3
Nanofabrication of Metal Structures

The effect of localized surface plasmon resonance (LSPR) requires metal nanostructures. The following chapter describes such structures and discusses the possibilities (and practicability) for their generation.

3.1 Introduction

There are a variety of definitions of nanostructures. They usually agree by taking the size as key value, but vary in the values and the dimension. A very narrow definition includes only structures with dimensions below 100 nm in all three dimensions; others extend at least one dimension up to 1 µm. On the other end there are definitions that include thin films with thicknesses below 100 nm but are otherwise without lateral limitations.

The localized surface plasmon resonance (LSPR) as basic effect of molecular plasmonics is best observed in structures with usually at least two dimensions below 100 nm, and only occasionally (e.g., for nanorod structures) extending in one dimension beyond this value.

What is the difference between nanoparticles and nanostructures? Again, there is no final conclusion about this. Often only the chemically (in solution) synthesized gold structures are termed nanoparticles, and the term “nanostructure” is used for all (both synthesized and lithographically prepared) nanoscale structures. The book follows this nomenclature. Other terms, which are less common, are nanospheres, apparently limited to the spherical nanoparticles, and nanobeads, probably derived from microbeads as the micrometer-sized (usually polymeric) particles are sometimes called. Another term used in the field is nanocluster. It usually describes the aggregation of just a few atoms, with overall diameter well below 5 nm. The book applies nanocluster for up to 150 atoms in one complex, and nanoparticle for larger structures.

The preparation of nanostructures can be divided into two general approaches according to the starting point for the fabrication: Either the process starts with larger pieces of the material, and removes parts of it until the final structure is realized. This approach is also denoted as top-down, coming from the large structure and realizing a smaller one. On the other hand, small, even very small subunits (such as molecules or atoms) can be assembled into larger structures. This resembles or includes chemical synthesis, and is denoted as bottom-up because it starts at the very bottom of the material size scale and builds up larger structures from there (Figure 3.1).

Figure 3.1 The two key approaches for the realization of nanostructures: Top-down (left) starts from bulk material and removes parts until the desired structures is realized. The rather chemical (or: molecular) bottom-up approach (right) connects small units such as ions or atoms often by some kind of self-assembly in order to realize the desired structures.

3.2 Nanofabrication: Top-Down

The traditional mechanical (and later precision-mechanical) approaches do not usually reach down to the nanometer range – mechanical (ball) milling is one exception. This technique can prepare metal particles with diameters well below 50 nm; however, the low homogeneity (size distribution) and contamination issues are limiting [1]. The most important technology for top-down nanostructure fabrication is lithography, as described in the following chapter. Other, usually more specialized technologies are discussed afterwards.

3.2.1 Lithography

Originally, lithography describes a printing approach where a stone plate with a wax-like pattern is utilized to transfer ink in that specific pattern onto paper. For microelectronic applications, this concept of pattern transfer was developed into photolithography, where light is applied in a pattern (created by a photo mask) onto a photosensitive polymer (resist) film. After removal of the irradiated photo resist areas, the resulting resist pattern is then transferred in underlying functional layer by etching. The different lithographic approaches described below differ in the way the nanoscale pattern (as kind of nano “mask”) is formed. Typically, this is done by structuring a resist accordingly (as in photo and electron lithography). However, the mask can also be represented by a self-assembly monolayer (SAM) of molecules transferred by a polymeric stamp (soft lithography) or a nanoscale structure like nanoparticle (nanosphere lithography) or (bio) molecule. Finally, a nanoscale pattern can also be realized by an accordingly nanopatterned stamp.

An important point to consider is the serial or parallel character of the approach, because it determines the throughput and thereby costs and applicability. In a serial approach, a tool (such as e-beam or the scanning probe tip) writes one point after another, leading to writing times in the range of minutes and even hours for square micrometers, which increase significantly in the case of waver scale. Parallel approaches transfer a whole patterned area either by light (photo lithography) or by a nanostructured stamp (soft lithography) in an instant. For application fields where disposable substrates are preferred such as medical diagnostics, only mass production will be able to fulfill the requirements for a sufficient number of dedicated substrates at a competitive price (Figure 3.2).

Figure 3.2 Stages of lithography: a photo resist layer (on top of the functional layer) is irradiated with patterned light (e.g., using a mask) and changes its behavior (solubility) upon light exposure. This is the effect which leads to selective removal (development) of the resist leaving a patterned surface (nano-mask), that can be used to structure the functional layer below (e.g., by dry or wet etching) in a pattern transfer step.

3.2.1.1 Thin Film Technology and Adhesion Layer

In order to fabricate metal micro- and nanostructures, usually thin film technology is applied, which includes the deposition of a thin layer (tens- to hundreds of nanometer thickness) of the functional material on a substrate. Sputtering and evaporation are typical techniques for this step. For plasmonic applications, mostly gold layers are utilized. In order to achieve optimal adhesion of this metal layer on typical substrate materials such as silicon (with its native oxide layer) or glass (both exhibiting –OH groups), additional adhesion layers are included (typically 2–3 nm adhesion layer and 50–100 nm gold) [2]. They are required because noble metals bind to other metals by metal bonds, but show a low affinity and, therefore, a low adhesion to oxide surfaces such as glass or silicon oxide. Transition metals, such as chromium or titanium, which are easily passivated, form stable bonds with oxygen, and exist in air with an oxide surface layer. However, they remain metallic when they are deposited in high vacuum, so that in a subsequent step, noble metals such as gold can be deposited directly on top of this adhesion layer, which forms stable connections both with the oxidic substrate surface below and also with the noble metal above.

However, this additional adhesion layer can have an influence on plasmonic properties. For example, in the case of plasmonic fluorescence enhancement on nanoholes in a gold layer with underlying adhesion layer, the adhesion layer influences significantly the plasmonic properties, which is explained by damping of coupled energy [3].

3.2.1.2 Optical Lithography

Today's production of integrated circuits (ICs) and memory chips is based on photolithography, where light is used to structure a photo resist before the transfer of this pattern into a functional layer below. In the case of negative resist, the irradiated areas become soluble in a subsequent development step, and will be removed, so that only the nonirradiated resist areas remain (as depicted in the scheme as nano “mask”). This pattern is then transferred into the functional layer. Usually an etching step is thereby utilized, such as with acids (wet etching) or with energetic particles (e.g., sputtering) as dry etching. One of the key advantages of the standard optical lithography compared to other nanofabrication methods is the parallel approach of fabrication: A whole area is irradiated and, therefore, patterned at once, leading to a high throughput not reached by serial (point-for-point) processes.

Owing to the refraction limit, the resolution of optical methods (microscopy as well as lithography) is limited by the wavelength to about 150–200 nm. However, as demonstrated by the resolution of today's ICs smallest structures, there are tricks to reach beyond this limit. In order to realize nanometer structure, the interference pattern that results from two interacting light beams is one approach. The feature size in the created pattern can be smaller than the used wavelength. However, this technique is rather limited to regular and periodic arrangements. Another recent microscopic development is also already tested for nanofabrication: stimulated emission depletion (STED) microscopy [4]. It relies on the fluorescence effect, where light excites certain molecules, which in turn spontaneously emit light in another wavelength in order to return to their ground state. This process can be suppressed by irradiation with the emitted light wavelength. If this suppressing light is ring shaped (with the inner diameter smaller than the wavelength) and overlaid with a fluorescence-exciting beam, then the fluorescence effect (which can be also used for fabrication) is...