Abstract:
A system for providing a seal between a rotating part and a stationary part that comprises two seals in series separated by a cavity is disclosed. The cavity may be at low pressure and failure of either seal may be detected by a change in cavity pressure. An alarm may be triggered when cavity pressure rises above a threshold, or when it remains above a threshold for more than a predetermined period of time. In a system comprising multiple cavities, a cavity may be selectively isolated to determine if a seal associated with that cavity is experiencing a leak.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 10/916,261 (the “Parent Application”), which was filed on Aug. 10, 2004, and claims the benefit of the Parent Application and of U.S. Provisional Patent Application No. 60/501,600 (the “Provisional Application”), which was filed on Sep. 9, 2003. The Parent Application and the Provisional Application are incorporated herein by reference in their entirety for all purposes, excepting any prosecution file history associated with same, any of same that is inconsistent with or in conflict with the present document, or any of same that may have a limiting affect as to the broadest scope of the claims now or later associated with the present document. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to cylindrical magnetrons and specifically to methods and systems for sealing endblocks used in cylindrical magnetrons. The cylindrical magnetron is used in a large coating machine for coating very large sheets of glass or other materials. One application where these sheets of glass are used is in construction of curtain wall buildings where a single glass sheet can be up to 15 feet wide by about 20 plus feet high. The sheets are run through the coating machine shortly after the glass is manufactured. Thus, these are large-scale machines, which must rapidly and evenly coat glass as quickly as it can be manufactured. In addition to the quality of the coating the magnetron deposits upon the glass, dependability and serviceability of the magnetron is of the utmost importance. 
         [0003]    This is not an easy task taking into account the constraints of the process that is involved. A cylindrical magnetron sputters material from a rotating target tube onto the glass as it is transported past the target. In order to coat such a large piece of glass the target tube can be up to 15 feet in length and 6 inches in diameter and can weigh 1700 pounds. Another complication is that the sputtering actually erodes the target tube during the sputtering process, so the target tube is constantly changing shape during its serviceable lifetime. The sputtering process can require that an extremely high AC or DC power (400 amps, 150 kW) be supplied to the target. This power transfer creates extreme heat in the target tube and the surrounding components, which must be cooled in order assure proper performance and to avoid catastrophic failure of the magnetron. Thus, water is pumped through the center of the rotating target tube at high pressure and flow rate. 
         [0004]    Rotating such a large target tube in such an environment is a difficult task.  FIG. 1A  depicts magnetron  100  for illustrative purposes.  FIG. 1B  shows magnetron  100  integrated into a large glass coating system  130 . Glass coating system  130  may be several hundred feet long and contain many magnetrons. Target tube  106  is supported by two endblocks  104  and  108  as glass sheet  110  passes by. The endblocks  104  and  108  generally supply cooling water, support and rotate the target tube, support a stationary magnetic array within the rotating target tube, and transfer the large amounts of electricity needed for the sputtering process. Effectively transferring electrical power to a rotating target tube is also a complex problem. Maintaining electrical isolation in a sputtering process is also crucial to continually laying down a uniform coating on the glass. If the drive system is not properly electrically isolated from the sputtering process, it will affect the quality of coating deposited upon the glass. The sputtered material may in fact also coat the drive and electrical components of the magnetron itself rather than the glass if they are not properly isolated. Aside from resulting in a poor coating, this has many other ramifications on the continuous reliable operation of the magnetron. For further information please refer to “Coated Glass Application and Markets” by Russell J. Hill and Steven J. Nadel, The BOC Group, 1999 (ISBN #0-914289-01-02). 
         [0005]    Efficient and effective sputtering also requires that the process take place in a vacuum or a reduced pressure relative to atmosphere. One or more vacuum pumps may be connected to provide vacuum within a coater. Thus, endblocks must have a very robust sealing system to prevent air or high-pressure water from leaking into the vacuum environment as the target is rotated. Typically sputtering takes place at a pressure of 2×10 −3  Torr and the chamber may be pumped to a base pressure of about 2×10 −6  Torr. 
         [0006]    Maintaining a good seal around a rotating part may be achieved using lip seals that seal against the rotating surface. However, such seals suffer from wear over time and may eventually fail allowing air to leak past the seal. Such a leak may not be detected in time to prevent damage to products in the coater. 
         [0007]    Therefore, there is a need for a robust sealing system that allows a vacuum seal to be maintained between a stationary part and a rotating part and that allows early detection of failure of a seal. There is also a need for a method of determining which seal has failed where multiple seals are present in a coater. 
       SUMMARY 
       [0008]    A sealing system has two seals that form a cavity between them. The seals may extend to seal the gap between a moving part (such as a rotating shaft) and a stationary part. The sealing system seals between a vacuum on one end and atmospheric pressure on the other end. The cavity is pumped through a restriction so that a leak into the cavity causes pressure in the cavity to rise or fall measurably. A gauge monitors cavity pressure. When a leak develops, the rise or fall in pressure may cause an alarm to be sent to a user. 
         [0009]    An alarm may be sent immediately when a threshold pressure is exceeded, or alternatively after the threshold pressure is exceeded for a predetermined period of time. Requiring an excess pressure for a predetermined period reduces the frequency of false alarms caused by momentary seat failures. Similarly, a drop in pressure may cause an alarm to be sent either immediately or after a predetermined period of time. 
         [0010]    Multiple seal cavities in a coating system may be connected together so that they may be pumped together. A manifold connects lines running to the cavities. The manifold has valves allowing individual cavities to be selectively connected to or isolated from a vacuum source. A restriction is located between the manifold and the vacuum source. When a leak is detected, individual cavities may be isolated to determine which cavity contains the leak. Based on the nature of the leak, an operator may determine whether a seal should be replaced immediately or whether the coater should continue running. 
         [0011]    A seal cavity may be pumped by a dedicated vacuum pump or a pump shared by multiple seal cavities. Alternatively, a pump used by the coating system may be used as a source of vacuum. In this case, no additional pump is required for the seal cavities. Another advantage of using a coater pump to provide vacuum for the seal cavities is that the seal cavities are automatically brought to the appropriate pressure for any coater condition. When the coater is under vacuum during operation, the seal cavities are under vacuum. When the coater is vented to atmosphere for maintenance, the seal cavities are at atmosphere. This automatic linking of pressure in the coater and in the seal cavity avoids problems caused by a pressure difference when seal cavity pressure is controlled directly by an operator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1A  shows a magnetron used to coat substrates. 
           [0013]      FIG. 1B  shows a coating system that includes the magnetron of  FIG. 1A . 
           [0014]      FIG. 2  shows a drive endblock used in the magnetron of  FIG. 1A . 
           [0015]      FIG. 3  shows a more detailed view of the drive endblock of  FIG. 2 . 
           [0016]      FIG. 4  shows a sealing system used in the drive endblock of  FIGS. 2 and 3 . 
           [0017]      FIG. 5  shows a water endblock used in the magnetron of  FIG. 1A . 
           [0018]      FIG. 6  shows a more detailed view of the water endblock of  FIG. 5 . 
           [0019]      FIG. 7  shows a sealing system used in the water endblock of  FIGS. 5 and 6 . 
           [0020]      FIG. 8  shows a sealing system used in either the drive endblock of  FIGS. 3 and 4  or the water endblock of  FIGS. 5 and 6 . 
           [0021]      FIG. 9  shows a sealing system similar to that of  FIG. 8  but with a restriction, pressure gauge and monitoring system. 
           [0022]      FIG. 10  shows four endblock seal cavities such as those shown in  FIGS. 4 and 7 . 
           [0023]      FIG. 11  shows a portion of the coating system of  FIG. 1B  incorporating multiple endblock seal cavities as shown in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Examples of endblocks that may be used for rotating water-cooled targets are given in U.S. Pat. No. 6,736,948, entitled “Cylindrical AC/DC magnetron with compliant drive system and improved electrical and thermal isolation,” by Richard L. Barrett, filed on Jan. 18, 2002. This patent is hereby incorporated by reference in its entirety. 
         [0025]    Typically, endblocks such as endblocks  104  and  108  of  FIG. 1A  are not identical and may serve different functions. One endblock may be a drive endblock used to rotate the target  106 . The other may be a water endblock used to supply cooling water to the target. These different endblocks have different design considerations but both hold a rotating part while maintaining a good vacuum seal between the rotating part and the static parts that surround it. 
         [0026]      FIG. 2  shows a drive endblock  200 . A drive endblock rotates the target during deposition. Thus, the drive endblock allows mechanical coupling between a drive motor  228  at atmosphere and a drive endcap  202  that is under vacuum. Some “wiggle room” is allowed between parts to allow for expansion of the parts at high temperature and to allow variation in part dimensions (within tolerance) between moving parts. 
         [0027]      FIG. 3  shows a portion of drive endblock  200  of  FIG. 2 . Bearings  212  and  214  allow rotation of drive cup  210  within isolation housing  216 . Isolation housing  216  is held within primary housing  224  in a manner that allows some flexing between these parts. The gap between drive cup  210  and isolation housing  216  and the gap between isolation housing  216  and primary housing  224  are sealed because vacuum is present on one end of these gaps while atmosphere is present on the other end. 
         [0028]      FIG. 4  shows a more detailed view of a sealing system  400  that may be used to maintain vacuum in an endblock such as drive endblock  200  of  FIGS. 2-3 . Reference numbers used in  FIG. 4  correspond to those in  FIGS. 2-3  where similar parts are referenced but numbers are incremented by  200  in  FIG. 4 . For example, drive cup  410  in  FIG. 4  corresponds to drive cup  210  of  FIGS. 2-3 . Two types of seals are used in sealing system  400 , lip seals (seal rings) and O-rings (compliant seal rings). Two lip seals  411  and  413  are located in the gap between drive cup  410  and isolation housing  416 . The two lip seals  411  and  413  are inserted so that if one fails, the other one will still seal the gap between drive cup  410  and isolation housing  416 . Atmosphere is to the right in  FIG. 4  with vacuum to the left creating a pressure differential from right to left. Lip seals  411  and  413  are C-shaped in cross-section. They are inserted with the opening towards the side with higher pressure. Thus, the pressure difference across each lip seal pushes the lip seal outwards to force it into compliance with the sealing surfaces. Lip seal  413  is held in position by a retaining ring  415 . Because the pressure differential is from right to left, the force on lip seal  413  is generally towards the left so lip seal  413  is restrained in this direction by retaining ring  415 . Lip seal  413  is separated from bearing  412  by a spacer  421 . Bearing  412  is separated from lip seal  411  by a spacer  425 . Between lip seal  411  and lip seal  413  a cavity  431  is formed that is isolated from both atmosphere on one side and vacuum on the other side. Cavity  431  is defined by drive cup  410  and isolation housing  416 , which form its inner and outer surfaces respectively, and by lip seals  411  and  413 , which form the end surfaces of cavity  431 . Cavity  431  is ring shaped, extending around drive cup  410 . Bearing  412  and spacer  421  are within cavity  431  but neither of these components forms a seal. Thus, gas may flow within cavity  431 . Therefore, cavity  431  may be treated as an undivided volume. 
         [0029]    Two O-rings  444   a  and  444   b  are located in the gap between isolation housing  416  and primary housing  424 . O-rings  444   a  and  444   b  are located so that if one fails then the other will still seal the gap between isolation housing  416  and primary housing  424 . A seal cavity  451  is formed between compliant seal rings  444   a  and  444   b . Cavity  451  is defined by isolation housing  416  and primary housing  424 , which form its inner and outer surfaces respectively, and by compliant seal rings  444   a  and  444   b , which form the end surfaces of cavity  451 . Cavity  451  is ring-shaped, extending around isolation housing  416 . 
         [0030]    A channel  455  connects cavity  451  and cavity  431 . Channel  455  allows gas to flow between cavity  451  and cavity  431  so that they form a single endblock seal cavity  461 . More than one channel may be used to connect cavities in this way to provide better communication between cavities (for example, two channels  255   a  and  255   b  are shown in endblock  200  of FIG.  3 . Endblock seal cavity  461  is connected to port  456 . Port  456  may be connected to a vacuum pump to maintain a reduced pressure in endblock seal cavity  461 . Thus, the pressure in endblock seal cavity  461  may be maintained at some intermediate pressure between atmospheric pressure and the high vacuum surrounding the target. 
         [0031]      FIG. 5  shows a water endblock  300 . A water endblock supplies cooling water to a target and provides a return path for cooling water from a target while a target is rotating and is under vacuum. It also allows electrical power to be fed to the target by means of a brush and commutator set-up maintained at atmospheric pressure. Seals  350  prevent water from entering the brush cavity while other seals maintain vacuum integrity from the brush cavity. The arrangement of bearings and seals in water endblock  300  is somewhat different from that used in a drive endblock, such as drive endblock  200 . However, both water and drive endblocks are designed to allow rotation of a part that extends from atmosphere to vacuum while maintaining a seal between atmosphere and vacuum. Both allow some “wiggle room” between parts to allow for expansion with increased temperature and to provide clearances necessary for rotation. 
         [0032]      FIG. 6  shows a portion of a water endblock  300  in more detail. Water spindle  320  is within bearing and seal carrier  360 . Bearing and seal carrier  360  is within water endblock isolation housing (WEIH)  304 . WEIH  304 , in turn, is within water endblock primary housing (WEPH)  308 . Between pairs of concentric parts gaps exist that extend from vacuum to atmosphere. These gaps are sealed to maintain vacuum in the area around the target. 
         [0033]      FIG. 7  shows a more detailed view of a sealing system that may be used in an endblock such as water endblock  300  of  FIGS. 5-6 . Reference numbers used in  FIG. 7  correspond to those in  FIGS. 5-6 , where similar parts are referenced but numbers are incremented by  400  in  FIG. 7 . For example, o-rings  712  (a and b) in  FIG. 7  correspond to o-rings  312  in  FIGS. 5-6 ; water spindle  720  in  FIG. 7  corresponds to water spindle  320  in  FIGS. 5-6 ; bearing  734  in  FIG. 7  corresponds to bearing  334  in  FIG. 5 ; lip seal  738  in  FIG. 7  corresponds to lip seal  338  in  FIG. 5 ; and port  756  in  FIG. 7  corresponds to port  356  in  FIG. 5 . Water spindle  720  is free to rotate within bearing  734 . Two lip seals  738  and  739  are located adjacent to bearing  734  in the gap between spindle  720  and bearing and seal carrier  760 . Lip seals  738  and  739  are separated by a spacer  737 . Lip seals  738  and  739  are located so that they are each individually sufficient to seal the gap between spindle  720  and bearing and seal carrier  760 . Thus, if one seal fails then the other is still capable of maintaining vacuum integrity. A seal cavity  770  is formed between lip seals  738  and  739 . Cavity  770  is defined by water spindle  720  and bearing and seal carrier  760 , which form its inner and outer surfaces respectively, and by seals  738  and  739 , which form the end surfaces of cavity  770 . Cavity  770  is ring shaped, extending around water spindle  720 . 
         [0034]    Bearing and seal carrier  760  is located within water endblock isolation housing (WEIH)  704 . The gap between bearing and seal carrier  760  and WEIH  704  is sealed by two O-rings  735   a  and  735   b . Between O-rings  735   a  and  735   b  a seal cavity  736  is formed. Cavity  736  is defined by bearing and seal carrier  760  and WEIH  704 , which form its inner and outer surfaces respectively, and by O-rings  735   a  and  735   b , which form the end surfaces of cavity  736 . Cavity  736  is ring shaped, extending around bearing and seal carrier  760 . 
         [0035]    WEIH  704  is located within water endblock primary housing (WEPH)  708 . The gap between WEIH  704  and WEPH  708  is sealed by two O-rings  712   a  and  712   b . Between O-rings  712   a  and  712   b  a seal cavity  719  is formed. Cavity  719  is defined by WEIH  704  and WEPH  708 , which form its inner and outer surfaces respectively, and by O-rings  712   a  and  712   b , which form the end surfaces of cavity  719 . Cavity  719  is ring shaped, extending around WEIH  704 . 
         [0036]    A channel  757  connects cavities  770 ,  736  and  719 . Channel  757  allows gas to flow between cavities  770 ,  736  and  719  so that they form a single endblock seal cavity  761 . More than one channel may be used to connect cavities in this way to provide better fluid communication between cavities. Endblock seal cavity  761  is connected to port  756 . Port  756  may be connected to a vacuum pump to maintain a reduced pressure in endblock seal cavity  761 . Thus, the pressure in endblock seal cavity  761  may be maintained at some intermediate pressure between atmospheric pressure and the pressure surrounding the target. 
         [0037]    One problem encountered with both drive and water endblocks is vacuum leakage due to failure of the seals. Leakage of air into the evacuated region around the target may affect the process by increasing the pressure and introducing contaminants. Large leaks may prevent sputtering because it is not possible to produce an adequate vacuum to create a plasma. 
         [0038]    Both lip seals and O-rings may fail. Mechanical wearing of a seal may cause the seal to fail. In the endblocks described above, lip seals usually fail before O-rings do because lip seals are used to seal a gap between a static part and a rotating part in these examples. To form an improved seal, and to reduce the effect of seal failure, two seals may be used to form a dual seal as described above. This provides redundancy in the sealing system so that if one seal fails, the second seal still ensures that vacuum is maintained. Between the seals a seal cavity, such as seal cavity  770 , is formed. A cavity may be connected to a vacuum pump to create a partial vacuum in the cavity. Two or more seal cavities in the same endblock may be connected together to form a single endblock seal cavity such as cavity  761 . 
         [0039]      FIG. 8  shows a simplified drawing of a differentially pumped sealing system  800 , which may incorporate sealing system  400  or  700  or a similar sealing system with a vacuum pump connected to differentially pump a sealing cavity. Differentially pumped sealing system  800  seals against a rotating spindle  882 . A vacuum pump  881  is connected to a seal cavity  883 . This differentially pumped arrangement produces a large pressure difference across seal  885  separating seal cavity  883  from atmosphere (the airside seal). As the seal cavity  883  is evacuated, this pressure difference approaches one atmosphere (760 Torr). Seal  887  separating seal cavity  883  from vacuum experiences a very small pressure difference when seal cavity  883  is evacuated because there is vacuum on both sides of this seal. Thus, as seal cavity  883  is evacuated the pressure difference across seal  887  approaches zero. Typically, the pressure in the cathode region around a target is about 2×10 −3  Torr during processing, while a seal cavity may be pumped down to 5-100 Torr. Thus, the actual pressure difference is less than 100 Torr across seal  887 . The pressure difference across seal  885  is more than 660 Torr. 
         [0040]    Generally, a seal experiencing a larger pressure difference will wear more rapidly and will fail sooner than a seal experiencing less pressure difference. A seal experiencing a small pressure difference should have an extended time to failure compared to the same seal if it were exposed to a large pressure difference. Thus, in  FIG. 8  airside seal  885  is likely to fail before vacuum side seal  887 . When seal  885  fails, seal  887  continues to function and continues to maintain vacuum in the cathode region. This avoids the need to shut down the entire system just to replace a failed seal. However, it is desirable to know that seal  885  has failed so that it may be replaced in a timely manner. 
         [0041]    According to one embodiment of the present invention, the failure of a seal may be detected by creating a restriction in a vacuum line connecting a seal cavity to a vacuum pump and by monitoring the pressure in the seal cavity or at a point in the vacuum line that is at a similar pressure to the seal cavity. 
         [0042]      FIG. 9  shows a seal cavity  983  connected to a vacuum pump  981  having a restriction  994  between cavity  983  and vacuum pump  981 . A pressure gauge  996  is connected to the vacuum line at a point that is between restriction  994  and seal cavity  983 . Thus, pressure gauge  996  is connected to seal cavity  983  by an unrestricted line so that the pressure measured by pressure gauge  996  is approximately the same as the pressure in seal cavity  983 . Under normal operating conditions, the vacuum pump  981  achieves a target pressure of about −27 inches of Mercury (76 Torr). This pressure is measured by vacuum gauge  996  and the output may be monitored directly by an operator or may be monitored by an automated monitoring system  998 . 
         [0043]    When seal  985  fails, air leaks into cavity  983 . Vacuum pump  981  continues to pump cavity  983 . However, because of the restriction  994  in the vacuum line between pump  981  and cavity  983 , the pressure in cavity  983  rises. This rise in pressure is detected by pressure gauge  996 . When a certain maximum pressure is reached, for example −20 inches of Mercury (250 Torr), an automated monitoring system  998  may begin a routine in response to the condition. This routine could simply involve sending a message to a user console  999  that the pressure in the seal cavity  983  was above a maximum pressure. However, false alarms indicating seal failure may occur if an alarm is sent every time the pressure exceeds some maximum. 
         [0044]    In an alternative routine, monitoring system  998  begins a timer when the pressure measured by pressure gauge  996  exceeds the maximum pressure. Air side lip seals that are used between moving parts such as lip seal  985  sometimes provide a “burp” in pressure in a seal cavity. This means that a momentary failure of seal  985  may allow some air to enter seal cavity  983  and cause the pressure to rise. However, such a leak does not persist and if no action is taken then pump  981  will pump out seal cavity  983  and the pressure in seal cavity  983  will return to a steady level below the maximum pressure. It is preferable that such an event should not cause an alarm to be sent by monitoring system  998  because no action is necessary. Instead of sending an alarm under these conditions, a timer may be initiated when the maximum pressure is exceeded. Pressure is monitored for a predetermined time (for example, 30 minutes) to see if the pressure remains above the maximum pressure. If the pressure in cavity  983  returns to a level below the maximum pressure within the predetermined time then no action is required because a “burp” has probably occurred. If the pressure in cavity  983  fails to return to a level below the maximum pressure then an “informational alarm” may be sent by monitoring system  998  to user console  999  to indicate that seal cavity  983  is leaking. An informational alarm indicates a condition that does not require immediate action but is of interest to an operator. Here, no immediate action is needed because seal  987  still maintains a seal, but the alarm informs an operator that seal replacement should be performed when there is an opportunity. For example, a message such as “Coat Zone X Source Bay Y has a high pressure,” might be sent to an operator console. 
         [0045]    In one embodiment, several endblock seal cavities are connected together so that they may share a common vacuum pump. In this embodiment an informational alarm indicating seal failure will only indicate that failure has occurred in one of several endblock seal cavities and will not tell an operator which seal is leaking. 
         [0046]      FIG. 10  shows four endblock seal cavities  1001 - 1004  connected together by valve manifold  1009 . Valve manifold  1009  is in turn connected to a vacuum pump  1081 . Vacuum pump  1081  is a diaphragm pump (Micro Dia-Vac® pump) that produces a vacuum in the range of less than 100 Torr. Valve manifold  1009  is connected to each of the seal cavities  1001 - 1004  by lines  1011 - 1014 . Lines  1011 - 1014  are ⅛ inch diameter polyflo tubing. In contrast, line  1015  connecting valve manifold  1009  to vacuum pump  1081  is 1/16 inch polyflo tubing. Line  1015  has an outer dimension of 1/16 inches, an inner dimension of 0.02 inches and is 12 inches in length. This smaller diameter line restricts the flow between valve manifold  1009  and vacuum pump  1081  compared with the flow between the seal cavities  1001 - 1004  and valve manifold  1009 . Thus, the line connecting valve manifold  1009  and pump  1081  is itself a restrictor in this example. The restrictor used for different situations depends on a number of variables including the number of seal cavities connected to a pump, the size of the pump, the distance between the pump and the manifold and between the manifold and the cavities. A restrictor could be a fixed orifice, a section of line or the entire line between manifold and pump  1081 . 
         [0047]    When an alarm occurs, an operator may wish to know which of endblock seal cavities  1001 - 1004  is leaking. The operator may isolate successive endblock seal cavities so that only one is connected to vacuum pump  1081  at a time and thus determine which endblock seal cavity has a leak. For example, in  FIG. 10 , valves  1021 - 1024  could all be closed to check the integrity of the lines between valve manifold  1009  and vacuum pump  1081 . Next, valve  1021  is opened so that only endblock seal cavity  1001  is connected to pump  1081 . If the pressure rises then endblock seal cavity  1001  is the source of the leak. If the pressure remains low then the leak is elsewhere. Valve  1021  is then closed and valve  1022  is opened to check endblock seal cavity  1002 . By checking each endblock seal cavity  1001 - 1004  in this manner, a leaking cavity may be identified. Once a leaking seal is identified a decision may be made on whether to valve off the corresponding seal cavity or keep pumping on it. A number of variables will aid in this decision as well as the decision to schedule an appropriate time for maintenance of this seal. The sensitivity of the process to leaks and the size of the leak are major factors. Generally, an operator will make the decision of how to deal with a leaking seal based on these and other factors. 
         [0048]    Vacuum side seals may also leak. This may be detected in a similar manner to that described above with respect to an airside seal leak. If a vacuum side seal leaks before the airside seal leaks then the pressure in the seal cavity will be reduced below its normal operating pressure. This occurs because the vacuum in the coater is much lower (2-5 mTorr) than the vacuum in the seal cavity (−27 inches of Mercury or 75 Torr). A minimum pressure may be set for the pressure in the seal cavity, for example, −29 inches of Mercury (25 Torr). When the minimum pressure is reached a timer is initiated. If the pressure remains below the minimum pressure for a predetermined period (e.g. 30 minutes) then an informational alarm is sent to an operator. An alarm such as “Coat Zone X Source Bay Y has a low pressure” may be sent to indicate the location and nature of the problem. The operator may then more precisely locate the seal failure by isolating all the seal cavities from the vacuum pump, then connecting them one-by-one to determine which one causes the pressure to drop in a procedure like that described above for an airside seal leak. A leak in a vacuum side seal is more serious than a leak in an airside seal because the vacuum integrity of the coater is compromised. Gas may enter the coater via this leak and cause contamination of the process within the coater. Thus, the sooner a leaking vacuum side seal is replaced the better. 
         [0049]    In some embodiments of the present invention, no dedicated vacuum pump is needed to provide vacuum in the seal cavity. Instead, a pump that provides vacuum to the coater is used to provide vacuum to the seal cavities also. Typically, a coater has several pumps to evacuate the large volumes within the coater. To achieve the high level of vacuum required a pumping stack of two or more pumps in series may be used. For example, a diffusion pump may be used in combination with a backing pump such as a mechanical pump to provide high vacuum. Several diffusion pumps may be connected to a single backing pump by a foreline. A gate valve may be between the pump stack and the coater so that the pump stack may be isolated from the coater for maintenance. Thus, a pump may be shut down and fixed without venting the coater to atmosphere. Within the coater there are several zones, each zone having one or more pump stack. 
         [0050]      FIG. 11  shows a portion of a coater including a coat zone  1173  and two interstage zones  1174  and  1175 . Four diffusion pumps  1176   a - 1176   d  are connected to coat zone  1173 . Diffusion pumps  1176   a - 1176   d  are attached to plenums  1123   a - 1123   d  which are attached to coat zone  1173 . Between each of diffusion pumps  1176   a - 1176   d  and coat zone  1173  are gate valves  1127   a - 1127   d . Two diffusion pumps  1176   e  and  1176   f  are connected to interstage zone  1175 . Diffusion pumps  1176   e  and  1176   f  are attached to plenums  1123   a  and  1123   d  respectively. Between diffusion pump  1176   e  and interstage zone  1175 , there is a gate valve  1127   e . Between diffusion pump  1176   f  and interstage zone  1175 , there is a gate valve  1127   f . Coat zone  1173  contains a target  1106  used to sputter onto a glass substrate. Endblocks  1104  and  1108  are shown at either end of target  1106 . Within each endblock  1104  and  1108  is an endblock seal cavity (not shown).  FIG. 11  shows lines  1111 ,  1112  and  1115  connecting the endblock seal cavities of endblocks  1104  and  1108  respectively to plenum  1123   d  of interstage zone  1175 . This arrangement provides vacuum to the endblock seal cavities without necessitating a separate pump. Because line  1115  connects upstream of gate valve  1127   f , the endblock seal cavities of endblocks  1104  and  1108  will be pumped-down as the interior of the coat zone  1173  and interstage zone  1175  are pumped down and will vent-up as they vent-up. This keeps the seal cavity at the desired pressure even when a pump is being serviced. Alternatively, line  1115  could be connected to vacuum elsewhere. For example, line  1115  could connect to foreline  1177 . However, this may require line  1115  to be isolated from foreline  1177  during servicing. Another possibility is to connect line  1115  to another zone. 
         [0051]    When a seal leaks in endblock  1108 , air is drawn into the endblock seal cavity and then through lines  1112  and  1115 , through plenum  1123   d  and gate valve  1127   f  to pump  1176   f . Some air may diffuse from plenum  1123   d  into interstage zone  1175 . However, because this is an interstage zone no sputtering takes place in interstage zone  1175 . Therefore, leakage will not directly affect the process. A restrictor  1178  is shown inserted in-line along line  1115 . This provides the necessary restriction to allow pressure in the seal cavities to rise. Alternatively, the size of line  1115  may be chosen to adequately restrict the flow between endblocks  1104  and  1108  and pump  1176   f  so that leakage in endblock  1108  will cause a rise in pressure. Pressure in the seal cavities of endblocks  1104  and  1108  is detected by pressure gauge  1196 . Valve manifold  1109  is connected where line  1115  connects to line  1111  from endblock  1104  and line  112  from endblock  1108 . Valve manifold  1109  may be used to identify which of endblocks  1104  and  1108  has a leak using the procedure described with respect to  FIG. 10 . 
         [0052]    Thus, using a vacuum gauge, an in-line gas-flow restricting device and a source of vacuum pumping, real-time monitoring of the status of the endblock seal cavity seals is accomplished. By differentially pumping the seat cavity, the airside seal ring will typically fail before the vacuum side seal ring. Once failed, the airside seal ring will let air pressure into the seal cavity. The increase of pressure will be monitored by use of a vacuum gauge. The retention of pressure is accomplished by using a narrow, short tube (gas-flow restrictor) between the valve manifold and the vacuum pump. The rate of pressure evacuation will be metered by the restrictive device, making it easy to monitor the pressure produced by the smallest leaks. 
         [0053]    It is to be understood that even though numerous characteristics and advantages of certain embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention.