The influence of fluid dynamics on plating electrolyte for the successful production of blind micro-vias: Laboratory investigati - Sponsored Whitepaper

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Successful copper plating of blind micro-vias is very strongly dependant on the effective mass transport of copper ions into the vias. This mass transport limitation is demonstrated by a rough or even in extreme cases at high aspect ratio or high applied current densities, a burnt appearance of the copper deposit at the base of the via. The poor quality of deposited copper has an obvious impact on productivity and also on the feasibility of filling the blind micro-vias required for highest packaging density. This paper describes the methods used to evaluate solution flow in micro-vias carried out in controlled laboratory conditions. The subsequent successful implementation of improved solution flow in simulated production equipment and also in full scale production is shown. Results are included from horizontal and also vertical processing, in both cases using production equipment operating with reverse pulse plating and insoluble dimensionally stable anodes.

Introduction With the increase in interconnect density the use of blind micro-vias has become the essen- tial technology to achieve the required circuit line and space tolerances. This development has been particularly driven by the demands from mobile telecommunication applications. The technology is also being used in packaging applications where components are directly bonded to the micro vias, in this case the re- quirements are for blind micro-vias completely filled after copper plating. Currently equipment is producing circuits for mobile telecommunications applications using pulse plating with insoluble, dimensionally sta- ble anodes achieving good results in copper plating at up to 9 A/dm2 effective current densi- ties and with blind micro-vias of aspect ratio up to 1:0.75. This equipment is most effective when used as part of a horizontal production line. As a comparison, for packaging applica-

tions the effective current density used is nor- mally much lower, in the region of 1–2 A/dm2 and the processing is usually carried out in vertical processing equipment due to the line and space requirements in the range of 30 µm. However as the required packaging density in- creases, the via aspect ratio also increases so creating more difficulties in achieving good copper plating results. Essentially the deeper and narrower the via the more difficult it is to get the required minimum copper thickness of the required quality. Figure 1 shows the effect of a decrease in the blind micro-via diameter on the copper plating results in an extreme example of blind via as- pect ratio. The effect shown can be extrapo- lated to other more normal production cases, the vias all have the same depth, 160 µm but varying diameter as shown. The copper plating conditions were identical in each case.
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