High-Sensitivity LEIS Principles & Applications
Transcript of High-Sensitivity LEIS Principles & Applications
ION-TOF GmbH
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Lehigh University
November 06,, 2014
High-Sensitivity LEIS
Principles & Applications
Hidde Brongersma TU/e, Imperial College, ION-TOF
1
Introduction to LEIS
Miscellaneous Applications
Imaging, NPs, segregation, surface modification, anti-wetting, NPs
Outer surface oxides
SOFC, membranes
Growth
Summary
Outline
Low Energy Ion Scattering
Ef (eV)
Energy spectrum of an
alloy
0 1000 2000 3000
S (
a.u
. ) 192Ir
103Rh
55Mn
51V
59Co 39K
Atomic composition of outermost
atomic layer
Energy 1 – 8 keV
Lateral resolution 0.01 – 1 mm
In-depth (0 – 10 nm)
No matrix effects
Characteristics of the technique
Example: 3 keV Ne alloy
Energy
LE
IS S
ign
al
θ
3He+, 4He+, Ne+, Ar+
1- 8 keV
Detector for Energy of Ions
1 3
2
1 2 3
Studying the outermost surface of any material
LEIS Enables Simple Quantification
Example: Coverage of Bromine adsorbed on Tungsten
100
200
300
400
500
600
0
0 100 200 50 150
SBr ( a.u. )
SW
(
a.u
.)
LEIS ‘sees’ only 1st atomic layer
No matrix effect !
θ w = 1
θ Br = 1
Bromine
Tungsten
Bromine
Tungsten
Bromine
Bromine Tungsten
LEIS can quantify the surface coverage θ in the 1st atomic layer
θ W + θ Br = 1
Noble gas ion source
Pulsing system
Position sensitive detector
Energy analyser
Focussing optics
Large acceptance angle ( 360o )
Parallel energy detection
only low ion fluence needed
STATIC LEIS
“ Analysis before Damage ”
“ Hit same place only once “
( Core/shell nanoclusters ! )
ToF filtering eliminates SIMS ions
Qtac: Unique new Analyser High – Sensitivity LEIS
Sample
LEIS Technique
Features of Low Energy Ion Scattering (LEIS)
He+, Ne+, Ar+, Kr+
1 - 8 keV
Reviews
Brongersma et al., Surf. Sci. Rep. 62 (2007) 63
H.H. Brongersma, Low-Energy Ion Scattering in: Characterization of
Materials, Ed. E.N. Kaufmann, pp. 2024-2044, Wiley (2012).
LEIS Features for elemental surface analysis:
Ultra-high surface sensitivity, top atomic layer
Reliable and straight-forward quantification
Non destructive (static) analysis
Detection limits:
Li - O ≥ 1 % of 1 ML
F - Cl 1 % - 0.05 % of 1 ML
K - U 500 ppm - 10 ppm of 1 ML
Static depth profiling (up to 10 nm)
LE
IS s
ignal
0.75 0.80 0.85 0.90 0.95
Pd
Pt
Ef / Ei
0.75 0.80 0.85 0.90 0.95
Pt
Pd
LE
IS s
ignal
Ef / Ei
4He 10,000 nC 4He 5.4 nC
NIM B 68 (1992) 207
Conventional LEIS High-Sensitivity LEIS
Promoters visible, but removed from spectrum
Conventional LEIS vs HS - LEIS
Pd / Pt-C ( 1000 m2/g )
Sample Cleaning
Atom Source for O- and H-atoms
O2 or H2
O or H atoms
Sample
CO CO2
H2O
CxHyOz
O atoms remove organics, coke
Chemical energy: no sputtering
Two possibilities:
1. Static LEIS + Sputter depth profiling with dual ion beam
( advantage of quantification, depth resolution LEIS)
2. Static LEIS (non destructive) for heavier elements
(analogous to RBS and MEIS, but better depth resolution)
In – Depth profiling
Depth info; non-destructive
Si
ZrO2
He+
Ef
3 keV
Heo
Zrsurf
Zrbck
Osurf
Sibck
Sisurf Fin
al E
ne
rgy
Depth resolution better for lower primary E !
He+
∆E
160 eV /nm
1st atom and Static Depth Profile
ZrO2 Atomic Layer Deposition on Silicon
Detection / quantification pinholes (still present after 70 cycles)
Thickness distribution ZrO2 layer (160 eV/nm)
No matrix effect
Example: calibration / quantification for a 2 component system
70 cycles
2 4
1 %
10 ppm
0.01 %
Depth Resolved
LEIS
SIMS
AES XPS
0.1 1 10 100
Information Depth ( Atomic Layers )
100 %
10 %
0.1 %
1 ppm
D
ete
ctio
n R
an
ge
Detection range vs Information Depth
for AES, LEIS, SIMS and XPS
Imaging
Surface segregation, anti wetting
Interdiffusion
Surface modification
Nano particles
Miscellaneous Applications
Surface Imaging
Lateral Resolution and Sensitivity required
Field of view: 2 mm
Ti
C
u
Imaging in surface analysis:
Probe or detector needs sufficient lateral resolution
Data rate has to be high enough (acquisition time)
Detection must be sensitive enough
sufficient information from each pixel before damage
detection sensitivity “useful lateral resolution”
With new ion source
lateral resolution
~ 10 um
Surface segregation: universal process
Example: Acrylonitrile-Butadiene-Styrene (ABS)
0 1000 2000 30000
1
2
3
4
ZnCa
O
N
LE
IS S
ignal (C
ts/n
C)
Energy (eV)
C
Na
Pb
Anti wetting
Fluorine density vs surface energy
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.45
10
15
20
25
30
35
40
45
Su
rfa
ce
en
erg
y (
mN
/m)
Fluorine density (*1015
/cm2)
P(MMA-co-F1MA) P(MMA-co-F6MA) P(MMA-co-F10MA) C
8F
17-PMMA/PMMA blend
Metal - polymer interface in ultra - thin layers
PLED: Ba evaporation on PPV
LE
IS S
ign
al (C
ts/n
C)
1500 2000 2500 3000
0
2
4
6
8
10
12
7 nm
During evaporation of
barium on PPV, most of
the Ba diffuses into the
PPV.
Compare the peakshape
of a sub-monolayer of Ba
(blue) with the actual
peak (red)
Peak shape ↔ depth
distribution
Measured
Ba for < 1 ML
Energy (eV)
PLED: higher light output
for narrow depth distribution
Surface modification
Aging of plasma oxidized HDPE
Aging (LEIS) faster than aging (XPS) !
“Straight line” diffusion process
Time1/2 (min)
1 .2 5
1 .7 5
2 .2 5
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0
Tim e½
log
(O s
ign
al)
0 .2 5
0 .7 5
1 .2 5
log
(O i
n a
t%)
XP S
L E IS
Lo
g(O
in
at%
)
XPS
LEIS
Nano particles (supported catalyst)
Average size
Core / Shell
TEM:
excellent catalyst characterisation
detailed info, but local
contrast required ( high Z cluster on low Z support )
Chemisorption:
requires known probe / surface interaction
HS - LEIS:
new technique; any material; clusters: 1 atom - 10 nm
Comparison: Richard A. P. Smith (J&M) et al., ECASIA 2009
T. Tanabe et al. (Toyota), Appl. Catal. A370 (2009) 108
Particle Size on Supported Catalysts
Diameter TON; size often related to failure
Example: Three-Way catalyst (exhaust)
Pt clusters on CeO2/ …../ γ-alumina
Loading = 0.004 g Pt / γ-alumina
Cluster diameter: 1.6 nm (average)
Accurate for d < 10 nm
Important / unique applications for catalysis
4. Nanoclusters
Average diameter nanoclusters
Surface segregation in alloy clusters
Core/shell particles
(verification, closure, thickness shell)
The diameter is derived from the ratio of the bulk loading (volume) to the
LEIS signal (surface area)
This method is possible where TEM fails ( d ≤ 2 nm; high Z support)
Low Energy Ion Scattering
Characterisation of Functionalised Core/Shell Nanoparticles
Courtesy of
M. Fuchs et al., Surf. Interface
Anal. 42 (2010) 1131
4 nm
The outer surface
of
mixed oxides
O Al
Zn
Energy
Inte
nsity
ZnAl2O4
ZnO
Zn in tetrahedral site below surface
LEIS of spinels AB2O4
LEIS reveals surface composition powders
Al
O
Zn
Powders: preferential exposure plane with B cations
(octahedral sites)
Important / unique applications for catalysis
Mixed oxides and catalysis
0
1
2
0 5 1 0 1 5
0
1
2
0 5 1 0 1 5
2
00
1
Yie
ld
15105
Co LEIS signal
CoAl2O4
ZnCo2O4
The atomic composition of the 1st atom layer controls catalysis.
In a spinel ( AB2O4 ) only the B-cations (octahedral site) are catalytically
active and visible for LEIS (1st at.).
The A-cations (tetrahedral sites) are in 2nd layer (not active, no LEIS peak).
Co3O4 = CoCo2O4
Test reaction:
only Co catalytically active
Co signals:
XPS: 1 : 2 : 3
LEIS: 0.3 : 1.9 : 2.0
LEIS Catalysis XPS
Cations in octahedral sites at surface ( B in normal spinel )
LEIS
Chemistry } 1 : 1 !
Cations in tetrahedral sites below surface ( A in normal spinel )
Chemistry
LEIS
----------------------------------------------------------------------------------
Perovskites, other oxides: more complicated !
Kilner et al.: LaSrCo oxides ….
Alumina: difference α – γ !
Spinels (AB2O4)
LEIS and chemistry
Performance relies on oxygen transport
Performance: “ Hampered by the surface ”
Why ? What is the surface ??
M. de Ridder et al., J. Appl. Phys. 92 (2002) 3056 - 3064
M. de Ridder et al., Solid State Ionics 156 (2003) 255 – 262
J.A. Kilner et al., J. Solid State Electrochem. 15 (2011) 861 - 876
Fuel Cells and Oxygen Membranes
Importance of the outer surface
0 1000 2000 30000
100
200
300
Hf/Ba
(Y,Zr)
Ca
SiNa
FO
LE
IS s
igna
l (a
.u.)
Energy (eV)
sample 2
sample 4
Calcination for 5 hours
at 1000 oC in an
oxygen flow of 1.5 bar.
Segregation of
monolayer of
impurities
For T > 700 C: No Y, Zr in 1st atom !
Fuel Cells
Yttria stabilized Zirconia (YSZ) after calcination
XPS: Ca not visible ( ↔ Zr )
0.0 0.2 0.4 0.6 0.8 1.00.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
YS
Z c
ove
rag
e
Iso
top
ic f
ractio
n 1
8O
CaO Coverage
M. de Ridder et al., Solid State Ionics 156 (2003) 255 - 262
Fuel Cells
CaO coverage blocks 16O –18O exchange
H. Téllez, R. J. Chater, S. Fearn, E. Symianakis, H. H. Brongersma, J.A. Kilner Appl. Phys. Lett. 101, 151602 (2012)
Isotopic exchange 16O – 18O
Growth of Ultra thin
LEIS and Microelectronics
Layer thickness versus cycle #
Max
Average
Min
Cycles
Thic
kness
Linear growth
Transient
Closure
The transient regime determines the uniformity of the layer
?
Width of the transient regime depends on nucleation of ALD
reaction, precursors and growth mode
- 2D growth: Preferential growth on substrate
CrOx/Al2O3, ZrO2/SiO2, HfO2/Si
- Random growth AlN/Al2O3, AlN/SiO2, Fe2O3/ZrO2
- 3D growth: Preferential growth on pre-deposited matter
Ni/Al2O3, HfO2/Ge, WNC/SiO2/Si, Ta(CN)/SiO2/Si, Ta(Si)/SiO2/Si, TiN/SiO2,
Al2O3/Si-H, ZrO2/Si-H
Controlling the transient regime is fundamental for continuity of
ultrathin ALD films. ALD + LEIS !!
Transient regime (growth before closure)
( 6 up to > 150 cycles !)
7 keV He+ LEIS 3.0 nm TiN / Si(100) nominal
0
1
1
2
2
3
3
4
0 1000 2000 3000 4000 5000 6000
Yie
ld
Ef (eV)
TRBS-I simulations
1. TiN (bulk) brown
2. TiO2 (bulk) green
3. 2.8 nm TIN / Si black
4. TiO2/TiO2+TiN /TiN /TIN+ Si red
5. LEIS (surf. peaks corr.) blue
Growth of Ultra-thin layers
LEIS: 1st atom + in-depth, quantitative, sensitive
Initial growth
Poisoning, activation
Pinholes
Diffusion
Thickness uniformity
Use LEIS in the optimization of growth processes (ALD!)
Sensitivity
1st atom + in-depth
Surf. segr. / anti wetting / M – polymer / surf. Modific. / NPs
/ graphene
Outer surface of oxides
SOFC , membranes
ALD growth
Summary
Miscellaneous applications
Catalysts, microelectronics, polymers, (bio-) sensors, ....
But also:
Candy wrappers
F 16 dome, windows of planes
Bone tissue, dental implants, stents, …...
Aging of linoleum ( “ Linowonder” )
Anti-wetting (watches, diapers, …..)
Floor wax
IBA 2013 – Seattle, WA – slide 41 [email protected]
Graphene analysis with LEIS ??
Wishes:
Determination layer thickness, layer integrity
Distinguish and Quantify carbon allotropes
Detection contaminants / dopants on/in and below the film (intercalation)
LEIS ??
Carbon: low Z Low sensitivity
Monolayer depth info? Carbon atoms very small
Distinguish allotropes ? No matrix effects (in general)
LEIS seems useless for studying graphene, but : ..........
Author: AlexanderAlUS, wikipedia Author: Mstroeck, wikipedia
Energy (eV)
Annealing time
5 nm
Yie
ld (
cts
/nA
)
Diffusion study with in-situ heating
Mo/Si layer – quantification of diffusion constant
FOM
5 nm Si
10 nm Mo
SiO2
Si Wafer
B4C
V. de Rooij-Lohmann, Appl. Phys. Lett. 94 063107 (2009)
V. de Rooij-Lohmann, J. Appl. Phys. 108 014314 (2010)
5 nm Si / 1.6 nm B4C / 10 nm Mo, annealing @ 500 deg. C
Diffusion coefficient without B4C : (8 2) • 10-20 m2/s
Diffusion coefficient with 1.6 nm B4C: (4 1) • 10-21 m2/s
High-energy edge of SAMs on Au
2200 2375 2550 2725 29000
4
8
12
16
20
Au
Norm
alised inte
nsity (
Cts
/nC
)
Energy (eV)
Au (x 0.02) Au (dirty) + C11 - thiols Au (clean) + C11 - thiols
11Å
7Å
overlayer
thickness
Fluorinated thiol on dirty Au surface
Fluorinated thiol on clean Au surface
7Å
11Å
0 5 10 15 20 25 300.0
0.5
1.0
1.5
2.0
Re
mo
ve
d la
ye
r th
ickn
ess (
nm
)
Oxidation time (minutes)
Erosion rate for atomic oxygen treatment
Erosion rate: 0.64 nm/min
C20 SAM on Au
Quantification of F with LiF(100)
LiF (100)750 1500 2250 3000
0
40
80
120
160
Au
C
F
LE
IS S
ignal (C
ts/n
C)
Energy (eV)
LiF
Teflon
F8 - thiol (*1/20)
LiF(100) : Teflon : s.-a. ML = 1.23 : 1.24 : 1.48 *1015 at./cm2
Outer and Near surface of Perovskites
by LEIS ( for Fuel Cells and Oxygen Membranes )
Outer surface: A-site termination (LEIS: no other cations)
Subsurface: Layered oxide (LEIS depth profiling)
Examples: Structure
La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ Perovskite
Gd Ba Co2 O5-δ Double Perovskite
La2 Ni O4-δ Ruddlesden - Popper
John Druce, Helena Tellez, 2014