Nanofabrication by laser irradiation of polystyrene particle layers on silicon

Abstract

Regular lattices of polystyrene (PS) microspheres formed by self-assembly processes were used for single-shot fabrication of large regular arrays of nanoparticles or nanobumps or nanoholes on Si surfaces. The morphologies of created nanofeatures were characterized by an atomic force microscope (AFM) and a scanning electron microscope (SEM). The near-field enhancements under the absorptive PS microsphere were studied and calculated. The ablation and thermal processes induced both by laser irradiation of the top surface of the PS microspheres and by the near-field enhancements underneath the microspheres were investigated. When the laser fluence was at a low value of 40 mJ cm−2, periodic PS nanoparticle arrays were produced. With a laser fluence in the range of 100–170 mJ cm−2, regular conical and circular-crater-shaped Si nanobump arrays were fabricated. When the laser fluence was high, up to 200 mJ cm−2, hole arrays were formed in the Si substrate. The mechanisms of this nanofabrication are discussed. Experimental results are explained and are consistent with those of theoretical calculations.

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

“…Bump formation in that case was ascribed to localized melting and, since the density of liquid silicon is higher than that of the crystalline solid, a temporary depression of the surface. The melt zone cools from the outside inwards, the re-expansion that accompanies solidification squeezes the remaining liquid material in the core of the melt zone, and this compression is relieved by vertical expansion -thereby yielding a localized bump [17,18]. Such a mechanism, though appealing, is seemingly unable to explain the obvious hole formation in the case of fs LILAC lithography when n liquid > n colloid .…”

Sub-micron parallel laser direct-write

“…For spheres with a radius similar to that of the incident radiation wavelength, r $ l, near-field optical effects become an important factor in the scattering of the spheres [25][26][27][28][29]. However, as the sphere's diameter exceeds several l, near-field effects are greatly reduced and the sphere's focal behavior follows geometrical optics [30].…”

The Near Field Properties of Colloidal Polystyrene Microspheres on Silicon

“…The ablation threshold for PS microspheres is about 33 mJ/cm 2 . 15 When the PS colloidal monolayer on the Si surface was irradiated with fluences of 40 and 50 mJ/cm, 2 slightly higher than the ablation threshold of PS microspheres, and ablation occurred on the top surface of the PS spheres, while for this low level laser fluence, the energy density after PS absorption is too low to cause any change on the Si surface. Therefore, upon irradiation with low laser fluence, the previous PS spheres on the Si substrate were partially ablated, and the particles left on the substrate shown in Figures 1(a and b) irradiation with a laser fluence of 50 mJ/cm 2 shown in Figure 1(b).…”

“…9 10 We have reported a method to use regular two-dimensional (2D) periodic lattices of micro-or nano-spheres for a single-step parallel surface patterning to generate hexagonal arrays of 2D holes, nanobumps and nanoparticles. [11][12][13][14][15] This nanofabrication method is based on the optical near-field effects under transparent or half transparent small sized spheres. The method is a high-speed parallel processing technique which permits single-step production of millions of holes/cones/nanoparticles with long-range order on the surface using a single or a few laser shots.…”

“…of previous theoretical investigations based on Mie theory have not well explored the field characteristics inside the particle or the field interactions and coupling between the neighbouring particles. [10][11][12][13][14][15][16][17] Until recently, several theoretical attempts have been made to analyze the near field optics of a 2D periodic system on a substrate. [18][19][20][21][22] For example, the optical transmission through metal or silicon coated monolayers of microspheres and the field distribution in these monolayers were calculated by FDTD modeling.…”