On the Growth, Percolation and Wetting of Silver Thin Films Grown by Atmospheric-Plasma Enhanced Spatial Atomic Layer Deposition
Authors: Mameli Alfredo , van den Bruele Fieke , Ande Chaitanya Krishna , Verheijen Marcel A. , Kessels W.M.M. , Roozeboom Fred 
Published: February 2020

Silver (Ag) thin-films have attracted interest for applications in sensing1, light management2, imaging beyond the diffraction limit3, interlayer in tri-layer transparent conductors (TCs)4 and even as possible replacement for Cu interconnects5. For tri-layer TCs, a narrow optimum exists between optical transparency and electrical conductivity6. The former decreases exponentially whereas the latter increases with thickness. The ability to precisely control both thickness and electrical percolation is crucial in tri-layer TCs, for which ultra-thin and uniform, yet conductive Ag layers are required over large areas. Here, Atomic Layer Deposition (ALD) can fulfill the requirements for precise thickness control and conformal step coverage over large area substrates.

In this work we studied the deposition of Ag by spatial-ALD on molybdenum (Mo) layers serving as a growth model substrate. Mo-layers were specifically selected for testing the suitability of spatial-ALD to fabricate MoO3/Ag/MoO3 tri-layer TCs for organic-based solar cells. Since Mo-films are known to have high surface energy (3 Jm-2), Ag is expected to wet a Mo surface. However, the presence of native MoO3 on the surface is likely to reduce the surface energy resulting in poor wetting. Therefore, an H2/N2 plasma pre-treatment was introduced before Ag deposition in order to reduce the native MoO3 and promote coalescence.

Ultrathin Ag films were deposited at 120 °C using Ag(fod)(PEt3) as the precursor and an H2/N2 plasma generated by a surface dielectric barrier discharge (SDBD) source as the co-reactant. Ag(fod)(PEt3) was evaporated into the deposition chamber using Ar as carrier gas flowing at 100 sccm, at the same temperature as that of the reactor. An H2/N2 co-reactant gas mixture was composed from 800 sccm H2 and 600 sccm N2. Silicon substrates with sputter-deposited Mo were used as a model substrate for Ag growth, and compared with the growth on bare silicon substrates.

Top view and cross-sectional SEM images of the Ag layers show that a more compact and closed Ag layer was achieved on Mo upon introduction of the plasma pre-treatment (Fig. 1). Spectroscopic ellipsometry measurements demonstrate that the use of an H2/N2 plasma pre-treatment significantly enhances the wetting, and therefore, the coalescence of Ag owing to the high surface energy of the Mo-layers (Fig. 2).

Furthermore, the better wetting and interfacial quality of the Ag deposited on plasma pre-treated Mo, compared to Ag on Si was probed by post-growth rapid thermal annealing experiments and XRD stress measurements. It was found that at 450 °C the Ag films de-wetted from Si, resulting in discontinuous films whereas the films deposited on plasma pre-treated Mo remained unchanged. Since both Ag films show similar stress content, the de-wetting most likely originates from interfacial properties rather than stress phenomena.

We can therefore conclude that a plasma pre-treatment of Mo layers promotes the Ag wetting. This leads to earlier film coalescence compared to that of Ag films grown on Si, as demonstrated by both morphological and optical inspections of the Ag thin films. Thus, upon plasma pre-treatment ALD deposited MoO3 layers can act as wetting layers for subsequent Ag spatial-ALD in tri-layer TCs. This can open up alternative pathways for the fabrication of TCs using ALD.

Figure 1