Abstract:
A method to improve the optical clarity of CVD diamond where the CVD diamond is single crystal CVD diamond, by raising the CVD diamond to a set temperature of at least 1500 degrees C. and a pressure of at least 4.0 GPa outside of the diamond stable phase.

Description:
[0001]     The present invention claims the benefit of priority from U.S. Provisional Application Ser. No. 60/486,435 filed Jul. 14, 2003, which is hereby incorporated by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST  
       [[0002]]     This invention was made with U.S. government support under grant number EAR-0135626 from the National Science Foundation. The U.S. government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to annealing diamond, and more particularly, to annealing single crystal CVD diamond.  
         [0005]     2. Discussion of the Related Art  
         [0006]     Chemical vapor deposition growth of diamonds is achieved by imparting energy into a gas-phase carbon containing precursor molecules. For example, microwave energy can be used to create plasma that deposits carbon so as form diamond upon a seed diamond. Up until recently, all CVD techniques for growing diamond resulted in polycrystalline diamond or very thin layers of single crystal diamond. The present inventors of this application have developed a microwave plasma CVD technique to grow large single crystal CVD that is disclosed in copending patent application Ser. No. 10/288,499 filed on Nov. 6, 2002, which is hereby incorporated by reference.  
         [0007]     The present inventors&#39; microwave plasma CVD technique can grow a single crystal diamond on a seed diamond, such as a yellow type Ib HPHT synthetic diamond, at rates of up to 150 micrometers an hour. The color of the diamonds produced by the present inventors&#39; microwave plasma CVD technique depends on the temperature at which the diamond is grown. More particularly, when diamond is grown within a certain temperature range, which is dependent upon the mixture of gases in the plasma, a colorless diamond can be produced. However, diamonds produced at temperatures outside of the certain range are yellow or brown in color.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, the present invention is directed to a annealing a single crystal CVD diamond that substantially obviates one or more problems due to limitations and disadvantages of the related art.  
         [0009]     An object of the present invention is to lighten or remove the color from a single crystal CVD diamond.  
         [0010]     Another object of the present invention is to eliminate defects from a single crystal CVD diamond.  
         [0011]     Another object of the present invention is to enhance the optical characteristics of a single crystal CVD diamond.  
         [0012]     Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned from practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims.  
         [0013]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, a method to improve the optical clarity of single crystal CVD diamond that includes raising the CVD diamond to a set temperature of at least 1500 degrees C. at a pressure of at least 4.0 GPa.  
         [0014]     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Reference will now be made in detail to the preferred embodiments of the present invention.  
         [0016]     The present inventors have used their microwave plasma CVD technique to grow single crystal diamonds with thickness of greater than one millimeter on a seed diamond, such as a type lb {100} HPHT synthetic diamond. In order to enhance the growth rate to about 50-150 μm/h and promote smooth {100} face growth, the single-crystal CVD diamond was grown in an atmosphere of N 2 /CH 4 =0.2-5.0%, CH 4 /H 2 =12-20% with a 120-220 torr total pressure, and at a temperature of 900-1500 degrees C. Raman spectra show a small amount of hydrogenated amorphous carbon (a-C:H) 4  and nitrogen-containing a-C:H(N:a-C:H) 4  that cause the single crystal CVD diamond to have a brown or yellow color when the diamond is grown at &lt;950° C. or &gt;1400° C. Further, photoluminescence (PL) spectra indicate nitrogen-vacancy (N-V) impurities.  
         [0017]     The CVD single crystal diamond grows faster at the higher temperatures of 1400-1500 degrees but the resulting diamond has a brown color. Regardless of the color, the single crystal CVD diamonds resulting from the present inventors&#39; microwave plasma CVD technique can still have impurities. Of course, a lighter colored diamond, a more translucent diamond or a diamond having no color is desired.  
         [0018]     The lightening of the brown color in brown natural diamonds, as well as a decrease in impurities by high pressure high temperature annealing has been reported by I. M. Reinitz et al. Gems &amp; Gemology 36, 128-137 (2000). However, the present inventors have discovered that HPHT annealing of a single crystal yellow or brown CVD diamond at a temperature of 1800-2900° C. and at a pressure of 5-7 GPa for about 1-60 minutes using a reaction vessel in a conventional high pressure high temperature apparatus so as to transform some single crystal brown CVD diamonds into transparent colorless single crystal diamond. More particularly, a single crystal yellow or light brown CVD diamond grown with high growth rate at a temperature of about 1400-1460 degrees C. in an atmosphere containing 4-5% N 2 /CH 4  ratio can be annealed to become a colorless single crystal diamond. Further, Raman and PL spectra of such an annealed CVD diamond demonstrate the disappearance of hydrogenated amorphous carbon and a significant decrease in N-V impurities in such a colorless single crystal diamond. These changes appear to be similar to the report by I. M. Reinitz et al. of transparency enhancement produced by HPHT annealing of brown natural diamond.  
         [0019]     The reaction vessel can be a cell, such as that described in U.S. Pat. Nos. 3,745,623 or 3,913,280, which are hereby incorporated by reference. The entire cell is subjected to pressures in excess of 4.0 GPa, such as 5-7 GPa, and heated to temperature in excess of 1500 degrees C., such as 1800-2900 degrees C., such that the pressure/temperature conditions in the cell or reaction vessel are within the graphite stable phase or just within the diamond stable phase for a time period that can be as short as 1 minute or as long as an hour. The cell is allowed to cool prior to the pressure being released so that the single crystal CVD diamond does not become graphite.  
       EXAMPLE #1  
       [0020]     A single crystal CVD diamond was grown with a N 2 /CH 4  ratio of 5% at a temperature of approximately 1500 degrees C. on a yellow type lb HPHT synthetic diamond at the Carnegie Institution of Washington. The dimension of the single crystal diamond CVD diamond was one centimeter square and a little larger than one millimeter in thickness. The color of the single crystal diamond CVD diamond was brown. The brown single crystal CVD diamond on the type lb HPHT synthetic seed diamond was then placed as a sample in the reaction vessel.  
         [0021]     The reaction vessel was placed in a conventional HPHT apparatus. First, the pressure was increased to a pressure of 5.0 GPa, and then the temperature was rapidly brought up to 2200 degrees C. The sample was maintained at these annealing conditions for five minutes, and then the temperature was decreased over a period of about one minute to room temperature before the pressure was released.  
         [0022]     The sample was removed from the reaction vessel and examined under an optical microscope. The brown single crystal CVD diamond had turned to a light green color and remained firmly bonded to the yellow type lb HPHT synthetic diamond. The yellow color of the synthetic type lb diamond became a lighter yellow or a more translucent yellow.  
       EXAMPLE #2  
       [0023]     Same as example #1 above, except the annealing conditions were maintained for 1 hour. The dark brown single crystal CVD diamond turned to a light green color, which was more translucent than the light green color resulting in example # 1, and remained firmly bonded to the yellow type lb HPHT synthetic diamond. The yellow color of the type lb HPHT synthetic diamond became a lighter yellow or a more translucent yellow.  
       EXAMPLE #3  
       [0024]     A single crystal CVD diamond was grown with a N 2 /CH 4  ratio of 5% at a temperature of approximately 1450 degrees C. on a yellow synthetic type lb diamond at the Carnegie Institution of Washington. The dimension of the single crystal diamond CVD diamond was one centimeter square and a little larger than one millimeter in thick. The color of the single crystal diamond CVD diamond was a light brown or yellow. In other words, a light brown that was not as dark as the brown of the single crystal CVD diamond in example #1 above. The yellow or light brown single crystal CVD diamond on type lb HPHT synthetic seed diamond was then placed as a sample in a reaction vessel.  
         [0025]     The reaction vessel was placed in a conventional HPHT apparatus. The pressure was increased to about to a pressure of 5.0 GPa, and then the temperature was rapidly brought up to about 2000 degrees C. The sample was maintained at these annealing conditions for five minutes, and then the temperature was decreased over a period of about one minute to room temperature before the pressure was released.  
         [0026]     The sample was removed from the reaction vessel and examined under an optical microscope. The light brown or yellow single crystal CVD diamond had become colorless and remained firmly bonded to the yellow type lb HPHT synthetic diamond. The yellow color of the type Ib HPHT synthetic diamond also became a lighter yellow or a more translucent yellow.  
       EXAMPLE #4  
       [0027]     Same as example #1 except a colorless microwave plasma single-crystal CVD-grown diamond in an atmosphere of N 2 /CH 4 =5% at a temperature of ˜1200 degree C. was annealed. After annealing, the microwave plasma single-crystal CVD-grown diamond was blue. This blue microwave plasma single-crystal CVD-grown diamond had a very high toughness of &gt;20 MPa m 1/2 . The hardness was about ˜140 GPa.  
       EXAMPLE #5  
       [0028]     Same as example #1 except a colorless microwave plasma single-crystal CVD-grown diamond in an atmosphere of N 2 /CH 4 =0.5% at a temperature of ˜1200 degree C. was annealed. The microwave plasma single-crystal CVD-grown diamond was still colorless. This colorless microwave plasma single-crystal CVD-grown diamond had a hardness of ˜160 GPa and toughness of ˜10 MPa m 1/2 .  
         [0029]     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the appended claims.