Plasma-Surface Interactions at a “Spinning Wall”
Vincent M. Donnelly
Department of Chemical Engineering
University of Houston
Houston, Texas
Students: Joydeep Guha (now at Lam Research), Rohit Khare
Postdocs: Peter Kurunczi (now at Varian), Luc Stafford (now at Univ. Montreal)
Visiting Professor Yi-Kang Pu (Tsinghua University, Beijing, China)
Supported by the National Science Foundation, the Department of Energy, the American Chemical Society’s Petroleum
Research Fund, the University of Houston, and Lam Research Corp.
Classes of Catalytic Reactions on Plasma Chamber Walls
(Catalytic means walls are not consumed)
Knowledge / Treatment
Ion Neutralization
and Fragmentation


wall


wall
A( g )  e  
  A( g )
A( g )  e  
  products
Good / Unit probability for this channel.
(g)
Poor / Usually ignored or adjustable
parameter
Neutral Recombination
and Reactions
2 A( g )  
  A2 ( g )
wall
A( g )  B ( g )  
  products
Fair to poor / A few published coefficients.
Usually an adjustable parameter
wall
(g)
Poor / A handful of studies. Usually ignored
or an adjustable parameter.
“SPINNING WALL” Method for Studying Plasma-Surface
Interactions
to
pump
to differentially-pumped
mass spectrometer, or
10-3P
plasma
Auger Electron
spectrometer
10-6P
pres.=P
highspeed
motor
spinning cylinder
surface exposed
to plasma
PLASMA REACTOR, SPINNING WALL AND MASS SPEC.
Feed gases
(Cl2 or Cl2/O2
in this talk)
Line-of-sight gas from spinning surface =
Chopper open signal – chopper closed signal
differential
pumping
Ionization gauge
mass spectrometer (Extrel)
tuning fork
chopper
pumping
differential
pumping
differential
pumping
anodized Al
reactor
AUGER SPECTRA OF “SEASONED” REACTOR WALL DURING LONG
EXPOSURES TO Cl2, O2, OR N2 PLASMAS, 5 mTorr, 600 W
120
Cl
100
LVV
O K L L (1 .1 )
(8 .1 )
N K L L (0 .9 2 )
Eb = 5 keV
N 2 p la s m a
60
A g M N N (3 .3 )
O 2 p la s m a
40
S i LV V (1 .5 )
E d I/d E (a .u .)
80
20
C l 2 p la s m a
0
A l K L L (0 .4 5 )
-2 0
S i K L L (0 .2 6 )
0
200
400
600
800
1000
1200
1400
1600
E (e V )
• Wall is AlySixOz with ~5% Cl, O, or N in Cl2, O2 or N2 plasmas –
Cl in Cl2 corresponds to ~1-2 x 1014cm-2 Cl at the surface.
• Si from erosion of quartz discharge tube; ~1% Ag from Ag-plated gaskets.
• Large amount of Cl in N2 plasmas, compared to O2 plasmas.
Cl2 MASS SPECTROMETER SIGNALS: PLASMA ON OR OFF
25
C l 2 M a s s S p e c . S ig n a l (a rb . u n its )
5 m T o rr C l 2
20
J. Guha, V. M. Donnelly, Y-K. Pu, J.
Appl. Phys. 103, 013306 (2008);
600 W
15
10
100 W
5
0 W
0
0
5
10
15
20
25
30
35
3
R o ta tio n F re q u e n c y (1 0 rp m )
• Plasma-ON signals are a result of desorption of Cl2 formed by recombination of Cl
on the spinning wall surface.
• Plasma-OFF signal is a result of desorption of physisorbed Cl2.
ATOM RECOMBINATION: Experiment Detects Delayed (L-H)
Recombination, not prompt (E-R)
Plasma (e.g. Cl2
plasma)
Cl atoms
Eley-Rideal (E-R) product (Cl2)
(if occuring, not detectable)
Langmuir-Hinshelwood
(L-H) product (Cl2)
Mass Spec.
Reaction time  1/(2f)
ABSOLUTE Cl2 DESORPTION FLUXES FROM ANODIZED-Al
EXPOSED TO A Cl2 PLASMA (plasma off removed)
Cl(g)  Cl(ads) in plasma, followed by 2Cl(ads) Cl2 in mass spec.
5 m T o rr
2 0 m T o rr
600 W , 400W , 200 W , 100 W
-1
15
C l 2 F lu x (c m s )
10
10
15
-2
-2
-1
C l2 F lu x (c m s )
600 W , 400W , 200 W , 100 W
fro m P 2 m e a s u re m e n ts
10
14
0 .0 0
fro m P 2 m e a s u re m e n ts
10
0 .0 1
0 .0 2
T im e (s )
0 .0 3
0 .0 4
14
0 .0 0
C l 2 F lu x (cm s )
0 .0 4
10
15
-2
-1
15
-2
-1
0 .0 3
600 W , 400W , 200 W , 100 W
600 W , 400W , 200 W , 100 W
C l 2 F lu x (cm s )
0 .0 2
T im e (s )
1 0 m T o rr
1 .2 5 m T o rr
10
0 .0 1
10
14
0 .0 0
10
0 .0 1
0 .0 2
0 .0 3
T im e (m s )
0 .0 4
14
0 .0 0
0 .0 1
0 .0 2
T im e (s )
0 .0 3
0 .0 4
TIME-RESOLVED AUGER SPECTRA OF SPINNING WALL
DURING Cl2 PLASMA EXPOSURE, 5 mTorr, 600 W
Eb = 1.5 keV
2 .5
___
___
P la sm a O n
P la sm a O ff
2 .0
___
krp m :
___
30
(1 m s)
P la sm a O n
krp m :
P la sm a O ff
1 .5
30
(1 m s)
25
25
20
7
E d I/d E (1 0 u n its )
1 .5
20
1 .0
15
15
10
10
1 .0
0 .5
5
5
3
3
0 .5
1 .3
(2 3 m s)
0 .0
1 .3
(2 3 m s)
0 .0
S i (L V V )
O (K L L )
C l (L V V )
-0 .5
40
60
80
100 120 140 160 180 200 220
E (e V )
440
460
480
500
E (e V )
520
540
560
TIME-RESOLVED PEAK-TO-PEAK AUGER INTENSITIES
Cl2 plasma, 5 mTorr, 600 W, Eb = 1.5 keV
1 .8
P la sm a o n
P la sm a o ff
[C l o n /C l o ff ]/[O o n /O o ff ]
1 .7
R a tio o f p e a k -to -p e a k C l/O
1 .6
1 .5
1 .4
1 .3
Dashed line
corresponds to
time-independent
Cl coverage.
1 .2
1 .1
1 .0
0 .9
CONCLUSION:
0 .8
Cl undergoing
recombination
accounts for <10%
of the total Cl
coverage.
0 .7
0 .6
0
5000
10000
15000
20000
ro ta tio n fre q u e n cy (rp m )
25000
30000
Extracting Cl L-H Recombination Probabilities
In P la s m a
Cl2 plasma
0 .4
 = 0 .1 s
C i (  /3 )
F a c in g M a s s S p e c .
 = 0 .0 1 s
{
(L-H) Cl2
C i(t)/A i, A d so rb e d C l 2
Cl atoms
Mass Spec.
O u t o f P la s m a
0 .5
0 .3
0 .2
0 .1
C i(2  /3 )
 = 0 .0 0 1 s
C i(0 )
C i (0 )
 /2 = t
0 .0
0 .0 0 0
r
0 .3 3 3
0 .6 6 7
1 .0 0 0
t/  , tim e
• When the sample is rotated much faster than the desorption rate, desorption and
coverage become independent of time and achieve their average values.
• Therefore as f   (i.e. t  0) it is as though the sample were continuously exposed to
a Cl flux of 1/3 that in the plasma, Cl.
• Therefore LH recombination probability,  Cl 
6D f 
Cl
R e c o m b in a tio n c o e ffic ie n t ( C l)
Cl Atom LH Recombination Probabilities on Anodized Al as a
Function of Cl Flux and Total Pressure
1 .2 5 m T
10m T
5m T
20m T
0 .1
0 .0 1
0
2
4
6
8
17
10
12
14
2
C l flu x (  C l) (1 0 a to m s /c m s )
• Cl is small and appears to both increase and decrease with increasing Cl flux
R e c o m b in a tio n c o e ffic ie n t (  C l)
Cl Atom LH Recombination Probabilities on Anodized Al as a
Function of Cl-to-Cl2 Number Density Ratio
1 .2 5 m T
10m T
5m T
20m T
0 .1
0 .0 1
0 .0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 .8
n C l/n C l2
• Cl scales with Cl-to-Cl2 flux ratio.
• Suggests Cl2 may block sites for Cl adsorption and recombination.
• See J. Guha, V. M. Donnelly, Y-K. Pu, J. Appl. Phys. 103, 013306 (2008); L. Stafford, R. Khare, J.
Guha, V. M. Donnelly, J-S. Poirier and J. Margot, J. Phys. D, Appl Phys. 42, 055206 (2009).
Cl Recombination on Anodized Aluminum vs. Stainless Steel
0 .1
Cl
A n o d iz e d a lu m in u m
0 .0 1
S ta in le s s s te e l
0 .0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 .8
0 .9
n C l / n C l2
• Similar recombination probabilities because they are both coated with a
SiOxCly layer.
• Stainless values actually lower, probably because the surface is smoother
(electropolished).
R e c o m b in a tio n c o e ffic ie n t  C l
Reported Cl Recombination Coefficients on Chlorine
Plasma-Conditioned Stainless Steel
M a lys h e v a n d D o n n e lly, IC P
10
-1
S p in n in g -s u b s tra te
(th is s tu d y)
10
C o rr e t a l., IC P
-2
S ta ffo rd , S W P
R ic h a rd s a n d S a w in , C C P
10
-3
0 .1
1
n C l / n C l2
10
Proposed Site Blocking Mechanism for Cl
Heterogeneous Recombination in Cl2 Plasmas
High Cl2/Cl density
Low Cl2/Cl density
“A global (volume averaged) model of a chlorine discharge”
E. G. Thorsteinsson and J. T. Gudmundsson
Plasma Sources Sci. Technol. 19 (2010) 015001
• Points: Experiments
(Malyshev and
Donnelly)
• Lines: Their model.
• No adjustable
parameters.
WHAT DOES TIME DEPENDENCE OF DESORPTION TELL US?
Proposed Mechanism for Cl Recombination, Cl2 Adsorption and Cl2
Desorption
Plasma On
Cl ( g )  a i  Cl ( ads )  a i
Cl ( g )  Cl ( phys )
Cl ( phys )  Cl ( ads )  a i  Cl 2 ( ads )  a i
Cl 2 ( ads )  a i  Cl 2 ( g )  a i
Plasma Off
Cl 2 ( g )  a i  Cl 2 ( ads )  a i
Cl 2 ( ads )  a i  Cl 2 ( g )  a i
Same
Process?
Time-Dependence of Observed and Modeled Desorption
AES or
MS side
plasma side
2/3
=
C

  s1    k d C
dt
A

dC
=0
tr=1/(2 f)
In P la s m a
D i/  s i, D e s o rb in g C l 2
spinning-substrate
O u t o f P la s m a
0 .4
 = 0 .1 s
C i (  /3 )
F a c in g M a s s S p e c .
 = 0 .0 1 s
{
C i(t)/A i, A d so rb e d C l 2
0 .5
0 .3
0 .2
0 .1
0 .0
0 .0 0 0
C i(2  /3 )
1
0 .1
 = 0 .0 0 1 s
C i(0 )
C i (0 )
 /2 = t
r
0 .3 3 3
0 .6 6 7
t/  , tim e
1 .0 0 0
0
20
40
60
k d ,e  , tim e
80
100
D i/  s i, D e s o rb in g C l 2
Predicted vs. Observed Cl2 Desorption Kinetics
10
O b s e rve d C l 2 d e s o rp tio n :
c h lo rin e p la s m a
1
M o d e l p re d ic tio n o f
C l 2 d e s o rp tio n
0 .1
0
20
40
60
k d ,e  , tim e
• Why so different?: Multiple rates.
 Distribution of surface sites
80
100
Time-Dependence of Observed and Modeled Desorption
Cl2 adsorption – desorption
P re ssu re
(Plasma OFF)
-2
-1
cm s )
10
(1 0
14
20m T
O FF
10m T
1
Df
O FF
, f
5m T
1 .2 5 m T
0 .1
0
5
10
15
20
25
30
35
40
• Adsorbed Cl2 formed by Cl2 adsorption.
• Adsorbed Cl2 also formed by Cl recombination.
• From our measured Cl recombination
probabilities we can calculate the amount of
adsorbed Cl2 due to Cl recombination.
• Lets assume Cl2 desorption is rate limiting.
• Use Cl2 desorption kinetics with plasma OFF to
compute kinetics with plasma ON. NOTE:
NOTHING CHANGED – just turn on plasma.
• Compare model to measurements.
D e ca y tim e (m s)
Assumed Gaussian distribution of binding energies for Cl2
adsorption and desorption, used to predict decays (lines).
5 .0
10 m
13
-2
S ite D e n s ity (1 0 c m )
4 .0
3 .0
2 .0
1 .0
Anodized Al
0 .0
6
7
8
9
10
11
12
13
14
15
16
B in d e n e rg y (kca l/m o l.)
17
18
19
20
Time-Dependence of Observed and Modeled Desorption
Assumed Gaussian distribution of binding
energies for Cl2 adsorption and desorption,
used to predict decays (lines).
5 .0
13
-2
S ite D e n s ity (1 0 c m )
4 .0
3 .0
2 .0
1 .0
• Adsorbed Cl2 formed by Cl2 adsorption.
• Adsorbed Cl2 also formed by Cl recombination.
• From our measured Cl recombination
probabilities we can calculate the amount of
adsorbed Cl2 due to Cl recombination.
• Lets assume Cl2 desorption is rate limiting.
• Use Cl2 desorption kinetics with plasma OFF to
compute kinetics with plasma ON. NOTE:
NOTHING CHANGED – just turn on plasma.
• Compare model to measurements.
0 .0
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
B in d e n e rg y (kca l/m o l.)
b ) 1 0 m T o rr C l , 6 0 0 W
2
-2
-2
-1
-1
(1 0 cm s )
(1 0 cm s )
a ) 1 .2 5 m T o rr C l , 6 0 0 W
2
1
Df
Df
re c
re c
15
15
1
0 .1
0 .1
0
10
20
30
t r (m s )
40
50
0
10
20
t r (m s )
30
40
50
O RECOMBINATION ON ANODIZED-Al EXPOSED TO AN O2 PLASMA
M a s s S p e c . In te n s ity (a rb . u n its )
14
O 2 P la s m a , 2 .5 m T o rr 6 0 0 W
12
n o ze ro rp m
su b tra ctio n
10
400 W
8
200 W
6
4
100 W
2
0 W
0
0
5
10
15
20
25
30
35
40
3
R o ta tio n F re q e n cy (1 0 rp m )
• Similar to Cl2 plasma, but no physisorbed O2 (i.e. no increase in O2 signal vs. rpm with
plasma off).
• P. F. Kurunczi, J. Guha, and V. M. Donnelly, J. Phys. Chem. B 109, 20989 (2005); P. F. Kurunczi, J.
Guha, and V. M. Donnelly, Phys. Rev. Lett. 96, 018306 (2006); J. Guha, P. Kurunczi, L. Stafford, and V. M.
Donnelly, J. Phys. Chem. C 112, 8963 (2008).
O Recombination in Oxygen Plasmas
Similar kinetics, different mechanism (no O2 site blockage)
O ( g )  a i  O ( ads )  a i
• Vary kd,I distrubution to get the
best fit of the model to the
experimental measurements.
O ( g )  O ( phys )
O ( phys )  O ( ads )
kd,i
 a i  O 2 ( ads )  a i
O 2 ( ads )  a i  O 2 ( g )  a i
1 .4
2 .5 m T o rr O 2 , 6 0 0 W
-1
(1 0 c m s )
-2
S ite D e n s ity (1 0 c m )
1 .2
12
-2
1 .0
14
0 .8
0 .6
2 .5 m T o rr O 2 , 1 0 0 W
Df
re c
0 .4
1
0 .2
2 0 m T o rr O 2 , 1 0 0 W
0 .0
-4
-3
-2
10 10 10 10
-1
0
1
2
3
4
5
6
10 10 10 10 10 10 10 10
-1
k d ,i (s )
7
0
5
10
15
20
t r (m s )
25
30
35
40
DESORPTION MASS SPECTRA OF Cl2/O2 PLASMAS
1400
35
C lO
re d = p rim a ry p ro d u c ts
b lu e = d a u g h te r io n s
+
• ClO and ClO2 are desorption products:
2 0 % C l2 / 8 0 % O 2
1000
800
Cl
600
O2
37
+
+
35
37
400
C lO
+
C lO 2
C lO 2
O(ads) + Cl(ads)  ClO(g)  ClO(ads)

+
2O(ads) + Cl(ads)  ClO2(g)  ClO2(ads)

+
35
C l2
+
200
-1
cm s )
In te n s ity (a rb . u n its )
1200
14
70
5 m T o rr, 6 0 0 W
2 0 ,0 0 0 rp m
-2
0
C l2
30
35
40
45
50
55
60
65
70
75
80
re d = p rim a ry p ro d u cts
b lu e = d a u g h te r io n s
1000
8 0 % C l2 / 2 0 % O 2
35
800
Cl
600
+
35
35
O2
C l2
+
400
200
C lO
14
1200
C lO
37
+
37
Cl Cl
+
+
35
C lO
+
C lO 2
+
37
C l2
+
D e s o rp tio n F lu x ( x 1 0
In te n s ity (a rb . u n its )
1400
12
10
8
C lO 2
6
4
O2
2
0
35
40
45
50
55
m /e
60
65
70
75
67
0
0
30
51
80
20
40
60
% O 2 /(C l 2 + O 2 )
80
100
Mixed Cl2 / O2 Plasmas: Recombination and
Reactions of Cl
i.e. Cl(g) + Cl(ads)  Cl2(g)
Cl(g) + O(ads)  ClO(g)
D e fin e  C l,to ta l a s th e p ro b a b ility
0 .3
C l 2 , C lO o r C lO 2
0 .1

Cl
 , 
C l,to ta l
th a t im p in g in g C l w ill fo rm
Cl
0 .0 3
0 .0 1
0
20
40
60
% O 2 /(C l 2 + O 2 )
80
100
Mixed Cl2 / O2 Plasmas: Why does O2 addition have little
effect on Cl, yet addition of Cl2 suppresses Cl?

C l,to ta l
0 .1

Cl
 , 
C l,to ta l
0 .3
Cl
0 .0 3
0 .0 1
0
20
40
60
% O 2 /(C l 2 + O 2 )
80
100
Proposed Site Blocking Mechanism for Cl
Heterogeneous Recombination in Cl2 Plasmas
Cl2 sticks and
blocks sites for Cl
recombination
O2 does not
-1
14
source
-2
Coat the sample with trace Cu while
it is exposed to an O2 plasma –
simulate contamination during via
etching Cu PVD
cm s )
Effect of Trace Cu on O-Atom Recombination in an O2 Plasma
12
-2
C u d o s e (1 0 c m )
0
4 .0 2
8 .0 4
4
3
D (x 1 0
O2
Plasma
Auger
spectrometer
8
7
6
5
2
1
0 .0
5 .0
1 0 .0
1 5 .0
14
-2
14
-2
C u d o s e : 9 .6 x 1 0 c m
4
In te n sity C o u n ts ( x 1 0 )
C u d o s e : 2 .4 x 1 0 c m
9 .4 0
R e co m b in a tio n co e fficie n t (  O )
t r (m s )
Cu
9 .3 5
9 .3 0
9 .2 5
0 .0 9
A fte r C u d o s e s
0 .0 8
0 .0 7
0 .0 6
0 .0 5
B e fo re C u
0 .0 4
0 .0 3
7
850
900
E n e rg y (e V )
950
8
9
10
C yc le #
11
12
13
O Recombination on Ti-contamined Surface in Oxygen
Plasmas
Si
O
0 .0 4 0
R ecom bination P robability (  O )
Ti
5
4 5 m in s o f T i d e p o s itio n
4
E*dI/dE (a.u.)
3
5 m in s o f T i d e p o s itio n
2
1
s ta rtin g s u rfa c e
0
-1
0 .0 3 5
5 m in s T i e x p o s u re
0 .0 3 0
4 1 % d e c re a s e
d u e to T i e x p o s u re
0 .0 2 5
4 5 m in s T i e x p o s u re
0 .0 2 0
0
200
400
600
800
1000
1200
1400
1600
1800
K .E . (e V )
• O recombination probability decreases by 41% after 5% Ti surface
coverage.
Proposed Mechanism
Cu+
Dangling bond
Cu(g) +
O
O
Si
O
Cu+
Si
O
OOO O
O
O2(g)
Si
O
Cu2+
O
Si
OOO O
Si Si Si Si
O
Ti
ref[1]
Si
O O O O + Ti(g)
Si Si Si Si
Cu2+
O + O
Ti
+ O
OOO O
Si Si Si Si
O
Cu+
O
Si
Ti4+
[1] J. Guha et. al. J. Appl. Phys. 105, 113309 (2009)
[2] J.P. Lafemina, Crit. Rev. Surface chemistry 3 (1994) 297
O
Si Si Si Si
O
Charge transfer
[2]
4+ (autocompensation)
Ti
OOO O
Si Si Si Si
SUMMARY
• Cl Langmuir-Hinshelwood (L-H) recombination seems to be limited by Cl2
desorption.
• The mean binding energy for Cl2 on anodized Al is 13 kcal/mol. and the range of
binding energies is ~9 to 17 kcal/mol.
• Cl recombination coefficient increases with Cl-to-Cl2 number density ratio.
• O recombination on anodized Al follows kinetics with a range of rates at
distributions of sites, but the mechanism is different from Cl recombination – no
O2 site blockage.
• Our  values have been used in a global model by Thorsteinsson and
Gudmundsson. With no adjustable parameters, their model reproduces Cl
densities measure by Maylshev and Donnelly in a chlorine ICP.
• Trace Cu surface contamination catalyzes O recombination.
• Small amounts of surface Ti suppresses O recombination.