Abstract:
A method of using ion implantation techniques to create graphene is disclosed. Carbon ions are implanted in a substrate, such as a metal foil, using a plasma doping system or a beam line implanter. The implant is performed at an elevated temperature, to allow a large number of carbon ions to be absorbed by the foil. As the temperature is reduced, the excessive number of carbon atoms causes the foil to be saturated, and the carbon atoms diffuse to the surface, thereby producing graphene. In another embodiment, a plasma doping system is used, where a plasma containing carbon and other species is created. These additional species are also implanted, thereby causing the diffused atoms to contain both carbon and the additional species.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Graphene has recently increased in importance due to its potential applicability for a variety of electronic uses. It has good diffusion barrier properties, making it corrosion resistant. Graphene has good antireflection property with low resistance, allowing it to be used for solar cells. It also has high carrier mobility, allowing it to be used to create transistor channels. Furthermore, it has an acute response to stress, making it suitable for sensor applications. Graphene&#39;s high conductivity and high optical transparency make it an excellent material for such applications as touch screens, and liquid crystal displays. Due to its high surface area to mass ration, graphene may also be used to create ultracapacitors. 
         [0002]    Graphene is a monolayer of carbon atoms arranged in a hexagonal shape, as shown in  FIG. 1 . Each carbon atom is bonded to three adjacent atoms via sp 2  bonding. Graphene synthesis has been achieved on a laboratory scale. One of the first successful attempts to create graphene was done in 2004 by extracting a single layer of carbon from a bulk piece of graphite. Since that time, others have reported creation of small graphene layers through the use of chemical vapor deposition (CVD), typically on nickel substrates. 
         [0003]    The main obstacle presenting the use of graphene in the aforementioned commercial applications is the ability to produce it on a large scale. The creation of large-scale patterns of graphene may be enhanced through the use of an ion implantation technology. Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create a beam of charged ions, which is then directed toward the wafer. As the ions strike the wafer, they impart a charge in the area of impact. This charge allows that particular region of the wafer to be properly “doped”. 
         [0004]      FIG. 2  is a block diagram of a plasma doping system  100 , while  FIG. 3  is a block diagram of a beam-line ion implanter  200 . Those skilled in the art will recognize that the plasma doping system  100  and the beam-line ion implanter  200  are each only one of many examples of differing plasma doping systems and beam-line ion implanters that can provide ions. This process also may be performed with other ion implantation systems or other substrate or semiconductor wafer processing equipment. While a silicon substrate is discussed in many embodiments, this process also may be applied to substrates composed of SiC, GaN, GaP, GaAs, polysilicon, Ge, quartz, or other materials known to those skilled in the art. 
         [0005]    Turning to  FIG. 2 , the plasma doping system  100  includes a process chamber  102  defining an enclosed volume  103 . A platen  134  may be positioned in the process chamber  102  to support a substrate  138 . In one instance, the substrate  138  may be a semiconductor wafer having a disk shape, such as, in one embodiment, a 300 millimeter (mm) diameter silicon wafer. In other embodiments, the substrate may be metal foil. The substrate  138  may be clamped to a flat surface of the platen  134  by electrostatic or mechanical forces. In one embodiment, the platen  134  may include conductive pins (not shown) for making connection to the substrate  138 . 
         [0006]    A gas source  104  provides a dopant gas to the interior volume  103  of the process chamber  102  through the mass flow controller  106 . A gas baffle  170  is positioned in the process chamber  102  to deflect the flow of gas from the gas source  104 . A pressure gauge  108  measures the pressure inside the process chamber  102 . A vacuum pump  112  evacuates exhausts from the process chamber  102  through an exhaust port  110  in the process chamber  102 . An exhaust valve  114  controls the exhaust conductance through the exhaust port  110 . 
         [0007]    The plasma doping system  100  may further include a gas pressure controller  116  that is electrically connected to the mass flow controller  106 , the pressure gauge  108 , and the exhaust valve  114 . The gas pressure controller  116  may be configured to maintain a desired pressure in the process chamber  102  by controlling either the exhaust conductance with the exhaust valve  114  or a process gas flow rate with the mass flow controller  106  in a feedback loop that is responsive to the pressure gauge  108 . 
         [0008]    The process chamber  102  may have a chamber top  118  that includes a first section  120  formed of a dielectric material that extends in a generally horizontal direction. The chamber top  118  also includes a second section  122  formed of a dielectric material that extends a height from the first section  120  in a generally vertical direction. The chamber top  118  further includes a lid  124  formed of an electrically and thermally conductive material that extends across the second section  122  in a horizontal direction. 
         [0009]    The plasma doping system may further include a source  101  configured to generate a plasma  140  within the process chamber  102 . The source  101  may include a RF source  150 , such as a power supply, to supply RF power to either one or both of the planar antenna  126  and the helical antenna  146  to generate the plasma  140 . The RF source  150  may be coupled to the antennas  126 ,  146  by an impedance matching network  152  that matches the output impedance of the RF source  150  to the impedance of the RF antennas  126 ,  146  in order to maximize the power transferred from the RF source  150  to the RF antennas  126 ,  146 . 
         [0010]    The plasma doping system  100  also may include a bias power supply  148  electrically coupled to the platen  134 . The bias power supply  148  is configured to provide a pulsed platen signal having pulse on and off time periods to bias the platen  134 , and, hence, the substrate  138 , and to accelerate ions from the plasma  140  toward the substrate  138  during the pulse on time periods and not during the pulse off periods. The bias power supply  148  may be a DC or an RF power supply. 
         [0011]    The plasma doping system  100  may further include a shield ring  194  disposed around the platen  134 . As is known in the art, the shield ring  194  may be biased to improve the uniformity of implanted ion distribution near the edge of the substrate  138 . One or more Faraday sensors such as an annular Faraday sensor  199  may be positioned in the shield ring  194  to sense ion beam current. 
         [0012]    The plasma doping system  100  may further include a controller  156  and a user interface system  158 . The controller  156  can be or include a general-purpose computer or network of general-purpose computers that may be programmed to perform desired input/output functions. The controller  156  can also include other electronic circuitry or components, such as application-specific integrated circuits, other hardwired or programmable electronic devices, discrete element circuits, etc. The controller  156  also may include communication devices, data storage devices, and software. For clarity of illustration, the controller  156  is illustrated as providing only an output signal to the power supplies  148 ,  150 , and receiving input signals from the Faraday sensor  199 . Those skilled in the art will recognize that the controller  156  may provide output signals to other components of the plasma doping system and receive input signals from the same. The user interface system  158  may include devices such as touch screens, keyboards, user pointing devices, displays, printers, etc. to allow a user to input commands and/or data and/or to monitor the plasma doping system via the controller  156 . 
         [0013]    In operation, the gas source  104  supplies a primary dopant gas containing a desired dopant for implantation into the substrate  138 . The gas pressure controller  116  regulates the rate at which the primary dopant gas is supplied to the process chamber  102 . The source  101  is configured to generate the plasma  140  within the process chamber  102 . The source  101  may be controlled by the controller  156 . To generate the plasma  140 , the RF source  150  resonates RF currents in at least one of the RF antennas  126 ,  146  to produce an oscillating magnetic field. The oscillating magnetic field induces RF currents into the process chamber  102 . The RF currents in the process chamber  102  excite and ionize the primary dopant gas to generate the plasma  140 . 
         [0014]    The bias power supply  148  provides a pulsed platen signal to bias the platen  134  and, hence, the substrate  138  to accelerate ions from the plasma  140  toward the substrate  138  during the pulse on periods of the pulsed platen signal. The frequency of the pulsed platen signal and/or the duty cycle of the pulses may be selected to provide a desired dose rate. The amplitude of the pulsed platen signal may be selected to provide a desired energy. With all other parameters being equal, a greater energy will result in a greater implanted depth. The plasma doping system  100  may incorporate hot or cold implantation of ions in some embodiments. 
         [0015]    Turning to  FIG. 3 , a beam-line ion implanter  200  may produce ions for treating a selected substrate. In one instance, this may be for doping a semiconductor wafer. In another embodiment, this may be for doping a metal foil. In general, the beam-line ion implanter  200  includes an ion source  280  to generate ions that form an ion beam  281 . The ion source  280  may include an ion chamber  283  and a gas box containing a gas to be ionized. The gas is supplied to the ion chamber  283  where the gas is ionized. This gas may be or may include or contain, in some embodiments, hydrogen, helium, other rare gases, oxygen, nitrogen, arsenic, boron, phosphorus, carborane, alkanes, or another large molecular compound. The ions thus generated are extracted from the ion chamber  283  to form the ion beam  281 . A power supply is connected to an extraction electrode of the ion source  280  and provides an adjustable voltage. 
         [0016]    The ion beam  281  passes through a suppression electrode  284  and ground electrode  285  to mass analyzer  286 . Mass analyzer  286  includes resolving magnet  282  and masking electrode  288  having resolving aperture  289 . Resolving magnet  282  deflects ions in the ion beam  281  such that ions of a desired ion species pass through the resolving aperture  289 . Undesired ion species do not pass through the resolving aperture  289 , but are blocked by the masking electrode  288 . 
         [0017]    Ions of the desired ion species pass through the resolving aperture  289  to the angle corrector magnet  294 . Angle corrector magnet  294  deflects ions of the desired ion species and converts the ion beam from a diverging ion beam to ribbon ion beam  212 , which has substantially parallel ion trajectories. The beam-line ion implanter  200  may further include acceleration or deceleration units in some embodiments. 
         [0018]    An end station  211  supports one or more substrates, such as substrate  138 , in the path of ribbon ion beam  212  such that ions of the desired species are implanted into substrate  138 . The substrate  138  may be, for example, a silicon wafer or a solar panel. The end station  211  may include a platen  295  to support the substrate  138 . The end station  211  also may include a scanner (not shown) for moving the substrate  138  perpendicular to the long dimension of the ribbon ion beam  212  cross-section, thereby distributing ions over the entire surface of substrate  138 . Although the ribbon ion beam  212  is illustrated, other embodiments may provide a spot beam. 
         [0019]    The ion implanter  200  may include additional components known to those skilled in the art. For example, the end station  211  typically includes automated substrate handling equipment for introducing substrates into the beam-line ion implanter  200  and for removing substrates after ion implantation. The end station  211  also may include a dose measuring system, an electron flood gun, or other known components. It will be understood to those skilled in the art that the entire path traversed by the ion beam is evacuated during ion implantation. The beam-line ion implanter  200  may incorporate hot or cold implantation of ions in some embodiments. 
         [0020]    As stated above, ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor substrates. A desired impurity material is ionized in an ion source, the ions are accelerated, and the ions are directed at the surface of the substrate. The energetic ions penetrate into the bulk of the material. Following an annealing process, the ions may become incorporated into the crystalline lattice of the semiconductor material to form a region of desired conductivity. 
         [0021]    An efficient, large scale graphene synthesis method is of immense interest to the electronic material industry. Accordingly, it would be beneficial if these proven ion implantation processes could be used to implant carbon atoms into a substrate, which then diffuse to form layers of graphene. It would also be beneficial if additional dopants can also be implanted simultaneously so as to form graphene-based compounds, such as graphane. 
       SUMMARY OF THE INVENTION 
       [0022]    The problems of the prior art are addressed by the present disclosure, which describes a method of using ion implantation techniques to create graphene. Carbon ions are implanted in a substrate, such as a metal foil, using a plasma doping system or a beam line implanter. The implant is performed at an elevated temperature, to allow a large number of carbon ions to be absorbed by the foil. As the temperature is reduced, the excessive number of carbon atoms causes the foil to be saturated, and the carbon atoms diffuse to the surface, thereby producing graphene. In another embodiment, a plasma doping system is used, where a plasma containing carbon and other species is created. These additional species are also implanted, thereby causing the diffused atoms to contain both carbon and the additional species. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which: 
           [0024]      FIG. 1  is a diagram showing the structure of graphene; 
           [0025]      FIG. 2  is a block diagram of a plasma doping system; 
           [0026]      FIG. 3  is a block diagram of a beam-line ion implanter; 
           [0027]      FIG. 4  is a sequence showing the deposition of carbon into a metal foil and the subsequent creation of graphene; 
           [0028]      FIG. 5  is a sequence showing the deposition of carbon into a metal foil when applied in the presence of a mark and the subsequent selective creation of graphene; and 
           [0029]      FIG. 6  is a sequence showing the cleaving process for a substrate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    As stated above, ion implantation is used to deposit ions into a substrate. In many applications, the substrate is a semiconductor material, such as silicon, however this is not a requirement. 
         [0031]    In the present disclosure, the substrate may be a metal or metal foil, such as but not limited to copper, nickel, ruthenium, iron and aluminum. In addition, the substrate can comprise alloys such as but not limited to bronze, brass, and invar, may also be used. 
         [0032]    In one embodiment, carbon ions, in the form of methane gas (CH 4 ) are implanted into the substrate. Other hydrocarbons, such as ethane, propane and others can also be used. The substrate is maintained at an elevated temperature, such as 200° C. to 600° C. or above. This increased temperature increases the solubility limits of carbon in the substrate.  FIG. 4   a  shows a representative substrate being implanted with methane. At elevated temperatures, hydrogen tends to quickly diffuse to the surface, and into the environment, thereby leaving only carbon atoms implanted in the substrate, as shown in  FIG. 4   b.  After the desired amount of atoms has been implanted, the temperature of the substrate is lowered, thereby causing the carbon atoms to precipitate to the surface, as shown in  FIG. 4   c.    
         [0033]    The implant of methane can be performed using a beam line implanter, as shown in  FIG. 3 , or a plasma doping system, as shown in  FIG. 2 . In one embodiment, the substrate is a metal foil, approximately xxx in thickness. The methane being implanted in the metal foil has a specific energy level, which is used to control the depth of the implantation of the carbon atoms within the substrate. In one embodiment, energy levels of between xxx and xxx are used. In addition, the dose of methane used can be varied as well. The dose that the substrate can absorb is dependent on its ambient temperature. Thus, at higher temperatures, more carbon can be introduced into the substrate. Typical doses of carbon atoms may be in the range of 1E15 to 1E17, at temperatures between 200° and 600° C. 
         [0034]    Variations in the dosages and energy level may affect the dopant profile of the carbon within the substrate. These changes in the profile can be used to accelerate or decelerate the precipitation of carbon out of the substrate. For example, a high dose of ions, implanted at a lower energy level will cause a large number of carbon atoms to be implanted just below the surface of the substrate. This amount can be further increased by further elevating the temperature of the substrate. As the temperature of the substrate is reduced, these carbon atoms will diffuse quickly from the substrate. In contrast, a higher implant energy will cause the carbon to be distributed deeper within the substrate, thereby slowing the time to diffuse to the surface. 
         [0035]    Furthermore, the creation and structure of the graphene layers can be tuned by varying the temperature profile during cooling. For example, graphene growth has shown a dependence on the metal substrate crystal orientation. For example, the temperature can be instantaneously decreased, or decreased more slowly at a constant rate. These changes will affect the thickness of the graphene and its growth orientation. 
         [0036]    The use of implantation technology allows for precise control of the carbon concentration and depth. This control allows for finer control of the graphene growth, as the diffusion rate and precipitation can be more tightly controlled. Furthermore, the use of implantation technology, such as beam line implanters and plasma doping systems allows for a variety of dopant profiles. For example, retrograde profiles, surface peak profiles, multiple peak profiles can all be achieved. Each of these may be advantageous in the precipitation of carbon and the creation of graphene. 
         [0037]    Additionally, implantation is commonly used to create doping patterns within a substrate. One such technique is to use a mask to block a portion of the substrate from being exposed to the incoming ions. This technique can also be used to create a specific pattern or shape. For example, as shown in  FIG. 5   a,  a mask can be placed over a portion of the metal foil. The carbon atoms can then be implanted in the exposed portion of the foil. Those portions of the substrate that are shielded by the mask are not implanted. As the temperature is reduced, carbon will precipitate from those portions that were exposed, thereby creating a specific shape or pattern of graphene layers.  FIG. 5   b  shows a cross-sectional view of the graphene layers produced over in those areas that were implanted. The shape and size of the pattern can be varied as desired. 
         [0038]    Since the carbon atoms are being implanted into the substrate, this technique allows the use of lower temperatures than can be used in other methods, such as CVD. Lower temperatures may be advantageous, as the substrate&#39;s grain growth is accelerated at high temperatures, which impacts the creation of graphene. 
         [0039]    Some of graphene&#39;s unique properties result from its atomic structure. In its natural state, there are unbonded electrons at each carbon atom. These unbonded electrons may be bonded to another species to create other useful compounds. Some examples may include graphane, where a hydrogen atom is attached to each carbon atom. Other examples include graphene oxide, where an oxygen atom is attached to each carbon atom. Other compounds may include a halogenized form of graphene. 
         [0040]    Ion implantation also allows the use of ions that contains many species. For example, as described above, methane is used to supply carbon and hydrogen atoms to the substrate. At elevated temperatures, the hydrogen quickly diffuses out of the substrate. However, at lower temperatures, the hydrogen may bond with these unbonded electrons in the carbon atoms to create graphane. 
         [0041]    In another embodiment, oxygen, in the form of xxx, is doped with carbon. This allows the oxygen atoms to attach to the unbonded carbon electrons, yielding graphene oxide. 
         [0042]    In another embodiment, a halogen, such as fluorine, chlorine, bromine, or iodine, is implanted with carbon to create biocompatible phases of graphene. For example, carbon tetrachloride (CCl 4 ) may be used as a source gas. Oxygen and nitrogen may also have the potential to create biocompatible phases of graphene. These altered graphene films could then be used as a passivating layer over implantable devices. 
         [0043]    These multiple species can be implanted in a number of ways. In one embodiment, the species are implanted sequentially. In one words, the methane may be implanted in the substrate first, followed by the additional species. In another embodiment, this order of implantation is reversed. In the case of a sequential implant, the source is simply changed during the implantation process. This can be done using either a plasma doping or beam line implanter. 
         [0044]    In a third embodiment, the carbon and the additional species are simultaneously implanted. In the case of a plasma doping system, the various sources are all combined in the chamber and turned into a plasma. This plasma will contain ions from all of the source gases. In the case of a beamline system, this may be accomplished by eliminating the mass analyzer and allowing all ions to pass from the implanter to the substrate. 
         [0045]    In another embodiment, additional species are implanted to help separate or cleave the graphene from the substrate. There are several methods of performing a cleave process, such as one referred to as “SmartCut”, which is shown in  FIG. 6 . This process is used for many applications, including the preparation of silicon-on-insulator (SOI). Briefly, a semiconductor substrate, such as a wafer  138 , receives a surface treatment to oxide the surface. This creates an insulating layer around the substrate. An ion implantation of hydrogen and/or helium  1000  is then applied to the substrate  138 , as shown in  FIG. 6   b.  The implanted hydrogen or helium ions tend to cause bubbles while the substrate is being annealed. These bubbles may aggregate to form a layer  1001  within the substrate. The depth of this layer is dependent on the concentration and energy of the hydrogen ions, as well as the anneal time. This layer weakens the substrate at that position, allowing it to be cleaved, as shown in  FIG. 6   c.  Either side of the cleaved substrate can be implanted with a second species, if desired, as shown in  FIG. 6   d.  This cleaved interface is then smoothed, using techniques such as chemical-mechanical polishing (CMP). The resulting film and handle substrate is then suitable for use in a SOI process. The remainder of the original semiconductor wafer can be reused to create another thin film, as shown in  FIG. 6   e.    
         [0046]    By introducing helium or hydrogen with, or after, the implantation of carbon, it may be possible to cleave layers of graphene from the substrate as they are formed. 
         [0047]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.