I regularly use some black wax to protect the front surface of my silicon
wafers during the KOH etch of the backside. Sticking a dummy wafer after
black wax application may be necessary. You should apply this process on a
hot plate above 120 C. After etch,  hot plate and/or trichlorethane (TCE)
solvent can be used for cleaning.
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Isa Kiyat