CEM 924 12.1 Spring 2001

5.16 Incident Ion Techniques for Surface Composition Analysis

5.16.1 Ion Scattering Spectroscopy (ISS)

At moderate kinetic energies (few hundred eV to few keV) ion scattered froma 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 (E0),scattered ion (E1) and displaced surface atom (E2)

E1

E0= 1

(1 + A)2cosθ1 ± A2 − sinθ1

2A =

M2

M1>1.0

E2E0

= 4A

1+ A( )2⋅cos2 θ2

CEM 924 12.2 Spring 2001

Since energy must be conserved

- E0 = E1 + E2

- unique relation between θ1 and θ2

Scattered particle emerges at particular angle with energy dependant only onratio of masses of incident and scattering particle

- technique called ion scattering spectroscopy (ISS) or low energyion scattering (LEIS)

If scattering angle (θ1) is 90°:

E1E0

= 1

(1+ A)2cosθ1 ± A2 − sin θ1

2

θ1 = 90°, cosθ1 = 0, sinθ1 =1

becomes

E1E0

= A −1A +1

If we use ion such that A>1 (choices: H+, He+, Ne+, Ar+, Kr+)

If E0, M1, θ1 fixed, measurement of E1 allows unique determination of M2,mass of scatterer

CEM 924 12.3 Spring 2001

Ion energy in ISS/LEIS 500 eV to 3 keV

Measure E1 with concentric hemispherical analyzer set to detect massivepositive 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 ionpeaks and poorer resolution

Resolving power:

ρ =

1

R=

M2∆M2

CEM 924 12.4 Spring 2001

where ∆M2 is range of masses indicated by width of scattered ion energydistribution

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 924 12.5 Spring 2001

CEM 924 12.6 Spring 2001

While energetics of collision depend only on mass, not atomic potentials,cross-section (probability) of scattering at one particular angle depends onatomic 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 largechange in direction and energy

- "grazing" collisions (small θ1) give small momentum transfer -produce smaller changes in direction and energy

CEM 924 12.7 Spring 2001

- possibility of double collisions increases towards grazing incidence- broader scattered ion energies

5.16.2 Sputtering

In ISS/LEIS surface atom (scatterer) recoils into surface - not accessible forspectroscopy

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 backscatteredparticles in solid

CEM 924 12.8 Spring 2001

CEM 924 12.9 Spring 2001

Process of removal of material by ion bombardment called sputtering or ionetching

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) surface cleaning

(ii) depth profiling

(iii) spectroscopy through fragment analysis

CEM 924 12.10 Spring 2001

5.16.3 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 ionsSputtering Yield

Sputtering is quite efficient, S>1 for Ar+ of few keV

- energy of sputtered atoms is low (1 keV in, 10-20 eV out)

CEM 924 12.11 Spring 2001

- energy dependence of S, S(E) peaks at 10-30 keV depending uponmaterial

- sputtering yields also vary with incident particle mass - incident iontype

CEM 924 12.12 Spring 2001

- sputtering yields vary considerably between elements (for fixedincident particle and energy) - depends on cohesive energy U (≈heat of sublimation)

- sputtering yield varies for element in different matrices - elements,oxides

CEM 924 12.13 Spring 2001

- sputtering yield varies as ~1/cosθ for moderate incidence angles(more energy concentrated at surface) but falls off at very grazingincidence (scattering dominates, no penetration)

- sputtering in elements best understood

- sputtering in single crystals, complex materials less well-understood

- quantitation of sputtering or sputtering rate difficult because oflarge number of variables

5.16.4 Depth Profiling

When used with other techniques (AES, XPS), sputtering can sequentiallyremove layers of material and build-up depth distribution of elements

CEM 924 12.14 Spring 2001

- straightforward to calibrate sputtering rate (Å·min-1) forpolycrystalline elements

- must use standards for complex materials

But differential sputtering in alloys, compounds leads to gradual modificationof elemental composition with prolonged sputtering

- XPS, AES will initially show true concentrations but change to newequilibrium value

CEM 924 12.15 Spring 2001

- Mixing, atom implantation and non-uniform ion beam limit depthresolution (resolution decreases with sputtering time)

CEM 924 12.16 Spring 2001

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 (laserionization) 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 924 12.17 Spring 2001

5.17.1 Secondary Ion Mass Spectrometry (SIMS)

Originally, a method developed to apply the high sensitivity/selectivity of massspectrometry 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 924 12.18 Spring 2001

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 A0.5B0.5 but with sputtering yieldsSA = 1, SB = 3

At time t = 0

AES or XPS analysis = A0.5B0.5 (correct bulk composition)

SIMS analysis = A0.75B0.25 (incorrect bulk composition)

CEM 924 12.19 Spring 2001

At time t > 0

New equilibrium surface concentration established in ratio of sputteringyields (cA·SA = cB·SB)

AES or XPS analysis = A0.75B0.25 (incorrect bulk composition)

SIMS analysis = A0.5B0.5 (correct bulk composition)

After short sputtering period, SIMS is unaffected by differential sputteringyields!

5.17.2 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" Destructive

Surface analysis 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 = 1015 atoms·cm-2

CEM 924 12.20 Spring 2001

Ion dose < 1013 ions·cm-2

1 nA⋅cm−2 = 1x10 −9 C

s ⋅cm2

=1x10−9 ions

1.6x10 −19 s ⋅cm2

= 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 924 12.21 Spring 2001

In case of polymers, mass spectrum is related to average MW andpolydispersity

CEM 924 12.22 Spring 2001

SSIMS ion yields dependent on "chemical environment" of surface atoms

Static SIMS can also be used to study adsorbed molecules:

CEM 924 12.23 Spring 2001

Change in ion yield at 250 K implies CO molecule dissociates to adsorbed Oand C at this temperature

Is there any structural information in SIMS?

CEM 924 12.24 Spring 2001

Azimuthal dependence of Cu+ and O- emission from Cu(100)-c(2x2)-Osuggests O lies in four-fold hollow

Evidence not tested - incident particles very damaging

5.17.3 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 - ionmicroscopy technique

Can focus high energy ion beam to <10 Å diameter but SIMS emission occursfrom 50-200 Å outside impact area

- typical minimum resolution ~100 nm

CEM 924 12.25 Spring 2001

↑ Au ore contaminated with pyrite (FeS2)

↑ 5 mm Ti bars on Si wafer

↑ Positive TOF-SIMS of Al microcontacts deposited onto GaAs showing defect presence

CEM 924 12.26 Spring 2001

5.17.4 Quantitation of SSIMS

I m+ = I p ⋅Sm ⋅ α+ ⋅θ m ⋅T

where

Im+ = secondary ion emission current

Ip = primary (incident) ion current

Sm = sputtering yield of element m (ions and neutrals)

α+ = proportion of positive ions

θm = fractional coverage of monolayer by element m

T = transmission function of mass analyzer

Absolute quantitation of SIMS very difficult, largely because of uncertaintiesin sputtering yields, ionization probabilities and mass spectrometer sensitivityto KE of sputtered particle

Is possible with sensitivity factors developed from standards

Generally not performed

5.17.4 Instrumentation for SIMS

CEM 924 12.27 Spring 2001

Energy filter necessary to "normalize" ion energy distributions - ions emergefrom surface with different KE's

Ion sources:

Use electronegative incident ion to increase emission of positive species (+veSIMS)

O2+ for metal analysis - but reacts with surface?

Use electropositive incident ion to increases emission of negative species (-veSIMS)

Cs+, Ga+, In+ for inorganics (F-, Cl-, O-) - liquid metal ion sources

(i) electron beam crossing noble gas or H beam - ions extracted and focussedelectrostatically

- 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 O2+ beam

CEM 924 12.28 Spring 2001

(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 fieldfrom liquid metal skin on fine needle

- brightest, most focusable beam

- difficult to operate

All sources can be pulsed (beam blanking) by sweeping deflection potential sobeam passes across small aperture

Mass analyzers:

(i) Magnetic sector relies on deflection of charged particle in fixed magneticfield by varying accelerating potential V

- ion radius is R = 1

B2 ⋅V ⋅m

z

0.5

CEM 924 12.29 Spring 2001

- high transmission (up to 50 % of ions), can be used with positionsensitive detectors (ion microscope)

- bulky, poor maximum m/z, cannot be baked

(ii) Quadrupole analyzers apply fixed and RF-varying DC fields to drive ionsof 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 sameKE as they drift along tube: high m/z ions travel slower

- t = L

m2 ⋅z⋅V

0.5

- needs short pulsed ion source

CEM 924 12.30 Spring 2001

- "infinite" mass range, practically 10,000 Da

- high transmission (up to 50 %), high resolution, parallel detectionof all m/z's

- "Reflectron" design eliminates differences in TOF for ions of samem/z but different initial KE

- need accurately pulsed, short ion pulse

Ion detectors:

(i) Microchannel plate (MCP) emits electrons when struck by ion. Multiplecollisions produce secondary electron cascade that is collected by positively-charged anode

CEM 924 12.31 Spring 2001

5.18 Summary

Excellent SIMS sensitivity (<109 atoms·cm-2) <10-4 ML for some elements

Excellent SNMS sensitivity and chemical selectivity for laser ionization (notEI)

Adaptable to imaging (microprobe or microscope modes) with good resolution(<0.1 µm)

Used in both depth profiling (dynamic SIMS) and "non-destructive" (staticSIMS) modes

Isotope sensitivity - labeled surfaces?

ISS and SSIMS sensitive to top atomic layer only

Rich spectra for organics - biological samples, polymers - spectra arefingerprints 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 yieldsand ionization mechanisms poorly understood)

Difficult to model

Rich spectra difficult to rationalize

Interlayer mixing

TOF-SIMS expensive ($75,000+)