4 - Scanning Electron Microscopy

What are the components of a scanning electron microscope?

• Electron source/gun produces beam of e^-. Typically W filament, LaB₆, or field emission gun (FEG).

• e^- beam passes through the central column, which comprises condenser lens(es) [determines beam current that impinges on sample] and objective lens [determines final spot size of the e^- beam].

• Scanning coil used to deflect the beam in X-Y directions → raster scan over the sample surface.

• Specimen chamber, hosts sample as well as a range of detectors, typically for secondary electron (SE), back-scattered electron (BSE), and X-rays. Typically under vacuum conditions.

How does the electron beam interact with samples?

• Typically interested in SE (topography), BSE (composition), CL (composition), and characteristic X-ray (chemical composition).

• Fluorescence and Bremsstrahlung important for background characterisation.

• Secondary electrons: inelastically scattered from the emission of valence e^-. Low energy.

• Backscattered electrons: elastically scattered; high Z elements backscatter e^- more strongly than low Z ones. High energy.

• Characteristic X-rays: ejection of inner shell e^- compensated for by e^- in higher orbital dropping down → excess energy given off as X-ray.

• Continuous X-rays: result from e^- decelerating when travelling close to target nucleus → looses energy as X-ray photon.

What is the electron interaction volume equation?

R= (0.0276×E^1.67×A)/(ρ×Z^0.89 )

E = electron beam energy (keV)

A = atomic weight (g/mol)

ρ = density of target (g/cm³)

Z = atomic number of target

→ R increases with electron beam energy E

→ R decreases with target atomic number Z

What are secondary electrons?

SE images are based on the characteristics of the surface topography (roughness) of the specimen, and are not dissimilar to images observed by eye.

• SE are e^- with energies of 0 to ~30 eV, which originate from within the target specimen.

• Target atomic number Z does not influence signal strength.

• Most SE originate <100 nm from the specimen surface.

• Divergence of primary beam  within target is thus almost zero → image resolution ~ incident beam diameter.

How are back scattering electrons used in minerology?

•       BSE imaging of polyphased material allows visualising different phases (e.g., minerals in a rock).

•       BSE imaging of single phases allows visualising chemical zoning:

→ Mg-Fe in olivine

→ Trace elements (U, Th, REE) in zircon

What are characteristic x-rays?

• Characteristic X-ray produced by electron transitions between bound electron orbits.

• Removal of an inner electron is necessary for production of characteristic X-ray.

• For characteristic X-ray line to be produced, incident electron energy E₀ must exceed critical excitation energy E_C required to excite the relevant shell.

• K_α X-ray is produced due to removal of K shell electron, with L shell electron taking its place. K_β occurs when K shell electron is replaced by electron from the M shell.

• L_α X-ray is produced due to removal of L shell electron, replaced by M shell electron.

• M_α X-ray is produced due to removal of M shell electron, replaced by N shell electron.

What is the critical excitation energy of characteristic x-rays?

• Critical excitation energy E_C = min. energy eject electron from atomic shell.

• Also known as excitation potential and X-ray absorption edge energy.

• With increasing atom size (increasing Z), E_C required to excite any particular transition line also increases.

• E_C is higher than associated characteristic X-ray energy.

• E_C K_a ~ Σ(line energies) ~ K_a + L_a + M_a

What is overvoltage?

•       Overvoltage U = E₀/E_C

→ ratio of accelerating voltage (gun) to critical excitation energy for selected line.

•       Max. efficiency at ~2-3 times E_C.

•       Fe: min. accelerating voltage needed to excite K_a ~ 15 kV.

•       U: min. accelerating voltage needed to excite L_a ~ 30 kV.

What is an Energy dispersive spectrometer (EDS)

•       Solid-state detector (semiconductor).

•       As each X-ray photon hits detector, small current produced.

•       Records X-ray of all energies simultaneously.

•       Low energy resolution, typically 80-150 eV wide.

→ good for broad qualitative characterisation of chemistry of samples investigated.

EDS allows producing X-ray element maps of whole sample → very useful for looking at their mineralogy.

Can combine several elements into composite X-ray map

How must samples be prepared for SEM and EPMA?

Secondary electrons

•       Information on topography – no need to be flat.

•       May need fixing to stabilise friable components.

•       Sample typically glued to a stub.

Backscatter electrons and X-rays

•       Need flat surface nicely polished to remove topographic effects.

•       May need impregnating as for thin section preparation.

Interaction between the incident e^- beam and target results in electric current developing on specimen surface. Unless target is a conductor (e.g., metal), electrical charges build up → apply thin layer (10’s nm) of conductive coating over the sample, typically C.

What is Electron Probe MicroAnalysis (EPMA)?

• Electron source/gun produces beam of e^-. Typically W filament or FEG.

• Electron beam focused by coils, condenser and objective lenses, and apertures → beam typically ~1 μm Φ.

• Scanning coil used to deflect the beam in X-Y directions → raster scan over the sample surface.

• Detectors for secondary electron (SE) and back-scattered electron (BSE), and X-ray energy dispersive (EDS) and wavelength dispersive spectrometers (WDS).

• High vacuum to limit interaction with gas molecules (~10^-4-10^-5 Pa).

What is Wavelength dispersive spectrometer (WDS)?

•       Typically 4-5 WDS spectrometers with 1-4 diffracting crystals.

•       Incoming X-ray dispersed by crystals, with a single wavelength diffracted to detector.

•       Each detector records one wavelength at a time.

•       Detector is gas-filled tube where gas is ionised by X-rays, yielding large multiplication factor.

•       High spectral resolution, typically 5-20 eV.

→ high precision quantitative chemical analysis of chemistry of samples investigated.

How does Bragg’s law impact crystal type for WDS?

nλ = 2d.sin(θ) → Braggs’ law

•       LIF, PET & TAP most common crystals.

•       H-crystals: smaller circle radius, more limited range, but higher count rates.

•       L-crystals: large analysing crystals for trace element analyses.

Compare EDS and WDS

• Molybdenite (MoS₂) EDS (yellow) and WDS (purple) spectra.

• In EDS spectrum, Mo Lα line @ 2.293 keV severely overlapped by S Kα @ 2.307 keV.

• These lines are clearly resolved in WDS spectrum. Also resolves S Kβ and Mo Lβ.

• EDS is cheaper, quicker, but some interferences impossible to resolve, and detection limits are fairly high → qualitative overview.

• WDS is more expensive and time consuming, but much better spectral resolution and detection limits → quantitative chemical analysis.

How is WDS spectra used to produce quantitative chemical data?

•       Check for possible peak overlaps.

•       Here want to quantify O Ka peak.

•       Precisely position peak centre.

•       Position background analysis positions on both side. Caution with interfering peaks.

•       Decide on appropriate model to fit background – linear?

•       Polynomial fit better here.

What equations are used for standardisation and corrections?

〖C'〗_unk=C_std×I_unk/I_std

• Apparent concentration in unknown is equal to concentration in standard multiplied by intensity ratio of unknown/standard.

C_unk=〖C'〗_unk×F_unk/F_std

• Real concentration in unknown is equal to apparent concentration in unknown multiplied by correction factor ratio of unknown/standard.

F=F_Z×F_A×F_F

• Correction factor is function of corrections resulting from backscattering (Z), absorption (A), and fluorescence (F). Known as matrix correction ZAF.

• We choose standards that are as similar as possible to the unknown to minimise the correction factor extrapolation → Funk /Fstd ~ 1.

What indicates a good analysis?

Totals: Having sum of all major elements analysed close to 100 wt.% suggests analysis is ok. Typically in range 98.5-100.5 wt.% for silicates and glasses.

Stoichiometry: calculated mineral stoichiometry from analyses makes sense (we’ll come back to this in practical).

What is stopping power correction?

•       Incident electrons lose energy by interacting with inner shell electrons.

•       Stopping power (= energy lost by electrons) drops with increasing Z.

→ higher number of X-rays produced in higher Z targets.

What is backscatter correction?

•       Fraction of incident electrons backscattered increase with Z.

•       Less incident electrons penetrate into targets with higher Z.

→ smaller number of X-rays produced in higher Z targets.