Patent Publication Number: US-2005115213-A1

Title: Fresh air ducts including downstream filters for clean rooms

Description:
CLAIM FOR PRIORITY AND REFERENCE TO RELATED APPLICATION  
      This application is a continuation application of U.S. application Ser. No., Ser. No. 10/272,319, which was filed Oct. 16, 2002 and claims priority to Korean Patent Application No. 2002-39160 filed on Jul. 6, 2002, the entire disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to the field of integrated circuit fabrication, and more particularly, to clean rooms used in the fabrication of integrated circuits.  
     BACKGROUND OF THE INVENTION  
      Integrated circuit devices are typically fabricated in what is commonly referred to as a clean room environment. As the term implies, a clean room environment can be essentially free of particles that can affect the yield of the integrated circuits produced in the clean room.  
      In particular, substances, either created during the manufacturing process itself or introduced to the clean room via external air supplied to the clean room, can contaminate the integrated circuits produced by the clean room thereby leading to their premature failure.  
      The types of devices fabricated in the clean room can dictate the degree of filtering of external air provided to the clean room. For example, studies have shown that in order to reliably produce 256 MB DRAMs having a design rule of 0.25 μm, particles having a size greater than 0.25 μm should be removed or filtered from the clean room. In addition, the temperature, humidity, and pressure of the air in the clean room may be controlled. Other types of environmental factors, such as illumination, noise, and vibration, may also be controlled.  
      In order to provide the filtering discussed above, air is provided to the clean room through an air conditioning apparatus having filter units that can filter external air (i.e., air obtained from an external environment) to the clean room. In particular, in some conventional approaches, fresh air produced by the air conditioner is introduced below the floor of the clean room which is then circulated above the ceiling of the clean room and into the clean room.  
      As the concentration of fine particles contained in the external air is increased, it may be difficult for a conventional air conditioner to filter the air to the level needed by the clean room which can increase contamination in the clean room resulting in a decrease in the yield of integrated circuits manufactured in the clean room. For example, a phenomenon referred to as “yellow sand dust” (or “dust”) can dramatically affect the yield of integrated circuits produced in a clean room. The yellow sand dust phenomenon can be characterized as when the external air includes an abnormally high level of sulfuric oxide, nitric oxide, and/or silicon. The yellow sand dust phenomenon can occur at particular times of the year and in particular geographic locations. For example, measurements in the area of Seoul, Republic of Korea in the year 2001, showed that density of dust in the air during the occurrence of the yellow sand dust phenomenon was about 7 to 14 times greater than the density at other times of the same year in the same location. In particular, the density of the dust was measured to be in a range between about 473 μg/m 3  and about 999 μg/m 3  during 2001 in Seoul. Furthermore, ozone, SO 2  and/or NO x  contained in the external air during the summer months can in turn be introduced into the clean room if proper filtering is not performed.  
       FIG. 1  is a greatly enlarged schematic view that illustrates a particle  10  lodged on a pattern  20  of an integrated circuit in a clean room. The particle  10  can cause the integrated circuit to be inoperative or to function improperly. The particle  10  can be about 0.05 μm in diameter and have a circular or annular ring shape.  
       FIG. 2  is a graph that illustrates measured compositions of the particle  10  shown in  FIG. 1 . In particular, an EDS (Energy Dispersive X-ray Spectrometer) shows that the particle  10  can include significant amounts of silicon and oxygen which are present in the composition of the dust produced during the yellow sand dust phenomenon. As shown in  FIG. 2 , many of the particles shown in the EDS have a size less than 0.05 μm.  
      It is known to use an air conditioner including an HEPA (high efficiency particulate air filter) and a chemical filter to filter air provided to a clean room. U.S. Pat. No. 5,890,367 to You et al. entitled  Air Conditioning System for Semiconductor Clean Room Including a Chemical Filter Downstream of a Humidifier , discusses the removal of chemical impurities from the air provided by the air conditioner, such as phosphoric acid generated by the process of manufacturing the integrated circuits, using a chemical filter located between the air conditioner, having a humidifier, and an ULPA (ultra low penetration air) filter.  
     SUMMARY OF THE INVENTION  
      Embodiments according to the present invention can provide fresh air ducts including separate particle filters downstream from air conditioning systems for use with clean rooms. Pursuant to these embodiments, a fresh air duct can be configured to direct fresh air filtered by an air conditioning system towards a clean room downstream from the fresh air duct, the fresh air having a first level of particle density. A separate particle filter in the fresh air duct, located downstream from the air conditioning system and upstream from the clean room, can be configured to further filter the fresh air to provide filtered fresh air having a second level of particle density therein that is less than the first level.  
      In some embodiments according to the present invention, the air conditioning system accepts external air having a third level of particle density that is greater than the first and second levels and is in a range between about 473μ/m 3  and about 999 g/m 3 . In some embodiments according to the present invention, the separate particle filter is in an outlet section of the fresh air duct adjacent the clean room.  
      In some embodiments according to the present invention, the separate particle filter can be at least one of an HEPA filter configured to filter about 99.97% of particles about 0.3 μm in size from the fresh air generated by the air conditioning system, an ULPA filter configured to filter about 99.99% of particles about 0.1 μm to about 0.17 μm in size from the fresh air generated by the air conditioning system, and an ozone filter.  
      In some embodiments according to the present invention, a second filter is located downstream from the first filter and upstream from the clean room. The second filter can be at least one of an HEPA filter configured to filter about 99.97% of particles about 0.3 μm in size from the fresh air generated by the air conditioning system, an ULPA filter configured to filter about 99.99% of particles about 0.1 μm to about 0.17 μm in size from the fresh air generated by the air conditioning system, a chemical filter and an ozone filter. In some embodiments according to the present invention, the second filter is included in an outlet section of the fresh air duct adjacent to the clean room.  
      In some embodiments according to the present invention, the separate particle filter can be a detachable filter and the fresh air duct can further include a first portion of the fresh air duct configured to direct air from an input of the fresh air duct to an output of the first portion and a second portion of the fresh air duct releaseably coupled to the output of the first portion and configured to direct air from the output of the first portion along a pathway through the second portion. The detachable filter is in the second portion of the fresh air duct.  
      In some embodiments according to the present invention, the detachable filter can be a first detachable filter. An outlet portion of the fresh air duct is located downstream from the second portion and a second detachable filter is in the outlet portion configured to provide filtered air downstream of the second detachable filter to the clean room. In some embodiments according to the present invention, the filter is located adjacent to an input of the second portion of the fresh air duct.  
      In some embodiments according to the present invention, the first portion can be a flexible section configured to allow the first portion of the fresh air duct to move away from an input of the second portion of the fresh air duct to expose the detachable filter. In some embodiments according to the present invention, a third portion of the fresh air duct is coupled to the second portion and has at least one output therefrom. A second detachable filter is included in the third portion and is configured to filter the fresh air conducted along the pathway to the at least one output from the fresh air duct into the clean room.  
      In some embodiments according to the present invention, a third portion of the fresh air duct is coupled to the second portion and has a plurality of outputs therefrom. A plurality of detachable filters is located in the third portion upstream from the plurality of outputs.  
      In further embodiments according to the present invention, a fresh air duct is located downstream from an air conditioning system and is configured to direct fresh air from the air conditioning system to a clean room. A detachable filter, is located in the fresh air duct and is configured to filter the fresh air to provide filtered fresh air downstream of the detachable filter.  
      In some embodiments according to the present invention, the fresh air duct can be a first portion of the fresh air duct configured to direct air from an input of the fresh air duct to an output of the first portion and a second portion of the fresh air duct that is releaseably coupled to the output of the first portion and is configured to direct air from the output of the first portion along a pathway through the second portion, wherein the detachable filter is in the second portion of the fresh air duct. In some embodiments according to the present invention, the detachable filter is inside the clean room.  
      In some embodiments according to the present invention, an outlet portion of the fresh air duct is lcoated downstream from the second portion. A second detachable filter is located in the outlet portion and is configured to provide filtered air downstream of the second detachable filter into the clean room.  
      Embodiments of methods according to the present invention can include providing fresh air filtered by an air conditioning system downstream towards the clean room, the fresh air having a first level of particle density, further filtering the fresh air downstream from the air conditioning system to provide filtered fresh air having a second level of particle density that is less than the first level and providing the filtered fresh air to the clean room. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a greatly enlarged schematic view of a particle lodged on a pattern of an integrated circuit device.  
       FIG. 2  is an EDS illustrating the composition of a particle commonly associated with the yellow sand dust phenomenon.  
       FIG. 3  is a schematic view that illustrates embodiments of clean room systems according to the present invention.  
       FIGS. 4A-4B  are schematic views that illustrate embodiments of fresh air ducts according to the present invention.  
       FIG. 5  is a schematic frontal view that illustrates embodiments of a filter on a filter frame and grill according to the present invention.  
       FIG. 6  is a bar graph that illustrates the density of ozone present in a clean room at different times of the year.  
       FIG. 7  is a schematic view that illustrates embodiments of clean room systems according to the present invention.  
       FIG. 8  is a schematic sectional view that illustrates embodiments of fresh air ducts according to the present invention.  
       FIG. 9  is a schematic view that illustrates embodiments of clean room systems according to the present invention.  
       FIG. 10  is a schematic sectional view that illustrates embodiments of fresh air ducts according to the present invention.  
       FIG. 11  is a graph that illustrates comparative measurements of particles in a conventional clean room system and in embodiments of a clean room system according to the present invention.  
       FIG. 12  is a bar graph that illustrates comparative measurements of the density of silicon in baths in a conventional clean room and in embodiments of clean rooms according to the present invention.  
    
    
     DESCRIPTION OF EMBODIMENTS ACCORDING TO THE INVENTION  
      The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which typical 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. In the drawings, the relative sizes of regions may be exaggerated for clarity. It will be understood that when an element such as a duct or portion of a duct is referred to as being “connected” or “coupled” to another element, it can be directly connected to the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Terms used herein are to be given their ordinary meaning unless explicitly defined otherwise herein.  
       FIG. 3  is a schematic diagram that illustrates embodiments of clean room systems according to the present invention. As shown in  FIG. 3 , the clean room system includes a clean room  320  in which integrated circuit devices are fabricated. An air conditioning system  300  accepts external air (i.e., air external to the clean room  320 ) and provides fresh air, via a fresh air duct  302 , downstream to a lower space  310 A of the clean room  320 . The fresh air is then circulated to an upper space  310 B of the clean room  320  as illustrated.  
      The air conditioner  300  filters particles in the external air using a plurality of filters  300 A,  300 B,  300 C and  300 D. The air conditioner  300  can also include a heater  300 E and a cooler  300 F, located downstream from the filters  300 A- 300 D, for controlling the temperature of the air. The air conditioner  300  can further include a humidifier  300 G, downstream from the heater  300 E and cooler  300 F, for adjusting the humidity of the air. The humidifier  300 G is located downstream from the filters  300 A- 300 D to avoid introducing excessive moisture into the filters  300 A-D which could otherwise adversely affect the efficiency of the filters. The air conditioner  300  can further include a blower  300 H, at an outlet of the air conditioner  300 , configured to force the fresh air from the air conditioner  300  into the fresh air duct  302 .  
      The air conditioner  300  will now be described in greater detail. The filters  300 A- 300 D can be selected to filter particles of different sizes. Moreover, the filters  300 A- 300 D can be arranged in a sequence according the size of the particles to be filtered. For example, filter  300 A can be selected to filter the largest particles whereas each of the remaining filters can be arranged to filter decreasingly smaller particles such that filter  300 D filters the smallest sized particles of the filters  300 A- 300 D.  
      External air is introduced into the air conditioner  300  through a roll filter  300 A which can remove impurities from the external air. A demister  300 B is located downstream from the roll filter  300 A and is configured to remove or reduce the moisture and impurities in the air. A medium filter  300 C is located downstream from the demister  300 B and is configured to remove minute dust from the air, preferably at an efficiency of at least about 90%. In some embodiments according to the present invention, the medium filter  300 C is an HEPA filter which can filter particles having a size of about 0.3 μm from the air at an efficiency of at least about 99.97%.  
      In some embodiments according to the invention, the heater  300 E and the cooler  300 F are located downstream from the filters  300 A-D. In some embodiments according to the present invention, the cooler  300 F is located downstream from the heater  300 E so that moisture introduced into the air by the heater  300 E can be reduced by the cooler  300 F.  
      In other embodiments according to the present invention, the heater  300 E and the cooler  300 F are upstream from the filters  300 A- 300 D. The air output from the heater  300 E flows through the humidifier  300 G before being provided to the clean room  320  so that the humidity of the air can be adapted if needed. A blower  300 H is located at the outlet of the air conditioner  300  to force the fresh air from the air conditioner  300  into the fresh air duct  302 . During times when the sand dust phenomenon occurs, the density of the particle impurities in the fresh air can be about 1,200,000 EA/cfm (cubic feet per minute).  
      The fresh air duct  302  is configured to conduct the fresh air from the air conditioner  300  to the lower space  310 A of the clean room. The fresh air duct  302  includes a filter  106  which provides secondary filtering of the fresh air provided by the air conditioner  300 . The filter  106  can be selected based on the types of particles and contaminants contained in the external air at a particular time. Because the fresh air has passed through filters  300 A- 300 D, the filter  106  preferably has an efficiency that is at equal or superior to the filter efficiency of the filter  300 D. In some embodiments according to the present invention, the filter  106  is an HEPA filter capable of filtering particles having a size of about 0.3 μm at an efficiency of at least 99.97%. In other embodiments according to the present invention, the filter  106  is an ULPA filter that is capable of filtering particles having a size in a range between about 0.12 μm and about 0.17 μm at an efficiency of at least about 99.99%. In other embodiments according to the present invention, when the external air includes contaminants, such as sulfuric acid or nitric oxide, a chemical filter can also be provided in the fresh air duct  302  (as described herein, for example, with reference to  FIG. 10 ). For example, during times when the sand dust phenomenon occurs, the density of 0.3 μm sized particles in the filtered fresh air provided by an HEPA filter  106  can be in a range between about 400,000 EA/cfm and about 500,000 EA/cfm.  
      According to the present invention, the inclusion of the filter  106  in the fresh air duct  320  downstream from the filters  300 A-D in the air conditioner  300  allows the particles in the air to be filtered in separate steps. When the density of the particles in the external air is exceptionally high, such as when the yellow sand dust phenomenon occurs, embodiments according to the present invention can nonetheless provide adequate filtering for clean rooms. Otherwise, the external air may be contaminated with so many particles that the efficiency of the filters  300 A- 300 D is reduced to the point that the air provided by the air conditioner  300  causes an unacceptable number of defects in the manufacturing process in the clean room. Therefore, the filter  106  can further filter the fresh air provided by the air conditioner  300  to provide fresh air having a lower density of particles than the fresh air provided by the air conditioner  300 , particularly when the yellow sand dust phenomenon occurs.  
      In some embodiments according to the present invention, the type of filtering provided in the fresh air duct  302  can be adjusted as the condition of the external air changes. For example, if the external air includes chemical contaminants, such as sulfuric oxide or nitric oxide, the filter  106  can be a chemical filter. In some embodiments according to the present invention, an HEPA filter or an ULPA filter can be used if the particle size is particularly small.  
      Furthermore, in the summer months in Korea, the density of ozone in the air may be higher than in other times of the year.  FIG. 6  is a bar graph illustrating differences in the density of ozone in a clean room measured in a summer season and in other times during the year. In particular, the bar graph on the left side of  FIG. 6  shows the minimum and maximum values of the density of ozone measured in the clean room during times of than the summer months (i.e., ordinary times). In contrast, the bar graphs on the right side of  FIG. 6  illustrates the minimum and maximum values of ozone density measured in a clean room during the summer season. As shown in  FIG. 6 , the density of the ozone measured in the clean room during the summer season is 4 to 4 ½ times greater than the density of the ozone measured in the clean room during other times of the year. Therefore, according to the present invention, an ozone filter may be attached to the fresh air duct during the summer season to provide additional filtering of ozone. If the increased amount of ozone in the external air is not filtered, the density of the ozone introduced into the-clean room may increase and thereby cause additional defects in the integrated circuits fabricated therein.  
      The fresh air duct  302  will now be described in further detail.  FIGS. 4A and 4B  are sectional views that illustrate embodiments of fresh air ducts according to the present invention. According to  FIG. 4A , the fresh air duct  302  is located downstream from the air conditioner system  300 . In some embodiments according to the present invention, the fresh air duct  302  is configured to conduct the fresh air provided by the air conditioner  300  from a first portion  101  of the fresh air duct  302  downstream to a second portion  108  of the fresh air duct  302  along a pathway defined by the fresh air duct  302  to the clean room.  
      The fresh air duct  302  includes a filter  106  positioned in the pathway. In some embodiments according to the present invention, the filter  106  is an HEPA filter capable of filtering particles having a size of 0.3 μm at an efficiency of at least about 99.97%, or an ULPA filter capable of filtering particles having a size of about 0.1 μm at an efficiency of at least about 99.99%. It will be further understood that an ozone filter capable of removing ozone and/or a chemical filter for filtering chemical contaminants such as sulfuric oxide or nitric oxide may be used in addition to the filters discussed above.  
      In some embodiments according to the present invention, the filter  106  is detachably coupled within the second portion  108  and filters the air provided through the first portion  101  to provide filtered fresh air downstream of the filter  106 . The first portion  101  is releasably coupled to the second portion  108  so that the filter  106  may be more easily removed from the fresh air duct  302  compared to conventional systems.  
      In some embodiments according to the present invention, a flexible section  101  A of the first portion  101  allows an output of the first portion  101  to be retracted from an input of the second portion  108  so that the filter  106  can be exposed and thereby accessed for detachment and replacement as shown, for example, in  FIG. 4B . It will be further understood that the flexible section  101 A may flex from side to side in addition to retracting from the input to the second portion  108  to further facilitate ease of replacement of the filter  106 . In some embodiments according to the present invention, the flexible section  101 A is in the form of bellows as shown in  FIG. 4A . In other embodiments according to the present invention, the flexible section  101 A can be an elastic material, such as rubber, and takes the form of a sleeve without the bellows type of arrangement shown in  FIG. 4A .  
      In some embodiments according to the present invention, an attachment section  104  includes first and second pieces that are releasably coupled to one another thereby allowing the first and second portions  101 ,  108  to be releasably coupled to one another. The first piece is coupled to the output of the first portion  101  whereas the second piece is coupled to the input of the second portion  108 . It will be understood that other configurations of the attachment section  104  may be used.  
      In particular, the first piece of the attachment section  104  includes a fixing frame  104 C and a fastening member  104 D. The second piece of the attachment section  108  includes a filter frame  104 A which positions the filter  106  along the pathway defined by the fresh air duct and a grill  104 B which overlies and can protect the filter  106 . The first piece of the attachment section  104  is configured to be releaseably coupled to the second piece of attachment section  104  by securing the filter frame  104 A to the fixing frame  104 C using the fastening member  104 D. In some embodiments according to the present invention, the fastening member  104 D includes a plurality of bolts or screws located near the edges of the fixing frame  104 C. A plurality of holes are located in the filter frame  104 A so that the fastening member  104 D, such as a plurality of bolts or screws, can be aligned and inserted into the holes. The plurality of bolts or screws can then be tightened to couple the first piece of the attachment section  104  to the second piece of attachment section  104 , thereby coupling the first portion  101  to the second portion  108 .  
      It will be understood that in other embodiments according to the present invention, the fastening member  104  can be other mechanisms, such as latches. It will be understood that in some embodiments according to the present invention, a series of attachment sections may be included in the fresh air duct  302  wherein each of the respective attachment sections holds a respective filter for a specific use in further filtering the fresh air.  
       FIG. 5  is a frontal view that illustrates embodiments of the second piece of the attachment section  104  according to the present invention. As discussed above, the filter frame  104 A provides a space in which the filter  106  can be inserted. The grill  104 B is detachably installed adjacent to a face of the filter frame  104 A, such as the face which faces upstream. The grill  104 B can be a lattice-type grill which supports and protects the filter  106 .  
      In operation, the filter  106  can be replaced by stopping or slowing the blower  300 H to reduce the air flow downstream to the filter  106 . In some embodiments according to the present invention, the filter  106  is replaced by loosening the fastening member  104 D thereby releasing the fixing frame  104 C from the filter frame  104 A as shown in  FIG. 4B . The flexible section  101 A can be retracted from the second portion  108  to expose the filter  106 . The grill  104 B is removed from the filter frame  104 A and the filter  106  is removed from the filter frame  104 A. A replacement filter  106  is mounted in the filter frame  104 A and the grill  104 B is replaced over the replacement filter  106  and secured to the filter frame  104 A. The flexible section  101 A is extended so that the fixing frame  104 C contacts the filter frame  104 A and the fastening member  104 D is used to fasten the first portion  101  to the second portion  108  of the fresh air duct  302 .  
      Returning now to the description of the clean room system shown in  FIG. 3 , air flowing from the fresh air duct  302  passes through the filter  106  and is introduced through an outlet into a lower space  310 A of the clean room. An air circulating section  312  circulates the air from the lower space  310 A to an upper space  310 B located above the ceiling of the clean room. The air circulating section  312  further circulates the air already existing in the clean room with the fresh air that is supplied via the fresh air duct  302 . The air circulating section  312  includes a pre-filter for filtering contaminated air in the lower space  310 A. The air circulating section  312  also includes a bag filter capable of filtering particles contained in the air and an efficiency of at least about 90% and an axial fan for circulating the air that is passed through the filters.  
      A clean room filter  330  filters the air supplied from the air circulating section  312  into the clean room  320 . The clean room filter  330  can be located in the ceiling of the clean room  320 . The air filtered by the clean room filter  330  travels from the upper space  310 B through a grating into the clean room  320 . The air exits the clean room  320  into the lower space  310 A. The air in the lower space  310 A can be recirculated into the upper space  310 B above the ceiling of the clean room  320  through the air circulating section  312 .  
      Accordingly, the clean room filter  330  filters the particles contained in the air before the air is introduced into the clean room  320 . The clean room filter  330  filters particles that are smaller than the particles filtered by the filters  300 A- 300 D. Because the particles contained in the external air can be removed or greatly reduced by the filter  106 , the clean room filter  330  can be used to filter the contaminants in the clean room  320  generated by the manufacturing process itself. In some embodiments according to the present invention, the clean room filter  330  can be an ULPA filter capable of filtering particles having a size and a range between about 0.12 μm and about 0.17 μm at an efficiency of at least about 99.99%.  
      In some embodiments according to the present invention, about 70% of the air introduced into the clean room  320  is circulated throughout the clean room  320 . Furthermore, in some embodiments according to the present invention, about 30% of the air is discharged from the clean room system through an exhaust apparatus that operates during the fabrication of integrated circuits in the clean room  320 . Therefore, a volume of air equal to 30% of air discharged by the exhaust system is introduced through the fresh air duct  320  according to the present invention.  
       FIG. 7  is a schematic diagram illustrating embodiments of a clean room system according to the present invention. The clean room system shown in  FIG. 7  is substantially identical to the clean room system disclosed above in reference to  FIG. 3 . Accordingly, a detailed description of many of the common elements of the clean room system shown in  FIG. 7  will not be repeated here.  
      In some embodiments according to the present invention, first and second filters  204  are included in an outlet section of a fresh air duct  402  through which the fresh air is provided to a lower space  410 A of the clean room system. In some embodiments according to the present invention, the first and second filters  204  are different from one another. For example, in some embodiments according to the present invention, the first filter  204  can be an HEPA filter whereas the second filter  204  can be an ULPA filter, an ozone filter, or a chemical filter.  
       FIG. 8  is a sectional view that illustrates embodiments of the outlets  201  of fresh air ducts according to the present invention as shown in  FIG. 7 . As shown in  FIG. 8 , the filter  204  is positioned in the outlet portion of the fresh air duct downstream from a damper  206  configured to control the flow of the fresh air through the outlet section  201 . In particular, the outlet section  201  is positioned downstream from an air conditioner system  400  and filters the fresh air provided to the clean room  420 .  
      In some embodiments according to the present invention, the filter  204  is positioned along the pathway defined by the outlet section  201  by an attachment section  202  and, in particular, is located in a filter frame  202 A. A filter grill  202 B overlies the filter  204  and is attached to the filter frame  202 A to protect the filter  204  and to hold the filter  204  in place. The attachment section  202  further includes a fixing frame  202 C and a fastening member  202 D which releasably couples the fixing frame  202 C to the filter frame  202 A. The fixing frame  202 C can be detached from the filter frame  202 A by removing the fastening member  202 D and the fixing frame  202 C to expose the filter  204  which then may be removed and replaced. The fixing frame  202 C and fastening member  202 D can be recoupled to the filter frame  202 A.  
      In will be understood that the first and second outlets  201  can provide filtered fresh air to different areas of the clean room  420 . For example, in some clean rooms, dedicated fresh air duct outlets are provided for different areas which perform photolithography, etching, implant, deposition, as each of these processes may require different degrees of filtering. Therefore, respective outlets  201  according to the present invention can be configured with different filters selected for the different areas of the clean room  420 .  
       FIG. 9  is a schematic diagram illustrating a clean room system according to the present invention including filters in the fresh air duct and the outlets. In particular,  FIG. 9  illustrates a fresh air duct  352  including a filter  154  according to the present invention and first and second auxiliary filters  164  according to the present invention in the outlet sections, as discussed above, for example, in reference to  FIGS. 3 and 7  respectively. Accordingly, a further detailed description of these filters is omitted.  
      Generally, the fresh air duct  352  includes at least one filter  154  for filtering particles contained in the air provided by an air conditioning system  350  located upstream. The auxiliary filters  164  are located at first and second outlets and can be different types of filters. For example, one of the auxiliary filters  164  can be an HEPA filter whereas the other auxiliary filter  164  can be an ULPA filter, an ozone filter, or a chemical filter.  
       FIG. 10  is a sectional view that illustrates embodiments of fresh air ducts according to the present invention. According to  FIG. 10 , a fresh air duct  150  including a filter  154  is substantially identical to the fresh air ducts illustrated in  FIGS. 4A and 4B  and further includes an auxiliary filter  164  located downstream from the filter  154 . According to  FIG. 10 , fresh air provided by an upstream air conditioner flows into a first portion  151  of the fresh air duct  150  which travels downstream to a second portion  158  of the fresh air duct  150  through the filter  154 . The elements shown in  FIG. 10 , including a first attachment section  152 , are substantially the same as those described above in reference to  FIGS. 4A and 4B  and will not be described further. The fresh air duct  150  further includes a damper  160  downstream from the second portion  158 . The damper  160  is configured to interrupt or reduce the flow of fresh air downstream. The auxiliary filter  164  is located downstream from the damper  160  and is positioned along the pathway defined by the second portion  158 . In particular, the auxiliary filter  164  is positioned in the second portion  158  adjacent to the output therefrom. The filter  164  is connected to a second attachment section  162 . The second attachment section  162  includes an auxiliary filter frame  162 A which holds the auxiliary filter  164  and a grill  162 B which overlies the auxiliary filter  164 . The auxiliary filter frame  162 A is releasably coupled to a fixing frame  162 C by a fastening member  162 D. In some embodiments according to the present invention, the auxiliary filter  164  can be an HEPA filter, and ULPA filter, a chemical filter, or an ozone filter. In some embodiments according to the present invention, the auxiliary filter  164  is an ozone filter or a chemical filter. During times when the sand dust phenomenon occurs, the density of 0.3 μm sized particles in the filtered fresh air provided by the auxiliary filter  164  can be in a range between about 1,000 EA/cfm and about 2,000 EA/cfm.  
       FIG. 11  is a graph illustrating the amount of particles measured in a fresh air duct of a conventional clean room system compared to the amount of particles measured in a fresh air duct according to embodiments of the present invention. The measurements were taken during the period from Mar. 20, 2001 to Apr. 10, 2001 in The Republic of Korea. The yellow sand dust phenomenon discussed above occurred during the period from Mar. 20, 2001 to Mar. 25, 2001 and from Apr. 7, 2001 to Apr. 10, 2001 (hereinafter referred to as the “interval”). During this time, the maximum dust density at the peak time was in a range between above 500 μg/m 3  and about 1000 μg/m 3 .  
      As shown by  600   a  in  FIG. 11 , the amount of particles in the fresh air duct of the conventional system was abnormally high during the interval. In contrast, as shown by  600   b  in  FIG. 11 , the amount of particles in a fresh air duct according to embodiments of the present invention, was not abnormally high and was less than that measured in the conventional fresh air duct during the interval. In  FIG. 11 , the increase in the amount of particles for the period from Mar. 30, 2001 to Apr. 2, 2001 was caused by a measurement error. As illustrated by  FIG. 11 , the fresh air provided to the clean room by embodiments according to the present invention, may be contaminated by fewer particles compared to conventional approaches, even during the yellow sand dust phenomenon.  
       FIG. 12  is a bar graph illustrating densities of silicon measured in a cleaning liquid bath in a conventional clean room and densities of silicon measured in a cleaning liquid bath in a clean room according to embodiments of the present invention. According to  FIG. 12 , the density of silicon detected in the cleaning liquid number  1  to number  4  in the bath in the conventional clean room was in a range between about 3 and about 8 parts per billion. In contrast, the density of silicon measured in the cleaning liquid bath of the clean room according to embodiments of the present invention (number  6  to number  15 ) was less than 1 part per billion. Accordingly, the contamination of the clean room due to particles in the external air was reduced in clean rooms utilizing embodiments according to the present invention.  
      It should be noted that many variations and modifications might be made to the embodiments described above without substantially departing from the principles of the present invention. All such variations and modifications are intended to be included herein within the scope of the present invention, as set forth in the following claims.