Utilizing Atomic Layer Deposition (ALD) for Surface Modification

As it is usually the part that experiences the most rigorous interactions, the surface of a material is crucial. It must be prepared with the utmost of care in highly engineered materials.

Image Credit: Shutterstock/ Yossef Zilberman

Atomic layer deposition (ALD) is a method which enables this high level of preparation and is seen over a wide array of industries at both the R&D and high-throughput manufacturing stages. ALD utilizes two or more precursor chemicals to build up a desired surface onto a substrate, these react with a surface one-by-one and in a well defined order.

Facilitated by an inert carrier gas like argon, alternating pulses of each precursor are deposited onto the substrate. This stops the chemicals being subjected to any undesirable chemical alterations before they reach the substrate. A so-called 'half reaction takes place when the chemicals reach the target surface, as it only makes up part of the material synthesis.

The pulse is timed to last just as long as the half-reaction takes to finish. This reaction is key to the ability to build the material monolayer by monolayer and it is self limiting, so it finishes when all of the available surface has been covered.

The thickness may be controlled easily via multiple angstroms to nanometers by controlling the number of self-limiting reactions that occur.1 Fine control can be achieved over the growth mechanism and so the final uniformity of the layers, by restricting the height of the layers to the distances between atoms, and working with cycle times that only take a few seconds.

Some other benefits are pinhole-free films and extreme surface conformity, scalability and repeatability. Surfaces that are prepared by ALD are characterized by their dense, ultrathin, conformal and smooth films.

ALD makes it possible to create a wide range of nanomaterials by utilizing different combinations of precursors. Originally developed for the production of thin film electroluminescent (TFEL) displays in Finland in 1971, ALD is now employed in numerous industries.

The ability to switch between various combinations of precursor chemicals makes it well-suited for R&D and prototyping work. Equipment may be designed to perform the process on an industrial scale after a process has been refined and optimized.

This can often be challenging, as specialized equipment is required for large quantity, high-throughput systems. Systems like Forge Nano’s Morpheus are now supplying a simple way for organizations to turn lab-scale ALD processes into high-throughput commercial operations.

Applications of Particle ALD

Particle Atomic Layer Deposition (PALD) is the utilization of ALD to coat individual particles. ALD can be employed to drastically alter the properties of particles down to the sub-micron scale by creating a near-perfect conformal coating, giving multiple potential near-term applications.2

The enhancement of the properties of polymer powders is one promising application area. High gas permeability of polymers hinders their utilization in numerous food, electronic and medical packaging applications.3

Employing ALD to coat polymer particles in an inorganic gas diffusion barrier – like an ultrathin alumina layer – can enhance polymer performance dramatically. ALD has shown promise in functionalizing or passivating a variety of particulate materials – including nanomaterials like carbon nanotubes – in big amounts, helping to address one of the key challenges in nanotechnology.4

Some other applications of particle ALD include altering absorbance spectra in a variety of optoelectronic and piezoelectric applications; changing the rheological properties of suspensions of particles in the nanometer to micron range; and heightening the stability of catalysts.2,5,6

Many high-profile industries have embraced and adapted ALD for their own utilization. The microelectronics industry is one of the biggest of these, where it is employed to build up high dielectric constant ceramics which from the transistor gate stack in microelectronics devices.7

The ceramic oxides which form the stack layers must be free of pinholes and highly uniform. This is so that there is no current leakage through it from the silicon underneath. ALD is also applied in fields of optoelectronics, like solar panels or photovoltaics.8

ALD enables manufacturers to control the composition of a compound of three or more elements in these applications. This permits properties like energy band levels, density, band gap, conductivity, and morphology to be tuned with a high level of precision.

The method can be employed in many areas of the photovoltaic, including passivation layers at the surface, which is similar to how it is employed in the transistor gate stack. Losses are minimized and the efficiency of the cell is enhanced by applying a custom Al2O3 layer to the front and back of the electronic panel.

Making ALD Scalable

ALD is clearly well-suited to various tasks, at each scale of process. When it developed its Prometheus and Morpheus systems, this ability for small scale R&D as well as large volume batch production was what Forge Nano was aiming for.

The former is an R&D tool, designed to bring the power of particle ALD to the masses, while the latter is designed to enable these prototypes to be created on large scales, economically.

The Prometheus system is an advanced platform which has been created to push forward the research into how sub-nano to nanoscale coatings can be applied to powders, making lab-scale ALD more achievable than ever before.9

With numerous reactor sizes available so that the batch size may be adjusted, it is able to handle powder volumes ranging from milligram to kilogram samples. At any one time, up to eight precursors can be input, with gas, liquid and solid precursor recipes attainable via the basic and low vapor pressure delivery draw systems.

The Prometheus system is designed to provide the ability to research and develop a coating process and the Morpheus system scales up this process to commercial levels.10 It keeps the same high precision nanoscale coating technology, but at industrial volume.

Morpheus may be customized to suit a desired output from kilogram to kiloton, and by taking on a modular design.

The world’s first virtual conference on PALD was on 21st of May 2020. Hosted by Forge Nano, the PALD summit included a range of live streams and other content from the world’s foremost experts in PALD technology.

References and Further Reading

  1. Pakkala, A. & Putkonen, M. Atomic Layer Deposition. in Handbook of Deposition Technologies for Films and Coatings 364–391 (Elsevier Inc., 2010). doi:10.1016/B978-0- 8155-2031-3.00008-9
  2. Weimer, A. W. Particle atomic layer deposition. Journal of Nanoparticle Research 21, (2019).
  3. Weaver, M. S. et al. Organic light-emitting devices with extended operating lifetimes on plastic substrates. Appl. Phys. Lett. 81, 2929–2931 (2002).
  4. Cavanagh, A. S., Wilson, C. A., Weimer, A. W. & George, S. M. Atomic layer deposition on gram quantities of multi-walled carbon nanotubes. Nanotechnology 20, 255602 (2009).
  5. Hakim, L. F. et al. Nanoparticle Coating for Advanced Optical, Mechanical and Rheological Properties. Adv. Funct. Mater. 17, 3175–3181 (2007).
  6. Oneill, B. J. et al. Catalyst design with atomic layer deposition. ACS Catal. 5, 1804–1825 (2015).
  7. Lee, F. et al. Atomic layer deposition: An enabling technology for microelectronic device manufacturing. in ASMC (Advanced Semiconductor Manufacturing Conference) Proceedings 359–365 (2007). doi:10.1109/ASMC.2007.375064
  8. Delft, J. A. van, Garcia-Alonso, D. & Kessels, W. M. M. Atomic layer deposition for photovoltaics: applications and prospects for solar cell manufacturing. Semicond. Sci. Technol. 27, 074002 (2012).
  9. Lab-scale Atomic Layer Deposition system. Forge Nano ALD tools for R&D. Available at: https://www.forgenano.com/products/prometheus/ . (Accessed: 23rd May 2020)
  10. Commercial-scale Atomic Layer Deposition. Nano Coating Surface Coating. Available at: https://www.forgenano.com/products/morpheus/. (Accessed: 23rd May 2020)

This information has been sourced, reviewed and adapted from materials provided by Forge Nano.

For more information on this source, please visit Forge Nano.

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