ALD2023 Session ALE2-WeM: Plasma and Energy-Enhanced ALE

We report the isotropic plasma atomic layer etching (ALE) of aluminum nitride using sequential exposures of SF₆ plasma and trimethylaluminum (Al(CH₃)₃, TMA). ALE was observed at temperatures greater than 200 C, with a maximum etch rate of 1.9 Å/cycle observed at 300C as measured using ex-situ ellipsometry. After ALE, the etched surface was found to contain a lower concentration of oxygen compared to the original surface and exhibited a ∼35% decrease in surface roughness. These findings have relevance for applications of AlN in nonlinear photonics and wide bandgap semiconductor devices. [arXiv:2209.00150]

Chlorine based plasmas are usually preferred to etch GaN because of the formation of GaCl₃ molecules at the surface, which are quite volatile¹. In fluorine-based plasmas, GaF₃ molecules are formed at the surface as well, but they are not volatile except at a very high temperature¹. However, in atomic layer etching, SF₆ plasma can be used to control the etching at the atomic scale. Indeed, the Ga surface can be saturated of fluorine, producing a monolayer of GaF_x sites, which are supposed to not desorb during the modification step. Then, this modified layer can be sputtered selectively during the argon plasma removal step if the sputtering threshold of this layer is lower than that of GaN.

In this presentation, we would like to highlight the advantage of using probes such as ion Retarding Field Energy Analyzer (RFEA) and Langmuir probes to control the ALE process. With such plasma diagnostics, it is possible to evaluate the energy of the ions bombarding the surface and adjust theprocess parameters in order to avoid direct sputtering of GaN, and selectively remove the modified layer.

We used an Inductively Coupled Plasma (ICP) reactor excited at a frequency of 13.56 MHz with an RF generator to perform the experiments. The self-bias voltage was varied using an independent RF power supply operating at the same frequency. Experiments were performed at different source powers and different pressures of argon. This study led to an optimized etching process, which enables the control of the etching at the atomic scale, monolayer by monolayer. An energy scan was carried out and the synergy of the ALE process was evaluated. In certain ion flux conditions during the removal step with the Ar plasma, a self-limiting regime could be obtained.

In parallel, we used and adapted the global model developed by Pascal Chabert’s team² at the “Laboratoire de Physique des Plasmas” in Palaiseau, France, to our reactor in order to simulate the plasma parameters for both Ar and SF₆ plasmas under different experimental conditions. Finally, some material characterization experiments (AFM, XPS, and ToF SIMS) were performed to analyze the GaN surface after the ALE process.

Acknowledgment

The authors appreciate the support provided by the European Union (IPCEI: Important Project of Commun European Interest) and Région Centre Val de Loire for the Nano2022 project. We also thank the CERTeM 2020 platform, which supplied the majority of the equipment.

¹ S.J. Pearton, J.C. Zolper, R.J. Shul, and F. Ren, J. Appl. Phys. 86, 1 (1999).

² C. Lazzaroni, K. Baba, M. Nikravech, and P. Chabert, J. Phys. Appl. Phys. 45, 485207 (2012).

Atomic layer etching (ALE) is a cyclical process that shows promise for precisely fabricating nanostructures in a variety of materials. Silicon carbide (SiC) is a wide bandgap semiconductor material known for its excellent electrical, thermal, and mechanical properties. These properties make SiC a common material for power electronics, optoelectronics, and quantum optics. Conventional dry etching techniques for SiC suffer from poor selectivity, roughness, and damage the crystalline structure. ALE offers a solution to these shortcomings by utilizing self-limiting chemical reactions to remove individual atomic layers of SiC, resulting in precise etching that leaves smooth surfaces.

We present a novel execution of ALE for SiC using a bias-pulsing scheme and demonstrate self-limiting single dimer (i.e. the Si-C doublet) removal with an Ar/Cl2 inductively coupled plasma reactive ion etching (ICP RIE) system by only pulsing the plasma DC bias. Gases are not purged between etch steps, which leads to etch cycles of merely 6 seconds, which are far shorter than those of conventional ALE methods. Our results show that this ALE process can achieve precise etching with sub-angstrom surface roughness. We believe that this technique will enable new SiC-based quantum devices, especially those that are sensitive to surface quality, and that bias-pulsed ALE can be applied to other material systems to provide a more rapid solution for ALE.