IV. Surface analysis for chemical state, chemical composition Probe beam Detect XPS Photon (X-ray) Photoelectron(core level electron) UPS Photon (UV) Photoelectron(valence level electron) AES electron Auger electron SIMS Energetic ion Secondary ion Schematic diagram for XPS, AES hν (X-ray) I e (5 kev) e (Auger electron) E Electron multiplier e (photoelectron) Electron energy analyser 1
1. XPS (X-ray photoelectron spectroscopy) or hυ (xray) ESCA (electron spectroscopy for chemical analysis) e (photoelectron) by Siegbahn Mg K α : 1253.6 ev half width: 0.8 ev Al K α : 1486.6 ev : 0.9 ev Cu K α : 8047.8 ev : ~ 3 ev L 2p 3/2 2p 1/2 2s 1/2 Photoelectron kinetic energy Photoelectron kinetic energy K 1s 1/2 E K.E = hυ E B photon energy (constant) Binding energy of the electron in the orbital in the atom 2
XPS spectra of silver Spin-orbit splitting parameters Sub shell j value Area ratio s 1/2 - p 1/2, 3/2 1 : 2 d 3/2, 5/2 2 : 3 f 5/2, 7/2 3 : 4 Detection limit: ~ 0.1 ~ 1 % Effective probe depth : 10 ~15 Å 2j + 1 : available # of states 3
XPS 1 identification of surface element 2 intensity surface concentration 3 core-level (binding energy) shift : chemical shift oxidation state The charge potential model 0 = + + qi Ei Ei kqi r i, j i, j q i : charge q on atom i high oxidation shift to high B.E 4 shake up and shake off 5 depth profiling is possible 4
Chemical shift e Oxygen i + Charge j Screening effect Charge transfer from atom i to j due to oxidation by atom j reduces screening effect on the hole decrease kinetic energy increase binding energy (E KE = hν E B, E KE E B ) Chemical Shift Chemical Bonding, Oxidation state 5
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Shake up peak 2p 3/2 2p 1/2 Cu 2 O Cu + : closed shell configuration CuO Shake up Cu 2+ : open shell configuration 931 943 949 Binding Energy (ev) During the photoemission process of core level electron, photoelectron excites a valence electron to an unfilled level at higher energy, thus suffers a discrete loss in energy K.E. decrease B.E. increase The presence or absence of the shake up peaks in a Cu spectrum is diagnostic of the oxidation state chemical shift of this sample is too small 7
N 1s binding energy 8
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2. AES ( Auger electron spectroscopy) in 1925 Primary e X-ray e (Auger) 2p 3/2 2p 1/2 2s ½ L III L II L I Excitation by primary electron (5keV) or X-ray 1s K De-excitation process (competitive) X-ray process Auger process (two electron process) XPS notation Auger notation e - KL I L III hν Kα 1 11
Auger electron energy is characteristic to elements ( Li ~ U) EK LI L = E III K EL E I LIII X-ray energy is also characteristic to elements (Na ~ U) E E K α I Kα 2 = E = E K K E E L L III II K α : L K K β : M K effective probe depth ~ 1 μm, detection limit ~ 0.1% Competitive reaction probability Auger electron emission X-ray fluorescence Li(3) Na(11) As (23) 12
Chart of Auger electron energy vs. atomic number Auger spectra - 1 st derivative of Auger signal 13
Auger electron energy : identification of element on the surface Auger intensity # of elements on the surface Shift of Auger peaks due to the change of binding energy of electrons in the core level chemical shift : chemical environment of element surface chemical bond plasmon loss peak photoelectron or Auger electron passing through a solid can excite the mode of collective oscillation of conduction electron such as plasmon hωb hω s = 1 + ε Dielectric constant hωb :15 ev for Al hω :10 ev for Al s Surface plasmon Bulk plasmon Auger Al E 14
AES detection limit ~ 0.1 1% effective probe depth 5 ~ 15 Å Possibility of depth profiling with sputter ion gun Coster-Kronig transition Energy is transferred between two subshells having the same principal quantum number very fast process E t > Auger parameter by Wagner h ; lifetime broadening ; ΔE ~ 10 ev, Δt =? is a combination of photoelectron and X-ray excited Auger peak position to give for many elements a more sensitive indicator of chemical state than the photoelectron peak energy alone i) It is independent of any surface charging ii) It is often more sensitive to the precise chemical state of an element than the energies of either photoelectron or Auger peaks on their own. 15
α = E K (Auger) E K (photoelectron) = E K ( Auger) + E B ( photoelectron) hυ sometimes α < 0 modified AP α* = α + hυ, α* >0 α* = E K ( Auger) + E B ( photoelectron) 16
3. Surface specificity for AES, XPS Interaction of electron with matter Secondary electron N(E) Primary electron Auger peak 0 E E p 17
Mean free path of electrons in metallic solids as a function of their energy λ : (monolayer) λ 1000 Independent of elements : universal curve 100 10 A λ = + E 2 B E A = 538 B = 0.210 For elements 1 10 100 1000 10000 (ev) E Minimum λ = a few Å at 40 ~ 100 ev The mean free path of created electron ( escape depth ) and not of the primary radiation(electron) is depth determining 18
I d = I d ( 1 λ e ) I d I : Peak intensity from a layer of material of thickness d : Peak intensity from infinitely thick layer for electrons of λ I d of 63 % from sampling depth λ I d of 87 % from sampling depth 2λ I d of 95 % from sampling depth 3λ ( I ) I d 1.0 0.8 0.6 λ should be small enough for secondary electrons to be surface specific λ : escape depth of electron (mean free path in the solid) hν e - (primary) θ e - θ d x 1 2 3 d/λ I d d λ sinθ = I ( 1 e sinθ = d x, x = d ) sinθ 19
three kinds of electron spectroscopy 20
4. SIMS (Secondary ion mass spectrometry) Ion gun Ar + 0.5 ~ 5 kev Surface Mass spectrometer S + /S - Ar + The most complex phenomena QMS (quadrupole mass spectrometer) TOF (time-of-flight) : Neutral atoms S, Sn, secondary electron Auger electron X-ray Surface sensitivity : 10-6 ~ 10-5 (a) collision cascade model by Sigmund(1977) total sputtered yield and energy distribution Good agreement between theory & experiment but charge state of sputtered materials no prediction # of ions E 21
(b) LTE(local thermodynamic equilibrium) model by Anderson(1973) Anal. Chem. 435, 1421 (1973) The surface region involved in the sputtering process a dense plasma in local thermodynamic equilibrium M 0 M + + e + e + n ne K = 0 n 0 : number of neutral particle n From statistical mechanics via the Saha-Eggert equation + + 2 p ( T ) 2 K = p / 0 p ( T ) n K : equilibrium constant : electron concentration ( 2 ) 3/ 2πmekT / h exp[ ( I E) kt ] Secondary ion formation p + (T), p 0 (T) : internal partition functions for ions and neutral atoms I p : the first ionization potential of neutral atom ΔE : reduction in the I p due to Coulomb interaction in the plasma E = e 3 ( 8πn / kt) e 1/ 2 22
The ionization coefficient n n + 0 n 1 e p p + 0 ( T ) T ( T ) 3 2 exp [ ( I E) / kt ] p = f ( n e, T ) ; T = 5 10 3 ~10 5 K From fitting theory to experimental curves: unrealistic (c) The kinetic model (d) The auto-ionizing model 23
Positive and negative secondary ion spectra from a cleaned nickel target 24
5. Applications of AES, XPS, and SIMS in Si microelectronics technology 1 Raw material evaluation 2 Oxidation Si/SiO 2 : SiO x 3 Photo-lithography, patterning, stripping 4 Wet chemical or plasma etching 5 Doping by diffusion or ion implantation 0.1 % 10 19 /cm 3 of surface atom, SIMS is used, 10 22 10-6 = 10 16 /cm 3 6 Metallization (Au/Cr/Si : Cr diffusion barrier) 100% Au Cr Si 100% Au Cr Si Sputter time 300 ºC 2 hrs Sputter time 7 spatial(lateral) distribution of element by SAM(scanning Auger Microscope) - focused electron beam, ~100 A spatial resolution 25