MICROWAVE PLASMA REACTOR AND METHOD OF OPERATION
FIELD OF THE INVENTION
This disclosure relates to a microwave plasma chemical vapour deposition (CVD) reactor, in particular a microwave plasma CVD reactor for the production of synthetic diamond, and methods of operating a microwave plasma chemical vapour deposition reactor.
BACKGROUND
Chemical vapour deposition (CVD) processes for synthesis of diamond material are well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found, for example, the review article by R.S Balmer et al. which 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 bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), in the form of a carbon containing gas, 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, synthetic diamond material can be deposited.
Atomic hydrogen is 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 for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene 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 stream 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 an effective method for driving CVD 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 CVD 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 microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave field 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 synthetic diamond film growth using a 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.
In order to maximise control of the growing diamond, the plasma must be carefully controlled. It is known that the plasma volume is directly proportional to the microwave power, but direct measurement of this is difficult. One way to address this problem is to measure the electric field magnitude, but such a measurement is dependent on where in the chamber the measurement is taken, owing to standing wave effects and variations in the field caused by carrier/substrate etc. SUMMARY OF THE INVENTION
It is an object to provide an accurate way to measure the power of a plasma in the chamber.
According to a first aspect, there is provided a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprises: a microwave generator configured to generate microwaves; a plasma chamber; a microwave 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; a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use; a cooling system configured to pass a cooling fluid around the plasma chamber; a first temperature sensor configured to measure a temperature of the cooling fluid at an inlet of the cooling system; a second temperature sensor configured to measure a temperature of the cooling fluid at an outlet of the cooling system; means for determining a flow rate of the cooling fluid in the cooling system.
By measuring the inlet and outlet fluid temperature of the cooling fluid, and knowing the flow rate of the cooling fluid, the power of the plasma can be calculated using calorimetry.
As an option, the means for determining a flow rate of the cooling fluid in the cooling system comprises a flow rate meter.
The microwave plasma reactor optionally further comprises a controller, the controller configured to, in use, calculate a power of a plasma in the plasma chamber on a basis of a temperature measurement from the first temperature sensor, a temperature measurement from the second temperature, and a flow rate measured by the flow rate meter.
The controller is optionally further configured, in use, to calculate a volume of the plasma using the calculated power of the plasma. As an option, the controller is further configured to dynamically adjust a microwave power on the basis of the calculated power of the plasma in the plasma chamber. This kind of dynamic control allows the growth conditions to be monitored and adjusted during a growth run if necessary.
As an option, the controller is further configured to dynamically adjust any of a gas pressure, a gas flow and a gas composition from the gas control system on the basis of the calculated power of the plasma in the plasma chamber. Again, this dynamic control allows the growth conditions to be monitored and adjusted during a growth run if necessary. Furthermore, if a cooling system is provided for cooling the substrate, this can be dynamically adjusted depending on the calculated power of the plasma in the plasma chamber.
As an option, the controller is configured to estimate an efficiency of the microwave generator on a basis of the calculated power of the plasma in the plasma chamber.
The microwave coupling configuration is optionally arranged to inductively couple microwaves into the plasma chamber.
The gas flow system optionally comprises one or more inlet nozzles disposed opposite the substrate holder for injecting process gas injecting process gas into the plasma chamber.
The cooling system optionally comprises a plurality of cooling fluid circuits, each cooling fluid circuit having an associated inlet, outlet and flow rate meter. These can be summed to give the overall plasma power, but where each cooling fluid circuit is used to cool a specific portion of the microwave plasma reactor, they can be used individually to get an indication of power dissipation in different specific portions of the microwave plasma reactor.
According to a second aspect, there is provided a controller for a microwave plasma reactor for manufacturing synthetic diamond material described above in the first aspect, the controller comprising: a first input to receive temperature measurements from the first temperature sensor; a second input to receive temperature measurements from the second temperature sensor; a microprocessor configured to calculate a power of a plasma in the plasma chamber on a basis of a temperature measurement from the first temperature sensor, a temperature measurement from the second temperature, and a flow rate of the cooling fluid in the cooling system. As an option, the controller further comprises a third input to receive flow rate measurements from the flow rate meter.
The microprocessor is optionally further configured to calculate a volume of the plasma using the calculated power of the plasma.
As an option, the microprocessor is further configured to calculate a desired microwave power on the basis of the calculated power of the plasma in the plasma chamber, the controller further comprising a first output in functional communication with the microwave generator for controlling the microwave power.
The microprocessor is optionally further configured to calculate any of a desired gas pressure, gas flow and gas composition from the gas control system on the basis of the calculated power of the plasma in the plasma chamber, the controller further comprising a second output in functional communication with the gas flow system for controlling process gas flow.
As an option, the microprocessor is further configured to calculate an efficiency of the microwave generator on a basis of the calculated power of the plasma in the plasma chamber.
According to a third aspect, there is provided a method of manufacturing synthetic diamond material using a chemical vapour deposition process, the method comprising: providing a microwave plasma reactor as described above in the first aspect; locating a substrate over the substrate holder; feeding microwaves into the plasma chamber; feeding process gases into the plasma chamber; measuring a first temperature of the cooling fluid at the inlet of the cooling system; measuring a second temperature of the cooling fluid at the outlet of the cooling system; determining a flow rate of the cooling fluid in the cooling system; calculating a power of a plasma in the plasma chamber on a basis of the first temperature, the second temperature and the determined flow rate; and forming synthetic diamond material on the substrate.
As an option, the flow rate of the cooling fluid in the cooling system is determined by measuring the flow rate of the cooling fluid in the cooling system. The method optionally further comprises calculating a volume of the plasma using the calculated power of the plasma.
The method optionally further comprises dynamically adjusting a microwave power on the basis of the calculated power of the plasma in the plasma chamber.
The method optionally further comprises dynamically adjusting any of a gas pressure, a gas flow and a gas composition from the gas control system on the basis of the calculated power of the plasma in the plasma chamber.
As an option, the method further comprises estimating an efficiency of the microwave generator on a basis of the calculated power of the plasma in the plasma chamber.
As an option, the cooling system comprises a plurality of cooling fluid circuits, each cooling fluid circuit having an associated inlet, outlet and flow rate meter, and the method further comprises calculating power dissipation using one of the cooling fluid circuits of the plurality of cooling fluid circuits.
BRIEF DESCIPTION 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 illustrates schematically a cross-sectional view of a microwave plasma reactor configured to deposit synthetic diamond material using a chemical vapour deposition technique in accordance with an embodiment of the present invention;
Figure 2 illustrates schematically an exemplary cooling jacket surrounding the plasma chamber shown in Figure 1 ;
Figure 3 illustrates schematically in a block diagram an exemplary controller for controlling the microwave plasma reactor illustrated in Figure 1 ; and
Figure 4 is a flow diagram showing exemplary steps for operating the microwave plasma reactor illustrated in Figure 1. The Figures are not drawn to scale. Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity.
DETAILED DESCTIPION
Figure 1 shows an embodiment of a microwave plasma reactor 1 according to an embodiment of the present invention. The microwave plasma reactor 1 comprises the following basic components: a plasma chamber 2; a substrate holder 4 disposed in the plasma chamber for holding a substrate 6; a microwave generator 8, for forming a plasma 10 within the plasma chamber2; a microwave coupling configuration 12 for feeding microwaves from the microwave generator 8 into the plasma chamber 2; and a gas flow system comprising a gas inlet 13 and a gas outlet 16 for feeding process gases into the plasma chamber 2 and removing them therefrom.
It should also be noted that while the microwave plasma reactor 1 illustrated in Figure 1 has a separate substrate holder disposed in the plasma chamber, the substrate holder may be formed by the base of the plasma chamber. The use of the term “substrate holder” is intended to cover such variations. Furthermore, the substrate holder may comprise a flat supporting surface which is the same diameter (as illustrated) or larger than the substrate. For example, the substrate holder may form a large flat surface, formed by the chamber base or a separate component disposed over the chamber base, and the substrate may be carefully positioned on a central region of the flat supporting surface. In one arrangement, the flat supporting surface may have further elements, for example projections or grooves, to align, and optionally hold, the substrate. Alternatively, no such additional elements may be provided such that the substrate holder merely provides a flat supporting surface over which the substrate is disposed.
The microwave coupling configuration 12 comprises a coaxial waveguide 14 configured to feed microwaves from a rectangular waveguide 16 to an annular dielectric window 18. The coaxial waveguide 14 comprises an inner conductor 20 and an outer conductor 22. The inner conductor 20 is a floating post in the illustrated embodiment which is not attached to an upper wall of the rectangular waveguide 16 but rather terminates within the waveguide at a transition region between the rectangular waveguide 16 and the coaxial waveguide 14.
The annular dielectric window 18 is made of a microwave permeable material such as quartz. It forms a vacuum-tight annular window in a top portion of the plasma chamber 2. The microwave generator 8 and the microwave coupling configuration 12 are configured to generate a suitable wavelength of microwaves and inductively couple the microwaves into the plasma chamber 2 to form a standing wave within the plasma chamber 2 having a high energy anti-node located just above the substrate 6 in use.
During operation, the plasma 10 generates a significant amount of heat, and this must be managed. Turning to Figure 2, the plasma chamber 2 is provided with a cooling system 28 to externally cool the plasma chamber 2. The cooling system 28 may be in the form of a jacket surrounding at least a portion of the external walls of the plasma chamber 2, or could be a series of cooling channels provided directly in the walls of the plasma chamber 2. The cooling system has an inlet 30 and an outlet 32 to allow a fluid to be passed through the cooling system 28 in order to dissipate heat from the chamber walls. Typically, water is used as the cooling fluid owing to its availability and cost.
An estimate of the plasma power and hence plasma volume can be made using calorimetry. By measuring the temperature of the fluid entering the inlet 30 and the temperature of the water exiting the outlet 32, and knowing the rate of flow of water through the cooling system 28, an estimate of the plasma power can be made.
The temperature Ti of the water entering the inlet 30 is measured using a thermocouple, and the temperature T2 of the water exiting the outlet 32 is measured. The change in temperature, AT is the difference between T1 and T2. AT provides information about the rate at which energy is being transferred from the chamber 2 to the surroundings.
Using the known heat capacity of the water and the measured temperature change, the amount of heat absorbed by the water can be calculated using the formula:
Q = m cAT Eq. 1
Where:
Q is the heat absorbed (joules); m is the mass of the water (grams); c is the specific heat capacity of water (J/g°C); and
AT is the temperature change (in °C)
This effectively provides a measure of the amount of heat absorbed by the water. The power output of the system can then be estimated using the formula:
P = Q /t Eq. 2 Where:
P is the power output (watts);
Q is the heat absorbed (joules), as calculated in equation 1 ; and t is the time over which the heat transfer occurred (seconds)
By using the flow rate of the water and calculating the heat absorbed by the water, the power output of the plasma in the plasma chamber 2 can be calculated.
As the power is directly proportional to the plasma volume, the plasma volume can be calculated. The plasma volume can be accurately measured to within around 2% using this technique.
Of course, multiple cooling systems 28 can be provided to cool different areas of the plasma chamber 2, and the total power can be obtained by summing the powers calculated from each cooling system 28. Furthermore, where multiple cooling systems 28 are used, the calculated power can be used to estimate the distribution of power in various portions of the plasma chamber 2.
The plasma power can also be used to provide dynamic active control to the microwave plasma reactor 1 . If the power calculations suggest that the plasma power is too high, then the power output from the microwave generator 8 is reduced. Similarly, if the power calculations suggest that the plasma power is too low, then the power output from the microwave generator 8 is increased. This feedback can be carried out manually or automated.
The uniformity and quality of diamond growth is determined to some extent by the uniformity of the plasma. This is affected by a combination of the pressure within the plasma chamber 2 and the power of the plasma. Again, this information can be used to control the pressure of the process gases input by the gas flow system and thereby provide a uniform plasma. This increases the uniformity and quality of the grown diamond.
The calculated power can also be used as a measure of the efficiency of the microwave generator 8. For example, magnetrons are commonly used to generate microwaves. Magnetrons can lose efficiency over time. If the measured plasma power decreases over time for a given input power to the magnetron, then this indicates that the magnetron is losing efficiency and at some point should be serviced or replaced. Turning now to Figure 3, there is illustrated schematically in a block diagram an exemplary controller 34 for controlling the microwave plasma reactor described above.
The controller 34 comprises a microprocessor 36. A first input 38 is provided for receiving a temperature measurement from a first temperature sensor arranged to measure a temperature of the cooling fluid at the inlet 30 of the cooling system 28. The first temperature sensor may be any suitable sensor, such as a thermometer or a thermocouple. A second input 40 is provided for receiving a temperature measurement from a second temperature sensor arranged to measure a temperature of the cooling fluid at the outlet 32 of the cooling system 28. The second temperature sensor may be any suitable sensor, such as a thermometer or a thermocouple.
In this embodiment, a third input 42 is provided for receiving a flow rate measurement from a flow rate meter configured to measure a flow rate of the cooling fluid in the cooling system 28. Note, however, that the flow rate is not necessarily measured if the flow rate is constant and already known.
The microprocessor 36 uses the measurements received at the inputs 38, 40, 42 to calculate the power of the plasma 10 in the plasma chamber 2.
As described above, the microprocessor 36 may also be used to calculate the volume of the plasma 10, and to provide control signals for controlling the power output from the microwave generator 8 and the pressure of the process gases input into the plasma chamber 2 by the gas flow system. In this case, a first output 44 is provided that is functionally connected to the microprocessor 36 for controlling the power output from the microwave generator 8, and a second output 46 is provided that is functionally connected to the microprocessor 36 for controlling the pressure of gases provide by the gas flow system.
A database 48 may be provided that contains historical data collected from the microwave plasma reactor 1. This may be used, for example, to compare current plasma power with historical plasma power to obtain an indication of the efficiency of the microwave generator 8, and to alert a user of any suspected problems with the output of the microwave generator 8.
It will be appreciated that the microprocessor 36 may be a single physical microprocessor or may have its functions split between different physical microprocessors. Similarly, the inputs 38, 40, 42 and the outputs 44, 46 may be separate physical inputs or may be embodied in one or more physical in/out devices. Turning now to Figure 4, there is shown a flow diagram showing exemplary steps for operating the microwave plasma reactor described above. The following numbering corresponds to that of Figure 4.
51. A microwave plasma reactor 1 as described above is provided.
52. A substrate 6 is disposed over the substrate holder 4. The substrate may have single crystal substrates attached to it for homoepitaxial growth of single crystal diamond, or may be seeded with diamond grit for growth of polycrystalline diamond, or may be a non-diamond substrate for heteroepitaxial growth of diamond.
53. A microwave generator 8 is used to generate microwaves which are fed into the plasma chamber 2 via the microwave coupling configuration 12.
54. A gas flow system is used to feed process gases into the plasma chamber 2. Typical process gases include hydrogen and a carbon-containing gas, along with any dopant gases.
55. A first temperature of the cooling fluid is measured at the inlet 30 of the cooling system 28 as described above.
56. A second temperature of the cooling fluid is measured at the outlet 32 of the cooling system 28 as described above.
57. A flow rate of the cooling fluid in the cooling system 28 is determined. This may be, for example, by using a flow rate meter to measure the flow rate.
58. The power of the plasma 10 in the plasma chamber 2 is calculated a basis of the first temperature, the second temperature and the determined flow rate. As described above, the calculated power may also be used to determine the plasma volume, and to provide active feedback control to the operating parameters of the reactor, such as the power of the microwave generator 8 of the pressure of the gases in the gas flow system.
59. Synthetic diamond material is formed on the on the substrate 6.
A microwave plasma reactor was provided with three water flow circuits as a cooling system. Note that the cooling system referred to above is described as a single entity with a single inlet, a single outlet and a single flow rate meter. In practice it may be desirable for the cooling system to be split into separate water flow circuits, each water flow circuit having its own inlet, outlet and flow rate meter.
From an engineering perspective, it may be desirable to have different water flow circuits to cool different parts of the microwave plasma reactor. For example, if separate water flow circuits are used then flow paths can be simpler and hence require less maintenance. Furthermore, flow paths can be designed to avoid portions of the microwave plasma reactor that are difficult to access.
A disadvantage of having a cooling system made of a plurality of water flow circuits is that cumulative errors in measurements can make the overall plasma power calculation less accurate.
An advantage of having a cooling system made of a plurality of water flow circuits is that power dissipation can be measured for different portions of the reactor, allowing for better control of the process, for example by identifying hot-spots.
Any description herein to a cooling system encompasses water cooling systems made up of a plurality of water flow circuits.
In this example, the cooling system was made up of three separate water flow circuits. A first water flow circuit was used to cool an upper portion of the plasma chamber, a second water flow circuit was used to cool a lower portion of the plasma chamber, and a third water flow circuit was used to cool an area of the plasma chamber around the substrate. Thermocouples were used to measure the inlet water temperature and the outlet water temperature of each water flow circuit. Table 1 lists the plasma power calculated over three different runs of the microwave plasma reactor.
Table 1 It can be seen that the calculated power was relatively stable over the three different runs, and as described above, these data can be used to provide dynamic feedback control to the input microwave power and gas pressure. While this invention has been particularly shown and described with reference to embodiments, it will be understood by 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 appended claims.