Scanning Electron Microscopy - Universität Stuttgart · Scanning Electron Microscopy Basics and Applications ... Principle of a “Einzel”-lens: The combination of 3 tubes at different

Scanning Electron MicroscopyBasics and Applications

Dr. Julia DeuschleStuttgart Center for Electron Microscopy

MPI for Solid State Research

Room: 1E15, phone: 0711/ 689-1193

email: [email protected]

Outline

J. Deuschle Scanning Electron Microscopy SS 2017

1.Setup and Instrumentation

2.Electron-Matter-Interactions

3. X-Ray Analysis

4. Literature:Goldstein et al.: Scanning Electron Microscopy And X-Ray Analysis,Springer Verlag 2003Reimer und Pfefferkorn: Rasterelektronenmirkoskopie,Springer Verlag 1973

1.1. The electron column

Additional components:

• vacuum system (pre-pump and turbo molecular pump for

chamber, ion getter pumps for column)

• electronic controls (high voltage, lens current etc.)

• software controls (software interface for the operator to

control the microscope)

• stage (for sample mounting and positioning)

1. Setup and Instrumentation


1.2. The electron gun

• generation of „free“ electrons

• acceleration of the electrons

• collimating or focusing of the beam



1.2. Electron guns


1.2.1 Tungsten Hairpin Cathode

• Thermoionic electron source, i.e. the emission is due to heating of the cathode• The emitted electrons are focussed into a bundle by the grid cap (= beam formation) and „crossover“. • The electrons are then accelerated towards the anode and enter the column.

1.2. Electron guns


Emission of Tungsten Hairpin Cathode

For a stable emission and hence a stable electron beam with constant beam current, the current through the filament must be sufficiently high, i.e. operated in saturation.

1.2. Electron guns


1.2.2 Lanthanum-Hexaboride-Cathode

• Thermoionic electron source

• lower work function compared to W, thus higher emission at the same temperature

• longer lifetimes but more expensive

•Degradation due to oxid formation and evaporation

LaB6 crystal mounted on C or Re, 500 x100 µm

Failed tip

1.2. Electron guns


1. Cold field emission gunEmission is due to strong electrostatic fields, which concentrate at the tip. The W-tip apex is typically several nm wide. UHV conditions are needed to allow tunneling of electrons.

2. Schottky field emission gunA combination of heating and electrostatic fields are used to set electrons free from a small region of 10-30 nm. A ZrO2-coating on the W-tip further lowers the work function. An additional suppresor grid is used to bundle the electrons to a beam.

1.2.3 Field-Emission Guns

1 2

1.2. Electron guns: comparision


Emitter Type Thermionic Thermionic Schottky TFE Cold FE

Tip image comparison

Cathode Material W LaB6 ZrO/W (100) W (310)

Operating Temp (K) 2700 1800 1800 300

Cathode Radius (nm) 60,000 10,000 <1000 <100

Effective Source Radius (µm) 25 10 0.015 0.0025

Emission Current Density (A/cm2) 3 30 5300 17,000

Total Emission Current (µA) 200 80 200 5

Brightness (A/cm2.sr.kV) 1x104 1x105 1x107 2x107

Maximum Probe Current (nA) 1000 1000 10 0.2

Energy Spread @ Cathode (ev) 0.59 0.4 0.31 0.26

Energy Spread @ Source Exit (eV) 1.5 – 2.5 1.3 – 2.5 0.35 – 0.7 0.3 – 0.7

Beam Noise (%) 1 1 1 5 – 10

Emission Current Drift (%/h) 0.1 0.2 <0.5 5

Operating Vacuum (hPa/mbar) <10-5 <10-6 <10-8 <10-10

Typical Cathode Life (h) 100 >1000 >5000 >2000

Cathode Regeneration (h) None None None 6-12

Sensitivity to External Influences Minimal Low Low High

Stability Standard High Very high Low

X-ray analysis EDS / WDS EDS / WDS EDS / WDS EDS Source: micro-to-nano



1.3. Electron lenses

1. Condensor lens(es)to demagnify the beam and adjust the spot size to the experimental needs

2. Objective lensto focus the beam on the sample surface

1.3 Electron lenses


Built up of a ferromagnetic mantle, which contains copper windings

When entering the inhomogeneous magnetic field inside the lens the electrons are bend towards the optical axis and the beam is focused. The electron path is a spiral.

1.3.1 Electromagentic lenses

1.3 Electron lenses


trajectory

1.3.2 Electro-static lenses

Principle of a “Einzel”-lens:

The combination of 3 tubes at different potentials acts as focusing device. The electrons travel towards positive potentials and are repelled by negative potentials.





2. Objective lensTo focus the beam on the sample surfacedesign must also containscanning coils, stigmator and the final aperture.



Scanning coils

for adjusting the position of the beam on the sample surface in x- and y-direction

Image formation

the beam is moved step by step. First along on „line“ in x-direction, then

one step down in y-direction etc.





2. Objective lensTo focus the beam on the sample surfacedesign must also contain scanning coils, stigmator and the final aperture.

1.4 Apertures


Aperture size is important for controlling the beam and imaging parameters as follows:

• determines the probe current

• controls the image depth of focus

• minimizies the effects of aberrations

The beam is characterized by

1. acceleration voltage2. probe current

3. convergence angle4. Spot size

1.4 Apertures


Aperture size

determines the probe currentin limiting the amount of electrons passing through

1.4 Apertures


Aperture size

controls the image depth of focus

Source: ammrf

1.4 Apertures


Aperture size

minimizes the effects of aberrations

Spherical aberration ds and aperture diffraction dd cause the point spot to blur to an enlarged spot of size d.

ds= 0.5Cs3

dd= 0.61/





2. Objective lensTo focus the beam on the sample surfacedesign must also contain scanning coils, stigmator and the final aperture.

1.5 Stigmator


Astigmatismof lenses is due to machining inaccuracies,

material or winding imperfections

When the probe cross-section that hits the surface is not circular, steaking of the image

features appears and needs to be corrected

Spot shape Octupole stigmatorcorrects asymmetric probe shape into a circular probe


1.5 Stigmator

Examples for astigmatic images and corrections



1.6. Electron detectors



Source: wikipedia

1.6.1 Everhart-Thornley detector

usable for SE and BSE detection depending on the bias of the grid

1.6.2 BSE (semiconductor) detector

BSE hitting the detector create electron-hole pairs, which are seperated. The

current is proportinal to the energy of the incident electron.



View inside the SEM chamber

BSE-detector:angular disc inserted betweenpolepiece and samlpe

SE-detector:positioned at the side of the chamber/sampleCollector grid is biased for effiecient collection of electrons

2. Electron-Matter-Interactions


Signal generation upon beam-solid-interaction

Signal used for imaging

Signal used for spectroscopy, i.e. chemical analysis of the material

Source: ammrf


2.1 The Interaction Volume

Vizualizing the interaction volume

PMMA sample irradiated for a certain time with a 20 kV beam, which has been chemically etched for successively longer time increments.

Depending on the energy deposition in the material, the molecular bonds weaken, resulting in faster etching rates than the surrounding material.

C, Z=6


The interaction volume is much lager than the beam diameter.It depends on

acceleration voltageThe higher the acceleration voltage the bigger the interaction volume will be

sample materialThe higher the amount of high atomic number elements inside the sample the smaller the interaction volume will be.

10 kV

20 kV

30 kVFe, Z=26

U, Z=91

Simulation of the interaction volume



Vizualizing the interaction volume as a function of atomic number

The possibility for backscattering of PE increases with increasing atomic number. Thus, the yield of BSE can be used for z-contrast imaging, i.e. material contrast.

N2

Z=7Ar Z=18

Gases „glow“ upon entering of the electron beam



2.2 Signals used for imaging

Primary electrons (PE): Incident (= beam) electrons hitting the sample

Secondary electrons (SE):Electrons that are created from inelastic scattering processes with beam electrons.They possess low energies and can only escape from shallow depths

Backscattered electrons (BSE): Incident electrons, that undergo elastic scattering and escape from the surface again.


2.2 Signals used for imaging

surface

BSE

BSE

to detector

envelope

range

escape depth

chamber

SE 3

SE 1

SE 2

J. Deuschle Übungen Materialcharakterisierungs- und Testmethoden Sommersemester 2010

acquired using SE

3D appearance due to „edge effect“

Useful imaging mode for topography

2.3 Examples

How does the signal type affect an image?

acquired using BSE

inclusions with different composition become more visible

useful imaging mode for z-contrast

J. Deuschle Übungen Materialcharakterisierungs- und Testmethoden Sommersemester 2010

How does the signal type affect an image?

2.3 Examples

acquired using SE

Surface topography is clearly visible

acquired using BSE

different compositions can be seen


2.3 Examples

beam incident angle = 0° Tilted: beam incident angle ≈ 80°

How does geometry affect an image?

SE are emitted only from very shallow depth

increasing the beam angle will raise the SE yield

Surface topography leads to changes in beam incident angle, thus the different SE yields will give „contrast“


2.3 Examples

Topological contrast and edge effect

Cleavage surface


3. X-Ray Analysis

Combining SE- and BSE- signal provides complementary information a more complete picture can be achieved

acquired using BSEacquired using SE

For a “full” characterization of the material, spectroscopy methods, using x-ray signals, can be used to add chemical information.


3. X-Ray Analysis

3.1. Creation of x-rays under electron beam irradiation

Characteristic x-rays are generated upon transitions between subshells. The energy difference is characteristic for each element, so detection of the energy of the x-ray photon gives information about the elements present in the sample.


3. X-Ray Analysis

3.2. Energy Dispersive Spectroscopy (EDS)

Capturing of a photon in the detection system creates a current pulse, which is analyzed. The height of the pulse is proportional to the energy of the incoming photon.

EDX: a complete spectrum is obtained from one measurement.

fast, but poor energy resolutionmostly used for qualitative analysisin scanning mode elemental distribution maps can be acquired


3. X-Ray Analysis

3.3. Wavelength Dispersive Spectroscopy (WDS)

The x-ray photons emitted from the sample undergo a diffraction process, which separates one specific wavelength, for which the Bragg condition is met. The “proper” photons are focused on the detector and captured for analysis.

WDX:Only one element can be measured at a time

Slow and complicated, but with 10 x better resolution than EDXmostly used for quantitative analysis or trace element identification


3. X-Ray Analysis

Quantification of a WDX-spectrum

The intensities (counts per second) measured on the sample of unknown composition can be compared to intensities from a standard with known composition.Thus, the weight fractions of elements in the sample can be calculated.

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Scanning Electron Microscopy - Universität Stuttgart · Scanning Electron Microscopy Basics and Applications ... Principle of a “Einzel”-lens: The combination of 3 tubes at different