Fabrication of high aspect ratio Si nanogratings with smooth sidewalls for a deep UV-blocking particle filter
Grating schematic. Collimated particles flow through narrow, high aspect-ratio grating slits while photons with wavelengths greater than the slit width are absorbed.
Simulated transmission characteristics of the transverse electric and transverse magnetic modes of 121.6 nm Lyman- light through Si and Au nanoscale gratings with 200 nm period and slits of 40 and 50 nm width. The dark lines correspond to Si gratings and the light to Au, solid lines for TE transmission, and dotted lines for TM.
Process flow for grating fabrication. Steps are: (1) Make or buy silicon-on-insulator wafer, (2) grow 200 nm mask oxide, (3) evaporate 10 nm chromium, (4) deposit thermal polymer (MRI 8030), (5) nanoimprint grating into polymer, (6) polymer residual etch and chromium etch, (7) remove polymer, (8) dry-etch mask oxide, (9) STS etch grating, stage 1, (10) dry oxidize sample to narrow grating lines and create second-stage etch mask, (11) STS etch to buried oxide etch-stop, (12) STS etch backside through wafer, (13) chemical etch through to etch stop, and (14) dry-etch oxide.
Scalloping of DRIE etches. (a) The standard Bosch process results in large, deep scallops. (b) Our initial tests with etch chemistry reduced scalloping significantly. (c) Our current process results in less than 7 nm scalloping over a vertical etch of 15 nm.
This grating was created from a 115 nm half-pitch mold following the process in Fig. 3 through stage 9. After the oxide hard-mask was created, a 7 min STS DRIE etch (described in Section III) resulted in deep grating lines (etch rate: ). The early 35 nm mask undercut widened the initial lines to 150 nm, leaving an aspect ratio of 8.5:1.
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