The present invention relates to a data storage unit, such as a compact disc.

A compact disc (CD) is formed using a reflective material in which a spiral of pits encode information digitally. In use the drive reads the data by shining light from a laser diode onto the disc. Laser light is used since all emitted light is in phase and has identical wavelength.

Reflected light is detected by a photodiode, usually after being deflected through a prism. The photodiode receives a light signal where the laser beam has been reflected from a non-pitted portion of the disc only. The pits present on the disc are designed to cause a half wavelength shift in the phase of the reflected light. Light reflected from a pitted portion of the disc is thus totally out of phase with light reflected from a non-pitted portion and destructive interference of the two signals ensues so that in this situation no light is detected by the photodiode.

Thus, the digital pattern of pits on the disc produces a pulsed light signal which is detected by the photodiode and converted into a digital electrical signal. The electrical signal is converted into visual or audial data by the drive electronics.

Generally a fast-acting servo-mechanism is incorporated in the CD drive, to compensate for irregularities in the surface of the disc and for vibrations so that the laser beam remains focused onto the correct area of the CD. Data in the form of bar codes etc is scanned in a similar manner.

However commercially available CDs cannot be erased, and neither can alternative data be written thereon and this is a reflection of the technical difficulties to be overcome since accuracy in data writing and reading is paramount.

There is also a need for other forms of erasable rewritable data storage units, such as bar codes, bank cards and the like.

There is provided a method of manufacturing a data storage unit (such as a compact disc), preferably an erasable rewritable data storage unit, said method comprising applying a liquid crystal polymer to a substrate, preferably composed of a reflective base material. Selected areas of the liquid crystal polymer can be converted between a substantially opaque liquid crystal phase and a substantially transparent or translucent phase by an energy transfer process, such as heating (thermal transfer) or by provision of energy in the form of a radiant or particle beam (for example an electron beam or an ion beam) .

The data is usually written onto the storage unit in digital format and is encoded by the presence and absence of translucent or transparent areas in the opaque liquid crystal polymer. Creation of the transparent areas within the liquid crystal polymer

depends in turn upon transfer of energy to those areas of the liquid crystal polymer. Thus it is important to be able to accurately convert very distinct areas of the LCP into the transparent/translucent form.

Compact discs are a preferred form of data storage unit. It should, however, be noted that the LCP, optionally with associated dye, can be deposited on to a wide range of substrates, including flexible substrates, such as plastic films or cloth to form a data storage unit. This opens up to a range of additional applications in information strips on credit cards, replacements for bar codes, etc. It has the advantage of storing much larger quantities of information than is possible by the use of a conventional information storage media, such as magnetic fields, and can optionally be erased and re- written.

Generally, the liquid crystal polymer (LCP) is heated and allowed to cool slowly so that it is present on said substrate in the liquid crystal phase throughout. In the liquid crystal phase the incoming light is multiply refracted within the film, thereby increasing its optical path length, and thus the LCP appears opaque. In this situation less of the light from a laser or other beam scanning the data storage unit is reflected to a photodiode. Contrast can be enhanced further by incorporating into the LCP a dye which absorbs the incident beam and this may be advantageous.

When writing data digitally onto the data storage unit, selected areas of the LCP receive transferred energy so that the LCP in such areas is converted into an substantially transparent or translucent phase. Energy

transfer may occur by heating using a laser until the LCP in the affected areas is converted to the isotropic phase, or by use of an electron beam until the LCP in such areas is made clear. Where heating is responsible for the change in appearance of the LCP, fast cooling to below the glass transition temperature (Tg) causes the structure of the isotropic phase to be locked in and the area appears transparent or translucent. In this situation light from a scanning laser passes through the LCP, is reflected by the base material and can thus be detected by the photodiode. Alternatively light may be transmitted through the transparent regions for detection by the photodiode.

The local heating required may be achieved by either electrical heating via buss bars under the LCP or by absorption of a laser beam. Such effects are known in the literature for Smectic A low molar mass materials (H Melchior, F J Khan et al, Appl Phys Lett 2J.: 392 (1972), F J Khan, Appl Phys Lett 22, 111 (1973) and reviewed in Thermotropic Liquid Crystals, Ed G W Gray, Chapter 4 by D Coates, Publ John Wiley & Sons) and in LC Polymers (GB-A-2248065) and "Side Chain Liquid Crystal Polymers", Ed C B McArdle, Publ Blackie, USA, ρ357).

An example of a suitable dye component which may be incorporated into the liquid crystal polymer is F593 (available from Merck) which has significant absorption at 633nm (see Figure 1) . Typically, 1-2% by weight of dye is incorporated into the LCP. Dyes with maximum absorbance of wavelengths up to 800nm are also acceptable and may be preferred in some circumstances. Such dyes are also known in the art and commercially available.

In one embodiment the energy transfer to the LCP occurs via a heating process. The LCP is most conveniently heated to its isotropic phase by the use of a laser. This allows dots of extremely high resolution, for example 20 μm to 0.1 μm, preferably 12 μm to 0.1 μ , especially preferably less than 2 μm, for example lμm to 0.05 μm, to be obtained on the data storage unit. Since a laser is also used to read the data on the unit, the read-only laser should be of low power which is insufficient to cause phase change in the liquid crystal. Alternatively a laser having a different wavelength may be used. Use of heat to energise the LCP results in a rewritable, erasable data storage unit. The facility for erasing and rewriting may be preferred in certain applications.

In order to keep the heat very localised, the laser beam may be moved relative to the data storage unit so that it passes across the areas to be heated very quickly and good results have been obtained by moving a lOmW He-Ne laser at speeds of 10-30cm/s. The laser beam may be switched on and off quickly in short pulses and this is the most convenient for writing digital data. Even with current technology, it is possible to pulse lasers very quickly. In the present case the pulse length required to achieve the phase transition in the LCP will vary depending on the power of the laser. The length of time required to achieve the isotropic phase will vary depending on the power of the laser, the type and amount of dyes in the liquid crystal polymer and the phase behaviour of the LCP.

The use of a laser to heat the LCP into the isotropic phase has the advantage that, by focusing the laser, the resolution of the area to be heated can be

carefully controlled. Moreover, since a laser beam is brightest in the centre portion of the beam the centre of the area struck by the laser undergoes most heating so that it is converted to the isotropic phase more completely than peripheral areas, further increasing resolution.

In a preferred aspect, the same laser is used to write the data onto the data storage unit and also to read the unit without destroying the data written thereon. Thus, when writing data onto the unit the laser should be of sufficient power to cause heating of the LCP above into its isotropic phase. However, when the laser is to be used in "read-only" mode the power of the laser should be reduced so that heating of the LCP into the isotropic phase does not occur.

To erase the data on the unit, the whole unit may be heated to convert the LCP into the isotropic phase and cooled down slowly, for example at a rate of 2 °C/minute so that all the LCP is converted into the liquid crystal phase. Further data may be written onto the unit when required.

Conveniently, where the data storage unit is a compact disc, the heating and slow cooling of the disc is achieved within the CD drive, for example by application of heating bars to the non-LCP coated side of the base material or by radiant heating. Thus, a further aspect of the present invention provides a CD drive with an apparatus to heat an LCP-coated CD into the isotropic phase followed by slow cooling.

For other types of data storage unit the data may be erased in an analogous manner to that described above.

An apparatus arranged to cause data erasion forms a further aspect of this invention.

In an alternative embodiment, the LCP may be selectively energised by use of a radiant or particle beam. One particularly useful example is an electron beam, which may be focused down to lμm or less. The areas of the LCP energised by the electron beam are converted to a transparent phase and thus storage of data occurs as described above. Where an electron beam is used to effect energy transfer of the LCP the data may be encoded permanently in the data storage unit; this can be advantageous for certain applications. Whether the alterations to the LCP caused by energy transfer from the particle beam are reversible or permanent may be controlled by careful selection of certain parameters, for example selection of the LCP and/or dye component used and/or selection of the flux and/or energy of the particle beam.

A number of liquid crystal polymers are known and the exact selection of a particular LCP will depend upon a number of other factors, such as the power of the laser or strength of electron beam to be used and/or dye component to be incorporated. Mention may be made of LCPs based on polyacylates and polyesters which have a Tg temperature above ambient. LCPs with a Tg temperature of 60°C or above, such as 65°C and above, are especially preferred since this allows easy storage at normal conditions without affecting the pattern contained on the CD. Generally, LCPs having a Tg temperature of 200°C or less are convenient since this allows the necessary heating stage to be performed by use of a laser. The LCP may be a main chain polymer, a side-chain polymer or a combination of the two, or

there may be side chain groups which may themselves be oligomers or polymers (see Finkelmann in "Thermotrophic Liquid Crystals" ed G W Gray, Chichester 1987, Page 159 onwards) .

Careful selection and substitution of the LCP will allow influence of the Tg temperature and thus the skilled man would be able to "engineer" an LCP with a suitable Tg temperature.

The LCP may comprise a single polymer or may be a mixture of two or more polymers admixed together.