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Optical chip-to-chip communication is a promising technology that can mitigate some of the performance short-comings of electrical interconnections, especially bandwidth. Moreover, future high-performance chips are projected to drain hundreds of amperes of supply current. To this end, it is important to develop a high-density and high-performance integrated electrical and optical chip I/O interconnection technology. We describe sea of polymer pillars (or polymer pins), which enables the simultaneous batch fabrication of electrical and optical I/O interconnections at the wafer-level. The electrical and optical I/O interconnections are designed to be laterally compliant to minimize the stresses on the die's low-k dielectric as well as to maintain optical alignment between the coefficient of thermal expansion (CTE)-mismatched board and die during thermal cycling. We demonstrate the fabrication and mechanical performance of various size and aspect ratio electrical and optical polymer pillars. We also describe methods of fabricating polymer pillars with nonflat tip surface area for optical interconnection.