Patent Application
20130272928
The invention relates to growing mono-crystalline diamonds and graphitic and non-graphitic inclusion-free diamonds in a microwave plasma chemical vapour deposition apparatus and such an apparatus. Furthermore, the invention also relates to growing white colour diamonds by using a substrate stage assembly that controls the ratio of gaseous methane molecules and excited methyl ions/radicals in the gas phase in the plasma.
Artificial single crystal diamonds have potential for a variety of scientific, industrial and commercial applications in particular as jewellery, heat sinks, electronic devices, laser window, optical window, particle detectors and quantum computing devices. As the commercial demand for single crystal diamond increase over the years, it is essential to increase the production of high quality optical and scientific grade single crystal diamonds without compromising the quality of the single crystal diamond. Factually the requirement of quality is very stringent on the single crystals for applications in scientific products especially for the purpose of semi-conducting devices and the particle detectors. The defects, inclusions, microscopic grain boundaries, other orientations are some prominent defects in single crystal diamonds and have to be deeply characterized in details.
The prior art so far has used one deposition chamber in which the suitable gases such as methane, hydrogen and other gases such as nitrogen, oxygen and diborane are passed to grow single crystal diamonds, with the exhaust gases exiting to the atmosphere. The gases are decomposed into various ionic forms and radicals using an intense microwave electric field at a frequency of 2.45 GHz. The impurities often get incorporated in to the diamond structure from the gas lines, chambers and other sources of contaminations. A significant point to be noted, however, is that the efficiency of the decomposition of the gases in their ionic form is substantially low and it is perhaps not realized that the exhaust still may contain the constituent gases for further diamond growth. Moreover the gas composition is also purified after passing through the plasma phase as most impurities would have been removed by the plasma. It is the endeavour to understand this and utilize this fundamental fact to which the present invention is directed.
The process of growing the poly-crystalline grains of diamond was first patented by W. G. Eversole in 19621. Subsequently, the poly-crystalline2-5 as well as mono-crystalline diamonds6-8 have been grown by variety of chemical vapour deposition (CVD) techniques using methane and hydrogen as precursor gases. The role of methane is to ensure the supply of carbon in the gas phase while the hydrogen plays an important role in the stabilization of diamond phase. Poly-crystalline diamond, despite having similar properties as mono-crystalline diamonds, is not a potential material for new applications due to the presence of the grain boundaries and defects8-10. For instance, the thermal conductivity of the poly-crystalline diamond still does not surpass thermal conductivity of mono crystalline diamond11,12. In poly-crystalline diamond, the grain boundaries play a deteriorating role and inhibits the exhibition of the superior properties unique to diamond because the grain boundaries act as scattering centres8,12. The presence of the grain boundaries are a major drawback in many applications of diamonds.
While there is a clear preference for using mono-crystalline diamonds in applications, mono-crystalline diamonds are difficult to grow with the same texture, clarity, purity and finish as natural diamond. Although, mono-crystalline diamond has superior properties compared to poly-crystalline diamond, microscopic and macroscopic graphitic and non-graphitic inclusions, feathers (long line defects) are very common in CVD grown mono-crystalline diamond.
Detailed characterization of defects in mono-crystalline CVD grown diamond by Raman spectroscopy and X-ray diffraction (XRD) reveals that the defects comprise graphitic regions having a size in the range of submicrons and several microns in otherwise mono-crystalline diamond.
The appearance of the graphitic and non-graphitic inclusions in the mono-crystalline chemical vapour deposited diamond (CVD diamond) may be due to the presence of un-reacted methane in the deposition chamber. Almost all techniques employ a mixture of methane and hydrogen gases for the CVD of diamond. The methane gas is electrically decomposed leading to the formation of excited methyl group species (CH3+ ions) due to the electric field of microwaves of 2.45 GHz frequency. The electrical discharge of the methane and hydrogen gases form a hot plasma consisting of CH3+ ions, atomic hydrogen, H2+ ions and a significant concentration of electrons. The plasma region of the prior art is of substantially ellipsoid shape and it engulfs the substrate stage assembly completely.
Prior art substrate stages are generally made of molybdenum in the shape of a flat disc which is used as a pedestal for loading the diamond seeds (substrates) of the sizes varying from 1×1 mm to 10×10 mm as the case may be. The pedestal can also be made of tungsten or any other suitable metal. As the methyl ions reach substrate at a temperature 900° C., their mobility is high and they start forming a sp3 bonded diamond network in presence of high concentration of hydrogen. The boundary (outer periphery) of the plasma region may contain the neutral molecular methane gas and it may decompose thermally. The thermal decomposition of the methane occurs at 800° C.13 and the result of the thermal decomposition is the formation of black carbon soot that can induce the graphitic and non-graphitic impurities in the diamond deposit.
We propose here in this patent application a substrate stage which provides uniformity of microwave electric field and increase the concentration of CH3+ ions in the plasma region and reduces the ratio of un-reacted methane in the plasma region. The substrate stage also ensures that the heat current flows in such a way so that the temperature of the periphery of the stage is much lower than the rest of the pedestal. As a result, the carbon soot formation can be entirely avoided.
In one form therefore although this may not be the only or broadest form the invention provides an apparatus for the manufacture of diamonds, the apparatus comprising:
In another alternative form the invention comprises an apparatus for the manufacture of diamonds, the apparatus comprising:
In an alternative form the invention comprises a substrate stage for the manufacture of gem grade diamonds, the substrate stage comprising
Certain forms of the invention have other steps or elements in addition to or in place of those mentioned above. The steps or elements will become apparent to a person skilled in the art from a reading of the following detailed description when taken with reference to the accompanying drawings.
This then generally describes the invention but to assist with understanding reference will now be made to the accompanying drawings which show preferred embodiments of the invention.
In the Drawings:
The apparatus 50 comprises a series of chambers 52 arranged in series with gag flow pipes 56 between each chamber. A gas supply supplies gases into a first chamber through a gas entry 54. A vacuum pump 58 after the final chamber 52g evacuates all the chambers and draws through the series of chambers process gases, as discussed below, from the gas entry 54 into the first chamber 52a. Each chamber has a substrate stage assembly 32 and a microwave arrangement 37 which will be discussed in more detail below.
In application the gases supplied are used in sequence with the gas exiting the first chamber becoming the input for second chamber. The vacuum in all the chambers is created by the same vacuum pump 58. Used gas is exhausted at gas exit 62. Each chamber has its own independent pressure measuring apparatus 60. As the purity of the gas improves significantly as it passes through each chamber the quality of the diamond improves significantly and leads to defect free creation of diamond single crystals. By this invention that it is possible to connect a number of chambers in this way where the number could be more than two depending upon the capacities of the pumping system and gas supply. The gas composition can be formulated in such a way that the diamonds are grown in each chamber with similar growth rate and by conserving the cost a larger quantity of diamonds can be grown at a higher yield.
It has been unexpectedly discovered that a reduction in high quality diamond manufacturing cost and a reduction in the amount of the exhaust gas emission are made possible by reusing the gases supplied to the chambers in aforementioned manner.
The chamber 52 has a casing 30 which has supported within it a substrate stage assembly 32. The substrate stage assembly 32 comprises a substrate stage 10 as discussed below in more detail and a peripheral reflector 34. The peripheral reflector 34 comprises a cylindrical body around the substrate stage 10 and is spaced laterally from the peripheral raised edge 13 of the substrate stage 10. The reflector 34 that can function as a heat shield is used on the outside of the substrate stage 10 so that the stage can reach the required temperature for an appropriate value of power to the chamber. The substrate stage 10 and reflector assembly are supported on metal plate 35 that is cooled by a fluid coolant such as water, liquid nitrogen, etc. The plate 35 is made of a metal with high thermal conductivity, such as copper, molybdenum and etc.
The peripheral reflector 34 is used mainly to contain the heat and has minor role of containing the microwave electric fields. Its construction is a very thin circular annular ring made of molybdenum with a shiny inside surface for the heat containment. It is kept about 2.5 mm away from the substrate stage 10. As the heat containment is effective, the substrate temperature can be reached at a lower microwave power and improve the power profile of the machine.
The microwave arrangement 37 for supplying microwave power to the chamber 52 generates a 2.45 GHz microwave and directs the microwave energy into the chamber 52 in the region of the substrate stage 10 to form an oblate spheroidal plasma region 14. Gases as discussed below are added into the chamber 52 to form the diamonds. Gas is provided from a gas supply or a previous chamber in the series of chambers via port 56a and is extracted from the chamber to a subsequent chamber via port 56b.
An embodiment of a substrate stage for use within the series of chambers according to the present invention is shown in
The applicants have carried out extensive experimental work on the design of new substrate stage and substrate stage assembly for controlling the temperature of the different sectors of the substrate stage such that the thermal decomposition of the methane gas is controlled in the vicinity of the substrates and the electric filed is uniform in the whole region of the location of the substrates.
The applicants have also analysed in details the heat flow in the substrate assembly stage and have found periphery of the stage is at much lower temperature than that of the bulk of the stage and therefore the formation of the carbon soot is significantly reduced.
The substrate stage 10 has a substantially circular planar base 12 and a peripheral raised edge 13 to the base 12. The peripheral raised edge 13 defines a central recessed substrate receiving surface 15. The central recessed substrate receiving surface 15 is substantially planar. The peripheral raised edge 13 comprises an outer edge 13a and an inner edge 13b and the inner edge 13b comprises a bevel 24 extending down to the recessed substrate receiving surface 15.
In use diamond seeds 19 that may vary in size between 1×1 mm and 10×10 mm are placed in an array or matrix onto the central recessed substrate receiving surface 15 as is discussed in more detail below.
The peripheral raised edge 13 to the base comprises an upper surface 13c and a lower surface 13d. In one embodiment of this invention, the peripheral raised edge 13 comprises an annular groove 18a, 18b in at least one of the upper and lower surfaces 13c and 13d. In another embodiment of present invention, there are annular grooves 18a and 18b in both the upper and lower surfaces 13c and 13d respectively.
Multiple diamond seeds are loaded in a recessed region 21 on the central recessed substrate receiving surface 15. The uniform size diamond seeds ranging from 1×1 mm and 10×10 mm are placed in a matrix layout. As the microwave power is coupled into the chamber in presence of hydrogen gas, a plasma region 14 (see
This embodiment of a substrate stage 10 is made of molybdenum. Molybdenum has a high thermal conductivity which assists maintaining an even temperature on the base 12.
The outer periphery 16 of the substrate stage 10 is isolated from the main bulk of the assembly by the annular or slotted grooves 18 which are preferably on both the top and bottom surfaces of the substrate stage 10. Heat conduction to the outer periphery is region is less because of the narrow flange 20 and as a result the temperature of the outer periphery 16 is lower than the bulk of the stage assembly 12. It is believed that the reduction of the periphery temperature avoids the thermal decomposition of methane and hence the formation of carbon impurities.
The presence of the slotted groove 18 and the bevelled edge 24 provides uniformity by increasing the concentration of CH3+ ions in the plasma region and reducing the ratio of un-reacted methane in the plasma region. The substrate stage 10 also provides the stability to the plasma by intensifying the electric field of the microwave radiation in the region. Last but not least, the substrate stage 10 ensures the heat current flows in such a way so that the temperature of the periphery of the stage is much lower than the rest of the pedestal.
The central idea in this invention is to make the inclusion-free diamond, preferably gem grade diamond, by using the substrate stage 10 such that the thermal decomposition of the methane gas is avoided in the region where the diamond seeds are located.
In a preferred embodiment of the invention methane, hydrogen, nitrogen and diborane containing gases are used as precursors for microwave plasma chemical vapour deposition process. The dominant concentration of the gases in the chamber is methane and hydrogen. Preferably the flow of hydrogen is 800 sccm (standard cubic centimetres per minute) and methane is 55 sccm. The plasma of these gases is generated in the region 14 above the substrate stage 10. As the electric field will be intense at the sharp edges, the plasma is more stable and uniform in the described configuration of the substrate stage 10.
As shown in
As shown in
Thus, chambers 52a1, 52a2, and the vacuum pump 58a form a first branch in a chamber network. A vacuum pump 58b is coupled to a chamber 52b1 in series so that the both of them together form a second branch in the chamber network. A gas flow pipe 56s in the first branch is coupled to a gas entry 54s of a chamber 52b1 in the second branch so as to allow gas flow between branches. The main gas line 500, the first branch and the second branch together form the chamber network, among which the gases flow. It should be understood that a chamber network may comprise any number of branches and a branch may comprise any number of chambers and the number of branches in a network and the number of chambers in a branch depend upon the capacities of the vacuum pump system and gas supply system.
Additionally, the applicants have found that, although a relatively small amount of nitrogen is required, there must be at least some nitrogen in combination with diborane gas to be present in the CVD gases to increase the growth rate of the diamonds deposited by a CVD process. In addition, by using very small quantities of nitrogen and in combination with the diborane, the colour and the clarity of the diamond crystals can be remarkably improved. The applicants have found that the presence of boron in the diamond structure containing nitrogen atoms will turn a yellow brown colour diamond colourless making it a gem grade diamond.
A method of growing mono-crystalline diamond using a substrate stage in accordance with one embodiment of the invention involving a CVD process that utilises microwave plasma is as follows.
Diamond is grown on a diamond seed 19 that may vary in size between 1×1 mm and 10×10 mm. The method is carried out in a microwave plasma chamber.
The crystallographic orientation of the diamond seeds 19 is determined and the diamond seeds 19 having an orientation other than (100) are rejected. The diamond seeds 19 having an orientation of (100) are polished to optical finish with roughness in the order of the wavelength of visible light in preparation for the CVD process.
Once the diamond seeds 19 are located in the chamber 52, the temperature inside the chamber 52 is increased from ambient temperature to a temperature in the range of 750 to 1200° C. and the pressure inside the chamber is reduced to a pressure in the range of 120 to 160 mbar.
The chamber is supplied with gases for growing diamond and the gases comprise methane (CH4), hydrogen (H2), nitrogen (N2) in combination with diborane (B2H6), and helium (He) and are passed through the chamber at a gas flow rate of 30 l/hr.
Nitrogen in combination with the diborane gas are supplied in a quantity that comprises 0.0001 to 0.1 vol % of the balanced gases for growing diamond. For the optimal mixture of the nitrogen and diborane, the growth rate of the diamond is about 18-20 microns per hour.
An electrical field is applied to surround the seeds such that plasma is generated from the gases in the chamber 52. The electrical field is generated by a magnetron operating at 6000 Watt and at 2.45 GHz. The generated electrical field causes the hydrogen gas to be ionised, thereby forming plasma in the vicinity of the diamond seeds 19. Under these process conditions, diamond is caused to grow on the diamond seeds 19.
The growth pattern of diamond is step-wise and therefore enables diamond to grow that is substantially defect and impurity free.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Singapore or any other country.
It is apparent to a person skilled in the art that many modifications, alternatives and variations may be made to the preferred embodiment of the present invention as described above without departing from the spirit and scope of the present invention. Accordingly, it is intended to embrace all such modifications, alternatives and variations that fall within the scope of the included claims.
It will be understood that the term “comprises” or its grammatical variants as used in this specification and claims is equivalent to the term “includes” and is not to be taken as excluding the presence of other features or elements.
