Materials for Erasable Optical disks

Materials

Chemistry

189

and Physics, 32 (1992) 189-195

Materials for erasable optical disks J. C. Bern&de Universite’ de Names, Faculte’ des Sciences, 44072 Nantes Ceder 03 (France)

(Received

February

26, 1992; accepted

Laboratoire

de Physique

des Mat&iaux

pour

I’Electronique,

2, me de la Houssinit%e,

March 6, 1992)

Abstract After a brief review of the optical disks which are at present on the market: Compact Disk Read Only Memory (CD-ROM), Write Once Read Many (WORM) disks, Write Erasable Magneto Optical (WEMO) disks, we give erasable optical recording the results of the laser Write Erasable (WE) materials. Disks based on phase-change media (chalcogenide alloys, dye polymers) are presented. Recent progress in realizing practical media for phasechange reversible optical recording are described. More information is given on new thin film materials for optical recording such as anthraquinone derivatives.

Introduction Optical disk systems using a laser beam for data recording and reading are foreseen for use as high density information storage systems. After the Compact Disk Read Only Memory (CD-ROM) and the Write Once Read Many (WORM) disks, erasable magneto-optical (WEMO) disk systems have become commercially available. There is a strong demand for higher data transfer rate systems, such as digital or analogic video image application. Progress in direct overwriting of WE-MO has been obtained which increases the data transfer rate. However the Laser Write-Erase cycle by a simple phase change is another possibility. Recent progress in the performances of the chalcogenide based alloys have made them very attractive. More explorative but not less interesting is the use of dye polymer media. All these possibilities will be described below.

Compact

disks on the market

[l-S]

Compact Disk Read Only Memories (CD-ROM) The technology of Compact Disk Read Only Memories (CD-ROM) is similar to that of an audio compact disk. Manufacturing is divided into two stages: mastering and replication. Mastering is a low volume operation and includes recording of format or data base information as

0254-0X4/92/$5.00

well as the microstructure into a master disk. From these masters, stampers are made by creating copies. Replication includes injection molding and vacuum coating. Then the final disks are assembled into protective cartridges. Distribution of data bases and document storage are some applications of CD-ROM, they will become replacements for paper storage. However, there are many applications where the number of distribution sites is not large enough to justify the CD-ROM mastering, or it may be desirable to record the information in house. Therefore Write Once Read Many (WORM) disks are acceptable for these applications and the information can be updated in the field. This is not possible with CDROM. Write Once Read Many (WORM) optical disks The problem was to develop an inexpensive disk recorder of digital information. It was to have direct read-after write capability and to be able to record in an ordinary room, not the usual clean room required for dust protection. The solution was an optical disk that records by laser. Optical disk systems that use laser recording to store encoded information at very high data rates with extremely high density is a very efficient technique because it allows instantaneous playback, fast random access, better recording density than magnetic recording and archival storage. In order to write the information, a bright beam of laser light is focused onto a spinning disk to make a recording, the laser beam being modulated by the

0 1992 - Elsevier Sequoia.

All rights

reserved

190

information. The recorded information is retrived by illuminating the disk with a somewhat less intense beam and then detecting the light reflected back from the disk. Information is recorded by turning the recording beam on and off at different rates, producing pit marks of varying lengths along a spiral or circular track on the disk. During playback, these marks will alter the amplitude of the reflected beam for different periods of time, producing a frequency-modulated signal. The heat mode laser recording on a thin film medium results in the formation of permanent marks - called pits - due to thermal structural changes. Therefore, one of the most critical elements of an optical storage system is the storage medium. Ideally an optical data storage medium should have such characteristics as high resolution to accommodate high density storage of data, high sensitivity to allow high speed recording with low power laser, and high stability for permanent storage. Thin films of tellurium and its alloys show fairly high laser sensitivity and excellent direct read-after write characteristics. Other media using ablation films of dye in polymer are also efficient [l-3]. Both these storage media families are actually in use on the market. WORM optical technology is best suited for the storage of computer data that needs to be archived. Files may be written on optical disks and retrieved as many times as needed. This is basically a random access replacement for sequential computer tape backup on a denser media. Write Erasable Magneto Optical (WE-MO)

disks

Erasable optical disks are best suited for replacing current magnetic media. Their key advantage is its mass storage. Its current disadvantage

Fig. 1. Magneto-optic processes [6]. (a) At elevated temperature caused by laser Focused spot size determines minimum rotated on reflection from M-O medium restores to original blank disk.

is that the reading and writing of data is currently a much slower process when compared to magnetic disk drives. Of the several technologies providing rewritable media, magneto-optics continues to be favored. The magneto-optic technology use a material with a high coercive coefficient at room temperature and therefore the recording process is that shown in Fig. 1. Usually the films used are amorphous [44-S] sputtered films [4, 51. Todays media are based on transition-metal/rare-earth alloys (Fe, Co, Tb...) alloys which are inherently corrosion sensitive. Protective and reflective layers are added to stabilize the magneto-optical layer and to enhance its performance. The first-generation magneto-optical (WE-MO) disk drives have been commercialized for use as an external storage for computers. The application of these drives is limited by their slow data processing time including data transfer rate and access time. Moreover, these drives do not have the function of overwriting. A solution of the overwriting problem would go a short way toward making WE-MO drives a possible replacement for rigid magnetic disk drives. There are many kinds of approaches for direct overwriting (Fig. 2). There are two classes of methods. One is by the optical modulation method and the other is by magnetic field modulation using an electromagnetic coil. In the optical modulation method, there are two techniques. One uses the exchangecoupled double-layered disk media [27], the other uses a conventional one magnetic layer but needs two optical heads.

Blank disk. At ambient temperature bias the field has no effect on medium. (b) Record. pulse, bias field exceeds coercivity of medium, resulting in local reversal of magnetization. feature size. (c) Read via Kerr effect. Polarization angle of low-power CW laser beam is according to orientation. (d) Erase. Reversed bias plus heat from high-power CW laser

191

Overwritable

Maaneto-ootic

1

disk

Mmnrtir

Fig.

2. WE-MO

over

write

schemes

field

[26].

Also, in the magnetic field modulation method, two techniques can be considered. One is where a laser light with a continuous light power illuminates the media, and data is recorded by magnetic field modulation. In the other method, a laser diode illuminates the media with pulsed lights, and data is recorded by magnetic field modulation. A good performance may be obtained [28-301.

Laser write erasable

optical media

While magneto-optic recording (MO-WE) is presently the most mature technology phase -change optical storage has several attributes which are attractive. The optical head needs fewer components than in the case of the ‘heavy’ magneto-optical head, which simplifies alignment and then decreases the data access time. The magnitude of the phase-change signal, which is several orders of magnitude greater than that from the magneto-optic media, provides the potential for much higher signal-to-noise ratio. The principle underlying optical recording using phasexhange media is the controlled, reversible switching of a spot in the disk between two states, usually the amorphous and crystalline states, but other phenomenon may be used as described later. There are essentially two kinds of media that are generally used: tellurium-based alloys and dye polymer media. Tellurium-based alloys Research of phase-change recording materials began with the works of Ovshinsky and co-workers [31]. The written amorphous state is created by heating the material above the melting temperature

with a laser beam and then letting it cool rapidly. Erasure is accomplished by heating the spot above the crystallization temperature for a sufficient time to cause complete recrystallization of the amorphous spot. Changes in electronic structure accompany these phase transformations and result in significant changes in optical properties such as reflectivity. The reflectivity difference is detected using a low-power, continuous laser beam to read the data. Direct overwrite is relatively easy with phase change technology, because the crystalline state is nearly modified back to the amorphous state. The material used needs some typical properties: fast writing and erasing, stable written and erasable states, large signal together with low noise and adequate number of reversible cycles. Progress in the technology makes available fast crystallization materials such as GeTe based alloys [32-341. The stability of the memorised state depends on the stability of the amorphous state against crystallization. Limited reversibility is a problem for phase change media. High temperature encountered during amorphization is one of the major causes of cycling failure. However, materials with higher melting temperatures (such as GeTe), are required to guarantee long-term structural stability of the amorphous state. Achieving a high number of cycles required accurate control of the laser power [35, 361. Direct overwrite may be obtained by using a two power level modulation laser spot (Fig. 3): the one is peak power level for melt amorphous mark making. The other is bias power level for crystallizing the mark easing. However as shown by Gotoh et al. [38] ‘erasing residuals’ may decrease

(a)

Time

(nsec) (b)

Fig. 3. Over-write signal (a) laser power modulation over-written track

and corresponding temperature level; (b) temperature profile

(99): of the

192

the SNR. In order to reset the accumulation of the crystallization level in the overwrite process they propose a melt erasing method which increases significantly the SNR. Progress in realizing operational phase change erasable media has been obtained by using GeTe based alloys. Many materials can be switched between the amorphous and crystalline state in less than 100 ns. This fast switching ability has been exploited to obtain single-beam direct overwrite of data. Long term amorphous state stability has been demonstrated. Good read out SNR has been achieved. Rapid progress is being make in increasing the number of reversibles cycles of phase-change media [39]. Dye polymer media Most experimental technology in the industry of write erasable disks concerns dye polymers. Two processes may be used for dye polymers topology change (‘bump’ formation) and phase change in dye film. Topology change in dye polymer The structure is based on a two layer structure, a retention layer and an expansion layer (Fig. 4). The process is a topology change based on the heat of the laser beam [40,41]. The rewrite erable media consists of two active layers which are dyed to selectively absorb in two spectral ranges corresponding to the separate write and erase laser wavelengths (Fig. 5). When applied, the written laser, the expansion layer swells and deforms the retention layer, creating a bump on the surface. Inversely when the erasing laser is applied, the retention layer is heated above its transition temperature of elasticity allowing the expansion layer to pull the retention layer flat, which restores the surface. Reading is obtained because the bumps scatter the laser light, resulting in a lower returned light level than the flat parts. As in phase change technology, the temperature change can influence

Fig. 4. Base structure

[40].

WRITE

READ

ERASE

Fig. 5. Write, read and erase processes [41]. To write: thermally expand the expansion layer, thereby visco-elastically deforming the retention layer. After cooling, the process leaves the retention layer in compression and the expansion layer in tension. To read: observe diffraction scattered light or phase contrast at lower power without exceeding the sharp marking threshold. To erase: heat the retention layer above its Ts to reduce the modulus, allowing the expansion layer to pull the retention layer flat. This restores the surface.

the structure of the layers or can affect the sensitive layer properties, reducing the life cycle. Phase changes in dye films The search for a reversible optical recording organic material is a long one. Dyes were selected from the known classes of dyes that are thermally and photochemically stable such as the anthraquinones, the photholocyanines and some of the squaryliums. They are known to undergo phase changes, but the last two families appear to be non reversible and therefore anthraquinones dyes were studied [42]. Some of them present a reversible phase change, but the erasure time is long and the cycle reproducibility poor. In order to avoid long erasure time, dyes with low-order parameters should be used, and to increase the life cycle the dye must be photochemically and thermally stable. Another way has been investigated by, Potember and co-workers [43-48]. They have shown that Ag TCNQ organometallic charge transfer complex may be an erasable optical storage medium. The Ag TCNQ films were produced by a vacuum deposition technique in which the donor (Ag) and acceptor (TCNQ”) were reacted in the solid state. High contrast patterns were produced on the material by CW and pulsed Nd:YAG and Ga Al As lasers, and erasure read accomplished by CW Nd:YAG and Ga Al As lasers or bulk heating. They have shown that the effect of the applied optical field in the Ag TCNQ charge transfert salt is to induce a phase transition resulting in the formation of a non-stoichiometric complex salt containing neutral TCNQ. However the switching is quite slow and the reproducibility poor. Since anthraquinones present good thermal, photochemical stability and may form charge transfer salts, we investigated some anthraquinone derivatives and their complex salts in the laboratory [49-521. 9, lo-Anthraquinone thin films have been deposited under vacuum [49, 501. IR absorption, X-ray diffraction and electron spectroscopy for chemical analysis (XPS) characterizations have

193

shown that anthraquinone may be easily evaporated. After deposition the films were amorphous. After annealing at T= 360 K for 24 h the films crystallized, with a monoclinic structure. The c axis of the crystallites is essentially perpendicular to the plane of the substrate. The interband transition energy was estimated by optical measurements to be about 3.25 eV. A similar study was done on dichloroanthaquinone and it was found that crystallized DCAQ thin films may be obtained for any kind of substrate after appropriate annealing or by deposition on a substrate heated at T, =365 K. The binding energies deduced from ESCA lines and the frequencies of the IR absorption bands were found to be in good agreement with the powder reference. Optical absorption may be associated with a gap of about 3.1 eV. The average grain size estimated from scanning electron micrographs is around 0.5 pm. After this study we proceeded to the study of the formation of the metallic complex salt Ag+DCAQ -. Two kinds of sample were studied in order to obtain metal-DCAQ complexes; DCAQ films deposited on Cu or Ag sheets or silver (copper) thin films deposited on DCAQ films, in the thickness ration of l/20. The metal complex was identified by X-ray diffraction (XRD), IR absorption and XPS measurements. Just after deposition no modification of the XRD and XI’S spectra was visible. By annealing at 370 K for 24 h, the spectra are

modified [52]. In the X-ray diffraction pattern, a peak caused by the Ag DCAQ complex was observed at an angle corresponding to an inter-planar spacing of 2.86 A. In the Cl 2p XPS pattern, the intensity of the peak situated at 198 eV has increased. The same results were obtained when silver film is deposited on DCAQ films heated at 365 K. Given the above XFS results, it is thought that part of the DCAQ has reacted with silver and formed an Ag DCAQ charge transfer complex. The 198 eV binding energy corresponds to Cl-ion (Fig. 6). The IR spectra of Ag/DCAQ thin films are also modified after annealing. The most striking feature of this curve is that the peak caused by the carboxyl substituent vibration (1677 cm-‘) changes. It is dedoubled with a new peak at 1685 cm- ‘. Therefore the Fourier transform IR spectra showed that silver has interacted not only with Cl but also with 0 since the C=O vibration has changed while a new peak absorption has appeared at about 1400-1340 cm-’ which may be attributed to C-O vibrations [52]. Therefore Ag DCAQ complexes may be obtained by sequential Ag and DCAQ evaporations. We have noticed that with Cu used instead of Ag, no charge transfer complex is obtained [52]. In Fig. 7 we can see that the transmission spectra of the films before and after annealing are very different. This is a very promising phenomenon in order to obtain optical recording material.

6.

6-

206

204

202

200

196

196

194 Binding energy E(eV)

Fig. 6. [49] (0). XPS spectra of Cl 2p peak at the surface (T=365 K for 24 h); (c) structure deposited at T,=365 K.

of a AgDCAQ

structure:

(a) before

annealing;

(b) after annealing

194

1-

-10

a

I

I 400

600

600

1000

1200

1400

1600

1600

2000

-h(nm) Fig. 7. Optical xnsity

of a Ag/DCAQ

structure

[49]; (a) before

Conclusions

While magneto-optic recording is today, the most mature technology, and successfully commercialized for use as an external storage in personnal computers, phase-change optical storage has several attributes which are attractive: simple optical head, good SNR, direct overwrite facilities. Recent progress in switching time (amorphous + crystalline) and in the increase of the number of reversibles cycles of phase change GeTe alloys media has made them nearly competitive. The dye polymer media has been less investigated and the studies in progress actually have to be developed. References

5 6 7 8 9 10 11 12 13 14 15

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