Patent Application
20150030786
The present invention relates to a microwave plasma reactor for manufacturing synthetic diamond material using chemical vapour deposition techniques.
Synthesis of diamond material using chemical vapour deposition (CVD) techniques is now well known. Useful 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 al. 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).
A range of possible microwave plasma reactors for diamond film growth via a chemical vapour deposition (CVD) process are known in the art. Such reactors have 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 generator into the plasma chamber; a gas flow system for feeding process gases 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 useful overview article summarizing various possible reactor designs is given in the previous mentioned Journal of Physics (see “Microwave engineering of plasma-assisted CVD reactors for diamond deposition” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364202).
Having regard to the patent literature, U.S. Pat. No. 6,645,343 (Fraunhofer) discloses an example of a microwave plasma reactor configured for diamond film growth via a chemical vapour deposition process. The reactor described therein comprises a plasma chamber with a substrate holder mounted on a base thereof. A cooling device is provided below the substrate holder for controlling the temperature of a substrate on the substrate holder. Furthermore, a gas inlet and a gas outlet are provided in the base of the plasma chamber for supplying and removing process gases. A microwave generator is coupled to the plasma chamber via a high-frequency coaxial line which is subdivided at its delivery end above the plasma chamber and directed at the periphery of the plasma chamber to an essentially ring-shaped microwave window in the form of a quartz ring. The invention as described in this document focuses on the ring-shaped microwave window and teaches that because the coupling is distributed over a large surface, high microwave power levels can be coupled without high electric field intensities developing at the microwave window thus reducing the danger of window discharge.
EP0480581 (Applied Science & Technology Inc.) describes an alternative arrangement to that described above. The arrangement also comprises a plasma chamber in which a substrate holder is mounted above a base thereof. The arrangement also comprises a microwave generator coupled to an upper portion of the plasma chamber via a wave guide. The major difference between the arrangements described in EP0480581 and U.S. Pat. No. 6,645,343 lies in how the process gas is supplied and removed from the plasma chamber. In U.S. Pat. No. 6,645,343, the gas inlet is provided in a base of the plasma chamber adjacent to the substrate holder. In contrast, EP0480581 discloses an arrangement in which the gas inlet is positioned in a top portion of the plasma chamber directly above the substrate holder and configured to directly feed gas at high velocity towards the substrate. Process gas is removed at an outlet in or near the base of the plasma chamber and recirculated to the inlet using a pump. As such, this document discloses a high gas flow system using recirculation and a single inlet disposed axially above the substrate. The apparent advantage of such a system is that the high velocity gas flow transports activated gas species from the plasma to the substrate by convection. This enables the growth of large area diamond films when compared with systems which rely upon diffusion of activated gas species from the plasma to the substrate as is the case, for example, in U.S. Pat. No. 6,645,343.
Variations on the axially oriented gas flow arrangement described in EP048581 are known. For example, the commercially available IPLAS microwave plasma reactor comprises four gas inlet holes disposed in a top portion of the plasma chamber over the substrate holder.
More recently, US2010/0189924 (Lockheed) discloses another system 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 a similar manner to EP0480581. The configuration disclosed in this document comprises a flattened bell-shape plasma chamber in which a substrate holder is mounted above a base thereof. A microwave generator is coupled to the base of the plasma chamber via a wave guide. Gas inlet ports are provided in a top portion of the plasma chamber above the substrate holder and gas outlet ports are provided in the base of the plasma chamber.
There are several differences between the arrangements described in US2010/0189924 and EP0480581. For example, the position at which microwaves are coupled into the plasma chamber is different, microwaves being coupled in at an upper portion of the chamber in EP0480581 and at the bottom of the chamber in US2010/0189924. Furthermore, the shape of the plasma chamber is significantly different, the chamber being substantially cylindrical in EP0480581 compared to the flattened bell-shape chamber of US2010/0189924 in which the chamber is relatively wide, having a width greater than its height. Further still, the inlet configuration in the top portion of the plasma chamber is different. EP0480581 discloses a single axially disposed gas inlet, positioned directly above the substrate holder and configured to directly feed gas at high velocity towards the substrate. In contrast, US2010/0189924 discloses an inlet arrangement comprising more than one gas inlet which is similar to the commercially available IPLAS reactor. One difference between the more recent reactor design described in US2010/0189924 and the arrangements suggested in the previously described prior art is that US2010/0189924 suggests providing an axially disposed nozzle and at least 3 or 4 further nozzles spaced evenly apart around the central nozzle and angled inwardly such that gas streams emanating from the nozzles cross above the substrate, interacting with each other and with the plasma discharge. It is described that this inwardly angled nozzle arrangement improves uniformity of the diamond film by managing the shape of the plasma discharge, for example intensifying and flattening the hemispherical shape of the plasma discharge, increasing the concentration of reaction species in close proximity to the substrate surface.
The present inventors have identified a number of possible problems with prior art reactor designs. The present inventors concur with the findings of EP0480581 and US2010/0189924 that high flow velocity arrangements can be advantageous for certain applications to improve the deposition rate of a synthetic diamond film using convection rather than diffusion. As such, the present inventors believe that positioning the gas inlet above the substrate so as to direct gas flow axially towards the substrate is a preferable arrangement. However, one problem with the single axially positioned gas inlet arrangement disclosed in EP0480581 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 aforementioned problem has been at least partially solved by arrangements such as those provided the IPLAS reactor and US2010/0189924 which utilize a plurality of gas inlets. For example, the IPLAS reactor comprises four parallel oriented gas inlets whereas US2010/0189924 teaches the provision of a central gas inlet nozzle and several further nozzles spaced evenly apart around the central nozzle and angled inwardly. It is described that this inwardly angled nozzle arrangement improves uniformity of the diamond film by managing the shape of the plasma discharge. That is, the gas streams which are angled inwards will push inwards against plasma forced outward by the central gas stream thus constraining the plasma in a lateral direction and preventing a central hole forming in the plasma. While such arrangements may present an improvement over the single gas inlet arrangement described in EP0480581, the present inventors have identified several possible problems with the described arrangements.
One problem is that if the gas streams are angled to laterally constrain the plasma discharge, this will limit the area over which a diamond film can be deposited. Alternatively, if the gas streams are angled to push against a more central region of the plasma such that the outer portions of the plasma are not constrained, then this will actually exacerbate the problem of punch-through in a central region of the plasma, such that the plasma is pushed towards the sides of the substrate leading to non-uniform diamond film formation.
Another possible problem with the arrangement described in US2010/0189924 is that the plasma must be constrained in a lateral direction to ensure that it does not impinge upon the corner portions of the plasma reactor located between the central, vertically extended portion in which the inlet nozzles are provided and the laterally extending side portions of the plasma chamber. These corner portions of the plasma chamber are disposed relatively close to the region in which the plasma discharge is located in use. If the plasma were to impinge on these portions of the wall of the plasma chamber, the wall may be damaged in these regions. Furthermore, contamination by process gases may build up on the wall resulting in a degradation of the diamond film quality formed in the reactor over time. As such, it is apparent that the angled gas streams described in US2010/0189924 are required to laterally constrain the plasma discharge for these reasons.
Another possible problem with the arrangement described in US2010/0189924 is that the plasma must be constrained such that it does not ignite at high energy microwave anti-nodes disposed within the laterally extending side portions of the plasma chamber. This would lead to a reduction in power efficiency and could also damage the wall of the plasma chamber and cause contamination to build up on the walls resulting in a degradation of the diamond film quality formed in the reactor over time.
Yet another possible problem with the arrangement described in US2010/0189924 is that the microwave window is positioned in a base of the plasma chamber down stream of the inlet nozzles. This may lead to contamination of the microwave window with process gases or wall contaminants falling onto the window resulting in a reduction in performance, such as might be associated with increased microwave absorption at the window area.
Although the IPLAS reactor comprises parallel oriented gas inlets rather than gas inlets which are angled inwardly as in US2010/0189924, it has been found that the relatively small number of gas inlets provided by this arrangement is insufficient to provide uniform diamond film formation at high rates over a relatively large area. There is no teaching that indicates having more holes, or even the position of the holes, has any significant influence on deposition conditions.
It is an aim of certain embodiments of the present invention to at least partially solve the aforementioned problems. In particular, it is an aim of certain embodiments of the present invention to provide a method and apparatus for achieving high quality, uniform diamond film formation at high rates over a relatively large area.
In addition to the above, certain embodiments of the present invention are also concerned with methods and apparatus for synthesising CVD diamond comprising one or more dopants. Doping of diamond material during CVD synthesis is known in the art. Common dopants in diamond which may have some desirable use include boron, nitrogen, silicon, and phosphorous. Boron doped diamond is of particular interest as boron doping can, for example, make the diamond material semi-conductive or, at high doping levels, full metallic conduction can be achieved. Boron doped diamond finds applications that range from mechanical applications to electronics and sensors.
There is a need to grow diamond which contains a uniform concentration of boron to maintain consistency of product. For example in 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 wafers which can be processed using electric discharge machining (EDM) methods. In order to achieve this, the boron concentration needs to be high enough to ensure a reasonable and viable cutting rate, but not so high that it begins to degrade the material properties. Further, 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 diamonds to contain similar boron concentrations.
There is also a need in some methods (particularly in single crystal {100} oriented growth) to find routes to achieve the highest boron concentration necessary for example, for metallic conduction.
A significant amount of work has been performed in this field in relation to boron doped polycrystalline and single crystal diamond material. For example, EP 0 822 269 B1 discloses the basic CVD chemistry required for achieving boron doping. EP1463849 teaches how to achieve uniform boron doping over a single crystal of synthetic CVD diamond material by utilizing a diamond substrate having a surface substantially free of crystal defects.
It is a further aim of certain embodiments of the present invention to provide a method and apparatus which is capable of achieving uniform doping of CVD diamond material 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. It is also an aim of certain embodiments to achieve very high levels of doping such as high boron doping concentrations for electronic and sensor applications.
The present inventors have found that in contrast to the teachings of US2010/0189924, it is advantageous to provide a gas inlet configuration in which gas inlet nozzles are not angled inwardly to interact above the substrate and constrain the plasma in a lateral direction. While the commercially available IPLAS reactor discloses such a non-angled gas inlet configuration, both the IPLAS reactor and the reactor disclosed in US2010/0189924 have a relatively small number of gas inlets. In contrast to both these arrangements, the present inventors have found that instead of providing a relatively small number of gas inlet nozzles, the problems of gas stream punch-through in a central region of the plasma discharge at very high gas stream velocities can be reduced by providing a higher number of inlet nozzles which are substantially parallel or divergent in orientation.
In addition, it has been found that the relatively high number of nozzles should 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, a first aspect of the present invention provides a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising:
The number density and area ratio parameters can be calculated over at least a majority of the nozzles in the gas inlet nozzle array, e.g. over at least 50%, 60%, 70%, 80%, 90%, or all of the nozzles in the array.
Furthermore, a second aspect of the present invention provides a method of manufacturing synthetic diamond material using the microwave plasma reactor described above, the method comprising:
These aspects of the invention capture four key features which are advantageously provided in combination to achieve the technical benefits of the present invention: (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 should be substantially parallel or divergent; (iii) the gas inlet nozzle number density should 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 substrate. 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 reactive 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 US2010/0189924.
Further still, it has been found that providing a gas inlet nozzle array as described herein 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 CVD 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 CVD 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 CVD diamond is the onset of arcing. The structural features of this invention allow growth of CVD diamond material at higher power densities and pressures than in a conventional synthesis system which might use only one axially symmetric gas inlet hole. 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, for example a single axially symmetric nozzle. Furthermore this increase in operating pressure is not at any expense of uniform deposition area. For example, in the case of an operating frequency in a range 800 to 1000 MHz, uniform growth may be achieved to form a disk of CVD diamond having uniform thickness over a diameter in the range 70 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 <20%, <15%, <10%, <5%, or <2% of the mean thickness of the CVD 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: 200, 220, 240, 260, 280, 300, 320, 340, 360, 380, or 400 Torr 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 Torr 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 Torr 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 Torr for a microwave frequency in a range 800 to 1000 MHz, or 80 to 140 Torr 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 CVD diamond growth.
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 embodiments of the present invention may be in the range 3.0 to 4.0 W/mm2 of the substrate growth surface.
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 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 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.
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.
In addition to the above, it has been surprisingly found that incorporation of dopants such as nitrogen, boron, silicon, phosphorous and other defects such as dangling bonds and vacancy clusters are very sensitive to total gas flow and geometry of gas flow. It has been found that the provision of an inlet nozzle arrangement as previously described enables uniform doping to be achieved over larger areas of polycrystalline diamond material and/or over a larger number of single crystal diamonds grown in a single growth run.
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:
As can be seen by reference to
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 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 nozzle 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.
The gas injection nozzles may be formed by a portion of the microwave/vacuum wall such that the nozzles form part of the microwave cavity wall of the plasma chamber, rather than being outside the mesh that defines the microwave cavity wall.
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.
The gas flow system 112 comprises source gas containers 117 and a gas inlet nozzle 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 nozzle 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 nozzle array 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 plurality of gas inlet nozzles 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.
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 nozzle 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.
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 generated in the plasma which is below the 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
The gas inlet nozzle array 120 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 120 comprises a plurality of gas inlet nozzles 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 inlet nozzles 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 inlet nozzles are integrally formed.
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 nozzles. The pre-mixing chamber may include a diffuser or an array of holes disposed prior to the 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 over the substrate. Reducing the distance between the gas inlet nozzle 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.
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, the 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.
A further 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, silicon incorporation, inclusions, and nuclei for defect formation in the CVD diamond material.
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 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 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 Re 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 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 preferred that the dimensions of the gas inlet nozzles 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 nozzle inlet array which is utilized.
The present invention has thus far been described with reference to an embodiment as illustrated in
Each nozzle in the array can be characterised by its lateral spacing (radius) away from a central axis of the plasma chamber. A central nozzle, if it exists, may be 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 for 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 follow 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 Rp, and then start to become divergent out to a maximum radius on which the nozzles are placed, Rm. In the region between Rp 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 uniform 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 from 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 the density of nozzles reduces towards the outer edge of the 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 for large nozzle arrays (e.g. greater than 100 nozzles), or at least particularly for the majority of the nozzles in such an array (e.g. at least 50%, 60%, 70%, 80%, or 90% of the gas inlet nozzles have the same diameter).
Projecting all the nozzles onto a plane whose normal lies parallel to the central axis, the density of nozzles in the nozzle array, particularly for large nozzle arrays (e.g. greater than 100 nozzles), given in nozzles/cm2, 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 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 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 CVD 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 75 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 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 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, π(Rm)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.
The total gas flow fed through the nozzle array may be equal to or greater 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 nozzle array may be equal to or less than 60000, 50000, 30000, 20000, or 10000 standard cm3 per minute depending on the specific inlet nozzle array 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 and substrate area.
The total gas flow fed through the nozzle array is preferably equal to or greater than 3, 10, 20, 50, 100, 200, 500, or 1000 standard cm3 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 substrate area may be defined as the useful deposition area, with radius Rs 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).
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 mm, 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 substrate, with a distance Dc 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 6Rs, 4Rs, or 2Rs, where Rs is the radius of the substrate or the substrate holder. 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 substrate 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 6Rs, 4Rs, or 2Rs, from the substrate.
The arrangement of nozzles 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.
A first example configuration is one which comprises at least seven nozzles, comprising at least one centre nozzle (more particularly only one) and disposed around these at least six nozzles (more particularly six nozzles), where the surrounding six 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 in accordance with the present invention, 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 Rp, 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 Rp=Rm 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 Rm×Fm is greater than or equal to Rs, where Fm 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 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 sees 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 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.
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 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.
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.
The provision of an inlet nozzle 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 inlet nozzle 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 inlet nozzle arrangements as described here.
For example, an inlet nozzle 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.
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.
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 Rs); 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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1021870.9 | Dec 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/072825 | 12/14/2011 | WO | 00 | 10/29/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61439322 | Feb 2011 | US |
