Thin film processing using S-layer proteins: Biotemplated assembly of colloidal gold etch masks for fabrication of silicon nanopillar arrays

Abstract

We explored the bionanofabrication of silicon nanopillar structures using ordered gold nanoparticle arrays generated from microbial surface layer (S-layer) protein templates. The S-layer template used for these thin film processing experiments was isolated from the Gram-positive bacterium Deinococcus radiodurans. In this preliminary work, S-layers preimmobilized onto chemically modified silicon substrates were initially used to template the fabrication of a nanolithographic hard mask pattern comprised of a hexagonally ordered array of 5-nm gold nanoparticles (lattice constant = 18 nm). Significantly, the use of the biotemplated gold nanoparticle mask patterns in an inductively coupled plasma (ICP) etching process successfully yielded silicon nanopillar structures. However, it was found that the resultant nanopillars (8–13 nm wide at the tip, 15–20 nm wide at half-height, 20–30 nm wide at the base, and 60–90 nm tall) appeared to lack any significant degree of translational ordering. The results suggest that further studies are needed in order to elucidate the optimal plasma processing parameters that will lead to the generation of long-range ordered arrays of silicon-based nanostructures using S-layer protein templates.

Introduction

Silicon nanopillars (SNPs) are a class of low-dimensional structures which have recently sparked considerable interest in a wide range of fields. Nanometer-scale silicon structures present novel electrical and optical properties, e.g., room temperature visible photoluminescence [1], that are different from those of bulk silicon materials. In particular, Si nanopillar arrays with sub-100 nm periodicities are expected to find many applications in, for example, solid state photovoltaic devices/electroluminescent displays [2], field emission devices [3] and 2-D photonic crystals/waveguide elements [4]. In the semiconductor electronics industry, SNP arrays may offer an attractive route to the creation of vertically oriented field-effect transistors [5], which permit decoupling of device density from the channel length.

Within the past several years, various technologies have been proposed for the nanofabrication of devices incorporating nanopillar structures. Regarding the patterning of ordered Si nanopillar arrays, the highly precise fabrication of ultrafine target features (<50 nm) with sub-100 nm periodicities remains a significant processing challenge. For example, Wellner et al. [6] recently reported the parallel fabrication of ordered arrays of SNPs (with pillar diameter ≈ 20–40 nm) using a patterned reactive ion etch (RIE) mask created by metal deposition through a self-assembled monolayer (SAM) of polymer particles. However, this colloidal lithography method required the use of relatively large particles (0.5 μm) to form the polymer mask, resulting in a relatively low density of pillars on the surface (with interpillar spacings >200 nm). Others [7] have successfully employed electron-beam lithography (EBL) (in combination with reactive ion etching) and focused ion beam lithography (FIBL) for the fabrication of submicrometer semiconductor nanopillar arrays. Unfortunately, EBL and FIBL methods have major disadvantages in being both relatively slow and extremely costly.

One promising alternative approach to the advanced fabrication of ordered nanostructures is the application of prokaryotic surface layer (S-layer) proteins [8], [9], [10] as biomacromolecular templates. S-layers are naturally occurring 2-D protein crystals which have a highly periodic morphological structure containing a well-defined arrangement of functional groups. These properties make S-layers ideal support matrices for the synthesis (‘bionanofabrication’) of metallic and semiconductor nanocrystal superlattices using a variety of methods [11], [12], [13], [14], [15], [16], [17], [18]. In this new work, we explore the fabrication of vertically aligned silicon nanopillars using ordered gold nanoparticle (NP) arrays generated from an S-layer template isolated from the Gram-positive radiotolerant bacterium Deinococcus radiodurans. The D. radiodurans S-layer – also known as the ‘hexagonally packed intermediate’ (HPI) – has a p6 rotational symmetry with a reported spacing of 18 nm between each protein core region [19]. The core region itself consists of six identical protein monomers (each with molecular weight = 98 kDa) enclosing a single central pore (Fig. 1A), and is in turn surrounded by six vertex regions of identical size [20]. In Fig. 1A, the black dotted lines indicate a regular hexagonal lattice model overlaid on top of a schematic illustration of the HPI layer. The lattice model is aligned in the figure illustration such that: (1) the corners of the hexagons correspond to the vertex regions in the HPI layer; (2) the geometric center of each hexagon corresponds to the location of the central pore of the S-layer. The blue double-headed arrows in Fig. 1A indicate the following dimensional parameters for the native HPI S-layer structure [17]: (i) diameter of the vertex region (7.1 ± 0.5 nm); (ii) spacing between adjacent core regions (“HPI lattice constant”) (18.5 ± 0.7 nm).

The set of procedures we followed to produce arrays of silicon nanostructures using the HPI S-layer protein lattice as a nanoarchitectural biotemplate is schematically illustrated in Fig. 2. In this bionanofabrication scheme, bionanomask patterns were first generated by exposing the HPI S-layer protein fragments (preimmobilized on chemically modified silicon wafer substrates) to an aqueous solution of 5-nm Au NPs. The resulting hexagonally ordered Au NP arrays were then employed as an etch mask in an inductively coupled plasma (ICP) etching process using either HBr or SiCl₄ gas chemistries. Finally, the morphology and translational ordering of the resulting nanopillar arrays were evaluated using scanning electron microscopy (SEM) and image processing techniques.