Abstract:
An apparatus of the invention comprises a tube with a vacuum chamber located in its middle portion and a rod inserted in the tube so that the gap between the rod and inner surface of the tube is narrow, e.g. within the range from 5 to 10 microns. Compressed air is injected to the gap between the inner surface of the tube and the rod via orifices at the ends of the tube to form air bearings, which allow moving the rod along its axis. A sample is placed in a recess in the rod, so that it can be inserted from the ambient atmosphere into the vacuum chamber by moving the rod along its axis. Since the gap between the rod and inner surface of the tube is narrow, the air leakage to vacuum chamber is insignificant. This leakage can be further decreased by evacuating an excess air from the gap in one or more areas between the vacuum chamber and orifices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable  
         FEDERALLY SPONSORED RESEARCH  
         [0002]    Not Applicable  
         SEQUENCE LISTING OR PROGRAMM  
         [0003]    Not Applicable  
         FIELD OF INVENTION  
         [0004]    The present invention relates to the field of vacuum technology, in particular to the systems, which require insertion of a sample into a high vacuum region. The invention may find application in many fields where reduction of the time required for insertion of a sample into a vacuum system can affect the overall throughput of the equipment.  
         BACKGROUND OF THE INVENTION  
         [0005]    There are a lot of various technological procedures performed under vacuum. The examples are sputtering, etching, electron microscopy, mass spectrometry. Two standard approaches are typically used to insert a sample from atmosphere into a vacuum chamber. The first most obvious approach consists of filling the chamber with air up to atmospheric pressure prior to sample insertion. Then the chamber with the sample can be evacuated to the required degree of vacuum. Depending on chamber size and depth of vacuum, evacuating the chamber can take a significant amount of time and affect the overall throughput of the equipment. According to the second approach, the equipment throughput can be improved if the sample is evacuated in an airlock chamber attached to the main vacuum chamber (see U.S. Pat. No. 5,354,380 to Zejda, 1994). Then the sample can be moved from the airlock chamber to the main chamber under vacuum. This approach eliminates waste of time for evacuating the main chamber, but requires additional complicated equipment, i.e. airlock chamber and means to manipulate with a sample under vacuum.  
           [0006]    A static air bearing is a well-known device, which allows rotational or linear motion practically without friction. It is widely used for precision positioning, high-speed rotation, in test equipment, etc. A feature of static air bearing is that to ensure high stiffness and load capacity the gap between its stationary and moving parts must be narrow (typically 5 to 10 microns). This feature allows using air bearing to transmit a motion into a vacuum chamber, so that the air bearing gap additionally works as a vacuum seal (see U.S. Pat. No. 4,118,042 to Booth, 1978).  
         SUMMARY  
         [0007]    An apparatus of the invention comprises a tube with a vacuum chamber located in its middle portion and a rod inserted in the tube so that the gap between the rod and inner surface of the tube is narrow, e.g. within the range from 5 to 10 microns. Compressed air is injected to the gap between the inner surface of the tube and the rod via orifices at the ends of the tube to form air bearings, which allow moving the rod along its axis. A sample is placed in a recess in the rod, so that it can be inserted from the ambient atmosphere into the vacuum chamber by moving the rod along its axis. Since the gap between the rod and inner surface of the tube is narrow, the air leakage to vacuum chamber is insignificant. This leakage can be further decreased by evacuating an excess air from the gap in one or more areas between the vacuum chamber and orifices.  
           [0008]    It is an object of the invention to provide a simple apparatus for insertion of a sample into a vacuum chamber.  
           [0009]    Another object of the invention is to minimize the time required for insertion of a sample into a vacuum chamber.  
           [0010]    Additional object of the invention is to allow insertion of one sample into a vacuum chamber simultaneously with removing another sample from the vacuum chamber.  
           [0011]    One more object of the invention is to provide an apparatus for insertion of a sample into a vacuum chamber which is easy to operate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1A and FIG. 1B show a preferred embodiment of the apparatus of the invention with a sample outside the apparatus and inside the vacuum chamber, respectively.  
         [0013]    [0013]FIG. 2 shows arrangement of orifices in the preferred embodiment of the apparatus of the invention with no orifice on top side of the tube, which allows recess on the rod to pass through this region between the orifices.  
         [0014]    [0014]FIG. 3 shows the first alternative embodiment of the apparatus of the invention with additional grooves between the orifices, which allow recess on the rod to remain under atmospheric pressure while passing through the region of orifices.  
         [0015]    [0015]FIG. 4 shows the second alternative embodiment of the apparatus of the invention with an additional recess on the opposite side of the rod, preventing asymmetrical pressure distribution when the recess with a sample is at the orifices.  
         [0016]    [0016]FIG. 5 shows the third alternative embodiment of the apparatus of the invention with an annular groove on the rod, preventing asymmetrical pressure distribution when the recess with a sample is at the orifices.  
         [0017]    [0017]FIG. 6A and FIG. 6B respectively show two working positions of the rod in apparatus of another embodiment on which one sample can be inserted into the vacuum chamber simultaneously with removal of another sample. 
     
    
     DETAILED DESCRIPTION  
       [0018]    [0018]FIG. 1A and FIG. 1B show a preferred embodiment of the apparatus of the invention with a sample outside the apparatus and inside the vacuum chamber, respectively. The apparatus consists of a vacuum chamber  10  intersected by a tube  12  with precision inner diameter and a precision rod  14 . The diameter of rod  14  can be, e.g., 30 mm. The rod has a recess  16  which contains a sample  18 . The length, width, and depth of recess  16  can be, e.g., 10 mm×10 mm×10 mm. The construction of the apparatus of the invention is symmetrical with respect to vacuum chamber  10 . For the sake of simplicity we will describe all the features of the apparatus on one side from vacuum chamber  10 . It is understood that for each of the described features there is a similar feature on the opposite side from vacuum chamber  10 .  
         [0019]    A narrow gap  20  between the inner surface of tube  12  and rod  14  is connected to a source of compressed air (not shown) via passages  22  and orifices  24  in proximity to the end of tube  12 . The thickness of gap  20  can be, e.g., 10 microns. The diameters of orifices  24  can be, e.g., 0.1 mm and their number can be, e.g., 3. FIG. 2 shows arrangement of orifices  24  in the preferred embodiment of the apparatus of the invention with no orifice on top side of tube  12 .  
         [0020]    There is an annular groove  26  on the inner surface of tube  12  between orifices  24  and vacuum chamber  10  (FIG. 1A,B). This groove is connected to ambient atmosphere by a passage  28 . There is an annular groove  30  on the inner surface of tube  12  between groove  26  and vacuum chamber  10 . This groove is connected to a vacuum pump (not shown) by a passage  32 . The distances between grooves  26 ,  30 , and vacuum chamber  10  can be, e.g., 20 mm.  
         [0021]    An apparatus of the invention operates in the following manner. Initial position of rod  14  (FIG. 1A) provides that recess  16  is outside tube  12  so that sample  18  can be placed into recess  16 . Compressed air at a pressure, e.g., of about five atmospheres is injected into gap  20  via passages  22  and orifices  24  and escapes from gap  20  to groove  26  and to the ambient atmosphere at the end of tube  12 . By flowing through gap  20 , compressed air detaches rod  14  from the inner wall of tube  12  to form an air bearing, which allows moving rod  14  inside tube  12 . From groove  26  air escapes to the ambient atmosphere via passage  28 , so that pressure in groove  26  remains equal to the atmospheric pressure.  
         [0022]    Gap  20  serves as an air bearing between the end of tube  12  and groove  26 . In view of its minimal thickness, the same gap  20  between groove  26  and chamber  10  serves as a vacuum seal. In order to decrease air leakage from the ambient atmosphere to vacuum chamber  10 , groove  30  is evacuated via passage  32 . Residual pressure in groove  30  can be, e.g., 100 Pa. Residual pressure in vacuum chamber  10  can be, e.g., 0.01 Pa.  
         [0023]    Air leakage through gap  20  occurs in viscous flow regime when the mean free pass of molecules is less than the gap thickness. Otherwise, air leakage occurs in molecular flow regime. For the gap thickness of 10 microns transition from viscous to molecular flow regime occurs at a pressure of about 10 4  Pa. Thus, between groove  26  and groove  30  air leakage occurs mainly in viscous regime. Between groove  30  and vacuum chamber  10  air leakage occurs solely in molecular regime. Taking into account the regime of air flow in each region, the following estimations of leakages can be made:  
         [0024]    from groove  26  to groove  30  leakage is about 1 l/s at 100 Pa,  
         [0025]    from groove  30  to vacuum chamber  10  leakage is about 1 l/s at 0.01 Pa.  
         [0026]    Leakages of these levels can be easily evacuated by conventional vacuum pumps.  
         [0027]    To insert sample  18  into vacuum chamber  10 , one can move rod  14  along its axis from position shown on FIG. 1A, where recess  16  with sample  18  is outside tube  12 , to the position shown in FIG. 1B, where recess  16  with sample  18  is inside vacuum chamber  10 . The above described air bearing allows to perform this motion easily. On its way to vacuum chamber  10 , recess  16  with sample  18  passes through the region of elevated pressure at orifices  24 , then through the region of atmospheric pressure at groove  26 , and the region of intermediate vacuum at groove  30 . When sample  18  is inside vacuum chamber  10  (FIG. 1B), its position can be adjusted along the axis of tube  12  by shifting rod  14  in the axial direction and across the axis of tube  12  by pivoting rod  14  around its axis. Both of these motions are made possible due to the use of the air bearing.  
         [0028]    Being in proximity of orifices  24 , recess  16  can affect air flow and pressure in gap  20 . As a result, rod  14  can be shifted from the axis of tube  12 . To minimize distortion of pressure field in gap  20 , orifices  24  are distributed around the axis of tube  12  in such a way that recess  16  can pass though this region between the orifices (FIG. 2). As the distortion of pressure field in gap  20  is small, rod  14  does not touch the inner surface of tube  12  and can continue its axial motion.  
         [0029]    Pressure in recess  16  with sample  18  takes the value of atmospheric pressure when the recess passes through the region of groove  26 . Region of groove  30  serves as an intermediate vacuum chamber. Passing through this region, recess  16  with sample  18  is evacuated to an intermediate degree of vacuum. Performance of the pump attached to groove  30  must be high enough to evacuate recess  16  while it passes the groove. For example, if rod  14  moves with a speed of 1 cm/s, performance of the pump attached to groove  30  can be 10 l/s at 100 Pa.  
         [0030]    There are several alternative possibilities to deal with distortion of pressure field in gap  20  when recess  16  is at orifices  24 . In the description of preferred embodiment of the invention this pressure distortion is minimized by proper distribution of orifices  24  around the axis of tube  12  (FIG. 2). As shown in FIG. 3, the distortion of pressure field can be completely avoided with the help of additional grooves  34  on the inner surface of tube  12  along its axis between orifices  24 . Grooves  34  connect groove  26  with the end of tube  12 , so that one of grooves  34  is aligned with the pass of recess  16  and provide that recess  16  with sample  18  remains under atmospheric pressure while passing through the region of orifices  24 .  
         [0031]    Another possibility to let the recess  16  pass through the region of orifices  24  freely is to keep pressure distribution in gap  20  symmetrical, so that no net radial force on rod  14  would arise. It can be done by supplementing recess  16  on the top side of rod  14  with an extra recess  17  of the same size as recess  16  on the bottom side of rod  14 . FIG. 4 shows the embodiment of the apparatus of the invention with recess  17 . Distribution of orifices  24  around the axis of tube  12  in this case must be symmetrical with respect to the horizontal plane passing through the axis of tube  12 . The ability of airflow from orifices  24  to support rod  14  in this position vanishes. To keep rod  14  at this position afloat, additional flow of compressed air is injected into gap  20  via passages  23  and orifices  25  located between orifices  24  and groove  26 .  
         [0032]    When recess  16  passes orifices  25 , rod  14  is supported by compressed air injected via orifices  24 . As shown in FIG. 5, asymmetrical pressure distribution in gap  20  can also be eliminated with a help of an annular groove  36  on rod  14  overlapping with recess  16 . In this case additional orifices  25  should also be installed to keep rod  14  afloat when recess  16  passes orifices  24 .  
         [0033]    An additional embodiment of the apparatus of the invention is shown in FIG. 6A and FIG. 6B. The device of this embodiment allows insertion of one sample into vacuum chamber  10  simultaneously with removing of another sample. This goal is achieved with the help of an additional recess  38  on rod  14 . The proper distance between recesses  16  and  38  provides that recess  16  is outside tube  12  when recess  38  is inside vacuum chamber  10  (FIG. 6A) and vice versa (FIG. 6B). Thus, when sample  18  in recess  16  is inserted into vacuum chamber  10 , sample  40  in recess  38  is removed from vacuum chamber  10 .  
         [0034]    On its way to vacuum chamber  10 , recess  16  with sample  18  must be evacuated at groove  30  to the intermediate vacuum level. At a given performance of a pump, attached to groove  30 , evacuating recess  16  with sample  18  takes a certain time. To speed up insertion of sample  18  into vacuum chamber  10 , one or both of two alternative means can be used. Namely, rod  14  can be moved non-uniformly, so that recess  16  with sample  18  moves fast before and after groove  30 , but stays for a certain time at groove  30  until pressure in recess  16  is decreased to the proper level. Alternatively, buffer chamber can be attached to groove  30 . In this case, pressure in recess  16  will instantly equalize with pressure in the buffer chamber when the recess is at groove  30 . If the volume of buffer chamber is high enough, common pressure of the buffer chamber and recess  16  will be close to initial pressure of buffer chamber.  
         [0035]    Thus it has been shown that the invention provides apparatus and method that simplify insertion of a sample into a vacuum chamber, minimize the time required for insertion of a sample into a vacuum chamber, and allow insertion of one sample into a vacuum chamber simultaneously with removing another sample from the vacuum chamber.  
         [0036]    Although the invention has been shown and described with reference to specific embodiments, it is understood that these embodiments should not be construed as limiting the areas of application of the invention and that any changes and modifications are possible, provided these changes and modifications do not depart from the scope of the attached patent claims. For example, although only one groove  30  was shown as an intermediate vacuum chamber, it is understood, that number of these grooves, distances between adjacent grooves, and residual pressures in the grooves are determined by the requirements to vacuum depth and by leakage allowed to vacuum chamber  10 . If requirements to vacuum and leakage in chamber  10  are minimal, the apparatus can be made even without groove  30  and/or without groove  26 . Otherwise, the number of grooves  30  can be one or more. Each subsequent groove on the way of recess  16  with sample  18  to vacuum chamber  10  is evacuated to the increasingly deep vacuum. The groove adjacent to vacuum chamber  10  can be evacuated to the same vacuum degree as vacuum chamber  10 . In this case there will be no leakage to vacuum chamber  10  from gap  20 , and no pressure spike in vacuum chamber  10  during insertion of sample  18 .