Verlag, Berlin 1973 Praxis der

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1 6. Scanning Electron Microscopy (SEM) Literature: Rasterelektronenmikroskopie L. Reimer und G. Pfefferkorn, Springer Verlag, Berlin 1973 Praxis der Rasterelektronenmikroskopie und Mikrobereichanalyse Peter F. Schmidt, expert verlag, Renningen-Malmsheim 1994

2 6.1 Introduction History First electron microscope in the 1930ies by Knoll and Ruska (transmission geometry) First SEM in UK in the 40ies Properties Big depth of focus Three-dimensional image vie shadow effects Resolution Priniple: Electron beam scans surface Image recorded pixel by pixel

3 Signels for imaging

4 6.2 Wave-Particle Dualism of Electrons de Broglie Beziehung, Wellenlänge eines Teilchens: h h λ = = p mv m Masse, v Geschwindigkeit des Elektrons, p Impuls Energie des Elektrons mv 2 = eu 2 U Beschleunigungsspannung Wellenlänge λ = h 2meU mit relativistischer Korrektur λ = 2m m o Ruhemasse des Elektrons o h eu eu 1 + 2moc Wellenlänge nm bei 10 kv und nm bei 100kV 2

5 6.3 Interaction between electrons and specimen Formation of a diffusion cloud by elastic and inelastic scattering ot the primary electrons (PE) in the solid Diameter of cloud >> diameter of beam Penetration depth x and diameter of cloud is function of Acceleration voltage (U x ) Specimen material (Z x ) Conductive specimen required

6 Elastic scattering Coulomb interaction High scattering angles No energy loss Back scattered electrons; energy: 50 ev U(PE) inelastic scattering Electron-electron-interaction Loss of kinetic energy by Ionization of atoms by electron loss from core level Electron from outer shells are moved to unoccupied states (Energy bands) Often energy loss by small amounts 5-50 ev Secondary elecrons created in depth of 1-10 nm N(E)

7 6.4 Setup of SEM Components Colum and Vakuum Chamber Electron gun Electron optics Stage Detectors (SE, BSE, X-Ray) Recording (Screen, Film, Computer)

8 Electron beam formation / Electron gun Thermionic (hot) cathode (W, LaB 6 ) Gun heated ( K für W) Electrons are emitted into vacuum; accelerated by high tension (1-50 kv) Current density ca. 5 A/cm 2 Life cycle of cathode (limited by evaporation) (10 bis 150 h) Vacuum required because of oxidation of W Workfunction of LaB 6 lower à lower T ( K),higher current density (50 A/cm 2 ), higher life time Field emission guns Cathode material W 1. Anode creates high electric field à tunneling effect 2. Anode accelerate electrons UHV required Cold emission possible; hot mostly used in technology Current densitys ca A/cm 2

9 Eelectron guns Comparison of the different guns: CFE cold field emission - TFE thermal field emission

10 Electron optic Forms a smaller image of cathode on specimen surface Electro-magnetic lenses, deflection vie Lorentz-force, cork screw shaped movement Scanning unit Magnification V: V = Size monitor Size of scanned area Adjustable focus length by objective lens current Beam spread angle < 1, Rayleigh-resolution ca. 60λ Field of depth higher in comparison to optical microscope

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12 Aberrations Spherical Aberration Chromatic Aberration (energy spread) Astigmatism Bsp.: Astigmatismus

13 Vacuum chamber and stage Vacuum: protection of gun and preventing of contamination Environmantal SEMs x-y movement for specimem z-movement for adjusting (working distance WD) Tilt and rotation High mechanical stability

14 Detectors Photo-multiplier SE mit Saugspannung RE möglich, mit Gegenspannung zur Ablenkung der SE Semiconductor (only BSE) Robinson-Detector (BSE)

15 6.5 Imaging with SE and BSE Image formation governed by yields: BSE, back scatter coefficient: η = n n RE PE function of SE: Material (change and increasing with Z) Angle of incident Crystal orientation function of Plane inclination Edges Material (higher for lower Z) Interpretation of image: Analogue to diffuse scattered light Detector resemble light source Inclined planes look brighter Shadowing because of geometry n δ = n SE PE

16 Contrast by plane inclination: SE-yield is function of α (angle between plane normal and beam) 1 δ cosα SE RE starke Schattenbildung

17 Contrast increase via shadowing BSE have higher kinetic energy à more shadowing Topography contrast in BSE image SE SE 100µm BSE BSE

18 Edge effects Stronger SE- (and BSE-Signal) at edges due to higher electron emission à edges often brighter Small edges: only SE effect, not seen by BSE à high magnification: SE images are better Detektor

19 Topography and Materilas contrast BSE-Image: sensitive on material RE BSE

20 Resolution SEM: Diameter of electron beam Dependent on current density à FEG: High current density, small electron source Low voltage possible Diameter of diffusion cloud!! SE better then BSE!!!

21 Auflösung am Beispiel von Au-Inseln auf Graphit SE BSE

22 Charging!! σ=δ+η Cu Reichweite der PE Austrittstiefe der SE

23 6.6 Electron-Channeling Channeling- or Orientation contrast PE parallel to crystal planes -> higher penetration depth channeling Lower BSE- and SE-yield (only seen on almost perfect surfaces) RE RE

24 Electron-Channeling-Pattern (ECP) Channeling-Effect for determination of: Lattice Orientation of known lattice Elektronenstrahl überstreicht mit großem Winkel einen einkristallinen Bereich (Standardverfahren, mehrere mm große Probe erforderlich) Alternativ: Elektronenstrahl wird auf einen Punkt fixiert und hin- und hergeschwenkt (Feinbereichsmethode, lateraler Auflösung von etwa 2 µm) Elektronenbeugung führt zu ECP, die charakteristisch für Kristallgitter und seine Orientierung sind

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26 Electron-Channeling-Pattern (ECP) example: Si-Einkristall Zonenachse: Richtung [uvw], der mehrere Ebenen {hkl} angehören. Zonengleichung: uh+vk+wl=0

27 6.6 Electron-Back-Scatter-Patterning (EBSP oder EBSD) Electron beam stable at one position (no tilt as in ECP) Distribution of BSE on CCD Electron diffraction leads to Kikuchi-Lines Indexing of crystal orientation Automatic recoring via software Lateral resolution nm Time per pixel: 1-10 s

28 Example links EBSP wie beobachtet und rechts mit Indizierung Kristall ist [101] orientiert Durchstoßpunkt nicht im Zentrum wegen Verzerrung der Abbildung

29 links: Bild mit Channeling-Kontrast rechts: zugehörige EBSP-Auswertung (markiertes (blaues) Korn hat [100]- Orientierung, andere graue (rote) Körner [111]-Orientierung, restliche sind weiß) [010] [110] 3 µm

30 oben: Bild mit Channeling-Kontrast unten: EBSP-Auswertung der gleiche Stelle zur Bestimmung der Korngrenzeigenschaften (Kleinwinkel- und Zwillingskorngrenzen: dicke Linien; Großwinkelkorngrenzen: dünne Linien)

31 Au/NaCl(100): Cube-on-cube orientation relation 500 nm Au on NaCl SEM EBSD NaCl (100) Au (100)