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Scanning Transmission Electron Microscope
Outline
• History• Interactions of Electrons• Background• STEM• Components• Specimen Preparation• Mode • Image formation• Comparison
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
Interactions of electrons
Ramiz Ahmad
Back ground
• Maximum resolution, d
• STEM an electron’s velocity approaches the speed of light, c
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
Cont…
• DETECTOR1. Small disk on the column axis
which detects the transmitted beam (BF STEM image) or diffracted beam (DF STEM image)
2. Annular detector (a plate with a hole) which detects all the diffracted beams except the transmitted one (ADF STEM)
• Resolution limited by the spot sizehave poorer resolution but better
contrast
Naveed Abbas
Scattered beam electrons
• In STEM signal is detected by I. back scattered electrons(BSE)II. 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
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
TEM =
STEM
Imran Doger
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
Source formation
• The STEM consists of an emission source tungsten filament, or a lanthanum hexaboride
• High voltage source (typically 100-300kV)
• Electrons emit by field emission.
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)
V1 V2
Field emission
Electromagnetic Lenses
Aperture Specimen stage
mesh
Specimen holder
Sami-Ullah
Specimen Preparation
• Preparation done in two steps• Pre-Thinning: Reducing the thickness to about 0.1mm• Final Thinning: Reducing the thickness to about 100nminvolveIon MillingElectrolytic ThinningUltramicrotomy
Ion Milling
• Uses a beam of energetic ions to bombard specimen surfaces to reduce the thickness by knocking atoms out of a specimen
• General procedurea) Dimple grindingb) ion milling ion beam of 1–10 keV
bombarded specimen is placed in the
center at an angle of 5-30◦
Electrolytic Thinning
• Reducing specimen thickness to 100nm
• General procedureA 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.
Ione milling
Electrolytic thinning
Ultramicrotomy
M Junaid Anjum
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
Image formation
• BF-STEM images are equivalent to TEM (reciprocity principle).
• Produced Bragg disks hitting the detector
• Give the bright field or phase signal
Image formation
Bright field STEM image of Au particles on a carbon film
ADF images
• Electrons which have scattered to high angles are collected
• Images contain Bragg diffraction
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.
HAADF better Z-contrast than BF
• HAADF is much less sensitive to local diffraction conditions than BF.
• Its sensitivity mainly to the atomic number
Bright and dark field STEM image of Au particles on a carbon film
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
THANK YOU