Multilayer phase-only diffraction gratings: Fabrication and application to extreme ultraviolet optics
SEM micrograph of multilayer deposited with dc magnetron sputtering on top of binary gratings. Sputter plasma having nonuniform radial angular distribution propagates topography along the average direction of incident flux denoted here by the angle. Also notice the smoothing effect of multilayer as features propagate throughout the multilayer volume.
SEM micrographs of an etched multilayer grating with period. Etch depth is set to approximately , yielding a phase shift on reflection (round trip). Embedded Cr etch-stop layer fabricated to exactly one multilayer period ensures suppression of the zeroth order and highest possible performance.
[(a) and (b)] Idealized model for the efficiency of square wave gratings and the relationship between the zeroth and the first orders.
Theoretical model for the performance of a multilayer with embedded Cr buried 16 layers below the surface.
(a) Schematic of trilayer cross section used to calibrate Cr deposition parameters. (b) Schematic of multilayer system, with embedded Cr etch-stop layer and Cr hard mask, ready for lithography and pattern transfer.
Simplified three-step lithography process for pattern transfer. (a) KRS-XE e-beam resist used to define features with e-beam lithography. (b) Resist development, plus chemistry to etch through Cr hard mask, and chemistry to etch through multilayer. (c) Removal of residual hydrocarbons using descum, and Cr wet etchant to remove Cr hard mask.
Results. (a) Measured grating efficiency, better than 88% with respect to the ideal. (b) Measured multilayer reflectivity, before and after processing, showing the result of increased surface roughness.
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