The Fabrication of Direct-Write Waveguides via the Glassy-State Processing of Porous Films: UV-Induced Porosity and Solvent-Induced Porosity
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The incorporation of porosity in a material potentially results in the changes in electrical, mechanical and electrical properties and has generated much interest by researchers. The development of new techniques for inducing porosity in thin films may prove advantageous if they lead to a decrease in processing complexity, or an increase in the processing flexibility by widening the window of compatible physical conditions, or the improvement of the final properties of the porous materials. Two processing techniques were developed to produce porosity in thin dielectric films at temperatures below the glass transition temperature of the host matrix. These glassy-regime processing methods relied on the susceptibility of hydrogen silsesquioxane (HSQ) to gelation in the glassy regime when exposed to polar substances. Both of these glassy-regime processing methods relied on the susceptibility of hydrogen silsesquioxane (HSQ) towards gelation in the glassy regime when exposed to polar substances. The first processing method made use of co-solvent mixtures of polar non-protic organic solvent to serve both as gelation catalysts and pore-generators. HSQ films were soaked in the polar organic co-solvents, which penetrated the films and initiated crosslinking throughout the matrix. Afterwards the films were baked, volatilizing entrapped solvents and producing air pockets within the rigid matrix. The second porosity method used UV-radiation to initiate acid-catalyzed decomposition of polycarbonate sacrificial polymers after first using bases to catalyze the gelation of HSQ. The radiation-based (direct-write) decomposition of the porogen enabled the selective patterning of regions porosity via the use of a photomask, which resulted in the creation of refractive index profiles in the direct-written films. Porous films that were produced by these two glassy-state processing techniques were used to build slab waveguide structures. Optical characterization experiments showed that the fabricated waveguides had average propagation losses of 16 - 27 dB/cm for the first guided TE mode and about 36-40 dB/cm, for the second TE guided mode. It is believed that the large propagation loss values were caused by a combination of the Rayleigh scattering from the relatively large UV-induced pores produced in the direct-write layers as well as scattering induced by surface roughness.