Hi Gary, Because I would like to etch 300nm in Silicon to do a Master. I did a E-Beam lithography in PMMA but PMMA is a bad kind of material in RIE to etch great depths in the silicon. Then we use a thin layer of SiO2 to the brokering of silicon etching. Judith 2011/6/6 Gary Hillman> Judith why so thin a layer of oxide. Gary > > Gary Hillman