Uziel Landau

Hydrogen and Oxygen Evolution on Boron‐Doped Diamond Electrodes

The evolution of hydrogen and oxygen was studied on diamond electrodes containing approximately 1021 boron atom/cm3. Voltammetry showed a wide potential window [−1.25 to +2.3 V vs. standard hydrogen electrode (SHE)] without significant water decomposition. This window was much narrower for poor quality diamond films with appreciable sp2 content. A redox couple observed at +1.7 V indicates oxidation of the diamond surface prior to oxygen evolution. The extent of surface oxidation increased with sp2 content. Anodic polarization made the diamond surface hydrophilic; x‐ray photoelectron spectroscopy showed an increase in oxygen coverage and the presence of carbon‐oxygen bonds. The estimated capacitance of the interface ranged from 0.05 μF/cm2 for high quality diamond to 5 μF/cm2 for low quality diamond. Preliminary measurements of the exchange current densities for oxygen and hydrogen evolution indicated slow kinetics compared to metals or highly oriented pyrolytic graphite.

A Polymer Electrolyte for Operation at Temperatures up to 200°C

In developing advanced fuel cells and other electrochemical reactors, it is desirable to combine the advantages of solid polymer electrolytes with the enhanced catalytic activity associated with temperatures above 100 C. This will require polymer electrolytes which retain high ionic conductivity at temperatures above the boiling point of water. One possibility is to equilibrate standard perfluorosulfonic acid polymer electrolytes such as Nafion, with a high boiling point Bronsted base such as phosphoric acid. The Nafion/H[sub 3]PO[sub 4] electrolyte has been evaluated with respect to water content, ionic conductivity and transport of oxygen, and methanol vapor. The results show that at elevated temperatures reasonably high conductivity (>0.05 [Omega][sup [minus]1] cm[sup [minus]1]) can be obtained. Methanol permeability is shown to be proportional to the methanol vapor activity and thus decreases with increasing temperature for a given partial pressure. Comparisons and distinctions between this electrolyte and pure phosphoric acid are also considered.

Electro‐osmotic Drag Coefficient of Water and Methanol in Polymer Electrolytes at Elevated Temperatures

The electro-osmotic drag coefficient of water in two polymer electrolytes was experimentally determined as a function of water activity and current density for temperatures up to 200 C. The results show that the electro-osmotic drag coefficient varies from 0.2 to 0.6 in Nafion{reg_sign}/H{sub 3}PO{sub 4} membrane electrolyte, but is essentially zero in phosphoric acid-doped PBI (polybenzimidazole) membrane electrolyte over the range of water activity considered. The near-zero electro-osmotic drag coefficient found in PBI indicates that this electrolyte should lessen the problems associated with water redistribution in proton exchange membrane fuel cells.

A time-dependent transport-kinetics model for additive interactions in copper interconnect metallization

Electrodeposition of copper in the presence of additives mixture that is typically used in bottom-up fill of sub-micrometer vias and trenches on semiconductor wafers is analyzed. The time-dependent additives interactions, accounting for their transport and adsorption kinetics, are incorporated in a via-fill model. Transient polarization measurements provide the adsorption time constants for polyethylene glycol (PEG) and bis(3-sulfopropyl) disulfide (SPS). Experiments indicate that the fast PEG adsorption on the electrode is diffusion controlled, while the slow SPS adsorption is controlled by the adsorption kinetics. The results are applied to a transport-kinetics model that provides the additives distribution inside a via. It is noted that the PEG diffusion to the via bottom is extremely slow due to the PEG adsorption on the sidewalls. The role of SPS in the bottom-up fill is characterized by simulating the transport within the via through analogous transport to a flat rotating disk electrode. It is observed that SPS is essential for maintaining fast deposition at the via bottom by preventing PEG adsorption. The critical necessity for a three-additive system comprised of chloride ions, PEG, and SPS is explained, and process parameters essential for bottom-up fill are identified.

Rapid Charging of Lithium-Ion Batteries Using Pulsed Currents A Theoretical Analysis

Lithium-ion batteries are typically charged using constant current that is applied until the cell voltage reaches about 4.2 V, at which time, charging continues at a constant voltage until the full battery capacity is attained. This process is slow, typically requiring 2-4 h. Empirically selected pulse-charging sequences have been shown to provide enhanced charging; however, no model exists to explain and optimize the pulse-charging protocols. Modeling the lithium diffusion into a homogeneous intercalant layer indicates that the lithium concentration reaches saturation at the graphite/electrolyte interface after about 1 h under conventional constant current charging, mandating the shift to the lower rate constant voltage charging. It is shown here that charging the lithium battery using non-dc waveforms with properly selected parameters may circumvent this lithium saturation, enabling charging at significantly higher rates. A nonlinearly decreasing current density profile which conforms to the mass transfer coefficient variation was shown to provide complete charging in less than 3 4 of an hour, faster than any other pulse-charging profile studied.

Voltammetry Studies of Single‐Crystal and Polycrystalline Diamond Electrodes

Boron-doped polycrystalline and near-single-crystal quality diamond electrodes were studied by voltammetry. A redox couple with E 1/2 = + 1.83 V vs. standard hydrogen electrode was detected on the polycrystalline electrodes but was absent on the single-crystal electrodes. The results strongly suggest that the couple is associated with reactivity at the grain boundaries. Plasma fluorination of polycrystalline diamond electrodes using CF 4 in a radio frequency plasma eliminated the redox couple at + 1.83 V but did not alter the potential range of water stability. Cathodic polarization of as-grown, polycrystalline diamond electrodes caused an irreversible addition of oxygen to the surface. Subsequent anodic polarization added additional oxygen and made the surface hydrophilic. Single-crystal electrodes also displayed an increase in oxygen coverage upon both cathodic and anodic polarization. Voltammetry studies of electrodes covered with a thin sp 2 carbon surface layer indicate that the redox couple at + 1.83 V corresponds to multiple processes including the etching of sp carbon in the grain boundaries.

Electrodeposition of Ni‐P Amorphous Alloys Observations Supporting the Indirect Mechanism of Phosphorus Incorporation

The mechanism for codeposition of nickel and phosphorus in electrodeposited Ni-P amorphous alloys has not been clearly established. This paper resents experimental observations supporting the «indirect mechanism» for Ni-P alloy formation by electrodeposition from a phosphorous acid-based bath. Specifically, the indirect mechanism proposes that phosphine, PH 3 , is an intermediate in Ni-P electrodeposition. The experiments described here are; (i) reaction of nickel in aqueous solution with gaseous phosphine, (ii) electrodeposition from a nickel phosphite bath, and (iii) titration studies

Polyether Suppressors Enabling Copper Metallization of High Aspect Ratio Interconnects

Void-free bottom-up fill in aggressive interconnect geometries, i.e., aspect ratio exceeding 20, requires suppression stronger than that provided by the conventionally used poly(ethylene glycol) (PEG). Reported here are polarization studies of copper electrodeposition in the presence of several polyethers, which heretofore were not studied in the context of bottom-up fill. Several of these compounds (including polyoxyethylated β-naphthol and polyoxyethylene lauryl ether) exhibit superior inhibition, thus enabling the bottom-up fill of high aspect ratio features, which cannot be metallized by PEG. Plating experiments of high aspect ratio features confirm the advantage provided by this class of additives. A first-order scaling model that predicts the bottom-up fill efficacy of an additive system in terms of readily measured polarization parameters and the wafer geometry is also presented.

Reducing Mass-Transport Limitations by Application of Special Pulsed Current Modes

Current pulsing, unlike potential pulsing, is generally considered ineffective in enhancing mass transport. However, we show here, that by properly selecting the pulsed current parameters, the depletion of the reactant atthe electrode during cathodic reduction, or its excess during anodic dissolution, can be reduced compared to the corresponding dc current application, while still passing the same amount of charge in an identical amount of time. The advantageous pulsed current modes include a sequence of decreasing current density amplitudes, or pulsing the current at the same amplitude, but with increasing relaxation intervals. By comparison, it is also shown that applying a sequence of constant-amplitude square current pulses at constant time intervals leads to an identical concentration profile as in dc. The application to the battery-charging process is briefly discussed.

Electrodeposition of radioactive rhenium onto stents to prevent restenosis.

Radioactive stents are currently being evaluated for preventing restenosis. A major difficulty to overcome is the need to load any pre-manufactured stents with defined amounts of radioactivity at the time of use. Using stents that are preloaded by the manufacturer is not ideal because the stent length usually differs from the length needed for a specific lesion and the amounts of radioactivity varies widely due to ongoing decay of the source. Thus, we have developed a novel method that allows any currently used stainless steel or tantalum stent to be coated with radioactive rhenium. The method involves placing the stent in a series of rinsing and electroplating solutions, one containing radioactive rhenium (186Re, 188Re, or both). The overall processing time is 15 min and the procedure may be conveniently applied just prior to the stent insertion. The plated stent contains radioactive rhenium in a 1.2 microm-thick cobalt layer, with an outer 2 microm layer of gold. The gold layer gives the radioactive stent excellent radiochemical stability, good bending and biocompatibility properties, and improves stent visibility during fluoroscopy.