Fabrication of annular nanostructures using ion beam proximity lithography.

摘要

Magnetic films can be patterned to have a ring shape, which supports a "vortex" magnetization state from which no stray fields emanate. This property allows for very high packing densities while ensuring minimal magnetic interaction with neighboring structures. The fabrication of ring-shaped magnetic bits is seen as a way to improve storage density in magnetic random access memory. The work described in this dissertation entailed (1) the development of a fabrication process for forming large areas of ring-shaped structures from permalloy films and (2) the study the switching behavior of these structures through measurement and simulation.;To support the development of this fabrication approach, silicon nitride stencil masks with high pattern uniformity and large areas were fabricated, the silicon oxide conformal coating was optimized, and an x-y mechanical stage with integrated piezo-electric nano-positioning system was constructed. Process integration tests ensured that each step was compatible with the magnetic films, and modeling using micromagnetic simulation software (OOMMF) assisted in process optimization.;These fabrication approaches were used to form 6x6mm 2 arrays of rings with a constant pitch (667nm), while the outer diameter was varied from 350nm to about 450nm with a fixed inner diameter of 150nm. This yielded three unique samples that could be characterized using a vibrating sample magnetometer. The magnetic switching curves showed the expected switching from an onion to a vortex and back to an onion state and were in good agreement with simulations. It was determined that the switching behavior of the structures is degraded by roughness in the features, which prevent proper propagation of the domain walls; size uniformity degraded performance to a lesser extent. It is therefore critical that one emphasize smoothness over size uniformity when patterning magnetic ring structures.;To fabricate these structures, ion beam proximity lithography was used to pattern an array of circular openings in photoresist, a silicon dioxide overcoat was used to conformally coat the resist patterns, and an anisotropic reactive ion etch was used to thin the oxide while leaving the sidewalls intact. The resulting circular structures were transferred into the underlying permalloy film to form the magnetic bits.