Atomic layer etching of sputter-deposited AlN thin films in radiofrequency Cl₂–Ar plasmas

Abstract

Atomic layer etching (ALE) of AlN is an important process for enabling high-precision patterning in advanced photonic and electronic devices. In this study, ALE of sputter-deposited AlN thin films was carried out using an ALE approach consisting of Cl2-based surface modification followed by an Ar ion bombardment step. The developed process exhibited highly self-limiting behavior with etch-per-cycle approaching the thickness of a single AlN monolayer, a process synergy of 82%, and a post-etch root mean square surface roughness as low as 0.6 nm. To define the ALE ion energy window, ion energy distribution functions were measured and calibrated by taking into account the voltage drop across the dielectric layer on the wafer surface. This calibration revealed a significant reduction in effective energy of ions reaching the wafer surface and a corresponding shift in the ALE ion energy window when expressed in terms of the peak-to-peak voltage. The ALE ion energy window for AlN was experimentally determined to be between 142 and 196 eV, which is in good agreement with molecular dynamics simulations predicting the lower threshold of the window to be 150 eV. These findings underscore the importance of considering dielectric stack thickness in ALE and conventional plasma processing.