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NEED: Plasma resistivity (××××10)
NEED: Resonance frequency (MHz)
20 nm a-Si-i as deposited after 70 s H2 plasma etching after 110 s H2 plasma etching
www.pvcomb.de
INTRODUCTION
PECVD Process MonitoringPECVD Process Monitoring
in Thin Film Silicon Solar Cell Manufacturingin Thin Film Silicon Solar Cell Manufacturing
Onno Gabriel1, Ivo Erkens
2, Matthias Zelt
1, Björn Rau
1, Bernd Stannowski
1, Rutger Schlatmann
1
1 PVcomB/Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
2 Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands
Email: [email protected]
ACKNOWLEDGEMENTS
PLASMA DIAGNOSTICS: A-SI/µC-SI PHASE TRANSITION
Competence Center Thin-Film- and Nanotechnology for Photovoltaics Berlin
Figure: OES measurement of the
SiH and H radical densities (top),
RGA measurement of H2 and SiH4
densities (middle) and plasma re-
sistivity and resonance frequency
(bottom) during H2 plasma etch.
PECVD SYSTEM & IN SITU PLASMA DIAGNOSTIC TOOLS
Figure left: NEED raw data example
showing higher harmonics in the rf
current signal.
PLASMA MONITORING: PROCESS STABILITY
Amorphous and microcrystalline layers for a-Si:H/µc-Si:H thin film silicon tandem cells are de-
posited at PVcomB in an fully automated Applied Materials AKT1600A PECVD system. Sub-
strates are 30x30 cm² glass panels coated with SnO2 or ZnO front contact TCO.
The PECVD chambers are equipped with several in situ diagnostics tools with the aim of
1. Plasma monitoring to control process stability and the determination of long term drifts,
2. Plasma diagnostics to measure plasma parameters for a better understanding of the
PECVD processes, i.e. the plasma chemistry and the plasma/surface interaction and film
growth.
AMAT
cluster tool
SEERS
SEERS
OES
OES
OES
spectra
Logfiles, Recipes
Mass
spectraChA
ChB
RGA
RGA
SEERS
data
Sun
Workstation
Input
folder
PVcomB Data Server:
Scripts
MySQL
data base
Web
Server
Desktops,
Laptops
Desktops,
Laptops
Desktops,
Laptops
200 300 400 500 600 700 8000
100
200
300
400
500
600
700
800
µc-Si:H BCi deposition
a-Si:H BCn deposition
Si
Si
Si
SiH
Hβ
Inte
nsity (
a.u
.)
Wavelength (nm)
H2 fulcher
Hα
400 500 600 700 800 900 10000
500
1000
1500
2000
F
F
F
N2
N2
Process start
Process end
F
N2
N2
NF
N2
Inte
nsity (
a.u
.)
Wavelength (nm)
NF3 plasma clean Species Transition Wavelength
Si 3s23p
2 – s
23p4s 288.3 nm
SiH X2Π – A
2Δ 409 – 422 nm
Hα n=3 – n=2 656.3 nm
Hβ n=4 – n=2 486.1 nm
H2 2s3Σ
+g – 3p
3Π
−u 570 – 640 nm
F 2s22p
4(
3P)3s - 2s
22p
4(
3P)3p 703.7 nm
F 2s22p
4(
3P)3s - 2s
22p
4(
3P)3p 624.0 nm
Ar 3s23p
4(
3P)4s - 3s
23p
4(
3P)4p 496.5 nm
SiHx
a-Si-H BCn Deposition
Si2Hx
SiF3
Hx
44CO2
48SiHF
49SiH2F16O
14N
18H2O
34PH3
89PHF3?
19F20HF
47SiF
Figure: Equivalent circuit of
the rf circuit.
NONLINEAR EXTENDED ELECTRON DYNAMICS (NEED)
RESIDUAL GAS ANALYSIS (RGA)
OPTICAL EMISSION SPECTROSCOPY (OES)
PEAVI: PECVD ON AKT VISUALIZATION
Figure: Scheme of an AKT1600 process chamber with in-
stalled in situ plasma diagnostic tools OES, NEED and RGA.
SUMMARY
The PECVD chambers for thin film silicon deposition at PVcomB are equipped with in situ
plasma diagnostic tools (OES, RGA and NEED), which deliver several complementary plasma pa-
rameters. These parameters give a better insight into the plasma chemistry and plasma/surface
interaction. Moreover, some of them are identified to be useful for plasma monitoring: they are
reproducibly indicators of the process stability, revealing short and long term drifts of the
PECVD processes in thin film solar cell production.
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
0 36.87 73.74 110.61 147.48
Time (ns)
Current @ sensor (a.u.)
Voltage @ generator (a.u.)
A Hercules N250 NEED system (Plasmetrex GmbH) delivers
absolute and reproducible parameters that characterize the
rf circuit. They depend strongly on the chamber history and
give valuable information about the process stability.
Table right: Overview about
plasma parameters derived from
NEED measurements.
Densities of various atoms and molecules within the plasma gas phase are measured by OES.
Since the emission lines originate from electronically excited species, the densities are relative
and depend not only on the ground state densities, but also on the excitation process (electron
collisions). Species detected: Ar, F, NF, N2, Si, SiH, H, H2. The gas temperature is derived, too.
A MKS Vision 2000C residual gas analyzer (mass
spectrometer) is installed close to the pump exit
at the chamber. It measures stable atom and
molecule densities (see example spectrum).
All measured data in combina-
tion with AKT process data
(gas flow rates, chamber pres-
sure etc.) is stored in a data
base and analyzed in PEAVi, a
an data analysis software de-
veloped at PVcomB.
PLASMA DIAGNOSTICS: NF3 CHAMBER CLEAN
[1] G. Dingemans, M. N. van den Donker, A. Gordijn, W. M. M.
Kessels, M. C. M. van de Sanden, Applied Physics Letters 91
(2007) 161902
Pressure (Torr)
Hα @ 656 nm (thick)
Hβ @ 486 nm (thin)
Ar @ 696 nm
F @ 703 nm
2 31
NEED: Resonance frequency (MHz)
NEED: Plasma resistivity (××××10)
The PECVD chambers are cleaned by an active NF3/Ar
etch plasma after each deposition (Fig. right).
The plasma chemistry during the clean is complex
and heavily influenced by the surface conditions.
Even a very thin a-Si:H film on the glass substrate can
strongly change the plasma (Fig. bottom).
To determine the a-Si:H/µc-Si:H phase transition, films grown under various SiH4 concentrations
are etched by a H2 plasma. It has been shown that the SiH radical density in this H2 etch plasma
depends on the degree of crystallinity of the previous grown film due to different etch rates [1].
We found, that several other plasma parameters are influenced by the film surface, too:
The a-Si:H/µc-Si:H phase
transition can be determined
fast and in situ by the usage
of plasma diagnostics for
various different plasma con-
ditions.
Figure right: Plasma resistivity (top) and resonance
frequency (bottom) measured by NEED during 9 dif-
ferent chamber cleans. The black curve is the first
clean of the day (different chamber condition).
01/10/2012 01/18/2012 01/24/2012
NEED is an excellent plasma monitoring tool
that delivers absolute and comparable pa-
rameters characterizing the process stability.
The slight drifts in NEED parameters over a day
are not fully understood. They are probably due
to temperature effects. However, parameters
during the first deposition of a day are very re-
producible.
Figure: Pressure, OES signals and NEED data dur-
ing Ar/NF3 etch: 1) - 3): electrode, susceptor and
remaining chamber walls completely cleaned.
Figure left: Plasma resistivity (top) and resonance fre-
quency (bottom) measured on three different days during i
-layer deposition.
Plasma resistivity = νeff ωgen
ωe (ne)2(basically the collision rate
over electron density)
Parameters derived from NEED measurements:
Resonance frequency of the total system
depending on
- bulk inductance Lp
- the sheath capacitances Cs
- the inductance of the electrode system/chamber LC. (all directly depending on electron density, chamber design and
gap between electrode and substrate)
1
2
3
4
Uniformity Edge = ratio of geometric dimension to skin
depth (RE/2.405∙δskin)
Nonlinearity= ability of the plasma to generate higher
harmonics (here: fixed to 0.125)
5 RF Fundamental = RF current at generator frequency
6 V1rms_mV = sensor signal amplitude of the first
harmonics (in mV)
This work was supported by the Federal Ministry of Education (BMBF) and the state
government of Berlin (SENBWF) in the framework of the program “Spitzenforschung
und Innovation in den Neuen Ländern” (grant no. 03IS2151) and by the BMBF and the
Federal Ministry for Environment, Nature Conservation and Nuclear Safety (BMU) in
the framework of the program “Innovationsallianz Fotovoltaik” (grant no. 0325317C).