Yoichi Hoshi

ITO thin films deposited at low temperatures using a kinetic energy controlled sputter-deposition technique

Abstract Low-temperature deposition of tin (Sn)-doped indium oxide (ITO) thin films was attempted using a kinetic energy controlled particle deposition (KECD) technique in which the kinetic energies of sputtered particles along with their incidence angles to the substrate can be controlled in the film forming process. It was expected that this new sputter deposition technique would be able to deposit ITO thin film having a dense structure as well as extremely smooth surfaces even at low substrate temperatures, which would result in the formation of a film having low resistivity. It was found that this method can produce ITO thin films having resistivity as low as 3.5×10 −4 cm and smooth surfaces ( R a ≦0.15 nm) even at temperatures below 50 °C. The sputtering must be performed at a voltage below −100 V, and the sputtering gas pressure in the sputtering source and the oxygen gas pressure during sputtering must be adjusted to an optimal value.

Crystallization of amorphous CoFeB ferromagnetic layers in CoFeB/MgO/CoFeB magnetic tunnel junctions

We demonstrate that the crystallization of ferromagnetic CoFeB layers originates at the interface with a MgO layer in CoFeB/MgO/CoFeB magnetic tunnel junctions by annealing using cross-sectional transmission electron microscopy and electron diffraction. The CoFeB layers, which are amorphous in the as-deposited state, crystallize with a (001) out-of-plane texture by annealing at 360 °C. Crystal grains of 15–20 nm in the CoFeB layers are observed at the interface with the MgO layer, but not at the interface with a Ta or Ru layer. Much smaller crystal grains with random crystal orientations are formed in a region away from the MgO interface in the CoFeB layers. The depth profiles obtained by X-ray photoelectron spectroscopy show that boron diffuses from the crystallized region at the interface into the MgO layer and the rest of the region in the CoFeB layers during crystallization, indicating that crystal grains have much lower B contents than the original composition.

UNIAXIAL MAGNETIC ANISOTROPY OF IRON THIN FILMS DEPOSITED BY OBLIQUE INCIDENCE OF DEPOSITION PARTICLES

Uniaxial magnetic anisotropy induced in iron thin films by the oblique incidence of deposition particles was investigated by computer simulation. In the simulation, the self‐shadowing effect was considered mainly to explain changes in the magnetic anisotropy of iron films with the incident angle. The simulated grain shape in the film changes significantly with the incident angle of deposition particles. Grains elongated in a direction normal to the incidence direction of depositing particles were clearly observed in simulated film deposited with an incidence angle around 60°. However, the film deposited at an incidence angle of 80° had columnar grains separated from each other and inclined to the incidence direction. Changes in the uniaxial magnetic anisotropy of the film can be qualitatively explained by the shape anisotropy of these grains in the film. The decrease in saturation magnetization of these films with incident angle seems to be mainly caused by the formation of a porous film.

Control of orientation and crystallite size of barium ferrite thin films in sputter deposition

Hexagonal barium ferrite thin films were deposited in a facing target sputtering system. Films with random orientation, and c-axis orientation were obtained by depositing the films on a thermally oxidized silicon wafer and on a c-axis oriented ZnO underlayer, respectively. The film deposited on the thermally oxidized silicon wafer had crystallite sizes as large as 1500 /spl Aring/, which was increased to about 2000 /spl Aring/ by depositing the film on the substrate with ZnO underlayer. The randomly oriented film had a large coercive force of about 4000 Oe and exhibited almost isotropic properties. In contrast, the c-axis oriented film deposited on ZnO underlayer had a large perpendicular magnetic anisotropy field above 10 kOe and exhibited an angular dependence of coercivity consistent with a fanning model, although the film had a smaller coercive force and smaller saturation magnetization. A strong negative magnetic interaction operated between the particles in the perpendicular direction of the film. These BaM films had a large dispersion in coercivity even in the film with excellent c-axis orientation (/spl Delta//spl theta//sub 50/=1.6/spl deg/).

Reactive synthesis of well‐oriented zinc‐oxide films by means of the facing targets sputtering method

Well‐oriented ZnO films are reactively synthesized by means of the facing targets sputtering (FTS) method. In this paper, the change in the crystallographic characteristics of ZnO films with respect to sputtering conditions are described in detail. Films obtained by means of the FTS method exhibit one of the best c‐axis orientations reported to date. The following conditions are necessary for formation of well‐oriented ZnO films with good surface smoothness: (1) High‐energy particle bombardment on the film surface during deposition should be suppressed and (2) the sputtered particles landing on the substrate surface should have an appropriate energy level of several eV.

rf and dc discharge characteristics for opposed‐targets sputtering

An rf opposed‐targets sputtering (OTS) system, which can also be used as a dc OTS one, has been constructed and its discharge and plasma characteristics have been investigated. The rf OTS system has discharge and plasma characteristics similar to those of rf magnetron sputtering. Ionization of sputtering gases using high‐energy γ electrons is the main mechanism for producing high‐density plasma in both rf and dc OTS, though the ionization of gases by the rf excitation of electrons in the rf electric field becomes prominent at low plasma density in rf OTS. The absolute value of the self‐bias potential (‖Vsb‖) of targets in rf OTS during sputtering is much higher than that of the applied target voltage (‖Va‖) in dc OTS. This is mainly caused by the fact that the trapping efficiency of γ electrons is much higher in dc OTS than in rf OTS, since the γ electrons are lost at the target surface in rf OTS but not in dc OTS. It was found that the bombardment of ions to the substrate surface is markedly heavier in r...

Photocatalytic Characteristics of TiO2 Films Deposited by Oxygen Plasma-Assisted Reactive Evaporation Method

Titanium dioxide (TiO2) films were deposited by the oxygen plasma-assisted reactive evaporation (OPARE) method. TiO2 films with an anatase structure were formed when activated oxygen plasma was supplied to the substrate during deposition, whereas the films with a rutile structure were formed when nonactivated oxygen gas was supplied. These results were explained by the promotion of the oxidization of titanium atoms by the supply of activated oxygen plasma, because the anatase film is obtained under conditions where sufficient oxidization of titanium atoms is achieved, and the rutile film is formed under the conditions where insufficient oxidization occurs. These results indicate that oxygen plasma assist is an effective method for promoting oxidization and reducing the amount of oxygen gas used in this method. Both films deposited with or without plasma assist at temperatures greater than 250 °C exhibited ultra-hydrophilicity. These films also showed definition methylene-blue (MB) decomposition photocatalytic performance indexes in the range from 7.8–11.9 µmol·l-1·min-1. The described results indicate that the TiO2 films obtained by this method exhibit adequate performance for photocatalytic applications.

MECHANISM OF COMPOSITION CHANGE IN SPUTTER DEPOSITION OF BARIUM FERRITE FILMS WITH SPUTTERING GAS PRESSURE

In this study, we used computer simulation to investigate changes in the composition of hexagonal barium ferrite films with sputtering gas pressure obtained by the sputter-deposition processes. The iron content in the film deposited by facing target sputtering increased as the sputtering gas pressure increased and reached a maximum value at a certain gas pressure. These changes in the film composition were explained as follows: sputtered particles scatter when they collide with sputtering gas atoms, and this scattering changes the ratio of the particles reaching the substrate. When the substrate was located to the side of the target, as in a facing target sputtering system, this scattering resulted in an increase in the amount of sputtered particles arriving at the substrate, although too much scattering caused the amount to decrease. When a magnetron sputtering system is used for the film preparation, this gas scattering leads to a decrease in the amount of sputtered particles arriving at the substrate w...

Suppression of Cone Formation on Carbon Target during Sputtering

The mechanism of cone formation on a carbon target in sputtering was investigated. Marked whisker growth was observed on the target surfaces of both graphite and glasslike carbon during sputtering when the target was not cooled sufficiently. This whisker growth causes significant cone formation on the target surface, which makes it difficult to perform sputter deposition at a gas pressure below 1 mTorr. On the other hand, cone formation was completely suppressed on the glasslike carbon target when target cooling was improved, which made it possible to perform sputtering at a gas pressure as low as 1×10-4 Torr. This indicates that the use of the glasslike carbon target and cooling of the target during sputtering are effective for suppressing cone formation on the target.

Study of low power deposition of ITO for top emission OLED with facing target and RF sputtering systems

Deposition of ITO as top transparent electrode was studied using two deposition systems with and without direct contact to working plasma; namely with conventional RF-magnetron planar (RSS) and pulsed-DC facing target sputtering systems (FTS). Test devices were made on glass substrates and consisted of (from bottom up) ITO/4 Organic Layers/ITO. Depositions were performed at low deposition powers; 30 and 60 watts, to reduce damages by energetic sputtered particles to underlying organic layers. Test devices from both sputtering systems were found to function well. Leakage current density at -5 V reverse bias were relatively constant from 0.3 and 0.4 mA/cm2 at 30 W and 60 W in FTS, while the values were found to increase from 0.001 to 0.2 mA/cm2 at 30 W and 60 W in RSS.