Silicon Interposer For Compact Right Angle Coupling Of Flip Chip Vcsel And Detector Arrays To Multi-Facet Endface Optical Fibers
Authors: Terry Bowen, Marcel Buijs, Jeroen Duis, Mirko Schurink Company: Tyco Electronics and DCD Date Published: 1/18/2011
Pan Pacific Symposium
Abstract: Silicon wafer-scale processing promises to provide a manufacturable approach for high volume, low cost, opto-electronic packaging. A silicon interposer with precise mechanical features is presented which accurately positions optical fibers with laser shaped fiber endfaces to Vertical Cavity Surface Emitting Laser (VCSEL) and optical photodetector array die. These optical array die and corresponding electrical driver and receiver array die are designed to be flip-chip mounted onto the top surface of silicon interposer sites. The electrical and optical die are passively aligned and electrically interconnected by simultaneous reflow of solder pads used for the flip chip mounting. Input and output electrical traces are routed on the silicon interposer top surface and transferred to the interposer back surface by through wafer vias. After dicing the populated silicon interposer sites from the wafer, termination is achieved with optical fibers that feature a multi-facet laser shaped endface. This optical fiber termination step also features passive alignment using precision placement of v-grooves for the optical fibers and a set of flip chip Au Sn solder pads formed on the top surface of the silicon interposer. The VCSEL and photodetector arrays have a matching set of solder pads that are precisely located relative to optical apertures on each die. When the wafer-scale solder reflow process is performed, surface tension of the molten solder pulls the two pad arrays into precise alignment. This places apertures of the optical devices into an aligned position over the v-grooves that receive the optical fibers. The result is a right angle optical coupling arrangement, which has a very compact structure. The optical coupling efficiency is enhanced by laser forming a multi-facet mirror directly on the endface of the optical fibers. This is achieved using a pulsed CO2 laser, which is programmed to cut the desired shape onto the fiber endface, and application of a reflective coating. One facet of the shaped fiber endface is allocated to provide a mechanical stop surface. This stop surface properly positions the fiber endface mirror relative to the VCSEL / detector optical apertures when it physically contacts the end facet of the Si interposer v-groove during fiber termination. The opposite end of the v-groove is widened to accommodate the coating of the optical fiber in order to provide a strain relief where the fiber exits the interposer. Finally, electrical termination is made to the back side of the silicon interposer using a high speed flex circuit. The result is a compact high speed Chip-to-World opto-electronic interconnect designed for greater than 10 Gbps operation giving optical coupling efficiency of less than 1 dB loss. Optical simulation using Zemax® is used to calculate coupling efficiency with respect to manufacturing tolerances.