Atomic Layer Etching of GaN and AlGaN Using CH4/H2, H2 and HCl Chemistry
Authors: Radehaus Marco , Wege Stephan , Miersch Christian , Ricken Elias , Krug Mario 
Published: February 2024

Modern microelectronic components for high-power and high-frequency applications require reliability and reproducibility of the production processes. Atomic layer etching (ALE) represents a key technology for achieving these requirements.
An ALE sequence is characterized by a chemical modification step that affects only the top atomic layer of the surface (adhesion) and an ion assisted non-reactive inert gas etching step that removes only this chemically modified area due to creation of volatile byproducts. These two steps are separated by inert gas purge. This grants the etching of single atomic layers. The etching step can be triggered by thermal energy or kinetic energy by impinging particles (e.g. ion bombardment by a plasma). The low energy required for ALE guarantees low-damage etching.
In literature ALE processes using Cl2 are often reported. In this work, the results of plasma-enhanced ALE of GaN and AlGaN using different chemistries of CH4/H2, H2 and HCl as etch gases are presented. The experiments were performed with a highly customized etch facility equipped with a capacitively coupled plasma source (CCP, 60 MHz) and a substrate bias generator (RF, 2 MHz). The investigations were carried out on epitaxially deposited GaN on sapphire with masks of protective resist and SiO2 as well as GaN-AlGaN heterostructures with patterned SiO2 maskings. For process analysis optical emission spectroscopy (OES) was carried out. The etching rate and roughness were determined by ellipsometry and atomic force microscopy (AFM) as well as scanning electron microscopy (SEM).
Observations by OES indicate a successful excitation and dissociation of the etching molecules. With rising plasma powers physical sputtering effects were revealed. At the beginning of the process development continuous etching processes were examined. With CH4/H2 chemistry using GaN samples it was found that the etching behavior is strongly influenced by the type of masking material. Protective resist resulted in a low etch rate of 7.5 nm/min and significant damage of the masking material. The cause of this behavior is assumed to be redeposition of material by interaction of methane, resist, and plasma. With the change to a hard mask of SiO2 etch rates of 45 to 72 nm/min were achieved, increasing with RF-power and CH4/H2 ratio. Adding methane led to detrimental side effects such as dendrite growth and wall formation on the transient region of unmasked to former masked sample areas. In the further development of the ALE process an etch chemistry using only hydrogen as the chemical etchant was used resulting in an etch per cycle (EPC) of about 3 Å for GaN samples. Further investigations of ALE with hydrogen etch chemistry on GaN-AlGaN samples revealed that AlGaN, in contrast to pure GaN, could not be etched successfully with this chemistry.
Substituting CH4/H2 or H2 to HCl enables the etch of AlGaN. The investigations carried out indicate that this etching process is also a self-limiting ALE. AFM measurements revealed an EPC of 1.2 Å for lower RF-powers (50 W). Increasing the RF power to 150 W only slightly enhance the EPC to 2.0 Å. The surface roughness tends to decrease with rising RF-power from 50 to 300 W.