Method of producing a data storage medium

Abstract

The present invention relates a method of producing a data storage medium comprising the steps of: a) coating a layer comprising a polymer material onto at least a part of a template surface thereby to obtain a modified template surface; b) clamping the modified template surface produced in step (a) with a target surface thereby to obtain an assembly; and c) introducing a liquid to an environment of the assembly obtained in step (b) thereby to transfer the layer comprising the polymer material of the modified template surface onto at least an adjacent region on the target surface.

Claims

1. A method for producing a data storage medium comprising: (a) coating a polymer material directly onto at least a part of a template surface so as to form a polymer layer, wherein the polymer layer and the template surface form a modified template surface, wherein the polymer material comprises a polystyrene-r-benzocyclobutene random copolymer and wherein the polymer layer comprises a thickness of 100 nm, wherein the polymer layer of the data storage medium has a root mean square (RMS) surface roughness of less than 0.2 nm when measured in a 0.1 m.sup.2 target area; (b) clamping the modified template surface produced in step (a) with a target surface so as to form an assembly; and (c) introducing a liquid to an environment of the assembly formed in step (b) so as to cause the polymer layer to separate from the template and remain on at least an adjacent region of the target surface; wherein the data storage medium comprises indentation marks having a pitch of 24 nm and a depth of 2 nm such that the data storage medium has a storage density of 1.4 Terabit/square inch and an overall signal-to-noise ratio (SNR) of 8 dB, wherein the data storage medium comprises a pixel to pixel distance of 4 nm, and wherein the data storage medium exhibits an amplitude of the power spectrum of 180 at a period of 0.02/nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Reference will now be made, by way of example, to the accompanying drawings in which:

(2) FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D, illustrate the steps in an embodiment of the present invention;

(3) FIG. 2A and FIG. 2B show AFM images of a layer of PS-BCB deposited on a silicon substrate by spin-coating (FIG. 2A and an embodiment of the present invention FIG. 2B);

(4) FIG. 3 shows the power spectra of the samples of FIG. 2A and FIG. 2B;

(5) FIG. 4 shows a data storage medium prepared in accordance with an embodiment of the present invention; and

(6) FIG. 5A, FIG. 5B and FIG. 5C illustrate the steps in another embodiment of the present invention.

DETAILED DESCRIPTION

(7) Within the description, the same reference numerals or signs are used to denote the same parts or the like.

(8) Reference is now made to FIG. 1A. FIG. 1B, FIG. 1C and FIG. 1D, which illustrate the steps in an embodiment of the present invention.

(9) As shown in FIG. 1A, a layer 2 comprising a polymer material is coated onto at least a part of a template surface 1 thereby to obtain a modified template surface 2. In an embodiment of the present invention, the modified template surface is a combination of the template surface 1 and the layer 2 comprising the polymer material coated thereon.

(10) As shown in FIG. 1B the modified template surface 2 is is brought into contact and clamped with a target surface 3 thereby to obtain an assembly. The target surface 3 may, for example, be a surface onto which deposition of the layer 2 comprising the polymer material is desired.

(11) As shown in FIG. 1C, the layer 2 comprising the polymer material of the modified template surface 2 is transferred onto at least an adjacent region on the target surface 3 by introducing a liquid 4 to an environment of the assembly. In an embodiment of the present invention, the layer 2 comprising the polymer material may be transferred onto a region on the target surface 3 that lay directly adjacent to the modified template surface 2. The layer 2 comprising the polymer material may be transferred onto the whole of the target surface 3 or a part thereof (for example, lying adjacent to the region of the target surface 3 onto which the layer 2 comprising the polymer material was transferred by, for example, heating the target surface 3.

(12) By way of example, the polymer material is polystyrene-r-benzocyclobutene random copolymer (PS-BCB), which is a cross-linkable polymer, has a hydrophobic character, and is not hygroscopic. The template surface 1 is a surface of freshly-cleaved mica. Mica is chosen on account of having a hydrophilic character and its unique property that, when cleaved, it yields a relatively defect-free surface. A layer 2 of PS-BCB is coated onto the mica surface 1 by spin-coating thereby to obtain a modified mica surface 2. Typically, the spin-coating is done by dropping a solution of the PS-BCB onto the mica surface 1 and then spinning the mica surface 1 at a speed of about 2000 revolutions/minute. In this way, the layer 2 of the PS-BCB is coated onto the mica surface 1 at a thickness of about 100 nm. Of course, the PS-BCB can be deposited at a desired thickness onto the mica surface 1 by varying the weight of the PS-BCB in the solution thereof. After spin-coating the layer 2 of the PS-BCB onto the mica surface 1, the modified mica surface 2 that is obtained is heated at 220 degrees centigrade for about 30 minutes. This is done to activate the cross-linking reaction of the constituents in the PS-BCB.

(13) The modified mica surface 2 produced is brought into contact and clamped with a target surface 3 thereby to obtain an assembly. The target surface 3 may, for example, be a surface of a silicon substrate. The clamping may be done by any clamping method and/or device suitable to the application of an embodiment of the present invention (for example, for the individual or mass scale production of a data storage medium).

(14) A polar liquid 4 such as, for example, water, is introduced into an environment of the assembly. Typically, this is done by immersing the assembly in water. Water molecules penetrate the interface between the layer 2 comprising PS-BCB and the mica surface 1 on account of being attracted to the hydrophilic mica surface 1. The attractive forces between the water molecules and the mica surface 1 result in a disjoining pressure to be exerted between the layer 2 comprising the PS-BCB and the mica surface 1, causing them to separate spontaneously. The separation of these surfaces is further aided by the hydrophobic character of the PS-BCB, which repels the water molecules. In this way, the layer 2 comprising the PS-BCB of the modified mica surface 2 is transferred onto at least an adjacent region on the silicon substrate 3. The mica surface 1 is then lifted off and the layer 2 comprising the PS-BCB transferred onto the silicon substrate is blown dry by using nitrogen gas.

(15) As can be seen from FIG. 1D, the layer 2 comprising the PS-BCB is transferred onto the silicon substrate 3 in a manner such that its surface that was previously in contact with the mica surface 1 is now exposed. The present invention exploits the fact that the surface roughness of the exposed surface 5 of the layer 2 comprising the PS-BCB is a near replication of the surface roughness of the mica surface 1 that it was previously in contact with and that it has now been separated from. Since freshly cleaved mica has the property that it is relatively defect-free, the exposed surface 5 of the layer 2 comprising the PS-BCB demonstrates the same degree of flatness as the mica surface 1 that it was previously in contact and, advantageously, demonstrates this over an area of several mm.sup.2.

(16) The present invention allows deposition of a layer 2 comprising a polymer material onto a target surface 3 in a manner that does not require complicated processing equipment and/or steps. In the above example, which was given to demonstrate the principle underlying the present invention, transfer of the layer 2 of PS-BCB onto the silicon substrate 3 was done by exploiting the surface forces acting between water 4 and, respectively, the hydrophilic mica surface 1 and the hydrophobic PS-BCB in the layer 2.

(17) As discussed earlier, the rms surface roughness values of a layer 2 comprising a polymer material deposited onto a target surface 3 in the above manner is approximately 0.2 nm when measured in a 0.1 m.sup.2 area of the target surface 3 whereas that obtained with previously-proposed techniques such as spin-coating is typically 0.5 nm to 1 nm when measured on the same scale. The improvement in surface roughness values that may be obtained with an embodiment of the present invention over spin-coating is demonstrated in FIGS. 2A and 2B, which show AFM images of a layer 2 comprising PS-BCB deposited on a mica surface 1 by spin-coating (FIG. 2A) and the present invention (FIG. 2B). The change in surface topography is illustrated by the variation in the gray-scale of these images. By comparing, it is evident that the variation in surface topography and, therefore, the surface roughness of the layer 2 comprising PS-BCB prepared using an embodiment of the present invention is less than that obtained with spin-coating.

(18) In order to draw a quantitative comparison between the layers shown in FIGS. 2A and 2B, reference is made to FIG. 3 that shows the power spectra of their surface topographies. In this regard, the spectrum of the spin-coated sample of FIG. 2A is denoted by x, the sample prepared in accordance with an embodiment of the present invention is denoted by y, and the electronic noise of the detection mechanism is denoted by z in FIG. 3.

(19) From FIG. 3, it can be seen that the amplitude of the power spectrum of the layer 2 comprising PS-BCB deposited on the mica surface 1 using an embodiment of the present invention (curve y) is up to one order of magnitude lower than that obtained by spin-coating (curve x). This translates to a threefold reduction of the indentation depths in the layer 2 comprising PS-BCB deposited on the mica surface 1 prepared using an embodiment of the present invention over that prepared by using spin-coating but which, despite being shallower, are capable of being detected with a comparable SNR as that employed for the layer 2 comprising PS-BCB prepared by spin-coating. Due to the reduced surface roughness of the layer 2 comprising PS-BCB prepared using an embodiment of the present invention as compared to spin-coating, shallower indentation marks can be formed on this layer without compromising on sensing margins and/or requiring complicated sensing equipment. Since the lateral dimensions of the indentation marks scale with their depth, the number of indentation marks that are formed may be increased for a layer 2 comprising PS-BCB that is produced using an embodiment of the present invention over spin-coating, which results in an increased recording density capability.

(20) The improvement that may be obtained with an embodiment of the present invention over, for example, spin coating is particularly evident from the amplitude of the power in the frequency region of a typical bit distance. Specifically, at around 0.02/nm (denoted by an arrow labeled relevant region in FIG. 3), the amplitude of the power spectrum associated to layer 2 comprising PS-BCB deposited on a mica surface 1 using an embodiment of the present invention is around 180 whereas that obtained with spin-coating is around 36 (these units being in accordance with FIG. 3). Additionally the latter signal is limited by electronic noise of the detection system, which is denoted by the curve z. Thus, an improvement of a factor of at least 5 is obtained with an embodiment of the present invention over spin coating in the wavelength region of a bit distance that is typical in data storage media.

(21) FIG. 4 shows a data storage medium prepared in accordance with an embodiment of the present invention. The x- and y-axes denote the number of recorded pixels. The pixel to pixel distance is, in this case, about 4 nm. The indentations were written with a pitch of about 24 nm, which translates to a storage density of 1.4 Terabit/square inch for a d=1 code, i.e. where there is at least one non-indentation mark (0) between indentation marks (1) in a data track. The depths of the indentations is about 2 nm and the overall SNR is about 8 dB, which is sufficient for obtaining a raw bit error rate of less than 10.sup.4, i.e. on average and without employing a correcting mechanism, one error is obtained for 10,000 bits. It is expected that the SNR is mostly limited by electronic noise of a system used in sensing the indentations. Advantageously, the reduced depth of the indentations results in reduction in rim formation around the indentations, which typically interferes with and distorts the sensing of indentations.

(22) For the template surface 1, the present invention is not limited to the use of mica. Indeed, other substrates that are hydrophilic and have the same/similar surface quality as mica can be used. For example, a surface of a flame-annealed glass substrate, a silicon oxide layer on a silicon substrate, or a (100) surface perovskite may be used, these being preferable for implementing the present invention in an environment for the mass production of data storage media. In the case of a (100) surface perovskite, this may be represented by ABO.sub.3 where the element A is a lanthanide alkaline earth metal, B is a transition metal, and O is oxygen. A specific example of the (100) surface perovskite is strontium titanate.

(23) Alternatively, the template surface 1 may comprise a sacrificial layer on a support. Referring to FIG. 5A, the layer 2 comprising the polymer material is coated onto at least a part of the sacrificial layer 6 provided on a support 7 thereby to obtain a modified sacrificial layer 2. In this case, the modified sacrificial layer 2 is taken to be a combination of the layer 2 comprising polymer material and the sacrificial layer 6 onto which it is coated. From FIG. 5B, it can be seen that) the modified sacrificial layer 2 is then brought into contact and clamped with a target surface 3 to obtain an assembly. As shown in FIG. 5C, in order to facilitate the transfer of the layer 2 comprising the polymer material from the modified sacrificial layer 2 onto a region of the target surface 3 that lay adjacent to the modified sacrificial layer 2 in the assembly, the sacrificial layer 6 is dissolved by the introduction of a liquid 4 suitable for this purpose.

(24) The support 7 may comprise, for example, a Si(111), Si(110), Si(100), Ge(100) crystal surface or the like, this being done on account of the profile of such crystal surfaces.

(25) In one embodiment, the sacrificial layer 6 may be a layer of a water-soluble salt, such as, for example, sodium chloride, potassium chloride, or the like. In an alternative embodiment, the sacrificial layer 6 may comprise a silicon oxide layer on a silicon substrate 7. In this case, removal of the silicon oxide layer 6 in step (c) may be facilitated by using a liquid 4 comprising hydrofluoric acid, which effectively etches the silicon oxide layer 6. In yet another embodiment, the sacrificial layer 6 may comprise a metal layer, in which case, the liquid 4 may comprise, for example, a suitable acidic etchant. In still another embodiment, the sacrificial layer 6 may comprise an organic material that has the property of forming a layer of ordered orientation i.e. a highly ordered thin-film. The organic material may be, for example, a material comprising self-ordering alkyl molecules, self-ordering block copolymers, or the like. The organic material may also be a material capable of forming a liquid crystal layer. Where the sacrificial layer 6 comprises an organic material, the liquid 4 may be a solvent suitable for use with and capable of dissolving organic materials.

(26) In an embodiment of the present invention, the polymer material may have a hydrophobic character (in which case, it is preferable that it is also hygroscopic) or a hydrophilic character.

(27) The template surface 1 and the target surface 3 may be chosen so that their respective surface energy is such that the molecules of the liquid introduced in step (c) is attracted with preference to the template surface 1 rather than the target surface 3. By way of example, the target surface 3 should be chosen so as to exhibit weaker hydrophilicity than the template surface 1 or to have a hydrophobic character. Specifically, when the target surface 3 and the polymer material both have a hydrophobic character, transfer of the layer 2 of polymer material onto the target surface 3 is aided. The target surface 3 and the layer 2 comprising the polymer material, due to their hydrophobic nature, serve to repel molecules of the polar liquid 4 in combination. This causes the exertion of a stronger disjoining pressure by the polar liquid 4 (that causes the separation of the layer 2 comprising the polymer material from the hydrophilic template surface 1) than if the target surface 3 did not have a hydrophobic character. Furthermore, firmer adhesion of the layer 2 comprising the polymer material onto the target surface 3 is facilitated by the Van der Waals forces acting between these hydrophobic surfaces. The target surface is, for example, a silicon substrate, a hydrogen-passivated surface thereof, or a polymer layer coated on a substrate having a hydrophobic character.

(28) An embodiment of the present invention is not limited to data storage applications and may, for example, be used in any other scanning probe applications such as, for example, high-resolution lithography, bio-assays, and the like.

(29) The present invention has been described above purely by way of example and modifications of detail can be made within the scope of the invention.

(30) Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.