Excimer laser machining of a polymer multilayer for optoelectronics

Optical waveguides are already preferably in operation as optical backplanes in multi-processor high-end computers or as on-chip interconnections between transmitter and receiver modules such as laser-diodes or detectors. Presently, the interconnection between these modules is mainly realized with optical fibres made of glass or polymer. The use of planar optical waveguides is an alternative method to the fibres technique. Polymeric optical waveguides can directly be structured with laser radiation on the substrate to their final shape. With fewer process steps than conventional patterning techniques (for e.g. reactive ion etching) excimer laser machining is regarded as a faster and reliable technology for structuring materials such as polymer waveguides in thin film packages. Polymers are employed because of their advantageous properties such as an adjustable refraction index, easy to process or the relatively low cost compared to other materials used for optical waveguides. For these investigations a special polyimide suitable for optical transmission in the range of 400-2000 nm was used. A three layer polyimide film (cladding, core, bottom) was spun-on a silicon wafer. Each layer had a thickness of 5-6 μm and the core layer had the highest refraction index. The waveguides were structured with a pulsed KrF-excimer laser source (Lumonics 888i) at λ=248 nm and τ_L=25 ns (FWHM) within a process chamber