Ensure that the BoronPlus sources are loose in their slots.
Tight slots can result in warped sources. Slot dimensions
and carrier fabrication dimensions can be found in Product
Bulletin Carrier Design. The BoronPlus™
and PhosPlus® sources have the same dimensions, permitting
the same boat design to be used for both types of sources.
Cleaning BoronPlus sources is not normally
necessary. If you want to clean them, the following procedures
- 8 minutes in NH4 OH/H202/H20(1/1/5)
at 80°C or
8 minutes in a megasonic cleaning system at room temperature
- 2 minutes in DI WATER
- 10 minutes DRY at 90°C
The BoronPlus sources
should not come in contact with HF or HCI at any time.
There are several techniques available
for oxidizing any boron-silicon phase that has formed between
the deposited boron glass and the silicon substrate. Listed
below are the three most popular methods. You may select the
approach best suited to your process.
- A high temperature oxidation process
that involves changing from nitrogen to 100% oxygen at the
predeposition temperature. While this technique can rapidly
remove the boron-silicon skin, the oxidation rate is so
high that it may also convert doped silicon to SiO2,
resulting in back-diffusing of the boron from the silicon
and in non-uniform doping of the silicon.
- Use a high oxygen concentration during
some or all of the cooling time form the deposition temperature.
this has been shown to work very well for high temperature
processes. It not only oxidizes the boron-silicon phase,
but it also minimizes silicon damage often associated with
the formation of the phase.
- Deglazing the silicon wafers and then
placing them back in the diffusion furnace for 20-30 minutes
at 800 - 900oC in steam. This technique is often preferred
for base processes instead of the in-situ LTO described
in (b) above because of the better sheet resistivity uniformity
it normally provides. This process is commonly referred
to as a low temperature oxidation (LTO) cycle.
Remember that BoronPlus
sources are not sensitive to oxygen. You may, therefore, tailor
the oxygen concentration in your carrier gas to produce the
best results for your devices.
Before using BoronPlus sources in production
for the first time, an initialization or aging period is required.
This ensures that all moisture has been vaporized, and it
enables the sources to achieve a constant rate of boron evolution.
Aging should take place at the predeposition temperature in
nitrogen with 25 - 50% oxygen. Aging may last from a few hours
for high temperature processes to as long as 24 hours for
low temperature processes. See Page 1 for the recommended
aging time for this process.
Remember that BoronPlus sources are not
sensitive to oxygen. You may, therefore, tailor the oxygen
concentration in your carrier gas to produce the best results
for your devices.
Any time BoronPlus sources
are exposed to temperatures above 600°C, silicon wafers
should be placed between each pair of sources and at each
end of the load.
Appropriate storage procedures should
be followed to protect BoronPlus sources from unnecessary
exposure to moisture. Proper storage will contribute to increased
uniformity and long life of the sources and will improve the
electrical properties of your devices.
For dedicated diffusion tubes, we recommend
that BoronPlus sources be stored in the hot zone of the tube
near 600oC. Sufficient dry nitrogen should flow through the
tube so that no backstreaming will reach the sources. In those
cases where the diffusion furnace is used for other processing
steps, or is somehow not suitable for storing BoronPlus sources,
the BoronPlus sources should be stored in an oven at a temperature
above about 200oC. The oven must be continuously purged with
dry nitrogen flowing at a rate sufficient to prevent room
air from entering the storage chamber.
When the sources are stored in room air or accidentally left
in room air for a considerable period of time, sufficient
moisture may be absorbed to affect their performance during
the following run. Their performance can be easily restored,
however, by inserting the sources 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.
1. J. E. Rapp, The Planar Diffusion Technique,
Semicon Technology Asia 1998/9, Nordica International.3.F
Block B, Quarry Bay, Hong Kong, p. 33.
2. R. Rogenski, The In-situ Low Temperature
Oxidation Processing Step, Global Semiconductor 2000, Sterling
Publications Limited, London, England, p. 29.
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