WO2006134530A1

WO2006134530A1 - Information layer with stabilized inclusions

Info

Publication number: WO2006134530A1
Application number: PCT/IB2006/051843
Authority: WO
Inventors: Joachim W. Hellmig; Liesbeth Van Pieterson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-06-14
Filing date: 2006-06-09
Publication date: 2006-12-21
Also published as: TW200703319A

Abstract

An information storage medium with stabilized inclusions comprised in an information layer and a method of stabilizing the inclusions in the information layer is disclosed. The 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 GeSbSn- based phase-change composition, the phase-change composition further being capable of supporting at least a first and a second structural phase, and wherein a shape of an inclusion of the second phase in the first phase is stabilized in a cooling process by having an atomic percentage of Sn in the phase-change composition in the range of 10 to 20%. The information storage medium may be an optical storage medium where the stabilization is a phase stabilization so that inclusions of an amorphous phase in a crystalline phase remain in their intended form, i.e. remain in the amorphous phase in the region as intended from the write laser irradiation.

Description

Information layer with stabilized inclusions

FIELD OF THE INVENTION

The invention relates to a rewritable information storage medium and to a method of stabilizing inclusions comprised in an information containing layer. The medium comprising at least one information layer in a stack of layers, the stack of layers comprising a phase-change composition layer. In particular the invention relates to stabilization of a shape of inclusions 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 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 in the information layer by way of formation of amorphous inclusions, or marks, and erased therefrom by crystallization of the amorphous marks. Information is read from a modulation of reflected light from the amorphous marks and the crystalline areas.

Recording speed is an important driver of innovation in recordable and rewritable storage media, and especially DVD products. The latest products and standards in the market include up to 4x speed for rewritable DVD and the rewritable media industry and the drive manufacturers are busy trying to increase the speed even further. In contrast to recordable (write-once) media, rewritable media must be capable of direct overwrite (DOW) at the highest recording speed. US patent 5,637,372 discloses a phase-change recording medium with a recording layer comprising: (Ag₅Au)(Sb₅Bi)(Te₅Se)(In₅Al₅P)(Si₅Ge₅Sn₅Pb) in specified atomic percentages, the medium being intended for high- velocity recording. When recording data in high- velocity phase change discs, it may occur that a small number of marks has a significant shorter length than the intended length, this induces problems in relation to readout of the recorded data. The inventors of the present invention have appreciated that in order to achieve higher erase velocities it is of benefit to provide higher crystallization speeds of the phase-change information layer, and have in consequence devised the present invention.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved information storage medium suitable for high-speed, i.e. multi-speed, recording. Preferably, the invention alleviates, mitigates or eliminates one or more of the above or other 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 GeSbSn-based phase-change composition, the phase-change composition further being capable of supporting at least a first and a second structural phase, and wherein a shape of an inclusion of the second phase in the first phase is stabilized in a cooling process by having an atomic percentage of Sn in the phase-change composition in the range of 10 to 20%.

The information layer may be part of stack of layers in a rewritable phase- change optical recording medium, such as a CD or DVD medium.

The information layer may be a layer comprising a phase-change composition capable of supporting at least a crystalline and an amorphous phase. The first structural phase may be a (poly)crystalline phase, whereas the second structural phase may be an amorphous phase. 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 only little light is reflected or transmitted from or by an area comprising the amorphous phase, or at least less light is reflected from the amorphous phase than from the crystalline phase, so that amorphous and crystalline areas may be distinguished optically. The phase-change composition may be a composition with a growth-dominated crystallization mechanism. The inclusions are also referred to as marks or optical effects, whereas the regions between marks are often referred to as spaces. In optical recording, data may be stored in inclusions and spaces of different lengths. The inclusions are on a CD medium or DVD medium aligned along a track spiralling from the centre and outwards. In a phase- change type disc, the inclusions are amorphous regions with low reflectivity, whereas the spaces are crystalline regions with high reflectivity. The stabilization may be seen upon as a phase stabilization so that inclusions of an amorphous phase in a crystalline phase remain in their intended form, i.e. remain in the amorphous phase in the region as intended from the write laser irradiation. 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 crystallization temperature crystallize in nanoseconds, in principle enabling such recording speed as linear velocities between 7 and 60 m/s, corresponding to 2x to 17x DVD-speed.

The phase-change composition comprises an atomic percentage of Sn in the range of 10 to 20%. The presence of Sn in the range of 10 to 20% in the phase-change composition is particularly but not exclusively advantageous since, in a cooling process, the shape of an inclusion of the second phase in the first phase is stabilized. When recording data in high-speed phase-change discs, it may occur that a small number of inclusions has a significantly shorter length. This is especially clear for carriers of 3T length, where it may be observed that a number of the 3 T inclusions have a length of period 2T. This effect is referred to as the '3T window error', generally indicating inclusion lengths lying outside their supposed length window. A possible cause of a resulting inclusion length shorter than the intended inclusion length may be that in the DOW process after the laser pulse has passed by, a nucleation event occurs in the still hot phase-change layer. When this nucleus grows, it erases part of the inclusion. By having Sn in the phase-change composition in the specified range, the nucleation and growth of crystalline areas in the created inclusions are impeded, thereby stabilizing the shape of inclusions as created in the DOW process. The GeSnSb-based composition may be on the general form Ge_xSn_xSb_{1 -x-y}.

The concentration of the various elements may be such that x is in the range of 0.1 to 0.2, such as in the range 0.12 to 0.18, such as in the range 0.14 to 0.16, and y is in the range of 0.1 to 0.2, such as in the range of 0.12 to 0.19, such as in the range 0.13 to 0.18, such as in the range 0.14 to 0.17, such as in the range 0.15 to 0.16. The recording properties, i.e. the kinetic of the structural phases, may depend upon the element concentration in a complex way. It may therefore be an advantageous to use the specified ranges to ensure optimum properties of the recording process.

The phase-change composition may further comprising an element selected from the group of In, Te, Ga, Se and S. The phase-change composition may comprise up to 5% of In, such as up to 4%, such as up to 3%, such as up to 2%, such as up to 1% of In. The phase-change composition may comprise up to 10% of Te, Se, S and/or Ga, such as up to 8%, such as up to 6%, such as up to 4%. The phase-change composition may be a GeSnΨΩSb-based composition on the general form Ge_xSn_yΨ_zΩ _wSb_1-x-y-z-w, with z up to 0.05 and w up to 0.1, where Ψ may be In and Ω may be Te, Ga, Se or S. Elements may be added to the phase-change composition for a number of reasons, e.g. Indium may be added in order to improve the reflectivity from the crystalline areas, and Te may be added to improve the shelf stability.

The optional feature as defined in claim 4 and 5 may be advantageous since, the kinetic of the structural phases, may depend upon the element concentration in a complex way. It may be advantageous to monitor the length distribution of the inclusions, or of a specific inclusion such as the 3 T inclusion, and determine the atomic percentage of Sn by optimizing the length distribution.

The optional feature as defined in claims 6 to 8 may be advantageous since the shape of the inclusion may be further stabilized by increasing the cooling rate of the phase- change composition. Being able to stabilize the shape of the inclusions by properly selecting the atomic percentage of Sn and by increasing the cooling rate of the phase-change composition facilitate optimal tuning of the recording properties of the storage medium in accordance with the large number of requirements to high-speed storage media. The optional feature as defined in claim 9 may be advantageous since by setting the erase level to a lowest allowed erase level, the minimum amount of power is provided to the phase-change composition, thereby facilitating cooling of the composition. The lowest allowed erase level may depend on the details of the stack of layers and it may be necessary to determine this level for a specific stack. Highly cooled stacks require higher erase levels than moderately cooled stacks. An example of a lowest allowed erase level for a typical stack may be such as 5 mW.

According to a second aspect of the invention is provided a method of stabilizing an inclusion in an information layer of an information storage medium according to the first aspect of the invention. 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. The first and the second aspect of the invention may be combined and coupled in any way possible within the scope of the 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 shows a schematic drawing of the 3T mark write strategy, FIG. 3 shows a distribution of measured 3 T mark lengths, FIG. 4 shows a TEM picture of 3T amorphous marks in a crystalline phase- change layer of GeInSnSb, FIG. 5 illustrates the effect of Sn-concentration with respect to the 3 T window error, and

FIG. 6 illustrates the effect of the I2-layer thickness with respect to the 3T window error.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 is an example of an optical storage medium 10 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. The dielectric layer being placed between the phase-change composition layer 1 and the metal layer 6 is also referred to as the 12 layer 4. Light 8 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 of a medium.

In its original state, the information layer of a RAV disc is in a polycrystalline reflective phase. During writing, a focused laser beam selectively heats areas of the phase- change material above the melting temperature (500-700 °C), so all the atoms in this area is reformed into an opaque amorphous state. If the phase-change layer is heated below the melting temperature but above the crystallization temperature (120-230 °C) for a sufficient time (at least longer than the minimum crystallization time), the atoms revert back to an ordered state (i.e. the crystalline state). The amorphous and crystalline states have different optical properties, such as different refractive indices, and can therefore be optically distinguished.

In FIG. 2 is shown a schematic drawing 20 of the 3 T inclusion write strategy. The laser power, P_L, is shown as a function of time, t. A write strategy defines the light pulse used to form the various inclusions (marks). The illustrated write strategy comprises a number of write parameters, four power levels (E, W, B, C) and a time duration for each. The laser starts with an erase level (E), then a writing pulse (W) is given, after which the power is reduced to bias level (B), in order to quench the phase-change material. Finally an erase pulse with power C is used to recrystallize a part of the amorphous mark in order to put the trailing edge on the right position. The puls shape of the write pulse W may be specified according to the pulse type (e.g. 3T mark, 4T mark etc.) by specifying the number of subpulses and the duration of each of the subpulses. It is pointed out that the invention is not limited to a write strategy of the type illustrated in FIG. 2, and the figure is provided only as an example of a write strategy.

FIG. 3 shows a distribution 30 of measured 3 T mark lengths, the distribution illustrates the '3T window error'. The majority of the mark lengths are comprised in the Gaussian part of the distribution 30, whereas a small fraction 32, 33 deviates from that.

In the distribution it is observed that about l%o of the (intended) 3 T marks is observed to have a length of period 2T whereas the rest lies in the Gaussian distribution. The exact number of 2T marks may depend upon several factors, such as upon the write strategy used to create the marks. FIG. 4 shows a transmission electron microscope (TEM) picture 40 of 3T amorphous marks in a crystalline phase-change layer of GeInSnSb. In the encircled area 41, a mark with a deviating shape is visible. A possibly scenario of the origin of the deviating shape is that after the laser pulse has passed by, a nucleation event of a crystalline nucleus has occurred in the still hot phase-change layer. When this crystalline nucleus 42 grows, it erases part of the amorphous mark. For materials with a crystallization mechanism dominated by fast crystal growth, as for GeInSnSb, nucleation probability is relatively low. Nevertheless it is important to impede recrystallization of the marks in order to ensure optimum retrieval of the recorded information. FIG. 5 illustrates the effect of Sn-concentration with respect to the 3 T window error in a GeSnSb phase-change disc. The number, N(2T), of 2T counts of 80000 3T marks is shown as a function of erase power between write pulses, Pe, of the recording laser (label E, FIG. 2). The erase power being a measure of the temperature in the phase-change composition in the recording process. Consequently, the graph also illustrates the 3T window error as a function of temperature in the phase-change layer. Three graphs are shown, one for a Ge₁₅Sn₁₅Sb₇₀-composition 51, a Ge₁₅Sn₂₅Sb₆₀-composition 52 and a Ge₁₅Sn₂₀Sb₆₅- composition 53. It is seen that the 3 T window error is more pronounced at higher erase powers (higher temperature), but smallest for the composition with the lowest amount of Sn present in the phase-change composition. At lower Sn-concentration (<10%) the crystalline structure of the phase-change material may change (from cubic to rhombohedral) and no homogeneous reflectivity of the crystalline phase can be obtained. At higher Sn- concentration (>20%) the bit error rate is high because of the 3 T window error effect.

FIG. 6 illustrates the effect of enhanced cooling of the phase-change layer with respect to the 3T window error in a GeSnSb phase-change disc. As in FIG. 5, the number of 2T counts of 80000 3 T marks is shown as a function of erase power between write pulses. Two graphs are shown both for a Ge₁₃Sn₂₀Sb₆₇-composition, and one for an I2-layer (reference numeral 4, FIG. 1) thickness of 18 nanometer 61 and one for an I2-layer thickness of 13 nanometer 62, the thickness of 12 includes the thickness of the capping layer of 3 nm. It is seen that by reducing the layer thickness the 3T window error is further decreased due to enhanced cooling of the phase-change layer.

It is seen from the graphs shown in FIGS. 5 and 6 that effective cooling of the phase-change material is important. Effective cooling can be achieved either by optimization of the write strategy or by optimizing the phase-change stack. In the write strategy, the applied erase power should be as low as possible without sacrificing erasability. Cooling of the phase-change stack can be varied by either the thickness or thermal properties of the 12 dielectric layer, or of the metal layer (reference numeral 6, FIG. 1). The thickness of the 12- layer, e.g. ZnS or SiO₂, may advantageously be chosen between 3 and 15 nm, such as between 5 and 11 nm, such as between 7 and 10 nm. This thickness includes a possible capping layer, e.g. Si₃N₄. The thickness of the metal layer, e.g. Ag, may advantageously be chosen to be larger than 120 nn, such as between 150 nm and 250 nm.

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 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 only for clarity reasons 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 GeSbSn-based phase-change composition, the phase-change composition further being capable of supporting at least a first and a second structural phase, and wherein a shape of an inclusion (41) of the second phase in the first phase is stabilized in a cooling process by having an atomic percentage of Sn in the phase-change composition in the range of 10 to 20%.

2. A medium according to claim 1, wherein the GeSnSb-based composition is on the form Ge_xSn_xSb_{1 -x-y} and wherein x is in the range 0.1 to 0.2 and y is in the range 0.1 to 0.2.

3. A medium according to claim 1, wherein the phase-change composition is further comprising an element selected from the group of In, Te, Ga, Se and S.

4. A medium according to claim 1, wherein a structural phase change in the composition is obtained by modulating a light intensity (20) of at least a strong and a weak level, so that the inclusion is created in response to a light beam incident on the phase-change composition, and wherein the atomic percentage of Sn is determined so as to stabilize the length of an inclusion.

5. A medium according to claim 4, wherein the length of a 3 T inclusion is stabilized.

6. A medium according to claim 1, wherein the shape of the inclusion is further stabilized in the cooling process by increasing the cooling rate of the phase-change composition.

7. A medium according to claim 6, wherein the stack of layers includes a dielectric layer (4) and a metal layer (6), the dielectric layer being placed between the phase- change composition layer (1) and the metal layer, and wherein the cooling rate of the phase- change composition is increased by having a thickness of the dielectric layer in the range of 3 to 15 nm.

8. A medium according to claim 6, wherein the wherein stack of layers includes a dielectric layer (4) and a metal layer (6), and wherein the cooling rate of the phase-change composition is increased by having a thickness of the metal layer larger than 120 nm.

9. A medium according to claim 1, wherein a structural phase change in the composition is obtained by modulating a light intensity according to a write strategy (20) comprising an erase level (E), and wherein the shape of the inclusion is stabilized in the cooling process by setting the erase level to a lowest allowed erase level.

10. A method of stabilizing an inclusion (41) in an information layer (1) of an information storage medium (10), the medium 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, the information layer being a layer comprising a GeSbSn- based phase-change composition, the phase-change composition further being capable of supporting at least a first and a second structural phase, and wherein a shape of an inclusion of the second phase in the first phase is stabilized in a cooling process by having an atomic percentage of Sn in the phase-change composition in the range of 10 to 20%.

11. Use of Sn in the range of 10 to 20% to stabilize a shape of an inclusion in an information layer of an information storage medium.