4. ContâŚ
⢠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
7. Back ground
⢠Maximum resolution, d
⢠STEM an electronâs 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
9. ContâŚ
⢠DETECTOR
1. 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
ďś limited by the spot size
ďśhave poorer resolution but
10.
11. 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
12. ContâŚ
⢠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
16. 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.
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
21. Specimen Preparation
⢠Preparation done in two steps
⢠Pre-Thinning:
ďś Reducing the thickness to about 0.1mm
⢠Final Thinning:
ďś Reducing the thickness to about 100nm
involve
ďśIon Milling
ďśElectrolytic Thinning
ďśUltramicrotomy
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
a) Dimple grinding
b) ion milling
ďś ion beam of 1â10 keV
bombarded
ďś specimen is placed in
the center at an angle of
23. Electrolytic Thinning
⢠Reducing specimen
thickness to 100nm
⢠General procedure
ďśA specimen placed in
an electrochemical
cell as anode
ďśA suitable reduce
specimen thickness
ďśCommon technique is
jet polishing
ďśElectrolytic thinning
completed in 3â15
minutes.
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
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
29. ADF images
⢠Electrons which have
scattered to high
angles are collected
⢠Images contain
Bragg diffraction
30. 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?
â˘For DF imaging the annular detector collects more electrons than an
aperture.
â˘STEM ADF images are less noisy then TEM DF images as no lenses are
used to form them.
â˘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