Vacuum ultraviolet enhanced atomic layer etching, and area selective atomic layer deposition for next generation nanofabrication
Creators: name: Coffey, Brennan Muir
name: 0000-0002-0669-3149
Publisher: name: The University of Texas at Austin
Year: 2021
Subjects: Atomic layer etching
Vacuum ultraviolet enhanced atomic layer etching
Oxidation
Atomic layer deposition
Area selective atomic layer deposition
Nanopatterning

description: Novel nanofabrication methods for area-selective (AS) deposition and atomic layer etching (ALE) are presented. The aims of these methods are to facilitate the integration of metal assisted chemical etching (MACE), and electroless Cu deposition into devices such as FinFETs, transparent conductive electrodes (TCEs), and patterned photonic structures, which are ubiquitous in next generation nanotechnology, and flexible electronics. MACE and electroless Cu deposition require high quality metal catalyst (e.g., Pd, and Ru) patterns for their implementation, however, ALE methods to remove metals are limited, and difficult to control on an atomic scale. Additionally, no ALE methods currently exist that offer selectivity to remove undesired metal deposition regions, while leaving desired regions largely untouched. Two solutions are presented to ameliorate these difficulties: the first demonstration of vacuum ultraviolet (VUV) enhanced ALE, which has the potential to remove one atomic layer of Pd and Ru at a time, while also being selective to undesired Pd and Ru deposition; and the second, is to control deposition using AS atomic layer deposition (ALD) for photonic devices to desired regions alone, circumventing the need for patterning the challenging to etch BaTiO₃, while still allowing surface phase epitaxy. The first demonstration of VUV ALE is shown, where etching of Pd and Ru is achieved at 100 – 150 °C, with approximate material removal rates of 2.8, and 1.2 Å/cycle, respectively. Etching is accomplished using an oxidation half-cycle, consisting of co-exposure of the metal substrate to VUV and O₂ at 1 Torr for 2 – 5 min, followed by an etching half-cycle, consisting of exposure to 0.50 Torr of HCOOH for 30 sec. Density functional theory (DFT) is used to explore oxidation mechanisms on Pd and Ru. DFT results indicate all oxidants readily incorporate into the surface of both metals, however, a large of concentration of atomic O in the near surface region is required to form an oxide that can be removed by HCOOH exposure. Nudged elastic band (NEB) calculations indicate this is due to difficulty in forming subsurface oxides when a surface oxide is present on Pd, as well as Ru. True self-limiting behavior is predicted, and observed for Ru ALE, while the amount of Pd etched is controlled with the temperature and time of VUV/O₂ co-exposure. Additionally, selectivity in oxidation, and thus material etched, is observed on Pd, and Ru, where at co-exposure times less than 2 min, Pd will not oxidize if it is a continuous (low surface area) thin film, while discontinuous Pd will oxidize. Ru etch rate is decreases as etching is performed, indicating that as roughness decreases (with ALE cycles), longer oxidation times are required to achieve oxidation. This is amenable to the fabrication of the TCE, which requires selectivity between undesired and desired deposition, and patterning of Pd or Ru catalyst layers. Finally, AS-ALD of difficult to pattern crystalline BaTiO₃ (BTO) is presented, where patterns are defined with one lithographic patterning step. Epitaxial crystallization to form a single crystal film with an out-of-plane c-axis orientation, should be observed after AS patterning. This process could eliminate the need for post-growth etching for devices in, for example, Si photonic device fabrication