High-aspect-ratio x-ray lithography for magnetic head fabrication

The development of the future-generation magnetic recording heads is based on availability of high resolution and high- aspect ratio lithography. A key step in the magnetic head fabrication process is the formation of high-aspect ratio trenches in photoresist that are subsequently used as a plating mask for the magnetic read-write heads. Currently, 1.2 to 1.5 micrometer wide and 10 micrometer tall trenches in the resist are formed using optical lithography. In the near future, more than 6 micrometer tall resist patterns with trenches of 0.5 micrometer or smaller will be required. A study of using X-ray lithography to generate patterns suitable for future-generation magnetic recording heads was undertaken at the Center for X-ray Lithography at UW-Madison. It was successfully demonstrated that 0.8 micrometer trenches in 15 micrometer thick resist and 0.4 micrometer trenches in 6 micrometer thick resist can be formed. The main steps in the fabrication of the high-aspect ratio resist patterns included (1) production of an initial (master) mask using e-beam lithography, (2) high-contrast replicated (final) X-ray mask manufacturing using X-ray replication process, and (3) actual patterning of thick PMMA resist using the final masks. Both X- ray masks were formed on a 2 micrometer thick silicon-nitride membranes as mask carrier. APEX-E resist 0.5 micrometer thick was used for e-beam writing, and 2 micrometer thick PMMA was utilized for the replicated mask. The absorber was electroplated gold: 0.4 micrometer thick for the master and 1.5 micrometer thick for the final mask. Details are given for 6 micrometer and 15 micrometer thick crack-free PMMA resist formation and characterization, exposure and development conditions.