Patent Number: 062657232
Section: description

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. 1B. 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.times.b, c, and d, respectively, in this embodiment, these sizes are set as follows: size of the opening portion 102: PA1 size of the tubular member 104: PA1 size of the flange portion 105: PA1 angle of the flange portion 105 with respect to the tube axis a=990 mm PA2 b=250 mm PA2 the sectional size is equal to that of the opening portion 102 PA2 c=100 mm PA2 d=10 mm to 20 mm PA2 .theta.=90.degree. 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.degree. 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 .theta. of the flange portion 105 with respect to the tube axis may be set to 90.degree.. 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 .theta. formed by the tangential direction at the edge of the flange portion 205 and the tube axis may be set to 90.degree.. 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.times.b=990 mm.times.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 .theta.=90.degree., 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 .theta.=90.degree., 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 104a of the tubular member 104, shown in FIG. 5A, by using rivets 305a, as shown in FIG. 5B. In this case, if the number of rivets 305a 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 305a, 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 104a of the tubular member 104 to connect the edges 104a to each other, thereby forming a flange portion continuously surrounding the opening portion 102 of the tubular member 104. Alternatively, no edges 104a 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.