As most of us are already aware, photoresist exposed to ion implantation will form a dehydrogenated amorphous carbon layer. The difficulty of removing this carbonized layer is determined by the implant conditions which can have a wide variation in species, energy, dosage, and other parameters. The challenge for both ashing and all-wet PR stripping is to have the process aggressive enough to remove the carbonized surface layer on the photoresist, but still maintain low material loss. We have found that the most challenging residue is formed where the photoresist pattern meets the wafer surface and especially at the edge of the wafer near the edge-bead removal region. Any photoresist stripping process must be closely examined for its ability to remove this edge removal as it is usually the last contamination to be removed from the wafer.
This work from many years ago, recently published in a new handbook by Doering and Nishi exemplifies the difficulty in removing the carbonized crust layer. The activation energy for crust removal by ashing was found to be much higher than that for un-implanted resist