Showing papers by "Hiroshi Yokoyama published in 1998"

Role of space charge in scanned probe oxidation

Abstract: The growth rate and electrical character of nanostructures produced by scanned probe oxidation are investigated by integrating an in situ electrical force characterization technique, scanning Maxwell-stress microscopy, into the fabrication process. Simultaneous topographical, capacitance, and surface potential data are obtained for oxide features patterned on n- and p-type silicon and titanium thin-film substrates. The electric field established by an applied voltage pulse between the probe tip and substrate depends upon reactant and product ion concentrations associated with the water meniscus at the tip-substrate junction and within the growing oxide film. Space-charge effects are consistent with the rapid decline of high initial growth rates, account for observed doping and voltage-pulse dependencies, and provide a basis for understanding local density variations within oxide features. An obvious method for avoiding the buildup of space charge is to employ voltage modulation and other dynamic pulse-sha...

Parasitic diseases of cultured marine fish in Japan

Abstract: More than 30 species of marine fish are cultured in Japan and a wide variety of parasites are recognized as serious pathogens of these fish. In most cases, marine fish are cultured in floating net cages, which can provide a suitable substrate for monogenean eggs to become entangled with. This characteristic is most typically exemplified by the monogenean Heterobothrium okamotoi of tiger puffer; the parasite deposits a very long string of eggs, almost all of which become entangled with the net meshing, resulting in an accumulation of eggs within the culture system. Life cycles of important parasites of cultured marine fish, such as myxosporeans (e.g. Kudoa amamiensis and Myxobolus buri of yellowtail), microsporidians (Microsporidium seriolae of yellowtail and an unidentified species from tiger puffer) or blood flukes (Paradeontacylix spp. of amberjack) have not been elucidated. This is a large obstacle to the establishment of control measures against these parasitic diseases. Generally, parasitic diseases of cultured marine fish are much more difficult to control than their counterparts in fresh water. No single method has proved sufficient for the effective control of parasitic diseases of marine fish. Future efforts to control marine infections should be directed not only to chemical treatments, but also to biological measures, and to prophylaxis and quarantine through parasite detection and early diagnosis.

Understanding scanned probe oxidation of silicon

Abstract: A model for scanned probe microscope (SPM) silicon oxidation is presented. The model was derived from a consideration of the space-charge dependence of this solid-state reaction as a function of substrate doping type/level and has been verified experimentally by integrating an in situ electrical force characterization technique, scanning Maxwell stress microscopy (SMM), into the SPM fabrication process. This system enables us to obtain SPM topographic, SMM capacitance, and SMM surface potential information of nanometer-scale oxide features as a function of ionic concentrations within the growing oxide film. SPM oxide properties are compared to those of anodic and thermal oxides. The predictive power of the resulting model is demonstrated by showing how the growth rate and electrical character of the SPM-oxide features can be altered dramatically by modulating the applied oxidation voltage.

Fabrication Of A Nanometer-Scale Si Wire By Micromachining Of A Silicon-on-Insulator Substrate

Abstract: Air-bridge-structured Si wires were fabricated by using a silicon-on-insulator (SOI) substrate and electrically characterized. The SOI substrate used had a sandwich structure of a 200-nm-thick p-type Si layer (SOI layer), a 400-nm-thick buried oxide layer (BOX layer) and a p-type Si substrate. The wires were made by conventional photolithography process followed by dry etching and thermal oxidation thinning of the SOI layer. The Si wires were isolated from the substrate by an air gap made in the BOX layer and had dimensions of typically 20 nm in width, 40 nm in height and 150 nm in length. It was found from the measurements of current-voltage characteristics that the current through the wires much increased by illumination with a He-Ne laser and reached about 20 nA at a bias of 1 V.

Experimentally induced hemorrhagic thelohanellosis of carp caused by Thelohanellus hovorkai (Myxosporea : Myxozoa)

Abstract: Hemorrhagic thelohanellosis in two varieties of carp (Cyprinus carpio) caused byThelohanellus hovorkai was investigated by experimental infection. One-year old coloured carp (koi-carp) and common carp were kept for five weeks in tanks with oligochaetes (Branchiura sowerbyi) collected from a pond where hemorrhagic thelohanellosis was enzootic. Another group of coloured carp was exposed only to the effluent from a tank containingB.sowerbyi. Four weeks post-exposure, both coloured carp and common carp maintained withB. sowerbyishowed clinical signs of infection and chronic mortalities. On the other hand, neither mortalities nor clinical signs were observed in the effluent group, though light infections withT. hovorkaiwere detected. Control groups of coloured carp and common carp maintained in clean tanks showed no signs of infection at all. Through a standardized trypsin digestion method, the intensity of infection withT. hovorkaiin carp maintained withB. sowerbyiwas determined to be more than 50, 000 spores/g-tissue. Histological observations revealed thatT. hovorkaiplasmodia were most frequently detected in connective tissues of the intestine and the ventral skin in the cohabitation group, whereas they were found most frequently in the gills of the fish of the effluent group. In conclusion, hemorrhagic thelohanellosis was experimentally induced in carp by maintaining hosts withB. sowerbyiregardless of host's varietal difference, and the ingestion ofB. sowerbyiinfected with the actinosporean may be an important factor inducing the disease conditions.

Occurrence of Beko Disease Caused by Microsporidium seriolae (Microspora) in Hatchery-Reared JuvenileYellowtail

Abstract: Hatchery-produced juvenile yellowtail (Seriola quinqueradiata) were transferred from an indoor tank to sea netpens on June 20, June 30, July 10 and July 31, 1995 (4 groups), and each group was monitored at about 10 day intervals for Microsporidium seriolae, the causative agent of beko disease. Water temperature gradually increased during the study period, ranging from 20.8 to 30.2°C. In the first 3 fish groups, almost all fish were found to be infected. However, prevalence of infection in the last fish group was much lower than in the other groups. Groups which were transferred later attained their maximum prevalence of infection within a shorter period. Furthermore, we investigated the effects of water temperature and fish size on the development of M. seriolae in juvenile yellowtail. Two different fish size classes (average body weight 0.3 g and 22.0 g) were infected with M. seriolae to a similar extent. In the histological examination, the development of M. seriolae and host recovery were accelerated in fish reared at 25°C compared to those kept at 20°C.

The relation between corrosion and surface potential measured with the scanning Maxwell stress microscope

Abstract: The topographies and the surface potentials on various corroded metal surfaces have been simultaneously measured with the scanning Maxwell stress microscope to investigate the microscopic relation between the topography of corroded surface and the surface potential in atmosphere. In the early stage of corrosion the potential was low at the corroded region. In the next stage, however, the region where the potential was low at the non-corroded area appeared since anodic region transferred from corroded area to non-corroded area. The microscope is very useful to study the time dependence of the relation between a corroding region and the surface potential.