Scanning electron microscopy Fei Quanta Tabletop Hitachi Example: - - PowerPoint PPT Presentation

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
• ur 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 probe formation, not for image formation directly, as in

• TEM. Two condenser lenses reduce the crossover

diameter of the electron beam. The objective lens further reduces the cross-section of the electron beam and focuses the electron beam as probe on the specimen surface.

Instrument: SEM is it like a TEM ?

Objective lens

Objective lens Cross section

The final probe-forming lens has to

• perate 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 SED SED

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 years 18 months 6 months 1 month Life time 0.2 eV 0.5 eV 1.5 eV 2.0 eV Energy spread, E 1000 2x109 500 5x108 10 1x107 1 1x106 Brightness [A/cm2sr] Room temp 1500 C 1500 C 2300 C Temperature 3 - 5 nm 10 – 25 nm 1 – 2 mm 1 – 2 mm Source size Cold FE Schottky FE LaB6 W 10-11 Torr 10-9 Torr 10-7 Torr 10-5 Torr Gun vacuum 20-30 nA >100 nA 50 nA -1 mA 50 nA -1 mA Probecurrent 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 to sequentially 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 picture points 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).

Diatome Courtesy of Gokhan Taken by Quanta SEM microscope Magnification: 30000x Sample: Diatome Detector: ETD Voltage: 4.0kV Vacuum: 4.01e-4Pa Horizontal Field Width: 9.93μm Working Distance: 7.7 Spot: 2.0 https://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

• ccur 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 information SE I SE Inner 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) Electron amount 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 voltage

high 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 energy the 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

500eV

30eV

100nm

BSE – Backscatter electrons

一 一 一 一 一 SE Surface information SE I SE Inner 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 Alcohol Accelerating Voltage : 3kV, Vacuum: 60Pa,

• Mag. : 1,000x

Sample : Polyvinyl Alcohol Accelerating 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

Top Upper Lower

STEM Detector

Signal type Signal name Detector information BSE HA- BSE T

• p

Composition, crystal BSE LA- BSE Upper Composition + T

• po

（Charge suppression） SE SE Upper Surface information （Including voltage contrast） SE Lower Lower T

• po

STEM

BF- STEM

※１

STEM Sample internal information + Crystal STEM

DF- STEM

※１

Lower Sample internal information + Composition SE BSE STEM Electrode

V ariety of signal detection system in SU8200-series

37

Upper

Electrode

Sample EXB

SE BSE

Top

Topographical image with shadow

Lower

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 ：SU8020

Surface 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 ：SU8020

Topographical + 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 ：SU8020

Compositional + 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 αi

Improved resolution

Normal mode

Top Upper

Accelerated BSE SE Accelerated SE

Vd 1 Both of the SE and BSE are accelerated by the 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

Microscopy Lecture II – EDS, EDX

Backscattered Electrons (BSE) Cathodoluminescence Auger Electrons Characteristic X-rays Secondary Electrons (SE) Transmitted Electrons Absorbed Electrons Electron Beam Heat

Parameters , imaging , conclusion?

We can change

• 1. Accelerating voltage
• 2. Magnification
• 3. Spot size
• 4. Working distance
• 5. Beam current
• 6. Detectors

We want: High resolution We have choosen low Accelerating voltage Ask questions in lab: We will repeat inlens detectors Next 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
• f 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 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 electron Emission 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) K1,2 2308 eV K 2464 eV  (K - K1,2) 156 eV S (16) Ca (20) K1,2 2308 eV 3692 eV K 2464 eV 4013 eV  (K - K1,2) 156 eV 319 eV S (16) Ca (20) Mn (26) K1,2 2308 eV 3692 eV 5900 eV K 2464 eV 4013 eV 6492 eV  (K - K1,2) 156 eV 319 eV 592 eV

The K-family of lines

Mo (42) Ce (58) L1 2292 eV 4839 eV L 2394 eV 5262 eV Ll 2014 eV 4287 eV Mo (42) L1 2292 eV L 2394 eV Ll 2014 eV

The L-family of lines

Line intensity relations (2)

Lα1 Lβ1 Lβ2 Lγ1 Ll Lγ2/3 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 Lγ1 Ll Lγ2/3 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

• f 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=0,8 Ni/Cr=0,7 Fe/Cr=0,5 Ni/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 µm Rd ≈ 1,3 µm Rd ≈ 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

C

E U 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 = 6.5x10-20 nsbs 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. correction Function of electron backscattering factor & electron stopping power - depend upon the average atomic number of unknown and standard Dependent on ionization cross section Varies with composition and accelerating voltage A – absorption correction Varies with m, takeoff angle, accelerating voltage F – fluorescence correction primary 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 coefficients Mass 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/e ds/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