Optical interconnects and high frequency electronic packaging : SBU-materials, material characterization, LAP- & microprocessing

Abstract: The main objective of this thesis is to discuss and demonstrate optical interconnect technologies. These can be used to enhance the data throughput capacity of future printed circuit boards and backplanes, i.e. exchange Copper conductors with light conductors. A large area panel sequential build up (SEU) technology of light waveguides on industry standard FR4 like large area panels (609.6 mm ' 609.6 mm) has therefore been developed for interconnect distances typically above 10 cm. Another objective has been to study the same or similar materials and thin film process technologies for high-frequency (HF) packaging and HF-devices, to also prepare for future integrated optical and HF-packaging. High performance embedded integral passive components (resistors, capacitors, inductors) with new material and process technologies is part of this strategy.An overview on the state-of-the-art of optical backplanes, micro mirror technology for in-/ out-coupling of light to/ from the optical waveguides is given. This includes free space optical interconnect, buried wave guides, on-top of printed circuit board waveguides, flex-foil based approaches, and bi-directional approaches. An update is given concerning materials useful to fulfill these approaches. Recent literature related to polymer materials suitable for optical interconnect is overviewed including data for optical loss (>. = 800/ 1300/ 1550 nm) and thermal stability. Polymer host materials suitable for rare earth light amplifying materials are also overviewed. Polymer and sol-gel based devices working at wavelengths >. = 600/ 1064/ 1550 nm have been found so far, but in general devices for>. = 800/ 1300 nm seems to be missing. For the implementation of optical waveguide amplifiers on optical backplanes a flip-chip approach was proposed.A flexible approach of producing optical interconnects on 609.6 mm ' 609.6 mm large area panels has been demonstrated. Stepwise projection patterning from 101.6 mm ' 101.6 mm masks has generated optical waveguide patterns over the whole panel using large area projection lithography equipment. For minimization of optical losses due to misalignment of the step-outs, four different waveguide ending structures were used in the overlap region of the mask. On a test wafer (152.4 mm) a misalignment of ≤ 5 μm could be achieved. Initial optical characterization of the large area panel demonstrator (FR4-substrate) gave optical losses of 0.5 dB/ cm at >. = 850 nm (ORMOCER® as waveguide material).The refractive index of the ORMOCER® systems used was measured over a broad composition range by using the prism coupler method.A literature survey on integral passives with main focus on polymer nanocomposites for integral capacitor applications has been performed. Composites containing ceramic particles were found to give a dielectric constant up to εr = 150 (load of the host: 85 vol%). Conductive fillers gave much higher values (e.g. εr. = 1000 for silver flakes in epoxy) but on the other hand suffer from unpredictable percolation behavior and reduction of the bulk resistance. A case study showed, that a rise in the dielectric constant of the polymer host can give a significant rise in the dielectric constant of the composite.The focus for polymer nanocomposite was also due to process compatibility for the PCB industry. Until now, the permittivity values proposed by NEMI for this material class could not be reached yet (εr ≃ 200).Process development has been conducted for Sequentially Build-Up high frequency test vehicles (ORMOCER® as dielectric material). The microwave demonstrator gave a permittivity of εr = 3.05 and a loss tangent tan~ = 0.024 in the frequency range of 10 to 40 GHz.The presented flexible manufacture approach of an optical backplane seems to be suitable for the PCB industry, because of a rather high production speed, guaranteed flexibility through step-out of small masks on large area substrates and the usage of photopatternable materials with PCB industry compatible process temperatures.Coupling structures are still not satisfying and is regarded as a weak link at present. In a case study it was shown that it is feasible to implement an optical waveguide amplifier on optical backplanes. The concept of a flip-chip amplifier in connection with micro-periscopes and cylindrical lenses, and sol-gel silica on silicon processes seem to be promising.