Fabrication of phase shifting masks employing multi-layer films

Abstract

The design and fabrication of phase shifting masks using multi-layer films are suniniarized. Mask blanks consist of fused silica substrates coated with a top chrome layer, an SiO2 phase layer and an etch-stop layer. The layer thickness's are designed to give a π-phase shift at 248 nm (or at 365 iiiii), and equal and maximal transmission at etched and non-etched regions. The chrome layer is patterned by c-beam resist and is etched by reactive ion etching (RIE) with Cl2 chemistry. The SiO2 layer is patterned by another e-beam resist in registration with first chrome pattern and is etched by RIE with CHF3 chemistry. Two materials were investigated as the etch stop layer, namely Al2O3 and HfO2, with the latter having advantageous properties. It is shown that a 20% processing window is achieved in this type of PSM for a 5 degree phase accuracy as in contrast to quartz etched PSM in which the etched depth and uniformity has to he controlled to within 2.5% for a similar phase accuracy.