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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
Plasma Science CenterPredictive Control of Plasma Kinetics2
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’
Erosion Redeposition
Different SurfaceCarbon or Silicon
SurfaceModified Surface Layer(intensity, extent)
Carbon or SiliconSurface
Modified Surface Layer(intensity, extent)
H2 PlasmaWall
MS
MS
ProbeProbe
OESOES
Real-timeellipsometry
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)
Plasma Science CenterPredictive Control of Plasma Kinetics3
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?
• ….
Plasma Science CenterPredictive Control of Plasma Kinetics4
Objectives• Surface characterization by
ellipsometry• Discharge properties
– Langmuir probe– Ion sampling– Optical emission
spectroscopy
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’
Erosion Redeposition
Different SurfaceCarbon or Silicon
SurfaceModified Surface Layer(intensity, extent)
Carbon or SiliconSurface
Modified Surface Layer(intensity, extent)
H2 PlasmaWall
MS
MS
ProbeProbe
OESOES
Real-timeellipsometry
• 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
Plasma Science CenterPredictive Control of Plasma Kinetics5
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
Plasma Science CenterPredictive Control of Plasma Kinetics6
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
Plasma Science CenterPredictive Control of Plasma Kinetics7
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
Plasma Science CenterPredictive Control of Plasma Kinetics8
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°
Plasma Science CenterPredictive Control of Plasma Kinetics9
ExperimentalD2 or H2
CapillaryAtom Source
D or H
Ellipsometry
Sample onTemperature Controlled
Stage
UHV ChamberRF Plasma Chamber
for a-C:H Films
Plasma Science CenterPredictive Control of Plasma Kinetics10
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
Plasma Science CenterPredictive Control of Plasma Kinetics11
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.
Plasma Science CenterPredictive Control of Plasma Kinetics12
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
Plasma Science CenterPredictive Control of Plasma Kinetics13
Characterization of Polymer Film
• Refractive index of film – or multilayer structure - is obtained by optical modeling of data obtained during processing
Plasma Science CenterPredictive Control of Plasma Kinetics14
Relationship of Refractive Index and Hydrogen Content, Film Density …
• Film properties are strongly reflected in refractive index
soft (polymeric)layer formation
Plasma Science CenterPredictive Control of Plasma Kinetics15
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
Plasma Science CenterPredictive Control of Plasma Kinetics16
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)
Plasma Science CenterPredictive Control of Plasma Kinetics17
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)
Plasma Science CenterPredictive Control of Plasma Kinetics18
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)
Plasma Science CenterPredictive Control of Plasma Kinetics19
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
D0 Fluence (1018 at/cm2)
Mod
ified
("So
ft") L
ayer
Thi
ckne
ss (n
m)
Plasma Science CenterPredictive Control of Plasma Kinetics20
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
D0 Fluence (1018 at/cm2)
Plasma Science CenterPredictive Control of Plasma Kinetics21
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
Plasma Science CenterPredictive Control of Plasma Kinetics22
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
Plasma Science CenterPredictive Control of Plasma Kinetics23
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
Plasma Science CenterPredictive Control of Plasma Kinetics24
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
Plasma Science CenterPredictive Control of Plasma Kinetics25
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 )
D0 Fluence (1018 at/cm2)
Plasma Science CenterPredictive Control of Plasma Kinetics26
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)
Plasma Science CenterPredictive Control of Plasma Kinetics27
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
Plasma Science CenterPredictive Control of Plasma Kinetics28
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
Plasma Science CenterPredictive Control of Plasma Kinetics29
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
Plasma Science CenterPredictive Control of Plasma Kinetics30
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
Plasma Science CenterPredictive Control of Plasma Kinetics31
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
Plasma Science CenterPredictive Control of Plasma Kinetics32
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
Plasma Science CenterPredictive Control of Plasma Kinetics33
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
Plasma Science CenterPredictive Control of Plasma Kinetics34
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
Plasma Science CenterPredictive Control of Plasma Kinetics35
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
Plasma Science CenterPredictive Control of Plasma Kinetics36
36
Ar Plasma : Multilayer Films
Plasma Science CenterPredictive Control of Plasma Kinetics37
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
Plasma Science CenterPredictive Control of Plasma Kinetics38
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
Plasma Science CenterPredictive Control of Plasma Kinetics39
39
Simulation of Multilayer Films for H2 Plasma
Plasma Science CenterPredictive Control of Plasma Kinetics40
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
Plasma Science CenterPredictive Control of Plasma Kinetics41
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.)
Plasma Science CenterPredictive Control of Plasma Kinetics42
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%
Plasma Science CenterPredictive Control of Plasma Kinetics43
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
Plasma Science CenterPredictive Control of Plasma Kinetics44
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).