WO2008056190A2

WO2008056190A2 - Ordered array of nanostructures and method of fabrication

Info

Publication number: WO2008056190A2
Application number: PCT/GB2007/050681
Authority: WO
Inventors: David Cockayne; Yizhong Huang
Original assignee: Isis Innovation Limited
Priority date: 2006-11-10
Filing date: 2007-11-12
Publication date: 2008-05-15
Also published as: WO2008056190A3; GB0622447D0

Abstract

The present invention relates to nanostructured materials. In particular, but not exclusively, the invention relates to an ordered array of needle-like nanostructures, and a method of forming such an ordered array. Embodiments of the invention provide a method of fabricating an ordered array of nanostructures comprising the steps of: forming a first layer comprising a first material on a substrate comprising a second material; patterning the first layer to form a patterned structure comprising an array of pillars; and subsequently irradiating the patterned structure with ions.

Description

ORDERED ARRAY OF NANOSTRUCTURES AND METHOD OF FABRICATION

The present invention relates to nanostructured materials. In particular, but not exclusively, the invention relates to an ordered array of nanoscale needle-like structures, and a method of forming such an ordered array.

Nanostructured materials are of increasing interest as elements of advanced devices having a variety of applications in the fields of electronics, optoelectronics, photonics and magnetics. Exploitation of physical phenomena associated with nanoscale systems offer a range of possibilities for the development of new devices.

For example, arrays of carbon nanotubes have been employed as field emitters in field emission display devices, and as memory elements in data storage devices.

Nanostructures can be fabricated in a variety of ways, including by vacuum deposition, by annealing, by self assembly from a melt during cooling from a liquid phase, and by catalysis as in the case of carbon nanotubes.

In a first aspect of the invention there is provided a method of fabricating an ordered array of nanostructures comprising the steps of: forming a first layer comprising a first material on a substrate comprising a second material; patterning the first layer to form a patterned structure comprising an array of pillars; and irradiating the patterned structure with ions.

This has the advantage that the locations on a substrate at which nanostructures are to be formed can be precisely determined. Furthermore, large ordered arrays of nanostructures may be fabricated in a convenient and efficient manner.

The step of patterning the first layer may comprise the steps of irradiating the first layer with a beam of ions.

The use of a beam of ions has the advantage that the first layer may be patterned without the need to expose the first layer to a wet chemical etchant. Use of a wet chemical etchant may result in disadvantageous reaction between the etchant and the first layer.

The step of irradiating the patterned structure with ions may comprise the steps of irradiating the patterned structure with a beam of substantially parallel ions.

Alternatively or in addition the beam of ions may comprise a beam of focussed ions.

The step of irradiating the patterned structure with ions may comprise the steps of scanning a beam of ions over the patterned structure.

This has the advantage that the first layer may be patterned directly, without the need for the use of an intermediate layer of a resist material which must itself be subjected to a patterning process.

The step of patterning the first layer to form a patterned structure may comprise the steps of forming a plurality of channels in the structure.

The channels may extend between the substrate and the free surface of the first layer, the channels having a depth of up to a full thickness of the first layer.

The channels are formed through at least a portion of a material underlying the first layer.

The channels may extend from the substrate to a free surface of the first layer.

Preferably, the step of forming a plurality of channels in the structure comprises the step of forming an array of substantially parallel channels.

A plurality of arrays may be formed. At least two of the plurality of arrays may be substantially orthogonal.

Two arrays may be formed, oriented orthogonal to one another. Respective parallel channels of each arrays may be generally equally spaced from one another. The method may further comprise the step of forming a hole in the first layer prior to irradiating the patterned structure with ions, said hole being formed in an island defined in the first layer by one or more of said plurality of arrays of channels.

This feature has the advantage that a generally hollow nanostructure may be formed following a subsequent step of irradiating the structure with ions or a beam of ions.

The hole may be formed in a substantially central portion of an island.

This feature the advantage that a nanostructure having a generally hollow central core may be formed.

The hole may be formed away from a central portion of an island.

The hole may be formed at an edge of an island.

The hole may be formed through the entire thickness of the first layer.

The hole may be formed through at least a portion of a material underlying the first layer.

A method as claimed in any one of claims 13 to 18 comprising the step of forming a third material in the hole.

The method may comprise the step of forming a plurality of holes in the first layer.

The method may comprise the step of forming a hole in each of the islands.

The method may further comprise the step of forming a third material in each of a plurality of the holes.

The method may still further comprise the step of forming a fourth material in each of a plurality of the holes.

The third and fourth materials may each comprise a magnetic material. In some embodiments the third material comprises a magnetic material whilst the fourth material does not comprise a magnetic material. Preferably, the nanostructures are formed to comprise a portion of the second material and a portion of the first material.

A multilayer structure may be provided between the first layer and the substrate.

The nanostructures may be formed to comprise a portion of the multilayer structure.

The step of patterning the first layer may further comprise the step of patterning the multilayer.

The step of patterning the first layer may further comprise the step of patterning the substrate.

Preferably the first layer is formed to comprise a magnetic material.

This has the advantage that nanoscale magnetic phenomena may be exploited.

The magnetic material may comprise iron.

Alternatively or in addition the first layer may comprise iron oxide.

The iron oxide may comprise Fe₃O₄.

The iron oxide layer may be formed by post growth oxidation of an iron layer. Alternatively or in addition the iron oxide layer may be formed by forming a layer of iron oxide directly on a substrate.

The substrate may be formed to comprise a semiconductor material.

This feature has the advantage that phenomena associated with semiconductor materials may be exploited in combination with the first layer. For example, metal: semiconductor junctions may be formed. Furthermore, conventional semiconductor circuit elements may be integrated onto the same substrate. The conventional semiconductor elements may form part of a system comprising nanoneedles or other nanostructures. The semiconductor material may comprise gallium arsenide. Alternatively or in addition the semiconductor material may comprise silicon.

Alternatively or in addition the substrate may be formed from an oxide material, a nitride material, or any other suitable substrate material.

The first layer may comprise an epitaxial material.

This has the advantage that an ordered layer may be formed having a relatively uniform structure. Thus, the properties of respective nanostructures may be relatively uniform. The nanostructures may be nanoneedle or nanoneedle-like structures.

In a second aspect of the invention there is provided a method of fabricating a nanoneedle at a predetermined location on a surface of a substrate, comprising the steps of: forming a pillar comprising a first material on a substrate comprising a second material; and irradiating the pillar with ions.

In a third aspect of the invention there is provided a structure comprising: an ordered array of nanostructures formed on a substrate, wherein the nanostructures comprise an upper portion formed from a first material and a lower portion formed from a second material.

In a fourth aspect of the invention there is provided a method of fabricating an ordered array of nanostructures comprising the steps of: forming a first layer comprising a first material on a substrate comprising a second material; patterning the first layer to form a patterned structure comprising an array of pillars; and irradiating the patterned structure with ions.

For a better understanding of the present invention, and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which: FIGURE 1 is a cross-sectional schematic diagram of a process of forming nanostructures according to an embodiment of the invention;

FIGURE 2 is a scanning electron micrograph of a Fe film formed on a GaAs substrate following a patterning process according to a first embodiment of the invention;

FIGURE 3 is a scanning electron micrograph of a structure formed according to the embodiment of FIG. 1 ;.

FIGURE 4 is a scanning electron micrograph of a structure according to an embodiment of the invention;

FIGURE 5 is a plan view of a patterned first layer wherein islands of first material are provided with a central hole therethrough prior to irradiation with ions; and

FIGURE 6 is a scanning electron micrograph of a generally hollow structure according to an embodiment of the invention.

According to a first embodiment of the invention, an array of nanoneedles is fabricated according to the method shown schematically in Figure 1.

Figure 1 (a) is a cross-sectional schematic diagram of a structure having a first layer of material 10 formed over a substrate 20. In the first embodiment of the invention the first layer 10 is an epitaxial layer of Fe, and the substrate 20 is an (001 ) GaAs wafer.

The epitaxial layer is deposited at room temperature to a thickness of about 50nm. According to the embodiment of FIG. 1 the first layer is deposited at room temperature by molecular beam epitaxy under ultrahigh vacuum (UHV) conditions. Prior to depositing the first layer, the substrate is heated under UHV conditions to a temperature of about 800⁰C for a period of 30 minutes in order to obtain a clean surface. Other methods of obtaining a clean surface are also useful.

It will be appreciated by the person skilled in the art, however, that an epitaxial layer of Fe may be formed on an (001 ) gallium arsenide substrate using a variety of techniques including sputtering, chemical vapour deposition (CVD), and any other suitable techniques.

Figure 1 (b) is a cross-sectional schematic diagram of the structure of Figure 1 (a) following patterning of the first layer 10 using a focused beam of Ga+ ions. Other focussed beams are also useful. Other methods of patterning are also useful.

In some embdodiments, lithographic techniques, such as patterning and etching are used. Dry of wet etching may be used.

The patterned layer comprises two arrays of mutually orthogonal channels cut in the first layer by systematically scanning the ion beam across the surface of the first layer 10. Discrete Fe pillars 12 remain on the substrate 20 following patterning.

Figure 2 is an electron micrograph of an epitaxial Fe film 50nm in thickness formed on an (00I )GaAs substrate that has been patterned in this manner. The structure of Figure 2 was produced using an ion beam current of 1 nA at a spot size of about 150nm. The ordered array of Fe pillars is formed to have a pillar: pillar pitch of about 0.9um.

In the embodiment of FIG.2 the pillars are of generally square cross-section when viewed in plan view. Other cross-sectional shapes are also useful.

End point detection means is employed to determine when a channel has been cut to the required depth, at which stage irradiation of that channel is terminated.

The end point detection means is configured to detect direct irradiation of the GaAs substrate with the ion beam, thereby indicating that a channel has been cut through the thickness of the first layer 10.

It will be appreciated that in embodiments of the invention end point detection is not used. Furthermore, it will be appreciated that in some embodiments of the invention endpoint detection means may be configured to allow channels to be formed to penetrate through both the first layer 10 and a portion of the substrate 20. Figure 1 (c) shows a cross-sectional schematic diagram of the structure of Figure 1 (b) following irradiation of the structure by Ga+ ions. In the embodiment of FIG. 1 (c) the Ga+ ions are supplied in the form of a beam.

Irradiation with Ga+ ions leads to removal of material from both the first layer 10 and the underlying substrate 20, resulting in the formation of nanoneedles 30.

The nanoneedles 30 have an upper portion 32 and a lower portion 34. The upper portion 32 is formed from the first layer 10 whilst the lower portion 34 is formed from the substrate 20.

It will be appreciated that the invention is not limited to irradiation by Ga+ ions, and that other ion species could be used instead. Similarly, the beam need not be a focussed ion beam. In alternative embodiments of the invention a broad, substantially parallel beam of ions is used to irradiate the patterned structure.

It will be further appreciated that a non-scanned source of ions can be used to irradiate the patterned structure. For example, a stationary source of substantially parallel ions wide enough to irradiate a patterned area of interest may be used. The stationary source is a stationary beam in some embodiments of the invention, such as that shown in FIG. 1 .

Figure 3 shows the structure of Figure 2 following a process of irradiation of the structure by a beam of Ga+ ions with an energy of 30keV, at a dose of 3x10¹⁷cm^"2. The ion beam was oriented substantially normal to the surface of the substrate, and raster-scanned across the surface of the patterned structure such that adjacent beam spots were set to have zero overlap. The spot size was approximately 150nm at a beam current of 1 nA. The dwell time of the ion beam at each point of the scan was 1.0 μs.

The formation of nanoneedle structures by irradiation of a uniformly patterned structure with a relatively broad beam of ions is a surprising effect. It is clearly different from conventional methods by which pre-determined structures are milled using a focused ion beam. The scale of the features resulting from the process described above is much smaller than the diameter of the ion beam used to produce the structures. Formation of the structure does not require variation of either the ion beam current or the angle of incidence of the ion beam with respect to the substrate. It will of course be appreciated that some variation in these parameters may be advantageous in some embodiments of the invention.

In the case of the structures described above, formed according to the embodiment of FIGs. 1 to 3, nanoneedles having tips of around 10-20nm in diameter were produced using a parallel beam of ions incident on the substrate substantially normal to the plane of the substrate. The diameter of the beam was approximately 150nm in the particular example of FIG. 3.

In alternative embodiments of the invention, ion beams of larger diameter may be used in order to form nanoneedle structures.

In some embodiments of the invention, the first layer 10 is an epitaxial iron oxide layer instead of an Fe layer; in some embodiments the epitaxial iron oxide is Fe₃O₄. Other layers are also useful.

In some embodiments the substrate is silicon (Si) instead of GaAs. In some embodiments of the invention, the first layer 10 is Fe₃O₄ and the substrate is (10O)GaAs.

In some embodiments of the invention the iron oxide layer is 10nm in thickness. In some embodiments the iron oxide layer is formed by forming a layer of Fe by molecular beam epitaxy followed by post growth oxidation to form Fe₃O₄. In some embodiments of the invention, post growth oxidation of an epitaxial Fe layer is performed.

In some embodiments of the invention the iron oxide layer is not formed initially as an iron layer and converted to iron oxide, but rather an iron oxide layer is formed directly on the substrate surface.

In some embodiments of the invention, a silicon substrate is used. In some embodiments of the invention, a (1 10) silicon substrate is used. In some embodiments an epitaxial layer of Fe is formed on a (1 10) silicon substrate to a thickness of around 100nm. In some embodiments of the invention, an interlayer (which may be a barrier layer) is provided between the substrate and the epitaxial layer. FIGURE 4 shows an example of a structure formed on a (110) silicon (or (11O)Si) substrate following a process of forming a first layer of epitaxial Fe, 100nm in thickness, on the (11O)Si substrate.

Following formation of the epitaxial Fe layer on the (11O)Si substrate, the epitaxial Fe layer (the 'first layer 10') was patterned to form two arrays of mutually orthogonal, generally equally spaced channels by systematically scanning the ion beam across the surface of the Fe layer. Discrete pillars of Fe remain on the substrate following patterning.

The structure was then irradiated by a beam of Ga+ ions, resulting in the formation of nanoneedles 40 (FIG. 4).

In the case of the formation of an Fe layer on a (11O)Si substrate, the aspect ratio of the nanostructures formed following ion irradiation is found to be generally lower than those formed in the case of Fe formed on (10O)GaAs substrates. In other words, the nanostructures formed in the case of Fe on Si(110) are less elongate than those formed in the case of Fe on (10O)GaAs. Thus, the nanostructures formed in the case of Fe on (10O)GaAs are more needle-like than those formed on (11O)Si.

In a variation on the embodiments described above, in some embodiments a hole is formed in the first layer 10 in the course of patterning the first layer 10.

FIG. 5 is a schematic plan view of a structure similar to that of FIG. 2 following a process of forming holes 50 in the first layer 10 at a position generally in the centre of each Fe pillar 12. As in the case of the embodiment of FIG. 2, an array of orthogonal channels

51 have been formed in the first layer 10 to define the Fe pillars 12. In the embodiment of FIG. 5 the channels do not penetrate the substrate surface. In some embodiments, the channels do penetrate the substrate surface.

In some embodiments of the invention the channels are not formed through a full thickness of the first layer 10.

FIG. 6 shows the structure of FIG. 5 following a process of irradiation the structure with a beam of gallium ions in a corresponding manner to other embodiments of the invention described above. It can be seen from the figure that truncated hollow needle-like structures are thereby formed. The structures may be referred to as 'hollow nanoneedles' or 'nanoring' structures.

It will be appreciated that the depth of the hole 50 may be varied in order to form structures of different size and shape. For example, in some embodiments the hole is formed to penetrate through a portion of the substrate. In some embodiments the hole is not formed through the entire thickness of the first layer. In some embodiments the hole is formed to expose the substrate surface.

In some embodiments of the invention the hole is filled with a third material. In some embodiments the hole is filled with a third material prior to exposing the structure to ions.

In some embodiments the third material comprises a magnetic material. In some embodiments the third material does not comprise a magnetic material. In some embodiments some holes are filled with a third material whilst other holes are filled with a fourth material. The fourth material comprises a magnetic material in some embodiments whilst in other embodiments the fourth material does not comprise a magnetic material.

In some embodiments the third material comprises a magnetic material whilst the fourth material does not comprise a magnetic material.

In some embodiments the spacing of the orthogonal channels is different along one direction compared with that along another direction. In other words, oblong pillars 12 (as viewed from above) may be formed. Similarly, respective arrays of generally parallel channels need not be orthogonal, but may be provided at an angle not equal to zero to one another. Thus, in some embodiments, pillars 12 in the shape of a parallelogram (as viewed from above) or other four-sided figure(s) may be formed.

In some embodiments pillars are formed not having a pair of mutually parallel sides.

In some embodiments, pillars may be formed to have a different number of sides, such as three sides, five sides, six sides, seven sides, eight sides, nine sides, ten sides, or any other suitable number of sides. In some embodiments the pillars are formed to be generally circular or elliptical in plan view. Nanostructures formed from pillars of different shapes may be configured to exhibit different respective magnetic properties. For example, in some embodiments magnetic structures having uniaxial magnetic properties may be formed.

In some embodiments, respective arrays of channels are formed, wherein channels of a given array are not all mutually parallel.

In some embodiments the pillars 12 are not formed in the first layer by cutting arrays of channels.

In some embodiments of the invention, pillars 12 are formed of different respective shapes on the same substrate. In some embodiments, the respective different shapes may be formed to provide magnetic properties tailored to a particular application or range of applications.

Nanostructures according to embodiments of the invention may be of utility in applications including field optical emission, tunnelling probe applications, atom probe applications and a variety of other optical, magnetic, electronic and chemical applications.

It is anticipated that in some embodiments of the invention, doping of the substrate 20, for example a semiconductor substrate such as gallium arsenide, would allow the fabrication of devices exploiting a variety of quantum size effects.

Of particular importance might be nanomagnets of a size to form magnetic vortices, such as nanoneedles having a portion of magnetic material such as those described above with reference to Figure 3. Such nanomagnetic structures or 'nanomagnets' are of potential utility as nanoscale memory elements. Arrays of nanomagnets such as those shown in Figure 3, 4 or 6 may form the basis for nanoscale memory arrays in advanced micro- and nanoelectronic devices and systems.

It will be appreciated that the word 'pillar' is intended to cover and not to exclude any outwardly extending formation of any shape including but not limited to cylindrical or cuboid formations of any height, aspect ratio, etc. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

CLAIMS:

1 . A method of fabricating an ordered array of nanostructures comprising the steps of: forming a first layer comprising a first material on a substrate comprising a second material; patterning the first layer to form a patterned structure comprising an array of pillars; and subsequently irradiating the patterned structure with ions.

2. A method as claimed in claim 1 , whereby the step of patterning the first layer comprises the step of irradiating the first layer with a beam of ions.

3. A method as claimed in claim 2, wherein the step of irradiating the patterned structure with ions comprises the step of irradiating the patterned structure with a beam of substantially parallel ions.

4. A method as claimed in claim 2 or claim 3 wherein the step of irradiating the patterned structure with a beam of ions comprises the step of irradiating the patterned structure with a beam of focused ions.

5. A method as claimed in any one of claims 2 to 4 wherein the step of irradiating the patterned structure with a beam of ions comprises the step of scanning the beam over the patterned structure.

6. A method as claimed in any preceding claim, wherein the step of patterning the first layer to form a patterned structure comprises the step of forming a plurality of channels in the structure.

7. A method as claimed in claim 6 wherein the channels extend between the substrate and the free surface of the first layer, the channels having a depth of up to a full thickness of the first layer.

8. A method as claimed in claim 7 wherein the channels are formed through at least a portion of a material underlying the first layer.

9. A method as claimed in any of claims 6 to 8 wherein the step of forming a plurality of channels in the structure comprises the step of forming an array of substantially parallel channels.

10. A method as claimed in claim 9 comprising the step of forming a plurality of arrays.

1 1 . A method as claimed in claim 10 wherein at least two of the plurality of arrays are non-parallel.

12. A method as claimed in claim 1 1 wherein at least two of the plurality of arrays are substantially orthogonal.

13. A method as claimed in claim 1 1 or claim 12 further comprising the step of forming a hole in the first layer prior to irradiating the patterned structure with ions, said hole being formed in an island defined in the first layer by one or more channels of said plurality of arrays.

14. A method as claimed in claim 13 wherein the hole is formed in a substantially central portion of an island.

15. A method as claimed in claim 13 wherein the hole is formed away from a central portion of an island.

16. A method as claimed in claim 15 wherein the hole is formed at an edge of an island.

17. A method as claimed in any one of claims 13 to 16 wherein the hole is formed having a depth of up to a full thickness of the first layer.

18. A method as claimed in any one of claims 13 to 17 wherein the hole is formed through at least a portion of a material underlying the first layer.

19. A method as claimed in any one of claims 13 to 18 comprising the step of forming a third material in the hole.

20. A method as claimed in any one of claims 13 to 19 comprising the step of forming a plurality of holes in the first layer.

21. A method as claimed in claim 19 comprising the step of forming a hole in each of the islands.

22. A method as claimed in claim 22 comprising the step of forming a third material in each of a plurality of the holes.

23. A method as claimed in claim 22 comprising the step of forming a fourth material in each of a plurality of the holes.

24. A method as claimed in any preceding claim wherein the nanostructures are formed to comprise a portion of the second material and a portion of the first material.

25. A method as claimed in any preceding claim, wherein a multilayer structure is provided between the first layer and the substrate.

26. A method as claimed in claim 25, wherein the step of patterning the first layer further comprises the step of patterning the multilayer.

27. A method as claimed in claim 25 or 26, wherein the nanostructures are formed to comprise a lower portion comprising a portion of the multilayer structure and an upper portion comprising a portion of the first layer.

28. A method as claimed in any preceding claim, wherein the step of patterning the first layer further comprises the step of patterning the substrate.

29. A method as claimed in any preceding claim, wherein the first layer is formed to comprise a magnetic material.

30. A method as claimed in claim 29, wherein the magnetic material comprises iron.

31 . A method as claimed in any one of claims 1 to 30 wherein the first layer is formed to comprise iron oxide.

32. A method as claimed in claim 31 wherein the iron oxide comprises Fe₃O₄.

33. A method as claimed in any preceding claim, wherein the substrate comprises a semiconductor material.

34. A method as claimed in claim 33, wherein the semiconductor material comprises gallium arsenide.

35. A method as claimed in claim 23 or 34 wherein the semiconductor material comprises silicon.

36. A method as claimed in any preceding claim wherein the substrate comprises at least one selected from an oxide material, a nitride material, or any other suitable substrate material.

37. A method as claimed in any preceding claim, wherein the first layer comprises an epitaxial material.

38. A method of fabricating a nanoneedle at a predetermined location on a surface of a substrate, comprising the steps of: forming a pillar comprising a first material on a substrate comprising a second material; and irradiating the pillar with ions.

39. A structure comprising: an ordered array of nanostructures formed on a substrate, wherein the nanostructures comprise an upper portion formed from a first material and a lower portion formed from a second material.

40. A structure as claimed in claim 39 wherein the first layer comprises a magnetic material.

41 . A structure as claimed in claim 40 wherein the magnetic material comprises iron.

42. A structure as claimed in any one of claims 39 to 41 wherein the first layer comprises iron oxide.

43. A structure as claimed in claim 42 wherein the iron oxide comprises Fe₃O₄.

44. A structure as claimed in any one of claims 39 to 43 wherein the substrate comprises said second material.

45. A structure as claimed in any one of claims 39 to 44 wherein the substrate comprises a semiconductor material.

46. A structure as claimed in claim 45 wherein the semiconductor material comprises gallium arsenide.

47. A structure as claimed in claim 45 wherein the semiconductor material comprises silicon.

48. A structure as claimed in any one of claims 39 to 47 wherein a nanostructure has a hollow portion.

49. A structure as claimed in claim 48 wherein the hollow portion is in the form of a generally central hollow core.

50. A structure as claimed in claim 49 wherein the hollow core intersects a free surface of the nanostructure.

51 . A structure as claimed in claim 49 or 50 wherein the hollow core is filled with a third material.

52. A structure as claimed in claim 51 wherein the third material comprises a magnetic material.

53. A structure as claimed in claim 51 or 52 wherein the third material comprises a non-magnetic material.

54. A structure as claimed in any one of claims 39 to 53 wherein the first layer comprises an epitaxial material.

55. A method of fabricating an ordered array of nanoneedles substantially as hereinbefore described with reference to the accompanying drawings.

56. A method of fabricating an ordered array of nanostructures substantially as hereinbefore described with reference to the accompanying drawings.

57. A method of fabricating a nanostructure at a predetermined location on a substrate substantially as hereinbefore described with reference to the accompanying drawings.

58. A method of fabricating a nanoneedle at a predetermined location on a substrate substantially as hereinbefore described with reference to the accompanying drawings.

59. A structure substantially as hereinbefore described with reference to the accompanying drawings.