Physical and chemical effects in directional atomic layer etching

Atomic layer etching (ALE) has received much attention in recent years as a viable state-of-the-art patterning technique for the fabrication of future generations of nanoelectronics. Thermal excitation or plasma activation, coupled with chemical reactions have been shown as different approaches to enable ALE. In this review, the importance of surface oxidation state is considered as a viable parameter to tailor the chemical contrast that is needed in realizing ALE. With the help of thermodynamic assessment of viable reaction pathways, an alternative approach that combines both plasma and thermal ALE concepts is proposed: the physical effect from energetic ions results in not only chemical conversion but also directionality, while the chemical effect dictates the selective removal of the converted material. This hybrid plasma-thermal ALE approach allows for a simultaneous control of selectivity and anisotropy and a wider ALE window. This approach is tested on a number of model systems and could be extended to more complex materials systems that are needed in future integrated circuits. (topical review)