Multilevel IL process starts by (a) patterning a reference grating at the periphery of the substrate with period . The substrate is then spincoated with ARC and resist. (b) The first grating level is exposed after aligning to the reference grating. (c) The grating pattern is transfered into the nitride layer and the substrate is cleaned. (d) The substrate is spincoated with ARC and resist. (e) The second grating level is exposed at a phase offset to the first exposure. (f) After transferring the pattern, the resulting pattern has period .
Image-reversal process starts with (a) a positive resist grating over ARC with linewidth . (b) The grating pattern is transfered into ARC and trimmed with high pressure oxygen PE. (c) A silicon-containing polymer is spin coated. (d) The ARC is then exposed with RIE etch back, and (e) the pattern is transfered into the substrate. The final linewidth has .
Linewidth as a function of plasma etch time. The line is a linear fit to the experimental data.
Top-view SEM images of a grating line after (a) lithography, and (b) PE trim step. (c) Micrograph of the image-reversed trench etched into the substrate.
Top-view SEM image of two overlaid period grating levels. The dark lines (A) and bright lines (B) are the first grating level etched into nitride and the second grating level in resist, respectively.
Map of overlay error (nanometer) between two period grating levels.
Top-view SEM images of period silicon nitride grating fabricated by overlaying two period grating levels.
Fabricated sample immersed in water and illuminated with a beam of white light to allow diffraction of blue wavelength from the period reference grating. No diffraction can be observed from the spatial-frequency doubled grating.
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