Helium ion microscopy (HIM) is being used both to create and to inspect nanostructures for photonic and plasmonic devices. As part of a program exploring photoelectrons as sources for multiple beam electron beam lithography, fractal apertures are being developed to generate a sub-20nm light source using surface plasmon resonance. The fabrication was carried out by direct write helium ion milling into a 90 nm thick aluminum film deposited by evaporation onto a fused silica substrate. A machining precision of 5 nm was obtained, allowing the creation of sharp, vertical edges and corners with the required radius of curvature. These results are compared to previous attempts to achieve these geometries with electron beam lithography and Ga FIB milling.
In order to complete the photoemitter, the surface of the Al film was subsequently coated with a CsBr thin-film (10 nm) photocathode. The 'C' shape of the aperture concentrates the light energy directly over the tongue, where the metal film is re-entrant into the aperture. This dimension is approximately 20 nm, far below the ordinary diffraction limit. The average photocurrent measured from the apertures is approximately 260 pA. Considering the 22 nm2 source size as calculated by Finite Difference Time Domain simulations, the current density appears to surpass that of thermionic electron emitters.
Larry Scipioni is the Director of Applications Research for Carl Zeiss SMT, Inc
Sponsors: AFM Workshop, American Vacuum Society, Bruker, Chemplex, JEOL USA, Kurt J. Lesker Company, Olympus, Oxford Instruments, Thermo Scientific, Zygo, Agilent Technologies, Angstrom Scientific, Cameca, FEI Company, Kratos Analytical, Oerlikon Leybold Vacuum, Ophir Photonics, Panalytical, Witec
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