Self-aligned chip-to-chip optical interconnections in ultra-thin 3D glass interposers

This paper presents the modeling, design and demonstration of a three-dimensional polymer waveguide (3D WG) that couples two optical through-package vias (TPVs) in a 3D ultra-thin glass interposer for chip-to-chip optical communications. Coupling of the device is enabled using positive and negative sloped, 45° total internal reflection (TIR) micro-mirrors. The simulated coupling efficiency is within 0.5 dB for 45±5°. A novel inclined UV photolithography process is proposed to fabricate the microstructures simultaneously with self-alignment. The alignment is inherent because it is resolved prior to inclined photolithography during the planar patterning of double-sided metallization layers. The new process is experimentally demonstrated using commercially available PCB manufacturing technologies. The measured alignment tolerance between the optical via and the polymer waveguide is within 2.5 um across the entire panel. Fifty micron tall polymer WGs at 20 ~ 60 um width with 45 degree entry and exit turning surfaces are fabricated on 150um thick glass substrate. Rounded waveguide sidewalls and inadequate adhesion are observed, which requires further process development to allow high quality optical measurements.