scanning electron microscopy techniques · 2019-07-09 · the object being imaged is the source...

Emad Oveisi I SEM techniques – April 2019

Scanning electron microscopy techniquesMS‐633 Material Science and Engineering (EDMX)

Emad Oveisi, Marco Cantoni

Lausanne, Spring 2019

http://cime.epfl.ch


Outline

2

• Introduction to electron microscopy (EM) (by E. Oveisi)Why use electrons?Wavelength and resolutionTypes of interactions

• EM setup (by E. Oveisi)Electron sourcesLensesVacuum systemDetection system

• SEM (by E. Oveisi & M. Cantoni)

• Operation, Signals• Contrast mechanism• Interpretation of images, Challenges• Related techniques• Advanced and high‐resolution SEM

• Chemical analysis and Monte Carlo simulations (by M. Cantoni)

• Focused ion beam (by M. Cantoni)


How the SEM works?

• Electrons are accelerated to high energies

• Condenser lens system defines probe size and control probe current

• Scanning coils above Objective lens raster beam on sample

• Objective lens focus probe on sample

• Various detectors surrounding sample collected radiated signals

Scan coils


• Image formed step by step by the sequential scanning of the sample with the electron probe (using pair of deflector or scan coils, controlled by the scan generator)

• Monitor and scanning coils are synchronized• (one‐to‐one correspondence between the rastering pattern on the specimen and the rastering pattern used to produce the image on the monitor)

• Intensity of each pixel is proportional to signal received (collected electrons)

• When changing the magnification, we just change the raster size (no change in optics)

Under sampling

Sample Screen

Magnification = Area scanned on the monitor / Area scanned on the specimen

How the SEM works?


Which to trust? The scale bar or the indicted magnification…?

Measure the size of the particle and calculate the magnification !

Particle size is around:200 nm on the image (scale bar)1 cm on your laptop screens20 cm cm on the TV screen/projector

Quiz: what is the magnification of this image?a) 50 kXb) 1 MXc) 35.46 kXd) depends

Magnification

Magnification = Image size / Raster size

200 nm


Operational parameters

1. Beam accelerating voltage (kV): the voltage with which the electrons are accelerateddown the column;

2. Probe convergence angle (αp): the half‐angle of the cone of electrons convergingonto the specimen;

3. Probe current (ip): the current that impinges upon the specimen and generates thevarious imaging signals;

4. Probe diameter or spot size (dp): the diameter of the final beam at the surface of thespecimen.

Looking at the diagram it would seem that all we would have to do to maintainadequate probe current in a small probe diameter would be to increase the probeconvergence angle. But this is not the case due to aberrations in the optic system.

A small probe diameter always comes with a decrease in probe current.

These parameters are interrelated in other ways. For example, a decrease acceleratingvoltage will result in a decrease in probe current as well as an increase in probe size.

Doing SEM involves understanding the trade offs


Lenses in a SEM

There are a number of points to emphasize about lenses when thinking about SEM:

Electromagnetic lenses are used to demagnify the image of the beam source and to focus the beam on the specimen.

Condenser lenses are involved in demagnification of the image of the beam source.The objective lens focuses on the specimen as well as demagnifies.

Condenser lens

Objective lens

Smaller probeBetter resolution/detailsLower current: less charging and beam damage

lower signal to noise ratio


Lenses in a SEM

The object being imaged is the source diameter (Gaussian intensity distribution) of the electron beam.

So, we are interested in demagnifying this beam source diameter Smaller probe size

Image from Scanning Electron Microscopy Primer, University of Minnesota


Lenses in a SEM ‐ Focusing

Workingdistance


Lenses in a SEM – Spot size

Condenser lens

Objective lens

Smaller probeBetter resolution/detailsLower current: less charging and beam damage

lower signal to noise ratio


Lenses in a SEM ‐ Condenser aperture

Aperture size (micron)

Probe current Purpose

30 Low Ultrahigh resolution; Low probe current; Large depth of field

70 Medium Usual observation

100 High Good resolution at high probe current; Reduced depth of field


Resolution in a SEM

Spatial resolution in the SEM depends on spot size Smaller spots give higher spatial resolution

Shorter electron wavelengths mean smaller spot sizes Higher electron voltages give higher spatial resolution

BUTMany different factors limit spatial resolution, “in practice”, with in the SEM,

e.g., combined signals from multiple scattering, size of interaction volume, columbic interaction, aberrations, specimen charging….


Scan parameters

10 pixels1px = 20 nm20 nm probe




200 nm

Over sampling – Blurry image

Under sampling – Poor resolution

Good sampling – Good resolution

Good sampling – Resolution?

More pixels

Smaller probe

Less current

15 sec

60 sec

240 sec

240 secOr more!

1 sec Dwell time

0.8 sec 1nA

0.8 sec 100pA50 sec 100pA


Scan parameters





200 nm

Over sampling

Under sampling

Good sampling

Good sampling

More pixels

Smaller probe

100 nm

Under sampling

You must find balance between spot size/pixel,

current, and scan speed!

Sample Drift, Beam Damage, and Charging influence your

choice of scan speed and current and thus are practical

conditions that determine resolution


Focus and depth of field

In focus Over focusUnder focus


d = depth of field = required spatial resolution= convergence angle

d

Region of image in focus

The picture can't be displayed.

For small angles, tan =


Can control depth of field (d) with convergence angle ()

Out of focusOut of focus


Depth of field



Depth of field

30 µm100 µm


600 um OLA 200 um OLA 100 um OLA

WD

10 m

m

20 m

m

30 m

m

30 m

m

10 m

m

Depth of field


Stereoscopy

Parallax: Brain compares the view from different locations in order to estimate the distance

Parallax

Left eye sees: Right eye sees:

How do we see things in 3D?


Stereoscopy


Stereoscopy

Image at ‐5 degree tilt Image at +5 degree tilt


Elastic scattering

Incident electron

scatteredelectron

Incident electron

Back scattered electron

Rutherford‐type scattering

• No energy transfer

• Low angle scattering: Coulomb interaction with the electron cloud

• High angle scattering, or back scattering: Coulomb interaction with nucleus

• Atom is not ionized

• Backscattered electrons have an energy range from 50 eV to nearly the incident beam energy.

• Most backscattered electrons retain at least 50% of the incident beam energy.


Inelastic scattering

Incident electron

Secondary electron

Scattered electron

• An incident electron ejects a bound electron and scatters with an energy lowered by the electron

bound energy.

• The ejected electrons having low energies (5‐50 eV) are called secondary electrons (SE) and carry

information about the surface topography

• The incident electron can be scattered by Coulomb interaction with the nucleus

• In the case of inelastic interaction, there is energy transfer, and the target atom can be ionized

Incident electron

Scattered electron

Continuum X-ray (Bremstrahlung)*

* Incident electron decelerated due to the electromagnetic field of the atom nuclei‐> Energy released in the form of X‐ray (White radiation)

Ionizazion ‐> Emission of charateristic X‐ray or Auger


Electron signals:

Secondary electrons SEElectrons ejected from material at low energiesTopography, low energy ≈ 5‐50 eV

Backscattered electrons BSEIncident electrons that elastically scatter and leave the sampleAtomic number ZEnergy ≈ eV0 (range from 50 eV to an energy close to initial energy)

Auger electronsEjected electrons with an energy characteristic of target elementsNot detected in conventional SEM, surface analysis

Incident electrons interaction with the sample produces:

SEM signals


Other signals:

Electromagnetic radiations:

• X rays with continuous energy, resulting from deceleration of incident electrons ("Bremsstrahlung" or “breaking radiation")

• Characteristic X‐rays with a distinct energy associated to the target atoms.

• Cathodoluminescence: Visible radiation mainly emitted by insulating or semi‐conducting materials.

Others:

• Absorbed current, electron‐holes pairs creation, EBIC• Plasmon• Sample heating (phonons)• Radiation damages: chemical bounding break, atomic

displacement (knock‐on) damage

Incident electrons interaction with the sample produces:

SEM signals


Surface signals:

• Secondary electrons (topography)• Auger electrons (electronic states, chemistry)

Sub‐surface signals:

• Backscattered electron (Z contrast, crystallographic information)• Characteristic X‐ray (compositional information)• Secondary florescence (Cathodoluminescence, band‐gap)

We DO care where the signal (electron) comes from

Spatial resolution depends on the size of the interaction volumeInteraction volume differs with material, accelerating voltage, spot size

Monte Carlo simulations

BSE spatial res.

SEM signals


SEM signals

Red trajectories = Primary and backscattered electrons

Blue trajectories = Secondary electrons (SE1, SE2, SE3)

NOTE: Image signal is displayed at the probe position NOT at the actual SE production position

SE2 + SE3 reduce resolution

SE1: from interactions of the incident probe withspecimen atoms.• SE1s are produced in close proximity to the

incident beam and thus represent a high lateralresolution signal

SE2 from interactions of the high energy BSEs withspecimen atoms.• Both lateral and depth distribution characteristics

of BSEs are found in the SE(II) signal and thus it isa comparatively low resolution signal

SE3 are produced by high energy BSEs which strikethe pole pieces and other solid objects within thespecimen chamber.


Interaction volume ‐ Effect of beam energy

30 keV 20 keV 10 keV 5 keV

Red trajectories = backscattering

Blue trajectories = Primary electrons

The more the beam energy,

• the less the probability for elastic scattering(larger mean free path) Probability of elastic scattering is inversely proportional to beam energy

• the trajectories near the surface become straighter, so, the more the penetration depth (i.e. larger interaction volume)

Cumulative effects of multiple elastic scatterings cause some electrons topropagate back towards the surface, thus widening the interaction volume.



Top surface images of a TiO2/perovskite/FA‐CN device acquired using electron beam energiesof 3 and 1 keV, respectively demonstrating the perovskite and covering HTM layer.

S. Paek et al. Advanced Materials (2017).

Sample courtesy of S. Paek , GMF‐EPFL



Effect of the accelerating voltage on penetration depth and SE signal

20 kV: Strong penetration: SE2/SE3 is a much

larger signal than SE1. It reveals the copper grid under the C

film via the electron backscattering, but the structure of the film itself is hidden

2 kV: Low penetration, only a few electrons

reach the copper grid and most of the SE2 are produced in the C film together with SE1.

The C film and its defects become visible

From D.C. Joy, Hitachi News 16 1989



1 KV 5 KV

10 KV 30 KV

30 keV imaging has higher spatial resolution. However, the larger interaction volume and increased edge effects masks the fine features of the surfaces.

30 kV – buried interfaces are visible though surface features are less resolved

1kV ‐ Surface features are resolved with high spatial resolution


Interaction volume – Effect of “Z”

C Si Cu Ag Au

Red trajectories = backscattering

Blue trajectories = Primary electrons

The more the “Z”,

• the more the probability for elastic scattering(shorter mean free path)

• the shorter the penetration depth.


SE/BSE yield

Ionization occurs more readily at lower incident electron energies

BSE yield increases with “Z”

BSE yield depends on “Z” of material, and SE yield depends mainly on the voltage

Energy (keV)

Low Z

HighZ ƞ

SE yield

• ƞ shows a monotonic increase with atomic number. Thisrelationship forms the basis of atomic number (Z)contrast.

• Areas of the specimen composed of higher atomicnumber elements emit more backscatter signal and thusappear brighter in the image.

• Z contrast is relatively stronger at lower atomic numbers(see the slope of the line).

• δ is relatively insensitive to atomic number (around 0.1).

(SE1)

ƞ

δ

• There is a general rise in δ as the beamenergy is decreased, primarily due to thereduction in the interaction volume.


BSE yield vs. beam energy and Z

10 nm

BSE=14%

BSE=34%

BSE=44%

200 nm

BSE=8%

BSE=33%

BSE=47%

1

BSE=6%

BSE=50%

BSE=33%

Cu

C

U

1 kV 5 kV 20 kV

Higher yield

NOTE: There is only a small change in η with accelerating voltage. As the accelerating voltage is reduced toward the very lower end (1 keV), η increases forlow Z elements and decreases for high Z elements


SE/BSE yield

Energy spectrum of electrons leaving the sample

Auger electrons

SE: secondary electrons 0‐50 eVBSE: backscattered electrons E > 50 eV

BSE energy range: BSEs follow trajectories which involve very different distances of travel in the specimenbefore escaping. The energy range for BSEs is thus wide (from 50 eV to that of the incident beams energy).The majority of BSEs, however, retain at least 50% of the incident beam energy (E0).Generally speaking, higher atomic number elements produce a greater number of higher energy BSEs andtheir energy peak at the higher end is better defined.


SE/BSE imaging

Images courtesy of Manuel Bianco, GEM‐EPFL

STEEL

SrCrO4

MnCo2O4

Specimen: Ferritic stainless substrate initially coated with a porous MnCo2O4

This materials were coupled at high temperature (750°C) with a perovskite material.

SE image BSE image

SE have low energies (5‐50eV), and thusare emitted only from surface andpossess information about topographicalfeatures.

BSE emission depends on “Z”, thusintensity in the BSE images scales withatomic number and depends on localcomposition


Top surface images of a TiO2/perovskite/FA‐CN demonstrating the perovskite and covering HTM layer.

Sample courtesy of S. Paek , GMF‐EPFL

SE image BSE image

SE/BSE imaging


SE/BSE imaging

Dust on WC (different Z materials)

low Z materialflat material rough material low Z material

thin layer of contamintion low Z material

SE 25kV BSE 25kV


Detectors

https://myscope.training/legacy/sem/background/whatissem/detectors.php


Detector geometry

The fact that SEM images can be (somehow!) readily interpreted by viewers derives fromthe “Light optical analogy”.

The direction of the detector within the image is analogous to where the “illumination” appears to come from.

Since we are used to having illumination from the overhead, we should rotate the scanned image so that the detector/sun appears to be at the top of the image.


Detector geometry

Pyramid Etched pit

Interpretation requires to know where the detector is located.

Since we are used to having illumination from the overhead, we should rotate the scanned image so that the detector/sun appears to be at the top of the image.


Detector geometry (SE)

Quiz: If is the image of an indentation‐like feature, where the detector is located w.r.t. the image?

A: Upper‐left B: Upper‐rightC: Lower‐left D: Lower‐right

In‐lens detector located directly above and centered


Detector geometry (BSE)

Segments A+B

Segment A Segment B

A‐B B‐A

Quiz: Which segment configuration for this image?

B

A

BA

B A

B

A

A B

C D


BSE yield vs. tilt


BSE yield vs. tilt

NOTE: Surface roughness (tilt) may affect thecontrast between the heavy and light elements

Less Z contrast

More Z contrast

Large angle BSE detector Small angle BSE detector

Pure silver with topographically irregular surface


• If a sample is titled, the interaction volume is tilted and closer to the surface. Thus, more SE escape from below the surface, giving higher signals

• The same principle is true for rough surfaces – Sloped surfaces and edges have an interaction volume that is effectively titled and have higher SE signals

1. Rough Surfaces have high SE image contrast2. Titling can improve SE image contrast

SE yield vs. tilt


I0I()

1‐10nm

At high kV: penetration depth >> SE escape length

(adapted from D.C. Joy Hitachi News 16 1989)

Relative yield of SE vs angle of incidence on the sample surface:

SE yield vs. tilt

At low KV: penetration depth = SE escape length


Do not forget, in SEM:The signal is displayed at the probe position, not at the actual SE production position!!!

intensity profile on image

Size and edge effects

R, Electron range

SE signal intensity across spheres with diameters larger and smaller than electron range R and increase of the signal at an edge caused by diffusion contrast


49

In‐lens SE detector Everhart Thornley detector

In‐lens SE Detector located directly above and centered

SE2 Detector located on the lower right

Size and edge effects

Why they are different?

Where is the detector for each image?


Relaxation processes of the excited state

Characteristic X ray

X-ray generationX ray energy characteristic interorbital electron transitions and thus of the element

Auger electron

Emission AugerThe relaxing process interacts with an electron with a characteristic energy

Incident electron

Secondary electron

Scattered electron


Energy of the X‐ray peak for carbon?

Other signals in SEM – X‐ray


Energy Dispersive Spectroscopy

Other signals in SEM – X‐ray

• From beryllium up to periodic table • Sensitivity 0.1 wt%• Quantitate analysis (many requirements)• Point, line scan, and mapping• Interaction volume (important for SEM)• About 130 ev energy resolution for Si(Li) detector


Example of EDX‐SEM

STRONTIUM POISONING OF STAINLESS STEEL

STEEL

SrCrO4

MnCo2O4

• SEM SE picture of a ferritic stainless substrate initially coated with a porous MnCo2O4

• This materials were coupled at high temperature (750°C) with a perovskite materials• The SEM/EDS analysis was fundamental to understand that Sr migrated from the

perovskite into the steel

Slide courtesy of Manuel Bianco, GEM‐EPFL


Electron-holes creation: An incident electron onto a semiconductor can excite a valence electron to the conduction band, creating an electron-hole pair.

Cathodoluminescence:The excited electron recombines with its hole, emitting a photon with the gap energy, in the visible. This technique allows to put forward the defects that modify the gap energy.

Egap

Conduction band

Valence band

Electron incident

Egap

Conduction band

Valence band

Conduction band

Valence band

Electron that has lost energy

Eg =h

Other signals in SEM – Cathodoluminescence


Examples of CL Images of perovskite solar cell materials using Attolight ROSA SEM‐CL

SEM-CL is useful for correlating/detecting

phase distribution and composition with luminescent properties

defect structure with optical properties

detection of trace elements, their valence and structural position

Other signals in SEM – Cathodoluminescence

Slide courtesy of Dr. Thomas Lagrange – CIME In collaboration with Prof. M. Graetzel’s group LPI‐EPFL


Challenges

Beam induced changes to the sample:

• atom displacement ("knock on")

Radiation damage

• chemical bound breaking

Radiolysis

• lattice atom vibrations (phonons)

Sample heating


Challenges

Contamination

Primary example: Hydrocarbon build‐up on surface

Masks surface features and information about the sample

Sources

Sample surface

SEM chamber

Beam induced degradation and migration of sample compounds

Plasma cleaning sample prior to observation

Use gloves when handling samples


Challenges

Charging

Occurs in non‐conducting samples (also in samples that are not well grounded)

Charging deflects the low‐energy secondary electrons causing image distortions and contrast changes

Ways to mitigate charging

Coat the sample

Work at low kV

Use low currents (noisy images)

Use the “magic” charge neutrality voltage

Use high working chamber pressures (environmental SEM)

Charge compensation devices

Positive charging

Negative charging

Accelerating voltage

Total yield (SE + BSE)


Challenges ‐ Charging

Dark halodecreased SE collection

Dust particles on a metallic substrate

SE SE


Challenges ‐ Charging

Uncoated quartz fragment – 1kV ETD with positive bias

Depends on:‐ material properties (surface resistivity)‐ beam conditions (beam energy, current, and scan rate)


Enviromental SEM

Modified FEI quanta 200:• Available gases (mass flow controller): H2, O2, N2, CO, NH3, C2H4

• Pressure up to 800Pa (8mbar)• Temperature: up to 1000°C• QMS Mass Spectrometer

Slide courtesy of S. Poitel, LSME‐EPFL


Enviromental SEM

Anode under Redox cycling

Redox Cycling between O2 and H2 at 800°CSample: SOFC anode made of YSZ (yttrium stabilised Zirconium) and Ni (which is reduced and oxidised at each cycle)



Enviromental SEM

48 hours oxidation under 33 Pa of oxygen at 880°CSample: SOFC interconnect made of steel and coated with Co and Cerium.


SOFC coated interconnect oxidation


Focused Ion Beam


Focused Ion Beam

Nano fabrication

AFM tip


Focused Ion Beam

Cross‐sectioning SEM imaging

Failure analysis


Focused Ion Beam

Insertion of electrical connection:1) Removal of isolating layer (milling)2) Pt deposition (FIB deposition)

Deposition


Electron backscatter diffraction ‐ EBSD

Electron backscatter diffraction is an electron diffraction technique in scanning electron microscopy used to obtain information about crystallographic phase, crystal orientation and defect densities.left: EBSD pattern of Nb (15 kV) right: Geometric set‐up of an EBSD measurement system

© Max‐Planck‐Institut für Eisenforschung GmbH


EBSD‐based orientation mapping

A (scanning) electron microscopy technique which measures crystal orientations on a regular grid by electron diffraction. The so‐determined data are used to produce orientation or phase maps of the scanned area on the sample.

lateral resolution: 20 to 100 nm (depending on the sample material); measurement rate currently up to hundreds of patterns per second; Investigated areas: few µm² to few cm². Angular resolution: 0.5°down to 0.01° with special techniques (‐>XR EBSD). The technique can also be extended into a 3D technique (‐> grain boundaries).

© Max‐Planck‐Institut für Eisenforschung GmbH


scanning electron microscopy techniques · 2019-07-09 · the object being imaged is the source...

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