Atomic Layer Deposition as a Novel Technique for the Fabrication of Magnetoplasmonic Metasurfaces—Manufacturing and Characterization of Ferromagnetic Nickel Nanoarrays

Among magneto optical (MO) metasurfaces, magnetoplasmonic systems based on nano-ferromagnets (FMs) have attracted significant attention thanks to the interplay between the magnetism and the Localized Plasmon Resonances (LPRs) of the metallic nanoparticles. The combination of these two functionalities enhances the magneto-optical response and adds tunability to the plasmonic system.<br/>In the present work we investigate the possibility of using atomic layer deposition (ALD) as a means for creating the ferromagnetic component of magnetoplasmonic metasurfaces. We aim to expand the spectrum of possibilities using the exceptional processing advantages and precision of ALD. Among the advantages are the vast range of materials, the Ångstrom scale precision during deposition, and the compatibility with different substrate types. Besides, ALD layers are covalently bound to the substrate, adding the dimension of enhanced stability and a higher degree of control and tunability of the final properties of the metasurface.<br/>In the case under study, the material of choice is nickel, one of the prototypical materials for magnetoplasmonic applications. Nickel thin films and structures were obtained after ALD of nickel oxide, using Bis(cyclopentadienyl) nickel (Ni(Cp)₂) and ozone, and subsequent reduction to nickel by annealing the substrate in a H₂/N₂ atmosphere.<br/>Nickel nanoarrays were fabricated by initially coating a quartz substrate and subsequent patterning. After the ALD deposition, a negative resist was spin coated, patterned by e-beam lithography (eBL), and finally used as a mask to transfer the pattern (nanodisks) from the resist to the underlying thin film by anisotropic etching. After removing the resist, the nickel oxide nanodisks were converted to metallic nickel by reduction in H₂/N₂.<br/>To characterize the magneto-optical response and the LPRs of the ALD material, nanoarrays with a fixed square lattice and a periodicity of 550 nm were fabricated with different nanodisks diameters ranging from 100 nm to 350 nm. The characterization with a magneto-optical Kerr effect (MOKE) microscope and the measurement of the optical extinction spectra of the plasmonic nanostructures confirmed the suitability of the material for technological applications.<br/>Alternative nanoarrays fabrication routes were also investigated. A bottom-up pathway, starting with the deposition of a positive resist and followed by ALD of the ferromagnet, was developed. This etch-free procedure can be easily integrated into multilayer device fabrication and is compatible with different kinds of substrates among which two-dimensional materials.<br/>To showcase the potential of the bottom-up fabrication, a two-dimensional flake of MoS₂ was patterned by e-beam litography and decorated with ultra-thin nickel magnetic domains using ALD.<br/>TEM analysis of the fabricated samples demonstrated that ALD can be used for the production for novel types of nickel based metasurfaces including two-dimensional nanocomposites.