Patent Publication Number: US-2007095654-A1

Title: Controlled multi-step magnetron sputtering process

Description:
RELATED APPLICATIONS  
      This application is a continuation of Ser. No. 11/166,836, filed Jun. 24, 2005, to be abandoned, which is a division of Ser. No. 10/934,231, filed Sep. 3, 2004, now issued as U.S. Pat. No. 6,991,709, which is a division of Ser. No. 10/197,680, filed Jul. 16, 2002, now issued as U.S. Pat. No. 6,787,006, which is a division of Ser. No. 09/703,601, filed Nov. 1, 2000, now issued as U.S. Pat. No. 6,451,177, which is a continuation in part of Ser. No. 09/518,180, filed Mar. 2, 2000, now issued as U.S. Pat. No. 6,277,249, which is a continuation in part of Ser. No. 09/490,026, filed Jan. 21, 2000, now issued as U.S. Pat. No. 6,251,242. The application is also related to Ser. No. 09/703,738, filed Nov. 1, 2000, now issued as U.S. Pat. No. 6,406,599. 
    
    
     FIELD OF THE INVENTION  
      The invention relates generally to plasma sputtering. In particular, the invention relates to the sputter target and associated magnetron used in a sputter reactor and to an integrated via filling process using sputtering.  
     BACKGROUND ART  
      A semiconductor integrated circuit contains many layers of different materials usually classified according to whether the layer is a semiconductor, a dielectric (electrical insulator) or metal. However, some materials such as barrier materials, for example, TiN, are not so easily classified. The two principal current means of depositing metals and barrier materials are sputtering, also referred to as physical vapor deposition (PVD), and chemical vapor deposition (CVD). Of the two, sputtering has the inherent advantages of low cost source material and high deposition rates. However, sputtering has an inherent disadvantage when a material needs to be filled into a deep narrow hole, that is, one having a high aspect ratio. The same disadvantage obtains when a thin layer of the material needs to be coated onto the sides of the hole, which is often required for barrier materials. Aspect ratios of 3:1 present challenges, 5:1 becomes difficult, 8:1 is becoming a requirement, and 10:1 and greater are expected in the future. Sputtering itself is fundamentally a nearly isotropic process producing ballistic sputter particles which do not easily reach the bottom of deep narrow holes. On the other hand, CVD tends to be a conformal process equally effective at the bottom of holes and on exposed top planar surfaces.  
      Up until the recent past, aluminum has been the metal of choice for the metallization used in horizontal interconnects and in the vias connecting two levels of metallization. In more recent technology, copper vias extend between two levels of horizontal copper interconnects. Contacts to the underlying silicon present a larger problem, but may still be accomplished with either aluminum or copper. Copper interconnects are used to reduce signal delay in advanced ULSI circuits. It is understood that copper may be pure copper or a copper alloy containing up to 10% alloying with other elements such as magnesium and aluminum. Due to continued downward scaling of the critical dimensions of microcircuits, critical electrical parameters of integrated circuits, such as contact and via resistances, have become more difficult to achieve. In addition, due to the smaller dimensions, the aspect ratios of inter-metal features such as contacts and vias are also increasing. An advantage of copper is that it may be quickly and inexpensively deposited by electrochemical processes, such as electroplating. However, sputtering or possibly CVD of thin copper layers onto the walls of via holes is still considered necessary to act as an electrode for electroplating or as a seed layer for the electroplated copper. The discussion of copper processes will be delayed until later.  
      The conventional sputter reactor has a planar target in parallel opposition to the wafer being sputter deposited. A negative DC voltage is applied to the target of magnitude sufficient to ionize the argon working gas into a plasma. The positive argon ions are attracted to the negatively charged target with sufficient energy to sputter atoms of the target material. Some of the sputtered atoms strike the wafer and form a sputter coating thereon. Most usually, a magnetron is positioned in back of the target to create a larger magnetic field adjacent to the target. The magnetic field traps electrons, and, to maintain charge neutrality in the plasma, the ion density also increases. As a result, the plasma density and sputter rate are increased. The conventional magnetron generates a magnetic field lying principally parallel to the target.  
      Much effort has been expended to allow sputtering to effectively coat metals and barrier materials deep into narrow holes. High-density plasma (HDP) sputtering has been developed in which the argon working gas is excited into a high-density plasma, which is defined as a plasma having an ionization density of at least 10 11  cm −3  across the entire space the plasma fills except the plasma sheath. Typically, an HDP sputter reactor uses an RF power source connected to an inductive coil adjacent to the plasma region to generate the high-density plasma. The high argon ion density causes a significant fraction of sputtered atoms to be ionized. If the pedestal electrode supporting the wafer being sputter coated is negatively electrically biased, the ionized sputter particles (metal ions) are accelerated toward the wafer to form a directional column that reaches deeply into narrow holes.  
      HDP sputter reactors, however, have disadvantages. They involve a somewhat new technology and are relatively expensive. Furthermore, the quality of the sputtered films they produce is often not the best, typically having an undulatory surface. Also, high-energy ions, particularly the argon ions which are also attracted to the wafer, tend to damage the material already deposited.  
      Another sputtering technology, referred to as self-ionized plasma (SIP) sputtering, has been developed to fill deep holes. See, for example, U.S. patent application Ser. No. 09/373,097 filed Aug. 12, 1999 by Fu, now issued as U.S. Pat. No. 6,183,614, and U.S. patent application Ser. No. 09/414,614 filed Oct. 8, 1999 by Chiang et al, now issued as U.S. Pat. No. 6,398,929. Both of these patent applications are incorporated by reference in their entireties. In its original implementations, SIP relies upon a somewhat standard capacitively coupled plasma sputter reactor having a planar target in parallel opposition to the wafer being sputter coated and a magnetron positioned in back of the target to increase the plasma density and hence the sputtering rate. The SIP technology, however, is characterized by a high target power density, a small magnetron, and a magnetron having an outer magnetic pole piece enclosing an inner magnetic pole piece with the outer pole piece having a significantly higher total magnetic flux than the inner pole piece. In some implementations, the target is separated from the wafer by a large distance to effect long-throw sputtering, which enhances collimated sputtering. The asymmetric magnetic pole pieces causes the magnetic field to have a significant vertical component extending far towards the wafer, thus enhancing and extending the high-density plasma volume and promoting transport of ionized sputter particles.  
      The SIP technology was originally developed for sustained self-sputtering (SSS) in which a sufficiently high number of sputter particles are ionized that they may be used to further sputter the target and no argon working gas is required. Of the metals commonly used in semiconductor fabrication, only copper has a sufficiently high self-sputtering yield to allow sustained self-sputtering.  
      The extremely low pressures and relatively high ionization fractions associated with SSS are advantageous for filling deep holes with copper. However, it was quickly realized that the SIP technology could be advantageously applied to the sputtering of aluminum and other metals and even to copper sputtering at moderate pressures. SIP sputtering produces high quality films exhibiting high hole filling factors regardless of the material being sputtered.  
      Nonetheless, SIP has some disadvantages. The small area of the magnetron may require circumferential scanning of the magnetron in a rotary motion at the back of the target to achieve even a minimal level of uniformity, and even with rotary scanning, radial uniformity is difficult to achieve. Furthermore, very high target powers have been required in the previously known versions of SIP. High-capacity power supplies are expensive and necessitate complicated target cooling. Lastly, known versions of SIP tend to produce a relatively low ionization fraction of sputter particles, for example, 20%. The remaining non-ionized fraction of sputtered particles has a relatively isotropic distribution rather than forming a forward directed column which results from metal ions being accelerated toward a biased wafer. Also, the target diameter in a typical commercial sputter reactor is only slightly greater than the wafer diameter. As a result, those holes being coated located at the edge of the wafer have radially outer sidewalls which see a larger fraction of the target and are more heavily coated than the radially inner sidewalls. Therefore, the sidewalls of the edge holes are asymmetrically coated.  
      Other sputter geometries have been developed which increase the ionization density. One example is a multi-pole hollow cathode target, several variants of which are disclosed by Barnes et al. in U.S. Pat. No. 5,178,739. Its target has a hollow cylindrical shape, usually closed with a circular back wall, and is electrically biased. Typically, a series of magnets, positioned on the sides of the cylindrical cathode of alternating magnetic polarization, create a magnetic field extending generally parallel to the cylindrical sidewall.  
      Another approach uses a pair of facing targets facing the lateral sides of the plasma space above the wafer. Such systems are described, for example, by Kitamoto et al. in “Compact sputtering apparatus for depositing Co—Cr alloy thin films in magnetic disks,”  Proceedings: The Fourth International Symposium on Sputtering  &amp;  Plasma Processes , Kanazawa, Japan, Jun. 4-6, 1997, pp. 519-522, by Yamazato et al. in “Preparation of TiN thin films by facing targets magnetron sputtering, ibid., pp. 635-638, and by Musil et al. in “Unbalanced magnetrons and new sputtering systems with enhanced plasma ionization,”  Journal of Vacuum Science and Technology A , vol. 9, no. 3, May 1991, pp. 1171-1177. The facing pair geometry has the disadvantage that the magnets are stationary and create a horizontally extending field that is inherently non-uniform with respect to the wafer.  
      Musil et al., ibid., pp. 1174, 1175 describe a coil-driven magnetic mirror magnetron having a central post of one magnetic polarization and surrounding rim of another polarization. An annular vault-shaped target is placed between the post and rim. This structure has the disadvantage that the soft magnetic material forming the two poles, particularly the central spindle, are exposed to the plasma during sputtering and inevitably contaminate the sputtered layer. Furthermore, the coil drive provides a substantially cylindrical geometry, which may not be desired in some situations. Also, the disclosure illustrates a relatively shallow geometry for the target vault, which does not take advantage of some possible beneficial effects for a concavely shaped target.  
      Helmer et al. in U.S. Pat. No. 5,482,611 describe a target having a groove or vault facing the substrate. Stationary magnets are arranged on the outside of the vault sidewalls with parallel magnetic polarities so as to create a magnetic field generally parallel to the vault walls within the vault and having a magnetic cusp or null spot near the opening of the vault. The magnetic cusp directs the metal sputter ions in a beam towards the wafer. However, Helmer et al. admit that uniformity of deposition with this magnetic configuration is not good. Lantsman in U.S. Pat. No. 5,589,041 discloses an plasma etch chamber having a dielectric roof that is formed with a vault so as to shape the plasma.  
      It is thus desired to combine many of the good benefits of the different plasma sputter reactors described above while avoiding their separate disadvantages.  
      Returning now to copper processing and the structures that need to be formed for copper vias, as is well known to those in the art, in a typical copper interconnect process flow, a thin barrier layer is first deposited onto the walls of the via hole prior to the copper deposition. The barrier layer prevents copper from diffusing into the insulating dielectric layer separating the two copper levels and also to prevent intra metal and inter metal electrical shorts. A typical barrier for copper over silicon oxide includes Ta or TaN or a combination thereof, but other materials have been proposed, such as W/WN and Ti/TiN among others. In a typical barrier deposition process, the barrier layer is deposited using PVD or other method to form a continuous layer between the underlying and overlying copper layers including the contact area at the bottom of the via hole. Thin layers of these barrier materials have a small but finite transverse resistance. A structure resulting from this copper interconnect process produces a contact having a finite characteristic resistance (known in the art as a contact or via resistance) that depends on the geometry. Conventionally, the barrier layer at the bottom of the contact or via hole contributes about 30% of the total contact or via resistance. Geffken et al. disclose in U.S. Pat. No. 5,985,762 a separate directional etching step to remove the barrier layer from the bottom of the via hole over an underlying copper feature but not from the via sidewalls so that, during the sputter removal of the copper oxide at the via bottom, the dielectric is not poisoned by the sputtered copper. This process requires presumably a separate etching chamber. Furthermore, the process deleteriously also removes the barrier at the bottom of the trench in a dual-damascene structure. They accordingly deposit another conformal barrier layer, which remains under the metallized via.  
      As a result, there is a need in the art for a method and apparatus to form a low-resistance contact between underlying and overlying copper layers and having a low contact resistance without unduly complicating the process.  
      A copper layer used to form an interconnect is conveniently deposited by electrochemical deposition, for example, electroplating. As is well known, an adhesion or seed layer of copper is usually required to nucleate an ensuing electrochemical deposition on the dielectric sidewalls as well as to provide a current path for the electroplating. In a typical deposition process, the copper seed layer is deposited using PVD or CVD methods, and the seed layer is typically deposited on top of the barrier layer. A typical barrier/seed layer deposition sequence also requires a pre-clean step to remove native oxide and other contaminants that reside on the underlying metal that has been previously exposed in etching the via hole. The pre-clean step, for example, a sputter etch clean step using an argon plasma, is typically performed in a process chamber that is separate from the PVD chamber used to deposit the barrier and seed layers. With shrinking dimension of the integrated circuits, the efficacy of the pre-clean step, as well as sidewall coverage of the seed layer within the contact/via feature, become more problematical.  
      As a result, the art needs a method and apparatus that improves the pre-clean and deposition of the seed layer. Further, the seed layer needs to be conformally deposited in all portions of the via hole even if the barrier layer is removed in portions of the hole.  
     SUMMARY OF THE INVENTION  
      The invention includes a magnetron producing a large volume or thickness of a plasma, preferably a high-density plasma. The long travel path through the plasma volume allows a large fraction of the sputtered atoms to be ionized so that their energy and directionality can be controlled by substrate biasing.  
      In one embodiment of the invention, the target includes at least one annular vault on the front side of the target. The backside of the target includes a central well enclosed by the vault and accommodating an inner magnetic pole of one polarity. The backside of the target also includes an outer annular space surrounding the vault and accommodating an outer magnetic pole of a second polarity. The outer magnetic pole may be annular or be a circular segment which is rotated about the inner magnetic pole.  
      In one embodiment, the magnetization of the two poles may be accomplished with soft pole pieces projecting into the central well and the outer annular space and magnetically coupled to magnets disposed generally behind the well and outer annular space. In a second embodiment, the two poles may be radially directed magnetic directions. In a third embodiment, a magnetic coil drives a yoke having a spindle and rim shape.  
      In one advantageous aspect of the invention, the target covers both the spindle and the rim of the yoke as well as forming the vault, thereby eliminating any yoke sputtering.  
      According to another aspect of the invention, the relative amount of sputtering of the top wall of the inverted vault relative to the sidewalls may be controlled by increasing the magnetic flux in the area of the top wall. An increase of magnetic flux at the sidewalls may result in a predominantly radial distribution of magnetic field between the two sidewalls, resulting in large sputtering of the sidewalls.  
      One approach for increasing the sputtering of the top wall places additional magnets above the top wall with magnetic polarities aligned with the magnets just outside of the vault sidewalls. Another approach uses only the top wall magnets to the exclusion of the sidewall magnets. In this approach, the back of the target can be planar with no indentations for the central well or the exterior of the vault sidewalls. In yet another approach, vertical magnets are positioned near the bottom of the vault sidewalls with vertical magnetic polarities opposed to those the corresponding magnets near the top of the vault sidewalls, thereby creating semi-toroidal fields near the bottom sidewalls. Such fields can be adjusted either for sputtering or for primarily extending the top wall plasma toward the bottom of the vault and repelling its electrons from the sidewalls. A yet further approach scans over a top wall a small, closed magnetron having a central magnetic pole of one polarity and a surrounding magnetic pole of the other polarity.  
      The target may be formed with more than one annular vault on the side facing the substrate. Each vault should have a width of at least 2.5 cm, preferably at least 5 cm, and more preferably at least 7 cm. The width is thus at least 10 times and preferably at least 25 times the dark space, thereby allowing the plasma sheath to conform to the vault outline.  
      Various magnetron configurations are possible for use with the vaulted target. A particularly advantageous design includes an annular inner sidewall magnet of one polarity, an outer sidewall magnet of the other polarity, and a roof magnet that rotates about the central axis. The roof magnet may be composed of an annular outer magnet of the second polarity surrounding an inner magnet of the first polarity. The inner sidewall magnet is preferably divided into two axial portions separated by a non-magnetic spacer, thereby smoothing the erosion pattern on the inner target sidewall because the magnetic field is curved towards the non-magnetic; however, although the non-magnetic spacer is not required for all aspects of the invention.  
      The invention also includes a two-step sputtering process, the first producing high-energy ionized copper sputter ions, the second producing a more neutral, lower-energy sputter flux. The two-step process can be combined with an integrated copper fill process in which the first step provides high sidewall coverage and may break through the bottom barrier layer and clean the copper. The second step completes the seed layer. Thereafter, copper is electrochemically deposited in the hole. For sputtering into a dual-damascene structure, the conditions are preferably set so that the first step sputters the barrier from the bottom of the via hole but not from the more accessible trench floor.  
      After forming a first level of metal on a wafer and pattern etching a single or dual damascene structure for a second level of metal on the wafer, the wafer is processed in a PVD cluster tool to deposit a barrier layer and a seed layer for the second metal level.  
      Instead of using a pre-clean step (for example, a sputter etch clean step), in accordance with one aspect of the present invention, a simultaneous clean-deposition step (i.e., a self-clean deposition step) is carried out. The inventive self clean deposition is carried out using a PVD deposition chamber that is capable of producing high-energy ionized target material. In accordance with one embodiment of the present invention, the high-energy ions physically remove material on flat areas of a wafer. In addition, the high-energy ions can dislodge material from a barrier layer disposed at the bottom of a contact/via feature. Further, in accordance with one embodiment of the present invention, wherein an initial thickness of the barrier layer is small, the high-energy ions can remove enough material from the barrier layer to provide direct contact between a seed layer and the underlying metal (for example, between a copper underlying layer and a copper seed layer). In addition to providing direct contact between the two copper layers, the inventive sputtering process also causes redeposition of copper over sidewalls of the contact/via to reinforce the thickness of the copper seed layer on the sidewall. This provides an improved path for current conduction, and advantageously improves the conformality of a layer subsequently deposited by electroplating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic cross-sectional view of a first embodiment of a magnetron sputter reactor of the invention using a stationary, circularly symmetric magnetron.  
       FIG. 2  is a schematic cross-sectional diagram illustrating the collimating function of the target of the invention.  
       FIG. 3  is a schematic cross-sectional view of a second embodiment of a magnetron sputter reactor of the invention using a rotating, segmented magnetron with vertically magnetized magnets.  
       FIG. 4  is a schematic cross-sectional view of a third embodiment of a magnetron sputter reactor of the invention using a rotating, segmented magnetron with radially magnetized magnets.  
       FIG. 5  is a schematic cross-sectional view of a fourth embodiment of a magnetron sputter reactor of the invention using an electromagnetic coil.  
       FIG. 6  is a cross-sectional view of a fifth embodiment of a magnetron of the invention using additional magnets at the roof of the vault to increase the roof sputtering.  
       FIG. 7  is a cross-sectional view of a sixth embodiment of a magnetron of the invention using only the vault magnets.  
       FIG. 8  is a cross-sectional view of a seventh embodiment of a magnetron of the invention using additional confinement magnets at the bottom sidewall of the vault.  
       FIG. 9  is a cross-sectional view of an eighth embodiment of a magnetron of the invention using a closed magnetron over the vault roof and separate magnets for the vault sidewalls.  
       FIGS. 10-12  are cross-sectional views of ninth through eleventh embodiments of magnetrons of the invention.  
       FIGS. 13 and 14  are respectively a cross-sectional view and a schematic plan view of a twelfth embodiment of the invention using stationary outer sidewall magnets and rotating inner sidewall magnets.  
       FIG. 15  is a schematic plan view of a variant of the twelfth embodiment.  
       FIG. 16  is a cross-sectional view of the target and magnetron of the twelfth embodiment illustrating the resultant magnetic field.  
       FIG. 17  is a graph of sputtering yield as a function of copper ion energy.  
       FIGS. 18 and 19  are cross-sectional views illustrating the effects of high-energy ionized sputter deposition, particularly the effect of a high-energy copper PVD deposition removing the barrier layer at the bottom of the via.  
       FIG. 20  is a cross-sectional view illustrating how one copper PVD reactor can be used to both remove the barrier at the via bottom and to deposit a copper layer in its place.  
       FIG. 21  is a sectioned orthographic view of a desired barrier layer in a dual-damascene interconnect.  
       FIGS. 22 and 23  are cross-sectional views of a desirable structure for a barrier layer and copper seed layer in a dual-damascene interconnect.  
       FIG. 24  is a flow diagram of a process usable for achieving the desired interconnects of  FIGS. 20 and 23  including a electroplating via fill. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The invention uses a complexly shaped sputter target and a specially shaped magnetron which have the combined effect of impressing a magnetic field producing a thick region of relatively high plasma density. As a result, a large fraction of the metal atoms sputtered from the target can be ionized as they pass through the plasma region. Sputtered metal ions can be advantageously controlled by substrate biasing to coat the walls of a deep, narrow hole and to selectively interact with the already deposited barrier layer dependent upon the local geometry.  
      The inventive apparatus has been used to achieve several novel processes involving selective removal of layers at the bottom of high aspect-ratio holes and the selective sputter deposition on areas dependent upon their geometries. Multi-step processes involving both removal and deposition can be performed in the same sputter reactor, for example, the inventive reactor with a novel target and associated magnetron.  
     Apparatus  
      A magnetron sputter reactor  10  of a first embodiment of the invention is illustrated in the schematic cross-sectional view of  FIG. 1 . It includes a specially shaped sputter target  12  and magnetron  14  symmetrically arranged about a central axis  16  in a reactor otherwise described for the most part by Chiang et al. in the above referenced patent application. This reactor and associated processes will be referred to as SIP +  sputtering in contrast to the SIP sputter reactor of Chiang et al., which uses a planar target. The shaped target  12  or at least its interior surface is composed of the material to be sputter deposited. The invention is particularly useful for sputtering copper, but it may be applied to other sputtering materials as well. It is understood that target may be composed of alloys, typically to less than 10% of alloying. For example, copper is often alloyed with silicon, aluminum, or magnesium. As is known, reactive sputtering of materials like TiN and TaN can be accomplished by using a Ti or Ta target and including gaseous nitrogen in the plasma. Other combinations of metal targets and reactive gases are possible.  
      The target  12  includes an annularly shaped downwardly facing vault  18  opposed to a wafer  20  being sputter coated. The vault  18  could alternatively be characterized as an inverted annular trough or moat. The vault  18  has an aspect ratio of its depth to radial width of at least 0.5:1 and preferably at least 1:1. The tested embodiment had a vault width of 7.5 cm and an aspect ratio of 1.4:1. The vault  18  has an cylindrical outer sidewall  22  outside of the periphery of the wafer  20 , a cylindrical inner sidewall  24  overlying the wafer  20 , and a generally flat, annular vault top wall or roof  25  (which extends across the space between the annular sidewalls  22 ,  24  and closes the bottom of the downwardly facing vault  18 ). The sidewalls  20 ,  22  in this embodiment extend generally parallel to the central axis  16 , and the roof  25  extends generally perpendicularly. The target  12  also includes a central portion forming a spindle  26  including the inner sidewall  24  and a generally planar face  28  spanning the space formed at the bottom terminus of the inner sidewall  24  in parallel opposition to the wafer  20 . The target  12  is continuous across its parts  22 ,  25 ,  24 ,  28  with no structure intervening between these parts and the process space between the target  12  and the wafer  20 . The target  12  also includes a flange  29  which extends radially outwardly from near the terminus of the outer sidewall  22  and which is vacuum sealed to the lower chamber body of the sputter reactor  10 .  
      The magnetron  14  of the embodiment illustrated in  FIG. 1  includes one or more central magnets  30  having a first vertical magnetic polarity and one or more outer magnets  32  of a second vertical magnetic polarity opposite the first polarity and arranged in an annular pattern. That is, one is N—S; the other, S—N. In this embodiment the magnets  30 ,  32  are permanent magnets, that is, composed of strongly ferromagnetic material and are stationary. The inner magnets  30  are radially disposed within or axially upward of a cylindrical central well  36  formed in the spindle  26  and between the opposed portions of the inner target sidewall  24  while the outer magnets  32  are disposed generally radially outside of the outer target sidewall  22 . A circular magnetic yoke  34  magnetically couples tops of the inner and outer magnets  30 ,  32 . The yoke is composed of a magnetically soft material, for example, a paramagnetic material, such as SS410 stainless steel, that can be magnetized to thereby form a magnetic circuit for the magnetism produced by the permanent magnets  30 ,  32 . Permanently magnetized yokes are possible but are difficult to obtain in a circular geometry.  
      A cylindrical inner pole piece  40  of a similarly magnetically soft material abuts the lower ends of the inner magnets  30  and extends deep within the target well  36  adjacent to the inner target sidewall  24  to produce, in this case, a N pole within the well  36 . (It is of course appreciated that the selection of an N or S pole is for the most part arbitrary because almost all practical magnetic effects depend only upon the relative polarities of different poles.) If the magnetron  14  is generally circularly symmetric, it is not necessary to rotate it for uniformity of sputter deposition. A tubular outer pole piece  42  of a magnetically soft material abuts the lower end of the outer magnets  32  and extends downwardly outside of the outer target sidewall  22 . The magnetic pole pieces  40 ,  42  of  FIG. 1  differ from the usual pole faces in that they and the magnets  30 ,  32  are configured and sized to emit a magnetic field B in the target vault  18  that is largely perpendicular to the magnetic field of the corresponding associated magnets  30 ,  32 . In particular, the magnetic field B is generally perpendicular to the target vault sidewalls  22 ,  24 .  
      This configuration has several advantages. First, the electrons trapped by the magnetic field B, although gyrating about the field lines, otherwise travel generally horizontally and radially with respect to the target central axis  16 . Plasma sheaths are formed on both vault sidewalls  22 ,  24  which reflect electrons traveling along the magnetic field lines toward the vault sidewalls. As a result, the electrons are substantially bound within the vault  18 , and electron loss is minimized, thus increasing the plasma density. Secondly, the vertical depth of the magnetic field B intensifying the plasma density is determined by the height of the target sidewalls  22 ,  24 . This depth can be considerably greater than that of a high-density plasma region created by magnets in back of a planar target. As a result, sputtered atoms traverse a larger region of a high-density plasma and are accordingly more likely to become ionized. The support structure for the magnetron  14  and its parts is not illustrated but can be easily designed by the ordinary mechanic. The support structure usually includes an overlying cover shielding and supporting the magnetron.  
      The remainder of the sputter reactor  10  is similar to that described by Chiang et al. in the above referenced patent application although a short-throw rather than a long-throw configuration may be used. Long throw is defined by Chiang et al. as the separation between the target and wafer as being at least 80% and preferably at least 140% of the wafer diameter. The target  12  is vacuum sealed to a grounded vacuum chamber body  50  through a dielectric target isolator  52 . The wafer  20  is clamped to a heater pedestal electrode  54  by, for example, a clamp ring  56  although electrostatic chucking is possible. An electrically grounded shield  58  acts as an anode with respect to the cathode target  12 , which is negatively biased by a variable DC power supply  60 . DC magnetron sputtering is conventional in commercial applications, but RF sputtering can enjoy the advantages of the target and magnetron of the invention and is especially advantageous for sputtering non-metallic targets. An electrically floating shield  62  is supported on the electrically grounded shield  58  or chamber  50  through a dielectric shield isolator  64 . An annular cylindrical knob  66  extending downwardly from the outer target sidewall  22  and positioned inwardly of the uppermost part of the floating shield  62  protects the upper portion of the floating shield  62  and the target isolator  52  from being sputter deposited from the strong plasma disposed within and slightly vertically outwardly of the target vault  18 . The gap between the upper portion of the floating shield  62  and the target knob  66  and flange  29  is small enough to act as a dark space preventing the plasma from propagating into the gap.  
      A working gas such as argon is supplied into the chamber from a gas source  68  through a mass flow controller  70 . A vacuum pumping system  72  maintains the chamber at a reduced pressure, typically a base pressure in the neighborhood of 10 −8  Torr. Although a floating pedestal electrode  54  can develop a desired negative self-bias, it is typical in high plasma-density sputtering for an RF power supply  74  to RF bias the pedestal electrode  54  through an isolation capacitor, which results in a controlled negative DC self-bias. A controller  76  regulates the power supplies  60 ,  74 , mass flow controller  70 , and vacuum system  72  according to a sputtering recipe prerecorded in it with recordable magnetic or optical media.  
      The target and magnetron structure has several advantages. As mentioned previously, secondary electrons are largely trapped within the vault  18  with little loss even upon collision with the target sidewalls  22 ,  24 , more specifically reflected from the plasma sheaths adjacent the sidewalls. Also, the plasma thickness is relatively large, determined by the sidewall heights, thereby increasing the ionization fraction of the sputtered target atoms. The separation of the inner and outer poles  40 ,  42  is relatively small, thereby increasing the magnetic field intensity within the vault  18 . The target  12  is continuous across the pole pieces  40 ,  42 , thus preventing the magnetic material of the poles from being sputtered and deposited on the semiconductor wafer  20 .  
      The relatively high ionization fraction allows this fraction of the sputtered target atoms to have their trajectories toward the wafer be controlled both by the magnetic field looping from the target toward the wafer and by the electric field induced by the DC self-bias applied to the pedestal. Increasing the DC self-bias draws the ions into high aspect-ratio holes, thereby allowing high bottom and sidewall coverage of such high aspect-ratio holes. On the other hand, the ionization fraction is less than 100%, and the remaining sputtered atoms are neutral. In some situations a finite neutral component is useful, and the ratio of neutrals to ions can be controlled by adjusting power levels and chamber pressures.  
      The high aspect ratio of the vault  18  also improves the symmetric filling of holes located near the edge of the wafer, particularly in configurations having a shorter throw than that illustrated in  FIG. 1 . As schematically illustrated in  FIG. 2 , a hole  78  located at the right edge of the wafer  20  is to have a conformal layer sputter deposited on its sides. The size of the hole  78  and the thickness of the wafer  20  are greatly exaggerated, but the geometry remains approximately valid. If a planar target were being used, the right side of the wafer hole  78  would see a much larger fraction of the target than the left side and would thus be coated with a commensurately thicker layer. However, with the vault-shaped target  12 , the hole  78  sees neither the inner sidewall  24  of the left side of the vault  18  nor the left vault top wall  25 . Even the upper portion of the outer sidewall  22  of the left side of the vault  18  is shielded from the wafer hole  78  by the inner sidewall  24  of the left side of the vault  18 . As a result, the two sidewalls of the hole  78  to be coated see areas of the vault-shaped target that are much closer in size than for a planar target, and the sidewall coating symmetry is thereby greatly increased.  
      The target structure, as a result, can provide sputtered particles having trajectories preferentially aligned perpendicularly to the wafer surface, but without an apertured collimator, which tends to become clogged with sputtered material. The effect is increased by a high aspect ratio for the vault, preferably at least 1:2, and more preferably at least 1:1. The tested target had a vault with an aspect ratio of 1.4:1.  
      A sputter reactor  80  of second embodiment of the invention is illustrated in the schematic cross-sectional view of  FIG. 3 . A magnetron  82  includes the previously described inner magnets  30  and inner pole piece  40 . However, one or more outer magnets  84  and an outer pole piece  86  extend around only a segment of the circumference of the target, for example between 15° and 90°. An asymmetric magnetic yoke  88  shaped as a sector magnetically couples the inner and outer magnets  30 ,  84  but only on the side of target well  36  toward the outer magnets  84 . In fact, a circular yoke  88 , although larger, would not affect the operative magnetic field. As a result, a high-density plasma is generated in only a small circumferential portion of the target vault  18 . For self-ionized plating (SIP) and particularly sustained self-sputtering (SSS), a high plasma density is desired. In view of the limited capacity of realistic power supplies  60 , the high plasma density can be achieved by reducing the volume of the magnetron  82 .  
      To achieve uniform sputtering, a motor  90  is supported on the chamber body  50  through a cylindrical sidewall  92  and roof  94  preferably electrically isolated from the biased target flange  29 . The motor  90  has a motor shaft connected to the yoke  88  at the target axis  16  and rotates the magnetron  82  about that axis  16  at a few hundred rpm. Mechanical counterbalancing may be provided to reduce vibration in the rotation of the axially offset magnetron  82 . The mechanical details are not accurately represented in  FIG. 3  but will be described more completely below.  
      Other magnet configurations are possible to produce similar magnetic field distributions. A sputter reactor  100  of a third embodiment of the invention is illustrated in the schematic cross-sectional view of  FIG. 4 A  magnetron  102  includes an inner magnet  104  having a magnetization direction generally aligned with a radius of the target  12  about the target axis  16 . One or more outer magnets  106  are similarly radially magnetized but anti-parallel to the magnetization of the inner magnet  104  with respect to the center of the vault  18 . A C-shaped magnetic yoke has two arms  110 ,  112  in back of and supporting the respective magnets  104 ,  106  and a connector  114  supported on and rotated by the shaft of the motor  90 .  
      The magnets  104 ,  106  may be advantageously positioned only on reduced circumferential portions of the sidewalls  24 ,  22  of the target vault  18  so as to concentrate the magnetic field there. Furthermore, in this configuration extending along only a small segment of the target periphery, the magnets  104 ,  106  may be conveniently formed of plate magnets.  
      Electromagnetic coils may replace the permanent magnets of the previously described embodiments. A sputter reactor  120  of a fourth embodiment of the invention is illustrated in the schematic cross-sectional view of  FIG. 5 . A magnetron  122  includes a magnetic yoke including a central spindle  124  fit into the well  36  of the target  12  and a tubular rim  126  surrounding the outer sidewall  24  of the target vault  18 . The magnetic yoke also includes a generally circular back piece  128  magnetically coupling the spindle  124  and the rim  126 . An electromagnetic coil  130  is wound around the spindle  124  below the back piece  128  and inside of the rim  126 . The coil  130  is preferably powered by a DC electrical source but a low-frequency AC source can be used. The coil  130  in conjunction with the magnetic yoke creates a generally radial magnetic field in the target vault  18 .  
      The previously described embodiments have emphasized sputtering the vault sidewalls  22 ,  24  preferentially to sputtering the vault top wall or roof  25  (see  FIG. 1 ) since relatively few of the magnetic field lines terminate on the vault roof  25 . The metal ionization fraction can be increased if sputtering is increased in the vault roof  25  since the plasma thickness experienced by the average sputtered atom is increased. Also, the directionality of sputtered material leaving the vault  18  is increased.  
      The increased roof sputtering can be achieved in a number of ways. In a fifth embodiment of a magnetron  140  illustrated in cross-section in  FIG. 6  with the remainder of the sputtering chamber being similar to the parts illustrated in  FIG. 3 . A target  142  is similar to the previously described target  12  except for a thinner roof portion  144 . Similarly to the magnetron  82  of  FIG. 3 , it includes the rotatable yoke  88  supporting the inner magnets  30  of a first vertical polarity magnetically coupled to the inner pole piece  40  and the outer magnets  84  of a second vertical polarity magnetically coupled to the outer pole piece  86 . These magnets  30 ,  84  and pole pieces  40 ,  86  produce a generally radial magnetic field B extending between the sidewalls  22 ,  24  of the vault  18 . The magnetron  82  additionally supports on the magnetic yoke  88  an inner roof magnet  146  of the first vertical polarization aligned with the inner magnets  30  and an outer roof magnet  148  of the second vertical polarization aligned with the outer magnets  86 . The opposed roof magnets  146 ,  148  magnetically coupled by the yoke  88  produce a semi-toroidal magnetic field B penetrating the vault roof  144  at two locations. Thereby, electrons are trapped along the semi-toroidal magnetic field and increase the plasma density near the vault roof  144 , thereby increasing the sputtering of the vault roof  144 .  
      In the illustrated embodiment, the outer magnets  84  and outer pole piece  86  occupy only a segment of the periphery of the target  142  but are rotated along that periphery by the motor  90 . Similarly, inner and outer roof magnets  146 ,  148  extend only along a corresponding segment angle. However, a corresponding non-rotating magnetron can be created by making the roof magnets  146 ,  148 , outer magnet  84 , and outer pole piece  86  in annular shapes. The same circularly symmetric modification may be made to the embodiments described below.  
      The roof sputtering can be further emphasized by a sixth embodiment of a magnetron  150 , illustrated in  FIG. 7 , which includes the inner and outer roof magnets  146 ,  148  but which in the illustrated embodiment includes neither the inner magnets within the well  36  nor the outer magnets outside of the outer sidewall  22 . This configuration produces a relatively strong semi-toroidal magnetic field B adjacent to the vault roof  144  and a weaker magnetic field B in the body of the vault  18  adjacent to the sidewalls  22 ,  24 . Therefore, there will be much more sputtering of the roof  144  than of the sidewalls  22 ,  24 . Nonetheless, magnetic field lines in the vault body terminate at the sidewalls  22 ,  24 , thereby decreasing electron loss out of the plasma. Hence, the magnetic field intensity may be low in the vault, but the plasma density is still kept relatively high there so that the target atoms sputtered from the roof  144  still traverse a thick plasma region and are accordingly efficiently ionized.  
      Since no magnets or pole pieces are placed in the target well  36  or outside of the outer target sidewall  22  and assuming the target material is non-magnetic, the inner and outer sidewalls  24 ,  22  may be increased in thickness even to the point that there is no well and no appreciable volume between the outer sidewall  22  and the chamber wall. That is, the back of the target  142  may have a substantially planar face  152 ,  154 ,  156 . However, the inventive design of this embodiment still differs from a target having a circularly corrugated surface in that the spacing of the opposed roof magnets  146 ,  148  is at least half of the radial vault dimension and preferably closer to unity. This is in contrast to the embodiments of  FIGS. 1, 3 , and  4  in which the two sets of magnets are separated preferably by between about 100% and 150% of the vault width. Alternatively stated, the width of the vault  18  in the radial direction should be at least 2.5 cm, preferably at least 5 cm, and most preferably at least 10 cm. These dimensions, combined with the vault aspect ratio being at least 1:2 assures that the vault width is at least 10 times and preferably at least 25 times the plasma dark space, thus guaranteeing that the plasma conforms to the shape of the vault  18 . These vault widths are easily accommodated in a sputter reactor sized for a 200 mm wafer. For larger wafers, more complex target shapes become even easier to implement.  
      A seventh embodiment of a magnetron  160  illustrated in the cross-sectional view of  FIG. 8  includes the inner and outer main magnets  30 ,  84 , although they are preferably somewhat shorter and do not extend below the vault roof  144 . The magnetron also includes the inner and outer roof magnets  146 ,  148 . However, neither the inner pole piece nor the outer pole piece needs to be used to couple the magnetic field from the main magnets  30 ,  84  into the vault  18 . Instead, all these magnets produce a horizontally oriented semi-toroidal field B adjacent the vault roof  144 . Some of these magnets may be eliminated as long as there are opposed magnets associated with the inner and outer target sidewalls  22 ,  24 . Instead of ferromagnetic or paramagnetic pole pieces, non-magnetic (e.g. aluminum or hard stainless steel) or even diamagnetic spacers  162 ,  164  are supported below the inner and outer main magnets  30 ,  84  respectively. Henceforth, non-magnetic materials will be assumed to include diamagnetic materials unless specifically stated otherwise. The inner spacer  162  supports on its lower end an inner sidewall magnet  166  of the second magnetic polarity, that is, opposite that of its associated main inner magnet  30 . Similarly, the outer spacer  164  supports on its lower end an outer sidewall magnet  168  of the first magnetic polarity, that is, opposite that of its associated main outer magnet  84 . Both the sidewall magnets  166 ,  168  are located near the bottom of the respective vault sidewalls  24 ,  22 . Because, they have polarities opposed to those of their associated main magnets  30 ,  84  they create two generally vertically extending semi-toroidal magnetic fields B′ and B″ near the bottom of the vault sidewalls  24 ,  22 . Because of their opposed magnetic orientations, the sidewall magnets  166 ,  168  create two anti-parallel components of radial magnetic field across the vault  18 . However, because of the relative spacings of the poles, the semi-toroidal magnetic fields B′ and B″ dominate.  
      In one sub-embodiment, the horizontally extending magnetic field B near the vault roof  144  is much stronger than the vertically extending magnetic fields B′ and B″ near the vault sidewalls  24 ,  22 . As a result, sputtering of the roof  144  predominates. Alternatively, increased sidewall fields B′ and B″ can increase the amount of sidewall sputtering in a controlled way. In any case, the vertically extending sidewall fields B′ and B″ are sufficient to support a plasma throughout much of the body of the vault  18 . Also, the sidewall fields B′ and B″ are oriented to repel electrons in the plasma flux from the roof  144 , thereby decreasing the electron loss of that plasma.  
      All of the previous embodiments have used magnets that extend generally along either the entire circumference or a segment of the circumference of various radii of the target. However, an eighth embodiment of a magnetron  170  illustrated in the cross-section view of  FIG. 9  treats the planar vault roof  144  distinctly differently than the band-shaped vault sidewalls  22 ,  24 . The sidewall magnetic assembly is similar to that of  FIG. 6  and includes the rotatable yoke  88  supporting the inner magnets  30  of a first vertical polarization magnetically coupled to the inner pole piece  40  and the segmented outer magnets  84  of an opposed second vertical polarization magnetically coupled to the outer pole piece. These produce a generally radially directed magnetic field B across the vault  18 . The rotating magnetic yoke  88  also supports a closed magnetron over the vault roof  144  including an inner magnet  172  of one vertical magnetic polarization and a surrounding outer magnet  174  of the other vertical magnetic polarization producing between them a cusp-shaped magnetic field B′ adjacent the vault roof  144 . In the simplest sub-embodiment, the inner magnet  172  is cylindrical, and the outer magnet  174  is annular or tubular, surrounds the inner magnet  172 , thereby producing a circularly symmetric cusp field B′. However, other shapes are possible, such as a radially or circumferentially aligned racetrack or a pair of nested segment-shaped magnets. The roof magnetron of  FIG. 9  is the general type of magnetron described by Fu and by Chiang et al. in the previously referenced patent applications for SIP sputtering of a planar target, and those references provide guidance on the design of such a closed unbalanced magnetron having a strong outer pole surrounding a weaker inner pole of the opposite polarity.  
      The figure does not adequately illustrate the magnetic yoke  88  which in the conceptually simplest implementation would magnetically isolate the roof magnets  172 ,  174  from the sidewall magnets  30 ,  84  while still magnetically coupling together the roof magnets  172 ,  174  and separately coupling together the sidewall magnets  30 ,  84 . However, in view of the large number of magnets, a more complex magnetic circuit can be envisioned.  
      As has been shown in the cited patent applications, such a small closed roof magnetron will be very effective in highly ionized sputtering of the target roof  144 . The sidewall magnets  30 ,  84  on the other hand will extend the plasma region down the height of the sidewalls  22 ,  24  as well as cause a degree of sidewall sputtering depending on the relative magnetic intensities.  
      The relative magnetic polarizations of roof magnets  172 ,  174  relative to those of the sidewall magnets  30 ,  84  may be varied. Also, the sidewall magnets  30 ,  84  and particularly the outer sidewall magnet  84  may be made fully annular so as to close on themselves so that optionally they do not need to be rotated and may be coupled by their own stationary yoke while the roof magnets  172 ,  174  do rotate about the circular planar area on the back of the vault roof  144  and are coupled by their rotating own yoke.  
      Other combinations of the closed roof magnetron and the sidewall magnets of other embodiments are possible.  
      A ninth embodiment of a magnetron  180  of the invention is illustrated in the cross-sectional view of  FIG. 10  and includes the inner and outer magnets  172 ,  174  overlying the vault roof  144 . Side magnets  182 ,  184  disposed outside of the vault sidewalls  142  have opposed vertical magnetic polarities but they are largely decoupled from the roof magnets  172 ,  174  because they are supported on the magnetic yoke  88  by non-magnetic supports  186 ,  188 . As a result, the side magnets  182 ,  184  create a magnetic field B in the vault  18  that has two generally anti-parallel components extending radially across the vault  18  as well as two components extending generally parallel to the vault sidewalls. Thus, the magnetic field B extends over a substantial depth of the vault  18  and further repels electrons from the sidewalls. In the illustrated embodiment, all the side magnets  182 ,  184  are segmented and rotate with the roof magnets  172 ,  174 . However, a mechanically simpler design forms the side magnets  182 ,  184  in annular shapes and leaves one or both of them stationary. As illustrated, the polarities are such that the top pole of the inner side magnet  182  has the same polarity as the bottom pole of the adjacent annular top magnet  174  while the outer side magnet  184  has the opposite relationship with the annular top magnet  174 . However, these polarities may be varied.  
      A tenth embodiment  190  illustrated in the cross-sectional view of  FIG. 11  is similar to the magnetron  180  of  FIG. 10  except that an inner side magnet  192  is smaller than the outer side magnet  184 , thereby allowing tailoring of the magnetic fields on the two vault sidewalls. The opposite size relationship is also possible.  
      An eleventh embodiment  200  illustrated in the cross-sectional view of  FIG. 12  dispenses with the top magnets and uses only the two side magnets  182 ,  184  which may be of the same size or of unequal size. In this case, the yoke  88  need not be magnetic.  
      A twelfth embodiment  210  illustrated in the cross-sectional side view of  FIG. 13  is the subject of U.S. patent application Ser. No. 09/703,738, filed on Nov. 11, 2000 by A. Subramani et al., now issued as U.S. Pat. No. 6,406,599, incorporated herein by reference in its entirety. This embodiment has similar functionality to that of  FIG. 10  but has further capabilities.  
      The illustrated upper part of the sputtering chamber includes a cylindrical wall composed of a bottom frame  212  and a top frame  214 , on which is supported a chamber roof  216 . The SIP +  vault-shaped target  12  is fixed to the bottom frame  212 . All these parts are sealed together to allow cooling water to circulate in a space  218  in back of the target  12 . The vault-shaped target  12  includes the annular vault  18  having the outer sidewall  22 , the inner sidewall  24 , and the vault roof  25 , all generally circularly symmetric with respect to the vertical chamber axis  16 . The inner and outer vault sidewalls  22 ,  24  extend generally parallel to the chamber axis  16  while the vault roof  25  extends generally perpendicularly thereto. That is, the vault  18  is annularly shaped with a generally rectangular cross section.  
      A magnetron  220  is placed in back of the vaulted target  12  in close association with the vault  18 . The magnetron  220  includes a stationary ring-shaped outer sidewall magnet assembly  222  placed outside the outer vault sidewall  22  and having a first vertical magnetic polarity. The preferred structure for the outer sidewall magnets  224  is more complicated than that illustrated, as is described in the patent application to Subramani et al., but the functions remain much the same. A rotatable inner sidewall magnet assembly  224  includes an upper tubular magnet  226  and a lower tubular magnet  228  separated by a non-magnetic tubular spacer  230  having an axial length at least half the respective lengths of the two tubular magnets  226 ,  228 . The two tubular magnets  226 ,  228  have a same second vertical magnetic polarity opposite that of the outer sidewall magnet  222 . However, the non-magnetic spacer  230  is not required, and other magnet configurations may be selected to achieve a desired erosion pattern. The bottom of the lower tubular magnet  228  is separated from the back of a central planar portion  232  of the vaulted target  12  by a small gap  234  having an axial extent of between 0.5 to 1.5 mm.  
      The magnetron also includes a rotatable roof magnet assembly  236  in a nested arrangement of an outer ring magnet  238 , generally circularly shaped, having the first magnetic polarity surrounding an inner rod magnet  240  having the second magnetic polarity and a magnetic yoke  242  supporting and magnetically coupling the magnets  238 ,  240 . It is preferred that the total magnetic flux of the outer ring magnet  238  be substantially greater than that of the inner magnet  240 , for example, having a ratio of at least 1.5. It is preferred, although not required, that the magnetic polarity of the outer ring magnet  238  be anti-parallel to that of the inner sidewall magnet  224  so as to avoid strong magnetic fields adjacent to the inner upper corner of the target vault  18  and instead to intensify the magnetic field at the outer upper corner, which is being more quickly scanned. The metal ions produced in the very high-density plasma adjacent the roof are focused by the outer sidewall magnets  222  and the inner sidewall magnet  224  into a column directed to the wafer.  
      Both the inner sidewall magnet  224  and the roof magnet assembly  236  are rotatable about the chamber axis  16 . The inner sidewall magnet  224  is connected to and supported by a shaft  244  rotated about the chamber axis  16  by a motor  246 . The magnetic yoke  242  supporting the roof magnets  238 ,  240  is also fixed to the rotating shaft  244 .  
      The motor shaft  244  and the inner sidewall magnet  224  includes an inner passageway  250  configured for passage of cooling fluid, usually water, supplied from a chiller  252  through an inlet port  254  to a rotary union  256  connected to the motor shaft  244 . The cooling water flows from the bottom of the inner sidewall magnet  224  through the gap  234  at the bottom of the inner sidewall magnet  224 . It then flows upwardly between the inner vault sidewall  24  and the inner sidewall magnet  224 . The rotating roof magnet assembly  236  stirs up the cooling water in the back of the target  12 , thereby increasing its turbulence and cooling efficiency. The cooling water then flows down next to the outer vault sidewall  22 . As explained by Subramani et al., the tubular outer sidewall magnet assembly  222  is composed of a large number of rod magnets, and they are separated from the actual walls of the target  12 . As a result, the cooling water can flow both through and below the outer sidewall magnet  222  to one of several outlets  257  in the bottom frame  212  and then through several risers  258  in the frames  212 ,  214  to an outlet port  259  in the upper frame  214 , whence the warmed cooling water is returned to the chiller  252 . This cooling design has the advantage of supplying the coldest water to the hottest, central portions of the target  12 .  
      Even though the inner sidewall magnet  224  is rotating, its circular symmetry causes it to produce the same magnetic field as that produced by a stationary cylindrical magnet. A schematic plan view of the magnets is shown in  FIG. 14 . This figure is intended to represent the effective magnetic poles rather than actual magnets. The labeled polarities correspond to the uppermost poles and do not necessarily reflect the effective polarities within the vault  18 . The inner sidewall magnet  224  is included within the inner vault sidewall  24  and is essentially circularly symmetric even though it may be rotating. Similarly the outer sidewall magnet  222  is positioned on the radial exterior of the outer vault sidewall  22 , and it also is substantially circularly symmetric. The roof magnet assembly  236  including the outer and inner roof magnets  238 ,  240  is positioned over the vault roof between the outer and inner sidewalls  22 ,  24 , and it rotates about the center of the target. It is apparent that the roof magnet assembly  236  occupies less than 20% of the area of the vault  18 , and its effective magnetic fields occupy less than 10%. These factors provide corresponding increases in effective target power densities. Nonetheless, circumferential scanning provides uniform sputtering of the target.  
      It is possible to increase the number of roof magnet assemblies. For example, as illustrated in the schematic plan view of  FIG. 15 , a second roof magnet assembly  260  has outer and inner magnets  262 ,  262  of sizes and polarities matched to those of the first roof magnet assembly  236 . The second roof magnet assembly  260  is disposed over the vault  18  diametrically opposite the first roof magnet assembly  236  and is rotated with it. Additional roof magnet assemblies may be added. While the multiple roof magnet assemblies increase the sputtering rate, particularly of metal ions, they require additional power to achieve the same peak plasma density.  
      The asymmetry between the roof magnet assembly  236  and the sidewall magnets  222 ,  234  for the embodiment of  FIGS. 13 and 14  produces distinctly different magnetic field strengths and distributions in different parts of the vault  18 , as is schematically illustrated in  FIG. 16 . In the portion of the vault  18  with the roof magnet assembly  236 , there is a strong magnetic field B adjacent the vault roof  25 . With the illustrated magnetic polarities, the magnetic field is relatively weaker at the upper corner of the inner sidewall  24  but much stronger at the upper corner of the outer sidewall  22 . The inclusion of the non-magnetic spacer  230  between the two tubular magnets  226 ,  228  of the inner sidewall magnet assembly  224  produces a magnetic field distribution that is more parallel to the inner vault sidewall  24 , thereby evening the erosion pattern there. In contrast, as illustrated on the left side, in portions of the vault  18  distant from the roof magnet assembly  236 , the magnetic field B′ has a reduced intensity, particularly near the vault roof  25 . As a result, there is relatively more sputtering of the vault roof  25  in areas of the roof magnet assembly  236  than elsewhere, and the metal ionization fraction in that portion is substantially higher. In contrast, distant from the roof magnet assembly  236 , there is relatively substantial sputtering of the vault sidewalls  22 ,  24  with an increased fraction of neutral metal atoms being produced.  
      It is known that low-pressure sputtering requires a relatively high magnetic field. It is thus possible to select chamber pressure and target power such that the plasma is supported only adjacent the roof magnet assembly  236  or to select another combination of chamber pressure and target power such that the plasma is supported throughout the vault  18 .  
      It is thus seen that the complex geometry of the magnetron and target of the various embodiments of the invention provides additional controls on the intensity, directionality, and uniformity of sputtering.  
      It is possible to include multiple concentric vaults and to associate magnetic means with each of them.  
      It is also possible to additionally include an RF inductive coil to increase the plasma density in the processing space between the target and wafer. However, the unique configurations of the target and magnetron of the invention in large part eliminate the need for expensive coils.  
      Although the described embodiments have included a magnetron with a vault having vertical sidewalls and producing a substantially horizontal magnetic field in the vault. However, it is appreciated that the magnetic field cannot be completely controlled, and inclinations of the magnetic field may extend up to about 25°. Furthermore, the sidewalls may form more of a V-shaped vault with sidewall slope angles of up to 25°, but a maximum of 10° is preferred.  
      Although the invention has been described with respect to sputtering a coating substantially consisting of the material of the target, it can be advantageously used as well for reactor sputtering in which a gas such as nitrogen or oxygen is supplied into the chamber and reacts with the target material on the wafer surface to form a nitride or an oxide.  
     Processes and Structures  
      The magnetron  180  of  FIG. 12  using stationary annular side magnets has been used in a number of experiments with sputtering copper and has shown unusual capabilities. We believe that the unusual results arise from the enhanced ionization fraction of the sputtered copper as it passes through the extended magnetic field in the vault and the restriction of the high-density plasma to only a portion of the vault. The copper ions can be attracted to the wafer by the inherent DC self-bias of a floating pedestal and the attraction can be increased by RF biasing the pedestal. The controlled attraction controls the energy and directionality of the copper ions incident on the wafer and deep into the via hole and allows controlled fractions of ionized and neutral sputtered atoms.  
      The sputtering yield for copper ions as a function of the ion energy is plotted in  FIG. 17 . Thus, the higher sputter particles energies possible with the inventive magnetron and other magnetrons can produce a high copper yield in the case that the underlying copper is exposed during sputter processing by high-energy copper ions. Furthermore, the ratio of sputtering yield of tantalum relative to copper is 1:4 and further lower for TaN, thereby providing selectivity over copper. The believed effect of high-energy sputtered copper is schematically illustrated in the cross-sectional view of  FIG. 18 . A substrate  270  is formed with a lower copper metal feature  272 . An inter-level dielectric layer  274  is deposited thereover and photolithographically etched to form a via hole  276 . After pre-cleaning, a thin barrier layer  278  is substantially conformally coated in the via  276  hole and over the top of the dielectric  274 . The subsequent high-energy copper ion sputter deposition and resultant resputtering of the copper already deposited on the wafer reduces the deposition on the field area on the planar top of the oxide  274  and at a bottom  280  of the via hole  276 . However, the copper atoms resputtered from the bottom  280  of the via hole  276  is of lower energy than the incident copper ions and are emitted generally isotropically. As a result, they tend to coat the via sidewalls  282  even more than the via bottom  280  because the sidewalls  282  are not exposed to the anisotropic high-energy copper ion column. The bottom sputtering further is likely to etch through the barrier layer  278  at the via bottom  280 , thus exposing the underlying copper  272 . Furthermore, the top layer of the copper  272  is cleaned in what is generally a PVD process. As a result, as illustrated in  FIG. 19 , the barrier layer  278  is removed at the bottom of the via hole  276  and a recess  284 , experimentally observed to be concave, is formed in the underlying copper  272 . Further, relatively thick copper sidewalls  286  of thickness ds are deposited while a copper field layer  288  of thickness dB is formed over the planar top of the dielectric  274 . Because of the high resputtering, overhangs do not form on the lip of the via  276 . The sidewall coverage ds/dB is observed to be in the neighborhood of 50 to 60% for high target power and low chamber pressure. The result may be described as selective PVD.  
      The removal of the lower barrier layer has two implications. The contact resistance is reduced because the barrier material is removed in the direct current path, specifically at the interface of the via metal being deposited with the underlying metal feature, and the copper of the upper-level metallization is in direct contact with the copper of the lower-level metallization, namely, the copper feature  272 . Furthermore, in prior systems, a high resistivity oxide of the underlying metal needed to be removed by a pre-metallization cleaning step. With the invention, the pre-clean that was necessary for that function to clean the oxide or residue at the top of the underlying copper  272  prior to depositing the barrier or seed layer is no longer required to assure direct contact between the two copper levels because the PVD step is itself removing the barrier and cleaning the underlying copper or other metal layer. Pre-cleaning on the sidewalls and top of the dielectric is much less of a requirement and may in some circumstances be eliminated.  
      It is noted that the structure illustrated in  FIG. 19  shows the removal of the barrier layer  278  on the horizontal bottom of the via hole  276  but its barrier field portion  290  remaining in the field area on the planar top of the dielectric layer  274 . This is possible if the metal ionization fraction is less than 100% so that a substantial number of unaccelerated metal neutrals are sputtered onto the field area. The neutrals, however, are shielded from reaching the via bottom. As a result, the high-energy metal ions can sputter the barrier layer  278  at the bottom of the via hole  276 , but they are overcome by the lower-energy metal neutrals at the top, and there is a net deposition of copper and no barrier removal in the field area on top of the dielectric layer  274 . This result differs from the process disclosed by Geffken et al. in the above patent in which all horizontally extending barrier layers are removed.  
      The sidewall coverage afforded by the high-energy ionized sputter deposition is sufficient for use as a seed layer. It is believed that about 5 nm sidewall coverage is required in 3 μm-deep vias having an 11:1 aspect ratio. However, the copper field coverage is reduced over the conventional sputtering process and does not provide a sufficient electrical path for the electroplating current. Therefore, a short, more conventional copper sputter process may be used to complete the copper seed layer and eliminate any voids in it and to thicken the field coverage. The more conventional sputtering produces not only lower-energy copper ions but a larger fraction of neutral copper sputter particles, which do not respond to wafer biasing. The two steps can be balanced to provide a balance between bottom coverage, sidewall coverage, and blanket thickness. That is, the conformality can be tailored. The more conventional copper sputter could be performed in a separate sputter reactor. However, in view of the small quantity of copper needed to complete the seed layer, the same reactor used for the high-energy sputtering can be adjusted to effect lower-energy sputtering. To accomplish this second step, for example, the target power can be reduced to reduce the plasma density and metal ionization fraction, the chamber pressure can be raised above 1 milliTorr, preferably about 1.5 milliTorr or higher, to reduce the wafer self-bias, thus reducing the ion energy, and to decrease the metal ionization fraction, the RF pedestal bias power can be reduced to decrease the acceleration of ions toward the wafer, or a combination of the three changes can be made between the two steps.  
      The structure of  FIG. 19  is accomplished by producing a relatively high but not very high copper ionization fraction. It is possible to perform within a single sputter reactor a two-step copper PVD process in which the first step produces the structure of  FIG. 19  and the second step is performed with chamber parameters adjusted to reduce the copper ion energy so that, as illustrated in the cross-sectional view of  FIG. 20 , the via bottom is coated with a second copper layer  292  covering all areas including a via bottom portion  294 . Further, it is noted that the two-step copper PVD process can be advantageously used even in the case where the first step does not leave a barrier field layer  290  and a first copper field layer  288  is not deposited. For single-level damascene, the field region is subjected to CMP, and these extra layers in the field region are not crucial.  
      Following the formation of the second copper layer  292 , the via hole is filled and overfilled by electro chemical plating of copper using the second copper layer  292  both as a seed layer and an electrode. Thereafter, the copper and typically the barrier layers exposed over the field area are removed by chemical mechanical polishing.  
      Although the description above is directed to removing the barrier layer at the bottom of the via hole, a similar two-step process may be used to produce a more conformal seed layer coating even if the bottom barrier layer is not removed. The chamber parameters for the first step are adjusted to emphasize middle sidewall coverage with little or no bottom and/or field coverage. The chamber parameters are then changed for the second step to emphasize bottom, upper sidewall, and field coverage. In most cases, this means that there is a substantial fraction of energetic metal ions in the first step and a larger fraction of neutrals relative to energetic ions in the second step. The two-step process is superior to a one-step process with intermediate chamber parameters because the latter tend to immediately begin producing an overhanging lip at the top of the via hole which would interfere with bottom and middle sidewall coverage.  
      The invention can be advantageously applied to more complex and demanding structures desired in advanced integrated circuits. A dual damascene structure is illustrated in the sectioned orthographic view of  FIG. 21 , which allows inter-level vias and horizontal interconnects to be metallized in a single metallization process. A generally dielectric underlayer  300  includes a copper feature  302  in its surface that needs to be electrically contacted through an overlying inter-level dielectric layer  304 . A horizontally extending trench  306  is formed at the top of inter-level dielectric layer  304 , and one or more vias  308  (only one of which is illustrated) are formed between the bottom of the trench  306  and the corresponding ones of the copper features  302 . A single sequence of metallization steps are used to simultaneously metallize the trench  306  (providing the horizontal interconnects) and the vias  308  to the lower-level metallization  302 . However, a barrier layer  310  is required between the metal and any neighboring dielectric materials, for example, a TaN barrier for copper metallization. The barrier layer  310  is divided into a field portion  312  on top of the upper dielectric layer  304 , a trench sidewall portion  314 , a trench floor portion  316 , and a via sidewall portion  318 . All these portions  312 ,  314 ,  316 , and  318  are desired for a reliable integrated circuit. However, it is desired that the barrier layer  310  not extend over the bottom of the via hole  308  in order to reduce the contact resistance to the metal feature. Accordingly, it is greatly desired that a sputtering process be available which has high bottom coverage in the trench  306  and no bottom coverage in the via hole  308 . The dual-damascene process disclosed by Geffken et al. in the above cited patent lacks this selectivity. It is noted that the trench  306  has a very low aspect ratio along its axis but may have a relatively high aspect ratio transverse to its axis. Chen et al. describe a somewhat similar selective removal and deposition in Ser. No. 09/704,161, filed Nov. 1, 2000, incorporated herein by reference in its entirety. Parent U.S. Pat. No. 6,277,249 discloses a similar process to that discussed here with respect to  FIGS. 14 and 15 .  
      Such a selective removal of the barrier layer and selective deposition of copper is possible by adjusting the copper PVD process parameters to assure a balance between energetic copper ions and low-energy copper neutrals to produce the structure illustrated in cross section in  FIG. 22 . A first copper seed layer  320  is deposited with relatively high copper ion energy but a substantial neutral fraction so that the barrier layer  310  at the bottom of the via hole  308  is removed and the underlying copper feature  302  is cleaned. However, the copper layer  320  is deposited over and thus protects the barrier layer  310  on the via sidewall  322 , the trench floor  324 , the trench sidewall  326 , and the field area  328  because these are either less exposed to the high-energy copper ions or more exposed to the lower-energy copper neutrals.  
      It is advantageous to perform the second copper seed deposition to produce a conformal second copper seed layer  330 , illustrated in the cross-sectional view of  FIG. 23 , to assure a thick sidewall and via bottom coverage as well as thick field coverage. The second copper seed layer  330  is in direct contact with the cleaned upper surface of the underlying copper feature  302 , thus assuring a good electrical contact.  
      Following the deposition of the second copper seed layer  330 , the via hole  308  and trench  306  are filled with copper by electrochemical plating using the second copper layer  330  as both a seed layer and a plating electrode. Thereafter, chemical mechanical polishing removes any copper exposed above the field area  328  outside of the trench  306  and typically also the barrier layer  310  in the area.  
      For a given PVD chamber, particularly one of the SIP +  chamber described above, the metal ionization fraction is increased by operating at a lower pressure or a higher target power. The metal ion energy can be increased by these same two techniques or by increasing the pedestal self-bias by any technique.  
      It has been observed that the DC self-bias on a floating pedestal depends on the chamber pressure. For example, at 0.85 milliTorr, a self bias of about −20 VDC develops; and at 0.64 milliTorr, about −100 VDC. Thus, the chamber pressure can be used to control the copper ion energy. Similarly, increases of the target power from 20 kW to 40 kW show about the same sequence of floating self-bias voltages, providing yet another tool for copper ion energy.  
      An alternative approach to differentiate between the bottom and top of the via hole is to use an auxiliary electromagnetic coil wrapped around the outside of the central axial portion of the chamber about its central axis to selectively generate an axial magnetic field between the target and wafer. When the field is turned on in the first step, the metal ions are preferentially guided toward the wafer compared to when the field is turned off or reduced in the second step. Wei discloses such an auxiliary electromagnet in U.S. patent application Ser. No. 09/612,861, filed Jul. 10, 2000, now issued as U.S. Pat. No. 6,352,629, incorporated herein by reference in its entirety.  
      We believe that a sputter reactor such as those of  FIGS. 11, 13 , and  14  having vaulted target and one or more nested top magnet assemblies and continuous inner and outer sidewall magnet can be operated in two distinct modes determined by a combination of target power and chamber pressure. At higher power and lower pressure, the self-bias on the pedestal is between −100 to −150 VDC while at lower power and higher pressure, the self-bias assumes the more normal value of −30 VDC. A related difference is that, below a certain argon pressure, the target voltage is between about −450 and −700 VDC while above that pressure the target voltage drops to about −400 VDC. Although we are not bound by our understanding of the invention, we believe that at lower pressure and higher power the plasma is maintained in the vault only in the area of the top nested magnet assembly. Elsewhere, the plasma is extinguished. The magnetic fields in the area of the localized plasma may be sufficient to funnel an ionized copper flux towards the wafer. The copper ionization fraction in this mode may be quite high, near 50%, and the high wafer self-bias draws highly energetic ions to the wafer and deep within high aspect-ratio holes. We believe that at higher pressure the sidewall magnets are sufficient to maintain a plasma throughout the entire length of the vault. The lower plasma densities and increased scattering produce a more neutral flux of copper atoms.  
      Applying RF bias to the pedestal through a coupling capacitor will also increase the DC self-bias.  
      Some of the more pronounced high-energy sputtering results were obtained with a chamber pressure of 0.5 milliTorr, 40 kW of target power, and 300 W of RF bias applied to the pedestal.  
      A process for accomplishing a copper via is illustrated in the flow diagram of  FIG. 24 . In step  340 , a inter-metal dielectric layer of, for example, TEOS silicon dioxide or a low-k dielectric whether carbon-based or silicon-based, is deposited, usually by a CVD process and photolithographically patterned with via holes using a plasma etching process. The dielectric patterning may be dual damascene, which includes both the vias and interconnect trenches in a common connecting structure. These steps are not directly part of the invention, and may be practiced in any number of ways. It is assumed that the material underlying the via holes is copper. Contact holes to underlying silicon require a somewhat more complex process.  
      Thereafter, the wafer is placed in a multi-chamber integrated processing system. In some circumstances, no plasma preclean need be performed. Instead, one PVD system is used in step  342  to deposit the barrier layer into the via hole and on top of the dielectric. Chemical vapor deposition (CVD) can instead be used for the barrier layer, or a combination of CVD and PVD can be used. In step  344 , the high-energy ionized copper deposition both cleans the bottom of the via hole and coats its sidewalls, as has been described. This step also cleans the interface of the underlying copper exposed beneath the barrier layer. Even in this mode, a substantial neutral flux is present that cannot penetrate to the bottom of the via hole but does deposit on the planar field portion above the dielectric. As a result, the barrier layer on the field portion is not sputtered away by the energetic copper ions but is protected by some deposition of neutral copper.  
      In step  346 , a lower-energy, more neutral copper sputter deposition is performed to complete the seed layer, also used as the electroplating electrode. Whatever copper ions are present are accelerated by a lesser self-bias voltage and thus do not significantly sputter. Therefore, the lower energy copper ions the bottom of the via to provide bottom coverage and the neutral copper effectively coats the exposed planar field portion.  
      The two steps  344 ,  346  can be at least partially separated by requiring the first step  344  to be performed at a pressure of less than 1.0 milliTorr, more preferably 0.7 milliTorr or less, and most preferably 0.5 milliTorr or less, while the second step  346  is performed at 1.5 milliTorr or above.  
      By proper timing of the two steps  344 ,  346  and their associated target powers and chamber pressure, not only is the bottom barrier layer removed but the conformality of the copper deposition at the via bottom, via sidewall, and field portion can be adjusted.  
      In step  348 , the copper metallization is completed with an electroplating or other electrochemical process.  
      Although this process has been described with reference to the inventive vault magnetron, similar high-energy ionized copper sputtering can be achieved in other ways. Achieving the desired selective PVD is believed to be eased by creating an energy distribution of the copper ions in the plasma with a peak energy of between 50 and 300 eV and/or by maintaining the ratio of argon ions to copper ions Ar + /Cu +  in the plasma at 0.2 or less. Of course, the ultimate low fraction is obtained with sustained self-sputtering. The low fraction of argon ions reduces the problems commonly experienced with HDP sputtering.  
      Further, it has been shown the inventive SIP +  reactors can be used for the sputter deposition of Ta, TaN, Al, Ti, and TiN and should be usable with W, especially for the effects of selective removal, selective deposition, and a multi-step process.  
      The inventive process need not completely remove the barrier layer at the bottom of the via to reduce the contact resistance. The outer portion, for example, of TiN while providing the barrier function has the highest resistivity. Hence, removing just the nitride portion would be advantageous.  
      Of course, the invention can be used with copper alloyed with a five percent of an alloying element such as silicon, aluminum, or magnesium. Further, many aspects of the invention are applicable as well to sputtering other materials.