Patent Application
20140060726
The present invention relates generally to nanotechnology, and, more particularly, to processing methods for graphene and like materials.
Synthesis of graphene on copper foils by chemical vapor deposition (CVD) is a promising method for the synthesis of high quality, large area graphene films. However, graphene applications frequently require a substrate different from copper (e.g., aluminum, nickel, silicon dioxide, sapphire, polyethylene terephthalate (PET)). As a result, a graphene film grown by CVD must frequently be transferred from the substrate on which it was originally grown to a different substrate that is suitable for a particular technological application. In performing this transfer from one substrate to another, it is critical that the graphene film not be degraded (e.g., cracked or torn) or contaminated.
One method of performing such a transfer between substrates utilizes a layer of poly (methyl methacrylate) (PMMA) to support and protect the CVD graphene during the transfer process, although this method is not admitted as prior art by its discussion in this Background Section. Briefly, after depositing a film of graphene on a copper foil substrate by CVD, a layer of PMMA is coated on the graphene to yield a PMMA/graphene/copper film stack. At this point, the copper foil is etched away by floating the film stack on the surface of a copper etchant (e.g., an aqueous solution of iron chloride or iron nitrate) with the copper foil substrate facing downward. After the copper is removed, the remaining PMMA/graphene film stack is lifted off the copper etchant's surface and sequentially floated on several different deionized water baths (e.g., three to ten different deionized water baths) with the graphene still facing downward in order to clean the graphene. Once the graphene is clean, a new substrate is then immersed in the deionized water bath under the PMMA/graphene film stack and lifted upward until the PMMA/graphene film stack rests on the new substrate. A PMMA/graphene/new-substrate film stack is thereby produced. The new film stack is allowed to dry and then the PMMA is selectively stripped (i.e., removed) by acetone to yield a layer of graphene on the new substrate.
While capable of producing the desired result, practicing the above-described substrate transfer process with PMMA has several disadvantages. During processing, the PMMA support layer needs to be thin enough (e.g., several hundred nanometers to several micrometers) to allow the graphene to obtain good adhesion between the PMMA/graphene film stack and the new substrate, and to also allow the PMMA/graphene film stack to be buoyant enough to float. As a result the PMMA support layer is very weak and easy to break. Thus, after the original copper foil substrate is removed and before the new substrate is introduced, the PMMA/graphene film stack needs to be handled very carefully to avoid damage. Generally, the PMMA/graphene film stack cannot be allowed to become freestanding, but instead needs to be left floating on the surface of a liquid bath. During those short periods of time when the film stack is being transferred from one liquid bath to another, the PMMA/graphene film stack needs to be supported by an additional transfer substrate such as a portion of a silicon wafer. In so doing, the PMMA/graphene film stack must stay wet so that it easily releases from the transfer substrate and does not adhere to the transfer substrate. These various constraints make the transport of the PMMA/graphene film stack over long distances difficult. Thus, the ability to produce the PMMA/graphene film stack at one location and then ship it to a remote location for deposition on a new substrate at that remote location is problematic.
PMMA, moreover, is not a particularly attractive material for use in these applications. When PMMA is purchased in solution, it is usually received in a solvent comprising chlorobenzene or anisole, both of which are harmful to human health. Moreover, PMMA tends to leave residues when removed by acetone. As a result, the use of PMMA may require additional processing (e.g., thermal annealing) to obtain a suitably clean graphene film after the PMMA is stripped.
For the foregoing reasons, there is a need for alternative methods of transferring graphene and like materials between substrates that address the above-identified disadvantages.
Embodiments of the present invention address the above-identified needs by providing novel methods of transferring a thin film such as graphene from one substrate to another substrate.
Aspects of the invention are directed to a method of forming a thin film adhered to a target substrate. The method comprises the steps of: (i) forming the thin film on a deposition substrate; (ii) depositing a support layer on the thin film; (iii) removing the deposition substrate without substantially removing the thin film and the support layer; (iv) drying the thin film and the support layer while the thin film is only adhered to the support layer; (v) placing the dried thin film and the dried support layer on the target substrate such that the thin film adheres to the target substrate; and (vi) removing the support layer without substantially removing the thin film and the target substrate.
Advantageously, the above-identified embodiments make the processing of thin films such as graphene easier, safer, and less expensive, while, at the same time, providing a means by which these thin films can be more readily transported and stored before being placed on a final substrate.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
The present invention will be described with reference to illustrative embodiments. For this reason, numerous modifications can be made to these embodiments and the results will still come within the scope of the invention. No limitations with respect to the specific embodiments described herein are intended or should be inferred.
Aspects of the invention are directed to novel methods for transferring a thin film from one substrate to another substrate (a “substrate transfer”). For purposes of this description, it will be assumed that the thin film comprises graphene, although this need not be the case and other materials would also come within the scope of the invention.
In general terms, a processing sequence for accomplishing a substrate transfer in accordance with aspects of the invention can be conceptually divided into two phases.
Notably, while processing sequences falling within the scope of the invention are entirely novel and nonobvious, they still utilize several fabrication techniques (e.g., chemical vapor deposition (CVD), oxygen plasma etching, spray coating, wet etching, and drying) that will already be familiar to one having ordinary skill in, for example, the semiconductor or nanotechnology fabrication arts. Many of these conventional fabrication techniques are also described in readily available publications, such as: W. Choi et al., Graphene: Synthesis and Applications, CRC Press, 2011; D. B. Mitzi, Solution Processing of Inorganic Materials, John Wiley & Sons, 2009; M. Kohler, Etching in Microsystem Technology, John Wiley & Sons, 2008; P. M. Martin, Handbook of Deposition Technologies for Films and Coatings: Science, Applications, and Technology, William Andrew, 2009; and E. Tsotsas et al., Modern Drying Technology: Product Quality and Formulation, John Wiley & Sons, 2011, which are all hereby incorporated by reference herein. The conventional nature of many of the fabrication techniques further facilitates the use of largely conventional and readily available tooling. The CVD described herein may, for example, be performed in a CVD tube furnace available from, for example, MTI Corporation (Richmond, Calif., USA). Oxygen plasma etching may be performed in tools available from several vendors including, as just one example, PlasmaEtch Inc. (Carson City, Nev., USA).
Formation of the support-layer/graphene film stack, and thus the beginning of the first phase of the substrate transfer process, starts in
Subsequent processing acts to deposit a support layer 115 on the structure shown in
Suitable materials for the support layer 115 include, but are not limited to, solutions comprising a polymeric material such as, but not limited to, nitrocellulose, polyurethane, and polycrylic in an appropriate volatile solvent. The support layer 115 may also benefit from the inclusion of a plasticizer such as camphor, which acts to give the hardened material added flexibility. Aspects of the invention were actually reduced to practice with excellent results, for example, utilizing a conventional nail polish acquired from a local drugstore as well as a lacquer acquired from a local hardware store. The nail polish comprised nitrocellulose dissolved in ethyl acetate, butyl acetate, tri benzoin, propyl acetate, acetyl tributyl citrate, and hyroxybenzoate, and further included camphor as a plasticizer. The lacquer, in turn, comprised nitrocellulose dissolved in propane, naptha, toluene, ethylbenzene, xylene, 2-propanol, acetone, methyl ethyl ketone, isopropyl acetate, ethyl 3-ethyoxypropionate, n-buyl acetate, and amyl acetate. Additionally, aspects of the invention were likewise demonstrated utilizing polyurethane- and polycrylic-based support layers. Coating (i.e., deposition) of the support layer 115 may be by spray coating, spin coating, or dip coating. Subsequent to deposition of the support layer 115 as a liquid, hardening can be allowed to occur at room temperature or may be enhanced by mild baking, for example, on a hot plate or under an infrared lamp.
With the support layer 115 now in place over the upper graphene film 100, subsequent processing acts to remove the lower graphene film 105 to yield the structure shown in
The upper graphene film 100 in the structure shown in
Once any remnants of the copper etchant 120 and any etching by-products are sufficiently removed, the upper graphene film 100 and the support layer 115 may be dried. Drying may be accomplished by placing the support layer 115 and the upper graphene film 100 on a drying substrate 135 that has a relatively rough upper surface, as shown in
As was indicated above, the second phase of processing is directed at completing the substrate transfer by utilizing the dried support-layer/graphene film stack 130 created in the first phase of processing to adhere the upper graphene film 100 to a new substrate and ultimately eliminate the support layer 115. Two alternative illustrative methodologies for performing the second phase of processing are now presented with reference to
The first of the illustrative processing sequences for performing the second phase of processing starts in
The second of the illustrative processing sequences for performing the second phase of processing instead starts in
Subsequently, the support-layer/graphene film stack 130 is placed on the now-wetted upper surface of the new substrate 300 with the upper graphene film facing downward 100. This structure is then allowed to dry by evaporation with or without mild baking to produce the structure shown in
It should again be emphasized that the above-described embodiments of the invention are intended to be illustrative only. Other embodiments can use different processing steps and materials for implementing the described functionality and the results would still come within the scope of the invention. These numerous alternative embodiments within the scope of the appended claims will be apparent to one skilled in the art given the teachings herein.
For example, in cleaning the upper graphene film 100 and its support layer 115 while forming the support-layer/graphene film stack 130 in
All the features disclosed herein may be replaced by alternative features serving the same, equivalent, or similar purposes, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
Any element in a claim that does not explicitly state “means for” performing a specified function or “step for” performing a specified function is not to be interpreted as a “means for” or “step for” clause as specified in 35 U.S.C. §112, ¶6. In particular, the use of “step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C. §112, ¶6.
