In order to create different structures, the semiconductor industry uses different bottom-up and top-down methods to deposit and remove materials at the nanoscale. Thermal ALE is one of the latter. By using this technique, researchers at MIT have built the smallest 3D transistor yet of only 2.5 nm across [i].

How does it work? [ii]

Thermal ALE was first reported in 2015 and can be viewed as the reverse of another important process known as atomic layer deposition (ALD). ALD was developed in 1977 and is used to add thin layers of compounds such as ZnS to a surface through controlled self-limiting steps [iii]. In ALE, the opposite happens where different materials can be etched away atomic layer by layer using mainly two processes. 

For materials such as Al2O3, HfO2, and ZrO2, a fluorination process is firstly used. This first process depends on the material, and in other cases needs a conversion or oxidation step. It can be viewed as a way to prepare the surface for the second process which uses ligand exchange reactions to remove the material.

Figure 1: Schematic illustration of an ALE cycle. Step (I) shows the fluorination process, while step (III) shows the ligand exchange interactions to remove a layer from the surface. The purge steps (II&IV) are used to remove excess reagents or products.

On the one hand, the widely used etching technique, reactive ion etching (RIE) which uses plasma, is a lot quicker, requires lower temperatures, and has the desired properties of being selective and directional. On the other hand, the plasma does some damage to the surface and is not as reliable as thermal ALE. 

A sort of in-between method developed in 1988 [iv], plasma ALE, has some of the same advantages as thermal ALE. Plasma ALE is a directional etch (only removing material in one direction), whereas thermal ALE is isotropic (etching in all directions at a time). Both etching types are needed and for certain 3d structures, the latter becomes especially important.

Outlook

As semiconductor devices become smaller, both thermal ALE and plasma ALE are becoming increasingly attractive methods to reach the demands of reliability and precision. Currently, they are expensive options, but this could be less of a concern with the increasing demand for more powerful electronic devices. Additionally, as a big research area, new ALE techniques are being developed for a range of materials that supports the wider use of the methods.

If you’d like to learn more about nanotechnology, please subscribe to our newsletter and stay tuned for upcoming posts.

References

[i] MIT News, Engineers produce smallest 3-D transistor yet, Dec 2018, accessed at https://news.mit.edu/2018/smallest-3-d-transistor-1207

[ii] Chang Fang et. al, Thermal atomic layer etching: Mechanism, materials and prospects, Dec 2018, accessed at https://www.sciencedirect.com/science/article/pii/S1002007118304623

[iii] Richard W. J. et al., A brief review of atomic layer deposition: from fundamentals to applications, June 2014, accessed at https://www.sciencedirect.com/science/article/pii/S1369702114001436

[iv] Keren J. K., et al., overview of atomic layer etching in the semiconductor industry, 2015, accessed at https://avs.scitation.org/doi/10.1116/1.4913379

The post Removing Atomic Layers from Nanostructures with Thermal ALE appeared first on The Nano Future.

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.

If you’d like to learn more about nanotechnology, please subscribe to our newsletter and stay tuned for upcoming posts.

References

[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

The post Nanofabrication: The Top-down and Bottom-up Approaches appeared first on The Nano Future.