Silicon Nanophotonic Packaging - Polymer Interface
The compliant polymer interface between standard optical fibers and nanophotonic waveguides is illustrated below. It includes a standard removable fiber connector interface, integrated flexible polymer waveguides, and a mechanically compliant extension interfacing with the nanophotonic die. A 12x1 MT fiber mechanical interface is shown here but other fiber connector standards could be used as well. In the MT standard, the large holes in the compliant interface receive matching metal pins from a fiber connector to provide self-alignment between fibers and mode-matched polymer waveguides.
The optical path in the compliant interface is drawn below. Standard single-mode optical fibers are butt-coupled to mode-matched polymer waveguides. The cross-section of the polymer waveguide is then adiabatically transformed from a fiber coupler to a higher confinement waveguide for routing. The routing in the compliant interface can be in principle arbitrary. A simple pitch conversion is shown here but more involved schemes are possible as well such as port shuffles and L-shaped connections with a 90 degree bend from fiber to the die. The polymer waveguides are then adiabatically coupled to nanophotonic waveguides on the photonic die.
The compliant interface can be assembled to nanophotonic dies using standard high-volume, low-cost microelectronic packaging equipment. To bridge the gap between the typical +/- 10 um accuracy of high-throughput pick and place tools and the required +/- 2 um accuracy for optimal optical perfromance, we use matching sets of lithographically defined self-alignment structures as shown in the cross-sectional sketch below. Alignment ridges are defined on the compliant interface with matching slanted grooves on the nanophotonic die.
Our first implementation of the compliant interface is shown below. It employs a flexible polymer ribbon with lithographically defined waveguides assembled to an injection-molded ferrule to create a standardized interface to single-mode optical fibers. A ferrule lid is also employed to symmetrize the thermo-mechanical properties at the fiber interface.
The ferrule and the polymer ribbon are assembled first. Matching self-alignment structures are precisely defined on the injection-molded ferrule and the lithographically patterned polymer ribbon. These allow for accurate alignment between ribbon and ferrule with low-accuracy, high-throughput assembly tools. Accurate alignment between the ferrule and the ribbon is required for accurate alignment between the polymer waveguides and optical fibers as the fiber connector alignment structures are located on the ferrule while the waveguides are on the ribbon.
Once the ferrule lid is placed and the facet of the ferrule/ribbon/lid assembly polished to MT specifications, the compliant interface is assembled to a nanophotonic die using standard microelectronic tools as part of a typical microelectronic chip packaging flow.
Pictures of the compliant interface and of an assembly to a silicon nanophotonic chip are shown below.
Our self-alignment strategy is central to the low-cost manufacturability of the compliant interface. Hence, this was our first focus of experimental demonstration. We have reported at ECTC 2014 that we achieve self-alignment to 1-2 um accuracy while starting from +/- 10 um purposeful misalignment. Hence, the self-alignment structures provide the required alignment for optimal optical performance (+/- 2 um) despite large initial misalignment (+/- 10 um) corresponding to the accuracy of standard high-throughput pick and place tools. Slides of our ECTC 2014 presentation are available in pdf format here.
Additionally, we have published a comprehensive feasibility study of the optical performance of the compliant interface. We have performed an exhaustive tolerance analysis using real-word fabrication and assembly uncertainties consistent with low-cost manufacturing. We found that the full optical loss from fiber to integrated silicon nanophotonic waveguide are expected to fall between 1.0 and 2.9 dB for all polarizations (depending on fabrication, assembly, and propagation loss). The optical bandwidth of the design is very large with a computed wavelength sensitivity not exceeding a 0.1 dB penalty over a 200 nm bandwidth centered at 1.55 um. The compliant interface feasibility study is available in free open-access here.
The first optical demonstration of the compliant interface was done without ferrule-to-ribbon assembly. Automated assembly between polymer ribbon and silicon chip was used with active alignment between the polymer ribbon and a standard MTP connector. This is shown above along with a typical transmission spectrum from fiber to silicon on the upper right. A wide bandwidth was found with an encouraging peak performance. However, an unexpected polarization dependent loss was measured at long wavelengths. Experimental trends and computational analysis suggested that the excess loss was mainly due to scattering at abrupt junctions near the chip edge. This can be addressed by tweaking our design and assembly. The detailed results were presented at OFC 2015 and are available in pdf format here.
In 2016, the optical performance was notably improved as shown on the spectral response in the lower right. A peak performance of 1.4 dB was reported at FiO 2016. The paper is available in pdf format here. These latest improvements stem from adjustments to polymer waveguide design, polymer fabrication, and assembly. The silicon side of the interface remained unchanged.
Our goal is the broad enablement of low-cost silicon photonic packaging. Please contact us if you would like to make use of any of the described technology.
- The polymer ribbons used in our work were fabricated by Asahi Glass Corporation under the technical leadership of Shotaro Takenobu.
- The ferrules used in our work were fabricated by Furukawa Electric under the technical leadership of Masato Shino.
Tymon Barwicz et al.