Level 4 Oxide Etch
Mask 4 Etching transfers the pattern defined in the photoresist to the underlying
- Acid fume hood
- Polypropylene boat with handle
- Buffered Oxide Etch: 6:1 NH4F:HF
- HF rinse tank filled with DI water
- Final rinse tank filled with DI water
- BOE etch time:
- HF Rinse time:
- Final Rinse time:
All equipment is located in rm50S (wet lab)
- Acid etch hood:
- BOE etch tub
- HF rinse tub
- Final rinse tub
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.
Don’t forget to hardbake before etching!!!
- Place wafers in a polypropylene wafer cassette.
- Turn on the right hand faucet – only use a latex gloved hand.
- Put on the heavy green Nitrile gloves – only handle the gloves from the inside.
- Open the etch and rinse containers (from the back to front) and position the lids
against the back of the fume hood.
- Grip the wafer cassette so that it is unlikely for drips from the glove to land
on the wafers.
- Slowly immerse the wafers in the acid and begin timing. Agitate the wafers slowly
back and forth - do not splash!
- When the etch time has elapsed, tilt the wafer cassette and then slowly raise it
out of the acid.
- Allow several seconds for the acid to drip off the cassette, then quench in the
- Agitate the cassette in the first rinse for
seconds, then transfer to the
final rinse tank for
- 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.
- If no other etches are needed, set the wafer cassette on a glass plate – and only
on a glass plate!
- Replace the lids on the etch tubs.
- Rinse the Nitrile gloves under the running faucet. Take them off (handle only from
the inside) and leave on the countertop to dry.
- 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.
- 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
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.
- 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
- 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 7.5 minutes without consulting your instructor.
- Record the wafer type (p or n) determined using the hot point probe.
- Make sheet resistance measurements on the wafer with the four point
probe as before for both the phosphorus and boron doped test areas. Consult
Appendix F if necessary.
Final Boron sheet resistance:
Type ____ Rsb = ____ Ω/sq.
Final Phosphorus sheet resistance:
Type ____ Rsp = ____ Ω/sq.
Enter your Rs value and furnace boat position into your logsheet ASAP
using the LDS 4PP software.
When entering data into the logsheet for final boron and phosphorus sheet resistance,
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.
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