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In typical semiconductor operations, the
best results will usually be obtained when water vapor in
the diffusion tube is carefully controlled and kept to a minimum.
Trace amounts of water vapor can cause silicon surface damage,
non-uniform doping of the silicon slices, increased boron
penetration into the field oxide, and other yield reducing
effects. This bulletin discusses storage techniques that will
minimize the amount of moisture from entering the diffusion
tube that originates from diffusion equipment and materials.
Water vapor can originate from a variety
of sources. Excluding gross equipment malfunctions such as
cracked diffusion tubes and air leaks in the gas lines, small
but significant amounts of water vapor can enter the diffusion
tube through various routine processing operations.
Those most often identified are:
- Incomplete drying of the silicon slices.
- Trapped air between the silicon slices
and planar diffusion sources.
- Air back-streaming down the diffusion
tube during insertion of the boat.
- Moisture as an impurity in the carrier
gas.
- Absorption of moisture from the atmosphere
by the diffusion boats,
paddles, etc. during the loading/unloading steps.
When diffusion boats have been used for a large number of
runs, they begin to accumulate a thin B2O3
or P2O5 glassy film. Proper storage
techniques must be used to prevent this film from absorbing
significant amounts of moisture. When the film becomes thick
enough, however, it also becomes more hygroscopic and begins
to absorb water vapor from the air during the silicon wafer
loading/ unloading operation. At this time the sources should
be removed from the boat, and the boat should be cleaned with
a light HF etch. The boat cleaning can be done as often as
desired when BoronPlus and PhosPlus sources are used since
they do not stick to the diffusion boats.
The dopant source itself can also be a
source of moisture if not properly stored while not in use.
There is a significant difference in the amount of moisture
the two types of boron planar diffusion sources available
to the industry will absorb when left exposed to ambient air.
The boron nitride source contains a thin surface layer of
B2O3 which is extremely hygroscopic
and will rapidly absorb moisture from the atmosphere, even
during the relatively short time involved in the silicon loading/unloading
operations. The BoronPlus source, however, has most of the
B2O3 stored within the bulk of the material and not on its
surface. This results in significantly less moisture absorption
from the atmosphere. Tests conducted on BoronPlus and boron
nitride sources have shown that boron nitride wafers can increase
in weight by as much as 1.1% [1] while BoronPlus
sources increased less than 0.02% when stored under similar
conditions.
Since the phosphorus is present within
the PhosPlus sources in the form of a complex crystal and
not as the extremely hygroscopic P2O5
material, the PhosPlus sources exhibit a minimum amount of
water absorption. However, the sources can absorb small amounts
of moisture which may affect the electrical properties of
certain devices such as high betas from the phosphorus emitter
diffusion of a bipolar transistor. It is therefore recommended
that the sources be stored in the diffusion boats in a dry
environment at an elevated temperature when the time between
runs exceeds about 45 minutes.
If either the BoronPlus or PhosPlus sources
were accidentally left out in a room for a long period of
time, they can be quickly prepared for the next run by merely
inserting them into the diffusion tube at the insertion temperature
for about 15 minutes. When they are withdrawn from the tube,
the boat is ready for loading with production silicon.
The solid sources and the diffusion boat
should not be allowed to absorb small amounts of moisture
during long term storage if optimum results are to be obtained.
The following discussion will therefore provide the process
engineer with several alternate storage techniques from which
to choose. He should select the technique that best suits
his diffusion process and the one that is compatible with
his available storage equipment.
Selection of the best method for storing
BoronPlus or PhosPlus sources when not in use depends to a
large extent on the sensitivity of the process to the presence
of water vapor. Generally, the base diffusions in bipolar
processes and the source/drain diffusions in MOS processes
require the greatest control of moisture. Less sensitive processes
include certain isolation diffusions and high temperature
etch stop applications.
For the most critical processes:
Store in the hot zone of the furnace after it has been turned
down to about 600°C. This technique is highly recommended
for optimum results. It has been found to be very effective
for applications that are extremely sensitive to minute traces
of water vapor.
For processes that are moderately sensitive to moisture:Store
at the mouth of the diffusion tube with hot nitrogen gas from
the diffusion tube passing over the boat. The gas flow rate
should be kept high enough to prevent air from back-streaming
into the diffusion tube through any holes in the end cap.
Care should be taken to be certain that the gas inlet (source)
end of the boat does not extend into the hot zone of the diffusion
tube. If the cool zone at the front of the diffusion tube
is too short for the length of the diffusion boat, the furnace
tube can be extended by the use of an "elephant"
Store in a dry box being held at 200°C
or higher and continuously purged with dry nitrogen. This
technique is commonly used by many companies as an alternative
to technique a).
For processes that have little
sensitivity to moisture: Store in an extension of
a diffusion tube or "elephant" that is capped and
continuously purged with dry nitrogen at room temperature.
This technique is normally considered to be the minimum care
that one should take when storing a diffusion boat and sources.
Store the sources in a laminar-flow clean
hood. Although this technique will keep the boat and sources
clean, they will be continuously exposed to any moisture that
is present in the room air. If this storage technique is selected,
the process engineer should evaluate his product very carefully
to insure that this method will not have adverse effects upon
it.
The storage technique that is selected
for a particular process must be determined by the process
engineer. Experience has shown that careful consideration
and selection of an appropriate storage procedure is a vital
element in the successful application of planar sources to
semiconductor fabrication.
For more information on this Product Bulletin or on the BoronPlus
and PhosPlus dopant sources, contact the Planar Dopants Team:
www.techneglas.com
References:
1. David Rupprecht and Joseph Stach, "Oxidized
Boron Nitride
Wafers as an In-Situ Boron Dopant for Silicon Diffusions",
J.
Electrochem Soc., Vol. 120, No. 9, Sept. 1973, pp. 1266-1271.
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