RESEARCH ACTIVITIES

Chemical Mechanical Planarization (CMP)

CMP is defined as the technique for planarizing different substrates by the combination of chemical and mechanical action. Substrates can be either a metal or a semiconductor or an insulator. Chemical action is provided by different chemicals, termed in general as “slurry” which usually consists of an oxidizer, chelating agents, an inhibitor (optional), metal ions (optional) and abrasives. Mechanical action is provided by the dynamic pad action on the substrate. The pad action removes any film formed on the substrate and thus fresh surface is always exposed to the chemical attack by the slurry. Concentrations of all the chemicals used in the slurry and dynamic parameters like pad speed, down force etc., determine the optimal removal rate, surface quality, dishing, erosion and other factors which are necessary for achieving a smooth planarized surface. In our group, we develop new chemicals/slurries for different metal CMP applications. For example, benzotriazole (BTA) widely used as an inhibitor for Cu CMP possess difficulties in post CMP process. Also BTA may not be a good inhibitor for ECMP applications (see below). Research is on the way to synthesize new inhibitors in our lab and primary results have been successful.

Electro Chemical Mechanical Planarization (ECMP)

ECMP uses a unique way of planarizing metal substrates. In conventional CMP, oxidation of metals is achieved by oxidizers such as hydrogen peroxide. In ECMP, electrochemical potential (anodic) acts as the oxidant. The advantage of this technique is metal substrates can be planarized at high rate but with low down force (< 1 psi). This provides the future for IC applications as low-K dielectric films used as substrates for Cu interconnects are very fragile and can break at high down force as applied in CMP. Besides Cu, currently we are investigating NiP hard disk substrates.

Preparation and Characterization of Liposomes

Chemical encapsulation and controlled release of active ingredients have been widely used in many industrial and consumer products. Micelles, liposomes, and surfactant coated particles are among the most adapted supramolecular structures for microencapsulation. Unlike micelles and other similar structures in which only one types of molecule can be encapsulated (hydrophobic or hydrophilic), liposomes can trap hydrophobic molecules in its bilayer region as well as water soluble species in its water core. Hydrotrope method is used to prepare liposome particles with controllable size. We have also synthesized liposomes encapsulating metal ions which can be used for tungsten CMP process.

Investigation of micelles using dynamic NMR

The phenomena of solubilization, aggregation and micelle formation are at the core of aqueous surfactant chemistry which can be investigated with a wide range of analytical techniques. Each of these techniques has its own unique advantages and, sometimes, limitations. For example, a dynamic light scattering method can effectively measure particle size and particle size change in a surfactant system.  However, the technique is non-chemical specific.  In other words, any dust particles and air bubbles can pose severe interference.  Pulsed field gradient NMR method, however, can discriminate those particles made of materials that do not have the targeted chemical composition. Furthermore, PFG-NMR can also provide information on the molecular dynamics which yield very valuable information about the surfactant system such as solubilization and partition coefficient of a particular solute. Pulsed Field Gradient NMR (PFG-NMR) is a powerful, non-destructive technique of measuring self-diffusion coefficients. The technique can be used for a variety of systems without the need for any special handling or labeling of the species of interest. PFG-NMR is used to study the aggregation of molecules and the molecular interactions. Besides PFG NMR used for micelles study, dynamic NMR can also be used to determine the longitudinal relaxation times (T1) of water in silica or alumina which serves as a good indicator for the abrasive-metal interaction during CMP. This can be directly applied to characterize material removal rate (MRR) from different slurries in CMP.

Emulsion and Microemulsion Polymerization

Microemulsion is an isotropic, optically transparent or translucent, and thermodynamically stable dispersion that forms spontaneously by mixing two ordinarily immiscible liquids (i.e., water and oil) and a surfactant with or without an alcohol of intermediate chain length as a cosurfactant.  Because of their well-defined nature, microemulsions have been used as potential media for polymerization to prepare nanosize thermodynamically stable lattices, which are not easily obtained from other systems. We have previously demonstrated the synthesis of polystyrene nanoparticles through microemulsion polymerization wherein the molecular weight and particle size are studied as a function of monomer and initiator concentration. Surfactant charges also determine the nature of microemul.polymerization. For example, anionic surfactant like sodium dodecyl sulfate (SDS) and cationic surfactant dodecyl trimethyl ammonium bromide (DTAB) can be compared when potassium persulfate (KPS) is used as initiator. KPS undergoes homolytic cleavage at high temperature to form radical anions which will be attracted more towards DTAB rather than SDS leading to high amount of initiators available within the micelles and consequently, this results in relatively low molecular weight polymer particles. 

Photocatalysis

Photo induced redox reactions on semiconductor particles have been extensively investigated for their applications in waste treatment and solar energy conversion. The study of their potentials in organic synthesis, however, was limited. With the funding from NSF, PRF, and CAMP, we have systematically investigated some synthetically useful chemical transformations on semiconductor particles. The research has yielded not only an understanding of the mechanistic aspects of these chemical transformations but also some useful supporting techniques, such as a new dosimeter for quantum yield measurements and a new synthetic-scale photochemical flow reactor. Our long term objective is to further develop these and more new chemical transformations as a source of for the environmentally benign alternatives. These new chemical processes may be used to replace their corresponding conventional processes in industry in order to reduce pollution at its source. The applications of our research results in other areas include waste degradations, evaluation of sunscreen products, and the monitoring of UV- and g-radiations. For inorganic materials synthesis, a new method has been developed to coat metals such as Pd, Ag, and Au onto fine particles such as BaTiO₃, TiO₂, SiO₂ and Al₂O₃. Unlike the conventional electrochemical plating method, the photo-assisted process eliminates the use of toxic or problematic chemical or electrochemical reducing agents to achieve co-deposition of metal ions with different reduction potentials. The coated particles are of interests of electronic components, catalysts, and conducting adhesives. There are only a limited number of water-soluble polymerization initiators available for water-based polymerization. With the increasing demand of water-based polymerization for various environmental reasons, the need for water soluble organic peroxides with various reactivities has also risen. We have synthesized and investigated the first group of water soluble dialkyl peroxides and peroxyesters. These surface active peroxides are designed to be water soluble initiators at a desired interface. Currently, these compounds are used in the preparation of higher molecular weight and more uniformly dispersed polymer particles and polymerized surfactant vesicles. The polymerized vesicles will be used as reaction media for controlling reaction rates or the uniformity of inorganic particles, and as membrane analogs for ultra filtration, reversed osmosis, and controlled drug delivery.

Benzoyl peroxide has long been used in the topical treatment of acne vulgaris. However, the stability and solubility problems associated with benzoyl peroxides actually limit its efficacy. Newly synthesized water soluble peroxides have overcome the stability problem and were found to be more effective against both gram-positive and gram-negative bacteria with a prolonged activity as compared to benzoyl peroxide. We have shown that the oxidizing properties of dioxy group of these peroxides are responsible for their antibacterial activity.

 

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Last updated: 11/30/2007 18:55:00 -0500