Angstrom-Scale Smoothing of Aluminum Nitride Film Surfaces Using Atomic Layer Etching

Roughness-induced light scattering is an important limitation of the performance of various on-chip optical technologies such as microresonators and quantum photonic integrated circuits. Here, we report a plasma-thermal atomic layer etching (ALE) method which enables etching and smoothing of aluminum nitride (AlN) films with Angstrom-scale precision. The ALE process employs sequential exposures of SF₆ plasma and trimethylaluminum (Al(CH₃)₃, TMA). In each cycle, the surface is fluorinated by flourine radicals, followed by a ligand-exchange reaction to yield etching. We observed a maximum etch rate of 1.9 Å/cycle at 300 ^○C as measured using ex-situ ellipsometry. After ALE, the surface roughness of AlN was measured to decrease ~35% from 4.3 Å to 2.7 Å. In addition, the etched surface was found to contain a lower concentration of oxygen compared to the original surface. We anticipate that the ALE process may be applied as a post-treatment to improve the performance of on-chip photonic devices which exploit the simultaneous second-order and cubic-order optical nonlinearities of AlN. ALE processes may be generally applicable to photonic materials to decrease losses arising from surface roughness scattering.<br/>Reference: Wang, Haozhe, et al. "Isotropic plasma-thermal atomic layer etching of aluminum nitride using SF₆ plasma and Al (CH₃) ₃" <i>arXiv preprint arXiv:2209.00150</i> (2022).