Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2001-07-17
2002-12-03
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S208000
Reexamination Certificate
active
06488768
ABSTRACT:
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. I
Hiteshew Felisa
SEH America Inc.
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