James F. Rohan

Selective electroless nickel deposition on copper as a final barrier/bonding layer material for microelectronics applications

A low cost, selective electroless metallisation of integrated circuit (IC) copper bond pads with nickel and gold is demonstrated. This metallurgy can function as a barrier layer/bondable material when deposited as a thin layer or as the chip bump for flip chip applications when deposited to greater heights. Four alternative activation steps for selective electroless nickel deposition on bond pad copper are demonstrated. Selective low cost deposition has been achieved with a proprietary electroless plating bath developed at NMRC and three commercial baths on both sputtered copper substrates and electrolessly deposited copper on titanium nitride barrier layer material.

Ammonia Borane Oxidation at Gold Microelectrodes in Alkaline Solutions

Borane-based electroless plating baths are of interest in many microelectronics applications such as barrier and capping layers for copper integrated circuit interconnect. To optimize the plating baths a thorough understanding of the role of the bath constituents is required. To this end we have employed microelectrodes to investigate the oxidation mechanism of boranes in alkaline solutions. In this paper we present data for the simpler ammonia borane (AB) oxidation and compare it with the previous analysis of dimethylamine borane (DMAB) oxidation. Both AB and DMAB are shown to oxidize in two steady-state mass transport-controlled oxidation waves for specific concentration ranges. Particular emphasis is placed on the analysis of the second oxidation wave observed at less negative potentials and the differences observed in the analysis of AB and DMAB in this potential region. The potential range for oxidation, the optimum concentration, and a suggested mechanism for oxidation are shown.

Microelectrode studies of the lithium/propylene carbonate system—Part I. Electrode reactions at potentials positive to lithium deposition

Abstract Voltammetry at nickel microdisc electrodes (radii 25 μm) in propylene carbonate has been used to investigated the chemistry which occurs at potentials positive to bulk lithium deposition and, hence, determine the surface for the commencement of lithium deposition. Presumably, similar reactions occur on the surface of lithium in contact with electrolyte. It is confirmed that a surface layer results from the reduction of oxygen and that water also reduces at a more negative potential; this reaction leads to another surface layer or a change in the composition of the layer resulting from oxygen reduction. It is also shown that the nickel substrate corrodes on open circuit in the presence of these surface layers. While other peaks are observed on the voltammograms, the evidence for assigning these to the reduction of solvent or electrolyte anion is weak.

Cu Electrodeposition from Methanesulfonate Electrolytes for ULSI and MEMS Applications

Methanesulphonic acid (MSA) is an alternative to sulphuric acid electrolyte for metal deposition. The electrochemical nucleation and growth of Cu on a glassy carbon (GC) electrode in the methanesulphonate was compared with sulphate baths. The overpotential for Cu deposition was much smaller in the MSA bath compared to the traditional sulphuric acid bath and Cu nucleation occurred at a higher rate in the MSA bath. The measured diffusion coefficient value for Cu deposition from the MSA bath was 6.82 x 10 6 cm 2 /s. UV-Vis spectroscopic results confirmed that the coordination of Cu species was the same in both electrolytes. Cu electrodeposition on Ni sputtered Si substrate from the high efficiency MSA bath was found to be photoresist compatible with no void formation. 1D Cu nanorods were also deposited through AAO template on a Ni evaporated seed layer substrate showing potential applications as electrical interconnects in ULSI and MEMS.

Nanotubes of Core/Shell Cu / Cu2O as Anode Materials for Li-Ion Rechargeable Batteries

A direct electrodeposition technique for Cu nanotube array fabrication and the subsequent conversion of the deposited Cu into Cu 2 O was developed. The Cu 2 O nanotube arrays showed high capacity, cyclability, and rate capability. The cycling performance of the Cu 2 O nanotubes showed a high level of structural integrity with capacity retention even after 94 cycles when cycled at 1C to 3C rates. The enhanced electrochemical performance of the Cu 2 O nanotubes came from a high surface area, electrolyte access, high electrical conductivity of Cu core support, and structural integrity of the oxide shell active material.

High efficiency Si integrated micro-transformers using stacked copper windings for power conversion applications

This paper details the design, fabrication, and characterization of silicon integrated micro-transformers. Two types of race-track shaped micro-transformers, single copper winding or single layer metal (SLM) and double copper winding or double layer metal (DLM) were designed and fabricated using standard CMOS processing. The DLM devices have higher inductance density than SLM devices realized within similar footprint area. The design study showed that the efficiency of micro-transformers increased from 37% for SLM designs to over 75% for DLM transformers at 20 MHz.

Nanoporous Gold Catalyst for Direct Ammonia Borane Fuel Cells

Nanoporous gold (NPG) electrodes were fabricated in film and wire array formats by selectively dealloying Ag from Au0.18Ag0.82. The ammonia borane (AB) oxidation reaction was studied by cyclic voltammetry at the NPG electrodes. The onset potential for the oxidation at NPG in a wire array format shifted to more negative potentials than that observed at a Au disc and higher currents were realised. An onset potential of -1.30 V vs. SCE was recorded which is 0.28 V lower than that at a Au disc. The oxidation current for 20 mM AB in 1 M NaOH increased from 2.65 mA cm -2 at a Au disc to 13.1 mA cm -2 at a NPG wire array. NPG is a viable candidate as an anode catalyst for a direct ammonia borane fuel cell.

Microelectrode studies of the lithium/propylene carbonate system—part II. studies of bulk lithium deposition and dissolution

Abstract The deposition and dissolution of lithium in propylene carbonate/lithium hexafluoroarsenate has been investigated using voltammetry at nickel microdisc electrodes. It is shown that lithium is deposited by a nucleation and growth mechanism and both deposition and dissolution occurs at low over-potentials. The kinetics of the Li/Li + couple are rapid and the I — E response may be fitted to a model involving simple electron transfer; there is no experimental evidence to support an important role for a surface film. Lithium layers up to 50 μm thick have been electroplated onto a nickel disc (diameter 5 mm) and studied using optical microscopy: the stability of lithium foil in the medium has also been monitored. This paper attempts to correlate the results at microelectrodes and larger electrodes.

Characterisation of the electroless nickel deposit as a barrier layer/under bump metallurgy on IC metallisation

Selective electroless nickel-phosphorus deposits on integrated circuit (IC) metallisation such as copper and aluminium were characterised using differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) for elemental analysis. Annealing the Ni-P deposits in nitrogen atmospheres at temperatures compatible with organic dielectrics for IC components, such as polyimide, was performed to characterise the deposits. The crystallisation behaviour of the electroless nickel deposits with different concentrations of co-deposited phosphorus was examined.

Energy Storage: Battery Materials and Architectures at the Nanoscale

Enterprise Ireland (Project CFTD07325). European Commission (EU Framework 7 project Nanofunction, (Beyond CMOS Nanodevices for Adding Functionalities to CMOS) www.Nanofunction.eu EU ICT Network of Excellence, Grant No.257375)