WO2006045332A1

WO2006045332A1 - Mastering process with phase-change materials

Info

Publication number: WO2006045332A1
Application number: PCT/EP2004/012156
Authority: WO
Inventors: Mark Edward Julian Van Der Veer; Roland Jozef Custers
Original assignee: Singulus Mastering B.V.
Priority date: 2004-10-27
Filing date: 2004-10-27
Publication date: 2006-05-04

Abstract

The invention provides a method for manufacturing an optical information carrier, in particular for producing a master disc by recording information comprising information units in the form of a pattern of geometrical structures in a stack of at least a recording layer (2) formed on a surface of a substrate (1). The recording layer comprises a phase-change material, in particular GeInSbTe. In the method of the invention, the recording layer is exposed to a laser beam to form an exposure pattern in the recording layer and the exposed layer is developed for creating the pattern in the recording layer corresponding to the exposure pattern.

Description

Mastering Process with Phase-Change Materials

The invention relates to a master disc for optical media and particularly to a mastering substrate and a method of manufacturing the master disc using the mastering substrate.

Today, the standard technology for making master discs for optical media uses a lithographic process based on an organic photoresist. This process is described, for example, in EP-A-O 660 314 disclosing a method for manufacturing optical information carriers, in which a photoresist layer on a master disc is exposed by a radiation beam. The photoresist mastering process is based on a photochemical process that takes place in the photoresist when it is illuminated with a focused laser beam. Every photon entering the photoresist has a certain chance of inducing a chemical change in the photoresist. This implies that every photon in the writing spot can result in a chemical change and that the region where chemical changes take place is theoretically infinite. Photoresist also has a memory: chemical changes cumulate when the photoresist receives multiple exposure doses. This means that earlier induced chemical changes increase due to, for instance, inter symbol interference and cross talk during mastering.

One of the problems of mastering using a photoresist is that it is difficult to make structures smaller than the width of the recording laser beam. Various tricks exist to solve this problem for photoresist, but they also introduce new shortcomings like enhanced sensitivity for laser intensity variations. With the continuous reduction of features sizes used in optical media to gain higher densities, the limits are reached what can be realized by using organic photoresist in combination with optical recording systems. Today laser beam recorders are used using lasers in the deep-UV regime (260 nm) and objectives with a high Numerical Aperture (NA) of 0.9 which is a practical limit. Systems using liquid immersion lenses are also under development to increase the NA above 1.0.

Recently, a new mastering technology was introduced which is based on a thermal recording process making it possible to use less sophisticated laser beam recording systems. In WO-A-2004/034391, such a process is described. The laser light is absorbed by the sensitive layer and the heat induced results in a change of the material properties. In WO-A-2004/034391, the use of incompletely oxidized transition metals as a recording material is introduced. Due to the heat of the absorbed laser light the state of the incompletely oxidized transition metal is changed. The changed material has a higher solubility for an alkaline solution making it possible to create a pit or a groove structure.

A disadvantage of this material is a complex deposition process in order to control the exact oxygen concentration.

It is thus an object of the present invention to overcome the above mentioned problems of the known processes and to provide an improved method of manufacturing an optical information carrier and particularly a master disc.

These objects are achieved with the features of the claims.

In the present invention, a stack of at least a recording layer formed on a surface of a substrate, wherein the recording layer comprises a phase-change material (PCM) is used for manufacturing an optical information carrier, in particular a master disc. As described above, the known photoresist mastering process is based on a photochemical process that takes place in the photoresist when it is illuminated with a focused laser beam. Exposing a PCM layer results in a fundamentally different process. If a certain amount of energy is put into the layer, a physical change occurs. The structure of the material will either change from amorphous to crystalline or vice versa, depending on the initial state of the material. This change results in a difference in solubility, between exposed and unexposed areas, in selected fluids. This physical change occurs only at a certain temperature level in the material. The temperature of the material depends on the photon flux into the material and on the material properties. A very small region of the exposure spot can be used for this physical change, as it is a temperature induced process. This property gives this material its high resolution. When using PCM for mastering a higher resolution at the same mastering wavelength can be obtained, when compared to a photoresist mastering process. The present invention thus allows to make mastering of high-density masters. In the method of the present invention a substrate, e.g. glass, Si-wafers or metal shells with or without additional layers to influence the recording process, is used with a recording layer on top of it which comprises a phase-change material, like e.g. GeInSbTe. Other phase change materials, like GeSbTe, AgInSbTe or Ag-doped GeSbTe may be used. This recording material can be easily deposited on the support layer, is stable and provides the advantages of thermal recording. This material is known as a material for rewritable optical media where only the change in reflection is used for read-out when the material changes from a crystalline to an amorphous phase (or vice versa) in the recording process (see, e.g., US-A-5,789,055).

The recording layer is exposed to a focused laser beam, having a wavelength depending on the absorption spectrum of the material used. The wavelength may be in the range of 250 to 460 run or in an even broader range; as a specific example, 405 nm may be useful. The at least one recording layer may be etched and developed with an alkaline solution, e.g. NaOH, or an acidic solution, e.g. nitric acid, after exposure. Before electroforming the substrate, the substrate can further be pre-treated in order to make cleaning of the substrate after electroforming easier.

Since the phase-change is associated with a change in reflectivity it is also possible to introduce some process control functions that are not possible with organic photoresists.

First of all it is possible to verify whether the required effect is written by making use of the reflected light of the write laser (see for example US-A-5, 504,732). Also a second laser can be used to read the just written effects. This is generally called Direct Read After Write

DRAW. The calibration of the writepower can be done in an area on the disc that is not used for the recording. This can be inside the recording area or just outside the recording area, e.g. at places that are within the centerhole of the stamper or just outside the outer perimeter of the replicated media.

This so-called power calibration is done as follows: In the areas that are not used for recording the format specific information one can do some recording tests. This is close to the center of the disc till a radius used for lead-in or other information, or beyond the lead- out area, i.e. in general at a radius > 60 mm. A short recording is made at various laser- powers and/or various write-strategies. Since the change from amorphous to crystalline (or vice versa) gives a change in reflection of the material, it is possible to read the written marks and to perform an analysis. In this case the exact length of the written marks will be an important quality parameter. Based on the outcome of the analysis, the write-power and/or write-strategies are determined for recording the master.

With the DRAW method it is possible to monitor the change in reflection of the written marks immediately after recording. In general, there are two possibilities, which are both known in other mastering/recording applications. First, the normalized reflected light of the write-laser itself can be used. When one starts writing a mark, the major part of the spot is on an unrecorded area, some time later the spot is on a recorded part. This gives a time- dependent change in reflected light that is related to the length and width of the recorded mark (see also US-A-5,504,732).

The second method is using a separate read-laser at low intensity (below the recording level) or with a wavelength for which the material is insensitive. This read spot is positioned behind the write-laser. By analysis of the retrieved signal, in a similar way as is done for power calibration, it is possible to correct the write-laser for small changes in material characteristics or to abort the recording once an error is detected.

In the following, the invention is described in more detail with reference to the Figures.

Fig. 1 shows a flowchart of the mastering process according to the invention.

Fig. 2 shows a phase-diagram of a material comprising Ge, Sb, and Te in different ratios.

Fig. 3 schematically shows an embodiment of a stack of recording layers for a master disc according to the invention. Fig. 4 is a diagram showing the pit/groove depth versus the width at various development times at different exposure doses.

Fig. 5 schematically shows a further embodiment of a stack o f recording layers according to the invention (a) before and (b) after processing with the mastering method of the invention.

Fig. 6 shows a stack of recording layers according to the invention after being developed and etched.

Fig. 7 (a) to (d) schematically show different steps of a method for directly producing a stamper according to the invention.

As shown in Fig. 1, the process according to the invention comprises the steps of A) preparing the support layer

B) applying the recording layer(s) to the support layer

C) recording the substrate

D) developing and etching the substrate

E) pre-electroforming and electroforming and F) stamper cleaning and finishing.

The individual steps of the process will be described in detail in the folloΛving.

In step A), the substrate should be cleaned well to increase the adhesion between the substrate and the recording layer(s) and to improve cosmetic appearance. The procedure in case of a glass-substrate can be a treatment wherein the substrate is cleaned and subsequently brushed and watersprayed. The adhesion can be improved even more by adding a metal layer between the support layer and recording layer(s). An example is the use of a thin NiV layer between a glass support layer and a GeInSbTe la;yer. The application of the recording layer(s) in step B) may be carried out using different known application methods depending on the type of master. Usually, the PCM is deposited by vacuum deposition, in particular by sputtering.

A phase-diagram of a material comprising Ge, Sb and Te is shown in Fig. 2. Usually, the materials along the phase line GeTe-Sb2Te3 are used for DVD-RAM because of high crystallization speed and precise mark formation because of easier control of the recrystallization process. These materials are using lower power for recording, for example around 1OmW at 650nm. The other material class at SbγoTe3θ is mostly used for DVD+RW or DVD-RW because the crystallization speed could be controlled by the ratio of Sb/Te. Due to overwriting, DVD+RW and DVD-RW materials disadvantageously degrade which results in an impaired control of the recrystallization. The typical recording power compared to the DVD-RAM material is around 2OmW. In general all these materials are "growth dominated" materials only, the influence of the recrystallization/formation of the marks is driven once by grain boundaries or nuclei. The recrystallization at the mark edge results in a somewhat smaller mark.

The material which is preferably used in the present invention is a DVD+RW 2.4x target with Ge3 4-In4 0-Sb70_,6-Te22.0 ^at%- The crystallization temperature for the material is 180°C and the crystallization time is around 2 μs.

The additional components of Ge and hi, Sn, Bi or others are used to tune the optical properties and the writing and erasing behavior of the material. The amount of Ge determines the erasability of the material, i.e. the power range to recrystallize the material. In general the erasing power is half of the power used to record amorphous marks in a crystallized disc. The other materials like In stabilize the crystal structure and influence the archival lifetime of the disc. In the DVD+RW materials, the minimum pit length which could be achieved is determinated by the crystallization speed which is controlled by the Sb/Te ratio and the writing power as well the optimum pulse length for writing at a defined recording speed. The minimum pit size which could be achieved is connected to the composition of the phase change material. A type of master that has pits or grooves with a depth of 0 to 40 nm can be mastered using a recording stack given in Fig. 3. The stack given in Fig. 3 comprises a layer 3 of a metal adhesion promoter (MAP) and a layer 2 of a phase-change material (PCM) on a support layer 1. The MAP layer preferably has a thickness of less than about 100 nm. The PCM layer 2 as a thickness of about less than 40 nm corresponding to the desired depth of the pits or grooves. The stack shown in Fig. 3 is, however, not adequate to master Blu-ray disc ROM (BD-ROM) masters but can be sufficient for other formats (with higher densities) that require pits that are not so deep.

BD-ROM masters have a pit depth of 75 nm. Trie absorbance of light by phase-change materials is in general high, which results in a situation where it is not possible to master pits with depths in the vicinity of 80 nm. This situation is illustrated in Fig. 4. In Fig. 4, the pit/groove depth versus the width at development times of 30 min, 45 min and 55 min is shown. The vertical solid line in Fig. 4 is the required pit width for BD-ROM. The dotted horizontal line is the required pit depth for BD-RDM. The intersection between the lines is the position where both depth and width are at tlieir required values. It is clear that the requirements of BD-ROM cannot be achieved wittα a PCM layer alone.

With the recording layer stack shown in Fig. 5 this problem can be overcome. The stack of this embodiment comprises an adhesive layer 3, a layer 4 of a selective etch material and the PCM layer 2. The adhesive layer 3' preferably has a thickness of less than about 40 nm. The layer 4 of the selective etch material has a. thickness of at least x - tpc_M, wherein x is the required pit depth and tpc_M nm is the thickness of the PCM layer 2. The PCM layer 2 is used as an etch mask for the selective etch material below the PCM layer 2, in the stack configuration given in Fig. 5(a). The property of^" the PCM that only a small part of the writing spot is being used for physical conversion of the material is the foundation for applying such a stack. Only a small part of the selective etch material will be exposed to the etching material, e.g. fluid or plasma, because of the high resolution of the PCM. Fig.

5(b) gives a schematic presentation of how such, a stack would look like after exposure, after dissolution of the locally exposed PCM and after etching of the selective etch material. Exposing a substrate with recording layer(s) comprising a PCM is done in step C) as shown in Fig. 1. A regular laser beam recorder with for instance a 405 nm laser can be used for this purpose. Exposure with the laser results in a change in the physical state of the PCM. The change is either from an amorphous- to crystalline composition or from a crystalline- to amorphous composition, depending on the initial state. For instance, GeInSbTe PCM changes locally from amorphous to crystalline during exposure. Write strategy is important for recording these materials to control crystal growth or the melting process. A mark is typically made with a single pulse in an amorphous material and it is typically made with a pulse train in case of a crystalline material.

Developing in step D) is done in that the exposed substrate is treated in a way that the structures are created in the material. It may take several solvent- and/or plasma etch steps.

Fig. 6 shows the result after exposing, dissolving PCM and etching of the substrate. In the case shown in Fig. 6, the stack only comprises a PCM layer 2 on a substrate 1. The exposed parts of the PCM layer 2 can be dissolved with an alkaline solution, like e.g. a 10% NaOH solution. Substrates can be etched with either a plasma, e.g. O₂ plasma for glass and silicon, or with a fluid, e.g. HF solution for glass or KOH for silicon.

It is thus possible to write crystalline marks in an amorphous layer which is developed by an acid. Writing amorphous marks in a crystalline layer is also possible, however, in that case an alkaline solution is used for developing the recorded marks. This means that by selecting the right developer in combination with the written marks, it is also possible to realize a negative process which is described below with reference to Fig. 7.

Furthermore, it is possible that at recording only part of the material is changed into the other phase. In such a mixed phase, the development is possible by either one of an alkaline or an acidic solution.

In step E), electro forming is generally done in the conventional way. The master is plated in a nickel-sulfamate bath. The plated nickel layer is grown to a thickness of 0.3 mm. After plating, this nickel layer is separated from the master and is called the stamper. A stamper is used in an injection molding machine for mass replication of media. One of the main differences compared to photoresist mastering is that no conductive layer has to be sputtered on the PCM. The PCM is conductive by itself when it is in its crystalline phase. Therefore, the PCM can be made crystalline in a pre-electroforming step, if its initial state is amorphous. The PCM can be for instance baked at a temperature where it crystallizes, for instance in case of GeInSbTe at about 2OO ⁰C. The PCM can also be exposed to IR light to achieve the desired temperature rise in the PCM. A second advantage of this pretreatment step is that it is easier to remove the PCM from the stamper during the stamper cleaning step F).

In this step, cleaning of the stamper which results from the previous steps is carried out by dissolving the PCM in an alkaline solution, e.g. 10% NaOH. Oxidized Ni, which can be formed during this treatment, can be removed by sulfamic acid. Stamper finishing comprises polishing and punching in a conventional way.

With the method of the invention, it is also possible to directly make a stamper. In that case a metal substrate with a thickness of 0.3 mm is preferred. This is the standard thickness of a stamper produced by electroforming.

On the metal substrate the phase-change material can be coated when features below about 40 nm are required. Optionally an adhesion promoter is used. For higher structures it is preferred to use the developed phase-change structure as an etching mask. Etching can be done in a metal layer, for example Ni, or an organic layer that can be crosslinked to a hard material after etching and/or development. In Fig. 7, different steps of the direct stamper production according to the invention are shown. This process is also referred to as "negative process". First, a stack comprising a metal substrate 1, an adhesion layer 3', an etching layer 4, and a recording layer 2 is provided. In Fig. Υ{Ά) the unrecorded mastering substrate is shown. Fig. 7(b) illustrates the recording of a recording layer with the phase transition. After recording, the phase-change layer is develop ed (Fig. 7(c)) and the etching layer is etched (Fig. 7(d)). hi case, the optional etching layer is used, it is further possible to remove the phase-change layer after the etching process. Above, an embodiment of the mastering process according to the invention is described in a general way. In the following, detailed examples of the mastering processes with a phase- change material of the invention are given.

Example 1

In this first example, the different steps of the mastering process of the present invention described above comprise the following:

A) Preparation of the support layer o Cleaning glass support layer in Glass Disc Recycler (GDR) o Brush support layer with water.

B) Applying Recording Layer(s) o Sputtering of 70 nm NiV on glass support layer. o Sputtering of 30 nm GeInSbTe on. NiV resulting in an amorphous phase of the sputtered layer.

C) Recording (the GeInSbTe is in an amorphous phase before recording) o Linear velocity v = 3.5 m/s. o Laser power P = 0.4 mW. D) Developing and substrate etching o Substrate developed in 10 g/1 NaOH solution for 30 min.

E) Pre-electroforming treatment o Baking of developed substrate on hotplate at 200 °C for 20 min. Electroforming o Rinsing of substrate with sulfamic acid and DKY (surfactant), o Electroplating in nickel sulfamate bath, o Separating the electroplated stamper from the master disc.

F) Stamper cleaning and finishing o Immersion of stamper in 10 g/1 NaOH solution for 30 min. o Rinse substrate with sulfamic acid, o Rinse substrate with water for 30 s. o Spin dry substrate, o Apply protective coating on stamper. o Polish and punch stamper.

Example 2

The steps of the mastering process according to the invention in this Example 2 comprise the following:

A) Preparation support layer o Cleaning glass support layer in Glass Disc Recycler (GDR). o Brush support layer with water.

B) Applying Recording Layer(s) a. Apply N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane. b. Spincoat photoresist on diaminosilane layer. c. Bake photoresist on hotplate for 5 min at 95 ⁰C d. Sputtering of 20 nm GeInSbTe on photoresist.

C) Recording (the GeInSbTe is in an amorphous phase before recording) a. Linear velocity v = 3.5 m/s. b. Laser power P = 0.3 mW.

D) Developing and substrate etching a. 10 min. etch in 5% nitric acid. b. 1 min developing in 0.25% buffered NaOH solution.

E) Pre-electroforming treatment a. 10 min. exposure 10 W infrared light. b. Rinsing of substrate with sulfamic acid and SNAP-AM (surfactant). Electroforming c. Electroplating in nickel sulfamate bath. d. Separating the electroplated stamper from the master disc.

F) Stamper cleaning and finishing a. Immersion of stamper in 5% nitric acid for 20 min. b. Rinse with water. c. Spin dry substrate. d. Apply protective coating on stamper. e. Polish and punch stamper.

Although the invention has been described and illustrated with reference to specific embodiments and examples, it is not intended that the invention be limited to those embodiments or examples. Those skilled in the art will recognize that variations and modifications can be made without departing from the scope of the invention as defined by the claims.

Claims

Claims:

1. A method of manufacturing an optical information carrier, in particular for producing a master disc by recording information comprising information units in the form of a pattern of geometrical structures, e.g. pits, grooves, bumps, and/or banks, in a stack of at least a recording layer (2) formed on a surface (Ia) of a substrate (1), with the following steps:

(a) exposing the recording layer (2) to a laser beam directed and modulated according to the information to be recorded to form an exposure pattern in the recording layer (2), and

(b) developing the exposed recording layer (2) for creating the pattern in the recording layer (2) corresponding to the exposure pattern,

wherein that the recording layer (2) comprises a phase-change material.

2. The method of claim 1, wherein a layer (3) of a metal adhesion promoter (MAP) is arranged between the substrate (1) and the recording layer (2).

3. The method of claim 1, wherein a layer (4) of a selective etch material is arranged between the substrate (1) and the recording layer (2).

4. The method of claim 3, wherein an adhesive layer (3') is arranged between the layer (4) of selective etch material and the substrate (1).

5. The method of claim 1, 2, 3, or 4, comprising the step of:

(c) etching the material underlying the exposed and subsequently developed areas of the recording layer (2).

6. The method of claim 1, 2, 3, or 4, further comprising a step (d) of electrofoπning the stack after step (b) to produce a stamper.

7. The method of claim 5, further comprising a step (d) of electroforming th.e stack after step (c) to produce a stamper.

8. The method of claim 6 or 7, comprising a step of an electroforming pre-treatment prior to the electroforming step (d) for making the phase-change material crystalline.

9. The method of claim 8, wherein the electroforming pre-treatment step is done by baking the stack at a temperature where the phase-change material crystallizes.

10. The method of any one of claims 6 to 9, further comprising the step of finishing the stamper after step (d).

11. The method of claim 10, further comprising the step of cleaning the stamper before finishing.

12. The method of any one of the preceding claims, wherein the laser beam has a wavelength of about 250 to 460 nm, preferably 400 to 410 nm and more preferred

405 nm.

13. The method of any one of the preceding claims, wherein said phase-change material is GeInSbTe, GeSbTe, AgInSbTe or Ag-doped GeSbTe.

14. The method of any one of claims 1 to 4, wherein a stamper is directly produced.

15. The method of claim 14, wherein the substrate is made of a metal.

16. The method of any one of the preceding claims, wherein said at least one ozf the at least one recording layer (2) has a thickness of less than about 40 nm.

17. The method of any one of claims 7 to 16, wherein said substrate is for manufacturing a Blu-ray ROM master disc or a Blu-ray ROM stamper.

18. The method of any one of the preceding claims, wherein the information recorded is controlled based on the recorded information read directly after recording.

19. The method of claim 18, wherein the recorded information is read by detecting reflected light of the laser beam.

20. The method of claim 18, wherein a second laser beam is used to read the recorded information.

21. The method of any one of the preceding claims, wherein before step (a) a write- power and/or write-strategy of the laser beam is determined based on an analysis of a recording made at various write-powers and/or write-strategies in an area of the recording layer (2) which is not used for recording format specific information.

22. A master disc for the manufacture of an optical medium, the master disc being obtainable with the method of any one of the preceding claims.