Masatoshi Sakairi

Nucleation and growth of the nanostructured anodic oxides on tantalum and niobium under the porous alumina film

Abstract Anodic oxidation of Ta–Al (aluminium deposited on tantalum) and Nb–Al (aluminium deposited on niobium) has been performed in organic and inorganic acid electrolytes for porous alumina formation. Arrays of tantalum nanoscale oxide ‘hillocks’ and niobium oxide ‘goblets’ derived from the anodised Ta–Al and Nb–Al bilayer samples have been investigated by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy depth profiling. Anodising proceeds in the sequence of growth of porous anodic alumina and, when the aluminium layer is consumed up to the underlying metal, growth of anodic tantalum (niobium) oxide under the bottoms of the alumina pores. The oxidation of the underlying metal results from metal ions migrating outward and oxygen ions transported through, and released from, the alumina barrier layer, which dissolves at the tantala/alumina interface, i.e. without tantalum being in direct contact with the electrolyte. The shape and mutual arrangement of the anodic oxide nanostructures depend on the nature of the underlying metal, anodising solution, and are consistent with the difference between the resistivities of the tantalum (niobium) oxide formed and the barrier layer of the overlying alumina cells, which is influenced by incorporated electrolyte-derived species.

Formation of Al–Zr composite oxide films on aluminum by sol–gel coating and anodizing ☆

Abstract Al specimens were covered with zirconium oxide film by sol–gel coating using zirconium alkoxide, and then anodized galvanostatically in a neutral borate solution. The time-variation in anode potential during anodizing was followed, and the structure and dielectric properties of the anodic oxide film were examined by TEM, EDX, RBS, and impedance measurements. It was found that the anode potential increases during anodizing, and that the slope of the potential–time curve becomes steeper with increasing number of dippings in sol–gel solution. An anodic oxide film was formed at the interface between zirconium oxide and the aluminum substrate during anodizing. This anodic oxide film was composed of an inner Al 2 O 3 layer and an outer Al–Zr composite oxide layer. The capacitance of anodic oxide films formed after sol–gel Zr-oxide coating and anodizing was about 20% higher than without Zr-oxide coating.

Formation and Breakdown of Anodic Oxide Films on Aluminum in Boric Acid/Borate Solutions

Highly pure aluminum was anodized at a constant current density of 25 A m -2 at 293 K in 0.5 M boric acid/0, 0.005, or 0.05 M sodium tetraborate solutions, to examine the effect of sodium tetraborate concentration on the formation and breakdown characteristics of barrier oxide films by using inductively coupled plasma atomic emission spectrometry, electroluminescence/photoluminescence measurements, scanning electron microscopy, transmission electron microscopy, and electrochemical impedance spectroscopy. In boric acid/borate solutions, a crystalline alumina formed locally in the middle of the amorphous oxide film. Above the crystalline alumina, a void is formed and may lead to a breakdown of the oxide film at 420 to 540 V. In boric acid solution, an amorphous oxide film grew until 1180 V with the formation and development of imperfections and with enhancement of electroluminescence and gas evolution. At imperfections. the oxide/solution interface was convex and the oxide/metal interface curved in the opposite direction. This deformation is attributed to high-pressure O 2 evolved in the pores of imperfections and to the local formation and dissolution of oxide. The breakdown of the oxide film started when the O 2 evolution and oxide dissolution at imperfections become predominant. The mechanism of formation and breakdown of the anodic film in the boric acid/borate solutions is discussed in terms of pH buffering of the anodizing solution, and the electronic structure of anodic oxide films is correlated with electroluminescence and photoluminescence spectrum results.

Anodizing of Aluminum Coated with Silicon Oxide by a Sol-Gel Method

Aluminum specimens were covered with SiO 2 film by a sol-gel coating and then anodized galvanostatically in a neutral borate solution Time variations in the anode potential during anodizing were monitored, and the structure and dielectric properties of the anodic oxide films were examined by transmission electron microscopy, Rutherford backscattering spectroscopy, and electrochemical impedance measurements It was found that anodizing of aluminum coated with SiO 2 films leads to the formation of anodic oxide films, which consist of an outer Al-Si composite oxide layer and an inner Al 2 O 3 layer, at the interface between the SiO 2 film and the metal substrate. The capacitance of anodic oxide films formed on specimens with a SiO 2 coating was about 20% larger than without a SiO 2 coating. In the film formation mechanism, the conversion of Al 2 O 3 to Al-Si composite oxide at the interface between the inner and outer layers is discussed in terms of inward transport of Si-bearing anions across the outer layer.

Structure, Morphology, and Dielectric Properties of Nanocomposite Oxide Films Formed by Anodizing of Sputter-Deposited Ta-Al Bilayers

Anodizing of Ta-Al bilayers (aluminum deposited on tantalum) was performed in 0.2 M H 2 C 2 O 4 solution to transform the aluminum metal into its nanoporous oxide followed by pore widening and reanodizing to different voltages in the range of 100-600 V. The anodic films consist of an upper layer of nano-sized tantala columns penetrating into the pores and a lower layer of continuous tantalum oxide under the porous alumina film. The columns are mainly composed of tantalum pentoxide and tantalum sub-oxides TaO 2 , Ta 2 O 3 , and TaO while the lower film layer is tantalum monoxide. At the boundary between the columns and alumina cells, a region of mixed (composite) Ta 2 O 3 and Al 2 O 3 is formed due to channeling of the ionic current through the outer part of the alumina cell walls. The relationship between the layers and the ionic transport during oxide growth depend on pore size and formation voltage. The dielectric properties of the anodic films are close to those of an ideal capacitor. Voltage-independent apparent dielectric constant of 12.6 was determined for the films formed by normal reanodizing. The relatively higher dielectric constant for the films formed by reanodizing through the widened pores rises from 17.6 to 24.0 in the voltage range of 270-400 V, which is due to the change in morphology, relative amount and chemical composition of anodic tantala in the complex film structure. The nanocomposite anodic films can be used as dielectrics for high-voltage low-leakage current electrolytic capacitors.

Copper Electroless Plating at Selected Areas on Aluminum with Pulsed Nd‐YAG Laser

Local deposition of copper on aluminum was attempted by the successive processes of anodizing, laser irradiation, and electroless plating. Aluminum specimens covered with anodic oxide films were immersed in Cu 2+ , Cu 2+ /H 2 PO 2 - , and diluted NaOH solutions, and irradiated with a pulsed yttrium aluminum garnet (YAG) laser. The time variation in the rest potential of the specimens was followed during laser irradiation by a potentiometer, and the change of the surface composition by laser irradiation was examined by X-ray photoelectron spectroscopy (XPS). After the laser irradiation, copper electroless plating was carried out in (Cu 2+ , Ni 2+ )/H 2 PO 2 - solutions with or without Pb 2+ or thiourea. The rest potential measurements and XPS analysis suggested that Cu particles nucleate at the film-removed area by laser irradiation in Cu 2+ and Cu 2+ /H 2 PO 2 - solutions. A copper layer was obtained at the irradiated area by the subsequent electroless plating, and the copper nuclei acted as catalytic centers at the very initial stage in copper electroless plating. The effects of Pb 2+ and thiourea concentration and the deposition temperature on the kinetics of copper deposition is also discussed.

Nanopatterning on aluminum surfaces with AFM probe

Abstract Aluminum specimens covered with anodic oxide films were scratched with a silicon probe tip of atomic force microscope in pure water, CuSO 4 solutions, Cu-electroless plating solutions and diluted NaOH solutions. The rate of groove development ( R gd ) was in the order NaOH>CuSO 4 >pure water>Cu-electroless solution, and increased with increasing probe load and scratch number. Wear of the silicon probe tip was also examined by scratching many times in pure water and NaOH solution. It was greater in NaOH solution than in pure water. A silicon tip coated with polycrystalline diamond showed a high processing capability and wear resistance.

Local surface modification of aluminum by laser irradiation

A novel method involving a combination of pulsed YAG laser irradiation and electrochemistry for chemical and physical modification of aluminum surface at a selected area is reviewed. Local metal deposition is carried out by successive steps of anodizing, laser irradiation, and electroplating (or electroless plating), and the fabrication of a prototype for printed circuit boards is attempted. Local organic film deposition with anodizing, laser irradiation, and electrophoretic deposition is introduced, and the formation of microtrenches and pores on aluminum surfaces with anodizing, laser irradiation, and electrochemical etching (or chemical etching) is also described. It is emphasized that the key technology in these procedures is to use aluminum as a target for the laser irradiation in solution, and that anodic oxide films on aluminum play an important role in the processes.

Local Deposition of Ni-P Alloy on Aluminum by Laser Irradiation and Electroless Plating

Electroless plating of Ni‐P has been widely applied, since it is not selective to substrate materials with respect to electric conductivity and can provide excellent coverage of the substrate, especially those with complicated shapes. Much effort has been made to understand the deposition process, the effects of additives, and the structure and properties of the deposits in Ni‐P electroless deposition. 1‐9 In electroless plating, activation of the substrate is an indispensable pretreatment to initiate the reduction of nickel ions and to improve adhesion between deposit and substrate. Successive immersion in SnCl2 and PdCl2 solutions is a conventional activating process before the Ni‐P electroless plating. Here the specimen is initially immersed in SnCl 2 solution for 1‐2 min at room temperature to adsorb Sn 21 ions on the surface. During the second immersion in PdCl2 solution, Pd 21 ions are reduced by Sn 21

Microfabrication of an anodic oxide film by anodizing laser-textured aluminium

A simple method for the fabrication of microstructures of an aluminium anodic oxide film (anodic alumina) by anodizing laser-textured aluminium is demonstrated. In the process, the aluminium substrate was first textured by a low power laser beam, and then the textured aluminium was subjected to anodizing, to develop a continuous, thick porous layer on the textured surface. Microstructures with a depth of a few to several tens of micrometres were fabricated successfully on the anodic oxide film by using various combinations of laser power density and laser scanning speed. Removing the film from the aluminium substrate enables the fabrication of various 2D and 3D microstructures from anodic alumina.