WO2009123332A1

WO2009123332A1 - Planar tape

Info

Publication number: WO2009123332A1
Application number: PCT/JP2009/057016
Authority: WO
Inventors: Thomas Heinz-Helmut Altebaeumer; Stephen Day; Christian Lang; Peter Neil Taylor; Jonathan Heffernan
Original assignee: Sharp Kabushiki Kaisha
Priority date: 2008-04-01
Filing date: 2009-03-30
Publication date: 2009-10-08
Also published as: JP2011519729A; CN102007068A; GB2458906A; GB0805846D0

Abstract

A method of manufacturing a structure comprising a plurality of low-dimensional structures comprising: providing a flexible element (1b, 14) that connects two or more of the low-dimensional structures to the substrate. The flexible element allows the low-dimensional structures to be re-oriented, for example to be substantially parallel to the substrate. Additionally or alternatively it allows low-dimensional structures that are not aligned with one another to be aligned in a common direction.

Description

DESCRIPTION

MANUFACTURING A STRUCTURE COMPRISING LOW DIMENSIONAL NANOSTRUACURES

TECHNICAL FIELD

This invention generally relates to the conversion of structures with a high aspect ratio from a substantially non- planar configuration into a substantially planar arrangement. This invention particularly relates to the conversion of structures containing low dimensional structures from a substantially non-planar configuration into a substantially planar arrangement. A planar configuration may be required to deliberately modify the properties of the above mentioned structures using processing techniques whose application would be otherwise impossible or at least impose considerable technological challenges.

BACKGROUND ART

For simplicity, we focus in this invention on examples of structures which are created with a large aspect ratio extending substantially perpendicular to a substrate surface (that is that their dimension extending perpendicular to the substrate surface is considerably larger than at least one of the dimensions extending parallel to the substrate surface) . Examples of such structures include but are not restricted to low-dimensional elongate structures such as nanowires, nanopillars and nanotubes or fin-type structures as described in, for example, co-pending UK patent application 0620 134.7(UK patent application publication No. GB2442768A) .

Nanopillars are fabricated using a suitable etch mask and a subtractive method such as reactive ion etching. Due to the nature of the etch, nanopillars will extend substantially perpendicular off the substrate surface . Nanowires on the other hand are usually fabricated using additive techniques such as chemical vapour deposition or molecular beam epitaxy in conjunction with a suitable metal catalyst and can grow along several distinctive crystal orientations which are determined by the crystal orientation of the substrate on which the nanowires are formed. However, reproducible and well controlled nanowire growth is often associated with the suppression of all but one growth direction. In the particular case of silicon nanowires grown off a ( 1 1 1 ) silicon surface only one out of the four possible [ 1 1 1 ] growth directions is perpendicular to the silicon surface and a high control over the process conditions is usually reflected by one prevailing growth direction which is highly perpendicular off the substrate surface. After the above mentioned structures are formed, it is usually desired to apply at least one of the following techniques in combination with a lithographic technique to modify their properties in order to obtain a particular device (e .g. a transistor, diode, sensor, laser, or light emitting device) :

1 . Additive (e.g. deposition, transfer) • Deposition methods include but are not restricted to direct or indirect thermal evaporation, sputter deposition, chemical vapour deposition, spin coating, and ink-jet printing. • Transfer methods include dry transfer methods such as stamp-based transfers, and device bonding as well as wet transfer methods where the transfer of the desired structures occurs out of solution. 2. Subtractive (e. g. etching, sputtering, dissolving)

• Etching includes wet-chemical etching and dry- etching (e. g. reactive ion etching) . Dry etching techniques may be combined with sputtering techniques . • Sputtering includes ion milling.

3. Selective (e .g. self assembly, chemical functionalisation, local heating, local exposure to particles, local exposure to mechanical stress)

• Local heating may occur due to a localised exposure to an energy source (e. g. a focussed laser beam, selective exposure using a mask) or due to the energy absorbing properties of the elongate low dimensional structures or sections within the elongate low dimensional structures.

• Chemical functionalisation may utilise particular surface properties of the elongate low dimensional structures being defined by the material composition. • Local exposure of particles include beyond the aforementioned lithographic methods also the use of a focussed ion beam. Local exposure to mechanical stress includes imprint technologies. We note that many of the techniques mentioned above require or benefit from a planar device configuration (exceptions being the use of isotropic etchants and solvents, conformal coating techniques, thermal oxidation) and the vast majority of lithographic techniques including photolithography as being the most common one certainly do.

In addition, it may be desired to apply any of the above techniques to obtain the desired device on a suitable substrate which is different to the substrate on which the convertible structures are formed. This will require the application of transfer methods such as stamp-based transfer or direct device bonding for which a planar configuration of the structure is preferred.

This transfer is typically required if the properties of the substrate on which the structure is formed are not advantageous for the desired application while the desired devices could not be fabricated directly onto the desired final substrate to achieve monolithic integration. Monolithic integration may be not feasible because it may be either technically not possible or not cost effective. An example where the monolithic integration may not be a cost-effective option is the monolithic integration of a Complementary-Metal Oxide Semiconductor (CMOS) interface on a MEMS sensor, which often requires running through a costly CMOS process which won't cover the entire substrate area.

The monolithic integration may not be technically viable because either the substrate onto which the devices are to be integrated cannot withstand the process conditions (e.g. high temperature steps) , the required material cannot be deposited with sufficient quality onto the foreign substrate (e.g. due to structural incompatibilities) or the process flow may be incompatible with devices previously fabricated on the receiver substrate (e.g. high temperature steps after metallisation of previous devices or contamination issues) . Display technologies are an example where the structural incompatibilities in conjunction with the low thermal budget of the glass substrate inhibit the formation of single-crystalline semiconductors on amorphous glass substrate and where it is advantageous to integrate high performance semiconducting devices with differing functionality. Examples of such devices include npn transistors and pnp transistors (e. g. to form Complementary Metal Oxide Semiconductor (CMOS) circuits) , pressure sensors (e.g. for haptic interfaces) , light sensors (e. g. for adapting the display to the ambient lighting conditions) and last but not least red, green and blue Light Emitting Devices (LEDs) (e .g. for emissive displays) on a transparent substrate such as a glass substrate or plastic substrates which may be flexible. These devices may contain elongate low dimensional structures, which are formed onto a suitable substrate but can be subsequently transferred onto a different substrate. Examples of devices which may contain elongate low dimensional structures are npn transistors, pnp transistors, sensors, capacitors, red, green and blue LEDs. The bulk of the receiver substrate may consist of glass, polymers, metals, or semiconductors.

The term "low dimensional structure" as used herein refers to a structure that has at least one dimension that is much less than at least a second dimension. The term "elongate structure" as used herein refers to a structure having two dimensions that are much less than a third dimension. The definition of an "elongate structure" lies within the definition of a "low dimensional structure", and a nanowire is an example of a structure that is both a low dimensional structure and an elongate structure.

Low dimensional structures that are not elongate structures are known. For example, 'platelets' or tapes, both have two dimensions of comparable magnitude to one another and a third (thickness) dimension that is much less than the first two dimensions constitute "low dimensional structures" but are not "elongate structures" .

Description of the prior art

Methods are known for transferring structural features from a first substrate to a second substrate . However, at present no suitable techniques are available for converting elongate/low dimensional structures from a substantially vertical orientation to a substantially planar configuration such that all of the following desiderata are met: 1 . The spatial arrangement and spacing of the structures consisting of or containing elongate / low dimensional structures within each group is substantially maintained;

2. No compromise on the physical properties of the elongate low dimensional structures is required; 3. No additional lithographic techniques are required;

4. The desired planar configuration including alignment and orientation is maintained while applying suitable subsequent processing techniques. Control over one or more (and preferably all) the factors set out above is necessary to permit the use of such elongate or low dimensional structures to improve existing and develop new nanotechnologies .

US patent No . 7067328 discloses a method for transferring nanowires from a donor substrate (for example the substrate on which they are formed) to a receiver substrate. This is achieved by disposing an adhesion layer on the receiver substrate, and mating it with the donor substrate. A degree of alignment and ordering of the nanowires on the receiver substrate is achieved by moving the donor substrate and receiver substrate relative to one another while they are in contact. This method suffers from poor control over the spatial arrangement of the nanowires which will become a severe challenge if less compliant semiconducting nanowires are to be transferred. For example nanowires with diameters exceeding 80nm have a tendency to snap off.

US patent No. 6872645 teaches a method of positioning and orienting elongate nanostuctures on a surface by harvesting them from a first substrate into a liquid solution and then flowing the solution along fluidic channels formed between a second substrate and an elastomer stamp. The nanostructures adhere to the second substrate from the solution with a preferred orientation corresponding to the direction of fluid flow. This invention suffers from the challenge related to any approach attempting to assemble nanowires out of solution: obtaining a high degree of control over the spatial arrangement, orientation and density.

US patent No. 7091 120 discloses a process in which a liquid material is disposed on a population of nanowires that are attached to a first substrate with their longitudinal axes perpendicular to the plane of the first substrate. The material is then processed in order to cause it to solidify into a matrix that is designed to adhere to the nanowires and act as a support for the nanowires during the process of separating the nanowires from a first substrate and transferring them to a second substrate.

US 709 1 120 also discloses an extension to this process whereby the composite of nanowires embedded in the matrix material is lithographically patterned into blocks . The blocks are then applied to a second substrate such that the embedded nanowires are aligned with their longitudinal axes parallel to the plane of the second substrate . However, this invention does not disclose how the reorientation resulting in a planar arrangement can be facilitated. To address this limitation, in one embodiment of the method of US 7091 120 the composite material is formed by unidirectionally disposing the matrix material on an ordered or random arrangement of nanowires. The directional flow of the matrix material induces the nanowires to orientate within the composite material parallel to the plane of the first substrate . This approach essentially suffers from the same constraints as US 7067328 as it is constrained to sufficiently compliant nanowires.

Angew. Chem. Int. Ed. 2005, 44 , 2-5 discloses a method of aligning anchored Germanium nanowires. A water droplet is positioned on the sample containing the nanowires and subsequently blown of with nitrogen. The nanowires aligned into the direction induced by the flow of the water droplet. This method requires nanowires with sufficient flexibility and length and can not be applied to thicker or considerably shorter nanowires. Hence it suffers from the same constraint as US 7067328 and US 709 1 120.

In addition to the above prior art, co-pending unpublished UK patent application 0620134.7 describes a method of making encapsulated low dimensional structures such that they are suitable to be transferred to a different substrate. During the transfer the number of elongate structures, their alignment, spacing, and their orientation are maintained. Furthermore, these structures can be subsequently processed into devices using conventional lithographic methods in combination with subtractive (e. g. dry etching) and additive techniques (e. g. metal deposition) . The number of elongate structures within each device is well controlled. In one embodiment, a weak joint is defined allowing the encapsulated structures to be more easily- released from the substrate. However, this application does not address how the nanostructures can be reoriented from a perpendicular orientation relative to the first substrate to a parallel orientation relative to the second substrate. US2006/ 01 10738 proposes a "molecular motor" which has a base structure and a molecular rotor which can rotate relative to the base structure. The molecular motor is formed of an F l -ATPase enzyme which has a base structure formed of alpha and beta subunits and also has a "gamma-subunit" which can rotate relative to the base structure.

US2006/ 01 10738 proposes attaching a gold nanowire to the gamma subunit in order to make detection of rotation of the gamma subunit easier - the nanowire scatters light and the scattering characteristics are dependent on the angle of rotation of the nanowire and hence on the angle of rotation of the gamma subunit. In another embodiment the nanoparticle is not attached directly to the gamma subunit, but is attached to an oligonucleotide or antibody which in turn is attached to the gamma subunit. DISCLOSURE OF THE INVENTION

The current invention addresses the challenge of converting structures consisting of or containing low- dimensional elongate structures from a non-planar orientation to a substantially planar configuration.

The current invention addresses in particular the challenge of converting structures consisting of or containing low dimensional elongate structures from a substantially vertical orientation to a substantially planar configuration such that some, preferably all of the following desiderata are met:

1. The spatial arrangement and spacing of the structures consisting of or containing elongate/low dimensional structures within each group is substantially maintained;

2. No compromise on the physical properties of those sections of the elongate low dimensional structures is required which subsequently determine the performance of the intended device, e.g. these sections do not need to be modified to accommodate the desired change in orientation;

3. No additional lithographic techniques are required;

4. The desired planar configuration including alignment and orientation is maintained while applying suitable subsequent processing techniques. For the avoidance of any doubt, references to a 'substantially perpendicular orientation' or to 'a substantially vertical orientation' means that the smallest angle between the low-dimensional elongated structures and a surface exceeds 45° . Furthermore, a planar configuration means that the largest dimension of the structure extends parallel to the substrate surface while the shortest dimension extends perpendicular to the substrate surface. If the two shortest dimensions are very similar or even identical (as in the case of elongate low dimensional structures) , only one of the two shortest dimensions needs to extend perpendicular off the substrate to obtain a planar configuration.

A first aspect of the present invention provides a method of manufacturing a structure comprising a plurality of low- dimensional structures, the method comprising: providing a flexible element that connects at least two of the low-dimensional structures to the substrate, the flexible element having different elastic properties to a body portion of the low-dimensional structure.

The invention may be used to re-orient a structure including a group of elongate structures, for example nanowires, which all extend along substantially along the same direction. Providing the flexible element allows the structure to be re-oriented by applying a suitable force to the structure. The flexible element continues to connect the low- dimensional structures to the substrate while the structure is re-oriented, and may be incorporated in a finally-obtained device (although the flexible element may alternatively be removed) . After re-orientation the low-dimensional structures may extend parallel to a surface of the substrate on which the re-orientation took place. The structure may then be further processed to form devices using well-established planar processing techniques. Alternatively, the structure may be transferred to another substrate after it has been re-oriented.

The flexible element is preferably sufficiently flexible to allow the structure to be re-oriented by 90° (i.e. , such that the low-dimensional structures are re-oriented from being perpendicular to the substrate to being parallel to the substrate) . In practice, however, when the low-dimensional structures are re-oriented it may be possible for the connection between some of the low-dimensional structures and the substrate to break, and/ or for some of the low- dimensional structures to break near their connection to the substrate, without preventing satisfactory re-orientation of the array. The flexible element may connect all, or substantially all, of the low-dimensional structures to the substrate . It is not necessarily the case that each and every low-dimensional structure need be, or will be, connected to the substrate by a flexible element; for example the connections between some of the low-dimensional structures and the substrate may break from the action of re-orienting the structure, or the connections between some of the low-dimensional structures and the substrate may be severed purposely (or otherwise) before the structure is re-oriented. At a minimum, it is necessary that only two of the low-dimensional structures are connected to the substrate by the flexible element, in order to constrain the directions in which the structure as a whole may be re-oriented.

Providing the flexible element makes it possible to change the orientation of the structure without loosing control over its position.

The invention also allows low-dimensional structures that extend in more than one direction to be substantially aligned along one common direction. For example, if nanowires have been grown such that they are mis-aligned with one another and extend along respective directions that vary from one another, provision of the flexible element of the invention allows the nanowires to be re-oriented so as to extend substantially along a common direction. This direction may extend parallel to a surface of the substrate on which the re-orientation took place. The low-dimensional structures may then be further processed to form devices or may be transferred to another substrate . (It is conceivable that the low-dimensional structures may be re-oriented in two or more steps, eg a first step in which the low-dimensional structures are re-oriented to extend substantially along the same direction as one another and a second step in which all the low-dimensional structures are re-oriented so as to lie substantially along a desired common direction. ) Again, the individual elongate structures may be re-oriented without loosing control over their positions.

The flexible element may be defined in two or more of the low-dimensional structures, or it may be additional to the low-dimensional structures . Use a method of the invention to align nanostructures or other low-dimensional structures that extend along different respective directions may be effected using, for example, a method as described in figure 4 or 5 below, in which the flexible element is defined without use of a mask. A method that requires use of a mask to define the flexible element may be difficult to apply to low-dimensional structures that extend along different directions or for low-dimensional structures whose longest dimension does not extend parallel to a substrate surface. A further advantage of the invention is that it provides the ability to engineer the mechanical and/ or elastic properties of the flexible element without having to compromise on the properties of the remaining portions of the nanowires and thus on the performance of any device in which the nanowires are incorporated. While nanowires having a sufficiently small diameter that they are flexible are known, for example from Angew. Chem. Int. Ed (above) , these nanowires have a uniform diameter along their length and this limits the performance of any device in which the nanowires are incorporated. In embodiments of the present invention, however, the sections of the low-dimensional structures which do not form part of the flexible element are unaffected by the processing steps needed to form the flexible element. Furthermore, even if sections of the low- dimensional structures which do not form part of the flexible element are affected by the processing steps needed to form the flexible element this joint, this may be compensated for in the initial fabrication of the nanowires (eg, by initially fabricating low-dimensional structures with a larger cross- section than required in the final device) .

A concomitant advantage is that the properties of the flexible element may be defined independently of the position of the flexible element along the nanowires or other structures. In principle, the flexible element may be formed as the low dimensional structures are grown. For example, the flexible element may, as described below, be realised by providing two or more of the nanowires with a portion having a reduced cross-section dimension. When nanowires are grown using a metal catalyst, the surface tension of the catalytic metal used during growth of nanowires affects the contact area between the catalyst and the already grown parts of the nanowires. This contact area determines the nanowire diameter. Varying the surface tension, for example by varying the temperature and/ or gas-composition, will thus affect the diameter of the nanowires and allow the nanowires to be grown with a section having a reduced diameter, compared to the diameter of the nanowires at other points along their lengths. In general, however, it is expected that it will be more convenient to form the flexible element after the elongate structures have been grown.

The method may comprise forming the flexible element having different elastic properties to a body portion of the low-dimensional structures . The method may comprise fabricating the low- dimensional structures on the substrate such that each low- dimensional structure extends along a respective direction off the surface of the substrate . It may comprise fabricating each low-dimensional structure on the substrate such that each low-dimensional structure extends substantially perpendicular to a surface of a substrate.

Providing the flexible element may comprise providing, in at least two of the low-dimensional structures, a first portion that has different elastic properties to a second portion, the first portion being at a different axial position along the low-dimensional structure to the second portion. Providing the flexible element may comprise reducing the stiffness of this portion, or forming the portion with a lower stiffness, in comparison to the other sections - by, either realising a reduced second moment of area of the first portion or by choosing a lower elastic modulus or both.

Forming the flexible element may comprise, in at least two of the low-dimensional structures, making a cross- sectional dimension of the first portion of the low-dimensional structure less than the corresponding cross-sectional dimension of the second portion of the low-dimensional structure, whereby the first portion of the two or more low- dimensional structures comprises the flexible element. For example, in the case of a cylindrical low-dimensional structure, forming the flexible element may comprise making the diameter of the first portion less than the diameter of a second portion. Reducing the diameter of a portion of the low dimensional structure is a straightforward way of obtaining the flexible element, and the properties of the flexible element can be selected by choice of appropriate values for the length and diameter of the reduced-diameter portion of the low dimensional structure.

This embodiment is not however limited to reducing the diameter of the first portion, ie to making the first portion smaller in two dimensions, and it may also comprise thinning only one dimension of the first portion. This may be achieved by using a directional etch (eg, physical sputtering, exploiting etches or oxidation steps whose rates depend on the crystal orientation) . When applied to a cylindrical low-dimensional structure, this would result in a first portion with a cross- section that is generally oval.

In formal terms, what is required is forming or providing the first portion with a cross-section that has a lower second moment of area than the cross-section of the second portion. Where this embodiment is applied to a group of low- dimensional structures, it may not be necessary to reduce the cross-sectional dimension of a portion of each low- dimensional structure in order to form the flexible element. Provided that a sufficient number of the structures are provided with a portion that is sufficiently flexible and strong so that the group of structures as a whole remains connected to the substrate during the re-orientation process it does not matter if others of the low-dimensional structures should fracture when the structures are re-oriented or if others of the low-dimensional structures have inadvertently been over- thinned.

The method may comprise oxidising a circumferential part of the first portion of the low-dimensional structure and removing the oxidised part. Alternatively, the method may comprise etching the first portion of the low-dimensional structure thereby to reduce its cross-sectional dimension.

The method may comprise providing an etch mask over the low-dimensional structure, the etch mask not extending over the first portion of the low-dimensional structure. The length of the portion of the low-dimensional structure that is etched is defined by the etch mask.

The method may comprise providing a first masking layer over the low-dimensional structure; providing a second masking layer over the first masking layer; etching the first masking layer using the second masking layer as a mask thereby to remove the first masking layer from the first portion of the or each low-dimensional structure; and etching the low-dimensional structure using the first masking layer as the etch mask.

Alternatively, forming the flexible element in a low- dimensional structure may comprise making a cross-sectional dimension of the second portion of the low-dimensional structure greater than the corresponding cross-sectional dimension of the first portion of the low-dimensional structure, whereby the first portion of the low-dimensional structure comprises the flexible element. For example, in the case of a nanowire, the nanowire may initially be fabricated with a cross-section that provides the desired flexibility and that is substantially uniform along its length. Additional material may then be deposited on part of the nanowire but not on another part, to increase the stiffness of the part on which material is deposited; the part on which material is not deposited forms the flexible element. The properties of the flexible element can be selected by choice of an appropriate initial cross-section and of an appropriate length for the portion on which no material is deposited.

The first portion and the second portion of the low- dimensional structure may have different compositions to one another. For example , the first portion may etch or oxidise at a greater rate than the second portion, so that the flexible element may be defined in an etching or oxidation step without the need for a mask.

Providing the low-dimensional structures on the substrate may comprise forming the low-dimensional structures on a formation substrate and subsequently attaching the low-dimensional structures to the substrate; and attaching the low-dimensional structures to the substrate may comprise attaching the low-dimensional structures to the substrate with an adhesive material having a lower elastic modulus than the low-dimensional structures whereby the adhesive material forms the flexible element.

The method may comprise detaching the low-dimensional structures from the formation substrate. The adhesive material may comprise a first layer with a low elastic modulus disposed on the substrate and a second layer with a high yield strength disposed on the first layer. This allows the flexible element to be both compliant and resistant to mechanical strain. The method may comprise re-orienting the structure so as to change the angle of inclination, relative to the substrate, of a body portion of the low-dimensional structures. It may comprise re-orienting the structure so that the body portion of each low-dimensional structure is substantially parallel to the surface of the substrate.

The method may comprise applying, to the structure, a force having a non-zero component parallel to substrate thereby to re-orient the structure.

The force may be derived from fluid (gas or liquid) flow, or it may be a mechanical force (including a force applied by a solid or a centrifugal force) or an electrostatic force, or it may be derived from surface tension of a liquid.

The method may comprise adhering the structure to the substrate after the step of re-orienting the structure. The low-dimensional structures may be encapsulated in a matrix. For example, the low-dimensional structures may have been fabricated in two or more groups, and the groups of low dimensional structures are then encapsulated in a matrix, such that one group of low dimensional structures is encapsulated separately from another group of low dimensional structures, according to the methods described in co-pending UK patent application No. 0620134.7, the contents of which are hereby incorporated by reference.

The low-dimensional structures in the structure may be identical or substantially identical to one another. For example, the low-dimensional structures may be nominally identical to one another, in that they are identical to one another within the limit of manufacturing tolerances in the fabrication process used to form the low-dimensional structures. Similarly, where the flexible element is provided by providing two or more low-dimensional structures with a joint portion having different elastic properties to a body portion of the low-dimensional structure, the joint portion, and the position of the joint portion in the low-dimensional structure, will be nominally identical between low- dimensional structures.

The method may comprise the steps of: forming a plurality of structures, each structure comprising: a plurality of low-dimensional structures extending along a respective direction to a surface of the substrate and a flexible element that connects two or more of the low-dimensional structure to the substrate; re-orienting the structures such that, for each structure, a body portion of the low-dimensional structures extends along a common direction relative to the substrate; and removing selected ones of the structures from the substrate .

This allows the resourceful use of structures by fabricating as many as possible in parallel on one substrate and transferring the desired number to another substrate.

The low-dimensional structures may be elongate low- dimensional structures, for example nanowires, nanopillars or nanotubes.

A second aspect of the present invention provides a structure comprising a plurality of low-dimensional structures, each low-dimensional structure extending along respective directions to a surface of a substrate; wherein the structure further comprises a flexible element that connects two or more of the low-dimensional structures to the substrate, the flexible element having different elastic properties to a body portion of the low- dimensional structures.

The flexible element allows the structure to be reoriented by applying a suitable force to the structure . The flexible element continues to connect the low-dimensional structures to the substrate while the structure is re-oriented, and may be incorporated in a finally-obtained device (although the flexible element may alternatively be removed after the structure has been re-oriented) . The low-dimensional structures may extend substantially along a common direction, for example, perpendicular to the substrate. Alternatively, the low- dimensional structures may extend along different directions to one another. The flexible element may comprise, in at least two of the low-dimensional structures, a first portion that has different elastic properties to a second portion, the first portion being at a different axial position along the low dimensional structure to the second portion. In at least two of the low-dimensional structures, the first portion of the low-dimensional structure may have a smaller second moment of area than a body portion of the low-dimensional structure. Alternatively the flexible element may comprise an adhesive material. The adhesive material may comprise a first, thicker, layer with a low elastic modulus disposed on the substrate and a second, thinner, layer with a high yield strength disposed on the first layer. (A low elastic modulus is required to make the film compliant and flexible, but a high yield strength is required to prevent it from rupturing. Generally, a low elastic modulus means low yield strength. To provide both a low elastic modulus and a high yield strength, a thick, low elastic modulus film can be covered with a very resistant (= high yield strength) coating.)

Each low-dimensional structure may be an elongate low- dimensional structure, for example a nanowire, nanopillar or nanotube.

The low-dimensional structures may be encapsulated in a matrix.

A third aspect of the present invention provides a device comprising a substrate; a plurality of low-dimensional structures, a body portion of each low-dimensional structure extending substantially parallel to a surface of the substrate; and a j oint portion connecting two or more of the low- dimensional structures to the substrate, the joint portion having different elastic properties to the body portions of the low-dimensional structures .

The device may be formable by the steps of providing each low-dimensional structure so as to extend substantially perpendicular to a surface of the substrate; forming, as the j oint portion, a flexible element that connects two or more of the low-dimensional structures to the substrate; and re-orienting the structure so that a body portion of each low-dimensional structure is substantially parallel to the surface of the substrate .

The joint portion may comprise, in at least two of the low-dimensional structures, a first portion that has a different cross-sectional dimension to a second portion, the first portion being at a different axial position along the low- dimensional structure to the second portion.

The active region of the device may comprise the joint portion.

The body portion of each low-dimensional structure may comprise an electrical contact to the device.

An electrical contact to the device may be provided on the substrate .

An active region of the device may alternatively be defined in the body portion of the low-dimensional structures. The device may comprise a transistor, sensor, or memory- device.

Each low-dimensional structure may be an elongate low- dimensional structure, for example a nanowire, nanopillar or nanotube . The low-dimensional structures may be encapsulated in a matrix.

In a first aspect of the current invention the flexible element provides a flexible joint between the elongate low dimensional structures and a substrate surface . It is a feature of this invention that the mechanical and/ or elastic properties of this joint can be engineered independently of the properties of the adjacent elongate low-dimensional structures. Thus, where the low dimensional structures are to be incorporated in a device, the properties and/ or dimensions of the portion of each low dimensional structure that does not include the flexible joint may be chosen to give a desired device performance, independently of the properties and/ or dimensions of the portion of each low dimensional structure that define the flexible joint being chosen to give desired mechanical and/ or elastic properties of the joint.

The elastic properties of the flexible element are different from the elastic properties of a body portion of the low dimensional structure(s) .

Furthermore, the elastic properties of the flexible element may be chosen independently of the position of the flexible element (for example its position along the low- dimensional structures) .

The flexible joint needs to meet the following requirements: 1. It must maintain sufficient physical strengths, e . g. guarantee a minimum degree of control over the special arrangement of the elongate low dimensional structures while they are turned from a substantially vertical configuration into a substantially planar one. 2. It must be flexible enough so that the force used to keep the low-dimensional structures in a planar configuration exceeds the restoring force of the flexible joint. Forces include but are not limited to van der Waals forces, covalent binding, metallic binding, and chemical binding or a second substrate keeping the structures in the desired position. In a particular feature of this invention, the mechanical properties of the flexible joint are engineered by choosing appropriate geometrical dimensions, e .g. a section of the elongate low-dimensional structures which is close to the substrate to which they are attached is thinned. This thinned part forms the flexible joint whose mechanical properties can by altered by changing a cross-sectional dimension (for example its diameter) and length. The thinning may be done, for example, by thermal oxidation followed by subsequent removal of the oxide or by chemical etching such as wet chemical etching or dry etching. To create a cross-section at the base of the elongate low dimensional structures that is smaller than the cross-section elsewhere along the low dimensional structures at least one of the following approaches can be applied:

1 . The part of the elongate low dimensional structures which is to be thinned may consist of a material which is much more sensitive to the thinning process to be applied. For example, the thinning process may take advantage of the fact that thermal oxidation of silicon is dependant on the doping concentration.

2. The part of the elongate low dimensional structures which is not to be thinned can be masked off by another material. In yet another embodiment of this invention, the flexible joint is formed by a material with an elastic modulus that is sufficiently lower than the elastic modulus of the structure to be tilted. For example if the structures to be tilted consist of semiconducting material, the flexible joint may consist of a polymer.

The current invention differs from the prior art by the inclusion of a flexible j oint allowing to convert structures consisting of or containing low dimensional elongate structures from one common orientation to another, for example from a substantially vertical orientation to a substantially planar configuration, or to put low dimensional elongate structures that do not have a common orientation into a common orientation, such that some or all of the following desiderata are met: 1. The spatial arrangement and spacing of the structures consisting of or containing elongate/ low dimensional structures within each group is substantially maintained;

3. No additional lithographic techniques are required to define the flexible joint. 4. The desired planar configuration including alignment and orientation is maintained while applying suitable subsequent processing techniques.

The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described by way of illustrative example with reference to the accompanying figures in which:

Figure 1 illustrates one way of fabricating elongate low- dimensional structures 1 by using a catalyst 3 enabling growth of the desired structures in well defined spots.

Figure 2 illustrates the fabrication of masking layers 5 and 6 allowing subsequent thinning of structure 1 at its base . Figure 3 illustrates the fabrication of a masking layer 6 allowing subsequent thinning of structure 1 at its base using a patterned sacrificial layer 7.

Figure 4 illustrates the fabrication of a thinned section I b without the use of a masking layer but using a material dependent subtractive method. Figure 5 illustrates the fabrication of a thinned section Ib without the use of a masking layer but using material dependent thermal oxidation.

Figure 6 illustrates the fabrication of a flexible joint which is not, as in the previous examples, predominantly engineered by its geometry but by its material properties. The flexible j oint is formed within layer 1 1.

Figure 7 illustrates the desired outcome of this invention facilitated by a flexible joint: Turning a substantially vertical structure (a) into a substantially planar structure (b) . Figure 8 illustrates how the structure shown in Figure 7 can be obtained using flow alignment.

Figure 9 illustrates how the structure shown in Figure 7 can be obtained using flow alignment and a planar surface.

Figure 10 illustrates how the structure shown in Figure 7 can be obtained using flow alignment and a tilted planar surface.

Figure 1 1 illustrates how the structure shown in Figure 7 can be obtained using flow alignment and a curved surface.

Figure 12 illustrates how the structure shown in Figure 7 can be obtained using the surface tension induced by a liquid-gas interface 17a.

Figure 13 illustrates how a non-uniform coating can be obtained using an additive method and taking advantage of the masking properties of structure 13. (b) and (c) of Figure 13 illustrate how the structure in (a) of Figure 13 would become planar if a sufficient potential difference would be applied to the layers 18a and 18b, which would need to be conductive in this example .

Figure 14 illustrates how a non-uniform coating can be obtained using an additive method to create layer 18 and subsequently using a subtractive method while taking advantage of the masking properties of structure 13.

Figure 15 illustrates how the structure shown in Figure 7 can be obtained using a planar surface . Figure 16 illustrates how a planar arrangement of structures 13 can be obtained on a curved surface 16b coated with a patterned adhesive 19 using flow alignment.

Figure 17 illustrates how a planar arrangement of structures 13 can be obtained on a structured planar substrate 16b coated with an adhesive 19 using flow alignment.

Figure 18 illustrates how a structure similar to the one shown in Figure 7 can be converted into a two terminal device. In this particular case, the thinned region 19a may determine the ultimate device performance; and

Figure 19 illustrates how a structure similar to the one shown in Figure 7 can be processed further to allow the implantation of selected areas along structure Ib.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention disclosure breaks up into three areas:

1. Concept and fabrication of a flexible joint

2. Methods to achieve a planar configuration 3. Fabrication techniques which benefit from a planar configuration

Preferred embodiments of the present invention will now be described by way of illustrative example with reference to the accompanying figures. First, we illustrate the fabrication of elongate low- dimensional structures:

Initially a plurality of low dimensional structures, in this example elongate structures 1 , are formed over a formation substrate 2 [Figure I ] . The low dimensional structures may be grown on the formation substrate 2 by an additive process, or they may be formed by subtractive methods, such as lithography and etching. In this embodiment the elongate structures 1 are nanowires, but the invention is not limited to this. According to the invention, the elongate structures that are formed over the formation substrate 2 may be arranged in groups, for example according to a method of co-pending UK patent application No. 0620134.7 according to which the nanowires shown in Figure 1 resemble such a group. In this method, where two or more groups of elongate structures are formed, the spacing between one group and a neighbouring group is greater than the maximum spacing between adjacent nanowires in a group . In principle, the spacing between a group and a neighbouring group may be any spacing that ensures that adj acent groups do not merge following the process of deposition of a matrix (to be described below) .

In one growth method suitable for use in the invention, a suitable catalyst 3 is initially disposed on the growth surface of the formation substrate 2 at every location where it is desired to grow a nanowire, as shown in Figure l (a) . The catalyst 3 may be, for example , a metal catalyst. The catalyst 3 may be deposited by, for example a combination of sub- micron lithography/ imprinting and lift-off, or by the deposition of a metal colloidal material. Next, as shown in Figure l (b) , nanowires 2 are grown at each location where the catalyst 3 was deposited on the growth surface of the formation substrate 2. Growth of nanowires does not occur at locations where the catalyst 3 is not present. Thus, if the locations where the catalyst 3 is deposited on the growth surface of the formation substrate 2 are arranged in groups, the result is that the nanowires 1 grown on the formation substrate 2 are also arranged in groups. Figure l (b) shows one group of nanowires arranged in a line. The low dimensional structures 1 grown on the formation substrate preferably have a substantially unidirectional orientation. In Figure l (b) the nanowires are shown as oriented with their longitudinal axes generally perpendicular to the formation substrate 2. Furthermore, the nanowires may be grown such that the upper section I a differs, for example in composition, from the lower section I b . This is to aid the thinning of the nanowires at their base and is optional in the first two embodiments which rely on the use of a masking layer [Figure 2 and Figure 3] but is essential where no masking layer is used [Figure 4 and Figure 5] .

Typically the nanowires will have a diameter of less than 200nm and a length of 0. 1 - 100 μm. The pitch of nanowires in a group will typically be less than l μm. Typically the nanowires will consist of a semiconductor, metal or insulator.

The basic principle of this invention is not restricted to groups of low-dimensional elongate structures and can be extended to low-dimensional structures such as fin-like structures whose smallest dimension extends parallel to the substrate surface. Hence, for simplicity, we explain all embodiments of this invention by illustrating its application to a single nanowire (which may also resemble the cross section of a tape-like structure) as shown in Figure l (c) . The terms "tape-like" and "fin-like" are used herein to denote a different orientation to a given substrate, as in co-pending UK patent application No. 0620134.7; a "fin-like" structure denotes a structure with a smallest dimension extending generally parallel to the substrate surface and a "tape-like" structure denotes a structure with a smallest dimension extending generally perpendicular to the substrate surface . Some workers refer to these structures as a "tape", regardless of their orientation.

The first two embodiments illustrated in Figure 2 and Figure 3 demonstrate how a mask can be created which protects most of the nanowire structure shown in Figure 2 (a) from the thinning process:

The nanowire 1 in Figure 2 (a) is encapsulated in a first masking layer 5 and a second masking layer 6 [Figure 2 (b) ] for example by using a substantially isotropic deposition method such as chemical vapour deposition. If groups of nanowires are to be rotated, for example the group shown in Figure l (b) , it may be advantageous to ensure that the combined thickness of both layers exceeds the spacing in between the nanowires creating a fin-like structure as explained in further detail in the co-pending UK patent application 0620134.7, the contents of which are hereby incorporated by reference.

Next, layer 6 is removed except on the side-walls of the nanowire [Figure 2(c)] , which can be achieved using an anisotropic etch. Layer 6 acts as a masking layer protecting layer 5 during the subsequent isotropic etch which exposes the base of the nanowire [Figure 2 (d)] . In most cases, this subsequent etch will also expose the top of the nanowire, as indicated in Figure 2 (d) . It is important to note that the thickness of layer 5 and the extent of the isotropic etch determines the height of the exposed base portion of the nanowire. Next, an isotropic etch is used to thin the exposed base of the nanowire to the desired diameter for creating a flexible element to act as a flexible joint between the remainder of the structure and the substrate [Figure 2 (e)] . Finally, if the remaining parts of layer 6 are too thick to rotate the structure without breaking the thinned base, they may be thinned or completely removed as shown in Figure 2 (f) . Optionally, also the remaining parts of layer 5 may be thinned or removed.

In a variation of this embodiment, layer 6 may be removed prior to the thinning process of the nanowire after the step illustrated in Figure 2(d) but before the thinning illustrated in Figure 2(e) .

The etch rate of the etches used to etch layers 5 and 6 need to be sufficiently selective with respect to the material chosen for the nanowire. Additionally, the etch rate of the etch used to etch layers 5 needs to be sufficiently selective with respect to the material chosen for the layers 6. If the nanowire consists of silicon, layer 5 may for example consist of silicon dioxide and layer 6 of silicon.

In a preferred variation of this embodiment, the nanowire shown in Figure 2 (a) is coated with a thin dielectric layer such as silicon dioxide (not shown) , layer 5 consists of a conductive material such as highly doped silicon while layer 6 may consist of any material suitable to sufficiently withstand the etch of layer 5. If the etch used to thin the nanowire base also etches layer 5, layer 5 needs to be sufficiently thick. In this case layer 6 may not be required during the subsequent thinning process and may be removed.

In a second embodiment [Figure 3] , the length of the thinned nanowire base that forms the flexible element is not predominantly determined by the thickness of layer 5 but by a dielectric layer 7 covering the growth substrate [Figure 3(a)] .

The catalyst 3 is positioned into openings fabricated into this layer, for example as described in co-pending UK patent application 0620 134.7. Layer 7 may consist of several layers.

Next, the nanowire 1 is fabricated and coated with a layer 5 [Figure 3(b)] and coated with layer 6 [Figure 3(c)] .

Layer 7 may be silicon dioxide or silicon nitride, layer 5 may be silicon dioxide . If the nanowire consists of silicon, layer 5 may be created by thermal oxidation. Layer 6 may consist of silicon. Next, layer 7 is exposed by choosing a suitable anisotropic etch to remove layer 6 everywhere except the sidewalls of nanowire 1 [Figure 3 (d)] . Now, layer 7 can be removed using an isotropic etch [Figure 3 (e) ] . At this point, the base of nanowire 1 may still be coated with some material of layer 6 which needs to be removed using an isotropic process [Figure 3(f)] . Next, layer 5 is removed at the base using an isotropic etch [Figure 3 (g)] . In this embodiment, the base is thinned by first converting the material of the nanowire at its base into a different material 8 which can be subsequently removed, while the remaining nanowire remains largely unaffected. For example, if the nanowire consists of silicon, layer 5 of silicon dioxide and layer 6 of silicon, silicon dioxide can be formed using a thermal oxidation process. In this particular example also layer 6 will be oxidised [Figure 3 (h)] . Finally, material 8 is removed without removing the non-converted nanowire material [Figure 3(i)] . The conversion of nanowire material (e.g. silicon) into a different material (e .g. silicon dioxide) and its subsequent removal may be repeated a number of times.

It should be noted at this point that thermal oxidation of silicon depends on doping impurities as well as the curvature of the silicon surface to be oxidised. To compensate for faster oxidation rates at the concave base and the risk of pinching through the nanowire at its very base (that is where the nanowire merges with the substrate) it may be beneficial to enhance the oxidation rate by choosing appropriate doping along section I a such that the oxidation rate along section I a is faster than the oxidation rate at the point where the nanowire merges with the substrate.

Different aspects of the approaches discussed in Figure 2 and Figure 3 can be combined. Especially the choice of different material properties ( Ib in Figure 3) at the nanowire base can be applied to the method of Figure 2 to aid the thinning process illustrated in Figure 2. Conversely, the method of Figure 3 may be applied with a nanowire having uniform material properties along its length. Likewise, choosing a deposition process for creating layer 5 such that it also covers layer 7 may increase the length of the thinned base . Furthermore, thermal oxidation may be utilised to facilitate the thinning process used in Figure 2 in place of or in addition to the etching process described above.

The two embodiments illustrated in Figure 2 and Figure 3 also indicate a different way of obtaining a flexible element: If the low-dimensional structure(s) 1 in Figures 2d and Figure 3g is/ are already flexible enough as grown, the additional layers 5 and 6 effectively increase the second moment of area in the upper section of the structure . The overall process shown in, for example, Figures 2 (a) to 2 (f) is equivalent to adding material to one portion of the low-dimensional structure (thereby decreasing its flexibility) while not adding material to the portion that is intended to form the flexible joint) . Terminologically, assuming that these two layers 5,6 are thin enough such that they could be considered a part of the elongated low-dimensional structure, low-dimensional structures with a flexible joint are obtained. Using a conformal deposition process also ensures that is possible to position the low-dimensional structures 1 in the centre of the overall structure. If the structure is unintentionally bent, the region where the low-dimensional structures are situated along the so-called neutral fibre and the region around this neutral fibre experience the lowest strain in the overall structure. Hence, a rigid matrix (minimising the strain) while low-dimensional structures are situated in the centre of this matrix results in the smallest impact on device performance if the substrate is bent (e.g. as it would be in flexible displays) .

Last but not least, if all low-dimensional structures are fabricated along one line as indicated in figure 1 and a conformal deposition process is used to encapsulate them [Figures 2 and 3] , then all low-dimensional structures will "sit" predominantly in the same plane in each fin-like structure with respect to the matrix material. This will ease subsequent processing steps once the fins are rotated to lie flat on a substrate (e.g. opening of contact areas to expose sections of the low dimensional structures and subsequent fabrication of contacts) . This accuracy will be challenging to achieve if the fins are fabricated using subtractive techniques which require the alignment of an etch resistant etch-mask and withstanding the required anisotropic etch to form high aspect ratio structures as described in Figures 1 to 3. In a second set of embodiments, the material properties at the base of the nanowire I b are chosen such that the nanowire base can be thinned faster than the rest of the nanowire. Therefore, if an appropriate nanowire diameter is chosen prior to the thinning procedure, both sections I a and Ib of the nanowire will have after the thinning the desired diameters.

Figure 4 illustrates the use of an etch which etches the section I a of the nanowire much slower than section I b . Figure 5 illustrates the use of a process which converts the nanowire material at its base I b much faster into a different material 9 than anywhere else along the nanowire . Subsequently, this material 9 is removed. In the case of silicon nanowires a suitable conversion process is thermal oxidation, which is accelerated if section Ib is doped appropriately. That the doping concentration impacts the oxidation rate is a well known phenomenon, and has been studied for planar structures. The oxidation rate depends on: temperature, partial pressure of oxygen, gas composition (e. g. presence of water) but also on the silicon material to be oxidised, especially crystal orientation, doping concentration, kind of dopants, surface curvature and the thickness of the oxide already grown. All these are well-studied phenomena.

In the methods of Figures 4 and 5 , the portion I a of the low dimensional structures that does not form the flexible element is likely to undergo a reduction in its cross-sectional dimension during the etching or oxidation process. It is possible to compensate for this when the low dimensional structures are grow, by growing them to dimensions that, after etching/ oxidation, are expected to lead to a portion I a having the desired cross-section for a particular application.

The approaches described above are used to define the mechanical properties of the desired flexible element by its geometrical dimensions. Alternatively, it may be possible to define a flexible element with desired properties by using a lower elastic modulus material. As low elastic modulus materials such as polymers are not compatible with the nanowire growth, the flexible element can't be formed along the original nanowire . Hence, a different approach is chosen [Figure 6] : First, a structure 10 with a weak (but not necessarily flexible) joint at its base is formed, on a suitable formation substrate [Figure 6(a)] . Also, a low elastic modulus adhesive 1 1 is used to coat a substrate 12. Once the low elastic modulus adhesive is brought into contact with the structure 10 and a sufficient force is applied, the structure 10 will be detached by fracture at its base from formation substrate and glued to the substrate 12 by layer 1 1 [Figure 6(b) ] . Here, the properties of the flexible element are determined by the properties of the low elastic modulus adhesive 1 1 in close proximity to the structure 10 , namely, its elastic modulus, its thickness and adhesive strength to the structure 10.

The low elastic modulus adhesive 1 1 may be a polymer and may be more specific a cross-linkable polymer.

The low elastic modulus adhesive 1 1 may consist of more than one polymer layer, for example two layers where the layer facing the structure 10 may be considerably thinner and have a larger yield strength than the layer sandwiched between this thin layer and substrate 12. This way, the adhesive bi-layer system maintains its compliance (determined by the thick and soft layer) while being more resistant to mechanical strain inserted by the structures 10

(determined by the thin and hard layer) .

The approach illustrated in Figure 6 is particularly suited if no sufficient process to thin the nanowires at their base is available, which might be the case for materials which are chemically very stable such as GaN.

The methods of forming a flexible element have been described above with reference to a single low dimensional structure, for simplicity of description. Where the invention is applied to a group of low dimensional structures to a substrate, all or substantially all of the low dimensional structures may be provided with a flexible element. As noted above, however, it may not be necessary to provide each low dimensional structure with a flexible element and, in principle, providing only two of the low dimensional structures with a flexible element would be sufficient to constrain the directions in which the low-dimensional structures may be re-oriented.

Next, methods to achieve a planar configuration will be discussed.

Preferred embodiments of the present invention will now be described by way of illustrative example applied to the structure 13 shown in Figure 7(a) .

To convert a structure 13 from a substantially vertical orientation [Figure 7(a)] into a substantial planar orientation

[Figure 7(b)] several requirements need to be met:

1. A flexible element that provides a flexible j oint that allows the structure 13 to move, for example about a virtual pivot, needs to be present. 2. A force component which is directed to move the structure parallel to the substrate surface and is applied to the structure to be converted into a planar configuration needs to be present. The structures to be tilted do not need to extend necessarily perpendicular off a surface in order to achieve a planar configuration. In particular, where the invention is applied to a structure in which a plurality of low- dimensional structures are encapsulated in a matrix as a tape or fin structure, the tape/ fin structure has a special geometry as it can only tilt in two directions due to the way it is anchored to the substrate. If flow alignment is used, it will tilt such that the smallest area is exposed to the flow (of course, the flexible j oint is like a charging spring so that in theory it should never lie perfectly flat) . A fin structure can tilt only perpendicular to its long dimension on the substrate, as the anchoring along its length means that it cannot tilt about an axis perpendicular to the fin but parallel to the substrate - the longest dimension of the fin structure is defined by the number and spacing of the nanowires contained in the fin. In other words, the nanowires will not tilt to anchoring points of other nanowires within the same fin/ tape (assuming that the length (in a direction perpendicular to the substrate) of the flexible joint is not too large and assuming no permanent deformation of j oints such as yielding or rupturing) . 3. Any force which may cause the structure to reorientate itself out of the desired planar configuration (e. g. the restoring force stemming from the bent flexible joint) must not exceed the forces which keep the structure in its planar configuration. How a flexible joint 14 can be defined which determines a virtual pivot point for the subsequent rotation of the structure was discussed by illustrative example in the previous embodiments.

A force component acting parallel to the substrate surface on which the orientation takes place might be facilitated but is not restricted to approaches which may utilise at least one of the following:

1 . Motion of a liquid (e. g. "flow alignment")

2. Motion of a gas 3. Motion of a solid (e.g. a ram- or stamp like structure)

4. Surface tension

5. Centrifugal force

6. Positive or negative linear acceleration 7. Electrostatic force

8. Gravitational force

9. Magnetic force

Forces which keep the rotated structure in its planar configuration include but are not restricted to: 1 . Van der Waals forces

2. Ionic Bonds

3. Metallic bonds

4. Hydrogen Bonds (a special case of Van der Waals)

5. Permanent Dipole interactions 6. Cation/ pi-electron interactions 7. Covalent bonds

8. A second substrate keeping the structures in their desired position

The flexible joint needs to be engineered such that the restoring force caused by its bend is smaller than any of the forces which keep the structure in place, e . g. by choosing a material with a sufficiently low elastic modulus [Figure 6] or by thinning the base of the nanowires down to an appropriate diameter in proportion to the length of the thinned section [Figures 2 - 5] .

The use of a gas or a liquid in order to bring the structure 13 into a more planar configuration is illustrated in Figure 8. The flow rate (indicated by the density of the arrows) of the medium used to bring the structure into a planar configuration will determine the tilt of structure 13 with respect to substrate 15 [Figure 8 (a) and 8 (b)] . If the flow rate is sufficiently large, structure 13 will take planar position [Figure 8(c)] .

If a constant gas or liquid flow (indicated by the arrows) is used, an increase in the flow rate and therefore an increase in the tilt of structure 13 may be realised by bringing a substrate 16 closer to the substrate 15 [Figure 9 (a) and 9 (b)] . In this example, it may not be necessary to increase the flow rate until the structures orient themselves in the desired planar orientation as the substrate 16 may be used to finally bring them into contact with substrate 16 [Figure 9 (c)] . If it is desired to remove substrate 16 after the structure 13 has been put in a planar orientation it might be beneficial to use instead of a planar substrate 16 as shown in Figures 9 (a) - 9)c_ a substrate with a patterned surface (not shown) which would reduce the contact area and mitigate the impact of attractive surface forces between substrate 16 and the structure 13 while removing substrate 16.

The succession of the images in Figure 10 demonstrates how the flow of a liquid could be generated by a tilted substrate 16a which is at one side in contact with substrate 15 to which the structure 13 which is to be tilted is attached. As the angle between substrates 15 and 16a is decreased, a flow of the liquid enclosed by these substrates is induced, whose flow rate depends on the angular velocity with which the angle between both substrates 15 and 16a is reduced. The big arrow indicates the direction of the motion of the surface of substrate 16a. The small arrows indicate the direction of the liquid flow. The succession of the images in Figure 1 1 demonstrates a variation of the previous embodiment using a curved substrate surface 16b (e.g. realised by a cylinder) instead of a flat substrate surface 16a. The big arrows indicate the direction of motion in which the curved surface 16b would move in a rolling action while the small arrows indicate the direction of the liquid flow.

In any of the embodiments above where a solid substrate ( 16, 16a or 16b) is used to induce the flow of a liquid it might be beneficial to use a compliant substrate or to coat the surface of substrates 16, 16a or 16b accordingly. This is to reduce excessive strain on the structure 13 if the substrate 16, 16a or 16b are in contact with structure 13 but also to form one continuous contact area between the substrates 15 and 16, 16a or 16b (protruding perpendicular to the plane of the drawings) , which would result in a more uniform distribution of the flow rate .

Figure 12 illustrates how the surface tension of a liquid/ gas interface 17a (dashed line) can be used to insert a force on structure 13. For this to happen the liquid surface needs to insert an asymmetric force on structure 13 , that is that the force to one side (e . g. the right side in Figure 12 (a)) exceeds the force to the other side. In the example shown in Figure 12 this is achieved by positioning the liquid 17 only on the right side of structure 13 resulting in a force acting on structure 13 whose direction is indicated by the arrow.

Positioning a liquid droplet only on one side can be implemented by changing the surface properties on one side of the structure for example by an anisotropic additive process [Figure 13(a)] where a coating 18 is added only on one side of structure 13 and only on the surface of substrate 15 which extends on the same side of structure 13 (the right side in Figure 13(a)) . Alternatively, a uniform coating 18 might be deposited, as in Figure 14 , and partially removed using a anisotropic subtractive process. In both cases, the structure 13 acts as a mask. Assuming the coated areas 18a and 18b

[Figures 13(a) and 14(b)] are hydrophilic on the right side of structure 13 and the uncoated areas are hydrophobic and assuming that a polar liquid (e.g. water) 17 in Figure 12 is used, a droplet of the polar liquid could be positioned only on the right side of structure 13 in figure 12 (a) .

As the volume of this droplet is reduced due to evaporation [Figures 12(a) and 12 (b)] , the liquid-gas surface becomes smaller changing the direction of the force acting on structure 13 and tilting it further towards substrate 15 until its largest dimension extends parallel to the surface of substrate 15.

Furthermore, an electrostatic force could be used to tilt structure 13 in Figures 13 (a) and 14(b) . Assuming the thinned base of the nanowire is sufficiently resistive and the layers 18a and 18b are sufficiently conductive, a fast voltage pulse applied to layer 18b would temporarily create an electric field (determined by the time constant R x C, with R = resistivity at base and C = capacitance of structure 13 assuming the entire body of structure 13 is conductive) between layer 18a and layer 18b. The resulting force can be utilised to bring layer 18a in contact with layer 18b as illustrated by the succession of images in Figure 13.

In another embodiment of this invention, a substrate 16 approaches the structures 13 while also being translated laterally with respect to substrate 15 [Figure 15(a) and 15(b)] .

Once the tilt of structure 13 is sufficient, the lateral motion may be terminated until the structure 13 is oriented planar on the surface of substrate 15 [Figure I 5 (c)] .

Now, if the adhesion between structure 13 and substrate 15 is sufficiently strong, substrate 16 may be removed [Figure

15(d)] . In this case, it might be beneficial to use instead of a planar substrate 16 as shown in Figure 15 a substrate with a patterned surface (not shown) which would reduce the contact area and mitigate the impact of attractive surface forces between substrate 16 and the planar structure while removing substrate 16.

Alternatively, if the adhesive forces between substrate 16 and structure 13 are engineered such that they not only exceed the adhesive forces between substrate 15 and structure 13 but also are sufficient to break the flexible joint, structure 13 may be transferred to substrate 16. This direct transfer may be facilitated by choosing for substrate 16 a material which acts as a suitable adhesive (e. g. a polymer) or by coating it with an adhesive material. (This may also be applied to substrate 16 of Figure 9, substrate 16a of Figure 10, and substrate 16b of Figure 1 1 . )

If a plurality of structures 13 is to be transferred from substrate 15 to a new substrate 16b, 16c, it may not always be desired to transfer all structures 13 to the substrate 16b, 16c but possibly only some . In that case it may be desirable to either modify the adhesive strengths of substrate 16b, 16c in well defined regions or to alter the topography of the surface that only those structures 13 are brought in contact with substrate 16b, 16c which are to be transferred. This is illustrated in Figure 16 where a curved surface 16b is coated with a patterned adhesive 19. Figure 16 marries the process described in Figure 1 1 to reorientate the structures 13 with the concept of figure 15(e) where the structure is transferred to the substrate which induces the planar orientation while using a patterned adhesive . Subsequently they may be transferred from the curved substrate to another planar substrate. This way, the final substrate is populated with structures 13 at a lower density than the one obtained on substrate 15. The illustrations shown in Figure 17 demonstrate the transfer of some of the structures 13 by using a patterned substrate 16c having a non-planar surface coated with an adhesive layer 19a. The arrow indicates the motion of the substrate 16c. Layer 19a might be a polymer. Alternatively, and especially if all structures 13 are already oriented planar with respect to substrate 15, it may be desired to transfer them, or to transfer selected structures, to a planar substrate . In the example illustrated in Figure 17, a patterned substrate 16c is coated uniformly with a suitable adhesive and is brought into contact with structures 13 that have already been oriented to be parallel to the substrate 15. In this example, therefore , the steps of orienting the structures to be parallel to the substrate 15 and their transfer to the substrate 16 are separate steps, whereas in the example of figure 16 these steps are combined in one process .

In Figure 17, the adhesive coating 19a is required to be sufficiently thin such that the recesses in the substrate 16c are not filled with adhesive during the process of applying the adhesive coating 19a, since this would effectively planarise the surface topography of the substrate 16c.

Figures 16 and 17 show two particular methods in which selected structures are transferred to a substrate . They both rely on ensuring that only the structures intended to be transferred make contact with the stamp which means either coating a structured surface with a sufficiently thin adhesive (Fig 17) structuring a sufficiently thick adhesive (fig 16) . Other methods may be used, for example either changing the surface chemistry/ adhesive properties of the stamp only in well defined regions (not shown) or changing the surface chemistry of the structures to be transferred (not shown) , or any combination of the strategies mentioned.

Figure 17a indicates a further advantageous feature of this invention, if it is used in combination with the technique described in co-pending UK patent application 0620134.7 for encapsulating low dimensional structures as groups. After being rotated such that the largest dimension of all elongated structures lies parallel to the substrate surface, all elongated structures are substantially coplanar. This has important implications as the knowledge of the exact position (not only laterally, but also vertically) and orientation of all elongated structures eases the implementation of subsequent processing steps.

Figure 18 is an example where the final active device region is formed within the thinned and strained section I a while the thicker region I b and layer 21 are used to form electrical contacts to I a. The contact to I b can be made by partially removing layers 23 and 24 (which correspond, for example , to layers 5 and 6 of Figure 2 (e) or 3 (i)) and adding a conductive material 25 [Figure 18(b)] . Layers 20 and 22 are electrically insulating while 2 1 can be patterned and needs to be sufficiently conductive . In the example of Figure 18 the layers 20, 2 1 , 22 are present on the substrate when the structure 13 is fabricated. However, it would alternatively be possible to provide a contact layer on the substrate after the low-dimensional structures have been re-oriented.

Figure 19 illustrates that the material 23 can be patterned into a section 23a and used as a mask to subsequently only implant doping atoms at the ends of the non-thinned section Ib while the region of Ib which is exactly beneath 23a remains unaffected. Afterwards, the structure may be processed into a working device such as a transistor, sensor, or memory device and either left on the substrate or transferred to the desired substrate. Although Figures 7- 19 illustrate re-orientation of structures obtained by the methods of Figures 2 to 5, similar methods may be applied to structures obtained by the method of Figure 6. If the low-dimensional structure(s) protrude beyond the main body of the structure 10 (not shown in Figure 7) , the protruding portion might break off as the structure is tilted such that only the thicker matrix is glued to the adhesive layer 1 1. As the structure is tilted over, the contact area with the adhesive layer 1 1 is expected to change, becoming increasingly larger, while the initial contact area (the top side) may in the final planar position no longer be in contact with the adhesive layer 1 1 . Use of a hard and/ or thin adhesive layer may cause the structures to separate from the adhesive when they are tilted so that a thicker and softer adhesive layer (or a higher temperature) is preferably used to avoid this (or, particularly preferably, the adhesive bi-layer system mentioned above is used) .

The invention being thus described, it will be obvious that the same way may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

C L A I M S

1 . A method of manufacturing a structure comprising a plurality of low-dimensional structures, the method comprising: providing a flexible element that connects at least two of the low-dimensional structures to a substrate, the flexible element having different elastic properties to a body portion of the low-dimensional structures.

2. A method as claimed in claim 1 and comprising providing the low-dimensional structures so as to extend substantially along a respective direction off a surface of the substrate.

3. A method as claimed in claim 2 and comprising fabricating the low-dimensional structures on the substrate such that each low-dimensional structure extends substantially along a respective direction off a surface of a substrate.

4 A method as claimed in claim 2 or 3 and comprising fabricating the low-dimensional structures on the substrate such that each low-dimensional structure extends substantially perpendicular to a surface of a substrate .

5. A method as claimed in claim 1 , 2, 3 or 4 wherein providing the flexible element comprises providing, in at least two of the low-dimensional structures, a first portion that has different elastic properties to a second portion, the first portion being at a different axial position along the low- dimensional structure to the second portion.

6. A method as claimed in claim 5 wherein providing the flexible element comprises, in at least two of the low- dimensional structures, making a cross-sectional dimension of the first portion of the low-dimensional structure less than the corresponding cross-sectional dimension of the second portion of the one low-dimensional structure, whereby the first portion of the one low-dimensional structure comprises the flexible element.

7. A method as claimed in claim 5 wherein providing the flexible element comprises, in at least two of the low- dimensional structures, making a cross-sectional dimension of the second portion of the low-dirnensional structure greater than the corresponding cross-sectional dimension of the first portion of the low-dimensional structure, whereby the first portion of the low-dimensional structure comprises the flexible element.

8. A method as claimed in claim 6, comprising oxidising a circumferential part of the first portion of the low- dimensional structure and removing the oxidised part.

9. A method as claimed in claim 6, comprising etching the first portion of the low-dimensional structure thereby to reduce its cross-sectional dimension.

10. A method as claimed in claim 9 , comprising providing an etch mask over the low-dimensional structure, the etch mask not extending over the first portion of the low- dimensional structure .

1 1. A method as claimed in claim 10 and comprising providing a first masking layer over the low-dimensional structure; providing a second masking layer over the first masking layer; etching the first masking layer using the second masking layer as a mask thereby to remove the first masking layer from the first portion of the low-dimensional structure; and etching the low-dimensional structure using the first masking layer as the etch mask.

12. A method as claimed in claim 6, 7, 8, 9 , 10 or 1 1 wherein the first portion and the second portion of the low- dimensional structure have different compositions to one another.

13. A method as claimed in claim 1 wherein providing the low-dimensional structures on the substrate comprises forming the low-dimensional structures on a formation substrate and subsequently attaching the low-dimensional structures to the substrate; wherein attaching the low- dimensional structures to the substrate comprises attaching the low-dimensional structures to the substrate with an adhesive material having a lower elastic modulus than the low-dimensional structures whereby the adhesive material forms the flexible element.

14. A method as claimed in claim 13 and comprising detaching the low-dimensional structures from the formation substrate.

15. A method as claimed in claim 13 or 14 wherein the adhesive material comprises a first layer with a low elastic modulus disposed on the substrate and a second layer with a high yield strength disposed on the first layer.

16. A method as claimed in any preceding claim and comprising re-orienting the structure so as to change an angle of inclination, relative to the substrate, of a body- portion of each low-dimensional structure.

17. A method as claimed in claim 16 and comprising re-orienting the structure so that the body portion of each low-dimensional structure is substantially parallel to the surface of the substrate .

18. A method as claimed in claim 16 or 17 and comprising applying, to the structure, a force having a nonzero component parallel to substrate thereby to re-orient the structure.

19. A method as claimed in claim 18 wherein the force is derived from fluid flow.

20. A method as claimed in claim 18 wherein the force is a mechanical force .

21. A method as claimed in claim 18 wherein the force is an electrostatic force.

22. A method as claimed in claim 18 wherein the force is derived from surface tension.

23. A method as claimed in any one of claims 16 to 22 and further comprising adhering the structure to the substrate after the step of re-orienting the structure.

24. A method as claimed in any preceding claim wherein the low-dimensional structures are encapsulated in a matrix.

25. A method as claimed in any preceding claim wherein the low-dimensional structures are substantially identical to one another.

26. A method as claimed in any preceding claim and comprising the steps of: forming a plurality of structures, each structure comprising: a plurality of low-dimensional structures, each low-dimensional structure extending along a respective direction to a surface of the substrate and a flexible element that connects two or more of the low-dimensional structures to the substrate; re-orienting the structures so that, for each structure, a body portion of the low-dimensional structures included within that structure extends along a common direction relative to the substrate; and removing selected ones of the structures from the substrate .

27. A method as claimed in any preceding claim wherein the low-dimensional structures are elongate low- dimensional structures.

28. A method as claimed in any preceding claim wherein each low-dimensional structure is a nanowire, nanopillar or nanotube.

29. A structure comprising a plurality of low- dimensional structures, each low-dimensional structure extending along a respective direction to a surface of a substrate; wherein the structure further comprises a flexible element that connects at least two of the low-dimensional structures to the substrate, the flexible element having different elastic properties to a body portion of the low- dimensional structure.

30. A structure as claimed in claim 29 wherein the flexible element comprises, in at least two of the low- dimensional structures, a first portion that has different elastic properties to a second portion, the first portion being at a different axial position along the low-dimensional structure to the second portion.

3 1 . A structure as claimed in claim 30 wherein the first portion of the at least two low-dimensional structures has a smaller cross-sectional dimension than a second portion of the at least two low-dimensional structures.

32. A structure as claimed in claim 29 wherein the flexible element comprises an adhesive material.

33. A structure as claimed in claim 32 wherein the adhesive material comprises a first layer with a low yield strength disposed on the substrate and a second layer with a high yield strength disposed on the first layer.

34. A structure as claimed in any of claims 29 to 33 wherein each low-dimensional structure is an elongate low- dimensional structure.

35. A structure as claimed in any of claims 29 to 34 wherein each low-dimensional structure is a nanowire , nanopillar or nanotube.

36. A structure as claimed in any of claims 29 to 35 wherein the low-dimensional structures are encapsulated in a matrix.

37. A device comprising a substrate; and a plurality of low-dimensional structures, a body portion of each low- dimensional structure extending substantially parallel to a surface of the substrate; and a joint portion connecting two or more of the low-dimensional structures to the substrate, the joint portion having different elastic properties to the body portions of the low-dimensional structures.

38. A device as claimed in claim 37 and formable by the steps of: providing each low-dimensional structure so as to extend substantially perpendicular to a surface of the substrate; forming, as the joint portion, a flexible element that connects at least two of the low-dimensional structures to the substrate; and re-orienting the structure so that a body portion of each low-dimensional structure is substantially parallel to the surface of the substrate .

39. A device as claimed in claim 37 or 38 wherein the joint portion comprises, in at least two of the low-dimensional structures, a first portion that has a different cross-sectional dimension to a second portion, the first portion being at a different axial position along the low-dimensional structure to the second portion.

40. A device as claimed in claim 37, 38 or 39 wherein an active region of the device comprises the joint portion of the at least two low-dimensional structures.

41 . A device as claimed in claim 40 wherein the body portion of the at least two low-dimensional structure comprises an electrical contact to the device.

42. A device as claimed in claim 40 or 41 wherein an electrical contact to the device is provided on the substrate .

43. A device as claimed in claim 37, 38 or 39 wherein an active region of the device is defined in the body portion of the low-dimensional structures .

44. A device as claimed in any of claims 37 to 43 and comprising a transistor, sensor or memory device .

45. A device as claimed in any of claims 37 to 44 wherein each low-dimensional structure is an elongate low- dimensional structure.

46. A device as claimed in any of claims 37 to 45 wherein each low-dimensional structure is a nanowire, nanopillar or nanotube.

47. A device as claimed in any of claims 37 to 46 wherein the low-dimensional structures are encapsulated in a matrix.