Analysis of throwing power for megasonic assisted electrodeposition of copper inside THVs

The deposition of increased volumes of Cu down an interconnect through-hole via (THV) of a Printed Circuit Board (PCB) is highly desirable for the fabrication of increasing component density and PCB stacks. A quality metric, called micro-throwing power, characterises the volume of Cu that can be deposited within a THV. In this paper, we analyse the influence of 1 ± 0.05 MHz megasonic (MS) assisted agitation applied to copper (Cu) electroplating baths on the micro-throwing ability of a standard, non-filling Cu electroplating solution. Our results indicate that megasonic agitation is shown to increase the Cu deposition volume within a THV by 45% for an increase of MS pressure from 225 W to 450 W, highlighting the significance of acoustic pressure as a key parameter to control MS THV plating volume. Bulk fluid flow rate within a 500 L plating tank is shown to increase by 150% due to Eckhart acoustic streaming mechanisms, compared to existing bath agitation techniques and panel movement. From MS plating experiments and COMSOL^TM finite element acoustic scattering simulations, transducer orientation is shown to influence plating performance, with higher-order acoustic resonant modes forming within THVs identified as the cause. Simulations indicate that higher potential acoustic energy was coupled into a 0.200 mm diameter THV cavity, width-to-length aspect ratio (ar): 8:1, than a larger cavity of diameter 0.475 mm, ar 3.4:1. The maximum acoustic energy coupled into THV cavity is observed for a wavefront propagating along the axis of the cavity entrance, indicating an ideal alignment for the MS plating setup.