Steamer vs. torch in PV manufacturing—a cost of ownership comparison
reduce this growth rate. Because furnaces the furnace and onto the wafers. In This is difficult and labor intensive. As
are not gas tight, some ambient air typically addition, the water vapor generated is the uniformity can be improved from
back streams around the load door into only as pure as the original gases. Any >5% to less than <1.2% rework can be
the furnace. The higher the flow rate of contaminant in the gases will transfer to eliminated, with a large reduction in
steam, the less likely air will be able to the furnace and the wafers. The thermal associated labor.
affect the wafer load. In addition, the water profile created by the torch can lead to
Maintenance costs
vapor must be completely vaporized to a large thermal shadow which affects
• Pyrolytic Torch. Maintenance costs
generate the same water vapor conditions wafer-to-wafer uniformity.
associated with pyrolytic torches are
throughout the furnace. • Steamer. The Steamer uses a non-
often unexpectedly high. This is due
porous hydrophilic membrane that
• Pyrolytic Torches. Both the cost of to several factors associated with their
selectively allows water molecules to
burning oxygen and hydrogen and design. Because of the safety issues
pass. The membrane is 1,000,000 times
the high temperature lead to process involved with the use of hydrogen
more selective for water molecules
decisions that ultimately adversely affect many redundant interlocks are used
compared to nitrogen molecules. In the
uniformity. High steam volumes lead to on torches to ensure safe operation.
vapor phase, the membrane only allows
high cost. High heat load leads to front- Unfortunately, these components
water. All other molecules are greatly
to-back problems. To minimize these must operate near extremely high
restricted, so contaminants in water
issues, the process engineer will run the temperatures and as a result their
such as dissolved gases, ions, TOCs,
minimum steam needed. The resulting failure rate is high. It is therefore typical
particles, viruses, bacteria, pyrons, and
low steam flow rate leads to uniformity to schedule maintenance at periodic
metals can be removed in the steam
problems at the load door, and these intervals to prevent failure. During
phase. All wetted components in the
problems are accentuated as the tools this scheduled maintenance various
liquid path are quartz or Teflon®. The
wear out. components (e.g., thermocouples, flame
purified steam path components are
• Steamer. The Steamer uses a membrane detectors, lamps, heaters, injectors)
quartz and Teflon® fittings and valves.
to deliver 100% saturated purified are replaced, the quartz components
steam into the process tube. The The Steamer maintains the water reservoir cleaned, and MFCs calibrated (H2).
flow rate of the steam is measured at a constant boil preventing the growth of The frequency of this is typically two
and controlled, enabling the Steamer bacteria. This system is also equipped with to three months and may take a day to
to ensure the highest oxidation rate an automatic purge feature to flush the complete. Injector’s can cost between
(shortest time) and the highest CpK boiler at start up. $200 and $2000. TCs cost $200 each
(best uniformity). The water vapor and the associated costs related to lost
ratio can be easily increased because Process costs productivity and labor can be over
the process of generating steam directly There are a variety of gases and liquids $100,000 per event.
from water removes the limits typical used to generate steam for oxidation. The • Steamer. Maintenance and labor
with torches, bubblers, and DLI. This combination required determines the associated with operating the Steamer
ability to substantially raise the flow rate relative cost of the process. is negligible. The membrane filter is a
can overcome process challenges such cross flow device that does not clog or
• Pyrolytic Torch. Both hydrogen and
as leaky furnace load doors and loading lose efficiency under normal operating
oxygen are required to generate vapor
concentricity issues. By maximizing flow conditions. The primary maintenance
using a torch. In addition the heat
rate with 100% steam the optimum task is to track heating element (lamp)
generated through combustion must
growth zone within the furnace is hours for predicted replacement
then be removed with compressed
increased, leading to more wafers per times. This is recommended to be
air or cooling water, increasing power
run that meet good die standards. every six months or longer and takes
costs. This is independent of the labor
approximately 30 minutes.
Contamination including warpage, particles, load associated with the hydrogen and
and electrical performance oxygen support required for the torches. Facilities costs
Any process equipment used to generate Some facilities will not allow torches to When looking at new installations, whether
an oxidizing ambient must first be capable run untended which requires personnel green field, facility expansion, or dry to
of producing a contamination free onsite throughout the run and limits wet oxidation conversion, it is important
environment. The devices being produced third shift and weekend facility to review both material and infrastructure
must not be exposed to metals, carbon, operation. costs as well as permitting requirements.
organic materials, and solids. Contaminants • Steamer. The Steamer requires DI water
• Pyrolytic Torch. Torches require
lead to damaged devices and loss of overall and CDA. The elimination of oxygen
either a bulk hydrogen and oxygen
throughput. The creation of the steam must typically keeps material costs to less
facility or point of use gas cabinets.
be 100% to prevent droplet formation that than $0.0025/hr. A key aspect to keep
Bulk facility installations of hydrogen
can carry ionic contamination as well as in mind is the cost savings generated
and oxygen use large truck trailers or
create localized cooling which will lead to by better uniformity. The elimination
cryogenic tanks. The bulk gases are
warping. of rework and inspection time to
then filtered, purified and piped to
sort through uniformity scatter can
• Pyrolytic Torches. As the torch each furnace. Many municipalities
be eliminated through the Steamer’s
burns, the injector itself is consumed highly regulate these installations
improved wafer-to-wafer uniformity.
at a rate determined by flow rate and generally prohibit permitting
Also, some thick oxide customers
and accelerating with continuous in areas where other businesses or
would have to run wafers to grow
operation. This silica makes its way into residences might be affected by a leak
additional oxide if the film was thin.
8 – Global Solar Technology – May/June 2009
www.globalsolartechnology.com
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