An interface substrate is disclosed which includes an interposer having through-semiconductor vias. An upper and a lower organic substrate are further built around the interposer. The disclosed interface substrate enables the continued use of low cost and widely deployed organic substrates for semiconductor packages while providing several advantages. The separation of the organic substrate into upper and lower substrates enables the cost effective matching of fabrication equipment. By providing an opening in one of the organic substrates, one or more semiconductor dies may be attached to exposed interconnect pads coupled to through-semiconductor vias of the interposer, enabling the use of flip chips with high-density microbump arrays and the accommodation of dies with varied bump pitches. By providing the opening specifically in the upper organic substrate, a package-on-package structure with optimized height may also be provided.