Nanofabrication: The Top-down and Bottom-up Approaches

When fabricating nanostructures, a major task is to find reliable, and inexpensive methods that can be used on an industrial scale. These methods fall into two categories: top-down and bottom-up. The improvement of products such as smartphones relies on the semiconductor industry to improve on their methods or develop new ones.

Content

–          Top-down approaches

–          Bottom-up approaches

–          Outlook

Top-down Approaches[i]

A good analogy to top-down approaches is a sculptor carving out a statue from a template and thus removing material. An important top-down method in the semiconductor industry is photolithography. Here, short wavelength light (or electrons in e-beam lithography) is used to form the desired pattern in a photoresist to afterward use etching to form a nanostructure by removing material underneath. Different etching methods include chemical, plasma, or reactive ion etching.

Other top-down methods used are chemical- or electropolishing to smoothen a surface, or nano-imprint techniques (using a miniature stamp pressed down into a material) to form the wanted nanostructure.

A disadvantage of top-down approaches is that they are often done layer by layer and are thus 2D techniques which can be a limitation for creating certain 3D structures.

Bottom-up approaches

A bottom-up approach can be described by assembling a larger object from smaller pieces. An analogy here could be making a car. If the car represents the nanostructure, the individual pieces such as screws and wires can be thought of as molecules and atoms.

Nature does this very well and most processes in our bodies work by self-assembly and self-organization. Chemical bonds that are favorable guide the formation of complex structures such as proteins.

Inspired by nature, a big research area is the self-assembly of nanostructures with desired properties. An example is the self-assembly of monolayers of molecules on certain metals such as cysteine on gold surfaces which result in highly ordered structures. In some cases, this gives a useful coating to the material. 

In the industry, self-assembled monolayers are used to make quantum dots stable while preserving their optical properties used in QLED displays. [ii] In addition, quantum dots can themselves be synthesized by the bottom-up method known as colloidal synthesis.

Outlook

New methods are being developed and commercialized in both categories. Start-up companies such as Atlant 3D Nanosystems [iii] are part of creating more choices when it comes to making products with nanostructures. The start-up enables atomic layer 3D printing with certain materials making prototyping faster and cheaper. This method is considered a bottom-up approach.

Today some of the smallest nanostructures (7 nm) on mobile chips are made using extreme ultraviolet (EUV) lithography. This top-down method, used by Samsung and other companies, is thought to enable even smaller nanostructures soon. [iv]

There will likely be a continuous need for different methods as each method offers its own benefits and disadvantages, matching the needs of different products. For complex structures, a combination of methods is likely to provide the best results.

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References

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[i] Britannica, Nanofabrication, accessed 2020-11-30 at https://www.britannica.com/technology/nanotechnology/Nanofabrication

[ii] Department of Chemistry – Technical University of Denmark, Chemistry at the Nanoscale, 2020

[iii] Atlant 3D Nanosystems, accessed at https://www.atlant3d.com/

[iv] Samsung, Samsung Electronics Begins Mass Production at New EUV Manufacturing Line, accessed at https://news.samsung.com/global/samsung-electronics-begins-mass-production-at-new-euv-manufacturing-line