(Electrodeposition Division Research Award) The Scanning Bipolar Cell: Design Principles for Patterning of Diverse Metals without Contact to the Substrate
Authors: Braun Trevor M , Schwartz Daniel T. 
Published: February 2020

Bipolar electrochemistry has recently been utilized for many new electrochemical applications from electroanalytics to electrodeposition (1). Bipolar electrochemistry is defined as spatially segregated, equal and opposite reduction and oxidation on an electrically-floating conductor. Passage of current in an electrochemical cell produces an ohmic potential drop through the electrolyte solution. When this potential drop is substantial, and a conductor is in that potential gradient, the path of least resistance for current flow can be through the conductor, inducing bipolar electrochemical reactions.
Here we describe how to transform the electrodeposition method we call “Electrochemical Printing” (EcP) so it can function as a scanning bipolar cell. EcP uses a microjet cell configuration with an upstream anode and the substrate to be patterned is the cathode. Localized electrodeposition occurs when the ohmic potential drop is large compared to charge transfer overpotential, i.e. when there is a small Wagner number. Our existing EcP tool can serve as a scanning bipolar cell (Figure 1a attached) when, instead of using the substrate as a cathode, it is allowed to float, and a new “feeder” cathode is placed distant from the microjet. Ohmic potential drop beneath the microjet induces bipolar electrochemistry. With carefully engineered electrolytes, localized and controllable electrodeposition occurs on the floating substrate, without direct electrical contact.
The concept for the scanning bipolar cell (SBC) was validated with the highly reversible copper chemistry, performing localized deposition beneath the nozzle while a copper substrate oxidized in the far-field (2). Figure 1b shows an optical micrograph of the copper on copper system, patterned with the SBC operated over a range of fly heights and applied currents. Secondary current distribution computations were used to determine the relative current flow pathways through the electrolyte and extent of bipolar electrochemistry through the substrate.
We describe the electrolyte and operating principles for patterning a wide range of materials with the SBC. Our design guidelines depend on the nobility, kinetics, and reversibility of the material deposited, and importantly, tailoring the far-field oxidation chemistry redox potential to that of the species being deposited. Figure 1c shows an optical micrograph of nickel deposits patterned on a gold substrate at varying tool operating conditions. Here, the oxidation chemistry to complete the couple on the floating substrate is ascorbic acid. In this system, both nickel reduction and ascorbic acid oxidation behave irreversibly.
For reversible materials, such as copper, any previously patterned copper material can be effectively erased during subsequent patterning, since the equal and opposite oxidation can be metal etching on the floating substrate. This challenge is overcome by designing pseudo-stable electrolyte formulations, similar to electroless deposition baths, that protect the material from being further oxidized. Figure 1d depicts an optical micrograph of copper patterned on a gold substrate using ascorbic acid for the far-field oxidation chemistry. The electrolyte here was designed so that the equilibrium potential of ascorbic acid is marginally higher than that of copper, creating a slightly unstable formulation, but where ascorbic acid oxidation is preferential to copper etching, allowing the pattern to be retained.
The SBC scaling behavior has been studied, and is mainly limited by fabrication and motion control for the microjet. However, given the similarity of the SBC to scanning ion conductance microscopy, which has motion-control and pipette dimensions at sub-micron resolutions, we are confident practical sub-micron systems can be achieved (3).
We have shown that bipolar electrochemical patterning requires careful co-design of the electrolyte, substrate, and SBC, but it is possible to achieve patterning of diverse materials. The scanning bipolar cell represents a new class of remote control electrochemistry.

References

[1] G. Loget, et al., Accounts Chem. Res., 46, 2513 (2013)
[2] T.M. Braun, et al., J. Electrochem. Soc.,
162, D180 (2015)
[3] R. Thakar, et al., Analyst,
138, 5973 (2013).

Figure 1