Molecular Layer Etching Can Help Semiconductors scaling Beyond Moore’s Law

Argonne National Labs have developed a new technique, molecular layer etching, that may help develop microelectronics and show the way beyond Moore’s Law. Molecular layer etching could enable fabricating and controlling material geometries at the nanoscale, which could open new doors in microelectronics and extend beyond traditional Moore’s Law scaling.

Chemistry Matters- Molecular Layer Etching of Metalcone Films Using Lithium Organic Salts and Trimethylaluminum

Together with molecular layer deposition (MLD), a deposition technique, MLE can be used to design microscopic architectures. These approaches are analogs of atomic layer deposition (ALD) and atomic layer etching (ALE), the more commonly applied techniques for fabricating microelectronics. However, unlike atomic layering techniques, which deal exclusively with inorganic films, MLD and MLE can be used to grow and remove organic films as well.

How it works
In principle, MLE works by exposing thin films, several nanometers or micrometers thick, to pulses of gas inside a vacuum chamber. The process starts with one gas (Gas A) which, upon entry, reacts with the surface of the film. Next, the film is exposed to a second gas (Gas B). This AB process is repeated until the desired thickness is removed from the film.

“The net effect of A and then B is the removal of a molecular layer from your film,” said Argonne chemist Jeff Elam, a co-author of the study. ​“If you do that process sequentially, over and over again, you can reduce the thickness of your film to achieve the desired final thickness.”

A key aspect of MLD is that the A and B surface reactions are self-limiting. They only continue until all of the available reactive surface sites are consumed, and then the reactions naturally terminate. This self-limiting behavior is extremely helpful in manufacturing since it is relatively easy to scale the process up to larger substrate sizes.

Researchers tested their approach using alucone, an organic material similar to silicone rubber that has potential applications in flexible electronics. Gas A in their experiment was a lithium-containing salt, and Gas B was trimethyl aluminum (TMA), an organometallic aluminum-based compound.

During the etching process, the lithium compound reacted with the surface of the alucone film in a way that caused the lithium to stick onto the surface and disrupt the chemical bonding in the film. Then, when the TMA was introduced and reacted, it removed the layer of film containing lithium. The lithium serves a sacrificial role — it is deposited on the surface temporarily to break chemical bonds but is then removed by the TMA.

“The process can go on layer by layer like that and you can remove the whole material if you wanted to,” Young said.

Opening new doors in microelectronics
Using this technique can help manufacturers and researchers develop new ways of making nanostructures. The process may also be a safer option for them to use because it is free of halogens, a harsh components of chemicals common in other etching processes. It also has the advantage of being selective; the etching technique can selectively remove MLD layers without affecting nearby ALD layers.

“MLE has the potential to help usher in new pathways for fabricating and controlling material geometries at the nanoscale, which could open new doors in microelectronics and extend beyond traditional Moore’s Law scaling,” Elam said.

Abstract

Advances in semiconductor device manufacturing are limited by our ability to precisely add and remove thin layers of material in multistep fabrication processes. Recent reports on atomic layer etching (ALE) have provided the means for the precise removal of inorganic thin films deposited by atomic layer deposition (ALD), opening new avenues for nanoscale device design. Here, we report on a new technique for the precise removal of metal–organic thin films deposited by molecular layer deposition (MLD), which we term molecular layer etching. This etching process employs sequential exposures of lithium organic salt (LOS) and trimethylaluminum (TMA) precursors to produce self-limiting etching behavior. We employ quartz crystal microbalance experiments to demonstrate (i) etching of alucone films preloaded with LOS upon TMA exposures and (ii) layer-by-layer etching of alucone films using alternating exposures of LOS and TMA. We also identify the selectivity of these etching mechanisms. We probe the mechanism for the layer-by-layer etching of alucone using a quartz crystal microbalance and Fourier transform infrared spectroscopy and identify that the etching proceeds via heterolytic cleaving of Al–O bonds in alucone upon LOS exposure followed by methylation to produce volatile species upon TMA exposure. The etching process results in the removal of 0.4 nm/cycle of alucone at 160 °C and up to 3.6 nm/cycle of alucone at 266 °C in ex situ etching experiments on silicon wafers. This halogen-free etching process enables etching of MLD films and provides new fabrication pathways for the control of material geometries at the nanoscale.

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