<figure><img src="image_1.png" /></figure>

HAL
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Scanning Electron Microscopy

Franck Carcenac

► To cite this version:

Franck Carcenac. Scanning Electron Microscopy. Master. Université de Technologie de Troyes, France. 2024, pp.67. hal-04630493

HAL Id: hal-04630493

https://laas.hal.science/hal-04630493v1

Submitted on 1 Jul 2024

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# Scanning Electron Microscopy

Université Technologique de Troyes - 2024

Scanning Electron Microscopy – Franck CARCENAC

LAAS - CNRS 7, avenue du Colonel Roche 31031 TOULOUSE - FRANCE
* Switch from microscopy to characterization

<figure><img src="image_2.png" /></figure>

Knowledges, concepts, tools, tips, tricks, ...

* Scanning Electron Microscopy:

* A technique with performances and limits

<figure><img src="image_3.png" /></figure>

* Learn how to use it to be able to quickly achieve the best possible result

<figure><img src="image_4.png" /></figure>

* Choose parameters

<figure><img src="image_5.png" /></figure>

* Get the best results

<figure><img src="image_6.png" /></figure>

* Quickly adapt your set of parameters if needed

* Characterization:

<figure><img src="image_7.png" /></figure>

* Gathering useful and valuable informations from your observation/analysis using SEM
* Histories

* SEM presentation

* Principle

* Electron beam generation

* Electron optics

* Electron/Matter interactions

* Phenomena involved

* Origin of contrasts

* Detectors - examples

* SEM in practice

* Observation parameters choices

* Influence of choices

* Procedures: alignments, astigmatism/focus adjustment
* Histories

* SEM presentation

* Principle

* Electron beam generation

* Electron optics

* Electron/Matter interactions

* Phenomena involved

* Origin of contrasts

* Detectors - examples

* SEM in practice

* Observation parameters choices

* Influence of choices

* Procedures: alignments, astigmatism adjustment
* 1932 Reinhold Rüdenberg – Patented electron microscope

* 1937 Manfred Von Ardenne - SEM 1st prototype

* 1938 First commercial electron microscope: Siemens

* Today ... world competition:

* Electron optics

* Electrons sources

* Detectors

* Software

* Additional capabilities:

* Correlated microscopies (optical, fluorescent, AFM, ...)

* Spectroscopies: electrons, photons

* Materials properties: crystallography, strength, corrosion, ...
- Entirely numerical microscopes

- Less “frightening” ergonomics

- Alignment coils

- Numerous useful automatic functions

- Remember: physics does not change with microscopes

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

today ...

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /></figure>

* Schottky - Field Effect Gun (S-FEG)

* IN-LENS detectors (secondary & backscattered electrons)

* Imaging at very low energy (<1kV):

* Polarizing sample up to ~4kV

* Imaging apparent voltage = electron beam accelerating voltage - sample polarization voltage

* Keeps resolution and limits charging effects

* Environmental / low vacuum microscopes:

* Direct observation on insulating samples without treatment:

* Variable pressure in the chamber

* Ionizing molecules (+) on the electrons' trajectory

* Neutralizing charges (-) from the beam (electrons) at sample's surface

* At the cost of resolution (electrons diffusion on their trajectory)

* Monochromator (increase resolution at low energy # ⌖ΔE/E)

* ...
* Histories

* SEM presentation

* Principle

* Electron beam generation

* Electron optics

* Electron/Matter interactions

* Phenomena involved

* Origin of contrasts

* Detectors - examples

* SEM in practice

* Observation parameters choices

* Influence of choices

* Procedures: alignments, astigmatism adjustment
* Focused electron beam

* 1 point on sample = 1 point on display

* Display point brilliance:

* f (interaction, detector)

* Screen/sample synchronous scanning

* Point per point reconstructed image

* Magnification = f (distance between pts)

* Resolution do not depend on magnification

* Resolution is mainly given by beam diameter

* Fixed point: screen center

<figure><img src="image_16.png" /></figure>

B. Reig - LAAS-CNRS

Source production for
the optical system

Size's source reduction
(beam/spot size)

Projection of the source
on the sample (focus)

Detection, amplification
and signal treatment

Display
<figure><img src="image_17.png" /></figure>

Principle: resolution driving parameters

<figure><img src="image_18.png" /></figure>

$$ \text{Beam diameter # resolution} = f (HT, i, \alpha, \beta, C_s) $$
# Electron beam generation: guns... choosing your SEM

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

* 3 different SEM with 3 different gun types and 3 different resolutions ...

* How to choose one for your characterization ?

* What do you want to characterize ? Do you need high resolution ?
* Electrostatic lenses

* 3 electrodes (minimum):

* Filament or tip (attached to a filament)

* Extractor: extract electrons from the filament

* Anode: fix the energy of electron beam
# Electron beam generation: guns... choosing your SEM

## 2 families of electron guns:

* **Thermo-electronic:**

* Electrons extracted from a heated filament (extraction: ~500V)

* 2 subfamilies:

* W filament

* LaB₆, CeB₆ crystal soldered on W wire

<figure><img src="image_22.png" /></figure>

* **Field effect guns (FEG):**

* Electrons are extracted from a sharp tip by a high electric field (extraction: ~5kV)

* 3 subfamilies:

* Cold FEG

* Thermal FEG

* Schottky FEG

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /><figcaption>Wikipedia Schottky FEG tip - Erwin Meier</figcaption></figure>
## Thermo-electronic guns:

* W filament:

* <10⁻¹ Pa ... easily achieved by a Turbo Molecular Pump (TMP) + Dry Pump (DP)

* Filament lifetime: few hundred hours

* LaB₆, CeB₆ crystal:

* ≤ few 10⁻⁵ Pa ... achieved by TMP + DP + ionic pump

* Filament lifetime: few thousands hours

## Field effect guns (FEG):

* Cold FEG:

* ≤10⁻⁸ Pa - TMP + DP + baking out + ionic pumps

* Tip lifetime ~15 years (emission unstable: depends on surface contamination)

* Schottky FEG:

* ≤10⁻⁷ Pa - TMP + DP + baking out + ionic pumps

* Tip lifetime: few thousands hours

* Tip heated at ~1700°C 🔧 1 supplementary electrode to repel thermo-electrons
Look at vacuum pipes connections :

* 4 screws = vacuum > $10^{-5}$ Pa = elastomer o-ring: cannot be baked out

* > 4 screws = vacuum < $10^{-5}$ Pa = high vacuum = metal o-ring: compatible with baking out

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>
<figure><img src="image_28.png" /><figcaption>W filament SEM
Hitachi S-3700 N
Low vacuum</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>FEG SEM
Hitachi S-4800
C-FEG</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>FEG SEM (+FIB)
ThermoFisher HELIOS 600i
S-FEG</figcaption></figure>
Ionic pump

<figure><img src="image_31.png" /><figcaption>LaB₆ filament SEM - Jeol 840 A</figcaption></figure>

Gun Type ?
* Lenses: electromagnets or electrostatic lenses (e.g.: gun)

* Definitions:

* Optical axis: neutral point for electrons # no deviation

* Paraxial approximation: small angles, close to optical axis

* Circular symmetry defects induce aberrations:

* Due to mechanical manufacturing

<figure><img src="image_32.png" /></figure>

* Make aberrations depending on lens parameters

* Adjusting any parameter on any lens:

(Electrons' energy, filament T°, extracting voltage, beam current, working distance)

* Modifies optical axis position (XY image position), focus/astigmatism

* Induces the need of re-alignments and re-adjustment of focus/astigmatism lenses
* Histories

* SEM presentation

* Principle

* Electron beam generation

* Electron optics

* Electron/Matter interactions

* Phenomena involved

* Origin of contrasts

* Detectors - examples

* SEM in practice

* Observation parameters choices

* Influence of choices

* Procedures: alignments, astigmatism adjustment
# Phenomena involved

* **Scattering of electrons from the beam**

<figure><img src="image_33.png" /></figure>

Backscattered electrons (5-50%) - $E \ge 50$eV

<figure><img src="image_34.png" /></figure>

Transmitted electrons (if sample is sufficiently thin)

* **Energy transfer to matter**

<figure><img src="image_35.png" /></figure>

Atoms relaxation = emission of particles

* Secondary electrons (~50-80%):

* $E \le 50$eV

<figure><img src="image_36.png" /></figure>

* Mean free path few nanometers

<figure><img src="image_37.png" /></figure>

* Close to the scattered electrons' trajectory

* Auger electrons

* Photons : X, UV, visible, IR

* Induced current

* ...

* From one point to another, all events are independent
# Phenomena involved: backscattered electrons

* Backscattered electrons (belong to primary beam)

* Monte-Carlo simulation :

* Elastic diffusion on atoms
    (Rutherford backscattering: $Z^2/E^2$)

* Energy loss between 2 elastic diffusion
    (Bethe slowing down law: $Z/E$)
- 5-50% of all electrons exiting from the sample

- $50\text{eV} \le E \le E_{\text{beam}}$

- Sensitive to the local chemical composition

- High kinetic energy

- High directivity

- Low sensitivity to topography

- 45° surface escape half-angle

- Detector located above the sample: can be broken easily
# Detectors: BackScattered Electrons Detector (BSED)

## "Objective" lens

### BSE Detector

Principle: creation of charges in a polarized diode

Current proportional to the number of BSE produced

<figure><img src="image_38.png" /></figure>

### X-Rays Detector

### Secondary Electrons Detector

Chamber view from inside
<figure><img src="image_39.png" /></figure>

http://www.gel.usherbrooke.ca/casino/
# Phenomena involved: backscattered electrons trajectories

<figure><img src="image_40.png" /></figure>

- Substrate: Si

- Beam Energy: 20keV

- Backscattered coefficient: 17.5%

- Interaction volume:

- Max depth: 4µm

- Max radius: 3µm
<figure><img src="image_41.png" /></figure>

* Substrate: Si

* Beam Energy: 10keV

* Backscattered coefficient: 15.5%

* Interaction volume:

* Max depth: 1.300µm

* Max radius: 950nm
<figure><img src="image_42.png" /></figure>

* Substrate: Si

* Beam Energy: 5keV

* Backscattered coefficient: 17%

* Interaction Volume:

* Max depth: 370nm

* Max radius: 280nm
<figure><img src="image_43.png" /></figure>

- Substrate: Si

- Beam Energy: 1keV

- Backscattered coefficient: 15.5%

- Interaction Volume:

- Max depth: 30nm

- Max radius: 30nm
<figure><img src="image_44.png" /></figure>

- Substrate: PMMA (Zav=3.6)

- Beam Energy : 20keV

- Backscattered coefficient: 4.5%

- Interaction volume:

- Max depth: 7µm

- Max radius: 5µm
<figure><img src="image_45.png" /></figure>

* Substrate: Si (Z=14)

* Beam Energy: 20keV

* Backscattered coefficient: 17.5%

* Interaction volume:

* Max depth: 4µm

* Max radius: 3µm
<figure><img src="image_46.png" /></figure>

* Substrate: Au (Z=79)

* Beam Energy: 20keV

* Backscattered coefficient: 50%

* Interaction volume:

* Max depth: 0.3µm

* Max radius: 0.4µm
# Phenomena involved: max depth of escape

<figure><img src="image_47.png" /></figure>
# Phenomena involved: secondary electrons

* Emitted by the sample

* Energy < 50eV

* Mean free path: few nm

* Close to scattered electrons trajectories

* Emitted from few nanometers near the surface

* Source of contrast:

* Intensity depends mainly on topography

* Depends also on:

* Atomic number

* Organic contamination of the surface
<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /></figure>
Origin of contrast: topography

<figure><img src="image_50.png" /></figure>
Origin of contrast: topography

<figure><img src="image_51.png" /></figure>
Sample without topography composed of materials with different Z:

<figure><img src="image_52.png" /></figure>

<figure><img src="image_53.png" /></figure>

Interaction is different:

- More backscattering from Au

- More secondary electrons in escaping position

<figure><img src="image_54.png" /></figure>

Secondary electrons emission intensity is different

<figure><img src="image_55.png" /></figure>

Chemical contrast (without topography)
> Everhart & Thornley Detector (ETD - 1960)

> If objective lens is close to the sample:

- ETD collector potential is screened

- Electrons are trapped between sample and lens

<figure><img src="image_56.png" /></figure>

No detection

<figure><img src="image_57.png" /></figure>

Use the detector inside the column

<figure><img src="image_58.png" /></figure>
<figure><img src="image_59.png" /></figure>

<figure><img src="image_60.png" /></figure>

Topographical image example: candle stick fume nanoparticles (carbon)
# Origin of contrast: chemical contrast with SED

<figure><img src="image_61.png" /><figcaption>Flat freshly cleaved epitaxial sample without chemical etching</figcaption></figure>
Origin of contrast: BSED vs SED (Au on Si)

<figure><img src="image_62.png" /><figcaption>SED</figcaption></figure>

<figure><img src="image_63.png" /><figcaption>BSED</figcaption></figure>
Origin of contrast: BSED vs SED

<figure><img src="image_64.png" /><figcaption>SED</figcaption></figure>

<figure><img src="image_65.png" /><figcaption>BSED</figcaption></figure>

<figure><img src="image_66.png" /></figure>

<figure><img src="image_67.png" /></figure>
* Surface contaminant cross-linking

* Coming from the substrate

* Coming from the vacuum

* Adding a material based on C:

* Low SE emission

* Low contrast

<figure><img src="image_68.png" /></figure>

* Black rectangles

<figure><img src="image_69.png" /><figcaption>Tilted view of epitaxially grown layers after local etching</figcaption></figure>
<figure><img src="image_70.png" /></figure>

<figure><img src="image_71.png" /></figure>

## Energy Dispersive X-ray Spectroscopy (EDS or EDX or ...)
* High energy electron beam: ≥10keV

* Max. depth penetration > X-Rays max. depth of escape

* Resolution depend on X-Rays max. depth of escape (~1µm for Si)

* Not a surface analysis

* Elementary analysis:

* Resolution: ~125eV (50 - 130 eV for the Mn Kα line (5.89 keV))

* Point analysis

* Mapping

* Qualitative

* Quantitative:

* 1% precision

* Calibration 📉 full-time job
# Detectors: low energy X-Rays detector

* Low energy X-rays detection

* High sensitivity for light elements: Li, Be, B, C, O, N

* High Resolution SEM:

* Low energy electron beam: $\le 3$keV

* Interaction volume < escape volume: $\sim 10$nm

* Spatial resolution depend on interaction volume: $\sim 10$nm

* Surface analysis with high resolution
<figure><img src="image_72.png" /></figure>

<figure><img src="image_73.png" /></figure>

Project
management

Peak intensity
at a given
point for all
elements
detected
* Histories

* SEM presentation

* Principle

* Electron beam generation

* Electron optics

* Electron/Matter interactions

* Phenomena involved

* Origin of contrasts

* Detectors - examples

* SEM in practice

* Observation parameters choices

* Influence of choices

* Procedures: alignments, astigmatism adjustment
## 1. Choices :

- SEM type

- High voltage

- Beam current

- Object orientation

- Working distance

## 2. Adjustments :

- Alignments of all elements on optical axis: lenses, apertures

- Focus

- Astigmatism

- Layout

... It's simple !
<table><thead><tr><th>Working distance ↗</th><th>Voltage ↗</th><th>Current ↗</th><th>Bad alignment</th></tr></thead><tbody><tr><td><strong>Resolution</strong> ↗<br>Depth of focus ↗<br><br>Resolution ↗<br>Tip effect ↗<br>Charges ↗<br>Damages ↗<br>Surface details ↗</td><td><strong>Resolution</strong> ↗<br>Charges ↗<br>Damages ↗<br>Noise ↗</td><td><strong>Resolution</strong> ↗<br>Noise ↗</td><td><strong>Resolution</strong> ↗<br>Charges ↗<br>Damages ↗<br>Noise ↗</td></tr></tbody></table>

Once parameters are chosen, the main resolution improvement is achieved by decreasing WD
<figure><img src="image_74.png" /><figcaption>Long working distance</figcaption></figure>

<figure><img src="image_75.png" /><figcaption>Short working distance
High resolution</figcaption></figure>
Influence of choices: High Voltage

<figure><img src="image_76.png" /><figcaption>C-FEG</figcaption></figure>

<figure><img src="image_77.png" /></figure>

High accelerating voltage

<figure><img src="image_78.png" /></figure>

<figure><img src="image_79.png" /></figure>

Low accelerating voltage

Candle stick fumes:

- Same sample

- ~ same parameters
<figure><img src="image_80.png" /><figcaption>High accelerating voltage</figcaption></figure>

<figure><img src="image_81.png" /><figcaption>Low accelerating voltage</figcaption></figure>
On insulator decrease

<figure><img src="image_82.png" /></figure>

decrease

<figure><img src="image_83.png" /></figure>

Image: local CNRS-LAAS 0.3kV-D 5.7mm x50 SE(M) 1.00mm mirror: 1mm inside
* Height on which the beam size does not vary

* DoF = f(WD, obj. aperture size)

* Built-in synchronous focus variation across field of view

Example:

- Electrolytic Au / Si

- Tilt angle: 45°

- Working Distance: 6mm

- Depth of Focus: ~5µm

<figure><img src="image_84.png" /></figure>
# Influence of choices: layout

* Brain is organized in horizontal and vertical analysis

* When an image does not fit brain organization:

* You induce constraints on the observer

* You do not ease communication

<figure><img src="image_85.png" /></figure>

Angle evaluation !?

<figure><img src="image_86.png" /></figure>

You are sensitive to <2.5°

<figure><img src="image_87.png" /></figure>

Use integrated function
to align your layout
* A SEM is not calibrated at better than ~2.5 - 10%

* Depending on:

* HV, I, WD, Magnification

* SEM quality

* Regular calibration

* Precise measurements need calibrated objects:

* Polystyrene beads deposited close to the area of interest

* Known objects coming from a calibrated equipment: e.g. E-Beam writer
<figure><img src="image_88.png" /></figure>

Defines optical axis

<figure><img src="image_89.png" /></figure>

<figure><img src="image_90.png" /></figure>

<table><tr><td></td><td>HV</td><td>curr</td><td>mag</td><td>WD</td><td>det</td><td>HFW</td><td>tilt</td><td></td><td>500 µm</td><td>Q1</td></tr><tr><td></td><td>1.00 kV</td><td>86 pA</td><td>100 x</td><td>4.2 mm</td><td>ETD</td><td>2.07 mm</td><td>0°</td><td></td><td></td><td>Act</td></tr></table>
* Purpose is to use electrons :

* With the same energy

* With the same trajectory

<figure><img src="image_91.png" /></figure>

Focus them at the same location # high resolution

<figure><img src="image_92.png" /></figure>

Misalignment induces bad resolution

* All elements are mechanically pre-aligned

* Mini-coils inside the column to push the beam
<figure><img src="image_93.png" /></figure>
<figure><img src="image_94.png" /></figure>
Procedures: astigmatism (adjustable)

sagittal focal
beam = line

main focal
beam = circle

transverse focal
beam = line

<figure><img src="image_95.png" /></figure>

<figure><img src="image_96.png" /></figure>

<figure><img src="image_97.png" /></figure>

<figure><img src="image_98.png" /></figure>

<figure><img src="image_99.png" /></figure>

<figure><img src="image_100.png" /></figure>
<figure><img src="image_101.png" /></figure>
# Procedures: adjustments flow chart

## Select electron beam energy and current:

* Starting energy: high energy for conductive samples (15keV), low energy for insulating samples (1keV)

* Starting current (beam size, spot size, ...):

* Mean value for conductive sample to minimize the noise

* Low value to minimize charging

## Adjust image brilliance/contrast (use buttons or automatism)

## If there is too much noise or no image: verify GUN alignment

* Use the built-in "gun", "beam", "source", "cross-over", ... alignment procedure

## Adjust focus roughly (use buttons / mouse)

## Adjust sample position: X, Y, Z, tilt, focus distance (use buttons / mouse)
# Procedures: adjustments flow chart

<figure><img src="image_102.png" /></figure>
> **Scanning Electron Microscopy characterization:**

- Imaging: topographical and chemical contrast

- Analysis: EDX, WDX, Auger, EBSD, ...

- Simple to implement: training ~1 day for simple use

- Surface characterization technique

- "non destructive" (surface modification by pollution - black rectangles)

> **2 main procedures:**

- Lenses/aperture alignment: source (gun), "objective", stigmator X and Y

- Focus/Astigmatism adjustment loop

- Trick: "As long as you can: zoom in !. Resolution does not depend on magnification ... but your capability to adjust does !"

> **Principal limiting factor: user !**

> **50% job ✧ knowledge of the sample (location of structures to characterize)**

> **Your efficiency is ecoresponsible**
# Bibliography ?

## Academic websites:

* Société Française de Microscopies (Sfµ)

* Réseau d'Imagerie en Microscopie Electronique (RIME)

* Plateforme Toulouse Réseau Imagerie (TRI Genotoul - Life Sciences)

## All manufacturers' websites:

* Hitachi

* Jeol

* Thermo Fisher Scientific

* Tescan

* Zeiss

* Webinars from manufacturers (free registration required)

* Wikipedia