GB2500485A - Controlling doping of synthetic diamond - Google Patents

Abstract

A synthetic polycrystalline CVD diamond wafer, said wafer having a longest dimension greater than 140 mm and comprising at least one dopant having a concentration which varies by no more than 50% of a mean concentration over at least 70% of the volume of the synthetic polycrystalline CVD diamond wafer.

Description

CONTROLLING DOPING OF SYNTHETIC DIAMOND MATERIAL

Field of Invention

The present invention relates to a method of controlling the concentration and uniformity of dopants in synthetic diamond material manufactured using a chemical vapour deposition (CVD) technique.

Background of Invention

CVD processes for manufacture of synthetic diamond material are now well known in thc art. Useftil background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R.S Balmer et at gives a comprehensive overview of CVD diamond materials, technology and applications (see "Chemical vapour deposition synthetic diamond: materials, technology and applications" J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).

Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bull thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, diamond can be deposited.

Atomic hydrogen is believed to be essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available fbr heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CYD diamond growth including arc-jet, hot filament, DC arc, oxy-acctylcnc flame, and microwave plasma.

Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas strcam and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.

In light of the above, it has been found that microwave plasma is the most effective method for driving C'VD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.

A microwave plasma activated CYD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing miero\vavc. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.

A range of possible microwave plasma reactors for diamond film growth via a chcmical vapour dcposition (CVD) proccss arc known in thc art. Such rcactors havc a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave gcncrator into thc plasma chambcr; a gas flow systcm for fccding proccss gascs into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.

A uscftil ovcrvicw articlc by Silva et al. (Paris Univcrsity's LIMHP group) summarizing various possible reactor designs is given in the previous mentioned Journal of Physics (see "Microwave engineering of plasma-assisted CYD reactors for diamond deposition" J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364202).

Having rcgard to thc patcnt litcraturc, 1156645343, EP0480581 and U52010/0189924 disclose various reactor designs including systems in which process gas is injected into the plasma chamber at high velocity to establish convective transfer of activated gas species from the plasma to the substrate in order to increase growth rate of a CVD diamond film and/or improve thickness uniformity of the CYD diamond film.

Doping of synthetic diamond material during CVD synthesis is also known in the art.

Common dopants in diamond matcrial which may havc some desirable usc includc boron, nitrogcn, silicon, sulphur, phosphorous, lithium and beryllium. Synthetic boron doped diamond material is of particular interest as boron doping can, for example, make the synthetic diamond material semi-conductive or, at high doping levels, full metallic conduction can be achieved. Synthetic boron doped diamond material finds applications that range from mechanical applications to electronics and sensors.

Thcrc is a nccd to grow synthctic diamond material which contains a uniform concentration of dopant to maintain consistency of product. For example, in boron doped polycrystalline diamond it is desirable to grow large area (e.g. greater than 120 mm diameter), thick (e.g. great than 0.5 mm), free-standing polycrystalline diamond wafcrs which can bc proccsscd using electric discharge machining (EDM) mcthods.

In ordcr to achicvc this, thc boron conccntration nccds to bc high cnough to cnsurc a reasonable and viable cutting rate, but not so high that it begins to degrade the material properties. Furthermore, the boron concentration must be within these limits over the majority volume of the disk.

A similar argument applies to single crystals, for example wherein a plurality of single crystals might be homoepitaxially grown in a single growth run. Specifications on the boron set by applications that include electronics require all of these single crystal diamonds to contain similar boron concentrations.

There is also a need in some methods (particularly in single crystal 1 00 oriented growth) to find routes to achieve higher boron concentrations necessary for example, for metallic conduction.

A significant amount of work has been performed in this field in relation to boron doped polyeiystalline and single crystal diamond material. For example, EP 0 822 269 Bl discloses the basic CVD chemistry required for achieving boron doping.

EP1463849 teaches how to achieve uniform boron doping over a single crystal of synthetic CYD diamond material by utilizing a diamond substrate having a surface substantially free of crystal defects.

J. Achard, F. Silva et at also discuss boron doping ofCVD diamond material using a reactor as described in the previous discussed Silva et at paper (see "Thick boron doped diamond single crystals for high power electronics", Diamond & Related Materials (2010), doi: 10.1016/j.diamond.2010.11.014). Here, the effect of boron concentration in the reaction gases and microwave power density is discussed in relation to boron doping of single crystal CYD diamond material. It is described that in order to increase the level of boron incorporation into a single crystal CVD diamond film it is necessary to increase the amount of diborane added to the reaction gases but for [B]![C]ga. ratios above 5000 ppm the plasma is unstable due to formation of soot that accumulates and prevents deposition longer than a couple of hours, and thus prevents the growth of thick films. It is also described that high microwave power densities are desirable for rapid growth of CVD diamond films but that higher microwave power densities result in lower boron incorporation. As such, it is concluded that a compromise must be reached by using a mid-range microwave power density (specifically disclosed as 60 Wcm3) and a [B]/[Cj, ratio of 5000 ppm to grow a 300 jim-thick heavily boron-dopcd film (1020 cm3) from which a freestanding plate can be formed.

It is an aim of certain embodiments of the present invention to provide a method and apparatus which is capable of achieving more uniform doping of CVD diamond material ovcr large arcas of polycrystalline diamond material. It is also an aim of certain embodiments to achieve higher levels of doping such as high boron doping concentrations for electronic and sensor applications. It is a further aim to achieve uniform and!or higher levels of doping while simultaneous achieving good growth rates given that some dopants such as boron have a tendency to reduce growth rates.

Summary of Invention

While it was previously known that growth rate and thickness uniformity of a synthetic CYD diamond film are sensitive to gas flow rate and geometry, the present inventors have now surprisingly found that incorporation of dopants is also very sensitive to gas flow rate and geometry of gas flow. In particular, it has been found that it is advantageous to: select a microwave plasma reactor configured to inject process gases towards a growth surface of a substrate (i.e. a gas flow configuration which is oriented axially with respect to the plasma chamber so that process gases are injected directly towards the substrate); operate the configuration at high velocity gas flow; and introduce a dopant such as boron into the process gases at a suitable concentration. It has been found that this combination of features enables uniform doping to be achieved over larger areas of polycrystaHine diamond material and!or over a larger number of single crystal diamonds grown in a single growth run and!or for achieving very high levels of doping such as high boron doping concentrations in synthetic single crystal CVD diamond material while maintaining good growth rates and good material quality. For example, following the teachings of J. Aehard, F. Silva et al. the present inventors consider that it is not possible to achieve boron incorporation into certain synthetic single crystal CYD diamond materials, e.g. {l00} oriented synthetic single crystal CVD diamond materials, significantly over 1020 em3.

In contrast, by using an axially oriented gas flow arrangement and operating at high

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velocity gas flow it has been possible to significantly exceed 1020 cm3 boron concentrations and access the metallic conduction regime in high quality single crystal CYD diamond material produced using a microwave plasma activated CVD technique. Previously, while such levels of boron incorporation were possible in thin, lower quality single crystal films and polycrystalline diamond material, such a high level of boron incorporation into high quality single crystal CVD diamond material formed using a microwave plasma, particularly using a substrate having a desirable crystallographic orientation for high quality single crystal CVD diamond growth, was not achievable. In addition, using an axially oriented gas flow arrangement and operating at high velocity gas flow, particularly when using multiple inlet nozzles, it is possible to uniformly incorporate boron into both single crystal and polycrystalline synthetic CVD diamond material over large areas.

In light of the above, it has been found to be advantageous to provide a method of manufacturing synthetic CVD diamond material, the method comprising: providing a microwave plasma reactor comprising: a plasma chamber; a substrate disposed in the plasma chamber providing a growth surface area over which the synthetic CVD 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 process gases into the plasma chamber and removing them therefrom, injecting process gases into the plasma chamber; feeding microwaves from the microwave generator into the plasma chamber through the microwave coupling configuration to form a plasma above the growth surface area; and growing synthetic CVD diamond material over the growth surface area, wherein the process gases comprise at least one dopant in gaseous form, selected from a one or more of boron, silicon, sulphur, phosphorous, lithium and beryllium, the or each dopant present at a concentration equal to or greater than 0.01 ppm, and/or a nitrogen dopant at a concentration equal to or greater than 0.3 ppm, wherein the gas flow system includes a gas inlet comprising a plurality of gas inict nozzles disposed oppositc the growth surfacc arca and configured to inject process gases towards the growth surface area, and wherein the process gases are injected towards the growth surface area at a total gas flow rate equal to or greater than 500 standard cm3 per minute and wherein the process gases arc injected into the plasma chamber through each gas inlet nozzle with a Reynolds number in a range Ito 100.

The aforementioncd method may be utilized to grow a large synthetic polycrystalline CYD diamond wafer having a substantially uniform concentration of dopant throughout the wafer. Further still, the method can be utilized to achieving very high levels of doping (e.g. equal to or greater than 2 x 10 cm) while maintaining good growth rates and good material quality. When uniform doping over large areas is dcsired, thc provision of multiplc gas inict nozzles with flow characteristics as defincd above has been found to be required.

Utilizing the above methodology and as described in more detail later in this specification, the present invention provides a synthetic polycrystalline CYD diamond wafer, said wafer having a longest dimension greater than 140 mm, a thickness of at least 0.1 n-nil, and comprising at least one dopant having a concentration which varies by no more than 50% of a mean concentration over at least 70% of the volumc of the synthetic polycrystallinc CVD diamond wafer.

Embodiments of the present invention use gas flow to control incorporation of dopants such as nitrogen, boron, silicon, phosphorous, lithium and beryllium.

Embodiments may also control the conccntration and distribution of othcr dcfccts such as dangling bonds and vacancy clusters. Particularly usethl embodiments use gas flow to control the concentration and distribution of boron in synthetic CVD diamond material to meet the needs previously described in the background section of this specification. For example: free-standing polycrystalline diamond wafers which can be reliably and consistently processed using electric discharge machining (EDM) methods, e.g. to form mechanical tool and wear parts; and free-standing polycrystalline diamond wafers with consistent and uniform conduction characteristics for use as electrodes, e.g. in water treatment and ozone generation applications.

Brief Description of the Drawings

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: Figure 1 shows a cross-sectional view of a microwave plasma reactor configured to deposit a synthetic CYD diamond film using a single axially disposed gas inlet nozzle arranged to inject process gases towards a growth surface of a substrate; Figure 2 shows a cross-sectional view of a microwave plasma reactor configured to deposit a synthetic CVD diamond film using a plurality of gas inlet nozzles arranged to inject process gases towards a growth surface of a substrate; Figure 3 shows a plan view of a gas inlet nozzle array; Figure 4 shows a cross-sectional view of a portion of the gas inlet nozzle array illustrated in Figures 2 and 3; Figure 5 shows a plot indicating the relationship between resistivity of a synthetic CYD diamond film and boron concentration -the metallic conduction regime is reached at a boron concentration of approximately 4 x 1020 cm3; Figure 6 shows a plot indicating how boron uptake varies as a function of the quantity of boron in the reaction gases -increasing boron in the reaction gases leads to a linear increase in the concentration of boron in a synthetic CVD diamond film up until approximately 4 x 1019 cni5 after which no significant increase in concentration of boron in a synthetic CYD diamond film is observed unless process gas flow velocity is increased to achieve metallic conduction as indicated by the ringed graph points;

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Figure 7 shows a plot indicating how boron uptake varies as a function of carbon containing gas flow; Figure 8 shows a plot indicating how boron uptake varies as a function of boron containing gas flow; Figure 9 illustrates how increasing flow rate can be used to access higher operating pressures without onset of arcing; Figure 10 illustrates how increasing flow rate can be used to access higher operating pressures without onset of arcing and how the range of suitable flow rates narrows at higher pressures; Figure 11 illustrates how changing gas flow Reynolds number can be used to access higher operating pressures without onset of arcing and how the range of suitable Reynolds numbers narrows at higher pressures; Figure 12 illustrates that growth rate can increase with flow rate and with an increase in the number of gas inlet nozzles; and Figures 13 to 16 show resistivity maps for synthetic polycrystalline CVD diamond wafers illustrating that doping uniformity can be improved by tailoring gas inlet nozzle diameter and spacing.

Detailed Description of Certain Embodiments

Reactor Hart/ware and Gas In let Con figurations Figure 1 shows a cross-sectional view of a microwave plasma reactor configured to deposit a synthetic CYD diamond film using a single axially disposed gas inlet nozzle arranged to inject process gases towards a growth surface of a substrate as a directed jet of high velocity process gas.

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 10 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 diclectric 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.

The plasma chamber 102 may have a number of different configurations suitable for supporting a standing microwave. However, it is found that this invention is best utilized in conjunction with simple modal synthesis chambers, for instance the TM011 mode is advantageous as it has been found to be the most compact (small) mode which can be practicably used in a diamond CYD 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 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.

The gas flow system 112 comprises source gas containers 117 and a gas inlet 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 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 120.

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 gas inlet is preferably disposed closer to the substrate holder than the microwave window. Such an arrangement can minimize the possibility of the microwavc window being contaminatcd with proccss gases while also ensuring that the process gas is injected at a location relatively close to the substrate.

In the arrangement shown in Figure 1, the gas inlet comprises a single gas injection nozzle located on a central rotational axis of the plasma chamber so as to direct a jet of process gas in an axial direction towards the substrate holder. The gas injection nozzle may be formed by a portion of the microwave/vacuum wall such that the nozzlc form part of the microwavc cavity wall of the plasma chambcr, rather than being outside the mesh that defines the microwave cavity wall.

One or more gas outlets 122 are provided in a base of the plasma chamber 102. The gas outlcts 122 arc prcfcrably locatcd in a ring around thc substratc holdcr 104 and most preferably form a uniformly spaced array around the substrate holder 104 to enhance continuous gas flow from the gas inlet 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.

Figure 2 shows a similar microwavc plasma reactor to that shown in Figurc 1. Thc arrangement shown in Figure 2 differs in that the gas inlet comprises a phirality of gas inlet nozzles arranged in a gas inlet nozzle array 124 to inject process gases towards a growth surface of a substrate in an axial direction. In other respects like reference numerals are used for like parts.

The gas inlet nozzle array 124 comprises a plurality of gas inlet nozzles disposed opposite the substrate holder 104 for directing process gases towards the substrate holder 104. The gas inlet nozzle array 124 comprises a plurality of gas inlet nozzles disposed in a substantially parallel orientation relative to the central axis of the plasma chamber 102. Thc gas inict array 124 also compriscs a housing 128 dcfining a cavity for receiving process gases from one or more gas inlet pipes. The housing 12 also defines the plurality of inlet nozzles for injecting process gases from the cavity into the plasma chamber 102 and towards the substrate holder 104. For example, the housing may comprise metallic walls in which the inlet nozzles are intcgrally formed.

The housing 128 and cavity 130 can flinction as a mixing chamber for mixing source gases prior to injection into the plasma chambcr. Such a prc-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 nozzles. The pre-mixing chambcr may include a dififiser or an array of holcs disposcd prior to thc gas inlet nozzle array to encourage gas mixing and/or provide an even flow of gas to the gas inlet nozzle array.

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 nozzle array and the substrate where diamond growth occurs affects the thickness of a boundary layer oyer the substrate. Reducing the distance between the gas inlet nozzle array and the substrate had bccn found to reducc thc thickncss of such a boundary laycr and lead to an increase in diamond deposition rate.

The present inventors have found that in contrast to the teachings of US2O1O/0189924, it is advantagcous to provide a gas inlct configuration in which gas inlet nozzles are not angled inwardly to interact above the substrate and constrain the plasma in a lateral direction. Furthermore, IJS2O 10/0189924 discloses a relatively small number of gas inlets. One problem with providing a single axially positioned gas inlet arrangement or one which uses a relatively small number of gas inlets is that at very high velocity flows, the gas stream can penetrate through the plasma, essentially punching a hole in the plasma discharge and pushing the plasma outwards towards the sides of the substrate, leading to non-uniform diamond film formation.

The present invcntors havc found that instead of providing a relatively small number of gas inlet nozzles, problems associated with the process gas stream punching-through a central region of the plasma discharge at very high gas stream velocities can be reduced by providing a higher number of inlet nozzles. The inlet nozzles may be oriented so as to be substantially parallel or divergent in orientation.

In addition, it has been found that the relatively high number of nozzles may be closely spaced to ensure a relatively uniform flow of gas. It has been found that providing a relatively high number density of nozzles in an array improves the uniformity of gas flow towards the substrate in use and allows the plasma to be uniformly flattened and shaped relative to the substrate to achieve uniform diamond film formation at high rates over a relatively large area.

It has also been found to be useful to provide relatively small area nozzles such that the area of the nozzle array is largely made up of the space in-between the nozzles rather than the area of the nozzle outlets themselves. As such, whereas it has been found to be advantageous to provide a relatively large number density of nozzles in relation to the area of the nozzle inlet array, it has also been found to be advantageous to provide an array in which the ratio of the area of the nozzle inlets divided by the area of the nozzle array as a whole is low. It has been found that small nozzles are advantageous for providing high velocity directed gas flows. However, it is also desired to have a relatively uniform gas flow over a relatively large area for uniform deposition of a diamond film over a relatively large area. Accordingly, a combination of relatively small inlet nozzle size and a relatively high number density of such nozzles has been found to be advantageous to achieve a balance between high velocity directed gas flows and uniformity of gas flow over a relatively large area.

In light of the above findings, it has been found to be advantageous to provide a gas inlet nozzle array comprising at least six gas inlet nozzles disposed in a substantially parallel or divergent orientation relative to a central axis of the plasma chamber (by substantially parallel we mean at least within 100, 5°, 2°, or 1° of a perfect parallel arrangement). Preferably, the gas inlet nozzle array comprises a gas inlet nozzle number density equal to or greater than 0.1 nozzles/cm2, wherein the gas inlet nozzle number density is measured by projecting the nozzles 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. Furthermore, the gas inlet nozzle array may comprise a nozzle area ratio of equal to or greater than 10, wherein the nozzle area ratio is measured by projecting the nozzles onto a plane whose normal lies parallel to the central axis of the plasma chamber, measuring the total area of the gas inlet nozzle area on said plane, dividing by the total number of nozzles to give an area associated with each nozzle, and dividing the area associated with each nozzle by an actual area of each nozzle.

The aforementioned arrangements capture four features which are advantageously provided: (i) a relatively high number of gas inlet nozzles (in the most simple embodiment, six nozzles arranged in a hexagonal arrangement, but preferably many more nozzles are provided for certain applications); (ii) the orientation of the nozzles may be substantially parallel or divergent; (iii) the gas inlet nozzle number density may be high (at least 0.1 nozzles/cm2 but preferably much higher for certain applications); and (iv) the ratio of the area associated with each nozzle relative to the actual area of each nozzle should be high (at least 10 but preferably much higher for certain applications).

It has been found that a gas inlet nozzle array comprising these four characteristics can be utilized to form a relatively solid curtain of process gas flowing towards the substrate. By "solid" we mean that the plurality of individual gas streams are so densely packed that they may be approximated to a single uniform mass of gas flowing towards the substratc. The solid gas stream may comprise a dense curtain of individual gas flows or an essentially continuous (in a radial direction) uniform flow of process gas. The nozzles may be configured such that individual gas streams are aimed at the substrate but do not significantly interact with each other before the substrate to cause unwanted turbulence. While the individual gas streams may merge to form a single "plug" of gas flowing towards the substrate, the streams are not configured to significantly cross each other above the substrate. This is advantageous to provide good gas flow characteristics creating a more laminar flow of the gas streams and preventing or at least reducing turbulence.

Such an arrangement can provide a relatively uniform flow of gas over a relatively large area. Furthermore, such an arrangement can reduce gas entrainment such that the majority, or preferably substantially all, the gas flows in a direction towards the substrate and out of outlets in a base of the chamber with reduced, or preferably substantially no, gas re-circulating within the chamber. It has been found that by preventing gas entrainment, the concentration of species in the activated plasma region is more controllable by direct control of the concentration of gases injected through the inlet nozzles. Furthermore, by limiting the possibility of gases re-circulating within the plasma chamber, it is possible to minimize the possibility of contaminating the walls of the chamber even when relatively large area plasmas are formed. That is, a high density of closely spaced high velocity gas streams functions both to prevent any gas flowing back towards the inlet nozzles via convection and can also provide a relatively even distribution of pressure on the plasma discharge to flatten it in an even manner and allow the possibility for very large area, flat, even plasmas to be achievable at very high flow rates.

For example, for flat substrate configurations it is has been found to be advantageous to provide a gas inlet configuration comprising a high density of inlet nozzles which are oriented in a direction substantially perpendicular to the substrate so as to provide a substantially uniform curtain of gas propagating towards the substrate and impinging upon the plasma discharge disposed between the gas inlet and the substrate.

Such an arrangement has been found to flatten the plasma discharge and increase the concentration of activated gas species in close proximity to the substrate surface.

Furthermore, the substantially uniform curtain of gas formed by a high density of the nozzles has been found to provide a substantially uniform deposition of rcactivc gas species from the plasma to the substrate via convection transport over large areas without unduly constricting the plasma in a lateral direction by angling the inlet nozzles inwardly as is suggested in 1J52010/0189924.

In some arrangements, it has actually been found to be advantageous to angle at least some of the gas inlet nozzles outwardly in a divergent configuration to achieve more uniform diamond film formation. For example, one central nozzle and six surrounding nozzles which are oriented to form divergent gas streams. This arrangement has been found to be particularly useful when a non-planar substrate is utilized. In one arrangement, a convex substrate is provided with a central portion which is closer to the gas inlet arrangement than side edge portions. The divergent nozzles are then useful to aid in pushing the plasma around towards the side edge portions of the substrate to achieve relatively uniform diamond film formation over the convex substrate. Such an arrangement is useful for forming non-planar diamond films.

While the aforementioned description specifies that at least six gas inlet nozzles may be provided to achieve more uniform diamond film formation over larger areas and/or over non-planar substrates, it has been found that for certain applications a much larger and more dense array of gas inlet nozzles is advantageous for many applications. For example, in certain applications it may be preferable to provide a gas inlet configuration comprising equal to or greater than 6, 7, 9, 10, 15, 20, 30, 40, 60, 90, 120, 150, 200, 300, 500, 700, 1000, 1200, 1500 or more gas inlet nozzles.

Particularly preferred arrangements comprise a close-packed array of gas inlet nozzles, for example, a hexagonal close-packed array of gas inlet nozzles has been found to be particularly advantageous in achieving uniform diamond film formation over large areas and at a high rate of deposition. As such, hexagonal close-packed nozzle configurations comprising 6, 7, 19, 37, 61, 91, 127, 169, 217, 271, 331, 397, 469, 547, 631, 721, 817, 919, 1027, 1141, 1261, 1387, 1519 or more nozzles may provide preferable arrangements.

Figure 3 shows a plan view of the gas inlet nozzle array 124. The array comprises a hexagonal close-packed array of gas inlet nozzles 126. The array may comprise a gas inlet nozzle number density much greater than 0.1 nozzles/cm2, wherein the gas inlet nozzle number density is measured by projecting the nozzles 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 nozzle 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.

1-lowever, 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.

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

Each gas inlet nozzle 126 may have an outlet diameter in the range 0.1 mm to 5 nun, 0.2mm to 3.0 mm, 2.0 mmto 3mm, 0.2 mmto 2 mm, 0.25 mmto 2 mm, or 0.25mm to 1.5 mm. The diameter of the gas inlet nozzles may be configured to achieve good laminar flow of the individual gas streams injected through and out of the nozzles into the plasma chamber 102. The dimensions of the gas inlet nozzles 126 also affect the Reynolds number R0 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. When calculating Reynolds numbers for a nozzle, a characteristic length scale may be taken to be a cross sectional dimension of the nozzle. 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 nozzles 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 prefcrrcd that thc dimcnsions of thc gas inlet nozzles 126 are selected to give a Reynolds number for gas injection equal to or less than 100, 80, 50, 40, or 35. Furthermore, the Reynolds number for gas injection may be equal to or greater than 1, 5, 10, 15, 20, or 25. The most preferred Reynolds number for operation will depend to some extent on the specific nozzle inlet array which is utilized.

Figure 4 shows a vertical cross-sectional view of a portion of the gas inlet nozzle array 124 shown in Figurcs 2 and 3. In the illustrated arrangcmcnt, each gas inlet nozzle 126 has an inlet portion 134 having a first diameter d1 and an outlet portion 136 having a second diameter d2, the fir st diameter d1 being larger than the second diameter d2. Such an arrangement can be advantageous as the fine bore of the outlet portion which is advantageous for operating in a low Reynolds numbcr rcgimc is only reqidred 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 and the outlet portion 136 may have a length 12 with the sum of li and 12 being equal to a wall thickness. Furthermore, this design aids in achieving clean laminar flow as a converging nozzle profile causes a parabolic velocity profile to develop more quickly. Of coursc, it is also possible to providc a gas inlct nozzle in a wall portion of the gas inlet nozzle array which consists only of a single continuous bore which may have a constant diameter along its length or a continuously varying taper.

The present inveiltion has thus far been described with reference to an embodiment as illustrated in Figures 1 to 4. 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 inlet nozzles as discussed below.

Each nozzle in the array can be characterisedby its lateral spacing (radius) away from a central axis of thc plasma chamber. A ccntral nozzle, if it exists, may bc disposed down a central axis of the plasma chamber. Nozzles at the same radius (lying on a ring centred on the central axis) may show periodic rotational symmetry around the central nozzle, although the rotational angle may vary fbr different rings of nozzles.

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

The nozzles may all be retained parallel to the central axis until some radius R, and then start to become divergent out to a maximum radius on which the nozzles are placed, Rm. In the region between R and Rm, the divergence of the nozzles may vary as a function of the radius, or the angle of divergence may be fixed.

The spacing of the nozzles may be unifbrm across the surface through which they emerge. Preferably the nozzles 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 am individual nozzles 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 nozzles may increase with radius, such that thc density of nozzles rcduccs towards thc outer edge of thc array. The nozzles may be arranged in discrete rings, with little obvious correlation in the position of nozzles lying in adjacent rings. It is indeed possible to have a random array of nozzles which provides some reasonably uniform average density of nozzles to perform an adequate performance and realise some of the benefits of this invention, although the best arrangement is one of a regular array.

The diameter of each nozzle is optionally the same, particularly lbr large nozzle arrays (e.g. greater than 100 nozzles), or at least particularly fbr the majority (e.g. greater than 50% of) the nozzles in such an array.

Projecting all the nozzles onto a plane whose normal lies parallel to the central axis, the density of nozzles in the nozzle array, particularly fbr large nozzle arrays (e.g. greater than 100 nozzles), given in nozzles/em2, is preferably equal to or greater than 0.1, 0.2, 0.5, 1,2, 5, or 10 and equal to or less than 100, 50, or 10.

Projecting all the nozzles onto a plane whose normal lies parallel to the central axis, the total area of the nozzles in the array (that is the sum of the areas of each nozzle outlet in the array), given in mm2, may be in a range I to 5000, 5 to 3000, 10 to 3000, to 2750, 30 to 2750, or 50 to 2700. The total area of the array over which the nozzles are spaced, given in mm2, 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 nozzles and the total area of the array will depend on the number of nozzles provided in the array and the area over which they are distributed, which will also be dependent on the area of CYD diamond to be grown. For example, a simple six nozzle array may have a nozzle diameter in the range 2 to 3 mm, a total nozzle area of 20 to 50 mm2, and a total array area of approximately 450 mm2. In contrast, a 91 nozzle array may have a nozzle diameter of approximately 0.5 mm, a total nozzle area of approximately 18 mm2, and a total array area of approximately 1000 mm2. Further still, a 1519 nozzle array may have a nozzle diameter in the range 0.25 to 1.5 mm, a total nozzle area of to 2700 mm2, and a total array area of approximately 8000 mm2.

A ratio of the total nozzle area / area of the nozzle 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 nozzle area / area of the nozzle 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 nozzles provided in the array and the area over which CVD diamond is to be grown. For example, a simple six nozzle array may have a ratio in the range 0.05 to 0.1, a nine nozzle array may have a ratio of approximately 0.007. a 721 nozzle array may have a ratio in the range 0.004 to 0.2, and a 1519 nozzle array may have a ratio in the range 0.01 to 0.35.

A ratio of the area of the nozzle array to an array of the growth surface of the substratemaybeintherangeo.05to2,0.ltol.5,0.Stol.25,0.8tol.1,oro.9tol.1.

For arrays which comprise a large number of nozzles (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 nozzles, the area of the array is preferably less than the area of the growth surface of the substrate.

Projecting all the nozzles onto a plane whose normal lies parallel to the central axis, the total area of the nozzle array, ir(R111)2, divided by the total number of nozzles gives the area associated with each nozzle. A ratio of the associated area of each nozzle divided by an actual area of each nozzle is preferably equal to or greater than 10, 30, 100, 300, 1000, or 3000 and equal to or less than 100000, 30000, or 10000.

Embodiments of the present invention provide inlet nozzle configurations which ensure: a relatively even flow of process gas towards the substrate thus improving diamond film uniformity; relatively little gas entrainment compared to the gas flow through the nozzles thus improving control of plasma chemistry; the possibility of plasma formation outside a region of interest near the substrate for diamond film formation is lowered; the possibility of plasma punch-through is lowered thus allowing higher velocities of gas flow and thus an increased rate of uniform diamond film formation over potentially larger areas; the nozzles are adapted to provide directed gas flow streams at high gas flow velocities and suitable operating pressures; the possibility of reactive species flowing towards the walls of the reactor by diffusion or convection is lowered thus lowering contamination during use and improving diamond film purity; and higher gas flow rates and operating pressures can be utilized without arcing occurring within the plasma chamber, enabling higher power densities to be achieved facilitating increased growth rates and improved quality of CVD diamond product.

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 nozzle array flow downwards with gravity. However, it is possible to invert the reactor such that the gas streams from the gas inlet nozzle 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 gcncrated in thc plasma which is below thc substrate in an inverted arrangement).

This inverted arrangement may have some benefits for certain applications.

It should also be noted that while the microwave plasma reactor illustrated in Figures I and 2 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.

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 nozzles. Furthermore, a plurality of gas inlet nozzles may have a non-uniform spacing and/or comprise non-uniform nozzle diameters. This may be advantageous as different gases will have different flow characteristics and so each of the nozzles, or a group of nozzles, 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 nozzles which have been specifically adapted for injecting the associated source gas. For example, one or more central nozzles may be configured to inject carbon and dopant containing gas species and optionally hydrogen gas while a plurality of outer nozzles may be configured to inject hydrogen with less or no carbon and dopant containing gas species. Such an arrangement can prevent or at least reduce contamination of the reactor walls by carbon and dopant containing species.

An alternative to the provision of a metallic housing in which the inlet nozzles are integrally formed is to form the nozzles 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 inlet nozzles are integrally formed. In such an arrangement, the high velocity laminar flow produced by a gas inlet nozzle 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 injection nozzles and the microwave window therefore reducing the problem of contaminants falling onto the substrate and causing problems such as black spots, inclusions, and nuclei for defect formation in the CYD diamond material.

Each nozzle is preferably of a sufficiently large diameter to allow sufficient gas flow volumes at reasonable operating pressures. Accordingly, the nozzles should not be made too small and this size limitation will limit the density of nozzles which can be provided over the area of the nozzle array. Conversely, each nozzle should be made small enough to achieve a highly directed gas stream with good flow characteristics.

As such, the diameter of each gas inlet nozzle 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 nim, 0.25 mm to 2 mm, or 0.25 mm to 1.5 mm.

The array of nozzles 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 nozzle arrays (e.g. greater than 100 nozzles), or at least particularly for the majority (greater than 50% of) the nozzles in such an array.

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

The arrangement of nozzles can be thought to fit into one of three example configurations as discussed below, ahhough in practice the three example configurations all lay along a continuous spectrum ofpossible configurations.

A first example configuration is one which comprises at least six nozzles. The nozzles 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 nozzles while limiting to a relatively low number of nozzles, simplifying the fabrication of the nozzle array and simplifying use of additional elements such as interchangeable nozzle bores to vary the nozzle diameter for different applications and flow rates. The technique can achieve substantially higher uniformity in deposition than a single nozzle, particularly in relation to processes involving doping with boron.

A second example configuration comprises a central disc of nozzles which are all essentially parallel to the central axis of the plasma chamber and disposed in some regular array out to a radius R1,, outside of which are one or more rings of increasingly divergent nozzles to soften' the edge of the nozzle array. This configuration seeks to achieve a balance of the benefits from the example discussed above and the example discussed below.

A third example configuration is one in which R = R13. and all, or substantially all, of the nozzles are parallel to the central axis. Ideally the nozzles lie in a close packed hexagonal array, and the maximum radius of the nozzle array Rm meets the criteria R111 x F111 is greater than or equal to R3, where F111 is preferably equal to or greater than 0.5, 0.6, 0.7, 0.8, 0.9, or I 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 nozzles 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 sccs gases which have just been injected (fresh' gases).

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

The position and uniformity of the plasma with respect to the substrate can easily be optimiscd 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.

Due to minimal gas flow outside of the solid gas stream between the nozzle array and the substrate, wall contaminants do not enter the depositing gas stream, so that the purity of the deposited diamond is improved.

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 nozzles, 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 nozzles 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 nozzles 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 nozzles to be proximal to the region where the microwaves are introduced, with the substrate in the distal region of the cavity.

Process Gas Composition and Effect of Gas flow on Dopant bptake By using a microwave plasma reactor comprising a gas inlet configuration as described in the previous section it is possible to achieve a high level of control in relation to the level and distribution of dopant uptake in a synthetic CYD diamond film. In order to achieve such control a dopant containing gas must be provided at a suitable concentration within the process gas fed through the gas inlet and the flow rate of the process gas must be high. As such, according to an aspect of the present invention the process gases comprise at least one dopant at a concentration equal to or grcatcr than 0.01 ppm and thc proccss gascs arc injcctcd towards the growth surface area at a total gas flow rate equal to or greater than 500 standard cm3 per minute.

The dopant may comprise one or more of nitrogen, boron, silicon, sulphur, phosphorous, lithium or beryllium provided in gaseous form. The dopant may intentionally be added into the process gas at a desired concentration and controlled to maintain the dopant at a desired level within the plasma reactor chamber. The type and concentration of dopant provided in the process gases will vary according to the dcsircd product. For cxamplc, to achicvc mctallic conduction in synthctic CVD diamond material it is required to provide a high concentration of boron containing gas, such as diborane, in the process gases. In contrast, to achieve semi-conductive synthetic CVD diamond material a lower concentration of boron within the process gas is needed. Low boron concentrations can also be utilized to achieve attractive blue coloured gems. Alternatively, the concentration and distribution of nitrogen dopant may be controlled to achieve, for example, synthetic OlD diamond material suitable for quantum applications. For example, synthetic CYD diamond material having a relatively low and uniform nitrogen concentration forming NV defects which can be optically addressed for sensing and information processing applications.

For example, at least one dopant may be provided in gaseous form into the process gascs at a conccntration cqual to or grcatcr than 0.01 ppm, 0.05 ppm, 0.1 ppm, 0.3 ppm, 0.5 ppm, 0.7 ppm, 1 ppm, 3 ppm, 5 ppm, 10 ppm, 20 ppm, 50 ppm, 100 ppm, ppm, 300 ppm, 500 ppm, 700 ppm, or 1000 ppm. The concentration of dopant and carbon in the process gas may be such that a ratio of dopant concentration / carbon concentration is equal to or greater than 1 x 10, 10 x 10, 100 x 106, or 1000 x irn6. Dopant concentrations are quoted as atomic concentrations of dopant to account for different possible molecular gas species comprising the dopant and carbon atoms, e.g. if a boron containing gas dopant species comprises two boron atoms, such as diborane, then the concentration of diborane in the process gas is multiplied by two to yield the atomic concentration of boron.

The one or more dopants may be injected into the plasma chamber in a controlled manncr cither separately or added to onc or more source process gases prior to controlled injection into the plasma chamber.

For certain applications it may be desirable to have substantially no nitrogen dopant while in certain other applications it may be desirable to have substantially only nitrogen dopant. Accordingly, the at least one dopant may comprise: one or more of boron, silicon, sulphur, phosphorous, lithium and beryllium with the process gases comprising less than 0.3 ppm nitrogen or substantially no nitrogen other than minor background impurities, i.c. no nitrogcn intentionally added; or nitrogen at a concentration of equal to or greater than 0.3 ppm with the process gases comprising less than 0.01 ppm of boron, silicon, sulphur, phosphorous, lithium and beryllium or substantially none of these dopant, i.e. only nitrogen intentionally added into the process gas as a dopant.

The total gas flow fed through the gas inlet may be equal to or weater than 500, 750, 1000,2000, 5000, 10000, 15000, 20000,25000, 30000, 35000, or 40000 standard cm3 per minute. The total gas flow fed through the gas inlet may be equal to or less than 60000, 50000, 30000, 20000, or 10000 standard cm3 per minute depending on the specific gas inlet configuration which is utilized. For example, for certain nozzle arrays a typical operating flow rate may lie in a range 500 to 40000, 1000 to 40000, or 2500 to 40000 standard cm3 per minute depending on the desired growth rate, growth surface area, and target dopant concentration.

The total gas flow fed through the gas inlet is preferably equal to or greater than 3, 10, 20, 50, 100, 200, 500, or 1000 standard em3 per minute per cm2 of the substrate area (i.e. growth surface area of the substrate) and equal to or less than 50000, 20000, 10000, or 5000 standard cm3 per minute per cm2 of the substrate area. The growth surface area may be defined as the useful deposition area, with radius R, achieved in the reactor. This may correspond to a substrate, e.g. for polycrystalline diamond growth over a single substrate.

It has been found that operating using a high velocity, axially oriented gas flow enables higher dopant uptake than previously possible. In addition, using an axially oriented gas flow arrangement and operating at high velocity gas flow, particularly when using multiple inlct nozzlcs, it is possible to uniformly incorporate dopants such as boron into polycrystalline synthetic CYD diamond material over large areas.

Optionally, the atomic partial pressure of dopant within the plasma reactor is equal to or greater than: 20 Pa, 100 Pa, 200 Pa, 400 Pa, 800 Pa, 1200 Pa or 1600 Pa at a microwave frequency in a range 2300 to 2600 MHz; 10 Pa, 50 Pa, 100 Pa, 200 Pa, 400 Pa, 600 Pa, 800 Pa, or 1000 Pa at a microwave frequency in a range 800 to 1000 MI-lz; 5 Pa, 25 Pa, 50 Pa, 100 Pa, 200 Pa, 300 Pa, 400 Pa, or 500 Pa at a microwave frequcncy in a range 400 to 500 MHz. It has bccn found that increasing the atomic partial pressure of dopant within the plasma reactor can lead to an increase in the concentration of dopant which is incorporated into synthetic CYD diamond material to achieve high levels of doping. Here, the partial pressure of the dopant is calculated as thc partial pressurc of the dopant containing gascous species multiplicd by thc number of dopant atoms per molecule of the dopant containing gaseous species.

Optionally, the flow rate of carbon containing gas in the process gases is equal to or greater than 5, 10, 15, 20, 30, or 40 sccm. It has been found that increasing the flow rate of carbon containing gas increases the uptake of dopants such as boron in the solid phase.

Figure 5 shows a plot indicating the rclationship bctwccn resistivity of a synthctic CYD diamond film and boron concentration. The metallic conductioll regime is reached at a boron coilceiltration of approximately 4 x 1020 cni3. Figure 6 shows a plot indicating how boron uptake varies as a function of the quantity of boron in the reaction gas (5% CH4 in H2) using a singlc axially oricntcd gas inlet nozzle and a microwave frequency of 2450 MHz. A total gas flow rate of approximately 600 sccm was utilized and boron containing species were introduced into the gas phase leading to a linear increase in boron uptake with boron concentration in the reaction gas until approximately 4 x 1019 cm5. However, further increasing boron in the reaction gas had no significant further impact on boron uptake as deduced using SIMS. The desired metallic conduction regime as indicated by the points in the red circle (approximately 4 x 1020 cm3 boron atoms in the solid phase) was only achieved by reducing the gas inlet nozzle diameter while maintaining a high total gas flow rate so as to increase the gas flow velocity.

Figures 7 and 8 illustrate the relative dependencies of carbon and boron containing gas flow on solid boron concentration in synthetic CVD diamond material.

Experiments were conducted with certain process conditions kept fixed (including 2 mm inlet nozzle diameter, 2.75 kW microwave generator power, a microwave frequency of 2450 MHz, and 90 ton process pressure). For a fixed boron containing gas flow and increasing carbon containing gas flow, the solid boron concentration as measured by secondary ion mass spectrometry (SIMS) increases from 2 1020 cm3 to 3x1021 cni3 as illustrated in Figure 7. Correspondingly, the resistivity of the synthetic boron doped CVD diamond material decreases from 8.6 x 102 0-cm to 7.7x104 0-cm. Furthermore, for a fixed carbon containing gas flow and increasing boron containing gas flow, the solid boron concentration as measured by SIMS increases from 2 1020 em3 to 2 x 1021 cni3 as illustrated in Figure 8. Correspondingly, the resistivity of the synthetic boron doped CYD diamond material decreases from 1.9x102 0-cm to i.1xi03 0-cm.

Pressure, Power Dens/tv and Substrate Temperature In addition to the above, it has been found that providing a gas inlet as previously described enables higher gas flow rates and operating pressures to be utilized within the plasma chamber without arcing occurring within the plasma chamber. Higher operating flow rates and pressures enable higher power densities which equates to a more reactive plasma, i.e. more atomic hydrogen is generated to facilitate increased growth rates and improved quality of CYD diamond product.

It has surprisingly been found that the usable ranges for the synthesis parameters of pressure and power density for the manufacture of synthetic CYD diamond material can be altered by changes in the gas dynamics (flow, geometry etc) for a given gas composition. Typically the upper limit for uniform diamond synthesis in terms of the parameters pressure and power is determined by the onset of mono-polar arcs. Those skilled in the art will know that this mono-polar arc limit is affected by experimental factors such as the operating frequency, pressure/power ratio and also the geometry (diameter/thickness) of the substrate.

The present inventors found it surprising that manipulating the gas flow can have a dramatic effect on increasing the operating parameter space in terms of pressure and power while simultaneously not reducing the area of CVD diamond deposition or the uniformity of said deposition. An often encountered limitation in the pressure/power parameter space for growing CYD diamond is the onset of arcing. Embodiments of this invention allow growth of CVD diamond material at higher power densities and pressures than in a conventional synthesis system. In practise the inventors have found that the maximum pressure for operation is increased by >5%, >10%, >15%, >20%, >25%, >30%, or >35% over that which would normally be possible with alternative gas inlet geometries/flows. Furthermore this increase in operating pressure is not at any expense of uniform deposition area. For example, in the ease of an operating frequency in a range 800 to 1000 MHz, uniform growth may be achieved to form a disk of CYD diamond having uniform thickness over a diameter in the range to 160 mm. Thickness uniformity may be calculated by measuring the thickness of the CVD diamond disk at various points and calculating the percentage deviation from a mean thickness. For example, at least 10, 15, 17 or 20 measurement points may be taken over at least 70% of a total area of the disk. In accordance with certain embodiments of the present invention the maximum growth thickness variation may be equal to or less than 30%, 25%, 20%, 15%, 10%, 5%, or 2% ofa mean thickness of the synthetic CYD diamond disk.

Using embodiments of the present invention it is possible to avoid the problem of arcing within the plasma chamber at operating pressures equal to or greater than: 100, 200, 220, 240, 260, 280, 300, 320, 340, 360, 380, or 400 Ton at a microwave frequency in a range 2300 to 2600 MHz; 120, 140 160, 180, 200, 220, 240, or 260 Torr at a microwave frequency in a range 800 to 1000 MHz; or 60, 70, 80, 100, 120, 140, or 150 Ton at a microwave frequency in a range 400 to 500 MHz. The operating pressure may be equal to or less than 550, 450, 400, 350, or 300 Ton depending on the specific reactor design. For example, a typical operating pressure using a gas inlet nozzle array according to certain embodiments of the present invention may be in the range 200 to 330 Torr for a microwave frequency in a range 2300 to 2600 MHz, 160 to 220 Ton for a microwave frequency in a range 800 to 1000 MHz, or 80 to 140 Ton for a microwave frequency in a range 400 to 500 MHz. Using embodiments of the present invention it has been found that it is possible to achieve a uniform stable plasma at these pressures and uniform CYD diamond growth.

Figure 9 shows how the threshold for formation of monopolar arcs can be a very sensitive fhnction of gas flow. In the example shown, the arcing threshold for substantially uniform growth of synthetic CYD diamond material on a 120 mm substrate in an 896 MHz microwave plasma reactor increased from 228 Torr to 262 Torr for a total gas flow change from 3 to 6.0 slpm (standard litres per minute) through six 3 mm diameter gas inlet nozzles arranged in a circular geometry having an array diameter of 37 mm.

Figure 10 illustrates a similar trend showing that substantially uniform synthetic CVD diamond growth over arge areas and at high pressure operation is only achievable by using high gas flow rates. As in the previously described example, a 120 mm substrate was with a process gas composition comprising 4% CH4 diluted in H2.

However, in this case a hexagonal array of 19 gas inlet nozzles was utilized, each nozzle having a diameter of 0.5 mm. Figure 10 also illustrates that the gas flow operating window for limiting arc formation decreases in size as the operating pressure increases. As such, the gas flow rate may be carefully selected and controlled so as to be maintained within the stability window for a particular gas inlet nozzle configuration to achieve substantially uniform synthetic CVD diamond growth over large areas and at high operating pressure without the onset of arcing.

Figure 11 illustrates how the threshold for formation of monopolar arcs varies with Reynolds number. The experimental conditions were as described for Figure 10.

This Figure shows that the Reynolds number operating window for limiting arc formation decreases in size as the operating pressure increases. As such, the Reynolds number may be carefully selected and controlled so as to be maintained within the stability window for a particular gas inlet nozzle configuration to achieve substantially uniform synthetic CYD diamond growth over large areas and at high operating pressure without the onset of arcing.

Figure 12 illustrates how growth rate of synthetic CVD diamond material varies with flow ratc and number of gas inlet nozzles up to a limit at which arcing occurs. The left hand line in Figure 12 is for a gas inlet nozzle array comprising 19 nozzles whereas the right hand line is for a gas inlet nozzle array comprising 91 nozzles. In both cases, the diameter of each nozzle was 0.5 mm. The Figure shows that growth rate increases with flow rate for both nozzle configurations but that a higher flow rate and a higher growth rate is achievable prior to onset of arcing using a larger number of nozzles.

The flow charactcristics of cmbodiments 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 morc powcr to bc introduccd at high pressurcs without thc plasma arcing.

Power densities which can be delivered to the substrate may be equal to or greater than 0.05, 0.1, 0.5, 1, 1.5, 2.0, 2.5, 2.75, 3.0, 3.2, or 3.5 W/mm2 of the substrate growth surface. The power density may be equal to or less than 6.0, 5.0, or 4.0 W/mm2 of the substrate growth surface depending on the specific reactor design. For example, a typical operating power density using a gas inlet nozzle array according to certain cmbodimcnts of thc present invention may be in the rangc 3.0 to 4.0 W!mm2 of the substrate growth surface.

The power delivered to the substrate also increases with high velocity, highly uniform gas flow, incrcasing efficiency. That is, a fraction of the total power which is delivered to a sibsftate is increased. Typically this 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.

While J. Achard, F. Silva et a!. have described that increasing the microwave power density leads to a decrease in boron uptake during CVD diamond growth, the present inventors have found that high boron uptake levels can be achieved at high microwave power densities by using high velocity axial gas flow. This allows better quality doped CYD diamond material to be produced. It may also be noted that the advantageous technical effects of increased pressure and/or power can be applied to plasma chemistries which do not require the dopant as specified in certain aspects of this invention.

In addition to the above, during CVD diamond growth it is advantageous to maintain the substrate on which the CYD diamond material is growing at a temperature in the range 600 to 1300°C, 700 to 1300°C, or 750 to 1200°C. Combining the gas flow conditions as described herein with such substrate growth temperatures allows relatively thick, high quality CYD diamond material to be formed which contrasts with lower temperature thin CVD diamond coatings of metal tools and the like.

Products Using the previously described apparatus and methods it has been possible to provide more efficient and consistent ways of producing existing products and also produce new products which have not previously been achievable.

According to the present invention it is possible to produce large synthetic polycrystalline CVD diamond wafers have a high dopant uniformity. As such, synthetic polycrystalline CVD diamond wafers are provided having a longest dimension greater than 140 mm, 145 mm or 150 mm, a thickness ofat least 0.1 mm, and comprising a dopant having a concentration which varies by no more than 50%, 40%, 30%, 20%, 10%, or 5% of a mean value over at least 70% 80%, 90%, or 95% of the volume of the synthetic polycrystalline C'VD diamond wafer. In this regard, it should be noted that individual concentration measurements arc taken over multiple grains of the polycrystalline material. That is, the measurements are taken on a macroscopic, multi-grain scale. At a scale approach the size of individual grains, concentration variations will occur, for example due to different crystallographic faces having different concentrations of dopant. As such, the technique used for individual measurements must be broad enough to provide a local average over multiple grains.

A plurality of such measurements may then be taken at different positions across the wafer. Preferably at least 70%, 80%, 90%, or 95% of the measurements fall within the previously stated concentration ranges. Secondary Ion Mass Spectroscopy (SIMS) may be utilized for measuring dopant concentrations. For polycrystalline samples, a spot size of 250 jtm in diameter may be used and the average of 4 separate measurements may bc taken to obtain a local average.

The synthetic polycrystalline CVD diamond wafer may have a thickness of at least 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.75 mm, 1 mm, 1.25 mm, 1.5 mm, or 2 mm.

Furthermore, the synthetic polycrystalline CVD diamond wafer may have a thickness variation equal to or less than 30%, 25%, 20%, 15%, 10%, 5%, or 2% of a mean thickness. Variations/uniformity values can be calculated as [(max-min)/mean] x 100.

The methods described herein are particularly useffil for manufacturing large area wafers, e.g. greater than 140 mm in diameter.

Depending on the desired application the dopant may be selected from one of nitrogen, boron, silicon, sulphur, and phosphorous and may be incorporated into the synthetic polycrystalline CVD diamond wafer at a concentration of at least 1014 cm3, -3 16 -3 17 -3 18 -3 19 -3 20 -3 20 -3 20 cm,10 cm,10 cm,10 cm,10 cm,10 cm,2x10 cm,4x10 cm3, 6 x 1020,8 x 1020 cni3, 1021 cm3, 2 x 1021 cm3, 4 x 1021 cni3, 6 x 1021,8 x 1021 cm, or 1022 cm.

Due to the high operating powers achievable using embodiments of the present invention it is possible to produce very high quality material with low levels of graphite and other undesirable impurities.

Figures 13 and 14 illustrate how boron uniformity may vary in synthetic polycrystalline CVD diamond material as a function of gas inlet nozzle diameter. The Figures show contour plots indicating variations in resistivity across a wafer of polycrystalline CVD diamond material. The resistivity will vary with a variation in boron concentration within the wafer. Figure 13 illustrates the results for a wafer formed using larger nozzle diameters (six 3.0 mm nozzles in this particular configuration) indicating some variation in resistivity and thus boron concentration.

In contrast, Figure 14 illustrates the results for a wafer formed in an analogous maimer but using a gas inlet configuration with smaller nozzle diameters (six 2.5 mm nozzles in this particular configuration) indicating substantially uniform resistivity and thus boron concentration across the wafer. Furthermore, it may be noted that the resistivity of the wafer formed using narrower diameter gas inlet nozzles is low indicating high boron uptake across the entire wafer.

Figures 15 and 16 illustrate how boron uniformity may vary in synthetic polycrystalline CVD diamond material as a function of gas inlet nozzle spacing.

Again, the Figures show contour plots indicating variations in resistivity across a wafer of polycrystalline CVD diamond material. Figure 15 illustrates the results for a wafer formed using relatively closely spaced nozzles (six nozzles disposed in a circular array having an array diameter of 25 mm in this particular configuration) indicating some variation in resistivity and thus boron concentration. In contrast, Figure 16 illustrates the results for a wafer formed in an analogous maimer but using a gas inlet configuration with less closely spaced nozzles (six nozzles disposed in a circular array having an array diameter of 37 mm in this particular configuration) indicating substantially uniform resistivity and thus boron concentration across the wafer.

The previously described resistivity measurements were made using a four point probe technique. Four point probe measurements are routinely used in assessing the resistivity of doped silicon wafers in the semiconductor industry. The four point probe technique removes the effects of any contact resistances encountered. The probe itself has Thur tungsten carbide pins arranged in a line. The four probes are placed in contact with the material to be measured and a current is applied between the two outer pins. The voltage is then measured between the two inner pins. The distance between the pins on the four point probe are large relative to the thickness of the material, for example the thickness of the wafer may be less than 40% of the spacing between each contact probe, and the polycrystallinc wafer is assumed to be semi-infinite in lateral dimensions. These two conditions allow a limiting case to be considered where the sheet resistivity is first calculated. This sheet resistivity is given by:

R

Where: * V is the measured voltage across the two inner pins of the four point probe * I is the current applied to the two outer pins of the four point probe * k is a geometric factor which in this case is equal to -n 2 This sheet resistivity (R) is then multiplied by the thickness of the material to give the resistivity of the material (p).

For each wafer, 17 points were measured using the four point probe. This consisted of one centre point, 8 points around the edge of the wafer and a further 8 points located on a circle midway between the centre and edge of the wafer. These same 17 points were used to measure the thickness of the wafer. Thus each point on the wafer has a thickness and an associated resistance measurement. Contour plots representing the resistivity of the wafer were then produced using these 17 measured points.

Synthetic polycrystallinc diamond wafers may have a mean resistivity equal to or less than 10 Ohm-cm, 1 Ohm-cm, 101 Ohm-cm, or 102 Ohm-cm. Furthermore, the wafers may have a resistivity which varies by no more than +30%, +20%, +10%, or +5% of a mean resistivity for at least 70%, 80%, 90%, or 95% of at least 17 measurement points taken across at least 70% 80%, 90%, or 95% of an area of the synthetic polycrystalline diamond wafer. Preferably, the measurement method is as previously described above.

It may be noted that when the dopant is boron, it is often easier to measure dopant uniformity indirectly using the aforementioned resistivity measurement technique rather than directly measure the concentration uniformity of boron atoms within the synthetic polycrystallinc CYD diamond wafer. Accordingly, a further aspect of the present invention provides a synthetic polycrystalline CVD diamond wafer, said wafer having a longest dimension greater than 140 mm or 145 mm and comprising boron dopant, wherein said wafer has a resistivity which varies by no more than +30%, +25%, +20%, +15%, +10%, or +5% of a mean resistivity for at least 70%, 80%, 90%, or 95% of at least 17 measurement points taken across at least 70% 80%, 90%, or 95% of an area of the synthetic polycrystalline diamond wafer. For example, at least 90% of the measurement points may fall with +25% of the mean resistivity, at least 80% or 90% of the measurement points may fall within ±20% of the mean resistivity, at least 60%, 70%, 80% or 90% of the measurement points may fall with +15% of the mean resistivity, and most preferably at least 60%, 70%, 80% or 90% of the measurement points may fall with +10% of the mean resistivity.

Summary

In light of the above, it will be evident that embodiments of the present invention enable uniform doping to be achieved over large areas of polycrystalline diamond material. Furthermore, very high levels of doping can be achieved for electronic and sensor applications.

Embodiments of the present invention have also 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.

In addition to improve dopant uniformity in synthetic CVD diamond material, certain embodiments of the present invention can also improve uniformity of other material parameters. For example, improvement in uniformity can be measured by one or more of the following parameters: thickness uniformity of a CYD diamond film (across the deposition area as defined by R); uniformity of one or more quality parameters of the diamond material (e.g. colour, optical properties, electronic properties); and in polycrystallinc diamond material, uniformity of texture, surface morphology, grain size, etc....

The key parameters chosen for assessing uniformity depend on the synthesis process, the economies of fabricating the final product from the synthesis product, and the requirements of the final product itself For example, 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 may be quite different to achieving uniformity in growth rate or morphology in intrinsic diamond where depletion of carbon containing species occurs much less quickly.

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.

Claims (10)

1. Claims 1. A synthetic polycrystalline CYD diamond wafer, said wafer having a longest dimension greater than 140 mm, a thickness of at least 0.1 mm, and comprising at least one dopant having a concentration which varies by no more than 50% of a mean concentration over at least 70% of the volume of the synthetic polycrystalline CVD diamond wafer.
2. 2. A synthetic polycrystallinc CYD diamond wafer according to claim 1, whercin the concentration varies by no more than 40%, 30%, 20%, 10%, or 5% over at least 70%, 80%, 90%, or 95% of the volume of the synthetic polycrystalline CYD diamond wafer.
3. 3. A synthetic polycrystalline CYD diamond wafer according to claim 1 or 2, wherein the dopant is selected from one or more of nitrogen, boron, silicon, sulphur, phosphorous, and lithium.
4. 4. A synthetic polycrystalline CYD diamond wafer according to any preceding claim, wherein the synthetic polycrystalline C'VD diamond wafer comprises said dopant at a concentration ofat least 1014 cm, loIs em3, 1016 cm3, 1017 cm3, 1018 em 3, 1019 cm3, 1020 cm3, 2 x 1020 cm3, 4 x 1020 cm3, 6 x 1020, 8 x 1020 cm3, 1021 cm3, 21 -3 21 -3 21 21 -3 22 -3 2x10 cm,4x10 cm,ôxlO,8x10 cm,or 10 cm.
5. 5. A synthetic polycrystalline CYD diamond wafer according to any preceding claim, wherein the synthetic polycrystalline CVD diamond wafer has a thickness of at least 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.75 mm, 1 mm, 1.25 mm, 1.5 mm, or 2 mm.
6. 6. A synthetic polycrystalline CVD diamond wafer according to any preceding claim, wherein the synthetic polycrystalline CYD diamond wafer has a longest dimension of at least 145 mm or 150 mm.
7. 7. A synthetic polycrystalline CYD diamond wafer according to any preceding claim, comprising a dopant ratio x! y equal to or greater than 2, 5, 10, 20, 50, 100, 500, or 1000, where x is said dopant andy is one or more different impurities selected from thc group consisting of boron, nitrogen and silicon.
8. 8. A synthetic polycrystalline CVD diamond wafer according to any preceding claim, having a resistivity equal to or less than 10 Ohm-cm, 1 Ohm-cm, 10' Ohm-cm, or 102 Ohm-cm.
9. 9. A synthetic polycrystalline CYD diamond wafer according to any preceding claim, having a rcsistivity which varics by no morc than +30%, +20%, +10%, or +5% of a mcan resistivity for at Icast 70%, 80%, 90%, or 95% of at Icast 17 measurement points taken across at least 70% 80%, 90%, or 95% of an area the synthetic polycrystalline diamond wafer.
10. 10. A synthetic polycrystallinc CVD diamond wafcr according to any preceding claim, having a thickness variation equal to or less than 30%, 25%, 20%, 15%. 10%, 5%, or 2% of a mean thickness.