All-water-based electron-beam lithography using silk as a resist

Abstract

Traditional nanofabrication techniques often require complex lithographic steps and the use of toxic chemicals. To move from the laboratory scale to large scales, nanofabrication should be carried out using alternative procedures that are simple, inexpensive and use non-toxic solvents. Recent efforts have focused on nanoimprinting and the use of organic resists (such as quantum dot-polymer hybrids, DNA and poly(ethylene glycol)), which still require, for the most part, noxious chemicals for processing. Significant advances have been achieved using 'green' resists that can be developed with water, but so far these approaches have suffered from low electron sensitivity, line edge roughness and scalability constraints. Here, we present the use of silk as a natural and biofunctional resist for electron-beam lithography. The process is entirely water-based, starting with the silk aqueous solution and ending with simple development of the exposed silk film in water. Because of its polymorphic crystalline structure, silk can be used either as a positive or negative resist through interactions with an electron beam. Moreover, silk can be easily modified, thereby enabling a variety of 'functional resists', including biologically active versions. As a proof of principle of the viability of all-water-based silk electron-beam lithography (EBL), we fabricate nanoscale photonic lattices using both neat silk and silk doped with quantum dots, green fluorescent proteins (GFPs) or horseradish peroxidase (HRP).

Tipping solutions: emerging 3D nano-fabrication/ -imaging technologies

“…The pattern commonly serves as a mask for lift-off/etching procedures or is used as a template for nanoimprinting. Various kinds of resists have been developed to meet specific applications [46,47] with the recent addition of silk [45,48] as a bio-compatibile and bio-degradable material.…”

Silk patterns made by direct femtosecond laser writing

“…7 It was demonstrated that a water-based silk fibroin solution can be used as a positive tone resist in electron beam lithography (EBL). 8,9 Furthermore, optical properties of silk fibroin can be modified by high MeV energy electron exposure and crystallization. 8,10 A spin-coated film of silk fibroin was first crystallized by methanol-ethanol treatment and then electron beam exposure caused unzipping of the b-sheet network, typical for the secondary structure of crystallized silk, rendering the exposed regions water-soluble.…”

Development of New Smart Materials and Spinning Systems Inspired by Natural Silks and Their Applications

“…The as-spun silk fibers are composed a hierarchical system of β-sheet crystal domains and α-helix/turn non-crystalline domains, which convey to the silk fibers non-linear material properties and outstanding mechanical properties. Although several reviews of synthetic silk protein morphologies have been published (Altman et al, 2003;Vepari and Kaplan, 2007;Kim et al, 2010Kim et al, , 2014Rockwood et al, 2011;Tao et al, 2012;Kundu et al, 2014), new materials inspired by silk protein and various spinning methods mimicking spider and silkworm have not been reviewed, despite their importance and extensive applications. In addition, how silk fibers assemble into 3D hierarchical macroscale structures (cocoons and webs) to meet various needs by silkworms and spiders are not summarized until now.…”