Electrodeposition of microstructures using a patterned anode

Abstract

In this paper we report the transfer of micro-scale patterns by electrodeposition on to substrates without requiring them to be coated with a photoresist mask. This approach makes uses of a patterned tool which is placed in close proximity to the substrate in an electrochemical reactor. With an appropriate choice of electrochemical parameters, electrodeposition can be confined to regions corresponding to the exposed regions of the tool. Experiments indicate that the electrodeposition of copper features on to conductive substrates is possible using this approach. Copper lines of 100 μm width have been successfully replicated, but with some increase in dimension due to current spreading. This effect can be minimised by reducing the inter-electrode gap and employing an electrolyte with a low conductivity. It is also demonstrated that the tool can be used to pattern multiple substrates.

Introduction

Electrochemical microfabrication techniques are routinely used in the manufacture of microsystem devices [1]. Broadly, these techniques can be classified as “additive” methods were electrodeposition is used to deposit material on to a substrate or “subtractive” where electrochemical etching is used to remove material. Additive processes are usually based on the technique of “through-mask” plating [2] while subtractive ones typically employ masked electrochemical etching [3]. In both of these approaches the substrate is patterned with a photoresist material which acts as a mask to define the areas to be etched or plated. More recently, techniques based on patterned electrodeposition using conformal masks have been developed [4], [5].

A wide range of other electrochemical techniques (including mask-less and direct-write processes) has also been developed to deposit micro- and nano-structures on metal and semiconductors substrates [6], [7], [8], [9], [10], [11]. Often these involve rastering a sharp electrode tool over the substrate, or using light or other forms of radiation to induce localised deposition [6], [7], [8]. Meniscus-confined electrodeposition of 3D microstructures has also been demonstrated [9]. These techniques are capable of high spatial resolution and accuracy, but many [6], [7], [8], [9] are essentially serial fabrication methods and are generally not suitable for volume production of micro-devices.

Roy [12], [13], [14], [15] has recently proposed an alternative electrochemical microfabrication technique known as “EnFACE” which can be used to etch micro-patterns on copper substrates. They used a tool patterned with a photoresist mask while the substrate remained fully exposed. The tool and substrate were mounted in opposite walls of a flow channel. By employing a very small gap (< 500 μm) and a suitable electrolyte, selective metal dissolution on the regions facing the exposed portions of the tool was achieved. In this manner, micro-scale patterns which were considerably smaller (i.e. 50–200 μm) than the electrode gap were transferred to the substrates.

This technique has the advantage that a single patterned cathode can be used to etch many substrates. This is contrast to conventional etching techniques where each anode substrate has to be individually patterned with a photoresist mask [3]. By minimising the demands for lithography, this technique has the potential of drastically reducing the cost of electrochemically micro-fabricated parts. It is also a parallel microfabrication method with corresponding advantages in cost and throughput. While EnFACE was originally developed for etching, patterned electrodeposition is also feasible.

The objective of this paper is to report the results of pattern transfer achieved by electrodeposition. In this study we report for the first time the electrodeposition of copper on to substrates using the EnFACE process and the electrochemical parameters required for good pattern transfer.