Many types of optical recording elements are known.

Some of these can be recorded in real time and then played back at any time; an example of such an element is the CD-R. These can be played back using a conventional CD player. CD-R technology is described in numerous prior patents, for example in US Patents Nos. 5,090,009; 5,080,946; 5,079,135; 5,075,147; 5,009,818; 4,990,388; and 4,577,291.

It is desirable for optical data storage elements to be compatible with the existing range of reading devices, e. g. CD players. It is therefore desirable that the properties of the storage element match the requirements of the playing equipment. This, in turn, means that parameters such as the substrate material, the substrate groove geometry, the dye used in the recording layer, and the depth of the recording layer at the grooves and lands need to be carefully controlled in order to maximise the performance characteristics of the optical storage elements.

Writable optical recording media are generally prepared by spin coating a dye solution on a grooved polycarbonate substrate. The coated substrate is then dried, and the resultant dye layer constitutes the recording layer. Typically, the recording layer is then overcoated with a reflective layer and with a

protective layer.

The selection of the solvent used to dissolve the dye is important because it has been found to have a significant effect on the final geometry of the recording layer. As well as possessing the requisite affinity for the dye, the solvent should not damage the substrate. This limits the range of solvents that are available. For example, l-methoxy-2-propanol is undesirable as a solvent because it attacks the polycarbonate substrate.

Numerous solvents have been suggested for use in the process of depositing a dye onto the surface of the grooved substrate during fabrication of an optical data storage element. For example, EP 0 511 598 (Mitsui Toatsu Chemicals) relates to optical recording media in which a nonpolar solvent-soluble substituted phthalocyanine dye is dissolved in a suitable solvent in the coating step, the solvent preferably having a solubility parameter of less than 8.5. The essential characteristic of the invention as disclosed in this document is that the finished product should contain not more than 5% by weight nonpolar solvent remaining in the recording layer. A long list of solvents is given in this document, and the possibility of using a mixture of solvents is contemplated. There is no disclosure in this document of any requirement in relation to the use of mixed solvents other than that the solubility parameter of the mixture should be less than 8.5.

According to one aspect of the present invention, there is provided a method of producing an optical data storage medium, in which a dye is deposited out of solution onto a grooved surface, characterised in that the extent to which the grooves are filled with dye is

controlled by formulating the solvent from which the dye is deposited so that it comprises a major constituent and a minor constituent, the major constituent being an alkane, an alcohol or an ether and being present in an amount by volume of at least 80%; and the minor constituent being an alkane, an alcohol, an ether or a ketone; and the boiling point of the minor constituent being higher than that of the major constituent.

According to another aspect of the present invention, there is provided a method of coating an optical recording layer on a transparent grooved support, comprising the steps of: (1) forming a solution of a laser recording dye, or dye mixture, in a solvent which is a mixed solvent containing a major constituent and a minor constituent; (2) coating the solution on the grooved surface of the support, characterised in that: (a) the major constituent is an alkane, an alcohol or an ether and is present in an amount by volume of at least 80%; (b) the minor constituent is an alkane, an alcohol, an ether or a ketone; and (c) the boiling point of the minor constituent is higher than that of the major constituent.

We have found that the selection of the components of the solvent as described above makes it possible to

produce optical data storage media conforming to the required industry standard specification; it also makes it possible to control the manufacturing process more reliably. This in turn leads to more uniform product quality.

We have also found that deep-filled grooves, which can be achieved by the method of this invention, lead to finished media which have improved reflectivity and increased sensitivity. Better control of tracking parameters is also achieved.

Preferably, the minor constituent is present in an amount of at least 0.5%. It is presently preferred to employ the minor constituent in an amount (by volume) of 0.7%-5.0%, more preferably 0.8%-3.0%. It is also preferred that the minor constituent be more polar than the major constituent.

The dye will generally be a cyanine or phthalocyanine dye; other dyes used in optical recording media, e. g. azo dyes, may also be used.

The dye solution may contain conventional additives, e. g. stabilisers, binders, surfactants and diluents.

The amount of dye present in the solution will generally be less than 10 grams of dye per 100 ml of solution; typically, there will be from 1-5 g dye per 100 ml solution.

The coating process used will normally be a spin coating process, using speeds of 200-3000 rpm for times of from 5 to 20 seconds.

After formation of the dye layer, a metal reflective layer is preferably deposited over the dye; this may be

achieved by sputtering or by resistive heating under reduced pressure. Suitable metals include platinum, gold, silver, copper, aluminium and alloys thereof.

A protective layer is preferably applied over the metal reflective layer as is conventional in the art.

The surface of the substrate can have an additional buffer or heat deformable layer.

According to a further aspect of the invention, there is provided an optical data storage medium which has been produced by a method as defined above.

According to a further aspect of the present invention, there is provided an optical data storage medium which comprises a grooved substrate having a dye layer deposited thereon, a reflective layer overcoating said dye layer and a protective layer over said reflective layer, wherein (i) said dye is a phthalocyanine dye; and (ii) the depth of said dye layer (a) within the grooves is in the range 50-150 nm, and (b) on the lands (i. e. between the grooves) is in the range 10-100 nm.

Preferably, an optical data storage medium (ODSM) in accordance with this invention comprises a polycarbonate substrate having grooves which are 80-300 nm in depth, more preferably 110-250 nm; and which are from 100-800 microns (um) wide, more preferably 300-600 pm wide. After coating, the resultant depth of dye within the grooves is preferably in the range 50-150 nm and the depthe of dye on the lands is preferably 10-100 nm, more preferably 10-60 nm.

When an ODSM of this invention is used, the recording process generates marks which have lower reflectivity

than the unmarked areas of the ODSM. Recording may be effected with a light source at, for example, 780 nm.

The ODSM may be read using a similar light source, but at a lower power.

Examples of suitable solvent constituents are given in Table 1 below, in which: MCH = Methylcyclohexane; DMCH = Dimethylcyclohexane; DMH = 2,6-Dimethylheptan-4-one; and ECH = Ethylcyclohexane.

Table 1 Solv Major B. Pt B. Pt Minor Constituent ent Constituent °C °C 1 MCH 101 142-Di-n-butyl ether 143 2 MCH 101 124 DMCH 3 MCH 101 130-ECH | 132 4 MCH 101 169 DMH 5 MCH 101 155 Propyl cyclohexane 6 MCH 101 169 2,6- dimethylpentanone 7 MCH 101 135 2-Ethoxyethanol 8 MCH 101 166 4-hydroxy-4- methylpentanone 9 n-propanol 97 197-2-nonanol 199 10 n-propanol 97 196 1-octanol 11 n-propanol 97 214 1-nonanol 12 n-propanol 97 169 DMH 13 Di-n-butyl 142- 169 DMH ether 143 Other solvents useful as the major constituent include ethylcyclohexane; and as the minor constituent, 2- butoxyethanol (B. Pt. 169°C) and 2-phenoxyethanol (B. Pt.

237°C). 1-Methoxy-2-propanol is not recommended as the primary constituent because it can damage the

polycarbonate substrate.

The invention will be illustrated by the following Examples: Example 1 The relationship between solvent composition and dye deposition was investigated using solutions of a phthalocyanine dye (HW 642/24) whose formula is: in which R = di-isobutyl (i. e. 2,4-dimethyl-3-pentyl); and X = H or Br, there being approximately three Br atoms per molecule. These solutions were coated onto Plasmon's substrates P3359 and P3348. These are polycarbonate disk masters having a grooved surface structure; in each case the sidewalls of the grooves are generally linear and have a wall angle of 70° (close to vertical); the grooves are thus in the form of a truncated"V"-shape. P3359 has grooves which are

180 nm deep while P3348 has grooves which are 150 nm deep. The width of the grooves is, in each of these disks, dependant on the radial position of the groove; the groove widths (as determined by scanning electron microscope) are shown in the following Table 2: Table 2 Disk Radius (mm) Width (microns) 34 0.400 P3359 36 0.335 54 0.462 24 0.611 P3348 34 0.400 46 0.279 The spin coating process deposited the solution initially on the inside of the disk and working outwards to the outside of the disk; the disk was spun during coating and afterwards until dry. Spin speeds were in the range 500 to 1100 rpm. After coating, the disks were examined to determine the optical density of the dye film by absorbence at kmax and the diffraction efficiency from the dye surface in reflection and the results were used to calculate the width and depth of the dye-coated groove.

Three solvent mixtures were employed, namely MCH/ECH; MCH/DMCH; and MCH/DBE, where MCH is methylcyclohexane (boiling point 101°C); ECH is ethylcyclohexane (boiling point 130-132°C); DMCH is dimethylcyclohexane (cis and trans; boiling point 124°C); and DBE is di-n-butyl ether (boiling point 142-143°C).

Each of the three solvent mixtures was used in three concentrations in which the proportions (by volume) between primary and secondary solvent were 17: 3,18: 2 and 19: 1.

Plots of the groove depth against absorption at Xmax were compared for different solvent mixtures and concentrations. The groove depth at a radius of 34mm of disks prepared from the solvent mixtures which resulted in an absorption of 0.8 were then plotted against the boiling point of the secondary solvent; an absorption of 0.8 approximates to the optimal value for the read/write parameters used. The result is shown in Fig. 1. This shows that the groove depth decreases (i. e. dye deposition increases) linearly with the boiling point of the secondary solvent. No significant difference in groove filling was observed at the different secondary solvent concentrations employed.

Extrapolation of the linear plot to a boiling point of 101°C-that of the primary solvent, methylcyclohexane (MCH)-gave a predicted groove depth of 117nm for the situation where the dye is deposited from pure MCH; the predicted result was found to be in accord with observed results for deposition of the dye from pure MCH under analogous conditions, which gave depths of 109nm and 112nm.

Example 2 In this Example, a phthalocyanine dye designated HW864/52 was used; this dye has the general formula given in Example 1 above, the substituent R being --PO (OEt) 2. This dye was dissolved in a solvent in which the major constituent was methylcyclohexane (B. Pt. : 101°C) and the minor constituent was 2,6- dimethylheptan-4-one (DMH). The solution contained 2%

by weight of the dye, and eight batches of the solvent were used, these containing an amount of DMH (by volume) equal to zero, 0.1%, 0.25%, 0.5%, 0.75%, 1.0%, 2.5% and 5.0%. Each of the eight solutions were used to coat a grooved substrate using conventional spin coating techniques. This resulted in the deposition of the dye onto the substrate.

After drying, the resultant coated disks were examined to determine the diffraction efficiency from the dye- coated surface and the results were used to calculate the depth of the dye-coated grooves at different positions across the disks. Graphs plotting the resultant groove depth (in microns) against concentration of the minor constituent (DMH) of the mixed solvent were drawn and are shown in Figures 2 and 3; Figure 2 is a plot with the concentration of DMH given on a linear scale, while in Figure 3 the concentration of DMH is given on an exponential scale.

The individual traces on each graph represent the results for disks having absorptions at Xmax of 0. 55, 0.6,0.65,0.7,0.75 and 0.8. These show that, at a concentration of 1% and above, the groove depth is essentially independent of the solvent concentration.