Abstract:
A system for controlling emissions of hazardous, toxic or otherwise undesirable components in a waste gas stream, while maintaining uptime through decreased maintenance and repair, is provided. The system oxidizes the waste gas stream at high temperatures with a thermal oxidizer ( 110 ), effectively removes particulates in the waste gas stream as well as moderate levels of acid gas through a cyclone scrubber ( 120 ), and removes the remainder of the acid gas content in the waste gas stream through he use of a packed column ( 130 ). Finally, a condenser ( 140 ) lowers the moisture content of the waste gas stream before leaving the system by way of a blower ( 150 ), reducing the chance of condensation and corrosion in the facility ductwork. As a result, the system can run for sustained periods of time, reducing downtime in semiconductor operations and associated loss of revenue.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of Provisional Patent Application Serial No. 60/200,959, filed on May 1, 2000, entitled “Treatment System For Removing Hazardous Substances From A Semiconductor Process Waste Gas Stream,” which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to waste gas treatment systems, and more particularly to a system and method for reducing or eliminating emissions of toxic or dangerous gases and particulate matter.  
           [0004]    2. Description of Related Art  
           [0005]    Semiconductor fabrication processes, such as chemical vapor deposition (CVD), utilize several chemicals that are highly toxic, corrosive, flammable, pyrophoric or otherwise dangerous. Typically, the process consumes only small portions of the chemicals. The unconsumed chemicals, together with particulate-phase reaction products, exit the processing equipment as a waste gas stream and flow into an exhaust system. Because certain components of the waste gas stream possess dangerous or noxious properties, it is desirable and/or legally required to treat the waste gas stream prior to discharge to the atmosphere in order to eliminate or minimize discharge of the objectionable waste gas components.  
           [0006]    The prior art includes a number of commercially available waste gas treatment systems for removing selected gas- and solid-phase substances from the waste gas stream. Because of the relatively high particulate loading and corrosive nature of the waste gas stream, users of prior art treatment systems often experience problems with clogging of the gas flow path and component wear. Remediation of these problems (e.g., removal of accumulated particulate matter or replacement of corroded components) frequently necessitates temporary shutdown of the associated process equipment, causing unscheduled downtime. Such unscheduled downtime increases overall manufacturing costs and thus is particularly problematic in the highly competitive and price-driven semiconductor fabrication industry. Users of prior art treatment systems therefore find themselves forced to choose between a trade-off of downtime versus abatement efficiency.  
           [0007]    In view of the foregoing discussion, there is a need for a waste gas treatment system which avoids the clogging and corrosion problems which lead to unscheduled downtime, while efficiently and effectively reducing concentrations of hazardous or toxic substances in the waste gas stream to acceptable levels.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with an aspect of the present invention, a system is provided for controlling emissions of hazardous, toxic or otherwise undesirable components in a waste gas stream, while maintaining uptime through decreased maintenance and repair.  
           [0009]    The system incorporates a highly effective technique for destroying selected gaseous species in the waste stream. By oxidizing the waste gas stream at high temperatures, combustible substances contained in the waste gas are removed. The gas then passes through a cyclone scrubber, which effectively removes particulates in the waste gas stream as well as moderate levels of acid gas, and is the principal heat removal device in the system. The waste gas then passes through a counter flow type packed column, which removes the remainder of the acid gas. Finally, the waste gas stream is passed through a condenser to lower the moisture content of the gas before it leaves the system by way of a blower.  
           [0010]    The system advantageously includes a means for reducing the accumulation of particulates on internal surfaces of the treatment system, thereby avoiding the clogging problems associated with prior art systems. The cyclone scrubber utilizes a wetting means to prevent adherence of particulate matter to the internal components of the system. Additionally, the reduced moisture in the waste gas caused by the condenser reduces the chance of condensation and corrosion in the facility ductwork.  
           [0011]    As a result of the foregoing improvements, the system effectively removes dangerous and noxious substances from the waste gas and can run for sustained periods of time, thereby reducing downtime in semiconductor operations and associated loss of revenue. The system is particularly well suited to abatement in semiconductor fabrication processes such as CVD.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 schematically depicts a waste gas treatment system according to the present invention.  
         [0013]    [0013]FIG. 2A depicts a side view of a cyclone scrubber used in the waste gas treatment system of FIG. 1.  
         [0014]    [0014]FIG. 2B depicts a top view of the cyclone scrubber used in the waste gas treatment system of FIG. 1.  
         [0015]    [0015]FIG. 3 depicts a top view and three side views of an inner tube of the cyclone scrubber of FIGS. 2A and 2B.  
         [0016]    [0016]FIG. 4 depicts a packed column component of the waste gas treatment system of FIG. 1.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    [0017]FIG. 1 schematically depicts a waste gas treatment system  100  constructed in accordance with one aspect of the present invention. The waste gas treatment system  100  is seen to generally include a thermal oxidizer  110  for oxidizing selected gas-phase species, a cyclone scrubber  120  for removing particulate matter and a portion of the acid gases, a packed column  130  for removing remaining acid gases, a condenser  140  for removing a portion of the water vapor, and a blower  150  for drawing the waste gas stream through treatment system  100 .  
         [0018]    A waste gas stream from a semiconductor fabrication process tool, such as a nitride process tool, flows initially to the thermal oxidizer  110  by way of a waste gas inlet  105 . In a typical implementation of the treatment system  100 , the waste gas stream emitted by the process tool will include a nitrogen or other inert gas carrier mixed with various gas-phase and particulate-phase components which must be removed or destroyed prior to release of the waste gas stream to an ambient exhaust system. Typical gas-phase components that must be abated include silane (SiH 4 ), ammonia (NH 3 ), flourine, and hydrogen fluoride (HF). Particulate-phase components of the waste gas stream may include silicon nitride (SiN), silicon dioxide (SiO 2 ), and tungsten hexafluoride (WF 6 ). During normal operation of the process tool, the waste gas stream will be periodically alternated with a clean gas stream, typically comprising nitrogen trifluoride (NF 3 ) or a Freon compound.  
         [0019]    At the thermal oxidizer  110 , the waste gas stream is mixed with an oxidizing gas stream, which is injected by way of oxidizing gas inlet  115 , and passed through a high-temperature reaction zone inside the thermal oxidizer  110 . The oxidizing gas stream, which will typically include air or an air/oxygen mix, is injected into the waste gas stream through the oxidizing gas inlet  115  at high pressure in order to induce turbulence and promote rapid mixing of the streams inside the thermal oxidizer  110 . The amount ofoxidizing gas added to the waste gas stream may be adjusted according to the composition of the waste gas and abatement requirements. The thermal oxidizer  110  includes a heated metal tube through which the mixed gas streams are passed. The tube is fabricated from a commercially available high-temperature alloy such as Inconel 600 or Hastelloy C22. The tube may be heated with a conventional radiative ceramic resistance heater or suitable alternative. Depending on the waste gas stream composition and abatement requirements, the tube surface is heated to a temperature of between 500° C. and 850° C. The tube dimensions are preferably selected to provide adequate reaction time for oxidation of silane and other toxic gaseous species to be substantially completed, while maintaining gas velocity sufficiently high to minimize deposition of particulates on the tube&#39;s inner wall.  
         [0020]    The waste gas stream then exits the thermal oxidizer  110  and is directed into the cyclone scrubber  120 . As mentioned above, the cyclone scrubber  120  is operative to remove particulate-phase components of the waste gas stream along with a portion of the highly water-soluble gas-phase components such as hydrogen fluoride. The cyclone scrubber  120  has an additional function of cooling the waste gas stream, which is heated to an elevated temperature inside the thermal oxidizer  110 . It has been observed that accumulation of particulate matter tends to occur at or proximal to the interface between the dry and wet zones of the cyclone scrubber  120 , unless at least one of two conditions is met: (1) surface temperature exceeds 300° C., or (2) the surface is coated with water.  
         [0021]    The features and operation of cyclone scrubber  120  may best be understood with reference to FIGS. 2A and 2B, which respectively depict side and top views thereof. As can be seen in FIG. 2A, the cyclone scrubber  120  includes an upper section  210  and a lower section  220 . The upper section  210  is constructed from an inner tube  212 , through which the waste gas stream flows, and an outer tube  214  having a substantially larger diameter than the inner tube  212  and being positioned generally coaxially therewith. An annulus  225  is defined in the space between the inner and outer tubes  212  and  214 , into which water is injected through an injection port  230  that extends through the wall of outer tube  214 . As may be seen with reference to FIG. 2B, the longitudinal axis of the injection port  230  is angularly offset with respect to the radial axis of tubes  212  and  214  so as to impart a swirling or rotational movement to the water contained in the annulus  225 , the purpose of which is discussed below. Referring again to FIG. 2A, the upper end of the outer tube  214  is provided with a flange  240  which mates with a corresponding flange located at the lower end of the thermal oxidizer  110  (FIG. 1). As can be seen, the upper end of the inner tube  212  is positioned slightly lower (about 1″ in the implementation depicted) than the upper end of the outer tube  214 . Water injected through injection port  230  rises up in the annulus  225  until reaching the upper end of the inner tube  212 . The annulus  225  continues to fill with water until the water spills over the upper end and flows down the inner wall of inner tube  212 . The water film coating the surface of the inner wall acts to prevent accumulation of particulate matter and eventual clogging of the gas flow path.  
         [0022]    The swirling motion imparted to the water by the angular positioning of the injection port  230  serves to ensure that all surfaces contacted by the waste gas stream within the cyclone scrubber  120  are wetted. In the absence of the swirling motion of the water within the annulus  225 , the upper margins of the outer tube  214  (which, as explained above, extends about 1″ above the upper end of the inner tube  212 ) would not be coated with water, and hence accumulation of particulate matter thereon would occur. By imparting a swirling motion to the water, a free surface  250  of the water is given a conical aspect (as indicated on FIG. 2A) owing to the higher velocity of the water at the outer radius relative to the water velocity at the inner radius of the annulus  225 . The water in the annulus  225  thus extends further upwardly along the surface of the outer tube  214  (to the bottom of the flange  240 ) such that all gas-contacted surfaces within the cyclone scrubber  120  are wetted. Because the waste gas stream passes immediately from the thermal oxidizer  110  (FIG. 1), wherein all gas-contacted surfaces are maintained at high temperature, to the cyclone scrubber  120 , wherein all gas-contacted surfaces are wetted, particulate deposition is minimized and clogging problems are avoided.  
         [0023]    The inner tube  212  extends downwardly into the lower section  220  of the cyclone scrubber  120 . As the waste gas stream passes through the inner tube  212  into the lower section  220 , water is injected into the waste gas stream near the entrance of the lower section  220  through one or more water spray inlets  260 .  
         [0024]    [0024]FIG. 3 depicts a top view and three side views of the inner tube  212 . As can be seen, the water spray inlets  260  are coupled to one or more spray atomizers  310 . The water spray inlets  260  and the spray atomizers  310  are preferably positioned on the inner tube  212  so as to prevent the water droplets emitted from the spray atomizers  310  from traveling upward into the thermal oxidizer  110 , which would cool the gas-contacted surfaces and cause accumulation of particulate material. As is known in the art, the water droplets injected into the waste gas stream by the spray atomizers  310  in this way contact and capture particulates in the waste gas stream. Alternatively, the spray atomizers  310  can inject recycled water from the packed column  130  into the waste gas stream, as will be explained below in reference to FIG. 4.  
         [0025]    The water droplets injected into the cyclone scrubber  120  also serve to absorb a portion of the highly water-soluble acid gas species (such as hydrogen fluoride) from the waste gas stream, forming particle-laden droplets. Referring back to FIG. 2A, the particulate-laden droplets, together with the water used to wet the inner tube  212  and the outer tube  214 , travel downwardly under the influence of gravity and are collected in a reservoir for further processing. The waste gas stream is then turned upwardly, exits the cyclone scrubber  120  via a side port  270 , and is passed to the packed column  130  (FIG. 1). The risk of fouling the packing of the packed column  130  is greatly reduced by removing particulate matter from the waste gas stream at the cyclone scrubber  120 .  
         [0026]    Referring to FIG. 4, the waste gas stream is directed to the packed column  130  for removal of the remaining acid gases and particulate matter. The packed column  130  is preferably of the counterflow type, wherein a water spray  410  is introduced at the upper end of the packed column  130  and travels downwardly, while the waste gas stream is introduced proximal the lower end of the packed column  130  and flows upwardly. A packing material  420  is utilized in the packed column  130 . In one aspect, the packing material includes alumina ceramic, because of its superior qualities of removing flourine gas. In other aspects, the packing material includes stainless steel, Teflon, and polypropylene. As the water flows downwardly through the packing material, it absorbs the remaining acid gases (typically hydrogen fluoride) in the waste gas stream, together with any particulate matter not captured in the cyclone scrubber  120 . The resultant acidic wastewater is thereafter collected and can be processed or alternatively used to provide water to the spray atomizers  310  as described in reference to FIG. 3. The advantage of using the wastewater from the packed column  130  in the spray atomizers  310  is that the acid gases (typically hydrogen fluoride) contained in the wastewater substantially eliminates the presence of such corrosives as silicon nitride from the system, further enhancing the anti-clogging benefits and uptime of the system. After processing by the packed column  130 , the waste gas stream exits through an exit duct  430  located at the upper end thereof.  
         [0027]    Referring back to FIG. 1, the waste gas stream leaving the packed column  130  flows to the condenser  140 . The condenser  140  is operative to reduce the water vapor concentration in the waste gas stream in order to prevent or minimize the occurrence of condensation in facility ductwork, which may lead to corrosion problems. The condenser  140  also serves as a water trap to prevent moisture from the packed column  130  from entering the blower  150 . The condenser  140  may be of any suitable design having cooled surfaces which contact the waste gas stream to cause water vapor to condense thereon, but will typically include thermally conductive tubing through which relatively cool water is circulated, the tubing having its outer surface contacting the waste gas stream. The condensed water is collected in the bottom of the condenser  140  and is removed via a drain line.  
         [0028]    The waste gas stream (having a substantially reduced water vapor concentration) leaves condenser  140  and is directed to the blower  150 . The cleaned waste gas stream is then conveyed at elevated pressure to a facility exhaust duct or equivalent for eventual release to ambient. The blower  150  is therefore operative to draw the waste gas stream through the various elements of treatment system  100  and to exhaust the waste gas stream from the system  100 .  
         [0029]    It will be recognized by those skilled in the art that, while the invention has been described above in terms of various aspects, it is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment and for particular applications, e.g., semiconductor fabrication plants, those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially utilized in any number of environments and implementations.