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

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

© 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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