Abstract:
Apparatus associated with air bearings which are suitable for use in an environment with high vacuum levels. According to one aspect, an air bearing linear guide that is suitable for use in a vacuum environment is arranged to substantially wrap around a portion of a guide beam without covering the ends of the guide beam. The air bearing linear guide includes a sleeve that has an inner surface, and a first air pad that is located on the inner surface of the sleeve. A first land is arranged on the inner surface of the sleeve at least partially around the first air pad, e.g., such that the land is substantially offset from the perimeter of the air pad. The first land and the air pad define a first channel therebetween. Finally, the air bearing linear guide includes a second land that is arranged to substantially seal the inner surface of the sleeve from the vacuum environment

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent application is related to co-pending provisional U.S. patent application Ser. No. 9/870,881, filed concurrently herewith, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to lithography. More particularly, the present invention relates to a stage for use in an electron beam projection lithography system. 
     2. Description of the Related Art 
     Lithography processes, e.g., photo-lithography processes, are integral to the fabrication of wafers and, hence, semiconductor chips. Conventional systems used for lithography include optical lithography systems and electron beam projection systems. Many optical lithography systems and electron beam projection systems may use a direct writing process to “write” on wafers. However, direct writing processes are often relatively slow, as will be appreciated by those skilled in the art. 
     In order to increase the speed at which wafers may be written to, electron beam projection systems, as well as optical lithography systems, may project beams of finite area through patterns. The patterns are generally resident on a reticle, which effectively serves as a mask or a negative for a wafer. For an electron beam projection system, a relatively broad beam of electrons may be collimated and provided to a reticle, which may be a silicon wafer, e.g., a wafer that is suitable for scattering with angular limitation projection electron beam lithography or a stencil-type wafer. Typically, rather than absorbing the beam, the pattern deflects portions of the beam in order to prevent electrons from being ultimately focused onto a wafer. 
     FIG. 1 a  is a diagrammatic representation of one standard configuration of a lens system of an electron beam projection system. In general, a lens system  10  includes an illumination lens  12  and a projection lens  18 . An electron beam is generated by an electron gas, and arranged to pass through illumination lens  12 , through a reticle  16  held in a reticle chamber  14 . It should be appreciated that reticle chamber  14  is typically a vacuum chamber. As an electron beam passes through reticle  16 , portions of the electron beam are allowed to pass through reticle  16 , while other portions of the electron beam may be scattered to prevent those portions from being focused onto a wafer  22  held in a wafer chamber  20 , i.e., a vacuum chamber. In other words, reticle  16  acts as a mask to effectively mask out part of an electron beam. Projection lens  18  is arranged to project the pattern of electrons onto wafer  22 . 
     In general, wafer  22  is mounted on a wafer stage (not shown) to facilitate the movement of wafer  22  beneath projection lens  18 . The design of a wafer stage for use in an electron beam projection system may be complicated, as an electron beam projection system generally must not include moving magnets or metals which alter the magnetic field associated with the electron beam projection system and, hence, the electron beam. 
     Although separate vacuum chambers may be used to house a reticle and a wafer, an entire lens system may generally be housed in a single vacuum chamber. FIG. 1 b  is a diagrammatic representation of a standard lens system of an electron beam projection system, which is contained within a vacuum chamber. Like lens system  10  of FIG. 1 a,  a lens system  50  includes an illumination lens  12 ′ and a projection lens  18 ′. An electron beam is arranged to pass through illumination lens  12 ′. The electron beam then passes from illumination lens  12 ′ to a reticle  16 ′ which masks out part of the electron beam. After passing through reticle  16 ′, portions of the electron beam pass through projection lens  18 ′ and onto wafer  22 ′. As shown, illumination lens  12 ′, reticle  16 ′, projection lens  18 ′, and wafer  22 ′ are all contained within a vacuum chamber  60 . 
     A characteristic of electron beam projection systems is the ability to dynamically move a projection image to follow a stage, which is generally not possible with an optical system, as will be appreciated by those skilled in the art. In addition, electron beam lens systems typically correct for relatively small errors in relative stage positions, whereas optical systems generally do not. 
     It is often desirable to have moving parts associated with an electron beam projection system. For example, a wafer may be placed on a wafer stage which enables the wafer to be positioned beneath a projection column as appropriate. The use of air bearing guides to facilitate the movement of a wafer stage within a vacuum would generally provide for relatively high stiffness, substantially no friction, and low noise while the air bearing guide was moving over a guide beam. However, conventional air bearings typically leak air around their perimeters. Hence, air flow leaks into a vacuum chamber from an air bearing guide would reduce the vacuum level in a vacuum chamber. In general, the allowable leakage flow from an air bearing depends upon the acceptable vacuum level in a vacuum chamber, as well as the vacuum pumping capability associated with the vacuum chamber. Typically, for an electron beam projection system, desired vacuum levels are relatively high, e.g., on the order of approximately 1e-6 Torr. A conventional air bearing generally leaks flow many orders of magnitude above a tolerable, or acceptable, level for an electron beam projection system. 
     Therefore, what is needed is a method and an apparatus for enabling wafers to be positioned efficiently within an electron beam projection system. That is, what is desired is an air bearing linear guide which is suitable for use in a relatively high vacuum environment. 
     SUMMARY OF THE INVENTION 
     The present invention relates to air bearings which are suitable for use in an environment with high vacuum levels. According to one aspect of the present invention, an air bearing linear guide that is suitable for use in a vacuum environment is arranged to substantially wrap around a portion of a guide beam without covering the ends of the guide beam. The air bearing linear guide includes a sleeve that has an inner surface, and a first air pad that is located on the inner surface of the sleeve. A first land is arranged on the inner surface of the sleeve at least partially around the first air pad, e.g., such that the land is substantially offset from the perimeter of the air pad. The first land and the air pad define a first channel therebetween which is arranged to be vented to a non-vacuum environment to allow the air pad to function as if it were in a non-vacuum environment, e.g., an atmospheric pressure environment. Finally, the air bearing linear guide includes a second land that is arranged to substantially seal the inner surface of the sleeve from the vacuum environment. Sealing the inner surface of the sleeve from the vacuum environment, e.g., by minimizing the distance between the edge of the sleeve and the guide beam, prevents significant leakage of flow from the air bearing linear guide into a vacuum environment. In one embodiment, the second land is arranged to substantially seal the sleeve against the guide beam. 
     In another embodiment, a third land is spaced apart from the first land to at least partially define a second channel which is arranged to contain gas at a relatively low vacuum. In such an embodiment, a first area on the inner surface of the sleeve, the first area may also be in communication with the second channel and is, further, arranged to-be in fluid communication with the guide beam. 
     According to another aspect of the present invention, a guide bearing, the guide bearing which is arranged to interface with a guide beam and is also arranged to move with respect to the guide beam includes a sleeve, a plurality of air pads, a plurality of raised areas, and first and second pluralities of lands. The sleeve has both an outer surface which is exposed to a vacuum and an inner surface. The air pads are included on the inner surface, and are of a first height. Each raised area is arranged to define an individual channel between each raised area and a corresponding air pad, and has a height which is approximately the same as the first height. Each individual channel is arranged to vent to an atmospheric pressure environment. The lands are each arranged to define a channels in cooperation with raised areas and other lands. Channels are defined between each land of the first plurality of lands and raised areas to accommodate relatively low vacuum flow, while channels are defined between each land of the second plurality of lands and each land of the first plurality of lands, to accommodate relatively high vacuum flow. 
     According to still another aspect of the present invention, a guide bearing system includes an air bearing linear guide and a first guide beam. The air bearing linear guide includes a sleeve with an inner surface on which an air pad is mounted. The air pad is separated from a first land by a first area which substantially surrounds a perimeter of the air pad. The first area is arranged to be in communication with a source of substantially atmospheric pressure. The air bearing linear guide is generally arranged to substantially wrap around a non-end section of the first guide beam and to slide with respect to the guide beam. The guide beam includes a first duct and a second duct which are each in fluid communication with the air bearing linear guide. In one embodiment, wherein a gap is defined between a top surface of the air pad and an outer surface of the first guide beam that is approximately equivalent to a flying height of the air bearing linear guide. 
     In one embodiment, the first land is separated from a second land by a second area, and the second area being arranged to be in fluid communication with the first duct. In such an embodiment, the second land may be separated from a third land by a third area which is in fluid communication with the second duct. The guide bearing system may also include a second guide beam which includes a first section and a second section. The second guide beam maybe coupled to the air bearing linear guide such that a first transfer port of the air bearing linear guide is in fluid communication with the second area and the first section, and a second transfer port of the air bearing linear guide in fluid communication with the third area and the second section. 
     These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 a  is a block diagram representation of one configuration of the lens system of an electron beam projection system. 
     FIG. 1 b  is a block diagram representation of a second configuration of the lens system of an electron beam projection system. 
     FIG. 2 is a diagrammatic representation of a wafer stage assembly suitable for use as a part of an electron beam projection system in accordance with an embodiment of the present invention. 
     FIG. 3 a  is a diagrammatic representation of a first section of an air bearing linear guide, e.g., air bearing linear guide  102   b  of FIG. 2, in accordance with an embodiment of the present invention. 
     FIG. 3 b  is a diagrammatic representation of a second section of an air bearing linear guide, e.g., air bearing linear guide  102   b  of FIG. 2, in accordance with an embodiment of the present invention. 
     FIG. 3 c  is a diagrammatic representation of a third section of an air bearing linear guide, e.g., air bearing linear guide  102   b  of FIG. 2, in accordance with an embodiment of the present invention. 
     FIG. 4 is a diagrammatic representation of a portion of a guide beam, e.g., guide beam  110   b  of FIG. 2, in accordance with an embodiment of the present invention. 
     FIG. 5 a  is a diagrammatic representation of a portion of a guide beam, e.g., guide beam  108   b  of FIG. 2, in accordance with an embodiment of the present invention. 
     FIG. 5 b  is a diagrammatic cross-sectional representation of a guide beam within an air bearing linear guide, e.g., guide beam  108   b  and air bearing linear guide  102   b  of FIG. 2, in accordance with an embodiment of the present invention. 
     FIG. 6 is a diagrammatic side-view representation of an air bearing linear guide and a guide beam, e.g., linear guide  102   b  and guide beam  110   b  of FIG. 2, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The configuration of a wafer stage which is suitable for use in an electron beam projection system generally may not include conventional air bearings, as conventional air bearings typically leak air into the surrounding environment. The leakage of air from air bearings into a vacuum chamber associated with an electron beam projection system would likely significantly reduce the vacuum level in a vacuum chamber, thereby adversely affecting an electron beam lithography process. In other words, a conventional air bearing typically leaks flow many orders of magnitude above an acceptable level for an electron beam projection system. 
     An air bearing linear guide which does not leak a significant amount may be used in a vacuum chamber. Such an air bearing linear guide may generally include air flow evacuation stages which are incorporated into the linear guide in order to reduce the volume of gas which reaches the vacuum chamber and the volume of gas that is pumped out of the vacuum chamber. In one embodiment of such a linear guide, any leakage is generally insignificant enough that there is substantially very little affect on the overall vacuum level. Each air pad of an air bearing linear guide may be surrounded by a channel which is vented substantially directly to atmospheric pressure. As such, each air pad may effectively operate as if it were in a non-vacuum environment, while the linear guide which includes the air pads operates in a vacuum. Therefore, the linear guide is able to operate within a vacuum with an essentially negligible amount of friction, a relatively high stiffness, and low noise. 
     FIG. 2 is a diagrammatic three-dimensional representation of a wafer stage assembly which includes a plurality of air bearing linear guides, and is suitable for scanning a wafer table along two axes in accordance with an embodiment of the present invention. A wafer stage assembly  100  is arranged to cause a wafer  112  situated on a table  114  to scan along an x-axis  132  and a y-axis  134 . As will be understood by those skilled in the art, in some embodiments, wafer  112  may also be moved along a z-axis  130 , e.g., through the use of servo motors (not shown) attached to either table  114  or entire assembly  100 . 
     Table  114  is coupled, generally substantially fixably, to an air bearing linear guide  104 . Air bearing linear guide  104 , which may be considered to be a slider bearing, is arranged to move over a guide beam  110   b  which, in turn, is coupled to an air bearing linear guide  102 b. Air bearing linear guide  102   b  and guide beam  110   b  will be discussed below with reference to FIGS. 3 a-c  and FIG. 4, respectively. 
     Air bearing linear guide is coupled to guide beam  110   b  and, further, is coupled to an extension section  118  that effectively moves within a guide section  106   b  which includes a magnet track  116   b  and an air guide  133   b.  Guide beam  100   b  is supported along z-axis  130  and rotationally supported about y-axis  134  by extension section  118  and linear guide  102   b.  In addition, guide beam  110   b  is constrained from rotation about x-axis  132  and z-axis  134 . A coil portion of extension section  118  moves within magnet track  116   b  to form a linear motor, while an extension of extension section  118  is arranged to move within air guide  133   b.  Hence, extension section  118  slides within guide section  106   b  and enables linear guide  102   b  to move over guide beam  108   b.    
     Air bearing linear guide  104  is substantially fixably coupled to a cross bearing  122  which is arranged to move over a guide beam  110   a.  Cross bearing  122  may be arranged on guide beam  110   a  such that guide beam  110   a  may experience yaw, e.g., rotation about z-axis  130 . In order for guide beam  110   a  to yaw, cross bearing  122  may include single air pads on substantially only the two vertical inner sides of cross bearing  122 . This substantially minimizes the yaw stiffness, i.e., rotational stiffness about z-axis  130 , between cross bearing  122  and guide beam  110   a.  In addition, air pads on horizontal inner surfaces of cross-bearing  122  may be added for stiffness, or eliminated such that a stage made up of substantially only cross bearing  122  and linear guide  104  is effectively constrained along x-axis  132 , along z-axis  130 , about x-axis  132 , about y axis  134 , and about z-axis  130  substantially only by linear guide  104 . 
     As shown, guide beam  110   a  is coupled to an air bearing linear guide  102   a,  which is arranged to move over a guide beam  108   a.  In one embodiment, air bearing linear guide  102   a  has a similar structure to that of air bearing linear guide  102   b,  while guide beam  108   a  has a similar structure to guide beam  108   b.  Like guide beam  110   b,  guide beam  110   a  also has an extension section  118  that is arranged to move within a guide section  106   a  which includes a coil  116   a  and an air guide  133   a.    
     Linear motors, as for example linear motors including moving coils and fixed magnet tracks  116 , used to enable wafer  112  to move, may be arranged such that magnet tracks  116  are substantially stationary, thereby effectively eliminating issues associated with moving magnets. While the linear motor coils are relatively large conductors, the linear motor coils may be positioned relatively far from an electron beam column, and may be well shielded by the magnetic tracks associated with the linear motors. Further, while the linear motor coils may generate relatively large magnetic fields when energized, during scanning, the amount of current used for accelerating may be small and, as a result, have very little effect on an electron beam. 
     Each linear guide in a wafer stage, e.g., each of linear guides  102 ,  104 , and  122 , generally operates in substantially the same manner. Specifically, the general flow configurations of linear air guides  102  and  104  are substantially the same, whereas the flow configuration of linear air guide  122  is similar to the configurations of linear air guides  102  and  104 . With reference to FIGS. 3 a-c,  linear guide  102   b  will be described in accordance with an embodiment of the present invention. Linear guide  102   b,  which is effectively an air bearing linear guide, is formed to include a rectangularly shaped sleeve  300 . In general, sleeve  300  may be formed using any suitable method. By way of example, sleeve  300  may be formed from four plates which are matched to a guide beam, e.g., guide beam  108   b  of FIG. 2, such that a desired gap between linear guide  102   b  and the guide beam is maintained. The desired gap generally corresponds to the flying height of air bearing pads  304  within linear guide  102   b.  While the size of the desired gap may vary widely depending upon the requirements of the overall electron beam projection system, the desired gap size is typically in the range of approximately five to approximately seven microns. 
     Air pads  304 , which are coupled to an air supply that typically supplies air at a pressure of between approximately 60 pounds per square inch (psi) and 80 psi, are positioned on each inner surface of linear guide  102   b.  It should be appreciated that air pads,  304  may be designed as any type of conventional air bearings. Conventional types of air bearings include, but are not limited to, porous air bearings. In the embodiment as shown, side surfaces of linear guide  102   b,  e.g., the sections of FIGS. 3 a  and  3   b,  may include four air pads  304  each, whereas a top surface or a bottom surface of linear guide  102   b,  as shown in FIG. 3 c,  may each include two air pads  304 . It should be appreciated, however, that the number of air pads  304  as well as the location of air pads  304  within linear guide  102   b  may vary widely. 
     Air pads  304  are generally spaced to substantially minimize the length of linear guide  102   b,  e.g. the dimension of linear guide  102   b  along x-axis  132 . By minimizing the length of linear guide  102   b,  the length of an associated guide beam, e.g., guide beam  108  of FIG. 2, may also be substantially minimized. Minimizing the length of the associated guide beam increases the torsional and bending stiffness associated with the guide beam. In the described embodiment, the length of linear guide  102   b  along x-axis  132  may be approximately the sum of the length of travel, i.e., the stroke of linear guide  102   b,  the length of an exhaust port associated with linear guide  102   b,  i.e., an exhaust port in guide beam  108   b  of FIG. 2, and the length of lands and channels at the edge of linear guide sleeve  300 , i.e., sealing lands  340 . 
     Air-flow evacuation stages  310 ,  314 ,  318 , or grooves, are incorporated into linear guide  102   b  in order to reduce the volume of gas which reaches a vacuum chamber and, as a result, is pumped out of the vacuum chamber. Grooves  310 , which directly surround air pads  304 , are effectively atmospheric pressure grooves in that grooves  310  are arranged to contain gas at atmospheric pressure, i.e., at approximately 15 psi absolute. The existence of grooves  310  enables air pads  304  to function as if air pads  304  were not in a vacuum. That is, grooves  310  provide air pads  304  with an atmospheric pressure environment such that air pads  304  effectively function as if air pads were surrounded by gas at an atmospheric pressure. Typically, grooves  310  may be vented directly to atmospheric pressure through ports and hoses (not shown) to a vent located outside of a vacuum chamber in which linear guide  102   b  operates. 
     In one embodiment, grooves  310  may contain a low vacuum, e.g., a vacuum at approximately half of an atmosphere (atm), instead of atmospheric pressure. The use of a low vacuum may increase the evacuation of gas from linear guide  102   b.  A low, or weak, vacuum may also be applied to the end of a hose that is coupled to grooves  310  in order to compensate for flow restrictions so that the resulting pressure in grooves  310  may be approximately atmospheric, or in the range of approximately 2 psi to approximately 15 psi. 
     Lands  332  separate grooves  310  from low-vacuum pump-out grooves  314 . In general, lands  332  each have substantially the same height, or thickness, as air pads  304  so that the gap between the guide beam, i.e., guide beam  108   b,  and lands  332  is substantially the same as the flying height of the overall air bearing. Lands  332 , in cooperation between the gap between lands  332  and the outer surface of guide beam  108   b  of FIG. 3, essentially create a flow restriction which serves to reduce the leakage flow from grooves  310  to grooves  314 . The length of lands  332  in the flow direction, e.g., the length of land  332   a  between grooves  310   a  and  314   a  along x-axis  132 , may be determined such that conductance between grooves  310   a  and  314   a  may be minimized, as will be appreciated by those skilled in the art. 
     Grooves  314  are in fluid communication with low vacuum areas  322 . Low vacuum areas, in turn, are in fluid communication with low vacuum transfer ports  344 . Low vacuum transfer ports  304  may be ducted through beam  110   b  that serves as a guide beam for linear guide  104  of FIG. 2, which may be considered to be a “slider.” In the embodiment as shown, low vacuum transfer ports  304  may be ducted through beam  110   b  and out a low vacuum opening  370   a.    
     Low vacuum areas  322  are also arranged to encompass, or effectively enable flow into, a low vacuum port in guide beam  108   b  of FIG. 2, which will be described below with respect to FIGS. 5 a  and  5   b.  That is, low vacuum areas  322  are arranged such that while linear guide  102   b  moves with respect to guide beam  108   b,  low vacuum areas  322  are substantially always in fluid communication with a low vacuum port in guide beam  108   b.  The size of low vacuum areas  322  is generally relatively large in order to substantially maximize the conductance of low vacuum flow and to substantially minimize the total force on the inside of sleeve  300 . Force on the inside of sleeve  300 , as will be appreciated by those skilled in the art, has the tendency to expand and distort sleeve  300 . 
     As shown in FIG. 3 b,  low vacuum areas  322  are in fluid communication with a low vacuum exhaust groove  328 . Low vacuum exhaust groove  328  is arranged to overlap a low vacuum port in guide beam  108   b  of FIG. 2 such that a portion of low vacuum exhaust groove  328  substantially “covers” the low vacuum port during the stroke, i.e., movement, of linear guide  102   b.  The depth of low vacuum exhaust groove  328 , or the dimension of low vacuum exhaust groove  328  along y-axis  134 , is generally chosen to be between approximately 5 millimeters (mm) and 10 mm larger than the depth of low vacuum areas  322 . Such a range in depth generally allows adequate conductance of low vacuum flow between low vacuum exhaust groove  328  and the low vacuum port. In general, low vacuum exhaust groove  328  facilitates the low vacuum flow between linear guide  102   b  and guide beam  108   b,  as will be discussed below with reference to FIGS. 5 a  and  5   b.    
     Grooves  314  are separated from high vacuum grooves  318  by lands  336 . Like lands  332 , lands  336  are approximately the same height as air pads  304 . High vacuum grooves generally contain a vacuum on the order of approximately 10 −4  Torr. The actual pressure is dependent upon a variety of factors. The variety of factors may include, but are not limited to, pumping speed, conductance of an evacuation path, the relative dimensions of lands  332 ,  336 , and the size or height of the gap between guide beam  108   b  and land  332 ,  336 . Grooves  318  are arranged to conduct leakage to a high vacuum exhaust groove  326  which overlaps a high vacuum port in guide beam  108   b  of FIG. 2, as will be described below with respect to FIGS. 5 a  and  5   b.  High vacuum exhaust groove  326  is also in fluid communication with a high vacuum transfer port  374  which transfers a high vacuum between high vacuum exhaust groove  326  and slider  104  of FIG.  2  through beam  110   b.  It should be appreciated that beam  110   b  is internally divided into two ducts, one which supports high vacuum flow and has a high vacuum opening  370   b,  and one which supports low vacuum flow and has low vacuum opening  370   a,  as will be described below with reference to FIG.  4 . Further, each duct may be connected to a pump, e.g., a high vacuum flow duct may be in communication with a high vacuum pump while a low vacuum flow duct may be in communication with a low vacuum pump. In general, pumps may be connected to either one of guide beams  108 , which are in communication with guide beam  110   b.    
     An extension  382  of beam  110   b  includes an air pad  384  which is arranged to interface with an air guide, i.e., air guide  133  of FIG. 2, which may be surrounded by an atmospheric pressure groove, a low vacuum groove, and a high vacuum groove. In one embodiment, extension  382  includes a second air pad (not shown) which is located on the opposite side of extension  382  from air pad  384 . It should be understood that in lieu of a single air pad  384  on two opposing sides of extension  382 , each side of the two opposing sides of extension  382  may include two or more air pads. Air pad  384  is similar to air pads  304 , and, like air pads  304 , is surrounded by an atmospheric pressure groove. Air pad  384  is arranged to move within an air guide, e.g., air guide  133   b  of FIG. 2, as will be described below with reference to FIG.  6 . 
     Lands  340 , i.e., flow-restriction lands or sealing lands, separate grooves  318  from the ends of sleeve  300 . In other words, lands  340  effectively seal off grooves  318  from the exterior of linear guide  102   b  such that the flow of high vacuum, or any other flow, is substantially restricted from flowing into a vacuum chamber in which liner guide  102   b  is used. 
     In the described embodiment, air bearing leakage may be exhausted through atmospheric pressure grooves  310  to a flexible hose which is connected to the air that is outside of a vacuum chamber in which linear guide  102   b  operates. Leakage from atmospheric pressure grooves  310  is exhausted from low vacuum grooves  314  through ports to a low vacuum duct in beam  110   b.  Finally, leakage from low vacuum grooves  318  is exhausted from high vacuum grooves  318  through ports to a high vacuum duct of beam  110   b.    
     As previously mentioned, beam  110   b,  which is substantially fixably attached to linear guide  102   b,  may include two ducts. FIG. 4 is a diagrammatic representation of a portion of guide beam  110   b  in accordance with an embodiment of the present invention. Guide beam  110   b  includes a low vacuum flow duct  402  and a high vacuum flow duct  406 . Generally, guide beam  110   b  includes at least one low vacuum opening  408  and one high vacuum opening  410 , in order to allow vacuum flow to pass to linear guide  102   b  from slider  104  of FIG.  2 . By way of example, in the described embodiment, guide beam  110   b  may include low vacuum opening  408  and low vacuum opening  370   a,  as shown in FIG. 3 a,  which are located on opposing sides of guide beam  110   b.  Similarly, guide beam  110   b  may include high vacuum opening  410  and high vacuum opening  370   b,  as shown in FIG. 3 b.  It should be appreciated, however, that the number of openings may vary widely, e.g., guide beam  110   b  may only include low vacuum opening  370   a  and high vacuum opening  370   b.  It should be appreciated that the configuration of openings may also vary widely. For example, guide beam  110   b  may include only opening  370   b  and opening  408 , or guide beam  110   b  may include only opening  370   a  and  410 . 
     Referring next to FIGS. 5 a  and  5   b,  a guide beam, i.e., guide beam  108   b  of FIG. 2, will be described in accordance with an embodiment of the present invention. Guide beam  108   b  includes two ducts or sections  504  which are substantially adjacent to each other along y-axis  134 . In other words, ducts  504  of guide beam  108   b  are arranged such that duct  504   a  is adjacent to duct  504   b  along y-axis  134 . Duct  504   a  is arranged to accommodate a high vacuum flow while duct  504   b  is arranged to accommodate a low vacuum flow. 
     Linear guide  102   b  is effectively wrapped around guide beam  108   b  such that a low vacuum exhaust port  512   b  is substantially always encompassed within low vacuum exhaust groove  328  while linear guide  102   b  moves with respect to guide beam  108   b.  Similarly, during the stroke of linear guide  102   b,  a high vacuum exhaust port  512   a  is substantially always encompassed by high vacuum exhaust groove  326 . It should be appreciated that for ease of illustration, although linear guide  102   b  includes air pads and grooves, air pads and grooves have generally not been shown in FIG. 5 b.    
     When linear guide  102   b  effectively slides over guide beam  108   b,  low vacuum gas flows between low vacuum exhaust groove  328  and duct  504   b.  As discussed above with reference to FIG. 3 a,  low vacuum gas also flows between low vacuum exhaust groove  328  and beam  110   b.  High vacuum gas flows from high vacuum exhaust groove  326  into exhaust port  512   a  and then into duct  504   a.    
     In one embodiment, duct  504   a  may be connected to a high vacuum pump while duct  504   b  maybe connected to a low vacuum pump. As guide beam  108   b  is generally stationary, e.g., does not move in any direction, pumps may be substantially directly connected to ducts  504  without the use of flexible hoses. However, it should be appreciated that in the event that guide beam  108   b  and, further, overall wafer stage  100  of FIG. 2, maybe mounted on an apparatus to move wafer stage  100  along z-axis  130 , ducts  504  may be connected to pumps using flexible hoses. The use of flexible hoses enables guide beam  108   b  to be moved along z-axis  130  without affecting the connections of ducts  504  to the pumps. 
     As previously mentioned, a linear motor may be used to drive the movement of a linear guide. By way of example, guide section  106   b  of FIG. 2 includes a linear motor which includes coil  116   b  and extension section  118 . With reference to FIG. 6, guide section  106   b  will be described in accordance with an embodiment of the present invention. Guide section  106   b  includes magnet track  116   b  and guide  133   b.  A coil  602  is arranged to move within magnet track  116   b  to form a linear motor. Extension  382 , as discussed above with respect to FIG. 3 a,  includes at least one air pad and moves within guide  133   b  along x-axis  132 . The air pads on associated with extension  382  are generally arranged such that the air pads are substantially surrounded by an atmospheric pressure groove, a low vacuum groove, and a high vacuum groove. In one embodiment, extension  382  may include four air pads, i.e., two air pads on the top side of extension  382  and two air pads on the bottom side of extension  382 . The use of four air pads enables stiffness, i.e., pitching stiffness about y-axis  134 , to be obtained with respect to guide beam  110   b  and linear guide  102   b.  The use of air pads on opposing sides of extension  382  effectively preloads air pads against each other and increases stiffness. 
     Air pads on extension  382  may be substantially directly supplied with air via an air hose. As extension  382  is directly coupled to guide beam  110   b,  and does not move with respect to guide beam  110   b,  low vacuum flow and high vacuum flow may effectively be directly vented between or exhausted between guide beam  110   b  and extension  382 . 
     Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or the scope of the present invention. By way of example, a wafer stage with air bearing linear guides has been described as being suitable for use in a vacuum chamber associated with an electron beam projection system. It should be appreciated, however, that the wafer stage or, more generally, the air bearing linear guides described above may be used in substantially any suitable vacuum environment. Suitable vacuum environments include, but are not limited to, vacuum environments associated with ion beam projection lithography and extreme ultraviolet lithography. Alternatively, the air bearing linear guide of the present invention may also be implemented in non-vacuum environments, e.g., where a pure gas environment should not be contaminated by gas flow from a bearing. 
     The design of air bearing linear guides which may be used in a vacuum environment, has been described as including air pads and various channels for vacuum flow. It should be appreciated that the configuration of air bearing linear guides described above is just one example of air bearing linear guides which are suitable for use in a relatively high vacuum environment. Further, the design of the air bearing linear guides may vary from the design described above without departing from the spirit or the scope of the present invention. For example, the number of air pads in an air bearing linear guide may vary depending upon the requirements of a particular system. In addition, the number of grooves or channels and, hence, lands may also vary, e.g., a high vacuum groove may be eliminated in some embodiments. 
     While the air bearing linear guide has been described as being suitable for use in a scanning reticle stage, the air bearing linear guide may be used with a variety of different stages. Different stages include, but are not limited to, reticle and mask stages. It should be appreciated, in addition, that the air bearing linear guide may be implemented for substantially any suitable use, and is not limited to use within a stage. 
     In general, a linear guide is formed from as a rectangularly shaped sleeve with an inner surface, on which air pads are incorporated, and an outer surface. The outer surface is exposed to relatively high vacuum levels, e.g., vacuum levels of approximately 1 e-6 Torr associated with an electron beam projection system, whereas different areas of the inner surfaces are subject to pressures ranging from air bearing supply pressure to approximately 10 −4  Torr. The assembly of a linear guide or, more specifically, the sleeve may vary. By way of example, the sleeve may be assembled from four plates, and matched to the guide beam to maintain a desired gap between the inner surface, including air pads, of the sleeve and the guide beam. The sleeve assembly may generally be held together, for example, using adhesives or screws. The four plates may be assembled such that the ends of a bottom plate and a top plate are exposed, e.g., not in contact with either side plate, while the ends of the side plates abut the bottom plate and the top plate. Similarly, the four plates may be assembled such that the ends of the side plates are exposed, e.g., not in contact with either the top plate or the bottom plate, while the ends of the top plate and the bottom plate abut the sides of the side plates. While such assemblies may be acceptable, such assemblies do not facilitate the adjustment of the gap between the inner surface of the sleeve and the guide beam. For instance, in order to alter the size of the gap, the size of at least some of the plates would need to be altered. 
     The joints of an assembled rectangularly shaped sleeve of the embodiment described above are arranged to restrict the flow of vacuum gases in areas  322  from leaking into the vacuum chamber that houses the overall wafer stage. Additionally, some length of a joint  392  of FIG. 3 a,  e.g., the intersection or junction between land  332   a  and land  332   f,  is subject to approximately atmospheric pressure gases and, hence, are arranged to restrict flow into the chamber. It should be appreciated that substantially all joints between adjacent lands  332  are arranged to restrict the flow of low vacuum gases into the chamber. Similarly, a joint  394 , which is located at a comer of land  336   a  as shown in FIG. 3 a,  is also arranged to restrict flow into the chamber. 
     In one embodiment, a sleeve may be assembled such that each plate has an end that is exposed and an end that abuts the side of another plate. Assembling the four plates of a sleeve such that each plate has an end that is exposed and an end that abuts the side of another plate enables the gap between the inner surface of the sleeve and the guide beam to be readily altered, without departing from the spirit or the scope of the present, invention. 
     Generally, if lower vacuum levels are needed, an air bearing linear guide may be designed such that any combination of atmospheric, low vacuum, and high vacuum exhaust grooves may be used with any combination of hoses and internal ducts. On the other hand, if higher vacuum levels are needed, then an additional exhaust groove with ultra-high vacuum may be added to a bearing assembly and exhausted through an additional duct. That is, the actual design of the air bearing linear guide may vary depending upon the requirements of a particular system, e.g., stage system. 
     The beams, or guide beams, of the present invention are generally dual-chambered. Both the design of the dual-chambered beams and dimensions of the dual-chambered beams may be widely varied. By way of example, while the chambers or ducts in a beam have been described as being side-by-side such that the chambers are adjacent to each other along a y-axis, the chambers and ducts in a beam may instead be aligned side-by-side such that the chambers are adjacent to each other along a z-axis. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.