MKS ASTRON G7 Remote Plasma Source Application Note

Application Note
ASTRON® Remote Plasma Source for Thin-Film
Photovoltaic Process Chamber Cleaning
PROBLEM
The layers in a thin film photovoltaic (TFPV) device are
deposited using deposition techniques, some of which have
been adapted from semiconductor device fabrication. For
example, antireflective coatings are fabricated using plasmaassisted chemical vapor deposition (CVD) processes that
produce thin films of silicon dioxide or silicon nitride. These
CVD processes are performed in sealed process chambers,
normally under vacuum conditions. The films deposit not
only on the substrate surface, but also on all surfaces within
the chamber during the process. If allowed to accumulate, the
wall deposits can produce contamination or particles on the
substrates that are unacceptable. In addition, wall deposits can
adversely affect the deposition process or in-situ metrology of
the process results.
BACKGROUND
The development and application of thin film photovoltaics
(TFPVs) has grown rapidly over the past decade. Silicon solar
cells are made from silicon wafers, either mono- or multicrystalline, with a protective, anti-reflective layer deposited
on the side that will be exposed to the sun (typically silicon
nitride, and conductive pastes that act as the conduction
path to the outside world. TFPVs all have the same basic
multilayered structure, either built upon a support layer/
substrate (built from back to front), or built on the glass that
will act as the front side of the finished module (built from
front to back). (Figure 1). Transparent conductive layers form
the front and back of the cell. The side exposed to the incident
light must also have antireflective properties, while the one
behind the cell must allow unabsorbed light to be reflected
back from the highly reflective backside of the cell. The
antireflective layer may also be patterned to create a topology
that further enhances light trapping in the active layers below.
The CVD processes used to deposit these various layers
indiscriminately deposit thin film material on all surfaces
in the process chamber. Therefore, these chambers must be
periodically cleaned in order to minimize contamination of
the substrates which can impact process results or in-situ
metrology methods.
Transparent Protective Layer
Antireflective Layer
Electrical Connection Layer
n-Type Semiconducting Layer
p-Type Semiconducting Layer
Electrical Connection Layer
Support Layer
Figure 1 - A schematic showing the typical multi-layered structure of
a photovoltaic cell.
Transition From Manual Cleaning Processes to
In-Situ Plasma Cleans
Initially, chamber cleans used wet chemistry to remove
thin film deposits; to do so, the process tools were often
disassembled and the chambers taken off site to be cleaned
using liquid solvents such as aqueous hydrofluoric acid
solution. This approach produced excessive system downtime
and incurred high labor costs and serious workplace safety
risks. For these reasons, in-situ cleaning methods were
developed that avoided dismantling the equipment and
exposing workers to corrosive solutions. The in-situ cleans
used in TFPV manufacture normally employ fluorine
chemistries in which a molecule containing fluorine (a
precursor) is dissociated to produce molecular fragments
that react with the wall deposits, converting them to gaseous
compounds that can be pumped away. In some thin-film
processes (i.e. thermal CVD) chamber cleaning precursors
can be dissociated thermally in the process chamber, creating
species that react and remove deposits. However, the thinfilm processing equipment used for the oxide and nitride
AR films in TFPV manufacturing cannot tolerate the high
temperatures required for thermal deposition and cleaning. In
these cases, the CVD tools employ µwave or RF-excitation in
low temperature plasma assisted deposition processes for SiO2
and Si3N4 thin films as well as for any etching steps involved
in the TFPV process. In-situ plasma cleaning methods were
therefore developed as a logical extension of these plasma
deposition tools. These cleaning methods use the available
plasma excitation to produce reactive species from molecular
precursors. The combination of plasma-assisted deposition
processes with in-situ plasma cleaning proved a significant
advance in semiconductor and TFPV manufacturing, resulting
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Application Note
in valuable reductions in system downtime and much reduced
labor costs as compared with wet cleaning.
Modern plasma-assisted cleaning systems use a variety of
chemistries to create ionic species and reactive radicals. The
choice of chemistry will depend on a mixture of considerations
that encompass the nature of the reactive chemistry, the
risks incurred in storage and use of the reactive precursor
and environmental issues. Depending upon the elemental
composition of the deposits to be removed, either fluorine
or chlorine radical species may be desired. For example,
the deposition chambers used for AR thin-film processes
are cleaned using fluorine radicals as the reactive species.
Fluorine radicals are produced by the dissociation of different
compounds, including F2, NF3, ClF3, and SF6. NF3 is the
most commonly used feed gas for chamber clean applications,
because it can be handled safely and it dissociates relatively
easily.
Even though chamber cleaning with an in-situ generated
plasma may appear to be simpler, that method can also
produce serious problems for thin film device manufacturers.
These same problems do not result when cleaning is done with
reactive gases generated in a remote plasma source. Cleaning
with plasma generated in the process chamber (in-situ
cleaning) can cause ion bombardment of the chamber walls
and other components inside the process chamber wherever
there is direct exposure of a surface to the plasma environment.
This exposure degrades device performance, reduces yield,
and damages the internal components of the process chamber.
Furthermore, a cleaning process using in-situ generated plasma
may be slower due to lower dissociation efficiency.
Figure 2 - (a) Two configurations that are typical of plasma process
deposition tools that use in-situ plasma exposure; (b) A chamber
configuration using remote plasma deposition and cleaning.
Dissociation Efficiency
Remote Plasma
Source
In-Situ RF
Manual
Excellent
Fair
N/A
Cleaning Uniformity
Good to Excellent
Fair
Fair
Cleaning Efficiency
Good to Excellent
Fair
Poor
Damage to Parts of
the Process Chamber
None
Yes, due to ion
bombardments
Yes, due to bead
blasting, and etc.
Impact to Deposition
or Process RF set-up
None
Need increased
power for clean
Yes, chamber
components must
be easily removable
Impact to Deposition
or Process RF Matching
None
Need wide range
RF match for clean
Yes
Add interface on
deposition or
process chamber
May need NF3
abatement in
exhaust
Life time of the
process chamber
Good
Fair
Poor in General
Other Impact
COO
Table 1 - Relative characteristics of a remote plasma source for chamber cleaning as compared to in-situ RF plasma generation or manual cleaning methods.
SOLUTION
The development of remote plasma technology for deposition,
etch and cleaning has effectively addressed the problem of
ion-bombardment induced damage during these processes.
Figure 2 shows schematics for conventional in-situ plasma
systems (2a) as compared with remote plasma sources (2b).
Using remote plasma sources it is possible to eliminate the
presence of electric fields and ionic species within the process
chamber. These sources use plasma excitation in a chamber
that is physically removed from the process chamber to
generate reactive ionic and radical species. The reactive gas
stream is then passed through an ion filter that removes any
ionic species.
These remote sources thus deliver only neutral excited-state
(i.e. radical) species to the substrate and chamber surfaces
where they either react to form the growing film, or, when
cleaning chemistries are employed, etch the deposited material
away. More recently, this technology has been adapted to
employ RF excitation in a move to lower the cost and to
increase the flexibility of these plasma sources. Table 1, below,
compares different characteristics of manual wet cleaning, insitu RF plasma cleaning, and remote source plasma cleaning of
CVD process chambers.
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SOLUTION (CONT.)
A Flexible Remote Plasma Source
The ASTRON remote plasma source from MKS Instruments
Inc. is an economical, reliable remote plasma system for
generating reactive radical streams that can be used in TFPV
chamber cleaning applications. The ASTRON source is shown
in Figure 3a. It is packaged such that control, power and
plasma generation are all contained in a single unit of near
desktop dimensions. The plasma source technology is based
on a transformer in which the primary circuit is powered by
an RF power supply and the plasma is enclosed in a loop (a
toroid), as can be seen from the schematic shown in Figure
3b. Current driven in the primary coil induces a current in
the plasma (secondary) in the opposite direction by Faraday’s
induction law. A ferrite core confines the electromagnetic fields
to improve magnetic field coupling and reduce stray RF fields.
Typically, this electric field is 4-8 V/cm. Since the electric fields
within the plasma are maintained at such low levels, sputtering
of the source chamber walls is avoided. Within the source,
the plasma is contained within a 2.5-cm diameter plasma
channel. In operation the ASTRON plasma source dissociates
a precursor gas, typically NF3, in the plasma toroid producing
atomic fluorine for the downstream chamber clean. For large
scale processing as required in some TFPV applications, gas
flows of up to 30 SLM NF3 may be used. The power input to
the plasma is dependent on the gas flow and pressure in the
plasma chamber, and it can range between 300 W and 20 kW,
depending upon the application requirements.
Figure 3 - Schematic layout of the ASTRON® source chamber
Application Note
Typically, ASTRON sources produce >95% dissociation of
NF3 precursor over the system operating flow and pressure
range. This characteristic has obvious relevance to the
suitability of the ASTRON source in cleaning applications.
The effectiveness of the ASTRON in dissociating molecular
species enables more latitude in the selection of the precursor
gas used in cleaning applications. As noted above, there are
concerns for the use of NF3 as a fluorine radical source owing
to the impact that NF3 releases can have on GHG emissions.
Indeed, GHG considerations have led to increased interest
in the use of much more hazardous source gases such as
molecular fluorine, F2 and anhydrous hydrofluoric acid, HF.
However, the ASTRON source nearly completely dissociates
NF3 (EPA reports suggest >99% dissociation). When this
observation is combined with the low recombination rates in
the ASTRON plasma source and transfer lines, it becomes
apparent that the risk of GHG emissions when using NF3
source gas is minimized. The EPA estimates that NF3 source
gases, when used in a remote plasma cleaning application, can
produce a reduction of 5,500 metric tons of carbon equivalent
(TCE) as compared with conventional chamber cleaning
approaches.
The ASTRON source produces very high concentrations of
neutral radical species and delivers these high concentrations
to the process chamber. Since radical species have limited
lifetimes, measured in 10’s of milliseconds, recombination
reactions on the plasma chamber and transport line walls
must be minimized. This is accomplished in the ASTRON
source through the appropriate selection of materials of
construction. The ASTRON employs anodized aluminum for
all of the wetted metal surfaces, since the recombination rate
of fluorine radicals on aluminum oxide is very low relative to
other material choices such as quartz or stainless steel. The
ASTRON system also maintains the concentration of reactive
species delivered to the process chamber through its ability to
process relatively high gas flow rates and through admixture of
Ar with the reactive gas precursor stream. High gas flow rates
and fast pumping speeds increase the gas velocity through the
ASTRON source and the gas transport lines, thereby reducing
the residence time of the reactive species in these system
components. As well, the high gas velocities force the flow of
reactive species to surfaces once in the process chamber (i.e.
transport of reactive species to the walls is flow driven rather
than diffusion driven) and this further improves the delivery of
Page 4
Application Note
SOLUTION (CONT.)
high concentrations of reactive species to those surfaces
needing cleaning. Process chamber temperatures are
maintained at relatively high levels during cleaning since
remote plasma reactions are driven by purely thermal
mechanisms and reaction rates follow an exponential
dependence on surface temperature. Cleaning rates are
proportional to the partial pressure of reactive gases for similar reasons.
Best Practice For Chamber Cleaning Using The ASTRON® Source
The best chamber cleaning sequence using the ASTRON
remote plasma source in TFPV tools involves a five step
process. Once the reaction chamber has been brought to
temperature, Ar gas is flowed through the system for up to 10
seconds to purge any residual gases due to previous processes.
When the chamber purge is complete, the plasma is ignited
using the purge Ar and the plasma allowed to stabilize for
up to 10 seconds. Following the ignition step, the Ar flow is
maintained while NF3 gas flow is introduced into the system
in two transitional steps, the first using 1/5th of the planned
total NF3 flow for 5 seconds, and the second increasing the
NF3 flow to 2/3 of the total value over another 5 second
period. Following these transition steps, the NF3 flow is
increased to its full value and the chamber clean is allowed
to proceed for a pre-determined time that has been shown
to produce clean chamber walls. Finally, at the end of the
cleaning step, the reactor is purged with an Ar/N2 mixture
for 10 or more seconds. Figure 5 shows typical results for
a silicon dioxide residue clean using the ASTRON remote
plasma source. Faster oxide etch rates (up to ~10 µm/min) are
possible at higher process pressures (up to 10 Torr), and there
is an optimum pressure for cleaning rates as is shown for SiO2
in Figure 6. Process chambers that have been used to deposit
other films for TFPV manufacturing can likewise be cleaned
using the ASTRON remote source. Figure 7 shows data for
the etch rate of silicon nitride at different temperatures and
pressures.
Figure 5 - Silicon dioxide etch rates as a function of temperature in a
typical chamber clean using the ASTRON remote plasma source.
Figure 6 - Etch rate as a function of chamber pressure for SiO2 using
an ASTRON source to dissociate NF3: T=150 °C, NF3 flow of 150
sccm and Ar flow of 750 sccm.
Figure 7 - Etch rate as a function of chamber pressure and temperature for Si3N4 using an ASTRON source to dissociate NF3:
T=150 °C, NF3 flow of 150 sccm and Ar flow of 750 sccm.
Application Note
Page 5
CONCLUSION
REFERENCES
Remote plasma cleaning has distinct advantages over the other
available technologies for removing wall deposits and other
contaminants from the surfaces of thin-film process chambers.
Etch rates for oxides and nitrides are significantly greater than
those achievable with in-situ thermal or plasma cleaning.
High process temperatures and ion bombardment are avoided,
eliminating damage to internal process chamber components.
By selecting a safe yet reactive precursor gas such as NF3, it is
possible to achieve the benefits of improved yield and reduced
cleaning downtime while maximizing workplace safety. While
NF3 is itself a potent greenhouse gas, the nearly complete
dissociation of NF3 in the remote plasma source results in
overall reductions in industry GHG emissions. The ASTRON
remote plasma source when coupled with the selection of an
inherently safe source gas can thus provide the safest, most
cost effective chamber cleaning solution for semiconductor
and photovoltaic chambers. For these reasons, solar equipment
makers are replacing manual cleaning methods with ASTRON
remote plasma sources for chamber cleans. Remote plasma
cleans are being deployed for many solar photovoltaic
manufacturing processes, especially for silicon nitride, silicon
oxide, and amorphous silicon deposition chambers.
Plasma Sources Line Card
ASTRON® G7 datasheet
For further information, call your local MKS Sales Engineer or contact the MKS Applications Engineering Group at 800-227-8766.
ASTRON® is a registered trademark of MKS Instruments, Inc., Andover, MA.
App. Note 02/13 - 4/13
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All rights reserved.
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