Abstract:
The embodiments of the invention generally relate to a method and apparatus for processing a substrate where reduced defect formation is desired. Embodiments of the invention may be beneficially practiced in chemical mechanical polishing and electrochemical mechanical polishing processes, among other processes where reduction in defect formation due to foam formation is desired. In one embodiment, a processing system for planarizing a substrate is provided that includes a platen, a pad disposed on the platen, a carrier head configured to retain the substrate against the pad while contacting an electronically conductive processing solution; and a foam removal assembly. The foam removal assembly is configured to remove foam from the electrically conductive processing solution, wherein a gap exists between a surface of the electrically conductive processing solution and a bottom edge of the foam removal assembly.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     Embodiments of the present invention generally relate to methods and apparatus for polishing a substrate in an electrochemical mechanical polishing system.  
         [0003]     2. Description of the Related Art  
         [0004]     Electrochemical Mechanical Processing (ECMP) is a technique used to remove conductive materials from a substrate surface by electrochemical dissolution while concurrently polishing the substrate with reduced mechanical abrasion as compared to conventional Chemical Mechanical Polishing (CMP) processes. ECMP systems may generally be adapted for deposition of conductive material on the substrate by reversing the polarity of the bias applied between the substrate and an electrode. Electrochemical dissolution is performed by applying a bias between a cathode and a substrate surface (acting as an anode) to remove conductive materials from the substrate surface into a surrounding electrolyte. The bias may be applied to the substrate surface by a conductive contact disposed on or through a polishing material upon which the substrate is processed. The polishing material may be, for example, a conductive polishing pad disposed on a platen. A mechanical component of the polishing process is performed by providing relative motion between the substrate and the polishing material that enhances the removal of the conductive material from the substrate.  
         [0005]     When a bias is applied to the cathode and the substrate surface, the conductive metal layer under anodic polarization is converted into metal ions. These metal ions complex with chelating agents in the surrounding electrolyte. During this process, individual bubbles form at both the anode and the cathode, with oxygen being the main anodic product and hydrogen the main cathodic product. These individual bubbles can adhere to the wafer surface during polishing and block the electrical dissolution path, leading to different polishing rates for the foam covered and the foam free areas. These different polishing rates lead to “bubble defects” on the polished surface. As removal rates increase, foam production also increases thus leading to increased “bubble defects.” These increased “bubble defects” pose technical roadblocks for developing high throughput processes. Further, the foam can be transferred from platen to platen with the wafer, thus causing cross-contamination between platens.  
         [0006]     Therefore, there exists a need for a method and apparatus for polishing a substrate while reducing the amount of foam in the electrolyte bath during the polishing process.  
       SUMMARY OF THE INVENTION  
       [0007]     The embodiments of the invention generally relate to a method and apparatus for processing a substrate with reduced defect formation. Embodiments of the invention may be beneficially practiced in chemical mechanical polishing and electrochemical mechanical polishing processes, among other processes where reduction in defect formation due to foam formation is desired.  
         [0008]     In one embodiment, a processing system for planarizing a substrate is provided that includes a platen, a pad disposed on the platen, a carrier head configured to retain the substrate against the pad while contacting an electronically conductive processing solution; and a foam removal assembly. The foam removal assembly is configured to remove the foam from the electrically conductive processing solution. A gap exists between a surface of the electrically conductive solution and a bottom edge of the foam removal assembly. In another embodiment, the processing system further comprises a fluid delivery arm, an electrode disposed between the platen and the pad, and a power source having a pole coupled to the electrode. In another embodiment, the foam removal assembly is positioned at an angle in a plane parallel to the platen between about 200 and about 70° relative to the edge of the platen. In another embodiment, the foam removal assembly is positioned at an angle in a plane parallel to the platen between about 30° to about 45°.  
         [0009]     In one embodiment, an apparatus for defoaming an electrochemical mechanical polishing bath is provided. The apparatus comprises a foam removal assembly and a fluid delivery arm attached to the foam removal assembly.  
         [0010]     In one embodiment, a method of electrochemically and mechanically planarizing a surface of a substrate is provided. The method comprises providing a basin containing an electrically conductive solution and an electrode disposed therein, disposing a polishing medium in the electrically conductive solution, positioning a substrate against the polishing medium so that a surface of the substrate contacts the electrically conductive solution, providing a relative motion between the substrate and the polishing medium, applying a potential between the polishing medium and the electrode, and skimming a surface of the electrically conductive solution with a foam removal assembly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0012]      FIG. 1  is a partial sectional view of one embodiment of a processing station that includes one embodiment of a foam removal assembly attached to a fluid delivery apparatus.  
         [0013]      FIG. 2  is a plan view of the processing station of  FIG. 1 .  
         [0014]      FIG. 3  is a plan view of another embodiment of the processing station of  FIG. 1 .  
         [0015]      FIG. 4  is a partial sectional view of one embodiment of the foam removal assembly.  
         [0016]      FIG. 5  is a plan view of an electrochemical mechanical processing system. 
     
    
       [0017]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.  
         [0018]     it is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       DETAILED DESCRIPTION  
       [0019]     A method and apparatus for defect reduction via foam removal in an electrochemical substrate polishing process is provided. The method and apparatus may be utilized in systems where foam removal from a processing solution on a rotating work surface is desired. Although the embodiments below focus on removing foam from an electrochemical mechanical polishing process, it is contemplated that the teachings within may also be used in other polishing processes as well as depositing materials on a substrate by reversing the polarity of an electrical bias applied between a substrate and an electrode of the system.  
         [0020]      FIG. 1  is a partial sectional view of one embodiment of a processing station  100  that includes one embodiment of the foam removal assembly  180  for removing foam from the electrolyte bath. The foam removal assembly  180  is attached to a fluid delivery arm assembly  126 . Although the processing station  100  illustrated in  FIG. 1  as an electrochemical mechanical processing station, it is contemplated that the invention may be practiced in other electroprocessing stations and conventional chemical mechanical polishing stations.  
         [0021]     Referring to  FIG. 1 , the processing station  100  includes a carrier head  102  and a platen  104 . The carrier head  102  generally retains a substrate  122  against a polishing pad  108  disposed on the platen  104 . At least one of the carrier head  102  or platen  104  is rotated or otherwise moved to provide relative motion between the substrate  122  and the polishing pad  108 . In the embodiment depicted in  FIG. 1 , the carrier head  102  is coupled to an actuator or motor  116  that provides at least rotational motion to the substrate  122 . The motor  116  may also oscillate the carrier head  102 , such that the substrate  122  is moved laterally back and forth across the surface of the polishing pad  108 .  
         [0022]     In one embodiment, the carrier head  102  includes a retaining ring  110  circumscribing a substrate receiving pocket  112 . A bladder  114  is disposed in the substrate receiving pocket  112  and may be evacuated to chuck the wafer to the carrier head  102  and pressurized to control the downward force of the substrate  122  when pressed against the polishing pad  108 . One suitable carrier head  102  is a TITAN HEAD™ carrier head available from Applied Materials, Inc., located in Santa Clara, Calif. Another example of a carrier head that may be adapted to benefit from the invention is described in U.S. Pat. No. 6,159,079, issued Dec. 12, 2001, which is hereby incorporated herein by reference in its entirety.  
         [0023]     In  FIG. 1 , the platen  104  is supported on a base  156  by bearings  158  that facilitate rotation of the platen  104 . A motor  160  is coupled to the platen  104  and rotates the platen  104  such that the pad  108  is moved relative to the carrier head  102 .  
         [0024]     In the embodiment depicted in  FIG. 1 , the polishing pad  108  includes an upper conductive layer  118  and an underlying electrode  120 . Optionally, one or more intervening layers  154  may be disposed between the electrode  120  and conductive layer  118 . For example, the intervening layers  154  may include at least one of a subpad, an interposed pad and a conductive carrier. In one embodiment, the subpad may be a urethane-based material, such as a foam urethane. In one embodiment, the interposed pad may be a sheet of mylar. In one embodiment, the conductive carrier may be a metallic foil. In one embodiment, the top conductive layer  118  may be comprised of one or more conductive films, such as one or more films comprised of a conductive material suspended in a polymer binder. Optionally, the films may be disposed on a conductive fabric for increased mechanical strength.  
         [0025]     The electrode  120  is generally fabricated from a conductive material and may optionally include two or more independently biasable zones. In one embodiment, the electrode  120  is fabricated from stainless steel.  
         [0026]     The conductive layer  118  and the electrode  120  are coupled to opposite poles of a power source  123 . The power source  123  is generally configured to provide a potential difference between the conductive layer  118  and the electrode  120  of up to about 12 volts DC. The power source  123  may be configured to drive an electrochemical process utilizing constant voltage, constant current or a combination thereof. The power source  123  may also provide power pulses.  
         [0027]     A plurality of holes  124  are formed through at least the top conductive layer  118  of the pad  108 , such that a processing fluid filling the holes  124  may establish a conductive path between the electrode  120  and the substrate  122  disposed on the top conductive layer  118 . The number, size, distribution, open area and pattern density of the holes  124  may be selected to obtain a desired processing result. Some examples of suitable pads which may be adapted to benefit from the invention are described in U.S. patent application Ser. No. 10/455,895 filed Jun. 6, 2003 and U.S. patent application Ser. No. 10/642,128 filed Aug. 15, 2003, which are hereby incorporated by reference in their entireties.  
         [0028]     A fluid delivery arm assembly  126  is utilized to deliver a processing fluid from a processing fluid supply  128  to a top or working surface of the conductive layer  118 . In the embodiment depicted in  FIG. 1 , the fluid delivery arm assembly  126  includes an arm  130  extending from a stanchion  132 . A motor  134  is provided to control the rotation of the arm  130  about a center line of the stanchion  132 . An adjustment mechanism  136  may be provided to control the elevation of a distal end  138  of the arm  130  relative to the working surface of the pad  108 . The adjustment mechanism  136  may be an actuator coupled to at least one of the arm  130  or the stanchion  132  for controlling the elevation of the distal end  138  of the arm  130  relative to the platen  104 . Some examples of suitable fluid delivery arms which may be adapted to benefit from the current invention are described in co-pending U.S. patent application Ser. No. 11/298,643, filed Dec. 8, 2005, entitled “Method And Apparatus For Planarizing A Substrate With Low Fluid Consumption,” which is hereby incorporated by reference in its entirety to the extent not inconsistent with this application.  
         [0029]     The fluid delivery arm assembly  126  may include a plurality of rinse outlet ports  170  arranged to deliver a spray and/or stream of rinsing fluid to the surface of the pad  108 . The ports  170  are coupled by a tube  174  routed through the fluid delivery arm assembly  126  to a rinsing fluid supply  172 . The rinsing fluid supply  172  provides a rinsing fluid, such as deionized water, to the pad  108  after the substrate  122  is removed to clean the pad  108 . The pad  108  may also be cleaned using fluid from the ports  170  after conditioning the pad using a conditioning element, such as a diamond disk or brush (not shown).  
         [0030]     The nozzle assembly  148  is disposed at the distal end of the arm  130 . The nozzle assembly  148  is coupled to the fluid supply  128  by a tube  142  routed through the fluid delivery arm assembly  126 . The nozzle assembly  148  includes a nozzle  140  that may be selectively adjusted relative to the arm, such that the fluid exiting the nozzle  140  may be selectively directed to a specific area of the pad  108 .  
         [0031]     In one embodiment, the nozzle  140  is configured to generate a spray of processing fluid. In another embodiment, the nozzle  140  is adapted to provide a stream of processing fluid. In another embodiment, the nozzle  140  is configured to provide a stream and/or spray of processing fluid  146  at a rate between about 20 to about 120 cm/second to the polishing surface.  
         [0032]     In one embodiment, the foam removal assembly  180  is attached to the fluid delivery arm assembly  126  by a shaft  186 . Other common attachment or mounting means known in the art may also be used, for example, the shaft  186  may be attached to the fluid delivery arm assembly  126  by a screw. It is also contemplated that the foam removal assembly  180  be readily detachable from the fluid delivery arm assembly  126 . In another embodiment the foam removal assembly  180  is configured for mounting to the base  156 . In another embodiment, the foam removal assembly  180  is an integral part of the fluid delivery arm assembly  126 .  
         [0033]     The foam removal assembly  180  is vertically adjustable along shaft  186 . The foam removal assembly  180  should be positioned above the liquid level where it can skim the foam off the surface of the electrolyte bath without creating turbulence in the electrolyte bath. Thus, the elevation of the foam removal assembly  180  relative to the pad  108  is dictated by the height of the electrolyte bath. A gap separates a bottom edge of the foam removal assembly  180  from a top surface of the pad  108 . In general, he distance across the gap is greater than the height of the electrolyte bath.  
         [0034]      FIG. 2  is a plan view of the processing station of  FIG. 1  illustrating the location of the foam removal assembly  180  with respect to the carrier head  102 , the substrate  122  and the fluid delivery arm assembly  126  in one embodiment. The foam removal assembly  180  is positioned so it does not interfere with the movement of the carrier head  102 . Arrow  202  defines the rotational movement of the platen  104 . Arrow  204  defines the rotational movement of the carrier head  102 . Arrow  206  defines the movement of foam as it is blocked by the foam removal assembly  180  and swept off the edge  208  of the platen  104  by the rotational movement of the platen  104  represented by arrow  202 .  
         [0035]     The angle (θ)  210  of the foam removal assembly  180 , located in a plane parallel to the platen  104 , relative to the edge  208  of the platen  104  should be fixed so that the foam will be pushed out to the edge  208  of and off the platen  104 . The angle (θ)  210  of the foam removal assembly  180  relative to the edge  208  of the platen  104  should also be fixed so that the foam removal assembly  180  removes the maximum amount of foam without interfering with the movements of the carrier head  102 . In one embodiment, the angle (θ)  210  of the foam removal assembly  180  relative to the edge  208  of the platen  104  is between about 20° to about 70°, and in another embodiment is between about 30° to about 45°.  
         [0036]     The foam removal assembly  180  is horizontally adjustable relative to the fluid delivery arm assembly  126 . The location and amount of foam formed is dependent upon the speed of the platen  104 . As the speed of the platen  104  increases, the electrolyte moves toward the edge of the platen  104  and thus the foam also moves toward the edge of the platen  104 . As a result, the angle (θ)  210  of the foam removal assembly  180  can be adjusted as the speed of the platen  104  either increases or decreases for maximum foam removal. Thus the foam removal assembly  180  should be positioned along the angle (θ)  210  so it removes a majority of the foam produced. In one embodiment, a motor (not shown) is provided to control the rotational movement of the foam removal assembly  180 .  
         [0037]      FIG. 3  is a plan view of another embodiment of the processing station of  FIG. 1  illustrating the location of the foam removal assembly  180  with respect to the carrier head  102 , the substrate  122  and the fluid delivery arm assembly  126 . In this embodiment, the foam removal assembly  180  is attached to the distal end  138  of the fluid delivery arm assembly  126 . The foam removal assembly  180  should be positioned so it does not interfere with the movement of the carrier head  102 . Arrow  302  defines the rotational movement of the platen  104 . Arrow  304  defines the rotational movement of the carrier head  102 . Arrow  306  defines the movement of foam as it is blocked by the foam removal assembly  180  and swept off the platen  104  by the rotational movement of the platen  104  represented by arrow  302 .  
         [0038]     The angle (β)  310  of the foam removal assembly  180 , located in a plane parallel to the platen  104 , relative to the fluid delivery arm assembly  126  should be fixed so that the foam will be pushed off the platen  104 . The angle (β)  310  of the foam removal assembly  180  relative to the fluid delivery arm assembly  126  should also be fixed so that the foam removal assembly  180  does not interfere with the movements of carrier head  102 . In one embodiment, the angle (β)  210  of the foam removal assembly  180  relative to the fluid delivery arm assembly  126  is between about 45° to about 120°, and in another embodiment is between about 60° to about 100°, and in a specific embodiment is about 90°. In this embodiment, the foam removal assembly  180  is horizontally adjustable and locks in place for processing.  
         [0039]     In another embodiment, the foam removal assembly  180  is positioned at an angle in a plane perpendicular to the polishing medium. In one embodiment, the foam removal assembly  180  is angled downward opposite the rotational movement of the platen defined by arrow  202 . In another embodiment, the blade can be curved toward the rotational movement of the platen defined by arrow  202 .  
         [0040]     The foam removal assembly  180  is fabricated from a material that is compatible with process chemistries. The foam removal assembly  180  can comprise a plastic material such as PPS, PEEK, and the like, or a conductive material selected from the group consisting of stainless steel, copper, gold, silver, tungsten, palladium, bronze, brass, polymers and the like or some combination thereof.  
         [0041]      FIG. 4  is a partial sectional view of one embodiment of the foam removal assembly  180 . In this embodiment, the foam removal assembly  180  comprises a top portion  402  and a bottom portion  404 . The top portion  402  comprises a straight bar having a rectangular cross section. The top portion  402  needs to be sufficiently rigid so it does not bend or flex. The bottom portion  404  is slidably attached to the top portion  402  of the foam removal assembly  180  and can be replaced as needed. The bottom portion  404  may comprise one or more blades extending along the underside of the top portion  402 . The blade is formed from a material that does not react with process chemistries. In one embodiment, the bottom portion  404  is thin enough to flex from side to side. In one embodiment, the foam removal assembly  180  comprises more than one material, for example, the top portion  402  of the foam removal assembly  180  comprises a material such as stainless steel and the bottom portion  404  of the foam removal assembly comprises a material such as PPS or PEEK. In another embodiment, the foam removal assembly  180  comprises a unitary piece comprising a single material.  
         [0042]     In another embodiment, the foam removal assembly  180  comprises a brush with a plurality of bristles. The bristles are preferably packed together in a high density that projects downward from the fluid delivery arm assembly  126 . The foam removal assembly  180  can comprise any shape or material that properly removes foam from the surface of the electrolyte.  
         [0043]      FIG. 5  is a plan view of one embodiment of a planarization system  500  having an apparatus for electrochemically processing a substrate. The exemplary system  500  generally comprises a factory interface  502 , a loading robot  504 , and a planarizing module  506 . The loading robot  504  is disposed proximate the factory interface  502  and the planarizing module  506  to facilitate the transfer of substrates  122  therebetween.  
         [0044]     A controller  508  is provided to facilitate control and integration of the modules of the system  500 . The controller  508  comprises a central processing unit (CPU)  510 , a memory  512 , and support circuits  514 . The controller  508  is coupled to the various components of the system  500  to facilitate control of, for example, the distribution of electrolyte, and the position of the fluid delivery arm assembly  126 , the position of the foam removal assembly  180 , the speed of the platen  104 , and positioning of the carrier head  102 . The control system can optimize the distribution of electrolyte to the surface of the polishing pad  108  and prevent collisions between the foam removal assembly  180  and the carrier head  102 .  
         [0045]     The factory interface  502  generally includes a cleaning module  516  and one or more wafer cassettes  518 . An interface robot  520  is employed to transfer substrates  122  between the wafer cassettes  518 , the cleaning module  516  and an input module  524 . The input module  524  is positioned to facilitate transfer of substrates  122  between the planarizing module  506  and the factory interface  502  by grippers, for example vacuum grippers or mechanical clamps.  
         [0046]     The planarizing module  506  includes at least the first electrochemical mechanical planarizing (ECMP) station  100 , with the foam removal assembly  180  and fluid delivery arm assembly  126  and optionally, at least one conventional chemical mechanical planarizing (CMP) stations  532  disposed in an environmentally controlled enclosure  588 . Examples of planarizing modules  506  that can be adapted to benefit from the invention include MIRRA®, MIRRA MESA®, REFLEXION®, REFLEXION® LK, and REFLEXION LK Ecmp™ Chemical Mechanical Planarizing Systems, all available from Applied Materials, Inc. of Santa Clara, Calif. Other planarizing modules, including those that use processing pads, planarizing webs, or a combination thereof, and those that move a substrate relative to a planarizing surface in a rotational, linear or other planar motion may also be adapted to benefit from the invention.  
         [0047]     In the embodiment depicted in  FIG. 1 , the planarizing module  506  includes the first ECMP station  100 , a second ECMP station  530  and one CMP station  532 . Bulk removal of conductive material from the substrate is performed through an electrochemical dissolution process at the first ECMP station  100 . After the bulk material removal at the first ECMP station  100 , residual conductive material is removed from the substrate at the second ECMP station  530  through a second electrochemical mechanical process. It is contemplated that more than one residual ECMP stations  530  may be utilized in the planarizing module  506 .  
         [0048]     A conventional chemical mechanical planarizing process is performed at the planarizing station  532  after processing at the second ECMP station  530 . An example of a conventional CMP process for the removal of copper is described in U.S. Pat. No. 6,451,697, issued Sep. 17, 2002, which is incorporated by reference in its entirety. An example of a conventional CMP process for the barrier removal is described in U.S. patent application Ser. No. 10/187,857, filed Jun. 27, 2002, which is incorporated by reference in its entirety. It is contemplated that other CMP processes may be alternatively performed. As the CMP stations  532  are conventional in nature, further description thereof has been omitted for the sake of brevity.  
         [0049]     The exemplary planarizing module  506  also includes a transfer station  536  and a carousel  534  that are disposed on an upper or first side  538  of a machine base  540 . In one embodiment, the transfer station  536  includes an input buffer station  542 , an output buffer station  544 , a transfer robot  546 , and a load cup assembly  548 . The input buffer station  542  receives substrates from the factory interface  502  by the loading robot  504 . The loading robot  504  is also utilized to return polished substrates from the output buffer station  544  to the factory interface  502 . The transfer robot  546  is utilized to move substrates between the buffer stations  542 ,  544  and the load cup assembly  548 .  
         [0050]     In one embodiment, the transfer robot  546  includes two gripper assemblies, each having pneumatic gripper fingers that hold the substrate by the substrate&#39;s edge. The transfer robot  546  may simultaneously transfer a substrate to be processed from the input buffer station  542  to the load cup assembly  548  while transferring a processed substrate from the load cup assembly  548  to the output buffer station  544 . An example of a transfer station that may be used to advantage is described in U.S. Pat. No. 6,156,124, issued Dec. 5, 2000 to Tobin, which is herein incorporated by reference in its entirety.  
         [0051]     The carousel  534  is centrally disposed on the base  540 . The carousel  534  typically includes a plurality of arms  550 , each supporting a planarizing head assembly  552 . Two of the arms  550  depicted in  FIG. 5  are shown in phantom such that a planarizing surface of the first ECMP station  100  and the transfer station  536  may be seen. The carousel  534  is indexable such that the planarizing head assemblies  552  may be moved between the planarizing stations  100 ,  532  and the transfer station  536 . One carousel that may be utilized to advantage is described in U.S. Pat. No. 5,804,507, issued Sep. 8, 1998 to Perlov, et al., which is hereby incorporated by reference in its entirety.  
         [0052]     A conditioning device (not shown) is disposed on the base  540  adjacent each of the planarizing stations  100 ,  530 , and  532 . The conditioning device periodically conditions the planarizing material disposed in the stations  100 ,  530 , and  532  to maintain uniform planarizing results.  
         [0053]     One exemplary embodiment of an electrically conductive processing solution includes an acid based electrolyte, a first chelating agent having a carboxylate function group, a passivating polymeric material, a second chelating agent having an amine function group, an amide function group, or combinations thereof, a pH adjusting agent to provide a pH between about 3 and about 8, and a solvent. Embodiments of the electrically conductive processing solution may be used for polishing bulk and/or residual materials. The processing solution may optionally include one or more corrosion inhibitors or a polishing enhancing material, such as abrasive particles. While the compositions described herein are oxidizer free compositions, the invention contemplates the use of oxidizers as a polishing enhancing material that may further be used with an abrasive material. It is believed that the polishing compositions described herein improve the effective removal rate of materials, such as tungsten, from the substrate surface during ECMP, with a reduction in planarization type defects and yielding a smoother substrate surface. This processing solution described herein is just one exemplary embodiment. This invention contemplates the use of other processing solutions.  
         [0054]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.