Optical sub-diffraction limited focusing for confined heating and lithography
Electronics and nanotechnology is constantly demanding a decrease in size of fabricated nanoscale features. This decrease in size has become much more difficult recently due to the limited resolution of optical systems that are fundamental to many nanofabrication methods. A lot of effort has been made to fabricate devices smaller than the diffraction limit of light. Creating devices that are capable of confining fields by means of interference patterns of propagating wave modes and surface plasmon, has proven successful to confine light into smaller spot sizes. Zone plate diffraction lenses generate spots with dimensions very close to the diffraction limit. We report the fabrication of zone plates to be used in laser direct writing of silicon nanowires. We show experimentally and with numerical models that a silicon substrate subjected to a focused spot is capable of reaching the necessary temperature for the synthesis of silicon nanowires with widths of 60 nm, which is considerably smaller than the diffraction limit of the processing laser. Nanoscale ridge apertures are devices with a great potential to confine light energy. Such apertures have been experimentally proven to create very small lithography features. We believe that these apertures can be further modified in order to achieve a practical smaller confinement in the near field region. In this thesis we discuss several attempts to design and fabricate apertures with sharp edges and implement them in a previously reported parallel lithography setup. In an attempt to use apertures for parallel fabrication of patterns, we developed a system to control the position of the near-field region with respect to a lithography substrate. To do this we use a method of interferometric-spatial- phase-imaging (ISPI). With the implementation of this method we were able to produce an array of 32X32 lines with confined widths as small as 22 nm. Nanoscale ridge apertures were also studied to be used as near field transducers for heat-assisted magnetic recording. They have the capability of transmitting and confining enough energy to increase the temperature of a recording medium without reaching detrimental temperatures themselves. Numerical methods are presented to prove theoretically that a well-designed aperture performs well as a near field transducer. The size of the spot region focused by the aperture could allow us to record data with higher area density than current conventional methods.
Xu, Purdue University.
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