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We review the state-of-art and issues of dry etching of masks for nano-device photo lithography. After introducing the
basics of photo-mask structures and their plasma etching, we discuss the specifics of mask etching as compared to the
etching of silicon wafers, focusing on processes for the two most-challenging cases: phase-shift masks and NGL where it
is achieved by etching the quartz exactly to the prescribed depth. State-of the art solutions for the etching of such masks
must provide stringent etching uniformity and accurate end-point monitoring. This can be addressed by either reducing
the lithography fields to half or even quarter of their present area, with two to four times worse productivity, or working
with larger masks of 9 and later 12 inch. The pressure for such development is becoming stronger with the wider
adoption of 450 mm wafers. The new masks may need to be thicker to prevent unwanted deformations. Etching
uniformity will have to be sustained over much wider areas. Furthermore, RF bias control for thicker substrates is not
straightforward due to the quartz impedance preventing effective RF coupling. Our etching experiments in Ar and Cl2
showed that well established mask etching conditions can be reproduced for moderately thicker substrates, while further
increase of substrate thickness may require essentially different approach, such as a series resonance of the inductive
plasma bulk and the capacitive sheath.
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