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Preparing and Storing BoronPlus Sources
The BoronPlus sources are cleaned of processing contaminants
before shipping. If additional cleaning is desired, the procedures
outlined in Table II should be followed.
minutes in NH4 OH/H202/H20(1/1/5)
at 80°C or
8 minutes in a megasonic cleaning system at room temperature
minutes in DI WATER
minutes DRY at 90°C
sources should not come in contact with HF or HCI at any time.
The sources should be held at the intended deposition temperature
before they are used to be sure all moisture has been removed.
Although some processors begin immediately to use the sources
after this initial drying period, others prefer to hold the
sources at the
use temperature for an additional time until a more constant
B2O3 evolution rate occurs. The recommended
minimum aging times can be obtained from Figure 8.
Since the B2O3 is contained within the
source and not totally on its surface, the BoronPlus sources
exhibit a minimum amount of water absorption. However, since
absorption of even small amounts of moisture can cause problems
in silicon processing, we recommend they be stored in diffusion
boats in a dry environment at an elevated temperature. The
best procedure is to hold the sources in the hot zone of the
diffusion furnace at 600°C in dry nitrogen.
Doping Procedures with
diffusion boats of various designs have been successfully
used with the BoronPlus sources, the best results for depositions
made at temperatures below about 1100°C are normally obtained
with a four-rail quartz boat with a design similar to Figure
9. When depositions are made above 1100°C, silicon carbide
or polysilicon boats, having a 1/3-round design, 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 0.020" per inch of diameter.
and Removal: We recommend that a furnace ramping
technique be utilized for all deposition cycles. This procedure
involved slowly inserting the boatload of wafers into the
diffusion tube at a temperature below about 900°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 temperatures at which time the boat is withdrawn.
The insertion and withdrawal rates should not be more than
4 in./min. for 100mm sources. Because of the greater mass
of material involved, slower insertion and withdrawal rates
should be used with the larger diameter sources.
Cycles,Gas Flow Rates
Gases: The BoronPlus sources can be used with the
conventional gases of nitrogen and argon without detrimentally
affecting their performance. Although nitrogen is the most
common gas, some users prefer to use argon, especially at
temperatures above 1000°C. Small amounts of oxygen are
sometimes blended with the gas during the deposition. The
oxygen concentrations are usually less than 1% below 1000°C
and could be as high as 5% at deposition temperature above
1100°C, as shown in Figure 10. The sources should not
be used in the presence of steam.
Cycles: High concentrations of oxygen can be used
during the deposition cycle in the presence of the BoronPlus
sources since oxygen has a negligible effect upon the subsequent
performance of the sources. The oxygen diffuses through the
deposited glassy film and oxidizes the boron-silicon phase
the forms on the silicon surface during the deposition. The
oxidized phase may then be easily removed with a conventional
HF etch. This in-situ LTO step significantly reduces the overall
processing time as schematically illustrated in Figure 11.
LTO cycle can be used with any of the BoronPlus sources. The
predictable results of using the in-situ LTO with the GS-278
BoronPlus sources for a typical p-type emitter diffusion are
shown in Figure 12.
the in-situ LTO has been successfully used in other p-type
emitter, isolation, etch stop (micromachining) and similar
types of applications, the conventional low temperature oxidation
cycle is usually recommended for high sheet resistivity base
and source/drain diffusions because of better sheet resistivity
control. A typical LTO cycle is to hold the silicon wafer
in steam for about 20-30 min. at 800°C after removing
the deposited glass in 10:1 HF. The sources should not be
present during any LTO cycle involving steam.
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 backsteaming
down the diffusion tube, flow rates ranging from as low as
2.0 I/min. to as high as 15 I/min. have been successfully
used in a 135mm diffusion tube. Satisfactory results are most
often obtained with a flow rate of 3-7 I/min. for this tube
contained herein is derived from in-house testing and outside
sources and is believed to be reliable and accurate. TECHNEGLAS,
Inc., however, makes no warranties, expressed or otherwise,
as to the suitability of the product or process or its fitness
for any particular application."
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