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A successful production
of semiconductor devices requires that the silicon wafers,
boats, paddles, tubes, etc. be carefully cleaned of all
contaminates before they are used. Usually these materials
are cleaned with one of the procedures that have become
somewhat standard in the semiconductor industry. However,
if the procedure is found to have a deleterious effect on
the performance of the material, some alternate cleaning
procedure must be used.
This is the case for the
BoronPlus sources. This bulletin gives recommended procedures
on how to clean the BoronPlus sources. It also gives cautions
on alternate cleaning solutions and procedures that may
harm the sources.
The BoronPlus sources
are cleaned of processing contaminates before shipping.
If additional cleaning is desired, however, the following
procedure, which is similar to the SC1 portion of the "RCA
Standard Clean" [1], can be used on any BoronPlus source:
After the cleaning process
is complete, the sources should be inserted into the diffusion
furnace to begin the aging cycle using the procedures outlined
in Product Bulletin 511. The aging cycle insures that the
BoronPlus sources are evolving B2O3 at a uniform rate, that
all moisture is vaporized and that any residual cutting
and cleaning solutions are oxidized.
When a BoronPlus source
is initially fired in the diffusion furnace, a few "black
spots" may occasionally show up on its surface. These
spots have been found to be harmless, but they can be removed
if desired by following certain steps in the preparation
of the sources.
The "black spots"
actually originate early in the manufacturing process of
the BoronPlus sources. This is because the sources are made
from a relatively unstable glass composition which may permit
several large crystals to grow within the billet during
the casting and heat treating steps. These crystals cannot
be seen before the billets are cut because the material
is opaque. However, if the saw blade happens to cut through
one of the crystals during the slicing operation, the crystal
will absorb some of the cutting fluid. Since this cutting
fluid cannot be totally removed from the crystal with any
of the cleaning processes, it will eventually turn to carbon
when the sources are inserted into the diffusion furnace.
The residual carbon makes the crystal appear as a "black
spot" in the wafer. On the other hand, if the crystals
do not touch the cut surface, the cutting fluid is not absorbed,
and the crystals remain, in this case, as "white spots"
when fired.
Before the sources are
used to dope silicon wafers, they are inserted into the
diffusion furnace in 25% oxygen for a period of time. This
aging cycle usually turns the "black spots" into
"white spots" because the oxygen in the carrier
gas diffuses into the crystals and oxidizes the carbon.
The crystals themselves, however, do not disappear since
they can still be seen in the wafer when examined under
transmitted light.
Extensive testing
has been done to determine if these crystals would have
any effect upon devices being manufactured across from them
[2]. After measuring a number of sensitive electrical properties
of 1700 diodes, it was concluded that the properties of
the diodes located across from the crystals were not significantly
different than the properties of the diodes located across
from other portions of the sources. The results of the tests
were the same whether the sources were used at 975°C
or at 1075°C, whether the crystals were black or white,
and whether the sources were new or had been used for hundreds
of hours. It is therefore concluded that the spots in the
sources, white or black, are merely cosmetic in nature and
should not be a reason for any serious concern.
The BoronPlus sources
were cleaned in a number of organic and inorganic cleaning
solutions and were then tested to see if the solutions had
any effect on their doping characteristics. The organic
solutions that had little or no effect on the doping characteristics
were acetone, freon, heptane, xylene, butyl alcohol, and
isopropyl alcohol. The only inorganic solvents that did
not affect the doping characteristics, however, were HNO3
(dilute) and the SC1 clean when used as described above.
HF, HCl and H2SO4 in concentrations of 1:1 to
10:1 all reacted negatively with the sources and in some
cases completely stopped all B2O3
from evolving from them.
The BoronPlus sources
that were cleaned in very dilute HF (40:1) initially showed
a decrease in B2O3 evolution rate,
but the sources eventually recovered after aging and began
evolving B2O3 at a normal rate. Consequently,
in the event of a serious contamination where the impurity
is not loose on the surface but is actually imbedded into
the surface of the source (i.e. by diffusion), cleaning
the sources for a few minutes in this dilute solution of
HF might be an effective way to remove the impurity. However,
this cleaning procedure is not recommended unless the only
alternative would be to discard the sources.
The BoronPlus sources
must be cleaned of processing contaminates if optimum results
are to be obtained. However, unless the proper cleaning
solutions are used, the doping characteristics of the sources
may be severely affected. The cleaning procedures outlined
in this bulletin will provide the process engineer with
the proper techniques for the BoronPlus sources.
References:
1. "Hydrogen Peroxide Solutions for Silicon Wafer Cleaning",
RCA
Engineer, Vol. 28-4, July/Aug 1983, pp.99-105.
2. Hur-Ling Hsiu, "Effect of Solid Source Impurities
on Silicon
Devices", Thesis at Arizona State University, Dec.,
1988.
