1. Scanning Transmission Electron Microscope

2. Outline History Interactions of Electrons Background STEM Components
Specimen Preparation
Mode
Image formation
Comparison

4. Cont…
The first STEM was built in 1938 by Baron Manfred von Ardenne.(distroyed in air raid)
Not developed further until the 1970.
Then Albert Crewe at the University of Chicago developed the field emission gun.
Then built a STEM able to visualize single heavy atoms on thin carbon substrates

5. Interactions of electrons

6. Ramiz Ahmad

7. Back ground Maximum resolution, d
STEM an electron’s velocity approaches the speed of light, c

8. STEM
The basic principle of image formation fundamentally different from static beam TEM
small spot size is formed on the sample surface with the condenser lenses
This probe is scanned on the sample surface
the signal is detected by an electron detector, amplified and synchronously displayed on CRT with the scan coils

9. Cont…
DETECTOR
Small disk on the column axis which detects the transmitted beam (BF STEM image) or diffracted beam (DF STEM image)
Annular detector (a plate with a hole) which detects all the diffracted beams except the transmitted one (ADF STEM)
Resolution
limited by the spot size
have poorer resolution but better contrast

10. Naveed Abbas

11. Scattered beam electrons
In STEM signal is detected by
back scattered electrons(BSE)
Transmitted beam electrons scattered at some angles
In both cases, BSE and TBE, the signal intensity is a function of the average atomic number of the sample volume and also phase contrast that interacted with the beam
Thus providing atomic number and phase contrast

12. Cont…
In STEM, the electron beam is rastered (scan coil) across the surface of a sample in a similar manner to SEM, however, the sample is a thin TEM section and the diffraction contrast image is collected on a solid-state (ADF) detector.
HAADF-high angle annular dark-field

13. TEM =
STEM

14. Imran Doger

15. Components Source formation - Field emission Electromagnetic Lenses
- Condenser lens
Aperture
Specimen stage
-1. Single-tilt 2. Double-tilt
Vacuum system
Scanning coils
Detectors
-1. BF 2.ADF 3.HAADF

16. Source formation
The STEM consists of an emission source tungsten filament, or a lanthanum hexaboride
High voltage source (typically kV)
Electrons emit by field emission.

17. Vacuum system STEM is evacuated to low pressure 10^ -4 Pa
It consists of multiple pumping systems and air locks.
Low or roughing vacuum is achieved with either rotary vacuum pump or diaphram pumps
For low vacuum turbomolecular pumps are connected to the chamber
Gate valve: for different vacuum levels in specific areas(10^-4 to 10^-7 Pa)

18. V V2
Field emission
Electromagnetic Lenses
Aperture
Specimen stage mesh

19. Specimen holder

20. Sami-Ullah

21. Specimen Preparation Preparation done in two steps Pre-Thinning:
Reducing the thickness to about 0.1mm
Final Thinning:
Reducing the thickness to about 100nm
involve
Ion Milling
Electrolytic Thinning
Ultramicrotomy

22. Ion Milling
Uses a beam of energetic ions to bombard specimen surfaces to reduce the thickness by knocking atoms out of a specimen
General procedure
Dimple grinding
ion milling
ion beam of 1–10 keV bombarded
specimen is placed in the center at an angle of 5-30◦

23. Electrolytic Thinning
Reducing specimen thickness to 100nm
General procedure
A specimen placed in an electrochemical cell as anode
A suitable reduce specimen thickness
Common technique is jet polishing
Electrolytic thinning completed in 3–15 minutes.

24. Electrolytic thinning
Ione milling
Ultramicrotomy

25. M Junaid Anjum

26. Modes
Transmitted electrons that leave the sample at relatively low angles with respect to the optic axis(bright field (BF).)
Transmitted electrons that leave the sample at relatively high angles with respect to the optic axis(annular dark field (ADF).)
High Angle ADF (HAADF) collects the highly scattered Z-contrast beam

27. Image formation
BF-STEM images are equivalent to TEM (reciprocity principle).
Produced Bragg disks hitting the detector
Give the bright field or phase signal

28. Image formation
Bright field STEM image of Au particles on a carbon film

29. ADF images Electrons which have scattered to high angles are collected
Images contain Bragg diffraction

30. Iimage (r )= Iprob (r ) V2proj (r )
HAADF images
Two (out of several more) ways to simulate HAADF-STEM images are
Incoherent Imaging Model:
The Image is the convolution of object potential and probe intensity.
Iimage (r )= Iprob (r ) V2proj (r )
Multiple Scattering Image Simulation:
the frozen phonon approximation.

31. HAADF better Z-contrast than BF
HAADF is much less sensitive to local diffraction conditions than BF.
Its sensitivity mainly to the atomic number

32. Bright and dark field STEM image of Au particles on a carbon film

33. Why use STEM?
•For DF imaging the annular detector collects more electrons than an aperture.
•STEM ADF images are less noisy then TEM DF images as no lenses are used to form them.
•Contrast in STEM images is greater than standard DF images. a) b) c)
• Comparison of TEM DF and STEM ADF images of the same sample shows clear contrast difference

34. THANK YOU