Hisato Ogiso

Ultrasonic force microscopy for nanometer resolution subsurface imaging

We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low‐frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.

Analysis of subsurface imaging and effect of contact elasticity in the ultrasonic force microscope

We examined, both theoretically and experimentally, the characteristics of subsurface imaging with nanometer resolution and the effect of contact elasticity in the ultrasonic force microscope (UFM). In particular, the effect of the surface energy and effective elasticity on the maximum tip-sample force and the shift of the averaged tip-sample distance were examined. Furthermore, kink formation in the cantilever deflection (z(a)) against the ultrasonic frequency vibration (UFV) amplitude (a) characteristics was predicted. This model was used to explain experimental observations in UFM, such as the features of the measured z(a)(a) curve and the damping of the cantilever torsion vibration by the UFV. Moreover. the previously reported lateral ultrasonic force microscope image of subsurface features was explained by the response of subsurface edge dislocation to a large instantaneous force enhanced by the UFV.

Evaluation of Domain Boundary of Piezo/Ferroelectric Material by Ultrasonic Atomic Force Microscopy

Ultrasonic atomic force microscopy (UAFM) was used to investigate the elasticity variation on domain boundary (DB) in lead zirconate titanate (PZT). The UAFM imaged the change in contact stiffness not only among grains but also on the DB. According to an analysis, the contact stiffness of the DB was approximately 10% lower than that within the domain. This is the first direct evidence of the variation of the elasticity due to the DB. The implication of this finding is that the low stiffness at the DB may affect the piezoelectricity of PZT and the easy mobility of the DB under a stress and electric field, which are important for not only actuator applications but also high-speed writing memory applications.

Investigation of local charged defects within high-temperature annealed HfSiON/SiO2 gate stacks by scanning capacitance spectroscopy

We studied the oxide charges and traps within nitrided Hf-silicate (HfSiON)∕SiO2 gate stacks processed with high-temperature annealing with a spectroscopic technique by using high spatial resolution scanning capacitance microscopy. Spectroscopy was performed by detecting the static capacitance (dC∕dZ) between a conductive probe and the sample while sweeping the sample bias. The dC∕dZ image and spatially resolved dC∕dZ-V spectrum revealed the existence of positive fixed charges within HfSiON and interface trap charges between the SiO2 underlayer and Si substrate. We also observed a transient electron trap process from the conductive probe to the HfSiON film as abrupt discontinuities in the dC∕dZ-V spectrum and with bias-induced topography change of the HfSiON surface. These oxide charges and trap sites distribute inhomogeneously within HfSiON∕SiO2 gate stacks, and the origin of these charged defects is ascribable to phase separation induced by high-temperature postdeposition annealing.

Compression test system for a single submicrometer particle

A compression test system was developed for measuring the strength of a single particle with a cross-sectional dimension of less than 1μm. The test system used a small diamond plate that has a flat platen to compress the particle on a diamond substrate. To avoid compressing multiple particles and to avoid direct contact between the platen and the substrate, the flat area was comparable in size to the tested particle. Mechanical processes such as polishing, however, have difficulty in fabricating such a small area. Here, fabrication of a micrometer-sized flat area was achieved by using focused ion beam technology. The resulting compression test system successfully measured the strength of a single submicrometer particle. Results showed that the mean strength of alumina particle with nominal mean diameter of 0.7μm was 2.9GPa.

4-fold enhancement in the critical current density of YBa2Cu3O7 films by practical ion irradiation

We report an up-to-4-fold enhancement in the in-magnetic-field critical current density at 77 K of epitaxial YBa2Cu3O7 films on CeO2-buffered SrTiO3 substrates by 3-MeV Au2+ irradiation. This indicates that irradiation using an industrially practical ion beam, which generally has kinetic energy less than 5 MeV, can provide a substantial increase in the in-field current performance of high-temperature superconductor films. Transmission electron microscopy results show that point-like defects smaller than 6 nm in diameter were created in the films by the irradiation.

Atomic Force Microscopy Cantilevers for Sensitive Lateral Force Detection.

In order to enhance the lateral force sensitivity of atomic force microscopy (AFM) to detect atomic or molecular scale interaction, two types of force sensors were fabricated by additional processing of commercial sensors with the help of focused ion beam (FIB) technology. In one type of cantilever, a hinge was fabricated in the middle of the commercial cantilever by FIB milling, resulting in the reduction of the lateral force constant from 1204 N/m to 118 N/m. For further enhancement of the lateral sensitivity without causing snap-in behavior, a novel force sensor was designed and fabricated through a combination of FIB milling and FIB deposition techniques. The lateral force constant was reduced, for example, from 3600 N/m to 20 N/m, and the measured lateral force resolution attained the order of 1 nN.

Significant stiffness reduction at ferroelectric domain boundary evaluated by ultrasonic atomic force microscopy

Two-dimensional resonance frequency mapping in the ultrasonic atomic force microscopy was applied to the investigation of the ferroelectric domain structure in lead zirconate titanate ceramics. This method can visualize the stiffness anisotropy due to the differently oriented domains. Moreover, the significant stiffness reduction at the ferroelectric domain boundary was discovered. The disorder of the lattice, the ability of the switching of the domain, and the reduction of the piezoelectric stiffening are possible explanations. The implication of this work is the characterization of novel functional materials on nanoscale and the nondestructive evaluation of the microelectromechanical systems and nanotechnology devices.

EXCIMER LASER ASSISTED CHEMICAL MACHINING OF SIC CERAMIC

A highly effective method of machining ceramic has been newly developed using a krypton fluoride (KrF) excimer laser with a 248 nm wavelength. The laser was irradiated on SiC in water to form a soft hydrous oxide layer by photochemical reaction. The softened layer was then cut with a diamond tool to form a mirror surface finish. The optimum conditions were found for both high machining rate and better surface integrity of SiC.

Charge trapping properties in TiO2∕HfSiO∕SiO2 gate stacks probed by scanning capacitance microscopy

The charge-trapping properties of the high-permittivity titanium oxide–hafnium silicate–silicon dioxide (TiO2∕HfSiO∕SiO2) gate stacks have been studied using scanning capacitance microscopy. From the bias stress examination of the gate stacks, we concluded that there were electron traps within the films, and these trap densities increased with an increase in the oxidation temperature used for the fabrication of TiO2 top dielectrics. Furthermore, we found that the distribution of these charged defects was inhomogeneous within the gate stacks. These results are attributed to Ti diffusion through the dielectric layers, which caused electrical defects within the gate stacks.