Manas Ojha

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.

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.

Role of solid surface structure on evaporative phase change from a completely wetting corner meniscus

The transport processes that occur at small length scales are greatly influenced by interfacial and intermolecular forces. Surface roughness at the nanoscale generates additional intermolecular interactions that arise due to the increased surface area. In this work, we have experimentally studied how the magnitude as well as the shape of surface roughness influences the microscale transport processes that occur in the contact line region of a liquid corner meniscus. The surface roughness contribution to the interaction potential was calculated and a direct relationship between the wetting properties of the liquid and the underlying surface properties was obtained. Since the underlying roughness alters the surface potential, the shape of the meniscus and in turn, the resulting capillary and disjoining pressure forces also changed. Atomic force microscopy was utilized to obtain a detailed characterization of the shape of the prepared surfaces. Surface morphology features were obtained from a height-height c...

Polymer Penetration and Pore Sealing in Nanoporous Silica by CHF3 Plasma Exposure

The polymerization and pore sealing that occurs during fluorocarbon plasma treatment of nanoporous silica xerogel was investigated experimentally by Rutherford backscattering spectroscopy and successfully modeled using a diffusion-reaction analysis. CHF 3 was used as a reactant gas to expedite the rate of polymerization due to the presence of hydrogen in its structure and its low C/F ratio. Knudsen diffusion was assumed to be the dominant mechanism for the motion of polymer precursor species through the nanoporous material over the range of pressures used in the plasma experiments. The amount of fluorine atoms deposited on the sidewalls of the pores was measured as a function of depth in the dielectric film and that amount was assumed to correspond with the mass of the polymer layer formed inside the pores. By applying a Thiele-type analysis to this system, we successfully matched model calculations with measured fluorine amounts, predicted the time required to reach a steady-state concentration of the polymer precursor in a pore (∼10 - 7 s) and predicted the time required to seal off pore necks at the surface of the dielectric (∼70 s). Both the model and experimental results show a greater depth of penetration and an enhanced deposition of polymer at higher porosities, confirming the need for pore sealing during back-end-of-the-line processing of nanoporous materials.

Optical Interconnect Components for Wafer Level Heterogeneous Hyper-Integration

Optical waveguides for three-dimensionally stacked chip fabrication technologies, in which optical connection between layers plays a central role, are described. CMOS-compatible approaches to optical via and waveguide fabrication are addressed. Detailed modeling is used for design optimization and also addresses how manufacturing variations from ideal design may affect device performance.

A Model for the Etching of Nanoporous Silica in C 4 F 8 Plasmas Based on Pore Geometry and Porosity Effects

Plasma etching of nanoporous materials (NPMs) is a complicated phenomenon and depends upon the NPMs parameters, such as the overall porosity, the pore size and structure, and the concentration of organic groups on the surface of the film and inside its pores. Polymerization during fluorocarbon plasma exposure is ubiquitous and suppresses the net etching rate. The model developed here accounts for the polymerization that occurs. In this study, a new plasma etching model is developed that applies in the high-polymerization-rate regime. This new model includes pore structure factors (pore shape and size) as well as mass and volume effects in the form of the films overall porosity. According to the model, at low porosities the etching rate varies directly with the total porosity. However, as the porosity of the film increases, surface effects become important and the etching rate is affected by both total porosity and pore geometry. Finally, we correlate the corrected etching rate, including the porosity and the average pore size effect, with the etching of solid SiO 2 over a wide range of bias voltage. In the fluorocarbon suppression regime, the corrected etching rate expression agrees with the experimental results and collapses all etch rate data onto a single curve.