Investigation of 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 1.5μm wide and 10μm tall trenches in the resist are formed using optical lithography. In the near future, more than 6μm tall resist patterns with trenches of 0.5μm 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 0.8μm trenches in 15μm thick resist and 0.4μm trenches in 6μm thick resist can be formed. The main steps in the fabrication of the high-aspect ratio resist patterns included a) production of an initial (master) mask using e-beam lithography, b) high-contrast replicated (final) X-ray mask manufacturing using X-ray replication process, and c) actual patterning of thick PMMA resist using the final mask. Both X-ray masks were formed on a 2μm thick silicon-nitride membranes as mask carrier. APEX-E resist 0.5μm thick was used for e-beam writing, and 2μm thick PMMA was utilized for the replicated mask. The absorber was electroplated gold: 0.4μm thick for the master and 1.5μm thick for the final mask. Details are given for 6μm and 15μm thick crack-free PMMA resist formation and characterization, exposure and development conditions.