Increased Storage Capacity for a Method for Long-Term Storage of Information and Storage Medium Therefor

Abstract

The present invention relates to an information storage medium and a method for long-term storage of information.

Claims

1-60. (canceled)

61. A method for storage of information, comprising: providing a ceramic substrate of a first material; coating the ceramic substrate with at least one layer each of a second material different from the first material; and creating a plurality of recesses in the at least one layer by using a laser in order to encode information in the at least one layer, wherein the plurality of recesses have different depths, and wherein each depth corresponds to a predefined bit of information.

62. The method of claim 61, wherein the ceramic substrate is coated with two or more layers, wherein the second materials of the two or more layers are different.

63. The method of claim 61, wherein the coated ceramic substrate is tempered before and/or after creating the plurality of recesses.

64. The method of claim 61, wherein the first material comprises at least 90% by weight of one or a combination of Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2, ZrO.sub.2, ThO.sub.2, MgO, Cr.sub.2O.sub.3, Zr.sub.2O.sub.3, or V.sub.2O.sub.3.

65. The method of claim 61, wherein the first material comprises at least 90% by weight of one or a combination of a metal nitride; a metal carbide; a metal boride; or a metal silicide.

66. The method of claim 61, wherein each second material comprises one or a combination of Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, or V.

67. The method of claim 61, wherein each second material comprises one or a combination of a metal nitride; a metal carbide; a metal oxide; a metal boride; or a metal silicide.

68. The method of claim 61, wherein the laser comprises a femtosecond-laser, and wherein the plurality of recesses are created as Coulomb explosions.

69. The method of claim 61, wherein each at least one layer has a thickness no greater than 1 μm.

70. An information storage medium, comprising: a ceramic substrate of a first material coated with at least one layer each comprising a second material different from the first material; and a sintered interface between the ceramic substrate and a bottommost of the at least one layers, wherein the sintered interface comprises at least one element from both the first material and the second material of the bottommost layer, wherein the at least one layer comprises a plurality of recesses encoding information, wherein the plurality of recesses have different depths, and wherein each depth corresponds to a predefined bit of information.

71. The information storage medium of claim 70, wherein there are two or more layers, and wherein the second materials of the two or more layers are different.

72. The information storage medium of claim 71, wherein the two or more layers each have a thickness smaller than 100 nm.

73. The information storage medium of claim 70, wherein a minimum depth difference between the different depths of the plurality of recesses is at least 10 nm.

74. The information storage medium of claim 70, wherein a minimum depth difference between the different depths of the plurality of recesses is at most 500 nm.

75. The information storage medium of claim 70, wherein the first material comprises at least 90% by weight of one or a combination of Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2, ZrO.sub.2, ThO.sub.2, MgO, Cr.sub.2O.sub.3, Zr.sub.2O.sub.3, or V.sub.2O.sub.3.

76. The information storage medium of claim 70, wherein the first material comprises at least 90% by weight of one or a combination of a metal nitride; a metal carbide; a metal boride or a metal silicide.

77. The information storage medium of claim 70, wherein each second material comprises one or a combination of Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, or V.

78. The information storage medium of claim 70, wherein each second material comprises one or a combination of a metal nitride; a metal carbide; a metal oxide; a metal boride; or a metal silicide.

79. The information storage medium of claim 70, further comprising an oxide layer on top of a topmost of the at least one layers.

80. The information storage medium of claim 70, wherein each at least one layer has a thickness no greater than 1 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:

[0077] FIG. 1 schematically depicts a cross section through an information storage medium according to a preferred embodiment of the present invention;

[0078] FIG. 2 schematically depicts an example of the process of physical vapor deposition coating of the ceramic substrate;

[0079] FIG. 3 schematically shows a perspective view of an example of encoding a writable plate with information using a laser;

[0080] FIG. 4 schematically depicts a cross section through an information storage medium according to a preferred embodiment of the present invention;

[0081] FIG. 5 schematically depicts the principle of interference in case of a metal/metal oxide layer system;

[0082] FIG. 6 depicts a graph of reflectance versus wavelength in case of a metal/metal oxide layer system;

[0083] FIG. 7 schematically depicts a cross section through an information storage medium according to a preferred embodiment of the present invention;

[0084] FIGS. 8a and 8b depict micrographs at two different magnifications showing an exemplary encoding;

[0085] FIG. 8c shows a 3D visualization of a section of the micrograph of FIG. 8b;

[0086] FIG. 8d shows the cross sectional height profile through the micrograph of FIG. 8a;

[0087] FIGS. 9a and 9b depict micrographs at two different magnifications showing an exemplary encoding; and

[0088] FIG. 10 depicts an SEM image taken from an exemplary encoding.

[0089] In principle, identical parts are provided with the same reference signs in the figures.

DETAILED DESCRIPTION

[0090] FIG. 1 schematically depicts a cross section through an information storage medium suitable for long-term storage of information according to a preferred embodiment of the present invention. The information storage medium comprises a ceramic substrate 150 coated with a layer of a second material 170, the second material 170 being different from the material of the ceramic substrate 150. As mentioned above, a sintered interface (not shown) may be present between the ceramic substrate 150 and the layer of the second material 170 due to the optional tempering process. The layer of the second material 170 comprises a plurality of recesses 10 (four of which are shown exemplary) having different depths, wherein each depth corresponds to a predefined bit of information. In the embodiment shown in FIG. 1, four bits of information can be encoded. For example, the smallest depth of a recess 10 (or, alternatively, a surface without any recess at all) may correspond to the code “0000”. The largest depth of a recess 10 extending, for example, all the way through the second layer 170 to the substrate 150 may correspond to the code “1111”. Analogously, each of the intermediate depths corresponds to a specific predefined bit of information as well. While the depth difference between subsequent codes is shown in FIG. 1 to be constant, this does not necessarily to be the case.

[0091] Of course, the 4-bit code shown in FIG. 1 is only one specific example. Depending on the thickness of the second layer 170 and the depth differences of the various recesses 10 which can be both reliably manufactured for encoding and reliably measured for decoding more or less bits may be encoded.

[0092] In order to produce such an information storage medium, a method for storage of information is described herein. Initially, a ceramic substrate 150 is provided. As schematically shown in FIG. 2, the ceramic substrate 150 is then coated with a layer of a second material 170. The layer of second material 170 is preferably no greater than 50 μm thick. The writable plate 110 comprising the ceramic substrate 150 and the layer of second material 170 may either be stored until ready for use or may subsequently be encoded with information 120 using, e.g., a laser or focused particle beam 190. The laser or focused particle beam 190 is directed toward the layer of second material 170 and then, e.g., heats localized areas of the second material 170 which fall within the focus of the laser or focused particle beam such that recesses are being formed at these localized areas. This method will now be described in more detail.

[0093] The ceramic substrate 150 which is initially provided may comprise the majority of the material by weight of the writable plate 110. A number of different materials may be used for the ceramic substrate 150. In certain configurations the ceramic substrate 150 comprises an oxidic ceramic comprising at least one of Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2, ZrO.sub.2, ThO.sub.2, MgO, Cr.sub.2O.sub.3, Zr.sub.2O.sub.3, V.sub.2O.sub.3 or any other oxidic ceramic material. Alternatively, the ceramic substrate may comprise a non-oxidic ceramic comprising at least one of a metal nitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si.sub.3N.sub.4, ThN, HfN, BN; metal carbide such as TiC, CrC, Al.sub.4C.sub.3, VC, ZrC, HfC, ThC, B.sub.4C, SiC; a metal boride such as TiB.sub.2, ZrB.sub.2, CrB.sub.2, VB.sub.2, SiB.sub.6, ThB.sub.2, HfB.sub.2, WB.sub.2, WB.sub.4 and a metal silicide such as TiSi.sub.2, ZrSi.sub.2, MoSi.sub.2, MoSi, WSi.sub.2, PtSi, Mg.sub.2Si, or any other non-oxidic ceramic material. The amount of the oxidic or non-oxidic ceramic present may vary. Preferably the amount of oxidic or non-oxidic ceramic makes up at least 90% by weight of the ceramic substrate 150. More preferably the amount of the oxidic or non-oxidic ceramic substrate makes up at least 95% by weight of the ceramic substrate 150. One preferred configuration is a ceramic substrate 150 comprising at least 90% Al.sub.2O.sub.3 or SiO.sub.2 measured by weight.

[0094] The second material 170 is formed as a layer on the ceramic substrate 150. The layer of second material 170 is a thin layer in comparison with the thickness of the ceramic substrate 150 (FIG. 1 not to scale), the second layer 170 being preferably at most 50 μm thick. The second material 170 may principally comprise at least one of a metal such as Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, V, a metal nitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si.sub.3N.sub.4, ThN, HfN, BN; a metal carbide such as TiC, CrC, Al.sub.4C.sub.3, VC, ZrC, HfC, ThC, B.sub.4C, SiC; a metal oxide such as Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2, ZrO.sub.2, ThO.sub.2, MgO, Cr.sub.2O.sub.3, Zr.sub.2O.sub.3, V.sub.2O.sub.3; a metal boride such as TiB.sub.2, ZrB.sub.2, CrB.sub.2, VB.sub.2, SiB.sub.6, ThB.sub.2, HfB.sub.2, WB.sub.2, WB.sub.4; a metal silicide such as TiSi.sub.2, ZrSi.sub.2, MoSi.sub.2, MoSi, WSi.sub.2, PtSi, Mg.sub.2Si or any other ceramic material; preferably wherein the second material comprises CrN, Cr.sub.2O.sub.3 and/or CrAlN.

[0095] One preferred configuration is a layer of second material 170 comprising principally CrN, Cr.sub.2O.sub.3 and/or CrAlN.

[0096] FIG. 2 illustrates an exemplary method for coating the second material 170 onto the ceramic substrate 150 using physical vapor deposition (PVD). In the PVD process the ceramic substrate 150 is placed into a physical vapor deposition chamber together with a source 160 of second material 162. A vacuum is drawn on the physical vapor deposition chamber and the source 160 of second material is heated until a significant portion of the second material 162 contained therein is evaporated or sublimated. The airborne particles 164 of second material then disperse throughout the physical vapor deposition chamber until they contact a surface 152 of the ceramic substrate 150 and adhere thereto.

[0097] Although physical vapor deposition is a method commonly used for coating metal substrates, coating ceramic substrates can prove challenging for particles to adhere to. Thus, in order to improve adherence of second material particles 164 to the ceramic substrate surface 152, a conductive wire mesh or conductive metal plate 180 may be placed on the far side of the ceramic substrate 150, such that the ceramic substrate 150 is positioned in between the wire mesh 180 and the source 160 of second material 162. Such a conductive mesh/plate 180 when conducting current may attract ionized particles of second material 164 which then encounter the surface 152 of the ceramic substrate 150 and are held there against such that they then adhere to the surface 152 of the ceramic substrate. This coating process may also be repeated in order to coat multiple different surfaces of the ceramic substrate as discussed further below.

[0098] Depositing a layer of second material 170 on the ceramic substrate 150 may be performed using other coating methods, such as sputtering or sublimation sandwich coating. Essentially, any method capable of producing a layer of second material 170 may be used. The second material 170 may not necessarily cover the entire ceramic substrate 150. Instead only portions of the ceramic substrate 150 or a singular side 152 of the ceramic substrate 150 may be coated with the second material 170.

[0099] Once the ceramic substrate 150 is coated with a second material 170, the coated ceramic substrate then preferably undergoes an optional tempering process. Tempering is generally understood to be a process which improves the strength and/or other qualities of a material. In the case of ceramics, tempering can involve heating a ceramic item such that the chemical components thereof undergo chemical and/or physical changes such that the item becomes fixed or hardened. Tempering of the coated ceramic substrate may involve heating the coated ceramic substrate 150 to a temperature within a range of 200° C. to 4,000° C., preferably within a range of 1,000° C. to 2,000° C. The tempering process may comprise a heating phase with a temperature increase of at least 10 K per hour, a plateau phase at a peak temperature for at least 1 minute and finally a cooling phase with a temperature decrease of at least 10 K per hour. The tempering process may assist in fixing the second material 170 permanently to the ceramic substrate 150. In some cases, a portion of the second material layer 170 may form a chemical bond to the underlying ceramic substrate 150. After tempering the ceramic substrate 150 with the second material 170, the writable plate 110 is formed. The properties of the writable plate 110 are determined by the exact materials used within the writable plate 110. The writable plate 110 may now be stored or directly encoded with information 120. As mentioned above, the coated substrate may, in addition or alternatively, be tempered before and/or after information encoding.

[0100] FIG. 3 depicts the encoding of information onto the writable plate 110. During encoding, a laser or focused particle beam 190 directs collimated laser light or focused particle beam onto a layer of second material 170 of the writable plate 110. The laser or focused particle beam alters the portion of second material 170 within the localized area 175 such that it is (e.g. optically) distinguishable from the surrounding second material 170. While FIG. 3 schematically shows the laser or focused particle beam imprint of a text, it is to be noted that the encoding of multiple bits according to the present invention is most suitable for digital encoding of information. Alternatively, however, the different depths may also be used to achieve a color effect which may be used for providing a colored text or a colored image on the writable plate 110.

[0101] Preferably the laser or focused particle beam heats the localized area 175 of the second material 170 to at least the melting temperature and/or decomposition temperature of the second material 170. The melting point of the second material 170 is dependent on the chemical composition thereof. Preferably, heating the localized areas 175 past the melting point may involve heating the localized areas to a temperature of at least 3,000° C., more preferably at least 3,200° C., and even more preferably at least 3,500° C., most preferably at least 4,000° C. Imparting these localized areas with such high temperatures may cause a rapid expansion of the second material 170 within the localized areas 175. This rapid expansion can cause the second material 170 within the localized areas 175 to be ablated and/or vaporized.

[0102] Suitable laser wavelengths for the laser encoding methods may include a wavelength within a range of 10 nm to 30 μm, preferably within a range of 100 nm to 2,000 nm, more preferably within a range of 150 nm to 1,500 nm. Of further importance is the minimum focal diameter of the laser light or focused particle beam which dictates the minimum size of each recess. Preferably the laser or focused particle source 190 is capable of focusing the laser light or focused particle beam to have a minimum focal diameter no greater than 50 μm, preferably no greater than 15 μm, preferably no greater than 5 μm, preferably no greater than 1 μm, preferably no greater than 100 nm, more preferably no greater than 10 nm.

[0103] The form of the writable plate 110 can be determined by the needs of the user and the types of information 120 to be encoded. In some instances, the writable plate 110 can be formed in a tablet shape for storage, preferably no larger than 200 mm by 200 mm, more preferably no larger than 100 mm by 100 mm, more preferably no larger than 10 mm by 10 mm. In other instances a computer readable disk-shape may be preferable with a diameter no larger than 30 cm, more preferably no larger than 12 cm, more preferably no larger than 8 cm.

[0104] The information storage medium 110 according to the present invention is resistant to environmental degradation and is preferably able to withstand temperatures between −273° C. (0° K) and 1200° C. without suffering information loss. The information storage medium 100 may also resist electro-magnetic pulses, water damage, corrosion, acids and/or other chemicals. It is envisioned that the information storage medium 100 as herein described could preserve information 120 for a time period of at least 10 years, preferably at least 100 years, preferably at least 1,000 years, more preferably at least 10,000 years, more preferably at least 100,000 years. Under certain conditions of storage, including storage of the information storage medium 100 within an underground salt dome, the information storage medium may be able to preserve information for at least 1 million years.

[0105] FIG. 4 schematically depicts a cross section through an information storage medium suitable for long-term storage of information according to a further preferred embodiment of the present invention. The information storage medium comprises a ceramic substrate 150 coated with four layers 171 to 174 of different second materials being different from the material of the ceramic substrate 150. Again, a sintered interface (not shown) may be present at least between the ceramic substrate 150 and the bottommost layer 171 of the four layers. The sintered interface may comprise at least one element from both the substrate material and the material of the bottommost layer 171. Similar to the embodiment shown in FIG. 1, the information storage medium of the embodiment shown in FIG. 4 comprises a plurality of recesses 10 encoding information on the information storage medium, wherein the plurality of recesses 10 have different depths and wherein each depth corresponds to a predefined bit of information. Again, 16 different depths are shown in FIG. 4 corresponding to a 4-bit code.

[0106] However, different from the embodiment shown in FIG. 1, in case of the embodiment shown in FIG. 4 four different bits are encoded (by means of different depths) in each of the four layers 171 to 174. If the four layers 171 to 174 are made from different materials, the optical response of each layer may be different. This allows for achieving high accuracy during decoding because the depth information achieved may be correlated with, for example, the optical response.

[0107] Of course, more or less than four layers of different second materials may be present depending on the number of bits to be encoded.

[0108] One particularly preferred example for the multi-layer coating shown in FIG. 4 is a two-layer coating with a metal layer 171 being coated on the substrate 150 and a metal oxide layer (of the same metal) 172 being coated on the metal layer 171. If such a two-layer coating is illuminated with incident white light as schematically shown in FIG. 5, a part of the incident light 1 is reflected (2) at the oxide layer, whereas another part of the incident light 1 is refracted (3) into the oxide layer and reflected (4) at the oxide/metal interface. The light beam having been reflected at the oxide layer and the light beam having been reflected at the metal layer can be in phase, which leads to a visible colour, or out of phase, which does not yield said colour to be visible. Accordingly, a certain colour (which depends on the indices of refraction of both the oxide layer and the metal layer and the thickness of the oxide layer) is visible wherever the oxide layer is present, yet is invisible if the depth of a certain recess leads to destructive interference at this particular spot.

[0109] FIG. 6 depicts an exemplary graph showing the reflectance of laser light depending on the wavelength for a Ti/TiO.sub.2 double layer with different TiO.sub.2 layer thicknesses (17 nm, 24 nm, 28 nm, 31 nm, 40 nm and 46 nm). As can be seen in FIG. 6, the minimum reflectance depends strongly on the layer thickness and is shifted from about 400 nm (thickness of 17 nm) to about 700 nm (thickness of 46 nm) changing the color impression from yellow to blue. Accordingly, a whole color spectrum may be encoded with multiple recesses of different depths corresponding to the respective reflectance minima.

[0110] Thus, it is in principle possible to create a polychrome microfilm utilizing a metal/metal oxide layer system and encoding different colours by means of different depths of recesses.

[0111] FIG. 7 schematically depicts a cross-section through an information storage medium suitable for long-term storage of information according to a further preferred embodiment of the present invention. The information storage medium comprises a ceramic substrate 150 coated with a layer of the second material 170. Again, a sintered interface (not shown) may be present between the ceramic substrate 150 and the layer of the second material 170, wherein the sintered interface comprises at least one element from both the substrate material and the second material. The surface of the layer of the second material 170 comprises a plurality of nanostructures 20, wherein the plurality of nanostructures 20 have different optical properties and wherein each optical property corresponds to a predefined bit of information. In the specific example shown in FIG. 7, the different optical properties of the plurality of nanostructures 20 correspond to different orientations of so-called nano-ripples. In the depicted example, four different orientations of such nano-ripples are shown corresponding to a 2-bit code. Such nano-ripples having different orientations can be manufactured as follows: A femtosecond laser can be used to create wavy nanostructures named nano-ripples on ceramic (e.g. CrN) or metallic (Cr) surfaces. Several dozen to hundreds of linearly polarized femtosecond laser pulses and energy flow far below the ablation threshold generate the above mentioned nano ripples parallel to the direction of the polarization.

[0112] Several examples will be described in the following.

[0113] As a first example, a ceramic substrate made of Rubalit 708s containing at least 96% Al.sub.2O.sub.3 having the dimensions of 20 cm×20 cm available at CeramTec GmbH (Germany) was used as the raw material.

[0114] A plate of said ceramic substrate having the size of 10 cm×10 cm and a thickness of 1 mm was coated with a layer of CrN using physical vapor deposition. For this purpose, the ceramic plate was mounted on an electrically conductive plate made from steel with a size of 10 cm×10 cm. The ceramic plate together with the electrically conductive plate was brought into a physical vapor deposition machine available from Oerlikon Balzers AG (Lichtenstein).

[0115] Physical vapor deposition was then performed using the enhanced sputtering process BALI-NIT® CNI from Oerlikon Balzers AG at a process temperature below 250° C.

[0116] After the deposition, a layer of CrN with a constant thickness of 5 μm was present on one side of the ceramic substrate (opposite to the side facing the electrically conductive plate).

[0117] Subsequently, the coated ceramic substrate was tempered in a batch furnace model “N 150/H” available from Nabertherm GmbH. For tempering, the temperature was ramped up from room temperature (20° C.) to 1,000° C. within 2 h. The temperature was then increased with a rate of 100 K/h from 1,000° C. to 1,200° C. and the maximum temperature of 1,200° C. was maintained for 5 min. Subsequently, the substrate was cooled down with a rate of −200 K/h over 6 h.

[0118] After tempering, the stack of material comprised the ceramic substrate made of Rubalit 708s containing at least 96% Al.sub.2O.sub.3, a coating layer of CrN having a thickness of about 5 μm and a further metal oxide layer of Cr.sub.2O.sub.3 having a thickness of about 1 μm. Similar metal oxide layers have been described in Z. B. Qi et al. (Thin Solid Films 544 (2013), 515-520).

[0119] The metal oxide surface had a green darkish, almost black appearance.

[0120] The surface of said stack of material was inscribed in thin lines of 10-20 μm width of different depths using the femtosecond laser “CARBIDE” available from the company Light Conversion. The laser parameters used for inscribing were 230 fs pulse width, 515 nm wavelength, 60 kHz and 100 kHz repetition rate.

[0121] The laser created recessions reaching several depth levels between 4 and 10 μm dependent on the number of pulses used. FIGS. 8a and 8b show micrographs of the surface of said probe taken with the Keyence VHX-7000 high-resolution 4K microscopes at different magnifications (the bar at the right bottom of the two micrographs corresponding to 1,000.00 μm and 100.00 μm, respectively) with the depth (and width) decreasing from left to right.

[0122] FIG. 8c shows a 3D visualization of a section through the micrograph of FIG. 8b. As may be taken from said figure, each recession has a substantially constant width and depth along its length. FIG. 8d shows the cross sectional height profile through a section of the micrograph of FIG. 8a. Again, the depth clearly decreases from left to right. As is clearly visible, the depth of each recession can be controlled by the number of pulses used for inscribing with each pulse creating a depth of 500-1,000 nm.

[0123] Interestingly, the edges of the inscription show no sign of molten coating material (CrN and Cr.sub.2O.sub.3) due to the cold ablation effect (Coulomb explosion) of ultra-short pulses.

[0124] As a second example, the same stack of material as described in the first example was produced.

[0125] The surface of said stack of material was inscribed in thin lines of 1.92 μm width of different depths using a Spectra Physics Femtosecond-Laser Spirit-1040 HE30 (1040 nm, <400 fs, up to 120 μJ) using a focal length of 56 mm. Each laser pulse engraved a line recess having a depth of 1 μm. Each subsequent pulse at the same spot increased the depth by about 1 μm. Thus, five different line recesses of 1.92 μm width with depths of 1 μm, 2 μm, 3 μm, 4 μm, and 5 μm could be achieved. FIGS. 9a and 9b show micrographs of the surface of said probe taken with the Keyence VHX-7000 high-resolution 4K microscopes at different magnifications (the bar at the left top of the two micrographs corresponding to 20 μm) with the depth increasing from left to right.

[0126] As a third example, a ceramic substrate made of Rubalit 708s containing at least 96% Al.sub.2O.sub.3 having the dimensions of 22 mm×7 mm available at CeramTec GmbH has been coated with 500 nm CrN in a Leybold Z400 deposition system with the following process parameters: [0127] 3-inch Cr target (Plansee Composite Materials GmbH) [0128] base pressure below 5×10−6 mbar [0129] working gas pressure: 0.36 Pa with a N.sub.2/Ar flow-rate-ratio of 16/16 sccm/sccm [0130] DC target power: about 200 W (current controlled with 0.5 A) [0131] no substrate heating [0132] no substrate bias (hence, floating potential).

[0133] The surface of said probe was inscribed in thin lines of 30 nm width of different depths using a FEI Quanta 200 3D DFIB (a focused ion beam—FIB—workstation, equipped with a Ga ion source) with 6.667 nC/μm.sup.3 at 0.1 nA and 30 kV (which corresponds to 2*1014 J/m.sup.3 or 0.2 mJ/μm.sup.3). The ion beam was focused to 11.5 nm spot size. The focused ion beam engraved in an initial passage a depth of 50 nm. Each subsequent inscription with a further ion beam passage increased the depth by about 50 nm. Thus, ten different line recesses of 30 nm width with depths of 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, etc. could be achieved. FIG. 10 shows an SEM image of the surface of said probe taken with a FEI Quanta 250 FEG (a field emission gun scanning electron microscope—FEGSEM) with the depth increasing from left to right. The spacing identified in the SEM image with two arrows corresponds to 30.0 nm.

[0134] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and non-restrictive; the invention is thus not limited to the disclosed embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality and may mean “at least one”.