Borosilicate Glass Etch

During the boron predep a layer of BSG 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 boron doped areas to the solid solubility limit.

For bipolar transistors the base region should be lightly doped to minimize base current and thus increase Β. In order to do this, the boron doped regions will go through a second diffusion without a source of dopant. Redistribution takes place with a limited supply of dopants, thus lowering the doping concentration.

In addition to the formation of BSG, a thin layer (150-300Å) of SiB alloy forms in the transition from the boron rich glass to the relatively low number of dopants in the silicon. If this layer is not removed, it too can act as a source of dopants. In order to do this, it must be removed through oxidation. Ideally, a two hour low temperature (600°C) oxidation would be performed after the initial BSG was etched away. Due to time constraints, a quicker but less effective chemical oxidation is used.

In order to remove the BSG and SiB, a three step etch will be performed:

remove BSG:	HF etch
chemical oxidation of SiB:	H2SO4/HNO3
remove chemically oxidized layer:	HF etch

Tools

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• Acid fume hood
• Polypropylene boat with handle

Supplies

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• 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

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• 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

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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

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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

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1. Remove the borosilicate 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 to oxidize the elemental boron.
3. Rinse thoroughly with DI water and return to the 50 DI:1 HF oxide etch for another 20 seconds to remove the oxidized boron.
4. Wash very thoroughly in DI water and dry carefully with N₂.
5. Perform a hot-point probe measurement on any open region in the test area of the wafer. You may want to map the wafer again for a more thorough test. Record whether it is P or N type. (Refer to Appendix E). Also, make sheet resistance measurements on the wafer with the four point probe as before.
   
   Boron predep Type ____________ R_s1 = ____________ Ω/sq.
   
   Enter your R_s value 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 borosilicate glass removal times may be reduced proportionately.
   
   Did the Borosilicate glass affect the four-point probe measurement?