Low-density polyethylene (LDPE) is ubiquitous at the macroscale but uncommon in nanofabrication, which generally requires featureless nanoscale films. However, as LDPE solidifies in a hot spin-coating process, its semi-crystallinity causes the self-assembly of spherulites. These features begin at nucleation sites and radiate out, creating microscale domains. A test of hot spin-coating of LDPE shows such spherulites, as well as film thickness variation (Figure 1). These initial results motivate a screening experiment to study six factors in the hot spin-coating process: LDPE melt flow rate; solvent type; LDPE concentration; spin temperature; spin speed; and annealing time.

Our next goal is to pattern nanoscale films of LDPE by electron-beam lithography, enabling the fabrication of novel structures such as nanoplastic arrays and nanophotonic cavities. We deposit hydrogen silsesquioxane (HSQ) resist on a test film of LDPE, pattern the HSQ resist by electron-beam lithography, and etch the LDPE with an oxygen plasma to form an array of LDPE nanopillars. The results are encouraging, demonstrating an array of LDPE nanopillars, as well as some spherulite boundary defects to mitigate (Figure 1). Our study promises new capability for nanofabrication of an important plastic material, and introduces a methodology to deposit and pattern other thermoplastics.

<i>Figure 1.</i> Ultrathin films and nanopillar arrays of LDPE. (<b>a</b>-<b>c</b>) Optical micrographs showing an LDPE film. Thickness variation is apparent in (a) and (b) as structural colors. Yellow regions are ridges. Blue regions are valleys. Spherulite microstructures are apparent as (b) domains with irregular boundaries containing (c) a nucleation center and radial striations. (<b>d</b>) Interferometric optical micrograph showing thickness variation and spherulites. (<b>e-f</b>) Atomic force micrographs showing arrays of LDPE nanopillars with nominal diameters of (e) 500 nm and (f) 100 nm, with a nominal pitch of 5 µm. The HSQ resist remains on top if the pillars.

Figure 1. Ultrathin films and nanopillar arrays of LDPE. (a-c) Optical micrographs showing an LDPE film. Thickness variation is apparent in (a) and (b) as structural colors. Yellow regions are ridges. Blue regions are valleys. Spherulite microstructures are apparent as (b) domains with irregular boundaries containing (c) a nucleation center and radial striations. (d) Interferometric optical micrograph showing thickness variation and spherulites. (e-f) Atomic force micrographs showing arrays of LDPE nanopillars with nominal diameters of (e) 500 nm and (f) 100 nm, with a nominal pitch of 5 µm. The HSQ resist remains on top if the pillars.

Kalman Migler

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