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 reaction:

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:

where:

N = concentration (cm^-3)
N_sl = 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)

Equipment

Lindbergh-Tempress 8500 manual oxidation furnace chamber 7D

Supplies

• Si:P₂O₇ solid source
• Solid source wafer boat

Operating parameters

• Furnace temperature: 1000 °C
• gasses used: N₂, O₂
• N₂ flow
  • standby: 100 ± 10
  • processing: 100 ± 10
• O₂ flow
  • standby: 0
  • 15 min after start: 5
• Predeposition time: 25 min

Equipment/controls/tools locations

• 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

Operating precautions

High temperatures

Use the high temperature gloves when handling hot equipment.

Contamination issues

• Quartware is easily contaminated by alkali ions. This leads to premature quartz failure (breakage) due to devitrification as well as unstable MOSFET V_t. 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.

Operating procedure

1. 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.
2. 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.
3. Perform a phosphorus predeposition diffusion at 1000 °C for 25 min . The gases are switched for you. Nitrogen flows at the standby rate for the first 15 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.
4. 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).
   
   R_s= _________ Ω/square.
   
   
   
   Which area should you measure?