Search

Recent progress in developing high-performance organic polymers for electro-optics and photonics is reviewed. A highly fluorinated hyperbranched aromatic polymer with the degree of branching around 0.51 was prepared by a mild one-step polyesterification of an AB2 type monomer. Further post-functionalization with and thermally cross-linking by aromatic trifluorovinyl ethers (TFVE) afforded thermally stable, low loss optical polymer with improved solvent resistance. By more precisely controlling the molecular nano-architecture, we have developed a series of highly fluorinated crosslinkable dendrimers. These materials possess most of the desirable properties needed for the fabrication of optical waveguides, such as high solubility in common organic solvents (up to 50 wt%), very low optical loss, and excellent thermal stability. To overcome the “nonlinearity-stability tradeoff,” a facile and reversibly crosslinkable NLO polymer system is developed that combines both advantages of high poling efficiency and good alignment thermal stability. By smartly controlling the poling and crosslinking processes through the reversible Diels-Alder (DA) reactions, it allows highly polarizable chromophores to be efficiently poled at the stage of low viscosity linear thermoplastic polymer. The resulting nonlinear optical polymer exhibits a combination of a very large r33 value (76 pm/V at 1.3 μm) and good temporal stability at 70°C.

Spectral absorption behavior of a series of FTC-like dyes of varying shape incorporated into amorphous polycarbonate (APC) is characterized by photothermal deflection spectroscopy. Previous Monte Carlo calculations by Dalton and Robinson predict a strong dependence of the macroscopic nonlinear optical susceptibility on the chromophore waist:length aspect ratio in electric field-poled films. This dependence arises from London interactions between chromophores, which are expected to influence the absorption characteristics of the composite both by changing the local polarity of the medium and through dipole interactions. It is expected that these interactions will play a role in the absorption characteristics of unpoled films as well. Of particular interest are the spectral characteristics of the red edge of the main dye electronic absorption peak, and the fine structure in the near-IR, dominated by overtones of fundamental C-H stretching and bending modes. The spectral structure in these key regions can be influenced by inter- and intramolecular interactions and conformational changes in the dye. The near-IR structure, in turn, will dictate absorption loss in optical devices prepared from these materials at key transmission wavelengths (1.3 and 1.55 um). In this study, a homologous series of spacer lengths, ranging from ethyl to hexyl, attached to an FTC-like NLO chromophore, LMCO-46M, is characterized by a combination of photothermal deflection spectroscopy (PDS) and UV-Vis spectroscopy to examine the effects of the molecular environment on near-IR loss at 1090 nm, 1300 nm and 1550 nm.

Understanding defects in ultrathin polymer films is of great importance to the microelectronics industry. We have taken the approach of trying to understand factors in polymer mobility by changes in glass transition temperature (Tg) and dewetting behavior. Incorporating fullerite into polystyrene (PS) films increased Tg relative to pure films. Photocrosslinkable PS showed a linear decrease in Tg with decreasing film thickness. Pyrene end-tagged polystyrene (PS-Py) and PS (both Mw = 4k) were compared on glass and gold substrates. Fluorescence data gave evidence of a higher probability of pyrene groups in proximity to the gold interface. This change in spatial distribution would be related to increases in Tg and suppression of dewetting for PS-Py films on gold.

A wide variety of aromatic trifluorovinyl ether monomers and highly fluorinated crosslinkable dendrimers have been developed via novel synthetic strategies. Through the thermal dimerization of trifluorovinyl ether moieties on the monomers or on the periphery of dendrimers, these monomers or dendrimers can be melt or solution polymerized to form perfluorocyclobutane(PFCB)-containing prepolymers with good processability for optical waveguide fabrication. By further thermal crosslinking, the resulting thermoset materials possess low optical loss (0.3-0.4 dB/cm at 1310 nm with 1% of DR-1 or DCM doping), high thermal stability (T g : 100-400 °C), good thermo-optic property, high solvent and humid resistance, and excellent mechanical flexibility. The combination of processability and performance in these PFCB-containing thermoset materials make them as ideal candidates for the fabrication of high-performance polymeric planar lightwave circuit components with the applications in the telecom and datacom optical networks.

Spectral absorption behavior of Disperse Red-1 and Disperse Red-19 dyes incorporated into a series of polymers by covalent attachment, representing various chemical structures, is characterized by photothermal deflection spectroscopy. Of particular interest are the spectral characteristics of the red edge of the main dye electronic absorption peak, and the fine structure in the near-IR, dominated by overtones of fundamental C-H and O-H stretching modes. The spectral structure in these key regions can be influenced by inter- and intramolecular interactions, or conformational or configurational changes in the dye. The NIR structure, in turn, will dictate absorption loss in optical devices prepared from these materials at key transmission wavelengths (1.3 and 1.55 um) for waveguide devices. A well characterized dye-polymer system, DR1-PMMA, is compared with two other polymer systems. Differences in spectral absorption behavior is assessed in terms of polymer host structure and bonding environment.

Encapsulated by highly-fluorinated dendrons, a nonlinear optical chromophore core, which is based on the phenyl-tetracyanobutadienyl (Ph-TCBD) thiophene-stilbene-based NLO chromophore, exhibits a large ~30-40 nm blue shift of the charge-transfer absorption maximum, 20 oC higher decomposition temperature, and most impressively, three times higher E-O coefficient. The combination of these appreciable improvements makes the molecular design of dendritic modification as a very promising molecular-engineering for next generation of E-O materials.

We have used ellipsometry to study the glass transition temperature (Tg) of thin films (thicknesses 4-300 nm) of monodisperse polystyrene (PS-M), polydisperse polystyrene (PS-P), and a Shipley photoresist base polymer (SBP) with and without photoacid generator (PAG) on silicon substrates. For films less than 40 nm thick, Tg values decreased with respect to their bulk values for PS-M and SBP films. Films containing PAG exhibited further reduction of Tg values as predicted by the Fox equation to within experimental error.

Residual solvent present in resist films following spin casting can significantly influence dissolution behavior. We have used radiolabeling to measure solvent remaining in spin cast films after they have undergone various annealing protocols. Films of poly(3-methyl-4-hydroxystyrene) were found to contain a significant fraction of propylene glycol methyl ether acetate (as much as 50 percent by weight) depending on the annealing conditions. 1100 angstrom films contained a higher percentage of residual solvent than 1.2 micrometers films. A diffusion plus evaporation model was used to deduce the diffusion coefficients of residual solvent as a function of film thickness and annealing temperature. We found that diffusion coefficients of residual solvent in 1100 angstrom polymer films are more than two orders of magnitude less than in 1.2 micrometers thick films.

The optical performance of Markle-Dyson projection optics is now well established. Here we describe options for 1X reflective optical masks that might achieve the desired linewidth control. One option is the use of aluminum as the reflecting material. A film less than 50 nm thick has nearly twice the reflectivity of the silicon used until now, and so it should be possible to develop an etching process (for such a thin film) that is adequately precise. Moreover options exist for repairing both opaque and clear defects. An interesting alternative configuration, that eliminates the need to etch the aluminum, is to use a patterned absorber on the substrate and to deposit the aluminum over the patterned absorber.

Residual casting solvent present in resist films can have a profound effect on their dissolution behavior. We have used a radiolabeling technique to make quantitative measurements of solvent remaining in spin cast films after they have undergone different baking protocols. Films of poly (3-methyl-4-hydroxystyrene) were found to contain a significant fraction of propylene glycol methyl ether acetate (as much as 50% by weight) depending on the baking conditions. Films that were 1000 angstroms thick contained a higher percentage of residual solvent than films that were 1 micrometers . The solvent concentration in films containing a mixture of polymer and photoactive compound was a maximum for films containing 5 wt% PAC.

The inhibition mechanism in diazonaphthoquinone positive resists has been widely studied. The ability of the photoactive compound to retard the dissolution of the matrix resin has been attributed to both chemical interactions between resist components as well as spatial distribution of sensitizer in a resist film. Our work has focused on understanding the physical distribution of the photoactive compound on the development behavior of this class of positive resists. We have used Langmuir-Blodgett deposition to fabricate resist structures with highly stratified sensitizer distributions. Dissolution measurements using a quartz crystal microbalance have demonstrated that preferential placement of very thin layers of sensitizer both at the surface of a resist film and embedded between two layers of polymeric resin can significantly affect development.

The inhibition mechanism is novolak/diazonaphthoquinone resists is widely studied. We are interested in evaluating the effect of sensitizer distribution in the resist on the dissolution properties. Langmuir- Blodgett film deposition allows us to control the spatial distribution of sensitizer and thus evaluate its 'sphere of influence' on the dissolution inhibition of the novolak resin. We have performed both dissolution studies and lithographic studies on films with sensitizer deposition only on the surface as well as films with an embedded layer of sensitizer between two layers of novolak polymer. The surface layer films with one monolayer of sensitizer do not demonstrate any inhibitive properties unless the films are thermally treated. However, for the analogous embedded layer films, inhibition does occur without any baking of the samples. Also, in the lithographic studies, pattern contrast is improved considerably upon baking the samples; indicating that intimate interactions between resist components which may be induced by the thermal treatment is necessary for dissolution inhibition to take place.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Langmuir-Blodgett (LB) techniques can be used to prepare very thin organic films, and we have investigated their use in lithographic applications. Resist films were prepared usmg novolac and poly(p-hydroxystyrene) resins and both near and deep UV sensitive photoactive compounds (PAC). LB resist films formed from mixtures of polymer and PAC behaved lithographically in a similar fashion to spin cast films. LB films as thin as 300 A were sufficient to protect 500 A of chromium during wet chemical etching. Initial attempts to use LB films as the top layer in a bilayer resist system indicated that silylation of the LB films may yield etch rate ratios of up to 10: 1 vs hard baked novolac. Investigation of the "sphere of influence" of PAC molecules by controlling their distribution throughout the film indicates that strong interactions may exist between consecutive LB layers of PAC. LB films may also be useful for modifying the surface concentration of PAC.

Ultrathin (0.9 nm - 15.3 nm) poly(methylmethacrylate) (PMMA) films prepared by the Langmuir-Blodgett (LB) technique have been explored as high-resolution electron beam resists. One-eighth micron lines-and-spaces patterns have been achieved by using a Perkin Elmer MEBES I pattern generation system as the exposure tool. The etch resistance of films with thicknesses greater than 4.5 nm is sufficient to allow patterning of chromium film suitable for photomask fabrication. Monolayer PMMA films containing 5 mol% pyrenedodecanoic acid (PDA) as a probe were prepared by transfer to the substrate at different surface pressures and characterized by fluorescence spectroscopy. The ratio of excimer to monomer emission intensity (Ie/Im) has a maximum value at ~ 10 dyn/cm, which may be related to a structural rearrangement in the film. Intrinsic bilayer LB PMMA films prepared at 1 and 19 dyn/cm have also been examined by transmission electron microscopy (TEM). The wrinkle-like surface topography observed in the 19 dyn/cm sample and not in the 1 dyn/cm sample suggests that the structure of the LB PMMA film depends on the transfer pressure.

We are investigating the structure and homogeneity of thin, multi-component resists prepared by spin casting and Langmuir Blodgett film formation technologies. Fluorescence spectroscopy has been used to evaluate various casting solvents for spin casting of multi-component films. Films of novolac containing pyrene as a small molecule probe were cast from various ethers and ether alcohols. The small molecules aggregated less when the dipole moment of the solvent closely matched that of the polymer. Langmuir-Blodgett techniques were used to create very thin (40 nm) films of novolac/diazoquinone. Images down to 1 jim were developed in these films and transferred into 50 nm of chrome. These very thin resist films could have various applications in advanced resist technologies, including mask making and deep UV lithography.