KR20050017009A - Optical recording and reading system, optical data storage medium and use of such medium - Google Patents

Abstract

There is provided an optical recording and reading system for use in an optical data storage medium (5). This system is provided with a medium 5 having a recording stack 9 formed on a substrate 8. The recording stack 9 is suitable for recording using a focused radiation beam 1 having a wavelength? In air. The recording stack 9 has a first optical surface 6 farthest from the substrate 8. On the recording stack 9 side of the optical data storage medium 5 is an optical head 3 having a numerical aperture NA and an objective lens 2 for emitting a focused radiation beam 1 during recording. Is placed. The objective lens has a second optical surface 7 closest to the recording stack 9 and is configured to record / read at a free working distance d _F from the first optical surface 6 smaller than 50 μm. At least one of the first optical surface 6 and the second optical surface 7 has a transparent hydrophobic layer 10 having a refractive index n and having a thickness of less than 0.5λ / n. According to this, reliable writing and reading is achieved, in particular, accumulation of contaminants on the second optical surface 7 is prevented or prohibited.

Description

Optical recording and reading systems, optical data storage media, and uses of the media {OPTICAL RECORDING AND READING SYSTEM, OPTICAL DATA STORAGE MEDIUM AND USE OF SUCH MEDIUM}

The invention is used in optical data storage media,

A storage medium formed on the substrate, suitable for recording using a focused radiation beam having a wavelength λ in air, the storage medium having a recording stack having a first optical surface furthest from the substrate,

Emit a radiation beam focused during recording, having a numerical aperture NA, disposed on the recording stack side of the optical data storage medium, and having a second optical surface closest to the recording stack, from the first optical surface An optical recording and reading system having an optical head having an objective lens configured to record / read at a free working distance d _F of less than 50 μm.

Moreover, the present invention is suitable for recording using a focused radiation beam having a wavelength λ in air and having a recording stack having a first optical surface farthest from the substrate. It relates to a storage medium.

The invention also relates to the use of the medium in such a system.

One embodiment of a system having the form mentioned at the outset is known from US Pat. No. 6,069,853.

Next generation optical recording discs have larger data capacity and smaller bit size. For each next generation of record carrier, there is a trend that the wavelength of optical readout decreases and the numerical aperture NA of the optical pickup device OPU increases. The focal length and working distance are reduced, and the tilt margin is becoming more and more strict. As a result, the thickness of the transparent layer passing through when the recording layer is recorded and read is decreasing, from 1.2 mm for compact discs (CD), 0.6 mm for digital multifunction discs, and 0.1 mm for Blu-ray discs (BD). For "fourth-generation" optical (magnetic) disks, it is down to a few microns. The purpose of this layer is at least three, that is, to protect the information layer to make the disk more durable, to serve as an antireflection film, and to help cool the storage layer.

For the next generation of optical storage systems, the numerical aperture of the objective system increases to NA = 0.85, or even NA = 0.95, increasing the resolution. However, in spite of this trend of increasing the size of the objective lens, the demand for high data rate and access time increases, so that the overall mass of the objective lens must be reduced. This can only be achieved if the focal length and hence the free working distance (FWD) are reduced.

However, as the FWD decreases, the thickness of the transparent substrate through which the focused radiation beam passes needs to be reduced. Furthermore, as NA increases, the tolerance (tilt angle) of the angle of deviation of the media surface from the perpendicular to the optical axis of the optical system is reduced under the influence of birefringence and aberration due to the thickness of the transparent substrate. Therefore, the reduction in the effect of the tilt angle at high NA is another reason to reduce the thickness of the transparent substrate. The reduced thickness of the transparent substrate is also called a cover layer, or more generally a cover laminate. Therefore, the purpose of the cover layer in 4th generation optical recording is mainly to protect the recording stack from damage and to enable a small FWD. In order to record information on the above-mentioned optical (magnetic) data magnetic medium, a focused radiation beam, for example, light is emitted from the optical head through the transparent substrate having a thickness of 0.6 to 1.2 mm, The recording layers are heated to the recording temperature. In the case of a magneto-optical (MO) medium, at the same time, a magnetic field is applied by the magnetic head. Such a magnetic field may be modulated with information using a magnetic field modulator. As a result, information can be recorded on the recording medium.

Another topic of small free working distance is the size of the coil in the case of magneto-optical data storage media. If a system with a high data rate is desired, a large bandwidth is needed to modulate the current through the coil. For data rates on the order of 100-200 Mbit / sec, the switching frequency of the current through the coil must be at least 1-2 GHz to form steep sides in the switching behavior of the magnetic field. This requires a coil with small magnetic inductance, low resistance and small parasitic capacitance. Besides the speed of the coil, the power consumption by the coil is a problem. Thus, for example, it is preferable to use a small coil having an inner diameter smaller than 100 mu m. Since larger coils have greater inductance and higher power consumption, the use of larger coils impairs data rate and energy efficiency. As the coil gets closer to the surface, the magnetic field in the data storage medium can be modulated more energy efficiently. However, since a magnetic field with a size of 15 kA / m per ampere of such a small coil penetrates only a few hundred microns of space, the coil should be kept close to the recording stack, for example, a cover layer thicker than 100 μm may be used. Will interfere. In order to reproduce information from the optical (magnetic) data storage medium, light is also emitted by the optical system through the transparent substrate. In such a state, the optical head is also arranged on the transparent substrate side of the disk.

For objectives of slider-based (see Fig. 4) or actuator-based optical heads with a small working distance (usually smaller than 50 μm), contamination of the optical surface of the objective lens closest to the storage medium occurs. This is due to the high radiation beam power and the high surface temperature (approximately 250 ° C.) attributable to the temperature required to write (or even read data from the recording stack) to the recording stack. This is due to the recondensation of the escaped moisture, see FIG. 3. Such contamination ultimately causes a malfunction of the optical storage system, for example due to distortion of the system control signal, see FIG. 2. This problem is even more severe in the following cases: high humidity, high radiation beam power, low light reflectivity of the recording stack of the data storage medium, low conductivity of the storage medium, small operating distance and high surface temperature. Become. From US 6,069,853 a technique is known for increasing the distance between the recording stack and the external optical surface of the storage medium using an insulator or cover layer to prevent contamination of the objective lens. If such a heat insulating layer is not present, contaminants from the medium will evaporate and condense on the objective lens of the optical head during recording. This pollutant may, for example, be mixed with moisture together with a small amount of other pollutants. Moisture containing other contaminants will probably be present as a thin (single) layer on the outer surface of the medium. If no thermal insulation layer is present, a thin (single) layer is present in close proximity to the recording stack, indirectly heated by the recording stack, evaporates, and then condenses on the objective lens containing other contaminants. Such a process can occur relatively rapidly, i.e. within 30 minutes or perhaps faster, leading to unreliable writing and reading of the system, and eventually to a complete write and read failure. The advantage of applying the heat insulating layer or cover layer is to prevent heating of the (single) layer with contaminants, thereby preventing the accumulation of contaminants on the objective lens. This is because the thermal insulation layer forms an effective barrier that prevents the (single) layer on the medium from being heated and evaporated. However, such a thermal insulation layer, which usually has a thickness of tens of nanometers or more, has a number of problems. This may cause unreliable writing and reading of data, for example due to optical aberration and interference of the thermal insulation layer. Moreover, it may happen that the optical head is focused on the outer surface of the thermal insulation layer, making data writing and reading impossible, after which the optical head needs to refocus on the next surface. Such a process can lead to interruption of the data stream and thus unreliable data writing and reading. Moreover, the relatively thick thermal insulation layer allows the MO magnetic coil to have a relatively large magnetic field distance range in the axial direction of the coil, thus limiting the switching speed of the coil and thus writing reliability at higher data rates.

Consequently, the object of the present invention is the kind described at the outset, which can perform reliable writing and reading of data on a recording stack and preventing contamination of the objective lens of the optical head without having the above-mentioned disadvantages. In providing a system.

It is a second object of the present invention to provide an optical data storage medium for the recording and reading of reliable data, which is used in a system having a kind as described at the outset.

These objects, according to the invention, are characterized in that at least one of the first and second optical surfaces has a hydrophobic layer having a refractive index n and having a thickness of less than 0.5λ / n. Achieved by a writing and reading system. The first new idea is that by applying a relatively thin and transparent hydrophobic layer on the objective lens, it is possible to prevent the condensation of moisture on the optics. A second new idea is that applying a thin hydrophobic layer onto the recording stack of an optical data storage medium prevents moisture from adsorbing on the first plane, so that this moisture cannot be released later. Thus, the pollutant is removed. If the thickness of the hydrophobic layer is kept smaller than the above limit value, optical aberration and interference are prevented. An important feature of this technique is that subsequent cleaning of the objective lens is not necessary. In addition, the range of humidity levels at which the optical drive operates reliably can be improved. Preferably, the second optical surface has a hydrophobic layer having a thickness substantially equal to 0.25λ / n, in which case the hydrophobic layer can serve as an antireflection film. In practice, it may be difficult to have a layer having sufficient hydrophobicity on the objective lens. In such a case, it is preferred that the second optical surface has a hydrophobic layer having a thickness of approximately 0.25 lambda n. When using a hydrophobic layer, moisture that may accumulate on the second optical surface forms a layer with a nearly uniform thickness, which may be, for example, a micron thickness. This is because surface wetting is uniform. The central portion of the fluid layer, ie the portion that passes when the radiation beam propagates, hardly disturbs the wavefront of the focused radiation beam. The hydrophobic layer may be achieved by layers whose hydrophobicity is usually generated from oxygen atoms located on the surface, as is the case with Al ₂ O ₃ or SiO ₂ layers. In most cases, the NA of such FWD systems is greater than 0.80.

In one embodiment, the optical head further comprises a magnetic coil disposed on the optical head side closest to the recording stack such that the optical axis of the optical head crosses the center of the magnetic coil, and the recording stack of the optical data storage medium. The sieve has a magneto-optical form. In such a case, a high NA, i.e., a small spot, is possible, and the magnetic coil can go close to the recording stack, and the magnetic field can be modulated energy efficiently, thus enabling reliable magneto-optical recording at high density and high data rates. Do.

It is particularly advantageous for the magnetic coil to have an inner diameter of less than 60 μm. Since larger coils have greater inductance and higher power consumption, the use of larger coils may compromise data rate and energy efficiency.

In one embodiment, the hydrophobic layer comprises one material selected from the group consisting of poly-para-xylene, fluorocarbons and copolymers thereof. Parylene is the generic name for the poly-para-xylene group. Four different forms are available commercially, parylene-N being the most basic form of straight chains of para-xylene monomers. Other forms include parylene-C, parylene-D and parylene-AF4. Although applicable to any parylene derivative, parylene-C is of particular importance for the purposes of the present invention. Another suitable material is AF1600, drafted by Dupont, which is very suitable for the present invention as a copolymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-diosol.

In one embodiment, a magnetic coil is included inside the slider configured to float at a distance greater than 0.5λ / n and less than 2 μm from the first surface. In such a case, the slider constitutes a part of the objective lens, and hydrophobic cm is present on the surface of the slider facing the optical data storage medium. Slider technology is known, for example, from hard disk drive (HDD) technology. Using this technique, a "near-field" arrangement is possible, in which case the outer surface of the objective lens and the outer surface of the optical data storage medium are spaced apart from each other at a distance much smaller than 1 wavelength [lambda], ie FWD≤λ. / 10. In such an arrangement, the coupling between the medium and the objective lens may be achieved by evanescent wave light coupling. NA in this arrangement may be greater than one. Far field, ie FWD >> λ / 10, arrangements are also possible using slider technology.

In one embodiment of the optical data storage medium, the first optical surface has a transparent hydrophobic layer having a refractive index n and having a thickness of less than 0.5λ / n. If the thickness of the hydrophobic layer is kept smaller than the above limit value, optical aberration and interference are prevented. An important feature of applying the hydrophobic layer on the medium is that the need for the formation of a hydrophobic layer on the objective lens can be made smaller since the source of contaminants, i.e., the contaminants on the medium are removed or greatly reduced. This arrangement allows the medium of the old optical recording system, which does not have any action taken, for example, hydrophobic layers, to minimize the effects of contamination on the objective lens of the optical head since each vehicle does not cause contamination or much less contamination. It has the advantage that it can still benefit from the hydrophobic layer of the phase. In addition, the range of humidity levels at which the optical drive operates reliably can be improved. Preferably, the first optical surface has a hydrophobic layer with a thickness of less than 0.25λ / n, in which case there is much less optical aberration and interference.

Preferably, the hydrophobic layer on the medium comprises one material selected from the group consisting of poly-para-xylene, fluorocarbons and copolymers thereof.

In order to prevent damage of the hydrophobic layer due to contact of the optical head with the hydrophobic layer, it is advantageous to use a material having a relatively high hardness. If at least one of the medium and the objective lens has a hydrophobic layer, due to the extremely low coefficient of friction between these two layers, damage due to shear forces is greatly prevented.

The invention is now described in more detail by way of example embodiments with reference to the accompanying drawings in which:

Figure 1a shows one embodiment of a system according to the invention with a small free-range optic used in a MO drive,

FIG. 1B shows the layer stack structure of the medium shown in FIG. 1A

2 shows oscilloscope trajectories before and after contamination,

3 shows micrographs of contamination of an objective lens as a function of time,

4 shows a slider-based optical recording system,

5 shows a transparent slider having a magnetic field modulation coil,

6 shows a micrograph of a second surface of an objective lens with a layer with insufficient hydrophobicity,

FIG. 7 shows a micrograph of the second optical surface of the objective lens without a hydrophobic layer and instead with a very high hydrophilic layer.

1A and 1B illustrate one embodiment of an optical recording and reading system for use in an optical data storage medium 5. The medium 5 is provided with the recording laminated body 9 formed on the board | substrate 8 by sputtering, for example. The recording stack 9 is suitable for recording by the focused radiation beam 1. The wavelength λ of the focused radiation beam 1 is 405 nm. The recording stack 9 has a first optical surface 6 farthest from the substrate 8. An optical head 3 having an numerical aperture NA = 0.85 and having an objective lens 2 for emitting a focused radiation beam 1 during recording is provided with a recording stack 9 of the optical data storage medium 5. It exists on the side. The objective lens 2 of the optical head 3 has a second optical surface 7 that is closest to the recording stack 9, and has a free operating distance d from the first optical surface 6 of the medium 5. _It is configured to record / read at _F = 15 mu m. Note that the transparent member comprising the magnetic coil 4 constitutes a part of the objective lens 2 and the second surface 7 corresponds to the surface of this member closest to the medium 5. . The first optical surface 6 and the second optical surface 7 are copolymers of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-diosol and have an index of refraction of about 1.35 AF1600. And hydrophobic layers 10, 11 prepared. The thickness of this hydrophobic layer is 30 nm. The manufacture of fluoropolymer coatings, such as AF1600, is as follows. Depending on the particular condition, a cleaning procedure of the disk surface may be necessary, which consists of sonication, cleaning and drying. If the disc is clean, at room temperature the layer of fluoride silane is deposited in advance in the vapor phase, improving adhesion of the fluoropolymer coating. Fluoropolymer coatings can be done using dip coating or spin coating. For this purpose, a fluorine polymer such as AF1600 can be dissolved in FC-75 (Perfluoro-2-butyltetrahydrofuran manufactured by Acros). Finally, the coated disc is dried by laminar flow of air. At this point, heat treatment is also possible. To coat a disk with parylene, the disk is required to be completely clean. On the metal surface, parylene usually shows good adhesion, but adhesion on oxide surfaces such as glass and aluminum oxide is usually poor. In this case, A174 (gamma-methacryl-oxypropyl-trimethoxysilane in the vapor phase at room temperature or by spin coating or dip coating of a mixture of A174: distilled water: iso-propanol at 1: 100: 100. ) Layer first. Excess A174 is washed off with pure isopropanol and the disc is dried at room temperature in clean air. Parylene lamination is done in commercially available parylene coating equipment such as PDS2010 (available from SCS Europe). Basically, parylene monomer is evaporated and deposited on a rotating substrate. See the literature for more generally known details above this. The optical head 3 further includes a magnetic coil 4 disposed on the optical head 3 side closest to the recording stack 9. The optical axis of the optical head 3 crosses the center of the magnetic coil 4, and the recording stack 9 of the optical data storage medium has a magneto-optical form. The recording stack 9 is made of a reflective metal layer known in the art, for example, 25 nm of Al, and other auxiliary layers, 24 nm of the magnetic material TbFeCo, starting from the substrate 8 and in the order of the substrate. And a 60 nm SiN or ZnS / SiO ₂ interference layer.

If the medium 5 does not have a hydrophobic layer 10, a (single) layer of moisture mixed with contaminants accumulates on the first optical surface 6. When using an objective lens having no hydrophobic layer 11 on the second optical surface for recording on such a medium, servo, for example, focus or tracking servo failure will occur within minutes. This is shown in FIG. 3 shows the accumulation of contaminants at the second optical surface 7 when the hydrophobic layers 10, 11 are not used, with at least the hydrophobic layer 10 on the first optical surface 6. The system was found to be stable and robust.

In Figure 2, for two different laser beam powers P, traces 22 and 23 with error signals of radial open loop and closed focus servos, respectively, are illustrated. The optics are focused, but the tracking servo is not closed. At low power (P = 0.5 mWatt), the signal is normal. At higher power (P = 1.0 mWatt), due to the accumulation of contaminants on the second surface 7, severe signal instability occurs.

In Fig. 3, the second optical surface 3 of the objective lens 3 has no hydrophobic layer 11 and is still clean (t = 0 minutes) and contaminant accumulation after 1 minute at a relatively high laser power recording. Photomicrographs are illustrated. You can clearly see the moisture indicated by the reference number 30. The objective was placed under the microscope and dried, and observed again after 12 and 40 minutes. At t = 40 minutes, the water has evaporated completely, showing the contaminants indicated by reference numeral 31. These contaminants are recondensates of the moisture / pollutant mixture layer evaporated on the first optical surface 6 of the medium 5 without the hydrophobic layer 10. Application of the hydrophobic layers 10, 11 on the first optical surface 6 and the second optical surface 7 of the medium 2 and / or the objective lens 2 results in the accumulation of contaminants at a suitable laser power. Since it is not observed, the problem of contamination is greatly reduced.

4, a slider-based optical recording system is shown. The floating height of the slider 2a on the disk is approximately 1 mu m. At this time, since it is 1 µm >> λ / 10, this is not a close recording defined above. In the present embodiment, the slider includes a magnetic field modulation (MFM) coil 4 used for magneto-optical recording. At this time, the part of the slider 2a passing through the focused radiation beam 1 crosses the part of the objective lens 2.

In Fig. 5, a transparent slider having a magnetic field modulation coil is illustrated. The laser beam of FIG. 4 is focused through the central opening.

In FIG. 6, a micrograph of the second optical surface 7 of the objective lens 3 with the (poor) hydrophobic layer 11 is illustrated. Insufficient refers to the value of the contact angle between the water droplet and the hydrophobic layer, which in this case is not much greater than 90, but close to 180 degrees or close to this value for the ideal hydrophobic layer. Due to the fact that the contact angle between the water and the surface is not much larger than 90, some droplets can be seen, as indicated by reference numeral 60. This group of droplets has a size similar to both the free working distance between the objective lens and the recording medium and the diameter of the focused laser beam. The collection of these droplets causes the laser beam to be substantially scattered. The free operating distance corresponds to the distance between the first optical surface 6 and the second optical surface 7.

In FIG. 7 a micrograph of the second optical surface 7 of the objective lens 3 with no hydrophobic layer but instead with a very large hydrophilic layer is illustrated. The hydrophobic property is usually due to oxygen atoms on the surface as in the case of Al ₂ O ₃ or SiO ₂ . At this time, the contact angle between the liquid (water) and the surface is very small, ie much smaller than 90 degrees. As can be seen in the photograph, unlike FIG. 6, the entire layer of water, having a substantially uniform thickness, indicated at 70, collects on the surface, which layer is about 1 micron thick and may even be thicker than this. . Since the surface wettability is uniform, at least the center portion of the fluid moisture layer has an optically constant thickness and hardly disturbs the focused laser wavefront.

At this time, the above-described embodiments are intended to illustrate rather than limit the present invention, and various other embodiments may be made by those skilled in the art without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The term "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The term "a" or "an" in front of a component does not exclude the presence of such a plurality of components. The simple fact that a particular arrangement is mentioned in different dependent claims does not suggest that a combination of these arrangements may not be used advantageously.

According to the present invention, there is provided an optical recording and reading system for use in an optical data storage medium. This system includes a medium having a recording stack formed on a substrate. The recording stack is suitable for recording using a focused radiation beam having a wavelength λ in air. The recording stack has a first optical surface farthest from the substrate. An optical head having a numerical aperture NA and having an objective lens that emits a focused radiation beam during recording is disposed on the recording stack side of the optical data storage medium. The objective lens has a second optical surface closest to the recording stack, and is configured to record / read at a free working distance d _F less than 50 μm from the first optical surface. At least one of the first optical surface and the second optical surface has a transparent hydrophobic layer having a refractive index n and having a thickness smaller than 0.5λ / n. According to this, reliable writing and reading is achieved, in particular, the accumulation of contaminants on the second optical surface is prevented or prohibited.

Claims (11)

Used for optical data storage media (5), Recording having a first optical surface 6 formed on the substrate 8 and suitable for recording using a focused radiation beam 1 having a wavelength λ in air and farthest from the substrate 8. A storage medium 5 having a laminate 9, A radiation beam 1 having a numerical aperture NA, which is focused during recording, disposed on the recording stack 9 side of the optical data storage medium 5, and closest to the recording stack 9; Optical recording with an optical head 3 having a second optical surface 7 and an objective lens 2 configured to record / read at a free working distance d _F from the first optical surface 6 less than 50 μm. And a reading system, At least one of the first optical surface 6 and the second optical surface 7 has a transparent hydrophobic layer 10 having a refractive index n and having a thickness of less than 0.5λ / n and Reading system. The method of claim 1, The optical recording and reading system, characterized in that the second optical surface (7) has a hydrophobic layer (11) having a thickness approximately equal to 0.25λ / n. The method of claim 1, The optical recording and reading system, characterized in that the second optical surface (7) has a hydrophobic layer (11) having a thickness approximately equal to 0.25λ / n. The method of claim 1, The optical head 3 is a magnetic coil 4 disposed on the optical head 3 side closest to the recording stack 9 so that the optical axis of the optical head 3 crosses the center of the magnetic coil 4. Further comprising: the recording stack (9) of the optical data storage medium (5) has a magneto-optical form. The method of claim 4, wherein Optical recording and reading system, characterized in that the magnetic coil (4) has an inner diameter smaller than 60 mu m. The method according to any one of claims 1 to 5, The hydrophobic layer (10, 11) is an optical recording and reading system, characterized in that it comprises one material selected from the group consisting of poly-para-xylene, fluoride carbides and copolymers thereof. The method according to any one of claims 4 to 6, Optical recording and reading system, characterized in that a magnetic coil (4) is included inside a partially transparent slider configured to float at a distance greater than 0.5λ / n and less than 2 μm from the first surface (6). A recording stack 9 formed on the substrate 8 and suitable for recording using a focused radiation beam 1 having a wavelength λ in air and having a first optical surface furthest from the substrate In the optical data storage medium having, Optical data storage medium (5), characterized in that the first optical surface (6) has a transparent hydrophobic layer (10) having a refractive index n and having a thickness of less than 0.5 lambda / n. The method of claim 8, Optical data storage medium, characterized in that the first optical surface has a hydrophobic layer (10) having a thickness of less than 0.25λ / n. The method according to claim 8 or 9, The hydrophobic layer comprises an optical data storage medium comprising at least one material selected from the group consisting of poly-para-xylene, fluoride carbides and copolymers thereof. Use of the optical storage medium (5) according to any one of claims 8 to 10 for reliable recording and reading in the system of any one of claims 1 to 5.