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

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

•  Signels for imaging

6.2 Wave-Particle Dualism of Electrons

•  de Broglie Beziehung, Wellenlänge eines Teilchens:

m Masse, v Geschwindigkeit des Elektrons, p Impuls •  Energie des Elektrons

U Beschleunigungsspannung •  Wellenlänge

•  mit relativistischer Korrektur

mo Ruhemasse des Elektrons

•  Wellenlänge 0.012 nm bei 10 kV und 0.0037 nm bei 100kV

mvh

ph==λ

eU2mv2

=

meU2h

=λ

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛+

=λ

2o

ocm2

eU1eUm2

h

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

•  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)

6.4 Setup of SEM

Components –  Colum and Vakuum Chamber –  Electron gun –  Electron optics –  Stage –  Detectors (SE, BSE, X-Ray) –  Recording (Screen, Film,

Computer)

•  Thermionic (hot) cathode (W, LaB6) –  Gun heated (2500-3000 K für W) –  Electrons are emitted into vacuum; accelerated by high tension (1-50 kV) –  Current density ca. 5 A/cm2 –  Life cycle of cathode (limited by evaporation) (10 bis 150 h) –  Vacuum required because of oxidation of W –  Workfunction of LaB6 lower à lower T (1500-2000 K),higher current density (50 A/cm2), 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. 106 A/cm2

Electron beam formation / Electron gun

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

Eelectron guns

•  Forms a smaller image of cathode on specimen surface

•  Electro-magnetic lenses, deflection vie Lorentz-force, cork screw shaped movement

•  Scanning unit

•  Magnification V:

•  Adjustable focus length by objective lens current

•  Beam spread angle < 1°, Rayleigh-resolution ca. 60λ

•  Field of depth higher in comparison to optical microscope

Electron optic

•V =Size monitor

Size of scanned area

•  Aberrations –  Spherical Aberration –  Chromatic Aberration (energy spread) –  Astigmatism

Bsp.: Astigmatismus

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

•  Photo-multiplier –  SE mit Saugspannung –  RE möglich, mit Gegenspannung zur Ablenkung der SE

•  Semiconductor (only BSE)

•  Robinson-Detector (BSE)

Detectors

6.5 Imaging with SE and BSE •  Image formation governed by yields:

–  BSE, back scatter coefficient:

function of •  Material (change and increasing with Z) •  Angle of incident •  Crystal orientation

–  SE:

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

PEREnn

=η

PESEnn

=δ

SE RE

•  Contrast by plane inclination: SE-yield is function of α (angle between plane normal and beam)

α∝δcos1

starke Schattenbildung

•  Contrast increase via shadowing –  BSE have higher kinetic energy à more shadowing –  Topography contrast in BSE image

SE SE

BSE

BSE

100µm

•  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

•  Topography and Materilas contrast –  BSE-Image: sensitive on material

RE BSE

•  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!!!

•  Auflösung am Beispiel von Au-Inseln auf Graphit

BSE SE

•  Charging !!

Reichweite der PE ≈ Austrittstiefe der SE

Cu σ=δ+η

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

6.6 Electron-Channeling

RE

RE

•  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

•  Electron-Channeling-Pattern (ECP)

example: Si-Einkristall

Zonenachse: Richtung [uvw], der mehrere Ebenen {hkl} angehören. Zonengleichung: uh+vk+wl=0

•  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 20 -200 nm •  Time per pixel: 1-10 s

6.6 Electron-Back-Scatter-Patterning (EBSP oder EBSD)

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

Example

links: Bild mit Channeling-Kontrast rechts: zugehörige EBSP-Auswertung (markiertes (blaues) Korn hat [100]-

Orientierung, andere graue (rote) Körner [111]-Orientierung, restliche sind weiß)

[110]

[010]

beam axis

3 µm

oben: Bild mit Channeling-Kontrast unten: EBSP-Auswertung der gleiche Stelle zur Bestimmung der

Korngrenzeigenschaften (Kleinwinkel- und Zwillingskorngrenzen: dicke Linien; Großwinkelkorngrenzen: dünne Linien)

Au/NaCl(100): Cube-on-cube orientation relation

SEM EBSD

NaCl (100) Au (100)

500 nm Au on NaCl