Micropattern transfer without photolithography of substrate :Ni electrodeposition using enface technology

Abstract

Since the standard photolithographic patterning technology possesses a number of sustainable issues, a “maskless” technology, Enface, has been proposed. Here, a patterned ‘tool’ placed opposite to the substrate within micrometre range is required. Etching or plating occurs by passing tailored current or voltage waveforms, provided that the electrolyte resistance is high. Enface is a resource efficient process as the use of chemicals is greatly reduced. This research project aimed to investigate the feasibility of Ni pattern transfer using Enface under stagnant conditions. It would be advantageous if Enface could be used for nickel deposition as it is a slow kinetic system and controlled by mixed mass transfer and kinetics which is a system where Enface has never been used before. An electrochemical cell has been specifically designed and an electrolyte was systemically developed as required by Enface. Polarisation experiments were carried out to determine applied current densities that would be used in galvanostatic plating experiments for pattern transfer of millimetre and micron scale features. Deposited features were comprehensively characterised to see the performance of the patterning process. Current distribution during the pattern transfer experiments was investigated by simulation and modelling using Elsy software. An electrolyte of 0.19 M nickel sulfamate was selected and shown to be capable of depositing nickel. Polarisation data from experiments in Enface system showed that each feature size requires a different applied current density. As expected, pattern transfers of metallic nickel were achieved for millimetre and micron scale features at a current efficiency of around 90 % with current spreading were observed. The deposited feature width broadens with increasing time and decreasing feature size. In addition, maximum thickness that could be achieved was around 0.5 μm due to entrapped gas bubbles leading to process termination. The gas bubbles were detrimental to the deposits resulting in a rough and inhomogeneous surface as well as photoresist degradation. Ultrasound agitation was shown to be capable of diminishing the effect of gas bubbles. However it requires an optimisation of applied power density to avoid negative effects of cavitation bubbles. The result of simulation showed a non-uniform current distribution across the feature width with the highest current density at the centre resulting in a bell-shaped surface profile which is in agreement with the experiments. However, the deposited shape evolution obtained from the experiments is consistently much better than those obtained from the simulation.