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Cleaning,
Preparing and Storing PhosPlus Sources
Cleaning: As part of the routine cleaning
process, the PhosPlus sources are acid etched to remove
any foreign matter and to expose a pristine surface. As
a result, no additional cleaning is necessary before putting
the sources into the diffusion furnace. If additional cleaning
is desired, the procedure in Table II is the only one recommended.
Table
II
| Cleaning
Procedures |
15
seconds dilute acid at room temperature:
| TP-250 |
4:1 |
HNO3 |
| TP-470 |
10-1 |
HF |
2
minutes rinse in fresh DI water
1 minute rinse in fresh DI water
Hold in a clean hood until dry
Store in nitrogen
This
procedure is only recommended for initial cleaning.
If the sources become contaminated after use, contact
your area technical representative for assistance |
Preparation:
Before using the PhosPlus sources in production for the
first time, they should be held at the intended deposition
temperature for a period of time. This will ensure that
all moisture has been vaporized, and it enables the sources
to achieve a constant P2O5 evolution
rate. The aging period may last from a few hours for high
temperature processes to as long as 24 hours for low temperature
processes. Figure 14 gives the recommended minimum aging
times for the two PhosPlus sources.
Storage:
Since the phosphorus is present within the 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
absorption of even small amounts of moisture can cause various
problems in silicon processing. It is therefore recommended
that the sources be stored in the diffusion boats in nitrogen
at an elevated temperature when the time between 
runs
exceeds about 45 minutes.
If
the sources were accidentally left out in a room for a long
period of time, however, 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When they are withdrawn from the tube, the boat is ready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.
Typical
Doping Procedures with PhosPlus Sources
Boats: Although diffusion boats of various
designs have been successfully used with the PhosPlus sources,
the best results are normally obtained with a four-rail
quartz boat having a design as shown in Figure 15. When
depositions are made above 1100°C, silicon carbide or
polysilicon boats are often preferred because of their increased
resistance to deformation. Boats made of any of these materials
fit on standard paddles and cantilever systems and can be
used in automatic transfer systems. The spacing between
the silicon surface and the source surface should be constant
and should be between 0.060" and 0.100". The slots
for the sources should be about 0.010" wider than their
thickness. The sources should fit loosely in the boat, allowing
room for expansion of at least 0.010" per inch of diameter.
Insertion
and Removal: A furnace ramping technique should
be utilized for all deposition cycles. This procedure involves
slowly inserting the boatload of wafers into the diffusion
tube at a temperature below about 800°C and at least
100°C less than the deposition temperature. After the
furnace and boat have reached thermal equilibrium, the furnace
is ramped to the deposition temperature. At the end of the
deposition time, the furnace is cooled back to the insertion
temperature, at which time the boat is withdrawn. The insertion
and withdrawal rates should not be more than 4 in./min.
for 100 mm sources. Because of the greater mass of material
involved, slower insertion and withdrawal rates should be
used with the larger diameter sources.
Ambient
Gases: Either of the two PhosPlus sources can be
used with the conventional gases of nitrogen or argon without
detrimentally affecting their doping performance. Although
oxygen does not adversely affect the sources, too high of
an oxygen concentration should be avoided because of the
potential for masking off the silicon surface to be doped.
Steam should also be avoided in the presence of the PhosPlus
sources at the deposition temperature. Steam causes the
P2O5 to rapidly evolve from the sources
resulting in shorter lifetimes of the sources. When using
the TP-470 sources above 1000°C, small quantities of
oxygen may be blended with the nitrogen or argon. The oxygen
concentration in the carrier gas is usually less than 1%
below 1050°C and could be as high as 5% at temperatures
above 1100°C.
When
the TP-250 sources are used near 900°C, the oxygen concentration
should be maintained below about 1%. This concentration
may be increased up to about 5% when depositions are made
at higher temperatures. No oxygen should be used below about
875°C, especially when polysilicon layers are being
doped.
Gas
Flow Rates: The gas flow rate utilized during the deposition
depends primarily upon the diffusion equipment such as tube
size and end cap design. Although the flow rate must be
high enough to prevent room air from backstreaming down
the diffusion tube, flow rates ranging from as low as 2.0
l/min. to as high as 15 l/min. have been successfully used
in a 135 mm diffusion tube. Satisfactory results are most
often obtained with a flow rate of 3-7 l/min. for this tube
size.
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