Synthesis And Characterization Of Fluorinated Polyimides, And Processes For Fabricating Its Optical Waveguides

Next generation internet switches and high-end computers are expected to process aggregate data rates in the order of Tbit/s and the interconnection between the processing units will have to handle data rates in the order of 10-40Gbit/s. Based on basic physical law the copper based electrical interconections will suffer from high transmission losses, severe signal integrity, and electro magnetic interference(EMI) problems at such data rates. In order to overcome the evident high-speed interconnection bottle-neck, optical interconnects are considered the prefered option. The integration of optical waveguides in printed circuit board or backplane is considered more economical for board- or chip-interconnection. And the materials and processes of waveguides have to meet the severe requirements compatible with PCB industry.Following the development of integrated optics, the advantages of polymer waveguide materials, performances reqirements for electro-optical printed circuit board, and the methods for waveguide fabrication are reviewed. The reaction mechanisms of "two-step" synthsis of polyimide were also analyzed to find out the factors that influence the properties of final products.In order heighten the molecular weight of fluorinated polyimide, the process of synthesis were optimized through modified molecular ratio of dianhydride and diamine, the order of adding reactant, reaction temperature, solvents. And the products were characterized by IR, TGA and DSC, when the differences in molecular aggregation between the chemical imidized polyimides and thermal imidized ones.Moreover, the relationship between thermal and optical properties of fluorinated polyimides (FPI) with their repeat units were discussed based on the structure-properties relationship of polymers. Then the influences of curing temperature on the optical properties (refractive index, birefringence, optical loss and thermo-optical coefficient) of FPI were demonstrated by IR spectrum and optical measurements through Metricon 2010 prism coupler. The optical properties of FPI thin films have their extremum at 240-270℃duo to the changing aggregation structure of FPI after thermal curing. And the optimized curing process was proposed.Two kinds of waveguide process were demonstrated, including CO₂ laser direct-writing and micropen direct-writing. The heat transfer during fabricating FPI waveguide by CO₂ laser direct writing was simulated through ANSYS. Then the effects of processes such as writing speed, the power of laser, pre-curing temperature on the profiles of waveguide were investigated experimentally, and the optical loss of FPI waveguide fabricated was measured by cut-off method, which can be larger than 5.6dB/cm. Furthermore, the processes of micro-pen direct-writing FPI waveguide were implemented after well understanding the viscous flow characteristics of poly(amide acid)s solution and its wetting behaviours on SiO₂ films. And the optical loss of FPI waveguide was also measured by cut-off method, which can reach about 0.1dB/cm.