Efficient subdiffraction focusing of light using zone plate and bowtie aperture

This work proposes an approach to produce high efficient subdiffraction spot especially for selective laser chemical vapor deposition (LCVD) and lithography applications. Our study indicates that Fresnel zone plate (FZP) operating in near field is different from that in the far field. The scalar methods accurately calculate the FZPs\u27 diffraction in far field, but do not capture the polarization-dependent behaviors in the near field. We present the vectorial diffraction theory to correctly predict the focus spot of the FZPs in the near field. In addition, plasmonic zone plates were studied and optimized for subdiffraction focusing. Surface plasmon polaritons (SPP)-related phenomena were explained using analytical solutions and numerical methods. Optimizations of bowtie apertures for highest transmission were discussed. Additional concentric grating grooves surrounding the bowtie aperture on the incident side boost its transmission and on the exit side improve the confinement. Nevertheless, the enhancement from the bowtie alone was insufficient for the LCVD application. A phase FZP was integrated in the mask to provide adequate power at the bowtie output. The fabrication process for a mask containing FZP and bowtie were given. FZP is fabricated by electron beam lithography and requires extensive characterization. Bowtie aperture can be made for a smaller optical spot with inverted fabrication. Finally, a demonstration of this near-field mask requires nanometer-level alignment method. Interferometric-spatial-phase-imaging (ISPI) was implemented to both the LCVD and the lithography setup. The ISPI has detection sensitivity better than 1 nm. Gap alignment between mask and substrate can be well maintained within 20 nm range for the current system noise of 15 nm