Our colleagues to the north seem to think that PMMA cannot be used as an etch mask. They are badly misinformed! Just keep the plasma power low (<= 100W) to prevent the resist from flowing. ZEP is a somewhat better etch barrier than PMMA, but only by 20% to 30%. That’s not very substantial, so you should be skeptical of exaggerated claims about ZEP. PMMA and ZEP are actually very similar polymers, and so you would not expect them to be terribly different as etch masks.