Abstract:
An asymmetrical focus ring varies the flow-field, which aids in normalizing pressure gradients across the wafer being processed, thereby improving the process. Embodiments of the present invention utilize a focus ring that either (1) contains a pattern of through holes that enhances pumping, or (2) does not contain any such pattern.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to and claims priority to U.S. Provisional Application Serial No. 60/406,345, filed Aug. 28, 2002. The entire contents of this prior application is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention is directed to a plasma processing apparatus and a method for using the apparatus to process substrates (e.g., semiconductor wafers or liquid crystal display panels), and in particular to the manufacture and use of an asymmetrical focus ring.  
           [0004]    2. Discussion of the Background  
           [0005]    As is known in the art, a fundamental step in the manufacture of semiconductor devices, such as integrated circuits (ICs), is the process of forming electrical interconnections. The formation of electrical circuits, such as semiconductor transistors, involves a series of steps starting with the formation of a blank silicon wafer. The blank silicon wafer is then processed using successive steps of depositing and etching away various materials to form the proper interconnections and therefore the electrical circuits.  
           [0006]    One method of depositing and etching metal layers to and from a silicon wafer includes the use of a plasma reactor system. In semiconductor manufacturing, plasma reactor systems are used to remove material from or deposit material to a work-piece (e.g., semiconductor wafer) in the process of making integrated circuit (IC) devices. A key factor in obtaining the highest yield and overall quality of ICs is the uniformity of the etching and deposition processes.  
           [0007]    In a narrow gap, high aspect ratio capacitively coupled plasma reactor, a multipurpose chuck assembly design is often employed that attempts to allow the chuck assembly (i.e. wafer work piece holder) to serve additional purposes other than supporting the wafer. The complexity of the mechanical design of the chuck assembly is such that a non-optimal vacuum system design is required. A vacuum pumping system is used to evacuate the reactor processing region to the low pressures necessary to create a clean environment, to which a specific gas chemistry is introduced, which provides an environment for the generation of plasma. Consequently, due to the complexity of the mechanical design of the chuck assembly, the symmetry of the vacuum system (relative to the wafer) is sacrificed such that the vacuum pump is usually positioned to access the reactor vacuum chamber from the side rather than from the chamber bottom or top.  
           [0008]    This type of multi-purpose chuck assembly can become a very cumbersome component of the reactor. In a multi-purpose chuck assembly design, in addition to supporting the wafer, the chuck assembly is typically configured to provide vertical translation in order to reduce the electrode-to-wafer spacing. This spacing control is necessary in order to produce a narrow gap for process conditions and to enlarge the gap for wafer exchange. In addition to the aforementioned capabilities, the chuck typically sustains a radio frequency (RF) energy bias. Moreover, the chuck assembly design further includes components for chuck block cooling, electrostatic clamping, and backside gas flow to improve thermal conduction (between the wafer and the chuck). Consequently, the vacuum design is often a secondary consideration to other various mechanical and electrical component designs.  
           [0009]    A reactor chamber that is equipped with a side mount vacuum port is considered an asymmetrical design in a nominally cylindrical system. An inherent drawback associated with an asymmetric design is that it often times produces an asymmetric process. One such asymmetry stemming from an asymmetric vacuum design is the observation of pressure field non-uniformity above the wafer when the chamber is evacuated from the side. That is, a pressure gradient with about 10-20% variation can occur across the wafer being processed. In general, for moderate to high pressures (e.g. P&gt;20 mTorr), a region of low pressure is observed at an azimuthal location adjacent the pump entrance or pumping duct entrance (the pumping duct interfaces the inlet of the pump, e.g. turbo-molecular pump, with the vacuum chamber). In known capacitively-coupled plasma reactors, attempts to solve the problem of an asymmetric chamber flow field introduced by pumping from the side have included the insertion of an orifice plate adjacent to the chuck.  
           [0010]    A processing chamber generally includes a single evacuated volume wherein a portion of that volume is proximate the wafer and is hereinafter referred to as the processing region. When an orifice plate is employed, the chamber volume is separated into two regions by the orifice plate. The first region is predominantly occupied by the wafer processing region and the second region, referred to as the pumping volume, is accessed by the vacuum pump. This solution tends to improve the flow-field uniformity in the upper chamber volume by providing sufficient flow resistance through the orifice plate.  
           [0011]    However, this improvement is achieved at the expense of flow conductance or pumping speed at the processing region. In addition to placing the orifice plate adjacent the chuck assembly, other known designs included locating the orifice plate adjacent other surfaces, e.g. any surface interfacing the processing chamber volume that allows the exhaust of chamber gases.  
           [0012]    There are several inherent problems with the known methods that use an orifice plate to control pressure uniformity in a chamber. For example, known orifice plates typically distribute the small openings equally in the azimuthal direction about the orifice plate in the hope that the resultant flow conductance will be azimuthally symmetric through the plate. However, in order to achieve flow-field uniformity, it is necessary to restrict the flow through the orifice plate to the extent that the pressure difference across the orifice is significantly greater than any pressure gradient in the processing or pumping regions. This requires making the holes in the orifice plate small and, hence, paying a penalty in chamber pumping speed at the wafer. This penalty in pumping speed directly results in an adverse effect on throughput.  
           [0013]    In addition to the problem of pressure field non-uniformity described above, an additional problem associated with plasma processing systems is the transport of plasma to the pumping duct and pump inlet. In general, the aforementioned orifice plate or a separate pumping duct screen is utilized to attenuate the plasma density prior to reaching the pump inlet. For example, in known systems a pump screen (with generally less than 50% solidity) is placed in the cross-section of the pumping duct. Unfortunately the pumping screen attenuates the plasma and also reduces the pumping speed delivered to the processing region by at least a factor of two. This approach results in at least 50% of the frontal area of the pumping duct cross-section being utilized for recombination surfaces. In conventional designs, there is a one-to-one relationship between the increase in recombination surface area and the decrease in the frontal (flow-through) area.  
         SUMMARY OF THE INVENTION  
         [0014]    According to one aspect of the present invention, an asymmetrical focus ring varies the flow-field, which aids in normalizing pressure gradients across the wafer being processed, thereby improving the process. Embodiments of the present invention utilize a focus ring that either (1) contains a pattern of through holes that enhances pumping, or (2) does not contain any such pattern.  
           [0015]    According to a second aspect, the position of the focus ring of the present invention may be adjusted by rotation on the centerline of the chuck assembly. This rotation allows further adjustment of pressure gradients in the chamber.  
           [0016]    The focus rings of the present invention optionally may be utilized with an orifice plate or pumping plenum as an added method to adjust pressure gradients in the chamber. This pumping plenum also may rotate with the focus ring as a single unit.  
           [0017]    A focus ring of the present invention may further utilize varying cross-sectional areas to restrict pumping flow in the chamber in varying amounts to reduce pressure gradients in the chamber.  
           [0018]    By utilizing asymmetrical or varying geometries, the present invention, even when utilized without a pumping plenum, allows greater pumping speeds in the chamber than designs with a pumping plenum. The present invention also modifies the pressure gradient to a higher degree than designs where asymmetrical or varying cross sectional area focus rings are not used. This invention adds tailoring features to adjust vacuum pumping characteristics. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    These and other advantages of the invention will become more apparent and more readily appreciated from the following detailed description of the exemplary embodiments of the invention taken in conjunction with the accompanying drawings, where:  
         [0020]    [0020]FIG. 1 is a plan view of a portion of a plasma processing system (without a pumping plenum) using an asymmetric focus ring according to the present invention;  
         [0021]    [0021]FIG. 2 is a cross-sectional side view of the plasma processing system of FIG. 1;  
         [0022]    [0022]FIG. 3 is a plan view of a portion of a plasma processing system (without a pumping baffle) using an asymmetric focus ring containing holes according to the present invention;  
         [0023]    [0023]FIG. 4 is a cross-sectional side view of the plasma processing system of FIG. 3;  
         [0024]    [0024]FIG. 5 is a plan view of a portion of a plasma processing system (without a pumping plenum) using a symmetric focus ring with asymmetrical holes according to the present invention;  
         [0025]    [0025]FIG. 6 is a plan view of a portion of a plasma processing system (with a pumping baffle) using an asymmetric focus ring containing holes according to the present invention;  
         [0026]    [0026]FIG. 7 is a cross-sectional side view of the plasma processing system of FIG. 6; and  
         [0027]    [0027]FIG. 8 is a cross-sectional side view of a plasma processing system using a variable cross-section focus ring. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    According to one aspect of the present invention, as shown in FIG. 1, a plasma processing system  100  is provided that includes a process chamber  105 . Housed within the plasma process chamber  105  is a chuck assembly  110  for supporting a substrate during processing and a focus ring  120  which encircles an upper portion of the chuck assembly  110 . As illustrated, the focus ring  120  is asymmetric and includes a minor side  123  and a major side  126 . The major side  126  is provided closer to the pumping duct  130  that evacuates the plasma chamber  105 . For example, the minor side  123  can range from 0.5 to 2.5 cm in width, and the major side  126  can range from 2.5 to 10 cm in width.  
         [0029]    The asymmetrical focus ring may be made from any number of materials (e.g., quartz, silicon, silicon carbide, carbon, ceramic or some dielectric or partially metal structure). In one embodiment of the present invention, the ring is made from a uniform surface roughness. In an alternate embodiment, the ring is made to have a varying surface roughness.  
         [0030]    Without an asymmetrical focus ring, a region of low pressure is located adjacent to the pumping duct entrance. Accordingly, the asymmetrical focus ring  120  is constructed and placed in such a manner as to restrict flow in the area near the major side (i.e., in the area of the least pressure) of the process chamber  105 . Proceeding around the focus ring  105  in either direction away from the major side  126 , the restriction is lessened to a point where no restriction occurs near the minor side  123  (i.e., where pressure in the process chamber  105  is the highest). Furthermore the focus ring  120  is rotatable about a central axis of the chuck assembly  110 .  
         [0031]    As shown in FIG. 2, the focus ring  120  is held atop the chuck assembly  110  and opposite an upper electrode assembly  140 . The upper electrode assembly  140  may further include an impedance matching network (e.g., a fast match assembly). The chuck assembly  110  may be brought closer to the upper electrode  140  during processing and moved further away when the substrate is being exchanged. The movement may be controlled in conjunction with an automatic or robotic substrate transfer system (not shown).  
         [0032]    According to an exemplary embodiment of the present invention shown in FIGS. 3 and 4, an asymmetrical focus ring  120  is provided with a pattern of holes  129  to enhance pumping. The configuration of holes  129  may be other than as shown in the figures, but generally the configuration is selected to reduce pressure in the chamber  105  at the minor side  123  as compared to at the major side  126 .  
         [0033]    Advantageously, the holes  129  in the focus ring  120  tend to confine a plasma to a processing volume versus significantly reducing pumping volume. This provides a good balance between pumping speed and uniformity which is lacking in known systems. Alternately, as shown in FIG. 5, focus ring  120  can comprise a minor side  123  substantially equivalent in width to the major side  126 , wherein the asymmetry in design is introduced by the size of the holes distributed about the focus ring  120 . Alternately, the number density of holes can be varied about the focus ring  120 .  
         [0034]    According to the exemplary embodiment of the present invention shown in FIGS. 6 and 7, an orifice plate or pumping baffle is used in conjunction with focus ring  120  either containing throughholes or not. The focus ring  120  is arranged and located as before. The focus ring  120  is again rotatable, but the pumping baffle  150  rotates with the focus ring  120  as a unit. The pumping baffle  150  has a number of holes  155  to enhance pumping and aid in normalizing pressure gradients across the substrate under process. As with the holes  129  of the focus ring  120 , the holes  155  of the pumping baffle  150  can be configured other than as illustrated.  
         [0035]    While the embodiments of FIGS.  1 - 7  have been illustrated as utilizing a focus ring  120  that has a substantially constant thickness, it is also possible to use a non-constant thickness as well. As shown in FIG. 8, the focus ring  120 ′ still is generally configured to fit around the chuck assembly  110  within the process chamber  105 . The focus ring  120 ′, however, is constructed with cross-sectional areas varying around the periphery of the element. Generally cross-sections are constructed and placed in such a manner as to restrict the flow in the area of least pressure in the chamber (i.e., near where the major side  126  is placed). By varying the cross-section around the focus ring  120 ′ towards the minor side  123 , the variation in cross section (and therefore the variation in restriction) helps to enable uniform pumping in the process chamber  105 .  
         [0036]    The focus rings  120  of FIGS.  1 - 8  can be retrofitted into existing process chambers  105  by exchanging the existing focus rings for the illustrated ones. By utilizing such focus rings, process chambers  105  also can be built without the added cost and complexity of a pumping baffle  150 . The focus ring  120  can be attached, affixed, or mounted to the chuck assembly using standard design practice for focus rings understood by those skilled in the art of plasma processing.  
         [0037]    To improve uniformity, the focus ring  120  may also be equipped with a rotating attachment point. By turning the focus ring  120  using the rotating attachment point, the focus ring  120  is rotated about a centerline of the chuck assembly  110 . This enables the system  100  to alter a direction of the pressure gradient. A number of pressure sensing devices, such as pressure manometers, can be coupled to the outer wall of the process chamber  105  and configured to provide information on the spatial variation of the pressure field about the periphery of the processing region. Implementation of a number of pressure sensing devices is understood by one skilled in the art of vacuum system design.  
         [0038]    Although only certain exemplary embodiments of this invention have been described 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.