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WO2007008088A1 - Procedes de lithographie a nano-echelle/micro-echelle et dispositifs resultants - Google Patents

Procedes de lithographie a nano-echelle/micro-echelle et dispositifs resultants Download PDF

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Publication number
WO2007008088A1
WO2007008088A1 PCT/NZ2006/000173 NZ2006000173W WO2007008088A1 WO 2007008088 A1 WO2007008088 A1 WO 2007008088A1 NZ 2006000173 W NZ2006000173 W NZ 2006000173W WO 2007008088 A1 WO2007008088 A1 WO 2007008088A1
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WIPO (PCT)
Prior art keywords
clusters
substrate
cluster
contacts
regions
Prior art date
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Ceased
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PCT/NZ2006/000173
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English (en)
Inventor
Simon Anthony Brown
Rene Reichel
James Gordon Partridge
Aruna Awasthi
Shaun Cameron Hendy
Peter Anthony Zoontjens
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Nano Cluster Devices Ltd
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Nano Cluster Devices Ltd
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Publication of WO2007008088A1 publication Critical patent/WO2007008088A1/fr
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
    • C23C14/04—Coating on selected surface areas, e.g. using masks
    • C23C14/048—Coating on selected surface areas, e.g. using masks using irradiation by energy or particles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/26—Processing photosensitive materials; Apparatus therefor
    • G03F7/40—Treatment after imagewise removal, e.g. baking

Definitions

  • the present invention relates to a method of preparing patterns or arrangements of particles (particularly atomic clusters) on a substrate surface. More particularly but not exclusively it relates to a method of preparing patterns or arrangements useful in lithography " and the preparation of electronic devices both on the nanoscale and optionally up to the micronscale.
  • a method of selectively pre-patterning the surface is desirable.
  • Parker et al. [3] used standard optical lithographic methods to achieve resist patterns/lines with a line width of 2 ⁇ m.
  • Gold clusters then were deposited onto the samples followed by a lift-off step. The clusters preferentially accumulated along the edges of the resist structures. Changing the surface from hydrophobic to hydrophobic resulted in the formation of a cluster film within the exposed patterns, but preferential accumulation at the resist edges was still observed.
  • Another example of surface patterning using photo resist is given by Liu et al [4].
  • atomic copper was sputtered onto 2 to 5 ⁇ m wide resist patterns. Cu clusters formed on the surface via aggregation of the deposited atomic material. Preferential nucleation sites were reported at the boundaries between the bare SiO x substrate and the resist lines.
  • Refs. [3] and [4] allow preferential cluster accumulation by pre- patterning the substrate. However, in both cases a lift-off step is required i.e. that the photo resist must be removed after the cluster deposition/formation due to the presence of a distribution of clusters over the entire sample, and in order to reveal a pattern of clusters. Also, Refs. [3] and [4] use photo resist as a patternable material which limits the minimum possible feature size.
  • HMW high molecular weight
  • PMMA polymethyl methacrylate
  • Liftoff can be a difficult procedure to execute properly, since large unexposed areas can be difficult to lift-off, and the edges of the remaining metal (which necessarily run up the sides of the apertures onto the unexposed PMMA prior to lift off) can contain vertical spikes or otherwise be relatively rough.
  • a method of depositing particles on a patterned region of a substrate comprising the steps of providing a patterned substrate, the pattern having one or more first regions (the first region) and one or more second regions (the second region) and directing a plurality of particles with an average diameter less than 1 micron towards the pattern to form an arrangement of particles on the patterned region, with a greater percentage of the particles retained by one of the first or second regions than is retained by the other of the first or second regions .
  • the particles are atomic clusters.
  • the method includes controlling the behaviour of the clusters on impact with the first and second regions to be one or both of plastic and/or elastic thereby influencing the probability that the clusters adhere to one or both of the first and second regions.
  • the method includes controlling the directing of clusters towards the pattern and/or the nature of the first and second regions such that upon impact of the clusters with the patterned substrate one or more of the following occurs: elastic deformation of the clusters resulting in sticking of one or more clusters to a region, and/or elastic deformation of the clusters resulting in reflecting or bouncing or sliding of one or more clusters from a region, and/or - plastic deformation of the clusters resulting in sticking of one or more clusters to a region, and/or - plastic deformation of the clusters resulting in reflecting or bouncing of one or more clusters from a region.
  • one or both of the regions comprises a plurality of substantially independent sections.
  • the region which retains the greater percentage of clusters is continuous.
  • the method includes depositing the clusters to form a pathway (as defined herein).
  • the method includes depositing the clusters to form a pathway capable of electrical conduction.
  • the method includes a further step of forming at least two contacts on the substrate with the pathway existing generally between the two contacts.
  • the contacts are separated by a distance smaller than 10 microns, more preferably smaller than 1 micron, more preferably smaller than lOOnm.
  • the method includes first forming the contacts and then depositing the clusters on the substrate between the contacts.
  • the method includes monitoring the steps of depositing and forming the arrangement of clusters by monitoring conduction between the two contacts where deposition is ceased at or near the onset of conduction.
  • the method includes forming the contacts after forming the arrangement of clusters.
  • the method includes including providing a patterned substrate with least one dimension of one of the regions of the pattern less than 1 micron, more preferably less than lOOnm.
  • the method includes directing a plurality of clusters with an average diameter between 0.3nm and l,000nm, more preferably between 0.5nm and lOOnm, even more preferably between 0.5nm and 40nm towards the pattern
  • first or second regions of the substrate comprise different materials.
  • one of the first and/or second regions comprise the same material as the substrate but modified.
  • the first and second regions have different surface hardness or softness characteristics.
  • the first and second regions have different surface roughness.
  • the first and second regions have different surface wettability.
  • the first and second regions have different reflectivity to the clusters.
  • the first and second regions have different surface elasticity characteristics.
  • the first and second regions are at different temperatures.
  • the method includes patterning the insulating or semiconductor substrate by one or more of lithography, etching or metalisation.
  • the method includes patterning the insulating or semiconductor substrate with a second material, which is preferably non-conducting.
  • the method includes patterning an insulating or semiconductor substrate with a developed or undeveloped polymeric material or a self-assembled monolayer (SAM).
  • SAM self-assembled monolayer
  • the method includes providing an insulating or semiconductor substrate, coating it with a polymeric material or having a SAM form thereon, and patterning the polymeric material or SAM to result in one or more first and second regions.
  • the method includes patterning the polymeric material or SAM by forming one or more apertures through the polymeric material or the SAM so that the insulating or semiconductor substrate is at least partially, if not completely accessible to the clusters through the one or more apertures.
  • the method includes patterning the polymeric material or SAM by forming at least one slot in the polymeric material or SAM running between and/or partially overlapping with the two contacts (when present).
  • the pattern is formed in a polymeric material selected from the group consisting of PMMA, photoresist, electron-beam resist and SU8. More preferably the pattern is formed in a polymeric material comprising a bi-layer of high molecular weight (HMW) and low molecular weight (LMW) PMMA.
  • HMW high molecular weight
  • LMW low molecular weight
  • the pattern is formed in a SAM selected from the group consisting of C12-SiCl 3 , C12-Si(OEt) 3 , and CF-Si(OEt) 3 .
  • the method includes forming the pattern in the polymeric material or SAM by lithography and/or by etching.
  • the method includes controlling one or more of: the incident momentum of the clusters during deposition of the clusters; and/or the kinetic energy of the incident clusters during deposition of the clusters; and/or the velocity of the incident clusters during deposition of the clusters; and/or ⁇ the identity of the clusters during deposition of the clusters; and/or the size of the clusters during deposition of the clusters; and/or the temperature of the clusters during deposition of the clusters; and/or the angle of incidence of the clusters during deposition of the clusters; and/or other factors affecting the degree of chemical bonding and/or strength of interaction between the clusters and a surface during deposition of the clusters; and/or the thermodynamic phase of the clusters; and/or the crystallinity of the clusters; and/or the shape of the clusters.
  • the method includes directing the clusters towards the pattern with a selected or controlled velocity.
  • the method includes directing the clusters towards the pattern with kinetic energy of the clusters selected so as to be sufficient to cause at least part or the majority or substantially all of the clusters incident upon the surface of one of the first or second regions to bounce from that region whilst at the same time low enough to cause at least part or substantially all of the clusters incident upon the surface of the other of the first or second regions to remain on the surface (whether substantially immediately upon contacting the surface or some time after first contacting the surface).
  • the method includes calculating the velocity thresholds between the regimes of plastic and elastic behaviour of the clusters and then controlling the velocity of the clusters to be within a selected regime upon impact with one or both or the first and second regions.
  • the method includes calculating the velocity thresholds between the regimes of elastic deformation with sticking behaviour, elastic deformation with bouncing behaviour, plastic deformation with sticking behaviour, and plastic deformation with bouncing behaviour for the given cluster and/or substrate and/or environment, and then controlling the velocity of the clusters to result in the behaviour of the atomic cluster upon impact with the first and/or second regions falling within a particular regime.
  • the step of calculating the thresholds between the regimes includes calculation of the required velocity for the given cluster and/or substrate and/or environment in accordance with measurements of the proportion of clusters that bounce from (or stick to) the first and the second regions.
  • step of calculating the velocity for the given cluster and/or substrate and/or environment in accordance with molecular dynamics simulations of the proportion of clusters that bounce from (or stick to) each of first and second regions.
  • the clusters directed towards the pattern are selected from one or more of the group consisting of platinum, palladium, bismuth, antimony, aluminium, silicon, germanium, silver, gold, copper, iron, nickel, and cobalt.
  • the substrate is selected from the group consisting of silicon, silicon nitride, silicon oxide, aluminium oxide, indium tin oxide, germanium, gallium arsenide, quartz, and glass.
  • the method includes preparing the clusters by a method which involves gas aggregation.
  • the method includes preparing the clusters by evaporating a cluster source material from a crucible or by sputtering a cluster source material from a target to produce a vapour and condensing the vapour by cooling through an inert gas to form clusters.
  • the method includes controlling the velocity and/or kinetic energy of the clusters produced at least partially by controlling the flow rate of an inert gas flow into a chamber of the cluster source material in which the clusters are prepared.
  • the method includes including imparting a kinetic energy to the clusters corresponding to a velocity in the range lm/s to 2000 m/s; more preferably in the range lOm/s to 300 m/s.
  • the method includes imparting a kinetic energy to the clusters corresponding to a kinetic energy per cluster atom in the range 5xlO "26 J to 2x10 "19 J, more preferably in the range 5x10 "24 J to 5x10 "21 J.
  • the method includes directing copper or palladium clusters towards the substrate with diameters in the range 5-20nm and with velocities is in the range 100- 400m/s.
  • the method includes directing bismuth or antimony clusters towards the substrate with diameters in the range 10-lOOnm and with velocities in the range 10-lOOm/s.
  • the method includes including depositing the clusters to form one pathway or wire.
  • the method includes depositing the clusters to form a plurality of wires.
  • the method includes depositing the clusters to form a percolating film.
  • the method includes a pre-step of forming (by any means whatsoever) a wire or configuration structure on the substrate followed by forming the pathway of clusters over or in addition to the pre-existing wire or configuration.
  • the pathway of clusters is formed at a pre-selected angle to the preexisting wire or configuration, preferably at right angles to the pre-existing wire or configuration.
  • one of the first or second regions is comprised of one material which is conducting and a second material which is insulating and encapsulates the first.
  • the method includes forming the conducting material so as to be useful as a gate.
  • the method includes encapsulating at least a portion of the deposited clusters in an insulating or dielectric material.
  • the method includes forming a further contact or other structure on the surface of the insulating or dielectric material which is isolated from the pattern of clusters and can act as a gate.
  • the method includes forming the pathway of clusters on a multi-layer substrate, one layer of which is electrically conducting and can act as a gate.
  • one of the first or second regions is angled with respect to the other of the first or second regions and the method includes directing the clusters substantially orthogonally to one of the first or second regions.
  • the first and second regions of the pattern define a V-groove or inverted pyramid and the method includes directing the clusters so that they eventually accumulate or aggregate at the apex of the V-groove or inverted pyramid.
  • the method includes imparting a cluster with a velocity component perpendicular to the angled surface at such a level that the cluster deformation on impact with the angled surface is weak, leading to sticking or sliding, while elasto- plastic or plastic deformation takes place on impact on the surfaces orthogonal to the cluster beam, resulting in that clusters are at least partially reflected from those surfaces.
  • the method includes including imparting a cluster with a velocity component perpendicular to the angled surface at such a level that elasto-plastic bouncing takes place, leading to accumulation of clusters at the apex of the V-groove or inverted pyramid while the clusters impacting on the orthogonal planar surfaces are fully plastically deformed and at least partially reflected from those surfaces.
  • the method includes directing clusters with a range of particle sizes and with a temperature such that at least some of the smaller clusters of the range are liquid while at least some of the larger clusters are solid and the smaller, liquid clusters preferentially accumulate in one region of the first or second regions while the larger solid, clusters bounce away from that region.
  • the method includes including patterning the substrate so that the smaller clusters of the range are retained by one of the first or second regions whilst bouncing from the other of the first and second regions and larger clusters of the range are not retained in either region.
  • an arrangement of particles on a patterned region of a substrate prepared substantially according to the abovementioned method.
  • the clusters form a conducting pathway between two contacts on the substrate surface.
  • the average diameter of the clusters is between 0.3nm and l,000nm.
  • the contacts are separated by a distance smaller than 10 microns.
  • a further aspect of the invention there is provided method of preparing a pathway of atomic clusters between two contacts on a substrate comprising the steps of providing a substrate with two contacts on its surface, modifying a region on the substrate substantially between and/or overlapping the two contacts, directing a plurality of atomic clusters with average diameter less than
  • the method includes monitoring the formation of a conducting pathway between the contacts by monitoring the conduction between the two contacts.
  • the method includes providing contacts separated by a distance smaller than 10 microns, more preferably smaller than lOOnm.
  • the method includes providing a substrate of an insulating or semiconductor material coated with a polymeric or self-assembled monolayer (SAM) layer, modifying the region in the area between the two contacts by forming one or more slots in the polymeric or SAM layer positioned substantially between the contacts, so that the insulating or semiconductor material is accessible through the slot.
  • SAM self-assembled monolayer
  • the method includes modifying a region between the two contacts by providing a (or taking advantage of a pre-existing) ridge, depression, step-edge or defect, or array or pattern of ridges, depressions, step-edges or defects, and forming a pathway between the two contacts, the clusters impacting on the modification experiencing a "soft-landing" site so that the clusters stick while bouncing away from the non-modified regions.
  • the modification comprises naturally occurring step edges.
  • the substrate is silicon and the modification occurs on an exposed Si(5 5 12) or Si(I 1 3) facet.
  • the method includes engineering the ridges, depressions, step-edges or defects.
  • the method includes directing the clusters towards the substrate with a velocity high enough so at least part of, if not the majority of, the clusters bounce away from the unmodified region and low enough so that at least part of, if not the majority of the clusters incident upon the surface of the modification remain on the surface of the modification (whether substantially immediately upon contacting the surface or some time after first contacting the surface).
  • the method includes calculating the velocity thresholds between the regimes of plastic and elastic behaviour of the clusters upon impact with the first and second regions and then controlling the velocity of the clusters to be within a selected regime upon impact with one or both or the first and second regions.
  • the substrate is selected from the group comprising silicon, silicon nitride, silicon oxide, aluminium oxide, indium tin oxide, germanium, gallium arsenide, quartz, or glass.
  • a pathway of atomic clusters between two contacts on a substrate prepared according to the abovementioned method is provided.
  • a method for performing lithography including the steps of providing a substrate, coating the surface of the substrate with an electron- or photo-sensitive polymer layer, exposing some regions of the polymer layer to electron (for an electron sensitive polymer) or photons (for a photon-sensitive polymer), developing the polymer to remove one but not both of the exposed or unexposed regions, and depositing clusters on to the substrate to substantially coat one but not both of the exposed or unexposed regions and wherein the method does not include a lift-off step.
  • the method includes depositing clusters onto the substrate according to the abovementioned method.
  • Particle as used herein has the following meaning - a particle with dimensions in the range 0.5nm to lOOmicrons, which includes atomic clusters formed by inert gas aggregation or otherwise.
  • Nanoparticle 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.
  • Development or “developed” as used herein has the following meaning — in relation to a polymeric material, having been treated by chemical means such as exposure to a solvent, in order to remove part or substantially all of the polymeric material.
  • Nanoscale as used herein has the following meaning - having one or more dimensions in the range 0.5 to 1000 nanometres.
  • Mocronscale as used herein has the following meaning - having one or more dimensions in the range 1 to 1000 micrometers.
  • 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 10 7 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 wire 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 another 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 or on a substrate that is made up of individual units which may or may not be wholly interconnected (i.e.
  • wire while it may be a connected network, there may also be some spaces between the units).
  • a wire 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 is a subset of "pathway".
  • aperture as used herein has the following meaning - a gap, space or opening in a layer of polymeric material (such as PMMA, photo resist, or SU8) on a substrate. It is not restricted to shape or dimension. It is usually (but not only) used in relation to fully enclosed openings.
  • deformation we mean a change in shape due to the impact between a particle and a surface or other particle, which may be reversible or irreversible, and / or due to the conversion of kinetic energy into elastic (stored) energy or due to the dissipation of kinetic energy by plastic work.
  • plastic deformation we mean a change in shape due to the impact between a particle and a surface or other particle, which is irreversible due to the dissipation of kinetic energy by plastic work.
  • elasto-plastic deformation we mean a change in shape due to the impact between a particle and a surface or other particle which is at least partially, but not entirely, irreversible due to the dissipation of kinetic energy by plastic work, and wherein there is a region of the particle which is at least partially, but not entirely, reversibly deformed.
  • elastic deformation we mean a change in shape due to the impact between a particle and a surface or other particle, due to the conversion of kinetic energy into elastic (stored) energy, which is substantially reversible.
  • Elastic as used herein has the following meaning — deforming in such a way that substantially the original shape of the object is recovered after the deformation or interaction with another object is completed.
  • the deformation of a particle is expected to be limited to a region neighbouring the particle surface or facet in contact with the surface.
  • “Fully Plastic” and “Plastic” as used herein has the following meaning - deforming in such a way that the original shape of the object is altered after the deformation or interaction with another object is completed. For example, in a plastic collision with a surface the deformation of a particle is expected to extend through a substantial volume of the particle and may include substantially the entire volume of the particle.
  • Elasto-plastic as used herein has the following meaning - a deformation of a particle which is partially elastic and partially plastic.
  • a particle may exhibit a region neighbouring the particle surface or facet in contact with the surface which is plastically deformed and a second region more distance from the particle surface or facet in contact with the surface which is elastically deformed.
  • PMMA polymethyl methacrylate
  • MIBK methyl isobutyl ketone
  • IPA iso propyl alcohol.
  • FE-SEM field emission scanning electron microscope
  • Figure 1 FE-SEM images of antimony clusters within 3x3 ⁇ m 2 partially-exposed patterns in PMMA produced with increasing electron-beam doses for an overall cluster-layer thickness (as read from the rate deposition monitor) of54 ⁇ 6 nm.
  • Figure 3 FE-SEM images of bismuth clusters within 3x3 ⁇ m 2 partially exposed patterns in PMMA produced with increasing electron-beam doses for an overall cluster-layer thickness (as read from the rate deposition monitor) of 21 ⁇ 3 nm.
  • Figure 4. a) Variation in antimony cluster coverage within 3x3 ⁇ m 2 partially exposed patterns in PMMA with varying electron-beam dose for two different overall cluster-layer thicknesses as measured from the deposition rate monitor (diamonds: 54 ⁇ 6 nm; triangles 34 ⁇ 1 nm).
  • Figure 7. FE-SEM image of Sb clusters assembled within an aperture-slot in the HMW-PMMA film and aligned to planar Au contacts separated by a l ⁇ m gap- Figure 8.
  • Figure 9. FE-SEM image of a contacted Bi cluster-assembled wire (contacts not shown).
  • Figure 10. FE-SEM image of a Sb cluster-assembled film within a 'New Zealand silhouette' aperture in HMW PMMA.
  • Figure 11. FE-SEM image of a Bi cluster-assembled film within a 'New Zealand silhouette' aperture in HMW PMMA.
  • Figure 17 FE-SEM image of antimony clusters on an aperture-slot sample which was exposed to an electron-beam prior to the cluster deposition experiment.
  • Figure 26 Multiple parallel cluster-assembled wires formed around an SU8 template and planar electrical contacts and supported on a SiO x passivated Si substrate.
  • Figure 27 I(V) characteristics of a cluster-assembled wire of width 600nm and length 1 OO ⁇ m at temperatures ranging from 300K, 330K, 370K, 400K,
  • Figure 28 FE-SEM images of (a) a cluster-assembled wire deposited and characterised on a surface held at room temperature and (b) a cluster- assembled wire deposited at room temperature and subsequently heated to 460K.
  • Figure 29 FE-SEM image of NiCr/Au four point contacts with cluster wire.
  • Figure 30 FE-SEM images of Voltage annealed wire.
  • Figure 31 FE-SEM image of Current annealed wire.
  • Figure 32 Examples of results of measurement of cluster velocity as a function of cluster diameter for a variety of source conditions.
  • Figure 33 Examples of results of measurement of cluster velocity as a function of cluster diameter for a variety of source conditions.
  • Figure 34 Examples of results of measurement of ion current as a function of a retarding potential applied to a Faraday cup, yielding cluster velocity data.
  • Figure 35 Bi cluster film formed within an aperture in AZl 500 photo resist and overlaying multiple NiCr/Au contacts on a SiO x passivated Si substrate.
  • Figure 36 Simulated snapshots showing a selection of 147-atom liquid (bottom) and solid (top) clusters after equilibration on surfaces with various C values.
  • Figure 38 Plot of the center of mass z cm of a 147 atom icosahedron colliding with the target surface, plotted versus time for three impact velocities, 0.4v c ,
  • Figure 40 Plot of radius gyration along z-direction, R 2 , of 147 atom clusters from Figure 38 as a function of time.
  • Figure 46 Plot of E a f N° 5 as a function of impact velocity for the three different cluster sizes.
  • Figure 48 Plot of probability of sticking (P(stick)) versus incidence velocity averaged over 50 trials for a 147-atom icosahedron on a surface with an adatom and a surface with a step edge. Also shown is the probability of sticking for a liquid 147-atom droplet incident on the flat (111) surface.
  • Figure 49 Plot of probability of sticking (P(stick)) versus incidence velocity (of 147-atom icosahedron for different C-values i.e. corresponding to different cluster-surface interactions.
  • Figure 50 Plot of probability of sticking (P(stick)) versus normal incidence velocity averaged for non-normal incidence of 147-atom icosahedron on the substrate.
  • Figure 52 Plot of final velocity of the cluster as a function of initial horizontal velocity.
  • Figure 53 Plot of maximum deformation of the cluster at the moment of peak reflection velocity versus impact velocity for different size solid clusters and a liquid droplet.
  • Figure 54 Onset of conduction for a Bi cluster-assembled wire produced using an aperture in a PMMA template layer with length lOO ⁇ m and minimum width 200nm.
  • the preferred form of the present invention relates to our method of fabricating patterns (preferably conducting) of clusters, (particularly nanoclusters) and particularly wire-like structures, including pathways (on the nanoscale or micronscale), by deposition of clusters capable of conduction into or around apertures or patterns present or formed on a substrate.
  • patterns preferably conducting
  • clusters particularly nanoclusters
  • wire-like structures including pathways (on the nanoscale or micronscale)
  • Apertures/patterns can be formed within the non-conducting material on the surface of the substrate. These can be formed using standard lithographical techniques, i.e. electron-beam or projection lithography and dry-etching. - No lift-off step is required after metallization because the clusters do not stick to the unexposed surfaces (usually a non-conducting layer) thus there is no need to remove excess metal deposited on the polymer.
  • the resulting wires are automatically connected to electrical contacts as part of the formation process. This enables electrical characterisation before, during and after the formation of the cluster- assembled films or wires or pathways. When desired electrical current can be passed along the wires as soon as they are formed. - No manipulation of the clusters is required to form the cluster-assembled films or wires or pathways.
  • the invention relies upon a number of steps and/or techniques:
  • Preparation of a substrate may be as simple as selecting the substrate material or it may include formation of electrical contacts on the surface.
  • the invention covers scenarios when contacts are not required. However in most embodiments electrical contacts will be employed to allow establishment of an electrical circuit. In most cases formation of the contacts will precede the step of depositing the clusters. However it is possible that the clusters are deposited first and the contacts formed at some point afterwards. The invention covers the different combinations.
  • 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.
  • Si wafers with SiO x or Si x Ny insulating top layers.
  • Alternatives may be GaAs, GaN, AlGaAs or SiGe substrates (amongst many others) with passivation provided by SiO x , Si x Ny, AlO, spin-on glasses and polymers (amongst many others), so long as they have the properties which allow preferably a cluster-assembled pathway to be formed on the substrate surface.
  • the preferred method of contact formation relies on evaporation or sputtering of a metal or alloy.
  • metals or alloys include Ti, NiCr, Al, Au, Ag, Cu, W, Mo, Pd, Pt, Bi, and Sb.
  • the shadow-mask is positioned between the substrate material and the chosen evaporation source so that the evaporated film replicates the features of the mask.
  • the preferred deposition for the contacts is of atomic vapour generated via thermal or electron beam evaporation. However, other deposition procedures as known in the art may be used.
  • the planar electrical contacts allow in-situ monitoring of the current through cluster-assembled wires/pathways/films.
  • 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.
  • the preferred pattern is formed using a layer of non-conducting material.
  • the preferred non-conducting materials used in this step are photo-patternable materials such as standard photo resists used in optical lithography, or electron-beam patternable materials used in electron beam lithography.
  • photo-patternable materials such as standard photo resists used in optical lithography, or electron-beam patternable materials used in electron beam lithography.
  • HMW PMMA which can be easily patterned using standard e-beam lithography but other suitable materials are within the scope of the invention as mentioned below.
  • a typical scenario is as follows:
  • HMW PMMA is spun onto the clean substrate at 3000 rpm for 1 minute followed by baking at 185 0 C for 30 minutes to evaporate off the solvents. Exposure is done using a Raithl50 Electron Beam Lithography system. The exposed patterns are then developed in 1:3 IPA:MIBK for 30 seconds.
  • resists which may be used include, but are not limited to AZ1500, S1813, UV-3, UV-5, PMGI 3 SU8, ZEP, NEB-31, EBR-9 and many others as would be appreciated by one skilled in the art.
  • the layer of material is non-conducting because it ensures that there is no additional conduction path between the contacts parallel to or parasitic to the final pattern of conducting clusters/the resulting cluster assembled wires.
  • many other non-conducting materials can be coated onto a substrate and subsequently patterned using standard lithography procedures. For example, SiO x , Si x N y and many semiconductor materials or layers of insulating or semiconducting materials would serve this purpose.
  • the patterned layer may be of a conducting material.
  • a layer of Si or Al or other material which has been oxidised on its surface would provide sufficient electrical isolation.
  • the conducting material which is isolated from the conducting cluster structure by the oxide, can be employed as a gate, providing very small gate — channel distances which may be advantageous for the construction of high performance transistors.
  • a metallic material to which the clusters stick may be used as an alternative method of achieving a cluster pattern on the substrate, if, for example, the clusters do not adhere well to the original substrate but adhere preferentially to the metallic layer.
  • the cluster structure may grow (by sequential sticking of clusters to those that have previously stuck to the surface) from one part of the metallic pattern to another part, forming a bridge or wire.
  • the metallic materials may be subsequently used as electrical contacts to the bridge or wire structure.
  • Standard electron-beam lithography or optical lithography is used to produce the electrical contacts and the template features on our samples.
  • the contacts may be entirely produced using optical lithography or a combination of optical and electron- beam lithography can be used where nanoscale contacts are required.
  • an optical photo resist layer such as AZl 500 is spun onto the substrate, baked and UV- exposed and developed in order to remove selected areas of the resist and expose an underlying (Si x Ny or SiO x ) passivated Si substrate in those areas.
  • Metallisation is added by thermal evaporation and subsequent dissolution of the photo resist translates the pattern of the photo resist layer into the metallic layer on the substrate surface.
  • Optical and electron-beam resists can be used as the patterned template layer which is formed over the substrate surface and planar contacts. As in the case of contact formation, the resists are exposed and developed in order to remove selected areas and expose the underlying surface/contacts. Positive or negative resist layers may be used for this purpose and arbitrary 2-D patterns can be produced in either. The limiting factors are the resolution of the exposure tool and the sensitivity and thickness of the resist layer. Patterns with minimum feature sizes of ⁇ 25nm may be produced in 50nm thick PMMA electron-beam resist and similar results have been achieved with SU8 layers. As the template-layer and contacts are formed with optical/electron-beam lithography the inherent accuracy in alignment of these methods may be used to accurately position the template pattern with respect to the contacts.
  • Our preferred method is 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 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], but most particularly by sputtering of the cluster material from a target).
  • Clusters can be of Si, Pd, Pt, Cu, Bi, Pb, Sb, Ag and Au or of many other materials. We prefer Si, Bi, Sb, Pd and Cu. Sizes of cluster can range from less than 0.5nm to lOOOnm in diameter. We prefer clusters with diameters in the l-50nm range and an apparatus as described in [6].
  • a feature of our technique is that the clusters deposited within apertures in nonconducting material on the substrate and between the electrical contacts (if they are in existence) may form a conducting chain or wire whilst those clusters which are deposited on top of, and happen to stick to the non-conducting material (generally ⁇ 1% in our best embodiments, see below), are isolated from the electrical contacts.
  • Deposition of atomic vapour from a standard evaporator would result in similar metallic layers blanketing the substrate; in a standard lithography process the metal on top of the insulating material would need to be "lifted-off ', in order to yield an observable metallic pattern (i.e. to reveal the metal that was deposited into the lithographically patterned apertures).
  • the key to the present invention is that the deposited clusters do NOT stick to the patterned insulating material, thereby eliminating the need for a lift-off process i.e. preferentially the non-conducting material and cluster materials are such that the clusters bounce from the non-conducting material, while sticking to the substrate (or within the apertures created in the non-conducting material).
  • the selectivity of the clusters for the surface is due to or contributed to by one or more of:
  • Hardness / softness - a soft layer may provide a 'feather bed' in which the clusters are able to nestle, while they are unable to settle on a hard surface
  • Roughness / smoothness - the texture of the substrate surface may affect the ability of a cluster to wet the surface, or, a large degree of roughness may provide effective soft landing sites for clusters.
  • Wettability the wettability of a surface determines the area of the interface between the substrate surface and a cluster adhered to it, and hence the energy of attachment to the surface.
  • Implantation depth clusters with sufficient incident momentum may embed themselves in the surface on which they land.
  • Chemical bonding chemical interaction may occur between the surface of the substrate on and the cluster causing a greater degree of binding to the surface.
  • a further embodiment of the invention relies on surface texturing of the substrate surface. Focused Ion Beam, Reactive Ion Etching or Sputter-Etching can be used to locally roughen areas of Si x N y or SiO x surfaces in accurately defined locations and with nano-scale dimensions. Similarly, the underlying silicon substrates can be roughened prior to formation of the passivation layer. In both ' cases, the low reflectivity of the roughened surface causes selective adhesion of clusters deposited onto it, whilst the untreated areas of the substrate, having higher reflectivity, remain free of clusters. The textured areas of the Si x N y /SiO x may or may not be aligned to electrical contacts formed on the insulating surface prior to the deposition process in order to cause localised pathways of cluster between the contacts.
  • the projection of the particles towards the surface is a distinguishing feature of the invention.
  • the differences in the abovementioned properties provide qualities which may cause particles to be unable to overcome their tendency to bind to the surface in one area of the substrate (so they stick), while causing the particles to have greater energy than that which might bind the particles to the surface in another area, causing the particles to be reflected, or at least not to stick efficiently in those areas.
  • a clear distinction between the present invention and those methods of the prior art wherein there is provided a chemically patterned surface to which chemically functionalised nanoparticles choose to adhere, due to the formation of chemical bonds, when said nanoparticles diffuse into contact while in a solution containing both nanoparticles and the substrate.
  • the velocity of the clusters can be controlled so that at low velocity the clusters experience elastic collisions with the surface, and are then held in contact with the surface by the attraction of the cluster to the surface. Then as the velocity of the clusters increases the clusters are elastically deformed sufficiently that they rebound from the surface. As the velocity of the clusters increases further they are at least partially plastically deformed so that they contact the surface over a larger area, increasing their tendency to stick to the surface. As the velocity of the clusters increases still further the clusters are further deformed and the energy of recoil is sufficient for the clusters to bounce from the surface.
  • the invention includes the possibilities 1) we can use preferential sticking at step edges or other defects such as ridges or depressions, to create wires/structures
  • Alternative or further embodiments may involve monitoring the formation of more than one wire structure where more than one wire may be useful.
  • 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.
  • LJ Lennard-Jones
  • V(r) 4 ⁇ [ ( ⁇ /r) 12 - C ( ⁇ /r) 6 ] (1)
  • the Mackay icosahedra are made up of 20 tetrahedrally shaped fee units which share a common vertex.
  • the surface slab consists of fixed bottom layer and 15 layers of dynamic atoms with about 8000 atoms arranged in fee crystalline structure and exposing a (111) surface facet.
  • the surface has the dimensions of 11.7 ⁇ x 11.3 ⁇ x 10.3 ⁇ to allow for substantial deformation and broadening of the cluster on impact.
  • Newtonian dynamics is applied to the central part of atoms while outer region follows Langevin dynamics [10] at a temperature T.
  • the friction parameter is varied linearly from 0 at the Langevin- Newtonian interface to 2 at the Langevin exterior in Langevin region.
  • This block of 5846 Langevin atoms regulates the temperature of the 1344 Newtonian atoms and absorbs energy from the cluster impact.
  • the surface computational cell is repeated periodically in the two dimensions parallel to the (111) surface plane, with no periodic boundary conditions applied in the z-direction. This arrangement of atoms was selected after a checking the convergence of the energetics of the collisions.
  • Examples Part II we give examples illustrating the usage of these techniques in preparing patterns of clusters and cluster devices.
  • the wafers Prior to the lithography processes, the wafers were coated with photo resist and cleaved into 10 x 10 mm 2 substrates. Cleaning was then performed by immersing the substrates into ultrasonically agitated acetone, methanol and isopropyl alcohol. After the three- solvent cleaning process, the substrates were dried using N 2 gas and oven-baked at 95 0 C.
  • planar NiCr/ Au contacts are formed on a wafer prior to the dicing stage using standard photolithography.
  • An array of 25 chip layouts is exposed and developed in a l-2 ⁇ m thick AZ1500 photo resist layer and the metal layers are thermally evaporated onto the whole wafer surface.
  • Acetone is used to dissolve the AZl 500 layer and remove the unwanted metallisation in a lift-off process.
  • the metallised wafer is then transferred to a dicing saw and twenty-five 10 x 10mm 2 chips with large-scale planar electrical contacts are produced.
  • Vias Vertical interconnects through the SiN layer on the substrate were formed using Reactive Ion Etching (RIE) with a CHF 3 /Ar etch-chemistry. These vias were eventually coated with metal and provide electrical contact to the Si substrate which provides a means to create a variable electric-field in close proximity to the deposited clusters i.e. a "back gate” contact which can be used to control the electron concentration in a device.
  • RIE Reactive Ion Etching
  • a UV-sensitive photo resist " (Clariant AZl 500) was spun onto the clean Si x N 3 , coated substrates at 3000 rpm.
  • a Karl Suss MA6 mask-aligner with a UV light-source was then used to expose the photo resist through a chrome/glass mask featuring the appropriate large-area contact patterns. After developing, the large-area contact patterns were translated into voids in the photo resist layer.
  • Ti (or NiCr) and Au layers were evaporated over the entire substrate using an Edwards Auto 306 thermal evaporator.
  • the large-area contact pattern was finally revealed using an acetone lift-off process to remove both the photo resist and the Ti (or NiCr) /Au adhered to the photo resist.
  • large area contacts have been produced with a contact separation of 100 ⁇ m.
  • HMW PMMA High Molecular Weight PolyMethyl MethAcrylate
  • E- beam resist High Molecular Weight PolyMethyl MethAcrylate
  • HMW PMMA offers highly selective development characteristics and can be spun to very thin layers ( ⁇ 50nm) using moderate spin-speeds. When exposed to an electron-beam, HMW PMMA transforms into Low Molecular Weight PMMA (LMW PMMA) which can then be dissolved in a solvent, leaving only the HMW PMMA in unexposed areas. This characteristic of PMMA is exploited in the bi-layer lift-off process used to produce the nano-scale metallised contacts used for the invention.
  • LMW PMMA Low Molecular Weight PMMA
  • A.1.3.1. Bilayer process for metallisation of small scale contacts The following bilayer process is used to form small scale metal contacts with separations between 200nm and lOOOnm. All examples of small scale contacts shown in the figures were created using the bi-layer process and subsequent metallisation.
  • LMW PMMA is spun onto the clean substrate and then baked at 185 0 C for 30 min.
  • HMW PMMA is spun on top of the LMW PMMA and baked again at 185 0 C for 30 min.
  • the differing solvent bases for the HMW- and LMW- PMMA ensure that the layers do not merge during spinning/baking.
  • an undercut forms due to the higher dissolution rate of the underlying LMW PMMA ( Figure 18).
  • resist-undercut is advantageous for clean removal of the excess metal and clean edge-profiles. This technique was used for defining the nanoscale contacts and could be used for the openings in the final passivation layer but in the examples presented here a single HMW PMMA layer is preferred for the passivation layer (with the exception of Fig. 9).
  • a single HMW PMMA layer is spun onto the contacted sample at 3000 rpm and then baked at 185 °C for 30 min. This also forms a passivation layer over the substrate/contacts which eliminates the possibility of parasitic conduction across the substrate through clusters deposited far from the contact-gap. Electron-beam lithography is then used to create an aperture (aligned over the contacts) in the HMW PMMA layer.
  • Figure 14 shows the process of forming a passivation layer, developing a pattern in that layer, and subsequent cluster deposition into the aperture- slots.
  • the passivation layer can also be formed using the bi-layer process described above.
  • a bi-layer passivation was used and the aperture-slot was formed in the bilayer material.
  • the underlying LMW PMMA is more easily developed that the HMW PMMA top-layer.
  • the LMW PMMA therefore has a larger opening in it and the LMW PMMA can be seen as a lighter grey region in Fig. 9 with an edge approximately lOOnm from the edge of the cluster wire.
  • the size of the cluster wire is governed by the size of the narrow slot/opening in the top HMW PMMA layer. Note that the dark regions surrounding the wire is the region where the HMW PMMA remains in place, but the LMW PMMA has been removed from underneath it.
  • the contacted, passivated samples were mounted on the sample-arm of a cluster-deposition system. Electrical contact to the samples was established using push-pin contacts and electrical feed- throughs in the deposition chamber enabled the necessary connections to a voltage source and current- and volt-meters required for electrical measurements.
  • A.I.3.3. SU8 process for passivation of substrate and formation of aperture-slots SU8 can be used in a very similar manner to PMMA to provide passivation and patterned template features for selective cluster-assembly.
  • SU8 2000.5 was spun on selected Si x N y passivated Si samples at 4000rpm to produce a layer thickness of 500nm.
  • the SU8 layer was baked on a hot-plate at 100°C for 60-seconds.
  • the SU8 layer could then be patterned optically or using an electron-beam.
  • Optical exposures were performed on a Karl Suss MJB-3 UV mask aligner equipped with a 200W UV-bulb and the exposure periods were typically 10-12s.
  • the SU8 is extremely sensitive to electron- beam exposure and the electron-beam dose used for nanoscale patterning of the SU8 layers was approximately 1/50 th the dose required to expose PMMA (approximately l ⁇ C/cm 2 ). After exposure the SU8 was baked at 100°C for 60-seconds in order to enhance the cross-linking of the exposed resist. The samples were then immersed in standard SU8 developer for 90-seconds in order to develop away the non-exposed regions.
  • Clusters are produced in an inert-gas condensation source.
  • the apparatus may be operated with a thermal source or a magnetron source.
  • 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.
  • 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 ⁇ 1(T 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, although different nozzles are sometime used.
  • a quartz crystal deposition rate monitor is used. The samples are mounted on a movable rod and are positioned in front of the quartz deposition rate monitor during deposition.
  • 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 or sputtered 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 connect the different chambers constituting the deposition apparatus.
  • the key parameter which controls the probability of adhesion of a cluster to a surface is the velocity of the cluster.
  • 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 [6]). Note that, as discussed in [12], 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. [12], for Ar inlet flow-rates of 30, 60, 90 and 150 seem, respectively.
  • 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 estimated kinetic energy per (25nm-diameter) cluster of 1.0 x 10 "16 Joules).
  • the measured Cu cluster velocity with source-inlet Ar and He gas flow-rates of 700sccm and lOOsccm respectively is 260m/s (corresponding to an estimated kinetic energy per (IOnm-diameter) cluster of 1.5 x 10 '16 Joules).
  • the nozzle was a 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 difference between the deflection-pulse and the detected cluster pulse (the time of flight) was converted into a cluster velocity.
  • Table 2 Summary of gas velocity calculation and the measured cluster velocity for a long nozzle with 4mm diameter opening.
  • 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 or a continuous wire.
  • the surface coverage of the deposited nanoparticle film can therefore be controlled and the cluster-assembled film can be electrically characterised immediately after formation and in-vacuum.
  • Clearly monitoring the deposition in this way requires that contacts are first prepared on the surface.
  • Figure 54 shows the measured onset of conduction for a Bi cluster-assembled wire which was formed between dual planar NiCr/ Au contacts on a Si x N y passivated Si substrate supporting a PMMA template layer.
  • the conductance is seen to rise sharply through two orders of magnitude approximately 310s after the deposition process started. This rapid and significant increase in the conductance between the contacts indicates the formation of a conducting wire.
  • the Bi cluster-assembled wire which produced the onset characteristic shown in Fig. 54 was lOO ⁇ m in length and had a minimum width of 200nm.
  • the constituent clusters were deposited using a source inlet Ar gas flow-rate of lOOsccm and the cluster-coverage on the PMMA layer was less than l% ofone-monolayer.
  • the selective cluster accumulation is achieved using a patternable, non-conducting polymer (HMW PMMA) as a passivation/reflection layer.
  • HMW PMMA patternable, non-conducting polymer
  • the examples in this section result from deposition of Bi clusters with mean diameter ⁇ 25nm, produced in an inert gas aggregation source with source inlet flow rate lOOsccm of Argon, crucible temperature 750°C to 820 0 C, source pressure 23 Torr.
  • FIG. 14 illustrates schematically the use of electron-beam lithography to generate patterns in HMW PMMA layers that were spun onto a Si x Ny (or Au coated Si x Ny - metal coated substrates are included within the description of substrate) substrate. After exposure of the PMMA to electrons development of the pattern in the PMMA thereby exposes the underlying films of Si x N y or Au. Clusters are then deposited onto the HMW PMMA layers and into the patterned areas and the resulting films can be inspected using a Field-Emission Scanning Electron Microscope. Typically the clusters stick to the substrate but not to the PMMA, resulting in the desired cluster patterns.
  • Figures 6-11 are SEM images showing examples of patterns formed by the deposition of Bi clusters into apertures in PMMA on Si x N y substrates.
  • the examples in Figures 10 and 11 show that arbitrarily shaped cluster patterns may be achieved by this method; in this case the patterns of clusters are maps of New Zealand.
  • the clusters in Figure 10 are
  • the exposure dose provided by the electron-beam is of particular importance when patterning the HMW-PMMA and affects the quality of the surface on which the incident clusters land. Dose tests have therefore been performed with the aim of finding the optimum dose required to completely remove the HMW PMMA from within the patterned areas whilst achieving nanoscale resolution.
  • the roughness of the patterned areas after development has been measured using Atomic-Force Microscopy (AFM) and the results of this analysis have been used to explain both the cluster reflection from the HMW PMMA layer and cluster accumulation in the apertures.
  • AFM Atomic-Force Microscopy
  • Figure 3 shows bismuth clusters (with a mean diameter of 30 to 40 nm; inert gas aggregation source with source inlet flow rates 1 OOsccm of Argon, crucible temperature 785°C, source pressure 22.4 Torr) deposited within partially exposed and developed patterns in HMW PMMA (i.e. the PMMA was exposed to electron beam doses smaller than required to fully remove the exposed PMMA).
  • the total deposited thickness (as measured at the rate deposition monitor) of clusters was fixed to be ⁇ 21 ⁇ 3 nm in each case but the cluster coverage on the surface clearly increases with increasing electron dose (from 40 ⁇ C/cm 2 in a) in steps of 10 ⁇ C/cm 2 to 80 ⁇ C/cm 2 in e)).
  • the electron dose is effective in controlling the adhesion of the clusters to the exposed region.
  • the surface coverages in the FE-SEM images shown in Figure 1, Figure 2 and Figure 3 were quantified using image processing software.
  • the images were binarised (i.e. converted into black and white) using a threshold grey value which preserved the shape of the cluster patterns in the original images.
  • the number of black and white pixels in the resultant image was then digitally counted and converted to a surface coverage.
  • Figure 4a shows the Sb cluster surface coverage measured by analysing binarised versions of the images in Figures 1 and 2, while Figure 4b shows similar results for the Bi clusters in Figure 3.
  • Increased electron-beam dose results in increased cluster coverage. As discussed in more detail below, this effect arises from a combination of increased surface roughness in the partially exposed patterns (dose 40-80 ⁇ C/cm 2 ) and the greater propensity for clusters to stick to the Si x Ny surface which is exposed at higher doses (>80 ⁇ C/cm 2 ).
  • FIG. 15 Surface roughness on the nanoscale is commonly measured using an atomic force microscope (AFM).
  • Figure 5 shows atomic force microscope images used to determine the RMS roughness within HMW PMMA patterns which have undergone electron-beam exposures ranging from 0-140 ⁇ C/cm 2 (Electron dosage was a) 0 i.e. unexposed HMW PMMA, b) 40 ⁇ C/cm 2 , c) 50 ⁇ C/cm 2 , d) 60 ⁇ C/cm 2 , e) 70 ⁇ C/cm 2 , f) 80 ⁇ C/cm 2 and g)
  • RMS roughness is plotted against electron-beam exposure dose in Figure 4c.
  • the roughness increases for increasing dose until it reaches a maximum when the dose is approximately 65 ⁇ C/cm 2 .
  • the increase of cluster- coverage for a dose of 40 — 65 ⁇ C/cm 2 in Figures 4a and 4b can therefore be explained by the increase in surface roughness within the void. Increased surface roughness
  • Electron-beam doses higher than 65 ⁇ C/cm 2 are sufficient to remove enough PMMA to cause parts of the underlying substrate (see Figure 5e and 5f) to be exposed after development.
  • the further increase in coverage in Figures 4(a) and 4(b) is then explained by the increased surface-wetting / surface adhesion behaviour for both the Bi and Sb clusters on the Si x Ny compared to the HMW PMMA surfaces. Put simply, both Bi and Sb clusters bounce from unexposed PMMA layers, but stick to bare Si x N y surfaces.
  • the strength of the interaction can be controlled using a single parameter, C.
  • a strong interaction e.g. between a cluster and a rough surface, or for Bi rather than Sb
  • C ⁇ 0 represents a very weak interaction.
  • the Sb clusters were deposited with lower average incident velocity than the Bi clusters due to the choice of lower gas inlet flow rates and the addition of He to the gas mix
  • Sb clusters typically wet the Si x N 5 , or SiO x surface appreciably less than Bi clusters[12]).
  • Figure 20 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-thickness-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 20 that a far higher proportion of the incident Bi clusters adhere to the Si x N y and SiO x surfaces than adhere to the PMMA and AZl 500 surfaces.
  • Bi clusters were reflected from all surfaces (the total coverage on the Si x N y and SiO x 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 x Ny or SiO x 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 21 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 IOnm.
  • 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.
  • Bi and Cu clusters can be assembled into conducting films on a Si x N y surface layer whilst there is minimal accumulation of clusters on a PMMA surface layer (or photo resist layer in the case of Bi clusters).
  • 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).
  • Figure 22 shows measured Bi cluster coverages achieved after three deposition experiments onto SU8, PMMA, Si x Ny, SiO x and Au.
  • 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 17 A, 34 A and 51 A on a quartz crystal film-thickness-monitor. Again a far higher proportion of the incident Bi clusters adhere to the Si x N y , Au and SiO x surfaces than adhere to the PMMA and SU8 surfaces.
  • the cluster-coverage measured on the PMMA and SU8 surface layers was less than 3% of one monolayer after a deposition which causes a percolating layer to be formed on Si x Ny or SiO x surface layers.
  • Figures 23 and 24 show Field-Emission SEM images of AZl 500 photo resist, PMMA Electron-beam resist, MBE grown Si x Ny and thermally grown SiO x surface layers supporting Bi clusters (Fig. 23) and Cu clusters (Fig. 24). (Measurements of surface coverage from these images were shown in the cluster-coverage data in Fig. 20 and Fig. 21).
  • Fig. 23 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. 24 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.
  • Figure 25 shows Field- Emission SEM images of SU8 photo resist, PMMA Electron-beam resist, MBE grown Si x N y and thermally grown SiO x surface layers supporting Bi clusters.
  • the estimated total deposited Bi cluster layer thickness was 51 A. (Measurements of surface coverage from these images were shown in the cluster-coverage data in Fig. 22).
  • the source conditions for the Bi cluster depositions (Fig. 23) 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-
  • the source conditions for the Cu cluster depositions (Fig. 24) 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.
  • FIG. 6-11 illustrate selective assembly of Bi and Sb clusters within apertures formed in HMW PMMA films.
  • the cluster-assembled films have been formed on electrical contacts thus enabling electrical characterisation of the films as soon as they become electrically conducting.
  • Figure 6 shows a Bi cluster-assembled film suitable for four-point electrical conductivity measurements and formed in patterned HMW PMMA. The Bi clusters are distributed evenly within the aperture and the cluster-assembled film is of uniform thickness.
  • FIG 7 An image of Sb clusters assembled within an aperture-slot is shown in Figure 7.
  • the Sb clusters have adhered to the Au contacts whilst clusters have been reflected from both the HMW PMMA film and the substrate area within the aperture, leading to low cluster coverage over these areas.
  • the source inlet gas flow (and therefore the average incident momentum of the clusters) was sufficiently high to prevent selective assembly within the aperture.
  • the cluster-assembled film has grown into the aperture as clusters aggregate on the Au contacts and with other clusters.
  • Figure 8 shows a similar aperture and contact arrangement with an electrically conducting Bi cluster-assembled film bridging the gap between the contacts.
  • the source inlet flow was optimised to produce selective assembly within the aperture and minimal cluster-coverage on the surrounding PMMA.
  • Patterns and/or contacts within the HMW PMMA are not limited to geometrical forms; Figure 10 and Figure 11 show silhouette-maps of New Zealand patterned in HMW
  • Figure 9 shows an FE-SEM image of a contacted Bi cluster-assembled wire (contacts not shown). As can be seen in Figure 9 devices with minimum dimensions as small as one cluster width can be produced and contacted.
  • the contacts may be used to characterise the electrical characteristics of the device.
  • Figures 12 and 13 show post-formation Current- Voltage (1(V)) characteristics taken from contacted cluster-assembled films formed within apertures in PMMA.
  • Figure 12 shows a nonlinear 1(V) characteristic for the sample shown in Figure 8.
  • the I(V) measurements were performed at room temperature and at 95 K.
  • the solid line in Figure 13 is a two-point 1(V) measurement of the innermost contacts in Figure 6, and therefore the measurement includes the resistance associated with the contacts to the cluster- assembled wire.
  • a four-point measurement (dotted line in Figure 13) de-embeds the contact resistance and therefore provides an I(V) characteristic for the cluster-assembled wire alone.
  • the difference in the two- and four- point resistance measurements is attributed to bismuth-oxide tunnelling barriers at the contact/cluster interface and/or potential barriers at the Bi/Au interface, caused by the differing work functions of these materials.
  • the device and the contacts are formed by the same method and same material; no contact potential or significant tunnelling barriers are expected.
  • HMW PMMA is a positive resist and so exposed patterns are developed away.
  • Figure 19 shows an aperture which has been patterned in conventional optical photo resist (Clariant AZl 500) over a SiO 2 passivated V-grooved silicon sample.
  • Sb clusters were deposited onto the sample and assembled in the dual V-grooves (running vertically in the centre of the image) between planar Au contacts (lighter shade at top and bottom, with diagonal edges).
  • the cluster deposition process was stopped immediately after conduction between the contacts was observed (indicating that a continuous cluster- assembled wire had formed in one of the V-grooves).
  • the image shows that a greater number of clusters have adhered to the SiO x covered areas (between the Au contacts), than have adhered to the optical photo resist layer (at each side of the image).
  • FIG. 35 shows a further example of a slot-aperture device fabricated using optical lithography and photo resist.
  • the image shows one end of a "Hall bar” device i.e. a device which has a contact at each end of an elongate slot-aperture and several contacts along each of the sides.
  • Figure 26 shows an array of cluster-assembled wires which have been formed between planar Au electrical contacts (at top and bottom of the image) on a SiO x passivated Si substrate.
  • An SU8 template layer with aperture-slots (width approximately 5 ⁇ m) enabled the selective assembly of incident Bi clusters into conducting cluster-assembled wires on the SiO x and Au surfaces whilst the measured coverage on the SU8 amounted to less than 5% of one-monolayer.
  • FIG 27 the I(V) characteristics of a single cluster-assembled wire (with minimum width l.O ⁇ m and length lOO ⁇ m) are shown.
  • a FE-SEM image of this wire is shown in Figure 28(b).
  • the Bi clusters were deposited with an Ar-mlet flow-rate of lOOsccm onto a SU8 templated substrate held at room temperature (293K).
  • a linear 1(V) characteristic was recorded and the two-point wire resistance was lOk ⁇ .
  • a heater and temperature controller were then used to raise the temperature of the sample-arm and sample to 300K, 330K, 370K 5 400K, 430K and finally 460K.
  • Figure 28 shows that a larger average grain size had resulted from the heating process:
  • Figure 28(a) shows a cluster- assembled wire which was subjected to temperatures no higher than 300K (Fig. 28(a)) next to the cluster-assembled wire which was heated to 460K (Fig. 28(b)).
  • Cluster wires can be accurately positioned over an arbitrary number of metal contacts and these can be arranged and shaped with equal arbitrariness.
  • the contacts on the sample shown in Figure 29 have been formed using electron-beam lithography, followed by a standard metal deposition and lift-off. The separations between the contacts are 750nm, 500nm, and 750nm respectively.
  • the two inner contacts are both l ⁇ m wide.
  • Bismuth clusters with a mean diameter ⁇ 30nm ininert gas aggregation source with source inlet flow rates lOOsccm of Argon, crucible temperature 805°C, source pressure 22.5 Torr
  • the wire is 230nm wide and approximately 6 ⁇ m long.
  • the wire itself forms in the pre-formed slot-aperture which may be accurately aligned to the contacts (in the present example the contacts are substantially wider than the slot, but the contacts could be much smaller in a commercial device.
  • the size of the wire is limited by the resolution of the electron- beam/resist used (which may be smaller than IOnm), by the combination of substrate and electron-beam resist materials, and by the cluster size.
  • Figure 36 shows a selection of snapshots of the solid and liquid 147-atom cluster after equilibration on the (111) surface for 5xlO 5 time steps.
  • To estimate each contact angle we fitted a spherical cap to the positions of the cluster atoms.
  • the contact angles ⁇ w found are given in as shown in Table 3 for a variety of cluster sizes and C-values. Note that the solid clusters also show wetting behavior. In Ref.
  • Z 0n decreases linearly with time during the free flight period before beginning to interact with the surface.
  • the total cluster potential energy per atom, E pot is the sum of cluster internal energy per atom, E c and cluster-surface interaction energy per atom, E cs .
  • Eo s at first decreases as the cluster approaches the surface due to the attraction between the cluster and surface.
  • E 0 O.4v c
  • E c is restored to its precollision value indicated that the collision is elastic (as was indicated by the deformation in Fig. 40).
  • the change in E c is permanent, indicating that the collision is largely plastic.
  • v o >O.5v c the sticking probability increases but once more starts to decrease for v o >1.5v c .
  • this is because there are essentially two deformation regimes. For v o ⁇ O.3-O.4v c , little deformation occurs, so that the area of contact (and hence the adhesion energy) depends only weakly on the incident velocity.
  • v o ⁇ O.3-O.4v c little deformation occurs, so that the area of contact (and hence the adhesion energy) depends only weakly on the incident velocity.
  • velocities i.e.
  • Figure 45 shows the variation of coefficient of restitution, e, with the impact velocity.
  • Each data point shown in the figure represents an average of 100 trials for each cluster size.
  • the data shows a rough trend for e to decrease as the cluster size increases, e is approximately constant for low velocities but shows a strong dependence on velocity at Vo>O.5v c .
  • the dependence of e on velocity varies as e ⁇ v 0 "0593 , e ⁇ vo " °' 588 and e ⁇ v 0 "0567 , for 147, 309 and 561 icosahedra, respectively. This dependence on velocity is much stronger than that predicted by small deformation contact mechanics [13].
  • the state of the cluster (liquid / solid) can be controlled by the source conditions or by a thermalisation stage subsequent to the source. Thus the amount of bouncing or sticking to the surface can be controlled.
  • the plurality of particles deposited on the substrate have a size distribution but are all at substantially the same temperature (this is typical of inert gas aggregation sources such as the one we use [6]).
  • this is typical of inert gas aggregation sources such as the one we use [6].
  • Figure 49 illustrates the effects of varying the strength of cluster-surface attraction, C, showing the adhesion probability of 147-atom icosahedron as a function of the impact velocity. It is seen that the adhesion probability strongly depends on C and the transition from adhesion to reflection of the cluster is observed as the value of C is decreased from
  • the normal coefficient of restitution is defined as the ratio of the maximum normal velocity components after and before impact (see Figure 51).
  • v Oz ⁇ 0.5v c a constant restitution coefficient close to 0.8 is observed.
  • V 02 > 0.5v C5 e varies as Vo z " ° '68 . This is a somewhat stronger dependence on velocity than in the normal case.
  • the invention involves deposition of nanoscale clusters onto patterned regions of a substrate.
  • the preferred patterning takes the form of micro- and/or nanoscale apertures formed in non-conducting layer on a substrate.
  • the substrate may include 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 wire).
  • the apertures in the non-conducting polymer are formed using standard lithographic and/or etching 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 is sufficient to prevent adhesion on the surface of the non-conducting polymer.
  • the source inlet gas flow can be adjusted so that the momentum of the incident clusters is sufficient for the clusters to reflect from the surface of the non-conducting polymer.
  • the bouncing of clusters from surfaces was studied extensively in [17].
  • 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 >100 ⁇ m.
  • 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).
  • 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. The onset of conduction indicates the production of a conducting cluster- assembled pathway.
  • the resist layer thickness must be significantly greater than the deposited layer thickness in order to produce a well defined break in the film around the perimeter of the desired feature and hence produce a clean lift-off. Furthermore, it is often of advantage to form a undercut in the resist layer, assisting a clean lift-off. The latter requirements significantly limits the ability to produce high-aspect ratio structures with nanoscale dimensions, and the method of the invention may be used to avoid these limitations.
  • the apparatus and the method according to the invention make it possible to selectively form metallised, insulating or semiconducting regions (with lithographically defined dimensions and location) on an insulating, semiconducting or conducting substrate with no requirement to dissolve or otherwise remove the surface template layer which causes the clusters to assemble.
  • the deposited material and / or the substrate may be susceptible to damage from resist stripping chemicals or wet/dry etchants and so the method and invention can be applied in order to eliminate any possibility of damage during the lift-off process.
  • Al side-gates for a cluster-assembled wire.
  • the Al side-gate layer is covered (or encapsulated) with a patternable polymer layer (eg. SU8) in order to prevent complete oxidation of the Al and the polymer layer is then patterned to leave a selected area of the Al open through an aperture.
  • the Al oxidizes in this region and forms an AlO layer on which the clusters are deposited.
  • the non- exposed and non-oxidised Al layer then lies in close proximity to the cluster wire/film and can serve as a side-gate electrode
  • the apparatus and the method according to the present invention allow the fabrication of cluster-assembled structures with feature sizes of less than 20 ran. This may include cluster-assembled wires with uniform widths below 20nm or cluster-assembled wires which feature sections with minimum dimensions of less than 20nm. Quantum effects have been observed in wires and films with similar dimensions, and the present invention enables efficient electrical characterisation of such effects.
  • 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 apertures within the non-conducting polymer 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.
  • wires formed by the method of the invention are sensitive to many different external factors (such as light, temperature, chemicals, magnetic fields or electric fields) which in turn give rise to a number of 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:
  • 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.
  • Pd particles By depositing Pd particles through apertures in a non-conducting polymer 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 provides a mechanism by which the conductivity of the device changes, providing a sensor. It is important to emphasize here that a pathway of clusters (which is not yet conducting but will conduct upon the absorption of hydrogen) is required. This is within the scope of the invention, as described and claimed.

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Abstract

Procédé d'élaboration de motif de particules de la taille du micron et de taille inférieure sur une surface de substrat. Le motif peut prendre la forme d'un trajet conducteur d'agrégats atomiques entre contacts. Le procédé décrit peut se substituer aux techniques d'élaboration de motif lithographique classiques, sans recours à une phase de soulèvement.
PCT/NZ2006/000173 2005-07-08 2006-07-04 Procedes de lithographie a nano-echelle/micro-echelle et dispositifs resultants Ceased WO2007008088A1 (fr)

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CN113249684A (zh) * 2021-04-16 2021-08-13 杭州电子科技大学 一种高密度功能团簇材料及其制备方法
TWI805179B (zh) * 2021-05-07 2023-06-11 台灣積體電路製造股份有限公司 提高對準標記的反射率的方法及對準標記

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WO2017023394A3 (fr) * 2015-05-13 2017-04-20 Stc.Unm Courbure de nanofil pour un processus de dispositif planaire sur des substrats à base de si (001)
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CN113249684B (zh) * 2021-04-16 2023-05-16 杭州电子科技大学 一种高密度功能团簇材料及其制备方法
TWI805179B (zh) * 2021-05-07 2023-06-11 台灣積體電路製造股份有限公司 提高對準標記的反射率的方法及對準標記
US12033951B2 (en) 2021-05-07 2024-07-09 Taiwan Semiconductor Manufacturing Company, Ltd. Alignment mark structure and method for making

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