Patent Publication Number: US-2010126406-A1

Title: Production of Single Crystal CVD Diamond at Rapid Growth Rate

Description:
This application claims priority to U.S. Provisional Application No. 61/117,793, filed on Nov. 25, 2008 hereby incorporated by reference. 
    
    
     STATEMENT OF INTEREST 
     This invention was supported by NSF-EAR, NSF-DMR, DOE-NNSA (CDAC) and the Balzan Foundation. The U.S. Government has certain rights to the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing a single crystal diamond at a high growth rate using Microwave Plasma Chemical Vapor Deposition (MPCVD). 
     2. Description of Related Art 
     Large-scale production of synthetic diamond has long been an objective of both research and industry. Diamond, in addition to its gem properties, is the hardest known material, has the highest known thermal conductivity, and is transparent to a wide variety of electromagnetic radiation. These and other characteristics, therefore, make diamond very valuable industrially and open up a wide range of applications in a number of industries, in addition to its well-established value as a gemstone. 
     For at least the last twenty years, a process of producing small quantities of diamond by chemical vapor deposition (CVD) has been available. See B. V. Spitsyn et al., “Vapor Growth of Diamond on Diamond and Other Surfaces,” Journal of Crystal Growth, Vol. 52, pp. 219-226. The process involves CVD of diamond on a substrate by using a combination of methane, or another simple hydrocarbon gas, and hydrogen gas at reduced pressures and temperatures of 800-1200° C. Hydrogen gas is included to prevent the formation of graphite as the diamond nucleates and grows. Growth rates of up to 1 μm/hour were reported with this technique. 
     Subsequent work, for example, that of Kamo et al. as reported in “Diamond Synthesis from Gas Phase in Microwave Plasma,” Journal of Crystal Growth, vol. 62, pp. 642-644, has demonstrated the use of MPCVD to produce diamond at pressures of 1-8 kPa and temperatures of 800-1000° C. with microwave power of 300-700 W at a frequency of 2.45 GHz. A concentration of 1-3% methane gas was used in the process of Kamo et al. Maximum growth rates of 3 μm/hour were reported using this MPCVD process. In the above-described processes, and in a number of other reported processes, the growth rates are limited to only a few micrometers per hour. 
     Until recently, the known higher-growth rate processes only produced polycrystalline forms of diamonds. However, single crystal diamonds offer a variety of advantages over polycrystalline diamonds. Accordingly, considerable interest has been shown in recent years towards developing procedures which enable the fast growth of single-crystal CVD diamond by MPCVD. 1-4  It has been reported, for example, that nitrogen addition to MPCVD reaction chemistry (methane/hydrogen plasma) can significantly enhance the growth of {100} facets and produce smooth and continuous diamond surfaces. 1,5  Yan et al. 1  originally reported high growth rates of up to 100 μm/hr, two orders of magnitude higher than standard processes for making CVD diamond at the time. Since then, efforts have been made to increase the growth rate 3  or expand the growth area 4  for single-crystal CVD diamond. 
     Plasma power density, which is directly associated with microwave power and operating pressure, has been acknowledged as the critical parameter for CVD diamond synthesis. However, because microwave power is normally regulated by the capacity of the microwave power supply, increasing the pressure appears to be the most likely way to increase the growth rate. Grotjohn et al. 6  studied the relationship between power density and pressure up to 80 torr and show a near-linear trend. Chin et al. 6  reported an improvement in growth rate at near 300 torr growth pressure. However, in general, most research groups have focused on growth processes at pressures around 150 torr. 4,8  The addition of nitrogen to the gas chemistry can enhance growth. However, this leads to diamond with a yellowish or light brown color due to broad UV-visible absorption. 1,9  Meng et al. 10,11  have reported that nitrogen-vacancy-hydrogen (NVH-) complex centers are associated with this coloration. The complex center concentration can be reduced by either high-pressure-high-temperature (HPHT) or low-pressure-high-temperature (LPHT) annealing. 
     Improved procedures for making single crystal diamonds using MPCVD are described and claimed in, for example, U.S. Pat. Nos. 6,858,078 and 7,235,130. Further improvements are described and claimed in application Ser. No. 11/438,260. The contents of these earlier filings are incorporated herein by reference. 
     Notwithstanding the various efforts directed towards developing processes which might provide useful forms of single crystal CVD diamond, there remains a need to provide a process wherein CVD diamonds are prepared at a commercially attractive growth rate. 
     An important object of the invention is to provide such a process. Other objects will also be apparent from the following. 
     SUMMARY OF THE INVENTION 
     Broadly stated, the present invention provides an improvement in prior MPCVD procedures enabling the growth of single crystal CVD diamond at a rate of at least 200 μm/hr. The invention is based, to a significant extent, in the finding that a highly useful growth rate of single crystal DVD can be realized by operating the MPCVD process at a pressure in excess of 400 torr or higher, e.g. 410 torr, up to 760 torr (equivalent to 1 atmosphere) and a temperature in the range of 1000°-1500° C. while maintaining the stability of the plasma at an appropriate intensity and power density sufficient to enable the indicated growth rate of over 200 μm/hr. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention is more fully described by reference to the accompanying drawings wherein: 
         FIG. 1  shows OES intensities of CN, C 2 , CH, H, and diamond growth rates at various pressures; 
         FIG. 2  shows the UV-visible absorption coefficient at room temperature (25° C.) for brown, near-colorless and colorless CVD diamonds (SCD-1, SCD-2 and SCD-3, respectively), and for natural type-IIa diamond with a photograph insert of the three CVD diamond crystals; and 
         FIG. 3  shows the photoluminescence (PL) spectra for natural type-IIa diamonds, light brown (SCD-1), near-colorless (SCD-2) and colorless single CVD diamonds (SCD-3). 
     
    
    
     Referring more specifically to the drawings, it is noted that the OES intensities given in  FIG. 1  are normalized to measured values at 80 torr. Diamond growth rates were measured ex-situ by micrometer and then inserted into the plot. 
     In  FIG. 2 , the brown, near-colorless and colorless CVD diamonds are samples SCD-1, SCD-2 and SCD-3, respectively, referred to below. The insert is a photograph of the three single-crystal CVD diamond crystals SCD-1, SCD-2 and SCD-3 with energetic green plasma in the background. Clockwise from the top-right: 1) SCD-1: Light brown, brilliant cut and polished single crystal containing nitrogen (−0.5 carat); 2) SCD-2: Near colorless, 0.2 carat brilliant cut and polished single crystal produced from a −1 carat block; 3) SCD-3: Colorless 1.4 carat bullet shape single crystal produced from a −2.2 carat block. 
     In  FIG. 3 , the intensity scales were normalized to the diamond first order Raman peak. The insert provides detail of the 570-610 nm range for PL spectra (514.5 nm excitation, 300K). 
     It will be appreciated that the invention requires the use of a microwave plasma in an atmosphere comprising hydrogen, a carbon source such as methane or ethane, and preferably oxygen. These are used in the ratios described in the aforementioned application Ser. No. 11/438,260. It is also possible to include a small amount of nitrogen in the atmosphere but, if colorless diamonds are desired, the MPCVD process should be carried out in a nitrogen-free atmosphere. Other materials may also be included in the deposition atmosphere as noted elsewhere in this disclosure. 
     To maximize the diamond rate, it is preferred that the plasma density be in the range of about 10 watt/cm 3  to about 10,000 watt/cm 3  while the power source preferably is operated at 3000 to 5000 W. In one embodiment, the power source is operated at greater than 5 kW or higher. In another embodiment, the power source is operated at 15 kW or higher. In yet another embodiment, the power source is operated at 75 kW or higher. The plasma density should be maintained at the higher end of the indicated range when operating at above 400 torr. It is also critical when operating at pressures above 400 torr to take precautions to maintain the plasma stability and avoid arcing. Measures that can be taken to avoid arcing at pressures over 400 torr can include, for example, using a pulse microwave, operating in a divergent/multi-pole magnetic field, operating with additional components such as argon in the gas stream, and using a waveguide/storage/cavity design with different wavemodes (and then disturbing the modes). This latter measure can sustain a homogeneous stable plasma with a uniform temperature gradient. 
     Operating at higher powers and pressures enables an expansion of the deposition area. For example, the area can be expanded from about 1 inch in diameter when operating at 3 kW to around 3 inches in diameter when operating at higher power. This allows for the use of more than one seed per run, thus promoting mass production of single crystal diamonds. 
     It must be noted that at higher pressures, the plasma ball will become smaller, and higher power is necessary to sustain the size of the plasma. Additionally, it is advantageous to use the holder design with cooling capacity as described and claimed in previously referenced U.S. Pat. Nos. 6,858,078 and 7,234,130. 
     As noted above, the process can be advantageously carried out at the desired pressure in the presence of a small amount of nitrogen (e.g. 0.2 to 3 parts nitrogen per 100 parts of carbon precursor). It has been found that by including nitrogen in the deposition atmosphere, growth rate can be increased by as much as three times the rate obtainable under otherwise similar conditions in the complete absence of nitrogen. The resulting CVD diamond has a brown color due to the presence of the nitrogen. This color can be removed by HTHP (high temperature high pressure) annealing of the diamonds. The brown color can be avoided so as to give colorless high quality diamonds at a somewhat lower growth rate by carrying out the CVD process in the absence of any added nitrogen, i.e. in an atmosphere consisting essentially of a carbon source, hydrogen and an oxygen source. 
     Typically the invention is used to prepare single crystal diamonds of varying dimensions. For, example, the product may be 1-2.5 karats in size. Typically, the diamond may be deposited to a thickness of 10-25 mm thickness, for example, a thickness of 18 mm. The deposition pressure can be varied over a relatively wide range, with 300-350 torr providing representative results although preferably, according to the invention, the pressure is greater than 400 torr to enable the fastest growth rate. The example given herein utilizes pressure varying from 200 to 300 torr as illustrative, it being understood that the growth rate can be increased by increasing the pressure. For example, a growth rate of 165 μm/hr. can be obtained at 300 torr at high power density while an even higher rate can be realized at a pressure over 400 torr. 
     Various substrates can be used in the methods of the invention to prepare single crystal diamonds. For example, the substrate can be a natural diamond or a synthetic diamond, and further can be single crystal or polycrystalline. In certain embodiments the substrate can be, for example, a natural diamond (single crystal or polycrystalline), an HPHT diamond (single crystal or polycrystalline) or a CVD diamond (single crystal or polycrystalline). In preferred embodiments, the substrate can be a single crystal natural diamond, a single crystal HPHT diamond or a single crystal CVD diamond. 
     In certain embodiments of the invention, the crystal orientation of the substrate is 0-15 degrees off {100}. This is thought to increase the nucleation rate and reduce the level of impurities. 
     The gaseous atmosphere may comprise in lieu of oxygen or in addition to it other gases including, but not limited to, argon, CO, CO 2 , boron hydride (B 2 H 6 ), boron nitride or other boron-related material for nitrogen-free deposition. It must be noted that the source of oxygen in the gaseous atmosphere can be any compound which contains an oxygen atom but does not contain a nitrogen atom, including but not limited to O 2 , CO 2 , CO, water and ethanol. 
     As noted above, it is important to maintain an atmosphere which is essentially free of nitrogen. To that end, the methods of the present invention include measures to control the air leakage rate of the deposition chamber to below 0.003 mtorr/min. One such measure is to ensure that the atmosphere surrounding points in the system vulnerable to air leakage (e.g., vacuum connection parts such as viton gaskets) consists of a nitrogen-free gas (e.g., argon or CO 2 ). One means of accomplishing this is to surround the sealing area of vacuum parts with a balloon-type barrier material (e.g., a plastic) filled with a nitrogen-free gas. This will prevent air, which consists largely of nitrogen, from leaking into the sealing. 
     In order to generate a stable, symmetrical and centered plasma in the deposition chamber, it is important to evenly distribute the gas (e.g., H 2 , CH 4 , O 2 ) input and exhaust lines around the chamber. When the gas lines are evenly distributed around the chamber, the plasma will be located in the center of the chamber, as opposed to a location off-center (i.e., closer to the perimeter). 
     The following example shows that high quality brown, near colorless, and colorless single-crystal CVD diamond were grown at optimized condition, evaluated by optical omission spectroscope (OES) and characterized by photoluminescence and UV-visible absorption spectroscopy. The measurements obtained reveal a direct relationship between residual absorption and nitrogen content in the gas chemistry. The high growth rate and colorless single-crystal CVD diamond thus obtained with optical properties comparable to that of natural type-IIa diamond confirms the potential of the process and the resultant product. The fabrication of high quality single-crystal diamond at even higher growth rates, e.g. at 200 μm/h or more, are contemplated with modified reactor design that allows the use of higher gas synthesis pressures of more than 400 torr, e.g. 425 torr. Such modified reactor design includes means for avoiding arcing and the maintenance of a stable plasma density. 
     EXAMPLE 
     A 5 kW, 2.45 GHz ASTEX MPCVD system was used for single crystal diamond synthesis. HPHT synthetic type-Ib and single-crystal CVD diamond with {100} surfaces and minimum surface defects were used as substrates for diamond growth. A hydrogen generator with a palladium purifier was used to produce clean hydrogen with 7N purity. High purity methane (99.9995%) was also used. 
     Brown, near-colorless and colorless single-crystal CVD diamond were synthesized at 500 sccm H 2 , 20-80 sccm CH 4 , and 250-300 torr total pressure at temperatures ranging from 1000° C. to 1500° C., with the microwave power ranging from 3000 W to 5000 W. As the diamond crystals became larger, a higher power/pressure combination was needed to heat up the diamond substrates and holder efficiently and maintain a stable ball-shaped plasma. 2  Details for these three samples can be found in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Growth Details for Single Crystal Diamond 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Weight, 
               
               
                   
                   
                   
                   
                   
                 Weight, 
                 after laser 
               
               
                   
                   
                   
                 Pressure 
                 Growth 
                 before laser 
                 cutting 
               
               
                   
                 Color 
                 N 2 /CH 4   
                 (torr) 
                 Rate (μm/h) 
                 cutting (carat) 
                 (carat) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 SCD-1 
                 Brown 
                 2% 
                 200-220 
                 100-120 
                 1.2 
                 0.54 
               
               
                 SCD-2 
                 Near-colorless 
                 0.02%   
                 220-250 
                 75-95 
                 0.75 
                 0.19 
               
               
                 SCD-3 
                 Colorless 
                 0% 
                 250-300 
                 50-70 
                 2.1 
                 1.4 
               
               
                   
               
            
           
         
       
     
     These three samples were laser cut and polished into gemstone shapes for further analysis. 
     One polished high grade natural type-IIa diamond was also used for comparison. UV-visible absorption spectra for these four samples are presented in  FIG. 3 , along with a photograph of the three single-crystal CVD diamonds used in this example. Nitrogen-doped single-crystal CVD diamond exhibits features typical of brown CVD diamond, including broad bands at 270 nm (substitutional nitrogen), and 370 nm and 550 nm (nitrogen vacancy center). 11  It is clear that the higher nitrogen content in the growth chemistry leads to a higher background in the UV-visible spectra, which implies that defect centers and dislocations induced by nitrogen addition can dramatically affect the color of diamond. With lower nitrogen content in the gas chemistry, the UV-visible line shape becomes more even and the intensities of nitrogen-related bands decreases. There is no significant difference in the Raman lineshape of SCD-3 and the type-IIa natural diamond. The absorption coefficient of SCD-3 is slightly higher, very likely due to nitrogen impurities within the methane gas used for the growth. However, the relative contribution to the residual color and broad spectral features due to point and extended defects induced by growth in the presence of nitrogen were not determined. 10    
     Optical emission spectra (OES) were recorded with an Ocean Optics spectrometer with a 3 mm diameter optical fiber. PL spectra were excited with an argon ion laser at 514.5 nm. The optical properties of these materials were further investigated by micro UV-visible absorption spectroscopy. A Q-switched YAG laser system was used to remove growth layers from diamond substrates. 
     OES is a useful tool for the characterization of CVD diamond growth. 12-15  It has been reported that the emission spectra of H 2 /CH 4 /N 2  plasmas are dominated by the C 2  (d 3 Π g →a 3 Π u ) Swan band system, together with atomic hydrogen emission (Balmer-α transition, H α ) at 656.3 nm, the CN (B 2 Σ+→X 2 Σ + ) system at around 388 nm, and a relatively weak yet detectable emission of CH (A 2 Δ→X 2 Π) at wavelengths of −431.5 nm. 12,13    FIG. 1  shows the measured variation of CN, C 2 , CH, and H α  with pressure ranging from 80 torr to 350 torr, with 10 torr increments. H 2 , CH 4 , and N 2  flow rates were fixed at 500, 50, and 10 sccm, respectively. Microwave power was fixed at 3000 W, and the diamond substrate temperature ranged from 1100° C. to 1300° C. To better understand data evolution, emission intensities for all bands were normalized by measured values at 80 torr. Single-crystal CVD diamond have been synthesized at selected pressures. 
     Visually, the plasma ball significantly shrank and became more intense with increasing pressure. The color of the plasma ball also turned from pale purple (dominated by emission of atomic hydrogen) to intensive green (emission dominated by C 2 ) during the pressure increment. Growth of intensity from CN, C 2 , and CH emissions, imply an increase in plasma density with increasing gas pressures. Growth rates also gradually increased with higher pressure, although a correlation between growth rate and detectable emission bands was not clear. The maximum growth rate of 165 μm/h was obtained at 310 torr, above which the growth rate stabilized and a long-lasting plasma ball was difficult to maintain without introducing a direct discharge between the microwave antenna and the substrate stage (i.e. arcing). It is interesting to note that H α  emission showed a negligible variation throughout the measurement. Several models have been constructed to explain the correlation between the presence of atomic hydrogen and diamond growth rates. 6,14  Atomic hydrogen can etch away the undesired sp 2  phase and help attach hydrocarbon species to the diamond substrate by means of C—H and H—H bonds. The OES observations in this study, however, reveal that, at pressures higher than 150 torr, with the concentration of atomic hydrogen remaining almost constant, the growth rate continued to increase as the pressure went up. On the other hand, it is important to note that up until very recently, most theoretical studies of diamond growth focused on low pressure processes (&lt;150 torr), with low CH 4 /H 2  feed gas ratios (&lt;2%). There is a lack of studies describing the modeling of growth mechanisms at pressures higher than 150 torr. One possible explanation is that CH 3 , another key species in diamond growth, is not detectable by the OES employed in this study. 15,16  From the general trend of carbon related species in the measurements taken, it is proposed that the CH 3  molecular density also increased considerably based on the significant increase in plasma density. This might explain the highly increased growth rate. This view is supported by theoretical calculations, which show a continuous increase of growth rate up to 200 torr. 16  It is worth noting that changes in emission intensities may not directly represent the density of individual species, and more detailed experimental and modeling work would be required to interpret the growth processes at even higher pressures. 
     Photoluminescence spectra measured for nitrogen-doped single-crystal CVD diamond show signatures of obvious nitrogen-vacancy centers at 575 (NV 0 ) and 637 (NV-) nm, along with a broad luminescence background. Similar to what we observed in the UV-visible absorption spectra, the decrease in nitrogen content in the gas chemistry leads to a decrease in the intensities of detectable NV centers, which finally diminish for sample SCD-3. For sample SCD-2, silicon related defects at 735 nm were also detected, and can be attributed to the quartz windows exposed to high heat inside the CVD chamber. 17  The natural type-IIa diamond had a negligible background with the prominent feature being the first-order diamond Raman peak. The second-order Raman feature between 575 and 600 nm was also observed for the type-IIa diamond. SCD-3 grown without nitrogen addition exhibited similar PL spectra to the IIa diamond. It is difficult to distinguish SCD-3 from high quality type-IIa diamond based on these spectra. 
     For natural brown IIa diamond, the color is normally considered to be the consequence of extensive plastic deformation, 18,19  and type-Ia diamond with high nitrogen content (&gt;100 ppm) can be found either brown or colorless. This indicates that nitrogen might not be a direct factor in determining diamond color. However, it is clear that the amount of nitrogen content in the growth chemistry directly determines the number of nitrogen induced defects and impurities, and consequently the diamond visible absorption. The nitrogen content in high growth rate CVD diamond is generally very low (&lt;10 ppm) (i.e., in the range of natural type-IIa diamond). 20  Meng et al. 10  correlated the evolution of visible absorption in diamond with the nitrogen-vacancy-hydrogen (NVH-) complex defects by means of low pressure high temperatures annealing. The present study indicates the decrease in nitrogen flow rate reduces the NVH-defect center concentration and improves the visible absorption in this type of diamond. 
     Based on the foregoing, it appears that increasing the growth pressure in excess of 400 torr, e.g. 410 torr, is effective to synthesize very large, high quality diamonds at still higher growth rates (e.g., 200 μm/h). The MPCVD reactor design is critical in this case as at higher pressure (e.g. 400 torr) direct arcing between the microwave antenna and substrate tend to occur. Such arcing is destructive for quartz components located in the path of microwave. Accordingly, as earlier indicated, the invention contemplates the use of means for preventing arcing and/or otherwise stabilizing the plasma when using relatively high pressure (greater than 400 torr). The importance of this has been demonstrated by an attempt to grow single-crystal CVD diamond at 1 atmosphere pressure using a reactor which was not designed to avoid arcing between the plasma and microwave antenna. However, deposition had to be cut short due to the instability of the plasma balls. It is, therefore, essential for the reactor to be capable of generating a stable plasma at chamber pressures in the 1-2 atmosphere range. 
     While the method of the invention has been illustrated above using a pressure of 300 torr, it is considered on the basis of the results obtained that increasing the pressure to above 400 torr, e.g. 410-425 torr, up to atmospheric pressure, the growth rate of CVD diamond can be increased on an essentially linear basis. To do this, it is important to provide a furnace and associated means that avoid arcing and provide a stable plasma. 
     As indicated above, the use of a limited amount of nitrogen, in combination with oxygen, is considered to be advantageous towards increasing the growth rate although this results in diamonds of a brownish color which can be eliminated by subsequent annealing. Colorless diamonds are obtained if the presence of nitrogen is avoided or kept to a minimum as aforesaid. 
     The foregoing description, using 300 torr pressure to grow high quality single crystal CVD diamond at a rate of 165 μm/h is given only to demonstrate that growth rate can be increased dramatically by increasing the deposition pressure. OES measurement confirm a relationship between plasma density and growth rate, namely, that increased plasma density resulting from increased pressure, increases the growth rate. It appears that atomic hydrogen, which has been considered critical to enhanced growth rates, is not the key factor towards enhancing growth rates at high synthesis pressure. 
     Photoluminescence and UV-visible absorption spectra reveal a general relationship between the brown color of diamond and nitrogen addition in the gas chemistry. The optical quality of colorless single-crystal diamond produced at high CVD pressure was found to be comparable to that of type-IIa natural diamond, as verified by PL and UV-visible spectroscopy. 
     Additional tests were successfully conducted for single crystal diamond synthesis using microwave power of 15 kW at 2.54 GHz and 75 kW at 915 MHz. High power is necessary to generate a stable and large area plasma at higher pressure. 
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     It will be recognized that various modifications may be made in the invention described herein. Accordingly, the scope of the invention is defined in the following claims wherein: