Dedy Ng

Numerical simulations of LNG vapor dispersion in Brayton Fire Training Field tests with ANSYS CFX

Federal safety regulations require the use of validated consequence models to determine the vapor cloud dispersion exclusion zones for accidental liquefied natural gas (LNG) releases. One tool that is being developed in industry for exclusion zone determination and LNG vapor dispersion modeling is computational fluid dynamics (CFD). This paper uses the ANSYS CFX CFD code to model LNG vapor dispersion in the atmosphere. Discussed are important parameters that are essential inputs to the ANSYS CFX simulations, including the atmospheric conditions, LNG evaporation rate and pool area, turbulence in the source term, ground surface temperature and roughness height, and effects of obstacles. A sensitivity analysis was conducted to illustrate uncertainties in the simulation results arising from the mesh size and source term turbulence intensity. In addition, a set of medium-scale LNG spill tests were performed at the Brayton Fire Training Field to collect data for validating the ANSYS CFX prediction results. A comparison of test data with simulation results demonstrated that CFX was able to describe the dense gas behavior of LNG vapor cloud, and its prediction results of downwind gas concentrations close to ground level were in approximate agreement with the test data.

Nanoparticle Removal Mechanisms during Post-CMP Cleaning

In this research, the synergistic interaction of a brush, an adhered particle, and a wafer surface was evaluated during post chemical mechanical polishing (CMP). The approach includes theoretical analysis combined with experiments. A theoretical model was used to predict the particle/wafer contact area and contact force in order to identify the maximum tangential force required to remove particle agglomerates. Experiments are carried out to validate the analysis and to further study the removal mechanisms. This research suggests that a combined effort between a brush and surfactant would be effective in removing small particles during cleaning. For large particles, the mechanical force would be the dominating factor.

Role of Surfactant Molecules in Post-CMP Cleaning

In order to understand the effects of surfactant molecules on post-chemical mechanical planarization (CMP) cleaning, we used a tribology setup to simulate the cleaning process. An anionic surfactant (alcohol ether sulfates) was used during cleaning. Effects of surfactant concentration (below and above its critical micelles concentration) and temperature on cleaning were experimentally tested. Results showed that increase in surfactant concentration (over 0.75 wt %) can promote bilayer interaction of micelles on the hydrophilic particles. An interactive explanation of surface molecules with the wafer surface and the role of nanoparticles through friction is discussed. This understanding serves as a guideline to achieve effective particle removal for microelectronic industries.

Common lessons learned from an analysis of multiple case histories

In recent years there has been increased emphasis on process safety as a result of major chemical incidents involving gas releases, major explosions, and environmental incidents. Spending some time to fix our eyes on the historical catastrophes of industrial processes is necessary for process safety improvement. While each case history presents an important foundation for understanding, identifying, and eliminating root causes, to prevent recurrence of these incidents there is a need to identify the common lessons learned. Root causes are usually deficiencies in safety management systems, but can be any factor that would have prevented the incident if that factor had not occurred. In this article, multiple case histories were analyzed to understand the common similarities between process incidents. The objective of this article is to focus on learning some common lessons from the historical incidents in order to prevent recurrences of similar incidents. Ten important lessons were identified and are described in this article.

Friction and Wear-Mode Comparison in Copper Electrochemical Mechanical Polishing

This paper investigates friction and wear mechanisms of copper electrochemical mechanical polishing (ECMP) in pH and potential parameters as illustrated in the Pourbaix diagram. Three approaches were used in this study. First, a unique experimental setup was designed to investigate the effects of pH and electropotential on planarization. Second, the friction was measured in situ during polishing and used as a monitoring process. Third, the worn areas correlated with the frictional behavior of post-chemical mechanical polishing samples were characterized to study ECMP removal mechanisms. Results showed that friction maps can be generated in conjunction with the equilibrium Pourbaix diagram. This map is helpful in monitoring the removal rate and wear during copper ECMP. These findings can be used as guidance in ECMP process design and optimization.

Comparison of Interfacial Forces During Post-Chemical-Mechanical-Polishing Cleaning

This research investigates the interfacial forces involved in tribological interactions while removing nanosized particles during post-chemical-mechanical polishing cleaning. Surface and interfacial forces are discussed to understand the particle adhesion and subsequent removal through physical and chemical interactions. Approaches include theoretical analysis combined with experimental study. The theoretical analysis was focused on the forces that exist between particles and a substrate. Surface interaction consideration includes applied pressure, frictional force, and hydrodynamic drag. The polishing experiments were carried out on silicon wafers with SiO 2 slurry. Cleaning experiments were performed in de-ionized water using a polyvinyl acetal brush to remove particles from a hydrophilic-silicon surface. The fluid-drag force was found to affect the lubricating behavior of cleaning through changing material properties. Values of interfacial forces and their effects on cleaning were discussed along with a lubricating model system.

New Approaches in Investigation of Removal Mechanisms during Copper Chemical-Mechanical Polishing

This paper discusses new techniques for elucidating removal mechanisms of copper during chemical-mechanical polishing. Two new approaches were used in this research. One is the linkage between electrochemical, passivation, and wear for removal mechanisms. The other is to use an atomic force microscope to analyze polished copper surfaces at a nanometer length scale. Effects of pH, concentration of oxidizer, cathodic current, and amount of abrasive particles on surface materials removal were studied. Our investigation indicates that a balance between passivation and removal is needed to achieve a good localized planarization.

Interfacial forces in chemical-mechanical polishing

It has been known that there are many types of forces involved in tribological systems. During chemical-mechanical polishing (CMP), those forces appear to be synergetic and unpredictable. We have recently investigated the forces, such as van der Walls, electrostatic, fluid drag, and friction using experimental approach combined with theoretical analysis. Results showed that material detachment was affected by the hydrogen and van der walls. The fluid-drag force was found to influence the surface quality. In this presentation, we will discuss the surface and interfacial forces occurred during CMP. The types of forces will be discussed individually and compared as a group.

Interfacial Force Analysis and Lubrication Behavior During Post-CMP Cleaning

This research investigates the tribological interactions in removal of nano-sized particles during post chemical-mechanical polishing (CMP) cleaning. Possible surface interactive forces are discussed in order to understand the particle adhesion and subsequent removal through physical and mechanical interactions. The investigation was carried out using theoretical analysis combined with experimental study. The theoretical analysis was based on the particle adhesion and removal. Surface interaction consideration includes brush load, frictional force, and hydrodynamic drag. Finite element modeling using ABAQUS was used. The polishing experiments were done on silicon wafers with SiO2 slurry. Cleaning tests were done using DI water to brush clean the adhered particles from hydrophilic silicon surface. An Atomic Force Microscope (AFM) was used to evaluate the effective cleaning topography. It was found that particles are removed through sliding and rolling. The fluid film behavior was successfully illustrated using a modified Stribeck behavior with considering of changing material properties. New particle removal mechanisms and lubrication model were proposed.Copyright

Nano-Particle Interaction During Chemical-Mechanical Polishing

We investigate the particle adhering and removal processes during CMP and post-CMP cleaning. The mechanical interaction between abrasive particles and wafer surface was studied using a microcontact wear model. This model considers the particle effects between the polishing interfaces during load balancing. Experimental results on polishing and cleaning are compared with numerical analysis. This study suggests that during post-CMP cleaning, a combined effort in chemical and mechanical interaction (tribochemical interactions) would be effective in removal small particles during cleaning. For large particles, more mechanical forces would be more effective.Copyright