( 1+ A) 2 cos2 θ Incident Ion Techniques for Surface Composition Analysis Ion Scattering Spectroscopy (ISS)

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1 5.16 Incident Ion Techniques for Surface Composition Analysis Ion Scattering Spectroscopy (ISS) At moderate kinetic energies (few hundred ev to few kev) ion scattered from a surface in simple kinematic collision - interaction distance short (<10 Å) - scattered from corrugated surface potential - interaction time short (<1ps) - little electronic energy transfer - ion scattered inelastically by simple momentum transfer Equations based on simple momentum transfer for energy of incident ion (E 0 ), scattered ion (E 1 ) and displaced surface atom (E 2 ) E 1 E 0 = E 2 E 0 = 1 (1 + A) 2 cosθ 1 ± A2 2 sinθ 1 4A ( 1+ A) 2 cos2 θ 2 A = M 2 M 1 >1.0 CEM Spring 2001

2 Since energy must be conserved - E 0 = E 1 + E 2 - unique relation between θ 1 and θ 2 Scattered particle emerges at particular angle with energy dependant only on ratio of masses of incident and scattering particle - technique called ion scattering spectroscopy (ISS) or low energy ion scattering (LEIS) If scattering angle (θ 1 ) is 90 : E 1 E 0 = 1 (1+ A) 2 cosθ 1 ± A2 2 sin θ 1 θ 1 = 90, cosθ 1 = 0, sinθ 1 =1 becomes E 1 = A 1 E 0 A +1 If we use ion such that A>1 (choices: H +, He +, Ne +, Ar +, Kr + ) If E 0, M 1, θ 1 fixed, measurement of E 1 allows unique determination of M 2, mass of scatterer CEM Spring 2001

3 Ion energy in ISS/LEIS 500 ev to 3 kev Measure E 1 with concentric hemispherical analyzer set to detect massive positive particles - reverse potentials on hemispheres Resolution R (ability to separate signals from scatterers of different masses) depends on - monoenergicity of incident ions (< few %) - collimation of incident ion (< few ) - angular acceptance of analyzer (< few ) - large angular acceptance more ions, better S/N but broad ion peaks and poorer resolution Resolving power: ρ = 1 R = M 2 M 2 CEM Spring 2001

4 where M 2 is range of masses indicated by width of scattered ion energy distribution Maximum resolving power if - A is close to unity - scattering angle is large (at least 90 ) Interaction between incident ion/surface atoms large - very surface sensitive (usually considered top atomic layer only) CEM Spring 2001

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6 While energetics of collision depend only on mass, not atomic potentials, cross-section (probability) of scattering at one particular angle depends on atomic potentials of incident and scatterer particle Scattering depends on incident angle - trajectory and "impact parameter" which is sensitive to atomic potentials - size of shadow cone greatest for low energy ions - "head on" collisions give large momentum transfer - produce large change in direction and energy - "grazing" collisions (small θ 1 ) give small momentum transfer - produce smaller changes in direction and energy CEM Spring 2001

7 - possibility of double collisions increases towards grazing incidence - broader scattered ion energies Sputtering In ISS/LEIS surface atom (scatterer) recoils into surface - not accessible for spectroscopy But does produce collisional cascade in near-surface region - may include incident ion - can lead to multiple secondary collisions - may lead to ejection of surface atoms or fragments by backscattered particles in solid CEM Spring 2001

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9 Process of removal of material by ion bombardment called sputtering or ion etching At very high rates of removal called ion beam milling - higher incident beam energies than ISS/LEIS - more massive particles - normal incidence Useful technique for (i) (ii) (iii) surface cleaning depth profiling spectroscopy through fragment analysis CEM Spring 2001

10 Surface Cleaning Various ways to "clean" material and expose virgin surface - cleaving - electrochemical/chemical/mechanical polishing - heating in vacuum/gas environment - sputtering Sputtering very popular but - leaves embedded incident particle in solid - substantial damage and interlayer mixing - preferential sputtering of one component S = # atoms sputtered # incident ions Sputtering Yield Sputtering is quite efficient, S>1 for Ar + of few kev - energy of sputtered atoms is low (1 kev in, ev out) CEM Spring 2001

11 - energy dependence of S, S(E) peaks at kev depending upon material - sputtering yields also vary with incident particle mass - incident ion type CEM Spring 2001

12 - sputtering yields vary considerably between elements (for fixed incident particle and energy) - depends on cohesive energy U ( heat of sublimation) - sputtering yield varies for element in different matrices - elements, oxides CEM Spring 2001

13 - sputtering yield varies as ~1/cosθ for moderate incidence angles (more energy concentrated at surface) but falls off at very grazing incidence (scattering dominates, no penetration) - sputtering in elements best understood - sputtering in single crystals, complex materials less wellunderstood - quantitation of sputtering or sputtering rate difficult because of large number of variables Depth Profiling When used with other techniques (AES, XPS), sputtering can sequentially remove layers of material and build-up depth distribution of elements CEM Spring 2001

14 - straightforward to calibrate sputtering rate (Å min -1 ) for polycrystalline elements - must use standards for complex materials But differential sputtering in alloys, compounds leads to gradual modification of elemental composition with prolonged sputtering - XPS, AES will initially show true concentrations but change to new equilibrium value CEM Spring 2001

15 - Mixing, atom implantation and non-uniform ion beam limit depth resolution (resolution decreases with sputtering time) CEM Spring 2001

16 5.17 Analysis of Sputtered Particles Bombard surface with high energy ions and desorb ions (<1 %) and neutrals (> 99%). Measure with a mass spectrometer Neutrals can be ionized postirradiation by electrons (EI) or photons (laser ionization) in technique called secondary neutral mass spectrometry (SNMS) For EI SNMS, ionization efficiency ~10-6 ionize ~ 10-4 % of sample For laser ionization SNMS efficiency up to 1 ionize ~ 99 % of sample Alternatively, use small fraction of ions produced during desorption (< 1%) - technique called secondary ion mass spectrometry (SIMS) CEM Spring 2001

17 Secondary Ion Mass Spectrometry (SIMS) Originally, a method developed to apply the high sensitivity/selectivity of mass spectrometry to the analysis of solids Later realized that SIMS ions originate from top one or two atomic layers - surface sensitive Primary information from ion signal can provide information on (1) Quantitative chemical composition (mass spectrum of ions) (2) Structural information (cracking pattern as conditions varied) Positive SIMS (cation) spectrum dominated by electropositive atom ions Negative SIMS (anion) spectrum dominated by electronegative atom ions spectra contain complimentary information CEM Spring 2001

18 Characteristics of SIMS spectra - small clusters abundant - no large clusters - no multiply-charged clusters - few multiply-charged ions - sensitive to isotopic abundance - often observe (K + ), Na +, Cl -, F - But SIMS ions yield depends on surface concentration and sputtering yield - what about differential sputtering in complex materials? Example: Compound with bulk composition A 0.5 B 0.5 but with sputtering yields S A = 1, S B = 3 At time t = 0 AES or XPS analysis = A 0.5 B 0.5 (correct bulk composition) SIMS analysis = A 0.75 B 0.25 (incorrect bulk composition) CEM Spring 2001

19 At time t > 0 New equilibrium surface concentration established in ratio of sputtering yields (c A S A = c B S B ) AES or XPS analysis = A 0.75 B 0.25 (incorrect bulk composition) SIMS analysis = A 0.5 B 0.5 (correct bulk composition) After short sputtering period, SIMS is unaffected by differential sputtering yields! Static versus Dynamic SIMS Two extremes: Static SIMS Dynamic SIMS Low sputter rates High sputter rates ~1 na cm -2 Up to 10 ma cm -2 <10 Å hr -1 Up to 100 µ hr -1 Essentially "non-destructive" Surface analysis Destructive Depth profiling Static and dynamic SIMS are divided at the static SIMS limit of damage Incident ions can cause chemistry and interlayer mixing Generally limit incident ion to < 1% of surface species sputtered (< 1% ML) If 1 ML = atoms cm -2 CEM Spring 2001

20 Ion dose < ions cm -2 1 na cm 2 = 1x10 9 C s cm 2 1x10 9 ions = 1.6x10 19 s cm 2 = 6.25x10 9 ions s 1 cm 2 or static SIMS limit reached in 1600 s or 25 mins In fact, have good sensitivity in SSIMS at much less than 1 % ML Static SIMS used to "fingerprint" polymers and biological adsorbates CEM Spring 2001

21 In case of polymers, mass spectrum is related to average MW and polydispersity CEM Spring 2001

22 SSIMS ion yields dependent on "chemical environment" of surface atoms Static SIMS can also be used to study adsorbed molecules: CEM Spring 2001

23 Change in ion yield at 250 K implies CO molecule dissociates to adsorbed O and C at this temperature Is there any structural information in SIMS? CEM Spring 2001

24 Azimuthal dependence of Cu + and O - emission from Cu(100)-c(2x2)-O suggests O lies in four-fold hollow Evidence not tested - incident particles very damaging Spatial Information from SIMS Obtained in two ways: (1) scan ion beam (raster) across surface - ion microprobe technique (2) scan ion collection optics across surface uniformly irradiated - ion microscopy technique Can focus high energy ion beam to <10 Å diameter but SIMS emission occurs from Å outside impact area - typical minimum resolution ~100 nm CEM Spring 2001

25 Au ore contaminated with pyrite (FeS 2 ) 5 mm Ti bars on Si wafer Positive TOF-SIMS of Al microcontacts deposited onto GaAs showing defect presence CEM Spring 2001

26 Quantitation of SSIMS + I m = I p S m α + θ m T where I + m = secondary ion emission current I p = primary (incident) ion current S m = sputtering yield of element m (ions and neutrals) α + = proportion of positive ions θ m = fractional coverage of monolayer by element m T = transmission function of mass analyzer Absolute quantitation of SIMS very difficult, largely because of uncertainties in sputtering yields, ionization probabilities and mass spectrometer sensitivity to KE of sputtered particle Is possible with sensitivity factors developed from standards Generally not performed Instrumentation for SIMS CEM Spring 2001

27 Energy filter necessary to "normalize" ion energy distributions - ions emerge from surface with different KE's Ion sources: Use electronegative incident ion to increase emission of positive species (+ve SIMS) O 2 + for metal analysis - but reacts with surface? Use electropositive incident ion to increases emission of negative species (-ve SIMS) Cs +, Ga +, In + for inorganics (F -, Cl -, O - ) - liquid metal ion sources (i) electron beam crossing noble gas or H beam - ions extracted and focussed electrostatically - not very bright or finely focussed but rugged (ii) RF/microwave plasma operated in high pressure noble gas - Bright, focussed but gradually decreases in brightness - can use for O 2 + beam CEM Spring 2001

28 (iii) surface ionization by spontaneous emission of ions from heated surface (difference in work function important) - bright, works for low IP metals - Cs + - fragile and difficult to operate (iv) field ionization liquid metal ion sources rely on ionization in high field from liquid metal skin on fine needle - brightest, most focusable beam - difficult to operate All sources can be pulsed (beam blanking) by sweeping deflection potential so beam passes across small aperture Mass analyzers: (i) Magnetic sector relies on deflection of charged particle in fixed magnetic field by varying accelerating potential V - ion radius is R = 1 B 2 V m z 0.5 CEM Spring 2001

29 - high transmission (up to 50 % of ions), can be used with position sensitive detectors (ion microscope) - bulky, poor maximum m/z, cannot be baked (ii) Quadrupole analyzers apply fixed and RF-varying DC fields to drive ions of one m/z into stable (helical) trajectories - small, can incorporate EI source for RGA - poor transmission (< 1% ions), poor maximum m/z, low resolution (iii) Time-of-flight (TOF) analyzers measure "flight time" of ions with same KE as they drift along tube: high m/z ions travel slower m t = L 2 z V - needs short pulsed ion source CEM Spring 2001

30 - "infinite" mass range, practically 10,000 Da - high transmission (up to 50 %), high resolution, parallel detection of all m/z's - "Reflectron" design eliminates differences in TOF for ions of same m/z but different initial KE - need accurately pulsed, short ion pulse Ion detectors: (i) Microchannel plate (MCP) emits electrons when struck by ion. Multiple collisions produce secondary electron cascade that is collected by positivelycharged anode CEM Spring 2001

31 5.18 Summary Excellent SIMS sensitivity (<10 9 atoms cm -2 ) <10-4 ML for some elements Excellent SNMS sensitivity and chemical selectivity for laser ionization (not EI) Adaptable to imaging (microprobe or microscope modes) with good resolution (<0.1 µm) Used in both depth profiling (dynamic SIMS) and "non-destructive" (static SIMS) modes Isotope sensitivity - labeled surfaces? ISS and SSIMS sensitive to top atomic layer only Rich spectra for organics - biological samples, polymers - spectra are fingerprints for adsorbed species Semiquantitative if standardized Some information about bonding geometry? Works for insulators or conductors BUT Absolute quantification extremely difficult (matrix effects on sputtering yields and ionization mechanisms poorly understood) Difficult to model Rich spectra difficult to rationalize Interlayer mixing TOF-SIMS expensive ($75,000+) CEM Spring 2001