Bipolar Electrodeposition: A Pathway for Direct-Write Patterning of Metals without Electrical Connections to the Substrate
Authors: Braun Trevor M , Schwartz Daniel T. 
Published: February 2020

Recent advances in additive manufacturing technologies have been primarily driven by the availability of simple, integrated software/hardware platforms capable of rendering computer aided design files into fabricated objects. These easy-to-use systems facilitate a wider range of users, allowing highly technological hardware (such as 3D-printing) to be used both by hobbyists and trained technicians [1]. Despite these advances in 3D-printing, a similar shift to fully software-reconfigurable electrodeposition-based prototyping has yet to emerge. The commercial electrodeposition patterning standard, through-mask plating, requires a mask and several deposition and etch steps to fully develop the pattern. More sophisticated electrochemical fabrication technology, such as EFAB or MICA, also need a series of deposition, planarization, and chemical etch steps to build 3D objects layer-by-layer. The reliance on masks and chemical etching limits the reconfigurability of these technologies. However, direct-write electrodeposition methods can bridge the advantages of electrodeposition-based fabrication to user friendly software reconfigurable additive manufacturing techniques. Microelectrodes and microjet nozzles have demonstrated local electrodeposition dictated by the geometry of the electrode or nozzle for direct-write patterning of materials [2-3]. Impinging jet electroplating was further expanded by our research group by implementing full software control of electrodeposition and mass transport conditions with a system called Electrochemical Printing (EcP) [4].
More recently, our lab modified the EcP system to operate in a bipolar manner, enabling direct-write electrodeposition and patterning without direct electrical connections to the substrate [5]. This configuration, which we call a scanning bipolar cell (SBC), is potentially advantageous for direct electrodeposition on surfaces that are difficult to connect to and allows for further automation of the software/hardware. Bipolar electrochemistry involves spatially segregated, equal and opposite oxidation and reduction reactions on a single substrate. In the SBC (shown schematically in Figure 1), reduction of metal ions to metal occurs beneath the microjet nozzle and oxidation of an electron-donating species in solution takes place across the far-field area of the conductive substrate. The constraint of connecting to the substrate in standard electroplating techniques is eliminated through more intricate electrolyte design.
In this presentation, we outline the design guidelines for bipolar electrodeposition of metals across a wide range of nobilities (Ni, Cu, Ag, and Au), and discuss how the kinetic reversibility affects the spatiotemporal stability of the electrodeposited patterns. Selecting a suitable electron-donating oxidation chemistry for a given reduction chemistry depends on the reversibility and thermodynamics of the reduction redox couple. Nickel deposition is mostly irreversible, and therefore an electron-donator such as ascorbic acid oxidation to dehydroascorbic acid with an equilibrium potential more positive than that of nickel is used. This bipolar pair is thermodynamically uphill and the reactions are driven by additional current supplied by the SBC. For reversible deposition chemistries, such as silver reduction, previously patterned material can be electrochemically erased during subsequent patterning, since silver etching can act as the equal and opposite oxidation chemistry. This challenge is overcome by designing pseudo-stable baths, similar to electroless formulations, which protect the metal from oxidation. For silver reduction, Fe2+ oxidation to Fe3+ is used as the far-field oxidation chemistry. The electrolyte is designed so the equilibrium potential for Fe2+/Fe3+ is marginally less than that of Ag+/Ag(s) (about 20mV), creating a system where Fe2+ oxidation is preferential to silver etching.
Electrodeposition manufacturing processes are unique for their ability to control vertical material resolution at the nanometer scale. Incorporating micropipettes and nanopipettes through direct-write electroplating with EcP or the SBC adds control of lateral resolution ideal for additive manufacturing and patterning. The highest value commercial application for metal patterning in this domain is in integrated circuit and printed circuit board manufacturing. Bipolar electrodeposition with the SBC offers a solution for automated direct-write patterning by eliminating the need to electrically connect to the multifaceted substrates in these industries. Further research in this area will explore scale-down of the SBC nozzles to sub-micron dimensions as well as the impact of substrate surface area on the spatiotemporal behavior of bipolar electrodeposition.

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Figure 1: Schematic of the key features of a scanning bipolar cell and regions where reduction and oxidation chemistries occur on the conductive substrate.

Figure 1