Mitsuya Okada

Spectroscopic Ellipsometry Studies on Optical Constants of Ge2Sb2Te5 Used for Phase Change Optical Disks.

Spectroscopic ellipsometry has been applied to Ge2Sb2Te5, which is commonly used as a recording layer for phase change optical disks, and the optical constants have been measured for a wavelength range from 400 to 900 nm. We have improved the accuracy of this measurement by means of inserting a transparent film between a Ge2Sb2Te5 film and a Si substrate. A calculation of the reflectivity change, due to the phase transition between the amorphous and the crystalline states, shows that the change is sufficient for optical optimization of the disk structure, down to the 400 nm wavelength.

Analysis of Scattering Light from Magnetic Material with Land/Groove by Three-Dimensional Boundary Element Method

A calculation method with three-dimensional Boundary Element Method (BEM) has been developed for analyzing scattering light from a magnetic material, in order to determine the contribution of land/groove shapes and incident beam polarization to Kerr rotation. A product of scattering beam intensity and the Kerr rotation angle (R.c k ) has been calculated in the case that 680 nm wavelength Gaussian beam is focused on 680 nm or 1360 nm width land/groove, which has 85 nm depth or height. The land or groove shapes have been varied from rectangular shapes to V-shapes. Regardless of the land/groove shapes, the R.θ k of lands is larger than that of grooves in TM-polarization and smaller in TE-polarization. The results in verification experiments well met the calculated ones, i.e., the carrier level of lands is higher than that of grooves in almost the entire range of recording frequency for TM-polarization light, and it is lower for TE-polarization.

Jitter improvement in mark edge recording for phase change optical disks with optical phase encoding

We have developed a novel absorptivity control method for phase change optical disks for improving overwriting jitter in mark edge recording. In order to control absorptivity more easily, we have designed a disk with a small difference in reflectivity between the amorphous and the crystalline states, and with a large optical phase difference between the reflected light from those two states, which produces a high carrier to noise ratio. We have obtained remarkably reduced overwriting jitter, down to 2.47 ns from 5.69 ns for the leading edge, and this has meant greatly increased applicability of mark edge recording to phase change optical disks.

Crystal Orientation and Surface Roughness of Bi Films Prepared in Ionized Cluster Beam Apparatus

We have studied the effects of deposition conditions and underlayers on the crystal orientation and surface roughness of Bi films deposited by the ionized cluster beam method as a means of reducing the noise of MnBi magneto-optical disks. Substrate temperature and applied voltage strongly affect the crystal orientation and the surface roughness of Bi films. The c-axis orientation and surface flatness drastically deteriorate at substrate temperatures above 100° C, and the degree of orientation/flatness decreases as the applied voltage is increased. Surface roughness of Bi films is inversely proportional to the degree of c-axis crystal orientation, indicating that the surface roughness is determined by the growth rate of Bi crystal planes. The c-axis orientation and the surface flatness can be improved by using low-wettability underlayers, since crystal reorientation of the Bi films easily occurs on such underlayers. We find that sputter etching of SiN underlayers can improve the surface flatness of Bi films at 20 nm thickness.

Low temperature crystal growth of MnBi films

The relation between deposition conditions of Bi and Mn layers and crystal growth of MnBi were investigated to reduce the MnBi annealing temperature for its application to a magneto-optical disk. We found that higher c-axis orientation of the hexagonal Bi layer and lower Mn oxide concentration in the Mn layer reduces the annealing temperature. Growth of MnBi below 150/spl deg/C, which is much lower than the decomposition temperature of photo-polymer (about 200/spl deg/C), was achieved by optimizing deposition conditions of Bi and Mn layers. >

High C/N magneto-optical disks using plastic substrates for video image applications

High carrier to noise ratio (C/N) magneto-optical disks for analog video image applications are described. Optimized TbFeCo recording layer and Si 3 N 4 protective and spacing layers are fabricated on 200mmφ plastic pre-grooved substrates. A protective cover is sandwiched with a 0.5mm air gap. Recording, reading and erasing are performed through the protective cover. More than 55dB C/N at a 5 MHz frequency, a necessary condition for analog video component recording, is achieved, and a 49 dB video S/N is obtained.

Reduction of cross-erase in phase-change media

We developed a phase change media that has less cross-erase at a track pitch of less than 0.6 micrometer. The phase change media has an absorptivity ratio Ac/Aa of 1.5 (Ac:absorptivity in a crystalline state, Aa:absorptivity in an amorphous state), while maintaining the rapid cooling structure. There was no significant cross-erase even at a track pitch of 0.52 micrometer. Feasibility of recording capacity of about 5 GB on a 120 mm disk has been confirmed.

High-density phase-change optical disk with a Si reflective layer

For high-density recording at a wavelength of 690 nm, we developed a phase-change optical disk with a Si reflective layer. We estimated the effect of interference layer by calculating optical properties. The absorption control required for mark edge recording and a 2 dB C/N improvement was obtained by forming a ZnS-SiO2 interference layer on the Si layer. Under recording conditions with a minimum bit length of 0.335 micrometers and a track pitch of 1.2 micrometers , a sufficient C/N and a BER less than 10-4 were confirmed. This result indicates that the recording capacity of the new disk is more than 4 GB.

Phase Change Optical Disks Having Recording Marks with Large Optical Phase Difference Suitable for Mark Edge Recording

We have developed a novel absorptivity control method for phase change optical disks with the aim of reducing overwriting jitter in mark edge recording without degradation of overwriting cyclability. To control absorptivity more easily, we have designed a disk with a small difference in reflectivity between the amorphous and the crystalline states, and with a large optical phase difference between the reflected light from those two states, which produces a high carrier-to-noise ratio. We have obtained significantly reduced overwriting jitter, down to 2.5 ns for the leading edge, as well as a sufficient overwriting cyclability of up to 105 times.

Effects of Silicon Nitride Chemical Composition on Magneto-Optical Properties of Non-Stoichiometric Silicon Nitride/TbFeCo Layers

The effects of various silidon nitride chemical compositions on the magneto-optical properties of non-stoichiometric silicon nitride/TbFeCo layers have been investigated. With increasing pressure of the sputtering gas, stoichiometric Si3N4 changes to silicon oxynitride SiON, and with decreasing in the N2/Ar ratio of the sputtering gas, it changes to nitrogen-defective silicon nitride ( SiNx ). The refractive index of films decreases with increasing oxygen concentration, and it increases with decreasing nitrogen concentration. Measured values for Kerr rotation angle and reflectivity of the SiNx /TbFeCo/SiNx layers were found to correspond quite well to those derived theoretically. Increase of the oxygen concentration of the SiON layer results in oxidation at the interface, which in turn results in an observable decrease in the Kerr rotation angle of SiON/TbFeCo bilayers. A SiNx /TbFeCo layer has been found to be just as effective as a Si3N4/TbFeCo layer for use in magneto-optical disks.