Scanning electron microscopy

Tabletop

Fei Quanta

Hitachi

Example: Tin soldier

Pb M

Sn L

Secondary electrons Backscatter electrons EDS analysis – Average composition

Learning goals: • Understanding the principle

– Of the instrument

– Of type of signals

– detectors

• Understand how the parameters that you can/must change influences the picture and chemical analysis.

• Lab: Demonstration and hands on experience with our three different SEMs– Table top SEM

– Environmental SEM

– High resolution SEM

Microscopes then and now

1930

1970

2018

Limits to resolution

Unaided eye ~ 0.1 mm

Light microscope ~ 0.2 mm

Scanning EM ~ 1.0 nm

Transmission EM ~ 0.1 nm

The higher the accelerating voltage, the smaller the wavelength of the electrons and the higher the possible achievable resolution.

In SEM, there are several electromagnetic lenses,including condenser lenses and one objective lens.Electromagnetic lenses are for electron probeformation, not for image formation directly, as inTEM. Two condenser lenses reduce the crossoverdiameter of the electron beam. The objective lensfurther reduces the cross-section of the electronbeam and focuses the electron beam as probe onthe specimen surface.

Instrument: SEM is it like a TEM ?

Objective lens

Objective lens

Cross sectionThe final probe-forming lens has to

operate with a relative long working

distance (WD= 10 mm), that is the

distance between the specimen and

lower pole-piece. This is necessary

so that the emitted radiation can be

collected and detected with desired

efficiency.

The long working distance increases

the spherical aberration of the probe-

forming lens, which increases the

size of the smallest attainable

electron-beam spot.

Final Lens

•Probe Forming

•Labeled as Focus

Objective lens designs

Electron beam

SED

Virtual lens

Specimen

Out lens In lens Semi-in-lens

☆Easy to observe magnetic sample

☆Possible to observe bigger sample

☆Topographical imaging☆Deep depth of Focus

☆Ultra high resolution☆High throughput observation

at sample exchange position☆Variety of signal detecting

system to optimize the contrast

Features Features Features

☆Ultra high resolution☆ Possible to observe bigger

sample at short WD☆Variety of signal detecting

system to optimize the contrast

SED

Electron beam

Virtual lens

Specimen

Specimen

Virtual lens

Electron beam

SED

SEDSED

Compare the three different designs of objective lens :

1:Conventional lens

Here the magnetic field of the lens is located inside the lens and the

specimen sits away from the field.

This leads to that the beam travels “unprotected” between lens and sample

and is more vulnerable to EM disturbance. On the other hand the distance

between lens and specimen only has limited effect on the signal strength.

2: Snorkel lens, semi-in-lens or immersion lens

Here the magnetic field is projected down below the lens to enclose the

specimen if it is located at short working distance. At the same time as the

beam is protected from EM disturbance the electrons are effectively captured

and led up through the lens to be detected by an in-lens detector. For longer

working distance or when a side illumination effect is wanted, a classical SE

detector is mounted in the chamber.

3: In-lens

Here the sample sits on a TEM-type holder and has a maximum size of ca.

4x4x9 mm.

Benefits here are excellent mechanical stability, high electron collection

efficiency and very high EDS count rates.

Relationship between resolution and focal length

Different e- sources and gun types

> 5 years18 months6 months1 monthLife time

0.2 eV0.5 eV1.5 eV2.0 eVEnergy spread, E

1000

2x109

500

5x108

10

1x107

1

1x106

Brightness

[A/cm2sr]

Room temp1500 C1500 C2300 CTemperature

3 - 5 nm10 – 25 nm1 – 2 mm1 – 2 mmSource size

Cold FESchottky FELaB6W

10-11 Torr10-9 Torr10-7 Torr10-5 TorrGun vacuum

20-30 nA>100 nA50 nA -1 mA50 nA -1 mAProbecurrent

2-3 %0.2 %0.2 %0.1 %Stability,%/h

Magnification of SEM is determined by the ratio of the linear size of the display screen

to the linear size of the specimen area being scanned. The linear magnification is given by

Electronbeam is scanned across the specimen and the procedure is known as Raster scanning. Raster scanning causes the beam tosequentially cover a rectangular area on the specimen. The signal electrons emitted from the specimen are collected by the detector,amplified and used to reconstruct the image according to one-to-one correlation between scanning points on the specimen and picturepoints on the screen of cathode ray tube (CRT). CRT converts the electronic signals to a visual display.

Condenser Lens Current and Resolution.The current in the condenser lens changes the spot size or

diameter of the beam of electrons that scans the sample.

More detailed information will be collected when the

electron beam scans the same area with a smaller spot size.

An increased current or a higher number for the condenser

lens (CL) setting, will produce a smaller spot size and in

general will result in a better resolution (A).

DiatomeCourtesy of GokhanTaken by Quanta SEM microscopeMagnification: 30000xSample: DiatomeDetector: ETDVoltage: 4.0kVVacuum: 4.01e-4PaHorizontal Field Width: 9.93μmWorking Distance: 7.7Spot: 2.0https://www.fei.com/image-gallery/

Magnification ?Same picture - different size

Beam samples interaction:

Scattering“Inelastic” scattering refers to a variety of physical processes that act to progressively reduce the energy of the beam electron by transferring that energy to the specimen atoms through interactions with tightly bound inner-shell atomic electrons and loosely bound valence electrons.

Although the various inelastic scattering energy loss processes are discrete and independent, Bethe (1930) was able to summarize their collective effects into a “continuous energy

where E is the beam energy (keV), Z is the atomic number, ρ is the density (g/cm3), A is the atomic weight (g/mol), and J is the “mean ionization potential” (keV) given by

Beam interaction volumes

Images are formed because of beam interaction with the sample

This happens in a volume, not in a point

The size of this volume varies with beam energy...

1μm

Vacc : 10kV Vacc : 1kV

Sample = Si

Beam Excitation Volumes

Elastic scattering

The elastic scattering crossection, can be used to estimate how far the beam electron must travel on average to experience an elastic scattering event, a distance called the “mean free path,” λ

Simultaneously with inelastic scattering, “elastic scattering” events occur when the beam electron is deflected by the electrical field of an atom (the positive nuclear charge as partially shielded by the negative charge of the atom’s orbital electrons), causing the beam electron to deviate from its previous path onto a new trajectory, as illustrated schematically in the figure.

The mean free path is of the order of nm. Elastic scattering is thus likely to occur hundreds to thousands of times along a Bethe range of several hundred to several thousand nanometers.

Accelerating voltage

一

一

一

一

一SE

Surface informationSE I

SEInner information

Electron beam

Z

SE escape depth

Low energy BSE (II)

sample

SE II

一

一一

一一

一

一

一一

一

SE

一一

一

High energy BSE (I)

Z

BSE escape depth

Signals in the SEM

Emitted electrons

1 10 100 1000 10000

Energy of signal electron (eV)

Ele

ctr

on

am

ou

nt

Secondary electron Back scattered electron

(SE) (BSE)

50eV

When a sample is hit by an electron beam a variety of electron emissions are available.(Notice that the scale is logarithmic)

“Huskelapp”

The group of secondary electrons (SE) can be divided into 4 groups.

There are SEI, SEII, SEIII, and SEIV.

These electrons are classed according to how they are generated.

A SEI is an electron is an electron that is generated at the point of primary beam impingement in surface of the

specimen. Thus, it carries the highest resolution information.

A SEII is an electron that is generated when a backscattered electron leaves the surface of the specimen. Due to

the energy of backscattered electrons, this SEII could leave the surface of the specimen microns away from the

primary beam impingement site. SEIIs hurt the resolution of the image, but add greatly to overall image brightness.

A SEIII is an electron released when an energetic backscattered electron strikes the interior of the specimen

chamber, causing a SE to be released.

SEIVs are formed when the primary beam strikes an aperature within the electron column.

SEIIIs and SEIVs contribute noise to the image. By understanding signal formation, the specimen can be properly

prepared for analysis.

SE yield variation

The rapid change in the incident electron beam range causes a large, characteristic variation in the SE yield

Typically the yield rises from ~0.1 at 30keV to in excess of 1 at around 1keV, and as high as 100 for some materials

Experimental SE yield data for Ag

Why the SE yield changes

SE escape depth is ~ 3-5nm

At high energies most SE are produced too deep to escape so the SE yield d is low

But at lower energies the incident range is so small that most of the SE generated can escape so the SE yield rises rapidly

At very low energies fewer SE are produced because less energy is available so the SE yield falls again

interaction volumes

low

voltagehigh

voltage

Do high and low kV SE images look the same?

compared to the high energy

•The image looks less 3-D

•Highlighting is absent

•Surface junk is more visible

28

Emitted electrons – SE:s

Occurrence of edge effect in fine structured surfaces.

Change in SE yield at different incident e-beam angles.

Resolution

At high energy the SE1 signal typically comes from a volume 3-5nm in diameter, but the SE2 signal from a volume of 1-3µm in diameter.

But at low energythe SE1 and SE2 electrons emerge from the same volume because of the reduction in the size of the interaction volume

Resolution at ultra-low energies

• Because Cs and Cc decrease with the landing energy the imaging resolution is only limited by diffraction

500eV30eV

100nm

BSE – Backscatter electrons

一

一

一

一

一SE

Surface informationSE I

SEInner information

Electron beam

Z

SE escape depth

Low energy BSE (II)

sample

SE II

一

一一

一一

一

一

一一

一

SE

一一

一

High energy BSE (I)

Z

BSE escape depth

BSE vs. SE detection

Sample : Polyvinyl AlcoholAccelerating Voltage : 3kV, Vacuum: 60Pa, Mag. : 1,000x

Sample : Polyvinyl AlcoholAccelerating Voltage : 3kV, Vacuum: 60Pa, Mag. : 1,000x

BSE detector SE

33

Emitted electrons – BSE:s

The count ratio of BSE:s depends on the mean atomic number of the specimen.

Detectors on the SEM•Electron Detectors:

•° Everhart-Thornley (E-T) detector

•Located on one side and has a small solid angle of detection.

Detectors on the SEM

•Electron Detectors:

•° Solid-State Diode detector•Located on pole piece and has a large solid angle of detection

•Electron-hole pairs are produced by the action of high energy

•backscattered electrons. Annular detector split into two

•semi-circles, A and B.

•A+B: compositional mode

•A-B: topographic mode

SU8000Series standard optics

TopUpper

Lower

STEM Detector

Signal type

Signal name

Detector information

BSEHA-BSE

Top Composition, crystal

BSELA-BSE

UpperComposition + Topo（Charge suppression）

SE SE UpperSurface information

（Including voltage contrast）

SE Lower Lower Topo

STEMBF-

STEM※１

STEMSample internal information +

Crystal

STEMDF-

STEM※１

LowerSample internal information +

Composition

SE

BSE

STEM

Electrode

Variety of signal detection system in SU8200-series

37

Upper

Electrode

Sample

EXB

SE BSE

Top

Topographical image with shadowLower

Sample ： Photocatalyst

Vacc ： 3.0V

Mag. ： x 50k

courtesy of ：

Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato

Model ：SU8020

Lower detector

LOWER(SE-L)

Upper

EXB

Lower

Top

SE BSE

UPPER(SE)

Sample ： Photocatalyst

Vacc ： 3.0V

Mag. ： x 50k

courtesy of ：

Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato

Model ：SU8020Surface information(incl. Voltage contrast)

Control electrode

Electrode

Upper detector: Pure SE

Sample

Upper

EXB

Lower

Top

SE BSE

Sample ： Photocatalyst

Vacc ： 3.0V

Mag. ： x 50k

courtesy of ：

Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato

Model ：SU8020Topographical + Compositional information

UPPER(LA-BSE)Electrode

Upper detector: SE Filtering, LA-BSE signal

Control electrode

Sample

Upper

EXB

Top

Lower SE BSE

Sample ： Photocatalyst

Vacc ： 3.0V

Mag. ： x 50k

courtesy of ：

Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato

Model ：SU8020Compositional + Crystal information(Less topographical information)

TOP(HA-BSE)Electrode

Top detector: High-angle BSE

Control electrode

Sample

LOWER (SE-L) UPPER (SE)

UPPER (LA-BSE)TOP (HA-BSE)

Deceleration (cathode lens)

-VR

Vi = Vo - VR

Objective lens

Specimen

Primary beam

VR = 0 V

Vi = 0.5kV

VR = 1.5kV

Vi = 0.5kV

In deceleration, a negative voltage is applied to the specimen to decelerate primary electrons before arriving at specimen surface.

Deceleration

Landing voltage: Vi

-VR: Deceleration voltage

Expands

Optimum αiImproved resolution

Normal mode

TopUpper

Accelerated BSE

SEAccelerated SE

Vd

1 Both of the SE and BSE are accelerated bythe deceleration voltage(negative bias)

2 Makes it more difficult to only select the low energy electrons

3 Sample bias and the accelerated BSE:s increase the signal and contrast

Top ： Low energy signal

Upper ： High energy signal

But if we accelerate the SE:s ?

Lecture 2

Scanning Electron Microscopy

Lecture II – EDS, EDX

BackscatteredElectrons (BSE)

Cathodoluminescence

AugerElectrons

Characteristic X-rays

SecondaryElectrons (SE)

Transmitted Electrons

Absorbed Electrons

Electron Beam

Heat

Parameters , imaging , conclusion?We can change1. Accelerating voltage2. Magnification3. Spot size4. Working distance5. Beam current6. Detectors

We want:High resolution

We have choosen low Accelerating voltage

Ask questions in lab:

We will repeat inlens detectorsNext week

Plan:

• X-rays

• Detectors

• Qualitative analyses

• Quantitative analyses

• Data from lab(?)

• Rapport

SE and BSE

Average

PbM

Sn L

Atomic energy levels and line transition

Transitions have different probabilities

Lines have different intensities

EDS spectra: Origin of Bremsstrahlung and characteristic peaks

1 keV = 1.602677·10-16 J

continuum or Bremsstrahlung (breaking radiation)

• results from deceleration of beam electrons in the electromagnetic field of the atom core

• combined with energy loss and creation of an X-ray with the same energy

EDS spectra: Origin of Bremsstrahlung and characteristic peaks

Hmmm …….

Observerte data fra SEM + EDS

- Characteristic X-rays are formed by excitation of inner shell electrons

- Inner shell electron is ejected and an outer shell electron replaces it

- Energy difference is released as an X-ray

2 4 6 8 10 12 14

keV

0

1

2

3

4

5

cps/eV

C Si Cr Cr Mn Mn

Fe

Fe

Ni Ni

EDS spectra: Origin of Bremsstrahlung and characteristic peaks

If beam energy E > EK then a K-electron may be excited

Energy of emitted photon can be calculated:

EPhot = E1 – E2

e.g.: Fe L → K

E1 = EK = 7.11 keV

E2 = EL = 0.71 keV

EKa = 6.40 keV

X-ray energy is the difference between two energy levels !

EDS spectra: Origin of Bremsstrahlung and characteristic peaks

0.71 keV

7.11 keV

6.40 keV

X-ray and AUGER generation process

Emission of Auger electronEmission of X-ray

Auger and X-ray yield are competing processes

C

Ge

Fluorescence yield (ω)

• ω= fraction of ionisation events producing characteristic X-rays (rest produce Auger electrons)

+ A = 1- ω increases with Z- ω for each shell: ωK ωL ωM- Auger process is favoured for low Z, - fluorescence dominates for high Z

0.005 for C K

0.5 for Ge K

L-familie

Kα

Kβ

Fe K-familie

• Energy of characteristic peaks is defined by element• The higher the atomic number Z the higher the peak energy

Characteristic peaks: K, L, M series

S (16)

K1,2 2308 eV

K 2464 eV

(K - K1,2) 156 eV

S (16) Ca (20)

K1,2 2308 eV 3692 eV

K 2464 eV 4013 eV

(K - K1,2) 156 eV 319 eV

S (16) Ca (20) Mn (26)

K1,2 2308 eV 3692 eV 5900 eV

K 2464 eV 4013 eV 6492 eV

(K - K1,2) 156 eV 319 eV 592 eV

The K-family of lines

Mo (42) Ce (58)

L1 2292 eV 4839 eV

L 2394 eV 5262 eV

Ll 2014 eV 4287 eV

Mo (42)

L1 2292 eV

L 2394 eV

Ll 2014 eV

The L-family of lines

Line intensity relations (2)

Lα1

Lβ1

Lβ2

Lγ1LlLγ2/3

Lα2

Spectrum Barium L series, 15 kV

α1 : β1 : γ1 = 100 : 52 : 10

Line intensity relations (3)

Spectrum Barium L-series, 15 kV

α1 : β1 : γ1 = 100 : 52 : 10

Ti-Kα1

Ti-Kβ1

BaTiO3

Barium

Spectrum BaTiO3, 15 kV

Overlapped Ba L-series and Ti K-series

Lα1 (100)

Lβ1 (31)

Lβ2

Lγ1 (5)Ll

Lγ2/3

Lα2

Lα1

Lβ1

Lβ2

Lγ1LlLγ2/3

Lα2

Intensity and energy of characteristic lines

- Energy of line is defined by

- Element

- Type of transition

- Intensity of line is defined by

- probability of producing a hole (vacancy)

- probability of electron transition

- probability of x-ray emission

- concentration

Probability of producing a hole: Ionization cross-section for electrons

• Ionization cross section: probability of excitation

• maximum ionization cross section: 2,5 x Ebind

Ionisation cross section for electrons

Important for the selection

of accelerating voltage.

Ionization cross section for electrons

Fe

Cr

Ni

Cr: 33%

Fe: 33%

Ni: 33%

U = 10 keV

:bind

exc

E

E Cr Fe Ni

1,847 1,561 1,337

A sample with equal amount of Cr,Fe and Ni.

30 kV and 10 kV

Fe/Cr=0,8Ni/Cr=0,7

Fe/Cr=0,5Ni/Cr=0,15

• Isolated Atoms

• When isolated atoms are considered, the probability of an

• energetic electron with energy E (keV) ionizing an atom by

• ejecting an atomic electron bound with ionization energy Ec

• (keV) can be expressed as a cross section, QI:

where ns is the number of electrons in the shell or subshell

(e.g., nK = 2), and bs and cs are constants for a given shell

(e.g.,bK = 0.35 and cK = 1)E= beam energy Ec= ionization energy

http://www.med.harvard.edu/jpnm/physics/refs/xrayemis.html

5.) X-ray range

Different excitation ranges for: - characteristic x-ray radiation and Bremsstrahlung,

- secondary electrons (SE)- back-scattered electrons (BSE)

sample surface

electron beam (E0)

secondary electrons

ca. 0.5 ... 5 µm

ca. 10 µm3

back-scattered electrons

bremsstrahlung

X-rays

MEF4010 Scanning electron microscopy

Detectors on the SEM

•X-ray Detectors

•° EDS Spectrometer

•° WDS Spectrometer

X-RAY-detectors

WDX wavelength dispersive EDX Energy dispersive

Energy resolution is 100 times better with the WDX.

125-150 eV EDX

10 eV WDX

Kvalitativ analyse

Hva har vi tenkt på

• Valg av akselerasjonsspenning

• Utsnitt på prøven

• Vinkel – tilting av prøven

• Arbeidsavstand (WD)

• Telletid

• Oppladning (?)

• Mål:

• Gode data med god tellestatistikk og god oppløsning

Vi kan finne ut mer

Quantitative

5.) X-ray range

• Monte Carlo electron-trajectory simulations of interaction volume in iron as function of primary beam energy

With higher primary electron energy penetration depth is increasing

Rd ≈ 2,5 µmRd ≈ 1,3 µmRd ≈ 0,4 µm

EHT = 10 kV EHT = 20 kV EHT = 30 kV

http://www.gel.usherbrooke.ca/casino/What.html

5.) X-ray range

• Monte Carlo electron-trajectory simulations of interaction volume as function of atomic number (EHT = 15 kV)

With higher density penetration depth is decreasing

Carbon Rd ≈ 2 µm Iron Rd ≈ 0,6 µm Gold Rd ≈ 0,2 µm

The kV compromise

Ichar increases with increasing E0/Ec

X-ray signal improves

Rx increases with increasing E0/Ec

X-ray spatial resolution degrades

00

C

EU 2 ... 2,5

E

Optimum overvoltage

Without sufficient overvoltage, x-ray production is dramatically lowered.

QE (cm2 keV2)

U

U = Eo/Ec

Optimum overvoltage U is around 2-2.5

The ionization cross-section describes the probability that a particular event will take place:

For X-rays: Q = 6.5x10-20nsbs

UEc

ln(csU)

ns = number of electrons in a shell

Ec = critical excitation voltage

bs & cs = constants related to the electron shell

U = overvoltage (Eo/Ec)

ZAF

Can't calculate ZAFs unless concentration is known. But we don’t know the concentration??

Use k values (I/I° = k) to estimate compositions of each element. Then calculate ZAFs, and refine by iteration

Z – at. no. correctionFunction of electron backscattering factor &electron stopping power - depend upon the average atomic number of unknown and standardDependent on ionization cross section

Varies with composition and accelerating voltage

A – absorption correctionVaries with m, takeoff angle,accelerating voltage

F – fluorescence correctionprimary fluorescent x-rays ––>secondary fluorescent x-rays

Varies with composition and accelerating voltage

CalcZAF

http://www.probesoftware.com/Technical.html

http://www.probesoftware.com/download/PROBEWIN.pdf

kV=15 vinkel = 40

Mass absorption coefficientsMass absorption coefficients are stored as a matrix of numbers of absorption of a particular X-ray line (the emitter) by an absorber: For example, a portion of the MAC matrix for Kα X-rays for Z = 23 to 29 is shown below.

http://www.ammrf.org.au/myscope/analysis/eds/quantitative/

Mass absorption coefficients

Emitter V 4952 eV

Cr 5415 eV

Mn 5899

eV

Fe 6403 eV

Co 6930

eV

Ni 7478 eV

Cu 8048

eV

V 94.6 73.8 498.4 403.4 328.2 268.5 220.8

Cr 111.1 86.7 68.4 454.8 370.8 303.9 250.4

Mn 125 97.6 76.9 61.3 401.9 330.1 272.4

Fe 145 113.2 89.3 71.1 57.1 370.2 306

Co 160.9 125.7 99.1 79 63.5 51.4 329.4

Ni 187.9 146.8 115.8 92.3 74.1 60 49

Cu 200.7 156.8 123.8 98.7 79.3 64.2 52.4

Note that the MAC for absorption of Fe Kα by Co (79.0) is different from the MAC for absorption of Co Kα by Fe (57.1).

Sample surface and absorption

rough surface

polished surface

tilted polished surface

X-rays

electron beam

take-off angle

negative tilt angle

-TA

SDD

d1

d2

d1: distance to (imaginary) polished surface

d2: actual distance to specimen surface