Nobuaki Yasuda

The origin of the inhomogeneity of electrical resistivity in magnetron-sputtered indium tin oxide thin films

The intensity distribution of the ion flux on a substrate surface has been measured to examine the origin of the inhomogeneity of resistivity in magnetron-sputtered indium tin oxide (ITO) thin films. Ion flux intensity was determined as the product of ion saturation current Ii and floating potential Vf which were measured by a cylindrical Langmuir probe placed near a glass substrate. IiVf varied depending on the substrate position and showed a maximum near the upper position of the target erosion line. The resistivity of the ITO films showed a minimum at a certain oxygen flow rate. The oxygen flow rate where resistivity showed a minimum decreased as IiVf increased. Carrier mobility increased as IiVf increased under low oxygen flow rate conditions. Carrier concentration decreased as IiVf increased under high oxygen flow rate conditions. The film microstructure and composition were related to the electrical properties.

Optimum structure of deposited ultrathin silicon oxynitride film to minimize leakage current

The role of oxygen inside silicon oxynitride (SiON) films respecting leakage current was investigated. We successfully controlled composition of bulk SiON film in accordance with the alloy model: (Si 3 N 4 ) x (SiO 2 ) 1-x regarding ultra thin composite SiON structure and stacked SiON/ SiO 2 structure and measured electrical properties of these SiON films. We, furthermore, studied the conduction mechanisms of the leakage current for these films by using a direct tunneling (DT) current simulation with Wentzel-Kramers-Brillouin (WKB) approximation, taking dielectric constant, band profile and effective mass into consideration. From these experiments and simulations, it was found that the leakage current of composite structure is lower than that of stacked structure, even if those structure have the same alloy ratio x. Furthermore, we found the optimum film composition to minimize DT current. The optimum structure was determined by the balance between the decrease in electron DT current and the increase in hole current due to the physical thickness and the valence band barrier height.

Observation of thermomagnetically written domains by Lorentz electron microscopy in Tb‐Co and Tb‐Fe‐Co disks

For better understanding of the read/write properties of magneto‐optical disks, thermomagnetically written domains were observed by Lorentz electron microscopy. The domains were written in the quadrilayered medium with 25‐nm‐thick Tb‐Co film and in the trilayered medium with 50‐nm‐thick Tb‐Fe‐Co film, which were sputtered on pregrooved resinous substrates. Recording power and frequency were varied at intervals of 3.5 mm in a radial direction of the disk, and each recording position was observed individually. Qualitative understanding of the read/write properties, such as carrier‐to‐noise ratio (C/N) and jitter, were well achieved from Lorentz images of the domains. It is found that the C/N value of the Tb‐Fe‐Co film with a higher figure of merit (Rθk ) is lower than that of the Tb‐Co film with a lower value of Rθk . The reason for this is believed to be due to a larger variation in domain shape in the Tb‐Fe‐Co film compared to the Tb‐Co film. A careful observation based upon the image processing of domain...

Tellurium-carbon (TeC:H) films for a write-once read many (WORM) optical disk memory

Abstract A tellurium-carbon (TeC:H) film was deposited by the reactive sputtering of a Te target in an atmosphere containing methane. Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), and FT-IR absorption spectroscopy have shown that the chemical state of carbon in the TeC:H film is not amorphous carbon but rather sp 3 -hybridized hydrocarbon. Thermal decomposition of the TeC:H film produced sp 2 -hybridized hydrocarbon gas, which was analyzed by FT-IR/TG (thermogravimetry) measurements. Raman and FT-IR spectroscopies have supported the view that the structure of the TeC:H film is a composite structure consisting of amorphous tellurium and hydrocarbon fragments produced in methane plasma. The ablative optical recording sensitivity of the TeC:H film has been confirmed to be superior to that of a Te film, and its long archival and shelf lives have been recognized by accelerated aging tests. The recording sensitivity of the co-sputtered film consisting of tellurium and graphite carbon was found to be insufficient compared with the sensitivity of the Te film.

Perpendicular Magnetic Anisotropy in Amorphous TbCo Sputtered Films

Effects of sputtering conditions on perpendicular magnetic anisotropy in amorphous TbCo films prepared by two target magnetron cosputtering have been studied. The perpendicular magnetic anisotropy of amorphous TbCo films depended strongly on negative substrate bias voltage and Ar gas pressure. TbCo films deposited at zero substrate bias voltage and low Ar gas pressure (2 ~ 5 mtorr) showed a positive uniaxial magnetic anisotropy (Ku) of 3 ~ 4 × 106 erg/cm3 in the range of 13 ~ 31 at% Tb.

Amorphous Te-C Films For An Optical Disk

A new recording medium for optical disks was investigated. The new medium, called Te-C film, is characterized by a high sensitivity and stability, compared to Te films. This paper presents the optical recording characteristics for Te-C films on PMMA substrates and its stability under stressed conditions. Then, the high sensitivity and stability origins are discussed.

Dry Process Fabrication Of Pre-Groove On Optical Disc

A new simple method was developed to make a groove on an optical memory disc. This uses the swelling of the Te-C material. A bulged line is formed by irradiating laser light onto the Te-C layer which has a thin metal layer over it. This dry process is called the Ω(OMEGA)-process from the sectional shape of the groove. The Ω-disc should have a lower error rate and a higher track density than the disc made through a conventional wet process.

X-ray photoelectron and vibrational spectroscopic studies of TeC:H films for optical disk memory

The valence band spectrum of a TeC:H film was found to be essentially the same as that of amorphous elemental tellurium using X-ray photoelectron spectroscopy. Raman and Fourier transform far-IR spectra were examined based on the phonon density of states for tellurium, and the far-IR peak corresponding to the vibration of amorphous tellurium was identified at 150 cm−1. Organic TeC bonding was found in the TeC:H film, and the origin of this bonding is discussed by taking into consideration the deposition process, including surface reaction on the tellurium target. The chemical sputtering process is suggested to contribute to the deposition of TeC:H film.

Substrate position dependence of characteristics of Te-C:H film prepared by DC magnetron sputtering in methane atmosphere

A valence band and a Raman spectrum of Te–C:H film deposited in 50% methane atmosphere with argon as a dilution gas have been confirmed to vary according to the substrate arrangement on the anode in dc magnetron sputtering processes. The effect of bombardment by neutralized recoil argon was discussed for the sputtering processes of the Te–C:H film. Optical emission spectroscopy of the sputtering processes on tellurium was carried out for three gas atmospheres: argon, 50% methane, and pure methane. The optical emissions from atomic and ionic tellurium were recognized even in pure methane atmosphere. A chemical reaction and chemical sputtering have been considered to be involved in the sputtering process between the tellurium target and hydrocarbon species in the plasma.

Compositional and Thickness Dependences of Thermo-magnetic Properties for TbCo Films

For a better understanding of the dynamic characteristics of TbCo magneto-optical disks, the thermo-magnetic properties of TbCo films with various compositions and thicknesses were investigated. The measured Ms dependence on film temperature suggested that the effect of the leakage field in the write/erase process for TbCo films would be larger than that for Fe-based films. And the spacial Ms profile of the heated spot for a thinner TbCo film was more steeper than that for a thicker film. The domain nucleation process and the wall stiffness were estimated from statically obtained thermo-magnetic properties. Qualitative relationship among dynamic characteristics, such as optimum recording power and jitter, were clearly obtained.