Bridging parallel electrodes by electrodeposition

: characterisation of nanoscale contacts using in-situ current measurements

Abstract

A new method was developed to study the roughness of an electrodeposited thin film in-situ, by monitoring the conductance across the top and bottom electrodes of a metal-insulator metal (MIM) pillar. The MIM pillar with exposed edges is fabricated and subsequently contacted in a customized 4-electrode electrochemical cell controlled by a bipotentiostat. While a constant bias voltage is applied between the two working electrodes, separated by a 50 nm thin dielectric layer, the current is measured during electrodeposition of a contact between them. For sufficiently small features, the current running between the two metals is dominant over the electrodeposition current and represents the tunnelling current and eventually the mesoscopic contact current. The change in conductance of the MIM structure during electrodeposition is indicative of the growth of the film. A rough film growth leads to step-like, quantized current transients typical of mesoscopic point contacts. We observe both quantum conductance steps and smaller steps in the tunnelling current due to surface reconstruction. On the other hand, a uniformly growing film leads to smooth, exponentially growing current transients due to the significant contribution of a tunnelling current before physical contact is made. We use this method to investigate the growth of Au thin films from a thiosulfate-sulfite bath, and the growth of Cu from a sulfate bath. By cycling the deposition and dissolution of these metals, using the changing inter-electrode conductance as feedback, we repeatedly grow nanoscale contacts. We also show the effect of common levelling additives for Cu electrodeposition on the measured conductance, such as chloride ions, polyethylene glycol (PEG) and benzotriazole (BTA). The addition of the strong levelling additive BTA shows a clear change in the shape of the measured conductance transient. Additionally, we have observed a significant dependence of the film growth on the bias voltage applied between the top and bottom metals of the MIM. A stronger bias is thought to lead to faster bridging of the two electrodes, and an increased roughness of the deposited film. We also studied the electropolymerization and transport properties of the conducting polymer polypyrrole in a similar 4-electrode cell, where the polymer bridges a gap of a few microns between the two working electrodes. We show the reversible doping and un-doping during oxidation and reduction of the polymer film, leading to large changes in conductance. We investigate the conductance of the polymer as a function of the electrochemical potential, both during cyclic voltammetry and potentiostatically. The growth characterization technique using the 4-electrode cell with MIM pillar provides a highly accessible platform to study the effect of additives on surface roughness in a wide range of electrodeposition solutions, and the real-time conductance measurement can be used as feedback for the fabrication of long nanogap structures.

Date of Award	21 Jun 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Walther Schwarzacher (Supervisor)