Boron Predeposition Diffusion

The boron predeposition transfers boron from a solid source to the wafer.

Boron, in the form of B₂O₃, diffuses from the source to the wafer. When it reaches the silicon, it will undergo the following chemical reaction:

2B₂O₃ + 3Si → 3SiO₂ + 4B (borosilicate glass)

The borosilicate glass (BSG) 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 8D

Supplies

• BN solid source: the BN wafer was oxidized at the beginning of the semester
• Solid source wafer boat

Operating parameters

• Furnace temperature: 950 °C
• gasses used: N₂
• N₂ flow
  • standby: 100
  • processing: 100
• Predeposition time: 15 min

Equipment/controls/tools locations

• Temperature controller: on the side of the furnace
• Gas panel: bottom rotameter 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

• Quartzware 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 the wafer.
2. Perform a 10-15 second etch in 50:1 DI:HF on your wafer if it has been more than an hour since opening the diffusion windows, DI rinse, and N₂ dry.
3. Check the boron predep furnace and support equipment (i.e., gas flows and temperature). The boron predep furnace should be at 950°C.
4. Follow the procedure for furnace loading in appendix G of the paper version. Use the Boron predep furnace and load the wafer so that the patterned side is facing the nearest BN wafer. Be sure to record which position your wafer is in (see Appendix G.4).
5. After a 15 min predeposition at 950°C, unload your wafer.
6. 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 by 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.
   
   

Note: The BN source transfers boron to the wafer via B₂O₃. The B₂O₃ reacts with the silicon to form a heavily doped SiO₂ layer (borosilicate glass), with a B:Si alloy layer at the BSG:Si interface. The BSG is easily removed with 50:1 DI:HF, but the B:Si layer must be oxidized chemically before it can be removed with the 50:1 DI:HF.

This transfer of boron using B₂O₃ is the ideal case for the ECE444 lab, but can be greatly accelerated by the presence of H₂ or H₂O. The hydrogen reacts with B₂O₃ to form HBO₂ (meta-boric acid), which has a vapor pressure much higher than B₂O₃. The higher vapor pressure of the metaboric acid accelerates the growth rate of BSG, therefore requiring a longer BSG etch.