Herman J. Borg

Phase-Change Media for High-Numerical-Aperture and Blue-Wavelength Recording

We have studied the feasibility of doped eutectic SbTe alloys for practical application in digital video recording (DVR) rewritable phase-change media for high-numerical-aperture (NA=0.85) and blue-wavelength (λ=405 nm) recording. Remaining issues such as thermal cross-erase in land/groove recording and the thermal stability of recorded amorphous marks have been investigated and resolved. This work has resulted in the realisation of 22.5 GB phase-change media complying with the DVR blue format. The future of doped eutectic SbTe alloys in the phase-change recording-speed race is also promising, as the crystallisation speed and archival life stability can be optimised more or less independently by tuning the Sb/Te atomic ratio and adding Ge, respectively. We have demonstrated that a user data transfer rate of 70 Mbit/s (DVR double speed) is within reach using our current phase-change composition, and are expecting to realise data rates of over 100 Mbit/s in the near future.

Trends in optical recording

Optical recording technology is evolving rapidly, driven by the demands of higher storage capacities and increasing data rates. In this contribution, we discuss the technology roadmap of optical disc recording. The emphasis will be on rewritable optical recording, in particular, phase change recording.

Numerical simulation of mark formation in dual-stack phase-change recording

Dual-stack phase-change recording is an option to further increase the data capacity of rewritable optical disks. Such disks consist of two recording stacks that are both recorded and read from the same side of the disk. Consequently, the first recording stack needs therefore to be semitransparent to allow recording in the second recording stack. Thick nontransparent metal layers can therefore not be used in the first recording stack, which makes the first recording stack the most challenging stack from a thermal point of view. A numerical model based on crystal growth was developed to study formation and erasure of amorphous marks in phase-change stacks that are based on fast-growth doped SbTe phase-change materials. The validity of the model was demonstrated from transmission electron microscopy analyses of recorded marks that showed a good correspondence with the calculated mark shapes in a conventional single-stack recording stack. The model was subsequently applied to analyze formation and erasure of marks in slow-cooling phase-change stacks for digital versatile disk, (DVD) and digital video recording (DVR) recording conditions. The effect of the recording velocity, the erase power, and the crystal growth velocity on the erasability of amorphous marks was simulated. The calculated phenomena are in good agreement with the phenomena observed from DVD and DVR erasability measurements. Mark formation in slow-cooling recording stacks is characterized by severe recrystallization during writing. Two possible solutions are indicated in this article, aiming at reducing the heat accumulation and the resulting recrystallization during writing of amorphous marks. Additional semitransparent heat sinks improve the mark formation considerably but also require higher write powers. Another solution is the application of modified write strategies. Modeling and recorder results are discussed for both approaches.

Dual-Layer Blu-ray Disc Based on Fast-Growth Phase-Change Materials

A dual-layer disc is developed for blue-laser phase-change recording. The two recording stacks are based on fast-growth phase-change materials (FGMs). Both layers show good recording performance at a total disc capacity of 46.6 GB and with a slightly high jitter at 50 GB. The thicknesses of both the cover and spacer layers are controlled such that the deviations from the reference thicknesses of both recording layers are less than 2 µm. This eliminates the need for dynamical spherical aberration correction in the drive. The absolute difference in transmission between the written and unwritten states of the upper layer is only 2%. It is shown that this transmission difference causes no problem for readout and writing of the lower layer.

Thermal Cross-Erase Issues in High-Data-Density Phase-Change Recording

We report on the numerical simulations of high-data-density phase-change recording (DVR-blue and DVR-red) in order to analyse the phenomenon of thermal cross-erase in land/groove recording. In particular, if a maximum data capacity is pursued, which involves a small radial track pitch, thermal cross interference may become an important issue. The question of whether or not the temperature distributions and the related thermal cross-erase probability scale with the optical spot size is addressed. Two different thermal models (a planar model and a land/groove model) were used in which laser heating was simulated. Reliable temperature predictions and related trend analyses require a proper model and accurate input parameters. The thermal conductivity of the different layers in the stack was obtained from measurements performed with a time-resolved static tester setup in combination with a novel melt-threshold method. Since thermal cross-erase is dependent on the temporal temperature distribution in the stack, we analysed the most important parameters that possibly contribute to a reduction of the temperature, and thus the cross-erase probability, in the adjacent track.

Development of Recording Stacks for a Rewritable Dual-Layer Optical Disc

In this paper results are presented on the development of recording layers for a rewritable dual-layer phase-change optical disc. The disc is based on the digital video recording (DVR) system utilising a blue laser and a high numerical-aperture objective lens. With such a disc the capacity of the DVR system could be increased to about 45 GB. Various stack designs, write strategies and recording characteristics of the media are discussed. For each recording layer a design with a fairly good performance is found.

Phase-Change Media for High-Density Optical Recording

Abstract : Rewritable optical-storage systems are quickly gaining market share in audio, video and data-storage applications. The development of new rewritable optical-storage formats with higher capacity and data rate critically depends on innovations made to the recording media incorporating so-called phase-change materials. These materials allow reversible switching between a low and high reflective state induced by laser heating. In this paper, we highlight phase-change media aspects as optical and thermal design, sputter-deposition, materials optimization, and the development of new recording strategies. Focus is on the speed race in optical recording.

High density phase-change recording beyond 2.6 GByte

We review the current status of high density phase-change optical recording at red wavelengths. A user bit capacity of 3.0 GByte has been realized on a 120 mm rewritable disk by using in-groove recording and a simple and reliable wobbled groove format. The headerless format shows excellent data and address integrity during cyclability tests and it has a high degree of compatibility with read-only DVD. In order to extend disk capacity even further, both radial and tangential densities must be increased. In increasing the radial density, land/groove recording appears more promising than in-groove recording because optical cross-talk between neighboring tracks can be largely cancelled. A reduction of the track pitch in land/groove recording below 0.74 micrometers results in thermal cross-talk, leading to partial erasure of data in the adjacent tracks. The minimum bit length which gives acceptable recording tolerances in both land and groove tracks is 0.32 micrometers . With the combination of a track pitch of 0.74 micrometers and a bit length of 0.32 micrometers , an areal density of 2.7 Gbit/in2 and a user capacity of 3.5 GByte should be possible in land/groove recording.

Advances in thermal modeling of dual-layer DVR-blue fast-growth media

Dual-layer phase-change recording is an option to further increase the data-capacity of re-writeable optical discs. A numerical model based on crystal growth was used to study formation and erasure of amorphous marks in dual-layer stacks based on fast-growth doped eutectic SbTe phase-change materials. The effect of the linear velocity, erase power, and crystal growth velocity on the erasability of amorphous marks was simulated. The calculated effects are in good agreement with the phenomena observed from DVD and DVR measurements. Mark formation in slow-cooling dual-layer stacks is characterized by severe re-crystallization during writing. Two possible solutions are indicated in the paper aimed at reducing the heat accumulation in the recording stack, and thus re-crystallization, during writing of amorphous marks. Additional transparent heat sinks improve the mark formation considerably but also require higher write powers. Another solution is based on an appropriate write strategy. Recording results obtained with this such a write strategy are shown.

Embossing of light trapping patterns in sol-gel coatings for thin film silicon solar cells

For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest as much as possible of the available sunlight. Typically, one uses randomly textured front TCOs to scatter the light diffusively in pin-cells on glass. Here, we investigate methods to texture the back contact with both random and periodic textures for use in nip-cells on opaque foil. We applied an electrically insulating SiOx-polymer coating on a stainless steel substrate, and textured this barrier layer by embossing. On this barrier layer the back contact is deposited for further use in the solar cell stack. Replication of stamps with various random and periodic patterns was investigated, and, using scanning electron microscopy, replicas were found to compare well with the originals. Masters with U-grooves of various submicrometer widths have been used to investigate the optimum dimensions of regular patterns for light trapping in the silicon layers. Angular reflection distributions were measured to evaluate the light scattering properties of both periodic and random patterns. Diffraction gratings show promising results in scattering the light to specific angles, enhancing the total internal reflection in the solar cell.