(Invited) Design and Realisation of Ultrasonic Reactor for Maskless Microfabrication
Authors: Roy Sudipta 
Published: February 2020

Introduction

Despite the growing importance of electrodeposition for the synthesis of new materials, currently most plating processes remain reliant on traditional agitation methods [1]. Despite the success of eductors to deliver coatings at the nano-scale [2], the deployment of paddle cell for fabrication of magnetic sensors [3], use of fountain platers for high-speed wafer plating [4], and jetting systems for selective deposition [5], the search for agitation schemes has been limited.
However, two new disruptive ideas, mainly, electrochemical printing (ECP) [6] and mask-less microfabrication [7], which have potential for 2-D or 3-D fabrication of metallic structures, have shown the need for unconventional apparatus and flow control as a core need for the process to work. The former method required the adaptation and re-thinking of ink-jetting technology to selectively deposit metallic structures at a controlled rate [6]. The second, electrochemical mask-less microfabrication, often termed as EnFACE, required the close placement of anode and cathode (<500 mm) [7], which needed focused stirring within a narrow gap.
These developments illustrated that new approaches for agitating fluids for electrochemical processes were required, and that a deep understanding of fluid agitation was needed to design novel systems. Ultrasonic (US), agitation can provide a way forward for some of these new techniques [8]. For example, it is possible that US agitation using miniature probes could be used to pulsate fluids through small channels which enable jet flow. When the gap between electrodes is small there is also the possibility of focusing the flow within the gap by using a miniature US probe.

Scope of the Presentation

In this paper we describe the deployment of US to increase agitation in constrained spaces. In particular, here we describe the adaptations required, and the step by step approach taken to implement US stirring between two electrodes separated by a narrow gap. The presentation will describe the development of a bespoke electrochemical cell where a measurable small gap between anode and cathode could be generated. Since this geometry corresponds to a very simple case, it enables one to visualise the flow mechanisms existing within the narrow electrode gap. The cell incorporated an US horn which was located to focus the agitation within the gap.
The enhancement in stirring within the narrow gap was measured using the limiting current technique. The results of these experiments were used to develop mass transfer correlations which allowed interpretation of fluid flow mechanisms within the gap. The issues faced during the measurement of limiting current, and problems arising during our measurements will also be mentioned. Although the gap between two electrodes was usually in the mm scale, the results were tested to verify if they could be applied to sub-mm gaps.
Based on the laboratory scale understanding, agitation within a similar gap, seperated by longer electrodes (up to 10 times longer than the lab-scale system) was examined. This system consisted of an 18 L Ultrasonic tank, within which, electrodes were placed at the centre. The connectors to the electrodes as well as the inter-electrode gap was maintained via specially designed electrode holders. The US probes were lined outside the tank, and the tank was operated at 30 Hz. Limiting current experiments were performed at specially designed electrodes at 30, 40 and 60 WL-1 to develop mass transfer correlations. In all cases, it was found that US provided substantial stirring which could not be achieved by traditional agitation methods.

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