Kouji Inagaki

Figuring with subnanometer-level accuracy by numerically controlled elastic emission machining

A numerically controlled elastic emission machining (EEM) system has been developed to fabricate ultraprecise optical components, particularly in x-ray optics. Nozzle-type EEM heads, by which a high shear-rate flow of ultrapure water can be generated on the work surface, have been newly proposed to transport the fine powder particles to the processing surface. Using this type of EEM head, the obtainable spatial resolution in figure correction can be changed by selecting the suitable aperture size of the nozzle according to the required spatial frequency. As a result of test figuring, 1 nm level peak-to-valley (p–v) accuracy is achieved throughout the entire spatial wavelength range longer than 0.3 mm. In addition, the microroughness of the processed surface is certified to also be approximately 1 nm (p–v).

Wavefront Control System for Phase Compensation in Hard X-ray Optics

A highly precise adaptive optical system that can be used in the hard X-ray region was developed. To achieve highly precise control of the wavefront shape, we discussed an optical system with a bendable mirror of deformation accuracy better than 0.4 nm RMS. Using the system, we demonstrated the controllability of the wavefront of a 15 nm hard X-ray nanobeam. The intensity profile of the wavefront-modified beam was in good agreement with the wave-optically calculated profile.

Effect of Particle Morphology on Removal Rate and Surface Topography in Elastic Emission Machining

Elastic emission machining is a surface preparation technique utilizing chemical reactions between the work piece surface and the powder particle surfaces. In this process, an atomically flat surface with an extremely high figure accuracy in the nanometer range is realized. However, the surface removal rate is extremely low. To enhance removal rate, the silica powder particles having a larger surface area than the ordinarily used particles are applied to smoothen a Si(001) surface. Experimental results indicate that removal rate improved significantly by about two orders of magnitude higher than the conventional removal rate due to the increase in the contact areas between the surfaces of the particles and the work piece. Moreover, the topography of the processed surfaces was found to improve in comparison with the initial surface from power spectral density analysis.

First-principles molecular-dynamics calculations and STM observations of dissociative adsorption of Cl2 and F2 on Si(001) surface

We have studied the dissociative adsorption process of halogen molecules (Cl2 and F2) on the Si(0 0 1) surface by first-principles molecular-dynamics (FPMD) calculations and scanning tunneling microscopy (STM) observations. From FPMD calculations, we demonstrate that Cl2 and F2 molecules adsorb dissociatively at dangling bonds of a buckled dimer with no energy barrier, so that the buckled dimer becomes geometrically flat. In addition, STM observations show that the dissociative adsorptions of Cl2 and F2 induce buckled dimers at the SA step on the Si(0 0 1)-(2×1) surface to become symmetric dimers, in good agreement with the results of FPMD calculations.

Self-consistent van der Waals density functional study of benzene adsorption on Si(100)

The adsorption of benzene on the Si(100) surface is studied theoretically using the self-consistent van der Waals density functional (vdW-DF) method. The adsorption energies of two competing adsorption structures, butterfly (BF) and tight-bridge (TB) structures, are calculated with several vdW-DFs at saturation coverage. Our results show that recently proposed vdW-DFs with high accuracy all prefer TB to BF, in accord with more accurate calculations based on exact exchange and correlation within the random-phase approximation. Detailed analyses reveal the important roles played by the molecule-surface interaction and molecular deformation upon adsorption, and we suggest that their precise description is a prerequisite for accurate prediction of the most stable adsorption structure of organic molecules on semiconductor surfaces.

Theoretical Study on the Scanning Tunneling Microscopy Image of Cl-Adsorbed Si(001)

An interpretation of the scanning tunneling microscopy (STM) image of a Cl-adsorbed Si(001) surface is reported based on first-principles calculations of its electronic structure. From the analysis of the spatial distribution of the local density of states, we show that the STM image of this surface calculated under the Tersoff–Hamann approximation is in disagreement with the observed STM image. Some remarks associated with the discrepancy are given to further studies.

Improvement of Removal Rate in Abrasive-Free Planarization of 4H-SiC Substrates Using Catalytic Platinum and Hydrofluoric Acid

We used catalyst-referred etching, which is an abrasive-free planarization method, to produce an extremely smooth surface on a 4H-SiC substrate. However, the removal rate was lower than that obtained by chemical mechanical polishing, which is the planarization method generally used for SiC substrates. To improve the removal rate, we investigated its dependence on rotational velocity and processing pressure. We found that the removal rate increases in proportion to both rotational velocity and processing pressure. A lapped 4H-SiC substrate was planarized under conditions that achieved the highest removal rate of approximately 500 nm/h. A smooth surface with a root-mean square roughness of less than 0.1 nm was fabricated within 15 min. Because the surface, which was processed under conditions of high rotational velocity and high processing pressure, consisted of a step–terrace structure, it was well ordered up to the topmost surface.

First-principles theoretical study of hydrolysis of stepped and kinked Ga-terminated GaN surfaces.

We have investigated the initial stage of hydrolysis process of Ga-terminated GaN surfaces by using first-principles theoretical calculations. We found that the activation barrier of H2O dissociation at the kinked site of the Ga-terminated GaN surface is about 0.8 eV, which is significantly lower than that at the stepped site of about 1.2 eV. This is consistent with the experimental observation where a step-terrace structure is observed after the etching process of Ga-terminated GaN surfaces with catalyst-referred etching method. Detailed analysis on the nature of the chemical interaction uring the hydrolysis processes will be discussed.

Intermolecular interaction as the origin of red shifts in absorption spectra of zinc-phthalocyanine from first-principles.

We investigate electronic origins of a redshift in absorption spectra of a dimerized zinc phthalocyanine molecule (ZnPc) by means of hybrid density functional theoretical calculations. In terms of the molecular orbital (MO) picture, the dimerization splits energy levels of frontier MOs such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the constituent molecules. Consequently, the absorption wavelength seems to become longer than the monomer as the overlap between the monomers becomes larger. However, for a ZnPc dimer configuration with its cofacially stacked monomer arrangement, the calculated absorption spectra within the time-dependent density functional theory indicates no redshift but blueshift in the Q-band absorption spectrum, i.e., a typical H-aggregate. The origin of the apparently contradictory result is elucidated by the conventional description of the interaction between monomer transition dipoles in molecular dimers [Kasha, M. Radiat. Res. 1963, 20, 55]. The redshift is caused by an interaction between the two head-to-tail transition dipoles of the monomers, while the side-by-side arranged transition dipoles result in a blueshift. By tuning the dipole-dipole interaction based on the electronic natures of the HOMO and the LUMO, we describe a slipped-stacked ZnPc dimer configuration in which the Q-band absorption wavelength increases by as large as 144 nm relative to the monomer Q-band.

An atomically controlled Si film formation process at low temperatures using atmospheric-pressure VHF plasma

To grow epitaxial Si films with atomic- and electronic-level perfection, a high-temperature chemical vapor deposition (CVD) process (>1000 °C) has been generally employed. To reduce the growth temperature below 600 °C but keeping a high deposition rate, other energy sources than thermal heating are required. Atmospheric pressure plasma CVD (AP-PCVD) is considered to be suitable for fabricating high-quality films at high deposition rates due both to the high radical density and to the low ion bombardment against the film surface, because the collision frequency among ions and neutral atoms is high. The present study focuses on the low-temperature growth of epitaxial Si, and experimentally demonstrates that AP-PCVD is capable of growing epitaxial Si films with high perfection applicable for semiconductor devices. It is found that the pre-growth cleaning of the Si surface by H(2) AP plasma is effective to grow high-purity Si films, and that the exposure of a film-growing surface to AP plasma during growth is important to form particle-free and defect-free Si films. From the experimental results and the first-principles molecular dynamics simulations of surface atomic reactions, it can be mentioned that both H atoms in the AP plasma and high-density He atoms having thermal kinetic energy contribute to the reduction of growth temperature by supplying considerable energy to the surface.