HK1180737B - Method of producing white color mono-crystalline diamonds - Google Patents

Description

Method for producing white single crystal diamond

Technical Field

The present invention relates to a method for producing white single crystal diamond of gem grade quality in a chamber capable of carrying out microwave plasma chemical vapour deposition.

Background

A process for growing polycrystalline grains of diamond is disclosed in U.S. patent No. 3,030,187. Since then, various Chemical Vapor Deposition (CVD) techniques have been developed in the art for producing polycrystalline diamond and single crystal diamond.

Although polycrystalline diamond has properties similar to single crystal diamond, polycrystalline diamond cannot be a recommended material for new industrial application fields due to the presence of grain boundaries and defects contained therein. Furthermore, polycrystalline diamond is not as thermally conductive as single crystal diamond. Also, the grain boundary in the polycrystalline diamond inhibits natural diamond from exhibiting its characteristic excellent properties because the grain boundary functions as a scattering center for phonons, thereby weakening thermal properties and other properties. The presence of large and small angles in the grain boundaries of polycrystalline diamond is a major drawback in industrial applications.

Therefore, there is a clear preference for single crystal diamond in industrial applications. However, single crystal diamond is difficult to grow to have the same structure, purity and polishing degree as natural diamond. Although single crystal diamond has superior properties compared to polycrystalline diamond, microscopic and macroscopic graphite and non-graphite inclusions, feathered defects (long line defects) are very common in single crystal diamond grown by CVD methods. Thus, single crystal diamond grown by CVD methods has a reduced potential for use as a gemstone-grade quality product.

The specific characterization of defects in single crystal diamond grown by CVD methods can be performed by raman spectroscopy and X-ray diffraction (XRD) which reveal that the defects include graphitic regions in the single crystal diamond with dimensions from submicron to several microns.

One problem in producing single crystal diamond by the CVD method is a slow growth rate. Although growth rates of 70 to 100 microns per hour can be achieved by adding higher concentrations of nitrogen to the gas supplied during the CVD process, defects are prevalent and defect density generally increases as growth rate increases.

Japanese patent publication JP07277890 discloses a method of synthesizing diamond used as a semiconductor, an electronic component or an optical component or diamond used in a cutting tool. Specifically, JP07277890 discloses a method of growing diamond in the presence of a gas containing nitrogen and having a ratio of nitrogen to hydrogen of 3ppm to 1000ppm or containing oxygen and having a ratio of oxygen to carbon of 3% to 100% to increase the growth rate.

The scientific article by Yan et al (PNAS, 10/1/2002, vol.99, No.20, 12523-. Specifically, the method discloses performing a CVD process at 150 torr and includes adding nitrogen gas to a gas supplied during the CVD process and making a ratio of nitrogen to methane (N)₂/CH₄) Is 1 to 5 percent. Yan et al believe that nitrogen in the above ratio increases the growth rate because the {111} crystallographic plane becomes {100} crystallographic plane, creating more available growth sites.

The importance of nitrogen content in the gas supplied during the CVD process is disclosed in us patent No. 5,015,494 (Yamazaki), which teaches a method of growing diamond with specific properties for use in industry specific fields. Us patent No. 5,015,494 discloses formation of diamond by electron cyclotron resonance CVD, in which it is disclosed that nitrogen is added so that the ratio of nitrogen compound gas to carbon compound gas is 0.1% to 5% to prevent growth of lattice defects due to external or internal pressure. The resulting diamond had a nitrogen concentration of 0.01 wt% to 1 wt%. Us patent No. 5,015,494 discloses that boron gas needs to be added to the gas supplied during the CVD process to form boron nitride which will deposit on the substrate to promote bonding to the resulting diamond substrate.

It is known from the article by Yan et al and U.S. patent No. 5,015,494 that nitrogen is used to increase the growth rate of single crystal diamond grown by CVD method and also to prevent lattice defects in single crystal diamond grown by electron cyclotron resonance CVD method.

The nitrogen-containing gas in combination with the diborane-containing gas plays a crucial role in the growth of single crystal diamond by CVD processes. A disadvantage of using nitrogen in the amounts disclosed in the Yan et al paper and in U.S. patent No. 5,015,494 is that the resulting diamond is subject to nitrogen-based defects such as micro-cracks, micro-inclusions, and the like. Such diamonds appear brown and are not suitable for gemstone applications.

The applicant believes that the very small amount of nitrogen-containing gas in combination with the diborane-containing gas and optionally oxygen in the gases supplied during the CVD process will result in a white single crystal diamond which is substantially defect free and of quality for gemstone applications. It is believed that the amount of nitrogen-containing gas and the amount of diborane-containing gas disclosed herein are significantly less than the amounts of nitrogen and carbon disclosed in U.S. patent No. 5,015,494.

Nitrogen-containing gases and diborane-containing gases play an important role in diamond growth. In particular, it is known in the art that nitrogen-containing gases are naturally doped in diamond structures. In the absence of a suitable amount of nitrogen-containing gas, many defect structures may be generated in the diamond structure, thereby significantly affecting the properties of the diamond. For example, the presence of nitrogen in a monosubstituted structure imparts a light yellowish brown color to diamond. As shown in fig. 1, the donor-type defect center corresponding to monosubstituted nitrogen is located in the diamond bandgap of about 1.72eV and partially has a positive charge. When white light is incident on the diamond, all wavelengths below yellow (i.e., blue, violet, and ultraviolet) are absorbed, and thus, the diamond appears red or brown.

In contrast, as shown in fig. 1, the presence of boron in the diamond structure produces a negatively charged acceptor state of 0.38eV above the valence band. Diamond produces a blue color when a hole in the valence band can fill the center neutralized by an electron from the conduction band. When white light is incident on the boron-doped diamond, all wavelengths below blue are absorbed, and blue light is emitted from the diamond.

It is an object of the present invention to provide a method for producing white diamond of gem quality substantially free of defects by adding very small amounts of dopants in the form of nitrogen and diborane. Nitrogen-containing gas and diborane-containing gas are supplied along with methane and hydrogen gas during a Microwave Plasma Chemical Vapor Deposition (MPCVD) process suitable for growing diamond, thereby increasing the chroma of the single crystal diamond to white and increasing the clarity of the single crystal diamond, such increase occurring due to the compensation of boron and nitrogen centers. The present invention recognises that heating the diamond to an elevated temperature of 2300 ℃ increases the colour of the diamond to white and also improves the clarity of the diamond.

The gases supplied during the CVD process are believed to contain a relatively small amount of nitrogen-containing gas in combination with the diborane-containing gas in the gas mixture which results in the formation of diamond with optical centers associated with the C-N and C-B-N bonds which result in reduced chroma and purity of the single crystal diamond. The higher concentration of nitrogen-containing gas in the gas mixture also produces trace inclusions and growth cracks in the crystals. The above defects act as phonon scattering centers due to the difference in bond length between nitrogen-carbon bonds and carbon-carbon bonds and boron-carbon bonds, thereby degrading the electrical, optical and mechanical properties of the resulting single crystal diamond.

The formation of inclusions is believed to be dependent on the concentration of nitrogen-containing gas in the gas mixture.

The present invention recognizes that although relatively small amounts of nitrogen-containing gas are required, at least some of the nitrogen-containing gas must be present in combination with the diborane-containing gas in the gas supplied during the CVD process in order to increase the growth rate of the diamond. In addition, by using very small amounts of nitrogen-containing gas in combination with diborane-containing gas, the color and clarity of the diamond crystal can be significantly improved. The present invention recognizes that the presence of boron in a diamond structure containing nitrogen atoms changes the diamond from yellow-brown to white and gives the diamond a gem quality.

The use of relatively small amounts of a nitrogen-containing gas in combination with a diborane-containing gas in the gas mixture used during the CVD process is believed to result in diamond formation in a step growth regime where a diamond layer having an edge defined by steps is grown at the leading edge. This step-wise growth mechanism is different from the currently existing layer growth mechanism that occurs in CVD processes.

Single crystal diamond grown by a step growth mechanism containing a predetermined amount of nitrogen-containing gas in combination with diborane-containing gas is believed to be free of micro and macro graphitic inclusions (particularly nitrogen-based inclusions) and free of defects associated with diamond grown by currently existing layer growth mechanisms. Therefore, at least some nitrogen-containing gas must be included in the gas mixture used during the CVD process to avoid the formation of graphitic inclusions in the growing single crystal diamond.

Although the crystallographic orientation of diamond grown from a diamond seed up to 2mm in thickness is not exactly the orientation 100, this orientation may not be maintained, but other crystallographic orientations are produced.

The present invention recognizes that small amounts of other crystallographic orientations may also be present in diamond grown to a thickness greater than 2 mm.

Disclosure of Invention

According to one aspect of the invention, there is provided a method of producing white single crystal diamond having gem grade quality, the method comprising:

(a) providing a substrate having diamond seeds of a predetermined size and a predetermined optical orientation disposed thereon,

(b) placing the substrate with the diamond seeds in a chamber capable of performing Chemical Vapor Deposition (CVD),

(c) the chamber is supplied with hydrogen gas and,

(d) adjusting conditions within the chamber to make it suitable for performing chemical vapor deposition,

(e) starting a chemical vapor deposition process within the chamber,

(f) supplying a hydrocarbon gas containing carbon to the chamber,

(g) supplying a nitrogen-containing gas and a diborane-containing gas to the chamber, both gases being adapted to accelerate the growth rate of diamond on the substrate,

(h) applying an electric field to the chamber to form a plasma in the vicinity of the substrate to produce a stepped growth of diamond on the substrate,

(i) ending the chemical vapor deposition process within the chamber,

(j) unwanted carbon is cut and removed from the grown diamond,

(k) cleaning and cutting the diamond annealed at a predetermined temperature for a suitable period of time,

(l) Final cutting, polishing and chroma classification of the diamond is performed.

In the step (g) of the above method, the content of the nitrogen-containing gas is 0.0001 vol% to 0.1 vol%, and the content of the diborane-containing gas is 0.00002 vol% to 0.05 vol%.

Drawings

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.

Fig. 1 is an energy band diagram of diamond grown during a CVD process showing the location of nitrogen donor levels and boron acceptor levels in the band gap. These donor and acceptor levels may be partially charged.

Figure 2 is a process flow diagram showing the use of optimum amounts of nitrogen (0.015 vol%) and diborane (0.005 vol%) in the gas mixture according to an embodiment of the invention.

Fig. 3 is a process flow diagram when only a nitrogen gas stream is used in the gas mixture.

Fig. 4 is a process flow diagram for growing diamond without the use of nitrogen and diborane and using only methane and hydrogen in the gas mixture.

Fig. 5 is an FTIR spectrum of diamond deposited in a CVD process in which nitrogen in the CVD gas used is 0.02% to 0.1% and diborane is incorporated at 0.01% to 0.05%. The IR peak associated with the C-B-N center is visible, indicating that the sample is doped with N and B.

Fig. 6 is a photoluminescence spectrum of diamond deposited in a CVD process using nitrogen in the range of 0.0001 to 0.02 vol% of gas supplied during the CVD process and diborane gas flow in the range of 0.00005 to 0.005% of the mixture according to an embodiment of the invention. Photoluminescence spectroscopy showed that diamond deposited using a specific volume percent of nitrogen in combination with diborane had a strong peak at 605nm and a broad band with a lower intensity at 700 nm. The peak at 605nm indicates that the sample is of good quality.

In sharp contrast, the photoluminescence spectrum of diamond grown using only 0.0001% to 0.02% nitrogen flow without diborane showed no peak at 605nm with a broad band of higher intensity at 700nm (fig. 7), indicating the presence of impurities in the diamond.

Fig. 8 is a raman spectrum of a sample grown in the process of the present invention. 1332cm^-1The stronger spectral lines indicate that the diamond grown in the process of the invention is of good quality.

Fig. 9 is a high magnification optical microscope image of diamond grown in a CVD process that includes 0.015% nitrogen and 0.005% diborane according to an embodiment of the invention and exhibits step growth of diamond.

Fig. 10 is a high magnification optical microscope image of diamond grown in a CVD process that includes 0.02% nitrogen without diborane and exhibits stepped growth of diamond. However, the steps are not pure and not straight, with the drawback of being non-uniform.

Detailed Description

The present invention provides a method of producing white single crystal diamond, the method comprising a CVD process using microwave plasma.

The diamond is grown by diamond seeds placed on a substrate. The diamond seed may have a size of 3mm x 3mm to 5mm x 5 mm.

Figure 2 shows a flow diagram for optimal supply of nitrogen-containing gas and diborane-containing gas during a CVD process according to an embodiment of the invention. The present invention recognizes that this process enables diamond growth at a rate of about 18 microns/hour to 20 microns/hour.

The process starts at 202.

In the next step 204, the crystallographic orientation of the diamond seeds is predetermined and diamond seeds not having an orientation of (100) are excluded. Thereafter, the diamond seed crystal oriented at (100) was polished to be polished (in an orientation range of less than 0.1 degree) to a roughness of a visible light wavelength level in the manufacturing method of the CVD process. A diamond seed is then placed on the substrate.

The substrate with the diamond seeds is then placed inside a chamber capable of performing a CVD process. At step 206, the CVD process is started. Hydrogen gas is first supplied into the chamber. Prior to the start of the CVD process, the conditions within the chamber are adjusted to conditions suitable for carrying out CVD. Specifically, the temperature inside the chamber is increased from room temperature to 750 ℃ to 1200 ℃, and the pressure inside the chamber is reduced to 120mbar to 160 mbar.

Then, according to a preferred embodiment of the invention, a gas suitable for diamond growth is supplied to the chamber. In step 208, a catalyst such as methane (CH)₄) Such as a carbon-containing hydrocarbon gas, is supplied into the chamber.

At step 210, a nitrogen-containing gas is supplied into the chamber, while at step 212, a diborane-containing gas is supplied into the chamber. Supplying diborane (B) in an amount of 0.0001 to 0.1% by volume of a gas used for growing diamond by diamond seed₂H₄) Gas bound nitrogen (N)₂) A gas.

It is contemplated by the present invention that the nitrogen-containing gas may be in the form of nitrogen in hydrogen, nitrogen in oxygen, nitrogen in helium, nitrogen in nitrous oxide gas, or nitrogen containing diborane gas.

Will also include helium (He) and oxygen (O)₂) Other gases are supplied to the chamber. These gases were passed through the chamber at a gas flow rate of 30 l/hour.

An electric field is applied in the peripheral region of the diamond seed crystal, so that plasma is generated inside the chamber by the gas. The electric field is generated by a magnetron operating at a power of 6000Watt and a frequency of 2.45 GHz. The generated electric field ionizes the hydrogen gas, forming a plasma in the vicinity of the diamond seeds, which results in the growth of diamond through the diamond seeds. As shown in fig. 9, the present invention recognizes that the growth pattern of diamond is a stepwise pattern in which diamond can be grown without defects and without impurities.

The CVD process ends at step 214.

At step 216, unwanted or parasitic carbon is cut and removed from the grown diamond.

At step 218, the cleaned and cut diamond is annealed at a predetermined temperature for a suitable period of time. Specifically, the diamond is heated to 2300 ℃ in the plasma, thereby significantly improving the color and clarity of the diamond.

At step 220, the diamond is subjected to final cutting, polishing and color sorting.

At step 222, the diamond exhibits final shades G and H according to diamond grading.

The process ends at step 224.

Figure 3 shows a similar flow diagram except that the nitrogen containing gas supply 310 is changed to supply 0.015% by volume of nitrogen containing gas without diborane. The present inventors have recognized that the resulting diamond crystals appear light brown and dark brown, which is undesirable.

Figure 4 shows a similar flow diagram except that the combination of nitrogen-containing gas and diborane-containing gas is not supplied. The present inventors have recognized that diamond crystals appear white but exhibit a large number of defects, which is undesirable.

Fourier transform infrared spectroscopy (FTIR) may be used to determine the concentration of nitrogen and boron and the bonding of nitrogen and boron in a diamond sample. The FTIR spectrum of the grown diamond sample is shown in fig. 5.

As shown in fig. 5, FTIR spectra of diamond samples grown under conditions of 0.02% to 0.1% nitrogen in the gas mixture and 0.01% to 0.05% diborane showed clear and strong signals of boron-nitrogen centers in the samples, accompanied by some typical nitrogen centers. In particular, the stronger band associated with the boron-nitrogen center occurs at 1370cm^-1To (3). 1210cm^-1And 1280cm^-1The band of (B) may belong to the nitrogen centre accompanied by 1978cm^-1、2026cm^-1And 2160cm^-1C-C band. The nitrogen centers in the diamond sample may exist in a number of configurations as described in detail below.

Single atom substitution:

the characteristic peak in the FTIR spectrum is located at 1130cm^-1And 1350cm^-1And EPR gives the "g" value of 2.0024 for this center. In the sample grown under the condition of nitrogen of 0.005% to 0.02%, the center appears to be 1100cm^-1Weaker signal in nearby samples.

Aggregate "A":

480cm in FTIR^-1To 490cm^-1And 1282cm^-1Characteristic peaks for A-aggregates. These peaks are evident in the method shown in figure 2 for samples produced at nitrogen concentrations much greater than the present invention. The a-aggregates are also present in the natural diamond sample in greater concentrations that are used as a basis in embodiments of the present invention.

Aggregate "B":

the B-aggregates in diamond are believed to be composed of 4/8 nitrogen atoms paired with carbon atoms. These peaks are most common in natural diamond and may not be present in the samples of embodiments of the present invention.

N3 center:

the N3 center is not FTIR active and therefore is not present in fig. 1 and 2. However, the N3 center shows a sharp band at 415nm in the Photoluminescence (PL) and UV spectra. The center is composed of three nitrogen atoms near the vacancy (V).

Sheet form:

the platelet morphology consists of one or two additional atomic layers embedded in a diamond lattice. The properties of the platelet morphology in the diamond lattice were analyzed in detail. However, the fact that the corresponding IR band was observed only in diamonds containing detectable amounts of nitrogen suggests that the platelet morphology contains nitrogen and may be partially or completely composed of nitrogen. The peak position of the sheet-like morphology varied with the sample at 1354cm^-1And 1384cm^-1To change between. This change in position is attributed to the sensitivity of the sheet-like morphology to strain, which is introduced into the crystal by the a-and B-aggregate defects. The presence of platelet-shaped absorption indicates that the a aggregates begin to diffuse to form B aggregates. The peak position of the platelet morphology is inversely related to the size of the platelet morphology.

The present inventors have recognized that nitrogen exists in mono-substituted form and a small amount of a-aggregates in samples grown under nitrogen conditions at a flow rate of 0.005% to 0.02%.

Photoluminescence spectroscopy was performed on samples generated under diborane gas flow conditions of 0.00005% to 0.005% in the combined mixture at 0.0001% to 0.02% by volume of nitrogen flow. The results are shown in FIG. 6, which shows a stronger peak at 605nm (2.05eV) and a less intense broad band at about 700 nm. The broad band represents an impurity that degrades the quality of gem grade diamond. In contrast, as shown in fig. 7, the photoluminescence spectrum of diamond prepared using only 0.0001% to 0.02% nitrogen flow without diborane showed no peak at 605nm and a broad band with higher intensity at 700 nm.

No boron centres were seen in the photoluminescence spectrum, probably because boron compensated for nitrogen, thereby improving the optical clarity and purity of the single crystal diamond.

Optical microscope images of samples grown under the conditions of the diborane bound nitrogen concentration range according to embodiments of the present invention are shown in fig. 9 and 10. The magnification of these images was 500 to 5000, and the diamond surface shown in the images exhibited stepwise growth of diamond.

As shown in fig. 9, step-grown diamond was grown at a high density on the surface of the sample under the nitrogen flow condition according to the embodiment of the present invention. These growth steps are present due to threading dislocations during crystal growth of many materials, and are a clear indicator that diamond according to embodiments of the present invention grows with the aid of dislocations through a stepped growth mechanism.

In contrast, the present invention recognizes that, according to one aspect of the invention, diamond grown in a gas using an optimal amount of a nitrogen-containing gas in combination with a diborane-containing gas exhibits regular equidistant steps and is substantially free of graphite inclusions during a CVD process.

The present inventors believe that nitrogen concentrations above 0.015 vol% in the gas phase may produce both micro and macro graphite inclusions as shown in figure 10. These inclusions and defects are formed on the steps and have an adverse effect on the properties of the formed diamond.

In the nitrogen concentration schemes specified in embodiments of the present invention, the step growth mechanism exhibits advantages because it is less prone to incorporate defects and inclusions in the formed diamond, and therefore, the formed diamond is substantially free of defects and inclusions. The diamond so formed is of gem quality and has excellent electrical, optical and mechanical properties relative to other forms of diamond grown by other methods. In addition, the properties of the formed diamond also approach those of natural diamond.

Many changes may be made to the above-described preferred embodiments of the invention without departing from the spirit and scope thereof.

Claims (11)

1. A method of producing white single crystal diamond having gem grade quality, the method comprising the steps of:

(a) providing a substrate having diamond seeds of a predetermined size and a predetermined optical orientation disposed thereon,

(b) placing the substrate with the diamond seeds in a chamber capable of performing Chemical Vapor Deposition (CVD),

(c) the chamber is supplied with hydrogen gas and,

(d) adjusting conditions within the chamber to make it suitable for performing chemical vapor deposition,

(e) starting a chemical vapor deposition process within the chamber,

(f) supplying a hydrocarbon gas containing carbon to the chamber,

(g) supplying a nitrogen-containing gas and a diborane-containing gas to the chamber, both gases being adapted to accelerate the growth rate of diamond on the substrate, and both gases forming boron-nitrogen centers and nitrogen centers, wherein the nitrogen centers are present in any one of the following structures: a monoatomic substitution structure, an "A" aggregate structure, a "B" aggregate structure, or a N3 central structure, wherein the nitrogen-containing gas is in the form of nitrogen in hydrogen, nitrogen in oxygen, nitrogen in helium, nitrogen in nitrous oxide, or nitrogen containing diborane gas, wherein the diborane-containing gas is present in an amount of 0.00002 vol% to 0.05 vol%, the nitrogen-containing gas is present in an amount of 0.0001 vol% to 0.1 vol%,

(h) applying an electric field to the chamber to form a plasma in the vicinity of the substrate to produce a stepped growth of diamond on the substrate,

(i) ending the chemical vapor deposition process within the chamber,

(j) unwanted carbon is cut and removed from the grown diamond,

(k) cleaning and cutting the diamond annealed at a predetermined temperature for a suitable period of time, wherein the annealing is heating the diamond to 2300 ℃ in a plasma, thereby improving the color and clarity of the diamond,

(l) Final cutting, polishing and chroma classification of the diamond is performed.

2. A method of producing white single crystal diamond as claimed in claim 1, wherein the conditions include raising the temperature to 750 ℃ -1200 ℃ and lowering the pressure to 120mbar-160 mbar.

3. A method of producing white single crystal diamond according to claim 1, wherein the hydrocarbon gas containing carbon comprises methane.

4. A method of producing white single crystal diamond as in claim 1, wherein the chemical vapor deposition is performed in the presence of microwave plasma and hydrogen.

5. A method of producing white single crystal diamond as claimed in claim 4, wherein the plasma in the form of microwave plasma is generated by a magnetron operating at a power of 6000Watt and a frequency of 2.45 GHz.

6. A method of producing white single crystal diamond as claimed in claim 1, wherein the gas flows through the chamber at a gas flow rate of 30 l/hour.

7. The method of producing white single crystal diamond of claim 6, wherein the gas comprises oxygen and helium.

8. A method of producing white single crystal diamond according to claim 1, wherein the crystal orientation of the diamond seed is (100).

9. A method of producing white single crystal diamond according to claim 1, wherein the diamond seed has a size of 3mm x 3mm to 5mm x 5 mm.

10. The method of producing white single crystal diamond according to claim 1, further comprising polishing the diamond seed to achieve a roughness of the order of visible wavelength by optical polishing, and then placing the diamond seed on the substrate.

11. A white single crystal diamond of gem grade quality produced by the method of any one of the preceding claims.