High‐resolution and high‐fidelity x‐ray mask structure employing embedded absorbers

Existing x‐ray masks often suffer from a variety of problems. These include poor adhesion of small absorber features to the substrate, severe undercut in the etching of sub‐half‐micron absorber lines, distortion of absorbing features formed by plating through a resist stencil, distortion of the absorber due to metal migration and due to local stress, and mechanical damage due to handling. Here we describe an x‐ray mask structure in which a high atomic number metal fills trenches etched into a single‐crystal silicon mask membrane. Thus the problems mentioned earlier for conventional x‐ray masks can be either effectively eliminated or greatly reduced. The trenches were formed in the silicon by a combination of ultrathin resist, high‐resolution electron beam exposure, and novel etching techniques involving metal masking. The trenches were half‐micron deep with a sidewall angle of 1.5° to the vertical, and the minimum trench width measured at the top of the trenches was 70 nm, which gives an aspect ratio of 7...