Applying sputtering theory to directional atomic layer etching

Plasma assisted atomic layer etching (ALE) has recently been introduced into manufacturing of 10 nm logic devices. This implementation of ALE is called directional ALE because ions transfer momentum to the etching surface during the removal step. Plasma assisted directional ALE can be described as sputtering of a thin modified layer on the surface of the unmodified material. In this paper, the authors introduce a collision cascade based Monte Carlo model based on sputtering theory which has evolved for over 50 years [P. Sigmund, Thin Solid Films 520, 6031 (2012)]. To test the validity of this approach, calculated near threshold argon ion sputtering yields of silicon and chlorinated silicon are compared to published experimental data. The calculated ALE curve for Cl2/Ar ALE of tantalum is in good agreement with the experiment. The model was used to predict the presence of salient sputtering effects such as ion mass and impact angle dependence, as well as redeposition in directional ALE. Finally, the authors investigate time dependence of the synergy parameter for ion energies above the sputtering threshold of tantalum for Cl2/Ar ALE. The calculations show that close to 100% synergy can be obtained for short periods of time which opens a path to accelerate directional ALE. Very precise control of all process parameters as a function of time is prerequisite to realize this process space.