Polymer coatings are finding new applications in optoelectronics where many of their attributes important for semiconductor and MCM fine-line delineations are also applicable to the fabrication of waveguides. Photolithographic processes and photocurable materials such as polyimides, BCB, and epoxies are used to form precise grooves in the coating for the transmission of light. Most of these processes have already been described in connection with via formation except that, in the case of waveguides, long narrow grooves are formed in the dielectric coating. Polymer technology for waveguides is still in a state of flux because of the wide variety of materials and processes that can be used and the need to achieve low losses within the structure and at the connections. However, the rewards can be great because of the potential for cost reduction over current methods.
Polymer coatings such as BCB are being investigated for the fabrication of planar waveguides as potentially lower cost alternatives to optical fibers. Waveguides based on polymeric materials are less expensive due to fewer steps in the fabrication sequence than commonly used silica-on-silicon waveguides. In one study, Multimode Interference (MMI) waveguides were fabricated using photodefinable BCB for the core material and a thermosetting BCB for the under- and over-cladding material on silicon wafers. Core widths and thicknesses were 7 µm.[37]
In fabricating the waveguides, photoimageable polymer coatings such as polyimides, BCB, and epoxies are first spin-coated onto a rigid substrate. Silicon wafers, fused quartz, epoxy-glass laminate circuit boards, and ceramic circuit board materials have been used as substrates. The coating is then exposed to uv light and developed to form the grooves. Other etching methods common to semiconductor manufacturing have also been used, such as RIE, plasma etching, and laser ablation of a cured coating. Some rather unique techniques have been developed to create high transmission paths or guides in a coating by manipulating the refractive indices of the coating through diffusion of low-molecular-weight compounds or monomers into the coating in selected areas. Conversely, they can be created by selectively leaching out low-molecular-weight constituents from a partially cured coating followed by full curing. A good overview of these processes has been given by Booth.[38]
• Ability to delineate fine and uniform grooves in the coating.
• Ability to tailor the refractive index through partial or thorough polymerization, adding formulation ingredients, or diffusing into, or leaching out certain low-molecular-weight constituents.
• Uniform and stable refractive index.
• Smooth and uniform sidewalls after etching to assure minimal loss due to light scattering.
• CTE closely matching that of the substrate to avoid stresses and delamination that could affect light scattering.
• Resistance to subsequent high-temperature processing such as solder reflow.