ION-TOF GmbHHeisenbergstr. 15D-48149 Münster / Germanywww.iontof.com

March 23, 2011Lehigh University

Unique opportunities for new insight in the outer surfaces and interfaces by

High Sensitiviy Low Energy Ion Scattering (HS-LEIS)

Hidde Brongersma

ION-TOF GmbH / Eindhoven University of Technology / Imperial College (London)

H.H.Brongersma@tue.nl

1

� Introduction

� Principles and features of LEIS

− 1st atom, In – depth ( 0 – 10 nm ), quantitative

� Applications

− Organics (surface modific., anti-wetting, SAMs)

− Catalysts (mixed oxides, coke, NP’s, oxid. states)

− Ceramics (SOFC, membranes)

− ALD growth (high-k, inter-diffusion)

− and many more .......

Outline

Why High-Sensitivity LEIS ?

CONTROL at the ATOMIC LEVEL

QUANTITATIVE ANALYSIS at the ATOMIC LEVEL

requires

Ef = f(M0, M1, M2, θ)*Ei

Low Energy Ion Scattering

Analytical Capabilities

Energy

Inte

nsity

θ

3He+, 4He+, Ne+, Ar+

Ef (eV)

Energy spectrum of an alloy

0 1000 2000 3000

S (

a.u

. )

Ne+ � alloyEi = 3000 eV192Ir

103Rh

55Mn

51V

59Co39K

� Atomic composition of the outermost atomic layer

� Energy 1 – 8 keV� Lateral resolution 0.01 – 1 mm� Static in-depth (0 – 10 nm)� Quantitative !!� No matrix effects

LEIS

sig

nal

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

LEIS

sig

nal

Ef / Ei

4He 10,000 nC 4He 5.4 nC

NIM B 68 (1992) 207

Conventional High-Sensitivity LEIS

Promoters visible, but removed

Conventional LEIS vs HS - LEIS

Pd / Pt-C ( 1000 m2/g )

Noble gas ion source

Pulsing system

Position sensitive detector

Sample

Energy analyser

Focussing Optics

Energy image:

� parallel energy detection

� only low dose needed

STATIC LEIS

“ Analysis before Damage ”

(Molecular Dynamics simulation)

“ Hit same place only once “

ToF filtering eliminates SIMS ions

Qtac : Unique new AnalyserHigh – Sensitivity LEIS

� Analysis before damage (static)

� 1st Monolayer

� Quantitative (peak concentration / coverage)

� Sensitivity

� Mass resolution

� ToF-filtered LEIS

� Imaging----------------------------------------------------------------------------------------------------------------------------------

� In-depth: Static (0 – 6 nm) or with sputtering

HS-LEIS: Principles and Key Features

LEIS and SIMSTime resolved: LEIS analysis before damage !

LEIS

SIMS

ION-TOF GmbHHeisenbergstr. 15D-48149 Münster / Germanywww.iontof.com

Quantification

Review:

Brongersma et al., Surf. Sci. Reports, 62 (2007) 63 – 109.

QuantificationBromine adsorption on Tungsten

100

200

300

400

500

600

0

0 100 20050 150

SBr ( a.u. )

SW

( a.

u.)

θ W + θ Br = 1

No matrix effect !

1st atom

Simple interpretation !!

θ = fraction 1st atom layer

θ w = 1

θ Br = 1

Bulk Composition Ag80 Al20

NO matrix effectsNO matrix effectsNO matrix effectsNO matrix effects

Surface Composition Ag66 Al34

( independent of primary energy )

1000 1500 2000 2500 3000 35000

20

40

60

80

100total

Ag

Sur

face

cov

erag

e (%

)

Primary energy (eV)

4He 3He

Al

Rough silica: 50 – 380 m2/g

HS-LEIS: Insensitive to roughness

LEIS Signals:

rough silica about 77%

of flat silica (quartz)

0 100 200 300 4000

600

1200

1800

LEIS

Sig

nal (

Cts

/nC

)

Specific surface area (m2/g)

Si O

W.P.A. Jansen et al., SIA 36 (2004) 1469 - 1478.

Monolayer sensitivity

XPS

LEIS

SIMSsubstratesubstratesubstrate

SIMS not quantitative for near-surface / interface

XPS average over 3 – 10 nm; chemical info

LEIS 1st atom and in-depth; quantitative, sensitive

SE imageTi

Sn Pb

Elemental mapping by LEIS and SE Image Solder bumps

Depth info for

Ultra thin layers and interfaces

LEIS and Microelectronics

Two possibilities:

1. Static LEIS + sputter depth profiling with dual ion beam

( advantage of quantification, depth resolution LEIS)

2. Static LEIS

(analogous to RBS and MEIS, but better depth resolution)

In – Depth profiling

Depth info

Si

ZrO2

He+

Ef

3 keV

Heo

Zrsurf

Zrbck

Osurf

Sibck

Sisurf Fin

al E

nerg

y

Depth resolution better for lower primary E !

He+

∆E≅ 160 eV/nm

1st atom and Static Depth Profile

ZrO2 Atomic Layer Deposition on Silicon

� Closure / 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

24

In LEIS only backscattered ions are detected

Peaks: Ions backscattered from 1st atom.(one well-defined collision)

Tails: Backscattering in deeper layers + reionization (Scattering by oxygen atoms: efficient reionization)

Shape: f ( in-depth distribution Zr )

Intensity: f ( oxygen concentration in 1st atomic layer )

INFO from LEIS spectra

LEIS TechniqueFeatures of Low Energy Ion Scattering (LEIS)

He+, Ne+, Ar+, Kr+

1 - 8 keV

LEIS Features

Ultra-high surface sensitivity, top atomic layer analysis

Static depth profiling information (up to 10 nm)

Reliable and straight-forward quantification

Detection of all elements > He

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

Sample Treatment Atom Source for Surface Cleaning

O2 or H2

O or H atoms

Sample

COCO2

H2O

CxHyOz

� O atoms remove organics, coke� Chemical energy: no sputtering

� Surface segregation

� Dendrimers

� Antiwetting

� Surface modification

� Metal / polymer interface

� SAMs

Organics

Polymers, SAMs, ...

� Inter - molecular segregation� Segregation impurities, additives (0.1 s - days - ..;

up to 108 x !! )

� Intra - molecular segregation (0.1 s - days - ..)� Aging plasma oxidized PE

� Anti - wetting layers

� Metal diffusion in polymers

� SAM’s

Acrylonitrile-Butadiene-Styrene (ABS)Surface segregation of additives

0 1000 2000 30000

1

2

3

4

ZnC a

O

N

LEIS

Sig

nal (

Cts

/nC

)

E nergy (eV )

C

N a

P b

Metal - polymer interface in ultra - thin layers

PLED: Ba evaporation on PPVLE

IS S

igna

l (C

ts/n

C)

1500 2000 2500 30000

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

High-energy edge of SAMs on Au

2200 2375 2550 2725 29000

4

8

12

16

20

Au

Nor

mal

ised

inte

nsity

(C

ts/n

C)

Energy (eV)

Au (x 0.02) Au (dirty) + C11 - thiols Au (clean) + C11 - thiols

11Å7Å

overlayer thickness

Fluorinated thiol on dirty Au surface

Fluorinated thiol on clean Au surface

7Å11

Å

Aging of plasma oxidized HDPE

� Aging (LEIS) faster than aging (XPS) !

� “Straight line” � diffusion process

Time1/2 (min)

1.25

1.75

2.25

0 20 40 60 80 100 120

0.25

0.75

1.25XPS

LEIS

Log(

O in

at%

)

XPS

LEIS

Selection of examples:

� Pt/Au

� Mixed oxides

� γ-alumina

� Poison (Coke on TWC)

� Poison (oxygen membranes, SOFC)

� Use of probe molecules

� NP’s , core/shell

� Oxidation state 1st atom

HS – LEIS and Catalysis, Ceramics

Important / unique applications for catalysisMixed oxides and catalysis

0

1

2

0 5 1 0 1 50

1

2

0 5 1 0 1 5

2

00

1Yie

ld

15105Co 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

� 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

Fuel Cells and MembranesImportance of the outer surface

0 1000 2000 30000

100

200

300

Hf/Ba

(Y,Zr)

Ca

SiNa

FO

LEIS

sig

nal (

a.u.

)

Energy (eV)

sample 2 sample 4

Calcination for 5 hours at 1000oC in an oxygen flow of 1.5 bar.

Segregation of monolayer of impurities

For T > 700 C: No Y, Zr in 1st atom !

Fuel CellsYttria 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

over

age

Isot

opic

frac

tion

18O

CaO Coverage

M. de Ridder et al., Solid State Ionics 156 (2003) 255 - 262

Fuel CellsCaO coverage blocks 16O –18O exchange

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

over

age

Isot

opic

frac

tion

18O

CaO Coverage

M. de Ridder et al., Solid State Ionics 156 (2003) 255 - 262

Fuel CellsCaO coverage blocks 16O –18O exchange

Cold start: 50% loss of Pt signal — sintering or coke formation ?

Room temperature oxidation with atomic oxygen gives complete recovery

of Pt signal loss is due to coke.

Detection of C with “any” surface technique. But: WHERE is the coke ??!

LEIS determines which fraction of Pt is covered by coke !

J.M.A. Harmsen, et al., Catal. Lett. 74 (2001) 133 – 137.

Applications:� Number of Pt atoms available for catalysis.

Quality control of catalysts !

� Detection of nucleation site for coke (active phase, support, binder, ... )

Coke Formation on Commercial TWC

Three Way Catalyst (TWC) ( Pt, Rh / CeO2 / γ – Al2O3 )

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), ECASIA 2009

T. Tanabe et al. (Toyota), Appl. Catal. A370 (2009) 108

Particle Size on Supported Catalysts

Diameter TON; size often related to failureDiameter TON; size often related to failureDiameter TON; size often related to failureDiameter 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 catalysis4. 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)

0 2 4 60

6

12

18

Zn/

Cu

atom

ic r

atio

Depth (monolayers)

CO/CO2/H

2 at 573 K

H2 at 473 K

H2 at 573 K

H2 at 673 K

bulk

Strong Zn(O) segregation

Zn(O) on top of Cu is thermodynamically favorable

bulk

1st atom

Cu / ZnO / SiO2 CatalystsSynthesis of Methanol, Fatty Acids

XPS:Oxidation states, BUT averaged over 10 – 20 atomic layers.

LEIS:Elemental composition outer atomic layer, BUT no chemical infoOxidation of metallic Cu, Zn gives shielding by oxygen.Signal decrease: factor 5 resp. 3.7.

Chemical titration:Information on oxidation states, BUT not only the outer surface (?)

? ? ?

Oxidation states Cu and Zn in outer surface ?

LEIS + chemical titration !

XPS:Oxidation states, BUT averaged over 10 – 20 atomic layers.

LEIS:Elemental composition outer atomic layer, BUT no chemical infoOxidation of metallic Cu, Zn gives shielding by oxygen.Signal decrease: factor 5 resp. 3.7.

Chemical titration:Information on oxidation states, BUT not only the outer surface (?)

LEIS + Chemical titration: oxidation states in the outer surface !

� N2O for oxidation

� LEIS for detection increase in shielding after N2O treatment

Oxidation states Cu and Zn in outer surface ?

LEIS + chemical titration !

Cu/Zn/SiO2 reduced at 473 K Cu/Zn/SiO2 reduced at 673 K

Outermost atomic layer:ZnO 0.42Cu1+ 0.54Zn0 0.02Cu0 0.02

SiO2 support

Subsurface:Cu: Zn = 9

Outermost atomic layer:ZnO 0.77Cu1+ 0.03Zn0 0.19Cu0 0.01

SiO2 support

Subsurface:Cu: Zn = 9

Cu / ZnO / SiO2 - CatalystDetermination of cluster size and oxidation states by LEIS

Atomic Layer Deposition (ALD)

“ Growth with Digital Accuracy “

LEIS and Microelectronics

ALD cartoons: (often) show closed layer after 1 cycleIn practice: closure after a few up to > 100 cycles !

Typical examples (depending on ALD conditions ! ):

o 6 cycles CrOx / Al2O3o 6-9 cycles HfO2 / Sio ~ 15 cycles ALN / SiO2o ~ 40 cycles Al2O3 / Sio ~ 70 cycles Fe2O3 / ZrO2o ~ 150 cycles TiN / SiO2

How many cycles for Closure ?

Layer thickness versus cycle #

Max

Average

Min

Cycles

Thi

ckne

ss

Linear growth

Transient

Closure

The transient regime determines closure and uniformity

？

Characterization of MOCVD vs. ALD HfO2 layer closure and growth mode on Silicon:a new model for preferential deposition

M.J.P. Hopstaken1, M.S. Gordon1, J. Schaeffer2, H. Jagannathan1,

T. Grehl3, H.H. Brongersma3,4, M. Copel1, M.M. Frank1, V. Narayanan1,

K. Choi2, M. Fartmann4, D. Breitenstein4

1IBM Research, 2GLOBALFOUNDRIES, 3ION -TOF, 4Tascon

ALD 2010, June 21 – 23, 2010 Seoul, Korea

0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

RBS HfO2 thickness (Å)

LEIS

sur

face

cov

erag

e (%

)

RBS areal Hf-dose (1015 at.cm-2)

MOCVD Si Hf F

0 5 10 15 20 25 30

0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

RBS HfO2-thickness (Å)

LEIS

sur

face

cov

erag

e (%

)

RBS areal Hf-dose (e15 at.cm-2)

HfO2

SiO2

0 5 10 15 20 25 30

HfO2 layer closure: MOCVD vs ALD

MOCVD ALD

Layer closure

Layer closure

Surface fractions (LEIS) as function of coverage

Earlier layer closure for ALD-HfO2

LEIS and Growth

� Initial growth; growth mode

� Poisoning, activation

� Closure, pinholes

� Thickness distribution

Conclusion IBM / Global Foundries / ION-TOF / Tascon study:

� Origin of the superior quality of the ALD grown layers

revealed by HS-LEIS

( other analytic tools have insufficient depth resolution )

Summary: Why do you need LEIS ?? !!

� Any material, any T� Quantitative� 1st atom and high-resolution in-depth !!

Unique applications of High Sensitivity LEIS ( NOT’s )

� Segregation, Anti-wetting � Adhesion: “ 5% vs 100% “� Follow ultrathin growth� Pinholes in ultrathin; Nano pinholes� Metal / polymer in-depth diffusion� Catalysis: poison, promoter, probe molecules, core-shell, …..� Nanoclusters (diameter; outer atoms) � Inorganics: oxidation states� Improve cleaning strategies

Complementarity to XPS, ToF-SIMS, …..

Miscellaneous applications

� Microelectronics, polymers, ceramics, catalysis, sensors, ……..

But also:

� Pinholes in coatings� Candy wrappers� Gold mining� F 16 Dome� Bone tissue, implants, stents, …...� Ageing of Linoleum ( “ Linowonder” )� Anti-wetting (watches, …..)� Floor wax

Complementary Cutting Edge techniques

HS-LEIS HR-XPS

Qtac100 Scienta ESCA 300

Lehigh University

+

Qtac 100 Photo

Thank you for your attention.