Phosphorus Predeposition Diffusion
The phosphorus predeposition transfers phosphorus from a solid source to the wafer.
Phosphorus, in the form of P2O5, diffuses from the source
to the wafer. When it reaches the silicon, it will undergo the following chemical
2P2O5 + 5Si → 5SiO2 + 4P (phosphosilicate glass)
The phosphosilicate glass (PSG) is in contact with the silicon surface. A concentration
gradient is formed, and since this process occurs at high temperature, diffusion
will occur. The surface concentration will be fixed at the solid solubility, and
the distribution will be in the form of a complimentary error function:
N = concentration (cm-3)
Nsl = solid solubility limit for dopant (cm-3)
x = position inside silicon relative to the surface
D = diffusion coefficient for dopant (cm2/s)
t = time (seconds)
Lindbergh-Tempress 8500 manual oxidation furnace chamber 7D
- Si:P2O7 solid source
- Solid source wafer boat
- Furnace temperature:
- gasses used: N2, O2
- N2 flow
- standby: 100 ± 10
- processing: 100 ± 10
- O2 flow
- standby: 0
min after start: 5
- Predeposition time:
- Temperature controller: on the side of the furnace
- Gas panel: bottom rotameters at the front of the furnace
- Quartz handling: covered cart is to the left of the furnace, tongs are inside
- Boat: at the center of the furnace
Use the high temperature gloves when handling hot equipment.
- Quartware is easily contaminated by alkali ions. This leads to premature quartz
failure (breakage) due to devitrification as well as unstable MOSFET Vt.
Once quartz is contaminated, little can be done to remove the contamination.
- Always wear latex gloves when working with the furnace.
- N2 should always be flowing in standby to minimize contamination by backstreaming
of air in the room into the hot chamber.
- Degrease your wafer using the instructions in Appendix C of the paper version if
you did not perform Mask 2 etch during the same lab period.
- If it has been more than an hour since opening the diffusion windows, perform a
10-15 second etch in 50:1 DI:HF, DI rinse, and N2 dry.
- Perform a phosphorus predeposition diffusion at
min . The gases are
switched for you. Nitrogen flows at the standby rate for the first
min , then
switches to oxygen for the remaining time (it leaves the nitrogen on the entire
time). A low oxygen concentration (~5%) is used in order to minimize the phosphorus
silicide formation described in 7.12 of Anner. The contribution to the field oxide
thickness may be ignored for prediction purposes. Record the actual flow rates in
your electronic logsheet.
- Use the LDS four-point probe to get a rough idea of the sheet resistance. Consult
the instructor if it's outside the range specified on the SPC chart, you may have to return
the wafer to the furnace.
Verify dopant type by using the hot point probe (Appendix E).
Rs= _________ Ω/square.
Which area should you measure?
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