1. Tools & Techniques
AES, XPS, SEM, EDX,TEM, FIB...

2. Index
AES, EPMA
XPS
SEM, EDX, TEM
13-Nov-18

3. © Bernhard, Electronic Materials Chemistry
Electron Probe
Electron interaction with nucleus
Electron interaction with electron
High energy
secondary
electron
Elastically
scattered
electron
Inelastically
scattered
electron
Inelastically
scattered
electron
(Bremsstrahlung,
Braking radiation)
© Bernhard, Electronic Materials Chemistry
13-Nov-18

4. Electron Sample Interaction
Electron interaction with electron
Electron from outer shell fills in the inner ‘empty’ location and...
Characteristic X Rays-EDX
Auger Electrons
OR Energy is given to another electron at outer shell, which comes out at a well defined energy
Energy is released as X Ray
© Bernhard, Electronic Materials Chemistry
13-Nov-18

5. AES Combined Probability of ( X-ray or Auger Electron) =1
Very sensitive (all elements except H2)
Very small quantities can be detected (10% of a mono layer)
Energy of Auger Electron = E1-E2-F
E1 = Energy of inner electron
E2= Energy of outer electron (that is jumping to the inner orbit)
F = work function (energy required for an electron to escape from surface)
Nomenclature: KLL (if K shell electron first goes out, L shell electron jumps to K and another L shell electron escapes)
For each material, certain transitions are favoured
Atomic Number < 15, KLL, between 15 and 40: LMM , for > 40, MNN
13-Nov-18

6. AES
Surface Sensitive
Many methods are bulk sensitive. Bulk signal will swamp surface signal
Some methods are surface sensitive and few are surface specific (also depends on the definition of the surface)
Monitor Auger Signal Intensity for various energies
Scattered electrons will have a ‘background’ spectrum, but Auger Signal usually clear. Primary electron energy ~ 10 or 20 keV
Use calibration for practical results (to identify energies)
Note: If the atom is bonded, the energy levels may shift a bit, and chemical shift can give some idea of the type of material
However, other methods are better
SAM- Scanning Auger Microscopy
13-Nov-18

7. AES Issues Small beam to analyze a particular well defined area
however if the intensity is too high and the material is insulator, may degrade
Defocus beam ==> lower spatial resolution
Auger Signal 2
Energy (keV)
AES is more routine for conductors
Electron Beam heating may cause diffusion etc... , sputtering may not be uniform for all species...Issue in depth profiling
==> corroborate with other methods, use reference sample with known profile
13-Nov-18

8. EPMA Instead of electrons, monitor the characteristic X Rays
Electron Probe Micro Analysis (EPMA)
Generated up to 2 um depth
low X ray intensity, detection takes few hours
Not as routine as Auger,
X Rays coming out of sample are detected by one of the two methods
WDS (wavelength dispersive spectrometer)
uses diffraction grating (Bragg’s law), tune the detector to some wavelength and count the intensity over time
tune for other wavelengths (serial information, slower)
higher resolution (5 eV), more expensive
EDS (energy dispersive spectrometer)
electron-hole pair produced. # of pairs proportional to energy of X Ray
parallel information gathering, faster
poorer resolution (150 eV), less expensive
© Univ Wisconsin
13-Nov-18

9. X Ray Probe Sample X Ray Probe
Photo Electrons - X Ray Photoelectron Spectroscopy (XPS)
X Ray Photon
Auger Electrons (XPS)
X-Ray Fluoroscence Spectroscopy
Sample
XPS = Electron Spectroscopy for Chemical Analysis = ESCA
© Bernhard, Electronic Materials Chemistry
13-Nov-18

10. XPS Use ‘soft’ x rays (about 1 keV energy) to excite the sample
less damage to material, less charging
still sufficient emission of photoelectron
Very surface sensitive (top 5 nm probed)
large spot size (less resolution)
Energy of secondary electron E = hn -EB-F
where hn incoming x ray energy
EB Binding energy
Elements in different environments (different bonds) will produce slightly different specta
13-Nov-18

11. XPS
Source: Usually referred to by the material (e.g. Mg K-alpha source, with 1.2 keV energy)
Instead of wavelength or frequency, energy (hn) is used
Sample excited with x ray, out coming electron detected
Plot count vs energy (reverse scale)
Auger Electrons come on the left side of the plot, usually small peaks (perhaps small probability of getting Auger Electrons with X-Ray excitation)
Counts 1
Binding Energy(keV)
This technique is sensitive to chemical environment and crystal structure
13-Nov-18

12. SEM: Principle Surface topography Scanning Electron Microscope
Focussed Electron Beam lit on the surface and detected
Beam Scanned over surface, to get the topography
100,000X magnification, very good depth of focus
Electrons fall on the surface and some electrons knocked off the sample
Secondary electron (usually < 50 eV energy)
Electron from a peak escapes easily. Electrons from a trough has less probability of escaping (absorbed by material)
Primary beam, after spreading out, does not produce that many secondary electrons with sufficient energy to escape
13-Nov-18

13. SEM : Operation
Good topographic sensitivity and spatial resolution (5 nm)
Back Scattered Electron (BSE), from nuclear interactions
some information about the material, but not good spatial resolution. BSE can also come from ‘deeper’ atoms
Primary electrons with sufficient energy and intensity
needle pointed metal tip in intense electric field and UHV (field emission gun)
use small energy to avoid charging insulators (better resolution)
otherwise coat materials with gold
Elemental composition using EDS (sometime WDS)
e.g. Applications
Porosity, grain size
Gate length, width, general feature sizes
Failure Analysis, contamination, void location
13-Nov-18

14. SEM: Operation Vacuum inside the chamber
sample has to be able to withstand vacuum (should not sublime, for example!)
Sample (first sputter coated with gold, if necessary), placed into the chamber, with air lock
Detector usually operated at low temp
to reduce noise
cooled using liquid nitrogen
Electrons focussed using magnetic ‘lens’ and scanned on the surface using electromagnetic field
Note: SEM is sometimes written as “Secondary Electron Microscope” !
The primary electrons are NOT the ones used for obtaining image (usually)
13-Nov-18

15. © Museum of Science, Boston
SEM Schematic
Schematic
© Museum of Science, Boston
13-Nov-18

16. EDX
Energy Dispersive X Ray Spectroscopy (EDX, EDAX - energy dispersive analysis of x rays...)
Attached to SEM Machine
Detects the X Rays emitted by an electron ‘dropping from outer shell to inner shell’
One element can provide many transitions ==> many peaks
Nomenclature:
K shell filled by L Shell electron == K-alpha x ray
K shell filled by M shell electron== K-beta X ray
and so on...
Usually, analysis is for KLM lines
Not very accurate below Carbon
very small sample size is sufficient
detection limit of about 1 atomic % (typical)
13-Nov-18

17. TEM Transmission of electrons Sample has to be very thin (sub micron)
allowed sample thickness depends on energy of primary electrons and the materials (dense materials will block electrons better)
Sample preparation is elaborate
Extremely high resolution (in theory 0.04 A, in practice 2A, because of tool limitations and lens aberrations)
Can be used to differentiate between crystalline and amorphous structures and also between different crystalline structures
SEM acceleration voltage ~ 30 kV, TEM ~ 300 kV
* Material need not be conductive, as in the case of SEM
13-Nov-18

18. AAS Atomic Absorption Spectroscopy To detect each element
UV-Vis used in chemistry lab, for example, to detect elements
depends on the electrons absorbing at a particular wavelength
extensive preparation may be necessary
e.g. Need to convert an element to a particular state (e.g. Trivalent state, etc)
AAS detects the elements, regardless of their state
Principle similar to UV
Equipment design is different
13-Nov-18

19. AAS UV Vis scans (e.g. From 200 nm to 800 nm)
IR will be 800 nm + (near IR, Far IR etc)
AAS does not scan
uses a separate ‘bulb’ for each element detection
tedious, but more accurate (micro gram per liter)
Detects usually metals and few non metals
e.g. Chlorine etc are not detected by AAS
Sample is present in gaseous state during analysis
Typically sample is in liquid (dissolved) form, for flame AA
Solid can be used for graphite ( electro thermal AA)
13-Nov-18

20. Sample image AAS Sample Compartment Detector Light Source
© Bette Kreuz
Univ Michigan
13-Nov-18

21. AAS Source Source Lamp (Hallow Cathode Lamp or HCL)
contains the element being measured (e.g. Aluminum)
Anode (typically Tungsten)
Cathode is cylindrical, either made of material of interest or coated with material of interest (surrounded by glass shield)
© Bette Kreuz
Univ Michigan
13-Nov-18

22. Source and Nebulaizer
Inert gas (Ar or Ne) at low pressure in the source
High voltage ==> ionization of gas
sputter on the cathode, release element either in neutral or in ionized state
even neutral atoms may go to excited state
When they return to ‘ground’ state, release a photon
Light source of few wavelengths
Sample is ‘sprayed’ in the sample chamber
Nebulizer (atomizer) creates a fine aerosol
mixes oxygen and fuel and the sample, in the sample chamber
Flame to destroy all the complexes
some atoms will be in elemental form and some in ionic form
13-Nov-18

23. Nebulizer and Flame
Fuel is usually acetylene and oxygen is supplied as nitrous oxide or air
Some refractory elements form ‘hard to break down’ oxide
use nitrous oxide instead of air
Suction created by nebulizer (controlled flow) and liquid is automatically taken in the chamber
Liquid that is not vaporized, is collected at the bottom of the chamber (connected to waste collector)
Samples tend to be dissolved in acids (corrosive)
Material in elemental form will absorb light
Atomic ABSORPTION spectroscopy
13-Nov-18

24. Sample Chamber and Detector
Material absorbs the wavelength
Flame has light of all the wavelengths
Monochromator to filter out all the other wavelengths and let only the wavelength of interest
Detector (usually photo multiplier tube or PMT)
Sample is ‘sprayed’ in the sample chamber
Nebulizer creates a fine aerosol
mixes oxygen and fuel and the sample, in the sample chamber
Flame to destroy all the complexes
13-Nov-18

25. AAS
Use ‘standard’ solutions, change the flame (fuel, oxygen ratio) until maximum absorption is seen for the ‘standard sample’
Use acid solution of same concentration (as in the prepared sample) as a ‘blank’
Calibrate and test sample (similar to UV Vis)
13-Nov-18