Design of an ultrasonic tank reactor for copper deposition at electrodes separated by a narrow gap

This work describes the design and testing of an ultrasonic reactor suitable for processes which require agitation within a narrow gap in a tank geometry. A maskless microfabrication process was used to validate the ultrasonic reactor design. This mask-less electrodeposition method requires the inter-electrode distance between the anode tool and the cathode substrate to be maintained at 300 μm, and sufficient stirring of the electrolyte by ultrasound agitation. A design was proposed allowing 74 mm x 105 mm size substrates to be mounted into an electrode holder and loaded into an 18 L ultrasonic reactor. Experiments were carried out to test the uniformity of the mass transfer within the narrow electrode gap at different locations on the substrate, and to validate the feasibility of a mask-less metal plating technique by depositing features of μm-scale. When increasing ultrasonic powers from 30 to 60 W L-1, increasing agitation was observed at the centre of the substrate, but not at its edges. A Sherwood number correlation showed developing turbulence within the narrow gap, even in the centre of the plate. Micron scale features were plated onto A7 substrates, but the deposited features were 2.5 times the original width. The work showed that sonic streaming can produce sufficient agitation in long sub millimetre channels which can be employed to overcome mass transfer limitations.