Russell A. Budd

160 Gb/s Bidirectional Polymer-Waveguide Board-Level Optical Interconnects Using CMOS-Based Transceivers

We have developed parallel optical interconnect technologies designed to support terabit/s-class chip-to-chip data transfer through polymer waveguides integrated in printed circuit boards (PCBs). The board-level links represent a highly integrated packaging approach based on a novel parallel optical module, or Optomodule, with 16 transmitter and 16 receiver channels. Optomodules with 16 Tx+16 Rx channels have been assembled and fully characterized, with transmitters operating at data rates up to 20 Gb/s for a 27-1 PRBS pattern. Receivers characterized as fiber-coupled 16-channel transmitter-to-receiver links operated error-free up to 15 Gb/s, providing a 240 Gb/s aggregate bidirectional data rate. The low-profile Optomodule is directly surface mounted to a circuit board using convention ball grid array (BGA) solder process. Optical coupling to a dense array of polymer waveguides fabricated on the PCB is facilitated by turning mirrors and lens arrays integrated into the optical PCB. A complete optical link between two Optomodules interconnected through 32 polymer waveguides has been demonstrated with each unidirectional link operating at 10 Gb/s achieving a 160 Gb/s bidirectional data rate. The full module-to-module link provides the fastest, widest, and most integrated multimode optical bus demonstrated to date.

Chip-to-chip optical interconnects

Terabus is based on a silicon-carrier interposer on an organic card containing 48 polymer waveguides. We have demonstrated 4times12 arrays of low power optical transmitters and receivers, operating up to 20 Gb/s and 14 Gb/s per channel respectively

Terabit/s-Class Optical PCB Links Incorporating 360-Gb/s Bidirectional 850 nm Parallel Optical Transceivers

We report here on the design, fabrication, and characterization of highly integrated parallel optical transceivers designed for Tb/s-class module-to-module data transfer through polymer waveguides integrated into optical printed circuit boards (o-PCBs). The parallel optical transceiver is based on a through-silicon-via silicon carrier as the platform for integration of 24-channel vertical cavity surface-emitting laser and photodiode arrays with CMOS ICs. The Si carrier also includes optical vias (holes) for optical access to conventional surface-emitting 850 nm optoelectronic devices. The 48-channel 3-D transceiver optochips are flip-chip soldered to organic carriers to form transceiver optomodules. Fully functional optomodules with 24 transmitter + 24 receiver channels were assembled and characterized with transmitters operating up to 20 Gb/s/ch and receivers up to 15 Gb/s/ch. At 15 Gb/s, the 48-channel optomodules provide a bidirectional aggregate bandwidth of 360 Gb/s. In addition, o-PCBs have been developed using a 48-channel flex waveguide assembly attached to FR4 electronic boards. Incorporation of waveguide turning mirrors and lens arrays facilitates optical coupling to/from the o-PCB. Assembly of optomodules to the o-PCB using a ball grid array process provides both electrical and optical interconnections. An initial demonstration of the full module-to-module optical link achieved >; 20 bidirectional links at 10 Gb/s. At 15 Gb/s, operation at a bit error ratio of <; 10- 12 was demonstrated for 15 channels in each direction, realizing a record o-PCB link with a 225 Gb/s bidirectional aggregate data rate.

Silicon Carrier with Deep Through-Vias, Fine Pitch Wiring and Through Cavity for Parallel Optical Transceiver

The design, fabrication, assembly and characterization of a novel silicon carrier package used for enabling a Tb/s parallel optical transceiver is reported. Electrical through-vias, high speed wiring and a through cavity for housing optoelectronic (OE) devices are critical features of the silicon carrier that allow high density integration of optical and electrical components on a single substrate, resulting in a small form factor system that is capable of meeting high bandwidth requirements of large computing systems. A novel hierarchical approach involving eutectic AuSn and SnPb solder systems and flip chip bonding technology is used to assemble the transceiver module. The optical system used for coupling light from the OE devices to waveguides is based on a relay lens that is integrated into the OE array. The measurement and model for alignment tolerance analysis showed constant coupling efficiency from the OE to waveguide over a range of plusmn 10 mum, giving an excellent margin for alignment. Electrical simulations and measurement of silicon carrier through-vias showed an insertion loss of better than 1 dB at 20 GHz. Simulations and measurements also exhibited an attenuation of 4.3 dB/cm at 20 GHz for high speed wiring on the silicon carrier, which was adequate for 20 Gbps data transmission over a channel length of 7 mm

Application of the aerial image measurement system (AIMS)TM to the analysis of binary mask imaging and resolution enhancement techniques

The newly developed Aerial Image Measurement System (AIMSTM*) was used to quantify the lithographic benefits of several resolution enhancement techniques as compared to standard binary mask imaging. This system, a microscope based stepper emulator, permits rapid characterization of mask images from both binary and phase shifted mask (PSM) patterns at multiple focal planes. The resultant images are captured digitally with a CCD camera and analyzed using an exposure-defocus tree technique to quantify the depth-of-focus as a function of exposure latitude. The AIMS is used to extract both phase and transmission errors from captured aerial images of all the masks evaluated. AIMS results are compared to wafer electrical linewidth data. A 0.5 numerical aperture (NA) DUV stepper was used with a partial coherence of 0.6 combined with IBM APEX-E resist process. Collected data were analyzed using techniques identical to the AIMS analysis, allowing for a high level of consistency. Comparative data focused on binary mask imaging for the verification of the AIMS results. Trends associated with feature sizes and types are discussed.

New tool for phase-shift mask evaluation: the stepper equipment aerial image measurement system--AIMS

The aerial image measurement system is an optical system for measurements on phase shift masks under chosen stepper characteristics of NA, sigma, wavelength and depth of focus. The present tool operates at I-line or DUV (248 nm) and commonly 5 or 6 inch reticles can be handled. The image obtained is optically equivalent to that incident on resist, but is highly magnified so that it can be recorded using an UV CCD camera. Typically, features of interest are recorded as a through focus series; image intensity is digitized and may be analyzed in a variety of ways so as to produce intensity contours or profiles. Combined with simple models for predicting resist behavior a great deal of information may be obtained on the expected printing performance of a given reticle as a function of intensity and depth of focus prior to actual resist tests.

Silicon Photonic Switch Fabrics in Computer Communications Systems

We discuss silicon photonic switch fabric designs that target data-intensive computing networks, reviewing recent results, and projecting future performance goals. We analyze the achievements of demonstrated hardware in terms of switching time, footprint, crosstalk, and power consumption, concluding that the most crucial metric to improve upon is net loss. We propose integrating semiconductor optical amplifiers into the switch fabric using either flip-chip or wafer-bonding technology, and investigate its potential merits alongside several challenges in implementation. Furthermore, we explore the dominant causes of crosstalk, and discuss manners for reducing it. We perform switch simulations that project a 7-dB reduction in crosstalk, when using a push-pull, rather than a single-ended phase shifter drive scheme. We also evaluate crosstalk effects on transmission performance using a full-link model that incorporates multiple crosstalk-accumulating photonic switch hops. The study demonstrates the degree to which crosstalk may degrade signal integrity after just a few occurrences. Finally, a comparison of four topologies highlights tradeoffs in physical-layer design and scheduling complexity, illustrating the scales that may be accomplished with the simplest topologies, and the device improvements required to achieve the more robust architectures.

160-Gb/s Bidirectional Parallel Optical Transceiver Module for Board-Level Interconnects Using a Single-Chip CMOS IC

We report here on the design, fabrication and high-speed performance of a novel parallel optical module with sixteen 10-Gb/s transmitter and receiver channels for a 160-Gb/s bidirectional aggregate data rate. The module utilizes a single-chip CMOS optical transceiver containing both transmitter and receiver circuits. 16-channel high-speed photodiode (PD) and VCSEL arrays are flip-chip attached to the low-power CMOS IC. The substrate emitting/illuminated VCSEL and PD arrays operate at 985 nm and include collimating lenses integrated into the backside of the substrate. The IC-OE assembly is then flip-chip attached to a high density organic package forming the transceiver optical module. The exclusive use of flip-chip packaging for both the IC-to-optoelectronic (OE) devices and for the IC-to-organic package minimizes the module footprint and associated packaging parasitics. The OE-on-IC assembly achieves a high area efficiency of 9.4 Gb/s/mm2 (Schow et al., 2007). The complete organic carrier transceiver package provides a low-cost, low-profile module similar to a conventional chip-carrier that can be directly surface mounted to a circuit board using a conventional BGA solder process. SLC transceiver modules with transmitter and receiver OE-IC arrays were assembled and characterized. Operation of all 16 transmitters in the transceiver module was demonstrated at data rates >10 Gb/s. Similarly, all 16 receiver channels operated error-free at >10 Gb/s. The receiver eye-diagrams were generated using a second transceiver source and therefore constitute a full transceiver optical link.

Terabus: a chip-to-chip parallel optical interconnect

Terabus is based on a chip-like optoelectronic packaging structure (Optochip) assembled directly onto an organic card with integrated waveguides (Optocard). To-date, Terabus has demonstrated 4times12-array optical transmitters and receivers operating up to 20 Gb/s and 14 Gb/s per channel

Characterization of parallel optical-interconnect waveguides integrated on a printed circuit board

The development of optical interconnects in printed circuit boards (PCBs) is driven by the increasing bandwidth requirements in servers, supercomputers and switch routers. At higher data rates, electrical connections exhibit an increase in crosstalk and attenuation; which limits channel density and leads to high power dissipation. Optical interconnects may overcome these drawbacks, although open questions still need to be resolved. We have realized multimode acrylate-polymer-based waveguides on PCBs that have propagation losses below 0.04 dB/cm at a wavelength of 850 nm and 0.12 dB/cm at 980 nm. Transmission measurements at a data rate of 12.5 Gb/s over a 1-m-long waveguide show good eye openings, independent of the incoupling conditions. In the interconnect system, the transmitter and receiver arrays are flip-chip-positioned on the top of the board with turning mirrors to redirect the light. The coupling concept is based on the collimated-beam approach with microlenses in front of the waveguides and the optoelectronic components. As we aim for large two-dimensional waveguide arrays, optical crosstalk is an important parameter to be understood. Accordingly, we have measured optical crosstalk for a linear array of 12 optical channels at a pitch of 250 um. The influence of misalignment at the transmitter and the receiver side on optical crosstalk will be presented as a function of the distance between waveguide and transmitter/receiver.