Nanolithography using nanoscale ridge apertures
There is a continuous effort to develop techniques for nanoscale feature definition below the diffraction limit. Nanolithography has been a key technique because of its precision and cost effective. A sub-wavelength hole in an opaque screen can be used to provide a small light source with the optical resolution beyond the diffraction limit in the near field. However, a nanometer-sized hole in circular or square shapes is plagued by low transmission and poor contrast. This drawback limits the nanoscale apertures from being employed in nanolithography applications. Ridge apertures in C, H and bowtie shapes, on the other hand, have been numerically and experimentally demonstrated to show the ability of achieving both enhanced light transmission and sub-wavelength optical resolution down to nanometer domain benefiting from the existence of waveguide propagation mode confined in the gap between the ridges. In this report, the detailed field distributions in contact nanolithography are analyzed using finite difference time domain (FDTD) simulations. It was found that the high imaging contrast, which is necessary for successful lithography, is achieved close to the mask exit plane and decays quickly with the increase of the distance from the mask exit plane. Simulations are also performed for comparable regular shaped apertures and different shape bowtie apertures. Design rules are proposed to optimize the bowtie aperture for producing a sub-wavelength, high transmission field with high imaging contrast. High resolution contact nanolithography was carried on a home constructed lithography setup. It has been experimentally demonstrated that nanoscale bowtie and C apertures can be used for contact lithography to achieve nanometer scale resolution due to its intrinsic advantages of achieving enhanced optical transmission and concentrating light far beyond the diffraction limit. It also has shown the advantages of bowtie and C apertures over conventional apertures in both transmission enhancement and nanoscale light concentration. Lithographic holes as small as 40 nm × 50 nm and 50 nm × 60 nm for bowtie and C apertures, respectively, has been achieved. To study the properties of nanoscale bowtie apertures, a home-made near-field scanning optical microscope (NSOM) is developed. AFM images of standard calibration samples are used to calibrate the piezoelectric stage and topography resolution. NSOM results of bowtie apertures are also presented to study their transmission enhancement and field localization. Near-field scanning optical microscopy (NSOM) probe integrated with nanometer scale bowtie aperture for enhanced optical transmission is demonstrated. The bowtie-shape aperture allows waveguide propagating mode in the bowtie gap region, which enables simultaneous nanoscale optical resolution and enhanced optical transmission. These unique optical characteristics of bowtie aperture are demonstrated by measuring optical near fields produced by apertures in metal film. It is shown that bowtie aperture probes have one order of magnitude increase in transmission over probes with a regular shape aperture. The imaging results using bowtie aperture are in agreement with those obtained from numerical calculations. Spectroscopic measurements of transmitted field through bowtie shaped nanoscale apertures in visible wavelength region were used to further calibrate the aperture. Resonance in these apertures and its relation with the aperture geometry are investigated. The near-field spectral response is also investigated using Finite Difference Time Domain (FDTD) computation and compared with the spectroscopic measurements. The dependences of the peak wavelength and peak amplitude on the geometry of the bowtie aperture are illustrated. Design rules are proposed to optimize the bowtie aperture for producing a sub-wavelength, high transmission field.
Shipsey, Purdue University.
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