Chlorine atom and molecule dynamics in an inductively ... - Leti

Chlorine atom and molecule dynamics
Laboratoire de Physique des Plasmas
in an inductively coupled plasma
in pure Cl 2
Jean-Paul Booth
Nishant Sirse, Yasmina Azamoum
Pascal Chabert

•Test/calibrate/improve models : HPEM from Mark Kushner
•Understand pulsed plasmas – for processing
Measure:
• Cl atoms - Absolute densities (time and space resolved)
• Cl 2 molecules ….. ……..
• Electron density
• Gas temperatures
•Dynamics in pulsed plasmas :
Laboratoire de Physique des Plasmas
Objectives
•Cl recombination at the reactor walls

Gas outlet grid
13.56 MHz
power supply
Al 2O 3 window
550mm chamber
200mm chuck
Laboratoire de Physique des Plasmas
Anodized aluminium chamber
ICP reactor at LPP:
Cl 2, HBr, O 2, Ar
Supplied via showerhead
4 turn spiral antenna
Pumping port
-Industrial Scale Reactor
(550mm, 100mm high)
-Industrial gases (Cl 2, HBr)
-All surfaces Al 2O 3
-Advanced diagnostics:
TALIF
Hairpin proben e
Laser photodetachement
IRLAS etc

n e ( x10 16 m -3 )
10
8
6
4
2
Laboratoire de Physique des Plasmas
Electron density (at centre)
50mm from top/Cl 2 /Centre
500W
400W
300W
200W
100W
0
0 20 40 60 80 100
Pressure (mT)
• Measure using microwave
hairpin resonator
• Maximum density at 10 mTorr
– 9x10 16 m -3

Electron density (10 10 cm -3 )
9
8
7
6
5
4
3
2
1
Laboratoire de Physique des Plasmas
Comparison simulation to experiment
Electron density at reactor centre
500W
Experimental
Simulation
0
0 10 20 30 40 50 60 70 80 90
Cl 2 pressure (mTorr)
• Good agreement at very low
pressure
• Increasingly wrong (low) at
higher pressure
• What is wrong in the
simulation?

Ono, JVSTA 1992
Laboratoire de Physique des Plasmas
Two-Photon Laser-Induced Fluorescence
(TALIF) of atomic Chlorine
Laser: 233.2nm,
1-3 mJ, 5ns
Detect Fluorescence
at 726nm (Gated redsensitive
PMT)
•Measures relative concentrations with
high spatial and temporal resolution

TALIF Laser:
233.2nm, 3
mJ, 5ns
Detect Fluorescence at 726nm
(Photomultiplier + filter)
Laboratoire de Physique des Plasmas
Calibration :
Photo-dissociation of Cl 2 at 355 nm
Pumping port
Absorptin cross-section (10 -24 m 2 )
Photolysis Laser
355 nm, 10 mJ 2mm
9% dissociation of Cl 2
30
25
20
15
10
5
Cl 2 absorption spectrum
= 15.9x10 -24 m 2 @355nm
0
250 300 350 400 450 500
Wavelength (nm)

TALIF signal (arb. units)
50 mTorr Cl 2:
5
4
3
2
1
0
Laboratoire de Physique des Plasmas
Compare TALIF signals Plasma/laser:
50 mTorr Cl 2
Laser
photolysis
233.200 233.205 233.210
Laser wavelength (nm)
500 W Plasma
Photolysis products have higher velocity :
Doppler broadening
-Ratio of integrated signals:
S plasma/S laser= 0.498
n Cl = 8.3x10 13 cm -3 @ 50 mTorr 500W
In practice, 355nm beam quality limits
precision (±20%)

Cl atom density, n Cl (10 13 cm -3 )
8
7
6
5
4
3
2
1
0
0 100 200 300 400 500
Laboratoire de Physique des Plasmas
RF Power (W)
Results : Cl density
Cl 2 pressure:
50 mTorr
20 mTorr
10 mTorr
5 mTorr
2 mTorr
Severe test of model, but:
Model : all parameter for one plasma
condition
Expt: one parameter for all plasma
conditions!

n /n Cl 0
Cl2
0.20
0.15
0.10
Laboratoire de Physique des Plasmas
Cl normalised to initial gas density
0.05
20 mTorr
50 mTorr
0.00
0 100 200 300 400 500
RF Power (W)
2 mTorr
5 mTorr
10 mTorr
Normalise to Cl 2 density before plasma:
•Dissociation fraction highest for lowest pressure
•20% @ 2mTorr 500W
• 5-7% at higher pressure, saturates with RF power
•Why so low? (n e peaks at 10 mT)
•What is Cl 2 density during plasma?

• Plasma + 355nm laser :
Laboratoire de Physique des Plasmas
Cl 2 molecule density in plasma
– produces an additional signal,
– n(Cl 2) during plasma
• Calculate Cl 2 density relative
• to “no plasma” from:
nCl I plasma laser I
2
n I
Cl
0
2
laser
plasma
TALIF intensity
20
15
10
5
0
20mTorr 500W
355nm laser
plasma
plasma + 355nm laser
233.200 233.205 233.210
Laser wavelength (nm)

Cl /Cl 2 0
2
1.0
0.8
0.6
0.4
0.2
0.0
Laboratoire de Physique des Plasmas
Cl 2 density
0 100 200 300 400 500
RF power (W)
Cl 2 relative density
2mT
5 mT
10 mT
20mT
50mT
Cl 2 density strongly depleted in plasma:
-increases with RF power, and pressure
-dissociation
-gas heating?

(nCl+nCl )/nCl 2 0
2
1.0
0.8
0.6
0.4
0.2
0.0
0 100 200 300 400 500
Laboratoire de Physique des Plasmas
RF power (W)
Total gas density
Total gas density
(relative to no plasma)
2mT
5 mT
10mT
20mT
50mT
? No other possible product in
pure Cl 2
Total gas density = n Cl + n Cl2
Gas density severely depleted in
the plasma at higher pressure
Strong gas heating?
(Electron pressure negligible
-Max = 0.3mT @10mT)

T (K)
2500
2000
1500
1000
500
Gas temp (without corr)
T(2mT)
T(5mT)
T(10mT)
T(20mT)
T(50mT)
Laboratoire de Physique des Plasmas
Estimate gas temperature
0 100 200 300 400 500
RF power (W)
From ideal gas law :
T plasma =T 0.n 0/n plasma
(total pressure unchanged by plasma)
Estimated T gas = 2300K!
Nb: TALIF only measures Cl density in
P 3/2 ground state.
If P 1/2 state (at 0.11eV) is populated by
electron collisions (up to 50% more
density) T gas 1900K

Cross-section(10 -20 cm 2 )
25
20
15
10
5
Temperature variation of Cl 2 absorption
291K
441K
554K
699K
853K
1038K
0
260 280 300 320 340 360 380 400 420
Laboratoire de Physique des Plasmas
Cl 2 absorption spectrum
355 nm
wavelength (nm)
(Gibson and Bayliss,
Phys. Rev., 44, 188 (1933))
absorption cross-secion (10 -20 cm 2 )
15
10
5
(354.7nm)
Cl 2 absorption cross-section
-Temperature dependence
- cross-section 30%
lower at 2300K ?
0
300 600 900 1200 1500 1800 2100
Temperature (K)
? assume exponential
dependence ?
Cl 2 density underestimated by 30%
at 2300K
Corrected Tgas 1900K

Temperature (K)
1200
900
600
Gas temperature from Ar m spectroscopy:
5mT
10mT
20mT
50mT
300
0 100 200 300 400 500
Laboratoire de Physique des Plasmas
Ar:Cl 2 /50:50/LIF
Power (W)
50% Cl 2/ 50% Ar
Laser-Induced Fluorescence : at reactor
centre
•T m1250K @ 50mTorr 500W
•TALIF : 2300/1900K in pure Cl 2
(Measurements not possible with less
Ar at higher pressures)

Laboratoire de Physique des Plasmas
Cl atom recombination coefficient, g
• Cl recombination at reactor walls has strong effect on plasma
physics and chemistry:
– Determines dissociation fraction Cl/Cl 2
– Impacts etch behaviour – Cl spontaneous etching, uniformity
– Electron collision rate –higher for Cl 2 then Cl (Te)
– Electron attachment e + Cl 2 Cl - + Cl
• plasma transport changed for electronegative plasmas
• Still one of the biggest factors of uncertainty in modelling

Previous measurements of g Cl on Al 2O 3
Kota et al, JVSTA (1997)
g Decreases with temperature
-poor precision at higher T
-ex-situ measurement
Laboratoire de Physique des Plasmas
Guha et al JAP (2008)
-spinning wall technique
-quasi-in-situ
g depends on n Cl/n Cl2 ratio
-but surface polluted by SiO 2
-large scatter for high Cl/Cl 2
Compare to in-situ measurements:

TALIF signal (arb. units)
Determine Cl lifetime in
afterglow of a pulse-modulated plasma by TALIF:
40
30
20
10
0
0.000 0.005 0.010 0.015 0.020 0.025
Laboratoire de Physique des Plasmas
20mT Cl 2
100W
200W
500W
time (s)
Excellent S/N ratio
-follow decays up to 50 ms
-pure Cl 2 :
-no gas-phase loss processes
-Directly deduce surface
recombination coefficients g Cl
first ms
-non-exponential decays
-gas cooling
Assume gas is cold after 2ms

(Chantry, J.Appl.Phys. 62,1141, (1987) For a cylinder:
1
k
g presumed equal on all surfaces
-Diffusion Coefficient for Cl in Cl 2 :
Laboratoire de Physique des Plasmas
Determining recombination coefficient
from decay rates
wall
2
D
0
V
S
D 0=0.149cm 2 s -1 @1 atm 298K
3
2 T
D D0
.
298
2(2 -g
)
.
vg
.
p
p
0
1
2
0
v
=
L
8kBT
m
2
+
2
2.4
R

surface loss coefficient g
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Loss rates and surface recombination coefficient
Cl surface loss coefficient
100W
200W
500W
0.00
0 5 10 15 20
Laboratoire de Physique des Plasmas
pressure (mT)
• Deduced from decay 2-10ms
• g very high ! 0.2-0.35
(compare 0.02 by Guha et al)
•Pressure : decrease in g
•RF power : only a small effect. Trend
seems to depend on pressure

TALIF signal
10
1
0.1
Laboratoire de Physique des Plasmas
Background signal at long times :
10 mT 200W
0.00 0.01 0.02 0.03 0.04 0.05
time (s)
Model Exponential
Equation y = y0 + A*ex
p(R0*x)
Reduced Chi
-Sqr
0.1523
Adj. R-Squar 0.99599
Value Standard Err
1 y0 0.23357 0.06247
1 A 33.5583 0.35147
1 R0 -406.348 7.69425
-almost constant background signal in
late afterglow (20 ms)
-has same spectral shape as TALIF
signal
-Increased strongly by small O 2
addition/low flow rates
g falls to low value in late afterglow
-Saturation of reaction sites?
-The value of g in the late afterglow is
more comparable to those measured by
Guha and by Kota…..

Laboratoire de Physique des Plasmas
Conclusions
• Cl atom and Cl 2 molecule absolute density determined by a new TALIF
calibration scheme
– Cl density is small compared to initial gas density (

• Determine n Cl2 in afterglow
• Measure Cl P 1/2 density by TALIF
Laboratoire de Physique des Plasmas
Future work
• Cl - negative ion density by laser photodetachment
• Model improvements
– Add missing gas heating terms
– Model the afterglow – test transport modelling
– Model the breakdown phase/ vompare to experiments
• Other gases:
– O 2, HBr, mixtures
• Work partly supported by ANR INCLINE (ANR-09 BLAN 0019) & AMAT

TALIF signal
10
1
0.1
Laboratoire de Physique des Plasmas
Effect of flow rate /residence time
Effect of flow rate/residence time
10mT 200 W
0.00 0.01 0.02 0.03 0.04 0.05
Time (s)
10 sccm
20 sccm
50 sccm
100 sccm
Low flow long residence time
increased importance of air leaks
Due to O 2 impurities?
- Add small amount of O 2 at high flow rate :

Cl TALIF signal (arb. units)
10
1
Laboratoire de Physique des Plasmas
Effect of impurities: small O 2 addition
Effect of O addition 2 O2 addition has same affect as
reducing flow rate
10mT 200W
50 sccm Cl 2
0.1
0.00 0.01 0.02 0.03 0.04 0.05
Time (s)
+3 sccm O 2
+2 sccm O 2
+1 sccm O 2
pure Cl 2
Flow rate effect appears to be linked
to O 2 impurities:
Possible origins:
•Saturation of Cl reaction sites by O 2
when no ion bombardment?
•Formation of O xCl y product,
photolysed by 233nm laser?

TALIF intensity
25
20
15
10
5
0
Laboratoire de Physique des Plasmas
Persistent signal in afterglow
10 mT 200W
10sccm Cl 2
3 sccm O 2
233.185 233.190
wavelength (nm)
0 ms
20 ms
20ms (x3.7)
Identical centre wavelength and line-width
as Cl TALIF at start of afterglow:
Doesn’t appear to be photolysis of a Cl xO y
product (would have high energy-large
Doppler width)
TALIF signal
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Decay of slow sigal:
50 Cl 2 + 3 O 2
0.01 0.02 0.03 0.04 0.05
Time (s)
Observed decay rate 5-10s -1
Pump-out rate 2.5s -1 @ 10sccm
-corresponds to g of 1-2%
(comparable to Guha et al)

nCl+ nCl2
1.0
0.8
0.6
0.4
0.2
Laboratoire de Physique des Plasmas
Effect of adding 50% Cl P 1/2:
Cl + Cl2 (without correction)
Tot(2mT)
Tot(5mT)
Tot(10mT)
Tot(20mT)
Tot(50mT)
0.0
0 100 200 300 400 500
RF power (W)
Total gas density
total density
1.0
0.8
0.6
0.4
0.2
tot with spin orbit
0.0
0 100 200 300 400 500
RF power (W)
Tot(2mT)
Tot(5mT)
Tot(10mT)
Tot(20mT)
Tot(50mT)

n Cl /n Cl +n Cl2 )
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Pressure
(mTorr)
2
5
10
20
50
Laboratoire de Physique des Plasmas
Effect of adding 50% Cl P 1/2:
mole fraction Cl/ (Cl + Cl 2)
Cl mole fraction
(without P 1/2 correction)
0.0
0 100 200 300 400 500
RF power (W)
n Cl /(n Cl +n Cl2 )
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Mole fraction n Cl /(n Cl +n Cl2 )
(with P 1/2 correction)
Pressure (mTorr)
2
5
10
20
50
0.0
0 100 200 300 400 500
RF power (W)

Temperature (K)
1200
900
600
Gas temperature from Ar m spectropscopy:
2mT
3mT
5mT
10mT
Laboratoire de Physique des Plasmas
Absorption:
Ar:Cl 2 /10:90
300
0 100 200 300 400 500
Power (W)
Cl 2/Ar 90/10%
Temperature (K)
1200
900
600
2mT
3mT
5mT
10mT
Absorption Fluorescence
No signal above 10 mTorr;
Tm 800K @ 10mTorr 500W (comparable to 50% Ar)
Ar:Cl 2 /10:90/LIF
300
0 100 200 300 400 500
Power (W)

Laboratoire de Physique des Plasmas
Absolute Cl calibration techniques
• Niemi et al. : compare to rare gas TALIF for H, O, N
– No suitable rare gas transition near 233nm
• Ono et al: use 233nm TALIF beam to photolyse CCl 4 (known
pressure, no plasma)
– Gave Cl densities 3x higher than total gas density!
– Depends on knowledge of spatio-temporal profile of 233nm beam
at waist
– Integrate rate equations for photolysis + simultaneous excitation
– Photolysed Cl fly out of the (very small) detection volume
– These effects lead to over-estimation of effective Cl density
• A better technique is needed!

n Cl /n Cl2
1.0
0.5
P (mTorr)
2
5
10
20
50
0.0
0 100 200 300 400 500
Laboratoire de Physique des Plasmas
RF power (W)
Ratio n Cl/n Cl2
without correction Maximum 1.2 at 2 mTorr 500W
(Cl mole fraction 0.55)
Increasing RF power causes more
gas heating, rather than
dissociation!

n e ( x10 16 m -3 )
10
8
6
4
2
Laboratoire de Physique des Plasmas
Electron density (at centre)
50mm from top/Cl 2 /Centre
500W
400W
300W
200W
100W
0
0 20 40 60 80 100
Pressure (mT)
• Measure using microwave hairpin
resonator
• Density at reactor centre unless
stated
• Maximum density at 10 mTorr
– 9x10 16 m -3

n e ( x10 16 m -3 )
10
8
6
4
2
0
Laboratoire de Physique des Plasmas
Electron density :radial profile
50mm from top/ Cl 2 / 500W / radial scan
3mT
5mT
10mT
0 50 100 150 200 250
Position (mm)
Peaked at centre for low
pressure ……

n e ( x10 16 m -3 )
10
8
6
4
2
0
Laboratoire de Physique des Plasmas
Electron density :radial profile
50mm from top/ Cl 2 / 500W / radial scan
10mT
20mT
50mT
90mT
0 50 100 150 200 250
Position (mm)
Peaked at centre for low
pressure
Peaks away from axis at higher
pressure
- Local density maximum under
coil

T (K)
2500
2000
1500
1000
500
Laboratoire de Physique des Plasmas
Effect of adding 50% Cl P 1/2:
Gas temp (without corr)
T(2mT)
T(5mT)
T(10mT)
T(20mT)
T(50mT)
0 100 200 300 400 500
RF power (W)
Gas temperature
T (K)
2500
2000
1500
1000
500
T(2mT)
T(5mT)
T(10mT)
T(20mT)
T(50mT)
-reduces peak temperature from 2300K to 1900K
Gas temperature (including P1/2)
0 100 200 300 400 500
RF power (W)

Laboratoire de Physique des Plasmas
Previous gas temperature measurements
Steady state
Cunge JVSTA 2009
700W Cl 2/Ar
centre
wall
Determined from Ar m Doppler width.
Line-integrated technique
-Strong axial T gradient
-estimated 1000K at the centre
Factor 2 lower than our observations
(& at higher power density!)
IRLAS measurements in our reactor….

TALIF signal
10
1
0.1
0.00 0.01 0.02 0.03 0.04 0.05
time (s)
Laboratoire de Physique des Plasmas
Decays vs pressure and power
5 mT
500W
200W
100W
TALIF signal
100
10
1
0.1
10 mT
500W
200W
100W
0.00 0.01 0.02 0.03 0.04 0.05
TALIF signal
10
1
0.1
20 mT
500W
200W
100W
0.00 0.01 0.02 0.03 0.04 0.05
•At long times : presence of an almost constant background signal in all cases.
•Reasonable fit with single exponential + offset
•RF power only has a small effect
•NB data taken after stabilisation; initial rates (when plasma first turned on) faster by
10-20% (surface temperature effect?)
time (s)
time (s)