The above sections provide a detailed discussion of electron beam lithography and electron beam induced deposition, examples of top-down and bottom-up nanofabrication methods, respectively. Al- though these are the two techniques used as part of this project and therefore have received most attention in this review, they are certainly not the only nanofabrication techniques available. Other al- ternatives include Focussed Ion Beam Lithography (FIBL) and Ion Beam Induced Deposition (IBID), which are similar to the EBL and EBID techniques but use a focussed ion beam instead of electron beam. In the following paragraphs a brief overview is provided of some of the other technologies used for the fabrication of nanoscale patterns [90].
The categorisation of fabrication technologies into either top-down or bottom-up techniques is based on how the nanoscale patterns are achieved. In top-down fabrication methods, a surplus of material is deposited, often in a continuous, even layer, after which the pattern is applied to the layer (e.g. via etching using a lithographically patterned mask). Bottom-up fabrication methods, in comparison, deposit the material immediately in the desired pattern. This can be achieved by having precise patterning equipment available (e.g. EBID), to have controlled deposition of materials, or the target materials can be delivered to a general location, where they are engineered to self-assemble as a result of the nature of the material and its environment itself [123].
Optical lithography
Optical lithography, also called direct laser writing, is a top-down fabrication technique similar to EBL, but instead of using an electron beam, the energy to break and create bonds within a resist is provided by photons in the UV range, with smaller wavelengths providing higher resolution. The resists in this case are referred to as a photoresists, and some resists can be used for both optical and electron beam lithography (e.g. SU8 [115]). It is possible to operate the optical lithography system in a patterning configuration similar to EBL, steering the beam across a sample surface to irradiate patterns in the photoresist, and potential sources of focussed, collimated light are plentiful with a wide selection of lasers available spanning across wavelength and energy ranges. However, as light is limited in the achievable spot-size to half its wavelength according to Abbe’s diffraction theorem, it is possible to use masks patterned with a chromium coating to block light in some areas and flood-exposure the entire sample [124] . Masks can be contact masks directly touching the resist of the sample, they can be placed in the focal plane of a beam, or additional optics can be added to achieve higher resolution [123]. Optical lithography is a cheaper alternative to EBL, and when using the flood exposure and mask settings, it is also faster and able to pattern larger areas, especially when a similar pattern is often repeated.
Soft Lithography
Soft lithography is a technology that combines top-down patterning of a pre-formed reusable pattern with bottom-up self-assembly of deposited layers. It uses a reusable master-mould which can be used to transfer molecules or other materials to a target surface in the desired pattern, where the molecules self-assemble into a pattern [128]. A popular material to form the mould is PDMS (poly-(dimethyl siloxane)), the mould itself is often produced using electron beam, ion beam, or optical lithography [129]. An advantage of soft lithography is that outside of the mould fabrication step, the patterning can be performed without requiring cleanroom facilities [123] [127] [129].
Scanning Probe Lithography
An example of a direct deposition technique as a potential alternative to EBID, Scanning Probe Lithography (SPL) is a fabrication technology on the expensive and slow end of the spectrum, but it is able to deposit materials with atomic precision [131]. It uses existing technologies like Scanning Near-field Optical Microscopy (SNOM), Scanning Tunneling Microscopy (STM), or Atomic Force Microscopy (AFM) to transfer a material pattern onto a substrate. A first example is dip-pen tech- nology, in which a solution of the target material is adsorbed onto an AFM tip and then transferred onto the substrate by scanning the AFM tip in the desired pattern [132]. Next to depositing material it is also possible to pattern a layer by removing material by one or multiple passes of the probe over the surface [133], making scanning probe lithography also suitable for precise pattern etching, instead of deposition. SPL can also pattern materials by locally chemically altering a material at the surface, e.g. by local oxidation of a surface [123] [129] [134]. EBID is also able to perform localised etching, by using a precursor gas producing a molecule on interaction with the electron beam which interacts with the target sample surface [90].
Self-Assembly of Molecular Structures
A bottom-up fabrication technique, self-assembling molecules are designed and developed to have the structure they form depend on their molecular properties, and spontaneously assemble as they are brought in contact on the intended surface. An example is the formation of a self-assembled mono- layer of MBP0 (4-methylbiphenyl-4-thiol), where the MBP0 molecules automatically order themselves into a Self-Assembled Monolayer (SAM) on a gold-coated substrate as the sulphur in the thiol-group weakly bonds to the gold. This layer can then be strengthened by crosslinking it through electron or photo-illumination to form a membrane, or patterned using electron or photon irradiation [123] [136], [137].
DNA-Guided Self-Assembly
One of the disadvantages of self-assembled molecular structures is that the pattern-forming infor- mation needs to be contained within the forming molecules themselves. Unless the desired pattern material is a particularly complex and chemically engineered molecule, the information that can be held by the material is limited. A solution to this problem is offered in the form of using DNA (de- oxyriboNucleic Acid) as complex molecule as a self-assembling carrier for a simpler target material to bind to. DNA-guided self-assembly materials make use of the concept of DNA origami, a pro- cess of folding DNA to form arbitrary planar shapes [139], and use the DNA structures as a carrier to order and form other material structures, including plasmonic nanostructures [123] [138] [140] [141].
Figure 2.3– MBP0 molecules automatically orient themselves on a gold surface by loosely binding the sulphur of the thiol-group to the gold, forming a Self-Assembled Monolayer of ordered molecules on the sample substrate.
Nanoimprint lithography uses a master cast of the desired pattern to mechanically emboss that pat- tern into the desired material. The use of a master cast allows large areas to be patterned quickly, making nanoimprinting a high-throughput, low-cost nanofabrication option [125] [126].
Block Copolymer Nanopatterning
Block copolymer nanopatterning is another technology combining top-down and bottom-up fabrica- tion techniques. Block copolymers can automatically form thin films with small-scale patterns with a periodicity as low as 10 nm, when coated over a substrate surface [130]. It is possible to pre-pattern the target substrate (e.g. using EBL, a top-down fabrication method) to contain markers which can guide and manipulate the block copolymers to self-assemble into complex nanoscale patterns when forming a thin film [123] [130].
Edge Lithography
Edge lithography uses edges to obtain nanoscale structures, either by using the edges as a location ot deposit nanostructures, or to cleave a material and use the properties of the created edge [129] [135].