Steamer vs. torch in PV manufacturing—a cost of ownership comparison
Steam generation options
rate is not maximized until the water vapor high temperature generated by the
The steam generation method affects
pressure is equal to 100% of the operating combustion of the hydrogen creates
all oxidation factors that affect cost of
pressure. several key reliability challenges.
ownership. The two primary options are:
The torch must be cooled either by
• Pyrolytic Torches. Torches cannot
• Pyrolytic Torch. High purity hydrogen
water or compressed air. Even with
provide 100% water vapor due to
and oxygen gas combine to form steam
this cooling feature the heat causes
accuracy limits of mass flow controllers
that then diffuses oxygen into the
torch components to break down and
(MFCs). The reaction rate (growth rate)
silicon wafer, forming the TOx layer.
require maintenance every six to 18
will vary over time because the MFCs
• RASIRC Steamer. De-ionized water is
months. The safety system also needs
will drift, again affecting repeatability
vaporized then passed through a non-
to be verified at this time. This usually
and throughput. In addition, for safety
porous hydrophilic membrane, creating
requires at least two days of down time
reasons there must always be excess
ultrapure steam at a controlled positive
per shutdown. In addition, the use of
oxygen to ensure against combustion.
pressure that then flows to the furnace.
hydrogen delays the startup of a new
As a result, it is not uncommon to
furnace system because permits are
Maximizing good die
have 10% oxygen and 90% steam.
required and a safety check needs to be
To provide a better understanding of how
This increase in oxygen flow reduces
completed. The safety interlock system
the choice of steam generation affects yield,
the partial pressure of steam, thereby
typically fails every 18 months. This can
each technique is discussed in terms of
reducing the growth rate.
lead to the loss of a complete load of
throughput and film quality.
• Steamer. Using a proprietary
wafers. Thick oxide conditions, which
membrane developed by RASIRC, the
require long run times, are especially
Throughput and growth rate
Steamer admits 100% saturated purified
difficult on torch components due
Oxide growth rate can be increased by
steam into the process tube. The flow
to prolonged thermal abuse. It is not
increasing the partial pressure of water
rate of the steam is measured and
unusual to lose four days per tube per
vapor
4
. While this may seem obvious,
controlled. Delivery of 100% saturated
year for planned and unplanned service.
technical difficulties interfere with
steam ensures the highest oxidation rate
• Steamer. The Steamer does not require
increasing the actual quantity of delivered
(shortest time). The positive pressure
hydrogen or oxygen to generate water
water vapor.
eliminates the need for a carrier gas
vapor. The non-porous membrane
According to the model of Deal and
and minimizes back streaming into
provides both control of the steam flow
Grove
4
, the growth rate of the oxide layer
the furnace. The Steamer compares
rate and a barrier between liquid water
is directly related to the effective diffusion
favorably to the torch, as illustrated in
and the furnace tube. The redundant
of the water molecules into the oxide layer
Figure 1.
thermal control system and the minimal
and the equilibrium concentration in the
amount of moving parts provide a
immediate area. When a carrier gas is
Throughput, process interruptions and reliable steam delivery system, especially
used to deliver water vapor, the carrier gas
maintenance when running long multiday to multi
molecules generate a partial pressure. This
The number of good die is not only week processes. Extensive field testing
partial pressure lowers the partial pressure
dependent on device yield, but also on tool has shown minimal maintenance
of water vapor and slows the diffusion
uptime. Planned and unplanned downtime and high reliability under demanding
of water into the oxide film. The result
directly reduces the amount of wafers process conditions.
is lower driving force and slower growth
that can be oxidized. Each steam delivery
rate. For a given temperature and process
Uniformity
method has inherent reliability levels.
pressure, oxide growth rates are fixed if the
For wet thermal oxidation, the partial
gas ratio is also constant. However, for a
• Pyrolytic Torch. Pyrolytic torches pressure at the wafer surface determines the
given operating temperature, this growth
burn hydrogen at 2000°C. This local growth rate. Any air or oxygen will
Figure 2. Steamer shows improved within-wafer and cross-load uniformity over
Torch source and is well within specification limits. Illustration is across 125 wafer
Figure 1. 7% improvement in growth rate of a Steamer over a pyrolytic torch. loading (175 wafer total).
www.globalsolartechnology.com
Global Solar Technology – May/June 2009 – 7
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