(352h) Remote Control Electrodeposition: Design Criteria for Patterning on Substrates without Direct Electrical Connections
More recently, our lab modified the EcP system to operate in a bipolar manner, enabling direct-write electrodeposition and patterning without needing direct electrical connections to the substrate . This configuration, called a scanning bipolar cell (SBC), is advantageous for direct electrodeposition on surfaces that are difficult to electrically 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 conductive substrate. In the SBC, reduction of metal ions to metal occurs beneath a rastering microjet nozzle and oxidation of an electron-donating species takes place across the far-field area of the conductive substrate. The constraint of connecting to the substrate in traditional electroplating techniques is thus eliminated, albeit through more intricate electrolyte design.
In this presentation, we outline the design guidelines for bipolar electrodeposition of metals across a wide range of nobilities, and discuss how the coupling of kinetic reversibility, thermodynamics of the bipolar pair, and ionic transport through the electrolyte affects the spatiotemporal stability of electrodeposited patterns . The kinetic reversibility and thermodynamic equilibrium potential of the reduction reaction dictates the selection of a suitable electron-donating oxidation chemistry. Metal deposition chemistries that are irreversible, such as nickel deposition, are paired with electron-donating species that have fast kinetics and a more positive equilibrium potential. This type of bipolar couple is thermodynamically uphill, and the reactions are driven by inputted current supplied by the SBC. For reversible metal deposition chemistries, such as silver deposition, previously patterned material can electrochemically erased during subsequent patterning. Being reversible, any silver metal deposited on the substrate can act as the equal and opposite oxidation chemistry for the bipolar reaction. This challenge is overcome by designing pseudo-stable baths, similar to those used in electroless deposition, which protect the deposited metal from oxidation. For silver deposition, Fe2+ oxidation to Fe3+ is used as the oxidation chemistry. This electrolyte is designed so the equilibrium potential for the Fe2+/Fe3+ couple is marginally less (about 20 mV) than that of the Ag+/Ag(s) couple, creating an environment where Fe2+ oxidation is preferential to Ag(s) 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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