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Plasma modelling & reactor designAlan Howling, Ben Strahm, Christoph Hollenstein
Ecole Polytechnique Fédérale de Lausanne (EPFL), SwitzerlandCenter 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
black boxPECVD
sila
nehy
drog
en
ΦSiH4
ΦH2
1) PECVD reactor setup
Plasma Box,Oerlikonprinciple:
Jacques Schmitt
sila
nehy
drog
en
p
butterflyvalve
processpumps
vacuum chamber
ΦSiH4
ΦH2
PRF[W]f [MHz]
heating glass substrate
RF connection
grounded box
showerhead RF electrode
gas flowplasma boxside view
1) Hydrogen dilution for plasma deposition of μc-Si:H
sila
nehy
drog
en
ΦSiH4
ΦH2
0
0.05
1c
4SiH
totalsilane concentration 5%
is commonly used
low cΦΦ
= <
plasma box
Surface diffusionmodel
Selective etchingmodel Chemical annealing
model
Need high ratio of H to SiHx fluxes
to deposit μc-Si:H
1) Reason for hydrogen dilution
"Add more hydrogen
than silane"intuitively
1) ...but the optimal plasma control parameters are less clear
sila
nehy
drog
en
0
0.05
PRF[W]
Φtot
f [MHz]
p[mbar]
gap area
1c
ΦSiH4
ΦH2
??plasma black box:parameterinter-dependence
1) ...because the plasma is complex.
p
ΦSiH4
ΦH2
PRF[W]
f [MHz]
{ }
{ }
{ }
{ }
2 4
ne e
y
molecules H SiH
electron diss : ,
SiH , H, cations, anions
sec reactions
polysilanes, dust
transport in the plasma and the sheath
sur
x
n T
{ } 2face reactions SiH H, Hx
2 4 H SiHpolysilanes, dust
1) Consider low pressures (< 2 mbar)
p
ΦSiH4
ΦH2
PRF[W]
f [MHz]
{ }
{ }
{ }
{ }
2 4
ne e
molecules H SiH
electron diss : ,
SiH , H, cations, anions
transport in the plasma and the sheath
surface reactions SiH
x
x
n T
2 H, H
2 4 H SiH
1) Consider only the majority species
p
ΦSiH4
ΦH2 { } 2 4molecules H SiH 2 4 H SiH
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
"The plasma composition (and deposition) is determined by the partial pressures of SiH4 and H2in the plasma"
1) How to measure the silane partial pressure in the plasma?
plasma box• heated• closed
• showerheadp
butterflyvalve
process
vacuum chamber
pumps
load lock
gate
val
ve
pumping line
"inaccessible" plasma reactor
substrate loading door
1) FTIR in pump line: Non-intrusive diagnostic for SiH4 pressure
p
IR source
IR detector
Fourier transform infrared (FTIR)absorption spectroscopy
via ZnSe windows.
1 m
sin
gle
pass
extendedpumpingline
4 40
SiH SiH
Silane pressure with plasma < silane pressure without plasma,
< ,
due to electron dissociation of silane ( ).
Hydrogen par
p p
irreversible loss
•
• tial pressure increases with plasma (surface recombination, & pump speed adjustment)
1) Silane partial pressure measurementa) Silane Q-branch absorbance; b) Integrate spectrum; c) Read off a calibrated curve
pΦSiH4
ΦH2
1) Silane input concentration (without plasma)
40SiHsilane input concentration,
Note 0 1
Define
pc
pc
=
≤ ≤
4 20 0SiH Hp p p+ = 4 2
0 0SiH Hp p p+ =
pΦSiH4
ΦH2
= 4SiHpl
silane concentration with plasma,
Definep
cp
4 2SiH Hp p p+ = 4 2SiH Hp p p+ =
1) Silane concentration with plasma
pΦSiH4
ΦH2
−=
≤ ≤
4 4
4
0SiH SiH
0SiH
silane fractional depletion,
Note 0 1
Define
p pD
p
D
4 2SiH Hp p p+ = 4 2SiH Hp p p+ =
1) Silane depletion due to plasma
pΦSiH4
ΦH2
( )= −plsilane concentration in plasma, 1Therefore
c c D
4 2SiH Hp p p+ = 4 2SiH Hp p p+ =
1) Silane concentration in plasma
2 parameters, c & D,define the plasmacomposition
1) The plasma "black box" becomes a {c,D} unit box
conc
entra
tion
00 1
1
silane depletion fraction, D
sila
ne in
put c
once
ntra
tion,
c
the "plasma dimension"
vani
shin
gly
wea
k pl
asm
a
infin
itely
str
ong
plas
ma
infinitely diluted silane
pure silane
1) Contours of constant plasma composition
00 1
1
silane depletion fraction, D
sila
ne in
put c
once
ntra
tion,
c
( )= − =plsame plasma composition for 1 .c c D const
1) Contours of constant plasma composition
00 1
1
silane depletion fraction, D
sila
ne in
put c
once
ntra
tion,
c
( )= − =pl 1 0.05c c D
5% silane95% hydrogen
strong input dilution,weak depletion
5% silane95% hydrogenNO input dilution,95% depletion
( )0.050.1cD==
( )10.95cD==
IN THEPLASMA
IN THEPLASMA
1) Same plasma composition = same film properties
Constant plasma composition, cpl, gives constant film properties
1) Same plasma composition = same film properties
transitionmaterial
1) Same plasma composition = same film properties
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.
cpl = 0.5%
cpl = 1.2%
1) Two regimes of operation
00 1
1
silane depletion fraction, D
sila
ne in
put c
once
ntra
tion,
c
REGIME 1 "conventional" :Film crystallinity governed byhydrogen dilution of silane,'independent' of plasma.
REGIME 2 "recent" :Film crystallinity governed bystrong plasma depletion, evenfor high silane concentration.
eH2SiHSiHe 24 1 ++⎯→⎯+k
2surf2 HSiSiH +⎯→⎯S
surf
2surf
H vacancyH
vacancyHH H
⎯→⎯+
+⎯→⎯+
21H H2
R⎯⎯→
Based on a reduced set of gas phase reactions…
…and simplified surface reactions.
e e+ + +
e e+ +eH2He H2 +⎯→⎯+k
+
1) Simple plasma chemistry model
p
1) Simple, analytical plasma chemistry model
• A zero-dimensional model is appropriate to showerhead reactors
( )
{ }
4 4 4
2 2 2 2
2
SiH e SiH SiH
H SiH H H e H H
SiH H
2
radical fluxes
su
kn n an
S n R n k n n an
S n R n
Φ
Φ
= +
+ + = +
{ }rface reaction depositionH+Hrecombination
IN Plasma reactions & deposition OUT
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.
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."
ΓΓ
⎛ ⎞ ⎛ ⎞= = + = − +⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠
2
2 2 4
HH H H H
SiH SiH SiH pl
12 2 2 1 2k k
nRn k kSn n c
radicalflux ratio
PRF[W]
Φtot
f [MHz]
p[mbar]
gap area??1) Depletion accounts for all of the plasma parameters
( )
1e1
1a knD
c
−⎛ ⎞
= +⎜ ⎟+⎝ ⎠
[ ]( )
Φ
ΦΦ
−⎡ ⎤ = = ⋅⎣ ⎦ ⋅ ⋅
⎡ ⎤ = =⎣ ⎦
= 4
gas total-1 6
-1e
SiH
total
s effective pumping speed 6.1 10 ;gap area
s silane dissociation rate , MHz ;
silane input concentration, .
RF
Ta
p
kn function of P f
c
plasma black box
{ } { }RF total gas if any of , gap, area, , , &/or , D p c P f TΦ
analyticalmodel
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
time (s)0 20 40 60
SiH
norm
aliz
ed in
tens
ity
van den Donker et al (2005) Feitknecht et al (2002)
... thick amorphous incubation layer
... deteriorates solar cell performance.
μc-Si:H
Almost a minute to reach the plasma
composition suitable to grow μc-Si:H...
time (s)
2) OES: Non-intrusive, rapid diagnostic for equilibration time
OpticalEmissionSpectrometer
pumpgrid
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!
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
directpumping
t=0-1 s
time to steady-state > 100 s ! time to steady-state < 1 s !
Silane back-diffusion from any intermediatedead volume increases the equilibration time.
t=0-100 s
2) Compare open and closed reactors (numerical)
But the door is normally closed, the amorphous incubation layer is then thinner than 10 nm. substrate
2) Practical consequence of the pumping configuration
409 nm96 nm
Amorphousincubation layers with open door
Two Conclusions1. Plasma composition and deposition depend on the silane concentration in the plasma,
cpl=c(1-D) , and not only on the silane concentration in the input flow, c.
Strong hydrogen dilution in the plasma, and μc-Si:H deposition, can be obtained with high input concentration of silane.
- monitored non-intrusively by FTIR in the pumping line.
2. Rapid equilibration of plasma chemistry requires a closed, directly-pumped showerhead
reactor with a uniform plasma - avoid gas circulation between the plasma and any dead volumes.
- monitored non-intrusively by OES in the pumping line.
Refs. B. Strahm et al, Plasma Sources, Sci. Technol. 16 80 (2007).
A. A. Howling et al, Plasma Sources, Sci. Technol. 16 679 (2007).
Acknowledgements: Swiss Federal Research Grant CTI 6947.1; and
Dr. U. Kroll and colleagues, Oerlikon-Solar Lab SA, Neuchâtel, Switzerland.