Microwave plasma reactor for manufacturing synthetic diamond material

Abstract

A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor includes a plasma chamber, a substrate holder, a microwave coupling configuration for feeding microwaves into the plasma chamber, and a gas flow system for feeding process gases into the plasma chamber and removing them therefrom. The gas flow system includes a gas inlet array having a plurality of gas inlets for directing the process gases towards the substrate holder. The gas inlet array includes at least six gas inlets disposed in a substantially parallel or divergent orientation relative to a central axis of the plasma chamber.

Claims

1. A synthetic diamond material manufacturing microwave plasma reactor comprising: a plasma chamber; a substrate holder disposed in the plasma chamber for supporting a substrate on which the synthetic diamond material is to be deposited in use; a microwave coupling configuration for feeding microwaves from a microwave generator into the plasma chamber; and a gas flow system for feeding synthetic diamond-forming process gases into the plasma chamber and removing them therefrom; wherein the gas flow system comprises a gas inlet array comprising a plurality of gas inlets disposed opposite the substrate holder for directing process gases towards the substrate holder, the gas inlet array comprising: at least six gas inlets disposed in a substantially parallel orientation relative to a central axis of the plasma chamber; a gas inlet number density equal to or greater than 0.1 has inlets/cm.sup.2 and equal to or less than 100 gas inlets/cm.sup.2, wherein the gas inlet number density is measured by projecting the gas inlets onto a plane whose normal lies parallel to the central axis of the plasma chamber and measuring the gas inlet number density on said plane; and a gas inlet area ratio of equal to or greater than 300, wherein the gas inlet area ratio is measured by projecting the gas inlets onto a plane whose normal lies parallel to the central axis of the plasma chamber, measuring the total area of the gas inlet array on said plane, dividing by the total number of gas inlets to give an area associated with each gas inlet, and dividing the area associated with each gas inlet by an actual area of each gas inlet, wherein the gas inlet number density and the gas inlet area ratio are calculated over at least 50% of all the gas inlets in the gas inlet array, wherein a maximum radius of the gas inlet array R.sub.m meets the criteria: R.sub.m×F.sub.m is greater than or equal to R.sub.s, where R.sub.s is a radius of the substrate holder and F.sub.m is equal to or greater than 0.5 and equal to or less than 1.5; and wherein the microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber is disposed at an end of the plasma chamber upstream of the substrate holder, wherein the gas inlet array is disposed in a central region of said end and the microwave coupling configuration comprises a dielectric microwave window disposed in a ring around the gas inlet array.

2. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein the gas inlet number density is equal to or greater than 10 gas inlets/cm.sup.2.

3. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein the gas inlet number density is equal to or less than 50 or 10 gas inlets/cm.sup.2.

4. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein the gas inlet area ratio is equal to or greater than 1000 or 3000.

5. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein the gas inlet area ratio is equal to or less than 100000, 30000, or 10000.

6. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein the gas inlet array comprises equal to or greater than 7, 9, 10, 15, 20, 30, 40, 60, 90, 120, 150, 200, 300, 500, 700, 1000, 1200, 1500 gas inlets.

7. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein each gas inlet has an outlet diameter in the range 0.1 mm to 5 mm, 0.2 mm to 3.0 mm, 2.0 mm to 3 mm, 0.2 mm to 2 mm, 0.25 mm to 2 mm, or 0.25 mm to 1.5 mm.

8. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein a ratio of total gas inlet area/area of the gas inlet array is equal to or less than 0.5, 0.35, 0.3, 0.2, 0.1, 0.05, 0.02, 0.01, or 0.007.

9. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein a total area of gas inlets in the gas inlet array, given in mm.sup.2, is in a range 1 to 5000, 5 to 3000, 10 to 3000, 20 to 2750, 30 to 2750, or 50 to 2700.

10. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein a total area of the gas inlet array over which the gas inlets are spaced, given in mm.sup.2, is in a range 100 to 15000, 200 to 15000, 400 to 10000, 800 to 10000, or 1000 to 8000.

11. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein a minimum distance D.sub.c between the gas inlet array and the substrate holder is less than or equal to 6R.sub.s, 4R.sub.s, or 2R.sub.s, where R.sub.s is a radius of the substrate holder.

12. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein F.sub.m is equal to or greater than 0.6, 0.7, 0.8, 0.9, or 1 and equal to or less than 1.3, 1.2, or 1.1.

13. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein each gas inlet has an inlet portion having a first diameter and an outlet portion having a second diameter, the first diameter being larger than the second diameter.

14. A method of manufacturing synthetic diamond material using the synthetic diamond material manufacturing plasma reactor according to claim 1, the method comprising: injecting synthetic diamond-forming process gases into the plasma chamber through the plurality of gas inlets; feeding microwaves from a microwave generator into the plasma chamber through the microwave coupling configuration to form a plasma above a substrate disposed over the substrate holder; and growing synthetic diamond material on a growth surface of the substrate.

15. A synthetic diamond material manufacturing plasma reactor according to claim 1, wherein the gas inlet array has an operating flow rate of from 2500 to 40,000 standard cm.sup.3 per minute.

16. A method of manufacturing synthetic diamond material according to claim 14, wherein the plurality of gas inlets forming a gas inlet array has an operating flow rate of from 2500 to 40,000 standard cm.sup.3 per minute.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a vertical cross-sectional view of a microwave plasma reactor configured to deposit a diamond film using a chemical vapour deposition technique in accordance with an embodiment of the present invention;

(3) FIG. 2 shows a plan view of a gas inlet array according to an embodiment of the present invention; and

(4) FIG. 3 shows a vertical cross-sectional view of a portion of the gas inlet array illustrated in FIGS. 1 and 2.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

(5) FIGS. 1 to 3 show various views of a microwave plasma reactor according to an embodiment of the present invention.

(6) As can be seen by reference to FIG. 1, the microwave plasma reactor comprises the following basic components: a plasma chamber 102; a substrate holder 104 disposed in the plasma chamber for holding a substrate 105; a microwave generator 106 for forming a plasma 108 within the plasma chamber 102; a microwave coupling configuration 110 for feeding microwaves from the microwave generator 106 into the plasma chamber 102 via a coaxial waveguide and through an annular dielectric window 119; a gas flow system 112, 122 for feeding process gases into the plasma chamber 102 and removing them therefrom; and a substrate coolant system 114 for controlling the temperature of a substrate 105.

(7) It is found that this invention is best utilized in conjunction with simple modal synthesis chambers, for instance the TM.sub.011 mode is advantageous as it has been found to be the most compact (small) mode which can be practicably used in a diamond CVD plasma reactor. Its compactness means that the impact of gas flow aspects on the near gas phase chemistry are maximized, however, this invention is not limited to this modal geometry. The use of a small plasma chamber having a compact microwave cavity is made possible by the flow characteristics of the gas inlet array according to embodiments of the present invention which ensures that process gas flows through a central portion of the plasma chamber without undue circulation of gases within the plasma chamber contaminating walls of the chamber which will be relatively close to the gas flow in a compact cavity arrangement.

(8) The gas inlets may be formed by a portion of the microwave/vacuum wall such that the gas inlets form part of the microwave cavity wall of the plasma chamber, rather than being outside the mesh that defines the microwave cavity wall.

(9) The flow characteristics of embodiments of the present invention also allow the plasma reactor to be operated at high power while constraining the plasma to avoid damaging the walls of the chamber and/or the microwave window. The high velocity, highly uniform gas flow within the plasma chamber achieved by embodiments of the present invention allows more power to be introduced at high pressures without the plasma arcing. The power delivered to the substrate also increases with high velocity, highly uniform gas flow, increasing efficiency. That is, a fraction of the total power which is delivered to a substrate is increased. Typically this invention enables at least 45%, 50%, 55%, 60%, 65%, or 70% of power fed into the plasma chamber to be transmitted through the base of the chamber (opposite to the gas inlet). It has been found that the power transmitted through the base of the chamber approximately equates to hydrogen flux towards the base of the chamber. Accordingly, increasing the power fed through the base of the chamber increases the hydrogen flux to the substrate over the base of the chamber which leads to better quality diamond material being formed over the substrate. It has also been found that utilizing high axial gas flows helps improve the stability of the plasma, leading to more uniform deposition in terms of both growth thickness and quality.

(10) The gas flow system 112 comprises source gas containers 117 and a gas inlet array 120 coupled to the source gas containers and positioned in a top portion of the plasma chamber 102 axially disposed above the substrate holder 104 and substrate 105 for directing process gases towards the substrate 105 in use. In the illustrated embodiment the process gas is fed from the source gas containers 117 to the gas inlet array through a central conductor of the microwave coupling configuration 110. However, other configurations are also possible for feeding the process gases to the gas inlet array 120.

(11) The microwave window 119 for feeding microwaves from the microwave generator into the plasma chamber is preferably disposed at an opposite end of the plasma chamber to the substrate holder. Furthermore, the plurality of gas inlets are preferably disposed closer to the substrate holder than the microwave window. Such an arrangement can minimize the possibility of the microwave window being contaminated with process gases while also ensuring that the process gas is injected at a location relatively close to the substrate.

(12) One or more gas outlets 122 are provided in a base of the plasma chamber 102. The gas outlets 122 are preferably located in a ring around the substrate holder 104 and most preferably form a uniformly spaced array around the substrate holder 104 to enhance continuous gas flow from the gas inlet array 120 towards the substrate 105, around the substrate 105, and out of the gas outlets 122 while minimizing turbulence and gas recirculation back up the plasma chamber 102. For example, it may be preferable to provide at least 6, 12, 18, or 30 gas outlets disposed around the substrate holder 104, preferably in a uniformly spaced array. In this regard, it should be noted that while embodiments of the present invention may function to reduce uncontrolled gas re-circulation within the plasma chamber, this does not preclude the possibility of using a controlled gas re-circulation system outside the plasma chamber for re-using process gas which is extracted from the plasma chamber through the gas outlets.

(13) It should be noted that while terms such as “top portion” and “base” are used in this specification when describing the plasma reactor, it is possible to invert the reactor so that in use the gas flow is in an upwards direction. As such, the terms “top portion” and “base” refer to the location of the reactor components relative to each other and not necessarily their location relative to the earth. In standard usage, the gas flow will be in a downwards direction such that the gas streams from the gas inlet array flow downwards with gravity. However, it is possible to invert the reactor such that the gas streams from the gas inlet array flow upwards against gravity. In the inverted orientation the gas flow will be parallel to principle thermally driven convection currents (which are in an upwards direction due to the large amount of heat generated in the plasma which is below the substrate in an inverted arrangement). This inverted arrangement may have some benefits for certain applications.

(14) It should also be noted that while the microwave plasma reactor illustrated in FIG. 1 has a separate substrate holder disposed in the plasma chamber, the substrate holder may be formed by the base of the plasma chamber. The use of the term “substrate holder” is intended to cover such variations. Furthermore, the substrate holder may comprise a flat supporting surface which is the same diameter (as illustrated) or larger than the substrate. For example, the substrate holder may form a large flat surface, formed by the chamber base or a separate component disposed over the chamber base, and the substrate may be carefully positioned on a central region of the flat supporting surface. In one arrangement, the flat supporting surface may have further elements, for example projections or grooves, to align, and optionally hold, the substrate. Alternatively, no such additional elements may be provided such that the substrate holder merely provides a flat supporting surface over which the substrate is disposed.

(15) The gas inlet array 120 comprises a plurality of gas inlets disposed opposite the substrate holder 104 for directing process gases towards the substrate holder 104. The gas inlet array 120 comprises a plurality of gas inlets disposed in a substantially parallel orientation relative to the central axis of the plasma chamber 102. The gas inlet array 120 also comprises a housing 128 defining a cavity 130 for receiving process gases from one or more gas inlet pipes. The housing 128 also defines the plurality of gas inlets for injecting process gases from the cavity 130 into the plasma chamber 102 and towards the substrate holder 104. For example, the housing may comprise metallic walls in which the gas inlets are integrally formed.

(16) The housing 128 and cavity 130 can function as a mixing chamber for mixing source gases prior to injection into the plasma chamber. Such a pre-mixing chamber has found to be useful to ensure efficient gas mixing prior to injection into the plasma chamber. Furthermore, a pre-mixing chamber is useful to ensure that there is a uniform gas flow over the entirety of the array of gas inlets. The pre-mixing chamber may include a diffuser or an array of holes disposed prior to the gas inlet array to encourage gas mixing and/or provide an even flow of gas to the gas inlet array.

(17) The housing 128 can also extend into the plasma chamber allowing gases to be injected closer to the substrate. The distance between the gas inlet array and the substrate where diamond growth occurs affects the thickness of a boundary layer over the substrate. Reducing the distance between the gas inlet array and the substrate had been found to reduce the thickness of such a boundary layer and lead to an increase in diamond deposition rate.

(18) A variety of modifications to the gas inlet array may be envisaged. For example, the gas inlet array may be configured to transport different gas compositions though different gas inlets. Furthermore, the plurality of gas inlets may have a non-uniform spacing and/or comprise non-uniform gas inlet diameters. This may be advantageous as different gases will have different flow characteristics and so each of the gas inlets, or a group of gas inlets, can be optimized for injection of a particular source gas. In this case, the mixing cavity should be segregated to prevent mixing of source gases and configured to direct each source gas to one or more gas inlets which have been specifically adapted for injecting the associated source gas.

(19) A further alternative to the provision of a metallic housing in which the gas inlets are integrally formed is to form the gas inlets in a microwave window which extends over a central region in an upper portion of the plasma chamber or even located closer to the substrate in a central portion of the plasma chamber. For example, the microwaves may be coupled into the chamber via a plate of microwave window material (e.g. quartz) in which the gas inlets are integrally formed. In such an arrangement, the high velocity laminar flow produced by a gas inlet array according to embodiments of the present invention will aid in keeping the plasma away from the microwave window. Using high gas flows will result in less contaminants being deposited near the gas inlets and the microwave window therefore reducing the problem of contaminants falling onto the substrate and causing problems such as black spots, silicon incorporation, inclusions, and nuclei for defect formation in the CVD diamond material.

(20) FIG. 2 shows a plan view of the gas inlet array 120. The array comprises a hexagonal close-packed array of gas inlets 126. The array comprises a gas inlet number density greater than 5 gas inlets/cm.sup.2, wherein the gas inlet number density is measured by projecting the gas inlets onto a plane whose normal lies parallel to a central axis of the plasma chamber and measuring the gas inlet number density on said plane. The gas inlet number density is measured in this way because the array may not necessarily be disposed in a plane. For example, the array may be disposed in a wall which is curved or otherwise angled relative to a plane whose normal lies parallel to a central axis of the plasma chamber. However, in the illustrated embodiment it will be noted that the array is disposed in a plane whose normal lies parallel to a central axis of the plasma chamber.

(21) The gas inlet array 120 may have a gas inlet area ratio much greater than 10, wherein the gas inlet area ratio is measured by projecting the gas inlets onto a plane whose normal lies parallel to a central axis of the plasma chamber, measuring the total area A of the gas inlet array on said plane, dividing by the total number of gas inlets to give an area associated with each gas inlet, and dividing the area associated with each gas inlet by an actual area of a gas inlet a. Where the gas inlets have different areas an average gas inlet area can be used for the area a. If the total area A of the gas inlet array is delineated by a line passing through the centre of each of the gas inlets in an outer ring of gas inlets in the array, it will be noted that half the area associated with the outer ring of gas inlets will be outside this area. This can be corrected for by dividing the number of gas inlets in the outer ring by two when calculating the total number of gas inlets and then using this corrected value in the aforementioned calculation so as to correctly calculate the area associated with each gas inlet. The actual area of each gas inlet may be calculated as an average gas inlet area by summing the actual area of every gas inlet in the array and dividing by the total number of gas inlets in the array. Alternatively, if all the gas inlets have the same area then the area of a single gas inlet may be used for the actual area of each gas inlet.

(22) Each gas inlet 126 may have an outlet diameter in the range 0.1 mm to 5 mm, 0.2 mm to 3.0 mm, 2.0 mm to 3 mm, 0.2 mm to 2 mm, 0.25 mm to 2 mm, or 0.25 mm to 1.5 mm. The diameter of the gas inlets may be configured to achieve good laminar flow of the individual gas streams injected through and out of the gas inlets into the plasma chamber 102. The dimensions of the gas inlets 126 also affect the Reynolds number R.sub.e for gas injection. The Reynolds number is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces acting in a gas stream and consequently quantifies the relative importance of these two types of forces for given flow conditions. The Reynolds number may be used to characterize different flow regimes, such as laminar or turbulent flow. Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities. In accordance with certain embodiments of the present invention it is preferable to operate at low Reynolds number to minimize turbulence. The effect of providing an array of smaller gas inlets compared to a small number of larger ones is to decrease the Reynolds number (if the mean velocity of the gas stream is maintained). This reduces the “inertial” component of gas injection, in comparison to the viscous forces operating. Accordingly, it is preferred that the dimensions of the gas inlets 126 are selected to give a Reynolds number for gas injection equal to or less than 100, 80, 50, 30, 20, 10, 5, 3, 2, or 1. In practice, the Reynolds number may be equal to or greater than 0.1. A typical operating range for the Reynolds number may lie in the range 1 to 20 depending on the specific gas inlet array which is utilized.

(23) FIG. 3 shows a vertical cross-sectional view of a portion of the gas inlet array 120 shown in FIGS. 1 and 2. In the illustrated arrangement, each gas inlet 126 has an inlet portion 134 having a first diameter d.sub.1 and an outlet portion 136 having a second diameter d.sub.2, the first diameter d.sub.1 being larger than the second diameter d.sub.2. Such an arrangement can be advantageous as the fine bore of the outlet portion which is advantageous for operating in a low Reynolds number regime is only required to be formed at a minimum length for achieving good gas flow characteristics. As such, for a wall thickness which is larger than a minimum length required for achieving good gas flow characteristics, the remainder of the wall thickness can be drilled out at a larger diameter. For example, the inlet portion 134 may have a length l.sub.1 and the outlet portion 136 may have a length l.sub.2 with the sum of l.sub.1 and l.sub.2 being equal to a wall thickness. Furthermore, this design aids in achieving clean laminar flow as a converging gas inlet profile causes a parabolic velocity profile to develop more quickly. Of course, it is also possible to provide a gas inlet in a wall portion of the gas inlet array which consists only of a single continuous bore which may have a constant diameter along its length or a continuously varying taper.

(24) The present invention has thus far been described with reference to an embodiment as illustrated in FIGS. 1 to 3. However, it is envisaged that various modification can be made within the scope of the invention. For example, certain embodiments of the present invention may conform to one or more general design principles for an array of gas inlets as discussed below.

(25) Each gas inlet in the array can be characterised by its lateral spacing (radius) away from a central axis of the plasma chamber. A central gas inlet, if it exists, may be disposed down a central axis of the plasma chamber. Gas inlets at the same radius (lying on a ring centred on the central axis) may show periodic rotational symmetry around the central gas inlet, although the rotational angle may vary for different rings of gas inlets.

(26) Gas inlets positioned at a particular radius from the central axis may be parallel to the central axis, or may be divergent from it. The gas inlets at any particular radius may be at least as divergent as any gas inlets lying on a smaller radius. That is not to say that a minority of gas inlets which do not follow this principle, or even are directed at a convergent angle, are not permitted.

(27) The gas inlets may all be retained parallel to the central axis until some radius R.sub.p, and then start to become divergent out to a maximum radius on which the gas inlets are placed, R.sub.m. In the region between R.sub.p and R.sub.m, the divergence of the gas inlets may vary as a function of the radius, or the angle of divergence may be fixed.

(28) The spacing of the gas inlets may be uniform across the surface through which they emerge. Preferably the gas inlets are in a consistent geometric arrangement, most preferably in a hexagonal array. While not being bound by theory, it is believed that such arrangements are advantageous as the gas jets from individual gas inlets converge such that their velocity profiles are well matched. This allows the gas jets to converge with little or no disturbance. Alternatively, the spacing of the gas inlets may increase with radius, such that the density of gas inlets reduces towards the outer edge of the array. The gas inlets may be arranged in discrete rings, with little obvious correlation in the position of gas inlets lying in adjacent rings. It is indeed possible to have a random array of gas inlets which provides some reasonably uniform average density of gas inlets to perform an adequate performance and realise some of the benefits of this invention, although the best arrangement is one of a regular array.

(29) The diameter of each gas inlet is optionally the same, particularly for large gas inlet arrays (e.g. greater than 100 gas inlets), or at least particularly for the majority of the gas inlets in such an array (e.g. at least 50%, 60%, 70%, 80%, or 90% of the gas inlets have the same diameter).

(30) Projecting all the gas inlets onto a plane whose normal lies parallel to the central axis, the density of gas inlets in the gas inlet array, particularly for large gas inlet arrays (e.g. greater than 100 gas inlets), given in gas inlets/cm.sup.2, is equal to or greater than 5, preferably equal to or greater than 10, and equal to or less than 100, 50, or 10.

(31) Projecting all the gas inlets onto a plane whose normal lies parallel to the central axis, the total area of the gas inlets in the array (that is the sum of the areas of each outlet of the gas inlets in the array), given in mm.sup.2, may be in a range 1 to 5000, 5 to 3000, 10 to 3000, 20 to 2750, 30 to 2750, or 50 to 2700. The total area of the array over which the gas inlets are spaced, given in mm.sup.2, may be in a range 100 to 15000, 200 to 15000, 400 to 10000, 800 to 10000, or 1000 to 8000. The actual total area of the gas inlets and the total area of the array will depend on the number of gas inlets provided in the array and the area over which they are distributed, which will also be dependent on the area of CVD diamond to be grown. For example, a simple six gas inlet array may have a gas inlet diameter in the range 2 to 3 mm, a total gas inlet area of 20 to 50 mm.sup.2, and a total array area of approximately 450 mm.sup.2. In contrast, a 91 gas inlet array may have a gas inlet diameter of approximately 0.5 mm, a total gas inlet area of approximately 18 mm.sup.2, and a total array area of approximately 1000 mm.sup.2. Further still, a 1519 gas inlet array may have a gas inlet diameter in the range 0.25 to 1.5 mm, a total gas inlet area of 75 to 2700 mm.sup.2, and a total array area of approximately 8000 mm.sup.2.

(32) A ratio of the total gas inlet area/area of the gas inlet array should preferably be relatively low, for example, equal to or less than 0.5, 0.35, 0.3, 0.2, 0.1, 0.05, 0.02, 0.01, or 0.007. The ratio of the total gas inlet area/area of the gas inlet array may be equal to or greater than 0.001, 0.004, 0.007, 0.01, or 0.02. The actual ratio provided will depend on the number of gas inlets provided in the array and the area over which CVD diamond is to be grown. For example, a simple six gas inlet array may have a ratio in the range 0.05 to 0.1, a nine gas inlet array may have a ratio of approximately 0.007, a 721 gas inlet array may have a ratio in the range 0.004 to 0.2, and a 1519 gas inlet array may have a ratio in the range 0.01 to 0.35.

(33) A ratio of the area of the gas inlet array to an array of the growth surface of the substrate may be in the range 0.05 to 2, 0.1 to 1.5, 0.5 to 1.25, 0.8 to 1.1, or 0.9 to 1.1. For arrays which comprise a large number of gas inlets (e.g. greater than 100, 500, or 1000), the area of the array may be set to be substantially equal to the area of the growth surface. For arrays which have a lower number of gas inlets, the area of the array is preferably less than the area of the growth surface of the substrate.

(34) Projecting all the gas inlets onto a plane whose normal lies parallel to the central axis, the total area of the gas inlet array, π(R.sub.m).sup.2, divided by the total number of gas inlets gives the area associated with each gas inlet. A ratio of the associated area of each gas inlet divided by an actual area of each gas inlet is preferably equal to or greater than 10, 30, 100, 300, 1000, or 3000 and equal to or less than 100000, 30000, or 10000.

(35) The total gas flow fed through the gas inlet array may be equal to or greater than 500, 750, 1000, 2000, 5000, 10000, 15000, 20000, 25000, 30000, 35000, or 40000 standard cm.sup.3 per minute. The total gas flow fed through the gas inlet array may be equal to or less than 60000, 50000, 30000, 20000, or 10000 standard cm.sup.3 per minute depending on the specific gas inlet array which is utilized. For example, for certain gas inlet arrays a typical operating flow rate may lie in a range 500 to 40000, 1000 to 40000, or 2500 to 40000 standard cm.sup.3 per minute depending on the desired growth rate and substrate area.

(36) The total gas flow fed through the gas inlet array is preferably equal to or greater than 3, 10, 20, 50, 100, 200, 500, or 1000 standard cm.sup.3 per minute per cm.sup.2 of the substrate area (i.e. growth surface area of the substrate) and equal to or less than 50000, 20000, 10000, or 5000 standard cm.sup.3 per minute per cm.sup.2 of the substrate area. The substrate area may be defined as the useful deposition area, with radius R.sub.s achieved in the reactor. This may correspond to a substrate (e.g. for polycrystalline diamond growth over a single substrate) or a substrate carrier (e.g. for single crystal diamond growth where the substrate carrier comprises a plurality of individual substrates) or the diameter of a table on which individual substrates may be placed (e.g. for coating loose components).

(37) Each gas inlet is preferably of a sufficiently large diameter to allow sufficient gas flow volumes at reasonable operating pressures. Accordingly, the gas inlets should not be made too small and this size limitation will limit the density of gas inlets which can be provided over the area of the gas inlet array. Conversely, each gas inlet should be made small enough to achieve a highly directed gas stream with good flow characteristics. As such, the diameter of each gas inlet is preferably in the range 0.1 mm to 5 mm, 0.2 mm to 3.0 mm, 2.0 mm to 3 mm, 0.2 mm to 2 mm, 0.25 mm to 2 mm, or 0.25 mm to 1.5 mm.

(38) The array of gas inlets is preferably formed into a surface which itself is rotationally symmetric around the central axis of the chamber. This surface may be planar, or it may curve, preferably in some smoothly varying manner. Preferably it is planar, particularly for large gas inlet arrays (e.g. greater than 100 gas inlets), or at least particularly for the majority (greater than 50% of) the gas inlets in such an array.

(39) The surface in which the gas inlets lie is preferably reasonably close to the substrate, with a distance D.sub.c from the central gas inlet (or where the central axis intersects the plane of the first ring of gas inlets) which is less than or equal to 6R.sub.s, 4R.sub.s, or 2R.sub.s, where R.sub.s is the radius of the substrate or the substrate holder. Preferably the central gas inlet, or a plane defining the first ring of gas inlets, is at least as close, or in some arrangements preferably closer, to the substrate as the plane defined by the next ring of gas inlets out from the central axis. Optionally, the plane defining the outer ring of gas inlets is also no more than 6R.sub.s, 4R.sub.s, or 2R.sub.s from the substrate.

(40) The arrangement of gas inlets can be thought to fit into one of three example configurations as discussed below, although in practice the three example configurations all lay along a continuous spectrum of possible configurations.

(41) A first example configuration is one which comprises at least seven gas inlets, comprising at least one centre gas inlet (more particularly only one) and disposed around these at least six gas inlets (more particularly six gas inlets), where the surrounding six gas inlets form a rotationally symmetric pattern and are either parallel to or divergent from (more particularly are divergent from) the central axis of the chamber. This configuration seeks to obtain the benefits of providing a plurality of gas inlets in accordance with the present invention, while limiting to a relatively low number of gas inlets, simplifying the fabrication of the gas inlet array and simplifying use of additional elements such as interchangeable gas inlet bores to vary the gas inlet diameter for different applications and flow rates. The technique can achieve substantially higher uniformity in deposition than a single gas inlet, particularly in relation to processes involving doping with boron.

(42) A second example configuration comprises a central disc of gas inlets which are all essentially parallel to the central axis of the plasma chamber and disposed in some regular array out to a radius R.sub.p, outside of which are one or more rings of increasingly divergent gas inlets to ‘soften’ the edge of the gas inlet array. This configuration seeks to achieve a balance of the benefits from the example discussed above and the example discussed below.

(43) A third example configuration is one in which R.sub.p=R.sub.m and all, or substantially all, of the gas inlets are parallel to the central axis. Ideally the gas inlets lie in a close packed hexagonal array, and the maximum radius of the gas inlet array R.sub.m meets the criteria R.sub.m×F.sub.m is greater than or equal to R.sub.s, where F.sub.m is preferably equal to or greater than 0.5, 0.6, 0.7, 0.8, 0.9, or 1 and preferably equal to or less than 1.5, 1.3, 1.2, or 1.1. This configuration provides a dense column of flow from the surface containing the gas inlets to the substrate, and which covers the entire substrate, and which permits essentially no intermixing of the existing gases in the chamber such that whilst there may be some convective currents in the peripheral regions of the chamber the substrate only sees gases which have just been injected (‘fresh’ gases).

(44) The aforementioned design criteria may provide one or more advantages for certain applications as discussed below.

(45) The position and uniformity of the plasma with respect to the substrate can easily be optimised by optimising the total flow. The solid gas stream of process gas towards the substrate can apply a substantially uniform ‘pressure’ across the plasma region.

(46) Due to minimal gas flow outside of the solid gas stream between the gas inlet array and the substrate, wall contaminants do not enter the depositing gas stream, so that the purity of the deposited diamond is essentially independent of any wall contaminants. This reduces spurious contaminants such as silicon and nitrogen which may originate from the wall or other surfaces in the reactor. This also enables deposition of high purity boron free layers in systems previously used for boron doping thus, for example, enabling continuous deposition of boron containing and then boron free layers. This also enables extreme isotope control, again enabling layers of different isotopic compositions to be precisely deposited in either a continuous process or in discrete processes.

(47) Furthermore, the solid gas stream ensures that no activated gases re-circulate within the plasma chamber and come into contact with the surface comprising the gas inlets, so that this surface stays free of deposits. This avoids any risk of such deposits breaking free and being pushed onto the substrate, which can be a source of defective growth. Further still, minimizing gas re-circulation within the plasma chamber by providing a solid gas flow of densely packed gas streams enables the gas inlets to be formed in a material which may be eroded by the plasma activated species, such as a quartz plate, without the plate becoming eroded or coated. Such a quartz plate providing the gas inlets may also be used as the entry point into the cavity of the microwave power, with its performance undiminished by the formation of coatings. Alternatively it is advantageous for the end of the microwave cavity in which the gases are introduced through the gas inlets to be proximal to the region where the microwaves are introduced, with the substrate in the distal region of the cavity.

(48) The provision of a gas inlet arrangement as previously described also enables uniform doping to be achieved over large areas of, for example, polycrystalline diamond material and/or over a large number of single crystal diamonds grown in a single growth run. Furthermore, the gas inlet arrangement as previously described has also been found to be advantageous for achieving very high levels of doping such as high boron doping concentrations for electronic and sensor applications. The present inventors found that prior art apparatus and methods, while being suitable for achieving uniform boron doping over small areas, were deficient in achieving uniform doping over larger areas. Investigating this problem, the present inventors have surprisingly found that incorporation of dopants such as boron is very sensitive to total gas flow and geometry of gas flow and that uniform doping and/or high levels of doping could only be achieved using the gas inlet arrangements as described here.

(49) For example, a gas inlet arrangement as described herein allows the total gas flow to be increased which has been found to increase the total boron incorporation measured in the solid. Furthermore, it has been found that adding boron from a side orifice leads to very poor boron uniformity in comparison with axial injection. Resistivity maps indicate that embodiments of the present invention are superior in achieving boron uniformity. Similar comments apply for other dopants such as nitrogen, silicon and phosphorus.

(50) Embodiments of the present invention have thus been able to achieve uniform and consistent product at high growth rates and over large areas. Furthermore, embodiments of the present invention have enabled the synthesis of products which have not been possible to produce using prior art methods such as high boron concentration single crystal diamonds, particularly {100} oriented single crystal material.

(51) Embodiments of the present invention improve uniformity in a CVD diamond growth process. Improvement in uniformity can be measured by one or more of the following parameters: thickness uniformity of a CVD diamond film (across the deposition area as defined by R.sub.s); uniformity of one or more quality parameters of the diamond material (e.g. colour, optical properties, electronic properties, nitrogen uptake, boron uptake, and/or boron activation level); in polycrystalline diamond material, uniformity of texture, surface morphology, grain size, etc . . . ; or in single crystal diamond material where growth takes place on an array of single crystal diamond substrates on a substrate carrier, uniformity of thickness, morphology, edge twinning, lateral growth, etc . . . , between each single crystal.

(52) The key parameters chosen for assessing uniformity depend on the synthesis process, the economics of fabricating the final product from the synthesis product, and the requirements of the final product itself. For example, for an array of single crystal diamonds, consistent morphology between adjacent crystals enabling efficient material utilisation may be more important than minor variations in colour, particularly when the material is used in cutting applications. Conversely, in boron doped material the uniformity of boron uptake may be the critical factor. The behaviour of boron in a synthesis reactor is of note here. The tendency is for boron containing gases to deplete rapidly to adjacent surfaces once the gas is broken down. Accordingly, achieving uniformity in boron incorporation in a diamond film is quite different to achieving uniformity in growth rate or morphology in intrinsic diamond where depletion of carbon containing species occurs much less quickly.

(53) While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendant claims.