Abstract:
An apparatus that uses a combination of mechanical contact bearings and air bearings is disclosed. The apparatus includes a fixed seal housing, attached to a process chamber and a floating seal cartridge, which is disposed in proximity to the fixed seal housing. A shaft is disposed with an aperture in the process chamber, the central opening in the fixed seal housing and the second central opening in the floating seal cartridge. A first air bearing is created between the shaft and the floating seal cartridge in the second central opening. A second air bearing is created between the floating seal cartridge and the fixed seal housing. In this way, the floating seal cartridge is free to move with the shaft radially, while still maintaining a seal between the process chamber and the external environment.

Description:
[0001]    Embodiments of the present invention relate to methods and apparatus for allowing motion to mechanisms and payloads within a vacuum chamber. 
       BACKGROUND 
       [0002]    Semiconductor workpieces are processed within process chambers. Many times, these process chambers are maintained at a pressure different, typically lower, than standard atmospheric pressure. In some embodiments, the pressure within a process chamber may be considered vacuum conditions, wherein the pressure in the chamber may be between 10 −3  and 10 −7  Torr. Maintaining this pressure requires adequate seals on all orifices and openings in the process chamber. 
         [0003]    In addition, there are often payloads or mechanisms within the process chamber that are required to move. This movement may be accomplished using a shaft, actuated outside the process chamber, which penetrates at least one of the walls of the process chamber.  FIG. 1  shows a typical embodiment, where the shaft  110  may enter the process chamber  100  through an opening in one of the walls  101 . This shaft  110  may be able to move linearly in one direction, as shown by arrows  111 , thereby changing the position of its payload  150  within the process chamber  100 . This linear movement may be created by an actuator  130 . This actuator  130  may be a linear motor, a ball screw, mechanical linkages or other suitable devices. In addition, the shaft  110  may be able to rotate, as shown by arrow  112 , about its center axis. This may be done by incorporating a rotary bearing and rotary actuator (not shown) between the linear actuator and the shaft  110 . In some embodiments, both linear motion  111  and rotary motion  112  are performed by the shaft  110 . In other embodiments, only one of these types of motion is utilized. Thus, actuator  130  may be capable of linear motion  111 , rotary motion  112  or both. An air bearing  120  may be used to maintain vacuum conditions within the process chamber  100 . The air bearing  120  may be constructed as an annular ring, where the cylindrical shaft  110  enters the process chamber after passing through the central opening  123  in the ring. The air bearing  120  uses a layer of pressurized air delivered to its central opening  123  to position the shaft  110  and hold it in the desired position. Pressurized air is delivered through channels  121  in the air bearing  120 , which terminate in the central opening  123  where the shaft  110  is disposed. The highly pressurized air serves to hold the shaft  110  in place, preferably so that the shaft  110  remains equally spaced from the sides of the central opening  123  of the air bearing  120 . Thus, the air bearing  120  serves to align and support the shaft  110  throughout its range of motion in direction  111 . In other words, the radial movement of the shaft  110  is minimized by the highly pressurized air which pushes against it in the central opening  123 . 
         [0004]    Additionally, the air bearing  120  may also have vacuum channels  122 . These vacuum channels  122  are in communication with a vacuum pump (not shown) and serve to evacuate the pressurized air from the space in the central opening  123  so that this pressurized air does not enter the process chamber  100 . In other words, the pressurized air in the volume between the air bearing  120  and the shaft  110 , when used with vacuum channels  122  in communication with a vacuum pump, act as a seal, effectively isolating the external environment from the environment within the process chamber  100  and maintaining the desired pressure differential. 
         [0005]    This configuration is useful in that the air bearing  120  serves two distinct purposes. First, it supports the shaft  110 , using a nearly friction-less interface, and maintains its position within the central opening  123 . Secondly, it provides a seal between the external environment and the process chamber  100 , allowing a pressure differential to exist therebetween. However, in some embodiments, the weight or load associated with the shaft  110  or payload  150  may be too great to be supported by an air bearing  120 . In this case, the highly pressurized air may not have enough load capacity to keep the shaft  110  properly aligned. Thus, the maximum weight of the payload  150  and the shaft  110  may be limited by the air bearing  120 . 
         [0006]    Therefore, it would be beneficial if there were an apparatus and method to allow a shaft and payload to penetrate a process chamber that does not impose limitations on the weight of these components. Furthermore, this apparatus should advantageously also provide the same sealing ability that is achieved by current air bearing systems. 
       SUMMARY 
       [0007]    An apparatus that uses a combination of mechanical contact bearings and air bearings is disclosed. The apparatus includes a fixed seal housing, attached to a process chamber and a floating seal cartridge, which is disposed in proximity to the fixed seal housing. A shaft is disposed with an aperture in the process chamber, the central opening in the fixed seal housing and the second central opening in the floating seal cartridge. A first air bearing is created between the shaft and the floating seal cartridge in the second central opening. A second air bearing is created between the floating seal cartridge and the fixed seal housing. In this way, the floating seal cartridge is free to move with the shaft radially, while still maintaining a seal between the process chamber and the external environment. 
         [0008]    In one embodiment, the apparatus for moving a payload within a process chamber comprises a shaft passing through an aperture in a wall of the process chamber; a mechanical contact bearing to support the shaft, disposed outside the process chamber; a fixed seal housing, having a first central opening through which the shaft passes, an upper surface affixed to an outer surface of the wall, and a lower surface; a floating seal cartridge, having a second central opening through which the shaft passes, an upper surface disposed proximate to the lower surface of the fixed seal housing wherein the space therebetween defines an interface, and a lower surface, the floating seal cartridge comprising a cartridge air channel, disposed in the floating seal cartridge, in communication with a source of pressurized air and terminating at the second central opening; and one or more cartridge vacuum channels, disposed in the floating seal cartridge, and terminating at the second central opening, wherein a first air bearing is created between the floating seal cartridge and the fixed seal housing at the interface and a second air bearing is created between the floating seal cartridge and the shaft at the second central opening. 
         [0009]    In another embodiment, a method of allowing a shaft to penetrate an aperture in a process chamber while maintaining a pressure differential between the process chamber and the exterior environment, is disclosed. This method comprises disposing a fixed seal housing against the process chamber, the fixed seal housing comprising a first central opening aligned with the aperture; disposing a floating seal cartridge beneath the fixed seal housing, where the space between the fixed seal housing and the floating seal cartridge defines an interface, wherein the floating seal cartridge comprises a second central opening; disposing the shaft through the second central opening, the first central opening and the aperture; and creating a first air bearing between the floating seal cartridge and the shaft in the second central opening and a second air bearing between the floating seal cartridge and the fixed seal housing at the interface. 
         [0010]    In another embodiment, an apparatus for moving a payload within a process chamber comprises a shaft passing through an aperture in a wall of the process chamber; a mechanical contact bearing to support the shaft, disposed outside the process chamber; a fixed seal housing, having a first central opening through which the shaft passes, an upper surface affixed to an outer surface of the wall, and a lower surface, the fixed seal housing comprising a housing pressurized air port for connection to a source of pressurized air; a housing differential vacuum pumping port for connection to a pump; one or more housing air channels in communication with the housing pressurized air port and terminating at the lower surface to deliver pressurized air; and one or more housing vacuum channels in communication with the housing differential vacuum pumping port and terminating at the lower surface to evacuate pressurized air; a floating seal cartridge, having a second central opening through which the shaft passes, an upper surface disposed proximate to the lower surface of the fixed seal housing wherein the space therebetween defines an interface, and a lower surface, the floating seal cartridge comprising one or more cartridge air channels, disposed in the floating seal cartridge, connecting the upper surface of the floating seal cartridge to the second central opening, such that pressurized air from the interface is supplied to the second central opening; and one or more cartridge vacuum channels, disposed in the floating seal cartridge, connecting the interface and the second central opening, wherein pressurized air is evacuated from the second central opening and delivered to the interface; wherein a first air bearing is created between the floating seal cartridge and the fixed seal housing at the interface and a second air bearing is created between the floating seal cartridge and the shaft at the second central opening. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0012]      FIG. 1  shows an air bearing of the prior art; 
           [0013]      FIG. 2  shows a system according to one embodiment; 
           [0014]      FIG. 3  shows the system of  FIG. 2 , where the shaft has moved radially; 
           [0015]      FIG. 4  shows the system according to another embodiment; 
           [0016]      FIG. 5  shows the system according to another embodiment; 
           [0017]      FIG. 6  shows an expanded view of  FIG. 2 , showing the air channels and vacuum channels; 
           [0018]      FIG. 7  shows the system according to another embodiment; and 
           [0019]      FIG. 8  shows the system according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    As described above, air bearings may be effective devices to support loads and create seals. However, air bearings may be ineffective in supporting very heavy loads, as the layer of pressurized air does not have the requisite force capacity to hold the load in place. Therefore, as payloads and shafts become heavier, it is likely that traditional air bearings, such as that as shown in  FIG. 1 , may become ineffective. 
         [0021]      FIG. 2  shows a first embodiment of a floating seal cartridge  200 . In this embodiment, as before, a shaft  110  penetrates an aperture in a wall  101  of a process chamber  100  and may be axially driven by an actuator  130  to allow movement along direction  111 . As described above, rotational movement along direction  112  may also be possible through the use of a rotary bearing and a rotary actuator. Thus, actuator  130  may be capable of linear motion  111 , rotary motion  112  or both. However, unlike the embodiment of  FIG. 1 , a mechanical contact bearing  210 , which may be a linear bearing, a rotary bearing, or a combination of a rotary bearing and a linear bearing, is used to support the shaft  110  and any associated payload  150 . The rotary bearing may be any suitable type, such as a ball bearing, cross roller bearing or other type of rotary bearing. The rotary bearings may be made of bearing grade steel but can be fabricated from ceramics as well. In the case of a linear bearing, the mechanical contact bearing  210  may be any suitable device, such as ball bearings, recirculating linear ball or roller bearings or other high load mechanical bearings. Traditional mechanical contact bearings can be designed to support far more weight than air bearings, making them preferable in this embodiment. However, mechanical contact bearings lack the dimensional precision needed to support the use of a differentially pumped non-contact seal. In other words, the shaft  110  will deviate from its center axis  113  as it moves along direction  111 . In other words, the mechanical contact bearing  210  allows axial motion, but also may cause or permit radial movement due to the inherent imprecision of the mechanical parts. Because of this radial movement, a mechanical contact bearing  210 , when used by itself, may be ineffective in maintaining isolation and pressure differential between the external environment and that within the process chamber  100 . 
         [0022]    An upper surface  205  of a fixed seal housing  200  is affixed to an outer surface of a wall  101  of the process chamber  100 . The fixed seal housing  200  may be made from aluminum or stainless steel with a bearing liner. The bearing liner is provided in case of contact that occurs when the seal is not pressurized. This bearing liner may be made from graphite, bronze or other suitable air bearing liner materials. The wall  101  may have an aperture in it, such as a round hole, which the shaft  110  passes through. The fixed seal housing  200  may be constructed as an annular ring, having a first central opening  201  dimensioned sufficiently large so that, accounting for the radial movement of the shaft  110 , the cylindrical shaft  110  does not contact the fixed seal housing  200  during its travel in direction  111 . In other embodiments, the fixed seal housing  200  may have another shape, with a central opening  201  disposed therein. The aperture in wall  101  and the first central opening  201  may be concentric, as shown in  FIG. 2 . However, because of the size of the gap between the shaft  110  and the inner wall of the fixed seal housing  200 , it may not be possible to create a seal using differential vacuum pumping in the first central opening  201 . 
         [0023]    A floating seal cartridge  220  is also employed in this embodiment, where, like the fixed seal housing  200 , the floating seal cartridge  220  may also be an annular ring. In other embodiments, the floating seal cartridge  220  may be a different shape, having a central opening. Like the fixed seal housing  200 , the floating seal cartridge  220  may be made from stainless steel or aluminum. Bearing surfaces may include a bearing liner, constructed of graphite or other suitable air bearing liner materials. However, as described in more detail below, the size of its central opening, also referred to as the second central opening  221 , is smaller than the central opening  201  of the fixed seal housing  200 . This second central opening  221  has a diameter slightly larger than that of the cylindrical shaft  110 . In some embodiments, the radial gap between the shaft  110  and the second central opening  221  is on the order of 10 microns. 
         [0024]    Housing air channels  202  are disposed in the fixed seal housing  200 . These housing air channels  202  terminate in one or more housing pressurized air ports disposed on the outer edge  204  of the fixed seal housing  200 . This housing pressurized air port is in communication with a pressurized air source (not shown). The housing air channels  202  deliver pressurized air from a pressurized air source to the lower surface  206  of the fixed seal housing  200 . The space between the lower surface  206  of the fixed seal housing  200  and the upper surface of the floating seal cartridge  220  defines an interface  208 . While  FIG. 2  shows the housing air channel  202  as containing two perpendicular parts, the housing air channel  202  may be disposed in any number of ways, which are not limited by this disclosure. This pressurized air creates a nearly friction-less surface at this interface  208 , allowing the floating seal cartridge  220  to move in radial direction  209  relative to the fixed seal housing  200 . 
         [0025]    Additionally, the fixed seal housing  200  may also have one or more housing vacuum channels  203  disposed therethrough, in communication with one or more housing differential vacuum pumping ports located on the outer edge  204  of the fixed sealing housing  200 . These housing vacuum channels  203  may disposed throughout the fixed seal housing  200  in any pattern. This housing differential vacuum pumping port may be in communication with a pump, such as a vacuum pump (not shown). The distal ends of the housing vacuum channels  203  terminate at the upper surface of the floating seal cartridge  220 . The housing vacuum channels  203 , in combination with the vacuum pump, serve to evacuate the pressurized air from the interface  208  so that this pressurized air does not enter the process chamber  100 . In other words, the layer of pressurized air in the interface  208  between the fixed seal housing  200  and the floating seal cartridge  220 , when used with housing vacuum channels  203  and a vacuum pump, acts as a seal, isolating the external environment from the environment within the process chamber  100  and maintaining the desired pressure differential between these environments.  FIG. 6  shows one particular embodiment including various vacuum ports  600 - 602 , each with a different vacuum pressure. These various vacuum ports  600 - 602  are collectively referred to as the housing vacuum channels  203  throughout this disclosure. As described above, pressurized air is pumped into the interface  208  through housing air channels  202 . This housing air channel  202  is disposed furthest from the shaft  110 . Adjacent to the housing air channel  202 , and disposed closer to the shaft  110  are multiple housing vacuum channels  600 - 602 . Vacuum channel  600 , is disposed adjacent to the housing air channel  202  and serves as a vent port which vents the pressurized air to atmosphere. Adjacent to the vent port  600  and closer to the shaft  110  is a rough vacuum port  601 , which is connected to a vacuum pump. This rough vacuum port  601  may be used to reduce the pressure to within 1 and 100 milliTorr, or to other pressures required by the design of the system. Adjacent to the rough vacuum port  601  and closer to the shaft  110  is a high vacuum port  602 , which is connected to a second vacuum pump and reduces the pressure to the sub-milliTorr range. This configuration of vacuum ports  600 - 602  may be used in any embodiment described herein where a differentially pumped non-contact seal is desired. 
         [0026]    The near vacuum conditions created by the housing vacuum channels  203  near the interface  208  may also draw the floating seal cartridge  220  toward the fixed seal housing  200 , thereby holding the floating seal cartridge  220  in place while maintaining the required gap space in interface  208 . 
         [0027]    As shown in  FIG. 2 , cartridge air channels  222  are embedded in floating seal cartridge  220 . These cartridge air channels  222  pass from the upper surface of the floating seal cartridge  220  near the interface  208  through the floating seal cartridge  220  and terminate at second central opening  221 . Again, although these cartridge air channels  222  are shown as two perpendicular paths, other patterns are possible. Pressurized air, which originates from the pressurized air source, passes through housing air channel  202  in fixed seal housing  200 , and is delivered to the interface  208 . It is then carried through cartridge air channels  222  in floating seal cartridge  220  and delivered to the second central opening  221 . Thus, the pressurized air source creates pressurized air at both the interface  208  and the second central opening  221 . Similarly, cartridge vacuum channels  223  are disposed in the floating seal cartridge  220 , with one set of openings disposed at the upper surface of the floating seal cartridge  220  near the interface  208  and the distal set of openings terminating at the second central opening  221 . As described above, these cartridge vacuum channels  223  may be disposed in any pattern in the floating seal cartridge  220 . These cartridge vacuum channels  223  serve to maintain the seal between the exterior environment and the process chamber  110  by evacuating pressurized air from the second central opening  221  and delivering it to the interface  208 .  FIG. 6  shows an embodiment where three vacuum ports  610 - 612  are in communication with vacuum ports  600 - 602  at the interface  208 . These vacuum ports  610 - 612  function in the same manner of vacuum ports  600 - 602 , respectively, as described above. Thus, the combination of a layer of pressurized air at interface  208  and a second layer of pressurized air at second central opening  221  allows a seal to be created between the external environment and the process chamber  100 , thereby maintaining the desired pressure differential. 
         [0028]    Furthermore, pressurized air in second central opening  221  allows the floating seal cartridge  220  to remain aligned with the shaft  110 , even during radial movement. Since interface  208  is nearly friction-less, the floating seal cartridge  220  may move radially to maintain its fixed relationship with the shaft  110 .  FIG. 3  shows, in an exaggerated view, a shaft  110  that has moved radially with respect to the fixed seal housing  200  and the process chamber  100 . In response, the floating seal cartridge  220  has moved radially with the shaft  110 , so that the shaft  110  remains concentric with the second central opening  221 . Note that the shaft  110  is no longer concentric with the aperture in the process chamber  100 , or with the first central opening  201  in fixed seal housing  200 . However, the movement of floating seal cartridge  220  allows the process chamber  100  to remain isolated from the exterior environment by maintaining seals at the second central opening  221  and at the interface  208 . 
         [0029]    Thus, unlike the embodiment of  FIG. 1 , the embodiment of  FIGS. 2 and 3  allows the shaft  110  to determine the positioning of the floating seal cartridge  220 . In  FIG. 1 , it was the air bearing  120  that determined the position of the shaft  110 . Because of this modification in operation, the limiting factor for load capacity is defined by mechanical contact bearing  210 , and not by air bearings. In this embodiment and those described below, two air bearings are created, one in the space between the shaft  110  and the floating seal cartridge  220  in the second central opening  221  and a second between the fixed seal housing  200  and the floating seal cartridge  220  at the interface  208 . Each of these air bearing provides a differentially pumped non-contact seal. 
         [0030]    The embodiments of  FIGS. 2 ,  3  and  6  assume that the layer of pressurized air supplied to the interface  208  can also be used to pressurize the second central opening  221 .  FIG. 4  shows another embodiment, where the cartridge air channels  301  are directly in communication with a source of pressurized air (not shown) via a cartridge pressurized air port disposed on an outer surface of the floating seal cartridge  300 . All components that remain unchanged from the previous embodiments have been given the same reference designators and will not be described again. As in the previous embodiment, the floating seal cartridge  300  may be an annular ring having an inner opening, or second central opening  221 , dimensioned such that the circular shaft  110  may pass therethrough. In this embodiment, a source of pressurized air (not shown) may be in communication with the cartridge pressurized air port disposed on the outer edge  308  of the floating seal cartridge  300 . This cartridge pressurized air port is in communication with the cartridge air channels  301 , which may travel from the outer edge  308  of the floating seal cartridge  300 , through the floating seal cartridge  300  to the second central opening  221 . In this way, the pressurized air supplied to interface  208  is completely independent of the pressurized air supplied to the second central opening  221 . 
         [0031]    As was described earlier, cartridge vacuum channels  302  may also be included to insure that the pressurized air in second central opening  221  does not reach the process chamber  100 . These cartridge vacuum channels  302  may be in communication with one or more vacuum pumps via a cartridge differential vacuum pumping port disposed on the outer edge  308  of the floating seal cartridge  300 . In some embodiments, the cartridge vacuum channels  302  comprise three vacuum ports, such as those described in connection with  FIG. 6 . In this scenario, the high vacuum port would be located closest to the process chamber  100 , with the vent port located furthest from the process chamber  100 , adjacent to cartridge air channel  301 . These cartridge vacuum channels  302  serve to evacuate pressurized air from the second central opening  221  and deliver it to the cartridge differential vacuum pumping port along the outer edge  308 . 
         [0032]    In a variation of this embodiment, the source of pressurized air may be connected to a cartridge pressurized air port disposed on the lower surface  303  of the floating seal cartridge  300 . Similarly, the vacuum pump may connect to the floating seal cartridge  300  via a cartridge differential vacuum pumping port on the lower surface  303 . In fact, the cartridge differential vacuum pumping port and the cartridge pressurized air port may be disposed on any exposed exterior surface of the floating seal cartridge  300 . In some embodiments, the exposed exterior surfaces include the outer edge  308  and the lower surface  303  of the floating seal cartridge  300 . 
         [0033]      FIG. 7  shows another embodiment, where the source of pressurized air and the vacuum pumps (not shown) are connected to an outer edge  308  of the floating seal cartridge  300 . However, in this embodiment, cartridge air channels  301  disposed in the floating seal cartridge supply pressurized air to both the interface  208  and to the second central opening  221 . Similarly, the vacuum pump evacuates air from both the interface  208  and the second central opening  221  and delivers it to a cartridge differential vacuum pumping port disposed on the outer edge  308  of the floating seal cartridge  300  through cartridge vacuum channels  302 . Therefore, in this embodiment, there are no housing air channels or housing vacuum channels disposed in the fixed seal housing  200 . While  FIG. 7  shows a cartridge air channel  301  having a T-connection, other configurations are possible. For example, separate sets of ports may be disposed on the floating seal cartridge  220 , where one set of ports services the interface  208  and a second set of ports services the second central opening  221 . Additionally, as described above, the cartridge pressurized air port and the cartridge differential vacuum pumping ports may be along any exposed surface, including the outer edge  308  and the lower surface  303 . 
         [0034]    In these embodiments, like that of  FIG. 2 , two air bearings are created, one in the space between the shaft  110  and the floating seal cartridge  220  in the second central opening  221  and a second between the fixed seal housing  200  and the floating seal cartridge  220  at the interface  208 . Each of these air bearing provides a differentially pumped non-contact seal. 
         [0035]    The embodiments of  FIGS. 2-4  and  7  assume that the negative pressure generated by the vacuum channels  203 ,  302  in interface  208  provides sufficient vacuum force in order to allow the floating seal cartridge  220 ,  300  to remain in position, separated by the fixed seal housing  200  by pressurized air in the interface  208 . In other embodiments, it may be necessary to support the floating seal cartridge  300  on both its upper surface  307  and its lower surface  303 .  FIG. 5  shows an embodiment where the floating seal cartridge  300  is supported on its upper surface  307  by fixed seal housing  200  and on its lower surface  303  by a second fixed surface  410 . The second fixed surface may also be an annular ring, or may be another shape having a central opening through which the shaft  110  passes. As described above in connection with  FIG. 4 , floating sealing cartridge  300  has cartridge air channels  301  for delivering pressurized air from a source (not shown) to the second central opening  221 . Similarly, cartridge vacuum channels  302  are used to draw air away from the second central opening  221  to maintain the vacuum conditions within process chamber  100 . 
         [0036]    In this embodiment, the second fixed surface  410  may also have second surface air channels  411  which may be used to create an air bearing along second interface  418 . The second surface air channels  411  are in communication with a second surface pressurized port disposed on the outer edge  415  of the second fixed surface  410 . This second surface pressurized air port is in communication with a source of pressurized air (not shown). The second surface air channels  411  are disposed within the second fixed surface  410  and terminate at the upper surface  413  which creates the second interface  418  between the second fixed surface  410  and the floating seal cartridge  300 . As before, second surface vacuum channels  412  may also be included to evacuate air from the second interface  418  and deliver it to a second surface differential vacuum pumping port disposed on the outer edge  415  of the second fixed surface  410 . In this embodiment, a nearly friction-less surface is created on the upper surface  303  and the lower surface  307  of floating seal cartridge  300 , allowing it to move radially in direction  209  as the shaft  110  moves. The use of air bearings in second central opening  221  and in interface  208  also serves to isolate the process chamber  100  from the external environment. Thus a seal between the central opening of the second fixed surface  410  and the shaft  110  may not be required. 
         [0037]    In this embodiment, as was true in the embodiment of  FIG. 4 , the source of pressurized air may be connected to a second surface port disposed on an exposed exterior surface of second fixed surface  410 , such as the outer edge  415  or the lower surface  414  of the second fixed surface  410 . Similarly, the vacuum pump may connect to the second fixed surface  410  via a second surface differential vacuum pumping port on any exposed surface, such as the outer edge  415  or the lower surface  414 . In this embodiment, three air bearings are created, one in the space between the shaft  110  and the floating seal cartridge  220  in the second central opening  221 , a second between the fixed seal housing  200  and the floating seal cartridge  220  at the interface  208  and a third between the floating seal cartridge  220  and the second fixed surface  410  at the second interface  418 . At least the first and second of these air bearings provide a differentially pumped non-contact seal. 
         [0038]      FIG. 8  is a variation of the embodiment of  FIG. 5 . In this embodiment, floating sealing cartridge  300  has cartridge air channels  301  for delivering pressurized air from a source (not shown) disposed on the outer edge  308  to the second central opening  221 . Additionally, pressurized air from the source is delivered to interface  208  and second interface  418 . Similarly, cartridge vacuum channels  302  are used to draw air away from the second central opening  221 , the interface  208  and the second interface  418  and deliver it to a cartridge differential vacuum pumping port disposed on the outer edge  308  of the floating seal cartridge  300 . This may be achieved through a set of cartridge air channels and a set of cartridge vacuum channels each in communication with a pressurized air source and vacuum pump, respectively and servicing the three destinations. In another embodiment, separate sets of ports may be disposed on the floating seal cartridge  300 , each associated with a different interface or opening. In this embodiment, there are no housing air channels or housing vacuum channels in the fixed seal housing  200 . Additionally, there are no second surface air channels  411  or second surface vacuum channels  412  in the second fixed surface  410 . 
         [0039]    This disclosure refers to upper surfaces and lower surfaces. This convention is used to correspond with the orientations shown in  FIGS. 1-8 . However, the disclosure is not limited to this embodiment and the shaft  110  does not need to enter from the bottom of the process chamber  100 . In fact, it may enter through any wall  101  in the chamber. In these embodiments, the term “upper surface” is used to denote that surface closest to the process chamber, while the term “lower surface” denotes the surface furthest from the process chamber. 
         [0040]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.