Alan Howling, Ben Strahm, Christoph Hollenstein. Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland. Center for Research in Plasma Physics ...
Plasma modelling & reactor design Alan Howling, Ben Strahm, Christoph Hollenstein Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland Center for Research in Plasma Physics (CRPP)
1) The "plasma dimension" for plasma deposition of μc-Si:H 2) Direct pumping of a plasma reactor ... and one non-intrusive plasma diagnostic for each.
IWTFSSC-2 Berlin April 2009
1) Plasma-Enhanced Chemical Vapour Deposition of μc-Si:H
2
silane
hydrogen
ΦH
black box PECVD
ΦSiH
4
1) PECVD reactor setup
ΦH
PRF[W] f [MHz] 2
hydrogen
Plasma Box, Oerlikon principle: Jacques Schmitt
butterfly valve process pumps
p
silane
vacuum chamber
ΦSiH
RF connection 4
showerhead RF electrode
gas flow
heating
grounded box
plasma box side view glass substrate
1) Hydrogen dilution for plasma deposition of μc-Si:H c 1
2
0.05 0
silane
hydrogen
ΦH
plasma box
ΦSiH
4
low silane concentration c = is commonly used
3
Φ SiH4
Φ total
< 5%
1) Reason for hydrogen dilution Need high ratio of H to SiHx fluxes
"Add more hydrogen intuitively
than silane"
to deposit μc-Si:H Selective etching model
Chemical annealing model
Surface diffusion model
1) ...but the optimal plasma control parameters are less clear c 1
ΦH
2
3 0.05 0
silane
hydrogen
f [MHz] gap
Φtot ΦSiH
4
PRF[W]
?
area
p[mbar]
plasma black box: parameter inter-dependence
1) ...because the plasma is complex.
ΦH ΦSiH
2
{molecules}
H2
SiH4
SiH4
polysilanes, dust
4
PRF[W]
H2
{electron dissn: ne, Te}
f [MHz]
p
SiHx , H, cations, anions
{sec y reactions} polysilanes, dust
{transport in the plasma and the sheath} {surface reactions}
SiHx
H, H2
1) Consider low pressures (< 2 mbar)
ΦH ΦSiH
2
{molecules}
H2
SiH4
H2
4
PRF[W] f [MHz]
{electron dissn: ne, Te}
p
SiHx , H, cations, anions
{transport in the plasma and the sheath} {surface reactions}
SiHx
H, H2
SiH4
1) Consider only the majority species
ΦH ΦSiH
2
{molecules}
H2
SiH4
H2
SiH4
4
All reactive species (SiHx, H, and ions) have very low density because of their volume and surface reactions
H2 and SiH4 have the dominant partial pressures in the plasma reactor
Only H2 and SiH4 leave the plasma reactor The partial pressure of SiH4 (and H2) is ~the same in the pumping line as in the plasma
p
"The plasma composition (and deposition) is determined by the partial pressures of SiH4 and H2 in the plasma"
1) How to measure the silane partial pressure in the plasma?
"inaccessible" plasma reactor
load lock
gate valve
vacuum chamber plasma box • heated • closed • showerhead
butterfly valve pumping line
substrate loading door
p
process pumps
1) FTIR in pump line: Non-intrusive diagnostic for SiH4 pressure IR source
1 m single pass
Fourier transform infrared (FTIR) absorption spectroscopy via ZnSe windows.
extended pumping line
p IR detector
1) Silane partial pressure measurement a) Silane Q-branch absorbance; b) Integrate spectrum; c) Read off a calibrated curve
• Silane pressure with plasma < silane pressure without plasma, 0 pSiH4 < pSiH 4
,
due to electron dissociation of silane (irreversible loss ). • Hydrogen partial pressure increases with plasma (surface recombination, & pump speed adjustment)
1) Silane input concentration (without plasma)
ΦH ΦSiH
0 0 pSiH + p H2 = p 4
2
0 0 pSiH + p H2 = p 4
4
p
Define silane input concentration, c = Note 0 ≤ c ≤ 1
0 pSiH 4
p
1) Silane concentration with plasma
ΦH ΦSiH
pSiH4 + pH2 = p
2 4
pSiH4 + pH2 = p
Define silane concentration with plasma, cpl =
p pSiH4 p
1) Silane depletion due to plasma
ΦH ΦSiH
pSiH4 + pH2 = p
2
pSiH4 + pH2 = p
4
p
Define silane fractional depletion, D = Note 0 ≤ D ≤ 1
0 − pSiH4 pSiH 4 0 pSiH 4
1) Silane concentration in plasma
ΦH ΦSiH
pSiH4 + pH2 = p
2
pSiH4 + pH2 = p
4
Therefore
p
silane concentration in plasma, cpl = c (1 − D )
2 parameters, c & D, define the plasma composition
1
0
the "plasma dimension"
infinitely strong plasma
concentration
pure silane vanishingly weak plasma
silane input concentration, c
1) The plasma "black box" becomes a {c,D} unit box
infinitely diluted silane
0
silane depletion fraction, D
1
silane input concentration, c
1) Contours of constant plasma composition 1
same plasma composition for cpl = c (1 − D ) = const .
0
0
silane depletion fraction, D
1
silane input concentration, c
1) Contours of constant plasma composition 1
(
c =1 D = 0.95
cpl = c (1 − D ) = 0.05
( 0
0
c = 0.05 D = 0.1
) IN THE 5% silane 95% hydrogen PLASMA NO input dilution, 95% depletion
)
silane depletion fraction, D
5% silane IN THE 95% hydrogen PLASMA strong input dilution, weak depletion
1
1) Same plasma composition = same film properties
Constant plasma composition, cpl, gives constant film properties
1) Same plasma composition = same film properties
transition material
1) Same plasma composition = same film properties
cpl = 0.5% cpl = 1.2%
Microcrystalline silicon can be deposited even with high silane concentration, provided that the silane depletion fraction is sufficiently high, because then the plasma is dominated by hydrogen.
silane input concentration, c
1) Two regimes of operation 1
REGIME 2 "recent" : Film crystallinity governed by strong plasma depletion, even for high silane concentration.
REGIME 1 "conventional" : Film crystallinity governed by hydrogen dilution of silane, 'independent' of plasma.
0
0
silane depletion fraction, D
1
1) Simple plasma chemistry model Based on a reduced set of gas phase reactions… k1 e + SiH 4 ⎯⎯→ SiH 2 + 2H + e
+ e
kH e + H 2 ⎯⎯→ 2H + e
…and simplified surface reactions.
SiH 2 ⎯⎯→ Si surf + H 2 S
H + H surf ⎯ ⎯→ H 2 + vacancy H + vacancy ⎯ ⎯→ H surf 1 R H ⎯⎯ → H2 2
e
+
+ +
+ +
e
e
1) Simple, analytical plasma chemistry model IN
Plasma reactions & deposition
Φ SiH4 ΦH2
= kne nSiH4 + S nSiH2
{radical fluxes} {surface reaction}
+ ( R 2 ) nH
S nSiH2 deposition
OUT + a nSiH4
= kHne nH2
+ a nH2
R nH
p
H+H recombination
• A zero-dimensional model is appropriate to showerhead reactors
1) Simple, analytical plasma chemistry model • Just one silane dissociation channel here: e + SiH4
SiH2 + 2H + e
• More detailed plasma chemistry only changes the numerical constants: • ... the general conclusions remain the same.
radical flux ratio
ΓH RnH = = Γ SiH2 SnSiH2
kH ⎛ nH2 2 ⎜ k ⎜⎝ nSiH4
⎞ ⎞ kH ⎛ 1 − 1⎟ + 2 ⎟⎟ + 2 = 2 ⎜⎜ ⎟ k c pl ⎝ ⎠ ⎠
Hydrogen and silane are the dominant partial pressures; so we can expect radical densities to depend on them.
"This shows why the plasma deposition is determined by cpl , the silane concentration in the plasma."
1) Depletion accounts for all of the plasma parameters plasma black box PRF[W] f [MHz] gap
Φtot
?
area
p[mbar]
analytical model
⎛ a kne ⎞ D = ⎜1+ ⎟ ⎝ (1 + c ) ⎠
−6 TgasΦ total -1 ⎤ ⎡ a ⎣s ⎦ = effective pumping speed = 6.1⋅ 10 ; p ⋅ gap ⋅ area
kne ⎡⎣s-1 ⎤⎦ = silane dissociation rate = function of ( PRF , f [MHz ]) ;
Φ SiH4 c = silane input concentration, . Φ total
D if any of { p, gap, area, c, PRF , f } &/or
{Φ total, Tgas }
−1
3
1) The "plasma dimension" for plasma deposition of μc-Si:H
2) Direct pumping of a plasma reactor
IWTFSSC-2 Berlin April 2009
2) Plasma chemistry equilibration time to steady-state depletion Feitknecht et al (2002)
SiH normalized intensity
van den Donker et al (2005)
Almost a minute to reach the plasma composition suitable to grow μc-Si:H...
0
20
40
60
time (s)
μc-Si:H
time (s)
... thick amorphous incubation layer ... deteriorates solar cell performance.
2) OES: Non-intrusive, rapid diagnostic for equilibration time
pump grid
Optical Emission Spectrometer plasma emission
2) Time-resolved optical emission spectrum from plasma ignition
Emission from silane radicals, SiH*, falls as the silane becomes depleted Emission from excited molecular and atomic hydrogen rises as its partial pressure rises
The plasma chemistry equilibration time is less than one second!
2) Compare open and closed reactors (numerical) Open laboratory reactor (axial symmetry)
Closed, directly-pumped plasma box (lateral view)
Plasma zone (20 cm in diameter) “Dead” volume
57 cm
Plasma zone
indirect pumping
t=0-100 s
time to steady-state > 100 s !
direct pumping
t=0-1 s time to steady-state < 1 s !
Silane back-diffusion from any intermediate dead volume increases the equilibration time.
2) Practical consequence of the pumping configuration Amorphous incubation layers with open door
409 nm
96 nm
plasma box side view