Abstract: Waveguides disposed in either an interposer layer or directly in the semiconductor package substrate may be used to transfer signals between semiconductor dies coupled to the semiconductor package. For example, inter-semiconductor die communications using mm-wave carrier signals launched into waveguides specifically tuned to optimize transmission parameters of such signals. The use of such high frequencies beneficially provides for reliable transmission of modulated high data rate signals with lower losses than conductive traces and less cross-talk. The use of mm-wave waveguides provides higher data transfer rates per bump for bump-limited dies as well as beneficially providing improved signal integrity even at such higher data transfer rates. Such mm-wave waveguides may be built directly into semiconductor package layers or may be incorporated into one or more interposed layers that are physically and communicably coupled between the semiconductor dies and the semiconductor package substrate.