Scanning Electron Microscopy (SEM)
SEM: A focused electron beam
(2-10 keV) scans on the surface,
several types of signals are
produced and detected as a
function of position on the
surface. The space resolution
can be as high as 1 nm.
Different type signal gives
different
information:
a.
Secondary electrons: surface
structure. b. Backscattered
electrons: surface structure and
average elemental information.
b. X-rays and Auger electrons:
elemental composition with
different thickness-sensitivity.
The image formation
TV screen
e-beam
The selection of signal
Advantages of Using SEM over OM
OM
SEM
Magnification
4x – 1000x
10x – 3000000x
Depth of Field
15.5mm – 0.19mm
4mm – 0.4mm
Resolution
~ 0.2mm
1-10nm
The SEM has a large depth of field, which allows a large amount of the sample
to be in focus at one time and produces an image that is a good representation of
the three-dimensional sample. The SEM also produces images of high resolution,
which means that closely features can be examined at a high magnification.
The combination of higher magnification, larger depth of field, greater
resolution and compositional and crystallographic information makes the SEM
one of the most heavily used instruments in research areas and industries,
especially in semiconductor industry.
OM
SEM
Schematic set-up
of SEM
Topographic contrast arises because SE
generation depend on the angle of
incidence between the beam and
sample.
Backscattered Electrons (BSE)
BSE yield: h=nBS/nB ~ function of atomic number Z.
BSE images show characteristics of atomic number contrast, i.e., high
average Z appear brighter than those of low average Z.
BSE image from flat
surface of an Al (Z=13)
and Cu (Z=29) alloy
Different surface sensitivity
Auger electron and X-ray can be the signal source for SEM and obtain
the elemental composition microscopy but with totally different surface
sensitvity.
Why suitable for
high energy e
magnetic lens
Magnetic lens
(solenoid)
(Beam diameter)
F = -e(v x B)
p
q
Lens formula: 1/f = 1/p + 1/q
Demagnification:
M = q/p
f  Bo2
f can be adjusted by changing Bo, i.e., changing the
current through coil.
TEM
Control
brightness,
convergence
binocular
screen
Control contrast
Interpretation of Images
bend
contour
Dislocations
loop
A buckled region of a thin foil and
bend contour occurs where the Bragg
condition is locally satisfied.
Bent
foil
image
0.5m
Dislocations are readily analyzed
and characterized by means of
diffraction contrast.
Why TEM
The uniqueness of TEM is the ability to obtain full morphological (grain
size, grain boundary and interface, secondary phase and distribution,
defects and their nature, etc.), crystallographic, atomic structural and
micro-analytical such as chemical composition (at nm scale), bonding
(distance and angle), electronic structure, coordination number data from
the sample.
TEM is the most efficient and versatile technique for the characterization
of materials. It has many mechanism for contrast in TEM for different
application purpose.
limitation
More or less bulk-like information, the
sample cannot be too thick, Sample
preparation can be difficult.
Comparison of OM,TEM and SEM
Source of
electrons
Light source
Condenser
Magnetic
lenses
Specimen
Objective
Projector
Eyepiece
Specimen
CRT
Cathode
Ray Tube
detector
OM
TEM
SEM
Principal features of an optical microscope, a transmission electron
microscope and a scanning electron microscope, drawn to emphasize
the similarities of overall design.
PEEM(Photoemission Electron Microscope)
Resolution reached < 20 nm
PEEM with integral
sample stage for precise
sample positioning via
remote controlled piezodrives.
Secondary
electrons generated by
the incident X-rays are
focused by the lenses to
form a magnified image
of the electron yield
distribution
emerging
from the sample surface.
•Topography contrast
•Work funktion contrast
•Chemical contrast (absorption edges)
•Magnetic contrast (magnetic dicchroism)
Secondary have relatively
long free path length
(10nm), very suitable
study multilayer
Low Energy Electron Microscopy (LEEM)
a
b
a) Si(001) atomic arrangement
b) the corresponding diffraction
pattern
Basically same as PEEM but with e
source
Green spots
Red spots
Localized structural information
Scanning Probe Microscopy (SPM)
1. Scanning tunneling Microscopy (STM)
I = U/d exp(-Kd f1/2)
Inverse decay length
K (reciprocal of
distance for density
to fall to
K = 2p/h
(2me f)1/2
1/e):
STM is based on the tunneling current
between a metallic tip and the sample
surface, which depends on voltage (U),
distance d, and work f average of the
two work functions of both sides , K is a
constant.
STM works only with conducting sample !
Tunneling between two metals
Without bias tunneling
Without bias tunneling
Spatial Resolution of STM
I = U/d exp(-Kdf1/2)
For a typical metal (K = 1 Å),
current falls about an order of
magnitude for an increase of
1.0 Å in d. very sensitive
dependence of tunnel current on
d - good "vertical“ resolution.
If one metal is sharp tip,
most of Itunnel will travel
through pinnacle atom good "lateral" resolution
How STM works
More compact
material
changes
length in
applied
electric field
- 10-4 to 10-7
% length
change per V
allows < 0.5
Å positioning
for in-plane
and vertical
resolution <
0.05 Å.
The image formation
I change
Constant Height Mode
Tip-sample distance fixed variation in Itunnel forms image
Fast but only works for flat
samples
How the scan works
height change
Itunnel constant by moving tip up and down
(feedback circuitry) - z
movement becomes image
Slower but works for rough surfaces
Most common
The tip sharpness
Sharpness
is essential
!!!
good
Etching: a
common
way
-
bad
+
Tip radius < 20 nm !!!
NaOH
Etched
not etched
STM
Vibration isolation of
the STM (SPM)
device
is
very
important
(spring,
eddy
current
damping
magnet
assemblies, etc). The
vertical resolution is
0.05 Å!! The electric
isolation
is
also
important,
the
tunneling
current
normally nA – pA.
tip
spring
STM on clean surface and with absorbent
Atom-resolved !!!
Ag(110) clean surface with step edge
O2 covered Ag(110)
Image of work function
Normal STM mode can not distinguish the change of morphology
(height) from work function (see the formula before), however, from
the formula
, which able to tell the difference.
Au on Si(111) line scan of both z and f, the change of z
is due to change of work function (where there is Au).
Tunneling from tip to substrate
Tunneling is current from occupied states of one side to unoccupied states
of another side of the STM. The direction of current depends on bias. The
current depends the filled or unfilled electron states near the Fermi surface.
Between metal tip to semiconductor
IV curve: Scanning Tunneling Spectroscopy
Unique
peak at
certain
bias
When tip is biased negative, one gets information about empty electronic states
on surface. When tip is biased positive, one gets information about filled
electronic states on surface. With the same height, one record the current as
function of bias voltage, in reality this measurement is often plotted as I/V,
dI/dV, or d2I/dV2 curves. This kind of plot provides atom-resolved
information about local electronic (and even vibrational) structure.
Simply one can can do STM at changing bias to resolve "location" of
individual electronic states.
Manipulation
Several methods:
- field effect - under
influence
of
high
electric
field,
polarizable molecule
or atom can be made
to jump from surface
to tip or vice versa.
- dragging - vdW
forces between close
tip and adsorbate can
be used to
drag species.
Tip
Atomic Force Microscope
Cantilever
Can work with insulator
Soft, flexible
(low spring constant)
The Force that is experience by the tip is mainly due to Van der Waals
force, typically 10-11 to 10-6 N at separation of ~ 1 Å.
Working modes
Contact Mode (Repulsive Mode): Tip makes a “soft physical” contact with the
surface and scan. Repulsive force on tip about 10-6 – 10-8 N. operates in ambient
or in liquid. frictional force that will distort the image. Works in two modes:
constant force or constant height.
non-contact mode (attractive Mode): Tip-surface distance 10-100 Å.
Attractive force of 10-12 N. uses AC driven oscillating cantilever (1001000 Hz frequency, 10-100 Å amplitude) to sense the changes in the
resonant frequency of a cantilever which reflect changes in the tip-to-sample
spacing. Fluid contamination may result in a quite different picture. best for
soft or elastic surfaces. least contamination. least destructive. long tip life
intermittent contact (tapping) mode: Similar to non-contact using vibrating
cantilever except at one extent tip "taps“(barely contact) into contact.
The amplitude > 20 Å, the change of the amplitude due to tap will be
sensed. Useful for soft surfaces - less prone to external vibration/noise
than noncontact, but will not image water layers (pierce). Less
destructive than contact AFM and can image rougher samples.
Working modes
a
b
c
Contact(repulsive) Non-contact (attractive) Tapping
a.Sliding the probe tip across surface, heavily influenced by frictional and
adhesive forces, which can damage samples and distort image data.
b.Sensing Van der Waals attractive forces between surface and probe tip
held above surface (50 - 150 Å), low resolution and can also be hampered
by the contaminant layer which can interfere with oscillation.
c.Tapping the surface with an oscillating probe tip, eliminates frictional
forces by intermittently contacting the surface and oscillating with
sufficient amplitude to prevent the tip from being trapped by adhesive
meniscus forces from the contaminant layer.
Fluid contamination
Non-contact AFM
contact AFM
AFM photos
Protein
Atom-resolved
50 x 50 m
NaCl(100) surface using a
variant of contact mode UHV
AFM – arrow shows defects
Creature with mouth
More Scanning Probe techniques
AFM
MFM
Lateral Force Microscopy (LFM): measures frictional forces
between the probe tip and the sample surface
Magnetic Force Microscopy (MFM): measures magnetic gradient
and distribution above the sample surface;
Electric Force Microscopy (EFM): measures electric field gradient
and distribution above the sample surface;
Scanning Thermal Microscopy (SThM): measures temperature
distribution on the sample surface.
Scanning Capacitance Microscopy (SCM): measures carrier
(dopant) concentration profiles on semiconductor surfaces.
Spin-resolved STM: atom-resolved magnetic microscopy.