Download Molecular growth

Dust formation : speculated mechanism
Ar+/H3+ sputtering/chemical sputtering/erosion
Gas phase
Chemistry
Coagulation
nucleation
Agglomeration
Surface growth
C, C2, C3
dN i ~ ~ ~ ~
 Ri  Gi  Wi  Ti
dt
Ni = density of particles with a size i
R = nucleation rate (estimated from the chemical kinetics model)
G = coagulation/agglomeration rate (two particles  larger particles)
W = growth rate (surface growth - heterogeneous chemistry)
T = particle losses due to transport : diffusion, thermophoresis, drag, ...
Model of nucleation, growth and transport
of dust in DC discharges ignited in Ar/H2 (2)
 Estimation of discharge main characteristics: flux and ion energy distribution or
ion average energy on the cathode
 Extraction of C1, C2 et C3 from the substrate surface
 Chemistry and molecular growth  Formation of Cn=1,nl clusters, where nl is
arbitrary chosen (nl=30 or 60)
 Nucleation of carbon dusts from clusters: Assumption of ‘Largest Molecular Edifice’
 Growth, charging, transport and wall losses of dusts
 Feed back on the gas phase chemistry  heterogeneous process
 Size distribution of dusts
Molecular growth modelling
of carbon clusters and dusts
Molecular growth
ni , z
t


   Di ni  i , z nz E  Wi
Diffusion
Mobility
Gas phase chemistry and molecular growth
Production rate of the Ci cluster
Nucleation
nnl , z
t
clusters
ni,z = density of the cluster Ci of charge z
 Wnl ( nl )  N
Dust Transport



 
n
   D.n   .n.z.E  N  C
t
N = nucleation
C = coagulation
A = condensation

   D.n   .n.z.E M  N  A
t
 Determination
of the average
diamater dp
Carbon cluster growth reactions**
Bernholc & Schweigert models (classical models)
(**):
• Growth = one single process (Cn + Cx  Cn+x), but take into account the
stability of the Cn clusters
• First version of the model took into account neutral clusters
•
Molecular growth of clusters
– Rates computed according to formation enthalpies
– Clusters have configurational isomers (chains, rings,
distinguished by cyclization entropy (20 kcal/mol/cycle)
multi-cycles)
– Extrapolation for unknown values according to cluster periodicities
Molecular growth modelling
of neutral carbon clusters and dusts
Low pressure discharge : p=1-10 Pa
Diffusion characteristic time =1-10 ms very short as compared to the growth
chemistry  no possibility for growth of neutral
 Need for species with higher residence time :
Negative clusters
And
Trapping electric field configuration
 Back to some basic discharge physics
Electric field reversal
and molecular growth of negative clusters
•
•
Charging of dust particles only effective if electric field is confining !
Where is the confining electric field ?  Kolobov & Tsendin, Phys. Rev. A
46 7837, Boeuf &PitchFord, J. Phys. D, (1994)
– Self-consistent electric field reversal: confinement
– Three electron populations: energetic, passing, trapped
2 V0/dc Energetic electrons (g)
Passing electrons (j)
E
ef
~<1V
Trapped electrons (ne)
NG
FDS
PC
and negative ions
E0
sheath dcx0
R
x1
x
NG: Negative glow / FDS: Faraday Dark Space / PC: Positive Column
Negative carbon cluster growth reactions
• Attachment Cn + e-  Cn– Rates computed according to
electronic affinities
• Charge exchange Cn- + Cx  Cn + Cx– Electronic affinities
Ti  j  Rij3 e
 
Ai  H j
kT
• Dust agglomeration (sticking)
From Y. Achiba et al., J. Elect. Spect. Related
Phen. 142, 231 (2005)
• Detachment Cn- + e-  Cn + 2e-
carbon particles aerosol dynamic in a DC dicharge
Particle charging is a key point :
==> Enhanced particle charging insures a significant trapping and
long residence time
U=zV
Z
==> Enhanced particle charging prevents coagulation and growth
kcoag
Z Z' ====> Z+Z'
kcoag ( z, z ' ) 
kcoag (0,0)
w( z, z ' )
U
 1
exp  el
U th 

w( z, z ' ) 
U el
U th
U el
U th
Kcoag(z,z’)
The only way to have growth ==> charge fluctuation and electron depletion
Possible because particle charg ing is a discrete process  Dynamic
fluctuation of small particles between positively and negatively charged
states
 Coagulation takes place between two particles that has opposite
instantanous charges or no charge  involve small particles.
tcoag<<tfluctuation<<ttrans
Transport feels the average charge








dqi
div ( J i  qi div ( Fi ) wqcoag  qi wcoag wqgrowth  qi wgrowth I  I




dt
ni
ni
ni
ni
Coagulation feels the fluctuations
Fluctuation
 Te U el 
 (q  q ) 2 
1


 ( q, q ) 
exp 
   f  T ,U 
2
2 
 2
th 


Molecular growth of negative clusters
Negative clusters have significant densities
 Growth rate is a function of the electric field profile in the discharge
 An accurate knowledge of the field profile is required
Dust density
11
10
Anode
10
10
9
10
180
10
160
10
140
10
11
9
120
10
100
10
10
40
<ee>=0.1 eV
6
10
5
10
4
10
np|max
3
10
=5x1011
cm-3
2
0
2
4
6
8
10
12
7
10
6
10
200
<ee>=1 eV
5
10
4
20
10
0
10
Field reversal
3
0
2
-20
10
-40
10
-60
10
-80
10
1
0
0
2
4
6
8
10
12
position (cm)
14
position (cm)
E
400
8
E (V/m)
60
7
np|max=1013 cm-3
10
80
8
10
600
12
dnp (cm^-3)
Cathode
12
10
dnp (cm^-3)
13
10
200
np
Electric field reversal <=> electron average energy in the NG
E  <ee>
14
E (V/m)
13
10
Dust average charge and diameter
Cathode
25
Anode
20
Cathode
-7
7.0x10
-7
6.0x10
0.03 eV
0.1 eV
1 eV
Anode
-7
0.03 eV
0.1eV
1eV
<ee>
10
diamètre (cm)
charge
15
5.0x10
-7
4.0x10
-7
3.0x10
5
-7
2.0x10
0
0
6
position (cm)
12
-7
1.0x10
0
6
12
position (cm)
It is indeed possible to explain particle formation through negative ion driven molecular
growth
 Discharge dynamic (field reversal) and sputtering kinetics are key-points
Pbs : we need better description of the growth kinetics : Model  1 hour for dust formation
(instead of few minutes)
Take into account the size and charge distributions
CASIMIR Device (Chemical Ablation, Sputtering, Ionization, Multi-wall
Interaction, and Redeposition)
3rd
module :
Redeposition chamber
- Collection of the
deposit : filter and
substrate)
2nd module :
1st module :
Microwave plasma source
Sputering/erosion of carbon susbtarte
"surfaguide"
(H2/Armicrowave
plasmas) discharge
Multipolar
Decoupling gas phase and
- Gaz = H2/Ar, Pressure 10-2 mbar
surface process
- carbon Substrate
(Controled temperature and voltage)
Measurement techniques
 Mass spectrometer / ion energy analyzer
- Detection of neutral and radivcalar species in the plasma (m/z 1-500 uma)
- Detection of positive et négative ions
- Measurement of IEDF (+/- 1000 eV)
 Optical Emission Spectroscopy (H/D et carbonated species) (temperature and density
measurements and characterization of plasma species in CASIMIR)
 Analysis of the deposit microstructure by SEM and Raman
Results
I. Mass spectrometry:
Polarisation
Sheath
Polarisation
Sheath
graphite disc
substrate
hotography of the negatively polarized disc
substrate in Ar/H2
Plane
Substrat
Photography of the plane polarized substrate in
Ar/H2 plasma
Resultts
I. Spectromètre de masse / analyseur d’énergie :
b) Mass spectrometry and IEDF measurements : Ions in the discharge
6
3,0x10
Ar
+
+
H3
D3
6
4x10
6
2,5x10
6
10
2+
Ar (Ar )
6
H
Intensité [c/s]
Intensité [c/s]
6
1,5x10
+
6
1,0x10
Intensité [c/s]
3x10
6
2,0x10
6
2x10
5
10
N2
H2O
6
1x10
4
10
5
5,0x10
H2
+
D
+
+
D2
0
0,0
0
10
20
30
40
50
0
0
10
20
Intensité [c/s]
2,0x10
1,5x10
1,0x10
5,0x10
D+, D2+, D3+ mass spectra
(0,60 kW, 100 sccm)
3
3
3
3
3
2
0,0
0
2
4
6
m/z [u.m.a]
8
10
20
30
40
50
m/z [u.m.a]
H+, H2+, H3+ mass spectra
(0,60 kW, 100 sccm)
2,5x10
40
m/z [u.m.a]
m/z [u.m.a]
3,0x10
30
10
D- mass spectrum
(0,60 kW, 100 sccm)
Ar2+, Ar+ mass spectra
(0,60 kW, 10 sccm)
50
Results
c) IEDF
D+
6
3,0x10
Ar/D2
Ar
D2
6
2,5x10
Intensité (c/s)
Ar+
6
2,0x10
6
1,5x10
6
1,0x10
5
5,0x10
0,0
0
5
10
15
Energie (eV)
D+ and Ar+ IEDF’s
20
25
Results
I. Carbon detection :
Detection of C, CH, CH3,CH4 et C2
I.2) deuxième études : sur la tête 1-500 uma
a) Hydrocarbon production
through erosion/sputtering in
CASIMIR
(1) : E between 9,8 and 14,25 eV
CH3 + e- => CH3+ + 2 e- (in the plasma)
(2) : E > 14,25 eV
CH4 + e- => CH3+ + H + 2 e- (in the analyzer)
seuil_plasma_Ar/H2_on_pol_115mA_960V
seuil_plasma_Ar/H2_off_pol
Ar
H2
7
10
Ar/H2
6
10
4
10
3
Intensité [c/s]
Intensité [c/s]
5
10
4
10
10
2
10
3
10
1
10
2
10
0
20
40
60
80
100
0
m/z [u.m.a]
10
5
10
15
20
25
30
35
40
Energie électronique [eV]
Mass spectra in H2, Ar, et Ar/H2
plasma
Threshold mode detection of CH3 radical
CH3
Results
I. Mass spectrometry:
b) Effetc of the polarisation on the erosion yield
Voltage contrôle  microarcs
Courant contrôle 300 mA –
1000 V
600 V – 2 A
plasma_Ar/H2_240V_Alim1
plasma_Ar/H _35mA_600V_Alim2
2
plasma Ar/H2 sans polarisation (Alim1)
Plasma A/H2 avec pol U=240V
6
10
6
10
5
10
5
Intensité [c/s]
Intensité [c/s]
10
4
10
3
10
4
10
3
10
2
10
2
10
1
1
10
10
10
11
12
13
14
15
16
17
18
19
20
m/z [u.m.a]
Mass spectrum in H2 plasma
With and without polarisation (Alim1)
10
11
12
13
14
15
16
17
18
19
20
m/z [u.m.a]
Comparaison of masse spectra obtained with
the two contrôle modes in Ar/H2 plasma