Printed Circuit Design & Fab - March 2009 - (Page 35) optoeLectronIcs taBLe 2. Polymer waveguide technology process comparison. Technology based on: Monomer Diffusion with clad lamination Polymerization induced monomer diffusion self development – [3] Optical CrossLinks, Inc. Pre coated photosensitive acrylate monomer &binder polymer on temporary carrier 1foot wide 100+ feet long. Contact photomask uv exposed waveguides. Monomer diffusion mass transfer creates higher density polymer guide. “Self-developed” guides are clad with similar polymer, interdiffused, fully exposed, and bake cured. [3] Technologies based on: Ridge Formation with Clad Backfill Image and develop with aqueous , etching (wet chemical or RIE) processing Rohm&Haas/Shipley, IBM, Siloxane based negative acting system that processes like a Photoresist. Compatible with CMOS and PWB manufacturing and waste streams. Loss @ 850nm 0.025dB/cm. Embedded waveguides demonstrated with minor increase in loss. Initial data shows good stability @85/85 for over 2500 hrs [2.5] Screen print or mold Gemfire / Dow Corning Embossed Technology-General Attribute Representative practitioners General process description fIGure 2. AMP Tyco Lightray Technology. ry is used. Beyond that, the losses from the skin effect and dielectric loss tangent become too great to compensate. Optical methods may be viable and, indeed, have been used in a few specialized applications over the years. Optical backplanes, however, have not yet entered the mainstream and are not expected to do so in the near future. fIGure 2 shows an AMP Tyco, LIGHTRAY OFX, a flexible solution for precision fiber management in high fiber count telecommunications network equipment applications. Designed using the latest fiber optic technology, LIGHTRAY OFX consists of a number of optical fibers encapsulated in thin protective film substrates. As well as saving valuable space in equipment and equipment racks, LIGHTRAY OFX also provides optimum optical performance. Optical fiber “crossovers” allow the optical fibers to be routed in both regular and irregular designs. Fibers are routed on 250 µm center lines. The displacement of copper by optical technologies for data transmission continues. The growing demand for greater data transmission capacity, smaller physical size and lower power consumption generally favors optical methods and results in optical connections replacing copper connections as soon as the total cost of the optical solution is competitive. With the rising costs of energy and the power draw and data transmission rates increasing, optical technologies will continue to replace copper. If we look at the use of optoelectronic technology across the spectrum, it might look something like: ■ Massive use in long haul, metropolitan communications. ■ Significant use in large LANs. ■ Growing use within data centers for rack-to-rack data transfer. MARCH 2009 OptoFoilFraunhofer Institute, IZM Hot Embossing of commercial available polymer foil using special negative tool made by high quality lithography and galvanic metallization. Filling of grooves with polymer of higher index of refraction. UV curing. Cladding lamination with same foil as used for embossing (undercladding). Several drying & tempering steps. Copper OK to 20 Gbit up to one meter in backplanes. ■ On circuit boards other than backplanes. • Static; awaits need & demand. • Much technology demonstrated but cost questionable. ■ On-to and off-of chip. • Exploring alternatives for lower power and more bandwidth. ■ On-chip. • Exploring alternatives for lower power and more bandwidth • A modulated light source is needed. Dr. Bruce Booth and I have discussed alternative waveguide manufacturing processes in detail in articles published in the March 2008 and April 2008 issues of PCD&F.3 Ridge technology and Diffusion technology are the two main types of processes, taBLe 2 compares these technologies. In the table, technology maturity is assigned for each of several technologies to be considered and includes important attributes using a scale of 1 to 4; namely: [1] literature, conceptual stage or early feasibility only. [2] laboratory demonstration, preliminary proof of concept, evaluation and testing. [3] prototypes constructed and delivered for evaluation, and/or demonstration or system design development, limited pilot production, initial ■ application specific testing. [4] commercial product deployment, extensive testing, manufacturability demonstrated. taBLe 3 discusses typical process characteristics of the different types of waveguides. The same maturity rating used in the previous table applies. Needs and Showstoppers What are the gaps and showstoppers for optoelectronic PCBs in production? We need a major OEM to say what its next generation of product X is going to be. Before committing resources to a technology, OEMs want to see the complete optical interconnect system demonstrated. Besides having a good waveguide technology, OEMs are looking to facilitate a complete optical interconnection system from off-board to the chip level, between chips on multiple boards and back out off board. Capabilities for a complete optical interconnection system involve at least: ■ Optical input and output connections at the edges of the motherboard or backplane. ■ Right angle connectorization at the daughter board (DB) junction with the MB. ■ Optical signal distribution on the DB or into Tx/Rx link packages. These capabilities must be costcompetitive and low-risk versus electriPRINTED CIRCUIT DESIGN & FAB 35
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