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Lecture 25 MNS 102: Techniques for Materials and Nano Sciences

• Scanning Electron Microscopy – SEM vs TEM, SEM signals

• Electron scattering: IMFP, energy distribution

• De-excitation via X-ray or Auger electron emission

• Modes of operation: Imaging (SE vs BSE) and EDX

• Energy-dispersive X-ray Spectroscopy

• SEM pros and cons

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Reference: #1 C. R. Brundle, C. A. Evans, S. Wilson, "Encyclopedia of Materials Characterization", Butterworth-Heinemann, Toronto (1992), Ch. 2, Ch. 3. References: http://www.charfac.umn.edu/sem_primer.pdf http://www.micro.magnet.fsu.edu/primer/virtual/virtual.html Virtual SEM http://www.microscopy.ethz.ch/sem.htm

SEM vs TEM

25- 3 Source: http://www.vcbio.science.ru.nl/en/fesem/info/principe/

SEM Signals: SE, BSE, X-rays

25- 4 Source: http://www.vcbio.science.ru.nl/en/fesem/eds/

Electron Scattering inside a Solid

• Elastic: no energy loss

• Inelastic: energy is lost to “excite” the material – this leads to deexcitation

• Forward vs backward scattering

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“Universal” Inelastic Mean Free Path of Electron

• IMFP, λ(E), is the average distance travelled by an electron inside a solid material before it suffers an inelastic collision.

• I(d) = I0 exp(-d / λ(E))

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SE Auger + BSE

Energy Distribution of Electrons

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Deexcitation by emission of X-ray or Auger electron

De-excitation can occur via one or both ways:

• “Radiation-less” decay via Auger electron emission – Auger electrons are usually quite high in energy > shorter IMFP than SE > only those near the surface can escape > very surface sensitive.

• X-ray fluorescence or X-ray emission – this leads to Energy Dispersive X-ray (EDX) or Wavelength Dispersive X-ray (WDX) Analysis. X-ray emission can occur along the entire electron scattering paths, leading to EDX probe volume size of a few microns.

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Electron-impact Ionization

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Photo-ionization

• A high energy electron can undergo inelastic collision with the atom and knocks out an electron inside. This could lead to creation of an cation.

• Cations can also be formed by the absorption of a photon – the so-called photoelectric effect discovered by Einstein. This leads to X-ray Photoelectron Spectroscopy (XPS).

• If the knock-out electron or the photoelectron comes from a core orbital in the formation of this cation, then several deexcitation processes could occur.

• These include: shake-up, X-ray emission, and Auger electron emission.

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X-ray Photoelectron Spectroscopy

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Einstein Equation

Secondary Electrons

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Modes of Operation: Imaging vs EDX

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• An electron beam (0.05 to 30 keV) is focussed by a condensor lens and is rastered across the surface of the specimen. Electrons from the specimen are used to construct the image.

• SE are detected by in-lens SE detector (inside electron column) or conventional SE detector (outside electron column).

• Back-Scattered electrons are detected by BSE detectors mounted along the primary electron beam axis.

• X-ray emission is detected by EDX detector.

Everhart-Thomley SE Detector

SE = Secondary Electron

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In-lens vs Conventional SE Detectors

• The in-lens detector is located inside the SEM electron column and is arranged rotationally symmetric around the optical axis. Due to a sophisticated magnetic field at the pole piece, the SEs are collected with high efficiency and high contrast at low voltages and small working distances.

• In-lens SE detector (right): Info about morphology and surface topography, differences in the work function (e.g., electronic variations) - graphite (low work function) with bright contrast vs polymer fiber (high work function) with dark contrast.

• Conventional SE detector (left): Topographic info.

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Contrast: Edge Effects

• Contrast is dominated by the edge effect: More SE can leave the sample at edges leading to increased brightness.

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Contrast: Voltage dependence

• The penetration depth of electrons increases strongly with increasing energy.

• Low energy electrons interact only with the surface of the specimen, leading to detailed images of the specimen, whereas high energy electrons pass through thin specimens, which consequently look transparent.

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BSE

• Larger atoms (with a greater atomic number, Z) have a higher probability of producing an elastic collision because of their greater cross-sectional area. Inelastic cross section is typically 1% of elastic cross section. The number of backscattered electrons (BSE) is proportional to the mean atomic number of the sample. A "brighter" BSE intensity correlates with greater average Z in the sample, and "dark" areas have lower average Z (so-called Z contrast).

• BSE images are useful for obtaining high-resolution compositional maps of a sample and for quickly distinguishing different phases.

• BSE is not quantitative.

• SE image (left) and BSE image (right) of Fe particles in Carbon. The BSE image shows the Fe particles with bright contrast. 25- 18

Contrast from BSD vs SE BSD:

• Substrate atomic number (Z)

• Angle of beam incidence

SE:

• Angle of beam incidence

• Energy of the incident beam

• Work function and surface condition

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Energy-Dispersive X-ray Spectroscopy

• X-ray spectroscopy is a valuable tool for qualitative and quantitative element analysis. Each element has characteristic peak positions corresponding to the possible transitions in its electron shell. The presence of zinc, e.g., is indicated by the K peak at about 8.6 keV and a L peak at 1.0 keV. O K line is located at 0.52 keV. In heavy elements like tungsten, a lot of different transitions are possible and many peaks are therefore present.

• EDX can be used in TEM and SEM instruments.

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De-excitation by Discrete X-ray Emission

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Intensity ratio K1:K2:K = 10:5:2

Fluorescence Yield

• Fluorescence Yield = = Number of X-ray photon produced / Number of ionization

• Fluorescence Yield + Auger Yield = 1

• Higher Z > higher

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EDX Maps

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Quantitative X-ray Analysis

Matrix effects (ZAF): Z (Atomic Number Effect): Z affects the backscatter coefficient and the stopping power; A (X-ray Absorption Effect): X-ray absorption cross section; F (X-ray Fluorescence Effect): secondary fluorescence.

Correction by Phi-rho-Z: Use the experimental fitted phi curve (known) and compare with the unknown one to generate a corrected fluorescence factor.

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Light Microscopy vs Electron Microscopy

Light Microscope Electron Microscope

Wavelength = 500 nm (150/V0) = 0.0055 nm at 50 kV

Refraction Index = n 1.5 (glass) 1.0 (vacuum)

Half-angle = 70 deg 1 deg

Resolution = 0.61 / NA where NA = n sin

200 nm 0.16 nm*

Depth of Focus (DOF) = distance parallel to the optical axis that a feature on the specimen can be displaced without loss of resolution.

DOF = λ𝑛2−𝑁𝐴2

𝑁𝐴2 +250

𝑀2 M = 10, DOF = 60 m M = 100, DOF = 8 m M = 1,000, DOF = 200 nm

DOF = 0.1 𝑚𝑚

𝑀 𝜃

M = 10, DOF = 1,000 m M = 100, DOF = 100 m M = 1,000, DOF = 10 m M = 10,000, DOF = 1 m

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*Less than 0.05 nm possible with Cs (spherical aberration) and Cc (chromatic aberration) lens correctors. Most TEM specimens are not thin enough to produce images with resolution that could benefit from Cs correction. For thicker specimens, Cc correction via energy filtering is much more useful.

Homework 6B: Does one expect to see charging effect of an insulating specimen in a TEM? Justify your answer. Homework 6C: In less than one page, summarize and compare the information that one can obtain from a material using the TEM and SEM techniques. Homework 6D: Generate the last column of the Table on the last slide for an electron beam with energy (V0) of 1, 5, and 20 keV.

SEM Pros and Cons

• Artefacts: Contrast depends on many mechanisms and the nature of the specimen, including surface morphology and conductivity of the local regions. Different detectors also have different and/or additional contrast mechanisms. By combining images from different detectors, a better picture of the specimen can be obtained.

• Electron beam damage: The energy of the e-beam at large kV (20 kV) is sufficient to induce damage (depending on the specimen) and to introduce dirt onto the sample (via electron dissociation of ambient inside the vacuum chamber). Low kV operation is helpful for soft materials.

• Specimen preparation: Sample preparation is much easier for SEM than for TEM. For nonconductive samples, charging may occur. Gold or carbon coating may be needed for discharging the samples. Operating at a lower voltage will also help. Another issue is sample cleanliness. Sometimes the image gets darken quickly due to deposition of C species (dissociated by the high-energy electron beam) onto the sample. Cleaning of the chamber and the sample by plasma cleaning will help.

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WATLab SEMs: Zeiss Merlin & UltraPlus

http://microscopy.zeiss.com/microscopy/en_de/products/electron-microscopy.html 25- 28