Abstract:
A sputtering target having an annular vault with a throat between two sidewalls and facing a substrate to be sputter coated. The vault is partially closed by a plate placed in the annular throat between the sidewalls. Thereby, the plasma density is increased within the vault. Furthermore, the position of the annular gap in the plate between the two sidewalls may be chosen to increase uniformity of sputtering deposition arising from the two sidewalls. The plate may be formed of one or more annular rings attached to the walls or a single plate having apertures formed therein may bridge the throat. Alternatively, the target may be formed as a cylindrical hollow cathode with the plate partially closing the circular throat. A rotating asymmetric roof magnetron may be combined with a hollow cathode without the restricting plate.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to plasma sputter reactors. In particular, the invention relates to complexly shaped sputter targets.  
         BACKGROUND ART  
         [0002]    Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. Most commercial sputter reactors rely upon magnetron sputtering in a plasma reactor. The most common commercial sputter reactor is a magnetron sputter reactor in which a metal target of the material to be sputter deposited is placed in opposition to the wafer to be sputter coated. The vacuum chamber containing the wafer and target is filled with a few milliTorr of argon. The target is then electrically biased to a few hundred volts DC, which excites the argon into a plasma. The resulting positively charged argon ions are attracted to the negatively biased target and dislodge (sputter) metal atoms from the target. Some of the metal atoms fall on the wafer and coat a thin metal layer on it. Typically, a set of magnets, called a magnetron, is placed in back of the target to create magnetic field lines parallel to the front face of the target, thereby trapping electrons and increasing the plasma density adjacent the target and thus increasing the sputtering rate. In reactive sputtering, a reactive gas such as nitrogen is also admitted to the chamber, and the reactive gas reacts with the sputtered metal atoms to form a metal compound, such as a metal nitride, on the wafer surface.  
           [0003]    The older, conventional magnetron sputter reactors produce a relatively low-density plasma of the argon ions and, as a result, the sputtered metal atoms are mostly neutral, only a few percent of them being ionized. It has become recognized in recent years that a higher fraction of metal ions would be very beneficial, particularly for coating the sides and bottoms of holes having high aspect ratios. Such holes may be via or contact holes or may be DRAM trenches. The mostly ballistic sputtering process described to this point is ill suited for reaching into holes having aspect ratios significantly larger than one at the same time that vias of modem integrated circuits often have aspect ratios of 5 and greater. However, it has been recognized that a negatively biased wafer can accelerate metal ions in the direction normal to the wafer surface, thereby draw the sputtered metal ions deep into the hole.  
           [0004]    Generally, increasing the density of the argon plasma increases the ionization fraction of the sputtered atoms. Several approaches have been used to produce a high density plasma. In one approach, additional RF energy is inductively coupled into a plasma source region remote from the wafer. In a second approach, often called a hollow cathode reactor, a non-planar target surrounds the top and sides of a plasma region adjacent the target, thereby reducing the plasma loss and increasing the plasma density. In a third approach, often called self-ionized plasma (SIP) sputtering, a small intense magnetron concentrates the target power in a reduced area, thereby increasing the power density and hence increasing the plasma density adjacent to the magnetron. The small magnetron is scanned around the target to produce more uniform sputtering.  
           [0005]    An advanced sputter reactor that advances on the second and third approach is the SIP+sputter reactor marketed by Applied Materials, Inc. of Santa Clara, Calif. and schematically illustrated in FIG. 1. Reactors of this type have been described by Gopalraj a et al. in U.S. Pat. No. 6,277,249 and U.S. patent application Ser. No. 09/703,601, filed Nov. 1, 2000, both of which are incorporated by reference herein in their entireties. The lower part of the reactor  10  includes an electrically grounded chamber including sidewalls  12  generally symmetric about a central axis  14 . A vacuum pumping system  16  reduces the base pressure within the chamber to the neighborhood of 10 −8  Torr. However, working gas is supplied from an argon source  18  through a mass flow controller  20  to maintain the argon pressure in a range of 0.1 to 10 milliTorr. If a nitride film is being formed by reactive sputtering, nitrogen is additionally supplied.  
           [0006]    A wafer  22  to be sputter coated is supported on a temperature controlled pedestal electrode  24 . The wafer  22  may be secured to the pedestal electrode  24  by a clamp ring  26 , but an electrostatic chuck may alternatively be used. A grounded shield  28  supported on the sidewalls  12  protects the chamber walls and sides of the pedestal  24  from being coated with sputtered material and further acts as a cathode for the diode sputtering process. The argon working gas is admitted into a processing space  30  over the wafer  22  through gaps between the pedestal  24 , the wafer clamp  26 , and the grounded shield  28 . The high density plasma being generated benefits from an electrically floating shield  32  supported on the grounded shield  28  through an isolator  34 .  
           [0007]    The SIP +  reactor  10  is most visibly distinguished by a target and magnetron assembly  40  including a vault-shaped target  42  supported on the chamber sidewalls  12  through a second isolator  43 . The target  42  is composed of the metal to be sputtered. Copper sputtering is the most prevalent initial use of the SIP +  reactor  10 , but other metals can be used in the target  42 . The vault-shaped target  42  includes an annular vault  44  extending around the central axis  14  with its open end or throat facing the wafer  22 . The vault  44  includes an outer sidewall  46 , an inner sidewall  48 , both extending generally parallel to the central axis  14 , and a roof  50  extending generally perpendicular to the central axis  14 . A central well  52  is formed on the back of the target  42  inside the annular inner sidewall  48 . The target  42  is supported on the isolator  43  by an outwardly extending flange  54 . A projection  56  extending downwardly from the outer sidewall  46  forms a plasma dark space in opposition to the floating shield  32 .  
           [0008]    A DC power source  58  electrically biases the target  42  to a negative voltage of about −600VDC with respect to the grounded shield  28 . This voltage is sufficient to maintain an argon plasma within the processing space  30 . If a substantial fraction of the sputtered atoms are ionized, it is advantageous to induce a negative DC bias on the pedestal electrode  24  by biasing it with an RF power supply  60  connected to the pedestal electrode  24  through an unillustrated capacitive coupling circuit. A controller  62  controls the sputtering process and may be programmed for a multi-step process according to which it separately controls the chamber pressure, target power and wafer bias.  
           [0009]    In magnetron sputtering, magnets are positioned in back of the target  42  to increase the plasma density adjacent to the face of the target  42 . The SIP +  target and magnetron assembly  40  includes both stationary and rotating magnetic parts. The stationary part includes a large number of permanent magnets  70  of a first vertical polarity arranged around the outside of the outer vault sidewall  46 . A cylindrical magnet  72  of an opposite second vertical polarity is disposed within the vault well  52  behind and inside the vault inner sidewall  48 . Although the cylindrical magnet  72  is rotating for reasons relating to unillustrated target cooling, its magnetic field is essentially stationary. The two sets of magnets  70 ,  72  create anti-parallel magnetic fields close to interior sides of the vault  44  adjacent the opposed sidewalls  46 ,  48 . The rotating part includes a nested magnetron  74  positioned over the vault roof  50  and including an outer annular magnet  76  of the first magnetic polarity surrounding an inner cylindrical magnet  78  of the second magnetic polarity. The nested magnetron  74  is unbalanced in that the total (spatially integrated) magnetic flux produced by the outer magnet  76  is at least 50% larger than that produced by the inner magnet  78 .  
           [0010]    The roof magnetron  74  is supported on a magnetic yoke  80  fixed to a shaft  82  extending along the central axis  14  and rotated by an unillustrated motor so as to sweep the roof magnetron  14  along the circumference of the roof  50  of the target vault  44 . The inner sidewall magnet  72  is also supported through a non-magnetic spacer  84  connected to the shaft  82  although this rotation is not immediately pertinent to the physics of the sputtering process.  
           [0011]    The described magnetron in conjunction with the annularly vaulted target offers many advantages. The vault creates a region closed on three sides so that plasma leakage out of the sputtering region is minimized and the plasma density is increased. The magnetic field components running parallel to the target sidewalls  46 ,  48  and to the roof  50  further increase the plasma density near the target areas being sputtered. The relatively small roof magnetron  74  concentrates the sputtering in the area of the vault  44  over which the roof magnetron is passing, thus concentrating the limited target power there and increasing the target power density. Sputtering into high aspect-ratio holes is facilitated by a large fraction of ionized sputtered metal particles which can be attracted into the holes by biasing the wafer. The SIP +  reactor is believed to be capable of a metal ionization fraction of about 50%. The combination of a stationary distributed magnetic field and a rotating localized magnetic field allows the magnetron to operate in two distinct sputtering modes, believed to be associated with sputtering around the entire annular vault and with sputtering in the area of the roof magnet.  
           [0012]    Nonetheless, the SIP +  reactor could be further improved. In at least some applications, particularly those involving extreme aspect ratios of ten and more, it is desired to further increase the ionization fraction since any neutral sputter component is approximately isotropic, a cosine distribution off the normal between the target and wafer being assumed. As mentioned before, SIP +  sputter reactors as presently implemented seem to be limited to about a 50% metal ionization fraction. The ionization rate in SIP +  reactors is practically limited by the plasma density produced by the still relatively low target power. The localized sidewall and magnetic field confinement still allows excessive plasma leakage from the high-density plasma region.  
           [0013]    Although SIP +  targets provide relatively good sputtering uniformity, the sputtering uniformity on sidewalls across the wafers could be improved. The geometry of the target  42  and the wafer  22  with its high aspect-ratio holes  90  is illustrated in the cross-sectional view of FIG. 2. The holes  90  will hereafter be referred to as vias because this type of vertical connection through an inter-level dielectric between two metallization levels is a major application. The thickness of the wafer  22  is greatly exaggerated, but the geometry of the vias  90  is approximately correct. It has been observed that the maximum target erosion occurs at the outer vault sidewall  46 . That is, the greatest sputtering rate occurs at the outer sidewall  46 . In the usual configuration, the diameter of the wafer  22  generally extends from approximately one side to the other of the middle of the annular vault  44 . If the vias  90  are located near the periphery of the wafer  22 , this geometry exposes the hole inner sidewall  92  to the full brnt of the target sidewall sputtering. As a result, the inner hole sidewall  92  is subject to a larger flux of neutral target atoms than is the outer hole sidewall  94 . This differential flux tends to form an overhang  96  on the inside hole rim  96 , which has the possibility of closing off the hole  90 . Other geometries may favor inner sidewall deposition. Sidewall coverage is critical for formation of a thin copper seed layer. To minimize seed deposition times and prevent premature via closure, the sidewall deposition should be uniform across the wafer.  
           [0014]    The annularly vaulted target is related to a well known hollow-cathode target, for example, that described by Lai et al. in U.S. Pat. No. 6,193,854 and by Lai in U.S. Pat. No. 6,217,716 although significant differences exist in both the geometry and effect of the magnetic fields. Such a hollow-cathode includes a single cylindrical vault arranged about the chamber axis and having a tubular sidewall and a roof bridging the sidewall. The cited references describe several magnet configurations. The hollow cathode is in turn related to an effusion cell or partially closed hollow sputtering target, such as that described by Glocker in U.S. Pat. No. 5,069,770. In the effusion cell, the throat of the cylindrical hollow target facing the wafer is partially closed with a narrow opening at its symmetric center facing the wafer. The effusion cell can be likened to a black-body radiator in which an intense plasma develops within the cell&#39;s interior with only a relatively small portion leaking through the central aperture towards the wafer. This geometry does not address the problem of sidewall uniformity. Glocker&#39;s effusion cell is distinguished from a more conventional hollow cathode in that the effusion cell includes both an anode and a cathode within the target cavity.  
         SUMMARY OF THE INVENTION  
         [0015]    A sputtering target having an annular vault arranged about its side facing the substrate being sputter coated. The throat of the vault is partially closed. The throat ring may be arranged adjacent the inner sidewall or the outer sidewall of the vault or adjacent both sidewalls so as to form a smaller annular throat. Alternatively, the throat ring may be formed with a circular arrangement of holes. The holes may be circular or circumferentially elongated, and they may be formed in multiple concentric circles.  
           [0016]    The invention may also be applied to a hollow cathode target having a cylindrical vault.  
           [0017]    The partially closed throat more effectively confines the plasma within the vault, thereby increasing the plasma density and the ionization fraction of sputtered atoms. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a schematic cross-sectional view of a magnetron sputter reactor including an annularly vaulted target.  
         [0019]    [0019]FIG. 2 is a cross-sectional view illustrating the asymmetric deposition produced by a prior annularly vaulted target.  
         [0020]    [0020]FIG. 3 is a schematic cross-sectional view of a magnetron sputter reactor in accordance with the invention including a first embodiment of a partially enclosed annularly vaulted target.  
         [0021]    [0021]FIG. 4 is a cross-sectional view of the first embodiment of the partially enclosed annularly vaulted target.  
         [0022]    [0022]FIG. 5 is a cross-sectional view of a second embodiment of the partially enclosed annularly vaulted target.  
         [0023]    [0023]FIG. 6 is cross-sectional view of a third embodiment of the partially enclosed annularly vaulted target.  
         [0024]    [0024]FIG. 7 is an axial plan view of a fourth embodiment including a perforated throat plate.  
         [0025]    [0025]FIG. 8 is an axial plan view of a fifth embodiment including a throat plate with multiple rings of perforated segments.  
         [0026]    [0026]FIG. 9 is a cross-sectional view of a partially enclosed hollow cathode target.  
         [0027]    [0027]FIG. 10 is a cross-section view of a fully open hollow cathode target having a rotating roof magnet. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    In one embodiment of the invention illustrated in the cross-sectional view of the entire reactor  40  in FIG. 3 and in the more detailed cross-sectional view of an inventive target  110  in FIG. 4, the target  110  includes an annularly arranged vault  112  similar to the vault  44  of FIG. 1.  
         [0029]    However, an annular outerly radially extending portion  116  of the target  110  partially closes the vault  112  at its bottom or throat  114  facing the wafer  22  and is electrically fixed to the remainder of the target  110 . In this embodiment, the radially extending portion  116  is located on only one side, the radially inner side, of the throat  114 . The radially extending portion  116  will be referred to as a throat ring  116 , which in the illustrated embodiment produces an annular throat  114  that is symmetric about the center axis  14  but is radially asymmetric with respect to the two sidewalls  46 ,  48 . The throat ring  116  is preferably composed of the same material as the rest of the target  110 , for example, copper with less than 10 wt % of alloying elements such as silicon or magnesium Preferably, the throat ring  116  causes the throat  114  to have an area extending circumferentially around the vault  112  that is no more than two-thirds the area of a throat without the throat ring  116 . More preferably, the area is no more than one-half the area with the throat ring  116 . However, the size of the throat must be large enough to not significantly reduce the formation of the plasma in the vault  112 . Although the magnetic confinement with the vault  112  intensifies the plasma there, it is also important that the argon plasma not be impeded from entering the vault  112 . Therefore, the minimum dimension of the throat  114  should be at least as great as the argon mean free path at the operational pressure. The argon mean free path at 1 milliTorr is about 1 cm.  
         [0030]    The restricted throat has several effects. When the throat ring is attached to the inner sidewall  48 , the path between the inner sidewall  48  and the wafer being sputter coated is partially blocked, thereby reducing the preferential coating from the inner target sidewall  48  onto the outer via sidewalls of the wafer. The partial closing of the throat  114  also reduces the plasma loss out of the vault  112 . In particular, the magnetic field lines closely parallel to the inner sidewall  48  may have both ends terminating on either the target roof  50  or the throat ring  116  so that electrons trapped on these field lines are confined to the vault  112 . Thereby, the plasma density within the vault  112  is increased, and the ionization fraction of sputtered atoms is also increased.  
         [0031]    The more conventional vaulted target  42  of FIG. 1 with the more open vault  44  can be easily formed by machining or forging. The inventive target  110  with a partially closed vault  112  is not so easily machined or forged. However, the target parts excluding the throat ring  116  can be machined or forged as a single piece. A separate annular throat ring  116  may be machined or otherwise formed and then permanently attached to the inner vault sidewall  48  by e-beam welding or other types of welding. Autogenous welding is preferred since the use of a non-copper filler (that is in the more general case, a material other than the composition of the target) will contaminate the target. Assembly and welding is facilitating by forming the throat ring  116  with a horizontally extending annular projection  118 , which supports the unwelded ring  116  in an exposed position on a corresponding ledge of the inner sidewall  48  of the inverted target.  
         [0032]    The target may be formed as an integral member by machining the throat ring from a wider sidewall. It is also possible to use screws, rivets, keys, force pins, or other fasteners to attach the throat ring to the sidewall. However, the fastening member is preferably made of the same material as the target.  
         [0033]    A second embodiment of a vaulted target  120 , illustrated in cross section in FIG. 5, includes a throat ring  122  fixed to and extending radially inwardly from the outer vault sidewall  46 . This structure creates a reduced throat  124  asymmetrically located closer to the inner sidewall  48  than to the outer sidewall  48 . As a result, the wafer is protected from particles sputtered from the outer sidewall  46  and decreases the formation of the lip  98 , illustrated in FIG. 2, on the inner via sidewall  92 .  
         [0034]    A third embodiment of a vaulted target  128 , illustrated in cross section in FIG. 6, includes both the inner throat ring  116  attached to the inner sidewall  48  and the outer throat ring  122  attached to the outer sidewall  46 . This structure creates a reduced annular throat  128  that is more centrally located. However, even if the throat  128  is symmetrically located between the two sidewalls  46 ,  48 , the cylindrical geometry results in a non-symmetric sputtering pattern.  
         [0035]    The various embodiments of the vaulted target of the invention may be used in combination with the magnetron illustrated in FIG. 3. However, other magnet distributions may be used. In particular, in view of the reduced plasma loss through the reduced-area throat, the close magnetic confinement afforded by the magnets of FIG. 3 may not be required. Instead, other and less confining magnetic field configurations may be used. For example, the sidewall magnetic fields may be parallel rather than anti-parallel, the roof magnets may be eliminated, or the sidewall magnets may be horizontally oriented in parallel and produce a fairly uniform radial field across the vault.  
         [0036]    The plasma confinement can be further increased by utilizing a throat ring  130  illustrated in axial plan view in FIG. 7 having a plurality of apertures  132 , for example circularly shaped apertures  132 , distributed around the circumference of the target. Preferably, the number of apertures is eight or greater, more preferably at least twelve or sixteen. The throat ring  130  underlies the target vault  112  and is fixed to both the outer and inner sidewalls  46 ,  48 . Such a throat ring  130  introduces some difficulties in uniform sputtering deposition but greatly increases the plasma density within the vault. The uniformity can be increased by forming the apertures  132  in more rectangular arc shapes, in which case the number of apertures may be decreased.  
         [0037]    Another variation illustrated in axial plan view in FIG. 8 includes a plurality of elongated arc-shaped segments  142   a ,  142   b  arranged in multiple circles. Within each circle, the plural segments  142   a ,  142   b  are separated by struts  144 , preferably offset between different circles. The circles are separated by a circular band  146  and are surrounded by other circular bands  148 ,  150  respectively adjacent the outer and inner sidewalls  46 ,  48 . The bands  146 ,  148 ,  150  more closely confine the plasma inside the vault. The width of the segments  142   a ,  142   b  is chosen to be wide enough to first be larger than a plasma dark space and secondly to assure that the sputtering occurs within the vault and not predominantly on the exposed face of the throat ring  140 . The second condition requires that the throat be wide enough that the throat ring not ground out the plasma and prevent a plasma depletion zone from forming within the vault. However, the sputtering rate is largely controlled by the magnetic field parallel to the target face. This field is configured to be much larger inside the vault than on the exposed throat ring. A combination of the embodiments of FIGS. 7 and 8 includes a single ring of arc-shaped segments  142   a.    
         [0038]    Although the invention has been described with respect to an annular vault with a generally rectangular cross section formed by parallel sidewalls and a roof, other vault shapes are possible. For example, the vault may be shaped as a partial toroid, as a triangle, or as a truncated triangle with the base partially closed by the throat plate.  
         [0039]    Furthermore, although the invention has been developed for the annular vault of the SIP +  sputter reactor, some aspects of the invention can be applied to a cylindrical vault, similar to the hollow cathode target of Lai et al. As illustrated in the cross-sectional view of FIG. 9, a constricted hollow cathode  160  includes a conventionally shaped principal hollow cathode target  162  having a disc-shaped top wall  164  and a tubular sidewall  166  arranged about a central axis  168  to define a cylindrical vault  170 . An annular throat ring  172  is mechanically and electrically fixed to the bottom of the target sidewall  166  opposite the top wall  164  to define a restricted throat  174  into the vault  170 . The area of the restricted throat  174  is preferably no more than two-thirds that of the full throat defined between the sidewall  166 , and more preferably no more than half.  
         [0040]    A flange  176  extending radially outwardly from the bottom of the sidewall  166  may be used to support the target  162  on the chamber body. However, throat ring  172  may be used for the same purpose. The relative axial positions of the throat ring  172  and flange  176  may be varied.  
         [0041]    A tubular sidewall magnet assembly  178  is arranged around the outside of the sidewall  166  and has a first magnetic polarity along the central axis  168 . The magnet assembly  178  may consist of a single tubular magnet or a set of similarly magnetized cylindrical magnets arranged in a circle. If the sidewall magnet assembly  178  surrounds only a central portion of the target sidewall  166  or more generally does not extend along the bottom portion of the sidewall  166 , its magnetic field is largely confined to within the vault  170 , thereby creating a strong magnetic field parallel to the target sidewall  166  and minimizing plasma loss out of the vault  170 .  
         [0042]    It may be advantageous to additionally include a small rotating roof magnetron  74  asymmetric with respect to the central axis  182  about which it rotates, as has been described with reference to FIG. 3. If the magnetron  74  is unbalanced and the magnetic polarity of the outer magnet ring  76  is the same as that of the sidewall magnet assembly  178 , a very strong magnetic field is created at the top corner between the roof  164  and the sidewall  166 . This effect as well as the localized high-density plasma produced by the roof magnetron are advantageously obtained in an open-throated hollow cathode  180  illustrated in FIG. 10 which lacks the throat ring  172 , thereby resulting in an unrestricted or open throat  182 . Also in this case, an elongated tubular magnet assembly  184  may extend along most of the vault sidewall  184  and particularly its bottom to better achieve Lai&#39;s magnetic null at a location below the non-occluded throat  182 .  
         [0043]    Other forms of the roof magnetron may be used, including balanced magnetrons and magnetrons formed of two bands of opposed poles. Nonetheless, with or without the roof magnetron  74 , the constricted throat  174  of FIG. 9 better confines the plasma to the vault  170  and thereby increases the plasma density and ionization fraction.  
         [0044]    Although the invention has been described with respect to the important application of copper deposition, other metal and metal compounds may be deposited using the novel target of the invention. Aluminum metallization is well known. Refractory metals such as tungsten, titanium, molybdenum, and cobalt are used in metallizing vias and other structures. Refractory nitrides are deposited by using a refractory metal target and a nitrogen ambient.  
         [0045]    The plasma confinement within the partially closed vault relaxes the need for magnetic confinement. As a result, other magnetic configurations may be used. In one modification, the rotating nested roof magnetron may not be needed to achieve a satisfactorily high plasma density, thereby greatly simplifying the magnetron design. In another modification. The sidewall magnets may have the same vertical polarity or may be arranged horizontally to produce a radial magnet field across the vault.  
         [0046]    The invention thus allows a significant increase in the capability of a vaulted target with only a small increase in the target&#39;s complexity.