WO2006038170A1

WO2006038170A1 - Dopant stabilised information layer

Info

Publication number: WO2006038170A1
Application number: PCT/IB2005/053227
Authority: WO
Inventors: Liesbeth Van Pieterson; Johannes H. J. Roosen; Antonius E. T. Kuiper; Wilhelmus C. Keur; Johannes C. N. Rijpers
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-10-06
Filing date: 2005-09-30
Publication date: 2006-04-13
Also published as: TW200625298A

Abstract

An information storage medium with a stabilised information layer and a method of stabilising the information containing layer is disclosed. The medium is comprising at least one information layer in a stack of layers, the information layer being a layer comprising a phase-change composition comprising Ge, Sb and Sn. The phase-change composition is further capable of supporting at least a first and a second structural phase, and wherein the structural phase of at least one of the least two structural phases is stabilised over time by adding a dopant to the phase-change composition, the dopant may be Se. The information storage medium may be an optical storage medium where the stabilisation is a stabilisation of a crystal phase so that the time development of the reflectivity of areas of the crystal phase is diminished, i.e. the jitter is diminished. Thereby reflectivity variations from areas crystallised at different time instants are diminished.

Description

Dopant stabilised information layer

FIELD OF THE INVENTION

The invention relates to a rewritable information storage medium and to a method of stabilising the information containing layer. The medium comprising at least one information layer in a stack of layers, the information layer comprising a phase-change composition. In particular the invention relates to stabilisation of a structural phase in the information layer.

BACKGROUND OF THE INVENTION

A variety of optical information storage media are well known in the art, particularly in the form of compact disc, CD, and digital versatile disc, DVD, media. In recordable or rewritable storage versions, a modulated laser beam incident on the record medium induces a change in the optical properties of the information layer. The change may optically be detected later during reading by a laser beam having a lower intensity. The use of a phase-change composition as the information containing layer may facilitate a rewritable medium where the user e.g. may erase and substitute old data with new data. •

In a phase-change composition use is made of a material which is capable of undergoing reversible phase changes between crystalline and amorphous phases under controlled irradiation. Information is recorded on the recording layer by way of formation of amorphous marks and erased therefrom by crystallisation of the amorphous marks. Information is read from a modulation of reflected light from the amorphous marks and the crystalline areas.

The present inventors have appreciated that in order to read recorded data in an error-free manner on a long time- scale, it is important that the reflectivity from crystalline areas and from the amorphous marks are constant through-out the storage medium. When overwriting old data, problems may arise from unstable crystallised areas where e.g. the reflectivity may decrease over time. Such problems may result in differences in the modulation width from different areas on a storage medium, and induce increased media noise and jitter. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a high-speed information storage medium with an improved shelf life, and consequently, the present invention seeks to provide an improved information storage medium. Preferably, the invention alleviates or mitigates one or more of the above disadvantages singly or in any combination.

Accordingly there is provided, in a first aspect, an information storage medium comprising a carrier substrate and a stack of layers supported by the carrier substrate, wherein at least one of the layers in the stack of layers is an information layer, the information layer being a layer comprising a phase-change composition comprising Ge, Sb and Sn, the phase-change composition further being capable of supporting at least a first and a second structural phase and wherein the structural phase of at least one of the least two structural phases is stabilised over time by adding a dopant to the phase-change composition. The information storage medium may be an optical storage medium, such as a rewritable CD or DVD, or the storage medium may be an electrical storage medium such as a phase-change memory medium (also called PCRAM or Ovonic memory). In the optical storage medium the information may be read by shining a laser onto the information layer and analyse the reflected light from areas of the first and second structural phases. In an electrical storage medium, the information may be read from e.g. resistance variations between currents running through areas of the first and second phases, respectively. The carrier substrate may be a polycarbonate plastic on which a stack of layers may be provided. The stack of layers may comprise the information layer, as well as protection layers, guide layers for electromagnetic radiation, etc., such that a storage medium fulfilling standard requirement in connection with size and reflectivity is provided, in order for standard CD-players, DVD-player, etc. may read and/or write data to and/or from the medium.

The information layer may be a layer comprising a phase-change composition capable of supporting at least a first and a second structural phase. The first structural phase may be a crystalline phase, whereas the second structural phase may be an amorphous phase. It is an advantage to provide a phase-change composition supporting at least two structurally different phases, since the two phases may exhibit different optical properties. The crystal phase may either be reflective or transparent. A transparent phase may be made reflective by providing a reflective layer below a layer comprising the transparent phase, such as a metal layer, e.g. an Al or Ag layer. The amorphous phase may be at least substantial opaque, i.e. no or very little light is reflected or transmitted from or by an area comprising the amorphous phase. The phase-change composition may be a composition with a growth-dominated crystallisation mechanism.

The phase-change composition comprises Ge, Sb and Sn. It may be an advantage to use a GeSnSb-based composition as the phase-change composition, since GeSnSb-based composition are suitable for high-speed recording, since the composition may when heated to the crystallisation temperature crystallise in nanoseconds, enabling such recording at linear velocities between 7 and 60 m/s, corresponding to 2x to 17x DVD-speed.

The structural phase of at least one of the least two structural phases may be stabilised over time by adding a dopant to the phase-change composition. It may be especially advantageous to use Se as the dopant in connection with a GeSnSb-based composition. Se may provide a stronger crystal bond strength and thereby stabilise the crystal phase. The amount of Se may be in the range of 1 to 10 percent, such as 2 to 9 percent, such as 3 to 7 percent, such as 4 to 6 percent, such as 5 percent. The Se may substitute for Ge. In this document the term percent should be read as atomic percent. It may be an advantage to stabilise the at least first structural phase since e.g. the amount of light reflected from an area of the first structural phase for a given light input level may vary over time. This may induce increased jitter when overwriting old data. A stabilisation of at least the first structural phase may be a stabilisation of the reflectivity of an area of the at least first structural phase, or may result in a stabilisation of an area of the at least first structural phase. By diminishing or even minimising the reflection variation with time, the shelf life of an information storage medium may be improved, this is of great importance since data may securely be stored for a longer period of time.

The GeSnSb-based composition may be on the general form Ge_xSn_ySbi_-x-y. The concentration of the various elements may be such that x is in the range of 0.03 to 0.3 and y is in the range of 0.1 to 0.3. The recording properties, i.e. the kinetic of the structural phases, may depend upon the element concentration in a complex way. It may be an advantage to use Ge to improve the stability of the amorphous phase, whereas it may an advantage to use Sn to ensure growth of small crystallites and Sb to ensure fast crystal growth. However, these and other properties depend upon a proper choice upon the element concentration and it may therefore be advantageous to chose x and y properly, i.e. in the specified ranges.

The phase-change composition may further comprising In, such as up to 5% of In, such as up to 4% of In, such as up to 3% of In, such as up to 2% of In, such as up to 1% of In. The GeSnSbln-based composition may be on the general form Ge_xSn_yIn_zSbi_-x-y_₂, with z up to 0.05. In may be added in order to improve the reflectivity from the crystalline areas.

The first and second structural phases may have a first and a second reflectivity upon exposure to electromagnetic radiation for visual or near-visual radiation. More specifically may the structural phases exhibit different optical properties for electromagnetic radiation in any one, or all, of the following wavelength or wavelength ranges: the present standard wavelength of 780 nm for CD's, the present standard wavelength of 658 nm for DVD's, a wavelength range used in the blu-ray disc format (BD) around 405 nm, and the wavelengths in the deep ultra violet range (DUV) of 230-300 nm. It is an advantage to provide a phase-change composition suitable to operate in the mentioned wavelengths or wavelength ranges since these wavelengths or wavelength ranges either are used in standard consumer equipment or foreseen to be used in future standard consumer equipment.

A first area may be formed into the first structural phase at a first time instant and a second area may be formed into the first structural phase at a second time instant. It may be an advantage to diminish or minimise the time development of the relative reflection from the first and second area. At least should the time development be kept below the norms for data storage. Such norm may be expressed as a requirement for the jitter, where in terms of shelf life the disk may have to fulfil: :

(J(DOW = 10,f = 12 -24))² -(J(DOW = 9,t = O))² < A

where J refers to the jitter as a function of the number of times old data is over written (DOW, direct overwrite) and the time. This means, if data is written and overwritten 9 times at t=Q, and then overwritten again (which makes it DOW=9+1+10) after 1 day (/=12-24 hours), the difference between the squares of these values should be less than A. The value of A may depend upon current standards and may consequently vary over time. A value of A may be e.g. 4.5 %.

According to a second aspect, the present invention relates to a method of stabilising a structural phase in an information layer in a storage medium, the method comprising the steps of:

- providing a carrier substrate

- providing on the carrier substrate a stack of layers, the stack of layers including the information layer, the information layer being a layer comprising a phase-change composition comprising Ge, Sb and Sn, the phase-change composition further being capable of supporting at least a first and a second structural phase, and wherein the structural phase of at least one of the least two structural phases is stabilised over time by adding a dopant to the phase-change composition. The method may be implemented in a fabrication method or in connection with running fabrication facilities in order to provide a medium according to the first aspect of the invention.

According to a third aspect, the present invention relates to use of Se to stabilise a structural phase in a GeSnSb-based phase-change composition. Se may be used in connection with fabricating an improved information storage medium according to the first and/or the second aspect of the present invention.

These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 is a schematic illustration of an optical storage medium,

Fig. 2 illustrates an example of writing and rewriting data in the phase- composition layer,

Fig. 3 illustrates reflection variations after overwriting of (old) data,

Fig. 4 illustrates the relative reflection as a function of time between successive crystallisation events of an area for different phase-change compositions,

Fig. 5 illustrates the dependence of the maximum erase velocity as a function of Se-concentration,

Fig. 6 illustrates the effect of the Se-concentration on the modulation- reduction, and

Fig. 7 illustrates the decrease in reflection over time for the phase-change disks with different Se concentrations.

DESCRIPTION OF PREFERRED EMBODIMENTS

In Fig. 1 an example of an optical storage medium 10 is provided. The information layer, i.e. the layer comprising a phase-change composition 1 may be provided in a stack of layers 2. The information layer is also referred to as the phase-change layer. The stack of layers is supported by the carrier substrate 3, such as a polycarbonate plastic carrier substrate 3. The stack of layers may comprise such layers as dielectric layers 4, 5, a reflective layer such as an Al or Ag layer 6 and a label or protection layer 7. Light is directed through the polycarbonate plastic layer 3 and focused on the phase-change layer 1. It is pointed out that the invention is not limited to storage media of the type illustrated in Fig. 1, and the figure is provided only as an example.

In Fig. 2 an illustrative example of writing and rewriting data in the phase- composition layer is provided. In Fig. 2 schematic illustrations of a track is provided. Such a track would in a CD or DVD medium be a single track spiralling from the centre to the periphery of the disk. The track is illustrated as seen from above along the centre of the track. Only the track is illustrated for clarity reasons, however other layer would be present as well as described in connection with Fig. 1.

A blank disk would typically be provided where the phase-change layer is in a transparent crystalline state, a blank track 20 is illustrated in Fig. 2A. In Fig. 2B the disk is rotated and thus the track would moves under a write laser 21 as indicated by the velocity arrow 22. Information is written in the phase-change layer by writing opaque marks in the layer along the track. The marks would typically be areas of material in an amorphous structural phase, such areas may be created by raising the temperature in a desired layer to a predetermined temperature where an amorphous structural phase is created and maintained upon cooling down again. In the figure two groups of data marks are provided, a first group marked 23 and a second group marked 24.

The information provided in FIG, 2B may be provided at a first time instant, whereas the user of the storage medium at a later time instant may decide to provide new data, to update data, erase data, etc. In Fig. 2C only the data provided in the first group 23 should be maintained whereas the data in the second group 25 should be rewritten. The so-called direct overwrite (DOW) method may be used, where both the old marks are erased and new marks are written in the same passage of the laser spot. This is made possible by optimising the write strategy, where erase levels are applied in-between the write pulses. Data may be erased by recrystallising the amorphous areas by raising the temperature to a predetermined temperature where a crystal phase is created and maintained upon cooling down again and new data may be provided by writing amorphous marks in the desired areas. _^

However a problem may arise in connection with overwriting of old data. The problem may arise due to reflection variation between reflection from "fresh" recrystallised areas and "older" crystalline areas, this may lead to increased media noise and hence increased jitter. It may, however, also be due to different crystallisation kinetics from the unstable crystalline phase.

The reason of the increased jitter is illustrated in Fig. 3. In the figure pristine areas 30 surround areas where data are provided and erased. In the areas marked 32 and 33 data have been erased at a first time instant, i.e. the areas have been recrystallised at a first time instant. These area have a certain reflectivity. At a later time instant, the area marked 34 is recrystallised. However, the reflectivity from this area is different from, or more specifically higher than, the reflectivity from the areas marked 32 and 33 which were crystallised, or written, first. As a consequence when reading data, the reflectivity from different areas may depend upon when the area was recrystallised.

(Doped) Ge_xSn_ySbi_-x-y-compositions (0.03<x<0.30, 0.10<y<0.30) are suitable for high-speed phase-change recording, e.g. up to 16x DVD+RW. These compositions combine high crystallisation rates with excellent amorphous phase stability. The crystallisation speed can be controlled by the composition, especially by controlling the Ge- concentration.

However, the Ge-concentration may influence other factors and the concentration may be important for providing a stable crystalline reflection. This is shown in Fig. 4A where the relative reflection is illustrated as a function of the time between successive crystallisation events of an area. It is seen that the crystalline reflection decreases upon storage at room temperature for all Ge concentrations 40-43. The decrease in reflection is more pronounced for high Ge-concentrations. This may have influence on the overwrite jitter. Namely, after a given time period (a few hours/weeks/years) the crystalline reflection has dropped somewhat, but upon overwriting the data a crystalline phase with a high reflection is regained and will induce media noise.

How fast the reflection of the crystalline phase decreases depends on the phase-change composition, and according to the present invention the crystalline areas may be stabilised, i.e. reflection from areas of the crystal phase may be stabilised. This is illustrated in Fig. 4B which illustrates the relative reflection as a function of time between successive crystallisation events of an area for different phase-change compositions 400-405 and the influence from dopants. By doping with a few percents Se (1-10 %) 400 greatly reduces the reflectivity decrease. Besides increasing the stability of the crystalline phase, adding dopants like Se will influence the crystallisation rate, in a way similar as Ge.

In the following, different aspects about the influence of Se doping of a GeSnSb-based composition is provided.

In Table 1 crystallisation temperatures of the phase-change composition Ge^s- _X)Se_xSn₂₀Sb₆₅ is provided. These were measured on glass substrate at a heating rate of 10 K/s. The crystallisation temperature is defined as the point where 50% of the material has crystallised.

Table 1

These data suggests that the amorphous stability decreases when Se is substituted for Ge.

The influence of archival life stability has been evaluated for the materials comprising 5% and 7% of Se. The evaluation is based upon an extrapolation of an Arrhenius plot of the time for complete crystallisation in an isothermal measurement at temperature T. From the extrapolation it is predicted that written marks will be stable (less than 4% crystallised) at 55 ⁰C for more than 10⁹ years at 5% Se and 7% Se. These values have a large margin of error because of extrapolation, but are several orders of magnitude above the requirements. For the other two Se-concentrations, not enough data is available about the archival life stability, but preliminary results suggest the material with 3% Se is stable enough, while the 9% Se material may be not.

The dependence of the maximum erase velocity as a function of Se- concentration for a (Ge+Se)isSn2oSb₆₅ composition is illustrated in Fig. 5. As the Se- concentration increases, the modulation drops. This may be due to changes in the optical constants or smaller amorphous marks. Since for materials with a growth-dominated crystallisation mechanism, amorphous marks of smaller radius lead to a higher measured value of V_e,ma_x,the speed increase 50 as shown in Fig. 5 may consequently be too large. To correct for this, the effect of modulation on V_e,_max was measured on one material, and the observed relation 51 was used to correct the results. These corrected results are also shown.

Still, an increase in erasability with Se-concentration is observed.

The effect of the Se-concentration on the modulation-reduction is illustrated in

Fig. 6 where the relative modulation is illustrated as a function of the relative gab. Even though the curves 60-63 are closely spaced, careful inspection shows that more back growth is observed for compositions with more Se.

In Fig. 7 the decrease in reflection over time for the phase-change disks is illustrated for different Se concentrations. It can be seen that the reflection of the 9% Se disk decreases the least, followed by the 7% disk. The 3 and 5% disks show the largest decrease. By doping with 2% to 4% Indium (In) in addition to Se a similar trend is found. Disks with 4-4.5% Se show a smaller decrease in reflection than those with 2.5-3.5%

Se.

Although the present invention has been described in connection with preferred embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims.

In this section, certain specific details of the disclosed embodiment such as number of layers, type of media, specific element concentrations, etc., are set forth for purposes of explanation rather than limitation, so as to provide a clear and thorough understanding of the present invention. However, it should be understood readily by those skilled in this art, that the present invention may be practised in other embodiments which do not conform exactly to the details set forth herein, without departing significantly from the spirit and scope of this disclosure. Further, in this context, and for the purposes of brevity and clarity, detailed descriptions of well-known apparatus, circuits and methodology have been . omitted so as to avoid unnecessary detail and possible confusion. Reference signs are included in the claims, however the inclusion of the reference signs is for clarity reasons only and should not be construed as limiting the scope of the claims.

Claims

CLAIMS:

1. An information storage medium (10) comprising a carrier substrate (3) and a stack of layers (2) supported by the carrier substrate, wherein at least one of the layers in the stack of layers is an information layer (1), the information layer being a layer comprising a phase-change composition comprising Ge, Sb and Sn, the phase-change composition further being capable of supporting at least a first and a second structural phase and wherein the structural phase of at least one of the least two structural phases is stabilised over time by adding a dopant to the phase-change composition.

2. A medium according to claim 1, wherein the GeSnSb-based composition is on the form Ge_xSn_ySbi._x-y and wherein x is in the range 0.03 to 0.3 and y is in the range 0.1 to

0.3.

3. A medium according to claim 1, wherein the phase-change composition is further comprising In.

4. A medium according to claim 1, wherein the dopant is Se.

5. A medium according to claim 4, wherein amount of Se is in the range 1 to 10 percent.

6. A medium according to claim 1, wherein the at least first structural phase is a crystal phase and wherein the at least second phase is an amorphous phase.

7. A medium according to claim 1, wherein the first and second structural phases is having a first and a second reflectivity upon exposure to electromagnetic radiation for visual or near- visual radiation and wherein the stabilisation is a stabilisation of the optical (and/or structural) properties of at least one of the structural phases.

8. A medium according to claim 1, wherein the reflectivity from an area in the first structural phase is higher than the reflectivity from an area in the second structural phase for visual or near-visual radiation, and wherein a first area (32,33) is formed into the first structural phase at a first time instant and a second area (34) is formed into the first structural phase at a second time instant and wherein the time development of the relative reflection from the first and second area is below 1% per day.

9. A rewritable phase-change optical recording medium (10) comprising the information layer (1) according to claim 1.

10. A method of stabilising a structural phase in an information layer (1) in a storage medium (10), the method comprising the steps of:

- providing a carrier substrate (3)

- providing on the carrier substrate (3) a stack of layers (2), the stack of layers including the information layer, the information layer being a layer comprising a phase-change composition comprising Ge, Sb and Sn, the phase-change composition further being capable of supporting at least a first and a second structural phase, and wherein the structural phase of at least one of the least two structural phases is stabilised over time by adding a dopant to the phase-change composition.

11. Use of Se to stabilise a structural phase in a GeSnSb-based phase-change composition.