Srini Raghavan

Treatment of alumina and silica chemical mechanical polishing waste by electrodecantation and electrocoagulation

Electrocoagulation and electrodecantation were investigated as methods to concentrate solids from the dilute suspensions typically encountered as waste streams of chemical mechanical polishing operations. Model silica and alumina suspensions containing particles of approximately 200 nm in diameter were studied. 3 L batches of suspension were subjected to electric fields of several volts per centimeter and monitored for particle removal. Electrodecantation was shown to clarify charge stabilized, low conductivity suspensions (20 mS/cm) without modifying the mean particle size or surface/charge characteristics. The technique is thus suited to circumstances where recycle and reuse of particles is desirable, though the results also show that Joule heating may disrupt the decantation process and so must be controlled in order to apply the method to higher conductivity suspensions. Electrocoagulation was shown to effectively destabilize and clarify high conductivity suspensions (1300 μS/cm), producing aggregates several‐fold larger than the primary particle size, which subsequently sedimented from suspension. Intermittent application of the electric field was found to yield superior rates of particle removal while consuming substantially less electrical power (ca. 1 Wh/L of clear solution).

Electrochemistry of Chemical Vapor Deposited Tungsten Films with Relevance to Chemical Mechanical Polishing

The electrochemical behavior of chemically vapor deposited tungsten films in solutions of interest to tungsten chemical mechanical polishing has been investigated using dc potentiodynamic polarization, linear polarization, and Tafel methods. It was found that in the absence of an oxidizer, the tungsten surface was passivated most effectively at acidic pH values. At pH 2 or 4, a WO{sub 2}/WO{sub 3} duplex oxide layer of less than 50 A thickness was detected over the tungsten layer by X-ray photoelectron spectroscopy. The oxide layer formed at pH 2 was much thicker, and had better passivity compared to the oxide formed at pH 4. Addition of H{sub 2}O{sub 2} at pH 2 or 4 resulted in a dramatic increase in tungsten dissolution.

Electrochemical Measurements during the Chemical Mechanical Polishing of Tungsten Thin Films

A polishing tool and a potentiostat were used to simultaneously polish and measure the direct current (dc) open-circuit potential and anodic polarization behavior of chemical vapor deposited tungsten films in the presence of various oxidants. Of the different oxidants tested at pH 1.5 or pH 4.4, (NH 4 ) 6 Mo 7 O 24 formed the most protective passive layer on tungsten. Even in the presence of the most aggressive oxidant, Fe(NO 3 ) 3 , the dissolution rates of chemical vapor deposited tungsten were approximately 3 nm/min during abrasion, which is a very small fraction of typical removal rates reported for chemical mechanical polishing of tungsten. This indicates that electrochemical oxidation followed by abrasive removal of the oxidation product and dissolution may not be the primary mechanism for tungsten removal. Atomic force microscopy scans of polis ed tungsten films indicate that corrosion assisted fracture may be an important removal mechanism for tungsten during chemical mechanical polishing.

Metallic copper corrosion rates, moisture content, and growth medium influence survival of copper ion-resistant bacteria

The rapid killing of various bacteria in contact with metallic copper is thought to be influenced by the influx of copper ions into the cells, but the exact mechanism is not fully understood. This study showed that the kinetics of contact killing of copper surfaces depended greatly on the amount of moisture present, copper content of alloys, type of medium used, and type of bacteria. We examined antibiotic- and copper ion-resistant strains of Escherichia coli and Enterococcus faecium isolated from pig farms following the use of copper sulfate as feed supplement. The results showed rapid killing of both copper ion-resistant E. coli and E. faecium strains when samples in rich medium were spread in a thin, moist layer on copper alloys with 85% or greater copper content. E. coli strains were rapidly killed under dry conditions, while E. faecium strains were less affected. Electroplated copper surface corrosion rates were determined from electrochemical polarization tests using the Stern–Geary method and revealed decreased corrosion rates with benzotriazole and thermal oxide coating. Copper ion-resistant E. coli and E. faecium cells suspended in 0.8% NaCl showed prolonged survival rates on electroplated copper surfaces with benzotriazole coating and thermal oxide coating compared to surfaces without anti-corrosion treatment. Control of surface corrosion affected the level of copper ion influx into bacterial cells, which contributed directly to bacterial killing.

Quantitative Analysis of Adsorbed Serum Albumin on Segmented Polyurethane Using FT-IR/ATR Spectroscopy

FT-IR attenuated total reflection (ATR) spectroscopy was applied to quantitatively determine the extent of bovine serum albumin adsorbed onto a biomedical-grade poly(ether)urethane film deposited on a ZnSe internal reflection element (IRE). The method of adsorption density determination was based on the optical principles for a stratified medium consisting of three layers. The spectral peak area due to bulk solution was less than 1% of total peak area. The measured adsorption density of albumin in a flowing system was 3.9 μg/cm2 at a solution concentration of 4.5 g/100 mL.

A Comparative Electrochemical Study of Copper Deposition onto Silicon from Dilute and Buffered Hydrofluoric Acids

An electrochemical direct current polarization method was used to investigate characteristics of copper deposition onto silicon from dilute and buffered hydrofluoric acid solutions. The corrosion current density and corrosion potential of silicon were not very sensitive to the Cu 2+ concentration, up to 1000 parts per billion, in buffered hydrofluoric acid. However, the extent of copper deposition, as measured by total reflection X-ray fluorescence, increased as the Cu 2+ concentration in solution increased. In dilute hydrofluoric acid, Cu 3+ addition had a significant and systematic effect on the corrosion potential and corrosion current density of silicon. However, in both types of solution, the cathodic current calculated from the measured copper deposition was found to be only a small fraction of the corrosion current (less than 1%). This indicates that the primary cathodic reaction is not copper ion reduction but hydrogen ion reduction. Illumination affected the electrochemical behavior of both p- and n-type silicon in Cu 2+ spiked dilute hydrofluoric acid, but only that of p-type silicon in buffered hydrofluoric acid.

Factors affecting the flotation recovery of molybdenite from porphyry copper ores

Abstract Investigations have been conducted to characterize the surface chemical and flotation properties of molybdenite in aqueous environments typical of those that exist in the copper circuits of plants which process porphyry copper ores. These investigations have revealed that molybdenite and quartz particles are negatively charged at the pH commonly used for bulk sulfide flotation, namely 11. While the adsorption of calcium ions reduces the magnitude of negative charge on molybdenite, the adsorption can reverse the surface charge on quartz particles when the calcium concentration in solution exceeds 1 × 10 −3 M . Heterocoagulation of molybdenite and quartz particles will thus be inevitable in solutions containing large amounts of lime and will impair the floatability of molybdenite. The deleterious effects of heterocoagulation can be somewhat overcome by an ultrasonic conditioning prior to flotation. Besides heterocoagulation, particle size is very critical for optimum molybdenite flotation. Coarse molybdenite particles (120 × 200 mesh) float extremely rapidly and completely without being influenced by heterocoagulation. The flotation of fine molybdenite particles (minus 200 mesh) is sensitive to particle size and heterocoagulation. Results of tests conducted to assess the effects of overgrinding of molybdenite have indicated that while there is no change in the crystal structure due to prolonged grinding, the floatability is significantly reduced due to the creation of very rough surfaces.

Characterization of highly hydrophobic coatings deposited onto pre-oxidized silicon from water dispersible organosilanes

The formation and quality of highly hydrophobic coatings deposited from water dispersible organosilanes onto pre-oxidized single crystal silicon were studied using atomic force microscopy, ellipsometry, dynamic contact angle measurements and electrochemical impedance spectroscopy (EIS). Highly hydrophobic films of a commercially available water dispersible silane and two different cationic alkoxysilanes were prepared by dip coating. It was found using atomic force microscopy that, in general, the structure of these highly hydrophobic films is a continuous film with some particulates attributed to bulk polymerization of the precursor molecule in water. Film defects were quantified using EIS by the value of charge transfer resistance at the hydrofluoric acid/silicon interface. Potential applications of this type of coatings include reduction/elimination of stiction in micro-electromechanical systems, contact printing in materials microfabrication, inhibition of corrosion and oxidation, prevention of water wetting, lubrication and protein adsorption.

Effect of Temperature on the Interaction of Silicon with Nonionic Surfactants in Alkaline Solutions

The interaction of silicon wafers with alkaline solutions of octylphenol polyethylene oxide nonionic surfactants of different ethylene oxide chain length has been characterized at 25, 50, and 75°C. Wettability of silicon wafers was improved significantly at higher temperatures. Surfactants with long ethylene oxide chains exhibited less adsorption than surfactants with short ethylene oxide chains, and increase in solution temperature resulted in increased adsorption. Generally, the addition of surfactants to alkaline solution decreased the surface roughness of silicon ; however, the degree of reduction of surface roughness was influenced by the length of ethylene oxide chain and conditioning temperature.

Formation and characterization of anodized layers on CP Ti and Ti-6Al-4V biomaterials

Abstract The biocompatibility of titanium and Ti-6Al-4V alloy materials has been attributed to the presence of a passive surface oxide layer. In this investigation, the feasibility of creating a thick, porous oxide layer by anodizing commercially pure Ti and the titanium alloy in sulfuric acid under potentiostatic conditions was examined. Characterization of the anodic oxide layers was carried out to determine their thickness, morphology and composition. The thickness of the oxide layers, as determined by Rutherford backscattering spectrometry, was found to be a function of applied potential, anodizing time and electrolyte temperature. Scanning electron microscopy investigations revealed that under suitably controlled experimental conditions, a very porous (10 μm diameter pores) surface layer could be formed. Incorporation of sulfate ions into the oxide layer was evident from depth profile analysis by Auger electron spectroscopy. The isoelectric point of the oxide layer formed on the alloy surface was measured to be 5.8.