Yasuhito Gotoh

Negative-ion implantation technique

Abstract Negative-ion implantation is a promising technique for charging-free implantation for the forthcoming ULSI fabrication, in which the water charging by positive-ion implantation will become a troublesome problem even with an electron shower. The negative-ion implantation technique remarkably ameliorates such a charging problem since the incoming negative charge of implanted negative ions is easily balanced by the outgoing negative charge of a part of secondary electrons. Thus, the surface charging voltage is maintained to within about ± 10 V for isolated conducting materials and insulators, and is free from space and time fluctuations. A high-current negative-ion source and a medium current negative-ion implanter developed for this technique are described with the design concepts. In addition, the fundamental measurements of interactions between the negative-ion beam and the gas/solid are also described.

Ion beam assisted deposition of niobium nitride thin films for vacuum microelectronics devices

Abstract We have deposited niobium nitride thin films by ion beam assisted deposition and evaluated their properties from the viewpoint of a cathode material for vacuum microelectronics devices. Substrate temperature and ion–atom arrival rate ratio were selected as deposition parameters. The film properties of nitrogen composition, crystallinity, electric resistivity, work function and sputtering yield against a low-energy argon ion bombardment were investigated. It was found that polycrystalline films could be obtained at the substrate temperature higher than 500°C, and the composition could be controlled by the ion–atom arrival rate ratio. The results also showed that the stoichiometric nitride film exhibited superior properties of a lower work function and a lower partial sputtering yield of niobium. The electron emission test also demonstrated a lower current fluctuation for the stoichiometric films. In summary, ion beam assisted deposition provided a low temperature process which could control the film properties suitable to a cathode material.

Influence of cathode material on emission characteristics of field emitters for microelectronics devices

In order to find out the cathode material suitable to vacuum microelectronics devices, dependence of cathode material of field emitters was investigated with respect to the emission characteristics. Since the field emitters for vacuum microelectronics devices are fabricated by thin film processes, the characteristics of the electron emission from deposited materials should be examined. In the present study, a dozen materials were deposited onto the tungsten needle fabricated by well‐controlled electrochemical etching. Measurement of the emission was performed at the pressure of 10−9 Torr range. The current–voltage characteristics and the stability measurements revealed that the gold emitters indicated excellent properties: stable and high current at low extraction voltage. The effective surface work function and the effective emission area were evaluated from the Fowler–Nordheim theory, assuming that the emission area rapidly decreases with reducing the apex radius. From this analysis, it is clarified tha...

CONTACT ANGLE LOWERING OF POLYSTYRENE SURFACE BY SILVER-NEGATIVE-ION IMPLANTATION FOR IMPROVING BIOCOMPATIBILITY AND INTRODUCED ATOMIC BOND EVALUATION BY XPS

Abstract Negative-ion implantation technique is expected as an effective surface modification method for insulators of polymer, since in negative-ion implantation into insulators the charge-up potential of the surface is in several volts. Polystyrene dishes were implanted with Ag − ions in an ion energy range under 30 keV in order to bring hydrophilic property to their surface for improving surface biocompatibility of cell adhesion property, and contact angles to water were measured. Contact angle was found to be lowered to 73° by the Ag-ion implantation from an original value of 86°, and it decreased with increase in both ion dose and ion energy below 20 keV. Atomic bonds of C–O, CO, and OC–O were introduced by ion implantation, these were increased in number with increase in dose and energy of implantation. These atomic bonds were considered to bring the hydrophilic property to the polystyrene surface. Human umbilical vascular endothelial cell (HUVEC) was cultured on each sample with a 199 medium. The cell growth and attachment were observed only for Ag-implanted surfaces.

Electron emission properties of Spindt-type platinum field emission cathodes

Electron emission properties of Spindt-type platinum field emission cathodes were investigated. The current–voltage characteristics together with the current fluctuation during long term operation were evaluated in ultrahigh vacuum. The changes of the emission properties in hydrogen, oxygen or carbon monoxide gas ambient were also investigated. Significant improvement of the emission properties was found when the cathode was operated in carbon monoxide ambient under a certain condition. It was found that the effect of the improvement lasted at least 200 h, and resistance against the oxygen exposure was also improved. The changes of the electron emission properties during long term operation in ultrahigh vacuum and in gas ambient were analyzed with the Seppen–Katamuki chart, of which ordinate and abscissa are slope and intercept of Fowler–Nordheim plot. The electron emission properties were distributed along a straight line in the Seppen–Katamuki chart, and those of the cathode improved by carbon monoxide ...

Surface modification of TiO2 (rutile) by metal negative ion implantation for improving catalytic properties

Abstract In order to improve photocatalytic property of rutile TiO 2 , metal negative ions were implanted to rutile substrates, and their optical absorption and photocatalytic efficiencies were investigated. Cu and Ag negative ions were implanted at 50 and 65 keV, respectively, with a dose in a range of 3×10 16 –1×10 17 ions/cm 2 . In optical absorption properties after annealed, Cu- and Ag-implanted rutiles showed a clear optical absorption with a peak near photon energy of 1.9 and 2.0 eV, respectively. This means that implanted metal atoms gathered to form metal nanoparticles of Cu or Ag in the surface layer. We tested decolorization of Methylene Blue solution by implanted rutiles under irradiation of a fluorescent light. Both Cu- and Ag-implanted rutiles after annealed at 400 °C showed better photocatalytic efficiencies by approximately 1.8 times than that of unimplanted rutile, although as-implanted samples had worse properties. This improved photocatalytic performance is contributed to the existence of metal nanoparticles in the surface layer as well as recovery of implantation damages in surface layer by heat treatment.

Measurement of work function of transition metal nitride and carbide thin films

Work functions of transition metal nitride and carbide thin films were measured. The materials investigated were ZrN, NbN, HfN, TaN, HfC, and TaC. The films were prepared either by radio-frequency magnetron sputter deposition or by ion beam assisted deposition. The work function was measured by Kelvin probe in air. The work functions of ZrN and HfN ranged between 4.6 and 4.7 eV, and were similar to or slightly lower than that of NbN and TaN, 4.7 or 4.8 eV. The work function of TaC was approximately 5.0 eV. The higher work function of carbide may be attributed to lower electronegativity of carbon as compared to nitrogen.

Emission characteristics of ZrN thin film field emitter array fabricated by ion beam assisted deposition technique

ZrN thin film field emitter arrays were fabricated by the ion beam assisted deposition technique. The work function of ZrN is controlled by the selection of the deposition parameters. We investigated the relationship between the work function and the emission stability, and the influence of the ambient gas on the emission stability by introducing the gases of N2 and O2 into the measurement chamber. It was found that the emission stability depended on the work function: the emitters with a lower work function were more stable than those with a higher work function. The dependence is explained qualitatively by the analysis of the Fowler–Nordheim equation. Emission noise increased with the increase of the ambient gas pressure. However, difference in the increase of noise between N2 gas and O2 gas was seen. O2 gas severely affected the emission stability of the ZrN emitter.

Slightly negative surface potential and charging model of insulator in the negative-ion implantation

Abstract In the negative-ion implantation into insulators, a slightly negative surface potential was obtained from the energy analysis of emitted electrons. Since emitted electrons offer an information of the surface potential, the surface potential was evaluated as an energy shift of the peak in the electron-energy distribution from the peak energy for a SiO2 (40 nm)/Si without charging effect. From the obtained results of surface potentials of many kinds of insulators during C− implantation as a function of the ion energy from 5 keV to 35 keV, it was found that the surface potential had a slightly negative value in a range from −2 to −12 V, and that the negative surface potential had a linear dependence on the ion velocity. This negative surface potential of insulators could not be explained by the charging model adopted for isolated electrodes. Therefore, taking account of the true electron-emission yield more than unity and of a quite low mobility in insulators, we have proposed a charging model of an electric double layer formed as a negative-charge layer at the surface and a positive-charge layer below the surface. The relationship between the surface potential and the true electron-emission yield, or the surface barrier are discussed.

Influence of surface treatment and dopant concentration on field emission characteristics of boron-doped diamond thin films

Field emission characteristics of B-doped diamond thin films terminated with oxygen and hydrogen were investigated. The diamond thin films were prepared by microwave plasma chemical vapor deposition. The dependence of emission characteristics on the surface treatment and on the B concentration was investigated. The turn-on voltage required to extract a current of 0.1 nA depended on these preparation parameters. The emitters with lower B concentration emitted electrons at a lower turn-on voltage, and the H-terminated emitters had a lower turn-on voltage than O-terminated emitters. The analysis of the slope and the intercept of Fowler–Nordheim plot revealed that the dependence of turn-on voltage on the surface treatment is due to the difference of emission barrier height, and that the dependence on B concentration is due not to the emission barrier height but to the surface morphology.