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30 The PCB Magazine • June 2016 quirements can only be fulfilled by increasing the throwing power (TP) of the applied electro- less copper bath. This paper introduces two new electroless copper baths developed for IC substrate manu- facturing based on semi-additive process (SAP) technology (hereafter referred to as e'less cop- per IC) and HDI production (hereafter referred to as e'less copper HDI) and optimized for high throw into BMVs. An introduction to reliable throwing power measurement methods based on scanning electron microscope (SEM) is giv- en, followed by a compilation and discussion of key performance criteria for each application, namely throwing power, copper adhesion on the substrate, dry film adhesion and reliability. State of the Art Technologies and Future Challenges The state-of-the-art production technol- ogy for high-end IC substrates—characterized by the smallest L/S at the outer redistribution layer (RDL)—is the SAP technology in vertical application mode. Contrary to the intuitive meaning of the expression 'semi-additive pro- cess,' the technology is in fact still a subtractive build-up technology and the L/S resolution is limited by the differential etch process step that is applied to form the desired pattern and that comes along with an inherent line width reduc- tion. However, the differential etch is reduced by the usage of bare laminates (base materials without a copper clad) resulting in a reduced copper thickness that needs to be etched. Lead- ing IC substrate manufacturers are etching ap- proximately 1.0 µm electroless copper plus ad- ditional 1.0–2.0 µm safety margin because of the rough surface (R z ~ 2.0 µm) and achieve 9/12 µm L/S with acceptable yield in mass pro- duction. A further reduction in L/S require- ments below this 21 µm track pitch could be theoretically fulfilled in different ways: Firstly, a fully additive process (FAP) would make the differential etch step superfluous be- cause the pattern is created before copper is plated. Unfortunately, there is no mass produc- tion proven FAP technology established yet in the industry. Secondly, new manufacturing ap- proaches in development like laser embedded conductors (LEC) could contribute to increased wiring densities. Thirdly—and closest to mass production—the state of the art subtractive SAP technology and all involved process steps could be optimized for minimized differential etch. Imperative for this capability extension of the SAP technology is a further reduction of the elec- troless copper layer thickness on the surface of the build-up layer. The electroless copper layer thickness in the wedge of the BMV on the other hand is limited to a certain minimum because of conductivity and process safety requirements of the following development and via filling process steps. These contradicting requirements of the layer thickness on the surface and in the BMV can only be solved by an increased throw- ing power of the electroless copper bath. In this context, throwing power is generally defined as the ratio between the deposited electroless cop- per thickness in the BMV compared to that on the surface. The impact on L/S resolution of a reduced electroless copper surface thickness is illustrated in Figure 1. As can be seen in the upper part of the schematic drawing, the state of the art technology, simulated for targeted 5/5 µm L/S, faces an inevitable line reduction due to the differential etch of about 3 µm (1 µm electroless copper plus an additional safety margin of 2 µm due to the roughness of the sur- face). In order to increase the actual line width all relevant process steps need to be pushed to the limits. The electroless copper thickness must be reduced and new base materials, character- ized by a lower surface roughness compared to standard materials like ABF GX-92, are required. HIGH-THROW ELECTROLESS COPPER—NEW OPPORTUNITIES FOR IC SUBSTRATES AND HDI MANUFACTURING " These contradicting requirements of the layer thick- ness on the surface and in the BMV can only be solved by an increased throwing power of the electroless copper bath. "