linear imaging approximations4 - cornell...
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David Muller 2006 1
Linear Imaging Approximations
Notes to accompany the lectures delivered by David A. Muller at the SummerSchool on Electron Microscopy: Fundamental Limits and New Science heldat Cornell University, July 13-15, 2006.
Reading and References:Chapter 1-3 of “Advanced Computing in Electron Microscopy” by E. J. Kirkland
David Muller 2006 2Single atom
Sensitivity:
Electron Energy Loss Spectrometer
Annular Dark Field (ADF) detector
yx
200 kV IncidentElectron Beam
(∆E=1 eV)
Incr
easi
ngen
ergy
loss
1 atom wide (0.2 nm) beam is scannedacross the sample to form a 2-D image
Elastic Scattering ~ "Z contrast"
Scanning Transmission Electron Microscopy
0 0.5 1 1.
ADF Signal Er M4 Edge
Distance (nm)
3 Å
P. Voyles, D. Muller, J. Grazul, P. Citrin, H. Gossmann, Nature 416 826 (2002)U. Kaiser, D. Muller, J. Grazul, M. Kawasaki, Nature Materials, 1 102 (2002)
David Muller 2006 3
ReciprocityReciprocity (or STEM vs. CTEM)(or STEM vs. CTEM)
CTEM STEM
Reciprocity (for zero-loss images):A hollow-cone image in CTEM an annular-dark field image in STEM.
Specimen
Illuminationangle
α
Collectorangle
β
Specimen
Objective Aperture
Objective Aperture
ViewingScreen Gun
Gun Detectors
However: In STEM, energy losses in the sample do not contribute to chromatic aberrations (Strong advantage for STEM in thick specimens)
David Muller 2006 4
Reciprocity
From L. Reimer, Transmission Electron Microscopy
Reciprocity: Electron intensities and ray paths in the microscope remain the same if (i) the direction of rays is reversed, and (ii) the source and detector are interchanged.
Proof follows from time-reversal symmetry of the electron trajectories and elastic scattering (to all orders).
Reciprocity does not hold for inelastic scattering:Sample is after probe forming optics in STEM - energy losses in sample do not cause chromatic blurring in the image
Sample is before the imaging optics in TEM – energy losses in the sample do cause chromatic blurring in the image. Imaging thick samples in TEM can be improved by energy filtering (so on the zero-loss image is recorded). This is not needed for STEM.
David Muller 2006 5
Reciprocity
From L. Reimer, Transmission Electron Microscopy
Image recordedIn parallel
Image recorded serially by scanning the source
Condensoraperture(before sample)
Objective aperture
(after sample)
Controls coherence
Controls resolution
Collectoraperture
Condensoraperture
STEMTEM
David Muller 2006 6
Geometric Optics Geometric Optics –– A Simple LensA Simple Lens
x x
Objectplane
imageplane
Backfocalplane
frontfocalplane
Lensat z=0
α
Focusing: angular deflection of ray α distance from optic axis
David Muller 2006 7
Geometric Optics Geometric Optics –– A Simple LensA Simple Lens
x x
Objectplane
imageplane
Backfocalplane
frontfocalplane
Lensat z=0
α1
α1
Wavefronts in focal plane are the Fourier Transform of the Image/Object
α1
David Muller 2006 8
α
λ1
0 =kλαrr
≈k
{ } xdexfxfkF xki rrrr rr 2.2)()()( π∫∞
∞−==F
All plane waves at angle αpass through the same pointIn the focal plane.
If the component of the wavevectorin the focal plane is ,thena function in the back focal plane, F(k)is the Fourier transform of a function in the image plane f(x)
kr
Fourier Transforms
{ } sdekFkFxf xki rrrr rr.21 )()()( π−∞
∞−
− ∫==F
Forward Transform: (image- > diffraction)
Inverse Transform: (diffraction ->image)
Note: in optics, we define k=1/d, while in physics k=2π/d
David Muller 2006 9
FT of A Circular Aperture
⎪⎩
⎪⎨⎧
>
<=
max
max
,0
,1)(
kk
kkkA r
rr
FT
rkrkJrA
max
max1
2)2(2)(
πππ=
An ideal lens would have an aperture A(k)=1 for all k. However, there is a maximum angle that can be accepted by the lens, αmax, and so there is a cut-off spatial frequency kmax= k0 αmax,
Focal PlaneImage Plane
David Muller 2006 10
The Aperture Function in the Image Plane
0
1
2
3
-3 -2 -1 0 1 2 3
A(x)
x
rkrkJrA
max
max1
2)2(2)(
πππ=
First zero at 2πkmaxr = 0.61
0
4
8
-3 -2 -1 0 1 2 3|A
(x)|2
x
2)(rA
The image of single point becomes blurred to (why?) 2)(rA
David Muller 2006 11
Linear Imagingf(x) g(x)h(x)
imageobject Blurring function
( ) )()(')'()'( xhxfdxxxhxfxg ⊗≡−= ∫∞
∞−
Image = object convolved (symbol ) with the blurring function, h(x)⊗
(Kirkland chapter 3)
( ) )()( kHkFkG =In Fourier Space, convolution becomes multiplication (and visa-versa)
Point spread function
Contrast Transfer Function
David Muller 2006 12
Contrast Transfer FunctionThe contrast transfer function (CTF) is the Fourier Transform of the point spread function (PSF)
The CTF describes the response of the system to an input plane wave.By convention the CTF is normalized to the response at zero frequency (i.e. DC level)
( ))0()(~
HkHkH =
A low-pass filter
k
( )kH~
A high-pass filter
k
( )kH~
(division by k in Fourier Space->integration in real space)
(multiplication by k in Fourier Space->differentiation in real space)
smoothing Edge-enhancing
David Muller 2006 13
High-pass filter
Edge-enhancing
Original image
A high-pass filter
Final image
Fourier Transform
x
Inverse Transform
=
David Muller 2006 14smoothing
Original image
A low-pass filter
Final image
Fourier Transform
x
Inverse Transform
=
Low pass filter
David Muller 2006 15
The Aperture Function in the Image Plane
0
1
2
3
-3 -2 -1 0 1 2 3
A(x)
x
rkrkJrA
max
max1
2)2(2)(
πππ=
First zero at 2πkmaxr = 0.61, with kmax = (2π/λ)θmax
i.e. r = 0.61/(2πkmax)~ 0.61 λ/θmax
0
4
8
-3 -2 -1 0 1 2 3|A
(x)|2
x
2)(rA
The image of single point becomes blurred to (why?) 2)(rA
David Muller 2006 16
ε
From Wyant and Creath
Adding a central beam-stopto the aperture increases the tails on the probe but only slighty narrows the central peak.
Is this the best we can do?
David Muller 2006 17
Coherent vs. Incoherent Imaging(Kirkland, Chapter 3.3)
Lateral Coherence of the Electron Beamfor an angular spread βmax (Born&Wolf):
max
16.0β
λ≈∆ cohx
Image resolutionmaxαλ
≈d
Combining these 2 formula we get:
Coherent imaging:
Incoherent imaging:
maxmax 16.0 αβ <<
maxmax 16.0 αβ >> (usually ) maxmax 3αβ >
Wave Interference inside allows us to measure phase changes
as wavefunctions add:
α
β
cohx∆
No interference, phase shifts are not detected. Intensities add
**222abbababa ψψψψψψψψ +++=+
22ba ψψ +
obj
David Muller 2006 18
Coherent Imaging(Kirkland 3.1)
Convolve wavefunctions, measure intensities
( ) )()( xAxx objectimage ⊗=ψψ
( ) ( ) 2xxg imageψ=
( )xobjectψ
( )kimageψ
Lens has a PSF A(x)
We measure the intensity, not the wavefunction
( ) )()( kAkk objectimage ⋅=ψψ
( ) 2)()( xAxxg object ⊗= ψ
α
β~0
David Muller 2006 19
Incoherent Imaging (Kirkland 3.4)
Convolve intensities, measure intensities
( ) 222)()( xAxx objectimage ⊗= ψψ
( ) ( ) 2xxg imageψ=
( )xobjectψ
( )kimageψ
We measure the intensity, not the wavefunction
( ) [ ] [ ])()()()( ** kAkAkkk objectobjectimage ⊗⋅⊗= ψψψ
( ) 22)()( xAxxg object ⊗= ψ
α
β Lens has a PSF2)(xA
Lost phase information, only work with intensities
CTF
David Muller 2006 20
Coherent vs Incoherent Imaging
Phase Object
Amplitude Object
Contrast Transfer Function
We measure
Point Spread Function
IncoherentCoherent
( ) 2)()( xAxxg object ⊗= ψ ( ) 22
)()( xAxxg object ⊗= ψ
)(xA
[ ])(Im kA
2)(xA
2* )()( kAkA ⊗[ ])(Re kA
David Muller 2006 21
Convolutions(from Linear Imaging Notes, Braun)
David Muller 2006 22
Resolution Limits Imposed by the Diffraction Limit(Less diffraction with a large aperture – must be balanced against Cs)
Lens
000
61.0sin61.0
αλ
αλ
≈=n
d
Gaussian image plane
The image of a point transferred through a lens with a circular aperture of
semiangle α0 is an Airy disk of diameter
0d
(for electrons, n~1, and the angles are small)
α0
(0.61 for incoherent imaging e.g. ADF-STEM, 1.22 for coherent or phase contrast,. E.g TEM)
David Muller 2006 23
Electron Wavelength vs. Accelerating Voltage
0.00871890.813521 MeV
0.0196870.77653300 keV
0.0250780.69531200 keV
0.0370130.54822100 keV
0.122040.01919410 keV
0.387630.0624691 keV
1.22630.0062560100 V
12.2640.00197841 V
λ (Ǻ)v/cAccelerating Voltage
0
0.01
0.02
0.03
0.04
0.05
0 200 400 600 800 1000
RelativisticNon-relativistic
λ (A
ngst
rom
s)
Electron Kinetic Energy (keV)
David Muller 2006 24
Resolution Limits Imposed by Spherical Aberration, C3(Or why we can’t do subatomic imaging with a 100 keV electron)
Lens
33min 2
1 αCd =
Plane ofLeast Confusion
Gaussian image plane
C3=0
C3>0
For Cs>0, rays far from the axis are bent too strongly and come to a crossover before the gaussian image plane.
For a lens with aperture angle α, the minimum blur is
mind
Typical TEM numbers: C3= 1 mm, α=10 mrad → dmin= 0.5 nm
David Muller 2006 25
Balancing Spherical Aberration against the Diffraction Limit(Less diffraction with a large aperture – must be balanced against Cs)
2303
2
0
220
2
2161.0
⎟⎠⎞
⎜⎝⎛+⎟⎟
⎠
⎞⎜⎜⎝
⎛=+≈ α
αλ Cddd stot
For a rough estimate of the optimum aperture size, convolve blurring terms-If the point spreads were gaussian, we could add in quadrature:
1
10
100
1 10
Prob
e Si
ze (A
ngst
rom
s)
α (mrad)
ds
d0
Optimal apertureAnd minimum
Spot size
4/34/13min 66.0 λCd =
David Muller 2006 26
Balancing Spherical Aberration against the Diffraction Limit(Less diffraction with a large aperture – must be balanced against C3)
4/34/13min 43.0 λCd =
A more accurate wave-optical treatment, allowing less than λ/4 of phase shift across the lens gives
Minimum Spot size:
4/1
3
4⎟⎟⎠
⎞⎜⎜⎝
⎛=
CoptλαOptimal aperture:
At 200 kV, λ=0.0257 Ǻ, dmin = 1.53Ǻ and αopt = 10 mrad
At 1 kV, λ=0.38 Ǻ, dmin = 12 Ǻ and αopt = 20 mrad
We will now derive the wave-optical case
David Muller 2006 27
Spherical Aberration (C3) as a Phase Shift
Lens
Gaussian image plane
C3 =0
C3 >0
Phase shift from lens aberrations:
∆s = path difference due to wave aberration
Wavefronts (lines of equal phase)
( ) ( )αλπαχ s∆=
2
α
For spherical aberration but there are other terms as well ( ) 434
1 αα Cs =∆
(remember wave exp[ i(2π/λ) x] has a 2π phase change every λ)
David Muller 2006 28J.C. Wyant, K. Creath, APPLIED OPTICS AND OPTICAL ENGINEERING, VOL. Xl
An Arbitrary Distortion to the Wavefront can be expanded in a power series(Either Zernike Polynomials or Seidel aberration coefficients)
=)',( θρχ
Zernike Polynomials
David Muller 2006 29
An Arbitrary Distortion to the Wavefront can be expanded in a power seriesSeidel Aberration Coefficients (Seidel 1856)
Or more generally
EM community notation is similar:
C5,0 and C7,0 are also important
David Muller 2006 30
An Arbitrary Distortion to the Wavefront can be expanded in a power seriesHere are some terms plotted out
David Muller 2006 31
Phase Shift in a Lens
( ) ( )αχαϕ ie=
Electron wavefunction in focal plane of the lens
⎟⎠⎞
⎜⎝⎛ ∆−= 24
3 21
412)( αα
λπαχ fC
Where the phase shift from the lens is
Keeping only spherical aberration and defocus
(Kirkland, Chapter 2.4)
( )
⎪⎩
⎪⎨⎧
>
<=
max
max
,0
,)(
kk
kkekA
ki
r
rr
χ
David Muller 2006 32
-2π
-π
0
π
2π
3π
4π
0 2 4 6 8 10 12
Phas
e Sh
ift (r
ad)
α (mrad)
α0
π/2
χ(α)
1/4C3α4
1/2∆fα2
Goal is to get the smallest phase shift over the largest range of anglesOptimizing defocus and aperture size for ADF
Step 1: Pick largest tolerable phase shift: in EM λ/4=π/2, in light optics λ/10Step 2: Use defocus to oppose the spherical aberration shift within the widest π/2 bandStep 3: Place aperture at upper end of the π/2 band
David Muller 2006 33
Goal is to get the smallest phase shift over the largest range of angles
Step 1: We assume a phase shift <λ/4=π/2 is small enough to be ignored
Step 2: Use defocus to oppose the spherical aberration shift within the widest π/2 band
4/34/13min 43.0 λCd =
Minimum Spot size:
4/1
3
4⎟⎟⎠
⎞⎜⎜⎝
⎛=
CoptλαOptimal aperture:
opt
dα
λ61.0min ≈
Step 3: Place aperture at upper end of the π/2 band & treat as diffraction limited
Optimal defocus: ( ) 21
λSopt Cf =∆
(The full derivation of this is given in appendix A of Weyland&Muller
Optimizing defocus and aperture size for ADF
David Muller 2006 34
(derivation is different, given in Kirkland 3.1)
4/34/13min 77.0 λCd =
Minimum Spot size:
4/1
3
6⎟⎟⎠
⎞⎜⎜⎝
⎛=
CoptλαOptimal aperture:
opt
dα
λ22.1min ≈
Optimal defocus: ( ) 21
5.0 λSopt Cf =∆
(The full derivation of this is given in appendix A of Weyland&Muller
Optimizing defocus and aperture size for TEM
Look for a uniform phase shift of ±π/2 across lens
David Muller 2006 35
Phase Shift in a Lens with an Aberration Corrector
( ) ( )αχαϕ ie=
Electron wavefunction in focal plane of the lens
⎟⎠⎞
⎜⎝⎛ +++∆−= K4
54
32
61
41
212)( ααα
λπαχ CCf
Where the phase shift from the lens is
O.L. Krivanek et al. Ultramicroscopy 96 (2003) 229
5th order spherical aberration
David Muller 2006 36
-2
-1
0
1
2
3
4
0 5 10 15 20 25 30
Phas
e Sh
ift (r
ad)
α (mrad)
α0
π/2
χ(α)
-1/4C3α4
1/2∆fα2
1/6C5α6
Goal is to get the smallest phase shift over the largest range of angles
Optimizing Aperture size with a C3 Corrector
Step 1: Pick largest tolerable phase shift: in EM λ/4=π/2, in light optics λ/10Step 2: Use defocus and C3 (now negative) to balance C5 within the widest π/2 bandStep 3: Place aperture at upper end of the π/2 band
David Muller 2006 37
Goal is to get the smallest phase shift over the largest range of angles
Optimizing defocus and aperture size
Step 1: We assume a phase shift <λ/4=π/2 is small enough to be ignored
Step 2: Use C3 to oppose the C5 shift within the widest π/2 band
6/56/15min 42.0 λCd =
Minimum Spot size:
6/1
5
6/1
5
47.1323
⎟⎟⎠
⎞⎜⎜⎝
⎛=⎟⎟
⎠
⎞⎜⎜⎝
⎛=
CCoptλλαOptimal aperture:
opt
dα
λ61.0min ≈
Step 3: Place aperture at upper end of the π/2 band & treat as diffraction limited
Optimal C3: ( ) 312
533 CC opt λ−=
David Muller 2006 38
Contrast Transfer Functions of a lens with Aberrations
Coherent Imaging CTF: [ ] ( )[ ]kSinkA χ=)(Im
( )
⎪⎩
⎪⎨⎧
>
<=
max
max
,0
,)(
kk
kkekA
ki
r
rr
χ
Aperture functionof a real lens
Generated with ctemtf
0 0.1 0.2 0.3 0.4 0.5 0.6-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Spatial Frequency (in 1/A)
MTF
E= 200keV, Cs= 1.2mm, df= 600A, Beta= 0mrad, ddf= 0A
coherent
partiallycoherent
David Muller 2006 39
Contrast Transfer Functions of a lens with Aberrations
Incoherent Imaging CTF:
( )
⎪⎩
⎪⎨⎧
>
<=
max
max
,0
,)(
kk
kkekA
ki
r
rr
χ
Aperture functionof a real lens
Generated with stemtf
2* )()( kAkA ⊗
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-0.2
0
0.2
0.4
0.6
0.8
1
1.2
k in inv. Angstroms
MTF
Cs= 1.2mm, df= 400A, E= 200keV, OA= 10mrad, d0=0A
550A
900A
0A
CTF for different defocii
Theorem:Aberrations will neverIncrease the MTFFor incoherent imaging
David Muller 2006
Phase vs. ADF Contrast
TEM: Bandpass filter:low frequencies removed = artificial sharpeningADF : Lowpass filter: 3 x less contrast at 0.3 nm than HRTEM
-1
-0.5
0
0.5
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
STEMHRTEM
Con
tras
t Tra
nsfe
r Fun
ctio
n
k (Inverse Angstroms)
(JEOL 2010F, Cs=1mm)
David Muller 2006
Phase vs. ADF ContrastRandom white noise
ADF CTFBF CTF
a-C support films look like this
David Muller 2006
Phase vs. ADF Contrast
David Muller, 2000
ADF : 40% narrower FWHM, smaller probe tails
(JEOL 2010F, Cs=1mm, Scherzer aperture and focus)
0
0.1
0.2
0.3
0.4STEM-CTEM
PSF
David Muller 2006 43
CTF
PSF
(200 kV, C3=1.2 mm)
Effect of defocus and aperture size on an ADF-STEM image
David Muller 2006
ADF of [110] Si at 13 mr, C3=1mm
Strong {111} fringes Strong {311} fringes
2 clicks overfocus
Best 111 and 311 fringes occur at different focus settingsIf the aperture is too large
What happens with a too-large aperture?
David Muller 2006 45
Aperture Size is Critical(200 kV C3=1mm)
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8
13 mrad10 mrad
CTF
k (1/A){111}
Si
{220}Si
30% increase in aperture size ~50% decrease in contrast for Si {111} fringes
David Muller 2006 46
Aperture Size is Critical
0
5
10
15
20
25
30
0 0.2 0.4 0.6 0.8 1
10 mrad13 mrad
Bea
m C
urre
nt E
nclo
sed
(pA
)
Radius (nm)
r80%
= 0.25 nm
r80%
= 0.8 nm
r64%
= 0.1 nm
All the extra probe current falls into the tails of the probe – reduces SNR
(200 kV C3=1mm)
David Muller 2006 47
Finding the Aperture with the smallest probe tails
(Kirkland, Fig 3.11)
4/1
3
22.1 ⎟⎟⎠
⎞⎜⎜⎝
⎛=
Coptλα
( ) 21
min 8.0 λSCf =∆
David Muller 2006 48
Depth of Field, Depth of Focus
200
061.0
tan αλ
α≈=
dD
For d=0.2 nm, α=10 mrad, D0= 20 nm For d= 2 nm, α=1 mrad, D0= 2000 nm!
For d= 0.05 nm, α= 50 mrad, D0= 1 nm!
David Muller 2006 49
Depth of Field in ADF-STEM: 3D Microscopy?
D=6 nm: θ0 = 25 mr
(300 kV, 25 mrad)
0
1
2
3
4
5
6
-300 -200 -100 0 100 200 300
25 mrad10 mrad
Peak
Inte
nsity
Depth (A)
David Muller 2006 50
Depth of Field in ADF-STEM: 3D Microscopy?
1
10
100
0 20 40 60 80 100
100 kV200 kV300 kV
Dep
th R
esol
utio
n (A
)
Probe Forming Semi-Angle (mrad)
D=1nm: θ0 > 50mr
Today: D~ 6-8 nm SuperSTEM: ~1 nm
Cs>0
Cs~0
C5~0
David Muller 2006 51
Does Channeling Destroy the 3D Resolution?(Multislice Simulation of [110] Si @ 200 kV, 50 mrad)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 5 10 15 20 25 30
Sign
al (F
ract
ion
of I 0)
Thickness (nm)
∆f=20 nm∆f=1 nm
NO! (at least in plane – relative intensities between different depths are still out)
Fringe contrast
David Muller 2006 52
Summary
Contrast Transfer Functions: Coherent:
Lower resolution, higher contrastEasy to get contrast reversals with defocusAperture size only affects cutoff in CTF
Incoherent:
Higher resolution, lower contrastHarder to get contrast reversals with defocusAperture size is critical – affects CTF at all frequencies
4/34/13min 43.0 λCd =
4/1
3
4⎟⎟⎠
⎞⎜⎜⎝
⎛=
Coptλα
4/34/13min 77.0 λCd =
4/1
3
6⎟⎟⎠
⎞⎜⎜⎝
⎛=
Coptλα