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Modelling of diluted silane-hydrogen VHF discharges
W.J. Goedheer1) and J.K. Rath2)
1) FOM-Instituut voor Plasmafysica „Rijnhuizen‟, Nieuwegein, www.rijnh.nl 2) Debye Institute, SID-Physics of Devices Department, Utrecht University, www1.phys.uu.nl/SID/
INTRODUCTION
Micro-crystalline silicon layers for solar cells can be deposited using diluted silane (SiH4) discharges. Because relatively thick layers are required, high deposition rates are needed for efficient production. A way to achieve this is by using VHF (typically 50MHz) high-pressure (a few Torr) discharges in a mixture of hydrogen and silane. Here we investigate the chemistry of these discharges with a onedimensional fluid model. Specific aspects studied are the influence of the fraction of silane in the inlet flow, the pressure, and the power. The frequency is 50 MHz in all cases. The reactor studied is the ASTER device that has an electrode distance of 27 mm and an electrode radius of 8 cm, so 1 sccm of SiH4 (4.5x107 Si atoms/s), uniformly deposited on both electrodes would give a deposition rate of 0.2 nm/s.
EFFECT OF ADDING MORE SILANE
Adding SiH4 implies: Generation of more silane radicals: increase of fluxes to walls More comsumption of atomic hydrogen: decrease of H flux
The ion flux increases more than linear with power Ion bombardment becomes more intense with power (Ion energy could be reduced by using higher RF frequency)
16
1.0 0.8
-3
Power (W)
2.5
)
Neutral-neutral reactions consume atomic hydrogen: SiH4 + H SiH3 + H2
0.6
2.4
Te 2280 mTorr 100/20 sccm H2/SiH4
Density (m
1.5x10
closed circles : 760 mT 1E17 closed triangles:1520 mT open triangles : 2280 mT
1 2 3 4 5 6 7 8 9 10 20
8.0e21
SiH4 density at 2280 mTorr 100 sccm H2 / 20 sccm SiH4
1.25x10
19
deposition rate (nm/s)
EFFECT OF INCREASE IN PRESSURE
Decrease of electron temperature: less radical production More atomic hydrogen reactions in volume: decrease of flux
16
Central electron temperature (eV)
1.0x10
16
0.4 0.2 0.0
2.3 2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 0 20 40 60 80 100
Reactions at the walls (d.b. is “dangling bond”): SiH3 + H(ads) SiH3 + d.b. H + H(ads) H + d.b. SiH4 + d.b. a-Si:H layer (growth by sticking) H2 + d.b. H(ads)
CHANGE IN ELECTRONEGATIVITY
More power favours production positive ions and electrons Recombination reduces negative ion density At high power ions consume more power in the sheath Increase electron density causes decrease electron temperature
1E20
Dep.rate at 2280 mTorr Dep. rate divided by power 2280 mTorr, 100/20 sccm H2/SiH4
1
maximum
60
Ion flux Plasma potential
1.00x10
19
2280 mTorr, 100/20 sccm H2/SiH4
0.01 10 100
10
Power (W)
0.00 0 20 40 60 80
0 100
Power (W)
10.0e21
-3 Density (m )
SOME IMPORTANT PROCESSES
Electrons colliding with silane produce radicals: SiH4 + e SiH3 + H + e (17%) SiH2 + 2H + e (83%)
2.5x10
16
6.0e21
sccm SiH4 in 100 sccm H2
4.0e21
1.4 1.2
2.0e21
Dep.Rate (nm/s)
H2 + e
760 mTorr 1520 mTorr 2280 mTorr
Ne Nn Np
0.0 0 20 40 60 80 100
2H + e
2.0x10
CONCLUSIONS
Power (W)
A high deposition rate requires a high pressure (H abstraction) and a high power (radical and ion production). Dust formation at high powers is hampered by SiH4 depletion and decrease of silane negative ion density
50
Average Plasma potential (V)
-2 -1 s )
40 7.50x10
18ห้องสมุดไป่ตู้
Total Ion Flux (m
0.1
30 5.00x10
18
-2 -1 s )
20 2.50x10
18
Flu x toward electrode (m
1E19
1E18
H SiH3 SiH2 Si2H5 H SiH3 SiH2 Si2H5 H SiH3 SiH2 Si2H5
5.0x10
15
1
2
3
4
5
6 7 8 9 10
20
0.0 0 20
2280 mTorr, 100/20 sccm H2/SiH4
40 60 80 100
sccm SiH4 in 100 sccm H2
Power (W)
Acknowledgement This work, supported by the European Communities under the contract of association between EURATOM/FOM, was carried out within the framework of the European Fusion Programme with financial support from NWO and NOVEM. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
EFFECT OF INCREASE IN POWER
More efficient: deposition rate per Watt first increases Increase rate levels off due to depletion
10
BEHAVIOUR IONFLUX AND POTENTIAL
W.J. Goedheer1) and J.K. Rath2)
1) FOM-Instituut voor Plasmafysica „Rijnhuizen‟, Nieuwegein, www.rijnh.nl 2) Debye Institute, SID-Physics of Devices Department, Utrecht University, www1.phys.uu.nl/SID/
INTRODUCTION
Micro-crystalline silicon layers for solar cells can be deposited using diluted silane (SiH4) discharges. Because relatively thick layers are required, high deposition rates are needed for efficient production. A way to achieve this is by using VHF (typically 50MHz) high-pressure (a few Torr) discharges in a mixture of hydrogen and silane. Here we investigate the chemistry of these discharges with a onedimensional fluid model. Specific aspects studied are the influence of the fraction of silane in the inlet flow, the pressure, and the power. The frequency is 50 MHz in all cases. The reactor studied is the ASTER device that has an electrode distance of 27 mm and an electrode radius of 8 cm, so 1 sccm of SiH4 (4.5x107 Si atoms/s), uniformly deposited on both electrodes would give a deposition rate of 0.2 nm/s.
EFFECT OF ADDING MORE SILANE
Adding SiH4 implies: Generation of more silane radicals: increase of fluxes to walls More comsumption of atomic hydrogen: decrease of H flux
The ion flux increases more than linear with power Ion bombardment becomes more intense with power (Ion energy could be reduced by using higher RF frequency)
16
1.0 0.8
-3
Power (W)
2.5
)
Neutral-neutral reactions consume atomic hydrogen: SiH4 + H SiH3 + H2
0.6
2.4
Te 2280 mTorr 100/20 sccm H2/SiH4
Density (m
1.5x10
closed circles : 760 mT 1E17 closed triangles:1520 mT open triangles : 2280 mT
1 2 3 4 5 6 7 8 9 10 20
8.0e21
SiH4 density at 2280 mTorr 100 sccm H2 / 20 sccm SiH4
1.25x10
19
deposition rate (nm/s)
EFFECT OF INCREASE IN PRESSURE
Decrease of electron temperature: less radical production More atomic hydrogen reactions in volume: decrease of flux
16
Central electron temperature (eV)
1.0x10
16
0.4 0.2 0.0
2.3 2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 0 20 40 60 80 100
Reactions at the walls (d.b. is “dangling bond”): SiH3 + H(ads) SiH3 + d.b. H + H(ads) H + d.b. SiH4 + d.b. a-Si:H layer (growth by sticking) H2 + d.b. H(ads)
CHANGE IN ELECTRONEGATIVITY
More power favours production positive ions and electrons Recombination reduces negative ion density At high power ions consume more power in the sheath Increase electron density causes decrease electron temperature
1E20
Dep.rate at 2280 mTorr Dep. rate divided by power 2280 mTorr, 100/20 sccm H2/SiH4
1
maximum
60
Ion flux Plasma potential
1.00x10
19
2280 mTorr, 100/20 sccm H2/SiH4
0.01 10 100
10
Power (W)
0.00 0 20 40 60 80
0 100
Power (W)
10.0e21
-3 Density (m )
SOME IMPORTANT PROCESSES
Electrons colliding with silane produce radicals: SiH4 + e SiH3 + H + e (17%) SiH2 + 2H + e (83%)
2.5x10
16
6.0e21
sccm SiH4 in 100 sccm H2
4.0e21
1.4 1.2
2.0e21
Dep.Rate (nm/s)
H2 + e
760 mTorr 1520 mTorr 2280 mTorr
Ne Nn Np
0.0 0 20 40 60 80 100
2H + e
2.0x10
CONCLUSIONS
Power (W)
A high deposition rate requires a high pressure (H abstraction) and a high power (radical and ion production). Dust formation at high powers is hampered by SiH4 depletion and decrease of silane negative ion density
50
Average Plasma potential (V)
-2 -1 s )
40 7.50x10
18ห้องสมุดไป่ตู้
Total Ion Flux (m
0.1
30 5.00x10
18
-2 -1 s )
20 2.50x10
18
Flu x toward electrode (m
1E19
1E18
H SiH3 SiH2 Si2H5 H SiH3 SiH2 Si2H5 H SiH3 SiH2 Si2H5
5.0x10
15
1
2
3
4
5
6 7 8 9 10
20
0.0 0 20
2280 mTorr, 100/20 sccm H2/SiH4
40 60 80 100
sccm SiH4 in 100 sccm H2
Power (W)
Acknowledgement This work, supported by the European Communities under the contract of association between EURATOM/FOM, was carried out within the framework of the European Fusion Programme with financial support from NWO and NOVEM. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
EFFECT OF INCREASE IN POWER
More efficient: deposition rate per Watt first increases Increase rate levels off due to depletion
10
BEHAVIOUR IONFLUX AND POTENTIAL