K. G. Lim

Temperature Dependence of Phase-Change Random Access Memory Cell

The temperature dependences of phase-change random access memory (PCRAM) cells on different Ge–Sb–Te phase-change recording materials are studied and compared. A Ge2Sb2Te5 phase-change film has a larger resistance margin and a higher thermal stability than Ge1Sb2Te4 and Ge1Sb4Te7 films. The set resistance, reset resistance, resistance margin and threshold voltage of PCRAM cells decrease with increasing temperature. A Ge2Sb2Te5 PCRAM cell has a higher thermal stability of threshold voltage than Ge1Sb2Te4 and Ge1Sb4Te7 PCRAM cells.

Dependence of Optical Constants on Film Thickness of Phase-Change Media

The optical constants of phase-change films are assumed to be constant regardless of film thickness in conventional optical design and thermal simulation of multilayer structure optical disks. However this assumption is not valid when the phase-change film thickness in the optical disks becomes very small. In this study, the dependence of the optical constants on the thickness of Ge1Sb2Te4 phase-change films is investigated. The changes of the extinction coefficient k and the refractive index n (especially k) become significant when film thickness is in the range of 10 nm–30 nm, and become larger at shorter wavelengths such as the blue region. This dependence of optical constants (n and k) on the film thickness can be explained and confirmed based on the discontinuous film model of very thin film. These results are significant in improving the accuracy of optical design and thermal simulation of phase-change optical disks, as well as in the study of phase-change optical disks at shorter wavelengths.

Computer-Aided Design and Analysis of Rewritable Phase-Change Optical Disk

The dependence of the optical and thermal characteristics of phase-change optical disks on the multilayer disk structure was analyzed and simulated. Using the integrated software developed, a phase-change optical disk structure for a blue laser is designed with both optical and thermal considerations, and a suitable disk structure is proposed. Based on the designed disk structure, writing and rewriting processes are simulated, and the results are satisfactory.

Properties and Reactive Sputtering Parameters of GeN Film for High-Density Phase-Change Optical Disk

The dependence of optical and thermal properties of phase-change optical disks on the diffusion of sulfur atoms from ZnS–SiO2 dielectric layers into a GeSbTe phase-change recording layer was simulated. The influence of reactive sputtering parameters on the properties of GeN films has also been studied. Significant improvement in the direct overwriting cycles was achieved in the 4.7 GB digital versatile disk-random access memory (DVD-RAM) disk with a GeN interface layer after optimizing the sputtering parameters of the GeN film.

Thermal Deformation Analysis of High-Density Optical Disks

The laser-beam spot size and track pitch of phase change optical disks are continuously decreasing. Hence any slight deformation in the disk will affect data storage performance efficiency. Thus said thermal deformation at high temperatures induced by laser irradiation becomes an important issue in optical disks. Thermal elastic deformation in blue-laser optical disks has been calculated by the finite element method (FEM). The associations of thermal deformation with disk structures and laser power have been investigated to supply methods to decrease thermal deformation. It was found that the peak temperature and peak deformation lies in different layers. Several methods of preventing deformation in blue-laser optical disks have been proposed. It was found that these methods can be used to optimize the structures of high density phase change optical disks.

Substrate Deformation Studies on Direct Overwriting of Phase-Change Rewritable Optical Disc with Germanium Nitride Interface Layers

The deformation of polycarbonate substrate during direct-overwriting (DOW) process of phase-change (PC) optical disc with germanium nitride (Ge-N) interface layers was studied. From scanning probe microscopy (SPM) results, the deformation of the land/groove caused by thermal energy during the writing process was highly reduced by the Ge-N interface layers. The Ge-N interface layers not only prevent sulfur atom diffusion into the PC layer during DOW, but also protect the substrate from thermal damage. Therefore, the optical disc with Ge-N interface layers achieved DOW cycles 2 orders higher than that of the conventional rewritable disc. The substrate deformation lifetime during the DOW process was multiplied by approximately 100 times.

Integrated Thermal and Optical Analyses of Phase-Change Optical Disk

An integrated analysis system based on thin film optics, thermal transfer and electromagnetics is developed. Thermal conductivity and generated heat are discussed for mark formation. The simulations of multilayer calorific sources and multibeam heating sources using the finite element method (FEM) are investigated. The readout of nanometer-scaled marks based on computational electromagnetics using the finite-difference time-domain (FDTD) analysis is discussed. The real marks captured with microscopes can be analyzed using this integrated analysis system combined with digital image technology. Material models in the electromagnetic vector method are discussed with reference to different layers of disk. It provides a powerful tool for structure design and failure analysis of phase-change optical disks.

Local Thermal Expansion in Super-Resolution Near-Field Structure

Investigation focuses on the local strain–stress field in the laser-beam-spot region in super resolution near-field structure. The three-dimensional finite element method is used to analyze the elastic thermal expansion in the readout process. The temperature distribution around the laser beam in a large laser power margin has been studied, and it may be related to the second phase transition. The investigation has revealed that the center of the laser beam is always ahead of the centers of the peak-temperature, peak-stress, and peak-strain regions so that the stress and strain are very different around the center of the laser beam. This may lead to the change of the optical properties. The super-resolution mechanism caused by the local thermal expansion is discussed.

Integrated Analysis and Design of Phase-Change Random Access Memory (PCRAM) Cells

An integrated software for analysis and design of PCRAM cells has been developed. The research focuses on the discussion on electric-thermal -mechanical analyses. The software involves in the materials, geometrical and layer structure design and electric pulse strategy. It aims to provide a powerful tool for structure optimization and failure analysis of PCRAM cells.

Study of Phase Change Random Access Memory (PCRAM) at the Nano-Scale

In this paper, phase change random access memory (PCRAM) cells at the nano-scale was studied. A hybrid patterning process integrating with electron beam lithography (EBL) and optical lithography was used to fabricate nano-PCRAM cell. PCRAM cells with different feature sizes ranging from 40 nm to 200 nm have been fabricated and tested by an in-house developed tester which was capable of generation of pulse with short width. Electrical testing including programming current and speed have been conducted on the nano-cells. The resistance-current curves have shown a good scaling effect on the programming current against the cell size. Besides the current reduction, it was found that nano-PCRAM cells have shown an improved programming speed when its size reduces. RESET speed as fast as 2 ns was achieved for PCRAM cell with 45 nm. The improved speed was possible attributed to the nano-size effect due to the increasing contribution of the interfaces.