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84 The PCB Magazine • March 2015 organic waveguides based on dry film photore- sist technology such as DuPont's Polyguide™. UV exposure changes the optical properties of the waveguide from the surrounding unexposed material. The material base for such waveguides was acrylate chemistry, which limited the trans- mission distances because of the lossy material and was limited to a transmission mode called multimode. The use of lower-loss photosensi- tive fluorinated polyimides was technically fea- sible but costly and raised concerns about the toxicity of the chemicals. Waveguides come in a variety of material constructions: organics, glass, silica, silicon, etc. The properties that are important to the perfor- mance of waveguides include: • Intrinsic absorption loss • Low optical scattering loss • Low waveguide fabrication loss • High thermal stability • Environmental stability • Precise control of refractive index • Low birefringence • Mechanical toughness Before waveguides were considered for opto- electronic integration into PWBs, notably back- planes, they could be found in wafer-scale pack- aging. A polymer cladding is typically applied onto either silica or silicon wafer supports using spin coating technology. Good coating unifor- mity and precision is achieved with this process over the surface of the wafer. Then a guide poly- mer of a higher refractive index than the clad- ding is spun on and patterned either by virtue of its own photosensitivity or with a layer of photoresist, sometimes over a metal mask. The thicknesses of the waveguide layer are in the range of 5–60 microns. The waveguide pattern is then formed by etch-back, usually by plasma, but sometimes by solvent. Then a final layer of cladding is spun on and the wafer is complete. It is diced, connected to fibers, mounted on a support such as glass or ceramic, perhaps on a heater or thermoelectric cooler, placed in a package and electrically connected. There are several processes to form poly- meric waveguides. One of them is a photoim- aging, all-dry process (Figure 1). Waveguide forming films with mobile monomers and figure 1: Dry process photolithography for waveguide formation. karl's teCh talk OPTICAL INTERCONNECTS continues

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