3D-to-2D Morphology Manipulation of Sputter-Deposited Nanoscale Silver Films on Weakly Interacting Substrates via Selective Nitrogen Deployment for Multifunctional Metal Contacts

Abstract

The ability to reverse the inherent tendency of noble metals to grow in an uncontrolled three-dimensional (3D) fashion on weakly interacting substrates, including two-dimensional (2D) materials and oxides, is essential for the fabrication of high-quality multifunctional metal contacts in key enabling devices. In this study, we show that this can be effectively achieved by deploying nitrogen (N2) gas with high temporal precision during magnetron sputtering of nanoscale silver (Ag) islands and layers on silicon dioxide (SiO2) substrates. We employ real-time in situ film growth monitoring using spectroscopic ellipsometry, along with optical modeling in the framework of the finite-difference time-domain method, and establish that localized surface plasmon resonance (LSPR) from nanoscale Ag islands can be used to gauge the evolution of surface morphology of discontinuous layers up to a SiO2 substrate area coverage of ∼70%. Such analysis, in combination with data on the evolution of room-temperature resistivity of electrically conductive layers, reveals that presence of N2 in the sputtering gas atmosphere throughout all film-formation stages: (i) promotes 2D growth and smooth film surfaces and (ii) leads to an increase of the continuous-layer electrical resistivity by ∼30% compared to Ag films grown in a pure argon (Ar) ambient atmosphere. Detailed ex situ nanoscale structural analyses suggest that N2 favors 2D morphology by suppressing island coalescence rates during initial growth stages, while it causes interruption of local epitaxial growth on Ag crystals. Using these insights, we deposit Ag layers by deploying N2 selectively, either during the early precoalescence growth stages or after coalescence completion. We show that early N2 deployment leads to 2D morphology without affecting the Ag-layer resistivity, while postcoalescence introduction of N2 in the gas atmosphere further promotes formation of three-dimensional (3D) nanostructures and roughness at the film growth front. In a broader context this study generates knowledge that is relevant for the development of (i) single-step growth manipulation strategies based on selective deployment of surfactant species and (ii) real-time methodologies for tracking film and nanostructure morphological evolution using LSPR.

Manipulation of thin silver film growth on weakly interacting silicon dioxide substrates using oxygen as a surfactant

“…We use in situ and real-time spectroscopic ellipsometry to monitor the evolution of optoelectronic properties of electricallyconductive layers and, in combination with ex situ microstructural analysis, we establish that presence of O2 throughout all film-formation stages, leads to a more pronounced 2D morphology, smoother film surfaces, but larger continuous-layer electrical resistivities, relative to Ag films grown in a pure argon (Ar) atmosphere. These trends are qualitatively consistent with the effect of N2 on Ag-layer morphology 42,43 . However, a ~10 times smaller O2 partial pressure is required for the surfactant effect to manifest itself; which can be explained by the higher reactivity of O2 toward Ag, as compared to that of N2 [44][45][46] .…”

“…Ar (purity 99.999%) is used as a sputtering gas. Films are deposited either in pure We have recently studied 42 the growth of magnetron sputter-deposited Ag layers on SiO2 in mixed Ar/N2 gas atmospheres, and we established that N2 affects the various film-formation stages in distinctly different ways. In order to explore whether the latter is also the case when O2 is used to manipulate growth, we deploy O2 using the following schemes: (i) O2 is present together with Ar throughout the entire deposition process; (ii)…”

“…1, f and ω ο are the oscillator strength and resonance frequency, respectively, and Γ represents the damping rate of the plasmon resonance. More details on the implementation and relevance of the Lorentz oscillator model for describing the optical response of discontinuous Ag layers can be found in our previous work 42 .…”

“…Besides the optical properties, the analysis of the ellipsometric data enable us to calculate the film height h f as a function of deposition time t. Using the continuous-layer h f value, the film deposition rate F is extracted (F ≈ 0.11 nm/s at all conditions used in this work), from which the nominal film thickness Θ (i.e., the amount of deposited material) at any given time during growth t is calculated as Θ = F × t. which a steady-state resistivity value ρ SS is reached. The nominal film thickness at which ρ SS is established marks to the formation of continuous layer (this Θ value is hereinafter denoted as Θ cont ) as we have previously shown 42,[53][54][55] .…”

“…Recently, we have contributed to the fundamental understanding of the mechanisms that govern surfactant-modified film growth on weakly-interacting substrates by studying the effect of nitrogen (N 2) gas on the morphological evolution of silver (Ag) films on silicon dioxide (SiO2) 42 . Our results showed that N2 affects the various film-formation stages in a complex manner: when N2 is present during island nucleation and coalescence, 2D growth morphology is promoted; while the opposite is observed when N 2 is deployed after island coalescence is completed, so that the stage of hole-filling is primarily affected.…”