Nanodroplet-Confined Electroplating Enables Submicron Printing of Metals and Oxide Ceramics

TL;DR

This study expands droplet confined electroplating (DCEP), derived from electrohydrodynamic redox printing (EHD-RP), to direct submicron deposition of both metals and metal-oxide ceramics using water-based electrochemistry. By controlling droplet chemistry, ion speciation, and on-the-fly material switching, the authors deposit a broad range of metals, realize nickel-phosphorus-oxide (Ni-P-O) doped structures, and demonstrate multi-material stacks such as Cu on Mg(OH)$_2$, all at nanoscale resolution. They propose a practical electrochemical framework with rules for metal deposition, address insulator deposition, and showcase a NiPO case study to illustrate dopant incorporation, highlighting opportunities for doped ceramics and metal-oxide architectures. Remaining challenges include achieving high metal-to-contaminant ratios, avoiding counter-ion co-deposition, and mitigating anode passivation, with future work pointing toward alternative ion sources and in-situ annealing to improve material quality and functionality.

Abstract

The fabrication of functional micro- and nano-electronic devices requires the deposition of high-quality materials of different electronic material classes, such as conductors, semiconductors and insulators. To establish ultra-high-resolution additive manufacturing as a viable addition to existing fabrication methods requires the combinatorial additive deposition of different electronic material classes. However, current techniques do not provide such a capability. Here, we demonstrate that droplet confined electroplating, an ultra-high-resolution AM technique initially developed for metals as electrohydrodynamic redox printing (EHD-RP), allows not only the direct deposition of many metals, but also of metal-oxides. Particularly, we demonstrate that applying fundamental electrochemical principles in combination with on-the-fly switching of the deposited material allows for the direct co-deposition of metals, metal-hydroxides and -oxides. Our results exemplify the feasibility of leveraging simple water-based electrochemical concepts to produce intricate and multi-material structures at the nanoscale.

Figures (9)

• Figure 1: The two-electrode setup: from standard electroplating to droplet-confined electroplating. (a) Meniscus-confined electroplating confines the electrodeposition of metals into a small volume by using a liquid bridge between on a solvent-filled capillary and a conductive substrate. This process is still very similar to normal electroplating, as both electrodes are connected through an electrolyte. (b) Droplet-confined electroplating is similarly based on a solvent-filled capillary in close proximity to a conductive substrate Reiser2019Multi-metalScale. Applying a high electric potential between an electrode immersed within the capillary and the conductive substrate leads to an electrohydrodynamic ejection of an ion-containing droplet. This droplet then impacts on the substrate, the contained ions are reduced and the droplet evaporates. A repetition of this cycle leads to the formation of an out-of-plane metal nanostructure. Notably, no liquid connection is maintained between the electrolyte reservoir and the individual droplets Nydegger2024Droplet-ConfinedNanowiresMenetrey2024OnPrinting.
• Figure 2: Extended range of elements available for deposition in droplet confinement. Scanning electron micrographs and EDX analyses of elements from a wide range of standard electrode potentials indicated in the top right of each box (relative to the standard hydrogen electrode, SHE). Elements marked with an asterix refer to the used electrode metal (substrate material) and are not part of the deposited structure. Bold indicates non-carbonaceous elements present in significant quantities that are neither the deposited element nor the substrate. Elements shaded on yellows contained traces of counter-ions or complexing elements, namely chloride for the utilized chloride salts. Note that Au could only be ejected by applying a negative potential, presumably due to the negatively charged Au tetrachloride ion. Elements shaded in red allowed for a facile deposition, while elements shaded in purple necessitated a pH between 3 – 5 to enable a deposition of a pure structure. Elements shaded in blue contained a significant amount of oxygen. The prevalence of carbon does not correlate with the electrode potential of the deposited material.
• Figure 3: Analysis of deposited magnesium. (a) Dark field TEM reveals a very porous microstructure of deposited magnesium compounds. (b) Selected area electron diffraction reveals a ring-like pattern that fits well with the expected pattern for nanocrystalline cubic MgO (spacegroup F$m\overline{3}$m). (c) The presence of a nanocrystalline material is further supported by high resolution TEM.
• Figure 4: Deposition and chemical analysis of Ni-P-O system. Deposited pillar from a mixture of an aqueous NiCl2 salt solution and an aqueous hypophosphoric acid solution (H3PO2) with EDS showing presence of P (blue), O (red), Ni (pink), and Cl (yellow) in the final structure and the Si (green) of the substrate.
• Figure 5: Co-deposition of a metal (Cu) and a dielectric material (Mg(OH)2). (a-b) A two channel nozzle as utilized to demonstrate the multi-material-classes deposition. One channel is filled with 1 mM CuSO4 and the other with 1 mM MgCl2. Best deposition results were obtained for orifice sizes around 160 nm.(c) EDX analysis of a Cu and Mg(OH)2 pad indicates that the materials can be deposited separately from the same nozzle with almost no intermixing (a very small Mg signal is observed in the Cu pads). (d) Low magnification image of a 5 by 5 µ m Cu pad on top of 10 by 10µ m magnesium hydroxide pad. The inset is shown in the EDX maps. The spatial distribution of Cu, Mg, and O indicates that Cu is only present in the square where it was deposited (the square at the top left in the Cu image originates from a shift in the position of the substrate during printing) and that Mg is mostly found in the larger pad surrounding the Cu. (e) Cross-section of a Cu-Mg(OH)2-Cu multi-layered structure. The first Cu layer delaminated from the Au substrate. Both Cu layers appear much brighter than the porous Mg(OH)2. Further, charging of the Mg(OH)2 layer can be observed as a brighter section within the layer.