Kenta Arima

Single-nanometer focusing of hard x-rays by Kirkpatrick-Baez mirrors.

We have constructed an extremely precise optical system for hard-x-ray nanofocusing in a synchrotron radiation beamline. Precision multilayer mirrors were fabricated, tested, and employed as Kirkpatrick-Baez mirrors with a novel phase error compensator. In the phase compensator, an at-wavelength wavefront error sensing method based on x-ray interferometry and an in situ phase compensator mirror, which adaptively deforms with nanometer precision, were developed to satisfy the Rayleigh criterion to achieve diffraction-limited focusing in a single-nanometer range. The performance of the optics was tested at BL29XUL of SPring-8 and was confirmed to realize a spot size of approximately 7 nm.

Very High Carrier Mobility for High-Performance CMOS on a Si(110) Surface

In this paper, we demonstrate CMOS characteristics on a Si(110) surface using surface flattening processes and radical oxidation. A Si(110) surface is easily roughened by OH- ions in the cleaning solution compared with a Si(100) surface. A flat Si(110) surface is realized by the combination of flattening processes, which include a high-temperature wet oxidation, a radical oxidation, and a five-step room-temperature cleaning as a pregate-oxidation cleaning, which does not employ an alkali solution. On the flat surface, the current drivability of a p-channel MOSFET on a Si(110) surface is three times larger than that on a Si(100) surface, and the current drivability of an n-channel MOSFET on a Si(100) surface can be improved compared with that without the flattening processes and alkali-free cleaning. The 1/f noise of the n-channel MOSFET and p-channel MOSFET on a flattened Si(110) surface is one order of magnitude less than that of a conventional n-channel MOSFET on a Si(100) surface. Thus, a high-speed and low-flicker-noise p-channel MOSFET can be realized on a flat Si(110) surface. Furthermore, a CMOS implementation in which the current drivabilities of the p-channel and n-channel MOSFETs are balanced can be realized (balanced CMOS). These advantages are very useful in analog/digital mixed-signal circuits.

Atomic-scale flattening of SiC surfaces by electroless chemical etching in HF solution with Pt catalyst

The authors present a method for flattening SiC surfaces with Pt as a catalyst in HF solution. The mechanism for flattening SiC surfaces is discussed. The flattened 4H-SiC(0001) surface is composed of alternating wide and narrow terraces with single-bilayer-height steps, which are induced by the rate difference of the catalytic reactions between adjacent terraces. Scanning tunneling microscopy images reveal a 1×1 phase on the terraces. The 1×1 phase is composed of coexisting of F- and OH-terminated Si atoms, which originate from the polarization of the underlying Si–C bonds.

Atomically resolved scanning tunneling microscopy of hydrogen-terminated Si(001) surfaces after HF cleaning

Atomic structures of hydrogen-terminated Si(001) surfaces after HF cleaning are investigated by scanning tunneling microscopy. It is revealed that the surface is macroscopically rough but is composed of terraces and steps. Inside a terrace, 1×1 structures are formed. This corresponds to the ideal 1×1 dihydride structure. The step edges run along the 〈110〉 direction. On the other hand, the 1×1 dihydride structure disappears when the surface is subsequently rinsed with ultrapure water, because every other dihydride row of the ideal 1×1 structure is preferentially etched in ultrapure water.

Atomic structures of hydrogen-terminated Si(001) surfaces after wet cleaning by scanning tunneling microscopy

Scanning tunneling microscopy observations are performed on a H-terminated Si(001) surface treated with HF solutions and ultrapure water with very low dissolved oxygen and total organic carbon contents. Over a large area, row structures are observed in [110] and [110] directions. Pyramidal-shaped etch pits are also observed, which are caused by anisotropic etching by OH ions. Detailed images clearly show 2×1 periodic structures. It is suggested that every other row of the ideally dihydride 1×1 surface is etched preferentially by OH ions. This explains the mechanism by which the smallest etch pits are formed.

Metal-assisted chemical etching of Ge(100) surfaces in water toward nanoscale patterning

We propose the metal-assisted chemical etching of Ge surfaces in water mediated by dissolved oxygen molecules (O2). First, we demonstrate that Ge surfaces around deposited metallic particles (Ag and Pt) are preferentially etched in water. When a Ge(100) surface is used, most etch pits are in the shape of inverted pyramids. The mechanism of this anisotropic etching is proposed to be the enhanced formation of soluble oxide (GeO2) around metals by the catalytic activity of metallic particles, reducing dissolved O2 in water to H2O molecules. Secondly, we apply this metal-assisted chemical etching to the nanoscale patterning of Ge in water using a cantilever probe in an atomic force microscopy setup. We investigate the dependences of probe material, dissolved oxygen concentration, and pressing force in water on the etched depth of Ge(100) surfaces. We find that the enhanced etching of Ge surfaces occurs only when both a metal-coated probe and saturated-dissolved-oxygen water are used. In this study, we present the possibility of a novel lithography method for Ge in which neither chemical solutions nor resist resins are needed.

Polishing characteristics of silicon carbide by plasma chemical vaporization machining

Silicon carbide (SiC) is expected to be a promising semiconductor material for high-temperature, high-frequency, high-power and energy-saving applications. However, it is so hard and so chemically stable that there is no efficient method of machining it without causing damage to the machined surface. Plasma chemical vaporization machining (PCVM) is a gas-phase chemical etching method in which reactive species generated in atmospheric-pressure plasma are used. PCVM has a high removal rate equivalent to those of conventional machining methods such as grinding and lapping, because the radical density in atmospheric-pressure plasma is much higher than that in normal low-pressure plasma. In this paper, the polishing characteristics of silicon carbide by PCVM are described. As a result, a high machining rate (approximately 0.18 mm/min) and a very smooth surface (below 2 nm peak-to-valley in a 500 nm square area) are achieved.

Atomic-scale analysis of hydrogen-terminated Si(110) surfaces after wet cleaning

Atomically resolved scanning tunneling microscopy observations are performed on hydrogen-terminated Si(110) surfaces after wet cleaning. When a Si(110) wafer is dipped into dilute HF-containing solution, the surface is constructed by piling small terraces and steps. When the sample is consequently rinsed with ultrapure water, some characteristic features such as a zig–zag chain inside a terrace, a single row at step edges and an isolated zig–zag chain on a terrace are clearly observed, and their atomic arrangements are determined. Excessive rinsing, however, creates the ridge-shaped structure of nanometer height, which is explained by anisotropic etching by OH− ions in water.

Hydrogen termination of Si(110) surfaces upon wet cleaning revealed by highly resolved scanning tunneling microscopy

The surface structure of the hydrogen-saturated Si(110) surfaces after wet cleaning is studied on an atomic scale by means of scanning tunneling microscopy. When a surface oxide layer is stripped using a HF-containing solution, the surface consists of nanometer-scaled terraces and atomic steps along various directions. Coupled monohydride lines are formed inside a small terrace, as predicted by infrared spectra. The Si(110) surface after subsequent rinsing for a short period is occupied by a long terrace along the [1¯10] direction in which the ideal 1×1 structure is formed. Atomic arrangements around step edges are determined in detail based on atomic images and first-principles calculations. A ridge-shaped structure is observed after excess rinsing with water, and infrared spectra indicate that the slope is composed of (111) microfacets. From these results, we present the mechanism underlying the formation of the hydrogen-terminated Si(110) surfaces during wet cleaning processes.

Atomic image of hydrogen-terminated Si(001) surfaces after wet cleaning and its first-principles study

Atomic arrangements of the hydrogen-terminated Si(001) surface after wet cleaning are investigated by scanning tunneling microscopy and by first-principles calculations. The hydrogen-terminated Si(001) surface after dilute HF cleaning is atomically rough with a structure of terraces and steps. In addition, it is confirmed that 1×1 dihydride structures are formed inside the terraces. First-principles calculations reveal that the maxima of the local density of states string parallel to the Si–H direction on the 1×1 dihydride structure, which makes it possible to determine the atomic arrangement of the observed atomic images. When the surface after HF cleaning is subsequently rinsed with ultrapure water, rows emerge along the 〈110〉 direction. Atomic images show that the rows are mainly formed by a 2×1 unit cell. To elucidate the atomic structure of the 2×1 unit cell, it is proposed that every other row of the ideal 1×1 dihydride surface is preferentially etched with ultrapure water. Total energy calculations...