Taeho Lim

Electrodeposited Ni dendrites with high activity and durability for hydrogen evolution reaction in alkaline water electrolysis

Different shapes of various nickel structures, including dendrite, particle and film are fabricated by electrodeposition under various conditions. The shape of nickel structures is definitely dependent on the deposition potential, leading to different electrochemical surface area and edge facets. The nickel particle which has a polycrystalline center and edge is obtained at high negative potential. On the other hand, the nickel dendrite deposited by relatively low negative potential exhibits large electrochemical surface area and a particularly active facet for hydrogen evolution reaction (HER) in alkaline water electrolysis. In fact the nickel dendrite shows the highest catalytic activity and stability for HER among the various nickel structures.

Pulse Electrodeposition for Improving Electrical Properties of Cu Thin Film

Pulse deposition, which has an advantage to apply relatively high current density by supplement of Cu ions during off-time, was applied to deposit 250 nm Cu film. The microstructural change during off-time was to be investigated. The differences between constant potential and pulse deposition were due to the change during off-time. The application of pulse deposition led to the increase in the Cu(111) intensity and the reduction in the film resistivity compared to constant potential deposition. The film characteristics were further improved as the duty cycle decreased. The change during the off-time was verified to be grain growth in contact with the electrolyte. Additionally, it was clarified that the grain growth completed in a second, unlike self-annealing process, which proceeded for tens of hours, and affected within about 2.0 nm of Cu film from the surface. Under optimum conditions, pulse deposition led to 50% enhancement in Cu(111) intensity and 30% reduction in resistivity compared to the constant potential deposition.

A Study on Seed Damage in Plating Electrolyte and Its Repairing in Cu Damascene Metallization

In this study, we observed the changes in the film properties of a Cu seed layer with its damage and repair. The immersion of the Cu seed layer in a sulfuric-acid-based plating electrolyte can result in damage to the Cu seed layer by the dissolution of the native Cu oxide and corrosion of Cu, leading to defects in the subsequent electrodeposited layer. The damaged seed layer was repaired using electroless plating. Cu re-covered the surface and the crystal structure of the seed layer was rebuilt and, finally, the filling characteristic was improved into superfilling in Cu electroplating for the damascene process. Electroless repairing, however, increased the seed roughness due to the low nucleation on the exposed barrier surface and the accompanying three-dimensional Cu growth. To refine the repairing process by inducing the nucleation on the barrier surface, Sn-Pd activation was adopted before the repair, and it reduced the surface roughness and improved the continuity of the seed layer effectively.

Deposit profiles characterized by the seed layer in Cu pulse-reverse plating on a patterned substrate

The Cu deposit profile from pulse-reverse plating is affected by the characteristics of the Cu seed layer. When a reverse current is applied to a patterned structure, the sidewall Cu film was dissolved preferentially in comparison to the top film. This is associated with the position-dependent film characteristics of a physical vapor deposition (PVD) seed layer. The sidewall seed layer tends to be less dense and has smaller grains than the top seed layer. It was found that a 90°-tilted oblique angle PVD Cu film, which serves as the sidewall seed layer, is less dense and has smaller grains, and is therefore more likely to be dissolved than an untilted film. This preferentiality led to a heavy deposit at the top of the trench in pulse-reverse plating. Thermal pretreatment of the PVD seed layer or electroless deposition of the seed layer help to reduce the preferentiality, and thus the heavy deposition on the trench top that occurs during pulse-reverse plating.

Electrochemical Preparation of Pt-Based Catalyst on Carbon Paper Treated by Sn Sensitization and Pd Activation

Direct methanol fuel cell (DMFC) is considered as a promising alternative power source for portable electronic devices due to the low operation temperature, no pollutant emission and high power density [1]. However, there are several problems to be solved for commercializing a DMFC, in an anode catalyst. One of the problems is the high cost of Pt catalyst which shows high catalytic activity for a methanol oxidation [2]. Thus many research groups are developing a Pt-based catalyst alloyed with transition metals such as Co, Ni and Pb for reducing catalyst cost while keeping or improving a catalyst activity [3]. Another problem is a poisoning effect of carbon monoxide generated from the dissociative adsorption of methanol [4]. It is well known that PtRu or PtSn catalyst shows high tolerance against carbon monoxide poisoning because adsorbed OH radical on Ru or Sn promotes the oxidation of carbon monoxide adsorbed on the Pt surface. Thus synthesis of the Pt-based alloy catalyst which has a high catalytic activity for a methanol oxidation and a strong tolerance against a carbon monoxide poisoning, is one of the keys to commercialize the DMFC. CoPtRu/Pd catalysts were prepared on a carbon paper using various electrochemical processes such as Sn sensitization, Pd activation, Co electrodeposition and galvanic displacements. Sn-Pd process is a surface treatment which guarantees more active sites on the carbon paper for following Co electrodeposition by modifying the surface to be hydrophilic [5]. Co particles were deposited on the Sn-Pd treated carbon paper by controlling deposition potential and time. Then Pt and Ru galvanic displacements were conducted on the Co particles to form CoRuPt/Pd and CoPtRu/Pd catalysts. The CoRuPt/Pd – 1, 2, 3 and CoPtRu/Pd – 1, 2, 3 catalysts are named according to last displacement time. For bare carbon paper, Co particles with the size of 72.5 nm (16.0 nm of STDEV) and the density of 7.5 x 10 #/cm were obtained for 2 seconds at -1.4 V as shown in Fig. 1a. Under the same condition, the Sn-Pd treated carbon paper led to the decrease of Co particle size and the increase of the particle density. Thus Co particle had the size of 27.9 nm with the deviation of 4.8 nm and the density of 8.5 x 10 #/cm which were two fifth smaller in size and 11.3 times denser in density, respectively (Fig. 1b). To investigate the catalyst shape, HAADF-STEM (high angle annular dark field-scanning transmission electron microscopy) line scanning with EDS (energy dispersive X-ray spectroscopy) was performed following a white arrow as illustrated in Fig. 2a and the result is shown in Fig. 2b. The Pt and Ru metals were well dispersed over the Co/Pd particle. Fig. 3a displays CV (cyclic voltammetry) curves of the methanol oxidation carried out with CoPtRu/Pd – 1, 2 and 3 catalysts. The peak potentials were observed at 746.2 mV (vs. NHE) in the forward sweep which was 75 mV lower than that of the commercial PtRu/C catalyst. All CoPtRu/Pd catalysts showed similar peak current densities each other due to the similar surface molar concentration of Pt. As considering the both of peak potentials and current densities, it could be concluded that the catalytic activities of CoPtRu/Pd catalysts were higher than that of a commercial PtRu/C catalyst. To confirm the tolerance of the catalysts against carbon monoxide poisoning, the carbon monoxide stripping was done through LSV (linear sweep voltammetry) as shown in Fig. 3b. The peak potential of CoPtRu/Pd – 1 catalyst, having the surface molar ratio of 1.17 was 647.2 mV. This value was 2 mV positively shifted compared to the PtRu/C commercial catalyst. On the other hand, the peak potentials of CoPtRu/Pd – 2 and 3 catalysts, which had 1.04 and 1.07 of Pt/Ru surface molar ratio, showed the same value of 639.2 mV which was 6 mV lower than that of the commercial PtRu/C commercial catalyst.

Electrodeposition of Cu-Ag Film in Cyanide-Based Electrolyte

Cu has been used for fabricating many of surface coatings and interconnections in the semiconductors. The properties of Cu films have been improved to enhance the characteristics of coatings or interconnections ever since it was introduced. In general, alloy or mixture films such as Cu-Ni, Cu-Pt, Cu-Au, and Cu-Ag, have good mechanical properties. Among them, Cu-Ag is known for adequate mechanical property, high electrical conductivity and suitable deformality. In this presentation, the Cu-Ag films, which were made by means of electrodeposition in cyanide-based electrolyte, will be introduced. Electrodeposition was performed in 3-electrode system. Cu blanket wafer, which has the structure of Cu seed layer (60 nm, PVD) / Ta (7.5 nm, PVD) / TaN (7.5 nm, PVD) / SiO2, was employed as a working electrode. The 99.9% Cu or Ag electrodes and Ag/AgCl electrode were used for a counter and reference electrode, respectively. The basic electrolyte contained KAg(CN)2, CuCN, KCN. The KCN played a role of complexing agent. Cu-Ag alloy plating was implemented in cyanide bath, because cyanide bath has a possibility to make Cu-Ag alloy without any problem such as precipitation of Ag caused by halide ion in halide bath. To analyze the characteristics of electrolyte, electrochemical analyses such as linear sweep voltammetry (LSV) and chronoamperometry were conducted with various concentrations of Ag, Cu, and KCN. Then, the Cu-Ag film was deposited with static potential method, and film resistivity, orientation, and composition were investigated through 4-point probe, xray diffraction (XRD), field emission scanning electron microscope (FESEM), and auger electron spectroscopy (AES). Finally, annealing process was carried out at 300°C for 1 hr in nitrogen atmosphere, to change the microstructure of the film. Figure 1 shows the LSV curves according to the CuCN concentration, and it revealed that the Cu reduction took place at near the -0.7 V. The reduction current density was proportional to the concentration of CuCN. On the other hand, the Ag was reduced at 1.0 V, which was lower than that of Cu, as shown in figure 2. Moreover, from figure 2, it could be known that the Cu reduction, compared to Ag reduction, was more favorable at higher potential than – 1.15 V, however it was overwhelmed by the Ag reduction current at lower than – 1.15 V. Based on the data, the electrodeposition was implemented varying the concentration of CuCN and KAg(CN)2, and the deposition potential in this study. Figure 3 shows one of the XRD results of Cu-Ag film. Before the annealing, the (111) orientations of Cu and Ag were detected, and it implied that the film was mixture of Cu and Ag. However, after annealing, the Cu peak disappeared and Ag peak shifted to the right. It might relate to the alloy formation. To investigate this phenomenon in depth, the experiments, performed varying the concentration ratio of Cu and Ag, KCN concentration, applied potential and annealing condition, are in progress.