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Coherent diffraction lithography: Periodic patterns via mask-based interference lithography
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Image of FIG. 1.
FIG. 1.

Depiction of the pattern of holes and posts in membranes to be stacked to form a 3D photonic crystal. For Si membranes, and operation at a frequency corresponding to a free-space wavelength of 1400 nm, each layer has a periodicity in both and of 600 nm and a layer thickness of 332 nm. In a given layer (a), the rods are located between two holes, and in a stack (b) the layers are shifted so that each post is located between the two holes in the layer above that do not already share an upper-layer rod between them.

Image of FIG. 2.
FIG. 2.

(a) Photograph of the experimental configuration for CDL; (b) schematic corresponding the photograph in (a). The diode laser is a temperature and current stabilized 405 nm, 60 mW GaN laser from Power Technologies Inc. The spatial filter consists of a 0.19 NA lens and a 10 μm pinhole. After the spatial filter, a 5 cm diameter biconvex lens collimates the light incident on the CDL mask. Interferometric-spatial-phase imaging (ISPI) is used to align the CDL mask relative to the substrate. The mask holder has tip/tilt control while the substrate stage provided the , and θ control.

Image of FIG. 3.
FIG. 3.

Depiction of the Talbot effect. Downstream of the phase mask, interference of the zero, +1 and −1 diffracted beams produce intensity distributions that reproduce the grating periodicity at discrete distances, referred to as planes of reimaging. The patterns associated with the even-numbered planes being laterally phase shifted by relative to the patterns of the odd-numbered planes.

Image of FIG. 4.
FIG. 4.

Micrographs of experimental results from CDL exposures. In (a) and (b) the substrate was tilted at an angle relative to the mask plane and in (c) the substrate was at a uniform 2 μm gap from the mask. (a) Exposure in resist with ∼5 mrad between the plane of a CDL grating mask and the substrate. The grating periodicity is reproduced in distinct regions as the gap is increased, with a phase shift of between adjacent such regions. The dark bands, separated by 772 nm, correspond to doubling of the mask’s spatial frequency (see Fig. 3 ). The grating is reproduced with maximum image contrast midway between the dark bands. Also shown, along the bottom of the micrograph, is a region of the mask that had no grating. This, in effect, is an interferogram of the wedge between the mask and substrate. (b) Micrograph at one of the reimaging planes for an angled exposure using a mask consisting of a 2D grid pattern. (c) Micrograph of a grating printed in resist at a uniform gap of 2 μm.


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Scitation: Coherent diffraction lithography: Periodic patterns via mask-based interference lithography