Abstract:
The invention provides methods and apparatus for supporting a substrate in a chemical vapor deposition reactor, and methods and apparatus for synthesizing large area thin films. The invention provides a substrate support assembly comprising at least two interdigitable substrate support fixtures, each fixture carrying at least one finger-like formation for engaging and positioning the substrate during the deposition process that creates the thin film. When two such fixtures are interdigitated, the substrate may be positioned not only in between and around the finger-like substrate engagement members, but also on the outside of each fixture, thus achieving a many-fold increase in the effective width of the substrate.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates broadly to the production of films via chemical vapor deposition, and in particular, to methods and apparatus for forming carbon films and other films using such deposition. More specifically, this invention relates to a substrate support assembly for processing and heating a substrate in a reactor chamber in order to form a large area thin film having an augmented width dimension. 
       BACKGROUND OF THE INVENTION 
       [0002]    Graphene and boron-nitride films are examples of useful large area thin films that may be beneficially produced using the methods and apparatus of the present invention. The invention is particularly useful in the synthesis of graphene, which is a one-atom thick, two-dimensional planar sheet or monolayer in which carbon atoms are bonded in a stable extended fused array, comprising polycyclic aromatic rings with covalently bonded carbon atoms having sp 2  orbital hybridization. The covalently bonded carbon atoms are densely packed in a honeycomb crystal lattice, and may form a 6-membered ring as the basic repeating unit, but 5-membered rings and/or 7-membered rings may also be formed. Graphene has distinctive electrical, mechanical and chemical properties that make it attractive for applications in flexible electronics. For example, electrons may move on a graphene sheet as though they have zero mass, and thus may move at the velocity of light. 
         [0003]    Graphene may be formed on the surface of a substrate by a variety of methods. An exemplary but promising method is set forth in U.S. Patent Application Publication No. 2011/0091647 (the disclosure of which is incorporated by reference herein in its entirety), in which graphene may be produced using a chemical vapor deposition (CVD) process. In general, CVD is a process by which a thin film layer is deposited onto a substrate. The substrate is supported in a vacuum deposition process chamber, and the substrate is heated to a high temperature, typically several hundred degrees Celsius. Deposition gases are then injected into the chamber, and are thermally activated such that a chemical reaction takes place by which a thin film layer is deposited onto the substrate. The substrate on which the thin film layer has been deposited is then cooled to room temperature, after which the thin film layer may be separated from the substrate. 
         [0004]    The aforementioned U.S. Patent Application Publication No. 2011/0091647 discloses a CVD process by which graphene may be formed on a metallic substrate such as copper foil, which is heat-treated in the presence of a gaseous carbon source, specifically, a mixture of a hydrocarbon gas and hydrogen gas. The metallic substrate is loaded into a tube furnace, usually comprising a cavity where the heat treatment is carried out at a specified temperature; the cavity is generally surrounded by heating elements and is in fluid communication with the gaseous sources. It is believed that during the treatment, some of the heated hydrogen gas disassociates into atomic hydrogen, which then reacts with the hydrocarbon gas, typically methane, to form one or more carbon growth species that, when they come into contact with the slightly cooler metallic substrate, form a deposit on the substrate as an thin carbon film (graphene). After a specified period of time the heat treatment is terminated, and the furnace, along with the coated substrate, is then cooled to room temperature, after which the thin film may used directly, with the substrate still attached, or may be separated or transferred from the substrate in a known manner, and then used. 
         [0005]    In a similar manner, thin films of boron-nitride can be obtained by CVD, from precursors such as boron trichloride or boron tribromide and either nitrogen or a source of nitrogen such as ammonia, using a metallic foil substrate. Moreover, in addition to utilizing conventional CVD, graphene films, boron-nitride films and other large area thin films can alternatively be produced using other related processes, such as plasma-enhanced CVD (PECVD), as well as by using similar processes such as atomic layered deposition (ALD). 
         [0006]    In each of these production techniques, the size (i.e., the surface area) of the thin film that is produced is determined by the size of the substrate on which it is grown, and the latter is limited, in turn, only by the dimensions of the reactor chamber or cavity of the CVD apparatus that is used. In general, that chamber is a horizontally-oriented cylindrical quartz tube, and the dimensions of the substrate which may be used are circumscribed by the dimensions of the chamber—the length of the substrate, which is defined as the dimension parallel to the axis of the cylindrical chamber, is determined by the length of that portion of the chamber which can be heated (that is, by the length of the heating zone of the furnace), and the width of the substrate, which is defined as the dimension perpendicular to the length and which is parallel to a radial dimension of the cylindrical chamber, is determined by the diameter of the chamber (that diameter being about equal to the maximum substrate width that can be obtained when the conventional technique of placing a flat sheet of the substrate into the cylindrical chamber is utilized). Although there are relatively few technical difficulties involved in manufacturing very long quartz tubes, and in manufacturing tube furnaces that would accommodate such tubes, the difficulty in manufacturing larger diameter quartz tubes increases dramatically with the increase in diameter of the tube. In addition, the connections between the quartz tube and the surrounding metal parts become more problematic with increased diameters because the manufacturing errors in the size and/or roundness of the quartz tube become magnified. Therefore, there are practical limits as to the diameter of the quartz tube reactor chamber, which in turn limit the width of the substrate that may be utilized therein. 
         [0007]    Accordingly, a technique by which to load a metallic foil substrate into the quartz tube reactor chamber, such that the width of the substrate, and therefore the width of the thin film subsequently produced, can be increased dramatically despite the limits imposed by the diameters of presently-existing reactor chambers for CVD furnaces, would be a useful addition to the technology by which such films are manufactured using CVD or similar processes. Yet, despite the existence and availability of CVD processes for many years, such a technique has eluded researchers. 
         [0008]    Although efforts have been made in the prior art to provide such techniques, those efforts are not completely satisfactory. For example, the prior art includes a technique by which the substrate can be wrapped around a cylindrical holder, but this provides an increase in substrate width of only about three times that of the chamber diameter itself. Moreover, although the aforementioned U.S. Patent Application Publication No. 2011/0091647 appears to disclose that a copper foil substrate about 2 meters in width may be utilized in the production of graphene, thus implying that the graphene film produced could have a comparable width, it has been determined that a flat substrate of such a width dimension would not be workable as a practical matter, due to the manufacturing and other difficulties mentioned above that are associated with producing quartz tube reaction chambers of increased diameter. 
         [0009]    It is therefore the principal object of the present invention to provide improved methods and apparatus for synthesizing large area thin films using a CVD or CVD-type furnace, in which the width dimension of the thin film produced may be greatly increased. 
         [0010]    It is another object of the present invention to provide improved methods of supporting a substrate, and an improved substrate support apparatus for heating a substrate, in the reactor chamber of a CVD or CVD-type furnace. 
       SUMMARY OF THE INVENTION 
       [0011]    These and other objects of the present invention are achieved by providing a substrate support assembly comprising at least two interdigitable substrate support fixtures, each fixture carrying at least one finger-like formation for engaging and positioning the substrate during the deposition process that creates the thin film. Illustratively, when each fixture carries at least three finger-like substrate engagement members, and when two such fixtures are interdigitated, the substrate may be positioned not only in between and around a total of six finger-like substrate engagement members, but also on the outside of each fixture, thus achieving a many-fold increase in the effective width of the substrate. The substrate support fixtures are also adapted to be joined or coupled with one another so as to secure the finger-like substrate engagement members in the interdigitaged position, and such that together the substrate support fixtures may be placed within the conventional cylindrical reaction chamber of a CVD or CVD-type furnace. 
         [0012]    Thus, one aspect of the present invention generally concerns apparatus that are useful for substrate support. In one embodiment of this aspect, an apparatus for synthesizing a large area thin film using CVD is provided, the apparatus including a substrate support assembly comprising at least two interdigitable substrate support fixtures, each carrying at least one finger-like substrate engagement member. 
         [0013]    In another embodiment of this aspect of the invention, an improved substrate support assembly for the synthesis of a large area thin film using CVD is provided, the substrate support assembly including at least two interdigitable substrate support fixtures, each carrying at least one finger-like substrate engagement member. 
         [0014]    In yet another embodiment of this aspect of the invention, an improved apparatus for processing a substrate is provided, the apparatus including at least two interdigitable substrate support fixtures, each carrying at least one finger-like substrate engagement member. 
         [0015]    In still another embodiment of this aspect of the invention, an improved CVD reactor is provided, the reactor including a reactor chamber consisting of a substrate support assembly comprising at least two interdigitable substrate support fixtures, each carrying at least one finger-like substrate engagement member. 
         [0016]    Another aspect of the invention generally concerns improved methods for thin film synthesis. In one embodiment of this aspect of the invention, a method for synthesizing a large area thin film is provided, the method comprising the steps of positioning a substrate for large area thin film synthesis in a substrate support assembly comprising at least two interdigitable substrate support fixtures, each carrying at least one finger-like substrate engagement member such that the substrate is positioned in between and around the finger-like engagement members, and generating a large area thin film on the substrate via a CVD reaction. 
         [0017]    In another embodiment of this aspect of the invention, a method of supporting a substrate in a processing chamber for depositing a large area thin film on the substrate is provided, the method comprising the steps of positioning the substrate in a substrate support assembly comprising at least two interdigitable substrate support fixtures, each carrying at least one finger-like substrate engagement member such that the substrate is positioned in between and around the finger-like engagement members, and generating a large area thin film on the substrate. 
         [0018]    In yet another embodiment of this aspect of the invention, a method of supporting a substrate in a processing chamber for depositing a CVD coating on the substrate is provided, the method comprising the steps of positioning the substrate in a substrate support assembly comprising at least two interdigitable substrate support fixtures, each carrying at least one finger-like substrate engagement member such that the substrate is positioned in between and around the finger-like engagement members, and generating a CVD coating on the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    These and other aspects, features, objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description of the presently most preferred embodiments thereof (which are given for the purposes of disclosure), when read in conjunction with the accompanying drawings (which form a part of the specification, but which are not to be considered as limiting its scope), wherein: 
           [0020]      FIG. 1  is a diagrammatic view depicting the conventional process by which a thin film such as graphene is deposited on a surface of a substrate in the reactor chamber of a CVD device; 
           [0021]      FIG. 2  is a schematic perspective view of a preferred embodiment of the interdigitable substrate support fixture of the present invention, in which three finger-like substrate engagement members are provided; 
           [0022]      FIGS. 3-5  are enlarged schematic cross-sectional views, showing two of the fixtures of  FIG. 2  and depicting the manner in which they may be brought together to form a preferred embodiment of the substrate support assembly of the present invention, as well as the manner in which a typical substrate may be loaded onto that substrate support assembly; 
           [0023]      FIG. 6  is a greatly enlarged schematic cross-sectional view of a portion of the substrate positioned adjacent the curved portion of one of the finger-like substrate engagement members following the deposition of a thin film on to both surfaces of the substrate; 
           [0024]      FIG. 7  is an enlarged schematic cross-sectional views, showing the manner in which a typical substrate may be unloaded from the substrate support assembly of  FIGS. 3-5  after the deposition thereon of a thin film; 
           [0025]      FIGS. 8-9  are schematic cross-sectional views showing the manner in which the a more planar shape may be imparted to the substrate upon which a thin film has been deposited; 
           [0026]      FIG. 10  is a schematic cross-sectional view of an alternative embodiment of the substrate support assembly of the present invention, in which a combined total of fifteen finger-like substrate engagement members are provided; and 
           [0027]      FIG. 11  is a schematic cross-sectional view of another embodiment of the substrate support assembly of the present invention, illustrating an alternative profile or shape. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]    The preferred and other embodiments of the present invention will now be further described. Although the invention will be illustratively described hereinafter with reference to the formation of a large area graphene film on a copper foil substrate in a conventional CVD furnace, in the manner described generally in U.S. Patent Application Publication No. 2011/0091647, it should be understood that the invention is not limited to the specific case described, but extends also to the formation of boron-nitride and other large area thin films, utilizing other metallic foils (including nickel foils or aluminum foils) or other substrates, and using alternative vapor deposition processes such as PECVD or ALD. 
         [0029]    Referring first to  FIG. 1 , the conventional prior art process by which a thin film such as graphene may be deposited on a surface of a flat substrate  10  in the reactor chamber  20  of a CVD furnace  30  having a gas inlet  40  and a gas outlet  50 , in the manner described generally in U.S. Patent Application Publication No. 2011/0091647, is depicted diagrammatically, but for ease of illustration, the substrate holder, heating elements and other components of a conventional CVD furnace have been omitted. It is to be understood that, except for the substrate holder and the configuration of the substrate itself, the present invention utilizes the same conventional process. 
         [0030]    Referring now to  FIG. 2 , a preferred embodiment of one of the interdigitable substrate support fixtures which comprise the substrate support assembly of the present invention is generally designated  100 . Fixture  100  is preferably fabricated of quartz, as are each of its components as hereinafter described. Fixture  100  comprises a base member  101 , which may be a “half-tube,” i.e., generally semi-cylindrical in shape (semi-circular in cross-section). Fixture  100  further comprises at least one finger-like substrate engagement member  102  projecting from and extending radially inwardly from base member  101 . Preferably, fixture  100  carries a plurality of engagement members  102  which are oriented generally parallel to one another, but the maximum number of engagement members (and the exact position of each engagement member) will depend on the diameter chosen for base member  101 . For example, if the diameter of base member  101  is chosen to be  90  mm., then as shown for purposes of illustration in  FIG. 2 , preferably the maximum number of engagement members  102  will be three. 
         [0031]    Each engagement member  102  comprises a tube or rod element  104  and a projecting plate member  106  which, at one edge, is affixed to and supports the associated tube or rod element  104 , and which, at the opposing edge, is affixed to the base member  101 . Preferably, each tube or rod element  104  has a diameter ranging from approximately 10 mm. to approximately 25 mm., and the thickness of each plate member  106  ranges from 2 mm. to 4 mm. The distance between adjacent engagement members  102  in each fixture  100  will depend on the diameter chosen for the tube or rod elements  104  that form a part of each engagement member, but should be chosen so that the engagement members  102  of a first fixture  100  may be selectively mutually interdigitated with the engagement members  102 ′ of a second fixture  100 ′ of similar structure. The base member  101  of each fixture  100  is also adapted to be joined or coupled with the base member  101 ′ of a second fixture  100 ′, such that the two semi-cylindrical base members together form a substantially completed cylinder. 
         [0032]    Referring now to  FIGS. 3-5  in additional to the aforementioned  FIGS. 1-2 , two fixtures  100  and  100 ′ may be brought together and joined or coupled with one another to form a substrate support assembly of the present invention. Engagement members  102  of the first fixture  100  are oriented so as to become interdigitated with the engagement members  102 ′ of the second fixture  100 ′, with sufficient clearance between adjacent interdigitated engagement members such that the tube or rod element  104  of each engagement member  102  of the first fixture  100  passes in between and does not collide with the tube or rod element(s)  104 ′ of the adjacent engagement member(s)  102 ′ of the second fixture  100 ′, as shown best in  FIGS. 3-4 . 
         [0033]    Furthermore, it will be understood that the length of the projecting plate members  106 ,  106 ′ will not be uniform, but will vary from one engagement member  102  to another, and will be chosen so that, when two fixtures  100 ,  100 ′ are joined or coupled with one another, and the substrate engagement members  102  of the first fixture  100  are interdigitated with the substrate engagement members  102 ′ of the second fixture  100 ′, the tube or rod elements  104  of fixture  100  extend to within 1-5 millimeters of, but do not touch, the opposing base member  101 ′ of fixture  100 ′, and similarly, the tube or rod elements  104 ′ of fixture  100 ′ extend to within 1-5 millimeters of, but do not touch, the opposing base member  101  of fixture  100 , all as shown best in  FIG. 4 . 
         [0034]      FIGS. 3-5  also illustrate the manner in which a substrate such as copper foil  108  may be loaded onto the illustrative embodiment of the substrate support assembly of the present invention. As shown in  FIG. 3 , a copper foil substrate  108  is oriented in a generally planar fashion, with one substrate support fixture  100  positioned alongside (or above) one surface of the foil, and with the other substrate support fixture  100 ′ positioned alongside (or below) the opposite surface of the foil. Thereafter the fixtures  100 ,  100 ′ are moved in the direction of the arrows A, A′ in  FIG. 3 , and may be brought together and joined or coupled, either by hand or using tools, to form the substrate support assembly, and as they are brought together and become interdigitated, the copper foil substrate  108  is caused to be wrapped around and engaged by the tube or rod element  104 ,  104 ′ of each of the engagement members  102 ,  102 ′. If the width of copper foil substrate  108  is chosen so as to exceed the diameter of fixtures  100 ,  100 ′, then only the central portion of foil  108  becomes wrapped around and engaged by the tube or rod elements  104 ,  104 ′ of engagement members  102 ,  102 ′, while the two end portions  109  of copper foil substrate  108  extend, on both sides, through and beyond the juncture of fixtures  100 ,  100 ′, as shown best in  FIG. 4 . Thereafter, preferably the entire substrate support assembly (that, is the joined fixtures  100 ,  100 ′) is rotated by substantially 180 degrees, so that each end portion  109  of substrate  108  becomes wrapped around the curved exterior surface of base members  101 ,  101 ′ of fixtures  100 ,  100 ′, respectively. This configuration is preferred, since it maximizes the width of the graphene film that can be produced. 
         [0035]    It is to be understood, however, that both the position of, and the width chosen for, the copper foil substrate  108  will affect whether, and the extent to which, a portion thereof will be wrapped around the curved exterior surfaces of base members  101 ,  101 ′ of respective fixtures  100 ,  100 ′. For example, the copper foil substrate  108  can be positioned such that it extends through and beyond the juncture of fixtures  100 ,  100 ′ on one side (either to the left or to the right, as seen in  FIG. 4 ) but not on the other, and in such case, upon rotation of the substrate support assembly, the copper foil substrate  108  will ultimately be wrapped around the curved exterior surface of only one of base members  101 ,  101 ′ of fixtures  100 ,  100 ′. Also by way of example, if the copper foil substrate  108  is positioned as shown in  FIG. 3 , but the width chosen for the foil is not as great as that shown in  FIG. 3 , but still exceeds the diameter of fixtures  100 ,  100 ′, then upon rotation of the substrate support assembly the substrate  108  will be wrapped only part way around the curved exterior surfaces of base members  101 ,  101 ′ of fixtures  100 ,  100 ′. As a further example, if the width chosen for the foil is substantially equal to the diameter of fixtures  100 ,  100 ′, then upon rotation the substrate  108  will not be wrapped around the curved exterior surfaces of base members  101 ,  101 ′ of fixtures  100 ,  100 ′ at all. 
         [0036]    In addition, and referring now to  FIG. 6  in addition to the aforementioned  FIGS. 1-5 , although the copper foil substrate  108  is wrapped around the respective tube or rod elements  104  of engagement members  102 ,  102 ′ such that, to the naked eye, there does not appear to be a gap between the foil  108  and each tube or rod element  104 , those of skill in the art will understand that a sub-microscopic gap  105  will always exist which will be sufficient to enable a one-atom thick graphene coating  110  to form during the CVD process on the surface of the copper foil substrate  108  that is proximal to the surface of each tube or rod element  104 , as shown best in  FIG. 6 . Thus, when subjected to a CVD process, a continuous graphene coating  110  will be formed across the entire width, and on both surfaces of the substrate  108 , even though the latter is wrapped around successive tube or rod elements  104  during the CVD process, as shown in  FIG. 5 . 
         [0037]    Referring now to  FIGS. 7-9  in addition to the aforementioned  FIGS. 1-6 , it will be seen that  FIG. 7  illustrates the manner in which the copper foil substrate  108  may be unloaded from the illustrative embodiment of the substrate support assembly of the present invention, following the deposition of a graphene coating using a CVD process. First, the ends  109  (if any) of the now-coated copper foil substrate are peeled and unwound from the curved exterior surfaces of base members  101  of fixtures  100 ,  100 ′ (this step is not shown in the drawings). Then, fixtures  100  and  100 ′ are disengaged from one another and are moved in the direction of arrows B, B′ in  FIG. 7 , leaving the now-coated copper foil substrate  108  free. However, the now-coated copper foil substrate  108  will, in general, retain the undulating shape imparted by engagement members  102 ,  102 ′ of fixtures  100 ,  100 ′. Thereafter; as illustrated in  FIGS. 8-9 , the ends of the now-coated copper foil substrate  108  may be pulled in opposite directions, as shown by the arrows C in  FIG. 8 , so as to impart a roughly planar shape. Finally, as shown in  FIG. 9 , the now-coated copper foil substrate  108  may be placed in between a pair of flattening members  112 , each of which is preferably a plate made of glass, and the flattening members  112  may be urged in the directions shown by the arrows D in  FIG. 9 , so as to compress, and impart a more planar shape, to the coated substrate  108 . 
         [0038]    Following the flattening step illustrated in  FIGS. 8-9 , the graphene coating  110  may be separated or transferred from either or both surfaces of the copper foil substrate  108  in a known manner. For example, a salt solution which is an oxidizing agent may be used to exfoliate the graphene coating from the copper foil substrate, after which the separated graphene layers may be utilized in a graphene application or otherwise further processed for ultimate use. 
         [0039]    As mentioned above, if the diameter of base member  101  is chosen to be 90 mm., then as shown in  FIGS. 2-9 , there will preferably be no more than three engagement members  102  in each fixture  100 ,  100 ′, so that there will be six engagement members in total. In this case, after the two fixtures  100 ,  100 ′ are joined or coupled to form a substrate support assembly, with the copper foil substrate  108  mounted therein as shown in  FIG. 5 , this substrate support assembly may be placed into a CVD furnace having a reactor chamber that is approximately 100 mm. in diameter. In this configuration, a copper foil substrate about 750 mm. in width may be used, and consequently, a graphene coating or layer that is about 750 mm. wide may be achieved, which is about 7.5 times the diameter of the CVD reactor chamber. 
         [0040]    In an alternative embodiment of the invention, as shown in  FIG. 10 , if base members  201 ,  201 ′, each with a diameter of 315 mm. is used, then a total of fifteen finger-like substrate engagement members can be accommodated, with eight engagement members in one fixture and seven engagement members in the other. In this case, after the two fixtures are joined or coupled to form a substrate support assembly, with the copper foil substrate mounted therein, this substrate support assembly can be placed into a CVD furnace with a reactor chamber that is approximately 330 mm. in diameter. In this configuration, a copper foil substrate that is about 3,000 mm. in width may be used, and consequently, a graphene coating or layer that is about 3,000 mm. wide may be achieved, which is more than nine times the diameter of the CVD reactor chamber. Hence, the invention provides methods and apparatus for forming graphene films and other thin films that have a greatly enhanced width dimension. 
         [0041]    Although the substrate support assembly of the invention has been described as having a cylindrical shape, it is to be understood that other shapes are possible. For example, as shown in  FIG. 11 , the base members  301 ,  301 ′ of the substrate support fixtures may be configured such that, when they are brought together, the substrate support assembly of the invention has a rectangular cross-section. This profile uses the available space less efficiently, but on the other hand, it presents fewer manufacturing challenges. 
         [0042]    While there has been described what are at present considered to be the preferred embodiments of the present invention, it will be apparent to those skilled in the art that the embodiments described herein are by way of illustration and not of limitation. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. Therefore, it is to be understood that various changes and modifications may be made in the embodiments disclosed herein without departing from the true spirit and scope of the present invention, as set forth in the appended claims, and it is contemplated that the appended claims will cover any such modifications or embodiments.