CMP Seminar MSU 10/18/ 2000CMP Seminar MSU 10/18/ 200011
What makes Surface Science “surface” science ?
What makes Surface Science “surface” science ?
R. J. Smith R. J. Smith Physics Department, Montana State Univ.Physics Department, Montana State Univ.
Work supported by NSF (DMR) Work supported by NSF (DMR) http://www.physics.montana.eduhttp://www.physics.montana.edu
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Outline
Motivation for surface sensitivity - thin film devicesMotivation for surface sensitivity - thin film devices General comments on surface sensitive techniquesGeneral comments on surface sensitive techniques Electron spectroscopy Electron spectroscopy
define the attenuation length (AL)define the attenuation length (AL) empiracle results for attenuation lengthempiracle results for attenuation length simple model for overall shape of ALsimple model for overall shape of AL
Recent analyses based on attenuation lengthsRecent analyses based on attenuation lengths
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Metal-metal Interface Structure
Understand overlayer growth and alloy formationUnderstand overlayer growth and alloy formation Chemical composition and structure of the interfaceChemical composition and structure of the interface Applications: magnetoresistive devices, spin electronicsApplications: magnetoresistive devices, spin electronics
Surface energy (broken bonds)Surface energy (broken bonds)
Chemical formation energyChemical formation energy
Strain energyStrain energy
A
B0int AB
energyformation ABBA
energystrain )()( equilobs dEdE
interface
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Metal-metal systems studied...
Substrates: Al(111), Al(100), Al(110)Substrates: Al(111), Al(100), Al(110) Metal overlayers studied so far:Metal overlayers studied so far:
Fe, Ni, Co, Pd (atomic size smaller than Al)Fe, Ni, Co, Pd (atomic size smaller than Al) Ti, Ag, Zr (atomic size larger than Al)Ti, Ag, Zr (atomic size larger than Al)
All have surface energy > Al surface energyAll have surface energy > Al surface energy All form Al compounds with All form Al compounds with HHformform < 0 < 0
Use resistively heated wires ( ~ML/min)Use resistively heated wires ( ~ML/min) Deposit on substrate at room temperatureDeposit on substrate at room temperature
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Comments on surface sensitive techniques...
Sensitivity to the surface is intrinsic to the Sensitivity to the surface is intrinsic to the technique - not based only on probing depthtechnique - not based only on probing depth
Electron spectroscopy: Distance traveled by Electron spectroscopy: Distance traveled by the electron before losing characteristic the electron before losing characteristic information (Attenuation length) is shortinformation (Attenuation length) is short
Low energy ions - neutralization and strong Low energy ions - neutralization and strong Coulomb interationCoulomb interation
High energy ions - geometric shadowingHigh energy ions - geometric shadowing
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Define the attenuation length
Measure decrease in beam intensity dI(x) after Measure decrease in beam intensity dI(x) after transmission through a film of thickness dxtransmission through a film of thickness dx
1( )
cos
dxdI x I
/( cos )( ) e xoI x I I
dx
I(x) < I
86% of the signal originates from depth of 286% of the signal originates from depth of 2 What is value of What is value of ? What determines this value?? What determines this value?
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Other discussion…and surprises
Idzerda, Journal Vac. Sci Technol A7(3), 1341 (1989)
S. Ossicini, et al. JVST A3, 387 (1985) - growth models S. Tanuma, et al. JVST A8, 2213 (1990) - attenuation lengths
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Observations of Attenuation Length for electrons in solids
Depends on Depends on electron kinetic electron kinetic energy (KE)energy (KE)
KE depends on KE depends on parameters of the parameters of the technique:Auger, technique:Auger, XPS, SEMXPS, SEM
Varies with Varies with materialsmaterials EEAL 54.0/1430 2
F
F
E
EEEdEEdEEEEEW
dEEEP
)'(')'(]''))'(''()''([
')',(
E
E’E’’+(E-E’)
E’’
EFFilled e-
states
Empty e-states
Low Energies: 1 eV < E < 100 eV
E = Kinetic Energy; Elastic Scattering
Transition rate (like Golden Rule)
W=(Transition matrix element)2
=density of filled or empty states
Total scattering probability/sec
[1/]E
EFdEEEPEP ')',()(
E
E’E’’+(E-E’)
E’’
EFfilledstates
emptystates
To go further might assume free-electron form for the density of states
Attenuation Length at low energies
2 2( ) ( / 8)oP E N A E
)()( 0EEAE
Distance between collisions is then
gv 1g
Ev
k
2 2
2
kE
m
Simple model (for homework) 0( '')E N
3
2( )const E
No
E
E’E’’+(E-E’)
E’’
EFFilled e-
states
Empty e-states
High Energies: E > few 100 eV
Consider scattering of point charges, i.e. Coulomb Scattering
Unscreened Coulomb Potential has cross section ~ 1/E2
21~ E
N
•For small E phonon scattering may dominate
•Insulators and semiconductors have energy gap so have long AL as approach twice the gap energy
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Ion scattering chamber Ion scattering chamber
High precision High precision sample goniometersample goniometer
Hemispherical VSW Hemispherical VSW analyzer (XPS, ISS)analyzer (XPS, ISS)
Ion and x-ray sourcesIon and x-ray sources LEEDLEED Metal wires for film Metal wires for film
depositiondeposition
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FM growth:layer-by-layerFM growth:layer-by-layer
Non-linear Non-linear growth curvesgrowth curves
Rapid attenuation Rapid attenuation of substrate signalof substrate signal
Breaks in slopeBreaks in slope =6,6 (green) =6,6 (green)
=20,20 (red)=20,20 (red)
substrate
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VW mode: Island growthVW mode: Island growth
Linear growth in Linear growth in signalssignals
Relatively slow Relatively slow decrease in decrease in substrate signalsubstrate signal
=6,6 (green) =6,6 (green) =20,20 (red)=20,20 (red)
substrate
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Co on Al (100): He+ backscatteringCo on Al (100): He+ backscattering
Ion channeling Ion channeling
Only near-surface Only near-surface
Al atoms are visible Al atoms are visible
to the ion beamto the ion beam
Increase of Al peak Increase of Al peak
means Co causes Al means Co causes Al
atoms to move off atoms to move off
lattice siteslattice sites
Coverage from Co Coverage from Co
peak area (RBS)peak area (RBS)
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Co on Al (100): HEIS intensitiesCo on Al (100): HEIS intensities
HEIS Al surface peak HEIS Al surface peak
vs. Co coverage vs. Co coverage
Number of visible Al Number of visible Al
increases up to 3 MLincreases up to 3 ML
Slope of 2:1 suggests Slope of 2:1 suggests
stoichiometry Alstoichiometry Al22Co Co
for the interface but for the interface but
AlAl22Co not in phase Co not in phase
diagramdiagram
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Co on Al (100): XPS intensitiesCo on Al (100): XPS intensities
Measured (o ,Measured (o , ) ) Simulation (lines)Simulation (lines)
Use HEIS for Use HEIS for coveragecoverage
0-3: layered CoAl 0-3: layered CoAl 3-10: Co islands3-10: Co islands CoAl particle CoAl particle
density is ~ 1.4 x density is ~ 1.4 x Al density Al density
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LEED patterns for Co on Al(100)LEED patterns for Co on Al(100)
LEED at 42.8 eV LEED at 42.8 eV (a) Clean Al(001)(a) Clean Al(001) (b) 0.5 ML Co (b) 0.5 ML Co
destroys pattern destroys pattern completelycompletely
(c) 7.6 ML A hint (c) 7.6 ML A hint of some long range of some long range order (1x1) is seen.order (1x1) is seen.
Co coverage from Co coverage from RBSRBS
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Co on Al(110): HEIS intensitiesCo on Al(110): HEIS intensities
Number of visible Al Number of visible Al
increases to 5 MLincreases to 5 ML
Slope 2.3:1, suggests Slope 2.3:1, suggests
stoichiometry of Alstoichiometry of Al22Co or Co or
AlAl55CoCo22
AlAl55CoCo22 exists in bulk phase exists in bulk phase
diagram, but crystal diagram, but crystal
structure is complex so structure is complex so
less likely to form at 30 less likely to form at 30 ooC. C.
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Co on Al(110): XPS intensitiesCo on Al(110): XPS intensities
Measured (o ,Measured (o , ) ) Simulation (lines)Simulation (lines)
Use HEIS for Use HEIS for coverage (change coverage (change at 5 ML)at 5 ML)
0-5: layered CoAl 0-5: layered CoAl growth growth
3-10: layered Co 3-10: layered Co metal growthmetal growth
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Comparison of XPS Intensities for Al(100) and Al(110)Comparison of XPS Intensities for Al(100) and Al(110)
Contract (110) Contract (110) coverage to coverage to (100) MLs(100) MLs
(100) Co(100) Co
(110) Co(110) Co
(100) Al(100) Al
(110) Al(110) Al
Co overlapCo overlap Al don’t !Al don’t !
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Snapshots from MC simulationsSnapshots from MC simulations
Al(110)+0.5 ML Ni Al(110)+0.5 ML Ni Clean Al(110)Clean Al(110) Al(110)+2.0 ML Ni Al(110)+2.0 ML Ni
MC (total energy) using EAM potentials for Ni, Al (Voter)MC (total energy) using EAM potentials for Ni, Al (Voter) Equilibrate then add Ni in 0.5 ML increments (solid circles)Equilibrate then add Ni in 0.5 ML increments (solid circles) Ion scattering simulations (VEGAS)Ion scattering simulations (VEGAS)
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Ion scattering simulations using VEGAS and the MC snapshotsIon scattering simulations using VEGAS and the MC snapshots
Measured (o) Measured (o) Simulation (Simulation ())
Slopes agreeSlopes agree Change of slope Change of slope
at 2 ML correct at 2 ML correct Good agreement Good agreement
so use snapshots so use snapshots for more insight for more insight
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XPS chemical shifts for Ni 2pXPS chemical shifts for Ni 2p
Shifts in BEShifts in BE Shifts in satelliteShifts in satellite Compare with XPS for Compare with XPS for
bulk alloys bulk alloys
(BE) (sat)(BE) (sat)NiAlNiAl33 1.05eV 1.05eV
NiNi22Al 0.75eV (8.0 eV)Al 0.75eV (8.0 eV)
NiAl 0.2 eV (7.2 eV)NiAl 0.2 eV (7.2 eV)
NiNi33Al 0.0 eV (6.5 eV)Al 0.0 eV (6.5 eV)
Ni 0.0 eV (5.8 eV)Ni 0.0 eV (5.8 eV)
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Simulated XPS intensity for Ni using EAM snapshotsSimulated XPS intensity for Ni using EAM snapshots
Ni coverage from Ni coverage from RBS RBS
Fit using model Fit using model with exponential with exponential attenuationattenuation
See a change in See a change in slope for all slope for all values of values of
Best fit:Best fit:NiNi= 5.2Å= 5.2Å
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XPS: Comparison of Calculated and Measured Al Intensities XPS: Comparison of Calculated and Measured Al Intensities
XPS intensity vs XPS intensity vs Ni coverageNi coverage
Best agreement Best agreement with data for with data for NiNi
= 5.2 Å = 5.2 Å AlAl = =
15 Å15 Å Universal curve Universal curve
NiNi = 13.5 Å = 13.5 Å
AlAl = 20.2 Å = 20.2 Å
Equilibrium?Equilibrium?
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SummarySummary
Surface sensitivity associated with short attenuation Surface sensitivity associated with short attenuation length for electrons in solidslength for electrons in solids
Long AL at low energy associated with decreased Long AL at low energy associated with decreased availability of final states for scatteringavailability of final states for scattering
Long AL at high energy associated with decreasing Long AL at high energy associated with decreasing scattering cross section for point chargesscattering cross section for point charges
Minimum AL for KE ~ 150 eVMinimum AL for KE ~ 150 eV Modeling of film morphology can be helpful for Modeling of film morphology can be helpful for
complex interface and alloy formationcomplex interface and alloy formation
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XPS: Comparison of Calculated and Measured Ni 2p Intensities XPS: Comparison of Calculated and Measured Ni 2p Intensities
XPS intensity vs XPS intensity vs Ni coverageNi coverage
Best agreement Best agreement with data for with data for NiNi
= 5.2 Å = 5.2 Å AlAl = =
15 Å15 Å Universal curve Universal curve
NiNi = 13.5 Å = 13.5 Å
AlAl = 20.2 Å = 20.2 Å
Equilibrium?Equilibrium?