WO1995018445A1

WO1995018445A1 - Data storage media

Info

Publication number: WO1995018445A1
Application number: PCT/GB1994/002803
Authority: WO
Inventors: Derek Paul Ashley Pearson; Philip George Pitcher
Original assignee: Johnson Matthey Public Limited Company
Priority date: 1993-12-29
Filing date: 1994-12-22
Publication date: 1995-07-06
Also published as: AU1320995A; GB9326508D0; ZA9410397B

Abstract

A materials system suitable for optical data storage in which data is stored as data bits on a thin film multilayer and the data bits are formed from localised intermetallic compounds.

Description

DATA STORAGE MEDIA
The present invention concerns improvements in data storage media and more especially it concerns improved optical data storage media.

WORM (Write Once Read Many Times) optical media are attractive for applications where once written, data is immutable. Fresh storage space is utilise as memory as new files are added or original files are updated. Its use may involve applications where data may be recalled many times or in archiving information. WORM media are available in two main types of geometries, as discs, primarily for use in computer peripherals, and as cards, primarily for use in holding personal details, eg banking transactions or health records. Of related

application are so called"SMART"cards which have an electronic memory and
data processing function. Microelectronic circuitry is incorporated in the card. In
addition there are cards of the magnetic strip type eg credit and bank teller machine
cards which are widely used.

Both smart cards and magnetic strip type cards have a limited data
capacity and suffer wear due to the handling experienced in use and through the
mechanical contact required in the data read-write process. Smart cards are also
expensive in the context of a mass market application. Optical data storage cards
are an attractive alternative data storage vehicle in applications requiring cheap or relatively large data storage capacity. Further, the data read-write process is contactless and therefore card wear problems are considerably reduced.

The essential requirements of an optical data storage medium are that data can be written readily to the medium surface by using a suitable write stylus eg a focused laser beam; that the data points written to the medium are completely stable and that the properties of the written compared with the unwritten regions are sufficiently different so that data can be read with ease. The most common optical WORM media system is"phase change"where the medium is changed from a crystalline to amorphous state, or visa versa, after the process of writing data bits. The data is read by comparing for example the optical differences between the two states. The general technical requirements of optical media systems are described in the review by A B Marchant"Optical Recording-a
Technical Overview", Addison Wesley 1990.

In published European patent application number 0 234 588 is
disclosed an optical memory system in which information is recorded by changing
the physical state of portions of a recording film. The recording film includes an
intermetallic. When information is recorded by a laser beam there is a phase
change in the intermetallic from an equilibrium phase to a non-equilibrium phase.
This system allows both writing and erasing of information. It is not suitable for archiving of information because information can be erased easily using a relatively low power laser beam.

An alternative method of recording information is described in US patent number 4,845,515. The recording layer is a bismuth/selenium layer and information is recorded by causing portions of the layer to be ablated by a spot heating technique such as using a laser beam.

The long term stability of current optical media, including WORM media, is an area where improvements are required. For example, a CD or
CDROM has a lifetime of some 5-8 years, which is an unacceptably short lifetime for archival storage.

This invention describes a new type of WORM medium, which has a high degree of environmental stability and physical ruggedness. This makes the media especially suitable for all WORM applications where repeated use, harsh environments or very long term data storage is required.

We have found a thin film WORM optical media system in which
very stable data bits can be written at low temperatures. The time-temperature
regime or"temperature of formation"for data bit information is believed to be
suitable for practical use as an optical storage medium. The"temperature of
formation"is the temperature above which a data bit can be written in a reasonable
length of time. A data bit can be written at very low temperatures but it may take
a matter of days which is not practical for manufacturing purposes. Further, a
significant differential in colour or reflectivity exists between written and unwritten
regions enabling data to be read easily.

The present invention provides a materials system suitable for optical
data storage comprising a substrate and one or more layers selected from a thermal
management, an optical enhancement, a tribological and a protective layer, and a
thin film multilayer, the multilayer being comprised of two or more precursor
layers, each precursor layer being a single component or a multi-component,
wherein a data writing treatment of said multilayer forms an admixture or
compound of the components to form written data bits, said data bits being
distinguishable optically from unwritten components.

Localised material synthesis (from thin film multilayer materials
systems) offers new applications as a data storage medium. Localised intermixing/
diffusion of the multilayer structure resulting for example in compound or alloy formation, through application of a thermal probe can produce regions of different
colour, reflectivity, absorbence, magnetic or magneto-optical property, thermal or
electrical conductivity etc which act as data storage bits in an unreacted (unwritten multilayer) matrix.

Preferably the thin film precursor layer is selected from aluminium, copper, gold, palladium, platinum or a combination thereof. The precursor layers can be stacked in any combination and number to form the multilayer.

Thin film deposition techniques for synthesis of multilayer systems include molecular beam epitaxy (MBE), various chemical vapour deposition (CVD) and electrochemical/electroless techniques, evaporation and sputtering. The latter technique is well suited to commercial scale production of these multilayer materials systems.

A significant colour and/or reflectivity change can occur on forming an intermetallic from a multilayer precursor background in some materials systems.

These differentials in optical characteristics can be used to advantage in producing large signal-to-noise ratios in the read-out signal from the media. Gold and purple coloured data bits formed from platinum and gold aluminide intermetallics respectively offer excellent optical differential from a silver colour multilayer precursor background.

The data bit may be an intermetallic and preferably has a "temperature of formation"of at least 100 C. A well defined materials transition in forming a data bit, reproducibility of bit geometry and property measurand are
important in data storage applications. Line compounds ie compounds with a
strictly limited regime of composition are generally good candidates for data bits
formed from multilayer systems. We have found platinum group metal
intermetallics for example can offer in addition to the above characteristics
excellent stability to environmental corrosion, are durable and thermally stable
under expected conditions of use. Furthermore such data bits are immutable and therefore excellent candidates for a WORM medium which may for example be based on a colour or reflectivity change.

The multilayer synthesis has the potential to allow data bits to be written at sufficiently low temperature of formation eg 200450 C at acceptable data rates.

Preferably the intermetallic is platinum aluminide or gold aluminide or a copper gold alloy. Other materials systems may exist which are suitable for this application provided the system results in a significant colour and/or reflectivity change when the data bits are written.

A chemically stable optical storage system results from forming chemically very stable intermetallic data bits in a chemically stable noble metal containing multilayer matrix. The stability of said intermetallic data bits is such that they are effectively immutable. Attempts to erase them in any practical manner would result in corruption of adjacent regions of the multilayer precursor background.

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7
An intermetallic such as PtAl2 can be formed for example on a hot plate at say 200 C or by the action of a focused electron beam. In the latter case compound formation is apparently"instantaneous". In the"pack aluminising" process platinum aluminides are typically formed at time-temperature regimes of 30 minutes to 2 hours at 650-850 C. The fine scale structure of the multilayer stack is used to enhance the effects of diffusion, therefore reducing the timetemperature regime required for compound formation.

Suitably, the data bits are written by a thermal stylus selected from a modulated electron or a laser beam and a hot mechanical probe. Data bits may be read by for example an optical probe, thereby in this instance read-out is mechanically non-contacting. Read-out can be direct by the properties of the reflected or transmitted beam. Indirect read-out can be achieved through differentials in film properties affecting measurands dependent on media-substrate interactions resulting from interrogation of the thin film media by the read optical beam eg thermo-mechanical or acousto-magnetic/magneto-optic effects. The data may be interrogated with the optical probe incident through the substrate or on the media-coated side of the substrate.

In multilayers of appropriate dimensionality the time-temperature regime for formation of a data bit can be controlled through the nature and thickness of individual layers forming the multilayer structure. The ratio of the thicknesses of the precursor layers and their composition determine the composition of a completely reacted region. The total thickness of the multilayer and the

presence of other additional layers will affect the above process by affecting the coupling of thermal energy to the media from the write stylus through such agents as thermal conduction and reflection.

The materials system may have regions specific for writing by laser or electron beam. Suitably the substrate supports a data storage medium which may be magnetic or magneto-optic and preferably is a platinum-cobalt or a rare earth transition metal data storage medium.

Preferably the substrate is selected from glass, aluminium, alumina, plastics, paper, card and combinations thereof. These substrates are all suitable for use with this materials system because the data bits can be written at relatively low temperatures of formation, eg up to 450 C.

The utility of the card system can be further enhanced through combination with erasable/re-writable Pt-Co magneto-optic data storage technology, eg multilayer precursor platinum aluminide intermetallic WORM medium and erasable magneto-optic medium eg Pt-Co multilayer medium. The medium combination can have associated read selectivity ie a polar Kerr or Faraday readout optical system for magneto-optic media regions with a different optical mode of detection or optical system required for read-out of WORM regions.

Although data is read (addressed) optically, the materials system can
be easily adjusted to enable production of card areas which can be written by laser
or only by electron beam, adding a further dimension to card security. Magneto
optic regions can only be written thermomagneto-optically. Thermal writing to
WORM regions can be made exclusive ie dependent on the nature of the thermal
stylus. The materials and optical design of the WORM system (medium plus
optical layers) specific to allow writing of data bits through for example:
a) an optical (laser beam) stylus or electron beam stylus,
b) alternatively regions of WORM medium can be fabricated such that only an electron beam stylus can suitably couple the stylus with medium for writing with regard to considerations of system stability and the requirements of a practicable data storage medium.

A combination of an optical data storage medium with other data storage media is particularly advantageous because unauthorised access to the data is much more difficult. In addition, greater amounts of information can be stored for longer periods of time.

The invention also provides a process for the manufacture of a materials system comprising depositing on a substrate one or more of a thermal management, optical enhancement, tribological and protective layer, and a thin film multilayer, said multilayer comprising two or more precursor layers, each layer being a single component or a multi-component, wherein a data writing treatment of said multilayer forms an admixture or compound of the components to create
written data bits capable of being distinguished optically from unwritten
components.

The invention will be described by way of example, such examples
are intended to illustrate but not limit the scope of the invention.

The following samples were all prepared in a modified Nordiko 3750
sputter deposition unit (Nordic Ltd, Havant, Hants, UK).

EXAMPLE 1
Rapid Low Temnerature Formation of PtAI, PGM Line Compound
This Example illustrates how a PGM line compound can be formed by a multilayer synthesis technique in which a colour or reflectivity change is produced.

Glass microscope slide substrates were mounted on a rotatable substrate table and the sputter chamber evacuated to a pressure < 5 x 10-6mb.

High purity Ar was metered into the sputter chamber establishing a pressure of 1.5 x 10-2mb.

The substrate table was rotated at 6rpm whilst 800W of rf power at
a frequency of 13.5MHz was applied to the table for 2 minutes establishing a dc
self-bias with respect to ground of-230V-substrate plasma etch.

The substrate table is centrally mounted between two horizontal configuration 8"x 4"planar magnetron sputter electrodes in direct opposition on either side of the substrate table.

DC excitation at 370V and 3.5A was applied to the Al sputter gun for 5 minutes to pre-condition the sputter target.

400W of rf power at 13.56MHz was applied to the Pt sputter electrode establishing a dc self-bias with respect to ground of 300V and the target allowed to pre-condition for 1 minute.

Shuttering obscuring the substrate from the sputtered flux was removed and the substrate table rotated at 1.7rpm for 5 minutes. A multilayer structure of alternating Pt and Al layers of approximate total thickness 920A was deposited through sequential exposure to the sputter sources. Each Pt layer of thickness 34 lA and each A1 layer 74 lA.

The temperature of compound formation was approximately 200 C.

Figure 1 shows the CIELAB reflectance spectra from the multilayer and
subsequently thermally reacted multilayer system. The colour change from silver
to gold colour is indicated by the CIELAB colour designation numbers:
Multilayer Reacted Multilayer
L 76.03 71.65
C 2.51 21.91
H 90.09 79.64
Real time writing PtAl2 was performed with a 30KeV electron beam focused to 5 microns diameter, beam current 19nA.

EXAMPLE 2
Adjustment of"Temperature of Formation"
In this Example, it is shown that the"temperature of formation"can be adjusted by varying the precursor component layer thickness.

The fabrication procedure described in Example 1 was followed with the exception that in the multilayer deposition stage the table rotation was 12rpm.

The component layer thicknesses were therefore 5 lA Pt and 11 1 A1 the total film thickness remaining the same.

The temperature of compound formation was 270 C.

EXAMPLE 3
"Temnerature of Formation"as a Function of Multilaver Height
This Example shows that the"temperature of formation"is not a
strong function of multilayer height in a rapid formation system.

The fabrication procedure described in Example 1 was followed with the exception that aluminium foil and glass substrates were used. The deposition time was eleven times that in Example 1 and thus the total layer thickness was correspondingly eleven times that of Example 1.

The temperature of compound formation was 200 C.

EXAMPLE 4
400 C"Temperature Formation"of AuAl, Compound in a
Ranid Formation Svstem
Alumina tiles and glass microscope slides were mounted on a rotatable substrate table and the sputter chamber evacuated to a pressure, 5 x 10-6mb.

High purity Ar was metered into chamber establishing a pressure of 1.5 x 10-2mb.

The substrate table was rotated at 6rpm whilst 800W of rf power at
a frequency of 13.5MHz was applied to the table for 2 minutes, establishing a self
bias of approximately-200V.

DC excitation at 365V and 3.5A was applied to the A1 sputter gun for 5 minutes to pre-condition the sputter target.

250W of rf power at 13.5MHz was applied to the Au sputter electrode establishing a dc self-bias with respect to ground of approximately-250V and the target allowed to pre-condition for 1 minute.

Shuttering obscuring the substrates from the sputtered flux was removed and the substrate rotated at 6rpm for 5 minutes. A multilayer structure of alternating Au and A1 layers of total thickness 0.1 microns was deposited through sequential exposure to the sputter sources. Each Au layer of thickness 11 1 and each Al layer 22 1 . A dc bias of approximately-15V was established with respect to ground was established over the substrate table.

A conversion from a silver metallic lustre to a mauve colour indicating compound formation was achieved after heating the substrates 400 C.

EXAMPLE 5
100 C Multilaver"Conversion Temperature"to AuAI, Comuound
The deposition procedure of Example 4 was followed with the following notable exceptions:
In the multilayer stage the substrate table was rotated at 1.7rpm for 15 minutes. The total layer thickness was 0.3 microns each individual Au layer thickness was 38.5A and each individual A1 layer thickness was 77A.

A visual colour change from silver metallic lustre to mauve was observed at approximately 100 C indicating formation of the intermetallic compound.

Claims

CLAIMS 1. A materials system suitable for optical data storage comprising a substrate and one or more layers selected from a thermal management, an optical enhancement, a tribological and a protective layer, and a thin film multilayer, the multilayer being comprised of two or more precursor layers each precursor layer being a single or multi-component, wherein a treatment of said multilayer forms an admixture or compound of the components to form written data bits, said data bits being distinguishable optically from unwritten components.

2. A materials system according to claim 1, wherein the precursor layer is selected from aluminium, copper, gold, palladium or platinum or a combination thereof.

3. A materials system according to claim 2 wherein the data bit is an intermetallic.

4. A materials system according to claim 3, wherein a"temperature of formation"for the intermetallic is at least 100 C.

5. A materials system according to claim 4, wherein the intermetallic is platinum aluminide or gold aluminide or a copper gold alloy.

6. A materials system according to any preceding claim wherein the data bits are written by a thermal stylus selected from a modulated electron, a laser beam and a hot mechanical probe.

7. A materials system according to any preceding claim, wherein a read-out detector of the data bits includes an optical probe.

8. A materials system according to any preceding claim, wherein there are regions specific for writing by laser or electron beam.

9. A materials system according to claim 8, wherein the substrate supports a data storage medium region selected from a magnetic or a magneto-optic medium.

10. A materials system according to claim 9 wherein the magnetic or magneto-optic region is a platinum-cobalt or a rare earth transition metal data storage medium.

11. A materials system according to any preceding claim, wherein the substrate is selected from glass, aluminium, alumina, plastics, paper, card and combinations thereof.

12. A process for the manufacture of a materials system according to any of claims 1-11, comprising depositing on a substrate one or more of a thermal management, optical enhancement, tribological and protective layer, and a thin film multilayer, said multilayer comprising two or more precursor layers, each precursor layer being a single component or a multi-component, wherein a data writing treatment of said multilayer forms an admixture or compound of the components to create written data bits capable of being distinguished optically from unwritten components.

13. A process according to claim 10, wherein the"temperature of formation"for the intermetallic is at least 100 C.