During the spinning of photoresist on a sample, there is a build-up of material at the wafer edges. In order to get the best resolution using contact lithography, it is necessary to have intimate contact between the sample and the photomask. Therefore, the edge bead must be removed prior to pattern exposure. Industrial facilities have integrated edge-bead removal processes within the spin stations. Our laboratory does not have this capability, so listed are 2 techniques to remove the resist edge bead prior to exposure.
- Technique #1
- Before baking, use razor blade to scrape off resist from edges of sample.
- Soft-bake resist as described in the particular recipe.
- Technique #2
- Expose the sample 1 mm in from its edge for 3 times the normal pattern exposure time at 7.5 mw/cm². Use aluminum foil/UV absorbing paper/another wafer/etc. as a mask.
- Develop in the resist specified developer for 60-120 sec.
- Rinse with DI water.
- Blow dry with dry nitrogen.
- Using yellow filtered light, look at sample under microscope to see if edge bead has been removed. You may need to perform steps 1-4 more than once.