Surface Chemistry and Reaction Dynamics of Electron Beam

Induced Deposition Processes

Howard FairbrotherJohns Hopkins University

Baltimore, MD, USA

Pt

CH3CH3CH3

e-

?

2nd FEBIP WorkshopThun, Switzerland 2008

Outline

• Background / Motivation• Experimental Approach

– Analytical techniques and electron source• Surface Chemistry and Kinetics• Electron Stimulated Dissociation Mechanism• Summary & Outlook

Substrate

Organometallicprecursor

Electron Beam

LowPressure

Electron Beam Induced Deposition

The ability to focus electron beams into small spots, controlelectron beam fluence and raster the beam makes EBID an

ideal method for growing a wealth of different nanostructures

Motivation

• Many questions about EBID process• Chemical reactions at the surface? • σreaction(E)?

•If we can better understand the chemistry, we can: • Choose precursors more selectively• Improve deposition purity (carbon)• Improve purification techniques• Increase metallic characteristics

The fundamental surface processes that are responsible for electron beam induced deposition of

nanostructures are not well understood

Outline

• Background / Motivation• Experimental Approach• Surface Chemistry and Kinetics (500eV)• Electron Stimulated Dissociation

Mechanism• Summary

Our Approach

• To understand the EBID process using well established surface analytical techniques

– Adsorbing a nanometer scale film of EBID precursor to a substrate provides a “clean” environment for in situ observation

– Surface coverage can be controlled

– An UHV environment enables analysis of gas phase products

– A film, on the order of cm2 in area, can be analyzed using common surface analytical techniques

Gold Substrate (~195K)

Electron Beam

Broad Beam Surface Irradiation

Production of a film over a large surface area enables traditional surface analytical techniques to probe the EBID process

Gold Substrate (~195K)

Pt

CH3CH3CH3

Why gold?

Electron Source: Flood Gun• Why use a flood gun?

– Uniform electron beam over a wide area (necessary for XPS and RAIRS)

– High target current– Relatively broad range of Energies (40 – 500eV)

Adjustable Filament Current

Extractor

Wehnelt

e-

Electron Flood Gun can produce:(a) 40 - 500 eV electrons(b) 5 -150 μA target currents

Tungsten wire

Sample

Type K thermocouple

Cu leads provide for heating and cooling (100K-450K)

Manipulator

Au Substrate(~195 K)

Instrumental Techniques

• We have studied the electron stimulated reactions of the well-known Pt precursor, Trimethyl(methylcyclopentadienyl)-platinum(IV), adsorbed onto gold using the above techniques:

QMS

neutrals

Electron Beam

X-ray Beam

hνe-

Electron Analyzer

FT-IR Beam

IR Detector

hνhν

Gold Substrate (~195K)

e-

XPS enables quantitative determination of chemical composition and effective oxidation state

Binding Energy (eV)0200400600

Cou

nts

(a.u

.)X-ray Photoelectron Spectroscopy

(XPS)

Binding Energy (eV)

707274767880C

ount

s (a

.u.)

0

60

1200

BE = hν - KEhν

BE

KE

1s

2s2p

Vacuum Level

hν

EjectedPhotoelectron

Pt(4f)

ElectronIrradiation

Time(sec)

Au(4p) Pt(4p) Au(4d) Pt(4d) C(1s) Au(4f) Pt(4f)

PtParent PtProduct

Reduction of Pt indicated by peak shift to lower BE

Reflection Absorption Infrared Spectroscopy (RAIRS)

Wavenumber (cm-1)

10001500200025003000

IR In

tens

ity (a

.u.)

0.01 Abs

FT-IR Beam IR

Detector

hν hν

Ultra-High Vacuum

Wavenumber (cm-1)

27002800290030003100

IR In

tens

ity (a

.u.)

0.001 Abs

240

1200

Pt

CH3CH3CH3

0

Reflective Au ElectronIrradiation

Time(sec)

ν(C-H)

ν(C-H)

MeCpPtMe3vibrational structure

Mass Spectrometry (MS)

M ass (a.m .u.)0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Cou

nts

(a.u

.)

H2

C H 3

H2OCH2 CH2

CH3

HC

CH2

CH2

CHHC

H2C

C H 3

Pt

CH3CH3CH3

Electron bombardment within the MS creates ionized fragments representative of species present in the gas phase

Electron Ionization

(70eV)

M+· + 2e- M+·

MassSelector

DetectorPt

CH 3CH3CH3 (g)

(g)

MS was used to:• Verify purity of organometallicprecursor

• Observe gaseous products of EBID

• Ensure cleanliness of UHV

Outline

• Background / Motivation• Experimental Approach• Surface Chemistry and Kinetics (fixed

electron energy = 500eV)• Electron Stimulated Dissociation

Mechanism• Summary

Adsorption of MeCpPt(IV)Me3 onto Gold Substrate – Controlling film

thicknessInfluence of Dosing Time on Film Thickness

Dosing Time (seconds)0 200 400 600 800 1000 1200 1400

Ads

orba

te L

ayer

s

0

1

2

3

4

5

6

7Pdosing = 1x10-6 Torr

XPS and MSExperiments Conducted

in this Regime

Pt

CH3CH3CH3

Film thickness, d, calculated from attenuation of Au(4f) signal

Au

dave =.96nm

0

*cos( )*ln( )IdI

λ θ=

d = adsorbate thicknessλ = ~2nm for Au(4f) photoelectronθ = 54° photoelectron take-off angleI = Au(4f) area

Binding Energy (eV)687072747678

Cou

nts

(a.u

.)

Pt (4f)

Production of Amorphous Platinum/Carbon Film

PtMeCpPtMe3

PtPtCx Pt

PtAu

XP spectra of sputter deposited platinum on gold

PtCxAu

Substrate heated to room temperature leaving electron beam irradiation product

AuPtCx

P t

C H 3C H 3C H 3

+Electron beam irradiation for 20 sec (20μA, 500eV)

P t

C H 3C H 3C H 3

Au

P t

C H 3C H 3C H 3

MeCpPt(IV)Me3 adsorbed onto gold substrate (~195K)

282284286288

Binding Energy (eV)707274767880

Counts (a.u.)

0

15

30

60

120

1200

IrradiationTime (sec)

Pt(4f)

8284868890

Au(4f)C(1s) PtMeCpPtMe3

PtPtCX

Influence of e- Beam Irradiation on Surface Composition of Adsorbate Layer

MeCpPt(IV)Me3 is stable under x-ray irradiation for >2hrs

The XP spectra of the C(1s), Au(4f), and Pt(4f) regions shows:

•No change in film thickness as determined by Au(4f) attenuation

•Shift in platinum environment from precursor to product

Irradiation Time (sec)0 50 100 150 200 250 300

ln [P

t(4f) t]/

[Pt(4

f) t=0]

-2.0

-1.5

-1.0

-0.5

0.0

10 mA50 mA100 mA

Irradiation Time (sec)0 100 200 300 400 500[P

t(4f) t]/

[Pt(4

f) t=0]

0

20

40

60

80

100

PtMeCpPt(IV)Me3

PtPtCx

Target Current

XPS Analysis of Deposition KineticsQuantification of the deconvoluted Pt(4f) region fit to exponential decay shows first order kinetics

Decay profiles show that the observed rate constant increases with increasing target current.

282284286288

Binding Energy (eV)707274767880

Counts (a.u.)

0

15

30

60

120

1200

IrradiationTime (sec)

Pt(4f)

8284868890

Au(4f)C(1s) PtMeCpPtMe3

PtPtCX

Signal Resolution vs. Signal Intensity

•High resolution / low intensity emphasizes elemental environment

•Low resolution / high intensity emphasizes stoichiometry

Film Thickness (nm)0 1 2 3 4

C:P

t (XP

S R

atio

)

6

7

8

9

10

11C

:Pt (

XPS

Rat

io)

6

7

8

9

10

11MeCpPtMe3 FilmPtCx Film

PtC9

PtC8

PtC7

Electron Beam Irradiation

PtC9

PtC8

PtC7

Evidence for Carbon Loss During Irradiation

• The C:Pt ratio of 10 XP spectra of the MeCpPt(IV)Me3 prior to e- beam irradiation is representative of the initial stoichiometric ratio of 9 carbon atoms to 1 platinum atomThe stoichiometric loss of 1

carbon atom as a result of irradiation is independent of film thickness

Pt

CH3CH3CH3

• The C:Pt ratio after e- beam irradiation decreases to a ratio of 8:1, indicating the loss of 1 carbon atom per molecule as a result of irradiation

(ads)Pt

CH3CH3CH3

Au (~195 K) Au

PtC8

500 eV e-

PtC9

Cou

nts

(a.u

.)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Mass (a.m.u.)

H2

C H 3

H2OCH2 CH2

CH3

HC

CH2

CH2

CHHC

H2CC H 3

CH4

Pt

CH3CH3CH3

Au

Pt

CH3CH3CH3(g)

e-

Methane Reference

CH4 (g) / H2(g)500 eV e-

(ads)Pt

CH3CH3CH3

Au (~195 K)

Gas Phase Products

Electron beam irradiation results in the production of hydrogen and methane

MS capable of measuring individual masses as a function of time

m/z = 15

Irradiation time (sec)0 200 400 600 800 1000 1200 1400 1600

Electron irradiation

MS

sign

al (a

rb. u

nits

)

Irradiation Time (sec)

0 20 40 60 80 100

ln[(C

H4)

t/(C

H4)

t=0)

]-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

9.88 A30.64 A59.89 A

μ μ μ

Target Current

Kinetic Analysis of Methane Production

•Tracking methane production during electron beam irradiation fit to exponential decay

•m/z = 15 is a unique mass representative of methane

•Methane loss fit to first order kinetics indicates an increase in the observed rate constant with increasing target current

MS

Target Current (μA)

0 10 20 30 40 50 60 70

k obs (

s-1)

0.0

5.0e-3

1.0e-2

1.5e-2

2.0e-2

2.5e-2

3.0e-2

XPS

k obs (

s1 )

1.0e-2

2.0e-2

3.0e-2

4.0e-2

5.0e-2

6.0e-2

μ

σ (cm2)

XPS 1.40E-16

MS (m/z=15) 9.75E-17

Complementary Techniques

arg

obs

t et

k AI

σ = ×

σave, 500eV = 2.28E-16 cm2

XPS, RAIRS, and MS provide similar σ values though they measure different processes

Gradient = kobs / Itarget

How does Film Thickness Influence the Process?

Adsorbate Layers0 1 2 3 4 5

Met

hane

Are

a

Methane area independent of film thickness

Adsorbate Layers0 1 2 3 4 5

σ (c

m2 )

0

1e-16

2e-16

3e-16

σ independent of film thickness

CH4 (g)/H2(g)

Au

PtC8500 eV e-

(ads)

Pt

CH3CH3CH3

Au (~195 K)

Pt containing amorphous C

films

Surface Chemistry

•Electron beam irradiation of surface adsorbed MeCpPt(IV)Me3 results in the formation of platinum atoms embedded in an amorphous carbon film via an electron impact process in which bond cleavage releases hydrogen and methane.

•Each precursor molecule that undergoes electron stimulated decomposition losses exactly one carbon atom.

One electronr.d.s

Outline

• Background / Motivation• Experimental Approach• Surface Chemistry and Kinetics (500eV)• Electron Stimulated Dissociation

Mechanism• Summary

Binding Energy (eV)707274767880

Cou

nts

(a.u

.)

1500eV

750eV

200eV

60eV

40eV

Dependence on Incident Electron Energy Observed Using XPS

Electron Beam

Energy

Above XP spectra: 25μA, t=60 sec

Electron Energy (eV)0 200 400 600 800 1000 1200 1400 1600

σ (c

m2 )

0.0

2.0e-17

4.0e-17

6.0e-17

8.0e-17

1.0e-16

1.2e-16

1.4e-16

1.6e-16

Optimal σ between 100-200eV suggests electron impact ionization process

Irradiation Time (sec)

0 100 200 300 400

ln (

CH

4 / C

H4(

t=0)

)

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0 60 eV200 eV2000 eV

Dependence on Incident Electron Energy Observed Using MS

Electron Energy (eV)0 500 1000 1500 2000 2500 3000

σ (c

m2 )

0.0

5.0e-17

1.0e-16

1.5e-16

2.0e-16

2.5e-16

σ60eV < σ200eV > σ2000eVMS supports XPS results

Dissociative Ionization Process

Pt

CH3CH3CH3

Pt

CH3CH3CH3

+

Decompositionproducts

Adsorbate Layers0 1 2 3 4 5 6

σ (c

m2 )

0

1e-16

2e-16

3e-16

4e-16

Pt/C SubstrateGraphite SubstrateGold SubstrateSilicon Substrate

Substrate Dependence?

NO

All experiments conducted at ~195K with 20μA target current and 200eV electron energy

Summary

Pt

CH3CH3CH3

Pt

CH3CH3CH3

+CH4 (g)/H2(g)

Au

PtC8

Pt atoms embedded in an

amorphous Cfilm

one electronr.d.s process

A UHV surface science approach can provided valuable information on reaction rates and

fundamental chemical processes involved in EBID

Exptl. Requirements

Electron Beam– Broad and defocused (uniform irradiation)

Substrate - Chemically unreactive towards precursor

Film- Thin!!! (1-3 Monolayer regime so that each precursor molecule experience the same electron flux)

Methods?Technique Pros Cons

XPS Reasonable quantification Central atom must change oxidation state

AES Destructive (Not applicable)

Mass Spectrometry(a) TPD Good Quantification Duty Cycle Slow

Decomposition may compete with desorption

(b) Analysis of Simple Indirect monitor of desorbing species reasonable quantification surface processes

IR (reflection) Parent can easily be monitored Poor SensitivityModest Quantification

Acknowledgements

• Joshua Wnuk, Johns Hopkins University• Justin Gorham, Johns Hopkins University• Willem van Dorp, Rutgers University• Ted Madey, Rutgers University• Kees Hagen, Delft University of Technology