120 Gb/s Optical Card-to-Card Interconnect Link Demonstrator with Embedded Waveguides

L. Dellmann1, C. Berger1, R. Beyeler1, R. Dangel1, M. Gmür4, R. Hamelin3, F. Horst1, T. Lamprecht1, N. Meier1, T. Morf1, S. Oggioni2, Mauro Spreafico2, R. Stevens3, B. J. Offrein1

1IBM Research GmbH, Zurich Research Laboratory, 8803 Rüschlikon, Switzerland Email: lau@zurich.ibm.com

2IBM Italy, EMEA Packaging Development, 20059 Vimercate, Italy 3INTEXYS photonics, 31240 L'Union, France

4Varioprint AG, 9410 Heiden, Switzerland

Abstract

We report on a card-to-card optical interconnect demonstrator with passively aligned butt-coupled optoelectronic modules onto waveguides embedded into the printed circuit board (PCB). After describing selected building blocks, we will present experimental results obtained with the demonstrator hardware consisting of a parallel 12-channel at 10 Gb/s (120 Gb/s) optical card-to-card link.

Optical interconnects The increasing performance of microprocessors leads to

higher bandwidth requirements for the data flow to and from the processor. Today, all signaling on a PCB is performed electrically, using copper lines that are integrated in the board. However, issues such as propagation loss and inter-channel crosstalk limit the scalability of electrical interconnects to ever higher bandwidth densities. Optical interconnects feature a higher bandwidth × length product, are more power-efficient and enable a higher bandwidth density than electrical interconnects do. It is especially the higher bandwidth density of optics that drives the research on optical PCB technology for inter-system interconnects [1] [2].

Several subsystems have to be developed to enable the integration of optical communication links in a PCB [3]: Multimode polymer waveguides route the optical signals in the board; optical connectors are required to couple the signals to other boards or to a fiber network, and transceiver elements perform the conversion of the signals from the electrical to the optical domain and vice versa. One major obstacle for the integration of optical elements, such as optical interconnects, into standard electronic systems, is the large gap in positioning tolerances between the optics and the electronics world. In standard PCB processing, manufacturing tolerances of as much as 100 µm, or even 200 µm for large backplanes, are acceptable. For our choice of using multimode optics, however the tolerances for mutual alignment between building blocks are only 5-10 µm. The tolerances for optics are more than one order of magnitude smaller than those for standard PCB technology.

The next sections will present the different building blocks needed to achieve a parallel card-to-card link based on integrated waveguides and passively aligned connectors and transceiver elements.

Polymer waveguide technology A key advantage of optical-polymer-based processes is the

possibility of using large-area, low-cost fabrication of optical

waveguides through simple deposition techniques and low-temperature processing.

However, for a successful adoption in the field of optical interconnects such optical polymers have to fulfill stringent requirements such as low intrinsic optical absorption enabling lowest propagation losses < 0.05 dB/cm (@ 850nm) as well as process compatibility with PCB manufacturing.

The process of the polyurethane-acrylate-based polymer we selected is described in Figure 1. First the bottom cladding layer is deposited on FR4 by doctor-blading and polymerized by UV flood exposure. Next the core layer is deposited by doctor-blading and photo-patterned by UV-laser direct writing using a high precision x-y-z table (Figure 2). Then a wet developing step of the core enables the patterning of the waveguide structures. Finally the top cladding layer is deposited by doctor-blading and polymerized by UV flood exposure. This process allows large-scale processing, i.e. board-size, of substrates up to 50 cm x 50 cm.

Lowercladding

Corelayer

Patterning

Uppercladding

Figure 1: Process overview for waveguide fabrication.

Figure 2: High-precision x-y-z table used for direct UV laser writing of the waveguides for PCBs of up to 50 cm by 50 cm.

Waveguides (Figure 3) with various core dimensions (e.g. 50×50 µm2 and 35×35 µm2), various pitches (e.g. 250 µm and 100 µm) and up to four layers have been fabricated. The waveguide propagation losses, which belong to the most critical material properties, were determined by the common

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waveguide cut-back method as well as by more accurate transmission measurements of up to 100-cm-long waveguide spirals. Typical attenuations of our waveguides were between 0.03 and 0.05 dB/cm at a wavelength of 850 nm [4].

Figure 3: Cross section of polymer waveguide

Passive alignment for optical elements A successful implementation of optics into PCBs requires

a precise passive alignment of optical elements relative to the optical waveguides in the board. Our concept [5] enables an accurate and simple plug-in of any kind of element, in particular of optical and opto-electronic elements, into a PCB. The concept is based on established PCB manufacturing processes, which is crucial for developing a low-cost optical interconnect technology. Markers structured into a copper layer during manufacturing are used as position reference for the polymer waveguide fabrication and for forming mechanical alignment features.

The fabrication of the optical PCB starts with a substrate, which can be just a thin foil or a complete multilayer FR4 board with various electrical traces inside. On the top of this substrate, there is a continuous layer of copper, onto which a thin RCC (resin-coated copper) foil is laminated. In the top copper layer of the RCC foil, the copper alignment markers are defined (Figure 4).

Figure 4: Substrate with marker structure defined in top layer of RCC foil.

On top of the substrate containing the markers, the optical waveguide layers (Figure 5) are fabricated using the process described above (Figure 1). The positions of the optical reference markers are measured using a camera system and image-recognition software. Based on these positions, the proper position and adjustment of the waveguide core pattern are determined, after which the pattern is exposed into the waveguide core layer using the laser-writing tool.

Figure 5: Waveguides built on top of substrate using optical reference structures in markers for positioning.

On top of the waveguide layer, the top FR4 multilayer

board is laminated, thereby completely burying the waveguide layers and marker patterns (Figure 6).

Figure 6: Waveguides buried in board by adding top FR4 multilayer.

To retrieve the markers, a first milling step removes the

board material below the markers. The milling depth is controlled by noting the moment of electrical contact between the milling tool and the continuous copper layer below the RCC foil. The milling then continues for a few more micrometers to remove the copper and stops in the resin of the RCC foil (Figure 7).

Figure 7: Removal of substrate below markers by milling until electrical contact occurs on continuous copper below RCC.

The remainder of the resin at the bottom of the markers is

ablated using a drilling laser (Figure 8). This laser stops on

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the copper, but continues to drill through the mechanical XY reference opening in the marker so that at this position we obtain a reference slot. However, the position of this reference slot is not defined by the inaccurate laser drill, but rather by the opening in the marker, which is accurately positioned relatively to the optical reference structures.

Figure 8: Cleaning away of resin from marker using a drilling laser. The laser stops on the copper, but drilling continues through the mechanical XY reference opening.

After the laser-drilling step, the bottom of the marker

structure serves as mechanical Z-reference and the edge of the marker-defined slot as mechanical XY reference (Figure 9 and Figure 10).

Figure 9: Bottom of marker structure serves as mechanical Z-reference, the edge of marker-defined slot as mechanical XY reference.

Figure 10: Photograph of a set of four mechanical alignment markers after processing.

We typically use the passive-alignment system with an intermediate adapter that provides a link between the

mechanical alignment structures in the PCB and the optical elements.

We chose to base our adapter on the industry-standard MT-pin interface, which accepts any element conforming to this standard, be it transmitters, receivers, fiber ribbon cables, or connectors. The injection-molded MT adapter is shown in Figure 11 and Figure 12. At the corners of the adapter there are alignment studs that mate with the mechanical alignment slots in the copper markers on the PCB. The two MT pins to the left and right of the waveguide array provide the reference structure for the optical element or cable.

We measured a standard deviation on the order of 5 µm for the in-plane and out-of-plane misalignments of the MT pins with respect to the optical waveguides.

Figure 11: MT adapter and its mating site in the optical PCB

Figure 12: MT adapter passively aligned onto the optical PCB, waveguides illuminated from the back with white light.

12 x 10 Gb/s transceiver elements Both 12 channels transmitter and receiver modules are

based on an MT butt-coupling interface to couple the light into and out of the PCB waveguides. These modules have been developed in collaboration with Intexys Photonics, with the goals of being of low cost as well as of achieving high-yield manufacturability, compactness, and versatility.

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The basic concept [6] is compatible with mass production thanks to the use of collective manufacturing technologies and passive optical alignment, such as standard wafer-level processing and assembly technologies.

Each element of the optical module is integrated on a highly resistive silicon motherboard. The module consists of a 10-Gb/s 850-nm VCSEL (or photodiode) array, a driver (or TIA IC), an MT-fiber ferrule interface with protruding fibers, some passive components, and an LCP flex-cable for signal transmission and commands.

As already discussed in the section passive alignment, we chose the MT interface to leverage the development costs and be compatible with established standards. The optical interface is therefore based on the following principle: the light is coupled into an MT-fiber ferrule in which optical fibers have been inserted and guided through a hole etched through the silicon substrate. DRIE etching is used to process this hole, whose shape is designed such that a 125-µm cladding diameter fiber can be inserted with minimum tolerances.

Figure 13 describes the optical interface concepts, where the MT adapter is passively aligned onto the waveguides and serves as connector for the optical module. The light is directly injected into the waveguides by butt-coupling.

Optical PCB

Heatsink

MT adapter

MT fiber ferrule Si-carrier with OE

& driver/TIA

LCP electrical flex

LGA Connector

Mechanical clamp

Optical PCB

Heatsink

MT adapter

MT fiber ferrule Si-carrier with OE

& driver/TIA

LCP electrical flex

LGA Connector

Mechanical clamp

Figure 13: Schematic of the optical module passively aligned onto the embedded waveguides.

Signal integrity is one of the key issues for proper

operation. Particular attention is therefore paid to the entire RF transmission-line design. To reduce parasitics and to maintain impedance matching, the entire transmission line from the silicon motherboard to the connector on the PCB has been optimized using a 3D simulation approach. To optimize the computational task of simulating a long and wide cable with tiny 3D vertical structures concentrated in a specific area, the study of the complete transmission path has been split into three blocks: the PCB interface with the flex-cable, the flex-cable, and the silicon motherboard with the interface to the flex-cable. The individual blocks have been modeled and optimized independently (Figure 14), and then merged to obtain the complete link. The results of the entire model with optimized dimensions are shown in Figure 15. It exhibits a loss of –1.5 dB and –25 dB reflected up to 10 GHz, indicating low losses and excellent impedance matching.

Figure 14: Schematic of the 3D model used for the transmission-line optimization.

Figure 15: Calculated attenuation and return loss for the entire transmission lines.

The silicon motherboard is manufactured at a wafer level. First aluminum pads and lines are deposited, and then covered with an array of Under Bump Metallizations (UBM) with gold finish. Next indium is deposited onto the UBM pads by means of a lift-off process. In this case 15-µm-diameter indium bumps are processed. To place the fibers, a row of 12 holes is DRIE-etched through the motherboard, adding the mechanical-guiding functionality to the silicon bench. Then, the indium bumps are collectively reflown to form regular spheres. The VCSELs (or photodiodes) are hybridized according to the process described above. After this step the laser driver (or photodiode TIA) is stud-bumped onto the motherboard, and the wafer diced into individual chips. These chips are connected to an LCP polymer flex-cable using the gold stud-bump technique (Figure 16).

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VCSEL – Driveror

Photodiode - TIA

LCP flex-cable

Silicon bench

VCSEL – Driveror

Photodiode - TIA

LCP flex-cable

Silicon bench

Figure 16: Populated silicon motherboard connected to the LCP flex-cable (front and back view).

The next step is to insert the MT ferrule interface

automatically and passively into the etched hole. The MT ferrule interface consists of a standard 12-multimode fiber protruding from a standard MT ferrule. The protruding fiber length corresponds to the silicon motherboard thickness.

Finally the assembly is glued onto a heat-sink. Figure 17 shows an assembled 12-channel transmitter module with the optical MT interface and heat-sink sitting on a PCB. The two pins of the passively aligned MT connector into the PCB can be seen in front of the transmitter module.

Heat-sink

Optical MT interface

Passively alignedMT connector

protruding from PCB

Heat-sink

Optical MT interface

Passively alignedMT connector

protruding from PCB Figure 17: Assembled 12-channel transmitter module with the optical MT interface and heat-sink sitting on a PCB with embedded waveguides and a passively aligned MT connector.

120 Gb/s card-to-card link characterization The card-to-card link demonstrator (Figure 18) is based on

the hardware presented in the preceding sections. It consists of a 12-channel transmitter module on the transmitter PCB and a 12-channel receiver module on the receiver PCB. Both cards were optically connected using a standard 12-fiber bundle with MT connectors. The optical modules and the fiber bundle were inserted into the MT connector (Figure 11), which has been passively aligned in the PCBs.

Data transmission has been demonstrated up to 10 Gb/s with open eye diagrams (Figure 19). Preliminary coupling loss measurements show a total link loss of 9 dB (transmitter to waveguide coupling loss: 2 dB; polymer waveguide propagation loss: 2 × 15 cm × 0.04 dB/cm = 1.2 dB; polymer waveguide to fiber coupling loss: 2 dB; fiber to polymer waveguide coupling loss: 1 dB, and polymer waveguide to receiver coupling loss: 3 dB).

12 channels @ 10 Gb/s

1m fiberbundle

StandardMT connector

StandardMT connector

Txm

odule

Rx module

All components passively aligned

12 channels @ 10 Gb/s

1m fiberbundle

StandardMT connector

StandardMT connector

Txm

odule

Rx module

All components passively aligned Figure 18: Photograph of the card-to-card demonstrator, consisting of two PCBs with embedded waveguides connected with a fiber bundle. Both the transmitter and the receiver module were passively aligned onto the embedded wave-guides using the MT adaptors.

Channel 1

Channel 12

Channel 1

Channel 12 Figure 19: Eye diagrams at 10 Gb/s of the 12 card-to-card links.

Conclusions All building blocks needed for a 12 channels card-to-card

optical interconnect link have been built and successfully tested with data transmission up to 10 Gb/s. The integrated polymer waveguide technology allows to use large-area, low-cost fabrication through simple deposition techniques and low propagation losses (< 0.05 dB/cm). Our PCB-manufacturing-based passive alignment concept enables an accurate and simple plug-in of any kind of element, in particular of optical and opto-electronic elements, into a PCB.

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A crucial factor for the acceptance of optical intra-box interconnects is costs. Our approach tackles this issue by, for example, using passive alignment concepts and batch fabrication and assembly technologies. However, also the market volume will be an important aspect in bringing down the costs to an acceptable level. We address this by focusing on a generic technology platform for a wide range of applications.

Acknowledgments We thank our colleagues F. Yamada and Y. Taira of the

IBM Tokyo Research Laboratory for the collaboration on the injection-molded MT connector.

References [1] D. A. B. Miller, “Physical Reasons for Optical

Interconnection”, Int. J. Optoelectron., Vol. 11 (1997) pp. 155-168.

[2] C. Berger, “Optical Interconnect Demonstrator with Embedded Waveguides and Butt-Coupled Optoelectronic Modules,” in Adaptive Optics: Analysis and Methods /Computational Optical Sensing and Imaging/Information Photonics/Signal Recovery and Synthesis, Topical Meetings on CD-ROM, Technical Digest, Paper IWD2, The Optical Society of America, Washington, DC (2005). ISBN: 1-55752-789-XR.

[3] C. Berger et al., “High-Density Optical Interconnects within Large-Scale Systems”, Proc. SPIE 4942 (2003), pp. 222-235.

[4] R. Dangel et al., “Development of a Low-Cost Low-Loss Polymer Waveguide Technology for Parallel Optical Interconnect Applications,” in Biophotonics/Optical Interconnects and VLSI Photonics/WBM Microcavities, 2004 Digest of the LEOS Summer Topical Meetings, San Diego, CA (2004), pp 29-30.

[5] T. Lamprecht et al., “Passive Alignment of Optical Elements in a Printed Circuit Board”, in: Proc. 56th Electronic Components and Technology Conf. 2006 “ECTC 2006”, San Diego, CA (2006), pp. 761-767.

[6] S. Bernabé et al.,“Highly Integrated VCSEL-Based 10Gb/s Miniature Optical Sub-Assembly”, in: Proc. 55th Electronic Components and Technology Conf. 2005 “ECTC 2005”, Orlando, FL (2005), Vol. 2, pp. 1333-1338.

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