I am trying to etch silicon using a standard Bosch DRIE process (SF6 - C4F8), but after a few minutes a black layer appears in the regions that I want to etch and apparently this layer prevents the regions to be etched. I use Al2O3 as masking layer for the process. Has anyone have this problem before?
Thank you in advance for your help.
Adding oxygen in you gas mixture may prevent the deposition of carbon onto your samples.
Did you look at this black layer with microscope or SEM? Could it be black silicon?
I would indeed expect this to be black silicon. It is caused by redeposition of passivation material (fluorcarbons) during the etch step. The redeposited material clumps are masking the silicon underneath, causing pillars to be formed underneath these nanomasks. It can be prevented by making your etch step more physical (because the nanomasking is based on a difference in chemical etch speed), so by increasing the ccp of the etch step. This will make sure that these clumps are etched also during the etch step, so that they cannot function as a mask for the following layers.
Thank you for your answers. Yes, it appears to be black silicon. I will try to increase the ccp of the etch step, hopefully I will get rid of it this way.
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