(Color online) (a) Schematic of the tilted exposure process flow: (1) and (2) sample exposure while the stage is tilted, (3) photoresist development to produce nanoholes, and (4) lift-off of deposited metal on the developed photoresist to generate metallic couple disk. (b) FDTD simulation of electric field intensity profile of UV source at perpendicular exposure. (c) The SEM image of developed thick photoresist 5214 at perpendicular exposure. (d) FDTD simulation of electric field intensity profile 15° tilted exposure, which is normalized to the maximum of electric field intensity of perpendicular exposure. The focal point is displaced by changing the angle of exposure. (e) The SEM image of developed thick photoresist 5214 at 15° tilted exposure.
(a) SEM image of cross section of single hole developed in a photoresist while is exposed in tilted angle. Partially positive profile of the thin photoresist, indicated by dashed lines in the figure, allows for direct lift-off. (b)–(f) SEM images of different configuration of holes developed in the exposed photoresist with 15° angle of tilting. Each of these configurations is a unit cell of a hexagonal array which the array period is determined by microsphere size.
Lift-off results of large arrays of (a) two, (b) three, and (c) five equally spaced metallic disks configurations using tilted exposure microsphere nanolithography. The inset in each figure is the unit cell of the corresponding array with a scale bar corresponding to 500 nm. Metal disks are less than 300 nm diameter. The microsphere used in fabrication of (a) has 1 μm diameter while for samples (b) and (c) is 4 μm. The gap size of coupled disk in inset of (a) is less than 50 nm, which is mostly achievable by e-beam lithography or FIB milling.
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