Phase Change Optical Recording Disks Research Articles

Laser-Induced Crystallization in AgInSbTe Phase-Change Optical Disk

Laser-induced crystallization in the initialization and erasing processes of a AgInSbTe phase-change optical recording disk was studied. After laser initialization, a AgInSbTe film formed a metastable face-centered cubic (f.c.c.) phase. Both the high-resolution transmission electron microscopy (HRTEM) cross-sectional image and the static test results showed the difficulty of nucleation. At low linear velocities, i.e., CD 1X and CD 2X, several clusters formed in amorphous marks due to the low cooling rate and the longer reheating of the subsequently applied pulses. These clusters enhanced nucleation and the amorphous marks probably crystallized via the nucleation and grain growth mechanism in the erasing process. The degree of order in the amorphous marks decreased with linear velocity, and no clusters existed in the amorphous marks at CD 4X. Due to the difficulty of nucleation, the erasing process was predominated by the direct grain growth mechanism at high linear velocities.

Deposition of diamond-like carbon film on phase-change optical disk by PECVD

With development of optical recording media oriented toward high-density optical systems, the protection of data storage becomes very critical. In this article, we report on the diamond-like carbon (DLC) films deposited by plasma-enhanced chemical vapor deposition on the polycarbonate (PC) substrate of phase-change optical recording disks. The mixed SiH4 in CH4∕H2 gases were used to reduce the internal compressive stress of the deposited films. The structural characteristics of the DLC films were analyzed as a function of film thickness by Raman spectroscopy. The variations of the G peak positions and the intensity ratio, ID∕IG, were investigated as a function of the DLC film thicknesses. The DLC film thickness is a very important characteristic related to the surface microroughness and transmittance. Several tests, including a tape pull test, a thermal shock test, and an alcohol rub test, were used to evaluate the adhesion of the DLC coatings on the PC substrates. The rms roughness and morphology of DLC films were inspected by an atomic force microscope. The dependence of optical transmittance, reflectivity, and the overwriting characteristics of jitter on the thickness of DLC films were investigated. Finally, our results indicate that DLC films can provide an effective capability for protecting stored data and exhibit a smooth enough surface and high transparency quality for the recording and overwriting characteristics of phase-change optical disks.

Structural and thermal analysis of Ag-Sb-Te alloy and its films for phase change optical memories

An Ag-Sb-Te alloy and its films are prepared as a new optical recording amorphous crystalline (a?c) phase transformation material. The crystallization process of Ag-Sb-Te films is systematically studied through measurement of recording characteristics to solve the trade-off problem between data (amorphous) stability and erasing sensitivity. phase change optical recording disks demonstrate long thermal stability of the amorphous recording marks. The crystallization process of Ag- Sb-Te material was studied using differential thermal analysis (DTA), and the nature of the material was studied by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The films were studied for both cases of before and after annealing. It was concluded that the alloy (Ag-Sb-Te) could be used as a phase change optical memory material.

Superlattice-like structure for phase change optical recording

In order to increase the crystallization speed and data transfer rate (DTR), a superlattice-like structure (SLL) was applied to the recording layer of phase change optical disks. Unlike the conventional phase change layer, the recording layer with the SLL structure consisted of alternating thin layers of two different phase change materials, i.e., GeTe and Sb2Te3. Although neither GeTe nor Sb2Te3 could be used as a phase change layer material for practical applications, present experimental results revealed that the phase change optical disk with the SLL structure demonstrated an excellent recording property that could meet practical recording requirements. X-ray photoelectron spectroscopy was employed to confirm that the SLL structure could be preserved after many times of melting and quenching. Dynamic properties of the optical recording disk with the SLL structure were investigated with a 1 T pulse duration of 8 ns and a constant linear velocity of 19 m/s. A clear eye pattern was observed. The carrier-to-noise ratio was about 58 dB and a DTR of 47 Mbit/s was achieved. The DTR would be as high as 140 Mbit/s if the blue light is used. It has been proven that the SLL structure is a useful means to increase the DTR of phase change optical recording disks.

Crystallization of Ag–In–Sb–Te Phase-Change Optical Recording Films

Crystalline phases formed on thermally annealed and laser-annealed Ag12.4In3.8Sb55.2Te28.6 four-element alloy films were observed to be different. After 1 h isothermal annealing at temperatures between 190°C and 450°C, hexagonal Sb and chalcopyrite AgInTe2 phases were observed, whereas laser annealing by initialization at laser power higher than 2.86 mW/µm2 yielded cubic crystalline Sb and AgSbTe2 phases. There was only one exothermic peak at 170°C determined by differential scanning calorimetry (DSC) measurement. Only the hexagonal Sb phase was observed by X-ray diffraction of samples subjected to DSC measurement. These experimental results suggest that the activation energy for crystallization derived from Kissinger's equation using DSC data may not be the same as that for crystallization during erasing of phase-change optical recording disks.

Effect of ZnS Adhesion Layer on Overwrite Cyclability of Phase Change Optical Recording Media

A high-performance phase change optical recording disk, which has a five-layered structure, was proposed. This disk has a ZnS adhesion layer between a GeSbTe recording layer and a ZnS–SiO2 second dielectric layer. A 2×105 overwrite cyclability was achieved with the original five-layered phase change optical recording disk. The cross-sectional transmission electron microscope observation and the in-plane X-ray diffraction measurement indicate that both the five-layered disk and the four-layered disk have almost the same crystalline structure. The contact angle measurements, however, indicate that the ZnS adhesion layer improves the adhesive force between the GeSbTe recording layer and the ZnS–SiO2 second dielectric layer. It is considered that the high overwrite cyclability results from this improvement of the adhesive force. Moreover, the electron probe microanalysis results indicate that the ZnS layer acts as a protective layer against the Ge diffusion from the GeSbTe recording layer.

Oxygen doping effect on Ge–Sb–Te phase change optical disks

High performance phase change optical recording disk has been developed by using an oxygen-doped Ge–Sb–Te recording layer. The 5×105 overwrite cycles were achieved with this 6 at. % oxygen-doped phase change optical recording disk. The transmission electron microscope observation shows that the crystal grain size was enlarged and the “self-sharpening effect” disappeared with increasing of oxygen concentration. It is thought that the oxygen doping made it easy to grow the uniform large grain size without the self-sharpening effect around the amorphous mark and therefore, the overwrite cyclability of the phase change optical recording disk was improved.

Thermal Modeling of Grooved Phase Change Optical Recording Disk

A new method of studying the laser induced temperature profiles of a multi-layered phase change optical disk is applied to the deep groove disk structure for the first time. The new model takes into consideration the thermal effects generated by both the incident and reflected light. Three-dimensional finite-element modelling and analyses of the new thermal model were carried out using commercial finite-element software MSC/PATRAN and ABAQUS. The calculated temperature differences between the new and the existing model are compared for grooves of depth 80 and 180 nm. Results showed that the reflected light gave rise to a significant temperature difference, especially in the deeper groove. Reduction of cross-erasure in adjacent tracks in a grooved phase change disk structure by using the deep groove method was also investigated. Findings showed that the reflected laser light gave rise to a significant temperature rise at the adjacent track.

PdGeSbTe Alloy for Phase Change Optical Recording

Phase change optical recording disks using a Pd–Ge–Sb–Te quaternary alloy demonstrated high crystallization speed and long-term thermal stability of the amorphous recording marks. This alloy film can be crystallized by a short duration laser pulse of less than 100 ns. It is applicable to a single beam overwrite optical recording system. The crystallized portion of this recording layer on the disk is assigned to single phase and polycrystalline face-centered-cubic (fcc) crystals by transmission electron diffraction. A small amount of Pd atoms (typically 0.2 to 3 at.%) in this alloy improve the thermal stability of amorphous marks.

Application of simulation on the design of phase-change optical recording disks

Optical absorption and thermal conduction are the two key factors affecting temperature distribution and, subsequently, the write, erase characteristics of a phase-change optical recording disk. Therefore, by using carefully measured film physical properties of each layer, this work simulates a transient temperature profile while simultaneously considering optical absorption and thermal conduction. Through the simulated transient temperature profile, cooling-rate and reflectivity, dependence of phase-changed spot size on the laser power and laser pulse duration was observed. The proper combination of the disk structure and the associated write, erase conditions are obtained as well. A disk structure can subsequently be designed on the basis of this information. In addition, a novel dielectric layer, i.e. hydrogenated amorphous carbon (/spl alpha/-C:II), is simulated and compared with the disk applying conventionally used ZnS-SiO/sub 2/ dielectric layers. The disk structures used herein are PC/ZnS-SiO/sub 2//GeSbTe/ZnS-SiO/sub 2//Al and PC//spl alpha/-C:H/GeSbTe//spl alpha/-C:H/Al. According to those results, /spl alpha/-C:H film is highly promising as a dielectric layer of the phase change optical recording disk for both wavelengths of 780 and 660 nm.

Optical and thermal analyses on multilayered thin films of a phase-change optical recording disk

A computer simulation method to study a laser-induced transient temperature profile of a multilayered phase-change optical recording disk (PCORD) has been developed. The simulation program consists of two parts. One part is an optical analysis program based on the optical characteristic matrix method, and the other is a thermal analysis program based on the finite-element method. A new estimation method for thermal conductivity of recording films was proposed. The estimated thermal conductivity of the thin-film recording media was found to be 50% lower than that of the bulk. Reflectivity, absorbance, and temperature values obtained with this simulation method were compared with those measured by a spectrophotometer and an infrared radiation thermometer when the samples were irradiated in an Ar+ laser beam. The accuracy using this simulator was within 8% for reflectivity and absorbance and within 10% for temperature. A method of optimizing the laser-absorbing layer was studied. With this method simulations showed that the PCORD had a maximum contrast ratio and minimum recording time when thickness, thermal conductivity, and complex refractive index of the laser-absorbing film were 60 nm, 4.2×10−3 W/cm °C, and 2.6−i⋅0.0, respectively. This simulation could provide better film designs for PCORDs and the other optical disks of heat mode recording.