<title>Nanoscale fabrication by interferometric lithography</title>

Abstract

ABSTRACT hiterferometric lithography (IL) techniques provide a demonstrated, low-cost, large area nanoscale patterning capability withfeature resolution to 50 nm. Combining IL with anisotropic etching (both by reactive-ion etching and by KOH wet etching)and with 3-D oxidation techniques provides a suite of techniques that accesses a broad range of Si nanostructures (as small as10 mu) over large areas and with good uniformity. Optical characterization includes measurements of reflectivity for a widerange of 1D grating profiles, and Ra.man scattering characterization of Si nanostructures. Three regimes are found for theRaman scattering: bulk (to linewidths of 200 nm), resonant enhanced (- 50 nm linewidths) and asymmetry and splitting(lmewidths < 20 nm).Keywords: interferometric lithography, Si nanoscale structures, Si quantum wires, Si photolummescence L INTERFEROMETRIC LITHOGRAPHY We have investigated interferometric lithography (IL) as a cost-effective alternative towards nanoscale patterning. IL tech-mques have been demonstrated to produce uniform nm-scale structures over large areas at low cost. The period for IL isAi2sinO, where 20 is the intersection angle between the two exposing laser beams. There is no constraint on the feature di-mension, only the period. Nonlinear response of photoresist and other processing techniques allow linewidths as small as 10urn, limited only by the uniformity and process controL For readily available laser sources from 0.488- to 0.l93-pnn at 0=75°,this translates into periods 0.25-0.1 jnn with linewidths of< 0.12-0.05 tm.Single exposure interferometric grating fabrication has a long history'2 Figure 1 shows a typical experimental configurationin which an expanded and collimated laser beam is incident on a Fresnel mirror (FM) arrangement mounted on a rotationstage3 for period variation. The mirror and sample stages have tilt & tip adjustments, the sample stage is equipped with in-plane rotation adjustment as well. The interference between two beams one directly incident on the wafer on one side of theFM and the second reflected from the mirror mounted on the other side of the FM, results in a periodic pattern )J2sinO, where0 can be precisely varied by the computer-controlled rotation stage. Grating patterns are first fonned in photoresist (PR) fol-lowed by a pattern transfer to the underlying substrate. Figure 2 shows examples of 360- and 250-nm period photoresistgratings fabricated on a bottom-ARC-coated Si wafer. These structures were fabricated with laser exposure at ?=355 am(third harmonic of a YAG laser) in Shipley 505-A positive photoresist. The grating linewidths were 180- and 130-nm re-spectively at a thickness of 500 urn4. At smaller periods, photoresist collapse5 and horizontal thinning during the develop-ment process limit the achievable aspect ratios and force the use of thinner ( 100-200 urn) PR films.