Patent Document

BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to air filtration for semiconductor manufacturing facilities, general business, or residential use, and more specifically, to the use of a liquid to remove impurities from circulated air in these situations. 
     2. Description of the Related Art 
     Currently, air filtration for both commercial and residential applications is accomplished by passing the air through a mesh material that traps a large percentage of the particulate matter in the air. Depending on the particular mesh used, the effectiveness of the filtration can vary. The current state of the art in air filtration is High Efficiency Particulate Arresting (HEPA) filtration. HEPA filtration was developed during World War II by the Atomic Energy Commission as a means to protect their researchers from radioactive particles in the air. HEPA filters will remove particulate matter down to the range of 0.3 micron at an efficiency of 99.97%. HEPA filters have become widely known and used in recent years in settings as diverse as clean rooms for the manufacture of semiconductor chips, hospital operating rooms, and home air cleaners for allergy and asthma sufferers. 
     However, as the filter becomes clogged with the captured materials, the flow of air through the filter becomes more restricted, while the fans used to circulate the air have to work harder. Periodically, either the filter must be replaced with a new filter or it must be removed, cleaned, and reinstalled. Many filters, such as HEPA filters, cannot be cleaned and reused. Throw away filters contribute to the ever-growing waste disposal problem, but cleaning and reusing the filters tends to degrade the filters over time to a point of ruin. 
     Air filtration systems utilizing a liquid to aid in removal of particulate matter or toxic gases have been introduced for use in specific activities that generate high levels of polluted air. One example is U.S. Pat. No. 5,408,834, which scrubs air from such sources as a spray-painting booth, resulting in an air stream that is heavily polluted with volatile organic compounds. This patent directs the air into a series of chambers where the air contacts a liquid containing active microbes that will convert the organic compounds into non-toxic substances. One chamber sprays the incoming air with a spray to cool the air to a proper temperature for the microbes to work. Then, in order to increase the contact between the polluted air and the microbial laden liquid, one chamber sprays the liquid over a filter medium, through which the air must pass. 
     A somewhat similar problem is faced in the use of vacuum cleaners, where it is desirable to clean a stream of very dirty air before it leaves the vacuum cleaner. One current solution is to pass the air stream through a container of water, where turbulence mixes the air and water, so that the collected dirt is transferred from the air to the water, where it may be disposed of. 
     It would be desirable to have an air filtration system that did not create waste, yet remained efficient. It would further be desirable if such an air filtration system was suitable for use in clean-rooms, such as for the manufacture of semiconductors, as well as in residential and other commercial uses. 
     SUMMARY OF THE INVENTION 
     The invention includes a device and a method for air filtration, using a liquid, rather than a mesh, to remove particulate matter from the air. The invention also provides for the removal of other contaminants, such as solvents, toxic gasses, or other fumes. All contaminants would be trapped in the liquid, from which they can be removed by current methods, such as centrifugal force, allowing the liquid to be reused. The specific embodiment of the filtration system can use water or another liquid, or multiple different liquids to remove the contaminants. Bubblers, sprays, and vapors are a few of the ways in which the air/liquid interface can be structured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts an air filtration system in accordance with a preferred embodiment of the invention; 
     FIG. 2 depicts an air/liquid chamber configured as a rain chamber in accordance with a preferred embodiment of the invention; 
     FIG. 3 depicts an air/liquid chamber configured as a bubbling chamber in accordance with a preferred embodiment of the invention; 
     FIG. 4 depicts an air/liquid chamber configured as a spray chamber in accordance with a preferred embodiment of the invention; 
     FIG. 5 depicts an air/liquid chamber configured as a vapor chamber in accordance with a preferred embodiment of the invention; 
     FIG. 6 depicts an air/liquid chamber configured as a waterfall chamber in accordance with a preferred embodiment of the invention; 
     FIG. 7 demonstrates a high-level diagram of a current typical air handling system for a clean room; 
     FIG. 8 shows the structure of FIG. 7, but with the air flow shown; and 
     FIG. 9 demonstrates a clean room air handling system in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     With reference now to the figures and in particular with reference to FIG. 1, one embodiment of filtration system  100  for air filtration is disclosed. Air enters the system at intake port  110  and exits at output port  120 , and between these two points is carried through ducting  195 . Air-handling units  130  and  132  move the air through the filtration system  100 . In air-handling units  130  and  132 , fans or other devices serve respectively to draw air into air filtration system  100  or force air out of air filtration system  100 , but though both types are shown, many applications will not require both input and output units. These items are conventional and do not require further explanation. 
     Incoming, dirty air is ducted into air/liquid chamber  140 . This chamber is the location in which the actual filtration takes place, and specific embodiments will be discussed below. Once filtered, the clean air proceeds, if necessary, through a de-vaporizer  150 , then exits the system. De-vaporizer  150  removes any vapor formed in air/liquid chamber  140 . A de-vaporizer is not necessary if the liquid used is water or another liquid with low vapor pressure. 
     The liquid used in air/liquid chamber  140  has its own separate circulation system, allowing the liquid to be cleaned and returned to use. Contaminated liquid from air/liquid chamber  140  is carried through pipes  187  to a liquid/dust separator  160 . There, the dust and other contaminates are removed from the liquid, which can then be re-circulated back to air/liquid chamber  140  through pipes  185 . Liquid that is captured in de-vaporizer  150  also flows with the other clean liquid into air/liquid chamber  140 . One embodiment for liquid/dust separator  160  utilizes centrifugal force to cause the liquid and particulate matter to separate. Alternate methods include boiling followed by condensation of the clean liquid, evaporation, settling tanks, mesh filters, skimming, and sand filters. Once separated, the particulate matter is then removed by appropriate means, while the liquid returns to circulation. One advantage of this system is that only the matter that is filtered out must be disposed of; with a liquid/dust separator, the liquid can be reused over and over without replacement. Alternatively, the liquid can be simply drained from filtration system  100 , either periodically or continuously, with the separation taking place elsewhere. When water is the liquid, installations can choose to use pre-existing waste water systems to handle this task. 
     Water is one obvious choice for the liquid used, as it is non-toxic, plentiful, and inexpensive. However, many other liquids can also be used to remove specific fumes or chemicals. Solubility tables for the specific chemical can aid in determining the optimum liquid for a given situation. 
     In some cases, it may be desirable to use several successive chambers to achieve the desired filtration, either multiple chambers of the same liquid or a series of chambers having different liquids. When a single liquid is used in all chambers, it may be desirable to duplicate only air/liquid chamber  140 , but multiple liquids would additionally require duplication of liquid handling portion  190  of the system. 
     In addition to the structures discussed above, accommodation is made for the removal of liquid from various points in the system. A lower liquid trap  168  keeps liquid from going into the air ducting from air/liquid chamber  140 . This liquid trap can be directly connected to the liquid circulation system, as shown, but this is not necessary. In either case, when filtration system  100  is in operation, air pressure will keep most, but not all, of the liquid out of this region. When the air circulation is stopped, lower liquid trap  168  can fill with liquid. Lower liquid trap petcock  164  allows removal of liquid in the trap. Petcock  162 , adjacent air/liquid chamber  140 , provides for drainage of this chamber for maintenance, as petcock  166  does from liquid/dust separator  160 . 
     We now turn to specific embodiments of air/liquid chamber  140 . With reference to FIG. 2, a first embodiment of air/liquid chamber  140  in FIG. 1 is rain chamber  200 . Air would preferably enter the chamber on the bottom  220  of the chamber, exiting from the top  230 , or another configuration that would have it traverse a large portion of the chamber. The liquid enters the chamber at the top or sides of the chamber through one or more low-pressure heads  210 , which cover as much of the chamber as possible. Low-pressure heads  210  can be fixed or can rotate and can have holes of a single pre-determined size or of many sizes. Droplets of liquid would capture dust particles and other contaminants as they fall, then carry them out through drain  240  near the bottom of rain chamber  200 . 
     With reference to FIG. 3, bubbling chamber  300  represents an alternate embodiment of air/liquid chamber  140  of FIG.  1 . In this embodiment, the air is bubbled through a pool  310  of liquid. As the air moves upward through the liquid, contaminates are removed by contact with the water. If a very low level of contaminants is collected in bubbling chamber  300 , e.g., in residential use, it may be desirable to drain chamber  300  periodically through drain  240  and replace the liquid through piping  185 , eliminating the need for liquid/dust separator  160  of FIG.  1 . However, in moderate or high levels of dust or other contaminants, drain  240  would continuously remove liquid and contaminants while pipe  185  brings in fresh liquid. 
     With reference to FIG. 4, spray chamber  400  represents a further alternate embodiment of air/liquid chamber  140  of FIG.  1 . In this embodiment, spray nozzles  410  inject the liquid under pressure into spray chamber  400 , where the spray traverses as much of spray chamber  400  as possible. The locations and number of nozzles  410  can vary, as can their spray patterns, which can be, for example, conical, stream, or flat ribbon in shape. Nozzles  410  can sweep across spray chamber  400  or remain stationary; the spray can be continuous or pulsed. Like rain chamber  200  in FIG. 2, the flow of liquid traps and carries the contaminants out through drain  240  near the bottom of spray chamber  400 . 
     With reference to FIG. 5, vapor chamber  500  represents a further alternate embodiment of air/liquid chamber  140  in FIG.  1 . Vapor chamber  500  uses a vapor to remove dust and other contaminants. Vapor generators  510  introduce vapor into the chamber at various points. Vapor generators  510  can use techniques like boiling or ultrasound to create the vapor. Any vapor that condenses will be removed from vapor chamber  500  via drain  240 , while any that is carried out with the airflow can be removed in de-vaporizer  150  in FIG.  1 . While this embodiment, more than others, is likely to require a de-vaporization chamber, care in designing vapor chamber  500  to allow for condensation can minimize this need. Alternatively, it may be desirable to add moisture to the air, in which case, the de-vaporizer may be unnecessary. 
     With reference to FIG. 6, waterfall chamber  600  represents a further alternate embodiment of air/liquid chamber  140  of FIG.  1 . Waterfall chamber  600  sends the liquid over a waterfall structure  610 , which in this embodiment resembles a staircase descending from both side walls toward the center of the chamber. Not only the stream of liquid, but also the vapor, mist and spray created by the waterfall, will serve to catch particulate matter, which is then carried out through drain  240 , which in this embodiment is partially seen behind the orifices through which the air travels from lower liquid trap  168  into chamber  660 . 
     FIG. 7 demonstrates a high-level diagram of a current typical air handling system for a clean room. Clean room  700  typically has an underlying concrete slab  717 . A raised floor  710  is built up over concrete slab  717 , using raised floor support posts  712 . Portions of raised floor  710  are built with floor tiles  715  which have numerous air holes through the tiles. Air is supplied to clean room  700  from ceiling  770  of the room, with the air directed toward raised floor  710 , where it is removed from the room. Ideally, air that touches wafers, their carriers, or the equipment that makes them should come directly from the overhead, filtered source and not have had a chance to pick up any contaminates on the way. From the space between concrete slab  717  and raised floor  710 , air from the clean room enters chase  720  carrying the air to an area above the clean room for handling and conditioning. Return ducts  725  direct the air from chase  720  to air handler/conditioner  730 , where temperature, humidity, and other air handling are adjusted. Supply duct  735  is followed by HEPA filter  740 , which removes particulate matter in the air, then the air moves into air plenum  750 , which allows the air to travel across the surface of ceiling  770 . Before the air is re-sent to clean room  700 , it once again passes through HEPA filters  760  set in ceiling  770  of clean room  700 . Note that numerous HEPA filters are used, both between air handler/conditioner  730  and air plenum  750  and between air plenum  750  and clean room  700 . For a typical fab making 3,000 wafers per week, clean room  700  would be about 55,000 square feet. HEPA filters  760  are typically installed in 50% of the ceiling area and cost $300 for a 2-foot by 4-foot filter. The filters  760  should be changed twice a year, giving a typical cost of $2,062,500 per year to replace. Some clean rooms that make 10,000 wafers per week have filters in 100% of the ceiling. These installations typically spend about $13 million for filters alone, although small fabs may cost as low as $500,000 per year. Additionally, these filters only remove dust and particulate matter from the air; they do not remove chemicals or fumes. Currently, semiconductor plants rely on individual exhausts for removing toxics from the air. If toxics are released, sensors are necessary to sound an alarm. 
     FIG. 8 is the same drawing as FIG. 7, but with the air flow shown. Air moves downward from air handler/conditioner  730 , through plenum  750  and into clean room  700 . Under floor  710 , the air travels outward until it enters chase  720 , to be carried upward to return ducts  725  and returned to air handler/conditioner  730 . 
     FIG. 9 demonstrates a clean room air handling system in accordance with a preferred embodiment of the invention. The basic airflow of clean room  700  is unchanged, but air handler/conditioner  730  is replaced by the liquid based air filtration system  100  of FIG.  1 . Air intake port  110  receives air from return ducts  725 , sends it though liquid based air filtration system  100 . Air leaving air output port  120  moves into air plenum  750 . Note that HEPA filters  740  and  760  are shown in this drawing, but they are optional. If they are still used, they should last a lot longer, and cost significantly less, because the air moving into them should be much cleaner. 
     In a wafer fab using a clean room as shown, some of the chemicals used that can enter the air stream include boron trifloride, isopropyl alcohol, acetone, sulfuric acid, hydrochloric acid, hydrofluoric acid, various metal organics, nitric acid, ammonium. All of the above contaminants are soluble or miscible in water; thus water would be an ideal liquid for removing them from the air. Other contaminants can include, gaseous arsine and phosphine, and phosphorous oxycloride (POCl3). These contaminants are insoluble or only slightly soluble in water, so either another air/liquid chamber would be necessary, with an appropriate liquid, or other means used to remove them when present. Still other fab contaminants, such as silane and tetraethylorthosilicate (TEOS), can spontaneously ignite in air, so they will be removed from the clean room air through separate venting to the outside, so they will not affect the present system. 
     The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Technology Category: 7