On the Growth, Percolation and Wetting of Silver Thin Films Grown by Atmospheric-Plasma Enhanced Spatial Atomic Layer Deposition

Silver (Ag) thin-films have attracted interest for applications in sensing 1 , light management 2 , imaging beyond the diffraction limit 3 , interlayer in tri-layer transparent conductors (TCs) 4 and even as possible replacement for Cu interconnects 5 . For tri-layer TCs, a narrow optimum exists between optical transparency and electrical conductivity 6 . 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 MoO 3 /Ag/MoO 3 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 MoO 3 on the surface is likely to reduce the surface energy resulting in poor wetting. Therefore, an H 2 /N 2 plasma pre-treatment was introduced before Ag deposition in order to reduce the native MoO 3 and promote coalescence. Ultrathin Ag films were deposited at 120 °C using Ag(fod)(PEt 3 ) as the precursor and an H 2 /N 2 plasma generated by a surface dielectric barrier discharge (SDBD) source as the co-reactant. Ag(fod)(PEt 3 ) 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 H 2 /N 2 co-reactant gas mixture was composed from 800 sccm H 2 and 600 sccm N 2 . 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 H 2 /N 2 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 pr