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标题: HDI Use in Optical Waveguides [打印本页]
作者: pjh02032121    时间: 2014-3-20 21:16
标题: HDI Use in Optical Waveguides
HDI Use in Optical Waveguides.pdf (7.66 MB, 下载次数: 107)
作者: pjh02032121    时间: 2015-3-31 13:04
Editor's Note: This article originally appeared in the January 2013 issue of The PCB Magazine.
Introduction
High-density interconnect (HDI) printed circuits play an important role for optical PCBs, and the challenge of optoelectronics and integration of photonics into the printed circuit. By 2012, expectations are that photonic PCBs will grow to a $2.5 billion industry. In this article, I look at the issues, materials and current processes being researched to create this integrated optoelectronic circuit board by European, Japanese and North American organizations. In addition to reviewing the global players in polymer photonics, the article reviews the current programs of three of the eight groups globally [1]:
Photonics and Electrical Performance
The performance of the conventional electrical interconnection technology is limited through the underlying physical properties. The most important disadvantages and problems respectively are that flowing electrons create a magnetic field; this introduces a myriad of problems at high frequency that makes up signal integrity and power integrity. These are major hindrances to the three growing challenges in advancing electronics:
Figure 1: Internet bandwidth and IP traffic trends place a greater load on communication infrastructures, especially from 2012 on. Current popularity of 4G wireless devices, as well as internet video and file sharing are the chief demands. (Source: CISCO VNI, 2010)
Figure 2: ITRS projected high-speed I/O data bandwidth trends for popular communication standards. (Source: ITRS)
The popularity of wireless devices and increasing applications that demand high bandwidth has placed an enormous burden on the Internet. Take, for example, the per-lane data rates of the PCIExpress interface that has increased from 2.5 Gbps for Gen 1.0, to 8.0 Gbps for the current Gen 3.0, and is expected to increase to 16 Gbps by Gen 4.0.
Figure 3: In 2006, IBM introduced the Mare Nostrum supercomputer with 27 Tflops.
To accomplish these higher speeds above 10 Gbps, optical connections seem to be the solution. Figure 3 shows the growth of optical interconnect cabling for IBM supercomputers, growing from 5,000 fiber cables in 2006 to more than 540,000 for the P775 in 2011. The next machine, Blue Water, will have over five million fiber-optic cables [2]. In these last two, instead of the fiber terminating at the PCB card edge, the Agilent and Avago MicroPOD optical connector actually extends onto the central processor module (Figure 3). The competition between electrons and photons (copper versus waveguide) is now down to the last 100 meters! As seen in Figure 4, a roadmap from the Terabus program [3], the number of optical lines is increasing, along with the density of those lines, correspondingly, the cost and power is coming down as well. Currently, the focus is on board-to-board channels of <1 meter, and for the “intra-system” of <10 m, and the “inter-system” of <100 m.
Figure 4: Evolution of optical interconnects from long-haul fiber, wide-area networks to systems, boards and then modules and chips. (Source: IBM [3])
Figure 5: Possible polymer waveguide materials, of which PDMS and haloacrylates are the most promising have their lowest loss in the 840-900 nm wavelengths while optical silica-glass used in fiber-optics-cables is lowest at 1550 nm. This is why the optical connections are 8x to 22x more productive than the older electrical connection.
Electrons Versus Photons
Why the focus on photonics? Probably because Gene Rodenberry told us that was the technology for Star Trek back in 1966! But aside from science fiction, the photon has inherent advantages over the electron. First, it does not generate magnetic fields, nor is it affected by magnetic fields, which is a big advantage in noise reduction.
The second advantage is its density!  As seen in Figure 5, a-d, a photon is much smaller and more compact than an electrical cable.
Thirdly, individual tracks (waveguides) and connectors can be packed closer together without generating noise or crosstalk. Figure 5a compares a one meter electrical and optical cable; a full rack of these cables (b); a 17.5 mm x 13.5 mm cross-section of an optical connector has 144 10+ Gbps interconnects while the same size for controlled impedance electrical connectors has only 21 (c); electrical traces on PCBs have only two differential pairs while the same size of optical waveguides contains 16 [4] (d).
These advantages are expressed by IBM’s Optical Roadmap (Figure 6 [4]). In 2006, their supercomputers used a single optical cable at the card edge only. By 2011, as in the P775 computer, they had multiple optical fibers to the card edge and across the board to individual connectors at the processors (Figure 3, lower right). In researchnow, but deployed by 2015 will be the integrated optical waveguides on the card and backplane with optical connectors and sockets. By around 2020, it is predicted that the optical interconnections will be on the chip and between cores and memory, as well as board and backplane.
For the foreseeable future, systems will be a mixture of electrical circuits with high-speed optical and slower electrical data busses (Figure 7 [5]). In the far future, the actual circuit elements may be photonic computing, memory, encryption and analysis as well as data distribution, effectively realizing the predictions of science fiction writers.
Photonics and Waveguides
It is difficult in this short article to expose fully all the progress and details in optical board developments since 2003. A very good summary is found in the e-Book, Handbook of Fiber OpticData Communication: A Practical Guide to Optical Networking, Chapter 26: Optical Backplanes, Board and Chips Interconnects, which contains a very good summary of the technology up to 2008, with 43 excellent references [6].
Optical Waveguide Materials
Five materials are mentioned in literature from five commercial companies:
There are a number of candidate polymer materials for waveguides, as seen in Figure 8 [1]. Two waveguide materials seem to predominate: PDMS (polydimenthylsiloxane) and halo-acrylates.The fabrication processes can be summarized in Figure 9.
PDMS Polymers and Photodefinable Polyacrylate
The PDMS polymers are a popular choice. The reported waveguide losses at 850 nm are only 0.05 dB/cm and had high thermal stability (>230°C). The acrylate waveguide polymers have been available in two forms, liquid and film. The liquid polymer is now distributed by Exxelis as Truemode. It has been the central material used in the UK Engineering and Physical Science Research Council’s (EPSRC) Innovative Electronics Manufacturing Research Centre (IeMRC), and Integrated Optical and Electronic Interconnect Printed Circuit Boards’ (OPCB) landmark program at Loughborough University. This included 10 industrial companies and four universities in the UK.
Figure 6: Proposed evolution of optical interconnections from 2006 until 2020, from single fiber, to multiple fibers, to integrated optical waveguides and eventually to optics integrated on the ICs [4]. (Source: IBM-Zurich)
Figure 7: A schematic of the proposed array waveguide evanescent couplers (AWEC) technique and its use for multicard backplane with electrical and optical busses [5].
A key objective of the various OPCB projects in the UK was the investigation of varied waveguide fabrication techniques and materials. A 10-layer board was designed by IBM-Zurich. It had eight conventional copper-clad layers and two optical layers on the outside, (Figure 9). The design rules for the 10 Gb/s optical channels was no less than 125 um with a vertical separation of no more than 250 um of the 70x70 um waveguides. When this is compared to a similar high-speed channel in copper, the separation grows to 1500 um, or a 36-fold increase in channel capacity (Figure 10 [8]).
In experiments on the inkjet deposition of polymer waveguide structures focusing
principally on the structuring of the core layer on top of the lower cladding (Figure 11). It was found that the UV cure optical polymer materials, photo-patternable polysiloxane and acrylates, and could be inkjet printed by controlling the viscosity and developing correct inkjet print-head waveforms. Using this technique, lines of optical polymer were initially deposited and then cured in a separate UV exposure unit. However, a key issue was preventing the spread of the material before curing, such that structures with a good height-to-width ratio (aspect ratio) were formed [6].
3D Waveguide Fabrication Techniques
Three ongoing programs seem to have the most publications, one from each major electronics infrastructure: Germany, the United States and the United Kingdom. However, many are studying the problem and government laboratories, institutes and universities from Austria, Australia, Finland, Canada, Korea, Japan and China have published on many of the topics brought up in this paper.
Figure 8: Six processes for defining polymer optical waveguides.
Figure 9: EOCB 10 Gb/s Data Demonstrator fabricated by Varioprint and demonstrating the PDMS polymer for waveguides [10].
EOCB
This work is part of the German projects “Electrical/Optical Circuit Board” and “OptoSys,” which are supported by the German government’s Department of Education and Research (BMBF) under grants 16SV802/6 and 01BP801/01. (Dortmund, Ulm, and the Fraunhafer Institute for Applied Solid-State-Physics in Freiburg are participating on a suborder basis.)
The progress in Germany is typical of that in the UK and USA, moving from the laboratory at universities to industrial companies building prototypes and the infrastructure. The Electrical Optical Circuit Board (EOCB) typified by C-Lab of Siemens in the last update is now replaced by commercial firms like Vario-Optics AG of Villingen, Switzerland (spin-off of Varioprint AG of Heiden, Switzerland), and tested/qualified by Microtec GmbH laboratories[Pusch-2006]. Microtec tested and reported on the EOCB performance along with failure mechanisms and failure predictions [11]. This provided new information for standards and pre-qualification actions.
Vario-Optics participated in a number of demonstration test vehicles. One is seen in Figure 9, an 8-channel, 10 Gb/s data link with damping rates of 0.05 dB/cm @ 850nm and coupling losses <1.2 dB per interface [10]. A second is shown in Figure 10a, and the fabrication process is shown in Figure 10b with waveguides of 50 x 50 μm.
Figure 10, a and b: Data link demonstrator daughter board (a); waveguide fabrication process (b) [10].
Integrated Optical and Electronic Interconnect PCB Manufacturing
In the UK, a large team of university and industrial companies collaborated under EPSRC’s leMRC, and OPCB: University College London, (UCL) Instigator, Principal Investigator and Technical Project Leader; the School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh; and the Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University--together with eight companies: Xyratex Technology (project manager and manufacturer of petabyte data storage systems), BAE Systems, Renishaw, Dow Corning USA, Exxelis (polymer supplier), Stevenage Circuits, Cadence Design Systems, and National Physical Laboratory.
Figure 11: Inkjet printing of waveguides was conducted at Loughborough University for both polyacrylate and polysiloxane using a Microfab Jetlab 4 with a Xaar 760 printhead. Various temperatures, viscosity and substrate pre-treatment were tested [14, 15].
The simple research in polymer waveguides that started in 1998 at Heriot-Watt University is now a very large program involving four UK universities and 10 industrial firms. The current reference [OPCB-12] has no less than 35 references published from 2005 to 2010. The purpose of this program is to develop manufacturing techniques for integrated optical and electrical interconnects in standard FR-4 PCBs. In particular, the purposes are:
Loughborough University is developing and testing polymer waveguide fabrication methods including direct laser write, laser ablation, and inkjet printing [14, 15].  This culminated with the building of a hybrid integrated optical and electrical interconnected test system backplane incorporating multiple layers of copper tracks and polymer waveguides to demonstrate bi-directional, error-free interconnections using 10 Gb/s ethernet digital traffic, as seen in Figure 12.
Figure 12: Hybrid integrated optical and electrical interconnected test system backplane incorporating multiple layers of copper tracks and polymer waveguides to demonstrate bi-directional error-free interconnections using 10 Gb/s Ethernet digital traffic [10].
Terabus Project (IBM’s Optocard)
IBM Research has invested heavily in the past five years in optical printed circuit board technology based on multi-mode polymer waveguides. This is now referred to as optocard. This research was partially funded by the U.S. government as the Terabus Program [3,12]. IBM believes this technology will be needed to provide the bandwidth for future server generations (clouds), allowing highly integrated electrical-optical links of waveguides, flex and fiber between systems, modules, boards and chips.
A typical high-performance computer (HPC) has a “chip-module-board” structure seen in Figure 13a. The bandwidth limits are characterized in Figure 13b. The electrical packaging technology is mature and there is not enough space or pins (including per-pin BW) to get past the chokepoint at the module-to-circuit board interface, which is only 12.6 Tb/s. What is needed is a more miniaturized structure proposed in Figure 13c, using optical TX and Rx along with waveguides (OE modules) and optical bussing on the PCBs. This raisesthe bandwidth to 76.8 Tb/s. The organic OE module is seen in Figure 13d [11]. IBM’s OE module progressionis detailed inFigure 14. Thegoal is exabitcomputing (1015bytes/sec) and inthe short-term from2008 to 2012, theprogress for theoptochip from 240Gb/s to 480 Gb/s.The second step usesthe O-PCB withthe polymerwaveguides andthe third stepincludes directoptical connectionsto the OE Module.
Figure 13, a-d: Structure (a) and performance (b) (bandwidth limits) of a “chip-module-board” structure, which is only 12.6 Tb/s for today’s structure. What is needed is a more miniaturized structure proposed in (c), using optical TX and Rx along with waveguides (OE modules) and optical bussing on the PCBs. This raises the bandwidth to 76.8 Tb/s. The organic OE module (d).
Back to Today
Figure 15shows the current architecture of the P775 supercomputer. The optical interconnects go directly to the processor module and use a 48-channel x 4 (48 x 4) operating at 12.5 Gb/s. These are supplied by Avago and soon, others. This allows the nearly 540,000 optical channel connections required for the system. Remember, Blue Water will require more than five million optical connections.
Figure 14: IBM’s pathway to exabit computing, optical data paths from board level to module level to chip level with increasing bandwidth and number of channels at each step [16].
The optical connections and cables are seen in Figure 16. This 192-channel flexible waveguide is an optical backplane operating at 850nm and 12.5 Gb/s. The flex material is polyimide with 12 WGs per tail with 250 mm pitch. The fabrication of the Optocard flex cables and rigid waveguides was conducted by Endicott Interconnect Technologies (EIT) of Binghampton, New York [12]. These were fabricated by direct deposition of the waveguide material (Dow Chemical XP-5202A) onto a hybrid PCB substrate of Upilex polyimide with a layer of resin coated copper (RCC) laminated on both sides and to a frame.
Figure 15: IBM opto packaging strategy (a); 2011 strategy of grouped waveguide cables to CPU modules (b); CPUs processor package and module that individual MicroPOD optical connectors (c); rack with 16 processor modules: four racks per system. (Source: IBM [11])
The Terabus project’s objective is to create a dense, hybrid packaging structure made up of optochips, optomodules connected with flexible waveguides to create the optocard with additional waveguides in a backplane between optocards. This is shown in Figure 17. The future vision is for an optically-enabled MCM as seen in Figure 6c,with performance projected in Figure 13cto the 2014 to 2016 era [9]. Other flex-optical cabling is ThunderBolt from Apple. Similar to Intel’s LightPeak, Thunderbolt is a combined electrical/optical connection standard.
Figures 18a-gshow these flex polymer waveguides in the optocard. A frame is employed to address the need of flat working surfaces that are required during optical fabrication, as seen in Figure 17a. The flex assembly is aligned to the optomodule and Pyralux and is used to laminate them together. The entire assembly is then baked under pressure to cure the Pyralux. The fully assembled structure is shown in Figure 17b. The attachment process is repeated at the other end of the waveguide flex to complete the optocard, as seen in Figure 14c. EIT also employed inkjet printing of the waveguides successfully [4].
The Terabus structure consists of the optomodule connected to the HDI PCB (Figure18a), with integral or flex waveguides (Figure 18b), patterned by the process (Figure 18c)and showing itscross-section (Figure 18d, e, f, g). The optomodule is an organic HDI chip carrier (known in IBM as SLC) with the optochip containing optical VCSEL and PD chips aligned by optochip lens array to waveguide lens array on the optoboard.
Figure 19shows a close-up of a possible silicon chip carrier containing all the optochip elements in addition to the organic chip carrier [20]. The next step is the ongoing work to integrate all of the optical TX (VCSEL) and Rx (photo-detectors) into the CMOS processors chips.
Figure 16: The details of the optical cabling and backplane consisting of 192 waveguides, fabricated as eight flex conductor sheets of 24 WGs, divided into four connectors with 48 waveguides each; the L-links and the D-links operating at 850 nm and 12.5 Gb/sec [20].
Figure 17: The details of IBM’s optical system: optochip, optomodule and optocard connected by the polymer waveguides [20].
Figure 18a - g: Optocard with flex waveguide attached (a); flex waveguide close-up (b); waveguide fabrication process (c); design rules and cross-sections (d); operating waveguides (e); laminated to optomodule (f); completed optocard integration with the polymer waveguide (g) [10].
Figure 19: The Terabus project optical waveguide interconnects and related work using a silicon carrier in addition to the organic SLC carrier with future vision and performance of 2014-2016 [9].
Conclusion
Embedded optical waveguides in printed circuits have progressed aggressively in the last few years due to cooperation between university and industrial partners. There are now several polymer materials available, and several successful methods of fabrication for the waveguide in a PCB, as well as design tools and test facilities, reliability data and assembly techniques. A number of successful optical demonstration boards were fabricated by industrial firms capable of building these boards in production. The 12.5 Gb/s threshold per channel has been achieved and now the 16 Gb/s and 25 Gb/s are in progress. Soon, the 40 Gb/s channel will also be achieved. Hopefully culminating by 2020 with optical interconnects integrated into the processors and system performance in the 1,000 petaflop/sec range or an exaflop [21, 22, 23, 24, 25]!
References:
1. Holden, H., "Optical Waveguides," CircuiTree Magazine, January, 2004.2. World’s Fastest 500 computers, www.top500.org.3. Doany, F., "IBM Optical Boards," ECTC, 2007.4. Offrein, B.J., "Optical Interconnects for Computing Applications,” IBM Swisslasernet Workshop, October, 2010.5. Flores, A. and Wang, M., "Soft Lithography Fabrication of Micro Optic and Guided Wave Devices," Lithography, Chapter 19, INTECH February, 2010, ISBN: 978-953-307-064-3, pp. 379-402.6. "Handbook of Fiber Optic Data Communication: A Practical Guide to Optical Networking," Copyright 2008, Elsevier Inc., ISBN: 978-0-12-374216-2, pp. 657-676.7. Pitwon, Hopkins, Milward, Muggeridge, Selviah, and Wang, "Passive  Assembly of Parallel Optical Devices onto Polymer-based Optical Printed Circuit Boards," Circuit World, Vol. 36 #4, March, 2011.8. Selviah, D. R., "Integrated Optical and Electrical Interconnect PCB Manufacturing," PCB007 Newsletter, March 16, 2010.9. Immonen, Karppinen, and Kivilahti, "Investigation of Environmental  Reliability of Optical Polymer Waveguides Embedded on Printed Circuit  Boards," Microelectronics Reliability, Vol. 47 (2007), pp. 363-371.10. Vario-Optics ag Presentation, SLN Workshop, 2010, www.vario-optics.ch. 11. Kash, Kuchta, Doany, Schow, Libsch, Budd, Taira, Nakagawa, Offrein, and Taubenblatt, "Optical PCB Overview," IBM Research Meeting Presentation, November, 2009.12. Chan, Lin, Carver, Huang, and Berry, "Organic Optical Waveguide Fabrication in a Manufacturing Environment," EIT Vendor Presentation, August 10, 2010.13. Das, Lin, Lauffer, and Markovich, "Printable Electronics: Towards Materials Development and Device Fabrication,"Circuit World, Vol. 37 #1 (2011), pp. 38-43.14. Selviah, Walker, Hutt, Wang, McCarthy, Fernandez, Papakonstantinou, Baghsiahi, et al, "Integrated Optical and Electrical Interconnect PCB  Manufacturing Research," Circuit World, Vol.36, #2, May, 2010, pp. 5-19.15. Selviah, Walker, Milward, and Hutt, “Integrated Optical and Electrical Interconnect PCB Manufacturing (OPCD)," leMRC Flagship Project Report.16. Schroder, "Planar Integrated Optical Interconnect for Hybrid Electrical-Optical Circuit Boards and Optical Backplanes," NeGIT Conference, Berlin, Germany, 2006.17. Offrein, B.J., "Silicon Photonics Packaging Requirements," IBM Silicon Photonics Workshop, Munich, Germany, 2011.18. An Optical Backplane Connection System With Pluggable Active Board Interfaces, www.xyratex.com.19. "Overcome Copper Limits with Optical Interfaces," www.altera.com, April, 2011.20. Libsch, F., "Optical PCB Overview," November 11, 2011.21. Benner, A., "Cost-Effective Optics: Enabling the Exascale Roadmaps," Hot Interconnects #17, August 27, 2009.22. Vasey, F., "Prospects for Future High Speed Optical Interconnects," IBM Workshop, November 16, 2010.23. Dellman, L., Berger, C. et al, "120 Gb/s Optical Card-to-Card Interconnect Link Demonstrator with Embedded Waveguides," ECTC, 2007.24. Schow, C.L.,Schares, L. et al, "Terabus and Beyond: Prospects of Waveguide-based Optical Interconnections," 57th Electronic Technology and Computer Conference, Reno, Nevada, May 29, 2007.25. Green, W., Assefa, S. et al, "CMOS Integrated Silicon Nanophotonics: Enabling Technology for Exascale Computational Systems," SEMICON 2010, Tokyo, Japan.26. Holden, H., "An Update on Developing Technologies of Integrated Optical Waveguides in Printed Circuits," The PCB Magazine, September, 2011.

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