Abstract:
Disclosed is a method of forming a graphene layer, including: putting a substrate in a chamber of an electron cyclotron resonance device, and then evacuating the chamber. Conducting a carbon-containing gas into the chamber, wherein the carbon-containing gas has a pressure of 10 −2  torr to 10 −4  torr in the chamber. Heating the substrate until the substrate has a temperature of 100° C. to 600° C., and using a microwave with an electron cyclotron resonance mechanism to excite the carbon-containing gas to deposit a graphene layer on the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based on, and claims priority from, Taiwan Application Serial Number 100149777, filed on Dec. 30, 2011, the disclosure of which is hereby incorporated by reference herein in its entirety 
     TECHNICAL FIELD 
     The technical field relates to a method of forming a graphene layer, and in particular relates to a method of forming a graphene layer utilizing an electron cyclotron resonance device. 
     BACKGROUND 
     In 2004, A. K. Geim et al. of the University of Manchester discloses a graphene layer transferred from a highly orientated pyrolytic graphite to a silica substrate by mechanical exfoliation. The graphene layer, having a thickness of an atom, is an actual single-layered graphite. Not only overcomes the law of thermodynamics, the method also provides a way to prepare a graphene layer. However, the method has two major disadvantages: the graphene layer positions are found by human effort with a lot of time, and the graphene connected to graphite (with a quite thickness) has too small of an area to be applied in the semiconductor industry. 
     In recent years, other methods of preparing the graphene layer have been developed, such as pyrolysis epitaxial growing a graphene layer on silicon carbide, oxidation-reduction, chemical vapor deposition, and the likes. Those methods only form a graphene layer with a small area on a substrate of a specific material. Transferring the graphene layer from the growth substrate to another substrate needs different solvents, catalyst, or ultra high temperature pyrolysis. In other words, the transfer process is expensive and time-consuming. 
     Accordingly, a novel method of forming a graphene having a large area on a substrate of any material is called-for. 
     SUMMARY 
     One embodiment of the disclosure provides a method of forming a graphene layer, comprising: putting a substrate in a chamber of an electron cyclotron resonance device, and evacuating the chamber; conducting a carbon-containing gas into the chamber, wherein the carbon-containing gas has a pressure of 10 −2  torr to 10 −4  torr in the chamber; heating the substrate until the substrate has a temperature of 100° C. to 700° C.; and using a microwave with an electron cyclotron resonance mechanism to excite the carbon-containing gas to deposit a graphene layer on the substrate. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIGS. 1A and 1B  show a deposited graphene layer on a substrate having a protrusion in one embodiment of the disclosure; and 
         FIGS. 2A and 2B  show a deposited graphene layer on a substrate having a cavity in one embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     In the disclosure, factors of an electron cyclotron resonance device are tuned to form a graphene layer having a large area on a substrate of an arbitrary material. First, the substrate is put into a chamber of the electron cyclotron resonance device. For a suitable electron cyclotron resonance device, reference may be made to, but is not limited to, ECR (electron cyclotron resonance) plasma for thin film technology,  Pure  &amp;  Appl Chem , Vol. 62, No. 9, p. 1751-1′756, 1990, Production of Large Diameter ECR Plasma,  J. PHYS IV FRANCE  7, Colloque C4, p. 235-246, 1997, or Multi-functional ECR plasma sputtering system for preparing amorphous carbon and Al—O—Si films,  Surface  &amp;  Coating Technology , Vol. 206, p. 1963-1970, 2011. A suitable substrate material can be, for example, metal (e.g. aluminum or copper), compound (e.g. silicon carbide), or non-metal (e.g. glass or plastic) if necessary. In addition, the substrate may have a flat surface, or a protrusion or a concave on its surface. In one embodiment, the substrate surface may have a film thereon to help form the graphene layer. The film can be a dielectric layer, a semiconductor layer, or a metal layer. In one embodiment, a silicon carbide film is disposed on the substrate surface. 
     In one embodiment, the electron cyclotron resonance device is an electron cyclotron resonance device having a single microwave source disclosed by the inventor of this application (See Taiwan Application No. 99121856), or an electron cyclotron resonance device having multi-microwave sources disclosed by the inventor of this application (See Taiwan Application No. 100101138). When the number of the microwave sources is more, the substrate and the graphene layer thereon have a larger area. For example, the electron cyclotron resonance device having a single microwave source can only deposit a graphene layer with an area of 1 cm×1 cm to 30 cm×30 cm on the substrate, and the electron cyclotron resonance device having the multi-microwave sources can deposit a graphene layer with an area of 1 cm×1 cm to 300 cm×300 cm on the substrate. The electron cyclotron resonance device includes a tunable electromagnet or a permanent magnet to provide a magnetic field of 875 Gauss for the electron cyclotron resonance phenomenon. 
     After the substrate is put into a chamber of the electron cyclotron resonance device, the chamber is evacuated. In one embodiment, the chamber is evacuated to have a pressure of 10 −6  torr to 10 −8  torr. An overly high chamber pressure will degrade a subsequently deposited film due to too much residue gas remaining in the chamber. An overly low chamber pressure takes too much time in the evacuating step while not improving upon following processes. 
     A carbon-containing gas is then conducted into the chamber, and the carbon-containing gas has a pressure of 10 −2  torr to 10 −4  torr in the chamber. The carbon-containing gas serves as a carbon source of the graphene layer. The carbon-containing gas can be a hydrocarbon compound such as methane, ethene, ethyne, or combinations thereof. An overly high pressure of the carbon-containing gas may cause an overly fast deposition, thereby forming a multi-layered graphene layer or a few-layered graphene layer. An overly low carbon-containing gas may cause unstable plasma and reduce the quality of deposited film. 
     The substrate is then heated to a temperature of 100° C. to 700° C. A graphene layer cannot be deposited on a glass substrate with an overly high substrate temperature. A graphene layer with a large area cannot be deposited on a glass substrate with an overly low substrate temperature. In one embodiment, the substrate temperature can be lower than 400° C. to utilize a glass substrate, a plastic substrate, or other low temperature substrate. 
     Subsequently, the carbon-containing gas is excited by the single microwave source or the multi-microwave sources to form a carbon-containing plasma. It should be understood that a substrate with a larger area (equal to the deposited graphene layer with a larger area) needs a higher power microwave. If the single microwave source cannot provide an ultra high power microwave, the multi-microwave sources can be adopted. Each source of the multi-microwave sources may provide a high power microwave, respectively, and the high power microwaves from different sources are simulated to the ultra high power microwave for large area deposition. Finally, the graphene with a large area is deposited on the substrate by the carbon-containing plasma. The carbon-containing plasma has a density of 10 10  ions/cm 3  to 10 12  ions/cm 3 . 
     In another embodiment, an inert gas and hydrogen can be further conducted into the chamber before, during, or after the step of depositing the graphene layer on the substrate. The inert gas and the hydrogen will be excited by the microwave to form an inert gas plasma and a hydrogen plasma, respectively. The inert gas can be argon, helium, or combinations thereof. The inert gas has a pressure of 10 −2  torr to 10 −4  torr in the chamber, and the hydrogen has a pressure of 10 −2  torr to 10 −4  torr in the chamber. The inert gas and hydrogen conducted into the chamber before the step of depositing the graphene layer on the substrate may clean the substrate surface and/or chemically reduce the substrate surface. The inert gas and hydrogen conducted into the chamber during the step of depositing the graphene layer on the substrate may avoid an impure phase or amorphous carbon from being formed on the graphene layer surface. The inert gas and hydrogen conducted into the chamber after the step of depositing the graphene layer on the substrate may modify the graphene layer to reduce its electrical resistance. 
     As shown in  FIG. 1A  (or  1 B), the substrate  10  has a protrusion  11 A (or a cavity  11 B), the described processes may deposit a graphene layer conformally on the substrate  10 . In one embodiment, the graphene layer on the surface of the substrate  10  and a top of the protrusion  11 A (or a bottom of the cavity  11 B) is slightly thicker than or equal to that on a sidewall of the protrusion  11 A (or a sidewall of the cavity  11 B). In another embodiment, a bias voltage greater than 50V is simultaneously applied to the substrate  10  when the graphene layer is deposited on the substrate  10 . As shown in  FIGS. 2A and 2B , the graphene layer  13 A on the surface of the substrate  10  and the top of the protrusion  11 A (or the bottom of the cavity  11 B) has a same thickness, which is largely thicker than the graphene layer  13 B on the sidewall of the protrusion  11 A (or the sidewall of the cavity  11 B) 
     Below, exemplary embodiments will be described in detail with reference to accompanying drawings so as to be easily realized by a person having ordinary knowledge in the art. The inventive concept may be embodied in various forms without being limited to the exemplary embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout 
     EXAMPLES 
     Example 1 
     A SiO 2  substrate with an area of 2 cm×2 cm was put into a chamber of an electron cyclotron resonance device of Taiwan Application No. 99121856. The chamber was evacuated to a pressure of 5×10 −6  torr, and ethyne, argon, and hydrogen were then conducted into the chamber until the pressure of the chamber reached 0.00075 torr. The substrate was then heated to 400° C., and a microwave of 1500 W was applied to excite the ethyne to form an ethyne plasma. As a result, a graphene layer with an area of 2 cm×2 cm was deposited on the SiO 2  substrate. The deposition process was performed for 5 minutes. The graphene layer was electrically tested to measure its sheet resistance (about 40kΩ/□). In a Raman spectroscopy of the graphene layer, the graphene layer had an obvious characteristic peak of 2650 cm −1 , and the graphene characteristic peak and a graphite characteristic peak (˜1570 cm −1 ) had an intensity ratio of about 0.1. 
     Example 2-1 
     Similar to Example 1, the only difference in Example 2-1 was the substrate being a quartz substrate with an area of 2 cm×2 cm (commercially available from Shih-Chuan Technology Company, Taiwan). The deposited graphene layer with an area of 2 cm×2 cm was electrically tested to measure its sheet resistance (about 20 kΩ/□). In a Raman spectroscopy of the graphene layer, the graphene layer had an obvious characteristic peak of 2650 cm −1 , and the graphene characteristic peak and a graphite characteristic peak (˜1570 cm −1 ) had an intensity ratio of about 0.15. 
     Example 2-2 
     Similar to Example 1, the only difference in Example 2-2 was the substrate being a glass substrate with an area of 2 cm×2 cm (Eagle2000, commercially available from Corning incorporated, Taiwan). The deposited graphene layer with an area of 2 cm×2 cm was electrically tested to measure its sheet resistance (about 30 kΩ/□). In a Raman spectroscopy of the graphene layer, the graphene layer had an obvious characteristic peak of 2650 cm −1 , and the graphene characteristic peak and a graphite characteristic peak (˜1570 cm −1 ) had an intensity ratio of about 0.13. 
     Example 2-3 
     Similar to Example 1, the only difference in Example 2-3 was the substrate being a pure copper substrate with an area of 2 cm×2 cm (commercially available from Nilaco Corporation, Japan). The deposited graphene layer with an area of 2 cm×2 cm was electrically tested to measure its sheet resistance (about 8 kΩ/□). In a Raman spectroscopy of the graphene layer, the graphene layer had an obvious characteristic peak of 2650 cm −1 , and the graphene characteristic peak and a graphite characteristic peak (˜1570 cm −1 ) had an intensity ratio of less than 1. 
     Example 2-4 
     Similar to Example 1, the differences in Example 2-4 were the substrates being a pure copper substrate with an area of 15 cm×15 cm (commercially available from Nilaco Corporation, Japan), and the electron cyclotron resonance device being that of Taiwan Application No. 100101138. The deposited graphene layer with an area of 15 cm×15 cm was electrically tested to measure its sheet resistance (about 8 kΩ/□). In a Raman spectroscopy of the graphene layer, the graphene layer had an obvious characteristic peak of 2650 cm −1 , and the graphene characteristic peak and a graphite characteristic peak (˜1570 cm −1 ) had an intensity ratio of about 0.6. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed methods and materials. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.