Multilevel diffractive elements in SiO2 by electron beam lithography and proportional etching with analogue negative resist

A negative low-contrast electron beam resist X AR-N 7700/18 is introduced, which provides a nearly linear dose-to-depth curve at electron acceleration voltages below 20 kV, and is therefore an excellent material for fabrication of multilevel diffractive optics. Direct electron beam recording at 12.5kV is used to pattern analogue surface profiles in this resist, and proportional reactive ion etching (RIE) is employed to transfer the profile into the SiO2 substrate. It is shown that errors in resist-profile scale can be corrected by proper adjustment of the radio-frequency power or chamber pressure in RIE. The reproducibility of the profile scale is better than 2%. Transmission-type blazed gratings, array illuminators, and pattern projection elements are demonstrated.