Metal Lift-off Using a Bi-layer Process
- Thomas Grimsley
- Sean O'Brien
Process Description This is a bi-layer photoresist process that is used for metal lift-off. The LOR 5A layer is not photosensitive and is spun on the wafer first. A layer of Fujifilm HPR-504, which is photosensitive, is spun on top of the LOR 5A. The wafer is patterned in the GCA Stepper and then developed. Since the LOR 5A has a higher dissolution rate in developer than the Fujifilm HPR-504, the resulting profile will have an overhang. A layer of metal is evaporated onto the wafer and then the excess metal is lifted off in a solvent leaving the desired patterns on the wafer. The thickness of the LOR 5A should be 1.3 times the thickness of the metal film.
| Recommended Process at RIT | |
| -HMDS Prime on the SVG Track Program 1. | |
| -Dispense the LOR 5A and spin at 2000rpm on the hand-spinner. The resulting thickness will be about 5000A. Do not coat LOR resist on the track. | |
| -Bake at 150C for 1 minute. | |
| -Dispense the HPR-504 photoresist on the SVG Track with Program 1, NO PRIME and soft bake as normal with Program 1. | |
| -Determine proper exposure with an exposure array and expose on the GCA Stepper. If your wafer has topography, the dose may need to be adjusted to clear the lower areas. The resolution is approximately 2 microns. | |
| -Develop at 30C on the SVG Track using the double puddle program 3. Verify the program before use. Do not bake the wafer after development or the profile may be destroyed. | |
-If a thicker LOR 5A layer is needed, multiple layers may be spun on top of each other. Each layer should get a 1 minute, 150C bake. |
SEM Cross Sections at Different Stages of Lift Off Process
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|  |
|---|---|---|
| Bi-layer Resist after Develop | Bi-layer Resist after Metal Evaporation | Remaining Metal after Resist Lift Off |
Microscope Views of Resist Stack
 |  |  |  |
|---|---|---|---|
| Scumming due to over bake or under develop. | Over baked or under developed LOR without undercut. | Properly baked and developed LOR with undercut. | Over developed or under baked LOR with excessive undercut. |
Example of lift off using sputtered metal.
Note the incomplete liftoff of the metal as well as the tails at the edge of the metal features.
