Looks like stage 3 die polishing. Note the die polishing alter the devices unevenly? The stage 2 shows the forming of the Ridge Ring. The ring makes the part of the devices rise. Thus then they are polished, the center areas of the devices are thinner and removed during the polishing. Now seen as weakness on the stage 3 outer devices. The next polishing on the stage 4 will reduce them even more when that happens to the dies. (Just more information for the training of the eyes)
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