Takayuki Takahagi

The formation of hydrogen passivated silicon single-crystal surfaces using ultraviolet cleaning and HF etching

We have tried to develop a new procedure to prepare the clean surface of a silicon single crystal. We successfully prepared the contamination free bare silicon surface with ultraviolet cleaning followed by HF dipping with low concentration HF obtained by dilution by organic free ultrapure water, at room temperature under the atmospheric condition. X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and ultraviolet photoelectron spectroscopy measurements proved thus prepared surface has a hydrogen monoatomic layer terminating the dangling bonds of silicon. The hydrogen termination was found to have remarkable passivation effect against surface oxidation reaction. A silicon thin‐film epitaxially grown on the prepared surface was confirmed to have perfect crystal structure and high‐purity level by scanning electron microscopy, reflection high‐energy electron diffraction, Raman spectroscopy and secondary ion mass spectroscopy.

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.

Self-organization of a porous alumina nanohole array using a sulfuric/oxalic acid mixture as electrolyte

The possibility of fabricating a highly regular nanohole array using different acid mixtures for the anodic oxidation of aluminum was investigated. The regularity of a nanohole array formed using a 1:1 sulfuric/oxalic acid mixture was quantified. Excellent regularity was obtained at around an anode voltage of 32 to 36 V. Cell pitch of the nanohole array at 36 V was 73 nm, which falls between those obtained using sulfuric acid (65 nm at 28 V) and oxalic acid (95 nm at 40 V). The present results strongly suggest that the pitch of the regular nanohole array can be varied by changing the ratio of the different acids.

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

Control of the chemical reactivity of a silicon single‐crystal surface using the chemical modification technique

A technique is developed to control the chemical reactivity of a silicon single‐crystal surface through chemical modification with atomic hydrogen. The reactivity of the reconstructed single‐crystal surface prepared by high‐temperature treatment in an ultrahigh vacuum is significantly decreased by capping the dangling bonds of top‐layer silicon atoms with hydrogen atoms. The Si—H bonds on the hydrogenated surface are found to be much more stable against oxidation than the Si—Si back bonds. The hydrogen‐passivated silicon surface is reactivated by electron beam irradiation. An ultrathin oxide layer pattern can be prepared using preferential oxidation of the area reactivated by a focused electron beam.

Fluorine-containing species on the hydrofluoric acid etched silicon single-crystal surface

The chemical structure and property of fluorine‐containing species on the hydrofluoric acid (HF) etched Si surface was examined by use of x‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The fluorine content on the surface was found to increase with increase of HF concentration. A silicon surface etched by 50% HF has fluorine of 2.6×1014 atoms/cm2 as Si–F. Most of the Si–F bondings are rapidly hydrolyzed to Si–OH by rinsing the wafer in water. Thus prepared Si–OH groups are found to be useful as active sites for chemical modification of the bare silicon single‐crystal surface. The Si–F was observed not to influence the oxidation rate of HF etched silicon surface.

Two-dimensional nanowire array formation on Si substrate using self-organized nanoholes of anodically oxidized aluminum

Abstract A highly ordered two-dimensional array of 48 nm Cu wires was successfully fabricated on Si substrate by the usage of anodic oxidation of aluminum (Al) for the first time. Anodic oxidation was carried out for Al sputtered film on Si substrate covered by a thin thermally oxidized SiO2 film, which was very effective to protect Si substrate from anodic oxidation. A highly ordered array of nanoholes was formed by the two steps Al anodic oxidation, and finally Cu was deposited by electroless plating in nanoholes which aspect ratio was 2.5. The present method suggests possibility of a large area two-dimensional array of quantum dots or wires on semiconductor substrate, which are considered to be a key technology for future ULSIs operated by single electron tunneling phenomena.

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.