Sally A. Swanson

High temperature nanofoams derived from rigid and semi-rigid polyimides

A means of generating high temperature polymer foams which leads to pore sizes in the nanometre regime has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block and a thermally labile material, such that the morphology provides a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon a thermal treatment the thermally unstable block undergoes thermolysis, leaving pores where the size and shape are dictated by the initial copolymer morphology. Multiblock and triblock copolymers, comprising rigid, semi-rigid and flexible polyimide matrices with either poly(propylene oxide) or poly(methyl methacrylate) as the thermally labile coblocks, were prepared. The copolymer synthesis was carried out through the poly(amic alkyl ester) precursor to the polyimide since this precursor is stable and allows for isolation and characterization prior to imidization. Microphase-separated morphologies were observed for all copolymers irrespective of block type or length by both dynamic mechanical and small-angle X-ray scattering techniques. Upon decomposition of the propylene oxide or methyl methacrylate coblock, reductions in the film thickness and the total integrated scattering were found for those copolymers derived from rigid and semi-rigid polyimide matrices, thus indicating a collapse of the foam as it was being formed. Conversely, copolymers based on the flexible polyimide produced stable foams upon decomposition of the labile coblocks.

Effect of precursor history on residual stress and relaxation behaviour of high temperature polyimides

Abstract Using a wafer bending technique, residual stress and its relaxation were investigated for four different kinds of high temperature polyimides: rodlike PMDA-PDA, semi-rigid BPDA-PDA, semi-flexible PMDA-ODA, and flexible BTDA-ODA. Residual stress was measured in situ on silicon wafers during thermal imidization of the polyimide precursors and subsequent cooling of the resulting polyimides as a function of temperature over the range of 25–400°C. The stress of the cured films at room temperature was significantly relaxed by the moisture uptake of the films rather than their creep behaviour. Both residual stress and moisture-induced stress relaxation in the cured polyimide films were strongly dependent upon the molecular nature (that is, molecular chain rigidity, degree of molecular orientation and packing), as well as the precursor origin. In addition, the diffusion coefficients of water in the cured polyimide films were estimated by best-fitting stress relaxation against time profiles.

Polyimide Nanofoams For Low Dielectric Applications

Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term “nanofoam”. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block leaving pores the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials, were investigated as well as a number of thermally labile polymers. The foams were characterized by a variety of experiments including, TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.

Admittance spectroscopy of polymer-nanoparticle nonvolatile memory devices

Nonvolatile resistive memory consisting of gold nanoparticles embedded in the conducting polymer poly(4-n-hexylphenyldiphenylamine) examined using admittance spectroscopy. The frequency dependence ...

5.2: High Performance Electrophoretic Displays

Electrophoretic (EP) displays have been proposed as a route to “paper-like” displays. A high performance EP display designed by IBM will be described along with aspects of electrode design, display characteristics, particle selection, and particle stabilization.

Crosslinked polyimide foams derived from pyromellitic dianhydride and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,3-trifluoroethane with poly(α-methylstyrene)

Abstract A new route for the synthesis of high glass transition temperature, thermally stable polymer foams with pore sizes in the nanometre regime has been developed, using compositionally asymmetric microphase-separated block copolymers where the minor component (poly(α-methylstyrene)) is thermally labile and the major component (polyimide) is thermally stable. The minor component can unzip to its monomer upon heating, and the decomposition products can diffuse out of the film, leaving behind pores embedded in a matrix of the thermally stable component. In this study, the polyimide block was crosslinked with ethynyl functionalities to obtain a stable porous structure. The decomposition of the α-methylstyrene in the block copolymer was studied by thermogravimetric, dynamic mechanical and thermomechanical analyses. Mild conditions were required to avoid rapid depolymerization of the α-methylstyrene and plasticization of the polyimide matrix. The foams showed pore sizes with diameters up to a micrometre in size as well as the expected reduction in the mass density. However, the crosslinking of the matrix as a means of stabilizing the expected nanofoams was not successful.

From electric field-induced second harmonic generation (EFISH) to electro-optic measurements of nonlinear chromophores

Experimental measurements of molecular hyperpolarizabilities can be useful in two ways. First, they allow comparison between different chromophores, so that structure-function relationships can be understood and increasingly nonlinear compounds synthesized. This application requires only good relative measurements. The second and ultimately more important application is prediction of the macroscopic nonlinear optical properties of custom materials. The latter application requires accurate absolute values for the molecular parameters. Examples of both types of comparisons are discussed below. We describe experimental measurements on several types of nonlinear optical chromophores, and the choice of conventions and reference standards that leads to accurate predictions of electro- optic coefficients in poled polymers. The degree to which nonlinearity and thermal stability are correlated is also discussed.

Imide—Dimethylsiloxane block copolymers: Design and synthesis of a permanent buried oxygen ion etch barrier

Imide—siloxane multiblock copolymers were investigated. A key feature of these copolymers is the preparation of bis(aminopropyl) oligomers via anionic ring-opening polymerization. The molecular weights of the oligomers ranged from 1000 to 5000 g/mol. The oligomers were coreacted with 4,4′-oxydianaline (ODA) and pyromellitic dianhydride (PMDA) diethyl ester chloride in a N-methyl-2-pyrrolidone/THF—solvent mixture in the presence of N-methylmorpholine. The resulting amic ethyl ester siloxane copolymers were isolated and washed to remove homopolymer contamination. Copolymer compositions, determined by 1H-NMR, ranged from 20 to 65 wt % siloxane content and the measured compositions were close to those charged. Solutions of the copolymers were cast and cured (350°C) to effect imidization, producing clear films. The films showed tough ductile mechanical properties with moduli varying with siloxane content. The copolymers displayed good thermal stability with decomposition temperatures in the proximity of 450°C. Multiphase morphologies were observed irrespective of siloxane block lengths or compositions.

Visualization of the develop process

Variations in critical dimension (CD) as a function of the proximity of an individual feature to other exposed areas are continuing to be a problem in the lithography process. For example, the CD uniformity (CDU) may degrade significantly depending on the proximity to densely or sparsely exposed areas. These pattern density effects will continue to get worse and become more complex as feature sizes decrease. Pattern density effects are believed to arise from several sources and may simultaneously contribute to a net observed CD variation [1]. One such source, develop loading, results in local depletion of developer in highly exposed regions, reducing the dissolution rate and thereby locally affecting CD. In this report we describe our results in visualizing develop loading by using pH sensitive dyes. Two different types of dyes are explored: acid/base pH indicators and a fluorescent dye bound to the resist polymer.

A silicon-containing resist for immersion lithography

We have developed a new silicon-containing resist for 193-nm immersion lithography. This resist is compatible with topcoats used in the industry today for immersion lithography. Most of the current topcoats contain 4-methyl-2- pentanol as a solvent. Our evaluations indicated that the previously developed silicon-containing resists are not compatible with the current topcoats because of their solubility in 4-methyl-2-pentanol. In the new resist polymers, we have incorporated high percentage (> 60 mol%) of lactone monomers to prevent them from dissolving in this solvent. In order to increase the lactone content in a silicon polymer, we have incorporated lactone containing acidlabile functionalities in addition to widely used acid-inert lactone monomers. Utilizing these polymers, we have demonstrated a functional silicon-containing photoresist for immersion lithography.