Boron Drive Diffusion

The boron drive performs two functions: it lowers the surface concentration and drives the junction deeper into the wafer.

The boron concentration near the surface after predeposition is too high and the junction depth is too shallow to act as a good base. After the BSG is removed from the surface of the wafer, a 'sourceless' diffusion (drive) will lower the surface concentration and simultaneously drive the dopant deeper into the wafer. The distribution can be approximated as a Gaussian profile:

and:

Where:

N₀ = surface concentration (cm^-3)
x = position inside silicon relative to the surface
D = diffusion coefficient for dopant (cm²/s)
t = time (seconds)

and

N₀ = surface concentration after the drive (cm^-3)
N₀₁ = surface concentration after the predep (cm^-3)
D₁ = diffusion coefficient of dopant during the predep (cm²/s)
t₁ = time of the predep (seconds)
D₂ = diffusion coefficient of dopant during the drive (cm²/s)
t₂ = time of the drive (seconds)

In addition to redistribution of the dopant, another oxide layer is grown to act as a diffusion mask for phosphorus predeposition.

Equipment

Lindbergh-Tempress 8500 manual oxidation furnace chamber 7B

Supplies

• N₂

Operating parameters

• Furnace temperature: 1100 °
• N₂ flow
  • standby: 110 ± 10
  • processing: 0
• O₂ flow
  • standby: 0
  • processing: 110 ± 10
• H₂ flow
  • standby: 0
  • processing: 40 ± 10
• The oxidation will consist of three steps:
  • 10 min dry oxidation (O₂ only)
  • 30 min steam oxidation (H₂ and O₂)
  • 5 min dry oxidation (O₂ only)

Equipment/controls/tools locations

• Temperature controller: on the side of the furnace
• Gas panel: second rack from the top in the back 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. Have your instructor check the boron drive furnace and support equipment (i.e., gas flows). The furnace should be at 1100 °C.
2. Degrease your wafer using the instructions in Appendix B of the paper version if you did not perform the glass etch during the same lab period.
3. Insert wafer into boron drive furnace using Appendix F of the paper version as a guide.
4. Perform the following drive recipe at T = 1100 °C.
   • Dry oxygen drive for 10 min .
     
     
   • Steam drive for 30 min .
     
     
   • Back to a dry drive for 5 min .