Shoso Shingubara

Fabrication of Nanomaterials Using Porous Alumina Templates

Nanofabrication by self-organization methods has attracted much attention owing to the fact that it enables mass production without the use of expensive lithographical tools, such as an electron beam exposure system. Porous alumina can be fabricated electrochemically through anodic oxidation of aluminum by means of such a self-organization method, yielding highly ordered arrays of nanoholes several hundreds down to several tens of nanometers in size. This paper is an overview of recent research on porous alumina science and technology, nanohole array self-organization conditions and mechanisms, various methods of nanostructure formation using porous alumina templates, optical and magnetic nanofabrication, perspectives on electronic nano device fabrication and chemical/biological sensors and membranes.

Ordered Two-Dimensional Nanowire Array Formation Using Self-Organized Nanoholes of Anodically Oxidized Aluminum

Self-organization of a two-dimensional array of nanoholes which were formed by anodic oxidation of aluminum was investigated quantitatively using fast Fourier transformation (FFT) analysis of scanning electron microscopy (SEM) images. The highly ordered array of nanoholes with diameters of 26 nm was obtained by two-step anodization at anodic voltage around 40 V, and oxalic acid concentration of 0.5 M. A two-dimensional ordered array of Au free standing nanowires was successfully fabricated by the deposition of Au using DC electroplating in nanoholes of aluminum oxide, by removal of the aluminum oxide barrier layer using wet chemical etching. The present method has a high efficiency to fabricate ordered nanowire array of a variety of conductive materials in a large area, and wide applications for fabricating quantum effect devices and materials would be expected.

Bottom-Up Fill of Copper in Deep Submicrometer Holes by Electroless Plating

Bottom-up fill of Cu in deep submicrometer via holes was achieved through electroless plating alone for the first time. We investigated the effect of addition of inhibitor molecules to electroless Cu plating solution, and found that sulfopropyl sulfonate ~SPS! was highly effective in promoting the bottom-up fill. The tendency for bottom-up filling was enhanced by shrinkage of the hole diameter. This suggests that the diffusion flux of SPS molecules to the bottom of holes was more suppressed for smaller holes.

Electromigration in a single crystalline submicron width aluminum interconnection

Electromigration properties in a single crystalline submicron width aluminum interconnection formed on Si(111) have been examined by resistance change measurements and in situ observations using scanning electron microscopy. It was observed that single crystalline aluminum has an extremely high resistance to electromigration‐induced open circuit failures, when compared to polycrystalline copper and aluminum. The mechanism for the high resistance is considered to be a large activation energy, resulting from lattice diffusion. A tendency for void formation to become parallel to the longitudinal direction of the interconnection assisted the life time prolongation.

Bottom-Up Fill for Submicrometer Copper Via Holes of ULSIs by Electroless Plating

In this report, the hole-filling characteristics upon addition of SPS were evaluated in detail by cross-sectional scanning electron microscopy ~SEM!, and the effects of SPS concentration on bottom-up fill ability, and fundamental film properties such as contaminant level, crystal texture, and surface morphology were investigated. Experimental ICB-Pd layers with a thickness of 1 or 2 nm were deposited on the surface of three types of TaN/SiO2 /Si substrates, hole patterns ~diameter, 0.31-1.0 mm; depth, 1.5 mm! for investigating filling viahole; trench patterns ~length, 100 mm; width, 0.21-100 mm; depth, 0.3 mm! for electrical resistivity measurement; and blankets for measurement of the deposition rate of electroless plating with SPS concentration. The thickness of the Pd layer was determined by a quartz microbalance placed on the substrate surface. Prior to electroless copper plating, all substrates were cleaned by ultrasonication in acetone at room temperature for 10 min. The composition of the electroless copper plating solution was CuSO4 i 5H2O ~6.6 g/L! ,C 10H16N2O8 ~EDTA; 70.0 g/L!, glyoxylic acid ~18.0 g/L! as a reducing agent, 2,28-dipyridine ~0.04 g/L! as stabilizer, polyethylene glycol ~4000 MW, 0.5 g/L! as the surface activator. The pH of the plating bath was adjusted to approximately 12.5 using tetramethylammonium hydroxide ~TMAH! and the bath temperature was maintained at 70°C. The interfacial structure and morphology of samples were characterized by field-emission scanning electron microscopy~FE-SEM! and field-emission transmission electron microscopy~FE-TEM! .A ll SEM and TEM samples were prepared by focused ion beam ~FIB!. To protect the surface of the Cu film for etching during FIB cutting, a fine film of epoxy resin was coated on the surface of Cu by spin

Well-size-controlled Colloidal Gold Nanoparticles Dispersed in Organic Solvents.

The preparation of well-size-controlled colloidal gold nanoparticles in organic solvent is presented. After the preparation of well-size-controlled aqueous colloidal gold particles, we changed the solvent to an organic one. This technique is required to enable a chemical reaction between gold particles and hydrophobic molecules, since a colloidal gold solution is typically prepared in water using a reduction process. We also investigated the stability of the gold particle suspension, and found that the stability decreases in the sequence of water, ethanol, chloroform, and benzene solution.

Experimental conditions for a highly ordered monolayer of gold nanoparticles fabricated by the Langmuir–Blodgett method

A highly ordered monolayer film of alkanethiol-encapsulated gold nanoparticles was fabricated on a silicon substrate by using the Langmuir–Blodgett (LB) method. The effects on the particle order, of the particle concentration and the type of solvent of the LB spreading suspension of encapsulated gold particles, were studied. We found that a low particle concentration of 0.06–0.3 mg/mL in chloroform is optimal for the fabrication of high quality gold particle monolayers. Since the proposed method is not restricted to gold particles, it is believed to be a practical process for fabricating quantum dot structures of various particle sizes and compositions.

Adhesion improvement of electroless copper to a polyimide film substrate by combining surface microroughening and imide ring cleavage

In order to enhance the adhesion strength of copper metal film to a polyimide (PI) film substrate, a method combining surface microroughness formation and imide ring cleavage was investigated. The results showed that imide rings were cleaved with a KOH treatment while carboxyl and amide groups were formed on the surface of the PI film. The surface micro-roughness did not change with the KOH treatment, and the adhesion strength of the copper metal film to the PI film was slightly improved to 30 g/mm, which could be attributed to the interaction of both carboxyl and amide groups with the copper atoms. When the PI films were successively treated with an alkaline permanganate and a KOH solution, many recesses were formed on the surface in an alkaline permanganate solution, and the size and depth of the recesses increased with alkaline permanganate treatment time. The results of the AFM measurements showed that the average roughness (R) increased from 3.54 to 10.23 nm after combined treatment with alkaline permanganate and KOH solutions. The adhesion strength of the copper metal film to the PI film reached 150 g/mm, which was five times greater than that achieved with the KOH treatment only.

Fabrication of nanohole array on Si using self-organized porous alumina mask

Formation of the ordered array of nanoholes on Si was investigated by the use of the self-organized porous alumina nanoholes array etching mask that is directly formed on the Si substrate by sputtering and subsequent anodic oxidation. Reactive ion etching using chlorine plasma against a porous alumina/thin-SiO2(10 nm)/Si substrate with a high self-bias of rf plasma was revealed to be very effective for pattern transfer to Si. After pattern transference a significant reduction of hole size was observed. In fact, the initial porous alumina hole size of 45 nm is reduced to 13 nm Si holes when the higher aspect ratio of porous alumina nanoholes mask is used. The etching characteristics strongly suggest that not only chemical etching but sputtering occurred, and that redeposition of nonvolatile materials in nanoholes plays an essential role in the reduction of the hole size.

Electromigration‐induced abrupt changes in electrical resistance associated with void dynamics in aluminum interconnections

Electromigration‐induced failure mechanisms were investigated by means of extremely sensitive resistance change measurements and simultaneous observations using scanning electron microscopy. Abrupt changes in resistance (ACRs), classified into three types: downward steps, upward steps, and oscillations, were found to occur frequently during the dc current stressing test. It was conspicuously observed that there was a rapid void annihilation associated with an abrupt increase in resistance, and a rapid void formation with a decrease in resistance. ACRs are considered to be caused by a rapid change in the number of excess vacancies concomitant with void annihilations or formations. The thermodynamical analysis on the stability of a void strongly suggests that the change of a local stress from tensile to compressive causes a rapid annihilation of voids, and the opposite one causes a rapid formation. Temperature dependence of the intensity of ACRs exhibited an activation energy of 0.43 eV, which implied that ...