Radical-Enhanced Atomic Layer Deposition of Silver Thin Films Using Phosphine-Adducted Silver Carboxylates

Influence of Substrate Materials on Nucleation and Properties of Iridium Thin Films Grown by ALD

Ultra-thin metallic films are widely applied in optics and microelectronics. However, their properties differ significantly from the bulk material and depend on the substrate material. The nucleation, film growth, and layer properties of atomic layer deposited (ALD) iridium thin films are evaluated on silicon wafers, BK7, fused silica, SiO2, TiO2, Ta2O5, Al2O3, HfO2, Ru, Cr, Mo, and graphite to understand the influence of various substrate materials. This comprehensive study was carried out using scanning electron and atomic force microscopy, X-ray reflectivity and diffraction, four-point probe resistivity and contact angle measurements, tape tests, and Auger electron spectroscopy. Within few ALD cycles, iridium islands occur on all substrates. Nevertheless, their size, shape, and distribution depend on the substrate. Ultra-thin (almost) closed Ir layers grow on a Ta2O5 seed layer after 100 cycles corresponding to about 5 nm film thickness. In contrast, the growth on Al2O3 and HfO2 is strongly inhibited. The iridium growth on silicon wafers is overall linear. On BK7, fused silica, SiO2, TiO2, Ta2O5, Ru, Cr, and graphite, three different growth regimes are distinguishable. The surface free energy of the substrates correlates with their iridium nucleation delay. Our work, therefore, demonstrates that substrates can significantly tailor the properties of ultra-thin films.

Two-Step Approach for Conformal Chemical Vapor-Phase Deposition of Ultra-Thin Conductive Silver Films

Conductive ultra-thin silver films are commonly fabricated by physical vapor deposition methods such as evaporation or sputtering. The line-of-sight geometry of these techniques impedes the conformal growth on substrates with complex morphology. In order to overcome this issue, volume deposition technologies such as chemical vapor deposition or atomic layer deposition are usually preferred. However, the silver films fabricated using these methods are generally non-electrically conductive for thicknesses below 20-50 nm due to island formation. Here, we demonstrate a novel approach for producing ultra-thin conductive silver layers on complex substrates. Relying on chemical vapor-phase deposition and plasma post-treatment, this two-step technique allows the synthesis of highly conductive and uniform silver films with a critical thickness lower than 15 nm and a sheet resistance of 1.6 Ω/□ for a 40 nm-thin film, corresponding to a resistivity of 6.4 μΩ·cm. The high infrared reflectance further demonstrates the optical quality of the films, despite a still large root-mean-square roughness of 8.9 nm. We successfully demonstrate the highly conformal deposition in lateral structures with an aspect ratio of up to 100. This two-step deposition method could be extended to other metals and open new opportunities for depositing electrically conductive films in complex 3D structures.

Synthesis of [{AgO2CCH2OMe(PPh3)} n ] and theoretical study of its use in focused electron beam induced deposition

The synthesis, chemical and physical properties of [{AgO₂CCH₂OMe} _n ] (1) and [{AgO₂CCH₂OMe(PPh₃)} _n ] (2) are reported. Consecutive reaction of AgNO₃ with HO₂CCH₂OMe gave 1, which upon treatment with PPh₃ produced 2. Coordination compound 2 forms a 1D coordination polymer in the solid state as evidenced by single crystal X-ray structure analysis. The coordination geometry at Ag⁺ is of the [3 + 1] type, whereby the carboxylate anions act as bridging ligands. The formation of PPh₃-Ag(I) coordinative bonds results in distorted T-shaped AgPO₂ units, which are stabilized further by an additional O-Ag dative bond. TG and TG-MS measurements show that 1 and 2 decompose at 190-250 °C (1) and 260-300 °C (2) via decarboxylation, involving Ag-P (2), C-C and C-O bond cleavages to give elemental silver as confirmed by PXRD studies. In order to verify if polymeric 2 is suitable as a FEBID precursor for silver deposition, its vapor pressure was determined (p_{170 °C} = 5.318 mbar, ∆H_vap = 126.1 kJ mol^-1), evincing little volatility. Also EI and ESI mass spectrometric studies were carried out. The dissociation of the silver(I) compound 2 under typical electron-driven FEBID conditions was studied by DFT (B3LYP) calculations on monomeric [AgO₂CCH₂OMe(PPh₃)]. At an energy of the secondary electrons up to 0.8 eV elimination of PPh₃ occurs, giving Ag⁺ and O₂CCH₂OMe^-. Likewise, by release of PPh₃ from [AgO₂CCH₂OMe(PPh₃)] the fragment [AgO₂CCH₂OMe]^- is formed from which Ag⁺ and O₂CCH₂OMe^- is generated, further following the first fragmentation route. However, at 1.3 eV the initial step is decarboxylation giving [AgCH₂OMe(PPh₃)], followed by Ag-P and Ag-C bond cleavages.

Deposition of metallic silver coatings by Aerosol Assisted MOCVD using two new silver β-diketonate adduct metalorganic precursors

This work reports two new silver metalorganic precursors for the chemical vapor deposition of Ag metallic coatings. Both precursors are based on β-diketonate adducts, namely, Ag(hfac)(L) (H-hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione), where L is 1,10-phenanthroline (phen) or 2,5,8,11-tetraoxadodecane (triglyme). Using these ligands, the designed precursors have better solubility in alcoholic solvents and are less toxic and costly than previously reported ones. The new precursors have been characterized and their crystallographic structure solved. With the new triglyme precursor, [Ag(triglyme)₂]⁺[Ag(hfac)₂]^-, pure metallic Ag coatings made of Ag nanoparticles about 20 nm in diameter were succesfully deposited on glass and Si substrates using Aerosol Assisted Metalorganic CVD (AA-CVD).

Atomic and molecular layer deposition: off the beaten track

ALD archetype and deviations from it.

Synthesis, structural characterization and thermal properties of copper and silver silyl complexes

A series of copper and silver-silyl complexes containing N-heterocyclic carbene or N-donor ligands were synthesized and characterized in the solid state. A number of different structural forms were observed and many compounds were shown to be volatile.

Direct excitation of dark plasmonic resonances under visible light at normal incidence

Dark plasmon resonance modes are optical modes that have small scattering cross-sections and are thus difficult to excite directly by light at normal incidence. In this paper, we propose to excite quadrupolar and higher-order modes with normal incident light (in the visible regime) on a continuous plasmonic metallic surface covering a dielectric pillar array, hence resulting in narrow-band perfect absorption. Different from the general electromagnetic means of inducing dark modes, our dark modes are due to charge densities that are electrically induced by the standing-wave resonance of current on the thin metal sidewall of pillars. This new means of exciting dark modes can significantly improve the excitation efficiency and also provides an easy way to excite strong higher-order modes.

Large-area plasmonic hot-spot arrays: sub-2 nm interparticle separations with plasma-enhanced atomic layer deposition of Ag on periodic arrays of Si nanopillars

Initial reports of plasmonic 'hot-spots' enabled the detection of single molecules via surface-enhanced Raman scattering (SERS) from random distributions of plasmonic nanoparticles. Investigations of systems with near-field plasmonically coupled nanoparticles began, however, the ability to fabricate reproducible arrays of such particles has been lacking. We report on the fabrication of large-area, periodic arrays of plasmonic 'hot-spots' using Ag atomic layer deposition to overcoat Si nanopillar templates leading to reproducible interpillar gaps down to <2 nm. These plasmonic 'hot-spots' arrays exhibited over an order of magnitude increase in the SERS response in comparison to similar arrays with larger interpillar separations.

Silver coated platinum core-shell nanostructures on etched Si nanowires: atomic layer deposition (ALD) processing and application in SERS

A new method to prepare plasmonically active noble metal nanostructures on large surface area silicon nanowires (SiNWs) mediated by atomic layer deposition (ALD) technology has successfully been demonstrated for applications of surface-enhanced Raman spectroscopy (SERS)-based sensing. As host material for the plasmonically active nanostructures we use dense single-crystalline SiNWs with diameters of less than 100 nm as obtained by a wet chemical etching method based on silver nitrate and hydrofluoric acid solutions. The SERS active metal nanoparticles/islands are made from silver (Ag) shells as deposited by autometallography on the core nanoislands made from platinum (Pt) that can easily be deposited by ALD in the form of nanoislands covering the SiNW surfaces in a controlled way. The density of the plasmonically inactive Pt islands as well as the thickness of noble metal Ag shell are two key factors determining the magnitude of the SERS signal enhancement and sensitivity of detection. The optimized Ag coated Pt islands on SiNWs exhibit great potential for ultrasensitive molecular sensing in terms of high SERS signal enhancement ability, good stability and reproducibility. The plasmonic activity of the core-shell Pt//Ag system that will be experimentally realized in this paper as an example was demonstrated in numerical finite element simulations as well as experimentally in Raman measurements of SERS activity of a highly diluted model dye molecule. The morphology and structure of the core-shell Pt//Ag nanoparticles on SiNW surfaces were investigated by scanning- and transmission electron microscopy. Optimized core-shell nanoparticle geometries for maximum Raman signal enhancement is discussed essentially based on the finite element modeling.