Periodic Reporting for period 2 - Teraboard (High density scalable optically interconnected Tb/s Board) Reporting period: 2017-06-01 to 2019-11-30 Summary of the context and overall objectives of the project TERABOARD proposes the demonstration of a scalable, low power, low cost photonic technology to sustain the continuous increase of bandwidth density by leveraging on combination of scalability and low energy consumption. TERABOARD is a complete solution for scalable low-energy optical interconnections to be used in multiple application scenarios, ranging from intra-board through intra-data center communication, such as high-speed switch/router line cards, baseband processing units in 5G radio base stations, and next generation data center multi-server blades. From the system point of view, TERABOARD is a new technology that enables very large aggregated bandwidth density (Tb/s/cm2) on board. Thus, the number of operations in a single board are increased, instead of achieving the same number of operations in many boards or in a rack. The most relevant advantage results in avoiding power supplying of multiple boards. The concentration of a large number of operations in a single board leads to a radical system innovation, reduction of total energy cost and reduction of hardware size and cost. The innovative solutions proposed by TERABOARD consist of developing advanced intra-board and edge interfaces, with ultra-high density and scalability in bandwidth, low insertion loss and low energy consumption. The target of energy cost per channel is 2.5 pJ/bit, with a manufacturing cost of 0.1 $/Gb/s in volumes. These indicators are 10 times better with respect to the commercial state of the art. The intra-board communication will be a novel concept based on a 3D passive interconnection platform with no intersections and no need of Wavelength Division Multiplexing (WDM). To reduce the overall power consumption of a data center, intra-rack communications will avoid the use of WDM, cutting the fibers cost and the footprint needed by fiber connectors. In both intra-board and intra-rack communications, TERABOARD will demonstrate a 10-times reduction of required power with respect to present commercial solutions. The structure of TERABOARD project was chosen to strongly reinforce the cooperation among several research institutions and large companies, covering the full value chain from R&D to industrial product manufacturing to system applications. TERABOARD will directly contribute to strengthening European telecommunications capability and R&D pushing CMOS photonics integration density on a chip beyond the state of the art, in order to give a strategic benefit both at component and system levels. Thus, TERABOARD will guarantee not only support of competitiveness of European telecom industry at the technological level via strengthening its manufacturing base in photonics, but also will widen market opportunities in communication for data center arenas. TERABOARD will enable European telecom industry to stay at the forefront of electronics and photonics development and applications. Furthermore, the higher energy efficiency will be the main point toward the reduction of CO2 emissions. The reduction in power consumption enabled by the TERABOARD technology is achieved due to the lower power required to switch and keep on the networking apparatus, as well as lower power required for the cooling. The resulting 10-times reduction in power consumption will have a significant impact in reducing CO2 emissions. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far TERABOARD has solved a number of challenges regarding high bandwidth density communications in a high confinement scenario, such as advanced datacom systems in data centers, as required by Ericsson, STM and Nokia. Silicon photonics active devices, developed by imec, were co-integrated with 3D-IC modules such as through-silicon vias (TSVs) of 10 µm diameter and 100 µm depth and 50 µm-pitch micro-bumps at wafer scale, opening the path for high density and low parasitic stacking of electrical and silicon photonics dies, to enable significantly improved high-speed interconnections compared to wire-bonding. The co-packaging of electrical drivers and TIAs with the silicon photonics SiGe EAM (electro absorption modulator) and/or Si microring modulators and Ge detectors has demonstrated very high performance at data rates up to 70 Gb/s for EAMs and 80 Gb/s and even 90 Gb/s for Ge waveguide detectors, with an overall energy cost of the transceiver of 3 pJ/bit for each electro-optical port (electrical and optical). The results have been also tested for typical data center distances of 3 km with almost unchanged performances at high data rate. On the demo board driven by an FPGA, error free communications at the receivers were tested by Ericsson and CNIT up to 50 Gb/s per channel. Important improvements have been accomplished on laser integration over the silicon photonic chips by means of a novel stamping technology (transfer printing). Considering that a large count of optical interconnects was one of the goals of the project, two further main technical advances have been reported. A low profile (1 mm) and low loss (1.7 dB) twelve-fiber optical connector, released by CNIT with CNR and UPV, directly mounted on the silicon photonics circuit has been demonstrated. A passive optical routing system based on an optical multilayer redistribution architecture was also shown. This latest system by CNIT is a stack, assembled by UPV, of at least two silica planar lightwave waveguide layers connected through vertical optical VIAs, developed by CNR by means of an innovative technology for the fabrication of low loss femtosecond laser written vertical waveguides. With the technology of TERABOARD, many optical links can be routed without resorting to complicated and lossy WDM systems requiring laser wavelength selection and control. All the results have been disseminated by EPIC in many events. TERABOARD has found immediate development demand by one of the partner companies, thanks to the high bandwidth density transceiver achievements and the promising resolution of connectivity limitations. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) TERABOARD is on the way of achieving progress which clearly goes beyond the state of the art. This is going to happen with the 3D passive interconnection platform which is one main objective of the project, and with the highly efficient silicon photonic components and electronic integrated circuits which are being developed within the project. TERABOARD is pushing the photonics integrated circuits based on silicon photonics technology toward the new frontier of 3D stacking to implement optical redistribution layers, suited for cost-effective volume manufacturing on semiconductor production lines merged with dielectric-based 3D photonic integration platforms. The proposed 3D interconnection technology will support radically new data center architectures requiring a reduced number of boards and backplanes, thus reducing the overall data center size and cost. This second challenge will reinforce the European industrial technology leadership through a relevant increase of market presence in high-bitrate optical communications for emerging exascale cloud data center, wireless and HPC applications. These two targets, once reached, will pave the way to Pb/s network throughput, enabled by scalable Tb/s board interfaces for optical intra-data centre transmission, and will let Europe to be more efficient and digital. The silica-based interconnection board, Starboard, between four processors and a switch
