To realize high density 3D packaging, various types of interposer including through vias are developed. Although the interposer should have a high wiring density not only horizontally but also vertically, the ability of the conventional interposers to provide high density through vias with a low cost is still quite limited. Copper-filled anodic aluminum oxide has been studied as an alternative interposer material. Stable electrical connection was confirmed with four-wiringlayer substrates. High density vias as fine as 35 μm pitch were realized as a ground-surrounded structure. This coaxial-like via structure was remarkably effective for reducing harmful noise that tends to increase with the increased via density required for high performance systems.

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