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.