Filling of microvia with an aspect ratio of 5 by copper electrodeposition

The filling of microvias with a diameter of 5 μm and a depth of 25 μm (aspect ratio of 5) by copper electroplating was investigated. Filling experiments were evaluated by analyzing cross-sections of filled vias with scanning electron microscopy and focused ion beam. The fill-up evolution shows a bottom-up mechanism, also known as superfilling mechanism. The evolution of potential with time (chronopotentiometric measurements) was recorded during the fill-up process of vias and is interpreted based on potentiodynamic polarization measurements.The bottom-up fill mode is affected by the concentration of leveler inside the vias. A differential plating rate that is responsible for bottom-up plating, develops along the profile of the via on depletion of the leveler inside the vias. Since the depleted via is less inhibited, the local electrodeposition rate increases in the via. At the top part and outside the via, the electrodeposition rate is strongly inhibited due to a higher leveler concentration comparable to the one in the bulk electrolyte, what results in a low local electrodeposition rate.In this paper, the contribution of levelers to the bottom-up mechanism during the electrodeposition of copper in microvias is investigated. The observed microstructure supports the superfilling mechanism.     

Filling behavior of ZnO nanoparticles into opal template via electrophoretic deposition and the fabrication of inverse opal

Combining colloidal crystal template (artificial opal) and electrophoretic deposition (EPD) process, well-ordered ZnO inverse opal can be formed by finding the optimum driving potential of EPD. Through providing the various driving potentials from −25 V, −10 V, −5 V to −2.5 V, the different mechanism of electrophoretically depositing ZnO nanoparticles into the colloidal crystal template was determined by the SEM observation of the filled templates. Because the nano-channels of colloidal crystal template are the network type, the results of surface jam, incomplete filling and perfect filling are found under specific applied voltages. The high-quality ZnO inverse opal can be only fabricated under the perfect nano-channel-filling condition. The filling behavior can be monitored dynamically by tracing the current transients, and the optimum conditions for filling the interstitial spaces of templates constructed from colloidal particles with 180 nm and 300 nm diameter can be obtained by applying a voltage of −5 V and −15 V, respectively. After the complete filling of ZnO nanoparticles into the colloidal crystal template consisting of 300 nm colloids, high-quality ZnO photonic crystal possessing an absorptive peak at the wavelength of 560 nm can be fabricated by removing the template. It is expected that the EPD can find extensive applications for preparing photonic crystals of various oxides only if their nanoparticles are available.     

Stress behavior of electrodeposited copper films as mechanical supporters for light emitting diodes

Thick copper films electrodeposited on vertically structured GaN-based LEDs play a critical role in supporting thin GaN multi-layers mechanically after laser lift-off (LLO) of the sapphire substrate. The intrinsic stress of electrodeposited copper was measured from the change in the substrate's curvature using a modified Stoney equation. 150-μm thick copper supporters showed very low intrinsic stress of 4-28 MPa (tensile stress), which developed during copper electrodeposition on Au seed. The stress generation was attributed to nuclei coalescence and formation of grain boundaries of which volume increased with the applied current density. The preferential texture plane of thick copper supporters was (2 2 0) at high current densities, while copper films thinner than 1 μm was strongly oriented along (1 1 1) plane. Various seed metals such as Cu, Ni, Au, Ag, and Al were employed to observe the influence of seed material on the stress of copper supporters. The effect of wetting agent on the stress of copper supporters was also monitored in the concentration range of 0.1-1.5 g/L of sodium lauryl sulfate (SLS).     

Electrodeposition of tungsten from ZnCl2-NaCl-KCl-KF-WO3 melt and investigation on tungsten species in the melt

The electrodeposition of tungsten in ZnCl₂-NaCl-KCl-KF-WO₃ melt at 250 °C was further studied to obtain a thicker deposit. In the ordinary electrolysis at 0.08 V vs. Zn(II)/Zn, the current density decreased from 1.2 mA cm⁻² to 0.3 mA cm⁻² in 6 h. A thickness of the obtained tungsten layer was 2.1 μm and the estimated current efficiency was 93%. A supernatant salt and a bottom salt were sampled after 6 h from the melting and were analyzed by ICP-AES and XRD. It was found that the soluble tungsten species slowly changes to insoluble ones in the melt. The soluble species was suggested to be WO₃F⁻ anion. One of the insoluble species was confirmed to be ZnWO₄ and the other one was suggested to be K₂WO₂F₄. Electrodeposition was carried out under the same condition as above except for the intermittent addition of WO₃ every 2 h. The current density was kept at the initial value and the thickness was 4.2 μm. The intermittent addition of WO₃ was confirmed to be effective to obtain a thicker tungsten film.     

Electrochemical reduction of silicon chloride in a non-aqueous solvent

The electrodeposition of silicon was engaged in propylene carbonate containing SiCl₄ and tetrabutylammonium chloride. Cyclic voltammograms showed the possibility of electrodeposition of Si at −3.6 V versus Pt quasi-R.E. Potentiostatic electrolysis yielded a porous and white deposit with the thickness of 50 μm at −3.6 V for 1 h. Raman spectroscopic analysis of the deposit immersed in pure propylene carbonate soon after the electrolysis confirmed that the electrodeposit was amorphous silicon. Moreover, EDX and XPS results indicated that the electrodeposited Si was so active that it was oxidized immediately in the air. Finally, the electrodeposition process of Si film is discussed based on chronopotentiometry and chronoamperometry.     

Template electrosynthesis of aligned Cu2O nanowires: Part I. Fabrication and characterization

Large arrays of aligned copper oxide nanowires were produced by electrodeposition, using anodic alumina membranes as template. We have studied the effect of two fundamental parameters involved in fabrication process: potential perturbation and bath composition. Performing electrodeposition from a copper acetate/sodium acetate bath (pH 6.5), we found that chemical composition of nanowires varied in dependence on the shape of the applied potential perturbation: pure copper oxide nanowires were produced by pulsed potential, whilst continuous electrodeposition resulted in a co-deposition of Cu and Cu₂O. In a copper lactate bath, buffered at pH 10, the shape of perturbation did not influence the chemical composition of nanowires, consisting of pure copper oxide. Besides, bath composition influenced the crystallographic structure of nanowires: in the acetate bath polycrystalline nanowires, having a maximum length of about 2.5 μm, were obtained whilst in the lactate electrolyte Cu₂O nanowires with a preferential orientation along the (2 0 0) plane were deposited. In every case, nanowire diameters were uniform with an average value of about 200 nm. Growth rate of nanowires was influenced by the shape of potential pulse.     

Development of a new graded-porosity FeAl alloy by elemental reactive synthesis

A new graded-porosity FeAl alloy can be fabricated through Fe and Al elemental reactive synthesis. FeAl alloy with large connecting open pores and permeability were used as porous supports. The coating was obtained by spraying slurries consisting of mixtures of Fe powder and Al powder with 3-5 μm diameter onto porous FeAl support and then sintered at 1100 °C. The performances of the coating were compared in terms of thickness, pore diameter and permeability. With an increase in the coating thickness up to 200 μm, the changes of maximum pore size decreased from 23.6 μm to 5.9 μm and the permeability decreased from 184.2 m³ m^− 2 kPa^− 1 h^− 1 to 76.2 m³ m^− 2 kPa^− 1 h^− 1, respectively, for a sintering temperature equal to 1100 °C. The composite membranes have potential application for excellent filters in severe environments.     

Tough organogels based on polyisobutylene with aligned porous structures

Macroporous gels with aligned porous structures were prepared by solution crosslinking of butyl rubber (PIB) in cyclohexane at subzero temperatures. Sulfur monochloride was used as a crosslinker in the organogel preparation. The reactions were carried out at various temperatures between 20 and −22 °C as well as at various freezing rates. The structure of the gel networks formed at −2 °C consists of pores of about 100 μm in length and 50 μm in width, separated by polymer domains of 10-20 μm in thickness. The aligned porous structure of PIB gels indicates directional freezing of the solvent crystals in the direction of the temperature gradient. The size of the pores in the organogels could be regulated by changing the freezing rate of the reaction solution. The results suggest that frozen cyclohexane templates are responsible for the porosity formation in cyclohexane. In contrast to the regular morphology of the gels formed in cyclohexane, benzene as a crosslinking solvent produces irregular pores with a broad size distribution from micrometer to millimeter sizes due to the phase separation of PIB chains at low temperatures. Macroporous organogels prepared at subzero temperatures are very tough and can be compressed up to about 100% strain without any crack development. The gels also exhibit superfast swelling and deswelling properties as well as reversible swelling-deswelling cycles in toluene and methanol, respectively.     

Effect of plating mode, thiourea and chloride on the morphology of copper deposits produced in acidic sulphate solutions

The influence of plating mode, chloride and thiourea (TU) on morphology of copper deposits has been studied. All experiments were conducted on disc electrodes rotating at 500 rpm and an average current density of 4 A dm⁻² to produce 10 μm thick deposits. In additive-free solutions, the use of pulsed current (PC) improved deposit morphology and brightness over DC plating. In the presence of thiourea (no Cl⁻), the deposits obtained by DC and PC plating were similar under most plating conditions. The presence of thiourea generally improved deposit quality over that obtained in additive-free solutions, but caused the formation of microscopic nodules and the deposits to appear slightly cloudy, resulting in lower reflectances than that of a polished uncoated copper surface. The addition of Cl⁻ to thiourea-containing solutions strongly influenced deposit morphology at both microscopic and macroscopic scales depending on chloride concentration and pulse conditions. It prevented nodule formation and created microscopically bright and reflective deposits, but caused extreme macroscopic roughness. Nevertheless, PC plating at 50 Hz in solutions containing appropriate amounts of thiourea and Cl⁻ was found to yield macroscopically and microscopically smooth deposits with reflectance similar to that of a polished uncoated copper substrate.     

Influence of ammonium salt on electrowinning of copper from ammoniacal alkaline solutions

In order to develop an energy-saving copper recycling process from wastes, electrochemical measurements were conducted in ammoniacal alkaline solutions containing Cu(I) ions and an ammonium salt of sulfate, chloride or nitrate. The results of each system were then compared. The polarization measurements suggested that the voltage required for the electrode process is lower in the chloride and nitrate systems than that in the sulfate system. The cathode current efficiency during the copper electrodeposition varied from 39 to 97% and increased with current density in the chloride and sulfate systems. In the nitrate system, the lowest cathode current efficiency of 30% was observed because of nitrate ion reduction. Based on these results, the power consumption required for the electrowinning stage of the copper recycling process was calculated. Among these three systems, the chloride system showed the lowest power consumption of 500 kWh t⁻¹ at the current density of 200 A m⁻², which is about 25% of the conventional copper electrowinning process from a copper sulfate-sulfuric acid solution.     

The stability of an acidic tin methanesulfonate electrolyte in the presence of a hydroquinone antioxidant

The electrodeposition of tin at a (0.28 cm²) copper surface from 0.014 mol dm⁻³ SnSO₄ and 12.5 vol.% methanesulfonic acid (MSA 1.93 mol dm⁻³) at 296 K was studied. Hydroquinone concentrations of 0.005, 0.05 and 0.5 mol dm⁻³ (corresponding to a molar concentration ratio of hydroquinone to stannous ions of 0.36, 3.6 and 36, respectively) were used. Cyclic and linear sweep voltammetry served to characterise the electrochemical behaviour of tin deposition and stripping. The effects of potential sweep rate and electrode rotation speed on the voltammetry were studied. The stability of the electrolyte with storage time was quantified by changes in the limiting current density for tin deposition at a smooth rotating disc electrode and the peak current density at a static disc electrode. The influence of hydroquinone on mass transport controlled tin deposition and suppression of hydrogen evolution was evaluated.     

Hydrometallurgical process for the recycling of copper using anodic oxidation of cuprous ammine complexes and flow-through electrolysis

Flow-through electrolysis for copper electrowinning from cuprous ammine complex was studied in order to develop a hydrometallurgical copper recycling process using an ammoniacal chloride solution, focusing on the anodic oxidation of cuprous to cupric ammine complexes. The current efficiency of this anodic oxidation was 96% at a current density of 200 A m⁻² under a batch condition. In a flow-through electrolysis using a sub-liter cell and a carbon felt anode, the anodic current efficiency increased with the flow rate and was typically higher than 97%. This tendency was explained by the backward flow of the cupric ammine complex, which was formed on the anode, through the diaphragm. The anodic overpotential was lower than 0.3 V even at an apparent current density of 1500 A m⁻². A similar current efficiency and overpotential were also achieved in a liter scale cell, which indicates the scale flexibility of this electrolysis. The power consumption requirements for copper electrowinning in this cell were 460 and 770 kWh t⁻¹ at the current densities of 250 and 500 A m⁻², respectively, which were much lower than that of the conventional copper electrowinning despite the longer interpolar distance.     

Electrodeposited Pd-Co catalyst for direct methanol fuel cell electrodes: Preparation and characterization

Pd-Co alloy has been recently proposed as a catalyst for the cathode of direct methanol fuel cells with both excellent oxygen reduction activity and methanol tolerance, hence electrodeposition of this alloy is an attractive approach for synthesizing porous metal electrodes with high methanol tolerance in direct methanol fuel cells. In this study, we electrodeposited two types of Pd-Co films onto Au substrates by applying different current density (−10 or −200 mA cm⁻²); and then characterized them in terms of morphology, composition, crystal structure, and catalytic activity. Pd-Co deposited at −10 mA cm⁻² was smooth and possessed smaller particles (ca. 10 nm), while that at −200 mA cm⁻² was dendritic (or rough) and possessed larger particles (ca. 50 nm). Both the Pd-Co alloys were found to be almost the same structure, i.e. a solid solution of ca. Pd₇Co₃ with Pd-skin, and also confirmed to possess comparable activity in oxygen reduction to Pt (potential difference at 1.0 μA cm⁻² was 0.05 V). As for methanol tolerance, cell-voltage was not influenced by addition of 1 mol dm⁻³ methanol to the oxidant solution. Our approach provides fundamental technique for synthesizing Pd-Co porous metal electrodes by electrodeposition.     

Zinc deposited polymer coatings for copper protection

Polypyrrole (PPy) and polyaniline (PAni) coatings were electrosynthesized on copper, by using cyclic voltammetry technique. Then, these coatings were modified with the deposition of zinc particles from aqueous zinc sulphate solution. The electrodeposition of zinc was achieved at a constant potential value of −1.20 V, in the amount of ∼0.75 mg/cm². The corrosion performance of zinc modified polymer coatings were investigated in 3.5% NaCl solution; by using the electrochemical impedance spectroscopy (EIS), and anodic polarization curves. The zinc particles improved the barrier property of polymer films, thanks to formation of voluminous zinc corrosion products within the pores of polymer coating. Also, the zinc particles provided cathodic protection to the substrate, where the polymer film played the role of conductance between zinc particles and copper.     

Nucleation of Sn and Sn-Cu alloys on Pt during electrodeposition from Sn-citrate and Sn-Cu-citrate solutions

The initial stages of Sn and Sn-Cu electrodeposition from Sn-citrate and Sn-Cu-citrate solutions on Pt were studied using both current-controlled and potential-controlled electrochemical techniques. For both Sn-citrate and Sn-Cu-citrate solutions, when the current density is controlled to lower than 15 mA/cm², potentials remain almost constant which is appropriate to plate dense and uniform films. When the current density is controlled to between 25 and 35 mA/cm², potentials drop quickly initially, followed by a gradual increase to a constant value. When current density is controlled to higher than 50 mA/cm², potential oscillation happens, and significant hydrogen evolution prevents the formation of dense and continuous Sn and Sn-Cu films. A constant transition time constant indicates a diffusion-controlled process. The diffusion coefficient calculated from the Sand equation is about 3.8 × 10⁻⁶ cm²/s for the Sn-citrate solution and 4.1 × 10⁻⁶ cm²/s for the Sn-Cu-citrate solution. The morphology of both Sn and Sn-Cu deposits plated under different potentials was examined by atomic force microscopy (AFM) and the distribution of each element were analyzed using Auger imaging. Analysis of both the electrochemical results at −0.72, −1.1 and −1.5 V and AFM images for both Sn and Sn-Cu deposits at −1.1 and −1.5 V suggested progressive nucleation controlled by diffusion for both Sn and Sn-Cu electrodeposition. Tin reacted with Pt to form PtSn₄, and co-deposited with Cu to form Cu₆Sn₅ during nucleation, with more Sn forming at higher applied potentials.     

Superstructure and mechanical properties of poly(L-lactic acid) microfibers prepared by CO2 laser-thinning

Poly(L-lactic acid) (PLLA) microfibers were obtained by a carbon dioxide (CO₂) laser-thinning method. A laser-thinning apparatus used to continuously prepare microfibers was developed in our laboratory; it consisted of spools supplying and winding the fibers, a continuous-wave CO₂-laser emitter, a system supplying the fibers, and a traverse. The laser-thinning apparatus produced PLLA microfibers in the range of 100-800 m min⁻¹. The diameter of the microfibers decreased as the winding speed increased, and the birefringence increased as the winding speed increased. When microfibers, obtained through the laser irradiation (at a laser power of 8.0 W cm⁻²) of the original fiber supplied at 0.4 m min⁻¹, were wound at 800 m min⁻¹, they had a diameter of 1.37 μm and a birefringence of 24.1×10⁻³. The draw ratio calculated from the supplying and winding speeds was 2000×. The degree of crystal orientation increased with increasing the winding speed. Scanning electron microscopy showed that the microfibers obtained with the laser-thinning apparatus had smooth surfaces not roughened by laser ablation that were uniform in diameter. The PLLA microfiber, which was obtained under an optimum condition, had a Young's modulus of 5.8 GPa and tensile strength of 0.75 GPa.     

Plasma electrochemistry: Absorption of flame borne species in platinum electrodes

Electrochemical potentiometric measurements have been shown to be possible in flame plasma using solid electrodes. The properties of the solid metal electrodes can effect the magnitude of the potential measured, due to the material's capacity to absorb flame borne species. Through this process, a surface alloy is formed and so alters the work function of the electrode. The study presented in this paper is concerned with understanding the mechanism by which copper, present in the flame, can absorb into platinum (Pt) electrodes at 1880 K. Electrodes were sectioned and the weight percent of copper was measured using an electron probe, to a distance of 100 μm from the surface. The distribution profiles of copper (Cu) measured for electrodes held in a flame doped with a solution containing copper chloride (5 × 10⁻² M) for 1, 5, 10 and 30 min modelled using finite element modelling. The predominant mechanism of absorption was unambiguously found to be instantaneous pulse, where a thin layer of copper was first deposited on the electrode surface by condensation followed by absorption. The layer was found to be 240 ± 30 nm and the copper diffusion coefficient was 8.0 ± 0.6 × 10⁻¹³ m² s⁻¹ in Pt at 1880 K.     

The influence of the ion-exchange groups nature and the degree of chemical activation by silver on the process of copper electrodeposition into the ion exchanger

The electrodeposition of copper into ion-exchange materials with different ionogenic groups is studied. We have worked out that the discharge of copper counterions in an ion-exchange matrix is characterized by a cathodic overvoltage that is higher than the overvoltage of the same process on a graphite substrate by 0.08 V, which is most probably connected with a limited mobility of counterions localized at ionogenic groups. It has been observed microscopically that the process of deposition begins at the graphite substrate/ion exchanger interface and passes into the volume of a polymeric matrix with the filling of nanodimensional pores by copper. Preliminary doping of the ion exchanger by silver leads to the jump of the deposition current of copper counterions that is caused by the appearance of sufficient electron conductivity of the doped polymeric matrix. The data of the local X-ray spectral microanalysis confirm the regular deposition of copper into the volume of an ion exchanger grain doped by silver.     

Superfast responsive ionic hydrogels with controllable pore size

A series of strong polyelectrolyte hydrogels was prepared from the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) as the monomer and N,N′-methylene(bis)acrylamide (BAAm) as a crosslinker in aqueous solutions. The gel preparation temperature (T_prep) was varied between −22 and 25 °C. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at T_prep=−8 °C. The hydrogels prepared below −8 °C exhibit a discontinuous morphology consisting of polyhedral pores of sizes 30-50 μm, while those formed at higher temperatures have a non-porous structure. The pore size of the networks increased by decreasing the charge density of the hydrogels, while addition of low molecular weight salts into the gelation system reduced the size of the pores. Calculations based on the equilibrium between the ice and unfrozen gel phases in the reaction system at low temperatures explain the results of observations. It was also shown that the hydrogels formed below −8 °C exhibit superfast swelling properties as well as reversible swelling-deswelling cycles in water and acetone.     

Real-time imaging of coating growth during plasma electrolytic oxidation of titanium

The microdischarge characteristics during dc plasma electrolytic oxidation (PEO) of titanium at 20 mA cm⁻² in orthophosphate electrolyte at 293 K have been investigated by real-time imaging. A relatively constant microdischarge rate of ∼100 cm⁻² s⁻¹ was revealed, with the sizes and lifetimes of microdischarges ranging between ∼70 and 380 μm and ∼35 and 800 ms, respectively. The average lifetime of the microdischarges increased with increased voltage, reaching ∼335 ms at 430 V in the main period of coating growth. The cumulative number of microdischarges during PEO was at least two orders of magnitude lower than the population density of pores on the surface of the final coating. Increased time of oxidation resulted in coarsening of the coating surface, due to the formation of relatively large pores. Microdischarges were observed with a bubble-free surrounding region indicative of generation of shock waves.