Photolithography - Manual Edge-Bead Removal Techniques
Purpose of Edge Bead Removal
Removing the thicker photoresist along the edges of your sample (aka. edge-bead removal, EBR) has multiple advantages:
- These techniques are required for loading full-wafers into etchers that use top-side clamps, to prevent photoresist from sticking to the clamp (and potentially destroying your wafer).
- For contact lithography, this improves the proximity of the mask plate and sample, improving resolution.
- For some projection systems, such as the Maskless Aligner, EBR can help with autofocus issues.
Razor Blade
Use a razor blade to scrape off the edge-bead regions. This works well for small samples, if the substrate material is not fragile (works well for Silicon, Sapphire, GaAs, Silica etc.).
Especially useful for photoresists that don't chemically dissolve easily, such as PMGI, SU8, BCB.
EBR100 Swabbing
EBR100 squirt bottles can be found on the spinner benches. (Do NOT bring Acetone onto the spin-benches! The fumes affect nearby PR.) Only use this chemical in a fume hood, and dispose in the waste in the fume hood waste containers.
- Squirt some EBR onto a COTTON swab (not plastic swab! They will dissolve.)
- Roll the swab on a cleanwipe to remove excess liquid.
- Use the swab like a paintbrush to remove PR from the edge of the wafer, re-soaking as needed.
Lithographic Edge Bead Removal
For small parts, you can cut pieces of tinfoil into small squares, and mask all but 2 edges and flood expose those two edges (on a contact aligner), then repeat for the other two edges. Expose with 3-5x the typical dose, since the PR is thick at the edge. Then develop the sample to remove the edge bead. Subsequent litho will work fine.
For larger parts (eg. full wafers), you will want to make a custom plastic or metal mask that you can place on top of the wafer during flood expose - then develop to remove the edge-bead.
We do have simple metal masks made for 4-inch wafers, that you can use on the Suss MA6 with Flood Expose mode to expose your edge bead.
See this procedure for performing lithographic Edge-Bead Removal with DUV Photoresists, which can easily be converted for I-Line:
ASML DUV: Edge Bead Removal via Photolithography
Demis D. John, 2022