25 nm mechanically buttressed high aspect ratio zone plates: Fabrication and performance

Abstract

High performance zone plates are critical for advancing the state-of-the-art in x-ray microscopy, both in terms of spatial and energy resolution. Improved resolution, increased energy bandwidth, and enhanced efficiency can be achieved through the fabrication of smaller, higher aspect ratio outer zones. Using electron beam lithography, we have fabricated and obtained initial performance data from a 25nm outer zone width zone plate, with a 7:1 aspect ratio, using a hydrogen silsesquioxane (HSQ)/cross-linked polymer bilayer process. We investigated the effectiveness of buttresses, i.e., mechanical supports perpendicular to the zones, on our ability to achieve higher aspect ratios which conventionally would be unreachable due to resist collapse. Optimum buttress spacing is affected by film thickness, linewidth, collapse mechanisms, and resist modulus. For 25nm zones, etched into 150nm cross-linked polymer (AZPN114), buttress spacings of approximately two times the resist thickness or ten times the zone width are sufficient to prevent collapse during plating. We find that high aspect ratio features not only have to be able to withstand collapse during liquid immersion, but also during dry etching processes. In addition, we show that a 50% feature bias and longer development times (8min in 1% TMAH based solutions) allow smaller dense feature sizes by eliminating resist webbing frequently observed in electron-beam imaging of HSQ.