That sounds about in line with a microlithography vendor (UK) I ordered zoneplates from. 7-10µm features were accessible and going to 0.7µm was an order of magn

That sounds about in line with a microlithography vendor (UK) I ordered zoneplates from. 7-10µm features were accessible and going to 0.7µm was an order of magnitude higher (and went from film to chrome on glass IIRC). File size was also a factor for more complex patterns. Do not hesitate to post results on cloudynights or your club's website, I'll refresh it from time to time ! I do not think I have enough diffraction experience to help though. Best of luck for your endaevor.