Joel L. Plawsky

Surface modified spin-on xerogel films as interlayer dielectrics

SiO2-based xerogels are highly porous materials that may enhance the performance of microelectronic devices due to their extremely low dielectric constants (e=1.36–2.2). Conventional xerogel and aerogel manufacturing techniques include an expensive and hazardous supercritical drying step to deposit crack free, high porosity films. Ambient drying techniques have recently been developed and in this article, we discuss how the process parameters in the ambient drying process affect the properties of a spin-coated film. Successful spin-on deposition of highly porous (>70%), thick (>1 μm), crack-free, xerogel films was accomplished using a solvent saturated atmosphere during spinning and aging. The saturated atmosphere allowed for the isolation of each processing step and a better understanding of the effects of process variable changes. The film porosity was controlled by varying the extent of silylation (surface modification), the aging time, or the initial water/silane ratio. Fourier transform infrared spec...

Thermal conductivity study of porous low-k dielectric materials

An experimental method based on the 3ω technique has been developed to measure thermal conductivity of porous Xerogel films as a function of porosity. The results show that the thermal conductivity of these porous dielectric films can be an order of magnitude smaller than that of SiO2. To account for the porosity dependence of thermal conductivity, two porosity weighted semiempirical models are introduced. These models suggest the scaling rule expressing the thermal conductivity as a function of porosity. The decrease observed in thermal conductivity of porous films suggests that the tradeoff between thermal and electrical performance is an important consideration when implementing porous dielectric materials as interlevel dielectrics for on-chip interconnects.

Experimental investigation of contact angle, curvature, and contact line motion in dropwise condensation and evaporation

Image-analyzing interferometry is used to measure the apparent contact angle and the curvature of a drop and a meniscus during condensation and evaporation processes in a constrained vapor bubble (CVB) cell. The apparent contact angle is found to be a function of the interfacial mass flux. The interfacial velocity for the drop during condensation and evaporation is a function of the apparent contact angle and the rate of change of radius of curvature. The dependence of velocity on the apparent contact angle is consistent with Tanners scaling equation. The results support the hypothesis that evaporation/condensation is an important factor in contact line motion. The main purpose of this article is to present the experimental technique and the data. The equilibrium contact angle for the drop is found experimentally to be higher than that for the corner meniscus. The contact angle is a function of the stress field in the fluid. The equilibrium contact angle is related to the thickness of the thin adsorbed film in the microscopic region and depends on the characteristics of the microscopic region. The excess interfacial free energy and temperature jump were used to calculate the equilibrium thickness of the thin adsorbed film in the microscopic region.

Spaceflight Promotes Biofilm Formation by Pseudomonas aeruginosa

Understanding the effects of spaceflight on microbial communities is crucial for the success of long-term, manned space missions. Surface-associated bacterial communities, known as biofilms, were abundant on the Mir space station and continue to be a challenge on the International Space Station. The health and safety hazards linked to the development of biofilms are of particular concern due to the suppression of immune function observed during spaceflight. While planktonic cultures of microbes have indicated that spaceflight can lead to increases in growth and virulence, the effects of spaceflight on biofilm development and physiology remain unclear. To address this issue, Pseudomonas aeruginosa was cultured during two Space Shuttle Atlantis missions: STS-132 and STS-135, and the biofilms formed during spaceflight were characterized. Spaceflight was observed to increase the number of viable cells, biofilm biomass, and thickness relative to normal gravity controls. Moreover, the biofilms formed during spaceflight exhibited a column-and-canopy structure that has not been observed on Earth. The increase in the amount of biofilms and the formation of the novel architecture during spaceflight were observed to be independent of carbon source and phosphate concentrations in the media. However, flagella-driven motility was shown to be essential for the formation of this biofilm architecture during spaceflight. These findings represent the first evidence that spaceflight affects community-level behaviors of bacteria and highlight the importance of understanding how both harmful and beneficial human-microbe interactions may be altered during spaceflight.

REVIEW OF THE EFFECTS OF SURFACE TOPOGRAPHY, SURFACE CHEMISTRY, AND FLUID PHYSICS ON EVAPORATION AT THE CONTACT LINE

Liquid-vapor phase-change processes are becoming increasingly important in a wide variety of fields ranging from energy conversion, to microelectronics cooling, MEMs devices, and self-assembly. The phase change in these systems is governed by processes that occur at the contact line, where three phases meet. Evidence suggests that alterations of the surface chemistry and surface topography on the nanoscale can be used to dramatically enhance the phase-change process. This article reviews the current state of the art in nanoscale surface modification as applied to the enhancement of evaporative processes.

Omnidirectional reflector using nanoporous SiO 2 as a low-refractive-index material

Triple-layer omnidirectional reflectors (ODRs) consisting of a semiconductor, a quarter-wavelength transparent dielectric layer, and a metal have high reflectivities for all angles of incidence. Internal ODRs (ambient materials refractive index n >> 1.0) are demonstrated that incorporate nanoporous SiO2, a low-refractive-index material (n = 1.23), as well as dense SiO2 (n = 1.46). GaP and Ag serve as the semiconductor and the metal layer, respectively. Reflectivity measurements, including angular dependence, are presented. Calculated angle-integrated TE and TM reflectivities for ODRs employing nanoporous SiO2 are R(int)/TE = 99.9% and R(int)/TM = 98.9%, respectively, indicating the high potential of the ODRs for low-loss waveguide structures.

High thermal conductivity epoxy-silver composites based on self-constructed nanostructured metallic networks

We demonstrate epoxy-silver nanoparticle composites with high thermal conductivity κ enabled by self-constructed nanostructured networks (SCNN) forming during the curing process at relatively low temperatures (150 °C). The networks formation mechanism involves agglomeration of the polyvinylpyrrolidone (PVP) coated nanoparticles, PVP removal, and sintering of the nanoparticles at suppressed temperatures induced by their small diameters (20–80 nm). Sintering and the SCNN formation are supported by differential scanning calorimetry and electron microscopy investigations. The formation of SCNN with high aspect ratio structures leads to enhancements in the measured thermal conductivity κ of the composite by more than two orders of magnitude versus the pure epoxy. However, κ enhancements are modest if microparticles (1.8–4.2 μm) are employed instead of PVP coated nanoparticles. The κ trends are qualitatively explained using a percolating threshold thermal conductivity model for the microcomposites. For the nano...

Internal high-reflectivity omni-directional reflectors

An internal high-reflectivity omni-directional reflector (ODR) for the visible spectrum is realized by the combination of total internal reflection using a low-refractive-index (low-n) material and reflection from a one-dimensional photonic crystal (1D PC). The low-n layer limits the range of angles in the 1D PC to values below the Brewster angle, thereby enabling high reflectivity and omni-directionality. This ODR is demonstrated using GaP as ambient, nanoporous SiO2 with a very low refractive index (n=1.10), and a four-pair TiO2/SiO2 multilayer stack. The results indicate a two orders of magnitude lower angle-integrated transverse-electric-transverse-magnetic polarization averaged mirror loss of the ODR compared with conventional distributed Bragg reflectors and metal reflectors. This indicates the high potential of the internal ODRs for optoelectronic semiconductor devices, e.g., light-emitting diodes.

Etching of xerogel in high-density fluorocarbon plasmas

The etching of various xerogel films has been studied in high-density fluorocarbon plasmas. The xerogel etch rate is in part enhanced by the porosity. In discharges resulting in low surface polymerization, such as CF4 or oxygen-rich fluorocarbon plasmas, an additional enhancement up to 60% is observed. When the polymerization of the discharge is increased, this additional enhancement disappears and the xerogel etch rate becomes more suppressed. The suppression is more pronounced for xerogel films with a higher porosity and a larger pore size. X-ray photoelectron spectroscopy analysis on partially etched samples shows that the suppression in etch rate is accompanied by an increasing amount of fluorocarbon material at the xerogel surface, especially in the pores of the xerogel structure. Finally, a 30% porous xerogel film was patterned using CHF3 as an etching gas. Slight bowing of the sidewalls was observed.

Effects of processing history on the modulus of silica xerogel films

Sintered xerogel films (porous SiO2) show a higher elastic modulus than other amorphous low dielectric constant (K) materials available for the same value of K. By comparing xerogels that were sintered, templated or made with ethylene glycol or ethanol as solvents, we show that process history is at least as important as the chemistry of the solid matrix or the porosity. The modulus extrapolated to zero porosity for the porous sintered and templated films is the same as those of the dense films made by chemical vapor deposition of SiO2. This suggests that the solid matrix for sintered xerogel films is close to ideal and their modulus is better because of the ordered arrangement of pores and fusion of particles making up the matrix. The modulus measured by nanoindentation on thick xerogel films (>0.8 μm) is well explained by the open cell foam model.