Mitsuru Konno

Incipient Space Weathering Observed on the Surface of Itokawa Dust Particles

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. The reflectance spectra of the most abundant meteorites, ordinary chondrites, are different from those of the abundant S-type (mnemonic for siliceous) asteroids. This discrepancy has been thought to be due to space weathering, which is an alteration of the surfaces of airless bodies exposed to the space environment. Here we report evidence of space weathering on particles returned from the S-type asteroid 25143 Itokawa by the Hayabusa spacecraft. Surface modification was found in 5 out of 10 particles, which varies depending on mineral species. Sulfur-bearing Fe-rich nanoparticles exist in a thin (5 to 15 nanometers) surface layer on olivine, low-Ca pyroxene, and plagioclase, which is suggestive of vapor deposition. Sulfur-free Fe-rich nanoparticles exist deeper inside (<60 nanometers) ferromagnesian silicates. Their texture suggests formation by metamictization and in situ reduction of Fe2+.

Oxygen Vacancy Induced Substantial Threshold Voltage Shifts in the Hf-based High-K MISFET with p+poly-Si Gates -A Theoretical Approach

A theoretical investigation has been made of the origin of substantial threshold voltage (Vth) shifts observed in p+poly-Si gate Hf-based metal insulator semiconductor field effect transistors (MISFETs), by focusing on the effect of oxygen vacancy (VO) formation in HfO2. It has been found that VO formation and subsequent electron transfer across the interface definitely causes substantial Vth shifts, especially in p+poly-Si gate MISFETs. Moreover, the theory also systematically reproduces recent experimental reports that large flat band (Vfb) shifts are observed, even in intrinsic poly-Si gates, and that the Vfb shifts exhibit a high dependence on HfSiOx thickness.

Atomic imaging using secondary electrons in a scanning transmission electron microscope: experimental observations and possible mechanisms.

We report detailed investigation of high-resolution imaging using secondary electrons (SE) with a sub-nanometer probe in an aberration-corrected transmission electron microscope, Hitachi HD2700C. This instrument also allows us to acquire the corresponding annular dark-field (ADF) images both simultaneously and separately. We demonstrate that atomic SE imaging is achievable for a wide range of elements, from uranium to carbon. Using the ADF images as a reference, we studied the SE image intensity and contrast as functions of applied bias, atomic number, crystal tilt, and thickness to shed light on the origin of the unexpected ultrahigh resolution in SE imaging. We have also demonstrated that the SE signal is sensitive to the terminating species at a crystal surface. A possible mechanism for atomic-scale SE imaging is proposed. The ability to image both the surface and bulk of a sample at atomic-scale is unprecedented, and can have important applications in the field of electron microscopy and materials characterization.

Observation of three-dimensional elemental distributions of a Si device using a 360°-tilt FIB and the cold field-emission STEM system

A technique for preparation of a pillar-shaped specimen and its multidirectional observation using a combination of a scanning transmission electron microscope (STEM) and a focused ion beam (FIB) instrument has been developed. The system employs an FIB/STEM compatible holder with a specially designed tilt mechanism, which allows the specimen to be tilted through 360 degrees [T. Yaguchi, M. Konno, T. Kamino, T. Hashimoto, T. Ohnishi, K. Umemura, K. Asayama, Microsc. Microanal. 9 (Suppl. 2) (2003) 118; T. Yaguchi, M. Konno, T. Kamino, T. Hashimoto, T. Ohnishi, M. Watanabe, Microsc. Microanal. 10 (Suppl. 2) (2004) 1030]. This technique was applied to obtain the three-dimensional (3D) elemental distributions around a contact plug of a Si device used in a 90-nm technology. A specimen containing only one contact plug was prepared in the shape of a pillar with a diameter of 200nm and a length of 5mum. Elemental maps were obtained from the pillar specimen using a 200-kV cold-field emission gun (FEG) STEM model HD-2300C equipped with the EDAX genesis X-ray energy-dispersive spectrometry (XEDS) system through a spectrum imaging technique. In this study, elemental distributions of minor elements with weak signals were enhanced by applying principal component analysis (PCA), which is a superior technique to extract weak signals from a large dataset. The distributions of elements, especially the metallization component Ti and minor dopant As in this particular device, were successfully extracted by PCA. Finally, the 3D elemental distributions around the contact plug could be visualized by reconstruction from the tilt series of maps.

Electron tomography of embedded semiconductor quantum dot

We performed an electron tomography for a single InAs quantum dot (QD) embedded in GaAs. A comprehensive three-dimensional image of indium distribution has been reconstructed by using a high-angle annular dark-field scanning transmission electron microscope. This was achieved by using a special nanopillar specimen prepared by a focused ion beam technique. The real structure of the embedded single QD has been found to have a complicated anisotropic structure reflecting the QD structure before being capped.

In situ high resolution electron microscopy/electron energy loss spectroscopy observation of wetting of a Si surface by molten Al

Electron energy loss spectroscopy was used to observe the segregation of Al on a Si surface above the melting point of Al. A mixture of Al and Si particles was heated above the melting point of Al in a vacuum of 1 × 10−5 Pa. The Si surface, which initially had been covered with an amorphous oxide layer before heating, became clean and atomically facetted when the Al melted. It was shown that the Si surface was segregated with Al.

Lattice imaging at an accelerating voltage of 30kV using an in-lens type cold field-emission scanning electron microscope.

We reported investigation of lattice resolution imaging using a Hitachi SU9000 conventional in-lens type cold field emission scanning electron microscope without an aberration corrector at an accelerating voltage of 30kV and discuss the electron optics and optimization of observation conditions for obtaining lattice resolution. It is possible to visualize lattice spacings that are much smaller than the diameter of the incident electron beam through the influence of the superior coherent performance of the cold field emission electron source. The defocus difference between STEM imaging and lattice imaging is found to increase with spherical aberration but it is possible to reduce the spherical aberration by reducing the focal length (f) of the objective lens combined with an experimental sample stage enabling a shorter distance between the objective lens pre-field and the sample. We demonstrate that it is possible to observe the STEM image and crystalline lattice simultaneously. STEM and Fourier transform images are detected for Si{222} lattice fringes and reflection spots, corresponding to 0.157nm. These results reveal the potential and possibility for a measuring technique with excellent precision as a theoretically exact dimension and established the ability to perform high precision measurements of crystal lattices for the structural characterization of semiconductor materials with minimal radiation beam damage.

Multidirectional observation of an embedded quantum dot

The authors succeeded in observing atomic scale images of undamaged single InAs quantum dots (QDs) embedded in the GaAs matrix using high resolution transmission electron microscope equipped with focused ion beam system. The QD can be viewed from multidirections, and a conclusive and comprehensible determination of the size and the shape anisotropy has been realized. Asymmetry of the structural properties has been confirmed between the [110] and [−110] crystal directions. The embedded QD is elongated along the [−110] axis. The strain-field pattern is also asymmetric according to the shape anisotropy. The results will enable the investigation of the exact structure anisotropy influencing the atomlike properties of QDs.

Application of lattice strain analysis of semiconductor device by nano-beam diffraction using the 300 kV Cold-FE TEM

The lattice strain of the semiconductor device was measured by the nano-beam diffraction method using a 300 kV TEM (HF-3300) equipped with a Cold-FE gun. The various samples thickness prepared by the Focused Ion Beam system. For the sample thickness measurement with accuracy, the mean free path of inelastic scattering (λ) was determined from the correlation between the In(It/I0) value from the Electron Energy Loss Spectrum and the thickness of the pillar-shaped sample measured by the TEM image. As a result, the rate of the lattice strain in device increased with changing sample thickness. In the sample thickness of 494 nm, the lattice strain has concentrated on the neighbourhood Si substrate by the NBD.

Boron Observation in p-Type Silicon Device by Spherical Aberration Corrected Scanning Transmission Electron Microscope

The boron detection in p-type metal oxide silicon (MOS) device had long been pursued with electron microscopy based analytical tools, but was not successful because of weak signal and interference from the matrix. With spherical aberration corrected electron microscope with newly designed sample holder, mesh and focused ion beam (FIB) damage removing process, boron in the extension area of the p-type MOS device was detected. This enables us to visualize the dopant distribution in silicon devices, which is indispensable to analyze transistor characteristics.