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Frostlike patterns emerge when acetone partially dissolves a plasticy layer of old photoresist.  This is the same sample as in my previous post, which used to be covered in jagged black mountains made of plasma-damaged photoresist. Now the mountains are mostly dissolved away, except for a few jagged peaks still visible in the upper left.  Even though the photoresist wasn’t completely gone, I learned what I wanted to from this sample: that the photoresist had successfully protected the underlying semiconductor material from the etching plasma.