Patent Publication Number: US-2011056433-A1

Title: Device for forming diamond film

Description:
RELATED APPLICATIONS 
     This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 200910190151.7, filed on Sep. 4, 2009 in the China Intellectual Property Office. 
     BACKGROUND 
     1. Technical Field 
     This disclosure relates to devices for forming diamond film, especially to a device for forming diamond film using Hot Filament Chemical Vapor Deposition (HFCVD). 
     2. Description of Related Art 
     Thin films of diamond are known to have great potential for use in a variety of applications due to their exceptional mechanical, thermal, optical and electronic properties. For example, diamond films can be used as semiconductors, transistors, heat sinks, optical coatings, optical devices, electronic devices, as coatings for drill bits and cutting tools, and as inert coatings for prosthetics. Thus, there is considerable incentive to find practical ways to synthesize diamond, especially in film form, for these many and varied applications. 
     Various methods are known for the synthetic production of diamond, including diamond in film form. In particular, the deposition of diamond coatings on substrates to provide films is known. One class of the methods currently developed for synthetic diamond deposition is the low pressure growth of diamond called the chemical vapor deposition (CVD) method. 
     Diamond films have been grown now by using a variety of deposition techniques, such as hot-filament chemical vapor deposition (HFCVD), microwave plasma CVD (MWCVD), plasma jet and flame jet. Among them, HFCVD is the most extensively used one. The advantage of this method is simplicity of the equipment, easiness of process control, relatively low cost of process and potential of scale-up. 
     The HFCVD technique involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen, wherein the hydrocarbon content usually is varied from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750° C. to about 2400° C. The gas mixture disassociates at the filament surface, and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is heated to a temperature in the region of about 500° C. to about 1100° C. 
     However, if metal materials are used as a hot filament at a high temperature, the hot filament is easy to carbonize, deform and become brittle, and the metal at high temperatures evaporates, thus, the diamond films obtained contains impurities. 
     What is needed, therefore, is to provide a device for forming diamond film using HFCVD. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the embodiments can be better understood with references to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a schematic view of one embodiment of a device using a HFCVD method for growing diamond films. 
         FIG. 2  is a schematic view of the device in use. 
         FIG. 3  shows a Scanning Electron Microscope (SEM) image of an untwisted carbon nanotube wire used in the device. 
         FIG. 4  shows an SEM image of a twisted carbon nanotube wire used in the device. 
         FIG. 5  is a schematic structural enlarged view of an outer surface of the carbon nanotube wire used in the device. 
         FIG. 6  is a schematic view of the device in use, wherein the device has a substrate connected in voltage. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
     Referring to  FIG. 1 , one embodiment of an device  10  for forming diamond films includes a reactor chamber  19 , a supporter  199 , a vacuum pump  18 , at least one hot filament  195 , a first electrode  192  and a second electrode  194 . 
     Referring to  FIG. 2 , a substrate  197  can be located on a top surface of the supporter  199  when the device  10  is in use. The substrate  197  is facing the at least one hot filament  195 . The first electrode  192  and the second electrode  194  are electrically connected with the at least one hot filament  195 , such that a voltage can be applied across the at least one hot filament  195  via the first and second electrodes  192 ,  194 . A smooth top surface of the substrate  197  is provided for growing a diamond film thereon. The supporter  199  is used to fix the substrate  197  in the reactor chamber  19 . 
     The reactor chamber  19  receives the supporter  199 , the substrate  197 , the at least one hot filament  195 , and the first and second electrodes  192 ,  194  therein. The reactor chamber  19  has an inlet  191  and an outlet  193 . The inlet  191  is configured to introduce a mixture of hydrocarbon gas and hydrogen into the reactor chamber  19 , thus producing or acting as a source of carbon atoms for growing diamond films (not shown) on the substrate  197 . The supporter  199  is facing and apart from the inlet  191 . The at least one hot filament  195  is located between the inlet  191  and the supporter  199 , and in front of the inlet  191 . The substrate  197  is located on the supporter  199  such that the at least one hot filament  195  is located between the inlet  191  and the substrate  197 . The outlet  193  is connected with the vacuum pump  18  and configured for allowing an exhaust gas to be evacuated/discharged from the reactor chamber  19 . 
     The reactor chamber  19  may have any shape, such as a circular, elliptic, triangular, rectangular, regular polygonal or irregular polygonal configuration. The reactor chamber  19  may be made of a material with a high temperature resistance and chemically stable performance. For example, the reactor chamber  19  may be made of quartz, ceramic, stainless steel or the like. In one embodiment, the reactor chamber  19  is a cylinder with a substantially circular cross section. 
     The supporter  199  can be a rectangular platform base body. The temperature of the supporter  199  can be controlled with a cooling system (not shown) located inside the supporter  199 . The cooling system can be used to control the temperature of the substrate  197  located on the supporter  199 . 
     The at least one hot filament  195  can face the inlet  191  to ensure the mixture gas can be heated sufficiently. In one embodiment, a distance between the at least one hot filament  195  and the supporter  199  is in a range from about 5 millimeters to about 15 millimeters. 
     The first electrode  192  and the second electrode  194  can be located inside the reactor chamber  19 . One portion of the first and second electrodes  192 , 194  can be fixed on and electrically insulated from the reactor chamber  19 . Another portion of the first and the second electrodes  192 , 194  can be connected to the at least one hot filament  195 . To provide a voltage on the least one hot filament  195 , the first electrode  192  and the second electrode  194  can be electrically connected to an electrical source, such as by conductive wires. The first electrode  192  and the second electrode  194  are made of conductive material. The shape of the first electrode  192  or the second electrode  194  is not limited and can be lamellar, rod, wire, and block shaped, among other shapes. A material of the first electrode  192  or the second electrode  194  can be metal, conductive adhesive, and graphite. In one embodiment, the first and the second electrodes  192 ,  194  are copper. 
     The hot filament  195  includes at least one carbon nanotube wire. The carbon nanotube wire comprises a plurality of carbon nanotubes joined with each other via Van der Walls attractive force. The carbon nanotubes are aligned end-to-end along an axis of the carbon nanotube wire and joined by van der Waals attractive force between them. The carbon nanotube wire can be a twisted carbon nanotube wire or untwisted carbon nanotube wire. The carbon nanotubes can be single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, or combinations thereof. 
     The untwisted carbon nanotube wire can be formed by treating the drawn carbon nanotube film with an organic solvent. Specifically, the drawn carbon nanotube film is treated by applying the organic solvent to the drawn carbon nanotube film to soak the entire surface of the drawn carbon nanotube film. After being soaked by the organic solvent, the adjacent paralleled carbon nanotubes in the drawn carbon nanotube film will bundle together, due to the surface tension of the organic solvent when the organic solvent volatilizes. Thus, the drawn carbon nanotube film will be shrunk into untwisted carbon nanotube wire. The organic solvent is volatile. Referring to  FIG. 3 , the untwisted carbon nanotube wire includes a plurality of carbon nanotubes substantially oriented along a same direction (e.g., a direction along the length of the untwisted carbon nanotube wire). The carbon nanotubes are substantially parallel to the axis of the untwisted carbon nanotube wire. Length of the untwisted carbon nanotube wire can be set as desired. The diameter of an untwisted carbon nanotube wire can range from about 0.5 nanometers to about 100 micrometers. In one embodiment, the diameter of the untwisted carbon nanotube wire is about 50 micrometers. Examples of the untwisted carbon nanotube wire is taught by US Patent Application Publication US 2007/0166223 to Jiang et al. 
     The twisted carbon nanotube wire can be formed by twisting a drawn carbon nanotube film by using a mechanical force to turn the two ends of the drawn carbon nanotube film in opposite directions. Referring to  FIG. 4 , the twisted carbon nanotube wire includes a plurality of carbon nanotubes oriented around an axial direction of the twisted carbon nanotube wire. The carbon nanotubes are spirally aligned around the axis of the carbon nanotube twisted wire. Length of the carbon nanotube wire can be set as desired. The diameter of the twisted carbon nanotube wire can range from about 0.5 nanometers to about 100 micrometers. Further, the twisted carbon nanotube wire can be treated with a volatile organic solvent, before or after being twisted. After being soaked by the organic solvent, the adjacent paralleled carbon nanotubes in the twisted carbon nanotube wire will bundle together, due to the surface tension of the organic solvent when the organic solvent volatilizes. The specific surface area of the twisted carbon nanotube wire will decrease, and the density and strength of the twisted carbon nanotube wire will increase. 
     Referring to  FIG. 5 , carbon nanotubes  1953  on an outer surface  1950  of the carbon nanotube wire has a first portion and a second portion. The first portion is fixed on the outer surface  1950 , and the second portion extends out of the outer surface  1950 . A part of the second portion of the carbon nanotubes  1953  may point to the substrate  197 . If there is no substrate  197  in the reactor chamber  19 , a part of the second portion of the carbon nanotubes  1953  can point to the supporter  199 . 
     Referring to  FIG. 6 , a voltage can be applied on the substrate  197  such that a voltage is higher than that of the least one hot filament  195 . Thus, the voltage difference between the at least one hot filament  195  and the substrate  197  can be controlled. The carbon nanotubes  1953  on the outer surface  1950  of the carbon nanotube wire can emit electrons to bombard the substrate  199 . The electron bombardment of the substrate  197  can allow the reacting gas to react easily and produce carbon atoms to form the diamond film on the substrate  197 . 
     Referring to  FIG. 2 , one embodiment of a method for forming a diamond film using the device  10  includes:
     (a) providing the substrate  197  and treating the substrate  197 ;   (b) providing the device  10  and positioning the substrate  197  into the reactor chamber  19 ; and   (c) forming a diamond film on the substrate  197  using a method of HFCVD.   

     In step (a), the substrate  197  can be treated with a diameter of about 0.5 micron diamond powder grinding for about 1 hour to about 2 hours, then placed in acetone solution in the ultrasonic wave for about 10 minutes to about 20 minutes. The material of the substrate  197  can be a material with a high temperature resistance and chemically stable performance. For example, the substrate  197  may be made of quartz, ceramic, stainless steel or the like. In one embodiment, the substrate  197  is a tungsten round sheet with a diameter of about 90 millimeters and a thickness of about 3 millimeters. 
     In step (b), the substrate  197  is positioned on the supporter  199  and facing the hot filament  195 . 
     In step (c), one embodiment of a method for forming the diamond film on the substrate includes:
     (c1) creating a vacuum in the reactor chamber  19  via the vacuum pump  18 ;   (c2) applying a voltage on the hot filament  195  to heat the hot filament  195  to a temperature of about 2200° C.; and   (c3) introducing a mixture of hydrocarbon gas and hydrogen into the reactor chamber  19 , controlling the temperature of the substrate  197  at about 900° C., and growing the diamond film on the substrate  197 .   

     In step (c1), any gases remaining within the reactor chamber  19  may be pumped out by the vacuum pump  18 . 
     In step (c3), the hydrocarbon gas can be a hydrocarbon gas, such as ethylene (C 2 H 4 ), methane (CH 4 ), acetylene (C 2 H 2 ), ethane (C 2 H 6 ), or any combination thereof. In one embodiment, a high purity methane gas diluted with hydrogen gas is introduced into the reactor chamber  19  via the inlet  191 , so as to flow over the hot filament  195  and the substrate  197 . The ratio of methane to the hydrogen gas may vary between about 0.4:99.6 to about 5.0:95. In one embodiment, conditions for the deposition reaction include a chamber temperature between about 700° C. to about 1050° C., a filament current of about 7 to about 9 amperes (amp), filament and substrate spacing from about 2 to about 15 millimeters (mm), total gas flow ranging from about 30 to about 100 Standard Cubic Centimeter per Minute (sccm), pressure ranging from about 20 to about 120 millibars (mB). Typically, the deposition time ranges from about 30 minutes to hundreds of hours, with a diamond film growth rate in the range of about 1 micron per hour. 
     The at least one hot filament  195  of the device  10  for forming diamond films includes at least one carbon nanotube wire. The at least one carbon nanotube wire includes a plurality of carbon nanotubes joined with each other via Van der Vaals attractive force. Due to carbon nanotubes having a property of ideal black bodies, the least one hot filament  195  has excellent electrical conductivity, thermal stability, and high thermal radiation efficiency. As an ideal black body structure, the carbon nanotube wire radiates heat when it reaches a temperature of about 200° C. to about 450° C. The radiating efficiency is relatively high. Furthermore, the carbon nanotube is very pure, and does not include other atoms other than carbon, thus the diamond films obtained is pure. 
     Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the disclosure. Variations may be made to the embodiments without departing from the spirit of the disclosure as claimed. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure.