Characterization of Buffered Electrolytes for Nickel Electrochemical Printing

Abstract

Electrochemical printing ͑EcP͒ is a maskless solid free-form microfabrication process that locally electrodeposits materials in a nonuniform current distribution beneath a rastering microjet electrode. To achieve high spatial resolution, EcP electrolytes are formulated such that ohmic resistance is large compared to the kinetic resistance. Extraordinary electrodeposition rates are common ͑Ͼ1 A cm −2 ͒ because a microjet electrode placed very close ͑16–31 ␮m͒ to the substrate delivers high convective-diffusive mass transfer rates. As a result, conventional electrolytes are generally not well suited for EcP. We describe a high-throughput approach for formulating and optimizing electrolytes in EcP, using nickel as a model material that displays interesting complications arising from pH changes driven by the coevolution of hydrogen. Two acetate buffered nickel sulfate electrolytes ͑pH 3.7 and 4.8͒ are used to show the interplay of bulk pH and tool operating conditions on overall current efficiency and the formation of complex deposit topographies induced by the local formation of soluble nickel hydroxides. An electrochemical quartz crystal microbalance is used to quantify current efficiencies ͑up to 45%͒ at current densities between 2 and 8 A cm −2 . Dense nickel patterns with grain sizes of Ͻ100 nm were electrodeposited at 3.6 A cm −2 .