Abstract:
A vertical rapid thermal processing system includes a processing chamber and an elevator structure providing for vertical movement of a workpiece within the processing chamber. The elevator structure includes a workpiece support for supporting the workpiece and an elevator shaft coupled to the workpiece support and extending externally from the processing chamber. An end portion of the elevator shaft, external to the processing chamber, has a selected magnetic polarity. The elevator structure also includes moveable carriage coupled to the shaft to provide vertical movement of the workpiece within the processing chamber. The carriage includes a shaft support having the selected polarity. The shaft support is positioned adjacent the polarized end portion of the elevator shaft such that repulsive magnetic forces maintain a gap between the end portion of the elevator shaft and the shaft support as the shaft support and elevator shaft move vertically along a path of travel.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the field of semiconductor thermal processing systems and, more specifically, to a vertical rapid thermal processing unit that includes an elevator assembly for positioning a workpiece within the processing unit. 
     BACKGROUND OF THE INVENTION 
     Thermal processing systems are widely used in various stages of semiconductor fabrication. Basic thermal processing applications include chemical deposition, diffusion, oxidation, annealing, silicidation, nitridation, and solder re-flow processes. Vertical rapid thermal processing (RTP) systems comprise a vertically oriented processing chamber which is heated by a heat source such as a resistive heating element or a bank of high intensity light sources. An elevator structure is controlled to move a workpiece such as a wafer workpiece on a workpiece support vertically within the processing chamber. The elevator structure of prior art RTP&#39;s typically included a vertically-oriented moveable elevator shaft which extended into the processing chamber. The workpiece support was affixed to an upper end of the elevator shaft. A lower end of the elevator shaft was coupled to a vertically moveable carriage that allowed the elevator shaft to move vertically with respect to the processing chamber and, therefore, allowed the workpiece to move vertically within the processing chamber. 
     In some RTP systems, the heat sources create a temperature gradient within the processing chamber and temperature ramp-up and ramp-down rates of the wafer being processed are controlled by the vertical location of the workpiece within the processing chamber. Therefore, to optimize the thermal processing of semiconductor workpieces it is important to accurately control the vertical position of the workpiece within the processing chamber. 
     In addition to accurately controlling the vertical position of the workpiece within the processing chamber, it is also desirable to minimize vibration imparted to the workpiece via the elevator structure. Vibration may cause the workpiece to move horizontally relative to the support, thereby losing concentricity with the support structure. Lack of concentricity results in temperature non-uniformity near the workpiece perimeter. 
     One approach to reducing vibration in elevator structures of prior art RTP&#39;s was providing a rigid base which was affixed to a lower end of the elevator shaft. The rigid base was thought to minimize vibration that may be transmitted through the elevator shaft and imparted to the workpiece. However, since the elevator structure does have to move vertically, the base necessarily had to be affixed to the vertically movable carriage. Thus, in spite of the rigid base affixed to the lower end of the elevator shaft, vibrations from external sources could be still transmitted through the vertically movable carriage, the base, the elevator shaft, the workpiece support and, finally, to the workpiece itself. 
     What is needed is a system that facilitates both accurate movement and positioning of a workpiece within the RTP processing chamber and minimizes vibration transmitted to the workpiece. What is also needed is a system that isolates the workpiece support within the processing chamber from the moveable carriage outside of the processing chamber. 
     What is also needed is a method that facilitates both accurate movement and positioning of a workpiece within the RTP processing chamber and minimizes vibration transmitted to the workpiece. What is also needed is a method that isolates the workpiece support within the processing chamber from the moveable carriage outside of the processing chamber. 
     SUMMARY OF THE INVENTION 
     A vertical rapid thermal processing system includes a processing chamber and an elevator structure extending into an interior region of the processing chamber. The present invention provides for mechanical isolation of a portion of the elevator structure extending into the processing chamber interior region and supporting a workpiece within the processing chamber and a carriage, external to the processing chamber that provides for vertical movement of the workpiece within the processing chamber. 
     In one exemplary embodiment of the present invention, a vertical rapid thermal processing system includes a processing chamber and an elevator structure extending into the processing chamber for movably supporting a workpiece within the processing chamber. The elevator structure includes a workpiece support for supporting the workpiece and an elevator shaft extending into the processing chamber and coupled to the workpiece support. An end portion of the elevator shaft, external to the processing chamber, has a selected magnetic polarity. The elevator structure also includes moveable carriage coupled to the shaft to provide vertical movement of the workpiece within the processing chamber. The carriage includes a shaft support having the selected polarity. The shaft support is positioned adjacent the polarized end portion of the elevator shaft such that repulsive magnetic forces maintain a gap between the end portion of the elevator shaft and the shaft support as the shaft support and elevator shaft move vertically along a path of travel. 
     In another exemplary embodiment of the present invention, an elevator tube positioning system includes a workpiece support for supporting a workpiece within a process chamber and an elevator tube coupled to the workpiece support and extending externally of the process chamber. An end portion of the elevator shaft, external to the processing chamber, has a selected magnetic polarity. The elevator tube positioning system also includes moveable carriage coupled to the elevator shaft to provide vertical movement of the workpiece within the processing chamber. The carriage includes a shaft support having the selected polarity. The shaft support is positioned adjacent the polarized end portion of the elevator shaft such that repulsive magnetic forces maintain a gap between the end portion of the elevator shaft and the shaft support as the shaft support and elevator shaft move vertically along a path of travel. 
     These and other objects, advantages, and features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-section view of a rapid thermal processing unit including a processing chamber and an elevator tube positioning system in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic exploded perspective view of the elevator tube assembly and a portion of a carriage assembly of the elevator tube positioning system of  FIG. 1 ; 
         FIG. 3  is a schematic cross-section view of a lower portion of the elevator tube assembly and a portion of the carriage assembly of  FIG. 2 ; 
         FIG. 4  is a schematic cross-section view of a gas bearing of the carriage assembly of  FIG. 2 ; and 
         FIG. 5  is a schematic cross-section view of a lower portion of an elevator tube and a portion of a carriage assembly in a second preferred embodiment an elevator tube positioning system of the present. 
     
    
    
     DETAILED DESCRIPTION 
     FIRST PREFERRED EMBODIMENT 
       FIG. 1  illustrates a rapid thermal processing (RTP) system  10  that uses a cylindrical hot wall system to thermally process workpieces, such as semiconductor wafers. The system includes a processing chamber, show generally at  12  and an elevator tube positioning system, shown generally at  14 . A workpiece  16  is placed on a workpiece support  18  of the elevator tube positioning system  14 . The workpiece  16  is moved, by the elevator tube positioning system  14 , vertically through the processing chamber  12  having a temperature gradient created by heating elements  20  behind the chamber walls  22 . In  FIG. 1 , two positions of the elevator tube positioning system  14  are shown, a lower vertical position of the workpiece support  18  is shown in solid line while a higher vertical position of the workpiece support  18  is shown in dashed line. 
     Heat is applied to the workpiece  16  via the heating elements  20 . Positioning and heating of workpiece  16  within the processing chamber  12  is controlled via control electronics (not shown). During processing of the workpiece  16 , the processing chamber interior region  15  is maintained at a desired reduced pressure condition by a pumping system (not shown). 
     In addition to the workpiece support  18 , the elevator tube positioning system includes an elevator tube assembly  24  (best seen in  FIG. 2 ), which supports the workpiece support  18  for vertical movement within the processing chamber  12 . The elevator tube assembly  24  includes an elevator tube  26  that protrudes through an opening  27  in a floor  28  of the processing chamber  12  and is supported vertical movement by a coupling  29  positioned in the processing chamber floor opening  27 . The elevator tube  26  is preferably made from a ceramic-based material to prevent metallic contamination within the processing chamber  12  and that can withstand high temperatures within the chamber  12 . Preferably, the ceramic-based material is quartz. Another possible ceramic-based material is alumina. 
     The coupling  29  ( FIG. 1 ) includes an air bearing  30  and a surrounding compliant member  34 . The air bearing  30  centers the tube  26  within the bearing and prevents the flow of gas into the processing chamber interior region  15 . The bearing  30  is mounted within the processing chamber floor opening  27  and is disposed concentrically about the elevator tube  26 . An inner bearing surface  32  is in close proximity to an outer surface of the tube  26 . A gas port supplies pressurized gas into a gas curtain defined by the gas flow between the tube  26  and the inner bearing surface  32 . The gas curtain prevents incursion of ambient air into the processing chamber. The compliant member  34  is adjacent to the bearing  30  and disposed between the bearing  30  and the processing chamber floor  28  for absorbing a force created if and when that the elevator tube  26  contacts the inner bearing surface  32 . The complaint member  34  may be a metal bellows. 
     If and when the tube  26  moves laterally with respect to a centerline CL of the coupling  29  during movement along its path of travel, the tube  26  pushes against the bearing  30  causing pressure variations within the gas curtain. The surrounding complaint member  34  deflects laterally in response and absorbs the force of the tube  26  while allowing the gas curtain pressure to actively center itself with respect to the tube  26 . In this manner, the lateral movement of the tube  26  relative to the bearing  30  is limited to reduce contact forces between the tube  26  and the bearing  30 . A suitable coupling is disclosed in U.S. application Ser. No. 10/646,228, filed on Aug. 22, 2003. Application Ser. No. 10/646,228 is assigned to the assignee of the present invention and is incorporated in its entirety herein by reference. 
     The elevator tube positioning system  14  also includes a carriage assembly  40  which is coupled to the elevator tube  26  and provides for movement of the tube  26 . In order to minimize vibration transmitted from external sources to the workpiece  16 , in the present invention, the elevator tube  26  is mechanically isolated from the carriage assembly  40 . Instead of a mechanical linkage, the carriage assembly  40  moves the elevator tube  26  vertically along a path of travel via repulsive magnetic forces, as will be explained below. Since the carriage assembly  40  and the elevator tube  26  are physically spaced apart, any vibration that the carriage assembly  40  is subjected to from external sources will not be transmitted to the elevator tube  26  and, therefore, will not be transmitted to the workpiece  16 . 
     The carriage assembly  40  includes a carriage  42  that, under the control of the control electronics, traverses a set of rails  46  ( FIG. 1 ) to move the elevator tube  26  and the workpiece support  18  vertically along a path of travel. Affixed to the carriage  42  is an elevator or shaft support  44 . The shaft support  44  includes a cylindrical or annular magnet  50 , shown in cross section in  FIG. 3 . A cylindrical or annular magnet  52  is also affixed to an end  53  the elevator tube  26 . The polarity of the magnets  50 ,  52  is arranged such that both magnets have the same polarity along the surfaces  54 ,  56  facing each other. In  FIG. 3 , for example, both surfaces  54 ,  56  are shown as having north pole polarities. Of course, both surfaces  54 ,  56  could be polarized to have south pole polarities. The strength of the magnets  50 ,  52  is sufficient such that the repulsive magnetic forces between the like-polarized surfaces  54 ,  56  cause a space or gap  57  between the surfaces  54 ,  56 . In essence, because of the repulsive magnetic forces between the magnets  50 ,  52 , the elevator tube  26  “floats” on and is mechanically isolated from the shaft support  44 . 
     The magnets  50 ,  52  are disposed within respective plastic shells  60 ,  62  to prevent contamination. The magnetic strength of the magnets  50 ,  52  is selected to be such that during maximum acceleration of the elevator tube  26 , the surfaces  54 ,  56  never contact each other. 
     Affixed to an upper surface  48  of the shaft support  44  is an air bearing  66  that functions to keep the elevator tube  26  vertically aligned and centered with respect to the centerline CL of the coupling  29  by forming a gas cushion or curtain between an outer surface of the tube  26  and an inner surface of the bearing  66 . As can best be seen in  FIG. 4 , to form the gas cushion, gas from a gas supply (not shown) is directed through an inlet port  68  and flows through a concentric ring of inlet orifices  70  into a gas curtain region  72  that is the gap between the outer surface of the elevator tube  26  and the bearing  66 . The supplied gas flows upward from each inlet orifice  70  to a corresponding concentric ring of exhaust orifices  74 . The gas removed via the exhaust orifices  74  is removed via an outlet port  76 . 
     SECOND PREFERRED EMBODIMENT 
     A second preferred embodiment of an elevator tube positioning system  14 ′ of the present invention is shown schematically in cross section in  FIG. 5 . In this embodiment, the shaft support  44 ′ is augmented with a second cylindrical or annular magnet  58 ′ positioned vertically above the first magnet  50 ′. The magnet  58 ′, like the magnets  50 ′,  52 ′ is encased in a plastic shell  78 ′. The polarity of the second magnet  58 ′ of the shaft support  44 ′ is selected such that a polarity of a surface  59 ′ is the same as a polarity of the surface  80 ′ of the magnet  52 ′ affixed to the elevator shaft  26 ′. For example, as can be seen schematically in  FIG. 5 , the facing surfaces  59 ′ and  80 ′ are both polarized with a north magnetic pole polarity, while the facing surfaces  54 ′,  56 ′ are both polarized with a south magnetic pole polarity. 
     The magnetic strength of the magnets  50 ′,  52 ′,  58 ′ is selected to be such that during maximum acceleration of the elevator tube  26 ′, the surfaces  54 ′,  56 ′ and the surfaces  59 ′,  80 ′ never contact each other, that is, there is a gap  57 ′ between the surfaces  54 ′,  56 ′ and there is a gap  82 ′ between the surfaces  59 ′,  80 ′. The respective surfaces never contact even during maximum acceleration of the elevator tube  26 ′. Essentially, because of repulsive magnetic forces, the magnet  52 ′ of the elevator tube  26 ′ “floats” between the magnets  50 ′ and  58 ′ of the shaft support  44 , thus, the elevator tube  26 ′ is mechanically isolated from the shaft support  44 ′. 
     The purpose of the second magnet  58 ′ affixed to the shaft support  44 ′ is to create a preload condition on the elevator tube  26 ′ because of the force exerted between the magnets  50 ′ &amp;  52 ′ and  52 ′ &amp;  58 ′. The preloading the elevator tube  26 ′ in this manner, the bandwidth of the control system can be increased because the preloading tends to filter out low frequency vibrations. Also, preloading advantageously decreases the effect of gravity on the payload since there are magnetic forces in both the upward and downward direction on the elevator shaft  26 ′ as a result of the four magnet combination. As in the first embodiment, affixed to an upper surface  48 ′ of the shaft support  44 ′ is an air bearing  66 ′ that functions to keep the elevator tube  26 ′ centered with respect to the centerline CL of the coupling (the coupling is shown as  29  in the first embodiment). 
     Although the present invention has been described with a degree of particularity, it is the intent that the invention include all modifications and alterations from the disclosed design falling within the spirit or scope of the appended claims.