Level 2 Oxide Etch

Mask 2 Etching transfers the pattern defined in the photoresist to the underlying SiO₂

Equipment

• Acid fume hood
• Polypropylene boat with handle

Supplies

• Buffered Oxide Etch: 6:1 NH₄F:HF
• HF rinse tank filled with DI water
• Final rinse tank filled with DI water

Operating parameters

• BOE etch time: 6 min
• 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:
  • BOE etch tub
  • HF rinse tub
  • Final rinse tub

Operating precautions

Chemical hazard

BOE is 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

Don’t forget to hardbake before etching!!!

Oxide Etch

1. Place wafers in a polypropylene wafer cassette.
2. Turn on the right hand faucet – only use a latex gloved hand.
3. Put on the heavy green Nitrile gloves – only handle the gloves from the inside.
4. Open the etch and rinse containers (from the back to front) and position the lids against the back of the fume hood.
5. Grip the wafer cassette so that it is unlikely for drips from the glove to land on the wafers.
6. Slowly immerse the wafers in the acid and begin timing. Agitate the wafers slowly back and forth - do not splash!
7. When the etch time has elapsed, tilt the wafer cassette and then slowly raise it out of the acid.
8. Allow several seconds for the acid to drip off the cassette, then quench in the first rinse.
9. Agitate the cassette in the first rinse for 5-10 seconds, then transfer to the final rinse tank for 10-20 seconds.
10. Rinse a Nitrile gloved hand under the faucet, then use it to spray the wafer cassette with fresh DI water - do this over the sink area.
11. If no other etches are needed, set the wafer cassette on a glass plate – and only on a glass plate!
12. Replace the lids on the etch tubs.
13. Rinse the Nitrile gloves under the running faucet. Take them off (handle only from the inside) and leave on the countertop to dry.
14. If acid drops are suspected on anything in the fume hood, gently spray with DI towards the back of the hood and N₂ dry – do not use wipes to dry! If you are uncomfortable with the situation, ask the instructor for assistance.
15. Turn off the faucet – only with latex gloved hands.

Wet etching requires the diffusion of the etchant to the surface and the diffusion of the reaction products away from the surface. The smallest windows on the wafer will etch at a rate closer to that of the large test areas with a little rotational agitation.

When coming out of the etch it is important to let the wafer carrier drip for a few seconds while no more than a few centimeters above the acid. Tilting the carrier in two opposing directions also helps return more acid to its container. This not only keeps the DI rinses cleaner, but also minimizes the depletion of the acid container.

Inspection

1. Use the hot point probe (see Appendix E) in the upper right window of the test areas to check for complete oxide removal.
   
   If no definitive reading (a few nanoamps or more) is obtained, etch in 30 second intervals until oxide is removed. Do not etch for more than 6 minutes without consulting your instructor.
   
   
2. Always follow up with a microscope inspection to insure that ALL the windows to be opened through the oxide are indeed etched to bare silicon. The test area should be uniform in color (metallic grey) and all the windows should match it.
   
   If oxide is found with the microscope, etch in 30 second intervals until oxide is removed. Do not etch for more than 6.5 minutes without consulting your instructor.
   
   
3. Record the wafer type (p or n) determined using the hot point probe.