Supercritical CO2 extraction of porogen phase: An alternative route to nanoporous dielectrics

DOI: 10.1557/JMR.2004.0413We present a supercritical CO2 (SCCO2) process for the preparation of nanoporous organosilicate thin films for ultralow dielectric constant materials. The porous structure was generated by SCCO2 extraction of a sacrificial poly(propylene glycol) (PPG) from a nanohybrid film, where the nanoscopic domains of PPG porogen are entrapped within the crosslinked poly(methylsilsesquioxane) (PMSSQ) matrix. As a comparison, porous structures generated by both the usual thermal decomposition (at approximately 450 °C) and by a SCCO2 process for 25 and 55 wt% porogen loadings were evaluated. It is found that the SCCO2 process is effective in removing the porogen phase at relatively low temperatures (<200 °C) through diffusion of the supercritical fluid into the phase-separated nanohybrids and selective extraction of the porogen phase. Pore morphologies generated from the two methods are compared from representative three-dimensional (3D) images built from small-angle x-ray scattering (SAXS) data.Professor Gangopadhyay and Professor Simon acknowledge the financial support of this work from the Semiconductor Research Corporation and from the National Science Foundation Grant No. CMS-0210230. The authors would also like to acknowledge initial support provided by the State of Texas Advanced Technology Program (ATP Grant No. 003644-0229-1999). The SAXS experiments were performed at the Advanced Photon Source at Argonne National Laboratory, which is supported by the United States Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the United States Department of Energy, Office of Basic Energy Sciences

Electric field and temperature-induced removal of moisture in nanoporous organosilicate films

doi:10.1063/1.1757019The effects of bias-temperature-stress (BTS) or simply temperature-stress (TS) on nanoporous low-k methylsilsesquioxane films are studied. Initially, the as-given and O2 ashed/etched films exhibit physical adsorption of moisture as revealed from the electrical behavior of the samples after 15 days. The temperature stressing at 170 °C volatilized the adsorbed water but is unable to remove chemisorb and hydrophillic Si-OH groups. As a result, the TS films remain susceptible to moisture. BTS at 170 °C also removes adsorbed water. More important, the surfaces under the metal-insulator structure were dehydroxylated by breaking the chemisorb Si-OH group facilitating the formation of siloxane bonds that prevents adsorption of moisture even after 60 days.The authors would like to acknowledge Dorel Toma of TEL for providing the samples, and SRC and NSF for funding this research

Effect of cellulose-based fibers extracted from pineapple (Ananas comosus) leaf in the formation of polyurethane foam

New polyurethane foams were fabricated utilizing cellulose-based fibers extracted from pineapple (Ananas comosus) leaf as raw material. The pineapple leaf fibers (PALF) were treated with alkali and subsequently bleached to enhance its fiber-matrix adhesion. Polyurethane composites have been prepared by incorporating 10% cellulose-based fibers extracted from PALF during polyurethane synthesis. The Fourier transform infrared (FTIR) spectra revealed that increase in the C-H and C-O vibrational modes absorption were observed when cellulose-based fibers were incorporated which might be attributed to the successful bonding of high purity cellulose-based fibers in the matrix of polyurethane foam. The physico-chemical, spectral and thermal properties of the polyurethane foam and the cellulose-based fibers reinforced polyurethane composites have been studied.Keywords: Cellulose-based fibers; Pineapple leaf fibers; Polyurethane foa

Characterization of porous low-k films using variable angle spectroscopic ellipsometry

doi:10.1063/1.2189018Variable angle spectroscopic ellipsometry (VASE™) is used as a tool to characterize properties such as optical constant, thickness, refractive index depth profile, and pore volume fraction of single and bilayer porous low-k films. The porous films were prepared using sacrificial pore generator (porogen) approach. Two sets of porous films with open- and closed-pore geometries were measured. Three models were used for data analysis: Cauchy, Bruggeman effective medium approximation (BEMA), and graded layer. Cauchy, a well-known model for transparent films, was used to obtain thickness and optical constant, whereas BEMA was utilized to calculate the pore volume fraction from the ellipsometric data. The Cauchy or BEMA models were then modified as graded layers, resulting in a better fit and a better understanding of the porous film. The depth profile of the porous film implied a more porous layer at the substrate-film interface. We found 3%-4% more porosity at the interface compared with the bulk for both films. This work shows that VASE™, a nondestructive measurement tool, can be used to characterize single- and multigraded layer porous films quickly and effectively.The authors would like to acknowledge the financial support of Semiconductor Research Corporation (SRC)

Isocyanate Reduction by Epoxide Substitution of Alcohols for Polyurethane Bioelastomer Synthesis

A phosphate ester-forming reaction was carried out by mixing epoxidized soybean oil with up to 1.5% o-phosphoric acid. In situ oligomerization took effect almost instantly producing a clear, homogeneous, highly viscous, and a low-acid product with a high average functionality. The resulting epoxide was used as a reactant for urethane bioelastomer synthesis and evaluated for rigid foam formulation. Results have shown that with a number of catalysts tested phosphoric acid significantly enhances a solvent-free oxirane ring cleavage and polymerization of the epoxidized soybean oil via phosphate-ester formation at room temperature. The resulting phosphoric acid-catalyzed epoxide-based bioelastomer showed an 80% decrease in extractable content and increased tensile strength at the same isocyanate loading relative to the noncatalyzed epoxide. The oligomerized epoxidized soybean oil materials exhibited ASTM hydroxyl values 40% less than the nonoligomerized starting material which translates to reduced isocyanate loadings in urethane applications

Soy based polyols [abstract]

Commercialization of Natural Oil Polyols, or NOPs, began in the late '90s to satisfy demand for sustainable products. One of the largest applications of NOPs is for urethane foams, where both Ford and General Motors are sparking demand by implementing NOPs into seats and other automotive foam applications. Such foams are also being introduced in shoe soles, mattresses, furniture, and other foam applications. The chemistry incorporated to make these foams is still emerging, and researchers at the University of Missouri have developed novel high molecular weight polyols and efficient means of production. An improved process for converting unsaturated vegetable oils into polyols is comprised of one or more of the following conversion mechanisms: i) bodying the vegetable oil to allow for increased hydroxyl equivalent weights, ii) partially oxidizing carbon-cabon [pi]-bonds to attach reactive moieties such as epoxy or alcohol moieties, iii) reacting carbon-carbon [pi]-bonds with monomers containing oxygen moieties, and iv) hydrolyzing ester bonds to replace ester moieties with alcohol moieties. A urethane foam recipe containing both alcohols and epoxies was particularly effective with the polyols of this invention. The useful molecules of this invention are not limited to polyols and applications generally include those applications where alcohols of carbon numbers greater than about 12 are applied. Additionally, two catalysts were identified to have significance. Potential Areas of Applications: * Anything in the wide range of urethane applications, but particularly consumer products where sustainability of raw materials is desirable * Insulation for appliances * Cushions for furniture/vehicle