SEM
MSE 421/521 Structural Characterization
Historical Development
1935 - Knoll publishes the earliest known paper presenting the concept of a scanning electron microscope
1938 - Manfred von Ardenne constructs first STEM. First micrograph was of a ZnO crystal imaged at
23kV at a magnification of 8000x. The resolution was 50 - 100nm and took 20 min to record onto
film.
1942 - First true SEM developed by Vladimir Kosmo Zworykin, J. Hiller, and R.L. Snyder at RCA Labs. The
gun was at the bottom of the column! Resolution of about 50nm.
Late 1940s - Charles Oatley at Cambridge selected Dennis McMullan to build an SEM as his PhD project –
the SEM1. He placed the gun at the top of the column.
1947 - James Hiller patents the idea of using electrons to produce analytical x-rays (EDS), but he never
constructs a working model
1952 - K.C.A. Smith develops method of detecting secondary electrons
1955 - T.E. Everhart develops what becomes the Everhart-Thornley detector
1961 - magnetic lenses are added for the SEM4
1965 - First four production models of the "Stereoscan" were delivered by Associated Electrical
Industries
http://www4.nau.edu/microanalysis/Microprobe-SEM/History.html
MSE 421/521 Structural Characterization
The Family of SEMs
All SEMs have an electron gun, condenser, scanning coils,
and a signal detection device
Add EDS and possibly WDS
it becomes an analytical SEM
Hitachi 4500 SEM
Add several WDS spectrometrs
and it becomes a Microprobe (EPMA)
MSE 421/521 Structural Characterization
TEM
SEM
Electron gun
Condenser system
(2 lenses + 1 aperture)
Scan coils
Specimen stage
Objective lens
Objective aperture
Scan generator
Detector
Principal image plane
Monitor
Projector lenses
Viewing screen
Vacuum system
MSE 421/521 Structural Characterization
SEM
HT ~20 – 30 kV
d ~ 2 – 10 nm
Electron gun
Scan generator
Condenser lens
Objective lens
Aperture
Scan coils
Monitor
Specimen stage
Detector
The objective lens in an SEM is really part of the condenser system,
as it is not used for magnification but to demagnify the beam.
Magnification involves NO lenses.
MSE 421/521 Structural Characterization
SEI Detector
Everhart-Thornley detector
Microscope chamber wall
Faraday cage
Electrons in
Scintillator
Light pipe
Photomultiplier
Electrical signal out
Screen
Quartz window
+200 V
+10 kV
Always contains some back-scattered component
Robinson detector – bulky, restricts WD, retracted for EDS
MSE 421/521 Structural Characterization
100µ
µm
SEM Optics
diameter of filament = d0
Strong C1 (small v1)
means large u2 and
small d.
Apertures used to
reduce spherical
aberration
I = I0(α1/α0)2
Physically fixed distance
(v1 + u2) = constant
demagnified source image
diameter d1 at cross-over
d1 = d0 x v1/u1
Minimised for
large αo (strong C1) or
small α1 (small objective
aperture)
final probe diameter d
v1WD
d = d1 x WD/u2 = d0
u1u2
½A
WD
α
tanα =
A
2WD
Can decrease probe size by either
decreasing v1 and increasing u2 (increase
strength of CL or increase V) or decreasing
WD (increase strength of OL or increase V).
~α
MSE 421/521 Structural Characterization
SEM Performance
Pixels (picture elements) ~ 100 µm, Specimen pixel size, p =
100 µm
Resolution can never be
better than p
M
Best to use a probe size, d, equal to p Probe size should be reduced as magnification increases.
s = beam diameter defocus
h = depth of field
s=hα
Assuming s ≤ p …
s 100 µm
h=α=
Mα
We already know that α ~ tanα ~
So, h =
Can increase h by:
WD
2WD
(A)
100 µm 2WD
M
½A
A
=
α
0.2WD
AM
tanα =
A
2WD
~α
mm
1. Increasing WD - Degrades resolution
2. Decreasing size of objective aperture – May degrade resolution
3. Decreasing M
MSE 421/521 Structural Characterization
Resolution–Probe Size
So, resolution is improved by:
1. Using a small λ (high kV)*
2. Using a large aperture (increasing α) – decreases h, increases Cc & Cs
3. Using a small probe current, i
4. Using a bright electron gun (typically FEG)
We have already seen that I can be reduced by increasing the strength of C1 or decreasing WD;
however, either increases spherical aberration (recall that rs = Csβ3). Minimum probe diameter for
best resolution (d1 = 0) is:
K ~ 1.22
Cs ~ 20 mm
dopt = Kλ¾Cs¼
dopt ~ 2.3 nm (20 kV)
As d decreases so does I. According to the Pease-Nixon equation:
[
3/8
( ) + ]1
d = dopt 7.92x109 IT
j
I = probe current [A]
T = temperature of filament [K]
j = current density at filament surface [A/cm2]
For thermionic emission
MSE 421/521 Structural Characterization
Resolution-Beam Current
To resolve two points, there must be a visible difference in the signal generated from them.
n = average number of electrons detected from a point, varies by ≤
C=
n1 – n2
∆n
=
(n1 > n2)
n1
n1
n
C = natural contrast (0 ≤ C ≤ 1)
According to Rose, the eye can only distinguish between two adjacent points if n1 > n2 + 5 n1
(signal 1 is greater than signal 2 by five times the noise in signal 1)
Minimum observable contrast level: C >
n0 = It
e =
IF x 10-6
n = qn0
Ic >
4 x 10-12
qFC 2
[A]
e
5 n1
n1
=
5
n1
n1 >
25
C2
n0 = number of electrons incident on a given pixel
I = beam current
t = time the beam stays at a given pixel
F = entire frame scan time (assuming 1000x1000 pixels)
n = number of electrons actually detected from a pixel
q = constant related to detector efficiency and electron yield
(0.1 < q < 0.2 for secondary electrons)
Ic = critical current needed to see a contrast level C
Actual contrast increases as n increases by: 1. Increasing beam current
2. Increasing scan time
MSE 421/521 Structural Characterization
Resolution = f(C)
Resolution-Voltage
photoresist on silicon
MSE 421/521 Structural Characterization
Topographical Images
Kanter, 1961
δ=
δ0
cosθ
δ = Secondary electron coefficient
δ0 = value of δ at normal incidence (θ = 0)
Smallest for normal incidence (20° - 40° is common)
Does not account for increased secondary electron emission caused
by backscatter, which also increases with θ.
δ
Not valid for very low kV, in which case all secondary electrons escape
Beam
δ0
0
20
40
60
80
R
θ
Tilt (deg)
MSE 421/521 Structural Characterization
R0
θ
Stereomicroscopy
Two images are taken, one tilted 10-15° with respect to the other.
Both are viewed simultaneously with a stereo viewer or anaglyph glasses.
h=
p
2Msin(θ/2)
h = vertical height difference
p = relative lateral displacement
M = magnification
θ = parallax angle
Also possible (but highly unusual)
to get topographical information
using a small backscattered
detector.
- Only line-of-sight electrons
detected, so emphasises shadows.
- Modern BSI detectors are
annular, so the effect is
cancelled.
MSE 421/521 Structural Characterization
End of Lecture
MSE 421/521 Structural Characterization