Patent Publication Number: US-2006003684-A1

Title: Grating and clean room system comprising the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a clean room such as that in which semiconductor devices and liquid crystal displays (LCD) are manufactured. More particularly, the present invention relates to the grating that forms a floor of a clean room system.  
      2. Description of the Related Art  
      Semiconductor devices and liquid crystal displays (LCD) must be manufactured under very precise processing conditions. Therefore, they are fabricated in extremely clean environments unlike many common products of manufacture. In this respect, several semiconductor processing apparatuses are typically provided in separate clean rooms in which the environments are maintained so as to be extremely clean.  
      In a clean room, technicians work in special dustproof clothes in order to minimize the production of foreign substances such as dust. Also, the upper portion of the clean room is maintained at a pressure slightly higher than the pressure prevailing at the bottom of the clean room such that air (clean air) flows downwardly in the clean room. Grates (panels having holes) provide the floor of the clean room such that the air is discharged through the floor. Therefore, contaminants entrained by the air in the clean room are directed toward the floor of the clean room and are discharged to the outside through the holes of the grating.  
      However, unlike micro-particles, molecular contaminants in the air of the clean room, referred to as nanoparticles, and airborne molecular contamination (AMC) are not easily removed. In fact, the differential pressure and velocity of the airflow required in the clean room for the nanoparticles and the AMC to be removed must be at least 18% higher than that under which micro-particles are removed. Needless to say, the operating costs of running the air conditioning system of the clean room system to provide such a high differential pressure and airflow velocity are very high. Also, a conventional clean room system that is capable of producing the differential pressure and airflow velocity required for the removal nanoparticles and AMC is very expensive to manufacture.  
      Also, as illustrated in  FIG. 1 , eddies are generated when the air passes vertically through the holes in the grating. The eddies are mainly generated at edges of the grates that define the entrances and exits of the holes, as shown with dotted lines.  
      To conform the effects of these eddies, particles (30,000 to 35,000 counter/cf) were produced at a height of 0.2 m above the conventional grating in a clean room.  FIG. 2  illustrates the results of counting the numbers of the particles (30,000 to 35,000 counter/cf) at respective heights in the clean room. As is clear from the results shown in  FIG. 2 , the particles were distributed to a height of 70 cm due to the eddies.  
      That is, the eddies generated around the holes of the grating prevent the air from being rapidly exhausted and cause contaminants (in particular, the nanoparticles and the AMC) to reach a height of up to 70 cm from the grating (the floor). As a result, the air is contaminated at the level at which the processes in the clean room are carried out.  
     SUMMARY OF THE INVENTION  
      An object of the present invention to provide a grate that facilitates a smooth and/or rapid discharging of air from a clean room.  
      Likewise, another object of the present invention is to provide a clean room system in which air can be discharged smoothly and/or rapidly from the clean room thereof, and which system does not require an expensive air conditioning system.  
      It is another object of the present invention to provide a grate capable of minimizing eddies in air traveling therethrough to prevent contaminants from being blown upwardly.  
      According to one aspect of the present invention, a grate used flooring in a clean room comprises a base plate having a plurality of holes that act as nozzles through which the air in the clean room is discharged.  
      According to another aspect of the present invention, a clean room system comprises a clean room, an air supplying portion including a fan and/or a filter in the ceiling of the clean room to supply clean air into the clean room, and an exhausting portion including grating disposed in the floor of the clean room, the grating defining a plurality of holes through which the air inside the clean room is discharged, and the cross-sectional area of each hole first decreasing and then increasing in a direction from the top (one side) to the bottom (other side) of the grating.  
      Each of the holes has a receiving portion through which the air is received and an exhausting portion through which the air is exhausted. The cross-sectional area of the receiving portion decreases in a direction toward the exhausting portion. The cross-sectional area of the exhausting portion decreases in a direction toward the receiving portion.  
      The surfaces that define the receiving portion and the exhausting portion of each hole are frusto-conical or curved. Also, the cross-sectional shape of the holes may be circular or elongate. Still further, each hole may also have a middle portion extending a finite distance between the receiving portion and the exhausting portion. The middle portion has a uniform cross-sectional area.  
      The grate or grating may also comprise a tile of polyvinyl chloride at the top thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will now be described in further detail with reference to the accompanying drawings. In the drawings:  
       FIG. 1  is a sectional view of a conventional grate forming the floor of a clean room;  
       FIG. 2  is a graph illustrating the distribution of certain particles throughout the height of a clean room whose floor is formed by conventional grating;  
       FIG. 3  schematic diagram of a clean room system according to the present invention;  
       FIG. 4  is a perspective view of an embodiment of a grate (panel) for use in the floor of a clean room according to the present invention;  
       FIG. 5A  is a plan view of a portion of the grate according to the present invention;  
       FIG. 5B  is a sectional view of the grate taken along line B-B′ in  FIG. 5A ;  
       FIG. 6  is a graph of the differential pressure in a clean room having a floor formed by the conventional grating and the differential pressure in a clean room having a floor formed by grating according to the present invention;  
       FIG. 7  is a perspective view of another embodiment of a grate (panel) according to the present invention;  
       FIG. 8  is a graph illustrating the turbulent kinetic energy K 1  of air passing through the conventional grating panel and the turbulent kinetic energy K 2  of air passing through the grating according to the present invention;  
       FIG. 9  is a graph illustrating the distribution of certain particles throughout the height of a clean room whose floor is formed by grating according to the present invention as compared with the distribution shown in  FIG. 3 ;  
       FIG. 10  is a partial sectional view of another embodiment of a grate according to the present invention; and  
       FIG. 11  is a sectional view of still another embodiment of a grate according to the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereinafter, the present invention will be described in more detail with reference to FIGS.  3  to  11 . Like reference numerals are used to designate like elements throughout the drawings.  
      Referring first to  FIG. 3 , a clean room system  100  includes a clean room, an air supplying portion and an air exhausting portion. The air supplying portion includes a fan filter unit  120  provided in the ceiling of the clean room  110 . The fan filter unit  120  includes a filter and a fan integrated as a unit so as to supply filtered air into the clean room. Although not shown, a ceiling filter, such as a high efficiency particulate air (HEPA) filter or an ultra low penetration air (ULPA) filter, may be provided in the ceiling of the clean room for removing from the air foreign particles, such as dust, whose average diameter is on the order of several microns.  
      The air passes through the fan filter unit  120  and into the clean room  110  where the air forms a vertical current. Such a vertical current forces the contaminants generated in the clean room  110  to the floor to prevent the contaminants from remaining at the level at which the manufacturing processes are performed. In particular, such contaminants are discharged together with the air through a grate  130  of the air exhausting portion. The grate  130  is supported by a supporting structure  114  above a sub-floor  112  of the air exhausting portion. Also, the grate  130  has holes therethrough so that the air can be discharged from the clean room into a region between the grate  130  and the sub-floor  112 . The air discharged through the grate  130  is drawn to the ceiling by the fan filter unit  120  and is re-circulated through the clean room by the fan filter unit  120 . Accordingly, the environment within the clean room is maintained extremely clean.  
      The grate  130  will now be described in more detail referring to  FIGS. 3 and 4 .  
      The grate  130  includes a base plate  132  made of steel, stainless steel, or aluminum. Other materials such as a composite material may be used. The grate  130  may be attached to the supporting structure  114  or may rest freely on the supporting structure  114 . In any case, the grate  130  may be easily removed from the supporting structure to allow access to wiring, ductwork, or other infrastructure  116  in the region between the grate  130  and the sub-floor  112 .  
      The grate  130  also includes a tile  133  made of polyvinyl chloride attached to the top surface of the base plate  132  so as to provide a protective surface. Also, the grate  130  may be processed so as to provide any number of desired surface characteristics. For example, the base plate  132  may covered with carpet or other flooring material for decorative and functional reasons such as to provide sound attenuation and conductivity control. Also, the base plate  132  may be coated with epoxy or may be gold-plated to provide desired characteristics such as static control, abrasion resistance, and protection against chemicals.  
      Referring to  FIG. 4 , the size of the grate  130  is typically 600 mm×600 mm. However, the grates may have other sizes such as 750 mm×750 mm or 500 mm×500 mm because the base plates, at least, can be easily formed by a molding process. In any case, the grate  130  may be sized according to the supporting structure  114  of the clean room system.  
      The grate  130  also has a plurality of holes  134  that extend therethrough. The holes  134  constitute channels through which the air inside the clean room is discharged to the outside (the region just above the sub-floor). The holes  134  are arranged in a pattern that is both attractive and facilitates the flow of the air through the grate. According to an embodiment of the present invention, the total area of the openings of the holes  134  at the top of the grate accounts for about 18% of the total area of the top.  
      Referring now to  FIGS. 5A and 5B , each of the holes  134  includes a receiving portion  136  open at one side of the grate and through which the air is received, and an exhausting portion  138  open at the other side of the grate and through which the air is exhausted. The receiving portion  136  is tapered such that the cross-sectional area thereof becomes smaller in a direction toward the exhausting portion  138 . Similarly, the exhausting portion  138  is tapered such that the cross-sectional area thereof becomes smaller in a direction toward the receiving portion  136 . The diameter of the narrowest portion L 2  of the hole  134  is about 8.5 mm (equal to the diameter of the holes of the conventional grate). The diameter of the widest portion L 1  of the hole is about 10 mm. Although the holes  134  are shown as being round in  FIG. 5A , the holes  134  may be elongate (in the form of slots) as illustrated in  FIG. 7 .  
      The flow of the air that passes through the holes  134  is illustrated with dotted lines in  FIG. 5 . As can be seen from  FIG. 5 , the hole  134  is formed such that a frusto-conical surface  136   a  defines the receiving portion  136  such that the surface  136   a  is inclined relative to the direction of flow of the air through the clean room. Accordingly, the air that collides with inclined surface  136   a  is introduced toward the exhausting portion  138  along the surface  136   a  such that the air flows smoothly in the receiving portion  136  of the hole. Similarly, a frusto-conical surface  138   a  defines the exhausting portion  138  such that the width of the exhausting portion  138   a  increases in the direction of the air flow. Accordingly, the air that flows from the receiving portion  136  to the exhausting portion  138  is rapidly exhausted.  
       FIG. 6  shows a comparison of the differential pressure in a clean room Q 1  whose floor is formed by the conventional grating and the differential pressure of a similar clean room Q 2  whose floor is formed by grating according to the present invention. As can be seen from  FIG. 6 , the pressure drop inside the clean room Q 2  is 2.379 Pa greater than the pressure drop inside the clean room Q 1  (an improvement of 42%). Such an improvement offered by the present invention is due to a nozzle (venture) effect provided by the holes  134 , wherein turbulence and pressure increases are minimized. In particular, eddies are hardly generated at the edges of the grate that define the entrances and exits of the holes  134 . Therefore, contaminants (in particular, nanoparticles and AMC) can be rapidly removed from the clean room through the grate  130 .  
      Also, the lack of turbulence prevents the contaminants from being blown up to a critical height in the clean room. In this respect,  FIG. 8  shows a comparison between the turbulent kinetic energy K 1  of air passing through the conventional grating and the turbulent kinetic energy K 2  of air passing through grating according to the present invention. As can be seen from  FIG. 8 , the turbulent kinetic energy K 2  is 37% less than the turbulent kinetic energy K 1 . Thus, the differential pressure in a clean room whose floor is formed by the grating according to the present invention is greater than that in a comparable clean room whose floor is formed by the conventional grating. Likewise, the mass flow of air in a clean room whose floor is formed by the grating according to the present invention is greater than that in a comparable clean room whose floor is formed by the conventional grating.  
       FIG. 9  illustrates a comparison of the numbers of particles at the respective heights in a clean room whose floor is formed by the grating according to the present invention and in a comparable clean room whose floor is formed by the conventional grating. For the purposes of providing this comparison, particles of 30,000 to 35,000 counter/cf were produced at a height of 0.2 m above the grating. Also, the air velocity in the fan filter unit  120  was 0.4 m/s. As can be seen from  FIG. 9 , the critical height when the grating according to the present invention was used was 40 cm less than the critical height when the conventional grating was used.  
       FIG. 10  illustrates a grate  130   a  similar to that of the above-described grate  130  with the exception of the shape of the holes  134 . The holes  134   a  of the grate  130   a  each have a receiving portion  136 , an exhausting portion  138 , and a middle portion  140  interposed between the receiving portion  136  and the exhausting portion  138 . The cross section of the middle portion  140  extends straight (parallel to the direction of air flow in the clean room) and is uniform between the receiving portion  136  and the exhausting portion  138 . The flow of the air in the hole  134   a  is similar to the flow of the air in the hole  134  described above in connection with  FIGS. 5A and 5B .  
       FIG. 11  also illustrates a grating panel  130   b  similar to that of the above-described grate  130  with the exception of the shape of the holes  134 . In this case, the holes  134   b  of the grate  130   b  are curved. More specifically, each hole  134   b  includes a receiving portion  136   b  through which the air is received and an exhausting portion  138   b  through which the air is exhausted. The surface of the base plate defining the receiving portion  136   b  is curved such that the cross-sectional area of the receiving portion  136   b  decreases in a direction toward the exhausting portion  138   b . Likewise, the surface of the base plate defining the exhausting portion  138   b  is curved such that the cross-sectional area of the exhausting portion  138   b  decreases in a direction toward the receiving portion  136   b . Such curved holes facilitate a smooth flow of the air (illustrated with dotted lines) like the holes  134 .  
      As described above, the turbulent kinetic energy of air flowing through the grate according to the present invention is minimized and hence, the air flow rate is maximized and the pressure drop loss is minimized. Therefore, the critical height of the particles above the grate is relatively low, e.g., lower by about 40 cm when compared to the prior art. Thus, the present invention is effective in controlling the nanoparticles and AMC in a clean room.  
      Finally, although the structure and function of the grating clean room system using the grating according to the present invention have been described above with reference to the preferred embodiments thereof, various changes in form and details thereto will be apparent to those of ordinary skill in the art. Accordingly, various changes can be made to the preferred embodiments without departing from the true spirit and scope of the invention as defined by the appended claims.