Abstract:
A water spray subsystem apparatus and method of treating discharge gas designed for use in the gas discharge zone of a Czochralski crystal growing apparatus. The subsystem composes a structure containing or defining a water spray body, which is capable of providing a water spray into the discharge gas stream from the CZ chamber. The water is supplied into the discharge gas in the form of a spray or a mist in order to provide for intimate contact between the droplets of sprayed water and the discharge gas stream. Contacting the discharge gas stream with the water spray cools the gas stream, causes the reaction of SiO to SiO 2 , and causes the precipitation of cooled SiO 2  and SiC out of the discharge gas stream.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a discharge gas treatment apparatus and method for use with a crystal growing chamber utilizing the Czochralski method, such invention comprising a water spray apparatus capable of limiting the amount of Si compounds released into the atmosphere while avoiding safety hazards related to unwanted siphoning of water into the Czochralski chamber. 
     BACKGROUND OF THE INVENTION 
     As industry drives demand for ever larger and higher quality crystals for use in microchip fabrication, crystal growers embrace an ever increasing body of technology which enables them to make crystals of increasing size and at an increasing rate. However, very real safety concerns still plague crystal growers as the techniques of today still require the control of potentially hazardous chemicals within a harsh high-temperature environment. Though hazardous waste streams produced during crystal growth are handled adequately by technology of today during its normal operation, the possibility of equipment failure mixed with the potentially extreme consequences of such an event could still be improved. 
     The majority of crystals today, particularly silicon crystals, are grown using the Czochralski method, known simply as the CZ method. In the CZ method, polycrystalline material is placed within a crucible along with any necessary dopants or other additives. The crucible, in turn, is placed within a closed chamber capable of being isolated from the atmosphere. Because of the extreme heat necessary for crystal production the primary components within a CZ chamber are typically graphite, with the exception of the crucible which is typical quartz. 
     The polycrystalline material is then heated and melted at around 1400° C., after which a seed crystal having the desired orientation is placed just above the surface of the melted polycrystalline material, known as the melt. Because of surface tension, molten polycrystalline adheres to the lower surface of the crystal. As the molten polycrystalline along the lower surface of the crystal cools, it adopts the crystalline structure of the seed crystal. As the seed crystal is slowly raised within the Czochralski chamber, the crystal is grown as additional molten polycrystalline adheres to the crystal and solidifies with the proper crystalline orientation. The practice continues until a crystal of suitable size is achieved. Also, it must be noted that the CZ method usually involves constant rotation of the crystal and/or the crucible to counteract problems associated with non-uniform distribution of dopants and additives within the melt. 
     During the production of silicon crystals, various hazardous chemicals can be produced. Perhaps the most hazardous byproduct of high temperature crystal production is silicon oxide (SiO) which is believed to be produced by reaction of the polysilicon melt with the quartz crucible. The silicon oxide is violently reactive with oxygen to form SiO 2 . Though oxygen is normally evacuated from the CZ chamber during the crystal growing process, the mere existence of SiO within the CZ chamber or within a gas discharge stream is cause for concern since any accidental breach of the CZ chamber during the crystal growing process would result in the potentially explosive oxidation of SiO. 
     Other byproducts of the crystal growing process are SiO 2 , which is partially vaporized and may thereafter solidify above the melt, and SiC, which forms as a solid residue upon the graphite components of the CZ chamber, and may thereafter flake off into the area above and around the melt. Although SiO 2  and SiC are relatively non-reactive, they are undesired components within the CZ chamber because of their ability to cause buildup within the chamber or, worse, to contaminate the melt and destroy the crystallinity of the growing crystal. 
     In order to minimize the adverse impact of the hazardous chemicals, and further to inhibit the production of the undesired byproducts discussed above, the CZ chamber is filled with an inert gas. Furthermore, crystal growth typically occurs under a slight vacuum. 
     In practice, the CZ chamber is repeatedly evacuated and supplied with inert gas as each crystal is produced. For instance, to insert a seed crystal within the CZ chamber, the chamber must necessarily be exposed to the atmosphere. After the seed crystal is put in place, the chamber is evacuated through use of a vacuum pump. Once the chamber has been evacuated, inert gas is supplied to the chamber, which is still under a slight vacuum. As the crystal growing process begins and continues, inert gas is continuously supplied to the chamber and continuously removed from the chamber through use of another vacuum pump. After the crystal has grown, the gas supply and discharge are closed, the chamber is opened to the atmosphere to allow access to the crystal, the melt is replenished if necessary, a new seed crystal is inserted, and the process is repeated. 
     The gas discharged from the chamber carries with it the various byproducts of the reaction, including SiO, SiO 2 , and SiC. As mentioned above, SiO is a potentially explosive component which is particularly dangerous when held in a hot, dry gas stream. The other components are also environmentally regulated and must be removed from the discharge stream before it is exhausted to the environment or otherwise reused. 
     Conventional discharge gas treatment methods often use a water bubbler to oxidize the SiO under controlled wet conditions, to solidify the SiO 2  and SiC components of the gas stream, and to cool the discharge gas. Use of a water bubbler involves discharging the contaminated gas stream into a water filled tank. After the discharge gas enters the tank, the gas is rapidly cooled as the gas rises through the water. As the gas is cooled, a large percentage of the SiO 2  and SiC precipitates out of the gas stream and falls to the bottom of the water bubbler, where it may be collected. The SiO within the gas stream reacts, in a controlled manner, with the oxygen present in the water to form SiO 2 , which precipitates to the bottom of the bubbler. The remaining gas stream is removed from the bubbler, still under slight vacuum, and either exhausted to the atmosphere or treated further. 
     Under normal operating conditions, the bubbler arrangement works quite well for removing contaminants from the gas stream. However, the CZ apparatus is operated at extreme temperatures and byproducts of the process, particularly SiO 2 , tend to solidify and readily form deposits within the discharge system which includes piping, vacuum pumps, and valves. Having SiO 2  deposits throughout the gas discharge system creates a likelihood that a vacuum pump will fail or that a valve will be unable to close during crystal growth. Furthermore, the gas discharge system including the vacuum pump and the valves may fail for some other reason. Given that the crystal growing process is conducted under a vacuum, it is possible that, due to equipment failure, water from the water bubbler could be siphoned back through the gas discharge system, perhaps invading the vacuum pump or even the CZ chamber itself. The most likely cause of such siphoning would be failure of a discharge gas vacuum pump or failure of the vacuum seal between the gas discharge system and the CZ chamber during evacuation. A result of water backflow is at least destruction of the pump or the crystal being currently grown, and could perhaps result in a steam explosion caused by large quantities of water encountering the 1400° C. chamber atmosphere. 
     Previous attempts to overcome the problem of water backflow have involved various methods of mechanically and physically isolating the water bubbler from the CZ chamber and vacuum pumps. Two approaches to isolating the bubbler from the chamber are found in U.S. Pat. No. 5,900,058, which discloses the use of a buffer tank between the CZ chamber and the bubbler. The buffer tank is a gas filled tank located in-line with the gas discharge system, between the bubbler and the CZ chamber and any vacuum pumps. In theory, any water siphoned back into the discharge system would be captured by the buffer tank before it was able to make its way into pumps or the CZ chamber. The same patent also discloses use of a vacuum breaker device which would be able to sense loss of vacuum within the gas discharge zone of the Czochralski apparatus and which would quickly bring the CZ chamber to atmospheric pressure thereby stopping any siphoning from taking place. 
     Conventional safety devices have lowered the possibility of catastrophic equipment failure during crystal growing using a CZ chamber. However, the safety measures related to use of the water bubbler are far from failsafe. Rapid buildup of SiO 2  and SiC deposits within the gas discharge zone means that equipment failure can occur, including equipment failure of the safety equipment mentioned in the prior safety techniques. 
     It would, therefore, be desirable to provide an apparatus and method of preventing water backflow into a CZ apparatus upon vacuum pressure disruption, wherein such apparatus is not subject to the same disabling deposits and corrosive effects of the CZ environment which typically cause the disruption. In other words, an improvement in safety during discharge gas treatment with water is desirable which does not involve exposing the safety equipment to the same disabling environment which likely caused an equipment failure in the first place. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method of treating discharge gas from a CZ chamber which dramatically reduces the risks of water backflow associated with traditional water bubblers. The reduction of risk of water backflow is achieved by replacing the traditional water bubbler with a water spray subsystem within the gas discharge system of the CZ apparatus. The water spray subsystem provides adequate interaction between the discharge gas and the cooling, reacting water, so that the SiO in the gas stream reacts to SiO 2 , and so that the SiO 2  and SiC are cooled and precipitated out of the gas stream. But, the water spray subsystem works without the necessity of a large accumulated volume of water that is maintained in communication with the CZ chamber, thus avoiding the possibility of water backflow and of all the catastrophic consequences related thereto. 
     The water spray subsystem is designed for use in the gas discharge zone of a Czochralski crystal growing apparatus. The subsystem composes a structure containing or defining a water spray body, which is capable of providing a water spray into the discharge gas stream from the CZ chamber. The water is supplied into the discharge gas in the form of a spray or a mist in order to provide for intimate contact between the droplets of sprayed water and the discharge gas stream. Contacting the discharge gas stream with the water spray cools the gas stream, causes the reaction of SiO to SiO 2 , and causes the precipitation of cooled SiO 2  and SiC out of the discharge gas stream. 
     In an embodiment, the subsystem is composed of a closed vessel having a discharge gas inlet for receiving discharge gas containing various waste components, from the CZ crystal growing chamber, and a discharge gas outlet which allows the wetted, cooled, and reacted discharge gas to leave the vessel and continue to another treatment process or to exhaust into the atmosphere. The spray body is preferably located within the vessel and provides a water spray to the inside of the vessel. There is at least one drain to allow for the escape of water from the vessel. 
     The vessel preferably has an upper region and a lower region with the upper region having a central area and a periphery, such as a cylindrical tank having a vertical axis of orientation or a conical tank having a vertical axis of orientation. The gas inlets and outlets are disposed within the upper region of the vessel, so that condensing water from the spray body naturally falls away from and is not sucked into either the inlet or outlet. The water spray body is disposed about the periphery of the upper region of the vessel so that water sprayed into the vessel is projected inward and downward into the vessel. The spray body most preferably is composed of a series of spray heads which provide a very dispersed water spray having a wide spray pattern within the vessel. The condensed water, along with the particulates condensed out of the discharge gas, are removed from the vessel through a water outlet located in the lower region of the vessel. 
     In operation, discharge gas is projected downwards into the main volume of the vessel where it intimately contacts water which is likewise projected downward and inward into the main volume of the vessel. Upon contact of the discharge gas with the water, SiO reacts with oxygen present in the water to form SiO 2 , and the SiO 2  and SiC within the gas stream are cooled and solidified, after which they fall to the bottom of the vessel. The cooled and reacted gas stream leaves the vessel through the gas outlet. The condensed water and particulates within the vessel settle to the bottom of the vessel, where they are removed through a water outlet. 
     By utilizing the water spray of the invention, adequate cooling and reaction of waste products within the discharge gas stream from the CZ chamber are provided. 
     Furthermore, use of the water spray and consequent lack of any large accumulated volume of water avoids the possibility of water being siphoned back into the CZ chamber in the event of an equipment failure. By eliminating the volume of water normally present in a bubbler, the present invention provides a dramatic improvement in safety by eliminating the possibility of water backflow into the discharge gas treatment system which has previously presented a hazard of mechanical destruction or steam explosion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an example of the invented discharge gas treating apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     In accordance with FIG. 1, an apparatus for treating discharge gas from a Czochralski crystal growing chamber is shown. In accordance with the invention, discharge gas from a Czochralski crystal growing chamber  10  is released into a duct  22  which carries the untreated discharged gas to a treatment vessel  20 . The treatment vessel  20  has a water spray body  32  which is capable of dispersing water about vessel  20  from an upper region of the vessel  20 . As the untreated discharge gas from the chamber  10  enters the treatment vessel  20 , chemicals within the discharge gas react favorably with the dispersed water within the vessel  20 . After treatment, the discharge gas exits the vessel  20  through gas outlet  24  for further treatment or exhaust. Water within the vessel, containing components from the treated discharge gas, is collected upon the bottom of the vessel  20  and released through a water outlet  34 . 
     The Czochralski chamber  10  is any of a series of specially designed apparatus that are capable of growing silicon crystals. Typical Czochralski chambers contain a quartz crucible capable of holding polycrystalline material, and heating elements capable of heating the quartz crucible and the polycrystalline material to a temperature of approximately 1400° C., the temperature about which the polycrystalline material becomes a melt. Typical CZ chambers also contain means for suspending a seed crystal above the polycrystalline melt, as well as a means of slowly rotating the seed crystal and for the quartz crucibly with respect to one another. 
     The CZ chamber  10  in accordance with this invention has at least one gas inlet for a supply of inert gas during operation of the CZ chamber  10 , and at least one gas outlet  12  for removal of the inert gas and other components that may be generated during operation of the CZ chamber from the inside of the chamber. Inert gas is supplied to the CZ chamber  10  during operation in order to surround the components and the reactants within the CZ chamber with an inert atmosphere. The inert atmosphere provided by the inhibits undesirable reactions within the chamber  10 . Undesired products include SiO and SiO 2 , which are products of the polysilicon contained within the crucible and residual oxygen within the CZ chamber  10 . Another, less prevalent, product which is inhibited by the inert gas atmosphere is SiC, which is produced by the reaction of hot silicon with carbon from carbon containing components within the chamber  10 . The volume of flowing through the CZ chamber and out of gas outlet  12  also acts as a vehicle to remove the SiO, SiO 2 , and SiC contaminants from the ultrasensitive polycrystalline melt and silicon crystal formation taking place within the CZ chamber  10 . 
     The contaminant filled discharge gas is carried away from CZ chamber  10  by piping  22 , which carries the discharge gas to the treatment vessel  20 . The CZ chamber  10 , as well as the entire treatment apparatus, is typically held at a vacuum during production of silicon crystals. The discharge gas moves from the CZ chamber  10  to the treatment vessel  20  because of a pressure differential between the chamber  10  and vessel  20 , most commonly due to a vacuum pump  42  in line with pipe  22  residing between the CZ chamber  10  and treatment vessel  20 . Alternatively, vacuum or pressure differential is provided by pumps residing outside of the treatment apparatus. Other apparatus  40  in line with duct  22  between CZ chamber  10  and the vessel  20  preferably include, but need not include, supplemental safety equipment such as back float check valves, isolation valves, buffer tanks, and vacuum breakers. 
     The discharge gas conducted through piping  22  enters the treatment vessel  20  through a gas inlet  23 . Though there is preferably a single gas inlet  23  located at a central region along the top portion of treatment vessel  20 , the discharge gas could enter the treatment vessel through a plurality of gas inlets  23  in accordance with another embodiment of this invention. The discharge gas may enter the vessel  20  through a simple opening  23  within the top of the vessel, or more preferably through an extended portion of discharge gas piping  22  which extends into the vessel  20 . The gas inlet  23  is located along an upper region of vessel  20  such that the gas inlet or gas piping  22  does not become submerged in residual water which may accumulate along the bottom of the treatment vessel  20 . The gas inlet  23  is most preferably located on the top of the treatment vessel  20  such that the untreated discharge gases projected downward into the vessel  20  are dispersed about the entire volume of the vessel  20  before being removed by the gas outlet  24 . 
     The vessel  20  is part of a water spray subsystem, which has a water spray body  32  capable of dispersing water within the treatment vessel  20 . The water spray body  32  is preferably a series of piping connected to an external water source, which resides along the internal walls of the vessel  20  and is in communication with one or more nozzles  38 . The nozzles  38  act to project and disperse water within the treatment vessel  20 . Alternatively, the water spray body  32  may be located predominately outside the treatment vessel  20  with the nozzles  38  simply extending through the walls of the vessel  20 . The number of nozzles  38  may practically range from one to any number more than one and is preferably four. The nozzles  38  may also be arranged in any manner to give adequate dispersion of water within the vessel  20 , but are preferably evenly spaced about periphery of the upper region of the vessel  20 . Preferably, the nozzles are arranged along the inner sidewalls and near the top of vessel  20 , and aligned such that a finely dispersed volume of water is projected inward and downward, thus occupying the vast majority of the volume of vessel  20 . In the embodiment shown in FIG. 1, the nozzles are shown projecting a volume of water downward upon extended piping  22  which extends into vessel  20  through gas inlet  23 . 
     Water released from the nozzles  38  is preferably projected in a light to medium mist so that water droplets are smaller and more dispersed. The well dispersed water droplets interact more favorably with the discharge gas within the vessel  20 . Spraying the water downward and inward within the vessel  20  acts as a safety measure to prevent large volumes of standing water or large volumes of sprayed water from being suctioned up extended piping  22  or out of gas inlet  23  in the event of a vacuum failure in operation of the CZ chamber  10  or gas treatment apparatus. A system of buffer tanks and/or check valves may optionally be disposed between the CZ chamber  10  and the vessel  20  as a backup system for preventing the backflow of any free volume of water from the vessel  20  to the chamber  10  or any pumps or similarly vulnerable equipment residing between the chamber  10  and the vessel  20 . 
     The sprayed water within vessel  20  interacts with the untreated discharge gas within the vessel  20  and accomplishes those goals which have heretofore only been accomplished by gas bubblers, which bubbled untreated discharged gas through a volume of standing water. The first desired effect of the interaction of the sprayed water with the discharge gas is the oxidation of SiO to SiO 2  in a relatively controlled environment. The compound SiO is extremely hazardous because it reacts violently with oxygen to form SiO 2 . Remember that the treatment vessel  20  is held in a vacuum just as the CZ chamber  10  and the other components of the treatment system. Limited amounts of oxygen are supplied to the vessel  20  as the dissolved oxygen naturally present in the water spray. The small amounts of oxygen provided by the water spray react with the SiO of the discharged gas in a controlled manner and neutralize the SiO to SiO 2.    
     The second goal accomplished by interacting the water spray with the discharge gas within the vessel  20  is the cooling and solidification of SiO 2  components within the discharge gas, and their subsequent removal from the discharged gas. The extreme temperatures of the CZ chamber  10 , usually above 1400° C., cause SiO 2  produced within the CZ chamber  10  to vaporize and be carried from the CZ chamber  10  by the gas stream to the treatment vessel  20 . The intimate interaction between the sprayed water and the discharge gas cools the discharge gas and its various components, including the SiO 2  component. Upon cooling, the SiO 2  is removed from the vapor phase and drops to the bottom of vessel  20  as a solid. Any SiC component within the discharged gas is also cooled and solidified and falls to the bottom of treatment vessel  20 . 
     Water that has been sprayed into treatment vessel  20 , and solids that have been removed from the discharge gas, are collected along the bottom of the treatment vessel  20 . The water and solid slurry is removed by water outlet  34  in communication with the bottom of the vessel  20 . It is preferred that water be supplied in a volume sufficient to dilute the solid components to an extent such that the liquid slurry flows freely from the vessel  20  through the water outlet  34 . Alternatively, the vessel  20  is equipped with an agitator  26 , which prevents the solid slurry from forming a cake on the bottom of vessel  20  and promotes the removal of the solids with the liquid leaving the vessel  20  through water outlet  34 . 
     In operation of the treatment system, it is desired that the water slurry level within the vessel  20  be kept well below the gas inlet  23 , the gas outlet  24 , or any piping from the gas inlet or outlet extending into the vessel. This is done to prevent a volume of standing water from being suctioned back into the gas inlet  23  and therefore the CZ chamber  10 , which could result in equipment damage and other safety hazards. As an additional safety measure, an alternative embodiment of the invention provides for an additional water outlet  36  positioned above the main water outlet  34 , but below any of the gas inlets or gas outlets such that, in the event of failure of main drain  34 , water and solid slurry material will be removed through secondary drain  36 . Secondary drain  36  is preferably disposed within the sidewall of the vessel  20 . 
     The discharge gas leaving through gas outlet  24  is cooled, has almost no SiO content, and has reduced SiO 2  and SiC content in comparison with the untreated discharge gas. The treated discharge gas stream may be monitored, further treated, or discharged to the environment. Similarly, the water slurry containing water, SiO 2  solids, and SiC solids is removed from the treatment apparatus and may be further tested, treated, or disposed of. 
     As shown above, the instant invention provides for the safe treatment of discharged gas from a CZ chamber with water without the possibility of a standing volume of water being sucked back into the CZ chamber upon equipment failure. The invention does not rely upon auxiliary safety equipment to prevent the large volume of water from being suctioned back into the apparatus upon equipment failure, such as vacuum breakers, auxiliary tanks, or safety valves. Instead, the invention provides a relatively fail safe apparatus that avoids the use of standing water in communication with the CZ chamber altogether. 
     Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.