Hi Steven, I had a same problem.I etched mesa on the GaAlAs/GaAs wafer in order to define QDs I used H2SO4:H2O2: DI as etchand solvent.I used N-415 photoresist.I saw a pink color on the mesa pattern( the area that wasn't etch) I couldn't remove this pink color as well but I think it's due to different high between 2 areas? good luck Steven Yangwrote: Hi, all Recently faced a strange problem during photoresist cleaning. I used AZ7220 to pattern electrode, the bake temperature for all is 95-100C. Because the pattern is broken during development, I clean the AZ 7220 with acetone for 5-10 mins which not completely remove the photoresist. Then I went through the Piranha 1:1 at room temperature for 20mins. However, there is still some pink color (in microscope) residual left in my DRIE etched pattern bottom. The strange thing is that I used all possible photoresit remove methods. But these pink color residual still left there with no visually removal at all. The methods I tried are: Prinha 3:1 (H2SO4:H2O2) for 30mins Resist remover RR2 for 20-30mins at 110C Remover PG for 20mins at 50C Even O2 plasma 100W 10sccm for 10mins. What probably you guys think this pink color layer is? and How can remove it completely with any possible method? Btw, I measured this thin pink layer is around 200nm thick.