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 BSG: 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: 600 seconds
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
Final Rinse time: 10-20 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 phosphosilicate 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.

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.
Immerse the wafer in 1 H₂SO₄ : 1 HNO₃ for 600 seconds to oxidize the elemental boron.
Rinse thoroughly with DI water and return to the 50 DI:1 HF oxide etch for another 20 seconds to remove the oxidized boron.
Wash very thoroughly in DI water and dry carefully with N₂.
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.

Type ____________
Average R_s2 = ____________ Ω/sq

Enter your R_s values and furnace boat position into your logsheet ASAP using the LDS 4PP software.

When entering data into the logsheet for boron predep, an SPC chart is generated showing the process limits. If your measured sheet resistance value lies between the top and bottom control limit lines then your wafer is within acceptable limits. If the measured value is out of this range consult your instructor. He or she may have you return your wafer to the boron predep furnace for an additional 10 minutes depending on the how far it's off. If this is required, the subsequent phosphosilicate glass removal times may be reduced proportionately.

Did the phosphosilicate glass removal affect the four-point probe measurement?