Nobuyuki Takeyasu

Multiple-spot parallel processing for laser micronanofabrication

A tightly focused femtosecond laser has been established as a unique tool for micronanostructure fabrication due to its intrinsic three-dimensional processing. In this letter, we utilize a microlens array to produce multiple spots for parallel fabrication, giving rise to a revolutionary augmentation for our previously developed single-beam two-photon photopolymerization technology [S. Kawata, H.-B. Sun, T. Tanaka, and K. Takada, Nature (London) 412, 697 (2001)]. Two- and three-dimensional multiple structures, such as microletter set and self-standing microspring array, are demonstrated as examples of mass production. More than 200 spot simultaneous fabrication has been realized by optimizing the exposure condition for the photopolymerizable resin, i.e., a two-order increase of yield efficiency. Potential applications of this technique are discussed.

Improving spatial resolution of two-photon microfabrication by using photoinitiator with high initiating efficiency

The lateral spatial resolution (LSR) in two-photon induced polymerization was improved to 80nm by using an anthracene derivative (9,10-bis-pentyloxy-2,7-bis[2-(4-dimethylamino-phenyl)-vinyl]anthracene (BPDPA)) as a highly sensitive and efficient photoinitiator. Photocurable resin containing 0.18mol% BPDPA exhibited a low polymerization threshold of 0.64mW at 800nm. Theoretical calculations showed that the LSR can be increased by reducing the laser power, indicating that the LSR could be improved using more sensitive initiators in the future.

3D Metallic Nanostructure Fabrication by Surfactant-Assisted Multiphoton-Induced Reduction†

A laser direct-writing technique employing multiphoton absorption processes has become a powerful and widely used tool in the fabrication of micro-/nanometer-scale structures in the past decade because of its 3D fabrication ability. Using this technique, a number of 2D and 3D microstructures have been successfully created with polymers, glasses, and metals. In particular, since nanoscale metals exhibit unprecedented and unique properties such as electromagnetic field enhancement, catalysis, photoemission, and electronic conductivity never found in their bulk states, metal structures with nanoscale geometries have received much attention from various fields such as plasmonics, electronics, bioscience, and

Three-dimensional fabrication of metallic nanostructures over large areas by two-photon polymerization

An experimental protocol for the realization of three-dimensional periodic metallic micro/nanostructures over large areas is presented. Simultaneous fabrication of hundreds of three-dimensional complex polymer structures is achieved using a two-photon photopolymerization (TPP) technique combined with a microlens array. Metallization of the structures is performed through the deposition of thin and highly conductive films by electroless plating. A chemical modification of the photopolymerizable resin and the production of a hydrophobic coating on the glass surface supporting the structures are realized. This process prevents metal deposition on the substrate and restricts adhesion on polymer. Our technique can produce periodic and/or isolated metallic structures with arbitrary shape, created by more than 700 individual objects written in parallel.

Selective electroless plating to fabricate complex three-dimensional metallic micro/nanostructures

We report on selective metal deposition over complex polymer structures formed by two-photon induced photopolymerization technique. Periodic three-dimensional micro/nanostructures are fabricated by means of a microlens array to produce multiple spots from a single-beam femtosecond laser. An electroless plating method is used to deposit a thin silver film onto the sample surface. The glass slide surface supporting the structures is chemically modified to avoid silver coating of the substrate. Our technique enables to produce complex metallic structures with arbitrary shapes under ambient conditions.

Conformal Chemical Vapor Deposition TiN(111) Film Formation as an Underlayer of Al for Highly Reliable Interconnects

Chemical vapor deposition (CVD) TiN film having the (111) preferred orientation and conformal step coverage was developed for the first time. The key processes are predeposition of the Ti(002) layer before the CVD TiN film deposition and optimization of the flow rate of reactants: TiCl4, NH3 and H2. Growth of (111) crystal plane seems to be induced due to the lattice matching of crystal plane of the Ti film. On the other hand, conformal step coverage is obtained by using a lower flow rate of NH3, resulting in the surface-limited chemical reaction. It was also demonstrated that both sputtering and CVD Al, deposited on the CVD TiN having stronger (111) orientation, showed stronger (111) orientation which is advantageous for obtaining high electromigration (EM) resistance. Furthermore, simultaneous contact hole filling and interconnects formation using CVD Al/CVD TiN stacked film are successfully demonstrated.

Metal Deposition Deep into Microstructure by Electroless Plating

Metal microfabrication has been actively studied and used in various fields because of the unique optical, electrical, and catalytic properties of the fine structures that it can produce. 1–6) The ability to simultaneously and precisely fabricate a number of micro-objects over a wide region is essential to the practical application of nanotechnology. In these processes, plating methods are occasionally used for metal deposition. Plating is a conventional method of depositing metal onto substrates, 7) and can be used not only with conductors, but also with insulators such as plastics and ceramics. Among known plating techniques, electroless plating is commonly used for metal deposition on insulators because it does not require any external electric field. Electroless plating is chemically performed with a solution, and it results in a uniform metal deposition over the entire surface area of micro-objects. 8–10) With this method, it is possible to deposit metal effectively even though the area is limited to less than a micrometer in width. 11,12) Moreover, a number of micrometer-scale metal patterns can be fabricated simultaneously over a very wide area much more easily than with sputtering or vacuum evaporation, which require large, complex equipment. These techniques have been long studied, and are used widely for metal fabrication processes. However, it is generally very difficult, using these methods, to deposit metal on a deep or normally occluded part of a structure, such as the inner wall of a long tubular structure. In this letter, we demonstrate electroless plating on three types of structure. First, gold was deposited on the inner wall of a fused-silica capillary, which is difficult to achieve by other methods. Second, silver was deposited onto glass beads. Third, gold was deposited inside a complex concave structure formed by polystyrene microbeads sandwiched between glass plates. As mentioned above, although it is generally difficult to coat metals onto the inside of a concave structure using other methods (such as vacuum evaporation and sputtering), with electroless plating the plating solution containing metal ions naturally goes into the inside of even a fine, complex structure, and therefore all surfaces contacting the plating solution are uniformly coated with metal. To determine the main features of our process and to verify its effectiveness, we first investigated the relationship between the amount of gold deposited onto the surface of a polystyrene substrate at room temperature (295 K) by measuring the transmission spectrum. We prepared 0.024 M tetra-chloro auric acid (HAuCl4), 0.75 M sodium hydroxide (NaOH), and 0.086 M sodium chloride (NaCl) in water as a gold ion solution, and 0.5 vol % glycerol (C3H8O3) in water as a reduction agent. Samples were obtained by terminating the reaction at 9, 12, 15, and 18 min from mixing the plating solution and the reduction agent. The transmission spectra were measured from 300 to 800 nm for these samples by an absorptiometer (Shimadzu, UV2500PC), and are shown in Fig. 1. For reaction times of less than 9 min, no noticeable deposition of gold was observed. A region with a lower transmission compared to the others was found from 500 to 600 nm; this was due to the local plasmon-mode resonance absorption of gold particles. 13) The

Design of high efficiency for two-photon polymerization initiator: combination of radical stabilization and large two-photon cross-section achieved by N-benzyl 3,6-bis(phenylethynyl)carbazole derivatives

Novel A–π–D–π–A V-shaped 3,6-bis(phenylethynyl)carbazole based chromophores were designed and synthesized as two-photon polymerization (TPP) initiators combining a large two-photon absorption cross-section with facilitated radical formation. 9-Benzyl-3,6-bis(4-nitrophenylethynyl)carbazole (4d) shows a strong two-photon absorption around 800 nm and exhibits very high two-photon polymerization initiating sensitivity with a threshold power of 0.8 mW at the concentration of 0.18 mol%, which is much lower than the threshold power of 6.37 mW found for benzil. The corresponding threshold of laser exposure intensity for TPP is 3.0 × 107 mJ cm−2. The lowest loading of 4d is up to 0.012 mol% with a threshold power of 3.2 mW.

Three-dimensional fabrication of metallic micro/nanostructures by two-photon polymerization for metamaterials

We report on selective metal deposition over complex polymer structures formed by two-photon induced photopolymerization (TPP) technique. Periodic three-dimensional (3D) polymer micro/nanostructures are fabricated by means of a microlens array to produce multiple spots from a single-beam femtosecond laser amplified by a regenerative amplifier. The photopolymerizable resin and the glass substrate are chemically modified, and a pre-treatment with SnCl2 is applied before realizing a uniform silver coating by electroless plating. This preparation enables a selective deposition of small silver particles only on the polymer surface all over the sample and to avoid metal deposition on the substrate. Electrical measurements show the structures to be highly conductive with typical resistivities ρ approx. 10-7 Ωm, only a few times larger than the value for bulk silver. By taking advantage of the high accuracy and arbitrary shape modeling of TPP fabrication, we can realize complex periodic and/or metallic micro-nanostructures which were so far out of reach. Thus, a straightforward application could be the realization of metamaterials. The processing efficiency of our technique is demonstrated with the fabrication of several large samples, created by more than 700 objects written in parallel and metallized with silver.

Oil-in-water emulsion as fabrication platform for uniform plasmon-controlled two-dimensional metallic nanoparticle array

Gold/silver nanoparticles were trapped at the oil/water interface of oil droplets dispersed in water. The metallic nanoparticles were self-assembled into a uniform two-dimensional large array structure through the aggregation and coalescence of the nanoparticle-covered oil droplets. The plasmon resonance of the array structure was tunable and a surface-enhanced Raman scattering measurement was performed with the silver nanoparticle array. The enhancement factor was ~105 and enhanced Raman signals were observed over the whole array () with high reproducibility, which is an advantage of a self-assembly method using a liquid/liquid interface.