WO2019089793A1 - Selective transfer of a thin pattern from layered material using a patterned handle - Google Patents

Selective transfer of a thin pattern from layered material using a patterned handle Download PDF

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Publication number
WO2019089793A1
WO2019089793A1 PCT/US2018/058508 US2018058508W WO2019089793A1 WO 2019089793 A1 WO2019089793 A1 WO 2019089793A1 US 2018058508 W US2018058508 W US 2018058508W WO 2019089793 A1 WO2019089793 A1 WO 2019089793A1
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Prior art keywords
handle
patterned
source material
transfer medium
pattern
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PCT/US2018/058508
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French (fr)
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Hayden TAYLOR
Hannah GRAMLING
Eric Yeatman
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The Regents Of The University Of California
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Publication of WO2019089793A1 publication Critical patent/WO2019089793A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/67—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere
    • H01L21/683—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere for supporting or gripping
    • H01L21/6835—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere for supporting or gripping using temporarily an auxiliary support
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/20—Deposition of semiconductor materials on a substrate, e.g. epitaxial growth solid phase epitaxy
    • H01L21/2003—Deposition of semiconductor materials on a substrate, e.g. epitaxial growth solid phase epitaxy characterised by the substrate
    • H01L21/2007—Bonding of semiconductor wafers to insulating substrates or to semiconducting substrates using an intermediate insulating layer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/41—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
    • H01L29/417—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions carrying the current to be rectified, amplified or switched
    • H01L29/41725—Source or drain electrodes for field effect devices
    • H01L29/41758—Source or drain electrodes for field effect devices for lateral devices with structured layout for source or drain region, i.e. the source or drain region having cellular, interdigitated or ring structure or being curved or angular
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/778—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/78—Field effect transistors with field effect produced by an insulated gate
    • H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
    • H01L29/78681—Thin film transistors, i.e. transistors with a channel being at least partly a thin film having a semiconductor body comprising AIIIBV or AIIBVI or AIVBVI semiconductor materials, or Se or Te
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2221/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof covered by H01L21/00
    • H01L2221/67—Apparatus for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components; Apparatus not specifically provided for elsewhere
    • H01L2221/683—Apparatus for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components; Apparatus not specifically provided for elsewhere for supporting or gripping
    • H01L2221/68304—Apparatus for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components; Apparatus not specifically provided for elsewhere for supporting or gripping using temporarily an auxiliary support
    • H01L2221/68359—Apparatus for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components; Apparatus not specifically provided for elsewhere for supporting or gripping using temporarily an auxiliary support used as a support during manufacture of interconnect decals or build up layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/24—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only semiconductor materials not provided for in groups H01L29/16, H01L29/18, H01L29/20, H01L29/22
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007—Multistep manufacturing processes
    • H01L29/66969—Multistep manufacturing processes of devices having semiconductor bodies not comprising group 14 or group 13/15 materials

Definitions

  • the technology of this disclosure pertains generally to van der Waals (vdW) materials and methods of device fabrication, and more particularly to systems and methods for the transfer of 2D layers from source crystals and vdW growth substrates onto target substrates for the van der Waals heterostructures. Transfer, rather than direct growth and patterning on the target substrate, enables low-temperature processing of the transferred materials and target substrate as well as the use of diverse target materials.
  • vdW van der Waals
  • Van der Waals crystals are a class of materials that are composed of stacked layers. Individual layers are single- or a few-atoms thick, covalently bonded in the plane and loosely bound to adjacent layers with van der Waals bonds. When isolated from adjacent layers, a single layer or a few layers of these materials are effectively two-dimensional. In this form, they exhibit unique mechanical, electrical, and optical properties, and are thus expected to see widespread adoption in devices across a range of fields.
  • the thinness of the layers results in extreme mechanical flexibility and exceptional properties, enabling many applications including in electronics, photonics, and chemical sensing. Interfaces between semiconducting, conducting and insulating 2D materials find applications in, for example, field-effect transistors (FETs), memristive memories, diodes, including light-emitting diodes (LEDs), photodetectors, photovoltaics, and catalysis.
  • FETs field-effect transistors
  • diodes including light-emitting diodes (LEDs), photodetectors, photovoltaics, and catalysis.
  • Graphene a single atomic layer of graphite, is capable of ballistic transport of electrons.
  • Other layered materials are also amenable to stable isolation as a single atomic layer (monolayer). Transition metal
  • TMDCs dichalcogenides
  • Such large lateral sheet dimensions are needed for at least two possible purposes: (1 ) to provide material on which can be created integrated circuits with many sub-micron devices in a pre-defined spatial arrangement; or (2) to define the boundaries of, e.g., powerful individual visible light emitters or sensitive detectors requiring dimensions in the tens of microns or larger.
  • a process which simultaneously achieves shape selectivity and monolayer selectivity is desirable for forming arrays of heterostructures, by enabling the deposition of a patterned monolayer array at the final substrate (which may already have patterned monolayer arrays on its surface).
  • the fabrication of various devices requires the deposition of layers of a few- or monolayer materials of specified size and shape and arranged in a pre-determined pattern on a desired substrate.
  • the material may originate from a bulk, mined source, or from a grown crystal, each composed of many layers. A small number of layers are removed for transfer to a substrate.
  • One embodiment of the manufacturing methods uses gold-mediated exfoliation in conjunction with a lithographically patterned handle layer to transfer arrays of monolayer regions with controlled shape, size, and separation.
  • the approach delivers a far higher areal density of usable, continuous monolayer material than unpatterned exfoliation, and does so in predictable relative locations so that arrays of devices can subsequently be created in a systematic way.
  • the technology can be used to remove nanometers-thick, laterally patterned material from an initial surface (potentially the material's original source), so that it can be subsequently deposited on a desired substrate.
  • the technology also can used for removal of few- or monolayer material from a many-layered van der Waals crystal, such as graphite, black phosphorus or molybdenite. Other applications will be readily appreciated by those skilled in the art from the disclosure herein.
  • a handle layer is formed over the patterned
  • a transfer medium is attached to the handle layer with an adhesive material, and the transfer medium is used to remove the handle layer and the patterned material.
  • the handle layer preferably has a thickness that is sufficient to prevent the adhesive material from contacting unpatterned portions of the source material when the transfer medium is applied, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
  • Transfer media including pressure-sensitive adhesives and other viscoelastic polymers, require applied pressure to adhere to their target.
  • the transfer medium is illustrated as attached to a possible backing layer, the presence of backing layer is not critical to the operation of the transfer medium.
  • An important feature of the methods is the ability to reliably transfer monolayers using a single, controlled exfoliation step.
  • This exfoliation process preferably begins with the deposition of a thin metallic film (e.g. Au) that mediates the process. It has been shown that the metal film can increase the monolayer selectivity of the exfoliation process and has the potential to exfoliate large-area samples.
  • a thin metallic film e.g. Au
  • the metallic film does two important things to the exfoliated material. First, it strains only the top layer of the film that is to be exfoliated. This strain leads to two important and sometimes competing effects. The strain changes the effective atomic density of the film. This alters the strength of the van der Waals force (on a per-area basis) between the top and subsurface layers of the exfoliated crystal. Tensile forces weaken the force, and compressive forces increase it. An increased force tends to pull the top layer closer to the second layer and decreases monolayer selectivity for the exfoliation process. A decreased force has the opposite effect.
  • the strain in the film also changes the stacking registry of the layer to be exfoliated. Because the lattice parameter of the strained to- be-exfoliated layer differs from that of the layer below, some regions of the exfoliated layer will be in unfavorable stacking positions relative to their initial positions. This weakens the bond between the exfoliated layer and layers beneath. The force density and stacking registry can sometimes compete against one another. In the case of the transition metal
  • a transfer system for removing a patterned material from a source material is provided utilizing a handle layer and a transfer medium such as an adhesive attached to the handle layer along with an optional backing layer.
  • Another aspect of the technology is to provide adaptable methods for removing a patterned material from a source material by forming a handle layer over the patterned material; attaching a transfer medium to the handle layer; and using the transfer medium to remove the handle layer and the patterned material.
  • the handle layer is configured to prevent removal of unpatterned source material while the patterned source material is removed.
  • Another aspect of the technology is to provide a laterally patterned material that is nanometers-thick and formed of a few- or monolayer material for placement on a target substrate where the source material is a many-layered van der Waals crystal, such as graphite or molybdenite.
  • FIG. 1 is a schematic functional flow diagram of a method for
  • FIG. 2A through FIG. 2K collectively depict intermediate structures for method steps for the production and transfer of one or more layers of M0S2 or WS2 to a silicon substrate according to another embodiment of the technology.
  • FIG. 3A is a schematic cross-sectional view of the handle for
  • FIG. 3B is a schematic cross-sectional view of the handle where the dimensions allow contact of adhesive with unpatterned source material. Under the applied pressure needed to ensure good adhesion, the vertical deformation of the transfer medium's adhesive layer is on the same size scale as gaps in the micropattern through which the adhesive may protrude.
  • FIG. 4A is a detailed schematic cross-sectional view of a back-gated M0S2 transistor according to one embodiment of technology.
  • FIG. 4B is a top view of an array of back-gated M0S2 transistors defined by the patterning process according to one embodiment of the technology.
  • FIG. 1 to FIG. 4B illustrate the characteristics and functionality of the devices, methods and systems within the context of a semiconductor adaptation.
  • Spatially-controlled transfer of arrays of single-layer M0S2 and WS2 sheets from multilayer crystals onto S1O2 substrates is used to illustrate the technology.
  • the methods may vary as to the specific steps and sequence and the systems and apparatus may vary as to structural details without departing from the basic concepts as disclosed herein.
  • the method steps are merely exemplary of the order that these steps may occur. The steps may occur in any order that is desired, such that it still performs the goals of the claimed technology.
  • FIG. 1 one embodiment of a method 10 for
  • the method enables a transfer medium to only adhere to patterned regions allowing the transfer medium to remove only the patterned material and to leave behind the unpatterned bulk.
  • the multi-layer source material to be transferred is selected and prepared.
  • the source material can be sized and shaped and the surface providing the layers of material can be leveled to provide an even surface and remove potentially degraded top layers.
  • Suitable materials are composed of many layers that are severable and amenable to stable isolation as a single atomic layer (monolayer) or a few monolayers.
  • the material may originate from a bulk, mined source as well as from a grown crystal.
  • Van der Waals solids composed of layers a few atoms thick and loosely bound by van der Waals bonds to adjacent layers, are particularly preferred.
  • Some van der Waals materials, including graphite, black phosphorous and molybdenite, occur naturally in bulk form, which can be thinned down to their single-layer (monolayer) forms of graphene, phosphorene and molybdenum disulfide, respectively.
  • Common 2D or single layer materials include Bismuthene,
  • Transition metal Di-chalcogenides such as M0S2, WS2, MoSe2, WSe2, MoTe2 and PtSe2, are atomically thin semiconductors that have a direct band gap and are materials that have important uses in electronics and optics as transistors or emitters or detectors respectively.
  • Other van der Waals-layered materials include layered insulators, like hexagonal boron nitride.
  • van der Waals-layered materials are preferred, many other layered materials may be suitable source materials for preparation at block 20 of FIG. 1 .
  • the surface of the material is optionally blanketed with a metal layer at block 30.
  • the metal can be coated onto the surface material using conventional techniques such as thermal evaporation, sputter deposition and chemical vapor deposition.
  • the preferred metal of the metal layer that is applied to the source material is gold.
  • other metals such as copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn) and silver (Ag) can be used as well.
  • the optional metallic film that is applied at block 30 creates strain in only the top layer of the source material. The strain changes the effective atomic density of the source material as well as changing the stacking registry of the layer to be exfoliated.
  • the metal film created strain weakens the bond between the exfoliated layer and layers beneath. It has been shown that the deposition of the thin metallic film can increase the monolayer selectivity of the exfoliation process as well as have the potential to allow the exfoliation of large-area samples.
  • a patterned photoresist layer is applied on top of the metal layer.
  • the photoresist layer can be applied and patterned using conventional materials and techniques. The pattern that is designed or selected will ultimately depend on the desired size, shape and
  • the photoresist layer is applied directly on top of the source material at block 40.
  • the patterned photoresist layer at block 40 is referred to as the
  • the handle layer separates a deformable adhesive transfer medium layer from the source material.
  • the handle layer preferably has a thickness sufficient to prevent the adhesive material from contacting unpatterned portions of the source material when the transfer medium is applied preventing removal of unpatterned source material when the patterned material is removed from the bulk source material with the handle.
  • the exposed points of the metal layer that are not covered by the patterned photoresist layer are etched, preferably without removing the photoresist. This step at block 50 exposes the source material that will not be transferred and the material that will be transferred will remain masked by the metal film and photoresist layers.
  • the source material that is exposed at block 50 with the removal of parts of the metal layer is then processed to remove at least one atomic layer at block 60.
  • What this processing step essentially achieves is control over where interlayer fracture initiates in vdW solids. It is more effective than simply blanketing the source material with metal because in that case there is no control of where inter-layer fracture events will initiate, and it is expected that exfoliated material edges would then correspond to naturally occurring steps in the crystal structure.
  • One key to maximizing yield may be to minimize the number of intersections of patterned gold metal regions with natural step changes in the height of the source material, so that opportunities for handles to contact multiple layers are reduced. This aim might be achieved by maximizing grain size relative to exfoliated feature size.
  • a transfer medium is then applied to the stack of patterned
  • photoresist layer etched metal layer and source material at block 70.
  • pressure-sensitive adhesives including thermal release tape or
  • Teflon tape or other viscoelastic materials, such as polydimethylsiloxane (PDMS) or Poly(methyl methacrylate)(PMMA), can be used as a transfer medium at block 70.
  • PDMS polydimethylsiloxane
  • PMMA Poly(methyl methacrylate)
  • a rigid, semi-rigid or flexible backing layer is applied to the top surface of the transfer medium.
  • the backing layer is on the opposite side of the transfer medium from the patterned photoresist layer.
  • the backer layer is a platform that distributes applied pressure to the stack and facilitates the transfer.
  • the stack of transfer medium, patterned photo resist, metal film and a small number of layers of source material are then mechanically separated from the bulk source material at block 80.
  • thermal release tape light pressure is applied, and the tape loaded now with the pattern is peeled from the bulk source material at block 80.
  • the stack with the few layers or monolayer of source material is then transferred via the
  • nanoscale-layered material to be isolated and transferred using standard pressure-sensitive adhesives or viscoelastic stamps.
  • the ability to use photoresist as a handle layer leverages established processing steps and is CMOS-process-compatible. It also enables the handle layer to be self- aligned to material isolated by etching, using the photoresist as a standard etch mask.
  • the handle layer approach is also amenable to stamp-based methods, where the pattern handle is itself the transfer medium.
  • the system and methods allow 3D spatial control over exfoliation that will enable complex integrated circuits to be fabricated more easily from 2D materials.
  • the ability to transfer monolayer sheets with areas >10 4 pm 2 makes the methods particularly appealing for the production of complex heterostructure-based circuits.
  • the methods can be used to transfer arrays of much smaller regions of material, e.g. to define many individual sub-micron transistor geometries prior to exfoliation and transfer.
  • the challenge in that case is to ensure a high enough feature yield to be able to construct the desired integrated circuit without missing devices.
  • higher functional yields may be achieved by transferring arrays of large monolayer sheets and then defining, e.g., conductive interconnect patterns to create one or more whole integrated circuits within each successfully transferred large monolayer region.
  • metal-mediated exfoliation may find itself used in conjunction with other emerging techniques for epitaxy and transfer of thin films to create semiconductor heterostructures.
  • FIG. 2A schematically in FIG. 2A through FIG. 2K.
  • the method 200 began with the preparation of multilayer source material 210 of M0S2 and WS2.
  • the flattest available sections of material were used.
  • M0S2 natural, mined crystals were obtained (eBay) and were manually cleaved to create a flake several mm in diameter and a fraction of a millimeter thick. This flake was mounted to a glass slide using double-sided Kapton tape for subsequent processing.
  • Both WS2 and additional M0S2 samples were also obtained as a multi-layer samples fabricated by chemical vapor transport (CVT), ⁇ 0.2 - 0.3 mm thick (HQ Graphene) and used as received.
  • CVT chemical vapor transport
  • HQ Graphene HQ Graphene
  • the coating of the source material 210 with blanket gold layer 220 was followed by a photolithography process yielding a patterned photoresist handle 230.
  • AZ 4620 photoresist 230 was spun onto the gold-evaporated surface of bulk M0S2 and WS2 flakes using a two-layer process.
  • the resist was baked at 1 10 °C for three minutes following each spin.
  • the resist was exposed with a chrome/glass contact transparency mask for 10 seconds at approximately 20 mW/cm 2 and subsequently developed for four minutes in AZ 400K developer predicted 1 :3 with deionized water.
  • Patterning of the photoresist layer 230 produces exposed areas 240 of the gold metal layer 220 as seen in FIG. 2C. Without removing the photoresist layer 230, the exposed areas 240 of gold layer 220 was etched for 1 minute in KI/I2 (Gold Etchant TFA, Transene Company, Inc.; used undiluted). This step removes the metal and exposed the M0S2 or WS2 source material 210 that was not going to be transferred, while the to-be- transferred material remained masked by the gold 220 and photoresist 230 layers. The sample was then rinsed in Dl water.
  • KI/I2 Gold Etchant TFA, Transene Company, Inc.
  • the exposed areas 250 of WS2 or M0S2 source material from the patterned removal of parts of the gold metal layer is processed further as shown in FIG. 2D.
  • the exposed portions 250 of the patterned source material were then treated by a 30-second etch in CF 4 plasma (20 seem, 100 W, Plasma Equipment Technical Services, Inc.) to remove at least one atomic layer of the M0S2 or WS2 from unmasked regions. Initially, a one minute etch time was used to ensure the removal of the top layer, per ref. 37; however, the longer etch time was correlated with large amounts of organic residue on the sample and was adjusted down.
  • an argon ion milling step was also used. Ion milling (Pi Scientific 8" system) was conducted using Argon ion (5 seem RF neutral, 15 seem ion source), with 100 mA beam current, 500 V beam voltage and a 20-degree incidence angle. The duration of the mill was 7 minutes and the pressure was 1 .9 ⁇ 10 ⁇ 4 Torr. In some cases, it was found that the photoresist was easier to remove later when ion milling had been used than when the CF 4 etch had been used, possibly because of fluoropoiymer deposition onto the photoresist during CF 4 processing.
  • the photoresist handle serves initially as an etch-mask for the gold metal layer, and then for a brief plasma etch or ion milling step, which was found to be beneficial.
  • the removal of a few atomic layers of portions of the source material by the etching/milling step shown in FIG. 2D create crack-initiation sites at the edges of the desired features.
  • the source crystal can be re-used, if there are enough layers remaining, by doing a blanket exfoliation with unpatterned thermal release tape to recover a close-to-flat surface on the source material.
  • an adhesive transfer medium layer 260 was applied to the stack.
  • the transfer medium layer 260 has an optional backing layer 270 positioned on the top surface.
  • backing layer 270 is not critical to the operation of the transfer medium 260.
  • thermal release tape is not critical to the operation of the transfer medium 260.
  • the exfoliated source material 280 is then mounted at a designated location to a target substrate 290 as shown in FIG. 2F. If a transparent transfer medium were chosen, such as the elastomer polydimethylsiloxane (PDMS), the technique would be compatible with optical alignment methods which would allow placement of sheets 280 in controlled positions on a target substrate 290.
  • a transparent transfer medium such as the elastomer polydimethylsiloxane (PDMS)
  • the silicon wafer target substrate 290 was prepared to receive the patterned source material 280 with treatment in O2 plasma for 5 minutes (120 W, Diener Electronic Nano). It was then placed on a hot plate at 80 °C for at least five minutes, and an IR gun was used to verify it had reached 80 °C.
  • O2 plasma 120 W, Diener Electronic Nano
  • the use of oxygen plasma and elevated temperature has previously been shown to enhance adhesion of exfoliated 2D materials to SiOx and thereby greatly increase the transferred area of material.
  • the transfer medium was then released from the patterned photoresist layer.
  • the thermal release tape was released by placement of the substrate 290 with the attached handle on to a hot plate at 160 °C to trigger the release of the thermal tape 260.
  • the substrate 290 was then ashed in O2 plasma (3 minutes, 20
  • the remaining gold layer 220 was then stripped in KI/I2 from the monolayers 280 that were adhered to substrate 290 and the sample is rinsed in Dl water.
  • the final result was a pattern of monolayer material 280 adhered to the silicon/silicon oxide substrate 290 as seen in FIG. 2K.
  • the handle layer preferably has a thickness that is sufficient to prevent the adhesive material from contacting any unpatterned portions of the source material when the transfer medium is applied, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
  • the transfer system 300 components are shown in FIG. 3A and the dimensions of the photoresist handle layer with unwanted transfer adhesive contact outside of the handles is shown schematically in FIG. 3B.
  • the dimensions of the metal sheet 310 thickness, photoresist layer 320 height and width and transfer medium 330 thickness can be optimized to avoid overflow on to exposed portions of the layered source material 350.
  • An optional backing layer 340 is also shown in FIG. 3A and FIG. 3B.
  • the transfer adhesive film 330 had a solid backing layer 340 of ⁇ 100 pm thickness coated with an adhesive layer that is ⁇ 50 pm thick.
  • the adhesive layer 330 is much less stiff than the backing layer 340, the adhesive was conservatively modeled as an elastic half-space having the properties of the adhesive layer to overestimate deflections and hence the likelihood of unwanted contact being made.
  • Nanoindentation measurements (Tl 900 Hysitron Tribolndenter) gave an adhesive layer mean Young's modulus of 3 MPa (measured at 30 ⁇ load) and a Poisson's ratio 0.5 was assumed, translating to a plane strain modulus of 4 MPa.
  • the dimensions of the photoresist handle layer 320 can be designed to prevent unwanted adhesive contact outside of the
  • the minimum height of the photoresist layer can be derived with modelling.
  • the thickness of the photoresist handle may be chosen based on contact mechanics modeling so that when thermal-release tape is attached to the top surface of the handle pattern prior to exfoliation, the ⁇ 300 kPa pressure needed to trigger adhesion does not deform the tape enough to touch the source material between the features. In this way, exfoliation occurs only within the desired features.
  • the required handle thickness depends on the elastic modulus of the thermal- release tape as well as the geometries of the features to be exfoliated, and particularly on their spacing.
  • back-gated MOSFET devices were fabricated using transferred material and evaluated.
  • a detailed cross section of a back- gated M0S2 transistor is shown schematically in FIG. 4A and a top view is of an array is shown in FIG. 4B.
  • present disclosure encompasses multiple embodiments which include, but are not limited to, the following:
  • the metal layer of the handle is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver (Ag) and gold (Au).
  • the method comprising: (a) forming a patterned handle over a source material; (b) attaching a transfer medium to the handle; and (c) separating the transfer medium and patterned handle from the source material to remove a pattern of one or more layers of material from the source material; (d) wherein the handle layer prevents removal of unpatterned source material at the same time that the patterned material is removed.
  • the patterned handle comprises: a patterned photoresist handle; and a patterned metal layer mounted to the patterned photoresist handle and coupled to the source material.
  • the metal layer of the handle is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver
  • a method for transferring a pattern of one or more monolayers of a source material on to a substrate comprising: (a) forming a patterned handle over a source material; (b) attaching a transfer medium to the handle; (c) separating the transfer medium and patterned handle from the source material to remove a pattern of one or more monolayers of material from the source material; (d) mounting the pattern of one or more monolayers of material to a substrate; and (e) removing the patterned handle from the mounted monolayers of material on the substrate.
  • the metal coating is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver (Ag) and gold (Au).
  • the transfer medium is selected from the group of a pressure sensitive adhesive, polydimethylsiloxane (PDMS), Poly(methyl
  • the mounting of material to the substrate comprises: placing the pattern of one or more monolayers bound to the patterned handle and transfer medium on a surface of the substrate; applying pressure to transfer medium, patterned handle and pattern of one or more monolayers to mount the pattern of monolayers to the substrate; heating the substrate to release the transfer medium from the patterned handle; and separating the patterned handle from the mounted pattern of one or more monolayers on the substrate.
  • the mounting of material to the substrate comprises: placing the pattern of one or more monolayers bound to the patterned photoresist handle and transfer medium on a surface of the substrate; applying pressure to transfer medium, patterned photoresist handle and pattern of one or more monolayers to mount the pattern of monolayers to the substrate; heating the substrate to release the transfer medium from the patterned photoresist handle; removing the patterned photoresist; and removing the patterned metal coating from the mounted pattern of one or more monolayers on the substrate.
  • the method comprising: forming a handle layer over the patterned material; attaching a transfer medium to the handle layer; and using the transfer medium to remove the handle layer and the patterned material; wherein the handle layer prevents removal of unpatterned source material at the same time that the patterned material is removed.
  • the patterned material comprises a few- or monolayer material and the source material comprises a many-layered van der Waals crystal, such as graphite or molybdenite.
  • a transfer system for removing a patterned material from a source material comprising: a handle layer; and a transfer medium attached to the handle layer; the handle layer attached to the patterned material; wherein the handle layer is configured to prevent removal of unpatterned source material at the same time that the patterned material is removed.
  • set refers to a collection of one or more objects.
  • a set of objects can include a single object or multiple objects.
  • the terms “substantially” and “about” are used to describe and account for small variations.
  • the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation.
  • the terms can refer to a range of variation of less than or equal to ⁇ 10% of that numerical value, such as less than or equal to ⁇ 5%, less than or equal to ⁇ 4%, less than or equal to ⁇ 3%, less than or equal to ⁇ 2%, less than or equal to ⁇ 1 %, less than or equal to ⁇ 0.5%, less than or equal to ⁇ 0.1 %, or less than or equal to ⁇ 0.05%.
  • substantially aligned can refer to a range of angular variation of less than or equal to ⁇ 10°, such as less than or equal to ⁇ 5°, less than or equal to ⁇ 4°, less than or equal to ⁇ 3°, less than or equal to ⁇ 2°, less than or equal to ⁇ 1 °, less than or equal to ⁇ 0.5°, less than or equal to ⁇ 0.1 °, or less than or equal to ⁇ 0.05°.
  • range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified.
  • a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

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Abstract

A transfer system and method for removing a designed pattern of one or more monolayers of material from a bulk layered source material like van der Waals layered materials for placement on a substrate. A stack of a transfer medium, patterned photoresist handle with an optional metal layer and backing layer is used to shape, separate and transfer atomically thin layers of source material. The dimensions of the patterned photoresist handle of the transfer apparatus may be selected to prevent an adhesive transfer medium from contacting unpatterned portions of the source material when the transfer medium is applied, thereby preventing removal of unpatterned source material when the patterned material is separated from the source material.

Description

SELECTIVE TRANSFER OF A THIN PATTERN FROM LAYERED MATERIAL USING A PATTERNED HANDLE
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to, and the benefit of, U.S. provisional patent application serial number62/579,963 filed on November 1 , 2017, incorporated herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under 1636256 awarded by the National Science Foundation. The Government has certain rights in the invention.
NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION
[0003] A portion of the material in this patent document is subject to
copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. § 1.14.
BACKGROUND
[0004] 1 . Technical Field
[0005] The technology of this disclosure pertains generally to van der Waals (vdW) materials and methods of device fabrication, and more particularly to systems and methods for the transfer of 2D layers from source crystals and vdW growth substrates onto target substrates for the van der Waals heterostructures. Transfer, rather than direct growth and patterning on the target substrate, enables low-temperature processing of the transferred materials and target substrate as well as the use of diverse target materials.
[0006] 2. Background Discussion
[0007] Van der Waals crystals are a class of materials that are composed of stacked layers. Individual layers are single- or a few-atoms thick, covalently bonded in the plane and loosely bound to adjacent layers with van der Waals bonds. When isolated from adjacent layers, a single layer or a few layers of these materials are effectively two-dimensional. In this form, they exhibit unique mechanical, electrical, and optical properties, and are thus expected to see widespread adoption in devices across a range of fields.
[0008] The thinness of the layers results in extreme mechanical flexibility and exceptional properties, enabling many applications including in electronics, photonics, and chemical sensing. Interfaces between semiconducting, conducting and insulating 2D materials find applications in, for example, field-effect transistors (FETs), memristive memories, diodes, including light-emitting diodes (LEDs), photodetectors, photovoltaics, and catalysis. Graphene, a single atomic layer of graphite, is capable of ballistic transport of electrons. Other layered materials are also amenable to stable isolation as a single atomic layer (monolayer). Transition metal
dichalcogenides (TMDCs) offer desirable properties as few-layer or monolayer films. For instance, it is only in monolayer form that molybdenum disulfide, a TMDC, exhibits a direct bandgap, which is ideal for applications in optoelectronics, including photovoltaics, and energy storage.
[0009] Accessing the monolayer form in a repeatable fashion, as part of a predictable and high-yield manufacturing process, will be critical to realizing the many potential applications of two-dimensional materials at scale. To fabricate devices made from few- or monolayer materials, layer(s) of material of specified size and shape, arranged in a pre-determined pattern, must be deposited on a desired substrate.
[0010] Fabrication of multi-material 2D structures through sequential vapor- phase deposition, lithography, and etching steps on a single substrate, as performed in conventional semiconductor manufacturing, is fraught with difficulties. First, the use of single-layer vapor-phase deposition
techniques, such as chemical vapor deposition, to deposit one specific 2D material on top of another, while possible, is time-consuming. Continuous layers of uniform thickness may prove impractical to produce because of lattice mismatches or chemical incompatibilities. Second, extremely high etch selectivity is needed when a particular layer of a heterostructure needs to be patterned without destroying those underneath to enable electrical contact, for example. Third, the high temperatures of typically 400-1000 °C that are required for vapor-phase deposition impose challenging thermal budgets and preclude the use of polymeric substrates, which are highly desirable for flexible electronics and would truly take advantage of the inherent flexibility of 2D materials.
[0011] Transfer-based assembly methods have been explored in view of the difficulties posed by conventional direct deposition techniques.
However, there has been no effective method for transferring regions of monolayer material of controlled shape and dimensions from a multilayer source to a substrate.
[0012] One known transfer-based assembly method uses the surface
tension of liquids to maneuver 2D monolayers into position offer limited spatial precision. However, these methods are prone to produce layers that wrinkle and fold and introduce residues at the monolayer-substrate interface.
[0013] Dry transfer (exfoliation) techniques have attempted to harness
normally-applied, shearing, and mixed-mode mechanical stresses to separate material from naturally-occurring and synthetic sources. Several of these methods provide some within-layer dimensional precision, but selectivity of layer thickness when exfoliating from multi-layer sources is typically limited. These techniques are not adaptable to applications where atomic monolayers are generally essential such as for achieving a direct bandgap in M0S2. [0014] What is needed is a technique with precision in all three dimensions, that can handle continuous sheets with lateral dimensions of many tens of micrometers or larger. Such large lateral sheet dimensions are needed for at least two possible purposes: (1 ) to provide material on which can be created integrated circuits with many sub-micron devices in a pre-defined spatial arrangement; or (2) to define the boundaries of, e.g., powerful individual visible light emitters or sensitive detectors requiring dimensions in the tens of microns or larger. Moreover, a process which simultaneously achieves shape selectivity and monolayer selectivity is desirable for forming arrays of heterostructures, by enabling the deposition of a patterned monolayer array at the final substrate (which may already have patterned monolayer arrays on its surface).
BRIEF SUMMARY
[0015] The fabrication of various devices requires the deposition of layers of a few- or monolayer materials of specified size and shape and arranged in a pre-determined pattern on a desired substrate. The material may originate from a bulk, mined source, or from a grown crystal, each composed of many layers. A small number of layers are removed for transfer to a substrate.
[0016] The transfer of 2D layers from source crystals and growth substrates onto target substrates enables low-temperature processing of the target substrates as well as the use of diverse target materials. These two attributes will allow the assembly of vdW heterostructures and the realization of devices exploiting the unique properties of vdW materials.
[0017] The technology provided herein includes methods, systems and
devices for the controlled transfer of 2D layers from source crystals and growth substrates onto target substrates. One embodiment of the manufacturing methods uses gold-mediated exfoliation in conjunction with a lithographically patterned handle layer to transfer arrays of monolayer regions with controlled shape, size, and separation. The approach delivers a far higher areal density of usable, continuous monolayer material than unpatterned exfoliation, and does so in predictable relative locations so that arrays of devices can subsequently be created in a systematic way.
[0018] The methods enable a transfer medium to adhere strictly to
patterned regions, thereby allowing the transfer medium to remove only the patterned material and to leave behind unpatterned bulk. The technology can be used to remove nanometers-thick, laterally patterned material from an initial surface (potentially the material's original source), so that it can be subsequently deposited on a desired substrate. The technology also can used for removal of few- or monolayer material from a many-layered van der Waals crystal, such as graphite, black phosphorus or molybdenite. Other applications will be readily appreciated by those skilled in the art from the disclosure herein.
[0019] In one embodiment, a handle layer is formed over the patterned
material, a transfer medium is attached to the handle layer with an adhesive material, and the transfer medium is used to remove the handle layer and the patterned material. The handle layer preferably has a thickness that is sufficient to prevent the adhesive material from contacting unpatterned portions of the source material when the transfer medium is applied, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed. Transfer media, including pressure-sensitive adhesives and other viscoelastic polymers, require applied pressure to adhere to their target. However, while the transfer medium is illustrated as attached to a possible backing layer, the presence of backing layer is not critical to the operation of the transfer medium.
[0020] An important feature of the methods is the ability to reliably transfer monolayers using a single, controlled exfoliation step. This exfoliation process preferably begins with the deposition of a thin metallic film (e.g. Au) that mediates the process. It has been shown that the metal film can increase the monolayer selectivity of the exfoliation process and has the potential to exfoliate large-area samples.
[0021] In brief, the metallic film does two important things to the exfoliated material. First, it strains only the top layer of the film that is to be exfoliated. This strain leads to two important and sometimes competing effects. The strain changes the effective atomic density of the film. This alters the strength of the van der Waals force (on a per-area basis) between the top and subsurface layers of the exfoliated crystal. Tensile forces weaken the force, and compressive forces increase it. An increased force tends to pull the top layer closer to the second layer and decreases monolayer selectivity for the exfoliation process. A decreased force has the opposite effect.
[0022] Second, the strain in the film also changes the stacking registry of the layer to be exfoliated. Because the lattice parameter of the strained to- be-exfoliated layer differs from that of the layer below, some regions of the exfoliated layer will be in unfavorable stacking positions relative to their initial positions. This weakens the bond between the exfoliated layer and layers beneath. The force density and stacking registry can sometimes compete against one another. In the case of the transition metal
dichalcogenides, the stacking registry changes dominate, and the strain, even though compressive, enhances monolayer selectivity of the exfoliation process.
[0023] The strain effects of the metallic film are present and play a role
even in the exfoliation of nominally infinite monolayers. However, the monolayer-selective exfoliation and transfer of patterned films is further assisted by the additional stiffness of the metallic film.
[0024] In view of these variables, one should be able to design patterned exfoliation processes with enhanced monolayer selectivity for a broad range of 2D van der Waals-bonded materials.
[0025] According to one aspect of the technology, a transfer system for removing a patterned material from a source material is provided utilizing a handle layer and a transfer medium such as an adhesive attached to the handle layer along with an optional backing layer.
[0026] Another aspect of the technology is to provide adaptable methods for removing a patterned material from a source material by forming a handle layer over the patterned material; attaching a transfer medium to the handle layer; and using the transfer medium to remove the handle layer and the patterned material. The handle layer is configured to prevent removal of unpatterned source material while the patterned source material is removed.
[0027] Another aspect of the technology is to provide a laterally patterned material that is nanometers-thick and formed of a few- or monolayer material for placement on a target substrate where the source material is a many-layered van der Waals crystal, such as graphite or molybdenite.
[0028] Further aspects of the technology described herein will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the technology without placing limitations thereon.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0029] The technology described herein will be more fully understood by reference to the following drawings which are for illustrative purposes only:
[0030] FIG. 1 is a schematic functional flow diagram of a method for
producing patterned few or monolayers and transferring them to a target substrate according to one embodiment of the technology.
[0031] FIG. 2A through FIG. 2K collectively depict intermediate structures for method steps for the production and transfer of one or more layers of M0S2 or WS2 to a silicon substrate according to another embodiment of the technology.
[0032] FIG. 3A is a schematic cross-sectional view of the handle for
removing one or more layers of a source material with dimensions configured to prevent overflow of adhesive and removal of unpatterned source material at the same time that the patterned material is removed.
[0033] FIG. 3B is a schematic cross-sectional view of the handle where the dimensions allow contact of adhesive with unpatterned source material. Under the applied pressure needed to ensure good adhesion, the vertical deformation of the transfer medium's adhesive layer is on the same size scale as gaps in the micropattern through which the adhesive may protrude.
[0034] FIG. 4A is a detailed schematic cross-sectional view of a back-gated M0S2 transistor according to one embodiment of technology.
[0035] FIG. 4B is a top view of an array of back-gated M0S2 transistors defined by the patterning process according to one embodiment of the technology.
DETAILED DESCRIPTION
[0036] Referring more specifically to the drawings, for illustrative purposes, embodiments of apparatus, system and methods for fabricating devices made from monolayer or multiple layer materials of specified size and shape and arranged in a pre-determined pattern and deposited on a desired substrate are generally shown. Several embodiments of the technology are described generally in FIG. 1 to FIG. 4B to illustrate the characteristics and functionality of the devices, methods and systems within the context of a semiconductor adaptation. Spatially-controlled transfer of arrays of single-layer M0S2 and WS2 sheets from multilayer crystals onto S1O2 substrates is used to illustrate the technology. It will be appreciated that the methods may vary as to the specific steps and sequence and the systems and apparatus may vary as to structural details without departing from the basic concepts as disclosed herein. The method steps are merely exemplary of the order that these steps may occur. The steps may occur in any order that is desired, such that it still performs the goals of the claimed technology.
[0037] Turning now to FIG. 1 , one embodiment of a method 10 for
transferring 2D layers from source crystals and growth substrates onto target substrates is shown schematically. The method enables a transfer medium to only adhere to patterned regions allowing the transfer medium to remove only the patterned material and to leave behind the unpatterned bulk.
[0038] At block 20 of FIG. 1 , the multi-layer source material to be transferred is selected and prepared. Optionally, the source material can be sized and shaped and the surface providing the layers of material can be leveled to provide an even surface and remove potentially degraded top layers.
[0039] Suitable materials are composed of many layers that are severable and amenable to stable isolation as a single atomic layer (monolayer) or a few monolayers. The material may originate from a bulk, mined source as well as from a grown crystal. Van der Waals solids, composed of layers a few atoms thick and loosely bound by van der Waals bonds to adjacent layers, are particularly preferred. Some van der Waals materials, including graphite, black phosphorous and molybdenite, occur naturally in bulk form, which can be thinned down to their single-layer (monolayer) forms of graphene, phosphorene and molybdenum disulfide, respectively.
[0040] Common 2D or single layer materials include Bismuthene,
Borophene, Germanene, Graphyne, Phosphorene, Silicene, and Stanene. Transition metal Di-chalcogenides (TMDCs), such as M0S2, WS2, MoSe2, WSe2, MoTe2 and PtSe2, are atomically thin semiconductors that have a direct band gap and are materials that have important uses in electronics and optics as transistors or emitters or detectors respectively. Other van der Waals-layered materials include layered insulators, like hexagonal boron nitride.
[0041] Although van der Waals-layered materials are preferred, many other layered materials may be suitable source materials for preparation at block 20 of FIG. 1 .
[0042] Once the source materials are selected and prepared in the step of block 20, the surface of the material is optionally blanketed with a metal layer at block 30. The metal can be coated onto the surface material using conventional techniques such as thermal evaporation, sputter deposition and chemical vapor deposition. The preferred metal of the metal layer that is applied to the source material is gold. However, other metals such as copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn) and silver (Ag) can be used as well. [0043] The optional metallic film that is applied at block 30 creates strain in only the top layer of the source material. The strain changes the effective atomic density of the source material as well as changing the stacking registry of the layer to be exfoliated. The metal film created strain weakens the bond between the exfoliated layer and layers beneath. It has been shown that the deposition of the thin metallic film can increase the monolayer selectivity of the exfoliation process as well as have the potential to allow the exfoliation of large-area samples.
[0044] The application of a blanket of metal to the source material surface is followed by a photolithography process yielding a patterned photoresist handle. This handle serves initially as an etch-mask for the metal film, and then for a brief plasma etch or ion milling step. This etching/milling step is thought to remove a few atomic layers in the source material that creates crack-initiation sites at the edges of the desired features.
[0045] Accordingly, at block 40 a patterned photoresist layer is applied on top of the metal layer. The photoresist layer can be applied and patterned using conventional materials and techniques. The pattern that is designed or selected will ultimately depend on the desired size, shape and
dimensions of the layers of source material to be transferred and the dimensions of the bulk. If a metal layer is not applied, the photoresist layer is applied directly on top of the source material at block 40.
[0046] The patterned photoresist layer at block 40 is referred to as the
"handle layer." The handle layer separates a deformable adhesive transfer medium layer from the source material. The handle layer preferably has a thickness sufficient to prevent the adhesive material from contacting unpatterned portions of the source material when the transfer medium is applied preventing removal of unpatterned source material when the patterned material is removed from the bulk source material with the handle.
[0047] At block 50 of FIG. 1 , the exposed points of the metal layer that are not covered by the patterned photoresist layer are etched, preferably without removing the photoresist. This step at block 50 exposes the source material that will not be transferred and the material that will be transferred will remain masked by the metal film and photoresist layers.
[0048] The source material that is exposed at block 50 with the removal of parts of the metal layer is then processed to remove at least one atomic layer at block 60. What this processing step essentially achieves is control over where interlayer fracture initiates in vdW solids. It is more effective than simply blanketing the source material with metal because in that case there is no control of where inter-layer fracture events will initiate, and it is expected that exfoliated material edges would then correspond to naturally occurring steps in the crystal structure. By introducing etched steps in the material at the edges of the photoresist handles, it is possible to
predetermine, at least to an extent, the locations of fracture.
[0049] One key to maximizing yield may be to minimize the number of intersections of patterned gold metal regions with natural step changes in the height of the source material, so that opportunities for handles to contact multiple layers are reduced. This aim might be achieved by maximizing grain size relative to exfoliated feature size.
[0050] A transfer medium is then applied to the stack of patterned
photoresist layer, etched metal layer and source material at block 70. Typically, pressure-sensitive adhesives, including thermal release tape or
Teflon tape, or other viscoelastic materials, such as polydimethylsiloxane (PDMS) or Poly(methyl methacrylate)(PMMA), can be used as a transfer medium at block 70.
[0051] Optionally, in one embodiment, a rigid, semi-rigid or flexible backing layer is applied to the top surface of the transfer medium. The backing layer is on the opposite side of the transfer medium from the patterned photoresist layer. The backer layer is a platform that distributes applied pressure to the stack and facilitates the transfer.
[0052] The stack of transfer medium, patterned photo resist, metal film and a small number of layers of source material are then mechanically separated from the bulk source material at block 80. In the embodiment using thermal release tape, light pressure is applied, and the tape loaded now with the pattern is peeled from the bulk source material at block 80.
[0053] After exfoliation from the source at block 80, the stack with the few layers or monolayer of source material is then transferred via the
combination of transfer medium and pattern to the target substrate such as S1O2 at block 90. The transfer medium is then removed from the
transferred stack at block 100 and the remaining photoresist and metal film layers are removed thereafter at block 1 10 of FIG. 1 . This process ensures that the interface between the exfoliated material and the target substrate remains dry and never touches any other materials. Moreover, the metal layer prevents direct contact between the transferred monolayer and any organic solids. These steps are used to minimize contamination of the final heterostructure.
[0054] It will be appreciated that this technology allows patterned
nanoscale-layered material to be isolated and transferred using standard pressure-sensitive adhesives or viscoelastic stamps. The ability to use photoresist as a handle layer leverages established processing steps and is CMOS-process-compatible. It also enables the handle layer to be self- aligned to material isolated by etching, using the photoresist as a standard etch mask. The handle layer approach is also amenable to stamp-based methods, where the pattern handle is itself the transfer medium.
[0055] The system and methods allow 3D spatial control over exfoliation that will enable complex integrated circuits to be fabricated more easily from 2D materials. The ability to transfer monolayer sheets with areas >104 pm2 makes the methods particularly appealing for the production of complex heterostructure-based circuits.
[0056] Although the capability for exceptionally large-area transfer has been emphasized, the methods can be used to transfer arrays of much smaller regions of material, e.g. to define many individual sub-micron transistor geometries prior to exfoliation and transfer. The challenge in that case is to ensure a high enough feature yield to be able to construct the desired integrated circuit without missing devices. In contrast, higher functional yields may be achieved by transferring arrays of large monolayer sheets and then defining, e.g., conductive interconnect patterns to create one or more whole integrated circuits within each successfully transferred large monolayer region.
[0057] Although the present process exploits Au-S binding to achieve
monolayer selectivity, the basic mechanism, which hinges on a lattice constant mismatch, is expected to be applicable to other material pairs. Additionally, metal-mediated exfoliation may find itself used in conjunction with other emerging techniques for epitaxy and transfer of thin films to create semiconductor heterostructures.
[0058] The technology described herein may be better understood with reference to the accompanying examples, which are intended for purposes of illustration only and should not be construed as in any sense limiting the scope of the technology described herein as defined in the claims appended hereto.
[0059] Example 1
[0060] To demonstrate the operational principles of the methods for
producing and placing patterned monolayers of a source material on a substrate, arrays of single-layer M0S2 and WS2 sheets from multilayer crystals were transferred onto S1O2 substrates. These sheets had lateral sizes exceeding 100 pm and were electronically continuous. The
preparation and use of the monolayer transfer system is shown
schematically in FIG. 2A through FIG. 2K.
[0061] The technique was demonstrated using chemical-vapor-transport- grown bulk WS2 as well as mined crystals of naturally occurring M0S2 as source materials. The results of the transfer process were examined with optical microscopy, photoluminescence (PL) imaging and spectroscopy, and characterization as a FET channel. Features that were printed using the method were predominantly composed of a monolayer material and included substantial continuous monolayer areas.
[0062] Turning first to FIG. 2A, the method 200 began with the preparation of multilayer source material 210 of M0S2 and WS2. The flattest available sections of material were used. In the case of M0S2, natural, mined crystals were obtained (eBay) and were manually cleaved to create a flake several mm in diameter and a fraction of a millimeter thick. This flake was mounted to a glass slide using double-sided Kapton tape for subsequent processing. Both WS2 and additional M0S2 samples were also obtained as a multi-layer samples fabricated by chemical vapor transport (CVT), ~0.2 - 0.3 mm thick (HQ Graphene) and used as received. The prepared source material 210 was then coated with a 100 nm-thick layer of gold 220 by thermal evaporation (Torr International, Inc.).
[0063] The coating of the source material 210 with blanket gold layer 220 was followed by a photolithography process yielding a patterned photoresist handle 230. As illustrated schematically in FIG. 2B, AZ 4620 photoresist 230 was spun onto the gold-evaporated surface of bulk M0S2 and WS2 flakes using a two-layer process. The resist was baked at 1 10 °C for three minutes following each spin. The resist was exposed with a chrome/glass contact transparency mask for 10 seconds at approximately 20 mW/cm2 and subsequently developed for four minutes in AZ 400K developer predicted 1 :3 with deionized water.
[0064] Patterning of the photoresist layer 230 produces exposed areas 240 of the gold metal layer 220 as seen in FIG. 2C. Without removing the photoresist layer 230, the exposed areas 240 of gold layer 220 was etched for 1 minute in KI/I2 (Gold Etchant TFA, Transene Company, Inc.; used undiluted). This step removes the metal and exposed the M0S2 or WS2 source material 210 that was not going to be transferred, while the to-be- transferred material remained masked by the gold 220 and photoresist 230 layers. The sample was then rinsed in Dl water.
[0065] The exposed areas 250 of WS2 or M0S2 source material from the patterned removal of parts of the gold metal layer is processed further as shown in FIG. 2D. The exposed portions 250 of the patterned source material were then treated by a 30-second etch in CF4 plasma (20 seem, 100 W, Plasma Equipment Technical Services, Inc.) to remove at least one atomic layer of the M0S2 or WS2 from unmasked regions. Initially, a one minute etch time was used to ensure the removal of the top layer, per ref. 37; however, the longer etch time was correlated with large amounts of organic residue on the sample and was adjusted down.
[0066] As an alternative to a plasma etch, an argon ion milling step was also used. Ion milling (Pi Scientific 8" system) was conducted using Argon ion (5 seem RF neutral, 15 seem ion source), with 100 mA beam current, 500 V beam voltage and a 20-degree incidence angle. The duration of the mill was 7 minutes and the pressure was 1 .9 χ 10~4 Torr. In some cases, it was found that the photoresist was easier to remove later when ion milling had been used than when the CF4 etch had been used, possibly because of fluoropoiymer deposition onto the photoresist during CF4 processing.
[0067] Accordingly, the photoresist handle serves initially as an etch-mask for the gold metal layer, and then for a brief plasma etch or ion milling step, which was found to be beneficial. The removal of a few atomic layers of portions of the source material by the etching/milling step shown in FIG. 2D create crack-initiation sites at the edges of the desired features.
[0068] Although the etching or ion milling process does change the
topography of the source material left behind after exfoliation, the source crystal can be re-used, if there are enough layers remaining, by doing a blanket exfoliation with unpatterned thermal release tape to recover a close-to-flat surface on the source material.
[0069] After the milling step shown in FIG. 2D, an adhesive transfer medium layer 260 was applied to the stack. As seen in FIG. 2E, the transfer medium layer 260 has an optional backing layer 270 positioned on the top surface. However, the presence of backing layer 270 is not critical to the operation of the transfer medium 260. Here, thermal release tape
(REVALPHA, Nitto) transfer media was brought into contact with the remaining photoresist pattern 230. Light manual pressure was applied by brushing rubber-tipped tweezers against the back side of the tape, and the tape, loaded now with the pattern, was peeled by hand from the bulk source material 210. Removal of the patterned handle structure from the source material includes the removal of one or more layers 280 of source material 210 attached to the gold metal layer 220 of the handle as seen in FIG. 2E. [0070] The exfoliated source material 280 is then mounted at a designated location to a target substrate 290 as shown in FIG. 2F. If a transparent transfer medium were chosen, such as the elastomer polydimethylsiloxane (PDMS), the technique would be compatible with optical alignment methods which would allow placement of sheets 280 in controlled positions on a target substrate 290.
[0071] The silicon wafer target substrate 290 was prepared to receive the patterned source material 280 with treatment in O2 plasma for 5 minutes (120 W, Diener Electronic Nano). It was then placed on a hot plate at 80 °C for at least five minutes, and an IR gun was used to verify it had reached 80 °C. The use of oxygen plasma and elevated temperature has previously been shown to enhance adhesion of exfoliated 2D materials to SiOx and thereby greatly increase the transferred area of material.
[0072] The separated stack, loaded with the patterned material 280, was positioned and placed onto the heated target substrate as seen in FIG. 2F. Pressure (indicated by arrows) was applied to the tape/substrate stack for 5 minutes using a 6.8 kg weight atop a rubber stopper (area: 1 1 cm2, thickness: 2.54 cm (1 ")). The purpose of the rubber was to distribute the load uniformly over the uneven micro-topography of the patterned tape's surface. The applied pressure was approximately 60 kPa.
[0073] As shown in FIG. 2G, the transfer medium was then released from the patterned photoresist layer. The thermal release tape was released by placement of the substrate 290 with the attached handle on to a hot plate at 160 °C to trigger the release of the thermal tape 260.
[0074] The remaining transferred stack of patterned photoresist 230, gold metal 220, source layers 280 that were adhered to the silicon/silicon oxide substrate 290 was placed in acetone for at least four hours to remove the photoresist 230 layer as shown in FIG. 2H.
[0075] The substrate 290 was then ashed in O2 plasma (3 minutes, 20
seem, 300 W, Plasma Equipment Technical Services, Inc.) to remove any organic residue on the surface. During the ashing step, the remaining gold layer protects underlying M0S2 or WS2 monolayers from damage or removal as seen in FIG. 21.
[0076] As shown in FIG. 2J, the remaining gold layer 220 was then stripped in KI/I2 from the monolayers 280 that were adhered to substrate 290 and the sample is rinsed in Dl water.
[0077] The final result was a pattern of monolayer material 280 adhered to the silicon/silicon oxide substrate 290 as seen in FIG. 2K.
[0078] Example 2
[0079] To further demonstrate the functional principles of the system and methods, the mechanical design of the handle was evaluated.
[0080] During exfoliation of the photoresist/gold/source material stack from the multilayer source, there is a risk of the transfer medium material deforming and penetrating the voids between the patterned photoresist handles. If any penetrating transfer medium material were to come into contact with exposed vdW-bonded material during transfer, it could result in transfer of material occurring outside of the desired locations. This condition is shown in FIG. 3B. Therefore, the handle layer preferably has a thickness that is sufficient to prevent the adhesive material from contacting any unpatterned portions of the source material when the transfer medium is applied, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
[0081] The transfer system 300 components are shown in FIG. 3A and the dimensions of the photoresist handle layer with unwanted transfer adhesive contact outside of the handles is shown schematically in FIG. 3B. The dimensions of the metal sheet 310 thickness, photoresist layer 320 height and width and transfer medium 330 thickness can be optimized to avoid overflow on to exposed portions of the layered source material 350. An optional backing layer 340 is also shown in FIG. 3A and FIG. 3B.
[0082] Therefore, the adhesive deformation process was modeled to ensure that the handle layer is thick enough to prevent unwanted contact. The transfer adhesive film 330 had a solid backing layer 340 of ~100 pm thickness coated with an adhesive layer that is ~50 pm thick. Here, because the adhesive layer 330 is much less stiff than the backing layer 340, the adhesive was conservatively modeled as an elastic half-space having the properties of the adhesive layer to overestimate deflections and hence the likelihood of unwanted contact being made.
[0083] Nanoindentation measurements (Tl 900 Hysitron Tribolndenter) gave an adhesive layer mean Young's modulus of 3 MPa (measured at 30 μΝ load) and a Poisson's ratio 0.5 was assumed, translating to a plane strain modulus of 4 MPa. The baked photoresist was an order of magnitude stiffer than the deformable component of the transfer medium (Etmnsfer medium = 3-4 MPa; Ep otoresist = 36-40 MPa; both measured by nanoindentation). Consequently, the photoresist deformation (<1 % strain expected under uniaxial applied far-field stress σ∞ = 300 kPa, which corresponds to < 150 nm of height compression) was ignored and the model considered only the deformation of the tape material into the regions between photoresist features.
[0084] The condition for roof collapse in a flexible 'stamp' with periodic rectangular protrusions is provided with reference to FIG. 3A and FIG. 3B. For a stamp of protrusion (handle) width 2a, spacing 2w, and a material of plane strain modulus E* exposed to a given far-field stress (σ∞) 360 the required protrusion height (photoresist handle height) h to avoid roof collapse (i.e. unwanted contact) is given by the equation:
Figure imgf000020_0001
[0085] For features of width 2a = 40 pm and inter-feature spacing 2w = 100 pm, under an applied exfoliation pressure σ∞ = 300 kPa (the minimum recommended pressure for ensuring good contact with a pressure-sensitive adhesive) a handle height of 9.9 m is required. A spin recipe was developed to exceed the minimum acceptable value of h and provide 12- 15 pm-thick handles. Layers with thicknesses within this range were produced using a two-layer spin-coat process with AZ 4620 photoresist.
[0086] Accordingly, the dimensions of the photoresist handle layer 320 can be designed to prevent unwanted adhesive contact outside of the
photoresist/metal film handles. The minimum height of the photoresist layer can be derived with modelling. The thickness of the photoresist handle may be chosen based on contact mechanics modeling so that when thermal-release tape is attached to the top surface of the handle pattern prior to exfoliation, the ~300 kPa pressure needed to trigger adhesion does not deform the tape enough to touch the source material between the features. In this way, exfoliation occurs only within the desired features. The required handle thickness depends on the elastic modulus of the thermal- release tape as well as the geometries of the features to be exfoliated, and particularly on their spacing.
[0087] For a given far-field applied stress (o∞) 360, and adhesive layer 330 material properties, a certain minimum h was calculated to prevent unwanted, direct contact between the transfer medium and the unmasked vdW-bonded, layered material 350. In this example, arrays of 320 pm χ 40 μηι features separated by 120 pm are found to need a handle thickness of at least 12 pm, which was readily obtained with spun-on commercial photoresists.
[0088] Example 3
[0089] To further demonstrate the operational principles of the apparatus and methods, back-gated MOSFET devices were fabricated using transferred material and evaluated. A detailed cross section of a back- gated M0S2 transistor is shown schematically in FIG. 4A and a top view is of an array is shown in FIG. 4B.
[0090] Generally, the test structures had a base silicon layer 410 with an oxide layer 420 on the top surface of the base 410. A thin layer 430 of MoS2 was placed over the oxide layer 420 and source and drain electrodes 440 were evaluated. The fabricated back-gated M0S2 transistors had a channel width 450 of up to 40 pm, defined by the patterning process, and a gate length of about 10 pm defined by the Ni contacts 440. The transferred monolayer material 430 was used as the channel, evaporated nickel electrodes 440 served as source and drain contacts, and the p-type silicon substrate and S1O2 functioned as a back gate and a gate dielectric, respectively. Source and drain electrode geometries separated by 10 pm- long channels were defined via photolithography using AZ 4620 photoresist (MicroChem), followed by 40 nm of nickel evaporation and liftoff in acetone.
[0091] A total of twenty devices with functional monolayer M0S2 and
channels were measured (Agilent 4155C Semiconductor Parameter Analyzer; Everbeing probe station and chamber) on two separate
substrates, and their switching characteristics were assessed.
[0092] The average characteristics of devices were shown to differ between the two chips. On Chip 1 , the on/off current ratio was between 104 and 106 orders of magnitude at the smaller VDS of 50 mV, rising to between 105 and 107 at VDS = 1 V. In Chip 2, however, the corresponding on/off current ratios were between 102 and 104 at VDS = 50 mV, and between 103 and 105 at VDS = 1 V.
[0093] Switching characteristics of six WS2 monolayer devices on a single substrate were also evaluated. At VDS = 50 mV, the on/off current ratio is at most 102, while at VDS = 1 V, the ratio is approximately 103 to 105.
[0094] In summary, back-gated FETs exhibit ID-VGS characteristics
confirmed that electrical continuity and semiconducting performance of the monolayers are maintained through the manufacturing process. All devices tested on monolayer material demonstrated switching behavior, with on/off current ratios at VDS = 1 V of between 103 and 107 for M0S2 and between 103 and 105 for WS2 devices.
[0095] From the description herein, it will be appreciated that that the
present disclosure encompasses multiple embodiments which include, but are not limited to, the following:
[0096] 1 . A transfer system for removing a patterned material from a
source material, the system comprising: (a) a patterned handle configured to attach to a source material; and (b) a transfer medium attached to a top surface of the patterned handle; (c) wherein the attached handle is configured to separate a pattern of one or more layers of material from the source material while preventing removal of unpatterned source material. [0097] 2. The transfer system of any preceding or following embodiment, the patterned handle further comprising: a metal layer coupled to a bottom surface of the patterned handle, the metal layer configured to attach to a source material.
[0098] 3. The transfer system of any preceding or following embodiment, wherein the metal layer of the handle is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver (Ag) and gold (Au).
[0099] 4. The transfer system of any preceding or following embodiment, further comprising: a backing layer mounted to the transfer medium.
[00100] 5. The transfer system of any preceding or following embodiment: wherein the transfer medium comprises an adhesive; and wherein the handle separates the transfer medium from the source material sufficiently to prevent the adhesive material from contacting unpatterned portions of the source material, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
[00101] 6. The transfer system of any preceding or following embodiment, wherein the transfer medium is selected from the group of a pressure sensitive adhesive, polydimethylsiloxane (PDMS), Poly(methyl
methacrylate)(PMMA), Teflon tape and thermal release tape.
[00102] 7. The transfer system of any preceding or following embodiment, wherein the source material comprises a many-layered van der Waals crystal material.
[00103] 8. A method for removing a patterned material from a source
material, the method comprising: (a) forming a patterned handle over a source material; (b) attaching a transfer medium to the handle; and (c) separating the transfer medium and patterned handle from the source material to remove a pattern of one or more layers of material from the source material; (d) wherein the handle layer prevents removal of unpatterned source material at the same time that the patterned material is removed.
[00104] 9. The method of any preceding or following embodiment, further comprising mounting a backing layer to a surface of the transfer medium.
[00105] 10. The method of any preceding or following embodiment, wherein the patterned handle comprises: a patterned photoresist handle; and a patterned metal layer mounted to the patterned photoresist handle and coupled to the source material.
[00106] 1 1 . The method of any preceding or following embodiment, wherein the metal layer of the handle is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver
(Ag) and gold (Au).
[00107] 12. The method of any preceding or following embodiment: wherein the transfer medium comprises an adhesive; and wherein the handle separates the transfer medium from the source material sufficiently to prevent the adhesive material from contacting unpatterned portions of the source material, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
[00108] 13. A method for transferring a pattern of one or more monolayers of a source material on to a substrate, the method comprising: (a) forming a patterned handle over a source material; (b) attaching a transfer medium to the handle; (c) separating the transfer medium and patterned handle from the source material to remove a pattern of one or more monolayers of material from the source material; (d) mounting the pattern of one or more monolayers of material to a substrate; and (e) removing the patterned handle from the mounted monolayers of material on the substrate.
[00109] 14. The method of any preceding or following embodiment, wherein the forming of a patterned handle comprises: coating a surface of a source material with a metal; applying a pattern of photoresist to the metal coating; removing portions of the metal coating that are not covered by the photoresist pattern to expose source material.
[00110] 15. The method of any preceding or following embodiment, wherein the metal coating is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver (Ag) and gold (Au). [00111] 16. The method of any preceding or following embodiment, wherein the transfer medium is selected from the group of a pressure sensitive adhesive, polydimethylsiloxane (PDMS), Poly(methyl
methacrylate)(PMMA), Teflon tape and thermal release tape.
[00112] 17. The method of any preceding or following embodiment, further comprising: etching the exposed source material to remove one or more layers of source material.
[00113] 18. The method of any preceding or following embodiment, further comprising: attaching a backing layer to a top surface of the transfer medium opposite of the patterned handle.
[00114] 19. The method of any preceding or following embodiment, wherein the mounting of material to the substrate comprises: placing the pattern of one or more monolayers bound to the patterned handle and transfer medium on a surface of the substrate; applying pressure to transfer medium, patterned handle and pattern of one or more monolayers to mount the pattern of monolayers to the substrate; heating the substrate to release the transfer medium from the patterned handle; and separating the patterned handle from the mounted pattern of one or more monolayers on the substrate.
[00115] 20. The method of any preceding or following embodiment, wherein the mounting of material to the substrate comprises: placing the pattern of one or more monolayers bound to the patterned photoresist handle and transfer medium on a surface of the substrate; applying pressure to transfer medium, patterned photoresist handle and pattern of one or more monolayers to mount the pattern of monolayers to the substrate; heating the substrate to release the transfer medium from the patterned photoresist handle; removing the patterned photoresist; and removing the patterned metal coating from the mounted pattern of one or more monolayers on the substrate.
[00116] 21 . A method for removing a patterned material from a source
material, the method comprising: forming a handle layer over the patterned material; attaching a transfer medium to the handle layer; and using the transfer medium to remove the handle layer and the patterned material; wherein the handle layer prevents removal of unpatterned source material at the same time that the patterned material is removed.
[00117] 22. The method of any preceding or following embodiment: wherein the transfer medium comprises an adhesive; and wherein the handle layer separates the transfer medium from the source material sufficiently to prevent the adhesive material from contacting unpatterned portions of the source material when the transfer medium is applied, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
[00118] 23. The method of any preceding or following embodiment, wherein the patterned material comprises nanometers-thick, laterally patterned material.
[00119] 24. The method of any preceding or following embodiment, wherein the patterned material comprises a few- or monolayer material and the source material comprises a many-layered van der Waals crystal, such as graphite or molybdenite.
[00120] 25. A transfer system for removing a patterned material from a source material, the system comprising: a handle layer; and a transfer medium attached to the handle layer; the handle layer attached to the patterned material; wherein the handle layer is configured to prevent removal of unpatterned source material at the same time that the patterned material is removed.
[00121] 26. The transfer system of any preceding or following embodiment: wherein the transfer medium comprises an adhesive; and wherein the handle layer separates the transfer medium from the source material sufficiently to prevent the adhesive material from contacting unpatterned portions of the source material when the transfer medium is applied, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
[00122] 27. The transfer system of any preceding or following embodiment, wherein the patterned material comprises nanometers-thick, laterally patterned material.
[00123] 28. The transfer system of any preceding or following embodiment, wherein the patterned material comprises a few- or monolayer material and the source material comprises a many-layered van der Waals crystal, such as graphite or molybdenite.
[00124] As used herein, the singular terms "a," "an," and "the" may include plural referents unless the context clearly dictates otherwise. Reference to an object in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more."
[00125] As used herein, the term "set" refers to a collection of one or more objects. Thus, for example, a set of objects can include a single object or multiple objects.
[00126] As used herein, the terms "substantially" and "about" are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ± 10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1 %, less than or equal to ±0.5%, less than or equal to ±0.1 %, or less than or equal to ±0.05%. For example, "substantially" aligned can refer to a range of angular variation of less than or equal to ±10°, such as less than or equal to ±5°, less than or equal to ±4°, less than or equal to ±3°, less than or equal to ±2°, less than or equal to ±1 °, less than or equal to ±0.5°, less than or equal to ±0.1 °, or less than or equal to ±0.05°.
[00127] Additionally, amounts, ratios, and other numerical values may
sometimes be presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.
[00128] Although the description herein contains many details, these should not be construed as limiting the scope of the disclosure but as merely providing illustrations of some of the presently preferred embodiments. Therefore, it will be appreciated that the scope of the disclosure fully encompasses other embodiments which may become obvious to those skilled in the art.
[00129] All structural and functional equivalents to the elements of the
disclosed embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Furthermore, no element,
component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed as a "means plus function" element unless the element is expressly recited using the phrase "means for". No claim element herein is to be construed as a "step plus function" element unless the element is expressly recited using the phrase "step for".

Claims

CLAIMS What is claimed is:
1 . A transfer system for removing a patterned material from a source material, the system comprising:
(a) a patterned handle configured to attach to a source material; and
(b) a transfer medium attached to a top surface of the patterned handle;
(c) wherein the attached handle is configured to separate a pattern of one or more layers of material from the source material while preventing removal of unpatterned source material.
2. The transfer system of claim 1 , the patterned handle further comprising:
a metal layer coupled to a bottom surface of the patterned handle, the metal layer configured to attach to a source material.
3. The transfer system of claim 2, wherein the metal layer of said handle is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver (Ag) and gold (Au).
4. The transfer system of claim 1 , further comprising:
a backing layer mounted to the transfer medium.
5. The transfer system of claim 1 :
wherein the transfer medium comprises an adhesive; and
wherein the handle separates the transfer medium from the source material sufficiently to prevent the adhesive material from contacting unpatterned portions of the source material, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
6. The transfer system of claim 5, wherein the transfer medium is selected from the group of a pressure sensitive adhesive, polydimethylsiloxane (PDMS), Poly(methyl methacrylate)(PMMA), Teflon tape and thermal release tape.
7. The transfer system of claim 1 , wherein the source material comprises a many-layered van der Waals crystal material.
8. A method for removing a patterned material from a source material, the method comprising:
(a) forming a patterned handle over a source material;
(b) attaching a transfer medium to the handle; and
(c) separating the transfer medium and patterned handle from the source material to remove a pattern of one or more layers of material from the source material;
(d) wherein the handle layer prevents removal of unpatterned source material at the same time that the patterned material is removed.
9. The method of claim 8, further comprising:
mounting a backing layer to a surface of the transfer medium.
10. The method of claim 8, wherein the patterned handle comprises: a patterned photoresist handle; and
a patterned metal layer mounted to the patterned photoresist handle and coupled to the source material.
1 1 . The method of claim 10, wherein the metal layer of said handle is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver (Ag) and gold (Au).
12. The method of claim 8:
wherein the transfer medium comprises an adhesive; and
wherein the handle separates the transfer medium from the source material sufficiently to prevent the adhesive material from contacting unpatterned portions of the source material, thereby preventing removal of unpatterned source material at the same time that the patterned material is removed.
13. A method for transferring a pattern of one or more monolayers of a source material on to a substrate, the method comprising:
(a) forming a patterned handle over a source material;
(b) attaching a transfer medium to the handle;
(c) separating the transfer medium and patterned handle from the source material to remove a pattern of one or more monolayers of material from the source material;
(d) mounting the pattern of one or more monolayers of material to a substrate; and
(e) removing the patterned handle from the mounted monolayers of material on the substrate.
14. The method of claim 13, wherein said forming of a patterned handle comprises:
coating a surface of a source material with a metal;
applying a pattern of photoresist to the metal coating;
removing portions of the metal coating that are not covered by the photoresist pattern to expose source material.
15. The method of claim 14, wherein the metal coating is a metal selected from the group consisting of Copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni), tin (Sn), silver (Ag) and gold (Au).
16. The method of claim 14, wherein the transfer medium is selected from the group of a pressure sensitive adhesive, polydimethylsiloxane (PDMS), Poly(methyl methacrylate)(PMMA), Teflon tape and thermal release tape.
17. The method of claim 14, further comprising:
etching said exposed source material to remove one or more layers of source material.
18. The method of claim 13, further comprising:
attaching a backing layer to a top surface of the transfer medium opposite of the patterned handle.
19. The method of claim 13, wherein said mounting of material to the substrate comprises:
placing the pattern of one or more monolayers bound to the patterned handle and transfer medium on a surface of the substrate;
applying pressure to transfer medium, patterned handle and pattern of one or more monolayers to mount the pattern of monolayers to the substrate;
heating the substrate to release the transfer medium from the patterned handle; and
separating the patterned handle from the mounted pattern of one or more monolayers on the substrate.
20. The method of claim 14, wherein said mounting of material to the substrate comprises:
placing the pattern of one or more monolayers bound to the patterned photoresist handle and transfer medium on a surface of the substrate;
applying pressure to transfer medium, patterned photoresist handle and pattern of one or more monolayers to mount the pattern of monolayers to the substrate;
heating the substrate to release the transfer medium from the patterned photoresist handle;
removing the patterned photoresist; and
removing the patterned metal coating from the mounted pattern of one or more monolayers on the substrate.
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