Lecture 11 Wed Microscopy › ... › 12141 › 11_Wed_Microscopy.pdf · Electron yied Secondary electrons Auger peak Energy Filter Page 32. 17 Electron energy E Electron energy E+

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WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN

X-ray Microscopy

Frithjof Nolting :: Head of LSC :: Paul Scherrer Institut

PSI Master School 2017

Basic research – electronic devices

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Hard discCars, sensors, displays

Modern communication devices are full of fascinating physics and advanced materials

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Aim of the lecture

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Basic concepts of X‐ray Microscopy, mainly soft X‐rays

• General considerations

• Scanning Transmission X‐ray Microscope (STXM)

• Photoemission Electron Microscope (PEEM)

• Contrast mechanism using XAS

• Be careful

• Hard X‐ray techniques

Basic research – electronic devices

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http://article.wn.com

Hard discCars, sensors, displays

Modern communication devices are full of fascinating physics and advanced materials

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The 1887 floral painting by van Gogh, “Patch of Grass”.

Dik et al., Anal. Chem. 80 6436 (2008).

“Seeing the invisible”

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Aim of the lecture

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• Be careful


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How?

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X-ray microscope

source optics detector

X-ray tube

Synchrotron

Bending magnet

Insertion device

Mirrors

Refractive elements

Diffractive elements

Electron optic

Photodiode

Phosphorscreen

…

sample

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5

Source, Optics, and Detectors

Source, optics and detectors see lecture from Laura Heyderman on Monday

Insertion Device

Beam defining aperture

Soft x-ray filter/window

Beam-defining slitsFocussing & monochromator optics

Clean-up slitsBeam intensity monitor

Secondary opticsSample

Detector

P. WillmottIntro to Synchr. Rad.

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Mircoscopy modes

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6

• Cross-sections for various processes involving interaction of x-rays with matter - primary scatterer is the electron

• Plot for Ba; orange area highlights upper energy range covered by synchrotron sources

Key scattering processes:

• Photoelectric Absorption

• Thomson Scattering (elastic)

• Compton Scattering (inelastic)

Total !

1 barn = 10-24 cm2

= 10-28 m2P. WillmottIntro to Synchr. Rad.

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Which contrast can we use?

vacuum

sample

sample surface

secondary electrons

Escape depth ~ 5 nmh

Fluorescence

Escape depth ~ 50 - 100 nm

Auger electrons

Escape depth ~ 0.5 nm

Soft X-ray penetration depth

~ 50 - 100 nm

X-ray microscope types

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Aim of the lecture

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• Be careful


vacuum

sample

sample surface

h

X-ray penetration depth

~ 50 - 100 nm

Photon in / Photon out

You need an optic for the (incoming) photons

Photon in / Photon out

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Scanning Transmission X-ray microscope

Focus X-rays

Scan sample

Detect X-rays

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δres ~ Δr = p/2

Fresnel

Zone Plate

Courtesy J. Vila‐Comamala (PSI)

Spatial resolution for STXM

Scanning Transmission X-ray Microscope

x

y

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STXM at the PolLux beamline at the SLS

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Primary electrons

Resist

Substrate

Isolated line exposure

secondary electrons

range ~ 10-15nm15 nm

Isolated 10 — 20 nm lines

Dense 10 — 20 nm lines

period, p = 2·w

period, p = 4·w

Limits of e-beam lithography FZP Fabrication

Resist

Substrate

Dense line exposure

Primary electrons

secondary electrons

range ~ 10-15nm


What limits the spatial resolution in STXM

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10

Low refractive index materialHigh refractive index material

Manufacturing

advantage:

One single EBL exposure:

No alignment required

template period, p = 4·w

Zone-doubled FZP

Ordinary FZP

δres ~ Δr = p/2

period, p = 2·Δr

δres ~ w = p/4

K. Jefimovs, J. Vila-Comamala, T. Pilvi, J. Raabe, M. Ritala, C. David. Physical Review Letters 99 (2007) 264801


Zone-doubling for high resolution

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STXM images of GaAs / AlGaAs heterostructures

SLS PolLux Beamline @ 22th September 2008


Resolution test

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3D chemical imaging

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3D chemical imaging

The technique is illustrated using mapping of a lowdensity acrylate polyelectrolyte in and outside of polystyrene microspheres dispersed in water in a 4 µm-diameter microcapillary. The 3-d chemical visualization provides information about the microstructure that had not previously been observed.

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Aim of the lecture

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• Be careful


vacuum

sample

sample surface

secondary electrons

Escape depth ~ 5 nmh

Auger electrons

Escape depth ~ 0.5 nm

You need an optic for the electrons

Photon in / Electron out

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13

Photonpenetrationdepth

50 nm~

Electronescapedepth 5 nm~

vacuum

sample

sample surfacesecondary electrons

h

e-hn

kinetic energyE

lect

ron

yied Secondary electrons

Auger peak

First layer

Second layer

Probing surface/interface

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16°

20kV

MCP

Phosphor

Energy analyzer

Lenses

(magnetic)

Photoemission Electron Microscope - PEEM

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Probe : slow electrons

Imaging : high energy electrons

(more stable and maintain spatial information)

Immersion lens: electrons have before and after the lens different velocity (different wavelength)

Cathode lens: Sample is cathode

electron microscope is anode

Sample High Voltage

0 eV 20 keV 20 keV

Slow electrons

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sample electrode

Accelerated electrons form parabolic trajectory

Tangents to parabolas are the incident rays

Extrapolated backwards form a virtual image at unit lateral magnification

virtual sample

l

2l

M = 1

Why is the accelerating field a lens?

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No, it is a very important lens in a PEEM, dominating the spatial resolution due to its spherical and chromatic aberrations.

Classical: electron in homogenous electric field

calculate electron trajectory

Ve

Vy

Vx

F=QE

Trajectory depends on emission angle and velocity

Just another lens?

e1 e2

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Spatial resolution PEEM – make an estimate

The spatial resolution (r) in PEEM can be approximated by

r ≈ (d ΔE) / (eU)

d: distance sample, objective lensΔE: energy spread of electronsU: acceleration voltage

The spatial resolution is given by the chromatic and spherical aberrations.

Chromatic aberrations are due to the energy distribution of the electron, with increasing energy spread is the spatial resolution decreased.

Spherical aberrations are due to the angular distribution of the electrons, which is reduced by the acceleration field (U/d).

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16

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

aperture diameter (micron)

spat

ial r

eso

luti

on

(n

m)

total

accel. field

chrom. lenses

spher. lenses

diffraction

PEEM 2 at the ALS, Simone Anders

Work function 4 eV, sample voltage 30 kV, X-rays

Calculated Spatial Resolution

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Electron energy EElectron energy E+E

kinetic energy

Ele

ctro

n yi

ed Secondary electrons

Auger peak

Energy Filter

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Electron energy EElectron energy E+E

Aperture cuts off transmission of electrons

with higher energy

850

900

950

1000

1050

1100

1150

1200

1250

0 5 10 150

50

100

150

200

250

300

350

400

energy (eV)

1025

5075

100200

500

Energy distribution is narrowed but transmission is reduced

Energy Filter

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Spatial resolution depends on aperture size ‐ limits pencil angle of transmitted electrons and

transmission

Highest resolution is achieved with 12 mm aperture for PEEM2, ALS

5 m

2mm 12 m20 m50 m

0.4 s

100 %

10 s

4 %

4.2 s

9 %

1s

39 %

Aperture

diameter

Exposure time

Transmission

Effect of Aperture Size on Resolution

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Topographical Contrast

Elemental Contrast

TiLa

Co

Fe

Photoemission Electron Microscope

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Aim of the lecture

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• Be careful


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Sample: Fe nanoparticles with diameter = 9 nm

690 700 710 720 730 7400.990

0.992

0.994

0.996

0.998

1.000

1.002

1.004

1.006

1.008

1.010

1.012

1.014

Inte

nsi

ty (

a.u

.)Photon Energy (eV)

Circ Plus Circ MinusL3

L2

XMCD image

Images with increasing Photon Energy

Spectra of individual Fe nanoparticles

A. Fraile Rodríguez et al. PRL 104, 127201 (2010)Page 37

L

775 780 785 790 795 800 805

L2

L3

Photon energy (eV)

TE

Y (

a.u.

)

Circular right Circular left

XMCD ~ <M> cos(M,L)

4 µm

e.g. J. Stohr et al Surface Rev. Lett. 5, 1297 (1998).

X-ray Magnetic Circular Dichroism (XMCD)

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0 degree 90 degree 180 degree

16°

In-plane system

Out-of-plane system

2D magnetization map

16°

Co structures (600 nm)

20nm GdFeCo film

X-ray Magnetic Circular Dichroism (XMCD)

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L3a L3b

E

XMLD ~ <M2>

2 µm

700 705 710 715 720 725 730

XM

LD

Photon Energy [eV]

domain a domain b (perpendicular to a)

Inte

nsity

[a.u

.]

L3a

L3b

e.g. A. Scholl et al Science 287, 1014 (2000)

X-ray Magnetic Linear Dichroism (XMLD)

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spectroscopy perpendicular coupling

x rays

Co

NiO

5 m

5 m 0.0

0.2

0.4

0.6

0.8

1.0

850 855 860 865 870 875

-0.05

0.00

0.05

Co/NiO(001)

XA

(ar

b. u

nit

s)

0707

25_N

iO_X

MLD

_Co_

XM

CD

.opj

XM

LD

(ar

b. u

nit

s)

photon energy (eV)

?

Co / NiO(001)

CoNiO

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Different contrast: strain and magnetic domains

Sensitivity axis for magnetization

=0° =90°

=0° =90°

Fe

XL

DF

e X

MC

D

- XMCD image is sensitive to one axis of magnetization

- XLD image is sensitive to an axis of anisotropy

Strong modification of anisotropy directly probed with XLD!

5 µm

CoFe2O4

BaTiO3

R.V. Chopdekar et al. Phys. Rev. B 86, 014408 (2012). Page 42

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Non resonant absorption

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Aim of the lecture

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• Be careful


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No drift

2 pixel drift

Don‘t forget to get pictures which one can publish

Be critical: Image drift!

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2 3 4 5 6 7 8 9

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Inte

nsi

ty (

a.u.

)

Position (a.u.)

spot 1 spot 2

2 4 6 8 10 12 14 16 18 200.0

0.2

0.4

0.6

0.8

1.0

15% Contrast

Co

ntr

ast

Spatial Frequency[LP/m]

20 kV sample Voltage 12 m aperture 20 m aperture 50 m aperture large aperture

450

400

350

300

250

200

150

100

50

25

40

Line Width [nm]

Be critical: spatial resolution

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Aim of the lecture

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• Be careful


1895 Discovery of X-rays by Wilhelm Röntgen

1901 Nobel prize in physics

Image of hand of Albert von Kölliker

this is the second image, the first one, very similar is said to be the hand of his wife

Wilhelm Röntgen – hard X-rays

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Grating interferometry

Courtesy Federica Marone

Absorption image

Conventional radiography

(since 1895…)

“Scatter image”“Differential Phase image”

Index of refraction: n = 1 - + i

Phase Absorption Page 49

From static to dynamical, quantitative imaging

Walker et al., submitted P. Donoghue et al., Nature 442, Aug. 2006

3D to 4D microscopy

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Scanning X-ray diffraction microscopy - Tomography

OMNY project

explore limits of X‐ray tomography

goal: sub‐10 nm 3D resolution

require sub‐10nm positioning accuracy!

Beam optics

Translation stages for scanning in 2D

Rotation stage

Detector

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Nano Tomography

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3‐D X‐ray imaging makes the finest details of a computer chip visible

Mirko Holler, Manuel Guizar‐Sicairos, Esther H. R. Tsai,

Roberto Dinapoli, Elisabeth Müller, Oliver Bunk, Jörg Raabe,

Gabriel Aeppli, Nature 16 March 2017, DOI:

10.1038/nature21698

First‐time 3D imaging of internal magnetic patterns

Claire Donnelly, Manuel Guizar‐Sicairos, Valerio Scagnoli, Sebastian Gliga, Mirko Holler, Jörg Raabe, Laura J. Heyderman, Nature 20 July 2017, DOI: 10.1038/nature23006

Micrometre‐sized pillar ( about 1 micrometer) made of gadolinium‐cobalt

Commercially available computer chip (about 10 micrometres)

CSAXS beamline at the SLS

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Conclusion: Scanning Transmission X-ray microscope

Focus X‐raysdetermine the spatial resolution (determined by the outer zone width)

Scan sampleprecision of scanning also important for the resolution

Detect X‐rayscan be large detector

x

y

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Conclusion: Photoemission Electron Microscope – PEEM

Components:

X‐ray (moderated focused)

Accelerating field

Electron optic

Maybe energy filter

Detector

Focus of X‐ray not determiningthe spatial resolution, full field technique

16°

20kV

MCP

Phosphor

Energy analyzer

Lenses

(magnetic)

The spatial resolution (approximation)

r ≈ (d ΔE) / (eU)d: distance sample, objective lensΔE: energy spread of electronsU: acceleration voltage

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Conclusion: Scanning X-ray diffraction microscopy -Tomography

Beam optics

Translation stages for scanning in 2D

Rotation stage

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Lecture 11 Wed Microscopy › ... › 12141 › 11_Wed_Microscopy.pdf · Electron yied Secondary electrons Auger peak Energy Filter Page 32. 17 Electron energy E Electron energy E+