Coverage and Stability of NHx Terminated Cobalt and Ruthenium Surfaces: a First Principles Investigation

Abstract

<div>In the atomic layer deposition (ALD) of Cobalt (Co) and Ruthenium (Ru) metal using nitrogen plasma, the structure and composition of the post N-plasma NHx terminated (x = 1 or 2) metal surfaces are not well known but are important in the subsequent metal containing pulse. In this paper, we use the low-index (001) and (100) surfaces of Co and Ru as models of the metal polycrystalline thin films. The (001) surface with a hexagonal surface structure is the most stable surface and the (100) surface with a zigzag structure is the least stable surface but has high reactivity. We investigate the stability of NH and NH2 terminations on these surfaces to determine the saturation coverage of NHx on Co and Ru. NH is most stable in the hollow hcp site on (001) surface and the bridge site on the (100) surface, while NH2 prefers the bridge site on both (001) and (100) surfaces. The differential energy is calculated to find the saturation coverage of NH and NH2. We also present results on mixed NH/NH2-terminations. The results are analyzed by thermodynamics using Gibbs free energies (ΔG) to reveal temperature effects on the stability of NH and NH2 terminations. Ultra-high vacuum (UHV) and standard ALD</div><div>operating conditions are considered. Under typical ALD operating conditions we find that the most stable NHx terminated metal surfaces are 1 ML NH on Ru (001) surface (350K-550K), 5/9 ML NH on Co (001) surface (400K-650K) and a mixture of NH and NH2 on both Ru (100) and Co (100) surfaces.</div>

Self-limiting nitrogen/hydrogen plasma radical chemistry in plasma-enhanced atomic layer deposition of cobalt

“…In our recent published work, the nature and stability of NH x -terminated Co(001) and Co(100) surfaces and the structures after the metal precursor CoCp 2 pulse were studied. 31,32 The results show that under typical ALD operating conditions from the literature 5 (temperature range 550 K to 650 K), the most stable termination of the low energy Co(001) surface is 0.56 ML coverage of NH-termination. On the high energy, zigzag Co(100) surface, a mixture of 0.67 ML NH and 0.67 ML NH 2 is the most stable surface termination.…”

“…40 The deposited Co films by PE-ALD are polycrystalline and have random surface orientations after low-temperature deposition. Based on our previous study 31 on the stability of NH/ NH 2 terminations of Co surfaces, we have chosen the most stable Co(001) surface (with the lowest surface energy) and to assess the role of surface structure and reactivity, we also consider the Co(100) surface, which has a higher surface energy but high reactivity in our investigation of the precursor reaction mechanisms. A similar comparison has been presented for copper.…”

“…From our previous studies, fixing these layers is sufficient to model these Co surfaces. 31 A k-point mesh 39 of 2 × 2 × 1 is used in the (4 × 4) supercell, and for the (3 × 3) supercell a 3 × 2 × 1 mesh is used. The van der Waals correction was applied with the PBE-D3 method to ensure an accurate description of the dispersion interactions in the computed adsorption energies.…”

“…On the Co(100) surface, mixed NH x terminations with NH occupying trench sites and NH 2 occupying surface sites are determined. 31 Trench NH species contribute to eliminating Cp ligands during the metal precursor pulse, resulting in bare N atoms on trench sites (Fig. 1(b)).…”

“…A complete PE-ALD process using N-plasma (NH 3 or a mixture of N 2 and H 2 ) 27 can be thought to follow the following process (Scheme 1): firstly, it is vital to note that at the postplasma stage, the metal surface is actually an NH x -terminated metal surface, where x = 1 or 2. 31 In the metal half-cycle, the metal precursor CoCp 2 reacts with the NH x -terminated metal surface. Cp ligands are eliminated by hydrogen transfer from the surface to form CpH, which desorbs from the surface, rather than migrating to another Co site on the surface.…”