Phosphosilicate Glass Etch

During the phosphorus predep a layer of phosphosilicate glass was grown on the surface of the silicon. This layer will act as a constant source of dopants for subsequent diffusion steps, thus fixing the surface concentration of the phosphorus doped areas to the solid solubility limit.

In order to remove the PSG, a three step etch will be performed:

remove PSG:	HF etch
chemical oxidation:	H2SO4/HNO3
remove chemically oxidized layer:	HF etch

Equipment

---

• Acid fume hood
• Polypropylene boat with handle

Supplies

---

• 50:1 DI water:HF oxide etch
• HF rinse tank filled with DI water
• Glass etch: 1:1 H₂SO₄:HNO₃
• Glass etch rinse tank filled with DI water
• Final rinse tank filled with DI water

Operating parameters

---

• 50:1 oxide etch time: 20 seconds
• HF Rinse time: 5-10 seconds
• Final Rinse time: 10-20 seconds
• Glass etch time: 10 minutes
• Glass etch rinse time: 5-10 seconds
• Final Rinse time: 10-20 seconds
• 50:1 oxide etch time: 20 seconds
• HF Rinse time: 5-10 seconds

Equipment/controls/tools locations

---

All equipment is located in rm50S (wet lab)

• Acid etch hood:
  • Etching boat
  • 50:1 oxide etch
  • Glass etch
  • Glass etch rinse tub
  • HF rinse tub
  • Final rinse tub

Operating precautions

---

Chemical hazard

The 50:1 oxide etch and glass etch are extremely hazardous. Use with extreme caution, and only with appropriate personal protective equipment (safety glasses, face shield, latex gloves, and nitrile gloves). Use only under a fume hood. Treat exposure as described in the safety sections.

Only handle the gloves from the inside. Assume anything that is wet is either extremely acidic or basic. Do not lean on fume hoods.

Operating procedure

---

Phosphosilicate glass should be considerably easier to remove than borosilicate glass, but we'll use the same procedure to remove it. High surface concentrations of phosphorus are detrimental to photoresist adhesion so it is imperative that we remove it all.

1. Remove the phosphosilicate glass by placing the wafer in the 50:1 DI:HF oxide etch for 20 seconds. Follow with a thorough DI rinse.
   
   
2. Immerse the wafer in 1 H₂SO₄ : 1 HNO₃ for 10 minutes.
3. Rinse thoroughly with DI water and return to the 50 DI:1 HF oxide etch for another 20 seconds.
4. Wash very thoroughly in DI water and dry carefully with N₂.
5. Re-measure the sheet resistance. Did it change? Enter your R_s value and furnace boat position into your logsheet ASAP using the LDS 4PP software.
   
   R_s= ________ Ω/square.
6. Use the hot point probe to verify that the largest test area (on left) has indeed been changed back to N-type. Consult your instructor if it did not.