Researchers at the U.S. Department of Energy’s Argonne National Laboratory, Chicago, have developed a new etching technique that could potentially help make small but complex semiconductors. The technique, known as molecular layer etching, is detailed in a new paper published in Chemistry of Materials.
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.”
Together with molecular layer deposition, MLE can be used to design microscopic architectures. These approaches are analogs of atomic layer deposition and atomic layer etching, 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.
A key aspect of MLD is that the A and B surface reactions are self-limiting. They continue only 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, 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 paper is titled “Molecular Layer Etching of Metalcone Films Using Lithium Organic Salts and Trimethylaluminum.”
