Low-Temperature H2 Plasma Interactions with a …doeplasma.eecs.umich.edu › files ›...

Plasma Science CenterPredictive Control of Plasma Kinetics1

Low-Temperature H2 Plasma Interactions with a-CH Surfaces

GottliebS. OehrleinDept. of Materials Science and Engineering, and Institute

for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742


Objective

TransportC, CHnSi, SiHn’

CHm or SiHm’

Erosion Redeposition

Different SurfaceCarbon or Silicon

SurfaceModified Surface Layer(intensity, extent)

H2 PlasmaWall

MS

Probe

OES

Real-timeellipsometry

TransportC, CHnSi, SiHn’C, CHnSi, SiHn’

CHm or SiHm’




Carbon or SiliconSurface

Modified Surface Layer(intensity, extent)

H2 PlasmaWall

MS

MS

ProbeProbe

OESOES


What is the approach to control of plasma distribution functions when plasma-surface interactions at plasma boundaries change, e.g. non-reactive to reactive surface?

• Example: H2 or D2 plasma with carbon or silicon (reactive or non-reactive surfaces)


Some Questions

• What is the impact of surface generated species on plasma distribution functions?

• What are the critical parameters that need to be measured to understand observed changes?

• Can we establish validated plasma-surface interaction models that can describe the fluxes of particles entering the plasma for different conditions?

• Can these observations be captured in comprehensive plasma/plasma surface interaction models and suggest approaches that will be valuable for control of plasma distribution functions?

• ….


Objectives• Surface characterization by

ellipsometry• Discharge properties

– Langmuir probe– Ion sampling– Optical emission

spectroscopy

TransportC, CHnSi, SiHn’

CHm or SiHm’




H2 PlasmaWall

MS

Probe

OES


TransportC, CHnSi, SiHn’C, CHnSi, SiHn’

CHm or SiHm’




Carbon or SiliconSurface

Modified Surface Layer(intensity, extent)

H2 PlasmaWall

MS

MS

ProbeProbe

OESOES


• Initial experiments with H2 plasma: Correlate ellipsometrymeasurements of surfaces with probe measurements of f(v,r,t)• Characterize a-C:H film erosion• Measure plasma I-V characteristics for modified situations• Characterize impact of surface-generated species on plasma

electrical characteristics


Outline

• D isotope exchange with a-C:H films

• Ar/H2 plasma interaction with a-C:H Films– Modification and erosion of a-C:H films– Coupling with MD simulations

• Conclusions

5


Outline

• Isotope exchange and erosion of a-C:H (a-C:D) films using D or H atoms in UHV system

AcknowledgementsInstitute for Plasma Physics, Garching GermanyT. Schwarz-SelingerK. Schmid M. Schlüter W. Jacob


Mechanistic Picture

CHCH

C

HD

CHCH

C

D

(a) Isotope exchangeand H elimination.

(b) Deuteration ofextra sites – formationof highly deuteratedlayer; simultaneous erosion (m’>>n’).

(c) Steady-state erosionof a-C:H film; modified layer (x>>y) of constant thickness is dynamicallyreformed on top of a-C:H film; for productsm>>n.

CH CDCCD CD2 CD3

CD CDxHy LayerCD

CHCH

C

DD CDm’Hn’ CDmHn


Experimental• Isotope exchange and erosion of a-C:H (a-C:D) films using D or

H atoms in UHV system (IPP Germany)– Ellipsometry characterization of surface processes– Ion Beam Analysis – for D content

• D(3He,p) 4He reaction

3He

p@ 690 keV, 5 µC

p, 4He @ 135°


ExperimentalD2 or H2

CapillaryAtom Source

D or H

Ellipsometry

Sample onTemperature Controlled

Stage

UHV ChamberRF Plasma Chamber

for a-C:H Films


1 10 100 10000

1

2

3

4

5

6

7

8

9

D0 on a-C:H

H0 on a-C:D

D0 on a-C:D

Exposure Time (min)

Cha

nge

of D

Are

al D

ensi

ty (1

0 15

at/c

m2 )

0.1 1 10 100 H0/D0 fluence (1018 at/cm2)

D (H) Areal Density vs. Exposure Time

deuteration of extra sites

isotope exchange only


Ellipsometry

Sensitive technique that is capable to monitor sub-monolayer changes in the optical properties of hydrocarbon layers

1. Hard a-C:H film on Si2. Growth of hard a-C:H film on Hard a-C:H film on Si3. Growth of soft a-C:H film on Hard a-C:H film on Si4. Formation of soft a-C:H layer and erosion of Hard a-C:H film on Si5. Formation of hard a-C:H layer on and erosion of Soft a-C:H film on Si

Si Si Si Si

1. 2. 3. 4.

Si

5.


Ellipsometry

• Change in polarization after reflection from surface and internal reflections is determined

• Change in amplitude ratio and of phase between Fresnel reflection coefficients of s and p components is expressed

Ambient

Film

Substrate


Characterization of Polymer Film

• Refractive index of film – or multilayer structure - is obtained by optical modeling of data obtained during processing


Relationship of Refractive Index and Hydrogen Content, Film Density …

• Film properties are strongly reflected in refractive index

soft (polymeric)layer formation


Ellipsometry Trajectories for D and H Steady-State Erosion of a-C:H

15 20 25 30 35

20

40

60

(a)

D

D

H

Psi (deg)

Del

ta (d

eg)

H

Soft a-C:D

Si

Hard a-C:H

D or H


Ellipsometry Data

32.4 32.6 32.8 33.017

18

19

20

21C

B

A

2

Psi (deg)

Del

ta (d

eg)

3

Data DuringInitial 20 min

1

density

decrease

(formation of soft

Layer)


Ellipsometry Analysis

32.6 32.8 33.0 33.2

18

20

2

a

b

Increasingly Soft a-C:H Overlayer (1.4 nm )on Hard a-C:H Film

8

Data DuringInitial 20 min

2 to 77

65

4

3

1

Psi (deg)

Del

ta (d

eg)

density

decrease

(formation of soft

Layer)


Change in D Areal Density and Modified Layer Thickness vs. Time

1 10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.60.1 1 10 100

0

2

4

6

8

Mod

ified

("So

ft") L

ayer

Thi

ckne

ss (n

m)

Time (min)

0

2

4

6

8

Modified ("Soft")Layer Thickness

D A

real

Den

sity

(10 1

5 at./

cm2 )

D Areal Density

D0 Fluence (1018 at/cm2)

Information on extra D (beyond isotope exchange)


Temporal Dependence of Modified Layer Thicknessand a-C:H Erosion

1 10 100 10000

1

2

3

4

200 400 600 800 1000 12000

1

2

3

4R

emov

ed ("

Har

d") L

ayer

Thi

ckne

ss (n

m)

Time (min)

0

1

2

3

4

Mod

ified

("So

ft") L

ayer

Thi

ckne

ss (n

m)

Modified ("Soft") Layer Thickness Removed ("Hard") Layer Thickness

Rem

oved

("H

ard"

) Lay

er T

hick

ness

(nm

)

Time (min)

0.1 1 10 100

0

1

2

3

4


Mod

ified

("So

ft") L

ayer

Thi

ckne

ss (n

m)


Rates: Modified Layer Formation vs. a-C:H Erosion

10 100 10000

1

2

3

4

1 10 100

Removed ("Hard") Layer Thickness

Rem

oved

("H

ard"

) Lay

er T

hick

ness

(nm

)

Time (min)

10 100 1000

0.000

0.005

0.010

0.015

Aver

age

Rat

e of

Cha

nge

of L

ayer

Thi

ckne

ss (n

m/m

in)

Average Rate of Change of ("Hard") a-C:HLayer Thickness



Different Erosion Rates for D and H Atom Interaction

0 5000 10000 1500010

20

30

40

50

60

70

(b)

H

H

D

D

Time (s)

Del

ta (d

eg)

• Detailed analysis showsabout 30% smaller erosion ratefor D atoms than for H atoms;

• Current interpretation in terms of reducedD flux due to reduced conductivityof capillary atom source for D


Mechanistic Picture

CHCH

C

HD

CHCH

C

D

(a) Isotope exchangeand H elimination.

(b) Deuteration ofextra sites – formationof highly deuteratedlayer; simultaneous erosion (m’>>n’).

(c) Steady-state erosionof a-C:H film; modified layer (x>>y) of constant thickness is dynamicallyreformed on top of a-C:H film; for productsm>>n.

CH CDCCD CD2 CD3

CD CDxHy LayerCH

CHCH

C

DD CDm’Hn’ CDmHn


D Exchange with a-C:H (at ~ 60 C)

• For simple isotope exchange we have

• Use Φ the flux of deuterium atoms (1.3x1015 D/cm2 s), measured film properties and thickness of modified layer

)()0()( tnntn HHD −=

( )[ ]Φ−−= σtntn HD exp1)0()(

Φ−= σ)()( tndttdn

HH


D Exchange with a-C:H (at 60 C)

• Assume that over 1.4 nm thickness of modified layer, deuterium exchange takes place in the hard films with known H content

• Deuterium exchange during first 2 min of D exposure -comparable to measured H abstraction cross section σ=2.0x10-18 cm2


Evaluation of Model

1 10 100 10000

2

4

6

8

0.1 1 10 100

D0 on a-C:H H0 on a-C:D D0 on a-C:D a) Isotope Exchange b) Ellipsometry (Extra D) c) Sum a) + b)

11 f7 OPJ [Fi 11 j]Exposure Time (min)

Cha

nge

of D

Are

al D

ensi

ty (1

0 15 a

t/cm

2 )



3 Stages of D Interaction with a-C:H (at ~ 60 C)

1. Hydrogen replacement• Complete after ~ 20 min (2×1018 cm-2)• Comparison of the cross-section for this process with

literature values for H interaction with graphite shows that this corresponds to the cross-section of hydrogen abstraction from the graphite surface

2. Creation of new C-D bonds• Soft a-C:D layer formation • Occurs over next ~ 200 min (~2×1019 cm-2)

3. Erosion of a-C:H, • A soft a-C:D layer “remains” on the substrate, with roughly

constant thickness (1.4 nm)


Outline

H2/Ar erosion of deposited a-C:H films• Ellipsometric modeling of deposition and erosion of a-C:H films• Growth and erosion of multilayer soft/hard films• Comparison with MD simulations• Questions and future work

AcknowledgementsN. Fox-LyonN. Ning D. B. Graves


Experimental Setup• Inductively coupled plasma (ICP) -

H2

– 600W source power– 3.7 MHz RF bias supply

• Surfaces– a-C:H by CH4 PECVD– Si substrates

• Future measurements– Ion sampling– Optical emission spectroscopy– Variable source-substrate

distance

Langmuir Probe


a-C:H Deposition and Erosion in H2

• Hard a-C:H Deposition• 300W SP, -200V,7 mTorr, 20

sccm CH4

• 33.5% H, density=1.9 g/cm3

• H2 Erosion• 600W, -50V selfbias• 90 sccm, 30 mTorr

• Interpretation: Formation of soft (n=1.6, ~45%H, 1 g/cm3, ~2-8 nm) polymer-like surface layer, then steady-state erosion

a-C:H Deposition

a-C:H Erosion


Hydrogen plasma effect on a-C:H

• a-C:H Film from CH4 Plasma• 20 sccm, 7 mTorr• 300W SP, -200V bias

– Ellipsometric/XPS characterization• Optical properties of films show H%

between 33-35% (Schwarz-Selinger, 1999)

• XPS can be used to characterize the a-C:H films’ sp2 to sp3 ratio (T.Y. Leung et al., “Determination of the sp3/sp2 ratio of a-C:H by XPS …”,1999)

• sp3 bonding percent using this method – 24.5%

30


Surface Processes

Soft a-C:H

SiHard a-C:H

• H2 plasma erosion – 600W, 30 mTorr; 90 sccm– RF bias (-50, -100, -200V

selfbias)• Modeling

• soft (n=1.6, ~45%H, 1 g/cm3 ) polymer-like surface layer on unmodified a-C:H underlayer

• Constant thickness of overlayer during steady-state erosion

• Higher ion energies lead to a thinner soft layer


Surface Processes• A higher RF bias voltage

produces thinner hydrogenated layer– Extent of modified surface

layer not solely controlled by H3

+ , H2+ and H+ energies

– Competition of hydrogenation and erosion

– Initial increase of total film thickness has been confirmed in MD simulations

• Erosion rates providecorresponding C fluxes into the plasma


Ar Plasma Effects On a-C:H

• Ar plasma on a-C:H– 300W SP, -200 V bias, 40 sccm, 10

mTorr• Densification and H loss from surface –

graphitic carbon with little H (~10%)

33

a-C:H (10%)

SiHard a-C:H


Competing Hydrogenation/Depletion in Ar/H2

34

• Different Ar/H2 gas mixtures lead to different degrees of modification– H2 effects dominant up

to high percentages of Ar

– Short‐term depletion of H seen with 5‐10% H2 gas mixtures, but long‐term slight loss of optical density observed.

– Long‐term H depletion during erosion only seen in pure Ar case

1: small instability in bias voltage during this time


35

Ar and H2 Plasma : Multilayer Films

• Change in growth conditions to produce sharply defined interface between hard and soft a‐C:H films

• A three layer stuctureis seen at t1, while further in erosion (t2) a two layer structure is modeled


36

Ar Plasma : Multilayer Films


37

Simulation of Ar Plasma : Multilayer Films

• ~ 1.75 nm densified a‐C:H in soft region; 1.25 nm in hard a‐C:H erosion region

• About 1‐1.5 nm away from the interface, the thickness begins deviating from steady state, indicating that the hard a‐C:H under layer begins seeing ion bombardment


38

H2 Plasma : Multilayer Films

New behavior for soft a‐C:H film

• H saturated film is losing H during H2

+ ion bombardment faster than it is losing C atoms


39

Simulation of Multilayer Films for H2 Plasma


40

Time Resolved Modified Layer Thicknesses

Ar

Very smooth, clear transitionInterface transition consistent with ~1.5 nm modified layer

Gradual, long-term changes, slight decrease with timeInterface transition consistent with ~8 nm modified layer

H2


MD Simulations: 50 eV H2+ on a-C:H

• The affected surface expands initially due to the changing chemistry.• After 8000 impacts (fluence~1017cm-2), 20 angstrom modified layer was formed where C:H ratio is

around 1 • Around 15 Angstrom thickness of film was etched away.

(N. Ning et al.)


MD - Ar on a-C:H

• a-C:H film has 30%H initially• Depth of modification

– 50 eV - ~0.5-1 nm– 100 eV - ~1-1.5 nm– 200 eV - ~2-2.5 nm

200 eV case

• Degree of modification -calculated from average %H of modified thickness

• -50 eV - ~18.9%• -100 eV - ~14.0%• -200 eV - ~10.8%


Conclusions• Ar/H2 plasma interacting with carbon-model surfaces are well-suited

for detailed characterization and understanding of surface processes

• Comparison of modified layer characterization with MD simulation– Good agreement for Ar plasma– For H2 plasma qualitative agreement, but quantitative

differences

• Other Activities:• Characterization of impact of gasified species from H2 ICPs

interacting with carbon-surfaces on electrical properties of plasma

• Application of additional diagnostics• Complex nature of problems provides excellent opportunities for

synergistic collaborations within PSC


Acknowledgments

We gratefully acknowledge support of this work by DOE’s Plasma Science Center

“Predictive Control of Plasma Kinetics: Multi-phase and Bounded Systems” (University of Michigan).

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