Abstract:
A split magnet ring, particularly useful in a magnetron plasma reactor sputter depositing tantalum or tungsten or other barrier metal into a via and also resputter etching the deposited material from the bottom of the via onto the via sidewalls. The magnet ring includes two annular magnet rings composed of the same axial polarity separated by a non-magnetic spacing of at least the axial length of one magnet and associated poles. A small unbalanced magnetrons rotates about the back of the target having an outer pole of the same polarity as the ring magnets surrounding a weaker inner pole of the opposite pole.

Description:
RELATED APPLICATION 
   This application claims benefit of provisional application 60/663,568, filed Mar. 18, 2005. 

   FIELD OF THE INVENTION 
   The invention relates generally to sputtering of materials. In particular, the invention relates to auxiliary magnets used to improve uniformity in a magnetron sputter reactor. 
   BACKGROUND ART 
   Sputtering, alternatively called physical vapor deposition (PVD), is commonly used in depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. Recent technology developed for copper metallization in silicon integrated circuits has emphasized sputtering of refractory metals such as tantalum for use as a barrier layer in a interconnect hole structure etched into a dielectric and sputtering of copper for use as a seed layer for the final electroplating of copper to fill the hole. The requirements placed upon sputtering have intensified as the diameter of the interconnect hole has shrunk to below 100 nm and as the aspect ratio of holes has increased to 5 and above. 
   Advanced sputter reactors have been developed with complexly shaped targets and inductive power sources, which for the most part are intended to increase the ionization fraction of the sputtered atoms. Thereby, biasing of the wafer attracts the ionized sputtered atoms to deep within holes and also to sputter etch overhangs and undesired layers at the bottom of the holes. However, fairly conventional sputter reactors continue to be used even in advanced applications because of their simplicity and low cost. The conventional sputter reactor is modified with sophisticated magnetics to achieve many of the performance characteristics of the more complex sputter reactors. 
   Gung et al in U.S. Pat. No. 6,610,184, incorporated herein by reference, hereafter Gung, disclose a plasma sputtering reactor  10  illustrated in the schematic cross-section view of  FIG. 1 . A vacuum chamber  12  includes generally cylindrical sidewalls  14 , which are electrically grounded. Typically, an unillustrated grounded replaceable shield and sometimes an additional floating shield are located inside the sidewalls  14  to protect them from being sputter coated, but they act as chamber sidewalls except for holding a vacuum. A sputtering target  16  having at least a surface layer composed of the metal to be sputtered is sealed to the chamber  12  through an electrical isolator  18 . A pedestal electrode  22  supports a wafer  24  to be sputter coated in parallel opposition to the target  16 . A processing space is defined between the target  16  and the wafer  24  inside of the shields. 
   A sputtering working gas, preferably argon, is metered into the chamber from a gas supply  26  through a mass flow controller  28 . An unillustrated vacuum pumping system maintains the interior of the chamber  12  at a very low base pressure of typically 10 −8  Torr or less. During plasma ignition, the argon pressure is supplied in an amount producing a chamber pressure of approximately 5 milliTorr, but as will be explained later the pressure is thereafter decreased. A DC power supply  34  negatively biases the target  16  to approximately −600 VDC causing the argon working gas to be excited into a plasma containing electrons and positive argon ions. The positive argon ions are attracted to the negatively biased target  16  and sputter metal atoms from the target. 
   The invention is particularly useful with self-ionized plasma (SIP) sputtering in which a small nested magnetron  36  is supported on an unillustrated back plate behind the target  16 . The chamber  12  and target  16  are generally circularly symmetric about a central axis  38 . The SIP magnetron  36  includes an inner magnet pole  40  of a first vertical magnetic polarity and a surrounding outer magnet pole  42  of the opposed second vertical magnetic polarity. Both poles are supported by and magnetically coupled through a magnetic yoke  44 . The yoke  44  is fixed to a rotation arm  46  supported on a rotation shaft  48  extending along the central axis  38 . A motor  50  connected to the shaft  48  causes the magnetron  36  to rotate about the central axis  38 . 
   In an unbalanced magnetron, the outer pole  42  has a total magnetic flux integrated over its area that is larger than that produced by the inner pole  40 , preferably having a ratio of the magnetic intensities of at least 150%. The opposed magnetic poles  40 ,  42  create a magnetic field inside the chamber  12  that is generally semi-toroidal with strong components parallel and close to the face of the target  16  to create a high-density plasma there to thereby increase the sputtering rate and increase the ionization fraction of the sputtered metal atoms. Because the outer pole  42  is magnetically stronger than the inner pole  40 , a fraction of the magnetic field from the outer pole  42  projects far towards the pedestal  22  before it loops back to behind the outer pole  42  to complete the magnetic circuit. 
   An RF power supply  54 , for example, having a frequency of 13.56 MHz is connected to the pedestal electrode  22  to create a negative self-bias on the wafer  24 . The bias attracts the positively charged metal atoms across the sheath of the adjacent plasma, thereby coating the sides and bottoms of high aspect-ratio holes in the wafer, such as, inter-level vias. 
   In SIP sputtering, the magnetron is small and has a high magnetic strength and a high amount of DC power is applied to the target so that the plasma density rises to above 10 10  cm −3  near the target  16 . In the presence of this plasma density, a large number of sputtered atoms are ionized into positively charged metal ions. The metal ion density is high enough that a large number of them are attracted back to the target to sputter yet further metal ions. As a result, the metal ions can at least partially replace the argon ions as the effective working species in the sputtering process. That is, the argon pressure can be reduced. The reduced pressure has the advantage of reducing scattering and deionization of the metal ions. For copper sputtering, under some conditions it is possible in a process called sustained self-sputtering (SSS) to completely eliminate the argon working gas once the plasma has been ignited. For aluminum or tungsten sputtering, SSS is not possible, but the argon pressure can be substantially reduced from the pressures used in conventional sputtering, for example, to less than 1 milliTorr. 
   An auxiliary array  60  of permanent magnets  62  is positioned around the chamber sidewalls  14  and is generally positioned in the half of the processing space towards the wafer  24 . The auxiliary magnets  62  have the same first vertical magnetic polarity as the outer pole  42  of the nested magnetron  36  so as to draw down the unbalanced portion of the magnetic field from the outer pole  42 . In the embodiment described in detail below, there are eight permanent magnets, but any number of four or more distributed around the central axis  38  would provide similarly good results. It is possible to place the auxiliary magnets  62  inside the chamber sidewalls  14  but preferably outside the thin sidewall shield to increase their effective strength in the processing region. However, placement outside the sidewalls  14  is preferred for overall processing results. 
   The auxiliary magnet array  62  is generally symmetrically disposed about the central axis  38  to produce a circularly symmetric magnetic field. On the other hand, the nested magnetron  36  has a magnetic field distribution is asymmetrically disposed about the central axis  38  although, when it is averaged over the rotation time, it becomes symmetric. There are many forms of the nested magnetron  36 . The simplest though less preferred form has a button center magnetic pole  40  surround by an circularly annular outer magnetic pole  42  such that its field is symmetric about an axis displaced from the chamber axis  38  and the nested magnetron axis is rotated about the chamber axis  38 . One such nested magnetron has a triangular shape with an apex near the central axis  38  and a base near the periphery of the target  16 . This shape is particularly advantageous because the time average of the magnetic field is more uniform than for a circular nested magnetron. 
   Gung describes the effects of their magnetic elements. The unbalanced magnetron  36  creates a semi-toroidal magnetic field B M  that is generally parallel to the sputtering face of the target  16  to thereby trap electrons, increase the plasma density, and hence increase the sputtering rate. Because of the imbalance, a substantial unmatched magnetic field emanates from the outer pole  42  creating both a return magnetic field B A1 , which projects into the chamber  12  near the chamber center  38  but returns to the back of the magnetron  36 , and a sidewall magnetic field B A2  near the chamber sidewall  14 . The sidewall magnetic field B A2  is drawn toward the similarly polarized auxiliary array  62  before it returns to the back of the magnetron  42 . Gung describes the beneficial effects of such an arrangement as extending the plasma and guiding the ionized sputter particles towards the wafer  24 . He further describes the improved radial uniformity of deposition of copper films. 
   The Gung configuration has been advantageously applied to copper deposition, particularly for a thin copper seed layer into a narrow via hole formed through an inter-level dielectric for forming a vertical interconnect as well as for horizontal interconnects in the commercially important dual-damascene structure. The copper seed layer is used as a seed and electroplating layer for the subsequent filling of the via hole by electrochemical plating (ECP). In this application, overhang is a considerable problem. On the other hand, when the Gung configuration is applied to sputtering a tantalum barrier layer between the walls of the via hole and the copper seed layer, the resulting uniformity was not completely satisfactory. In this barrier application, sidewall coverage and uniformity deep in the via hole is more important. 
   SUMMARY OF THE INVENTION 
   An auxiliary magnet assembly is positioned around the processing area of a plasma sputter reactor, preferably outside of the chamber wall. It includes at least two magnet rings of the same magnetic polarity separated by a spacing of non-magnetic or reduced magnetic material or space, preferably having an axial length at least as long as either of the rings and more preferably at least twice as long. 
   The two magnet rings are conveniently formed in a two-piece non-magnetic collar fixable together on the chamber exterior. The collar has two inwardly facing ribs with recesses for magnets. Two pairs of ring-shaped magnetic capture the magnets within the recesses and act as magnetic yokes. 
   A sputtering method for a metal such as a refractory metal such as tantalum, titanium, or tungsten may use the split magnet ring and the inter-ring spacing may be optimized for the process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a magnetron sputter reactor of the prior art including a single auxiliary magnet ring. 
       FIG. 2  is a schematic illustration of the magnetic field produced by the single magnet ring of  FIG. 1 . 
       FIG. 3  is a schematic illustration of the magnetic field produced by the split magnet ring of one embodiment of the invention. 
       FIG. 4  is a cross-section view of magnetron sputter reactor of the invention including the split magnet ring of  FIG. 3 . 
       FIG. 5  is an orthographic view of a two-piece collar incorporating a split magnet ring of the invention. 
       FIG. 6  is a graph illustrating the improved uniformity achievable with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   I believe that the improved uniformity achieved by Gung is in part achieved by the magnet ring  62  producing a generally semi-toroidal magnetic field  64  that resembles a dipole field adjacent the chamber sidewall  14  or the shields on the side of the chamber away from the rotating magnetron  36  but also exists on the side of the chamber  12  temporarily aligned with the rotating magnetron  36 . As shown in more detail in the schematic elevation view of  FIG. 2 , the magnetic field  64  produced by the magnet ring  60  is a magnet dipole field except for unimportant secondary effects due to annular form of the magnet ring  60 . Inside the chamber sidewall  14 , the dipole field  64  creates a magnetic barrier against the diffusion of the plasma, in particular its electrons, to the grounded chamber sidewall  14 . As a result, the plasma containing the sputtered metal ions which diffuses from the target  16  near the magnetron  46  is prevented from diffusing to the grounded wall  14 . Such a diffusing plasma results in a plasma that is stronger at the chamber center  38  than nearer its edge. Such a non-uniform plasma as it approaches the wafer  24  being sputter deposited and otherwise plasma processed insults in a strong radial non-uniformity on the wafer  24 . With the reduced sidewall diffusion, the plasma becomes more uniform in the radial direction resulting in more uniform wafer processing. 
   However, the dipole field  64  suffers some disadvantages. As illustrated, it bulges inwardly into the chamber near the midline of the magnet ring  60 . That is, the dipole field  64  bulges towards the chamber central axis  38  and creates a significantly concave barrier. As a result, the plasma is confined within the inwardly concave barrier and the ionized sputter particles are somewhat focused towards the center of the wafer  24 , resulting in uneven sputter deposition but more particularly sputter etching of the wafer  24 . 
   The sputter etching of the wafer  24  is particularly important for coating the sidewalls of a narrow deep via of high aspect ratio. Barrier metals such as refractory metals including titanium, molybdenum, tantalum, tungsten, cobalt, chromium, and ruthenium have reduced but significant electrical conductivity and their nitrides, which may be sputter deposited in the magnetron sputter reactor by reactive sputter, are poor conductors. If the sputter flux has a high ionization fraction and the wafer is strongly biased, the ions are drawn deep within the via to coat the bottom via sidewalls. What portion of the flux strikes and is deposited on the bottom of the via is likely to be resputtered simultaneously or subsequently and be deposited on the bottom via sidewalls. Hence, the process reduces or eliminates the barrier layer at the bottom, where it is not required against the underlying metal level, and increases the sidewall coverage. 
   The effect of a bulging magnetic barrier seems not to not be significant for the present generation of copper sputtering (though it may become so in future generations). However, the sputtering tantalum with the configuration of  FIG. 1  produces poor radial uniformity of sidewall asymmetry and bottom deposition and resputtering. Copper and tantalum are distinctly different materials. The target resputtering yield is significantly different between the two results in a significantly higher ionization fraction for copper to the extent that sustained self-sputtering is possible with copper but not tantalum. That is, for copper sputtering, after plasma ignition, the argon sputtering gas may be turned off and the sputtered copper ions will act as the sputtering gas to support the plasma. Also, the significantly different masses of copper and tantalum will produce significantly different rates of sputter etching within the vias. 
   The plasma can be better confined and produce more uniform sputter deposition and etching by flattening the magnetic field adjacent the chamber sidewall  14  or associated shield. The flattening can be achieved by splitting the magnet ring into two or more magnet rings separated by space or other dielectric. As schematically illustrated in the elevational view of  FIG. 3 , a split magnet ring  70  includes two magnet sub-rings  72 ,  74  of the same polarity with a separation or axial spacing  76  that is non-magnetic or at least of substantially reduced magnetic permeability from that of the two magnet sub-rings  72 ,  74 . Each sub-ring  72 ,  74  produces a respective substantially dipolar magnetic field. However, a resultant combined split ring magnetic field  78  is substantially flattened, especially on the interior of the sidewall  14 , because of the non-magnetic spacing  76 . As a result, the combined magnetic field  78  acts as an effective barrier adjacent the chamber sidewall  14  to prevent plasma from diffusing to the grounded sidewall  14  or shield, but with significantly reduced focusing of the plasma toward the center  38  of the chamber  12 . 
   This configuration has the further advantage that magnetic saturation of the magnet rings  72 ,  74  is reduced. As a result, the average magnetic field density produced by the split magnet ring  70  is increased over what would be produced if the magnet rings  72 ,  74  using the same magnets were continuous or placed adjacent each other with no spacing  76  between them. 
   A sputter reactor  80  of the invention is illustrated in the schematic cross-sectional view of  FIG. 4  including the split magnet ring  70 . An estimated magnetic field distribution  82  underlying the unbalanced roof magnetron  36  combines the unbalanced field from the magnetron  36  and the split magnet ring  72 . 
   The magnetron  36  is preferentially the unbalanced LDR magnetron having an arc shape of a closed plasma loop, as disclosed by Gung et al. in U.S. patent application Ser. No. 10/949,735, filed Sep. 23, 2004 and now published as U.S. Published Patent Application 2005/0211548, incorporated herein by reference. In its sputtering position, the convex side of the arc shape is close to the periphery of the target  16  so that its magnetic field is concentrated near the target periphery. The magnetron  36  can be switched by a centrifugal mechanism so the arc shape more closely aligns with the target radius to thereby clean the central portions of the target  16  between depositions. 
   A split magnet ring assembly  90  illustrated in the orthographic view of  FIG. 5  includes two half collars  92 ,  94  composed of non-magnetic material such as aluminum. The two half collars  92 ,  94  can be joined together with alignment pins  96  and screws  98  around the exterior of the chamber sidewall  14  and screwed to supports on the sidewall  14  through vertical through holes  100 . Each half collar  92 ,  94  includes two annular inwardly facing ribs  102  having recesses to accommodate a plurality, for example, eight vertically polarized rod magnets  104 . Each magnet  104  has an exemplary length of about 15 cm and an exemplary diameter of 6 mm and may be composed of NdBFe. That is, there are two sets of sixteen magnets  14  (divided between the two half collars  92 ,  94 ) arranged about the central axis for a chamber configured for 300 mm wafers. The vertical spacing between the magnets  14  may be varied to optimize deposition uniformity. A typical range is 25 to 44 mm, that is, greater than the length of the individual magnets and preferably at least twice the magnet length but less than four times the magnet length, plus the thickness of the associated pole faces. Screws capture the magnets  14  on the ribs  102  through two pairs of washer-shaped holders  106  composed of magnetic material, for example, SS410 stainless steel, and disposed on opposing vertical spaced sides of the ribs  102  to act not only as holders but also as magnetic pole faces. 
   In general, the sidewall magnets are effective only in the presence of significant wafer biasing, for example, 800W RF power for a 300 mm wafer, in order to resputter the tantalum deposited on the via bottom onto the lower via sidewalls. The biasing draws the ionized sputter ions also affected by the auxiliary sidewall magnets while neutral sputter atoms are primarily unaffected by either wafer biasing or sidewall magnets. Sputtering uniformity tests were performed using various ring magnets for sputtering tantalum. Sheet resistance R S  was measured for a deposited tantalum film to determine the deposition uniformity across the wafer radius. As shown by the graph of  FIG. 6 , either a single sidewall magnet ring, as taught by Gung, or a split magnet ring with no spacing between the two rings produce about the same high non-uniformity, generally considered unsatisfactory. Split magnet rings with spacings of 25 mm and 44 mm significantly reduce the non-uniformity. Further experiments have demonstrated that the split magnet ring is effective at increasing the resputtering near the wafer edge relative to the generally higher resputtering at the wafer center. 
   The split magnet ring has also been applied to sputtering titanium. In this case, the spacing between the two magnet rings was reduced by 2 mm to optimize the performance. The design freedom of varying the spacing in different applications is one advantage of the split magnet ring. 
   It is possible to have three or more magnet sub-rings with non-magnetic spacings between them. 
   Although the invention has been described with reference to sputtering tantalum and titanium, it is applicable to sputtering other materials, particularly barrier metals. Experiments have shown the usefulness of the invention to sputtering tungsten.