Elucidating the Exceptional Passivation Effect of 0.8 nm Evaporated Aluminium on Transparent Copper Films

Abstract

Slab-like copper films with a thickness of 9 nm (∼70 atoms) and sheet resistance of ≤9 sq −1 are shown to exhibit remarkable long-term stability toward air-oxidation when passivated with an 0. 8 nm aluminium layer deposited by simple thermal evaporation. The sheet resistance of 9 nm Cu films passivated in this way, and lithographically patterned with a dense array of ∼6 million apertures per cm 2 , increases by <3.5% after 7,000 h exposure to ambient air. Using a combination of annular-dark field scanning transmission electron microscopy, nanoscale spatially resolved elemental analysis and atomic force microscopy, we show that this surprising effectiveness of this layer results from spontaneous segregation of the aluminium to grain boundaries in the copper film where it forms a ternary oxide plug at those sites in the metal film most vulnerable to oxidation. Crucially, the heterogeneous distribution of this passivating oxide layer combined with its very low thickness ensures that the underlying metal is not electrically isolated, and so this simple passivation step renders Cu films stable enough to compete with Ag as the base metal for transparent electrode applications in emerging optoelectronic devices.

Enhanced Oxidation Stability of Transparent Copper Films Using a Hybrid Organic‐Inorganic Nucleation Layer

“…It is evident from Figure that using a mixed APTMS/MPTMS seed layer reduces the rate of oxidation of a 9 nm Cu film on glass by a factor of 4 from 0.0198 to 0.0048 Ω sq −1 hr −1 . This large improvement in stability can be rationalized in terms of the more compact slab‐like structure of the Cu film deposited onto seed layer modified glass, which impedes the diffusion of oxygen along the grain boundaries between crystallites …”

“…We have previously shown that a mixed molecular monolayer of MPTMS and APTMS co‐deposited from the vapour phase is an effective seed layer for the formation of Cu films on both glass and plastic substrates ,. APTMS catalyses the coupling reaction with the substrate, while both APTMS and MPTMS bind strongly to the incoming Cu atoms as they arrive at the substrate resulting in more slab‐like film formation and vastly improved film quality at <10 nm ,,,. Using this approach the surface roughness of a 9 nm evaporated Cu film on glass is reduced from 1.44±0.12 to 1.02±0.05 nm and the initial sheet resistance is reduced from 13.8 to 10.8 Ω sq −1 .…”

“…This stabilizing effect is also achieved on flexible plastic substrates (SI, Figures S3 and S4). Remarkably the rate of oxidation of the Cu film with a buried Al layer is comparable to that achieved using an Al or Ni deposited to the top surface of the Cu film ,. Also included in Figure is the evolution of the sheet resistance of a 9 nm Ag film fabricated using a polyethylenimine (PEI) adhesion layer, which serves as a benchmark against which the stability of the Cu films can be judged, since optically‐thin Ag films supported on PEI modified plastic substrates are the best performing Ag film electrodes to date ,…”

“…To enable the formation of uniform slab‐like Cu and Ag films at sub‐10 nm metal thickness a variety of different inorganic and organic nucleation layers have been proposed whose primary function is to suppress metal atom diffusion during early stages of film growth ,,,. For evaporated Cu films the most successful seed layers to date are based on the use of molecular monolayers that chemically bind both to the substrate and Cu, including 3‐mercaptopropyl(trimethoxysilane) [MPTMS] and 3‐aminopropyl(trimethoxysilane) [APTMS] …”

“…Surprisingly, despite the high potential of optically thin Ag and Cu films as transparent electrodes for optoelectronic applications, studies of the long‐term stability of Ag or Cu window electrodes are sparse . Stability towards air oxidation is a particularly important consideration because oxidation of the surface of very thin metal films can have a large detrimental effect on the electrode sheet resistance, as well as forming a barrier to charge transport between the electrode and an adjacent semiconducting layer in a device .…”