Patent Application
20130000552
The present disclosure relates to a device and method used to produce bulk single crystals. In particular, the present disclosure relates to a device and method used to produce bulk single crystals of a metal compound by an elemental reaction of a metal vapor and a reactant gas.
Crystalline metal compounds such as aluminum nitride and zinc oxide possess desirable properties for semiconductor applications including wide energy bandgaps, high-breakdown electrical fields, and high thermal conductivity. These properties are prominent in crystalline series III-V metal compounds having few defects. As such, it is desirable to produce large crystals for use in semiconductors; however, it may be difficult to produce large crystals having few defects.
Currently, a number of different methods are used, to produce large, low-defect crystalline metal compounds; however, each method has inherent limitations. A conventional sublimation method, shown in
Another method is the hydride vapor phase epitaxy (HVPE) method, shown in
Furthermore, other methods may facilitate other unwanted chemical reactions. For example, nitrozation at the surface of a melted metal feedstock, such as aluminum, in contact with a reactive gas, such as nitrogen or ammonia, may form more stable compounds at the exposed surface of the metal feedstock. These stable compounds greatly reduce the amount of vaporized feedstock produced relative to an initial rate of vaporization. In addition, as existing methods typically make use of a closed reactor, the crystals are formed using a batch process limited by the holding capacity of the crucibles used to vaporize the crystal compound or metal.
There exists a need for devices and methods of growing single low-defect crystals of metal compounds at a high growth rate. Further, a need exists for an efficient reactor to produce crystals that are not limited in size by the batch processing of a limited amount of feedstock.
An open-flow reactor for producing single crystals of a metal compound that overcome the limitations described above has been developed. The open-flow reactor incorporates a continuous addition of replenishable feedstock, making possible the continuous production of the single crystals to any desired size, unlimited by the availability of feedstock. Furthermore, the design of the open-flow reactor incorporates repositioning the growing crystal within the reactor to maintain the exposed surface of the crystal at a temperature conducive to rapid crystal growth. The exposed surface of the crystal is positioned to avoid exposing the already formed portion of the crystal to unnecessary mechanical or thermal stresses that may induce undesired voids within the crystal. Using the open-flow reactor, high-purity single crystals having a diameter of up to about six inches may be grown. The maximum crystal diameter may be limited only by the center-to-edge temperature uniformity within the crystal.
The open-flow reactor provides for the growth of a single crystal of a metal compound by an elemental reaction of a metal vapor and a reactant gas. The reactor includes a metal vapor system to continuously provide the metal vapor to the reactor. For example, the metal vapor system may include an electrical arc which vaporizes a metal wire as the wire is inserted into the reactor. In addition, the metal wire may be provided from a spool, providing essentially unlimited metal vapor to the reactor.
The reactor further provides a rotation-translation-weighing system to monitor the weight of the single crystal as it grows and to reposition the crystal to a position suitable for continued growth within the reactor, as the crystal increases in size. The rotation of the single crystal may reduce variations in the thickness of the crystal as it condenses from the metal compound vapor within the reactor. The movement of the rotation-translation-weighing system allows the crystal to be repositioned within the reactor to a location having reaction conditions at which rapid growth may be achieved. The weight of the growing crystal obtained by the rotation-translation-weighing system may be used to monitor various process parameters, such as overall crystal size or crystal growth rate.
The reactor is an open-flow reactor that includes a metal feedstock and a metal heating source for vaporizing the metal feedstock. The metal vapor reacts with a reactive gas to grow the crystals. The metal feedstock may include one or more pure metals, metal nitrides, and/or a doping metal. A doping metal may be used to intentionally dope the crystal during growth. The reactor includes a crystal growth substrate that is mechanically connected to an actuator for positioning the crystal growth substrate within the open-flow reactor. A force transducer is mechanically connected to the crystal growth substrate to monitor the weight of the single crystal as it grows.
The reactor further includes a metal heating source that includes a vaporization region situated between a vaporization inlet and a vaporization outlet. A metal feedstock that includes a metal storage container and a protruding free end is also included in the reactor, and the protruding free end is inserted through the vaporization inlet into the vaporization region.
One embodiment of the reactor also includes a gas supply tube that incorporates an exit port at one end and a gas supply connection at an opposite end of the gas supply tube. The exit port is situated near the vaporization inlet and is aimed at the vaporization region. A connection fitting of a gas source, which also includes a pressurized gas container, is hydraulically connected to the gas supply connection of the gas supply tube.
A hollow thin-walled heater can be included in the reactor. The heater defines a continuous internal volume that opens to a heater inlet formed at one end of the heater and to a heater exit formed at an opposite end of the heater. The heater inlet is hydraulically sealed to the vaporization outlet of the metal heating source. The continuous internal volume defined within the heater includes a mixing chamber, a crystal growth chamber that includes a growth inlet and a growth outlet, and a post-reaction chamber. The mixing chamber opens to the heater inlet at one end and to the growth inlet at its opposite end. The crystal growth chamber opens to the growth inlet at one end and the growth exit at its opposite end. The post-reaction chamber opens to the growth exit at one end and to the heater exit at its opposite end.
A thin sheet recrystallization substrate is situated within the crystal growth chamber. The substrate has a recrystallization face and an opposing mounting face. The recrystallization face is exposed to the growth inlet of the crystal growth chamber. The mounting face of the recrystallization substrate is attached to the attachment face of a substrate chuck. The substrate chuck has a rod attachment face opposite to its attachment face. The rod attachment face of the substrate chuck is attached to an attachment fitting at one end of an elongate support rod. The opposite end of the support rod has an actuator fitting that is attached to a weight sensor and at least one actuator.
The reactor of this embodiment also includes a reactor housing that includes a thin-walled hollow reactor body, an inlet end cap, and an outlet end cap. The reactor body defines a reactor volume sealed at one end by an interior surface of the inlet end cap and by an interior surface of the outlet end cap at an opposite end of the reactor body. The vaporization inlet of the metal heating source is sealed to the interior surface of the inlet end cap and opens to the exterior of the reactor housing through a reactor inlet channel running through the inlet end cap. The heater exit is sealed to the interior surface of the outlet end cap and opens to the exterior of the reactor housing through a reactor outlet channel running through the outlet end cap. At least one purge tube runs through the outlet end cap from the exterior of the reactor housing to the reactor volume external to the continuous internal volume of the heater.
Each purge tube includes a purge tube inlet at the end of the purge tube in contact with the reactor volume, and a vacuum attachment fitting at an end of the purge tube external to the reaction housing. At least one vacuum tube of a vacuum source is hydraulically connected to the vacuum attachment fitting of the one or more of the one or more purge tubes. Another of the at least one vacuum tubes may be connected to the heater exit. The opposite ends of the at least one vacuum tubes are hydraulically connected to a vacuum pump.
Embodiments of the disclosure may further provide a method of producing a single crystal of a metal compound by an elemental reaction of a metal vapor and a reactant gas that includes vaporizing a metal to produce the metal vapor. A reactant gas is contacted with the metal vapor, and the resulting mixture is heated to produce the metal compound. The metal compound is contacted with a crystallization substrate to precipitate a single crystal.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.
In various embodiments, the present disclosure provides an open flow reactor system 400 for the growth of a single crystal of a metal compound by an elemental reaction of a metal vapor and a reactant gas. The disclosure includes an open flow reactor designed and dimensioned to provide for continuous growth of bulk crystals. The disclosure further relates to a method of producing bulk crystals.
The reactor housing 402 is the encasement where the reaction between the metal vapor and reactant gas occurs to produce the resulting crystal product. A rotation/translation/weighing system 404, connected to the reactor housing 402, obtains real-time feedback about the production quality and amount of production of the single crystal. A metal vapor system 406 includes a metal heating source to vaporize the metal feedstock. The resulting metal vapor flows to the reactor housing 402 where it is mixed with the reactant gas supplied by a gas delivery system 408.
The gas delivery system 408 provides one or more reactant gases, such as nitrogen, to both the reactor housing and the metal vapor system. The reactant gases may be used to cool the one or more seals of the reactor housing 402. The reactant gases may also be used to purge the interior volume of the reactor housing 402 by removing any reactive oxygen present in the reactor housing 402. The gas delivery system 408 also connects to the metal heating source of the metal vapor system 406 to provide one or more reactant gases, such as nitrogen and/or a doping gas, before and/or after providing the metal vapor and reactant gas to the reactor housing 402.
Connected to the reactor housing 402 is a vacuum system 410 used to remove the one or more reactant gases provided by the gas delivery system 408 to purge the reactor housing 402. The amount of vacuum pressure applied to the reactor housing 402 may be modified to control the rate at which gases flow through the reactor housing. The heater system 412 heats the interior of the reactor housing 402 using for example, a radio frequency induction heater.
In various embodiments, the reactor system 400 may be configured as an upflow reactor in which the metal is vaporized and injected into the bottom of the reactor housing 402, in which the gas flow direction is toward the top of the reactor housing. In other embodiments, the reactor system 400 may be configured as a downflow reactor system 400 in which the metal is vaporized at the top of the reactor housing 402 and the gas flow direction is in the downward direction. In other embodiments, any other angular orientation of the reactor system 400 may be used ranging between 0° (upflow design) and 180° (downflow design). In yet other embodiments, the angular orientation may vary during the operation of the reactor system 400. In the various embodiments, the metal vapor, reactive gases, and other components used in the crystal growth are introduced to the reactor housing 402 through an inlet end and removed via an exit end.
Alternatively, the reactor system 400 may configured in any suitable orientation including a horizontal orientation, a vertical orientation, a 45° orientation relative to a vertical reference, and any intermediate orientation depending on the desired use of the reactor system 400.
In addition to the specific systems described above, the processes performed by the systems of the open flow reactor system 400 may be controlled by a control system (not shown). The control system may include manual inputs to the various devices included in the systems of the open flow reactor system 400, or the control system may be an automated control system that autonomously controls the processes of the open flow reactor system. The control system may be an open-loop system, in which predetermined commands to modify the processes of the reactor system 400 are issued sequentially to perform the functions of the reactor system. Alternatively, the control system may process feedback information measured from one or more sensors including a weight sensor, a temperature sensor, a pressure sensor, or a flow velocity sensor, to develop commands used to perform the functions of the reactor system 400.
The systems and components of the reactor system 400 described above, as well as exemplary embodiments of the reactor system are described in detail below.
An internal view of an embodiment of the reactor system is generally indicated as 500 and shown in
a. Reactor Sidewall
The sidewall 504 provides mechanical support for the related components of the reactor housing 502. The sidewall 504 also forms a hydraulically sealed container within which the crystal formation takes place. In addition, the sidewall 504 insulates the heating components located inside the reactor housing 502 and protects the environment outside of the reactor housing 500 from the high temperatures present inside the reactor housing 502. The sidewall 504 is constructed of at least a single layer of material, but can be constructed of two or more layers of material. The material of the sidewall 504 can be a single uniform material or a combination of a number of materials.
Materials suitable for use in the construction of the sidewall 504 are selected to maintain structural integrity at pressures within the reactor housing 502 ranging from about 0.01 ATM to about 20 ATM and temperatures up to about 2500° C. that may occur during the crystal production process. In addition, suitable materials are selected to resist thermal fatigue induced by the thermal cycles between room temperature and the operating temperatures of the reactor housing 502. Materials suitable for use in the construction of the side wall include stainless steel, quartz, aluminum borate and similar glasses, Ni, Ti, and ceramic materials such as alumina, zirconia, and magnesia that are capable of tolerating the high temperature conditions of the reactor housing 502. Combinations of these materials may also be used.
The sidewall 504 may also include active cooling elements to cool the reactor housing 502 and/or provided greater protection to the environment surrounding the reactor housing 502. Non-limiting examples of active cooling elements include radiative cooling elements, such as heat vanes attached to the outer layer of the sidewall 504, and conductive cooling elements, such as one or more heat sinks in contact with any one or more of the layers of the sidewall 504. Other examples are convective cooling elements, such as one or more fluid-cooled layers of the sidewall 504. An example of a fluid-cooled layer within the sidewall 504 is illustrated in
The fluid-cooled sidewall 504, as illustrated in
The fluid used to provide convective cooling may be any cooling fluid known in the art. The cooling fluid may be a gas such as air, nitrogen, helium, carbon dioxide, or a combination of gases. Alternatively, the cooling fluid may be a liquid such as water, ethylene glycol, diethylene glycol, propylene glycol, betaine, polyalkylene glycols, mineral oils, castor oil, silicone oils, fluorocarbon oils, or a combination of liquids. In an exemplary embodiment, water is used as the cooling fluid.
The sidewall 504 may also include seals 520A and 520B to hydraulicly seal of the interior of the reactor housing 502. The material used in the construction of the seals is selected to maintain function under the pressure and temperature ranges described above. The material of the seals 520A and 520B is also selected to maintain some degree of flexibility and maintain a seal in the presence of thermal expansion and contraction, vibrations, or other movements of components of the reactor housing 502. Any material known in the art for forming seals around devices having wide ranges of operating temperatures and pressures may be used. Non-limiting examples of materials suitable for use in the construction of the seals 520A and 520B include copper, rubber, and silicon.
b. End Caps
End caps 506 and 508 seal the respective ends of the reactor housing 502. The end caps 506 and 508 are attached to the opposed ends of the side wall 504 that enclose the interior space of the reactor housing 502 and provide sealed openings through which the metal vapor, one or more reactant gases, and unprecipitated crystalline metal compound may enter and exit the reactor housing 502. The end caps 506 and 508 also provide fittings to which other systems of the reactor housing 502, such as the metal vapor system 406 or heater system 412, may be attached.
The reactor housing 500 may include an inlet end cap 508 that includes openings 522 and 524 through which electrodes 528 and 530 may be inserted to connect the metal vapor system 406 to the reactor housing 402 (see
Suitable materials for the construction of the end caps are selected to maintain function at temperatures of at least 300° C. and up to about 2500° C. and pressures ranging from about 0.01 ATM to about 20 ATM, and to possess material properties for mounting fittings such as threaded male or female fittings. Non-limiting examples of materials suitable for the construction of end caps 506 and 508 include stainless steel, Ni, Ti, Mo, and combinations thereof.
Any alternative device for sealing high-temperature pressurized or evacuated vessels known in the art may be used in various embodiments without departing from the spirit of this disclosure.
The metal vapor system 406 supplies metal vapor used in producing the single crystal of the metal compound. In general, the metal vapor is produced by heating an amount of one or more metals to a temperature above the boiling point of the metal(s), causing the metal(s) to vaporize. Non-limiting examples of suitable metals include Mg, Al, Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, In, Sn, Ta, W, metal nitrides, and alloys thereof. Depending on the particular metal(s) to be vaporized, the temperature may range from about 750° C. to about 5600° C. The metal vapor system is designed to minimize undesired chemical reactions at the surface of the vaporizing metal, such as nitrozation, which may potentially limit the vaporization rate of the metal(s).
Various embodiments of the metal vapor system 406 may include a metal heating source including but not limited to an electrical arc, a DC plasma source, an induction coupled plasma source, a laser source, and a microwave source. The metal heating source may be any device that can heat metal to a boiling point. In some embodiments, one or more metal heating sources are used to vaporize the metal(s). In one example, an electrical arc and a DC plasma source may be used. In another example, a combination of multiple electrical arcs, DC plasma sources and an induction coupled plasma source may be used to vaporize the metal(s). In these embodiments, the metal heating sources may have varied and/or multiple spatial orientations within the reactor system 400 and remain within the scope of the disclosure.
The heating source(s) may be situated between a vaporization inlet and a vaporization exit. A metal feedstock is inserted into the metal heating source(s) through the vaporization inlet, and the vaporized metal exits the heat source(s) through the vaporization exit. In addition to the metal feedstock, at least one reactive gas may be introduced into the vaporization inlet and help remove the vaporized metal by convection.
The elements of an exemplary metal vapor system 406 are illustrated in
An external arc power supply (not shown) may generate the power for the electrical arc 540. The metal wire 538 may be drawn up to the electrical arc 540 area by an actuator such as a stepper motor (not shown). The electrical arc 540 and the feed of the metal wire 538 may be pulsed and timed together so that a new section of metal wire 538 is vaporized by each new pulse of the electrical arc 540. Alternatively, the electrical arc 540 may be constant and the metal wire 538 may be fed step-wise into the electrical arc 540.
Any suitable material may be used to construct the electrodes 538 and 540. Conductive materials with relatively high boiling points may be selected to avoid vaporization of the electrode material and inadvertent contamination of the single crystal produced by the reactor housing 502. Non-limiting examples of materials suitable for the construction of electrodes 538 and 540 include tungsten, copper, zirconium, carbon, and boron.
In other embodiments, one or both electrodes 538 and 540 may be replaced with one or more separate metal feedstocks. In these embodiments, the one or more metal feedstocks are connected to the electrical power source and act as consumable electrodes.
An exemplary embodiment of a metal vapor system 406A in which both electrodes 538 and 540 are replaced by metal feedstock is illustrated in
The material of the metal feedstock is selected based on the desired single crystal metal compound to be produced in the reactor housing 502. Non-limiting examples of suitable metals for the metal feedstock include Mg, Al, Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, In, Sn, Ta, W, and combinations thereof. In a preferred embodiment, the metal feedstock is aluminum or any alloy thereof.
The metal feedstock may consist of a single metal element. Alternatively, the metal feedstock may include a metal element and a second doping metal. The doping metal may be introduced into the reactor housing 502 by using a number metal feedstocks, where one metal feedstock is a pure metal and a second feedstock contains a selected doping metal. When vaporized, the pure metal and the doping metal form a metal vapor mixture which may condense into a single crystal of a doped metal compound. Alternatively, the metal and the doping metal may be combined within the same metal stock as an alloy. The metal feedstock may be introduced into the electrical arc 614 as a uniform composition to form a single crystal of a single homogeneous composition. Alternatively the composition of the metal feedstocks may be changed either abruptly or gradually to produce single metal compound crystals having discrete layers of changing composition or a gradually changing composition within the single crystal, respectively.
The metal feedstock may be supplied in any suitable form including but not limited to metal powder, a metal wire, a metal tube, a metal rod, and a metal bar. The metal feedstock may be supplied in the form of a container of metal powder, a spool of metal wire or metal tubing, or a sequence of metal rods or bars. If the metal feedstock is supplied in the form of a metal wire, the wire diameter may range from about 0.001 to about 1 cm. In a preferred embodiment, the metal feedstock is supplied as a spool of metal wire. In yet another embodiment, a liquid metal source, including but not limited to Al, Ga, and In, may be injected into the reactor without departing from the spirit of the disclosure.
In other embodiments, the metal vapor may be produced by vaporizing a metal nitride powder, or a mixture of metal(s) and metal nitride powder(s). By way of example and not limitation, the metal vapor may be produced by vaporizing a mixture that is 25% Al wire and 75% AlN powder. In this example, the Al wire and A1N powder may be introduced onto reactor through the same or different inlets. The metals may also be vaporized by the same or different metal heating sources.
In another example, a mixture that is 10% Al powder and 90% AlN powder may be vaporized. In this example, the powders may be mixed prior to being introduced into the reactor. In other examples, any number of metal powders and/or metal nitride powders may be mixed and vaporized. In a preferred embodiment, the powder particles range in diameter from 1 mm to 1 nm.
An exemplary embodiment of a spool-fed metal vapor system 406B is illustrated in
Electrodes to generate the electric arc may be any orientation so long as a small gap remains between the electrodes to form an electrical arc.
Other sources of heat may be used to vaporize the metal feedstock in addition to an electrical arc. In one embodiment, an induction coupled plasma source may be used to produce the metal vapor.
An exemplary embodiment of an induction coupled plasma metal vapor source is illustrated in
The gas delivery system 408 delivers at least one reactive species to the reactor housing 502. The gas delivery system 408 includes a number of devices that deliver gas to the reactor housing 502. The at least one reactive species may form the vapor mixture with which the metal compound will be condensed to form the single crystal, and may also be used to purge the reactor of unwanted gases such as air or oxygen. The gases may also cool the inlet end of the reactor housing 502. The particular gas composition supplied to the reactor may be selected depending on the desired composition of the singe crystal to be produced. The gas mixture may include a reactive gas as well as a doping gas used to produce a doped single crystal composition. The doping gas may be selected to enhance the p-type or n-type conductivity of the bulk crystals produced by the process described herein.
Non-limiting examples of reactive gas species suitable for use in the gas delivery system 408 in the production of single crystals include nitrogen, oxygen, carbon dioxide, NH3 CH4, C2H2, C3H8, B2H6, AsH3, H2S, SiH4, GeH4, GeCl4, dimethylberyllium, trimethlylgallium, trimethylindium, triethylgallium, triethylindium, di-isopropylmethylindium, ethyldimethylindium, Isobutylgermane, dimethylamino germanium trichloride, tetramethylgermane, tetraethylgermane, and combinations thereof.
The composition of the gas mixture introduced to the reactor may be uniform to grow a single crystal with a uniform composition. Alternatively, the composition of the gas mixture may be abruptly varied from one composition to another to grow a single crystal with discrete layers of different compositions. In one embodiment, the composition of the gas mixture can be abruptly changed, in real-time, during the growth process, thereby changing the composition of the crystal. In another embodiment, the composition of the gas mixture may be varied gradually over time to produce a gradient within the composition of the crystal.
The gas delivery system 408 may include a gas supply tube connected to a gas source. The gas source may include a pressurized gas container and a connection fitting hydraulically connected to one end of the gas supply tube. The exit port at the opposite end of the gas supply tube is inserted into the reactor and reactive gases are directed toward the vaporization inlet of the metal vapor system 406. In other embodiments the reactive gases may be directed further downstream of the vaporization inlet. The gas delivery system may further include a manual or electrically controllable pressure valve to control the rate of flow of reactant gases into the reactor. Depending on the particular crystal production process, the reactive gases may be delivered at a total pressure ranging from about 0.01 ATM to about 20 ATM and a temperature ranging from about room temperature to about 2500° C. The gas supply tube may be constructed of any suitable material known in the art, and may further include insulation to prevent heating of the reactive gas mixture as it enters the reactor housing 502.
The vacuum system 410 may be attached to an exit end of the reactor housing 502 in various embodiments. The vacuum system removes oxygen and other unwanted atmospheric gases from the interior of the reactor housing 502 prior to starting a crystal formation process. Additionally, the vacuum system 410 may be used to generate a gas flow within the reactor housing 502 by exhausting the gases introduced at the inlet end of the reactor housing 502 out through the exit end of the reactor housing 502.
The vacuum system 410 may include a vacuum source that may include at least one vacuum pump and at least one vacuum tube attached to the vacuum pump at one end. The opposite end of each of the vacuum tubes is hydraulically connected to a vacuum attachment fitting at the exit end of the reactor housing 502.
The vacuum system 410 may further include a vacuum valve to control the amount of vacuum pressure applied to the reactor. The vacuum valve may be set and adjusted manually by an operator of the reactor housing 502, or the vacuum valve may be electronically adjusted based on measured properties of the reactor housing 502 including temperatures, pressures, and the weight or size of the growing single crystal measured by the rotation/translation/weighing system 404.
The vacuum system 410 may include a two-stage pump. The first stage may be a mechanical vacuum pump used to reduce the base pressure within the reactor housing 502 to about 10−4 ton and to maintain the internal pressure during the crystal growth process by way of a butterfly valve. The second stage may be either a turbo pump or a diffusion pump used after completion of purging of the reactor to reduce the internal pressure of the reactor to about 10−9 torr.
The elemental reaction between the metal vapor and the reactant gas, as well as the condensation of the metal compound vapor to form the single crystal of the metal compound takes place within the heater system 412 of the reactor housing 502. The heater system 412 provides a high temperature mixing chamber within which the metal compound vapor is formed. The heater system 412 also provides a growth chamber containing a temperature gradient within which the metal compound vapor condenses to form the single crystal. The heating system 412 includes a post-reaction chamber, held at temperature lower than the mixing chamber, to which the formed crystal may be moved to gradually cool down and through which unused reactants may be removed by the vacuum system 410.
Any method of generating heat may be used within the heater system 412. Non-limiting examples of suitable methods of heating for the heater system 412 include induction heating, resistive heating, and focused microwave heating. In a preferred embodiment, the heater system 412 is a radio frequency induction heater system. An exemplary heater system 412 is illustrated in
An inductively heated main heater 510 may be held in place by a simple thread and nut system in which a threaded heater end 564 is secured to a mating heater connection fitting 534 formed within the inner surface of the outlet end cap 506. The opposite end of the heater may be sealed to the metal vapor system 406 near the electrical arc 540. The upward flow of gases through the heater from the electrical arc 540 to the outlet end cap 506 is enhanced by the thermal convection inside the main heater 510.
The temperature profile of the heater provides a spatial variation in temperature that may be used to perform the single crystal formation process.
Reaction precursors including the metal vapor and the one or more reactive gases 1108 enter the heater 510 through the heater inlet 1110 into the mixing chamber 1102. The mixing chamber 1102 is maintained at a higher temperature than the other chambers 1104 and 1106 of the heater 510 in order to achieve a sufficiently high temperature to produce the metal compound vapor, such as an aluminum nitride vapor or a mixture of metal vapor and reactive gases, including but not limited to aluminum and nitrogen species. In one embodiment, the temperature of the mixing chamber 1102 is maintained at a temperature of at least 1700° C. In another embodiment, the temperature of the mixing chamber 1102 is maintained at a temperature of at least 2000° C.
The growth chamber 1104, is maintained at a lower temperature than the mixing chamber 1102, making possible the crystallization of the metal compound vapor 1112 onto the substrate 1114. The growth chamber 1104 may also include a throttle 1116 to decrease the gas velocity close to the substrate giving the reactive species and precursors more time to migrate on the surface of the substrate 1114 and the exposed surface of the growing single crystal 1118.
The post-reaction chamber 1106 has a temperature gradient that gradually decreases from the exit of the growth chamber 1104 to the exit 1120 of the reactor housing 500 This thermal gradient allows the previously grown single crystal material to cool down slowly and reduce any stresses produced during the crystal growth process. This section may also be held at a higher temperature to suppress build-up on the sidewalls of the post-reaction chamber 1106.
In one embodiment, the heater 510 includes a single stage heating system that has a single inductively heated region within the growth chamber 1104. Alternatively, the heater 510 may include a multiple stage heating system. For example, a three-stage heating system is illustrated in
The heater 510 may be a RF induction heating susceptor constructed from a single piece of a refractory material. Suitable refractory materials may have low chemical reactivity with any of the single crystal precursors including the metal vapor, the one reactant gases, or the metal compound vapor. Alternative, a particular material used to produce the heater may be selected in order to react with unused metal vapor to produce additional metal compound vapor. For example, in a reactor housing 502 producing aluminum nitride, zirconium nitride may be used to construct the heater 510 because aluminum vapor may react with zirconium nitride to produce aluminum nitride. In addition, suitable refractory materials may have good thermal conductivity and/or susceptibility to inductive heating. Non-limiting examples of refractory materials suitable for the construction of a heater 510 include tungsten, tungsten carbide, tantalum, tantalum carbide, zirconium, zirconium carbide, zirconium nitride, zirconium boride, Molybdenum, Niobium, and any alloys thereof.
The rotation/translation/weighing (RTW) system 404 of the reactor system 400 measures continuously, the weight of the single crystal to assess the growth of the crystal. Additionally, the RTW system 404 repositions and rotates the growing crystal to enhance the quality, speed, and control of the crystal growth process. The RTW system 404 may include a substrate chuck on which a single crystal may be grown.
The substrate chuck housing 1304 may be constructed from hot-pressed boron nitride covered by tantalum carbide, ZrN, W, or graphite coated with tantalum carbide. In a preferred embodiment, the substrate chuck housing 1304 is constructed from boron nitride. Boron nitride is preferred because this material is not susceptible to radio-frequency induction heating, resulting in a similar same heating profile throughout the growth process from the beginning of the growth process (when the temperature of the substrate chuck 1300 is important), to the end of the process, where the temperature of the bulk of the single crystal 1306 is more important.
The use of a polycrystalline ingot as the substrate material for crystal growth is also advantageous for further thermal control of the crystal growth process. Because the substrate and single crystal materials are matched in composition, a more native heat transfer occurs at the interface between the substrate 1302 and the substrate chuck housing 1304. As a result, the thermal stress placed on the substrate 1302 and the single crystal 1306 and related dislocations within the single crystal 1306 are reduced. The substrate 1302 may be attached to the substrate chuck housing 1304 using any suitable means known in the art. In one embodiment, a retaining pin 1308 may be used to attach the substrate 1302 to the substrate chuck housing 1304. The retaining pin 1308 may be produced using a material selected from tungsten, boron nitride, tantalum carbide, or any other suitable material known in the art. In another embodiment (not shown) the substrate 1302 may be attached to the substrate chuck housing 1304 using a fast-acting cyanoacrylate adhesive including but not limited to methyl 2-cyanoacrylate, ethyl-2-cyanoacrylate, n-butyl cyanoacrylate, or 2-octyl cyanoacrylate.
The end of the substrate chuck housing 1304 downstream of the substrate 1302 may be tapered or otherwise shaped to reduce the flow resistance of the substrate chuck 1300 as well as the maximum flow turbulence experienced by the substrate chuck 1300 in the post-reaction chamber of the heater (not shown).
The substrate chuck 1300 may be connected to the RTW system 404 via a thread and nut system 1312, in which the substrate chuck housing 1304 may be a threaded female fitting and a refractory metal support rod 1310 may end in a matching threaded male fitting. Non-limiting examples of suitable materials for the construction of the support rod 1310 include boron nitride, a refractory metal, a refractory metal nitride, and refractory metal carbide.
The end of the support rod 1310 opposite to the thread and nut system 1312 may be connected to a weight sensor and one or more actuators. The one or more actuators of the RTW system 404 may rotate the substrate chuck 1300, substrate 1302, and the attached single crystal 1306 during the crystal growth process, resulting in a more uniform crystallization across the growth surface 1314. Further, the one or more actuators may also translate the substrate chuck 1300, substrate 1302, and the attached single crystal 1306 during the crystal growth process within the growth chamber during the crystal growth process. A longitudinal thermal gradient is created in the downstream direction of the growth chamber. Therefore, the temperature at the growth surface 1314 of the crystal 1306 is greater than the temperature of the crystal near the substrate due to the increasing thickness of the growing crystal. The temperature gradient in the growth chamber influences the growth rate of the crystal 1306. For example, a larger temperature gradient allows a greater growth rate, but may also result in the increased formation of defects such as voids and dislocations. A smaller temperature gradient produces a higher quality crystal with a slower growth rate and fewer defects, however, the slower growth rate also results in decreased crystal size.
As shown in
The position of the exposed crystal growth surface 1314 may be adjusted as the crystal grows in order to maintain an essentially constant growth rate during crystal production. Alternatively, the position of the exposed crystal growth surface 1314 within the growth chamber may be changed during crystal production, resulting in a crystal growth rate that varies during the production of the single crystal 1306. For example, the crystal may be grown at a very low rate initially, to insure that the single crystal contains relatively few defects to ensure strong bonding to the crystal growth substrate 1302, and to minimize internal stresses that may result in defects in the finished single crystal.
The processes executed within the reactor may be actively controlled by a feedback control system 414. The feedback control system 414 includes at least one sensor that monitors at least one process condition within the reactor. Non-limiting examples of a process condition includes a temperature, a pressure, and a flow rate measured at one or locations within the reactor, and a weight of the single crystal weight. Any suitable sensor may be used to monitor the process conditions, including but not limited to optothermometers, thermocouples, thermistors, piezoelectric pressure sensors, force transducers, and strain gages. The signal produced by the at least one sensor may be monitored by at least one processor included in the feedback control system 414. In addition, the processor may calculate and monitor a mathematical function of a process condition, including but not limited to the time rate of change of a process condition or the acceleration with respect to time of a process condition.
The at least one processor of the feedback control system 414 may generate at least one control command to alter a process condition based on any control scheme known in the art, including but not limited to any linear or non-linear control strategy. Any control command may be transmitted to one or more particular elements of the reactor in order to modify one or more operating conditions of the reactor. The modification to the operating conditions may result in a desired change in one or more process conditions, or in the maintenance of one or more process conditions at an essentially constant level. For example, the feedback control system 414 may generate a command to the heater system 412 to reduce the voltage of the applied electric field in response to an increase in the temperature within the mixing chamber to a temperature above a desired temperature range. Non-limiting examples of an operating condition that may be modified by commands from the feedback control system 414 include gas supply pressure, metal stock feed rate, vacuum pressure, and heater temperature.
Nitrogen and other dopant gases are flowed through the nitrogen gas inlet 1520 and dopant gas inlet 1522 into the middle of the electrical arc 1516. The resulting metal vapor and gas mixture 1524 is transported past the vacuum sealing top flange 1526 into the main heater 1528 through the mixing chamber 1530 and on to the substrate 1532 in the growth chamber 1534 where the resulting crystal 1536 is produced. The main heater 1528 may be separated from the main body of the reactor housing 1538, setting up inner and outer chambers inside the reactor housing.
Each chamber may be controlled by different vacuum systems. The flow is exhausted from the reactor out the post growth chamber 1540 to set up proper gas velocities and allow volumetric addition of additional Al vapor and nitrogen gas. This flow exhaust is achieved by the inner chamber vacuum control system 1542. The outer chamber is fed with nitrogen gas via the outer nitrogen gas feeds 1544 and 1546. The outer nitrogen gas feeds 1544 and 1546 cool the main heater 1528 ensuring that the heat from the induction coil 1548 does not travel up the main heater unit and destroy the integrity of the top flange's vacuum seal. The nitrogen gas also purges the outer chamber. The outer chamber uses a separate vacuum control system 1550 to purge and set the reaction pressure outside the main heater. The end of the reactor housing is sealed with the bottom flanges 1552 and 1554.
The bottom flange 1554 holds the substrate support system 1556 that in turn holds the substrate chuck 1558. The substrate chuck 1558 may be raised or lowered as needed during crystal production. The system temperature may be maintained with the inclusion of either graphite foam, refractory metal or other suitable insulation 1560 on the outside of the main heater unit.
The devices and system can be used to provide and carry out a method of producing a single crystal of a metal compound by an elemental reaction of a metal vapor and a reactant gas. The method allows the growth of single crystal of a metal compound such as AlN using the reaction of pure or alloyed Al with N2 gas or any other nitrogen bearing gas, while avoiding the parasitic formation of a polycrystalline nitrozation layer on the metal source that limits previous crystal growth methods.
The metal reactant is vaporized or melted by a heat source, such as an electrical arc. Because only a small cross-sectional area of the metal is heated and vaporized/melted at any given time, the surface area exposed to nitrozation or other surface chemistry effects is minimized. By reducing nitrozation of the metal, the rate of vaporization will not be greatly reduced. Furthermore, the bulk of the metal feedstock is held far away from the vaporization/melting site and is not held at high temperatures, allowing the storage of a large amount of metal feedstock in the reactor for an essentially continuous supply of metal feedstock.
The metal vapor is transported via gas flow, thermal convection, and or gravity into a heated mixing area that is maintained at a temperature above about 1700° C. and preferably above about 2000° C. The metal vapor reacts with the reactive gases, forming a metal compound sub-species. The metal compound sub-species then moves via gas flow, thermal convection, and/or gravity to a growth zone that is held at a lower temperature then the heated mixing area so that the metal compound vapor may crystallize.
The lower temperature in the growth zone allows the metal vapor compound to crystallize and form a single bulk crystal. The bulk crystal formation and subsequent growth may be initiated through seeded or non-seeded crystallization in the growth zone.
In a preferred embodiment a crystal seed or substrate, preferably composed of the same metal compound subspecies as the metal compound vapor, is placed in the growth zone. In this embodiment, the metal compound vapor precipitates onto the substrate to grow the single crystal.
In another embodiment, the single crystal may be formed through spontaneous self-nucleation of the metal compound sub-species once a sufficient density of the metal compound vapor has been accumulated in the growth zone. For example, the metal vapor compound is accumulated in the growth zone until a supersaturated vapor is formed. In this embodiment, the metal vapor compound may condense and form a single crystal on surfaces, such as the substrate chuck, within the growth zone.
The crystal formation process is velocity and volumetrically controlled by adjusting the exhaust pressure produced by the vacuum system hydraulically attached to the exit end of the reactor. Vaporized/melted metal may be continuously added into the heated mixing and growth chambers. The exhaust pressure regulates and sets the driving force for the movement of the metal vapor, reactive gas, and metal compound vapor mixture, which in turn governs the gas flow rates in addition to as well as thermal convection and or gravity for the transport of reactants through the open flow reactor.
While the disclosure has been explained in relation to exemplary embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the description. Therefore, it is to be understood that the disclosure disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.
