Abstract:
Magnetically shielded enclosures (e.g., rooms) and associated methods are disclosed for containing and magnetically shielding a field-sensitive system such as a charged-particle-beam (CPB) microlithography system without having to make the enclosure excessively large. The CPB microlithography system includes a lens column and substrate chamber collectively forming an internal shielding barrier. The shielded enclosure is in external surrounding relationship to the internal shielding barrier. The shielded enclosure includes a wall that defines an aperture through which some of the stray external magnetic field can penetrate to inside the enclosure. Some of the external magnetic field also leaks through the enclosure walls. The aperture is situated and configured such that the external magnetic flux leaking through the aperture (which flux tending to be directed toward the enclosure walls) and other external magnetic flux leaking through the enclosure walls (which flux tending to be directed toward the internal shielding barrier) at least partially cancel each other. This cancellation reduces the amount of the leakage flux that penetrates the internal shielding barrier to the field-sensitive system inside the barrier.

Description:
FIELD 
     This disclosure pertains to magnetic shielding of an enclosure such as a room containing equipment that is sensitive to the effects of external magnetic fields, and to enclosures shielded in such a manner. Exemplary field-sensitive systems that can be contained in such a magnetically shielded enclosure include systems for performing microlithography using a charged particle beam. 
     BACKGROUND 
     The growth of modern processing and analytical technology has included more extensive use of techniques that employ a charged particle beam (e.g., electron beam or ion beam). Accompanying more extensive use of these techniques generally has been a demand for progressively more accurate and precise performance from the systems that perform the techniques. For example, increasingly greater image resolution is being demanded from electron microscopy. Also, increasingly greater pattern-transfer resolution and accuracy is being demanded from charged-particle-beam (CPB) microlithography, which is a key “next-generation lithography” technology being actively developed for fabricating microelectronic devices. 
     Obtaining greater pattern-transfer accuracy from a CPB microlithography system requires application of more stringent measures to prevent the charged particle beam from being influenced uncontrollably by stray external and internal magnetic and electrical fields. External stray magnetic fields include magnetic fields produced by the earth, events in outer space, and by nearby man-made equipment such as power equipment, power cables, and elevators, for example. Similarly, external stray electrical fields can be produced by any of various sources, both natural and man-made. “Internal” stray fields usually are produced by components of the CPB microlithography system located, for example, inside the “lens column” (vacuum chamber that houses the CPB optical system) and/or inside the “substrate chamber” (vacuum chamber that houses the substrate stage and peripheral components). Even if the magnitude or fluctuation amplitude of a stray field is very small, the field nevertheless can cause an undesired change in the trajectory and/or position of the charged particle beam sufficient to destroy any prospect of achieving a desired accuracy and precision of pattern transfer. For example, if the charged particle beam is being used to transfer a pattern having linewidths of, e.g., 70 nm, the importance of reducing the effect of a stray magnetic and/or electrical field, even an extremely small-magnitude field, on the beam is readily appreciated. 
     As noted above, potentially troublesome fields can be electrical or magnetic, static or dynamic (fluctuating), strong or weak, man-made or natural, internal or external. An example of an internal field is a field generated by a component of the system, such as a stray magnetic field produced by an electron lens or deflector or by a reticle stage or substrate stage. An example of an external field is a field produced by the earth or by nearby industrial activity. 
     For shielding purposes, conventional CPB microlithography systems usually include one or more magnetic shields located inside the lens column and inside the substrate chamber. For example, shielding may be associated with certain peripheral components located in or near these chambers, such as wafer loaders, reticle loaders, electromagnetic lenses, stage motors, vacuum pumps, etc. Another conventional manner of shielding CPB microlithography lens columns and substrate chambers is the application to the chambers of a single, double, or triple coating of a material having a high initial magnetic permeability such as Permalloy. Alternatively or in addition, the lens columns and substrate chambers themselves are made of a material having high initial magnetic permeability, such as Permalloy. 
     Reference is made to FIG. 6 that schematically depicts exemplary conventional magnetic shielding used in association with a CPB microlithography system. The subject system  100  comprises an electron gun  1  that generates an electron beam that propagates in a downstream direction (downward in the figure). A substrate stage  24  includes a “wafer chuck” on which the lithographic substrate is mounted for exposure by the electron beam. The electron gun  1  and an electron-optical system (not detailed) extending along the trajectory of the electron beam are contained in a “lens column”  31 , and the substrate stage  24  is contained in a substrate chamber  33 . The lens column  31 , typically made of invar or soft iron, is connected via a duct  37  to a vacuum pump (not shown). The substrate chamber  33  typically is made of aluminum or non-magnetic stainless steel. The lens column  31  and substrate chamber  33  are conjoined and thus communicate with each other, allowing their respective internal spaces to be shared. Although not detailed, the microlithography system  100  includes one or more condenser lenses that direct the electron beam onto a reticle, one or more beam-trimming apertures, a reticle stage, a projection-lens assembly that demagnifies and projects the electron beam (propagating downstream of the reticle) onto a lithographic substrate, and one or more beam deflectors for beam positioning and aberration correction. 
     The microlithography system  100  is enclosed within a shielded enclosure  21 . The enclosure  21  is effectively a chamber made of a material having high initial magnetic permeability. The enclosure  21  houses the entire lens column  31  and substrate chamber  33  of the system  100 . 
     The enclosure  21  defines various openings. For example, a vacuum duct  37  extends through the wall of the enclosure  21  to allow evacuation of the lens column  31  and substrate chamber  33 . Other openings  22  in the enclosure  21  correspond with respective feed-through apertures  39  in the lens column  31  to allow passage of wires and the like to and from the lens column  31 . Another opening (not shown) corresponds with a respective opening in the lens column  31  through which the reticle is moved to and from the reticle stage. Although not detailed in FIG. 6, yet another opening in the enclosure  21  allows passage of the lithographic substrate through a respective opening in the substrate chamber  33  through which the substrate is moved to and from the substrate stage  24 . In addition, a gap  40  usually is associated with the location of a connecting member coupling the lens column  31  to the substrate chamber  33 . Since the enclosure  21  typically is not unitary, other gaps in the enclosure  21  typically are provided at respective conjunctions of shield segments. 
     Openings and gaps in the magnetically permeable material of the enclosure  21  usually reduce the magnetic-shielding performance of the enclosure, sometimes to a level at which the shielding effect is inadequate. Consequently, an aperture or gap is provided in the enclosure usually only when necessary. To offset the consequences of providing apertures and gaps in the enclosure  21 , it frequently is necessary to shield the walls of the room containing the enclosure  21  (with the CPB microlithography system  100  or other field-sensitive system inside). The shielded room can be “passively” shielded, wherein the room walls simply are covered with a magnetic-shielding material. Alternatively or in addition, the room can be “actively” shielded, wherein the room walls include respective coils that generate respective magnetic fields extending usually in a selected direction normal to the plane of the wall. The coil-containing walls typically are separated by a distance from the system enclosed in the room (e.g., separated from the lens column and substrate chamber). By appropriate energization of one or more of the coils, a portion of an external magnetic field leaking into the room is cancelled by a countervailing magnetic field generated from by the coil(s). This technique is termed “active cancellation,” and the coils are termed “active cancellers.” 
     A conventional shielded room  81  including active cancellers is shown in FIG. 7, comprising three active cancellers each comprising a respective pair  83  and  83 ′,  85  and  85 ′,  87  and  87 ′ of opposing coils collectively arranged three-dimensionally. The coils can be situated on the inside or outside of the walls of the room  81 . The arrows associated with the coils in the figure denote the respective directions of electric currents that flow in the coils. As a result of such current flow in the respective coils, the three pairs  83  and  83 ′,  85  and  85 ′,  87  and  87 ′ of coils generate respective magnetic fields in mutually perpendicular directions. The magnitude and direction of the electrical currents applied to respective pairs of coils can be adjusted as required to create, inside the room  81 , a net magnetic field having a magnitude and direction that serve to cancel at least a portion of a stray external magnetic field penetrating into the room. 
     In a shielded room  81  configured as shown in FIG. 7, the best field-cancellation performance generally is obtained at or near the center of the room. As a result, despite energization of the active cancellers  83  and  83 ′,  85  and  85 ′,  87  and  87 ′, some stray magnetic flux leaking into the room  81  from outside tends to remain not canceled inside the room, especially near the wall surfaces. If a portion of a field-sensitive system is situated near a wall under such a condition, the residual non-canceled magnetic flux leaking through the respective wall into the room  81  penetrates into the system and adversely affects system performance. Preventing this effect conventionally requires that the system be situated at the center of a room  81  that is many times larger than the system. If the system is large, then the room  81  must be extremely large, which may be impractical or impossible to construct from the standpoint of cost and/or ability of the site to accommodate such a large room. Also, the maximum size of a wall member made of a shielding material such as Permalloy is limited. When constructing a shielded room, the complexity of joints between walls and the processing required to configure such joints increase greatly with increased room size. Furthermore, in larger active cancellers, the increased size of the coils and of the equipment required to power them can be serious problems. 
     SUMMARY 
     In view of the shortcomings of the prior art as summarized above, the present invention provides, inter alia, magnetically shielded enclosures (e.g., rooms) and magnetic-shielding methods that produce a desired more complete shielding effect without having to make the enclosure prohibitively large. Also provided are magnetic-field-sensitive systems (e.g., charged-particle-beam lithography systems) enclosed in such enclosures. 
     According to a first aspect of the invention, magnetically shielded enclosures are provided for containing and magnetically shielding a field-sensitive system. (The field-sensitive system includes an internal shielding barrier substantially surrounding the system.) An embodiment of such an enclosure comprises multiple walls made of a material including a magnetically permeable material. The walls are configured relative to each other so as to define an internal magnetically shielded space that encloses the internal shielding barrier and thus the system. The enclosure embodiment also includes an aperture defined in at least one of the walls. The aperture is configured relative to a profile of the internal shielding barrier as shadowed on the apertured wall such that a first portion of an external magnetic flux leaking through the aperture into the internal magnetically shielded space at least partially cancels a second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space. The cancellation serves to reduce a net external magnetic flux incident on the system. 
     The aperture can be configured to have a ring shape substantially surrounding the profile of the internal shielding barrier as shadowed on the apertured wall. The aperture desirably has a diameter (or other cross dimension) greater than the corresponding width of the profile of the internal shielding barrier. 
     The apertured wall further can comprise at least one thickening member extending around an edge of the aperture. The thickening member serves to increase the thickness of magnetically permeable material adjacent the aperture, relative to the thickness of the magnetically permeable material in the apertured wall. 
     The aperture can be surrounded by a peripheral region defined from the apertured wall. In this configuration the peripheral region desirably comprises a magnetically permeable material having a magnetic permeability that is greater than the magnetic permeability of the magnetically permeable material of the wall. 
     According to another aspect of the invention, magnetically shielded enclosures are provided for containing and magnetically shielding a field-sensitive system. An embodiment of such an enclosure comprises an internal shielding barrier substantially surrounding the system, wherein the internal shielding barrier is made of a material including a magnetically permeable material. The enclosure also includes an outer magnetically shielded enclosure substantially surrounding the internal shielding barrier. The outer enclosure comprises: (a) multiple walls made of a material including a magnetically permeable material, wherein the walls are configured relative to each other so as to define an internal magnetically shielded space containing the internal shielding barrier, and (b) an aperture defined in at least one of the walls. The aperture is configured relative to a profile of the internal shielding barrier as shadowed on the apertured wall such that a first portion of an external magnetic flux leaking through the aperture into the internal magnetically shielded space at least partially cancels a second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space. The cancellation serves to reduce a net external magnetic flux incident on the system. 
     In this enclosure the field-sensitive system can be, for example, a charged-particle-beam microlithography system. With such a system, the internal shielding barrier can be configured collectively as a lens column and substrate chamber of the system. 
     According to another aspect of the invention, methods are provided for magnetically shielding a field-sensitive system substantially surrounded by an internal shielding barrier. In an embodiment of such a method one step involves configuring multiple walls so as to define an internal magnetically shielded space, wherein each wall is made of a material including a magnetically permeable material. Another step involves situating the system, surrounded by the internal shielding barrier, in the internal magnetically shielded space. Another step involves defining an aperture in at least one of the walls, wherein the aperture is configured relative to a profile of the internal shielding barrier as shadowed on the apertured wall such that a first portion of an external magnetic flux leaking through the aperture into the internal magnetically shielded space at least partially cancels a second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space. The cancellation serves to reduce a net external magnetic flux incident on the system. 
     The step of defining the aperture can comprise configuring the aperture to have a ring shape substantially surrounding the profile of the internal shielding barrier as shadowed on the apertured wall. The diameter (or other cross dimension) of the aperture can be greater than the diameter or other cross-dimension of the internal shielding barrier. 
     The step of defining the aperture further can comprise extending at least one thickening member around an edge of the aperture, so as to increase the thickness of magnetically permeable material adjacent the aperture, relative to the thickness of the magnetically permeable material in the apertured wall. Alternatively, the aperture can be surrounded by a peripheral region of the apertured wall. In this alternative configuration the step of defining the aperture further can comprise providing the peripheral region with a magnetically permeable material having a magnetic permeability greater than the magnetic permeability of the wall material. In either of these configurations, magnetic flux entering the thickened or peripheral region from the aperture increase the proportion of the leaking flux directed toward the wall of the enclosure rather than toward the internal shielding barrier. In addition to or alternatively to performing a field-canceling role, the aperture can be situated and configured such that a combination of the first portion of the external magnetic flux leaking through the aperture into the internal magnetically shielded space and the second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space collectively are oriented in direction(s) in which the leaked flux penetrates the internal shielding barrier only poorly at best. This achieves the result of minimizing the stray magnetic flux that penetrates the internal shielding barrier to the system. 
     The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 (A)- 1 (B) schematically depict the magnetic-shielding action of a shielded enclosure (e.g., room) according to a first representative embodiment, wherein FIG.  1 (A) is (for comparison purposes) an elevational section of a lower corner of a conventional shielded enclosure, with arrows indicating local directions and magnitudes of a stray external magnetic field penetrating into the enclosure, and FIG.  1 (B) is an elevational section of a lower corner of the shielded enclosure of the first representative embodiment, with said arrows. 
     FIG.  2 (A) is a schematic elevational section of a magnetically shielded enclosure, according to the first representative embodiment, containing a charged-particle-beam (CPB) microlithography system as an exemplary “field-sensitive” system. 
     FIG.  2 (B) is a “lower” orthogonal view of the enclosure shown in FIG.  2 (A). 
     FIG. 3 is a schematic elevational section showing an exemplary configuration of the illumination-optical system and projection-optical system of the CPB microlithography system situated inside the enclosure of FIG.  2 (A). Also shown are certain imaging relationships of the microlithography system. 
     FIG. 4 is an elevational section of a “lower” corner of a magnetically shielded enclosure according to a second representative embodiment. 
     FIG. 5 is an elevational section of a “lower” corner of a magnetically shielded enclosure according to a third representative embodiment. 
     FIG. 6 is a schematic elevational section of the lens column and substrate chamber of a conventional CPB microlithography system, including conventional magnetic shielding situated externally of the lens column and substrate chamber. 
     FIG. 7 is an isometric view schematically depicting a conventional magnetically shielded enclosure including respective active cancellers in each wall of the enclosure. 
    
    
     DETAILED DESCRIPTION 
     The invention is described below in the context of representative embodiments that are not intended to be limiting in any way. 
     A first representative embodiment is shown in FIGS.  1 (A)- 1 (B),  2 (A)- 2 (B), and  3 . FIGS.  1 (A)- 1 (B) schematically depict the magnetic-field-shielding action of a magnetically shielded enclosure (e.g., room) according to this embodiment. For comparison purposes, FIG.  1 (A) shows the status of an exemplary stray magnetic field relative to a magnetically shielded enclosure  53  lacking a wall aperture (in the manner of a conventional shielded enclosure). FIG.  1 (B) shows the status of an exemplary stray magnetic field relative to a shielded enclosure  43  including a wall aperture, according to the embodiment. FIGS.  2 (A)- 2 (B) schematically depict the magnetically shielded enclosure  43  of FIG.  1 (B) with a magnetic-field-sensitive lithography system  100  situated inside the enclosure, wherein FIG.  2 (A) is a “front” elevational section of a corner of the enclosure, and FIG.  2 (B) is an “underside” orthogonal view of the enclosure. FIG. 3 is a schematic elevational section of a microlithography system  100 , showing certain details of the CPB optics and imaging relationships of the system. The depicted system  100  utilizes an electron beam as a representative charged particle beam. However, it will be understood that the general principles of the system as described below are applicable with equal facility to use of an alternative charged particle beam such as an ion beam. 
     Referring first to FIG. 3, an electron gun  1  is situated at the extreme upstream end of the depicted system  100 . The electron gun  1  emits an electron beam that propagates in a downstream direction (downward in the figure, generally along an optical axis) toward a reticle  10 . Typically of reticles used for charged-particle-beam (CPB) microlithography, the reticle  10  is divided into multiple regions, termed “subfields,” each defining a respective portion of the pattern defined by the reticle, and is mounted on a reticle stage  11 . The electron beam (termed an “illumination beam” IB upstream of the reticle  10 ) passes through a condenser lens  2  and an illumination lens  3  (constituting respective portions of an “illumination-optical system”) and illuminates a selected region of a reticle  10 . The illumination-optical system also includes other components such as a beam-shaping aperture, a blanking deflector, a blanking aperture, and a subfield-selection deflector (not shown). Thus, the illumination beam IB formed by the illumination-optical system is scanned over the reticle  10  so as to illuminate, in a sequential manner, the subfields of the reticle  10  situated in the optical field of the illumination-optical system. 
     The reticle  10  includes a far greater number of subfields than can be illuminated by the illumination-optical system at a given instant. To illuminate subfields situated outside the optical field of the illumination-optical system, the reticle stage  11  moves the reticle  10  as required, relative to the illumination-optical system, in a “reticle plane” or “object plane” extending perpendicularly to the optical axis of the system. 
     Passage of the illumination beam through the illuminated region of the reticle  10  causes the beam to acquire an aerial image of the respective pattern portion defined by the illuminated region. Hence, the beam propagating downstream of the reticle is termed a “patterned beam.” Provided downstream of the reticle  10  is a “projection-optical system” comprising a first projection lens  15 , a second projection lens  19 , and deflectors  16  (note deflectors  16 - 1  to  16 - 6  in the figure). The deflectors  16  are used for aberration-correction and for making adjustments in image position on a downstream lithographic substrate  23 . The patterned beam is focused by the projection lenses  15 ,  19  and deflector  16  at a predetermined location on the lithographic substrate  23  (e.g., semiconductor wafer). So as to be imprintable with the focused image, the substrate  23  is coated with an exposure-sensitive material termed a “resist.” Typically, as a result of demagnification imparted by the projection-optical system, the image formed on the wafer is smaller (by an integer ratio such as ¼ or ⅕, termed the “demagnification ratio”) than the corresponding illuminated region on the reticle  10 . 
     A crossover C.O. is formed at a point on the optical axis situated, between the reticle  10  and wafer  23 , as determined by the demagnification ratio. A contrast aperture  18  of the projection-optical system is provided at the location of the crossover C.O. The contrast aperture  18  blocks portions of the patterned beam that were scattered by passage of the illumination beam through non-patterned portions of the reticle  10 . Thus, the scattered portions of the patterned beam are prevented from propagating to the substrate  23 . 
     The substrate  23  is mounted on an electrostatic chuck on a substrate stage  24  that moves the substrate in an “imaging plane” or “substrate plane” that extends in the XY direction perpendicular to the optical axis. Exposure of the reticle subfields occurs in a sequential manner. To such end, the reticle subfields are arranged in multiple parallel rows each having a length equal to the width of the optical field of the illumination- and projection-optical systems. To expose the subfields sequentially in a particular row, the illumination beam is deflected laterally across the optical field (roughly in the X direction) as the patterned beam also is deflected laterally (roughly in the opposite X direction) to place the respective subfield images at desired respective locations on the substrate  23 . To expose the rows sequentially, the reticle stage  11  and substrate stage  23  are moved in a synchronous, continuously scanning manner (in opposite Y directions). Thus, the subfield images are formed row-by-row on the substrate  23  in a manner such that the subfield images are “stitched” together in a contiguous manner. 
     Reference now is made to FIGS.  2 (A)- 2 (B), depicting the magnetically shielded enclosure of this embodiment. In FIG.  2 (A) only the electron gun  1 , the beam trajectory, and the substrate stage  24  of the electron-beam microlithography system  100  of FIG. 3 are shown. The system  100  includes a lens column  31  containing the electron gun  1  and electron-optical system (illumination-optical system and projection-optical system) and a substrate chamber  33  containing the substrate stage  24  and peripheral components. The lens column  31  desirably is made of soft iron or invar, and includes a passive shield made of, e.g., Permalloy bonded to it (e.g., with a suitable adhesive). The substrate chamber  33  desirably is made of aluminum or non-magnetic stainless steel, and includes a passive shield made of, e.g., Permalloy bonded to it (e.g., with a suitable adhesive). The lens column  31  and substrate chamber  33  are connected together such that their interior spaces are contiguous. 
     The lens column  31  and substrate chamber  33  collectively form an “internal shielding barrier”  35 . The internal shielding barrier  35  defines a vacuum duct  37  that is connected to a vacuum pump (not shown). The internal shielding barrier  35  also defines multiple apertures  39  as required for, e.g., insertion and removal of the reticle and substrate and for wiring feed-throughs. A gap  40  also may be present at the junction of the lens column  31  with the substrate chamber  33 . The substrate chamber  33  has a “bottom” wall  41  having a width of, by way of example, 3 meters. 
     The internal shielding barrier  35  is enclosed inside a shielded space defined by the magnetically shielded enclosure  43 . The enclosure  43  has walls  45  made of a material desirably comprising Permalloy B or analogous magnetically permeable material, and desirably has a height sufficient for allowing a suitable clearance above the top of the internal shielding barrier  35 . The enclosure  43  also is sufficiently wide for accommodating the substrate chamber  33 . The enclosure  43  has a “bottom” wall  45  having a width of, for example, 5 meters. 
     An aperture  47  (“ring”-shaped in this embodiment) is defined in the bottom wall  45  of the shield room  43 , as shown in FIG.  2 (B). As can be seen in the figure, the aperture  47  need not be round but desirably has a rectilinear profile or other profile desirably conforming to the general shape of the outline of the bottom wall  41  (see below) of the substrate chamber  33 . The aperture  47  need not be an actual void in the wall  45 , but rather can be a corresponding region of the wall having low initial permeability (e.g., a region lacking magnetically permeable material). Thus, the aperture  47  can be defined without compromising the strength or rigidity of the wall  45 . Also, the aperture  47  need not be configured as a continuous ring. Alternatively, the aperture can be defined collectively by a series of discrete apertures arranged, for example, in a circle. 
     The aperture  47  desirably has an inside cross dimension greater than the width of a “shadow region”  49  (enclosed by the broken line in FIG.  2 (B)) corresponding to the cross dimension of the bottom wall  41  of the substrate chamber  33 . Desirably, a distance L 1  (e.g., 20 to 30 cm) between the inner edge of the aperture  47  and the outer edge of the shadow region  49  is about ⅖ to ⅗ of a height dimension H 1  (e.g., 50 cm) between the bottom wall  45  of the enclosure  43  and the bottom wall  41  of the internal shielding barrier  35 . Thus, the aperture  47  desirably has a profile that conforms to the profile of the outer edge of the bottom wall  41  of the internal shielding barrier  35  (i.e., the bottom wall of the substrate chamber  33 ). Based on the other exemplary dimensions given above, the width B 1  of the aperture  47  desirably is about 30 to 50 cm, and the distance B 2  between the outer edge of the aperture  47  and the outer edge of the enclosure  43  desirably is 50 to 100 cm. 
     In any event, by providing the aperture  47  with a cross dimension greater than the cross dimension of the shadow region  49 , stray external magnetic flux entering the enclosure via the aperture  47  tends to bend more toward the wall  45  than toward the internal shielding barrier  35 , as discussed in detail below. 
     The substrate chamber  33  is supported relative to a “floor” F or other base surface by one or more legs  51  or analogous support members that extend through the aperture  47  outside the enclosure  43  to the floor F. Whereas the aperture  47  desirably accommodates the leg(s)  51 , the aperture need not have a complete-ring shape, as noted above; rather, it can be defined as multiple spaced-apart apertures sized and spaced apart from each other sufficiently to accommodate respective leg(s)  51  extending therethrough. 
     The magnetic-shielding action of the enclosure  43  is explained with reference to FIGS.  1 (A)- 1 (B), in which the large arrows have respective directions indicating the direction of respective portions of a stray magnetic flux. The respective sizes of the arrows denote relative magnitudes of the magnetic field at the respective locations. 
     If, as shown in FIG.  1 (A), no aperture  47  were defined in the wall  45  of the enclosure  43 , then respective portions of an external magnetic field M 1  would permeate into the wall  45  of the enclosure  43 . This magnetic flux permeating the wall is denoted M 2 . This permeating flux extends from the wall into the enclosure  43  as a “leakage” magnetic flux denoted M 3 . In general, the magnetic flux tends to flow toward regions having high magnetic permeability and to form a magnetic circuit with such regions. Hence, the permeating magnetic flux M 2  and the leakage magnetic flux M 3  are oriented toward the internal shielding barrier  35  in regions that are near the internal shielding barrier  35 , but curve back toward the wall  55  of the enclosure  53  in regions that are distant from the internal shielding barrier  35 . Hence, in the configuration of FIG.  1 (A), the closer the internal shielding barrier  35  to a wall (side wall or bottom wall  55 ) of the enclosure  53 , the greater the proportion of leakage magnetic flux M 3  oriented toward the internal shielding barrier  35 . 
     The leakage magnetic flux M 3  oriented toward the internal shielding barrier  35  permeates and passes through the wall  41  of the internal shielding barrier  35  (the permeating flux is denoted M 4 , and flux transmitted through the wall  41  is denoted M 5 ). The transmitted flux M 5  can perturb the magnetic fields of the electron-optical system and in the region of the substrate situated inside within the lens column and substrate chamber, respectively, defined by the internal shielding barrier  35 . 
     Note that the magnitude (strength) of the magnetic flux decreases as the flux progresses from outside the internal shielding barrier  35  to inside the space defined by the internal shielding barrier. 
     In contrast to the configuration shown in FIG.  1 (A), the magnetically shielded enclosure room shown in FIG.  1 (B) defines an aperture  47 , as described above with reference to FIG.  2 (B), in the bottom wall  45  of the enclosure  43 . An external magnetic field M 1 , as in FIG.  1 (A), permeates the wall  45  and passes into the enclosure  43 . The external magnetic field M 1  also enters the enclosure  43  through the aperture  47  (the magnetic flux passing through the wall  45  is denoted M 2 , the flux transmitted through the wall  45  is denoted M 3 , and the flux passing through the aperture  47  is denoted M 10 ). The magnitude of the flux M 10  passing through the aperture  47  is greater than the flux M 3  leaking through the wall  45 . As described above, the aperture  47  is defined so as to be larger than the “shadow” of the bottom wall  41  of the internal shielding barrier  35  as projected on the bottom wall  45  of the enclosure  43 . Consequently, it is difficult for the magnetic flux M 10  passing through the aperture  47  to enter the surface of the bottom wall  41  in a perpendicular manner. Whereas some of the magnetic flux M 10  passing through the aperture  47  bends toward the internal shielding barrier  35 , most of the flux M 10  bends back into the wall of the enclosure  43  (this bending-back flux is denoted M 10 ′). 
     Hence, both the magnetic flux M 10 ′ and the magnetic flux M 3  are present inside the space defined between the enclosure  43  and the internal shielding barrier  35 . In FIG.  1 (B) the respective Y-direction components of the fluxes M 10 ′, M 3  tend to cancel each other, leaving mainly an X-direction component. The X-direction component has a direction essentially parallel to the bottom wall  41  of the internal shielding barrier  35 . As a result, a reduced amount (compared to FIG.  1 (A)) of the leakage magnetic flux M 3  is directed toward the internal shielding barrier  35  (the small amount of the flux M 3  actually reaching the internal shielding barrier  35  is denoted M 3 ′). Thus, the magnitude of the leakage magnetic flux M 3  oriented toward the internal shielding barrier  35  is reduced significantly by the magnetic flux M 10 ′, and the direction of the magnetic flux M 3  is changed, which reduces the magnetic flux M 4  permeating the internal shielding wall  35 , and correspondingly reduces the leakage magnetic flux M 5  passing through the bottom wall  41 . 
     In the enclosure  43  the height of pedestals (not shown) used for attenuation of vibrations of the microlithography system  100  need not be great. Also, the bottom wall  41  of the internal shielding barrier  35  need not be distant from the bottom wall  45  of the enclosure  43 . The legs  51 , attached to the wafer chamber  33 , support the microlithography system  100  (and internal shielding barrier  35 ) relative to the floor F, thereby avoiding imposition of a substantial mechanical load on the bottom wall  45  of the enclosure  43 . Normally, Permalloy, which is the desired magnetically permeable material for use in making the walls of the enclosure  43 , needs to be magnetically annealed because it tends to lose its magnetic characteristics if mechanically stressed. But, in this representative embodiment, magnetic annealing of the enclosure  43  is unnecessary because it receives no significant mechanical stress. Also, the enclosure  43  need not be excessively large, which provides better shielding with smaller and less massive walls  45 . 
     When manufacturing a shielded enclosure according to this embodiment, the respective locations of apertures (provided for inserting and removing reticles and substrates, for example) can be established at locations of joints in the shield material. As a result, the number of joints can be reduced and construction correspondingly simplified, compared to the configuration in FIG.  6 . 
     The magnitudes of magnetic fields in the shielded enclosure of this embodiment were determined by computer simulation. The simulation results indicated a 4- to 5-fold improvement in magnetic shielding, compared to conventional enclosures lacking the aperture  47 . In addition, the amount of material required to fabricate the shielded enclosure was reduced by approximately 20% compared to conventional enclosures lacking the aperture  47 . 
     A partial sectional view of a magnetically shielded enclosure  63  according to a second representative embodiment is shown in FIG.  4 . In the enclosure  63  the thickness of magnetically permeable material around the aperture  67  defined in the bottom wall  65  is increased relative to the normal thickness of the magnetically permeable material in the wall  65  itself. By way of example, thickening members  61  are provided, each having a thickness approximately 2 to 3 times greater than the normal thickness of the wall  65  (which, in one example, is 0.5 to 1 mm). Each thickening member  61  can be formed by layering additional shielding material on the magnetically permeable material of the wall  65  at the outer and inner periphery of the aperture  67 . 
     Whenever the thickness of magnetically permeable material around the aperture  67  is increased in this manner, the magnetic flux entering through the aperture  67  is attracted easily to the thick portion  51 , which further increases the proportion of the entering magnetic-flux component that bends toward the wall  65  of the enclosure  63 . Thus, more of the leakage magnetic flux is effectively cancelled. 
     A partial sectional view of a magnetically shielded enclosure  73  according to a third representative embodiment is shown in FIG.  5 . In the enclosure  73  a portion  71  of the wall  75  surrounding the aperture  77  is made of a magnetic material having a higher magnetic permeability than the magnetic shielding material in the wall  75 . An exemplary material for the higher-permeability portion  71  is Permalloy C. The maximum magnetic permeability (μ m ) of Permalloy B used for the magnetically permeable material of the walls  75  is about 40,000 to 80,000, whereas the maximum permeability of Permalloy C is about 100,000 to 200,000. 
     By configuring the higher-permeable region  71  with such high magnetic permeability, magnetic flux passing through the aperture  77  is attracted easily to the region  71 , which increases the proportion of the transmitted magnetic flux that bends back toward the wall  75  of the enclosure, thereby producing the same effect as in the embodiment of FIG.  4 . 
     As is clear from the foregoing, superior magnetic shielding in an enclosure is achieved without having to configure the enclosure excessively large compared to field-sensitive system contained in such an enclosure. Also, improved magnetic shielding is achieved using less magnetically permeable material, while providing an enclosure that is easy to configure and construct. Also provided is improved performance of a microlithography system contained inside the enclosure. 
     Whereas the invention has been described in connection with multiple representative embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.