Abstract:
To protect a dielectric window in an inductively coupled plasma reactor from depositions of coating or etched material from the plasma, a dielectric insert is placed inside of the chamber closely adjacent the window. Where a slotted shield inside of the window protects the window from deposition, but has slots through which some material can pass in a direction toward the window, the insert is placed between the window and the shield. The insert is formed of a material that is compatible with the process being carried out on a semiconductor wafer within the chamber. Where the window and shield are planar, an unprocessed wafer of the same type and material as the wafer being processed is used for the insert.

Description:
[0001]    This invention relates to the semiconductor wafer plasma processing, particularly to plasma deposition and etching processes, in which dielectric chamber walls or windows bound the chamber. In particular, the invention relates to inductively coupled plasma (ICP) generators in which RF energy is coupled into a vacuum chamber through a dielectric wall or window to energize a plasma inside of the chamber.  
         BACKGROUND OF THE INVENTION  
         [0002]    Inductively coupled plasma (ICP) sources are finding increased use in the semiconductor processing industry. Ionized physical vapor deposition (iPVD) is performed, for example, in machines such as that disclosed in U.S. Pat. No. 6,080,287, U.S. patent application Ser. No. 09/442,600, and PCT Application No. PCT/US00/31756, all hereby expressly incorporated herein by reference. In such machines, metal ions are formed in a vacuum processing space by ionizing material sputtered from a target through a very dense, low electron temperature ICP formed in the processing space by coupling RF energy from an external coil through a dielectric window in the wall of the chamber, such as, for example, a planar window at the end of the chamber. A slotted deposition shield inside the chamber protects the window from the deposition of metal from the processing space. The metal, if allowed to deposit onto the window, would form an electrically conductive layer in which currents would be induced that would shield the processing space from the coil and prevent the coupling of the RF energy into the plasma. Slots in the shield prevent the formation of current paths in the film that would shield the coil, by conduction in the shield itself, if it is made of metal or other conductive material, or by conduction in the material that deposits onto the shield, whether the shield itself is conductive or not. The existence of the slots in the shield, however, eventually results in some accumulation of material on the window. This accumulation can, if too great, require a cleaning of the window more often than replacement of the sputtering target in the chamber, resulting in additional interruptions in the productive use of the machine. This is undesirable in that it reduces productivity and increases the cost of maintaining the machine and of the products produced. Even when the window does not require cleaning more often than target replacement is required, deposition onto the window can increase the thermal load on the window which can reduce the life of the window. Failure of the window results in process interruption and part damage, requiring its replacement. This is especially the case in larger machines, such as 300 millimeter wafer processing tools where the window is subjected to a substantial atmospheric pressure load.  
           [0003]    Furthermore, in ICP etch machines and processes, for example as described in the commonly assigned and copending U.S. patent application Ser. No. 09/875,339, filed Jun. 6, 2001, hereby expressly incorporated by reference herein, the etching of conductive material from a substrate can lead to deposits onto dielectric windows that ultimately produce contamination problems similar to those that exist in physical vapor deposition (PVD) applications. Even non-conductive deposits onto such windows can require cleaning of the windows and thereby increase the cost of maintaining such etch machines or can lead to local temperature gradients and local stresses that can cause the windows to break.  
           [0004]    Accordingly, the need exists for the prevention of contamination of dielectric windows in ICP deposition machines and processes and also in ICP etching machines and processes.  
         SUMMARY OF THE INVENTION  
         [0005]    A primary objective of the present invention is to reduce chamber cleaning time and frequency in plasma processing, particularly in chambers having dielectric walls through which energy is coupled into the plasma. A further objective of the invention is also to protect dielectric walls and windows from accumulating coatings on their interior surfaces, particularly electrically conductive coatings, which, if permitted, can increase the mean time between the failures of such walls or windows, and to reduce the overall cost of operation of a plasma processing apparatus where such walls and windows are employed.  
           [0006]    A more particular objective of the invention is to economically protect and reduce the need to clean dielectric walls or windows in ICP reactors in which energy is coupled from outside of the reactor chamber into the vacuum processing space within the reactor to sustain a plasma.  
           [0007]    According to the principles of the present invention, an ICP deposition or etching apparatus having a dielectric wall or window is provided with a replaceable protective insert located inside of the wall or window to intercept material sputtered or etched in the chamber that would otherwise deposit on the inside surface of the dielectric wall or window. The insert may be provided between the window and any slotted or other type of primary shield that is provided on the inside of the chamber as the primary protection against coating of the wall or window, where the insert intercepts any material that passes through the slots toward the wall or window or otherwise bypasses the primary shield.  
           [0008]    In accordance with the preferred embodiments of the invention, a dielectric window that separates a vacuum processing space within a plasma processing chamber from a coil or other antenna that is located outside the chamber is provided with a dielectric insert that covers the interior surface of the window. Where a deposition barrier is provided inside of the window in the form of a shield that is slotted to prevent electrical currents from being induced in the shield by RF energy from the antenna, the insert is positioned between the shield and the window. The invention is particularly practical for planar ICP sources where the inserts may be in the form of a disc or sheet. An unprocessed semiconductor wafer, or bare semiconductor wafer that has not been subjected to coating or etching processes, may be used for the insert to insure process compatibility with the process.  
           [0009]    These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a diagrammatic view of an iPVD reactor adaptable according to principles of the present invention.  
         [0011]    FIGS.  1 A- 1 B are enlarged cross-sectional views through the portion  1 A of the window of the reactor of FIG. 1 respectively with and without an insert according to the present invention.  
         [0012]    FIGS.  2 A- 2 D are graphs illustrating the average electromagnetic energy density along a line normal to and through the center of the window showing that the levels are essentially the same for various insert materials in the reactor of FIG. 1, and in which:  
         [0013]    [0013]FIG. 2A shows the energy levels with 0, 1, 2 and 3 standard thickness (0.3 mm) aluminum oxide wafers;  
         [0014]    [0014]FIG. 2B shows the energy levels with 0, 1, 2 and 3 standard thickness silicon wafers;  
         [0015]    [0015]FIG. 2C shows the energy levels with 0, 1, 2 and 3 standard thickness aluminum nitride wafers; and  
         [0016]    [0016]FIG. 2D shows the energy levels comparing no insert and inserts made of three standard thickness aluminum oxide, aluminum nitride and silicon wafers.  
         [0017]    [0017]FIG. 3 is a disassembled perspective view of the window assembly the iPVD reactor of FIG. 1 illustrating a standard semiconductor wafer used as the protective insert.  
         [0018]    [0018]FIG. 4 is a diagrammatic cross-sectional view illustrating one manner of mounting the insert in the embodiment of FIG. 3.  
         [0019]    [0019]FIG. 5 is a disassembled perspective view of an alternative window assembly to that of FIGS. 3 and 4 for use in an ICP etch reactor.  
         [0020]    [0020]FIG. 6 is a cross section through the window assembly of FIG. 5. 
     
    
     DETAILED DESCRIPTION  
       [0021]    [0021]FIG. 1 illustrates an ionized physical vapor deposition reactor  10  of the type described in U.S. Pat. No. 6,080,287 and U.S. patent application Ser. No. 442,600, filed Nov. 18, 1999, both expressly incorporated by reference herein. The reactor  10  has a chamber wall  11  that encloses a vacuum chamber  12 . A substrate support  13  is disposed within the chamber for holding an upwardly facing semiconductor wafer or other substrate  14  for processing. The chamber wall  11  is formed of, or lined with, metal and is electrically connected to a system ground  15 . In the top of the chamber wall  11  is an opening  16  that is sealed by an iPVD source  50 . The iPVD source  50  includes a target  24  of coating material and an RF energy source  20  for supplying energy to a plasma within the chamber  12 . The target  24  is frusto-conical annular and is sealed around its outside edge to, and electrically insulated from, the wall  11  of the chamber  12 . A generally circular dielectric window  17 , preferably fabricated of ceramic, is sealed around its edge to the inner edge of the target  24 , which completes the enclosure of the chamber  12  to support the maintenance of a vacuum within the chamber  12 .  
         [0022]    Behind the window  17  and outside of the chamber  12  is situated the RF source  20 , which includes a coil or other antenna  21  connected through a matching network  22   a  across an RF power generator  22 , which typically produces RF energy in the range of from 1 to 13.56 MHz. The antenna  21  is so positioned and configured to couple RF energy through the window  17  into the chamber  12  to form a high density plasma  23  in a low pressure processing gas within the chamber  12 . The annular frusto-conical target  24  is located either entirely within the chamber  12  or in the upper portion of the wall  11 , as shown, with a sputtering surface  25  thereof in communication with the inside of the vacuum chamber  12 . A permanent magnet pack  26  is positioned behind the target  24  to form a closed magnetic field to trap a high energy sputtering plasma  27  over the surface  25  of the target  24  when energized with DC power from a power supply  28 .  
         [0023]    Ions from the high density plasma  27  sputter material from the surface  25  of the target  24  into the processing chamber  12 , where they are energized by the high density plasma  23 . A bias power source  29  is usually provided, which is typically an RF source, which creates a negative DC bias on the wafer  14  that is mounted on the substrate support  13 , so that positive ions from the plasma  23  are attracted toward the substrate  14 . Without the target  24 , and the opening  16  in the wall of the chamber being fully covered by a window, the apparatus  10  described above would essentially be an etch reactor etching the substrate  14  with ions of gas from the plasma  23 . The configuration of an etch reactor is illustrated in FIGS. 5 and 6 described below.  
         [0024]    In an iPVD reactor such as reactor  10 , material sputtered from the target  24  is ionized by the high density plasma  23  and attracted by bias on the substrate  14  toward the substrate  14  so that the ionized material impinges onto the substrate  14  more perpendicularly than would otherwise occur were the sputtered material not ionized, thereby more effectively entering high aspect ratio features on the substrate  14 . In order to prevent material from the chamber  12 , such as material which is commonly metal sputtered from the target  24 , from depositing onto the window  17 , a deposition baffle  30  in the form of a metallic shield is positioned inside of the window  17  spaced approximately 0.8 to 1.0 millimeters from its inside surface, as illustrated in FIG. 1A. The baffle  30  intercepts material from the chamber  12 , which is thereby deposited onto the baffle  30  instead of being deposited on the inside surface of the window  17 . The baffle  30  has slots  31  therein to prevent electrical currents from being induced in the baffle that would inhibit coupling of energy from the coil  21  into the plasma  23 . The baffle is typically metal, but even if it were not, the sputtering of metal from the target  24  would quickly result in deposition of a conductive metal coating on the baffle  30 , so that making the baffle out of a similarly conductive material stabilizes the process parameters that would otherwise change if the baffle  30  were to go from non-conductive to conductive with the accumulation of conductive deposits on its surface. The slots  31  are preferably configured to provide no direct line-of-sight paths from the chamber  12  to the window  17 . But even with the slots so configured, small amounts of material eventually accumulate on the inside surface of the window  17 .  
         [0025]    Traces of metal contamination that build up over the long-term operation of the apparatus  10  can be seen on the surface of the dielectric window  17  that is exposed to the interior of the chamber  12 . The processes that rely on the ICP require a high RF power level, for example, reaching approximately 5 kW (kilowatts) at 13.56 MHz (megahertz), to achieve optimal process output. When the thickness of metal coating on the inside of the window  17  exceeds about 10 μm (microns) in thickness, more than 50% of RF power can be diverted from the plasma and instead coupled into the metallic coating on the window. Increased RF power coupling into residual metal deposits on the window, which is typically ceramic, causes localized thermal load on the window  17 , which, under certain circumstances, can shorten window life and cause shorter MTBF (mean time between failures) of window  17 .  
         [0026]    To eliminate contamination of the window and thus its shorter MTBF, a thin protective insert  40  is placed between the deposition shield  30  and window  17 , as illustrated in FIG. 1B. With the insert  40 , sputtered metal material passing through slots in the shield build up on the surface  42  of the thin protective insert  40 , which is now exposed to plasma, instead of on the surface  18  of dielectric window  17 . The material of the protective insert  40  is preferably the same as or has coupling properties comparable with the ceramic material of which the dielectric window  17  is made so that it has no effect on the plasma properties inside of the chamber  12 . The insert cam be made, for example, of alumina, AIN or silicon. Table 1 compares the properties of materials useable for such an insert.  
                                                                   TABLE 1                           Properties of Materials for Protective Insert.                Al 2 O 3     AlN   Si   SiC   PTFE                        DIELECTRIC CONSTANT    9.0-10.1   8.9   11.7   6.52   2.1-2.8       DIELECTRIC STRENGTH, kV mm 1     10-35   10   25-30   220-240   18.9       VOLUME RESISTIVITY @ 25° C., Wcm   &gt;10 14     10 11 -10 13     2.3 × 10 5     &gt;10 5  (Si)   &gt;10 18         THERMAL CONDUCTIVITY @ 20° C., Wcm −1  K −1     28-35   165   131-150   230-380   —       THERAML EXPANSIVITY @ 20-1000° C., 10 −6  K −1     8.0   5.3   2.6   2.9   2.5       SOFTENING (OR MELTING) PONT, ° C.   2050   —   1412   2830   327       UPPER CONTINUOUS USE TEMPERATURE, ° C.   1800   1200   —   —   260       CHEMICAL RESISTANCE AGAINST METAL   Good   Good   Good   Good   Good                  
 
         [0027]    The utilization of SiC substrates (grade SI—semi-insulating) may also have advantages for the insert  40 . The SiC is an excellent thermal conductor, so heat will flow more readily through SiC than other semiconductor materials. At room temperature, SiC has a higher thermal conductivity than any metal. This property enables SiC devices to be used at extremely high power levels and still dissipate the large amounts of excess heat generated. SiC can withstand a voltage gradient or electric field over eight times greater than can Si without undergoing avalanche breakdown.  
         [0028]    Advantages can be realized by using a standard semiconductor substrate as the insert  40 . Such substrates are produced in large quantities and can serve as consumable components in the semiconductor processing  10 . They are 100% compatible with the process being performed on such semiconductor substrates in the chamber  12 , since they are a standard on which process compatibility is based. A very practical protective insert is a silicon wafer having a diameter of 200 mm and a thickness of about 300 μm. Other ceramic substrates used in the electronics industry having comparable dimensions may be useful. For low RF power applications or where the deposition shield  30  is provided with adequate cooling, a PTFE (TEFLON™) insert may be an appropriate material for the insert  40 . Preferably, however, high temperature ceramic materials are more advantageous.  
         [0029]    The average electromagnetic energy density of the RF magnetic field after passing the protective insert is shown on graphs of FIGS.  2 A- 2 C for various materials and different thicknesses. The graphs show the average electromagnetic energy density vs. distance from the antenna  21  for protective inserts to be essentially the same for the various materials for the inserts  40  of (a) Al 2 O 3 , (b) AlN and (c) Si, and at the three different total thicknesses of 0.9 mm, 0.6 mm and 0.3 mm, (using 3, 2 and 1 standard 0.3 mm thick wafers respectively) as well as without the insert  40 . Graph (d) of FIG. 2D shows a comparison for the materials noted above at a total thickness of 0.9 mm. The graphs represent simulation results obtained by Maxwell EM 3D modeling of antenna made of a tubular conductor of 5 mm in diameter radiating RF power through an alumina window having a total thickness of 9 mm into a conductive plasma. The positions of the antenna  21  and dielectric window  17  are depicted in graph (a) of FIG. 2A. The graphs show that at a thickness of approximately one millimeter, the protective insert  40  produces substantially no reduction in RF power transfer into plasma, and the use of the protective insert  40  does not materially affect other process parameters.  
         [0030]    [0030]FIG. 3 illustrates a standard silicon semiconductor wafer being used as the protective insert  40  and positioned in relation to the deposition shield  30  as configured for an iPVD reactor. The shield  30  has a circular recess  32  therein defined by the inwardly facing inner edge of an annular rim  33 . The insert  40  nests in the recess  32 . An annular shoulder or step  35  lies inwardly of the rim  33 , which allows the insert  40  to be supported with a space between the insert  40  and the shield  30 , as illustrated in FIG. 4. A pair of channels  34  are formed through the rim  33  of the shield  30 . The channels  34  communicate with spaces formed by respective cut-off portions  41  of the insert  40 . The cut-off portions  41  allow the space between the insert  40  and the dielectric window  17  to be pumped to vacuum to avoid a pressure gradient across the insert  40  that could crack it. In this arrangement, typically the window  17  is sealed around its edges to the rim of an opening in the wall of the chamber. Alternatively, as shown in FIG. 5, a shield in the form of shield  30   a  may include a rim  33   a  that is sealed around the opening  16  to the chamber wall  11 . The dielectric window  17   a  may be provided with an O-ring or other seal  37 , and sealed to the shield  30   a  with seal  38  to seal a space between the window  17   a  and the shield  30   a  that contains the insert  40 .  
         [0031]    The protective insert  40  is not attached to or in contact with ceramic window  17 , 17   a  thereby avoiding direct thermal contact between the insert  40  and the window  17 , 17   a . The insert  40  is placed in recess on the top side of deposition shield. With this arrangement, even an accidental break of the protective insert  40  will not cause failure of plasma source since the vacuum will not be lost to the atmosphere as in the case of a dielectric window failure. To improve further thermal insulation between the insert  40  and the window  17 , 17   a , a gap is maintained of approximately 1 mm between the protective insert  40  and deposition shield  30 , as shown in FIG. 4. The shoulder  35  around the recess  32  provides an additional recess in deposition shield  30  that creates a stepwise surface on deposition shield  30 . The shape of protective insert  40  allows for the gas in the space between protective insert  40  and dielectric window  17 ,  17   a  to be pumped out of the chamber, for example, through the shallow channels  34  machined in deposition shield  30  and through openings  41  provided by the shape of the protective insert itself (FIG. 3).  
         [0032]    The above described structure provides several advantages. First, cost savings are realized because the protective insert reduces the need for the machined ceramic window  17  to be replaced or cleaned due to premature contamination, which, because of sealing surfaces that must be machined on the window  17 , is costly. In the case of an AlN window, this represents significant cost savings. In the case of Al 2 O 3  windows, the cost is about US$300, compared to the cost of about US$20 for a simple silicon wafer (200 mm in diameter).  
         [0033]    The invention also provides other material cost savings. For example, the protected window  17  can be re-used, since it is not significantly contaminated by sputtered process products. Furthermore, a window  17  protected by a shield  40  does not need to be aggressively cleaned, so sensitive sealing surfaces will not be damaged, thereby maintaining the original vacuum integrity of the system.  
         [0034]    Furthermore, such a protective insert  40  can be replaced more easily than the robust ceramic window  17  can be removed and cleaned. Hence, a blank silicon wafer can be used as an insert  40 , thereby providing a consumable insert having a much lower cost than the cost of maintaining an unprotected window.  
         [0035]    Test and metrology cost savings also results. A contaminated protective insert  40  can be used as a sample that will be evaluated after replacement, typically after the lifetime of the target in ionized PVD, or during any maintenance services on the source  50  allowing removal of the protective insert  40 , so that data can be compared on the thickness and composition of deposited contaminated layer. The standard semiconductor metrology instrumentation can be used for such evaluation since the shape and dimensions of protective insert  40  are consistent with semiconductor substrates. Obtained data on thickness and composition of the layer provides feedback to a manufacturer of semiconductors and of the processing apparatus  10  and may be used for continuous improvements of the plasma source and the processes performed therein. The protective insert  40  can be used also in other plasma processing systems that do not produce conductive coatings. In such case, a replaceable protective insert  40  reduces the time required for cleaning the chamber and the chamber cleaning frequency.  
         [0036]    While the above description and accompanying drawings set forth various embodiments of the invention, it will be apparent to those skilled in the art that additions and modifications may be made without departing from the principles of the invention. Accordingly,