Patent Document

CROSS REFERENCE  
       [0001]    This is a continuation of U.S. application Ser. No. 09/521,799, filed Mar. 9, 2000, which is a continuation of U.S. application Ser. No. 09/263,001, filed Mar. 5, 1999, which is a continuation-in-part of U.S. application Ser. No. 08/766,119, filed Dec. 16, 1996, which is a continuation of now-abandoned U.S. application Ser. No. 08/590,998, filed Jan. 24, 1996.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field  
           [0003]    The invention is related to plasma reactors for processing semiconductor wafers, and in particular confinement of the processing plasma in the reactor within a limited processing zone.  
           [0004]    2. Background Art  
           [0005]    Plasma reactors, particularly radio frequency (RF) plasma reactors of the type employed in semiconductor wafer plasma processing in the manufacturing of microelectronic integrated circuits, confine a plasma over a semiconductor wafer in the processing chamber by walls defining a processing chamber. Such an approach for plasma confinement has several inherent problems where employed in plasma reactors for processing semiconductor wafers.  
           [0006]    First, the walls confining the plasma are subject to attack from ions in the plasma, typically, for example, by ion bombardment. Such attack can consume the material in the walls or introduce incompatible material from the chamber walls into the plasma process carried out on the wafer, thereby contaminating the process. Such incompatible material may be either the material of the chamber wall itself or may be material (e.g., polymer) previously deposited on the chamber walls during plasma processing, which can flake off or be sputtered off. As one example, if the chamber walls are aluminum and the plasma process to be performed is plasma etching of silicon dioxide, then the material of the chamber wall itself, if sputtered into the plasma, is incompatible with the process and can destroy the integrity of the process.  
           [0007]    Second, it is necessary to provide certain openings in the chamber walls and, unfortunately, plasma tends to leak or flow from the chamber through these openings. Such leakage can reduce plasma density near the openings, thereby upsetting the plasma process carried out on the wafer. Also, such leakage can permit the plasma to attack surfaces outside of the chamber interior. As one example of an opening through which plasma can leak from the chamber, a wafer slit valve is conventionally provided in the chamber side wall for inserting the wafer into the chamber and withdrawing the wafer from the chamber. The slit valve must be unobstructed to permit efficient wafer ingress and egress. As another example, a pumping annulus is typically provided, the pumping annulus being an annular volume below the wafer pedestal coupled to a vacuum pump for maintaining a desired chamber pressure. The chamber is coupled to the pumping annulus through a gap between the wafer pedestal periphery and the chamber side wall. The flow of plasma into the pumping annulus permits the plasma to attack the interior surfaces or walls of the pumping annulus. This flow must be unobstructed in order for the vacuum pump to efficiently control the chamber pressure, and therefore the pedestal-to-side wall gap must be free of obstructions.  
           [0008]    It is an object of the invention to confine the plasma within the chamber without relying entirely on the chamber walls and in fact to confine the plasma in areas where the chamber walls to not confine the plasma. It is a related object of the invention to prevent plasma from leaking or flowing through openings necessarily provided the chamber walls. It is an auxiliary object to so prevent such plasma leakage without perturbing the plasma processing of the semiconductor wafer.  
           [0009]    It is a general object of the invention to shield selected surfaces of the reactor chamber interior from the plasma.  
           [0010]    It is a specific object of one embodiment of the invention to shield the interior surface of the reactor pumping annulus from the plasma by preventing plasma from flowing through the gap between the wafer pedestal and the chamber side wall without obstructing free flow of charge-neutral gas through the gap.  
           [0011]    It is a specific object of another embodiment of the invention to prevent plasma from flowing through the wafer slit valve in the chamber side wall without obstructing the ingress and egress of the wafer through the wafer slit valve.  
         SUMMARY OF THE DISCLOSURE  
         [0012]    The invention is embodied in a plasma reactor including a chamber enclosure having a process gas inlet and including a ceiling, a sidewall and a workpiece support pedestal capable of supporting a workpiece at a plasma processing location facing the ceiling, the workpiece processing location and ceiling defining a process region therebetween, the pedestal being spaced from said sidewall to define a pumping annulus therebetween having inner and outer walls, to permit process gas to be evacuated therethrough from the process region. The invention further includes a pair of opposing plasma confinement magnetic poles arranged adjacent the annulus within one of the inner and outer walls of the annulus, the opposing magnetic poles being axially displaced from one another the opposite poles being oriented to provide maximum magnetic flux in a direction across the annulus and a magnetic flux at the processing location less than the magnetic flux across the annulus.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a cut-away side view of a plasma reactor in accordance with a first embodiment of the invention employing open magnetic circuits.  
         [0014]    [0014]FIG. 2 is an enlarged view of the magnetic confinement apparatus near the pedestal-to-side wall gap.  
         [0015]    [0015]FIG. 3 is an enlarged view of the magnetic confinement apparatus near the wafer slit valve.  
         [0016]    [0016]FIGS. 4A and 4B correspond to a side view of a plasma reactor in accordance with a preferred embodiment of the invention employing closed magnetic circuits having pairs of opposed magnets.  
         [0017]    [0017]FIG. 5 is a perspective view of a pair of opposing ring magnets juxtaposed across the pedestal-to-side wall gap.  
         [0018]    [0018]FIG. 6 is a perspective view of a pair of opposing magnets juxtaposed across the wafer slit valve.  
         [0019]    [0019]FIG. 7 is a cut-away side view of a plasma reactor in which the closed magnetic circuit is a single magnet whose opposing poles are juxtaposed across the pedestal-to-side wall gap and which are joined by a core extending across the pumping annulus.  
         [0020]    [0020]FIG. 8 is a top view of the single magnet of FIG. 7 and showing the gas flow holes through the core joining the opposite poles of the magnet.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Conventional Reactor Elements:  
         [0022]    Referring to FIG. 1, an RF plasma reactor for processing a semiconductor wafer has a vacuum chamber  10  enclosed by a cylindrical side wall  12 , a ceiling  14  and a floor  16 . A wafer pedestal  18  supports a semiconductor wafer  20  which is to be processed. A plasma precursor gas is injected into the chamber  10  through a gas injector  22  from a gas supply  24 . Plasma source power is coupled into the chamber  10  in any one of several ways. For example, the reactor may be a “diode” configuration, in which case RF power is applied across a ceiling electrode  26  and the wafer pedestal  18 . This is accomplished by connecting the pedestal  18  and the ceiling electrode  26  to either one of two RF power sources  28 ,  30 . Alternatively, a cylindrical side coil  32  wound around the chamber side wall  12  is connected to an RF power source  34 . Alternatively to the foregoing, or in addition thereto, a top coil  36  is connected to an RF power supply. As is conventional, the wafer pedestal  18  may have its own independently controllable RF power supply  28  so that ion bombardment energy at the wafer surface can be controlled independently of plasma density, determined by the RF power applied to the coil  32  or the coil  36 .  
         [0023]    A vacuum pump  40  is coupled to the chamber  10  through a passage  42  in the floor  16 . The annular space between the periphery of the wafer pedestal  18  and the floor  16  forms a pumping annulus  44  through which the vacuum pump  40  evacuates gas from the chamber  10  to maintain a desired processing pressure in the chamber  10 . The pumping annulus  44  is coupled to the interior of the chamber  10  through an annular gap  46  between the periphery of the wafer pedestal  18  and the chamber side wall  14 . In order for the pump  40  to perform efficiently, the gap  46  is preferably free of obstructions.  
         [0024]    A conventional slit valve opening  50  of the type well-known in the art having a long thin opening in the chamber side wall  14  provides ingress and egress for a semiconductor wafer  52  to be placed upon and withdrawn from the wafer pedestal  18 .  
         [0025]    The walls  12 ,  14  confining the plasma within the chamber  10  are subject to attack from plasma ions and charged radicals, typically, for example, by ion bombardment. Such attack can consume the material in the walls  12 ,  14  or introduce incompatible material from the chamber walls  12 ,  14  into the plasma process carried out on the wafer  52 , thereby contaminating the process. Such incompatible material may be either the material of the chamber wall itself or may be material (e.g., polymer) previously deposited on the chamber walls during plasma processing, which can flake off or be sputtered off. Plasma reaching the chamber walls can cause polymer deposition thereon.  
         [0026]    The openings from the interior portion of the chamber  10 , including the pedestal-to-side wall gap  46  and the slit valve opening  50 , permit the plasma to leak or flow from the chamber  10 . Such leakage can reduce plasma density near the openings  46 ,  50 , thereby upsetting the plasma process carried out on the wafer  52 . Also, such leakage can permit the plasma to attack surfaces outside of the chamber interior. The flow of plasma into the pumping annulus  44  through the gap  46  permits the plasma to attack the interior surfaces or walls of the pumping annulus  44 . Thus, the designer must typically take into account not only the materials forming the chamber ceiling  12  and side wall  14 , but in addition must also take into account the materials forming the pumping annulus, including the lower portion  56  of the side wall  14 , the floor  16  and the bottom peripheral surface  58  of the wafer pedestal  18 , which complicates the design. Such a loss of plasma from the chamber  10  also reduces plasma density or requires more plasma source power to maintain a desired plasma density over the wafer  52 .  
         [0027]    Magnetic Confinement:  
         [0028]    In order to prevent plasma from flowing from the chamber  10  into the pumping annulus, a magnetic field perpendicular to the plane of the gap  46  and perpendicular to the direction of gas flow through the gap  46  is provided across the gap  46 . This is preferably accomplished by providing an opposing pair of magnetic poles  60 ,  62  juxtaposed in facing relationship across the gap  46 . In the embodiment according to FIG. 2, the magnetic pole  60  is the north pole of a magnet  64  located at the periphery of the wafer pedestal  18  while the magnetic pole  62  is the south pole of a magnet  66  next to the inner surface of the side wall  14 . The embodiment of FIG. 2 may be regarded as an open magnetic circuit because the returning magnetic field lines of flux  68  in FIG. 2 radiate outwardly as shown in the drawing.  
         [0029]    In order to prevent plasma from flowing from the chamber  10  through the slit valve opening  50 , a magnetic field perpendicular to the plane of the slit valve opening  50  and perpendicular to the direction of gas flow through the slit valve opening  50  is provided across the slit valve opening  50 . This is preferably accomplished by providing an opposing pair of magnetic poles  70 ,  72  juxtaposed in facing relationship across the slit valve opening  50 . In the embodiment according to FIG. 3, the magnetic pole  70  is the north pole of a magnet  74  extending across the bottom edge of the slit valve opening  50  while the magnetic pole  72  is the south pole of a magnet  76  extending along the top edge of the slit valve opening  50 . The embodiment of FIG. 3 may also be regarded as an open magnetic circuit because the returning magnetic field lines of flux  78  in FIG. 3 radiate outwardly as shown in the drawing.  
         [0030]    One potential problem with the returning lines of magnetic flux  68  (FIG. 2) and  78  (FIG. 3) is that some returning flux lines extend near the wafer  52  and may therefore distort or perturb plasma processing of the wafer  52 . In order to minimize or eliminate such a problem, a closed magnetic circuit (one in which returning magnetic lines of flux do not extend into the chamber) is employed to provide the opposing magnetic pole pairs  60 ,  62  and  70 ,  72 . For example, in the embodiment of FIGS. 4 and 5, the opposing magnetic poles  60 ,  62  across the gap  44  are each a pole of a respective horseshoe ring magnet  80 ,  82  concentric with the wafer pedestal  18 . The horseshoe ring magnet  80  has the north pole  60  and a south pole  81  while the horseshoe ring magnet has the south pole  62  and a north pole  83 . The poles  60 ,  81  of the inner horseshoe ring magnet  80  are annuli connected at their inner radii by a magnetic cylindrical core annulus  85 . Similarly, the poles  62 ,  83  of the outer horseshoe ring magnet  82  are annuli connected at their outer radii by a magnetic cylindrical core annulus  86 . The magnetic circuit consisting of the inner and outer horseshoe ring magnets  80 ,  82  is a closed circuit because the lines of magnetic flux between the opposing pole pairs  60 ,  62  and  81 ,  83  extend straight between the poles and, generally, do not curve outwardly, at least not to the extent of the outwardly curving returning lines of flux  68 ,  78  of FIGS. 2 and 3.  
         [0031]    In the embodiment of FIGS. 4A, 4B and  6 , the opposing magnetic poles  70 ,  72  across the slit valve opening  50  are each a pole of a respective long horseshoe magnet  90 ,  92  extending along the length of the slit valve opening  50 . The long horseshoe magnet  90  extends along the top boundary of the slit valve opening  50  while the other horseshoe magnet extends along bottom edge of the slit valve opening  50 .  
         [0032]    The advantage of the closed magnetic circuit embodiment of FIG. 4 is that the magnetic field confining the plasma does not tend to interfere with plasma processing on the wafer surface.  
         [0033]    In the embodiment of FIGS. 7 and 8, the lower annuli  81 ,  83  of the two horseshoe ring magnets  80 ,  82  are joined together as a single annulus by a magnetic core annulus  96 , so that the horseshoe ring magnets  80 ,  82  constitute a single horseshoe ring magnet  94  having a north pole  60  and a south pole  62 . The core annulus  96  extends across the pumping annulus  44  and can be protected by a protective coating  98  such as silicon nitride. In order to allow gas to pass through the pumping annulus  44 , the core annulus  96  has plural holes  100  extending therethrough.  
         [0034]    One advantage of the invention is that plasma ions are excluded from the pumping annulus  44 . This is advantageous because the pumping annulus interior surfaces can be formed of any convenient material without regard to its susceptibility to attack by plasma ions or compatibility of its sputter by-products with the plasma process carried out on the wafer. This also eliminates reduction in plasma density due to loss of plasma ions through the pumping annulus. Another advantage is that gas flow through the pedestal-to-side wall gap  46  is not obstructed even though plasma is confined to the interior chamber  10  over the wafer. Furthermore, by so confining the plasma to a smaller volume (i.e., in the portion of the chamber  10  directly overlying the wafer  52 ), the plasma density over the wafer  52  is enhanced. A further advantage is that stopping plasma ions from exiting through the slit valve opening  50  eliminates loss of plasma density over portions of the wafer  52  adjacent the slit valve opening  50 .  
         [0035]    In one example, each of the magnetic pole pair  60 ,  62  has a strength of 20 Gauss for a distance across the gap  46  of 5 cm, while each of the magnetic pole pair  70 ,  72  has a strength of 20 Gauss for a width of the slit valve opening  50  of 2 cm.  
         [0036]    While the invention has been described with reference to preferred embodiments in which the plasma confining magnets are protected from attack from plasma ions and processing gases by being at least partially encapsulated in the chamber walls or within the wafer pedestal or within a protective layer, in some embodiments (as for example, the embodiment of FIG. 6) the magnets may be protected by being located entirely outside of the chamber walls. Alternatively, if the reactor designer is willing to permit some plasma interaction with the magnets, magnets may be located inside the chamber in direct contact with the plasma, although this would not be preferred.  
         [0037]    While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.

Technology Category: 5