Ali R. Mazaheri

A Model for Effect of Colloidal Forces on Chemical Mechanical Polishing

It is well known that the variation of slurry pH could significantly affect the chemical-mechanical polishing (CMP) process. In this study, a particle-scale mechanical model for surface removal rate that includes the double layer (dl) forces of abrasives and wafer surface is developed. It is shown that the van der Waals force and the electrical dl attraction and repulsion play a major role in CMP processes. Furthermore, the magnitudes and signs of the zeta potential of the surface and the abrasives significantly affect the removal rates. The results show that the removal rate increases sharply for the cases that the zeta potentials of the surface and abrasive have opposite sign. On the other hand the removal rate decreases when the zeta potentials have the same sign. The removal rates for polishing of tantalum with silica and alumina abrasives are compared with the available data and qualitative agreements are observed.

Modeling the Effect of Bumpy Abrasive Particles on Chemical Mechanical Polishing

In this study, the effect of course surface roughness of abrasive particles on the removal rate in chemical mechanical polishing is analyzed. The mechanical contact theory is used and the relationship between the polishing rate and properties of pad, wafer, and abrasive particle is studied. Particular attention is given to the case where abrasives are compact bumpy particles and the pad is soft. It is found that the polishing rate depends on the size of abrasive particle bumps. The penetration depth of bumpy particles is larger than that of the spherical particles of the same diameter; the removal rate by the abrasive wear mechanism for bumpy particles, however, is less than the corresponding smooth spherical particles.

Hot-gas flow and particle transport and deposition in a candle filter vessel

Abstract Hot-gas flow and particle transport and deposition in an industrial filtration system are studied. The special example of the Siemens-Westinghouse filter vessel at the Power System Development Facility at Wilsonville, Alabama is treated in detail. This tangential flow filter vessel contains clusters of 91 candle filters, which are arranged in two tiers. The upper tier containing 36 candle filters is modeled by six equivalent filters. Seven equivalent filters are used in the computational model to represent the 55 candle filters in the lower tiers. The Reynolds stress turbulent model of FLUENT™ code is used, and the gas mean velocity and root mean square fluctuation velocities in the filter vessel are evaluated. The particle equation of motion used includes drag and gravitational forces. The mean particle deposition patterns are evaluated and the effect of particle size is studied. The computational results indicatethat large particlesof the order of 10 μm or larger are removed from the gas due to the centrifugal forces exerted by rotating flow between the shroud and the refractory.

Aerosol transport and deposition analysis in a demonstration-scale hot-gas filter vessel with alternate designs

Abstract In this paper the effect of design alternation on the particle transport and deposition in industrial hot-gas filter vessels are studied. Particular attention was given to the Siemens—Westinghouse filter vessel at the Power Development Facility, Wilsonville, AL, USA. The gas flow and particle deposition patterns for the current vessel are first evaluated. It is shown that in the present vessel, the majority of large particles (10–30 μm) are removed from the gas stream in the shroud. Then, three alternative filter vessel designs including using a short shroud, one with no shroud and a vessel with a deflector plate are considered. The effects of design alternations on the gas flow and transport and deposition of particles of different sizes are evaluated. The simulation results suggest that it is possible to modify the shroud in a way to allow large particles to deposit on the filters. Thus, the back-pulse process could more easily remove the filter cake.

Temperature Distribution in a Demonstration-Scale Filter Vessel With and Without Ash Bridging

The influence of ash bridging on the temperature distribution of the ceramic filters of a demonstration-scale filter vessel is analyzed. The Reynolds stress turbulence model of FLUENT™ code is used to study the gas flow behavior inside the filter vessel. Particle equations of motions are employed, and transport and deposition of the micron-size aerosols are studied. Computational results predict that ash bridging leads to a non-uniform temperature distribution along the ceramic candle filters in the bridging region. The analyses of ash bridging deposition on the internal surfaces of the filter show that the absence of ash bridging tends to promote the deposition of particles of 10 μm on the surfaces.

Single-phase and multi-phase fluid flow through an artificially induced, CT-scanned fracture

Single- and multi-phase flow through rock fractures occurs in various situations, such as transport of dissolved contaminants through geological strata, migration of dense non-aqueous phase liquids through fractured rocks, sequestration of carbon dioxide in brine-saturated strata, and oil recovery. The presence of fractures in a reservoir plays a major role in the fluid flow patterns and the fluids transport. In this study the Brazilian test technique was employed to induce an extensional fracture with dimensions of about 9.2 (cm)× 2.7 (cm)×0.77 (cm) in a layered Berea (calcite-cemented) sandstone sample. High-resolution X-ray micro-tomography (CT) imaging was used to determine the geometry of the fracture. A post-processing code was developed and used to computationally model the fracture geometry; Gambit was then used to generate an unstructured grid of about 1,000,000 cells. Single-phase and two-phase flow through the fracture were studied using FLUENT. The Volume of Fluid (VOF) model was employed for the case of two-phase flow. Flow patterns through the induced fracture were analyzed. In geological flow simulations, flow through fractures is often assumed to occur between parallel plates. The combination of CT imaging of real fractures and computational fluid dynamic simulations may contribute to a more realistic and accurate description of flow through fractured rocks.

Transonic and Supersonic Inviscid Flow Equations in 3-D Eccentric Nozzles

Three dimensional unsteady inviscid flows in convergent-divergent nozzles is of importance in understanding the stability of rockets and jet propulsion. A computer program for evaluating unsteady inviscid flow conditions in three-dimensional eccentric as well as concentric nozzles is developed. The program uses the cell-centered finite-volume method based on Roe’s approximate Riemann solver scheme. The flow simulation results in concentric circular nozzles are compared with the one-dimensional analytic solutions and the accuracy of the computation model is verified. The results for steady and unsteady flows through eccentric and concentric convergent-divergent nozzles are then presented. A range of exit to throat areas, pressure ratios, and inlet Mach number are considered.Copyright

Effects of Bounce on Particle Transport, Deposition and Removal in Turbulent Channel Flow

Effects of bounce on particle transport, deposition and removal in turbulent channel flow are studied. The pseudo-spectral method is used to generate the instantaneous turbulent fluid velocity field by Direct Numerical Simulation (DNS) procedure. The particle equation of motion includes all the relevant hydrodynamic forces. In addition, simulation accounts for particle adhesion, resuspension and rebound processes. For particle bounce from the surface, the critical velocity is evaluated and is used in the analysis. Effects of bounce during particle-wall collisions on the deposition rate are also studied.Copyright

Multiphase Flows Through Poro-Elastic Media: A Continuum Model

Based on the basic balance laws and the second law of thermodynamics, a model for multiphase fluid flows through poro-elastic media is presented. The basic conservation laws. Including the balance of phasic equilibrated forces are are described. Based on the thermodynamics of the multiphase mixture, appropriate constitutive equations are formulated. It is shown that the present theory leads to the extension of Darcy’s law and contains, as its special case, Biot’s (1957) theory of saturated poro-elastic media. The special case of gas-liquid flows in porous media is discussed.Copyright