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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 Rs2 = ____________ Ω/sq
Enter your Rs 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?
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