Biomedical subjects

Leonard F Pease

Publications and source records attributed to Leonard F Pease.

4 recordsLinked to original sources

PubMed2006

Charge driven, electrohydrodynamic patterning of thin films.

In electrohydrodynamic patterning, electrical forces and surface tension acting at the interface between two fluids sandwiched between silicon wafers compete to set the period of pillar arrays, gratings, and concentric rings. Shrinking the period to deep submicron lengths requires a precise understanding of the source of the electric field. Previous modeling efforts have assumed that applied voltages, contact potentials, and static charge drive the flow. Here we show the location of that charge and the tangential stress it engenders to impact profoundly how the period and growth rate depend on the dielectric contrast and the relative film thickness. The pillar-to-pillar spacing scales inversely proportional to the charge density, and densities of approximately 1 mC/m(2) (approximately 1 charge/100 nm(2)) suffice to produce micron sized pillars.

Electrochemistry↗

PubMed2005

Electric-field-induced patterns in thin polymer films: weakly nonlinear and fully nonlinear evolution.

A thin polymer melt on a substrate can be unstable to an electric field normal to the interface, a phenomenon that can be harnessed as a patterning technique with a range of potential applications. Motivated by the variety of patterns observed in experiments for polymers under both unpatterned and patterned masks, we describe here, from theoretical and numerical analyses, how nonlinear effects govern the growth of the instability and determine the final patterns. In particular, we discuss the nonlinear growth in terms of interactions among different Fourier modes and show that the second- and third-order nonlinearities favor the growth of hexagonal patterns under a featureless mask, in agreement with experimental observations. Also, numerical simulations based on the fully nonlinear model validate the prediction of the weakly nonlinear analysis: hexagonal patterns do emerge under an unpatterned mask. Furthermore, in one-dimensional simulations, we demonstrate the energetic evolution of this patterning process and reveal several "kinetically stable structures" along the path to the thermodynamically stable state. Two-dimensional simulations allow us to study the effects of both mask patterns and the initial film thickness. Generally, patterns on the mask guide the growth such that the pattern conforms to the geometric shapes. Interestingly, a small cylindrical protrusion at the center of the mask can produce exactly the same pattern as a large, flat, circular protrusion. The initial film thickness or the thickness ratio of the polymer layer to the air gap plays an important role in determining the final pattern formed. Finally, we demonstrate, by two simple examples, that the simulations can provide insights on "smart" mask designs for producing large areas of well-ordered patterns.

Journal Article↗

PubMed2004

Cylindrically symmetric electrohydrodynamic patterning.

Cylindrically symmetric structures such as concentric rings and rosettes arise out of thin polymeric films subjected to strong electric fields. Experiments that formed concentric rings and theory capable of explaining these and other cylindrical structures are presented. These rings represent an additional member of a class of structures, including pillars and holes, formed by electrohydrodynamic patterning of thin films, occasionally referred to as lithographically induced self-assembly. Fabrication of a set of concentric rings begins by spin coating a thin poly(methyl methacrylate) film onto a silicon wafer. A mask is superimposed parallel to the film leaving a similarly thin air gap. Electric fields, acting in opposition to surface tension, destabilize the free interface when raised above the glass transition temperature. Central pillars nucleate under small cylindrical protrusions patterned on the mask. Rings then emerge sequentially, with larger systems having as many as 10 fully formed rings. Ring-to-ring spacings and annular widths, typically on the order of a micron, are approximately constant within a concentric cluster. The formation rate is proportional to the viscosity and, consequently, has the expected Williams-Landel-Ferry dependence on temperature. In light of these developments we have undertaken a linear stability analysis in cylindrical coordinates to describe these rings and ringlike structures. The salient feature of this analysis is the use of perturbations that incorporate their radial dependence in terms of Bessel functions as opposed to the traditional sinusoids of Cartesian coordinates. The theory predicts approximately constant ring-to-ring spacings, constant annular widths, and growth rates that agree with experiment. A secondary instability is observed at higher temperatures, which causes the rings to segment into arcs or pillar arrays. The cylindrical theory may be generalized to describe hexagonal pillar/hole packing, gratings, and rosettes with the first being of particular importance given the ubiquitous observation of hexagonal packing. The perturbation analysis presented here is relevant to any system with cylindrical symmetry, for which the radial dependence can be described in terms of Bessel functions.

Journal Article↗

PubMed2004

Limitations on length scales for electrostatically induced submicrometer pillars and holes.

Thin leaky and perfect dielectric films can be driven electrically to form well-ordered patterns, typically of pillar arrays. While the technique appears to promise nanometer scale features, this paper begins to examine some of the limitations. The process, sometimes referred to as lithographically induced self-assembly, begins by spin coating a polymer onto a silicon wafer generating an initially featureless film and then overlaying a mask, which may be patterned, leaving a small gap. This configuration is heated above the glass transition temperature of the polymer, upon which flow ensues with a characteristic wavelength set by a combination of electrical forces and surface tension. The authors recently examined the initial stages of this process under pattern-free masks by deriving a generalized linear stability analysis not restricted to the lubrication approximation (J. Chem. Phys. 2003, 118, 3790). Herein comparison of this model with experimental data from the literature finds good agreement over a wide range of conditions including applied voltages and oxide layers on the mask and substrate. A significant discrepancy at the highest fields may be due to dielectric breakdown, suggesting that the minimum feature size may be limited. Viscous effects may also limit the effectiveness of large decreases in surface tension or large increases in electric field, leading to lower limits for the feature size. Long-range ordering seems to decrease as surface tension decreases and the potential increases, indicating that smaller pillars come with decreased quality. In the absence of an electric field, consideration of the viscosity-dependent time scale suggests an explanation for the apparent conflict between observations by Schäffer et al. (Nature 2000, 403, 874) and those of Chou and Zhuang (J. Vac. Sci. Technol., B 1999, 17, 3197).

Journal Article↗