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Source Preparation:
Aging
Initialization of the BoronPlus
and PhosPlus sources is a necessary step in their preparation
for use in production. Although the sources have an indefinite
shelf life, it is recommended that the sources be aged at
the predeposition temperature before the first production
run. This will insure that the sources are evolving B2O3
or P2O5 at a uniform rate, that all moisture is vaporized,
and that any residual cutting and cleaning solutions are
oxidized.
The
shape of a typical weight loss curve for a BoronPlus source
(Figure 1) indicates that a B2O3 concentration
gradient develops from the interior of the source to its
surface during use and that the B2O3
is evolving from the source at a diffusion-controlled evolution
rate. Although this results in the BoronPlus sources evolving
B2O3 at a high rate when they are
new, the rate falls off exponentially and quickly becomes
relatively constant with continued use time. A similar P2O5
evolution rate occurs with new PhosPlus sources. Therefore,
for optimum results, the sources should not be used in production
until the relatively constant evolution rate has been established.
This bulletin gives the recommended procedures that will
develop this evolution rate and that will properly prepare
the sources for use in production.
Aging Procedures
It is recommended that
the BoronPlus and PhosPlus sources be aged at the predeposition
temperature prior to the first production run using the
following procedures:
Place the dopant sources
into the diffusion boat in the normal production configuration.
One dummy silicon wafer should be placed between each pair
of sources for the purpose of conducting heat.
Adjust the nitrogen gas
flow rate to the rate that will eventually be used in the
production runs. The gas should contain about 25% oxygen
to oxidize all residual organic residues that may have been
left behind from the cutting and cleaning operations.
Insert the loaded carrier
into the diffusion furnace and ramp the furnace to the aging
temperature (same as the intended predeposition temperature)
using the procedures outlined in Product
Bulletin Processing Procedures.
Hold
the sources at the aging temperature for the minimum aging
times suggested in Figure 2 for the BoronPlus sources and
in Figure 3 for the PhosPlus sources. If longer aging times
are desired, the above steps can be repeated with or without
the oxygen in the nitrogen.
At the end of the aging
period, replace the dummy silicon wafers with production
silicon wafers or store the boat until needed (see Product
Bulletin Storage)
Once
this initial aging process has been completed, no additional
aging cycles are necessary. The boatload of sources is now
ready to be placed into production, and it can be run continuously
at the aging temperature (now the predeposition temperature)
until the diffusion engineer has determined that the sources
have reached the end of their useful life.
If it is desired to use
a set of BoronPlus or PhosPlus sources at a significantly
higher or lower predeposition temperature (generally, more
than about 50°C), the aging cycle should be repeated
at the new temperature. This will establish a different
B2O3 concentration gradient within
the BoronPlus sources that is characteristic of the new
temperature. Repeating the aging cycle for the PhosPlus
sources also insures that they are evolving P2O5
at a constant rate characteristic of the new temperature
of use.
Conclusion
Aging is a necessary and
important step in the preparation of the dopant sources
for use in production. The suggestions in this bulletin
will help to insure that the optimum performance of the
sources will be obtained in the production of semiconductor
devices.
The
shape of a typical weight loss curve for a BoronPlus source
(Figure 1) indicates that a B2O3 concentration
gradient develops from the interior of the source to its
surface during use and that the B2O3
is evolving from the source at a diffusion-controlled evolution
rate. Although this results in the BoronPlus sources evolving
B2O3 at a high rate when they are
new, the rate falls off exponentially and quickly becomes
relatively constant with continued use time. A similar P2O5
evolution rate occurs with new PhosPlus sources. Therefore,
for optimum results, the sources should not be used in production
until the relatively constant evolution rate has been established.
This bulletin gives the recommended procedures that will
develop this evolution rate and that will properly prepare
the sources for use in production.
Hold
the sources at the aging temperature for the minimum aging
times suggested in Figure 2 for the BoronPlus sources and
in Figure 3 for the PhosPlus sources. If longer aging times
are desired, the above steps can be repeated with or without
the oxygen in the nitrogen.
Once
this initial aging process has been completed, no additional
aging cycles are necessary. The boatload of sources is now
ready to be placed into production, and it can be run continuously
at the aging temperature (now the predeposition temperature)
until the diffusion engineer has determined that the sources
have reached the end of their useful life.