WO2006135261A2

WO2006135261A2 - Nansoscale patterning and fabrication methods

Info

Publication number: WO2006135261A2
Application number: PCT/NZ2006/000154
Authority: WO
Inventors: James Gordon Partridge; David Michael Angus Mackenzie; Simon Anthony Brown
Original assignee: Nano Cluster Devices Ltd
Priority date: 2005-06-15
Filing date: 2006-06-15
Publication date: 2006-12-21
Also published as: WO2006135261A3

Abstract

The invention disclosed relates to the formation of patterns on the surface of a substrate prepared by the deposition of clusters through a shadow-mask. In a preferred form the pattern is nanoscale and comprises an electrical connection between contacts on the substrate.

Description

NANOSCALE PATTERNING AND FABRICATION METHODS

FIELD OF THE INVENTION The present invention relates to a method of preparing cluster-assembled patterns between electrical contacts on an insulating substrate using a shadow-mask. More particularly but not exclusively the invention relates to a method of preparing patterns such as pathways or wires, both on the nanoscale and up to the micron scale.

BACKGROUND TO THE INVENTION

The shadow-masking (or 'stencil-evaporation') technique is a well known alternative to conventional photolithography. Amongst other applications, it can be used to provide selective deposition of metals, semiconducting layers and insulators to semiconductor wafers, MEMS substrates and optical components. Efficient placement of material in well defined locations, with nanoscale dimensions, and on planar and non-planar substrates is possible in a single-step deposition process. Provided it is undamaged, the shadow-mask may be used for multiple depositions.

In the deposition process, particles are passed through the apertures of the shadow-mask and deposited onto a substrate, thus replicating the aperture-pattern. The aperture- patterns are normally replicated on substrates using an atomic deposition process. The state of the art is described by Brugger et al in US 6,313, 905. [1] Prior art methods, including that of Brugger et al suffer a major drawback. During the deposition process, atomic material collects on the sidewalls of the aperture causing a reduction of the aperture dimensions. If the aperture is narrow (as is the case in shadow-masks for nanoscale device production), the reduction of the aperture dimensions is particularly detrimental and can lead to a completely filled (or 'clogged') aperture and incomplete pattern formation on the substrate. Previous experiments have demonstrated successful generation of Field-Emission tips by cluster-deposition through shadow-masks [2]. However this technique does not provide electrically conducting cluster-assembled structures (particularly in the plane of the substrate) and there was there was no demonstration of any method of avoiding clogging of the aperture.

Kolbel et al. [3, 4] have demonstrated a method which reduces clogging within the apertures of Si_xN_y membrane nanostencils. The Si_xNy membrane surface and aperture walls of the nanostencil are coated with either an alkyl or a perfluoroalkyl self-assembled monolayer (SAM) prior to the atomic deposition process. The authors claim a 100% increase in the amount of material that can be successfully deposited before clogging of the aperture occurs when either CF-Si(OEt)₃ or C 12-SiCl₃ SAMs are employed. We note that a 100% increase corresponds to only a factor of 2 increase in the lifetime of the stencil.

In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.

OBJECT OF THE INVENTION

It is an object of the invention to provide a method of preparing cluster-based films and/or wires and/or electronic devices and/or coatings formed therefrom which overcome one or more of the abovementioned disadvantages, or which at least provide the public with a useful alternative. SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of forming a pattern of clusters on a substrate comprising the steps of: a. positioning a shadow-mask having one or more apertures, with respect to the substrate or vice versa or both, b. directing a plurality of clusters towards the substrate in such a manner that one of more of the clusters passes through one or more apertures of the shadow mask, c. depositing one of more of the clusters which have passed through the apertures onto the substrate to form a pattern of clusters, wherein the pattern of clusters includes at least one pathway of clusters on the substrate.

In one embodiment, prior to the step of depositing the clusters there is a step of formation of two or more contacts on the substrate, and the step of deposition of the clusters comprises forming at least one pathway between two contacts.

Preferably the step of depositing one or more clusters results in one or more conducting pathway(s) capable of functioning as a wire or electrical connection between the two contacts.

Preferably the step of deposition of the clusters is monitored by monitoring the conduction between the two contacts, and deposition is ceased at or near to the onset of conduction between the two contacts, or at a time subsequent to the onset of conduction chosen to achieve a desired surface coverage or film thickness.

In an alternative embodiment there is a post-step of formation of one or more contacts on the substrate and the contacts are so positioned to ensure at least one pathway of clusters exists from one or between at least two of them. Preferably the step of deposition results in at least one pathway of clusters which is/are a conducting pathway(s) capable of functioning as a wire or electrical connection and the step of formation of the contacts comprises forming two contacts connected by the pathway of clusters.

Preferably in all embodiments the average diameter of the clusters is between 0.3nm and l,000nm. More preferably the average diameter of the clusters is between 0.5nm and 40nm.

Preferably wherein the contacts are separated by a distance smaller than 10 microns. More preferably the contacts are separated by a distance smaller than 200nm.

Preferably at least one of the dimensions of the aperture in the shadow-mask is less than 100 microns. More preferably at least one of the dimensions across the aperture in the shadow-mask is less than 1 micron. More preferably at least one of the dimensions across the aperture in the shadow-mask is less than 200 nanometres.

Preferably the step of directing the clusters towards the substrate includes imparting kinetic energy to the plurality of clusters sufficient to cause at least part or substantially all of the clusters incident upon the shadow-mask surface to bounce from the surface and/or the sides of the aperture(s), whilst also low enough to cause at least part or substantially all of the clusters passing through the one or more apertures of the shadow- mask towards the substrate to remain on the surface of the substrate.

Preferably in the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to a velocity in the range InVs to 2000 m/s.

More preferably the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to a velocity in the range lOm/s to 300 m/s. Alternatively the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to kinetic energies per cluster atom in the range 5x10^~26 J to 2x10^"19 J. Preferably in the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to a kinetic energies per cluster atom in the range 5xlO^"24 J to 5xlO^"21 J.

Preferably the clusters directed towards the substrate are copper or palladium clusters with diameters in the range 5-20nm and in the step of directing the clusters towards the substrate the velocity of the clusters is in the range 100-400m/s.

Alternatively the clusters directed towards the substrate are bismuth or antimony clusters with diameters in the range 10-lOOnm and the velocity of the clusters is in the range 10- 100m/s.

Preferably the method of control of the cluster kinetic energy is by control of the velocity of the clusters via control of the flow rate of an inert gas into or out of the source of the clusters.

Preferably in the steps of directing the clusters towards the substrate, and deposition on the substrate, the clusters aggregate on or stick to the substrate, but there is minimal aggregation or adherence to the shadow-mask.

Preferably the surface of the substrate is altered to decrease the reflection of clusters from its surface by one or both of: - coating with a material which decreases the reflection of clusters from its surface; - roughening of the substrate surface.

Preferably the shadow-mask is coated with a material which enhances the likelihood of clusters bouncing from or not adhering to the surface of the shadow mask due to the clusters having a weak interaction with the surface, or a low tendency to wet the surface. Preferably the shadow mask is coated with a layer of polymeric material. Preferably the shadow-mask is coated with a layer of PMMA or SU8 or photoresist or a surface assembled monolayer (SAM). Preferably the SAM comprises one of the materials C 12- SiCl₃, 012-Si(OEt)₃, or CF-Si(OEt)₃. Preferably the layer is has a thickness less than 1 micron, more preferably less than lOOnm.

Preferably the method includes applying an electrical charge to clusters (either in the cluster source, as part of or subsequent to the cluster production process, or by ionisation subsequent to creation in the source prior to passing through the shadow mask) and the step of directing the clusters towards the substrate includes applying a voltage to the substrate and/or the shadow-mask and/or a part of the cluster source so as to accelerate or decelerate the charged clusters.

Preferably the clusters have been prepared via inert gas aggregation. Preferably the method of creation of the vapour which aggregates to form the clusters is by thermal evaporation, sputtering, magnetron sputtering (either AC or DC), arc discharge, electron beam heating or laser irradiation.

Preferably the shadow-mask includes at least one aperture having the general shape of a slot so that in the step of directing the clusters towards the substrate, passage of clusters through the slot and deposition on the substrate gives rise to a pattern in the shape of a wire or substantially linear pathway (whether or not conducting) on the substrate.

Preferably wherein the step of positioning of the shadow-mask with respect to the substrate includes positioning the shadow-mask between the source of the plurality of clusters and the substrate and includes aligning one or more features of the shadow-mask with one or more features of the substrate (or vice versa). Preferably the plane of the shadow-mask is parallel to the plane of the substrate. Alternatively the plane of the shadow-mask is at an angle to the plane of the substrate, so as to decrease the effective size of the aperture or so as to change the probability of reflection of a cluster from the shadow-mask.

Preferably after the step of depositing the pattern of clusters on the substrate, the substrate and/or the shadow-mask can be rotated relative to each other, and a further step of depositing the pattern of clusters can be performed.

Preferably one or more of the apertures of the shadow-mask is/are aligned with the position of the one or more contacts where present (or vice versa), so as to ensure the formation of a pattern of clusters between the contacts on the substrate after deposition. Preferably the method of alignment of the shadow-mask and substrate includes optical microscopy or the mechanism of a scanning probe microscope.

Preferably there is more than one deposition of material though the shadow-mask. The more than one deposition can include alternating depositions of insulating and conducting materials though the shadow-mask, and at least one deposition comprises the deposition of clusters.

Preferably the pattern is aligned at right angles to a pre-existing wire or pathway. In one embodiment the pre-existing wire or pathway had been formed by any means whatsoever, in a pre-step prior to the method of the invention.

In one embodiment the substrate is planar. Alternatively the substrate is non-planar, or includes three-dimensional structures or is itself a three dimensional object.

Preferably the substrate is an insulating or semiconductor material, more preferably the substrate is selected from silicon, silicon nitride, silicon oxide aluminium oxide, indium tin oxide, germanium, gallium arsenide or any other III-V semiconductor, quartz, or glass.

Preferably the clusters are selected from platinum, palladium, bismuth, antimony, aluminium, silicon, germanium, silver, gold, copper, iron, nickel or cobalt clusters.

The contacts may be formed by lithography or alternatively by deposition of the contact material through a contact shadow-mask.

In one preferred embodiment of the invention the shadow-mask has one or more apertures corresponding to the contacts (the contact aperture(s)) and one or more apertures corresponding to the pattern to be deposited on the substrate (the pattern aperture(s)), and in no particular order, the step of forming the contact on the surface consists of directing clusters through the contact aperture(s) whilst blocking or avoiding the pattern apertures, and the step of forming the pattern consists of directing clusters through the pattern aperture(s), whilst blocking or avoiding the contact aperture(s). Preferably the contacts are of a different species to the pattern of clusters and the source of clusters is altered between these two steps.

In an alternative embodiment of the invention the shadow-mask has one or more apertures corresponding to the contacts (the contact aperture(s)) and one or more apertures corresponding to the pattern to be deposited on the substrate (the pattern aperture(s)), and the contacts and the pattern are prepared in a single step of directing clusters towards the substrate (the contacts and the pattern being prepared from the same species of cluster).

Preferably one of more of the temperature, surface smoothness, and/or identity of the substrate are such to discourage diffusion of the clusters on the substrate surface so that the deposited clusters remain in the general shape of the slot. Preferably the method involves a further step of encapsulating at least a portion of the pattern of clusters in an insulating or dielectric material, which may provide protection from oxidisation, it may provide electrical insulation or be for any other purpose.

Preferably a further contact or other structure may be prepared on the surface of the insulating or dielectric material (in a pre-step or a post step)which is isolated from the pattern of clusters and may act as a gate.

Preferably the pattern of clusters is fabricated on a multi-layer substrate, one layer of which is electrically conducting and can therefore act as a gate.

In a second aspect of the invention there is provided a pattern of clusters on a substrate prepared substantially according to the method above

In a third aspect of the invention there is provided a method of forming a conducting pathway of bismuth, antimony or copper clusters between two contacts on a substrate comprising the steps of: a. substrate or vice versa or both; b. directing a plurality of clusters towards the substrate in such a manner that one of more of the clusters passes through one or more apertures of the shadow mask; c. depositing one of more of the clusters which have passed through the apertures onto the substrate to form a pattern of clusters wherein the pattern of clusters includes at least conducting pathway of clusters on the substrate, wherein the two contacts are either formed before step a and the shadow-mask is so positioned that the resultant pattern will be in the region between the contacts, or the two contacts are formed after step c and are positioned to have at least a portion of the pathway running between them, wherein the contacts are separated by a distance smaller than 1 micron, and wherein at least one of the dimensions of the aperture in the shadow-mask is less than 1 micron. Preferably at least one of the dimensions across the aperture in the shadow-mask is less than 100 nanometres.

Preferably the shadow-mask is coated with a layer of polymeric material with a thickness of less than lOOnm.

According to a further aspect of the invention there is provided a conducting pathway of clusters between two contacts on a substrate formed according to the above method.

According to a further aspect of the invention there is provided a device including a conducting pathway between two contacts wherein the conducting pathway is formed substantially as claimed in any one of the preceding claims.

Preferably the device includes Pd or Pd-alloy clusters and is capable of operating as a Hydrogen sensor.

Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings. As used herein the term "and/or" means "and" or "or", or both.

As used herein "(s)" following a noun means the plural and/or singular forms of the noun.

The term "comprising" as used in this specification and claims means "consisting at least in part of, that is to say when interpreting independent paragraphs including that term, the features prefaced by that term in each paragraph will need to be present but other features can also be present.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7).

The entire disclosures of all applications, patents and publications, cited above and below, if any, are hereby incorporated by reference.

To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

DEFINITIONS "Nanoscale" as used herein has the following meaning - having one or more dimensions in the range 0.5 to 1000 nanometres.

"Micronscale" as used herein has the following meaning - having one or more dimensions in the range 1 to 1000 micrometres. "Cluster" as used herein has the following meaning - a particle with dimensions in the range 0.5 to 1000 nanometres, which includes atomic clusters formed by inert gas aggregation or otherwise. It is typically composed of between 2 and 10⁷ atoms. "Wire" as used herein has the following meaning - any nanowire, microwire, or wire of larger dimensions. It includes chains, cluster-assembled wires and lithographically defined wires. A wire formed by the assembly of nanoparticles may be electrically conducting partially, substantially or entirely via ohmic conduction or substantially or entirely by tunnelling conduction. Such a pathway is not restricted to a single linear form but may be direct, or indirect. It may also have side branches or other structures associated with it. The nanoparticles may or may not be partially or fully coalesced. The definition of wire may even include a film of particles which is homogeneous in parts but which has a limited number of critical pathways.

"Conducting" as used herein has the following meaning - conducting electrical current (i.e. a flow of electrons) partially, substantially or entirely via ohmic conduction or substantially or entirely by tunnelling conduction.

"Contact" as used herein has the following meaning - an area on a substrate, usually but not exclusively comprising an evaporated metal layer, whose purpose is to provide an electrical connection between the cluster deposited pattern and an external circuit or an other electronic device. "Substrate" as used herein has the following meaning - an insulating or semiconducting material comprising one or more layers which is used as the structural foundation for the fabrication of the device. The substrate may be modified by the deposition of electrical contacts, by doping or by lithographic processes intended to cause the formation of surface texturing. "Pathway" as used herein has the following meaning - a structure which lies between at least two regions of a substrate that is made up of individual units which may or may not be wholly interconnected (i.e. while it may be a connected network, there may also be some spaces between the units). Like a wire it is not restricted to a single linear form but may be direct, or indirect. It may also have side branches or other structures associated with it. The particles may or may not be partially or fully coalesced. The definition of pathway may even include a film of particles which is not homogeneous. The pathway may or may not conduct. "Wires" as formed according to the method of the invention are a subset of "pathway".

"Shadow-Mask" as used herein has the following meaning - a sheet or membrane with one or more apertures through which clusters may pass, the clusters being prevented from passing through other parts of the shadow-mask.

"Contact Shadow-Mask" as used herein has the following meaning - a sheet or membrane with one or more apertures through which atoms may pass, the atoms being prevented from passing through other parts of the shadow-mask. We use the term contact shadow-mask to distinguish a mask used in the deposition of the contacts on a substrate, from a mask used in the deposition of clusters in forming the pattern. "Aperture" as used herein has the following meaning - a gap, space or opening in a shadow-mask. It is not restricted to shape or dimension. It is usually (but not only) used in relation to fully enclosed openings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is further described with reference to the accompanying figures:

Figure 1. FE-SEM images of (i) the Si_xN_y aperture-slot used to produce the self- contacting cluster-assembled wire in Figure 2 and (ii) accumulation of clusters on PMMA residue and etch-damaged silicon at the edge of the membrane.

Figure 2. A cluster-assembled wire (width lμm) formed using the shadow-mask shown in Figure 1. Figure 3. Schematic cross-sectional diagram showing the structure of the

' commercial Si_xN_y membrane which forms the basis for the shadow-mask. Figure 4. Schematic cross-sectional diagram showing the membrane inverted on a Si earner and back-side evaporated with NiCr which provides an electrical contact to the reverse-side of the membrane necessary for the dry-etch process.

Figure 5. Schematic cross-sectional diagram showing the reverse-side NiCr-coated membrane mounted on an Al-coated Si carrier and featuring dual-layer PMMA which is spun over the membrane prior to the electron-beam write stage. Figure 6. Schematic cross-sectional diagram showing the membrane, patterned PMMA layer and angle-evaporated NiCr film which serves as the barrier to the aperture-forming dry-etch process. Figure 7. Schematic cross-sectional diagram showing the completed shadow-mask secured on the contacted substrate. Figure 8. Schematic plan view showing (a) the contacted substrate and (b) the membrane aligned with respect to the contacted substrate.

Figure 9. Image of a completed Si_xNy membrane shadow-mask with single aperture- slot opened at the ends to assist both in alignment and in verifying that the Si_xN_y film has been etched through. Figure 10. (i) Additional (secondary electron) image of the cluster-assembled wire shown in Fig. 2 and (ii) a SEM image of a cluster-assembled wire, also produced using the method of the invention, spanning a gap of lOOμm between two planar contacts. Figure 11. Conduction measured between the contacts of a substrate during a shadow- masked cluster deposition process.

Figure 12. The Current- Voltage characteristic of the shadow-masked, cluster- assembled wire shown in Figure 10.

Figure 13. High resolution image of an aperture-slot formed using reactive ion etching through a void in an evaporated NiCr film. The void was achieved using angled-evaporation onto a patterned PMMA layer.

Figure 14. An aperture-slot formed in a Si_xN_y membrane (thickness 200nm) using Focused Ion Beam etching. The minimum width of the slot is ~400nm. The openings at the ends of the slot assist in alignment and in verifying that the Si_xNy film has been etched through.

Figure 15. Bi cluster coverage on substrates with differing coatings measured from

FE-SEM images. Figure 16. Cu cluster coverage on substrates with differing coatings measured from

FE-SEM images. Figure 17. Field-Emission SEM images of Bi clusters deposited on (a) PMMA (b)

Photoresist (c) Si_xN₅, and (d) SiO₂ substrate layers. Figure 18. Field-Emission SEM images of Cu clusters deposited on (a) PMMA (b)

Photoresist (c) Si_xN_y and (d) SiO₂ substrate layers.

Figure 19. Conduction measured between the contacts of a substrate during the formation of an Sb cluster-assembled wire array.

Figure 20. Example of narrow cluster "necks" in a Bi cluster-assembled wire with a minimum width of approximately lOnm, formed in a PMMA aperture- template and supported on a Si_xN_y passivated Si substrate.

Figure 21 Examples of results of measurement of cluster velocity as a function of cluster diameter for a variety of source conditions. Figure 22 Examples of results of measurement of cluster velocity as a function of cluster diameter for a variety of source conditions.

Figure 23 Examples of results of measurement of ion current as a function of a retarding potential applied to a Faraday cup, yielding cluster velocity data.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to our method of fabricating patterns of clusters,

(particularly nanoscale clusters, (nanoclusters)) and particularly patterns incorporating or including pathways, on the surface of a substrate. These pathways are on the nanoscale and up to the micronscale, and are formed by deposition of clusters through a shadow- mask and onto a substrate. In preferred forms of the invention the pathways are capable of acting as wire-like structures which may have useful functionality due to their electrical conductivity.

The invention relates particularly to the deposition of conducting or semiconducting clusters. Potential advantages of forms of our invention (compared with many competing technologies) may include the following:

Electrically conducting wires (nano and microscale) can be formed using relatively simple techniques, i.e. cluster deposition together with electron-beam or projection lithography,

The resulting wires (nano and microscale) may be automatically connected to electrical contacts. This enables electrical characterisation before, during and after the formation of the cluster-assembled film or wire. - Electrical current may be passed along the wires as soon as they are formed. - No manipulation of the clusters is required to form the wire

Apertures in the shadow-mask may be formed using standard lithographical techniques, i.e. electron-beam or projection lithography and dry-etching. Apertures in the shadow-mask are not subject to clogging (due to the momentum of the incident clusters) Cluster-assembled or wires or pathways may be formed on planar or non-planar substrates

Cluster-assembled wires or pathways may be formed on chemically and/or biologically sensitive substrates.

A. METHOD OF THE INVENTION

One preferred method of the invention relies upon a number of steps and/or techniques, not necessarily in the following order:

1. the formation of contacts on the substrate surface; 2. the formation of a shadow-mask,

3. placement of the shadow-mask in front of the substrate,

4. the formation of atomic clusters,

5. deposition of the clusters onto the substrate though the shadow-mask,

6. monitoring the formation of the pathway or wire.

1. Formation of Contacts on the substrate

In most cases this will precede the step of forming the pattern. However it is possible that the pattern is formed first and the contacts formed afterwards.

Substrates can be any surface which is capable of supporting a cluster-assembled film and evaporated contact materials and which can be installed into a vacuum deposition chamber. We often use Si wafers with SiO_x or Si_xNy insulating top layers. Alternatives are GaAs, GaN, AlGaAs or SiGe substrates (amongst many others) with passivation provided by SiO_x, Si_xN_y, AlO, spin-on glasses and polymers (amongst many others), so long as they are capable of supporting a cluster-assembled pathway to be formed on the substrate surface.

These substrates may have three-dimensional features and/or be sensitive to chemical or biological agents. For example, chemically terminated Si [100] surfaces which may be damaged during standard lithographic photoresist application/development processes may successfully support electrical contacts achieved through cluster deposition. Further, a complex 3D surface such as a mechanical component could be coated by deposition of clusters through a shadow-mask e.g. a nanowire sensitive to magnetic fields and intended as a read-head for a hard disk drive could be deposited onto a complex structure such as the gimball assembly / write head of a standard commercial hard disk drive.

The preferred method of contact formation relies on evaporation or sputtering of a metal/alloy through a shadow-mask which is hereinafter called the "contact shadow- mask" with large scale features so as to define contact pads as well as the electrical contacts to the device. Metals or alloys which can be used include Ti, NiCr, Al, Au, Ag, Cu, W, Mo, Pd, Pt, Bi, Sb. Our preferred contacts are formed from NiCr and Au. The contact shadow-mask is positioned between the substrate material and the chosen evaporation source so that the evaporated film replicates the features of the mask. It should be noted that a standard shadow-mask process is typically used at this stage of the process: the feature sizes are much greater than in the following steps, and the preferred deposition is of atomic vapour generated via thermal or electron beam evaporation. However, other deposition procedures as known in the art may be used.

This method provides suitable contact structures and because it is both chemical-free and non-contacting, it offers an unmodified, ultra-clean substrate surface. The planar electrical contacts allow in-situ monitoring of the current through cluster-assembled wires/pathway/films and serve as an alignment aid when the substrate and shadow-mask are brought together.

Electron beam lithography and photolithography are well-established techniques in the semiconductor and integrated circuit industries and offer an alternative means of contact formation. These techniques are routinely used to form many electronic devices ranging from transistors to solid-state lasers. We emphasise that in one preferred method a shadow-mask process is used to produce both the contacts to our devices and the active component of the device. The contacts are conveniently formed by standard atomic evaporation through a large-scale contact shadow-mask and the cluster-assembled wires/films/pathways are formed by the deposition of atomic clusters through a small-scale shadow-mask.

As will be appreciated by one skilled in the art, other techniques of the art which allow for nano-scale or micron-scale contact formation will be included in the scope of the invention in addition to electron beam lithography and photolithography, for example nanoimprint lithography.

It should also be noted that at this stage of the process other features could optionally be formed on the substrate surface- particularly the formation of other lithographically defined 2 -D or 3-D patterns on the substrate. These could be alignment features for the shadow-masks, other features which assist with the deposition of the clusters such as V- grooves, inverted pyramids, trenches. It could also include substrate heating elements, other wires, circuits/circuit components/ electrical contacts and/or electrodes which establish electric-fields.

2, The formation of a shadow-mask

In one shadow-mask process, lithography, film-deposition and dry-etching are used to form apertures through a suspended low-stress membrane. Lithography is used to pattern features into a photo/electron sensitive film (resist-film) which is transferred to the membrane by etching (most preferably dry etching) in order to create apertures through the Si_xN_y membrane. Once the shadow-mask is completed, atomic clusters can be deposited through these apertures and onto a substrate. The membrane could be any suspended metallic/semiconducting or insulating film which may be wet- or dry-etched to form apertures. We prefer Si_xNy membrane films suspended across a Si frame. The photo/electron sensitive film coating used to produce the aperture patterns may be AZl 500, SU-8, UV3, UV5, ZEP or PMMA (PolyMethyl MethAcrylate). We prefer PMMA (PolyMethyl MethAcrylate). The resulting cluster-assembled film/wire/pathway on the substrate thus replicates the aperture pattern in the Si_xN_y film.

In an alternative method, a Focused Ion Beam (FIB) is used to etch the aperture-slot in the Si_xN_y membrane with no requirement for Electron-beam patterning and greatly simplified processing. The FIB is capable of etching through bilayers (eg. PMMA/ Si_xN_y) as well as the membrane and as a result, the high cluster-reflection property of cured PMMA or other polymer layers can be exploited (in similar fashion to the preceding EBL/RIE methods).

3. Placement of the shadow-mask in front of the substrate

The shadow-mask is aligned relative to the substrate and electrical contacts (if present) on the substrate. Most typically, the membrane edge of the shadow-mask is then aligned on the sample relative to the clearly visible metallised substrate contacts and secured using a small quantity of soluble adhesive. A variety of alignment tools may be employed, including optical and atomic force microscopy.

It is possible that the shadow-mask can be translated or rotated manually or automatically (e.g. electromechanically or with piezo-electric drives or motors) between deposition processes. This would allow multiple patterns to be deposited onto the same substrate, for example to allow several parallel or crossed wires to be fabricated, or to allow fabrication of several independent nanoelectronic devices.

4. Formation of clusters

In one preferred method the clusters are formed by a process whereby metal vapour is evaporated into a flowing inert gas stream which causes the condensation of the metal vapour into small particles. The particles are carried through a nozzle by the inert gas stream so that a molecular beam is formed. Particles from the beam can be deposited as atomic clusters onto a suitable substrate. This process is known as inert gas aggregation (IGA), but clusters could equally well be formed using cluster sources of any other design (see e.g. the sources described in the review [5]). In place of the evaporation method we have also use a magnetron sputtering source i.e. one in which a metal vapour is created by sputtering of the cluster material from a target. Clusters could be of Si, Pd, Pt, Cu, Bi, Pb, Sb, Ag, Au or of many other materials or alloys of materials (e.g. Ni, Au, Ag, Rh Cu, or Pb alloyed with Pd in order to fabricate improved hydrogen sensors) or doped semiconducting materials (e.g. Boron (p-doped) Si). We prefer Si, Bi, Sb, Pd and Cu for the specific applications investigated in our group recently e.g. Cu for interconnects, Bi and Si for transistors, Pd for hydrogen sensors).

Sizes of cluster can range from less than 0.5nm to lOOOnm in diameter. We prefer clusters with diameters in the l-50nm range. Additionally we prefer charged clusters from a magnetron sputtering source which may be focused and/or accelerated and/or decelerated using potentials applied to lenses and/or the substrates and/or the shadow mask.

5. Selective cluster deposition using a shadow-mask

The basic design of a cluster deposition system with the basic components of an IGA source (with thermal or magnetron source), differentially pumped vacuum stages with cluster-beam shaping nozzles, and deposition chamber and optional mass spectrometers, mass filters and / or electrostatic lenses or other elements is described in Refs [6] and [13], the contents of which are hereby incorporated by way of reference.

Following the cluster source are a series of differentially pumped chambers that allow ionisation, size selection, acceleration and focussing of clusters before they are finally deposited on a substrate. In fact, while such an elaborate system is desirable, it is not essential, and the invention can be performed using relatively poor vacuums without ionisation, size selection, acceleration or focussing. The essential feature of our technique is that the clusters must pass through the shadow- mask before being deposited on the substrate, between the electrical contacts (if present. If not the contacts can be formed after the cluster deposition) and that the shadow-mask prevents deposition on other areas of the substrate.

Preferentially the nature of the shadow-mask the cluster materials and the features and parameters of the deposition process are such that the clusters bounce from the shadow- mask so that the aperture does not clog, and so that minimal cleaning of the mask is required between depositions. Figure 1 shows a shadow-mask which was imaged (using a scanning electron microscope) immediately after it was used to produce a cluster- assembled wire. There is a complete absence of clusters in or around the aperture slot (shown in Figure l(i)), whilst clusters have clearly adhered to an area which is defected in that it contains PMMA residue and etch-damaged silicon found at the edge of the membrane (shown in Figure l(ii)). Note that the primary feature of Fig. l(ii) is the absence of clusters from the majority of the mask; in this case the residue / damage to the mask was unintentional but intentional roughening of PMMA and/or Si can be performed by RIE or FIB and hence cause deposited clusters to be preferentially assembled in prescribed areas on these materials (particularly on the substrate). An image of the cluster-assembled wire formed using the shadow-mask is shown in Figure 2.

For a given cluster material there exists a range of kinetic energies that the cluster can have which will cause it to be reflected from surfaces with low cluster-surface attachment energy whilst adhering to surfaces with higher cluster-surface attachment energy. The material parameters of the clusters (wetting), substrate (surface energy) and shadow-mask (surface energy) and the IGA source parameters (inert gas-flow and inert gas mix, controlling the kinetic energy of the clusters) are thus selected in order to ensure that reflection of the clusters occurs on the surface of the shadow-mask whilst adhesion of the clusters occurs on the surface of the substrate. Some specifics are given in 7. below. The result is deposition through the shadow-mask aperture with minimal adhesion to the shadow-mask. This is particularly important in the areas around the aperture, resulting in reduced clogging.

6. Monitoring the Formation of the Wire This step is used in one embodiment of the invention. It requires that the contacts are already present. It involves the monitoring of the conduction between a pair of electrical contacts and ceasing deposition of atomic clusters upon the formation of a conducting connection between the contacts (or indeed at some other controlled time after the onset). Alternative or further embodiments may involve monitoring the formation of more than one wire structure where more than one wire may be useful.

We monitor the formation by checking for the onset of conduction between two contacts. This requires incorporating into our deposition system electrical feedthroughs into the deposition chamber, to allow electrical measurements to be performed on devices during deposition.

It should be emphasised that monitoring of conduction is an optional step which may be omitted from the process. This step provides greater control over the deposition process, but is not essential in many applications.

7. Parameters affecting the Method of the Invention

There are a number of important parameters used in the invention which relate to the nature of the pattern formed, minimising clogging of the apertures whilst ensuring that sufficient numbers of clusters adhere to the underlying substrate.

a) identity of the clusters and the shadow mask / substrate

Extensive previous experimental work in our group (see for example [8,9,10,13]), together with molecular dynamics simulations [7], have shown that clusters of a wide variety of materials can be made to bounce or slide on a variety of surfaces. The significance of the bouncing or sliding of a cluster is that the cluster is unlikely to stick to the faces of the aperture in the shadow mask, and therefore unlikely to clog the stencil.

We have shown that, in simple terms, a cluster will bounce from a surface if its kinetic energy remaining after impact (i.e. as it rebounds) is sufficient to overcome the strength of the attraction to the surface. The cluster size and velocity are key parameters which determine whether the cluster bounces, as discussed in more detail in subsequent subsections.

We have shown [7] that a more detailed consideration of the deformation of the clusters on impact with a surface reveals two regimes in which clusters can bounce from a surface i.e. an elastic regime and a plastic regime (in the latter the cluster is significantly deformed on impact).

For example, for clusters of materials such as Bi, Sb, Si, Cu, and Pd we have seen different bouncing and sticking behaviours dependent upon velocity and the identity of the shadow-mask or substrate with which they are colliding. Specifically, at sufficiently high velocities (see below) we have observed bouncing of the clusters from various solid surfaces typical of the semiconductor industry such as Si, Si_xNy and SiO_x. In comparison bouncing occurs at significantly lower velocities for various polymers used by the semiconductor industry (e.g. photoresist, PMMA, SU8) to which clusters have a low tendency to adhere. This means when we use a particular cluster type incident on both a shadow-mask coated with a polymer, and a semiconductor substrate (through the shadow-mask apertures) we prefer to select a velocity of particles which will give rise to bouncing from the shadow-mask but also sticking or accumulation on the substrate.

Such a scenario gives rise to one preferred form wherein the clusters do not adhere to the material of the stencil, but do adhere to the material of the substrate on which they are intended to be deposited. Hence the coating of the shadow-mask with a polymeric material in order to reduce adherence of the clusters is a preferred embodiment of the invention, as discussed below. Conversely the preferred embodiment of the invention is that the substrate is not coated by a polymeric material, at least within the regions in which it is desired that the clusters should adhere. This is a specific embodiment of the invention but the same principle extends to the fabrication of shadow masks from other (for example, uncoated) solid materials to which clusters have a low tendency to adhere.

We prefer a thin layer of polymeric material (ideally PMMA or SU8), of thickness smaller than 1 micron, but ideally down to less than lOOnm; much thicker coatings will also cause cluster bouncing although of course the ability to produce fine features in a electron or optical resist is limited in thick layers.

We believe these materials to be examples of, and a subset of, a wide range of cluster and shadow-mask material combinations that may be used in the invention, as would be envisaged by one skilled in the art of cluster formation and/or lithography.

b) cluster size

Increasing the cluster size increases the mass of the cluster and hence, for a fixed velocity, its kinetic energy and momentum. As noted above, in general clusters with greater kinetic energy and momentum are more likely to bounce than clusters with lower kinetic energy and momentum, and so control of cluster size is an important parameter which can be used to control the fraction of clusters which bounce from a particular surface. Control of cluster size is considered in some detail in the modelling in [8] (for the elastic regime) and [7] (for the plastic regime).

The specific size we prefer to use will clearly depend upon the identity of the cluster, the particular system we are operating within and the desired dimensions of the pathway. By way of example only (and the invention is not to be restricted to these examples) if we use copper clusters we have had good results with diameters in the range 5-20nm. Alternatively with bismuth or antimony clusters we have had good results with diameters in the range 10-1 OOnm. c) velocity of the clusters incident on the shadow-mask/substrate As mentioned above, for a fixed cluster mass, increasing the cluster velocity increases its kinetic energy and momentum, and hence its likelihood of bouncing. Control of cluster velocity is considered in some detail in the modelling in [8] (for the elastic regime) and [7] (for the plastic regime).

We prefer to control the cluster velocity by controlling the flow rate of inert gas into the cluster source chamber [8], as this is simple and effective. However, for ionised clusters produced by a sputtering source (for example) or for clusters from an inert gas aggregation source with thermal evaporation which have been ionised by electron beam impact or laser ionisation (for example) it is possible to use electrostatic fields to accelerate or decelerate the clusters. Hence the application of a potential to either the substrate, the source or the shadow-mask to accelerate or decelerate the clusters is included within the scope of the invention

Again the specific velocities/kinetic energies we use will depend upon the system we are operating within and what we are trying to achieve. Again by way of example only (and the invention is not to be limited to this) we have typically used velocities in the range lm/s to 2000 m/s, though with very small clusters we expect that we may get into a different regime where the velocities may need to be higher. More specifically the kinetic energy imparted to the clusters in our experiments corresponds to a velocity in the range 10m/s to 300 m/s.

For the sake of example, if we look at the clusters discussed above, when the clusters are copper clusters with diameters in the range 5-20nm, the velocity of the clusters is ideally in the range 100-400m/s. Alternatively when clusters are bismuth or antimony clusters with mean diameters in the range 20-50 nm, the velocity of the clusters is ideally in the range 10-lOOm/s. It is sometimes useful to consider the collision between a cluster and a surface in terms of the kinetic energy per cluster atom. In the illustrative examples above, the kinetic energies per atom for the Cu clusters were typically in the range 5x10^"22J to 1x10^"20J and the kinetic energies per atom for the Sb and Bi clusters were in the range 1x10^"23J to 2xlO^"21J.

d) angle of deposition relative to the plane of the shadow-mask

Any aperture in a shadow mask can be considered to have a projected area on the surface of the substrate. If the shadow mask is mounted parallel to the surface of the substrate and orthogonal to the beam of clusters the projected area on the substrate is maximal and the area of the substrate covered by the clusters after deposition will be approximately equal to the area of the aperture (assuming a reasonably well collimated cluster beam, which is typical of our deposition system [13]). However the angle of the shadow mask can be rotated so that the projected length of the aperture remains constant while the projected width of the aperture decreases. In this case the area of the substrate (and width of the pattern) coated with clusters will be decreased. In the case of a simple slot-aperture this will result in a wire on the substrate with narrower width.

The angle through which the shadow mask may be rotated is limited by various factors such as the increase in exposed area of the sidewalls of the aperture with increased angle of rotation, and change in fraction of clusters that stick to a given surface with angle (see ^e-g- [7]), and obviously for sufficiently large angles the aperture may be entirely occluded.

It is further noted that it has been demonstrated [8] that clusters incident on an angled surface may bounce preferentially compared to clusters incident on flat surface. This effect may be employed to reduce the number of clusters sticking to a shadow-mask which has been rotated so that clusters are not normally incident on it. e) shadow-mask coatings

The data in figures 15-18 (discussed in more detail in the examples section below) show clearly that coating of the stencil with a polymeric material such as PMMA or photoresist may significantly increase the reflectivity of the clusters from the surface of the stencil, and hence reduce the accumulation of deposited material on the stencil, which could otherwise lead to clogging. In a preferred embodiment of the invention the process for fabricating the shadow mask involves the use of a PMMA or photoresist layer to pattern the membrane, and in an even more preferred embodiment the PMMA or photoresist is not removed after patterning the membrane, so as to leave behind on the surface of the membrane a layer which discourages accumulation of deposited material.

As mentioned above we prefer a thin layer of polymeric material (ideally PMMA or SU8), of thickness smaller than 1 micron, but ideally down to less than lOOnm.

j) substrate surface

One requirement for the achievement of cluster-assembled films and wires which accurately replicate shadow-mask features is that the clusters which remain on the substrate and contacts do not move significantly. This will almost always be the case for relatively large clusters (greater than about 10 nanometers in diameter), even at room temperature. For smaller clusters, or in the rare cases where even large clusters diffuse across the insulating surface of interest, the sample can be cooled down prior to deposition to eliminate surface diffusion. Similarly (and as discussed more extensively in our previous patent applications [8, 9]) control of other surface parameters such as surface roughness, defect density, surface energy, amount and type of interaction between the cluster material and surface material (or coating applied to the surface), may all be used to control surface diffusion, bouncing or sliding of the clusters on the surface.

B. PREFERRED FORM OR BEST MODE OF THE INVENTION In the preferred embodiment, the invention will result in the formation of a conducting pathway or wire between contacts on a surface. The method involves deposition (micro or nano-scale) of particles (clusters) through micro- and/or nano-scale apertures formed in a membrane (such as Si_xN_y) and onto a substrate featuring electrical contacts which are monitored throughout the cluster deposition process thereby indicating the exact time at which the cluster-assembled wire is completed. In-situ monitoring of the conduction between the contacts also provides precise control over the duration of the deposition process (and therefore the thickness of cluster-assembled pathway or wire). The apertures in the Si_xN_y membrane are foπned using standard lithographic techniques.

The invention is applicable to the fabrication of self-contacting cluster-assembled wires and films on planar and non-planar substrates.

The invention is applicable to a variety of cluster/substrate systems and the size of the incident clusters is unimportant, although preferably the average cluster momentum as well as cluster identity, size and shadow-mask identity and dimensions is sufficient to prevent adhesion and clogging within the aperture(s) of the shadow-mask. Replication of the shadow-mask features in the cluster-assembled film will occur as long as the incident clusters are smaller than the aperture's smallest dimension. The size and velocity of the clusters can be adjusted so that the momentum of the incident clusters is sufficient to prevent clogging of the membrane aperture during deposition, and so as to ensure that the majority of clusters bounce off the membrane, leaving it clean for repeated usage. The bouncing of clusters from surfaces was studied extensively in [8] and Figure 1 shows a shadow-mask which remains free of clusters after a deposition, because the clusters have bounced from the mask material while adhering to the substrate material (as shown in Figure 2). This is believed to be due to the fact that while some PMMA is removed while etching away the membrane to form the aperture in the shadow-mask, there remains a thin film of PMMA on the surface of the shadow-mask, and this film of PMMA causes clusters to bounce from the shadow-mask more efficiently than from the substrate, or from the bare material of the shadow mask. PMMA has a low surface energy and high elasticity (when compared with Si_xN_y, Si, SiO₂, Au, NiCr₅ Al etc.) and these two material properties are believed to cause the higher probability of cluster-reflection from PMMA than from Si_xN_y, Si, SiO₂, Au, Al.

In the case of Figures 1 and 2 the clusters were incident perpendicularly on the Si_xNy membrane and substrate, however it is important to note that the plane of the shadow- mask can be tilted relative to the plane of the substrate so as to both decrease the effective size of the aperture, and so as change the angle of incidence of the clusters on the shadow-mask so as to decrease the clusters' probability of sticking to the shadow-mask. This approach will provide an effective method of reducing the clogging of the aperture(s) in the shadow-mask when the substrate is of the same material as the shadow- mask. It should be noted that faces of the aperture in the shadow-mask are normally mounted parallel to the direction of the incident clusters, and that this is significant in reducing the clogging of the aperture in comparison to standard shadow-mask evaporations using atomic material. It is expected that there will be an optimal angle of rotation of the plane of the mask with respect to the plane of the substrate which will maximise the bouncing of the clusters from both the planar surface of the shadow-mask and the surfaces comprising the faces of the aperture.

The apparatus and the method according to the invention make it possible to fabricate self-contacting single or multiple, parallel or non-parallel cluster-assembled wires with widths from ~20nm to >100μm. The technique is not limited to wire-like patterns; also possible are arbitrarily shaped 2D cluster-assembled films (and arrays of arbitrarily shaped 2D cluster-assembled films). Provided the aforementioned structures are deposited between suitably arranged planar electrical contacts, monitoring of the conduction of the cluster-assembled structures is possible throughout the deposition process. Figure 11 shows the conduction across the contacts of a Si_xN₅, passivated Si substrate measured during a shadow-masked cluster-deposition process. The sharp rise in the conduction after a deposition time of 350s indicates the production of a conducting cluster-assembled pathway. C. APPLICATIONS OF THE INVENTION - INDUSTRIAL APPLICABILITY

An important characteristic of the wires formed by the method of the invention is that in general their conductivity will be sensitive to many different external factors (such as light, temperature, chemicals, magnetic fields or electric fields) because of either the intrinsic properties of the material forming the wire, or the physical or chemical properties of a wire formed from such a material, which in turn give rise to a number of electronic device and sensing applications. Devices of the invention may be employed in any one of a number of applications. Applications of the devices include, but are not limited to:

- Transistors or other switching devices,

Magnetic field sensors,

Chemical sensors,

Light emitting or detecting devices, - Temperature sensors.

Such devices are described in more detail in [8,9], and this description is herein incorporated by reference.

A particular application of interest is the formation of a hydrogen sensor using the methods described herein. Pd nanoparticles are known to expand on absorption of hydrogen such that a Pd nanoparticle film with coverage initially slightly below the percolation threshold will become conducting on absorption of hydrogen. In addition, embodiments which incorporate percolating, percolating-tunneling or tunneling pathways have been described [10]. By depositing Pd particles through a shadow-mask it is straightforward to define patterns of any shape of Pd nanoparticles located between 2 or more electrical contacts. The expansion of the particles on absorption of hydrogen then provided a mechanism by which the conductivity of the device changes, providing a sensor. This is within the scope of the invention, as described and claimed. The apparatus and the method according to the present invention allow the fabrication of cluster-assembled structures with feature sizes of less than 20nm. The electron beam lithography processes used to produce narrow aperture-slots here (see e.g. Fig 1 and Fig 9) are standard processes which can be extended down to dimensions of order 20nm. The 5 clusters used in the invention have typical dimensions in the range 5-30nm and it is noted that even in pathways comprising 30nm clusters there are constrictions (or "necks", see e.g. Fig. 2) which may dominate the properties of the overall device. (See also Fig. 20 which shows an example of necks formed in a cluster-assembled wire, in this case however the wire was produced using a PMMA template cluster-assembly method as

10 described in [H]). Therefore the cluster-assembled structures of the invention may include cluster-assembled wires with uniform widths below 20nm (produced using a shadow mask with a narrow aperture slot, or with a wider aperture tilted at an angle to the cluster beam) or cluster-assembled wires which feature sections with minimum dimensions of less than 20nm. Quantum effects have been observed in wires and films

15 with similar dimensions, and the present invention enables efficient electrical characterisation of such effects.

In the present invention, deposition of metallic or semiconducting cluster-assembled wires is possible in a parallel-write process ie. with a multiple-aperture membrane a 20 single deposition process will produce multiple cluster-assembled structures on a substrate. The technique therefore has an inherent advantage in speed-of-fabrication over techniques requiring scanned beams to produce micro- and nano-scale features (eg. Electron-Beam Lithography).

25 The invention can provide multilayered cluster-assembled films and wires, formed by successive depositions of clusters of different materials through the same shadow-mask. The mask may be moved or rotated between depositions to create gaps or variable overlaps between the films, or held in position to create a stack of identical layers at an accurately defined location. These films and wires can have dimensions in the sub-

30 100 nm region. The rotation of the mask (or substrate) between depositions can be used to provide 'cross bar structures' wherein a pair of crossed wires provide a device, for example a memory element or a transistor. In these applications it may be necessary to deposit an additional material (such as a layer of magnetic or molecular material), either through or in the absence of the shadow-mask, in order to provide the active material which is sandwiched between the two wires.

The invention requires no chemical treatment of the contacted substrate or application of photo- or electron-sensitive polymer (resist). Hence, cluster-assembled structures can be deposited through a shadow-mask onto non-planar substrates and onto substrates which are chemically or biologically reactive (where application of electron-beam or optical resist is impossible). As an example, the technique would provide a means to fabricate naiTOw and/or high-density interconnects on micro-machined devices and substrates with bio applications.

Typical applications include data storage by nm-sized structures such as magnetic storage elements using quantum bits, fabrication of electronic logic and memory devices and chemical/bio sensing devices.

An important application of the technique is in the provision of a device where the electrical contacts are formed by deposition of cluster material through the shadow-mask i.e. the step of formation of electrical contacts is omitted, and a large area of deposited clusters provides the contact to the wire or other structure that is formed.

EXPERIMENTAL

The following discloses our preferred experimental set up along with specific examples.

Formation of Contacts on substrates For convenience, Si wafers with SiO_x or Si_xNy insulating top-layers have been used as substrate material. The shadow-mask/cluster method can work equally well with any solid insulating material. The preferred method of contact formation relies on evaporation or sputtering of NiCr or NiCr/Au films through a contact shadow-mask with large scale features so as to define contact pads as well as the electrical contacts to the device. The contact shadow-mask is formed in stainless steel sheet (approximate dimensions 80mm x 80mm x 0.5mm) and the contact-apertures are cut out of the sheet using a standard commercial laser-cutting or wire-cutting process. The dimensions of the contact-apertures in the contact shadow-mask ranged from 0.1mm to 6mm. Simple dual contact arrangements were used and the gap between these contacts was produced using the shadow of a lOOμm or 25 μm diameter wire which was positioned across the contact- apertures in the stainless steel shadow-mask. 3-5nm of NiCr and 15-25nm of Au were evaporated through the shadow-mask and onto the SiO_x or Si_xNy passivated Si wafer to produce the planar electrical contacts.

The contact shadow-mask is positioned between the substrate material and the chosen evaporation source so that the evaporated film replicates the features of the mask. It should be noted that while a standard shadow-mask process is used at this stage of the process, the feature sizes are much greater than in the following steps, and the preferred deposition is of atomic vapour generated via thermal or electron beam evaporation, or by sputtering.

Formation of the Shadow-Mask - Aperture-slot formation using lithography and Reactive Ion Etching (RIE). The following deals with the formation of an aperture-slot in a Si_xN₅, membrane.

A commercially available Si_xNy membrane is used for the shadow-mask. This consists of a 200-500nm thick Si_xN_y film supported on a 200-400μm thick Si frame. As illustrated in Figure 3 the Si_xN_y membranes are produced using deep KOH etching to remove the underlying Si wafer 31 from behind the Si_xNy film 32 in order to form a region of unsupported Si_xN_y.

The Si_xN_y membrane is first mounted face-down on a silicon substrate 41 (used simply to support the membrane during processing) using photoresist as shown in Figure 4. The reverse side of the membrane and the Si frame are then coated with NiCr (thickness lOnm) 42, by thermal evaporation. Once the NiCr deposition is complete the sample is removed from the carrier using acetone. During the RIE processing which is used to form a slot 131 (shown in Figure 13) in the insulating Si_xN_y film, this NiCr coating serves to provide an electrical path for incident reactive ions.

The Si_xNy membrane is then attached to an Al coated carrier 51, with the back-side of the Si frame attached to the carrier (and the Si_xN_y film face upwards) as shown in Figure 5. The Al coating is required in a subsequent wet-etching process, as described below. PMMA (standard electron-beam resist, polymethyl methacrylate) is used to provide adhesion between the membrane and carrier and it is hardened in an oven (held at 100⁰C for 1-hour). PMMA is used rather than photoresist (PR) because the sample must be baked at 185⁰C and PR would be too difficult to remove after that baking step.

The membrane is now spun with bi-layer PMMA in readiness for the Electron Beam Lithography stage required to define an aperture-slot for the etching process. Dual spins with LMW (Low Molecular Weight) and HMW (High Molecular Weight) PMMA produce a total PMMA layer thickness of 200nm. The upper HMW layer is patterned using an electron-beam writing system (eg. Raith 150) and developed in 3:1 IPA:MIBK. The width of the e-beam written pattern may be less than lOOnm.

At this point in the process, two alternative methods for forming the aperture may be employed. In the first method, the electron-beam patterned PMMA is used as an etch mask. Since the etch-rates of PMMA and Si_xN_y are approximately equal (when performing CHF₃ZAr based RIE), this process can be used to produce apertures in the Si_xNy film provided its thickness is less than that of the PMMA layer. After the RIE process, the electron-beam patterned aperture in the PMMA is faithfully translated into the underlying membrane. In the second method, after electron-beam patterning of the PMMA, the membrane and carrier are once again installed in an evaporator with a NiCr source. The PMMA coated Si_xNy film is positioned above the source at an angle so that certain parts of the membrane are shadowed from the evaporated NiCr and void(s) (61) are formed in the evaporated film, as shown in Figure 6. (This technique is a well known variant of the shadow-evaporation method). The angled-evaporation stage has been incorporated into the shadow-mask fabrication process primarily because the NiCr layer protects the PMMA film during the dry-etch process, but also because it provides size- reduction of the aperture. (The width of the void is controlled by the thickness of the PMMA film and the angle of evaporation, rather than the width of the e-beam written pattern). Usually the membrane is orientated above the evaporation source so that the void created within the PMMA aperture-slot extends along the long side of the aperture. Realisable void widths using this method range from ~20nm upwards.

Using either PMMA or angle-evaporated NiCr as an etch mask, Reactive Ion Etching (RIE) is used to create a slot-aperture in the Si_xN_y film, as shown in Figure l(i). Various gas mixes have been used in the RIE system to etch the aperture-slot. CHF₃/ Ar etch- chemistry is preferred when etching with patterned PMMA as the etch-mask. When using a NiCr etch-mask, both CHF₃ZAr and CHF₃/O etch-chemistries provide highly selective etching of the Si_xNy with respect to the NiCr. Etch-rates were obtained from Si_xNy /Si test samples.

The membrane is removed from its carrier by wet-etching the sacrificial aluminium layer on the carrier. Once removed from its carrier, the membrane is immersed in a wet NiCr etchant to remove the top- and reverse-side NiCr layers. The membrane can then be mounted on a contacted substrate and installed in the cluster-deposition apparatus as shown in Figure 7. After the NiCr etch process is performed, the PMMA layer 71 is the uppermost layer ie. the layer on which the clusters will be incident. A Field-Emission SEM image showing a shadow-mask after the CHF₃/Ar RIE process is shown. in Figure 9. The aperture-slot is approximately lμm wide. The openings at the ends of the aperture-slot assist in verifying that the Si_xN_y film has been etched through and also in the subsequent mask-substrate alignment.

Formation of the Shadow-Mask - Aperture-slot formation using Focused Ion Beam (FIB) etching.

In an alternative method, a Focused Ion Beam is used to etch the aperture-slot in the Si_xNy membrane with no requirement for electron-beam patterning and greatly simplified processing. The Si_xN₅, membrane sample is attached to a carrier substrate using PMMA and placed in the FIB system. The desired pattern is stored as a 2-D CAD file and the system uses this CAD file to steer the ion beam over and through the Si_xN_y membrane. The FIB is capable of etching through bilayers of PMMA/ Si_xN₃, and as a result, the high cluster-reflection property of the cured PMMA layer can be exploited (in identical fashion to the preceding EBL/RIE methods). If desired, the PMMA is spun onto the substrate and baked prior to the FIB process and the duration of the etch process is adjusted to account for the additional layer thickness. Figure 14 shows an aperture-slot created in a Si_xN_y membrane (with a thickness of 200nm) using a XT Nova Nanolab 200 Ga FIB system with probe current settings of 5OpA (narrow section) and 1.OnA (wider sections), and an accelerating voltage of 3OkV. The minimum width of this slot is approximately 400nm and the aperture-slot was fully etched in 12-minutes. The openings at the ends of the aperture-slot assist in verifying that the Si_xN₅, film has been etched through and in the subsequent mask-substrate alignment.

Fabrication of contacted substrates

The clusters which pass through the aperture-slot in the Si_xN_y shadow-mask, land on an insulating substrate. With reference to Figure 8 the shadow-mask 85 is positioned relative to the substrate 87 so that the membrane window 84 and aperture-slot or aperture-slots 83 span the gap or gaps 88 separating two or more planar electrical contacts 81, 82. (The dashed lines 86 in Fig. 8(b) show the location of the planar electrical contacts (shown in their entirety in Figure 8(a)) beneath the aligned shadow-mask. These contacts may be formed on the surface of the substrate using a large-scale shadow-mask and evaporation/sputtering of conducting material, or indeed by any one of a number of standard lithography methods.

The substrate and shadow-mask are aligned using an optical microscope. The large-scale planar contacts on the substrate serve as alignment marks for this procedure. Alternative methods for alignment include micro-machined locators (formed in ultra-thick photoresist such as SU8) which would be formed on the substrate and could provide automatic alignment of the shadow-mask with micron-scale accuracy. Luthi et al. [12] have demonstrated a method using a Scanning Probe Microscope (SPM) stage and an ion beam milled cantilever to selectively evaporate through apertures and onto a substrate with nano-scale precision. An adapted SPM/cantilever could equally be used to provide nano- scale alignment accuracies in shadow-masked cluster depositions. Piezo drives (a standard component in many high precision translation stages) could also be used to move and position the shadow-mask with respect to the substrate inside the deposition chamber. After the cluster deposition process a cluster-assembled wire spans the gap or gaps separating the planar electrical contacts 81, 82.

Cluster formation and deposition

Our preferred apparatus is described in Ref. [13] and the details are incorporated herein by reference. Clusters are produced in an inert-gas aggregation source with either thermal evaporation of the cluster material or a magnetron sputtering of the cluster material. When operated with the thermal source, metal contained in a crucible is heated and evaporated. The sputter source produces metallic or semiconducting vapour from a magnetron sputter head and can therefore produce clusters from materials with very high- melting points. In both the thermal and magnetron source the metallic/semiconducting vapour is mixed with inert gas which causes clusters to nucleate and grow. The cluster/gas mixture passes two stages of differential pumping (from ~1 Torr in the source chamber down to ~10^"6 Torr in the main chamber) such that most of the gas is extracted. The beam enters the main chamber through a nozzle having a diameter of about 1 mm and an opening angle of about 0.5 degrees. In order to determine the intensity of the cluster beam, a quartz crystal deposition rate monitor is used. The samples are mounted on a movable rod and are positioned in front of the quaitz deposition rate monitor during deposition.

Note that the specific range of source parameters appears not to be critical: clusters can be produced over a wide range of pressures (0.01 torr to 100 torr) and evaporation temperatures and deposited at almost any pressure from 1 torr to 10^"12 torr. Any inert gas, or mixture of inert gases, can be used to cause aggregation, and any material that can be evaporated may be used to form clusters. The cluster size is determined by the interplay of gas pressure, gas type, metal evaporation temperature and nozzle sizes used to connected the different chambers of decreasing pressure. As discussed above, the source inlet Ar and/or He flow-rates are chosen to produce clusters which have sufficient kinetic energy to be reflected from a PMMA layer which is the preferred shadow-mask material.

The evaporative source conditions which were selected to produce the Bi cluster- assembled wires shown in Figs. 2 and 10 were as follows: source-inlet Ar gas flow-rate lOOsccm, source pressure approximately 25mbar, crucible temperature 800-820°C and deposition rate 0.6-0.7 A/s.

Parameters affecting cluster deposition through a shadow mask

In order to demonstrate the importance of a polymer layer on the surface of the shadow mask, experiments were devised and performed in order to demonstrate the selective adhesion and reflection of Bi and Cu clusters from substrate surface layers of AZl 500 photoresist, PMMA electron-beam resist, MBE grown Si_xNy and thermally grown SiO₂. Figure 15 shows measured Bi cluster coverages achieved after three deposition experiments onto the aforementioned surface layers. The source-inlet Ar flow-rate was lOOsccm and the average cluster diameter was approximately 25nm. The deposition periods were selected in order to deposit cluster films of thickness 15 A, 41 A and 140 A on a quartz crystal film-thiclcness-monitor, which are then the nominal thicknesses of material deposited on the substrate, corresponding to surface coverages of 35%, 95%, and 330 % respectively. It is clear from Figure 15 that a far higher proportion of the incident Bi clusters adhere to the Si_xN_y and SiO₂ surfaces than adhere to the PMMA and AZ 1500 surfaces. Under these deposition conditions, Bi clusters were reflected from all surfaces (the total coverage on the Si_xN_y and SiO₂ layers amounts to a significantly lower volume of material than that recorded by the FTM crystal). Percolating Bi cluster films (with a coverage of ~70% of one monolayer) were however formed on Si_xN_y or SiO₂ surface layers whilst the cluster-coverage measured on the PMMA surface layer after the same deposition process was less than 3% of one monolayer. Figure 16 shows Cu cluster- coverage data collected after Cu clusters were deposited onto similar samples. The Cu clusters were deposited with combined Ar and He flow-rates of 700sccm and lOOsccm and the average diameter of these clusters was approximately lOran. The deposition periods were selected in order to deposit cluster films of thickness 65 A, 118A and 140 A on a quartz crystal film-thickness-monitor, which are then the nominal thicknesses of material deposited on the substrate, corresponding to surface coverages of 40%, 70% and 85% respectively. Similar results to those obtained for the Bi clusters were obtained for the Cu clusters. The central result is that Bi and Cu clusters can be assembled into conducting films on a Si_xN_y surface layer whilst there is minimal accumulation of clusters on a PMMA surface layer. The source inlet Ar and/or He flow-rates are chosen to produce clusters which have sufficient kinetic energy to be reflected from a PMMA layer (see discussion of resultant velocities below).

Figures 17 and 18 show Field-Emission SEM images of AZl 500 photoresist, PMMA Electron-beam resist, MBE grown Si_xN_y and thermally grown SiO₂ surface layers supporting Bi clusters (Fig. 17) and Cu clusters (Fig. 18). (Measurements of surface coverage from these images were shown in the cluster-coverage data in Fig. 15 and Fig. 16). Fig. 17 shows the Bi cluster-coverage obtained on each surface after a deposition process with an estimated total deposited Bi cluster layer thickness of 4lA. Fig. 18 shows the Cu cluster-coverage obtained on each surface after a deposition process with an estimated total deposited Cu cluster layer thickness of 65 A.

The source conditions for the Bi cluster depositions (Fig. 17) using the standard inert gas aggregation source based on thermal evaporation were as follows: source-inlet Ar gas flow-rate lOOsccm, source pressure approximately 25mbar, crucible temperature 800-

820°C and deposition rate 0.6-0.7 A/s. The source conditions for the Cu cluster depositions (Fig. 18) using the gas aggregation source based on magnetron sputtering were as follows: source-inlet Ar and He gas flow-rates 700sccm and lOOsccm respectively, source pressure approximately 3.0 Torr, sputter-head power IOOW and deposition rate 0.2 A/s.

The data in figures 15-18 show clearly that coating of the stencil with a polymeric material such as PMMA or photoresist may significantly increase the reflectivity of the clusters from the surface of the stencil, and hence reduce the accumulation of deposited material on the stencil, which could otherwise lead to clogging. In a preferred embodiment of the invention the process for fabricating the shadow mask involves the use of a PMMA or photoresist layer to pattern the membrane, and in an even more preferred embodiment the PMMA or photoresist is not removed after patterning the membrane, so as to leave behind on the surface of the membrane a layer which discourages accumulation of deposited material.

Cluster velocities

Our favoured method of controlling the cluster velocity is to control the flow rate of gas into the cluster source chamber (the deposition system design is described in [13]). Note that, as discussed in [14], whilst the velocity of the inert gas leaving the source can be calculated (given the nozzle diameter and inlet flow rate), the unknown size of the velocity slip effect (clusters are accelerated by the gas flowing through the source chamber exit nozzle but are unlikely to reach the speed of the gas flow) means that precise calculation of the cluster velocity is not possible. We therefore prefer to quote the experimental source inlet gas flow rates when describing this work, but estimate that the average velocity of the clusters incident on the V-grooved substrates is approximately equal to the source exit gas velocity. Source exit gas velocities of 36, 41, 47 and 55 m/s were calculated for the source configuration used in Ref. [14], for Ar inlet flow-rates of 5 30, 60, 90 and 150 seem, respectively.

Using the standard inert gas aggregation source, and associated nozzles and pumping configuration [13], the estimated Bi cluster velocity with the current source configuration and using a source-inlet Ar gas flow-rate of lOOsccm is 50m/s (corresponding to an

10 estimated kinetic energy per (25nm-diameter) cluster of 1.0 x 10^"16 Joules). Using the gas aggregation source based on magnetron sputtering, and associated nozzles and pumping configuration [13], the measured Cu cluster velocity with source-inlet Ar and He gas flow-rates of 700sccm and lOOsccm is 260m/s (corresponding to an estimated kinetic energy per (IOnm-diameter) cluster of 1.5 x 10^"16 Joules). In this case, the nozzle was a

15 10mm long Laval nozzle with inlet/outlet diameters of 5.5mm and 4.9mm and a throat diameter of 3.3mm, and measurement of the Cu cluster velocity was performed using a deflector plate and a Faraday cup arrangement housed in the deposition system. Ionised Cu clusters were deflected using a voltage pulse applied to the deflector plate. A current pulse associated with the clusters was then detected on the Faraday cup and the time

20 difference between the deflection-pulse and the detected cluster pulse (the time of flight) was converted into a cluster velocity. We have characterised the cluster velocities for clusters of a wide range of masses produced using a range of magnetron sputtering source conditions, including aggregation length L, gas flow rate F (for He and Ar and mixtures thereof) and sputter power P. Some examples of these characterisation studies are shown

25 in Figures 21 and 22.

Further extensive measurements of cluster velocity have been performed for a variety of source conditions using a combination of a mass filter [15] to select individual cluster sizes and a retarding potential applied to a Faraday cup on which the clusters are incident

30 [13]. An example of the data from this method is shown in Figure 23, resulting in the velocity and mass data in Table 1 below. In Figure 23 the ion current on the Faraday cup is measured as function of the retarding potential (U), which has been converted into an equivalent cluster velocity (v) using eU=V₂mv², and the mean cluster velocity is inferred from the point of inflexion in each curve. The ion current changes sign due to an uncorrected offset current. Note that in Table 1 the calculated gas velocity is an estimate based on the gas flow rate into the source chamber [14], and the actual velocity is less than this due to the "velocity slip effect" referred to above.

Table 1. Summary of gas velocity calculation and the measured cluster velocity for a long nozzle with 4mm diameter opening.

Measurement during Deposition

This step is an optional step in the method and relies upon the existence of previously formed contacts (as discussed above). However one preferred form of the invention requires this step.

The measurement of the current flowing in the device during deposition is important to the realisation of several of the device designs since the onset of conduction marks the formation of a percolating film. [9] The surface coverage of the deposited nanoparticle film can therefore be controlled. Figures 11 and 12 show respectively the onset of conduction and post-deposition current-voltage characteristic for a shadow-masked, cluster-assembled wire. References

1 Brugger et al, US6,313,905 Bl .

2 E. Magnano, Surf. SciXett 544, 1709 (2003).

3 M. Kolbel, Nano Letters 2, 1339 (2002).

4 M. Kolbel, Advanced Functional Materials 13, 219 (2003).

5 W. A. deHeer, Rev. Mod. Phys. 65 611 (1993).

6 I. M. Goldby et al, Rev. Sci. Inst. 68, 3327 (1997).

7 A. Awasthi, S. C. Hendy, P. Zoontjens, and S. A. Brown, 'Particulate Deformation And Resultant Nanoscale And Micronscale Patterning Methods', New Zealand Patent Application No. 547784. Provisional Patent Application Lodged 7 June 2006.

8 S. A. Brown and J. G. Partridge, 'Templated cluster assembled wires'. New Zealand Patent Application No. 524059. International Patent Application number PCT/NZ2004/00012.

9 International Patent Application number PCT/NZ02/00160; NZ Patent Application number 51367, "Nanoscale Electronic Devices and Fabrication Methods".

10 S. A. Brown, A. Lassesson and J. van Lith, 'Fluid Sensors And Fabrication Methods', PCT/NZ2006/000101 filed 9 May 2006.

11 R. Reichel, J. G. Partridge, and S. A. Brown, "Nanoscale And Microscale Lithography Methods And Resultant Devices', New Zealand Patent Application No. 541209. Provisional Patent Application Lodged 8 July 2005.

12 R. Luthi, R.R. Schlittler, J. Brugger, P. Vettiger, M.E. Well and, J.K. Gimzewski, Parallel Nanodevice Fabrication Using a Combination of Shadow-Mask and Scanning Probe Methods, Appl. Phys Lett. 75, 1314 (1999)

13 R. Reichel, J. Nanoparticle Research DOI-10.10077s.l 1051-005-9021-1 (2006).

14 J. G. Partridge et al, Nanotechnology 15, 1382 (2004).

15 B. von Issendorff and R. E. Palmer, Review of Scientific Instruments 70, 4497 (1999); US patent 6,078,043.

Claims

WHAT WE CLAIM IS:

1. A method of forming a pattern of clusters on a substrate comprising the steps of: a. positioning a shadow-mask having one or more apertures, with respect to the substrate or vice versa or both, b. directing a plurality of clusters towards the substrate in such a manner that one of more of the clusters passes through one or more apertures of the shadow mask, c. depositing one of more of the clusters which have passed through the apertures onto the substrate to form a pattern of clusters, wherein the pattern of clusters includes at least one pathway of clusters on the substrate.

2. A method as claimed in claim 1 wherein prior to the step of depositing the clusters there is a step of formation of two or more contacts on the substrate, and the step of deposition of the clusters comprises forming at least one pathway between two contacts.

3. A method as claimed in claim 2 wherein the step of depositing one or more clusters results in one or more conducting pathway(s) capable of functioning as a wire or electrical connection between the two contacts.

4. A method as claimed in claim 3 wherein the step of deposition of the clusters is monitored by monitoring the conduction between the two contacts, and deposition is ceased at or near to the onset of conduction between the two contacts, or at a time subsequent to the onset of conduction chosen to achieve a desired surface coverage or film thickness.

5. A method as claimed in claim 1 wherein there is a post-step of formation of one or more contacts on the substrate and the contacts are so positioned to ensure at least one pathway of clusters exists from one or between at least two of them.

6. A method as claimed in claim 5 wherein the step of deposition results in at least one pathway of clusters which is/are a conducting pathway(s) capable of functioning as a wire or electrical connection and the step of formation of the contacts comprises forming two contacts connected by the pathway of clusters.

7. A method as claimed in any one of the preceding claims wherein the average diameter of the clusters is between 0.3nm and l,000nm.

8. A method as claimed in claim 7 wherein the average diameter of the clusters is between 0.5nm and 40nm.

9. A method as claimed in any one of the preceding claims wherein the contacts are separated by a distance smaller than 10 microns.

10. A method as claimed in claim 9 wherein the contacts are separated by a distance smaller than 200nm.

11. A method as claimed in any one of the preceding claims wherein at least one of the dimensions of the aperture in the shadow-mask is less than 100 microns.

12. A method as claimed in claim 11 wherein at least one of the dimensions across the aperture in the shadow-mask is less than 1 micron.

13. A method as claimed in claim 12 wherein at least one of the dimensions across the aperture in the shadow-mask is less than 200 nanometres.

14. A method as claimed in any one of the preceding claims wherein the step of directing the clusters towards the substrate includes imparting kinetic energy to the plurality of clusters sufficient to cause at least part or substantially all of the clusters incident upon the shadow-mask surface to bounce from the surface and/or the sides of the aperture(s), whilst also low enough to cause at least part or substantially all of the clusters passing through the one or more apertures of the shadow-mask towards the substrate to remain on the surface of the substrate.

15. A method as claimed in claim 14 wherein in the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to a velocity in the range InVs to 2000 m/s.

16. A method as claimed in claim 15 wherein in the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to a velocity in the range 10m/s to 300 m/s.

17. A method as claimed in claim 14 wherein in the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to a kinetic energy per cluster atom in the range 5x10^~26 J to 2xlO^~19 J.

18. A method as claimed in claim 17 wherein in the step of directing the clusters towards the substrate the kinetic energy imparted to the clusters corresponds to a kinetic energy per cluster atom in the range 5x10^"24 J to 5x10^"21 J.

19. A method as claimed claim 14 wherein the clusters directed towards the substrate are copper or palladium clusters with diameters in the range 5-20nm and in the step of directing the clusters towards the substrate the velocity of the clusters is in the range 100-400m/s.

20. A method as claimed in claim 14 any one of the preceding claims wherein the clusters directed towards the substrate are bismuth or antimony clusters with diameters in the range 10-lOOnm and the velocity of the clusters is in the range 10-lOOm/s.

21. A method as claimed in claim 14 wherein the method of control of the cluster kinetic energy is by control of the velocity of the clusters via control of the flow rate of an inert gas into or out of the source of the clusters.

22. A method as claimed in any one of the preceding claims wherein in the steps of directing the clusters towards the substrate, and deposition on the substrate, the clusters aggregate on or stick to the substrate, but there is minimal aggregation or adherence to the shadow-mask.

23. A method as claimed in any one of the preceding claims wherein the surface of the substrate is altered to decrease the reflection of clusters from its surface by one or both of: - coating with a material which decreases the reflection of clusters from its surface; - roughening of the substrate surface.

24. A method as claimed in any one of the preceding claims wherein the shadow- mask is coated with a material which enhances the likelihood of clusters bouncing from or not adhering to the surface of the shadow mask due to the clusters having a weak interaction with the surface, or a low tendency to wet the surface.

25. A method as claimed in claim 24 in which the shadow mask is coated with a layer of polymeric material.

26. A method as claimed in claim 25 wherein the shadow-mask is coated with a layer of one of PMMA or SU8 or photoresist or a surface assembled monolayer (SAM).

27. A method as claimed in claim 26 in which the shadow mask is coated with a surface assembled monolayer comprising one of the materials C 12-SiCl₃, C 12- Si(OEt)₃, or CF-Si(OEt)₃.

28. A method as claimed in claim 27 wherein the layer is has a thickness less than 1 micron.

29. A method as claimed in claim 28 wherein the layer has a thickness less than lOOnm.

30. A method as claimed in any one of the preceding claims wherein the method includes applying an electrical charge to clusters and the step of directing the clusters towards the substrate includes applying a voltage to the substrate and/or the shadow-mask and/or a part of the cluster source so as to accelerate or decelerate the charged clusters.

31. A method as claimed in any one of the preceding claims wherein the clusters have been prepared via inert gas aggregation.

32. A method as claimed in any one of the preceding claims wherein the shadow- mask includes at least one aperture having the general shape of a slot so that in the step of directing the clusters towards the substrate, passage of clusters through the slot and deposition on the substrate gives rise to a pattern in the shape of a wire or substantially linear pathway (whether or not conducting) on the substrate.

33. A method as claimed in any one of the preceding claims wherein the step of positioning of the shadow-mask with respect to the substrate includes positioning the shadow-mask between the source of the plurality of clusters and the substrate and includes aligning one or more features of the shadow-mask with one or more features of the substrate (or vice versa).

34. A method as claimed in claim 33 wherein the plane of the shadow-mask is parallel to the plane of the substrate.

35. A method as claimed in claim 33 wherein the plane of the shadow-mask is at an angle to the plane of the substrate, so as to decrease the effective size of the aperture or so as to change the probability of reflection of a cluster from the shadow-mask.

36. A method as claimed in claim 34 or 35 wherein after the step of depositing the pattern of clusters on the substrate, the substrate and/or the shadow-mask can be rotated relative to each other, and a further step of depositing the pattern of clusters can be performed.

37. A method as claimed in claim 36 wherein one or more of the apertures of the shadow-mask is/are aligned with the position of the one or more contacts where present (or vice versa), so as to ensure the formation of a pattern of clusters between the contacts on the substrate after deposition.

38. A method as claimed in any one of the preceding claims wherein there is more than one deposition of material though the shadow-mask.

39. A method as claimed in 33 wherein the pattern is aligned at right angles to a pre-existing wire or pathway.

40. A method as claimed in claim 1 wherein the substrate is an insulating or semiconductor material

41. A method as claimed in claim 41 wherein the substrate is an insulating or semiconductor material, more preferably the substrate is selected from silicon, silicon nitride, silicon oxide aluminium oxide, indium tin oxide, germanium, gallium arsenide or any other III- V semiconductor, quartz, or glass.

42. A method as claimed in claim 1 wherein the clusters are selected from platinum, palladium, bismuth, antimony, aluminium, silicon, germanium, silver, gold, copper, iron, nickel or cobalt clusters.

43. A method as claimed in any one of the preceding claims wherein the substrate is planar.

44. A method as claimed in any one of claims 1 to 42 wherein the substrate is non- planar, and/or includes three-dimensional structures or is itself a three dimensional object.

45. A method as claimed in claim 43 or 44 wherein there is more than one deposition of material though the shadow-mask.

46. A method as claimed in any one of the preceding claims wherein the contacts are formed by lithography.

47. A method as claimed in any one of claims 1 to 45 wherein the contacts are formed by deposition of contact material through a contact shadow-mask.

48. A method as claimed in claim 2 or claim 5 wherein the shadow-mask has one or more apertures corresponding to the contacts (the contact aperture(s)) and one or more apertures corresponding to the pattern to be deposited on the substrate

(the pattern aperture(s)), and in no particular order, the step of forming the contact on the surface consists of directing clusters through the contact aperture(s) whilst blocking or avoiding the pattern apertures, and the step of forming the pattern consists of directing clusters through the pattern aperture(s), whilst blocking or avoiding the contact aperture(s).

49. A method as claimed in claim 1 wherein the shadow-mask has one or more apertures corresponding to the contacts (the contact aperture(s)) and one or more apertures corresponding to the pattern to be deposited on the substrate (the pattern aperture(s)), and the contacts and the pattern are prepared in a single step of directing clusters towards the substrate (the contacts and the pattern being prepared from the same species of cluster).

50. A method as claimed in claim 31 wherein one of more of the temperature, surface smoothness, and/or identity of the substrate are such as to discourage diffusion of the clusters on the substrate surface so that the deposited clusters remain in the general shape of the slot.

51. A method as claimed in any one of the preceding claims wherein the method involves a further step of encapsulating at least a portion of the pattern of clusters in an insulating or dielectric material.

52. A method as claimed in claim 51 wherein a further contact or other structure may be prepared on the surface of the insulating or dielectric material (in a pre- step or a post step)which is isolated from the pattern of clusters and may act as a gate.

53. A method as claimed in any one of the preceding claims wherein the pattern of clusters is fabricated on a multi-layer substrate, one layer of which is electrically conducting and can therefore act as a gate.

54. A pattern of clusters on a substrate prepared substantially according to the method claimed in any one of claims 1 to 53.

55. A method of forming a conducting pathway of clusters between two contacts on a substrate comprising the steps of: a. positioning a shadow-mask having one or more apertures, with respect to the substrate or vice versa or both; b. directing a plurality of clusters towards the substrate in such a manner that one of more of the clusters passes through one or more apertures of the shadow mask; c. depositing one of more of the clusters which have passed through the apertures onto the substrate to form a pattern of clusters wherein the pattern of clusters includes at least a conducting pathway of clusters on the substrate, wherein the two contacts are either formed before step a and the shadow-mask is so positioned that the resultant pattern will be in the region between the contacts,, or the two contacts are formed after step c and are positioned to have at least a portion of the pathway running between them, wherein the contacts are separated by a distance smaller than 1 micron, and wherein at least one of the dimensions of the aperture in the shadow- mask is less than 1 micron.

56. A method as claimed in claim 55 wherein at least one of the dimensions across the aperture in the shadow-mask is less than 100 nanometres.

57. A method as claimed in claim 55 wherein the clusters directed towards the substrate are copper or palladium clusters with diameters in the range 5-20nm and in the step of directing the clusters towards the substrate the velocity of the clusters is in the range 100-400m/s.

58. A method as claimed in claim 55 wherein the clusters directed towards the substrate are bismuth or antimony clusters with diameters in the range 10- lOOnm and the velocity of the clusters is in the range 10-lOOm/s.

59. A method as claimed in claim 55 wherein the shadow-mask is coated with a layer of polymeric material or photoresist with a thickness of less than lOOnm.

60. A device including a conducting pathway between two contacts wherein the conducting pathway is formed substantially as claimed in any one of the preceding claims.

61. A device as claimed in claim 60 which is formed using palladium or palladium alloy clusters and which is capable of operating as a hydrogen sensor.

62. A conducting pathway of clusters between two contacts on a substrate formed according to the method claimed in claims 55 to 61.

63. A method of preparing a pattern of clusters substantially as herein described and with reference to any one or more the accompanying drawings and/or examples.

64. A pattern of clusters substantially as herein described and with reference to any one or more the accompanying drawings and/or examples.