Lithography

The primary method for fabrication of nanoscale array structures is by use of a lithography process. Many industries working in the nanoscale use photolithography to produce structures in large quantities with incredibly high accuracy, and it is a technology that is quickly improving to push the lower limit of resolution of manufacture, by using shorter and shorter wavelengths of light to reduce the diffraction limit of the features, and utilising other physical phenomenon such as photo interferometry to use interference patterns to further increase resolution[160].

Photolithography works by the process of coating a substrate with a photosensitive polymer material, known as a photo-resist. Deep-Ultraviolet (DUV) radiation is used to pattern the photo-resist, breaking or forming new bonds in the polymer chains, after which a developer solution removes the more soluble, short chain material. Though photolithography does have many applications in the manufacturing of nanoscale structures the resolution is limited, and surpassed by electron[170], and ion[171] beam lithography.

Ion beam lithography utilises charged particles, such as Helium or Neon ions which are accelerated to high velocity by a high voltage bias[172,173]. A typical Helium Ion Microscope column is shown in Figure 3.7. Similarly, electron beam lithography uses electrons. These are used in place of photons, and follow a similar principle to that of photolithography. A resist material is deposited to the surface in a thin layer (typically 100-400nm)[6,12,174] and a pattern is exposed, with the ions or electrons impacting the substrate and breaking polymer bonds this is then developed to selectively remove areas of resist, before further processing, such as deposition of metals.

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Figure 3.7: Schematic of a Zeiss Orion nano-fab column, used for Helium ion lithography[175].

In Electron Beam Lithography (EBL), electrons are accelerated at high vacuum by high voltage of typically 30kV. The electrons are focused and directed through magnetic lenses. This focused electron beam is then fired at the substrate and causes a change in the resist material deposited, making it more, or less soluble to a developer solution by changing the polymer length. Patterning is achieved through a combination of magnetic lenses that deflect the beam, so that it can be directed to specific parts of the target, and a beam blanker mechanism, that blocks the electron beam from striking the surface when engaged.

This combination of beam manipulation makes it possible to pattern separated structures on the target in a 2D plane. The advantage of using Helium ions, or similar ions such as Neon over electrons, is that they can impart more of their momentum to the resist layer, and are more massive, reducing the required dose. HIL is a powerful technique, that has been utilised to extremely high-fidelity structures[33,176–178], and has demonstrated a resolution on the order of 10 nm[179], with dense array structures possible due to the reduction in secondary

59 electrons from adjacent array cells. However, HIL is not utilised in this thesis, as structures were demonstrated to be within the resolution range of EBL.

Figure 3.8: Schematic of a Zeiss Gemini column, as used in the Supra FE-SEM for electron lithography and imaging[180].

Field Emission Scanning Electron Microscopes (FE-SEMs) are often used for SEM imaging[181–183], and lithography of patterned nanostructures[20,58], as they provide high voltage electrons in a controlled stream that can maintain precise beam current[184]. The Carl Zeiss Supra FE-SEM is the primary lithography tool used for Chapter 5 in the Hybrid nanostructure, and a schematic is shown in Figure 3.8. The Field Emission source is a very sharp single crystal Zirconium tip that has a high bias voltage applied, with the tip acting as a cathode, and an anode located further down the column[184]. This voltage can vary from 1kV for imaging, to over 30kV for lithography. Electrons are ripped from the tip in a very controlled fashion, with very little stray electrons being produced. This results in a beam that is highly collimated, and capable of higher resolutions than that of the more common Tungsten filament thermal source method, particularly at low voltages. However,

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these systems have been utilised at extremely high voltages (100kV) for nanoscale array fabrication[6,185,186].

Laser alignment stages are also available in some of the EBL tools. An Elionix 7700 is a thermal source SEM tool with a laser stage, used to fabricate larger samples by stitching small sub-sections together. These stages use laser interferometry to closely monitor the position of the positionof the sample being processed. Detectors monitor the position of the stage, and correct any drift that occurs over the course of exposure. This leads to less misalignment in an exposed pattern over the course of exposure, and allows for much larger arrays to be produced with better stitching between smaller elements of the overall design. Though the resolution is not as sharp in thermal SEM systems, this tool is used in chapter 4 on the large scale 5 mm Aluminium nanostructure sample. This is done as the features and unit cell of the Aluminium structure are within the operating parameters of the Elionix system, and fabrication on this SEM allows for the creation of a very large sample.