Abstract:
Ionized physical vapor deposition (IPVD) is provided by a method of apparatus for sputtering coating material from a compound sputtering source formed of an annular ring-shaped target with a circular target at its center, increasing deposition rate and coating uniformity. Each target is separately energized to facilitate control of the distribution of material sputtered into the chamber and the uniformity of the deposited film. The sputtered material from the targets is ionized in a processing space between the target and a substrate by generating a dense plasma in the space with energy coupled from a coil located outside of the vacuum chamber behind an annular dielectric window in the chamber wall in the central opening of the annular target and surrounding the circular target. A Faraday type shield physically shields the window to prevent coating material from coating the window, while allowing the inductive coupling of energy from the coil into the processing space.

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 09/073,141, of John S. Drewery and Thomas J. Licata, inventors herein, filed May 6, 1998, now U.S. Pat. No. 6,080,287 entitled Method and Apparatus for Ionized Physical Vapor Deposition, hereby expressly incorporated by reference herein. 
     FIELD OF THE INVENTION 
     This invention relates to Ionized Physical Vapor Deposition and, more particularly, to methods and apparatus by which coating material is supplied in a vapor state and ionized to improve the directionality and uniformity of the deposition of the coating material onto substrates. 
     BACKGROUND OF THE INVENTION 
     Ionized physical vapor deposition is a process which has particular utility in filling and lining high aspect ratio structures on silicon wafers. In ionized physical vapor deposition (IPVD) used for the deposition of thin coatings on semiconductor wafers, materials to be deposited are sputtered or otherwise vaporized from a usually solid source and then a substantial fraction of the vaporized material is converted to positive ions before reaching the wafer to be coated. This ionization is accomplished by a high density plasma which is generated in a process gas in a vacuum chamber. The plasma may be generated by reactively (capacitively or inductively) coupling RF energy from an RF powered excitation element into the vacuum processing chamber. Such a plasma may be produced, for example, by inductively coupling RF energy from a coil to produce an inductively coupled plasma (ICP) within the chamber. 
     To ionize the coating material, the dense plasma is generated in a region between the source and the wafer. When sputtered material passes through the region containing the dense plasma, a substantial fraction of the material becomes ionized. Once the material is ionized, electromagnetic forces applied to the ionized coating material, such as by applying a negative bias on the wafer, attract positive ions of the coating material toward the wafer. Such a negative bias may either arise with the wafer electrically isolated by reason of the immersion of the wafer in a plasma or by the application of an RF voltage to the wafer or wafer support. The bias potential causes ions of coating material to be accelerated toward the wafer, which increases the fraction of the coating material that arrives at the surface of the wafer at angles that are approximately normal to the wafer. This allows deposition of metal over wafer topography such as deep and narrow holes and trenches on the wafer surface, providing coverage of the bottoms and sidewalls of such topography. 
     Certain IPVD systems proposed by the assignee of the present application are disclosed in: U.S. patent application Ser. Nos. 08/844,751 entitled Method and Apparatus for Ionized Sputtering; 08/837,551 entitled Apparatus for Ionized Sputtering, now U.S. Pat. No. 5,800,688; and 08/844,756 entitled Apparatus for Ionized Sputtering of Materials. Each of these applications were filed on Apr. 21, 1997 and each is hereby expressly incorporated by reference herein. Such systems include a vacuum chamber which is typically cylindrical in shape and provided with part of its curved outer wall formed of a dielectric material or window. A helical electrically conducting coil is disposed outside the dielectric window and around, and concentric with, the chamber, with the axial extent of the coil being a significant part of the axial extent of the dielectric wall. In operation, the coil is energized from a supply of RF power through a suitable matching system. The dielectric window allows the energy from the coil to be coupled into the chamber while isolating the coil from direct contact with the plasma. The window is protected from metal coating material deposition by an arrangement of shields, typically formed of metal, which are capable of passing RF magnetic fields into the interior region of the chamber while preventing deposition of metal onto the dielectric window that would tend to form conducting paths for circulating currents generated by these magnetic fields. Such currents are undesirable because they lead to ohmic heating and to reduction of the magnetic coupling of plasma excitation energy from the coil to the plasma. The purpose of this excitation energy is to generate high density plasma in the interior region of the chamber. A reduction of coupling causes plasma densities to be reduced and process results to deteriorate. 
     In such IPVD systems, material is, for example, sputtered from a target, which is charged negatively with respect to a high energy sputtering plasma formed close to the target, usually by means of a DC power supply connected between the target and a chamber anode such as the grounded chamber wall. The target is often of a planar magnetron design incorporating a magnetic circuit or other magnet structure which confines the sputtering plasma over the target for sputtering the target. The material arrives at a wafer supported on a wafer support or table to which a negative bias is typically applied by means of an RF power supply connected through a matching network to the substrate support. 
     A somewhat different geometry employs a plasma generated by a coil placed internal to the vacuum chamber. Such a system does not require dielectric chamber walls or special shields to protect the dielectric walls. Such a system is described by Barns et al. in U.S. Pat. No. 5,178,739, expressly incorporated by reference herein. Systems with coils outside of the chamber, as well as systems of the type disclosed in the Barnes et al. patent, involve the use of inductive coils or other reactive coupling elements, either inside or external to the vacuum, that are physically positioned and occupy space between the planes of the sputtering target and the wafer. 
     Whether a coupling element such as a coil is provided inside or outside of a vacuum chamber, dimensions of the system are constrained by the need for adequate source-to-substrate separation to allow for the installation of the RF energy coupling elements between the source and the substrate. Adequate diameter must also be available around the wafer for installation of coils or other coupling elements. As a direct result of the increased source-to-substrate spacing due to the need to allow space for the coupling element, it is difficult to achieve adequate uniformity of deposition with such systems. If the height of the chamber is reduced to improve uniformity, there is a loss of plasma density in the central region of the chamber and the ionization percentage of the coating material is reduced. Also, in practice, the entire system must fit within a constrained radius. As a result, there are frequently problems due to heating arising from the proximity of the RF coils to metal surfaces, which may necessitate extra cooling, which increases engineering and production costs and wastes power. 
     An IPVD apparatus with the coil in the chamber has the additional disadvantage that the coils are eroded by the plasma and must, therefore, consist of target grade material of the same type as that of which the target is made. Moreover, considerable cooling of coils placed in the vacuum chamber is needed. If liquid is used for this cooling of the coils, there is danger that the coils will be penetrated by uneven erosion or by arcing, causing a resulting leak of liquid into the system, which is highly undesirable and will likely result in a long period of cleaning and requalification of the system. Furthermore, an excitation coil in the chamber also couples capacitively to the plasma, leading to inefficient use of the excitation power and to the broadening of the ion energy spectrum, which may have undesirable effects on the process. 
     As a result of the above considerations and problems, there remains a need for more efficiently coupling energy into the dense coating material ionizing plasma in IPVD processing systems, and to do so without interfering with the optimum dimensions of the chamber and, preferably, without placing a coil or other coupling element into the vacuum chamber. Furthermore, the solution to these problems must further provide uniformity of the deposited film on the substrate. 
    
    
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to provide an IPVD method and an IPVD apparatus in which the placement of the coil or other coupling element does not adversely affect the geometry of the chamber of the processing apparatus. Another objective of the present invention is to provide an IPVD method and an IPVD apparatus in which the placement of the coil or other coupling element does not adversely affect the uniformity of the deposited film on the substrate. A still further objective of the present invention is to provide a more efficient and effective method and apparatus for the performance of IPVD. 
     According to the principles of the present invention, an IPVD apparatus is provided with a ring-shaped source of coating material having a central opening at its center with a central source of coating material that matches the material of the ring-shaped source situated in the opening, preferably at the center. The sources produce a vapor formed of atoms and minute particles of the coating material in a processing space within a vacuum chamber. An annular coupling element is provided at the central opening of the ring-shaped source surrounding the central source. The coupling element operates to reactively couple RF energy into the chamber to produce a high density, reactively coupled plasma in the processing space. The high density plasma ionizes coating material passing through the processing space. The ions of coating material drift toward a substrate, which is supported in the chamber at the opposite end of the processing space from the source, in paths influenced by electrostatic or electromagnetic fields that are present in the processing space. Ions that arrive within a certain distance of the substrate, for example, in the order of a centimeter from the substrate, encounter a sheath field and are accelerated toward the substrate by the potential between the plasma and the substrate. As a result, a high percentage of the coating material arrives on the substrate at angles normal to the substrate, thereby more effectively lining the bottoms and sides of, or filling, small and high aspect ratio features on the surface of the substrate. 
     In accordance with certain objectives of the invention, an IPVD apparatus and method are preferably provided with two independently powered metal sputtering targets, a coil and a Faraday shield, concentrically arranged. One target is a central circular plate. The Faraday shield is annular and surrounds the plate. The second target is also annular and surrounds the Faraday shield. The coil is situated behind the shield and couples RF energy through the shield into the processing space. The Faraday shield and coil are optimized to work in cooperation with the two concentric targets to deliver coating material to the substrate at high deposition rates and with uniformity that can be optimized through adjustment of the relative powers to the targets. 
     In a preferred embodiment of the invention, a coating material source, preferably including a ring-shaped sputtering target, is provided with an annular dielectric window placed in its central opening. Behind the window, outside the vacuum of the chamber, is located a ring-shaped plasma source which includes a coupling element. The element is connected to the output of an RF energy source. The coupling element is preferably a coil configured to inductively couple energy supplied from the energy source through the window at the opening at the center of the ring-shaped material source and into the region of the chamber between the coating material source and the substrate on a substrate support at the opposite end of the chamber from the coating material source. 
     In accordance with the preferred embodiment, the coating material source includes a central source, preferably, a circular sputtering target, situated in the center of the annular window to provide coating material that originates from the center of the source. The central source cooperates with the ring-shaped source to provide a uniform flux of material onto the substrate and particularly, supplements the coating of the sides of features on the substrate that face radially inward toward a central axis of the chamber on which the substrate and source are centered. 
     Preferably, the apparatus of the present invention includes an annular sputtering target and a central sputtering target between which is the annular dielectric window that seals an opening in the wall of the chamber. Behind the window is a coil or other coupling element. In certain preferred embodiments, the annular and central targets are flat or nearly flat and lie in a common plane. A magnetron magnet assembly is preferably positioned behind the targets to produce a plasma confining magnetic field over the targets. the magnet assembly preferably includes an annular magnetic tunnel over the ring-shaped target. The targets are simultaneously energized with a negative voltage, which are preferably produced by a DC or pulsed DC power supply. High energy sputtering plasma is generated, which is generally confined to the surfaces of the targets, to sputter material from the targets. Separate DC power supplies for each of the targets allow the relative sputtering rates of the targets to be separately controlled. 
     The coupling element is preferably a coil positioned behind and close to the back, outside surface of the dielectric window at the central opening of an annular sputtering target. RF energy of, for example, 13.56 MHZ, is applied to the coil to excite a high density inductively coupled plasma in the chamber between the targets and the substrate. The main sputtering plasma, which is trapped under a field of the magnetron magnets at the surfaces of the targets, sputters coating material from the targets and into the region of the processing space occupied by the dense secondary plasma, where a substantial portion of the material is stripped of electrons to form positive ions of the coating material. A negative bias voltage applied to a wafer on the substrate holder attracts the positive ions of sputtering material from the region of the secondary plasma and toward and onto the surface of the substrate. The angles of incidence of the material arriving on the substrate are nearly perpendicular to the substrate and enter holes and trenches on the substrate surface to coat the bottoms of these holes and trenches. The central target provides increased material to the radially inward facing sides of the features, enhancing the uniformity of the film. The relative powers applied to the targets are separately adjusted to achieve uniform coverage on the substrate. These powers are readjusted over the life of the target to maintain uniformity as the target erodes or as other parameters change. 
     According to the apparatus of the invention, the processing chamber is dimensioned to provide optimum spacing between the coating material source and the substrate to provide both optimal ionization of sputtered species as well as optimal uniformity of deposition on the wafers. 
     The present invention provides greater freedom of design choice in configuring the processing chamber to optimize the IPVD process and does so while overcoming the difficulties set forth in the background above, providing a uniform film on substrates having sub-micron sized high aspect ratio features thereon. In particular, the present invention achieves higher deposition rates, better flat field uniformity and more uniform step coverage on recessed features than systems of the prior art. These benefits allow the source to be conveniently used for flat field depositions as well as for step coverage and fill depositions. 
     These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional diagram illustrating an IPVD apparatus according to one preferred embodiment of the present invention. 
     FIG. 2 is a plan view illustrating the cathode and electrode assembly of the IPVD apparatus of FIG.  1 . 
     FIG. 3 is a cross-sectional view of the shield of a cathode and electrode assembly of FIG. 2 taken along the line  3 — 3  of FIG.  2 . 
     FIG. 3A is a cross-sectional view, similar to FIG. 3, illustrating an alternative embodiment of the shield of the cathode and electrode assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An IPVD sputtering apparatus  10  according to a preferred embodiment of the present invention is illustrated diagrammatically in FIG.  1 . The IPVD apparatus  10  includes a vacuum chamber  11  bounded by a chamber wall  12 . The chamber  11  is provided with a vacuum pumping system  13  for evacuation of the chamber  11  to high vacuum and a supply gas control valve  14  which vents into the chamber  11  process gas, including an inert gas such as argon, for use in physical sputtering, or including reactive gases for use in reactive sputtering applications. A system for loading and unloading wafers for processing (not shown) is also provided. 
     At one end of the chamber  11  is situated a wafer holder  15  to which an RF generator  16  is connected through a matching network  17  for application of RF-generated negative bias on the substrate or wafer holder  15  and to a wafer  18  supported thereon for processing. A large opening is provided in the wall  12  at the opposite end of the chamber  11  from the wafer support or holder  15 . This opening is sealed by a source arrangement that includes a cathode and electrode assembly  20 . The support  15  is preferably axially moveable relative to a cathode and electrode assembly  20  so that the spacing between targets on the cathode and electrode assembly  20  can be adjusted relative to the wafer  18  on the support  15  both initially and during the target life, primarily to maintain desired uniformity as the targets erode. Where such an adjustable support is provided, a seal in the form of a metal bellows (not shown), for example, may be provided between the mount of the support  15  and the wall  12  of the chamber  11 . The geometry of the chamber  11 , particularly the location of the sides of the chamber wall  11  with respect to the outer edge of the wafer  18 , has an effect on the uniformity of the deposition. The effect is greater where the walls  12  are close to the wafer, so that wide spacing of the sides of the wall  12  from the wafer  18  is preferably maintained. Accordingly, the geometry of the chamber  11  is empirically configured in relation to other parameters of the process to optimize deposition uniformity and other properties of the film being deposited. 
     As further shown in FIGS. 1 and 2, the assembly  20  includes a generally annular target  21  shown on a backing plate  23 , which in operation, is energized to a negative potential by a cathode power supply  22 . The power supply  22  is, preferably, in the form of a DC power supply or a pulsed DC power supply for metal deposition, but may be an RF supply connected to the target  21  through a suitable matching network (not shown). The erosion of the target  21  is controlled by inner and outer dark space rings  24 , 25  and, preferably, also by magnetic fields  27  generated by a magnetron magnet assembly  26 . Cooling of the target  21  is achieved by the use of internal water channels (not shown) or by a suitable external cooling system (not shown), which are conventional. The target  21  is electrically isolated from the electrically grounded surfaces of the apparatus  10  by insulators  28  and  29 . 
     The assembly  20  also includes a generally circular target  31 , shown on a target backing plate  33 . In operation, the target  31  is charged negatively by a cathode power supply  32  similar to power supply  22 , but which is controllable independently of the power supply  22 . The erosion of the target  31  is controlled by an annular dark space shield  39  and, preferably, also by a magnetic field  34  generated by a magnetron magnet assembly  35 . Cooling (not shown) of the target  31  is also provided. The target  31  is electrically isolated from electrically grounded surfaces of the apparatus  10  by annular insulator  36 . The inner circular target  31  is concentric with the annular outer target  21 . 
     Between the outer annular target  21  and the inner circular target  31  is mounted an RF plasma excitation system  60 . The RF excitation system  60  may be implemented in a number of ways. One preferred embodiment is that illustrated, in which the plasma excitation system  60  includes a plane annular dielectric window  61  which seals the annular opening in the wall  12  of the chamber  11  between the targets  21  and  31 . The side of the window  61  that is facing the substrate holder  15  is in contact with the vacuum of the chamber  11  while the opposite side or outer face of the window  61  is in contact with a nominally atmospheric environment. Near the outer face of the window  61  is an excitation coil  65  which preferably extends in a direction generally parallel to the window  61  and may be planar or substantially planar. Suitable coils  65  can be adapted from coils, for example, as described in U.S. Pat. No. 4,948,458 granted to Ogle and in U.S. Pat. No. 5,669,975 granted to Ashtiani, both hereby expressly incorporated by reference herein. 
     The coil  65  is energized by an RF power supply  62  to which it is connected through a matching network  63 . 
     On the interior side of the window  61  is a shield  66 , which is a combination Faraday shield and physical shield. As a Faraday shield, the shield  66  permits RF energy from the coil  65  to couple into the chamber  11  without capacitively shielding the chamber  11  from the coil  65 . As a physical shield, the shield  66  prevents coating material, which may be metal and electrically conductive, from depositing onto the window  61 . The shield  66  is mounted a small distance from the window, close to the inside surface of the dielectric window  61 . The shield  66  is preferably slofted and may be electrically grounded to, and maintained in, good thermal contact with the dark space shields  24  and  25 , which are preferably water cooled. Alternatively, the shield  66  may be electrically floating, at least with respect to RF energy on the coil  65 , as, for example, by being grounded through a low pass filter  67 . In the preferred embodiment, the shield  66  is cooled by conduction from the edges, either to the dark space shield  25  which is water-cooled, or through an optional electrically insulating and thermally conductive supporting ring (not shown) that would allow cooling of the shield  66  while electrically insulating the shield  66  from the dark space shield  25 , if desired. Other shield cooling techniques and arrangements of shields can be employed. 
     In operation, an RF voltage is supplied by the power supply  62  through matching network  21  to the excitation coil  65 . Techniques for design of the matching unit and certain connections for minimizing undesirable capacitive coupling to the shield  66  are described in U.S. Pat. No. 5,669,975 to Ashtiani. Process gases are supplied to raise the pressure in the chamber to a pressure of from about 1 to about 100 mTorr, usually in the 1-40 mTorr range. A high density plasma can then be ignited in the chamber. DC or RF power is supplied to the sputtering targets  21  and  31 , which is eroded by ion bombardment from the main plasma. The material sputtered from the target by the main plasma is ionized as it passes through the high density plasma supported by the inductively coupled energy from the coil  65 . 
     A negative bias is applied to the substrate  18  from an RF source  16  through a matching network  17  to electrostatically attract and steer positive ions of sputtering material from the plasma onto the substrate  18  in a direction nearly perpendicular to the substrate  18 . The bias energy source  16  employs RF energy or other form of electrical energy that is effective to cause a potential to develop on the substrate that is sufficiently negative relative to the potential of the plasma to attract positive ions of sputtered material from the plasma toward the substrate  18  so that they preferably impinge upon the substrate  18  at or nearly at right angles to the surface, thereby enabling the coating of the bottoms of high aspect ratio features on the substrate  12 . For deposition, the bias potential on the substrate  12  should not be so high as to attract the ions onto the substrate at energies that are high enough to substantially etch or otherwise damage the films or the devices being formed on the substrate  12 . 
     As illustrated in FIGS. 2 and 3, the shield  66  is provided with an array of radially oriented slots  70  therethrough that divide the shield  66  into an array of radially disposed fins  71  connected at their inner and outer edges by narrow circumferential rims  72 , 73 . The circumferential extent of the rims  72 , 73  is interrupted at one slot  74  which extends the entire transverse radial extent of the shield  66  at one angular position on the circumference of the shield  66 . The interruption of the rims  72 , 73  prevents the formation of a circumferential current path in the shield  66  that would short out coupled fields from the coil  65  and thereby reduce the coupling of energy from the coil  65  to the plasma within the chamber  11 . The plurality of slots  71  prevent the induction of eddy currents in the shield that would also reduce coupling of energy from the coil  65  to the plasma. 
     As illustrated in FIG. 3, the slots  71  are preferably provided with an angular bend or other configuration that will block direct paths for the coating material to pass through the slots  71  of the shield  66  from the processing space within the chamber  11  and onto the window  61 , Such coating on the window  61 , if it were allowed to deposit, would coat the window with conductive film and electrically shield the plasma from the coil  65 , reducing or eliminating the coupling of RF energy to form the plasma. The slots  70  of the shield  66  of FIG. 3 are formed by any suitable method, such as by casting, by milling a plate from opposite sides or by electric discharge machining (EDM). 
     FIG. 3A illustrates an alternative shield  66   a,  which is formed of two layers or plates  75   a,    75   b,  each having respective arrays of slots  70   a,    70   b  therein and intervening fins  71   a,    71   b,  each interconnected at their inner and outer edges by respective inner and outer rims (not shown). The slots  70   a  are staggered in relation to the slots  70   b  so that the fins  71   a  and  71   b  interrupt direct paths for coating material from the processing space of the chamber  11  to the window  61 . 
     An advantage of locating the plasma excitation system at an opening in the center of an annular target or other ring-shaped source is that dimensional constraints imposed by the need to interpose helical coils concentrically around an axis between the center of the sputtering source and the center of the wafer are eliminated. A further advantage is that benefits arise from the use of the annular target or ring-like source, which enhances film uniformity at a greater target to wafer separation than in the case of a flat solid planar source. The ability to achieve uniformity at greater target-to-substrate distance allows more opportunity for ionization of the sputtered material. When ring-shaped sources have been used with sputter coating devices of the prior art, such devices usually required special shielding in the central region of the source to prevent the redeposition of sputtered material at this region. In such a case, heating of this central region often was necessary to ensure good adhesion of this material and to prevent flaking off of the deposited material, which would increase particulate contamination of the chamber. In embodiments of the present invention, the central opening of the ring-shaped source is used as a location for equipment used to excite a high density plasma and so becomes a useful part of the system rather than a liability by contributing to the ionization of sputtered material for directional deposition onto the substrate. 
     A further advantage arising from various embodiments of the present invention is that the cathode and the secondary plasma exciting structure can be made more compact than in the prior art and does not require extra feed-through or other modifications to the process vacuum chamber, making it practical to fit the source to existing modules. 
     An advantage of providing the two part target, particularly one that includes the annular target ring  21  and the inner target circular disc  31 , is that higher deposition rates can be achieved and greater uniformity of deposited film realized. By providing a controller  80  through which the relative powers and other operating parameters of the two targets  21  and  31  can be controlled differently, the uniformity of coating material on the substrate can be precisely maintained. With a cathode according to the present invention, greater freedom of design choice in configuring the processing chamber is provided which enables the optimizing of the IPVD process. The use of the inner and outer target according to the present invention overcomes the difficulties of the prior art and provides uniform film coverage on substrates having sub-micron sized high aspect ratio features thereon. In particular, the present invention achieves higher deposition rates, better flat field uniformity and more uniform step coverage on recessed features than systems of the prior art. These benefits allow the source to be conveniently used for flat field depositions as well as for step coverage and fill depositions. 
     Those skilled in the art will appreciate that the implementation of the present invention herein can be varied, and that the invention is described in preferred embodiments. Accordingly, additions and modifications can be made without departing from the principles and intentions of the invention.