3-D patterning of silicon by laser-initiated, liquid-assisted colloidal (LILAC) lithography

Abstract

We report a comprehensive study of laser-initiated, liquid-assisted colloidal (LILAC) lithography, and illustrate its utility in patterning silicon substrates. The method combines single shot laser irradiation (frequency doubled Ti-sapphire laser, 50fs pulse duration, 400nm wavelength) and medium-tuned optical near-field effects around arrays of silica colloidal particles to achieve 3-D surface patterning of silicon. A monolayer (or multilayers) of hexagonal close packed silica colloidal particles act as a mask and offer a route to liquid-tuned optical near field enhancement effects. The resulting patterns are shown to depend on the difference in refractive index of the colloidal particles (ncolloid) and the liquid (nliquid) in which they are immersed. Two different topographies are demonstrated experimentally: (a) arrays of bumps, centred beneath the original colloidal particles, when using liquids with nliquid<ncolloid, and (b) a combination of holes, created in the interstices between the colloidal particles, and bumps when using liquids with nliquid>ncolloid - and explained with the aid of complementary Mie scattering simulations. The LILAC lithography technique has potential for rapid, large area, organized 3-D patterning of silicon (and related) substrates.

Pattern formation on silicon by laser-initiated liquid-assisted colloidal lithography

“…These simulations highlight some of the changes in intensity distribution once R≤λ. The intensity patterns in the case that ncolloid>nliquid (figures 3(a) and (c)) are broadly similar to that reported previously for the case of a R=1.5 μm particle [14], but the region of maximum |Sz| moves to progressively smaller Z/R with decreasing R. Conversely, in the case that ncolloid<nliquid (figures 3(b) and (d)), Mie scattering effects progressively dominate the 'defocusing' seen for larger particles (recall figure 2(b)) and, in the R=195 nm case, the maximum |Sz| values are clearly under the sides of the particle. Nevertheless, the value of the |Sz| for the low index regime is smaller than 1 (the value of |Sz| when the field is propagating in free space, i.e.…”

“…We also note that ray tracing calculations for much smaller (R=350 nm) spheres yield results that are identical to those shown in figures 1(b) and 2(b) apart from the expected geometrical scaling factor. Our previous FDTD simulations illustrated that the near field enhancement effects are also sensitive to the difference between nliquid and ncolloid, and the substantial increases in the depth of focus that can be achieved in liquid assisted laser processing [14]. The false colour plots in figure 3 are the results of numerical Mie theory analysis using the Fullwave 9.1 software (RSoft Design).…”

“…The laser beam was controlled using WaveRunner software (Nutfield Technology) and a galvo scanner combined with a telecentric lens with focusing distance f=103 mm. The scan pattern was designed to ensure a series of equally spaced exposed areas on the substrate, each of which had been subject to a single shot of focussed (Gaussian spatial beam profile [14]) radiation from the laser operating at 10 kHz. Power levels used in the present work spanned the range 5P100 mW, which translate into incident fluences at the substrate surface in the range 0.28F5.66 J cm −2 .…”

“…We have recently reported first illustrations of the laserinitiated, liquid-assisted colloidal (LILAC) lithography technique wherein irradiating the substrate of interest through an array of colloidal particles in different liquid media (rather than air) can yield a diverse range of surface patterns [13,14]. The technique employs single pulse laser irradiation, together with self-assembled arrays of colloidal particles on a substrate (Si in the present case) immersed in a liquid.…”

“…The corresponding patterns produced in exposed and developed photoresist show feature dimensions as small as ∼100 nm [12], and topographies that are sensitive to the polarization, coherence and intensity distribution of the incident illumination. We have recently reported first illustrations of the laser-initiated, liquid-assisted colloidal (LILAC) lithography technique wherein irradiating the substrate of interest through an array of colloidal particles in different liquid media (rather than air) can yield a diverse range of surface patterns [13,14]. The technique employs single pulse laser irradiation, together with self-assembled arrays of colloidal particles on a substrate (Si in the present case) immersed in a liquid.…”