Abstract:
A system and method is provided for using an ionized physical vapor deposition (iPVD) source for uniform metal deposition having uniform plasma density at relatively low (5 mTorr) and relatively high (65 mTorr) operation. Magnet structure is combined with an inductively coupled plasma (ICP) source to shift the plasma toward the chamber periphery during low pressure operation to enhance uniformity, while plasma uniformity is promoted by randomization or thermalization of the plasma at higher pressures. Accordingly, uniformity is provided for both deposition and etching in combined sequential deposition-etch processes and for no-net-deposition (NND) and low-net-deposition (LND) deposition-etching processes.

Description:
[0001]     This invention relates to inductively coupled plasma (ICP) sources for use in the manufacture of semiconductor wafers. This invention particularly relates to relatively high pressure ionized physical vapor deposition (iPVD) and relatively low pressure etch sequential processes and systems where plasma uniformity is desirable over a wide pressure range as well as deposition and etching processes that result in no-net-deposition (NND) or low-net-deposition (LND).  
       BACKGROUND OF THE INVENTION  
       [0002]     For the deposition of films onto high aspect ratio, submicron-featured semiconductor wafers, ionized physical vapor deposition (iPVD) has proved most useful. Apparatus having the features described in U.S. Pat. Nos. 6,287,435, 6,080,287, 6,197,165, 6,132,564 are particularly well suited for the sequential or simultaneous deposition and etching processes. Sequential deposition and etching processes can be applied to a substrate in the same process chamber without breaking vacuum or moving the wafer from chamber to chamber. The configuration of the apparatus allows rapid change from ionized PVD mode to etching mode or from etching mode to ionized PVD mode. The configuration of the apparatus also allows for the simultaneous optimization of ionized PVD process control parameters during the deposition mode and etching process control parameters during the etching mode.  
         [0003]     Of the advantages of ionized PVD systems, there are still some constraints to utilization of the system at the maximum of its performance. For example, existing hardware does not allow optimizing uniformity for both deposition and etch processes simultaneously over a wide process pressure window. While an annular target provides excellent conditions for flat field deposition uniformity, the use of large area inductively coupled plasma (ICP) to generate a large size low-pressure plasma for uniform etch process is geometrically limited. While an ICP source that is axially aligned with the substrate is optimal to ionize metal vapor sputtered from a target and to fill features in the center of a wafer, it can produce an axially peaked high-density plasma profile that does not provide a uniform etch in a combined deposition and etch process or in a no-net-deposition (NND) process or low-net-deposition (LND) process. In these processes, etching occurs at an increased bias at the wafer so deposited metal is simultaneously removed from the flat field area of the wafer during deposition while remaining deposited at the sidewalls of the feature. The net process leaves the deposition of a thin film at the bottom of the feature.  
         [0004]     The iPVD source of U.S. Pat. No. 6,080,287 provides a high metal ionization fraction and uniform metal deposition. Etching can be combined with iPVD processes as in U.S. Pat. No. 6,755,945 . When this combination is used to produce low-net-deposition or no-net-deposition processes, either a continuous or pulsed process step of sputter-etching of the wafer can be used. However, with a compact and centrally located RF coil and baffle, a non-uniform plasma can result during etching due to the tendency of the plasma to concentrate toward the chamber center at the lower pressures that are typically preferred for etching.  
         [0005]     Researchers have investigated the effects of chamber geometry and pressure on the plasma profile in an inductively coupled plasma source. To achieve a uniform plasma profile at high pressure (several tens of mTorr), RF coils have been placed toward the periphery of the cylindrical chamber. It has been also shown that, during low pressure operation, the plasma profile tends to be domed irrespective of the location of the RF coils, with the edge-to-center plasma density ratio being about 0.4 - 0.5.  
         [0006]     Accordingly, there remains a need to provide an iPVD source that can generate a uniform plasma at both relatively low pressures (e.g., at about 5 mTorr) for sputter-etch and relatively high (e.g., at about 65 mTorr) pressures for uniform metal deposition and for LND and NND processes at some common pressure, often but not necessarily, in the range of 20 - 60 mTorr.  
       SUMMARY OF THE INVENTION  
       [0007]     An objective of the present invention is to provide an iPVD source that can generate a uniform plasma at both relatively low pressures and relatively high pressures.  
         [0008]     A further objective of the invention is to provide a uniform plasma for metal deposition for sputter-etching.  
         [0009]     In accordance principles of the present invention, an iPVD source is provided with an ICP antenna and a peripheral magnetic field configured to trap high energy electrons towards the chamber periphery, thereby reducing the concentration of high energy electrons at the chamber center at lower chamber pressures or during etching, and reduce chamber diameter. Embodiments of the invention employ the peripheral magnetic field to improve plasma uniformity iPVD and etching processes, particularly in sequential deposition and etching processes.  
         [0010]     These and other objects and advantages of the present invention will be more readily apparent from the following detailed description of illustrated embodiments of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is cut-away perspective view of a processing apparatus having a source according to one embodiment of the invention.  
         [0012]      FIG. 2  is cut-away perspective view of a portion of a deposition baffle of the source of the processing apparatus of  FIG. 1 .  
         [0013]      FIG. 3  is diagrammatic perspective view illustrating a cooling channel configuration for the baffle of  FIG. 2 .  
         [0014]      FIG. 4  is a cross-sectional view through a portion of  FIG. 1  illustrating the baffle of  FIG. 2 .  
         [0015]      FIG. 5  is a perspective view illustrating an alternative magnet configuration to the embodiment shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0016]     One embodiment of an iPVD processing apparatus  10  is illustrated in  FIG. 1 . The apparatus  10  includes a vacuum processing chamber  12  having a wafer support  14  at the bottom thereof for supporting a wafer  15  thereon for processing, and a source  20  that includes a plasma source  30  and coating material source  40 . The coating material source  40  includes a sputtering target  42  at the top of the chamber  12  and having a sputtering surface  44  in communication with the vacuum chamber  12 . The target  42  is mounted in an opening in a chamber wall  11  that encloses the chamber  12  and which is either non-electrically-conductive or insulated form the target  42 . A target cooling system (not shown) is typically also provided. The material source  40  may also include magnetron magnets (not shown) on the top (back) side of the target  42 , which may including fixed or moving magnets such as rotating magnets. The material source  40  is also provided with a sputtering power source (also not shown) of typically DC electrical energy to form a sputtering plasma confined closely to the sputtering surface  44  of the target  42 .  
         [0017]     The plasma source  30  includes a dielectric window  32  which forms the cylindrical side-wall portion of the chamber wall  11 , an RF antenna  34 , shown as a helical coil that surrounds the outside of the dielectric window  32 , and a cylindrical axially-slotted, electrically-conductive deposition baffle  36 , which shields the dielectric window  32  from contamination by coating material from within the chamber  12 . The antenna  34  is configured to inductively couple RF energy into the chamber  12  to form a high density plasma in the chamber  12 .  
         [0018]     The plasma source  30  has spaced around the outer periphery of the plasma source  30  outside of the chamber  12  an array of magnets  50 . In the illustrated embodiment, the magnets  50  are closely spaced circumferentially around the chamber  12  with opposing poles  51  and  52 , with the polar axes of the magnets extending axially between their respective poles and aligned in the same direction to enclose within a magnetic field  70 , extending between the poles  51  and  52 , portions of the chamber wall  11  at the dielectric window  32 . The magnets  50  may be formed, for example, in a horseshoe shape and include a pair of bar magnets  53  and  54 , each having a pair of poles arranged such that one of the poles is a respective one of the poles  51  or  52  located close to the dielectric window  32 , with the other of the poles being adjacent a bar of magnetic core material  56 . The magnets  50  are preferably RF shielded by a thin copper, silver or nickel layer, and at least air cooled. The magnets  50  may also be provided with a cooling system (not shown). For example, the magnets  50  may be placed inside of or proximate to a water jacket.  
         [0019]     In the embodiment illustrated in  FIG. 1 , a permanent magnetic field  70  extends axially between the poles  51 , 52 , arcing around the conductors of the antenna  34  inside of the chamber  12  and inside the shield  36 , forming a circumferential magnetic tunnel around the inside of the window  32 . It is believed that, at low pressures, at the levels used for etching in particular, for example below about 20 mTorr, the magnetic field captures energetic electrons near the coil  34 , and deters them from flowing across the chamber  12  where they might concentrate near the center of the chamber  12 . These electrons would then do their ionizing more at the chamber periphery. This edge-weighted ionization would provide a more uniform plasma distribution throughout the chamber  12 , with the plasma ion density less domed or concentrated at the center.  
         [0020]     It is further believed that, at higher pressures, at the levels used for iPVD in particular, for example at pressures above about 30 mTorr, the frequent collisions randomize the electron motion sufficiently, so they do not feel the effects of the magnetic field and the plasma density distribution remains unchanged by the addition of the magnet assemblies. However, in that case, it would be the frequency of collisions with the background gas that would keep the energetic electrons from streaming across the chamber  12  from the region near the coil toward the chamber center. Instead, they would do a random walk that would eventually lead them throughout the chamber, but at such a slow pace that they would dissipate most of their energy near the coil, again providing an edge enhanced ionization.  
         [0021]     If a lower pressure coating process is employed or if there are other reasons for removing the magnetic field during deposition, permanent magnets or parts thereof can be made moveable to switch into or out of position during etching and deposition respectively. However, the presence of the magnets during higher pressure iPVD processes is unlikely to be detrimental and should in many cases be beneficial. The magnetic field strength should be at least about 50 Gauss, for example, up to 200 Gauss or above.  
         [0022]     The presence of a magnetic field near the coil  34 , rather than the field&#39;s configuration, should provide similar advantages described above. For example, a magnet  55   a  made up of segments as illustrated in  FIG. 5  can be provided around the chamber  12 , spaced outward so that its field  55   a  produces an array of magnetic cusps defining axially oriented tunnels that enclose a more limited portion of the coil  34 . The field of magnet  55   a  would have some effect within the chamber  12  of retaining electrons near the inside of the window  32  inside the shield  36  so as to flatten the plasma at lower pressures. Other magnet configurations can be used to produce a plasma flattening effect.  
         [0023]     As designed, the maximum radius of the source  20 , for wafers up to 30 cm in diameter, can be 50.5 cm, which is considerably less than many current iPVD modules. Such a source  20  may include targets of various shapes, including planar targets and inverted frusto-conical targets. Frusto-conical targets having cone angles of approximately 10 degrees to the horizontal are expected to be particularly useful. The size of the current iPVD module was driven by the desire to keep the plasma as uniform as possible above the wafer, and to reduce the radial ambipolar electric field. In order to achieve that goal, a large empty space was provided around the wafer  15 . With sources  20  according to the above described embodiment of the present invention, the plasma is uniform by design, and the radial ambipolar electric field is very small. The only constraint on the radius of the chamber is metal transport and loss to the wall, where reduction in the chamber diameter increases the fraction of the metal that is deposited on the baffle.  
         [0024]     Because of the smaller processing volume, the required RF power can be less than the 5.5 kWatt, which is typical in current iPVD systems. The smaller size also reduces coil inductance, making operation at 13.56 MHz easier to attain. The number of turns of the coil or antenna  34  can also be optimized. Operation at 2 MHz is expected to be particularly useful.  
         [0025]     The baffle  36  is preferably provided with slots  38  having chevron-shaped cross-sections to impede the flow of coating material through the slots  38  to the window  32 . The cylindrical baffle  36  has a much larger surface area than the circular baffles used with sources having antennas at an end of the chamber. This, combined with the reduced power flow through it reduces the heat load on the baffle  32 . Such a baffle  32  can be adequately cooled by contact with a cold sink, which can be part of the chamber wall. Optionally, the baffle can be cooled by water flow through channels along the baffle top and bottom, as illustrated in  FIG. 2 .  
         [0026]     The baffle  36  can also be provided with an upper support flange  60  which connects the baffle  36  at the chamber wall  11 , as illustrated in  FIG. 4 . At the wall  11 , the baffle  36  may be insulated from or electrically connected to the wall  11 , depending on whether the baffle  36  is to be maintained at a potential different than the chamber wall  11 . Typically, the baffle flange  60  is between the window  32  and the wall  11  and is well RF grounded from the chamber wall  11 .  
         [0027]     The flange  60  has an upper cooling fluid channel  61  around the top thereof to which liquid cooling fluid is supplied through an inlet  62 . The channel  61  is connected through a vertical channel  63  between two of the slots  38  in series with a lower cooling fluid channel  64  in the bottom rim of the baffle  36 , as illustrated in  FIG. 3 . The lower channel  64  connects further through another vertical channel  65  between a different two slots  38  to a fluid outlet  66  in the rim  60 . With the inlet  61  and outlet  66  in the rim of the flange  60 , the water or other liquid feed can be in atmosphere at standard pressure rather than in the vacuum of the chamber  12 . Water first flows in the inlet  62  and through the upper ring  61  of the baffle, then to to the lower ring  64  along vertical channel  63  in one of the baffle ribs. After completing the traversal of the lower ring  64 , the water flows along vertical channel  65  to the top ring  61 , where it finally flows out of the baffle  36  via outlet  66 .  
         [0028]     The source  20  needs no chamber shields. Instead the exposed portions of the wall  11  can be made of aluminum and be water cooled, with the inside surface thereof treated to promote material adhesion. The wall  11  can then be periodically cleaned, which is usually done by replacing the wall with a cleaned wall and sending the removed wall out for cleaning and reconditioning.  
         [0029]     This source  20  has several advantages from the point of view of maintenance. The target  42  is decoupled from the RF source  30 . Thus, changing the target  42  is much simpler, and much quicker than with a design in which the plasma and material sources are combined. Similarly, the chamber wall  11  can be removed and cleaned. Also, the parts are sufficiently light to eliminate the need for a hoist to remove and replace a target. The small footprint and simple coil design also reduce costs.  
         [0030]     Examples of semiconductor wafer processing machines of the iPVD type are described in U.S. Pat. Nos. 6,080,287, 6,287,435 and 6,719,886. Embodiments of the present invention are described in the context of the apparatus  10  of  FIG. 1 , even though applicable to other types of systems.  
         [0031]     Although only certain exemplary embodiments of this invention have been describe in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.