TiN growth on Si by hybrid radical beam PLD

High-quality (good crystallinity and stoichiometry) titanium nitride (TiN) thin films were grown on Si(100) substrates by pulsed laser deposition (PLD) using a high-purity titanium target (99.99%) and nitrogen radical beam. The crystallinity, chemical composition, and depth profiles of the grown films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS), respectively. The XRD pattern indicated that the preferred growth of TiN(200) with an orientation parallel to the Si(100) direction was obtained and the nitrogen radical drastically improved crystallinity compared with that grown in ambient nitrogen gas. RBS spectra indicated that the combination of PLD and the nitrogen radical beam suppressed silicidation at the interface between the Si substrate and TiN thin film during growth. The XPS analysis revealed that this method achieved the synthesis of stoichiometric TiN films.     

Dual-beam ablation of fused silica by multiwavelength excitation process using KrF excimer and F2 lasers

A new technique of dual-beam laser ablation of fused silica by multiwavelength excitation process using a 248-nm KrF excimer laser (ablation beam) coupled with a 157-nm F₂ laser (excitation beam) in dry nitrogen atmosphere is reported. The dual-beam laser ablation greatly reduced debris deposition and, thus, significantly improved the ablation quality compared with single-beam ablation of the KrF laser. High-quality ablation can be achieved at the delay times of KrF excimer laser irradiation shorter than 10 ns due to a large excited-state absorption. The ablation rate can reach up to 80 nm/pulse at the fluence of 4.0 J/cm² for the 248-nm laser and 60 mJ/cm² for the F₂ laser. The ablation threshold and effective absorption coefficient of KrF excimer laser are estimated to be 1.4 J/cm² and 1.2᎒⁵ cm^-1, respectively.     

Metal-Containing Initiator Systems. 30. Vinyl Polymerization Initiated with Bis(ethyl acetoacetato)copper(II) and Maleimide

Some electron-accepting compounds such as maleimide (MIm), maleic anhydride (MAn), and tetracyanoquinodimethane were found to show pronounced accelerating effects on vinyl polymerization initiated with metal chelates. The polymerization of methyl methacrylate (MMA) initiated with bis(ethyl acetoacetato)-copper(II) (Cu(eacac)₂) and MIm was studied kinetically in benzene. The overall activation energy of the polymerization was calculated to be 11.5 kcal/mol. This value was much lower than that (17.6 kcal/mol) for the polymerization of MMA with Cu(eacac)₂ alone. The polymerization rate (R_p) was expressed as R_p =k[MIm]^1/2 [Cu(eacac)₂]^1/2 [MMA] The first-order dependence of R_p on the monomer concentration indicated that the monomer had no participation in the initiation step, in contrast with polymerization in the absence of MIm (where a monomer concentration dependence of 1.4th order was observed). Electronic spectroscopic study revealed that a complex between MIm and Cu(eacac)₂ had been formed. The ligand radical, an acetylcarboethoxymethyl radical, was trapped by 2-methyl-2-nitrosopropane in the reactions of Cu(eacac)₂ with MIm and with MAn in benzene. From these results the mechanism of the initiation of polymerization is discussed.     

Rapid prototyping of three-dimensional microfluidic mixers in glass by femtosecond laser direct writing

The creation of complex three-dimensional (3D) microfluidic systems has attracted significant attention from both scientific and applied research communities. However, it is still a formidable challenge to build 3D microfluidic structures with arbitrary configurations using conventional planar lithographic fabrication methods. Here, we demonstrate rapid fabrication of high-aspect-ratio microfluidic channels with various 3D configurations in glass substrates by femtosecond laser direct writing. Based on this approach, we demonstrate a 3D passive microfluidic mixer and characterize its functionalities. This technology will enable rapid construction of complex 3D microfluidic devices for a wide array of lab-on-a-chip applications.     

Optical and magneto-optical properties in Fe-doped glasses irradiated with femtosecond laser

Optical, magnetic, and magneto-optical properties have been investigated for the α-Fe₂O₃-doped transparent glasses irradiated with an infrared fs laser and subsequently annealed. The values of the saturation magnetization at room temperature for the irradiated glasses were increased compared with the as-prepared samples, which is due to the precipitation of the ferrimagnetic ferrite nanoparticles (NPs). By adding further dopants as precursors of plasmonic metals, Au or Al NPs were space-selectively precipitated together with the ferrite NPs in a confined region after irradiation and thermal annealing. In the case of the glass codoped with Al, magneto-optical Faraday effect was plasmonically enhanced and exhibited a negative distinct peak ascribed to a coupling between the ferrimagnetism of ferrite NPs and the localized surface plasmon resonance of Al NPs, while the glass, in which ferrite and Au NPs were precipitated, showed a positive enhancement of Faraday effect due to a coupling of plasmon resonance with diamagnetism of glass matrix.     

Photochemically triggered reaction of glucose oxidase in a fused vesicle containing Malachite Green leuconitrile derivative

Glucose oxidase (GOD) was encapsulated in vesicles containing a photoionizable Malachite Green leuconitrile derivative (MGL). Subsequent UV irradiation of MGL afforded the fusion of GOD- and glucose-encapsulating vesicles and thus decreased the concentration of glucose in the vesicles. The time dependence of the vesicle fusion was studied using fluorescent probe molecules. This phototriggered fusion could be instrumental in the development of a system for the production of nanometer-sized bioreactors.     

Laser-induced plasma-assisted ablation of fused quartz using the fourth harmonic of a Nd+:YAG laser

+ :YAG laser (266 nm) is reported. With the assistance of plasma resulting from laser irradiation on a metal target, the fused-quartz substrate is easily etched by the 266-nm laser beam in spite of the fact that the substrate is transparent to this wavelength. In contrast, no ablation takes place without the metal target, but damage is generated on the substrate surface. The ablated region is observed by optical microscopy and scanning probe microscopy (SPM), which reveal a fine grating structure (line spacing of 20 μm) without any severe damage. A series of experiments on the dependence of the ablation rate and the threshold laser fluence on ablation parameters, such as laser fluence, the number of pulses, and the distance between the fused quartz and the metal target is performed. On the basis of the results, three possible mechanisms of direct plasma interaction, plasma heating, and metal film deposition are discussed. Received: 27 February 1998/Accepted: 5 June 1998     

Global analysis of heat transfer in CZ crystal growth of oxide taking into account three-dimensional unsteady melt convection: Effect of meniscus shape

A global model of heat transfer analysis for the Czochralski crystal growth of oxides, in which a three-dimensional unsteady melt flow was taken into account, was developed in our recent study. In the model, however, the focal point was the methodology of formulating the model by coupling a conventional global model of heat transfer, which is based on a pseudosteady axisymmetric assumption, with a model of a three-dimensional, unsteady melt flow via two interface models. Therefore, for simplicity, the shape of the melt free surface was assumed to be flat. In this study, the global model was further developed by considering the meniscus of the melt free surface. It was found that the meniscus of the melt free surface caused the melt flow to be more unstable and shifted the critical Reynolds number at which the melt/crystal interface inversion occurs toward a much lower value.     

Formation of p-type layer by KrF excimer laser doping of carbon into GaAs in CH4 gas ambient

Carbon doping of GaAs using a KrF excimer laser to form a p-type active layer is described. Methane gas (CH₄) was used as a source of the C acceptor. Various quantities such as sheet resistance, surface carrier density, Hall mobility, and depth profile of C-doped GaAs are measured as the functions of laser fluence and laser pulse. It is shown that C atoms are doped only within a limited depth as shallow as 50 nm or less and with extremely high concentration exceeding 1×10²¹ cm^–3. The maximum activation efficiency is found to be 69.0%. Laser induced changes of surface morphologies and electron diffraction patterns are also discussed. Furthermore, non-alloyed ohmic contacts using laser-doped p-type GaAs are demonstrated.