Abstract:
A magnetic shield apparatus includes a magnetic shield room, a tubular member, and a flange portion. The magnetic shield room has an opening to shield external magnetism. The tubular member is made of a magnetic shield material and attached to the opening to project from the magnetic shield room by a first predetermined length. The flange portion is made of a magnetic shield material and formed around a distal end portion of the tubular member to be spaced apart from it by a second predetermined length.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetic shield room and, more particularly, to a magnetic shield room having an opening portion through which wafers and the like are unloaded and loaded. 
     Conventionally, as a unit for forming a resist pattern on a sample such as a wafer or mask, an EB (Electron Beam) exposure unit which draws a pattern on a photoresist by using an electron beam is used. 
     An EB exposure unit of this type has a problem that the electron beam irradiation position varies by the influence of the external magnetic field to undesirably cause a distortion in the drawn pattern. In order to prevent this, such an EB exposure unit is arranged in a shield room to operate while it is shielded from the external magnetic field. 
     Concerning a wafer or mask to be processed by the EB exposure unit, a cassette which stores a plurality of wafers or masks is usually handled as one unit. Therefore, the EB exposure unit has a loading portion for extracting a wafer or mask as a processing target from the cassette and supplying it to the processing chamber, and an unloading portion for storing a processed wafer or mask in the cassette. Also, a means for attaching and detaching the cassette to and from the loading and unloading portions is necessary. 
     When such loading and unloading portions are arranged in the magnetic shield room, conventionally, the cassette is manually loaded and unloaded by the operator or the like through an inlet/outlet port formed in the magnetic shield room and provided with a normally closed door. 
     When the door is opened to allow the operator to enter or leave the room, the EB exposure unit is influenced by the external magnetic field. Therefore, when loading/unloading the cassette, operation of the EB exposure unit must be temporarily stopped, leading to a decrease in throughput. 
     FIG. 6 shows a conventional magnetic shield room. As shown in FIG. 6, a maintenance door  3  is provided to the side wall of a magnetic shield room  1 , and a magnetic field shield material is adhered to the inner wall of the magnetic shield room  1 . An EB exposure unit (not shown) or the like is arranged in the magnetic shield room  1 . 
     With this arrangement, to prevent the influence of the external magnetic field, an opening portion having such a size that it does not allow the external magnetic field to influence the EB exposure unit may be formed in the magnetic shield room  1 , and the cassette may be loaded/unloaded through this opening portion. 
     Based on the demands for a higher micropatterning degree and a higher integration degree in recent semiconductor integrated circuits, a strict pattern drawing precision of 0.1 μm or less has been required, and the magnetic field around the EB exposure unit must be suppressed as low as possible. 
     If an opening portion having a size required for loading/unloading a cassette (e.g., one having a size of 180 mm×180 mm×180 mm) is formed, the external magnetic shield enters the magnetic shield room  1  through the opening portion to adversely affect the EB exposure unit. Therefore, the EB exposure unit must be installed to be sufficiently remote from the opening. As a result, the area occupied by the magnetic shield room  1  with respect to the area occupied by the EB exposure unit becomes considerably large. 
     In order to set the strength of the entering external magnetic field not to influence the EB exposure unit, the length of the short sides of the rectangular opening portion must be decreased to 100 mm or less. With this size, however, at most only one wafer or mask can be passed through this opening portion, and wafers and masks stored in a cassette cannot be loaded/unloaded at all. 
     As a method of suppressing entrance of the external magnetic field into the magnetic shield room  1  through the opening, one in which a tubular magnetic field shield material is provided to the outside of the opening portion is proposed, as shown in Japanese Patent Laid-Open No. 59-197198. 
     In order to improve the magnetic shield effect without decreasing the opening ratio, a technique as shown in Japanese Utility Model Laid-Open No. 3-12497 is proposed, in which a stereoscopic shield lattice having a depth is further arranged in a tubular shield material arranged outside the opening portion, such that the interval pitch is smaller than the depth. 
     In order to sufficiently shield the external magnetic shield by providing a tube made of a magnetic shield material at the opening portion, the length of the tubular member must be increased in accordance with the size of the opening. If, however, the tubular member is long, it interferes with the operability of loading/unloading the cassette in/from the loading and unloading portions in the magnetic shield room. For example, when the operability of placing the cassette on the loading or unloading portion is considered, the length of the tubular member is preferably as small as possible. 
     When a shield lattice is arranged in the tubular member, it is suitable for an application such as a vent port. For an application, e.g., a case that includes loading/unloading of a cassette, the lattice interval must be increased. For this reason, the length of the tubular member must be increased in accordance with the size of the opening, thus interfering with the operability. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a magnetic shield room which suppresses entrance of an external magnetic field and allows loading/unloading of a cassette. 
     It is another object of the present invention to provide a magnetic shield room in which operability of loading/unloading the cassette is improved. 
     In order to achieve the above objects, according to the present invention, there is provided a magnetic shield apparatus comprising a magnetic shield room having an opening to shield external magnetism, a tubular member made of a magnetic shield material and attached to the opening to project from the magnetic shield room by a first predetermined length, and a flange portion made of a magnetic shield material and formed around a distal end portion of the tubular member to be spaced apart therefrom by a second predetermined length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view of a magnetic shield room according to an embodiment of the present invention, and 
     FIG. 1B is an enlarged perspective view of a tubular member shown in FIG. 1A; 
     FIG. 2A is a sectional view taken along the line I—I of FIG. 1B, and 
     FIG. 2B is a sectional view showing another tubular member; 
     FIG. 3A is a front view of the magnetic shield room shown in FIG. 1, and 
     FIG. 3B is a sectional view of the magnetic shield room taken along the line II—II of FIG. 3A; 
     FIG. 4 is a graph showing the relationship between the distance from the opening and the strength of magnetic field in the magnetic shield room; 
     FIG. 5A is a perspective view of the tubular member and a flange portion before assembly, and 
     FIG. 5B is a perspective view of the tubular member and the flange portion after assembly; and 
     FIG. 6 is a perspective view of a conventional magnetic shield room. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described in detail with reference to the accompanying drawings. 
     Referring to FIG. 1A, a rectangular opening  120  is formed in the side surface of a magnetic shield room  101  having an inner wall adhered with a magnetic shield material. A tubular member  104  made of a magnetic shield material (e.g., permalloy) and having a rectangular section is attached to the opening  120  with rivets or the like. A portion of the tubular member  104  extending from its distal end for a predetermined length is outwardly bent at a right angle with respect to the tube axis to form a flange portion  105 , as shown in FIG.  1 B. More specifically, the flange portion  105  is constituted by the bent portions of the four sides of the distal end portion of the tubular member  104 . An opening portion  102  is formed by the distal end portion of the tubular member  104  to communicate with the opening  120 . 
     As shown in FIG. 2A, the flange portion  105  is formed by bending the edges of the tubular member  104  outwardly and perpendicularly. When the size of the opening portion  102 , i.e., the sectional size of the tubular member  104 , the length of the tubular member  104 , and the length of the flange portion  105 , are defined as a×b, c, and d, respectively, in this embodiment, these sizes are set as follows: 
     size of the opening portion  102 : 
     a=990 mm 
     b=250 mm 
     size of the tubular member  104 : 
     the sectional size is equal to that of the opening portion  102   
     c=100 mm 
     size of the flange portion  105 : 
     d=10 mm to 20 mm 
     angle of the flange portion  105  with respect to the tube axis 
     θ= 90 ° 
     When the size a of the opening portion  102  is set to 990 mm mentioned above, three cassettes can be arranged horizontally. A description will be made on an assumption that the tubular member  104  has no thickness. 
     Note that the present invention is not limited to these values. It suffices if at least a portion of the tubular member  104  extending from its distal end for a predetermined length is inclined outwardly of the tubular member  104  at an angle of almost 90° with respect to the tube axis. Preferably, the size d of the flange portion  105  may be set to 10 mm or more and the angle θ of the flange portion  105  with respect to the tube axis may be set to 90°. 
     As shown in FIG. 2B, a portion of the tubular member  104  near its distal end may be bent outwardly to have a certain radius of curvature, thereby forming a flange portion  205  having an arcuated section. In FIG. 2B as well, it suffices if the distal end of the tubular member  104  is inclined outwardly of the tubular member  104  with respect to the tube axis. Preferably, a size d of the flange portion  205  may be set to 10 mm or more and an angle θ formed by the tangential direction at the edge of the flange portion  205  and the tube axis may be set to 90°. 
     As shown in FIG. 3A, the opening  120  is formed in one of the four side surfaces of the magnetic shield room  101 . As shown in FIG. 3B, a magnetic shield material  111  is adhered to the entire inner wall of the magnetic shield room  101  without any gap to form a tubular member  104  projecting from the opening  120 . A loading/unloading portion  106  for loading/unloading wafers or masks is arranged near the opening  120 . A cassette loaded in the magnetic shield room  101  through the opening portion  102 , the tubular member  104 , and the opening  120  is mounted on the loading/unloading portion  106 . 
     The wafers and the like stored in the cassette are conveyed into a column  109  in an EB exposure unit  110  with an arm  107  of a convey arm portion  108 , and are exposed. Thereafter, the exposed wafers are mounted on the cassette again by the arm  107  in an order reverse to that described above. The cassette mounted with the wafers is unloaded outside the magnetic shield room  101  through the opening  120 , the tubular member  104 , and the opening portion  102 . 
     If the tubular member  104  Is excessively long, it causes a trouble when mounting the cassette on the loading/unloading portion  106 . The length of the tubular member  104  is preferably 200 mm or less. 
     The relationship between presence/absence of the tubular member  104  and the influence of the external magnetic field will be described. 
     FIG. 4 shows the relationship between the distance from the opening portion  102  and the strength of magnetic field in the magnetic shield room  101  when the external magnetic field has a strength of 5 mG. The size of the opening portion  102  is a×b=990 mm×250 mm. 
     As shown in FIG. 4, when the opening  120  is not formed, the magnetic field in the magnetic shield room  101  is 0.3 mG near the shield wall, 0.25 mG at a position separate from the shield wall by 500 mm, and 0.17 mG at a position separate from the shield wall by 1,000 mm, thus being attenuated gradually. 
     In contrast to this, when only the opening portion  102 , i.e., the tubular member  104 , is formed, the magnetic field near the opening  120  exhibits a value near about 3 mG but is 0.35 mG at a position separate from the opening  120  by 500 mm, thus being attenuated sharply. At a position farther separate from the opening  120 , the magnetic field is attenuated gradually. However, even at a position separate from the opening  120  by 1,000 mm, the magnetic field has a strength of 0.23 mG, which is higher than the value obtained when the opening  120  is not formed by about 0.06 mG. 
     In the first example provided with the tubular member  104  of c=200 mm which has the flange portion  105  of d=10 mm and θ=90°, the strength is 0.6 mG immediately inside the opening  120 , but at a position separate from the opening  120  by 500 mm, the magnetic field is attenuated sharply to a value almost equal to that obtained when the opening  120  is not formed, and at a position separate from the opening  120  by 1,000 mm, the magnetic field is attenuated gradually to 0.17 mG. 
     In contrast to this, when the tubular member  104  having no flange portion  105  is used, to obtain a shield effect almost equal to that described above, the tubular member  104  must have a length of 600 mm or more. This suggests effectiveness of the present invention in decreasing the length of the tubular member  104 . 
     In the second example provided with the tubular member  104  of c=100 mm which has the flange portion  105  of d=10 mm and θ=90°, the strength is about 1.4 mG immediately inside the opening  120 , but at a position separate from the opening  120  by 500 mm, the magnetic field is attenuated sharply to a value almost equal to that obtained when the opening  120  is not formed, and at a position separate from the opening  120  by 1,000 mm, the magnetic field is attenuated gradually to 0.17 mG. 
     The flange portion  105  may be formed by bending the edge of the distal end of the tubular member  104 , as described above, or by mounting a rectangularly annular flange member  305  on flange-like edges  104   a  of the tubular member  104 , shown in FIG. 5A, by using rivets  305   a , as shown in FIG.  5 B. In this case, if the number of rivets  305   a  is increased or the flange member  305  is connected and fixed to the distal end portion of the tubular member  104  in accordance with another mounting method, e.g., welding, in place of the rivets  305   a , the adhesion strength of the connecting portion can be increased. This decreases the impedance of the connecting portion so that the internal magnetic field can be emitted outside more easily. 
     Connecting portions made of a magnetic shield material may be mounted to the notched portions between edges  104   a  of the tubular member  104  to connect the edges  104   a  to each other, thereby forming a flange portion continuously surrounding the opening portion  102  of the tubular member  104 . 
     Alternatively, no edges  104   a  may be formed on the tubular member  104 , but a rectangularly annular flange member  305  made of a magnetic shield material may be attached to the distal end of the tubular member  104  with a known method. Alternatively, a tubular portion may be formed on the flange member  305  and be fixed to the tubular member  104  by fitting. Alternatively, instead of the flange member  305 , strip segments made of a magnetic material and constituting a flange portion may be separately attached to the respective sides of the distal end portion of the tubular member  104 . 
     As has been described above, according to the present invention, a tubular member having a flange portion is formed on the opening of a magnetic shield room. Even if the size of the opening is increased, the influence of the external magnetic field on the interior of the magnetic shield room can be decreased. More specifically, when compared to a case using only a tubular member, the same effect to that obtained by using a long tubular member can be obtained with a short tubular member. For example, when a tubular member having a flange portion and a length of about 100 mm is formed on the opening, the same effect as that obtained when no opening is formed can be obtained at a position separate from the opening by 500 mm. This allows loading/unloading of the cassette through the opening, and accordingly the loading/unloading portion can be arranged in the magnetic shield room, thus increasing the throughput. 
     Since entrance of the external magnetic field through the opening can be suppressed more than in the conventional case, the distance between the EB exposure unit and the opening can be decreased. Since no extra space is required unlike in the conventional case, the magnetic shield room can be made compact.