Characterisation of ion transportation during electroplating of high aspect ratio microvias using megasonic agitation

This paper presents the characterisation of the flow streams of an electrolyte solution during the electroplating of high aspect ratio microvias using megasonic agitation. The electrolyte fluid flow without acoustic streaming is firstly examined using micro-particle imaging velocimetry (micro-PIV) to establish the velocity of the flow at the mouth and within the microvias. Secondly, the average acoustic pressure developed by the transducer used for the megasonic agitation of the solution is measured using a fibre optic hydrophone. The fibre optic hydrophone is also used to determine the pressure in the mouth of microvia in a test Printed Circuit Board (PCB). The experimental results are then used to validate the flow and acoustic models created to optimise the set of experimental parameters which ensure the successful electroplating of high aspect ratio microvias in a PCB. The lack of fluid flow and, hence, transport of ions is demonstrated in blind microvias. It is also proved that acoustic pressure directed into the via promotes ion transport.