Abstract:
In one example, an apparatus for dispensing copper into a plating solution is provided which includes a cartridge containing an inlet and an outlet and comprising a copper metal source therein, a dosing device containing an oxidizing agent in fluid communication with the inlet, a tank for containing the plating solution in fluid communication with the outlet, a pH electrode adapted to contact the plating solution, and a system controller which receives input from the pH electrode and sends output to the dosing device. In another example, a method for replenishing copper in a plating solution is provided which includes flowing the plating solution from a plating cell to a replenishing system comprising a dosing device and a cartridge, dosing an oxidizing agent from the dosing device to the plating solution, exposing the plating solution to a copper metal source contained in the cartridge, enriching the plating solution with copper ions derived from the copper metal source, and flowing the enriched plating solution to the plating cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. Provisional Patent Application No. 60/580,255, filed Jun. 15, 2004, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Embodiments of the invention generally relate to a metal plating apparatus and process, namely for the replenishment of chemical components used to electroplate copper.  
         [0004]     2. Description of the Related Art  
         [0005]     Semiconductor substrates can be plated with copper by electroplating or electroless plating processes. During the electroplating, an anode is usually placed into an electrolyte solution and the substrate is conductively coupled to a cathode. As current flows, dissolved copper ions from the electrolyte solution are reduced and plated (or deposited) on the surface of the substrate as copper metal. Traditionally, the anode is made from consumable copper metal and is continuously oxidized to provide copper ions to the plating process. Due to the consumption of the copper anode, the dimension of the copper anode is changed. Therefore, the directional electrical fields produced by the anode also change accordingly. This alteration in the electric field presents a challenge to precisely control the electroplating process, especially within vias with high aspect ratios.  
         [0006]     Another electroplating process utilizes an inert or stable anode in place of a consumable anode. The use of an inert anode provides excellent control for precision plating since the anode is not consumed during the plating process. However, the inert anode does not supply a source of copper into the electrolyte solution. As the copper ions are reduced and plated from the electrolyte solution to the substrate surface, the copper ion concentration in the electrolyte solution is diminished. Therefore, as the plating process progresses, a copper source, namely copper ions, must be added to the electrolyte solution in order to continue the plating process. Copper sources are generally chosen from a variety of copper salts that include copper sulfate, copper hydroxide, copper oxide and copper phosphate.  
         [0007]     The prior art discloses a method to maintain an alkaline copper plating solution with a desired concentration of copper ions and hydroxide ions. Generally, copper hydroxide powder is added from a conduit to a dissolving tank containing an alkaline, pyrophosphate solution. Once the solution has been heated and agitated to insure that the copper hydroxide has been dissolved, the pyrophosphate solution is transferred via a pump to the plating solution. The plating solution is monitored with a pH meter and maintained with a basic pH between 7 and 10 by adding the alkaline, pyrophosphate solution. Though the addition of copper hydroxide powder is adequate in the realm of electroplating wires, this technique is unacceptable in a clean environment, such as a semiconductor fabrication room equipped to plate substrates. The dumping of a powdery precursor into a solution would present contamination issues for semiconductor processing in a cleanroom environment.  
         [0008]     Other prior art realizes the shortcomings of using copper hydroxide as a copper source in a cleanroom and discloses a method to replenish copper ions in a plating solution by dissolving metallic copper wires in an acidic solution. The plating solution also contains iron ions (Fe 3+/2+ ) used as an oxidizing/reducing agent. While the iron ions are useful electron donors/receivers, there are several undesirable characteristics to have iron ions in copper plating solutions. Iron ions can not be used with cation-exchange membranes since the ions will poison the membranes causing a dramatic performance drop to their selectivity and conductivity. Also, iron ions in plating solutions may be undesirable during many semiconductor processes due interfering with various organic additives as well as causing iron contamination in the deposited films.  
         [0009]     Therefore, there is a need for an apparatus and method to replenish chemical compounds, including copper ions and/or a pH adjusting agent, within an electrolyte solution in a consistent and reliable manner.  
       SUMMARY OF THE INVENTION  
       [0010]     In one example, an apparatus for dispensing copper into a plating solution is provided which includes a cartridge containing an inlet and an outlet and comprising a copper metal source therein, a dosing device containing an oxidizing agent in fluid communication with the inlet, a tank for containing the plating solution in fluid communication with the outlet, a pH electrode adapted to contact the plating solution, and a system controller which receives input from the pH electrode and sends output to the dosing device.  
         [0011]     In another example, an apparatus for dispensing metal ions into a plating solution is provided which includes a cartridge containing an inlet and an outlet and comprising a metal source therein, a dosing device containing an oxidizing agent in fluid communication with the inlet, a tank for containing the plating solution in fluid communication with the outlet, a sensor adapted to contact the plating solution, and a system controller which receives input from the sensor and sends output to the dosing device.  
         [0012]     In another example, a method for replenishing copper in a plating solution is provided which includes flowing the plating solution from a plating cell to a replenishing system comprising a dosing device and a cartridge, dosing an oxidizing agent from the dosing device to the plating solution, exposing the plating solution to a copper metal source contained in the cartridge, enriching the plating solution with copper ions derived from the copper metal source, and flowing the enriched plating solution to the plating cell.  
         [0013]     In another example, a method for monitoring and controlling a pH setting of a plating solution in an electroplating system is provided which includes electroplating a substrate with the plating solution within a first pH range, monitoring the plating solution to determine when the plating solution is within a second pH range, dosing an oxidizing agent into the plating solution, exposing a copper metal source to the plating solution to form an enriched plating solution, and ceasing the doses of the oxidizing agent once the plating solution is within the first pH range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     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.  
         [0015]      FIG. 1  shows a flow diagram for a two-sectional electrochemical cell with catholyte and anolyte, as described in the prior art;  
         [0016]      FIG. 2  shows a system to replenish chemicals in a plating solution according to one embodiment described herein;  
         [0017]      FIG. 3  shows a cartridge for replenishing chemicals according to one embodiment described herein;  
         [0018]      FIG. 4  shows a metal bundle for use in a cartridge for replenishing chemicals as described according to one embodiment described herein; and  
         [0019]      FIG. 5  shows another system to replenish chemicals in a plating solution according to one embodiment described herein. 
     
    
     DETAILED DESCRIPTION  
       [0020]     The present invention comprises apparatuses and methods to replenish chemical compounds, such as copper ions, in plating solutions in a consistent and reliable manner while overcoming the shortcomings of the related art as described in the background. Therefore, by utilizing the various embodiments of the apparatuses and methods of the present invention, each substrate experiences more consistent plating times and anolyte chemical concentrations.  
         [0021]     Embodiments of the present invention are useful in a variety of plating systems, including electroplating and electroless plating systems. Further, various embodiments are also applicable to electroplating with soluble anodes and with insoluble anodes.  FIG. 1  shows a schematic arrangement of an electroplating system  10  with a cell  11  containing an insoluble anode  12 . The insoluble anode  12  is made from relatively inert materials, such as platinum, titanium, titanium with a Pt-coating, palladium, nickel, stainless steel and/or carbon. The material of the insoluble anode  12  is generally configured to withstand the various process conditions involved while plating to a wafer or substrate  14 . Process conditions may have acidic or basic pH, oxidative/reductive potentials and an assortment of chemical compounds throughout the solution. In one embodiment, the insoluble anode  12  endures process conditions such as acidic plating solutions and an oxidative potential. The substrate  14  is attached to the cathode  13 , usually by a contact ring, pins, and the like (not pictured).  
         [0022]     The insoluble anode  12  and the cathode  13  are separated by a membrane  16  extending through cell  11 . The membrane  16  is an electroconductive membrane, such as an ion-exchange membrane, nano-filtration membrane, ultra-filtration membrane and others known in the art. The portion of the cell  11  containing the cathode  13  is in fluid communication with the catholyte tank  17  to recirculate the catholyte within. The catholyte is a mixture of compounds for copper plating may be a sulfuric copper plating electrolyte or a pyrophosphoric copper plating electrolyte. A sulfuric copper plating electrolyte will generally include a mixture of copper sulfate, sulfuric acid, water and various organic and inorganic additives including suppressors, accelerators, levelers and brighteners. Catholyte may pass through a diffuser  15  to be more evenly distributed while flowing towards the substrate  14 .  
         [0023]     The portion of the cell  11  containing the insoluble anode  12  is in fluid communication with the anolyte tank  18  and recirculates the anolyte within. For copper plating, the anolyte is usually an aqueous solution containing copper ions, often derived from dissolved copper salts, such as copper sulfate. Other copper ion sources include copper hydroxide, copper carbonate, copper oxide and copper phosphate.  
         [0024]     Under copper plating electrolysis, the half reaction in scheme (i) occurs on the insoluble anode  12 : 
 
H 2 O→2H + +2 e   − +½O 2(g) ,   (i) 
 
 while Cu 2+  ions migrate through the membrane  16  from the anolyte to the catholyte and are reduced according to the half reaction shown in scheme (ii): 
 
Cu 2+ +(SO 4 ) 2− +2 e −→Cu 0 +(SO 4 ) 2− .   (ii) 
 
 The combined half reactions in schemes (i) and (ii) are represented in reaction scheme (iii): 
 
CuSO 4 +H 2 O→Cu 0 +H 2 SO 4 +½O 2(g) .   (iii) 
 
 Therefore, as the electroplating process proceeds, the anolyte becomes depleted of copper ions due to the precipitation of metallic copper from the reduced copper salt, as well as more acidic due to the production of sulfuric acid. Also, water is consumed making the electrolyte more concentrated. 
 
         [0025]     The sulfuric acid formed in the anolyte increases the acidity of the catholyte due to protons penetrating membrane  16 . Therefore, the sulfuric acid in the anolyte lowers the pH of the catholyte. The more acidic anolyte and catholyte is not desirable since the unbalanced chemical concentrations are not constant and adversely effect the plating process. To prevent the lowering of the pH of the catholyte and to maintain a substantially constant copper concentration, an oxidizing agent is dosed into the anolyte upstream from a metallic copper source. The reaction between the metallic copper source, an oxidizing agent (e.g., H 2 O 2 ) and sulfuric acid generates copper ions (e.g., CuSO 4 ) while neutralizing the sulfuric acid, as shown by the reaction scheme (iv): 
 
Cu 0 +H 2 O 2 +H 2 SO 4 →CuSO 4 +2H 2 O.   (iv) 
 
 Therefore, the half reactions shown in schemes (iii) and (iv) are combined and the summed reaction is depicted by scheme (v), such as: 
 
Cu 0 +H 2 O 2 →Cu 0 +H 2 O+½O 2(g) ,   (v) 
 
 where metallic (source) copper is consistently oxidized to form copper ions enriching the anolyte and subsequently reduced to form metallic (deposited) copper. Also, scheme (v) reveals that hydrogen peroxide is consumed while water and oxygen are formed as byproducts. Sulfuric acid is formed and consumed in situ, therefore does not appear in scheme (v). 
 
         [0026]     In one embodiment, the plating solution is a copper sulfate based plating solution with a copper concentration from about 50 mM to about 1.5 M, preferably from about 100 mM to about 1.0 M. However, other plating solutions may be used, such as copper phosphate, copper chloride, copper acetate and combinations thereof. For example, as a copper phosphate or copper sulfate plating solution is used to electroplate, copper ions are depleted as the acidity increases, therefore an oxidizing agent may be pulsed into the system. The excess acid that is formed in situ (e.g., phosphoric or sulfuric) reacts with the metallic copper and the oxidizing agent to consume some of the acid while forming copper ions. Therefore, the copper ion concentration is increased and the acid concentration or acidity of the plating solution is decreased.  
         [0027]     The term plating solution herein may refer to the catholyte and/or the anolyte in an electroplating solution. In one example, the plating solution (e.g., anolyte) has a pH from about 2.5 to about 4.5, preferably from about 2.8 to about 3.5. In another example, the plating solution (e.g., catholyte) has a pH from about −0.5 to about 3, preferably from about 0 to about 1.5. Plating solutions with other non-acid pH ranges are anticipated to work with the present invention, including neutral and basic.  
         [0028]      FIG. 2  shows system  100  that may be used to replenish chemical components, such as copper ions, within a plating solution. System  100  may be used to replenish chemical components in electroless plating solutions, as well as electroplating solutions, including anolyte and/or catholyte solutions. In the preferred embodiment, system  100  is used on an anolyte portion an electroplating system.  
         [0029]     System  100  includes cell  111 , dosing device  130 , cartridge  120  and anolyte tank  118 , each in fluid communication by conduit  102 , such as pipes, tubes, hoses and the like. Cell  111  is used to plate materials to substrate surfaces, such as copper or copper-containing alloys during an electroplating process. For example, cell  111  may be the electroplating cell  11  shown in  FIG. 1 . The anolyte flows from anolyte tank  118  to cell  111 . A pump  114  may be situated between tank  118  and cell  111 , as shown, or pump  114  may be positioned in other sections of system  100 . After or during an electroplating process, depleted anolyte flows into the cartridge  120  via dosing device  130 . The dosing device  130  may be directly connected in-line to system  100  or fluidly connected on a spur  103 . The anolyte flows from cartridge  120  to anolyte tank  118  to complete the cycle.  
         [0030]     A system controller  124  communicates with dosing device  130  and a pH electrode  122 . System controller  124  may be a computer or connected to a computer and monitors the pH value of the anolyte that accumulates in anolyte tank  118  via pH electrode  122 . The pH electrode  122  is usually continuously exposed to the anolyte such that system controller  124  can determine the anolyte pH at any time during processing of the substrate. System controller  124  may be connected with other pH electrodes or sensors positioned throughout system  100  (not shown).  
         [0031]     The system controller  124  may pulse an oxidizing agent via the dosing device  130  as the pH value of the plating solution reaches a predetermined pH range. For example, if the anolyte is a copper sulfate based solution, then the preferred pH value of the anolyte is in the range from about 2.5 to about 4.0, preferably from about 2.8 to about 3.5. In one embodiment, the system controller  124  is preset to start administering an oxidizing agent, such as H 2 O 2 , when the pH value of the anolyte reaches about 2.7 or less. The system controller  124  is preset to stop administering the oxidizing agent when the pH value of the anolyte reaches about 3.0 or higher. In another example, once the anolyte has a pH value in the range from about 2.0 to about 2.7, the oxidizing agent may be administered until the anolyte has an increased pH value, such as in the range from about 2.8 to about 3.5. The exact point at which to start and/or stop the administration of an oxidizing agent is determined with routine experimentation and dependant to the plating solution composition as well as the desired plated material. In one embodiment, a system controller  124  is a pH controller and may be selected from a variety of commercially available models, such as the dTRANSpH 01 from JUMO Process Control Inc., DP24-E Process Meter from Omega, the EMIT-pH from Pathfinder Instruments, and the LED pH/ORP indicator/controller from Kemko Instruments.  
         [0032]     Dosing device  130  is triggered to administer the oxidizing agent by the system controller  124 . In some configurations, dosing device  130  may be serially connected with the pH electrode  122  and the system controller  124 . Dosing device  130  is an apparatus commonly used to administer chemicals in a controlled manner, such as a syringe pump or dosing pump, for example the CERAMPUMP®, available from Fluid Metering, Inc., located in Syosset, N.Y.  
         [0033]     Dosing device  130  administers an oxidizing agent as a solid, liquid or gas, but preferably a liquid. The oxidizing agent may be stored in a reservoir within the dosing device  130  or stored nearby, such as an external reservoir or supplied by in-house feed lines, and plumbed to dosing device  130  by piping or tubing. Oxidizing agents may include hydrogen peroxide, carbamide peroxide, organic peroxides, inorganic peroxides (e.g., calcium peroxide), various metal compounds (e.g., Fe 2+/3+  or cobaltocene), ozone solutions, chlorites (e.g., hypochlorites), bromites, derivatives thereof, and combinations thereof. Organic peroxides useful as oxidizing agents are described with the formula ROOR′, wherein R and R′ are each independently an organic group, such as methyl, ethyl, propyl, butyl, penta, alkyl, benzyl, aryl, and derivatives thereof, for example, benzoyl peroxide, tert-butyl peroxide, di-tert-amyl peroxide. The oxidizing agent may be a solution comprising an active oxidizer dissolved in a solvent and may contain an optional stabilizer. For example, benzoyl peroxide may be dissolved in an aqueous solution. The oxidizing agent is usually within the plating solution at a concentration in the range from about 0.01 vol % to about 5.0 vol %, preferably, from about 0.03 vol % to about 3.0 vol %. In the preferred embodiment, the oxidizing agent is hydrogen peroxide at a concentration in the plating solution in the range from about 0.03 vol % to about 3.0 vol %.  
         [0034]      FIG. 3  depicts one embodiment of the plating solution flowing along a pathway from inlet  121  to outlet  123  through cartridge  120 . The plating solution may also flow from the top to the bottom through cartridge  520 , as depicted in  FIG. 5 . Cartridges may be horizontally or vertically positioned. Cartridge  120  contains a metal bundle  125 , such as a copper bundle. In  FIG. 4 , a metal bundle  125  comprises at least one sheet of metal foil  126  and a separator  127   a . The metal foil  126  is preferably copper or a copper-containing alloy. Separator  127   a  is used to maintain metal foil  126  from touching itself while being rolled to form metal bundle  125 . Therefore, metal bundle  125  maintains metal foil  126  with high surface area and assures a consistent exposure of plating solution across metal foil  126 . In order to allow plating solution to pass freely, separator  127   a  has ridges  128  protruding against metal foil  126  while separator  127   b  has squared geometries  129  protruding against metal foil  126 . Separators are usually composed of materials that do not react with the plating solution, such as plastics, polymers, elastomers, rubbers, and inert metals, for example, polyethylene, polypropylene, nylon, PVC, fluoropolymers and PTFE and may include geometries such as meshing, screening and netting. Separators are commercially available from InterNet, Inc., located in Minneapolis, Minn.  
         [0035]     In one example of the embodiment, metal foil  126  is the preferred geometry for a metal source due to the high surface area as well as the ability to evenly dissolve into solution upon oxidation. In other examples, cartridge  120  contains metal sources with alternative geometries, such as sheets, spheres, beads, powder, granules, wires, springs, mesh, derivatives thereof and combinations thereof. In the preferred embodiment, a copper-containing foil is rolled with a meshing to form metal bundle  125  and supplied to cartridge  120 .  
         [0036]     However, in other examples, supplemental copper reagents may be useful for copper ion replenishment in a plating solution as described herein. Supplemental copper reagents include copper hydroxide, copper oxides, copper carbonate, copper sulfate and copper phosphate and combinations thereof, preferably copper hydroxide. Generally, plating solutions, enriched or depleted, have a copper ion concentration in a range from about 50 mM to about 1.5 M.  
         [0037]      FIG. 5  depicts a system  500  useful to replenish chemical components, such as copper ions, to a plating solution. System  500  includes cell  511 , dosing device  530 , cartridge  520  and anolyte tank  518 , each in fluid communication by conduit  502 , such as pipes, tubes, hoses and the alike. System  500  also contains a bypass line  503  and valves  531  and  532 . Valves  531  and  532  may be three-way valves which are positioned to segregate dosing device  530  and cartridge  520  from cell  511  and anolyte tank  518 . In one embodiment, valves  531  and  532  are positioned to allow anolyte to flow through dosing device  530  and cartridge  520  and not flow through bypass line  503 . In another embodiment, valves  531  and  532  are positioned to not allow anolyte to flow through dosing device  530  and cartridge  520 , but allow anolyte to flow through bypass line  503 .  
         [0038]     Cell  511  is used to plate materials to substrate surfaces, such as copper or copper-containing alloys during an electroplating process. Anolyte flows from anolyte tank  518  to cell  511 . A pump  514  may be situated between tank  518  and cell  511 , as shown, or pump  514  may be positioned in other sections of system  500 . After or during electroplating, depleted anolyte may flow from cell  511  through dosing device  530 , through cartridge  520  and into anolyte tank  518 . Alternatively, the depleted anolyte may flow from cell  511  through bypass  503  and into the anolyte tank  518 . The dosing device  530  may be directly connected in-line to system  500 , as shown, or fluidly connected on a spur.  
         [0039]     A system controller  524  communicates with dosing device  530 , valves  531  and  532 , and sensor  523 , such as a pH electrode, a photo sensor (FT-IR or UV-vis spectrometers), or other sensors commonly used to measure chemical concentrations. System controller  524  may be a computer or connected to a computer and monitors chemical concentrations of the anolyte composition that accumulates within anolyte tank  518  via sensor  523 , such as monitoring acidity with a pH electrode. The sensor  523  may be continuously exposed to the anolyte such that system controller  524  can determine chemical concentrations of the anolyte composition at any time during processing of the substrate. System controller  523  may be connected with other sensors, such as pH electrodes (not shown), positioned throughout system  500 .  
         [0040]     The system controller  524  may pulse an oxidizing agent via the dosing device  530  as the pH of the plating solution reaches a preset pH range. For example, if the anolyte is a copper sulfate based solution, the preferred pH value to maintain the anolyte is in the range from about 2.5 to about 4.0, preferably from about 2.8 to about 3.5. In one embodiment, the system controller  524  is preset to start administering an oxidizing agent, such as H 2 O 2 , when the pH value of the anolyte reaches about 2.7 or less. The system controller  524  is preset to stop administering the oxidizing agent when the pH of the anolyte reaches about 3.0 or higher. In another example, once the anolyte has a pH value in the range from about 2.0 to about 2.7, the oxidizing agent may be administered until the anolyte has an increased pH value, such as in the range from about 2.8 to about 3.5. The exact point at which to start and/or stop the administration of an oxidizing agent is determined with routine experimentation and dependant to the plating solution composition as well as the desired plated material. In one embodiment, a system controller  524  is a pH controller and may be selected from a variety of commercially available models, such as the dTRANSpH 01 from JUMO Process Control Inc., the DP24-E Process Meter from Omega, EMIT-pH from Pathfinder Instruments, and the LED pH/ORP indicator/controller from Kemko Instruments.  
         [0041]     Dosing device  530  is triggered to administer the oxidizing agent by the system controller  524 . In some configurations, dosing device  530  may be serially connected with the sensor  523  and the system controller  524 . Dosing device  530  is an apparatus commonly used to administer chemicals in a controlled manner, such as a syringe pump or dosing pump, for example the CERAMPUMP®, available from Fluid Metering, Inc., located in Syosset, N.Y. Dosing device  530  administer an oxidizing agent as a solid, liquid or gas, but preferably a liquid. In one example, the oxidizing agent is in a liquid state, such as hydrogen peroxide. In another example, the oxidizing agent is in a solid state, such as benzoyl peroxide.  
         [0042]     System  500 , containing bypass line  503  and valves  531  and  532 , function similar to system  100  when valves  531  and  532  are positioned to exclude anolyte circulation from bypass line  503 , herein, “full mode.” However, valves  531  and  532  may be positioned to exclude anolyte circulation through dosing device  530  and cartridge  520  and direct all anolyte through the bypass line  503 , herein, “bypass mode.” The full or bypass modes are controlled by positioning valves  531  and  532  via the system controller  524 . System  500  may be placed in bypass mode to replace the copper source and/or the oxidizing agent while continuing to operate the plating system. Also, system  500  may provide additional control to administer the oxidizing agent and copper ion concentration. For example, an oxidizing agent may be dispensed from the dosing device  530  by flowing anolyte in full mode and then switching to bypass mode to cease the administration of the oxidizing agent. This latter example may be applied to an oxidizing agent in the solid state that slowly dissolves into the anolyte.  
         [0043]     The above apparatus and process describes replenishing metal ions in a plating solution as well as decreasing the acidity (e.g., increasing pH) of the plating solutions. In the preferred embodiment, the plating solution is an anolyte within an electroplating system used to plate copper or copper-containing alloy. However, plating with other metals are within the scope of the present invention, such as zinc, cadmium, tungsten, nickel, platinum, palladium, gold, silver, vanadium, cobalt, alloys thereof, as well as other metals. In an alternative example, at least one secondary element may be added with a copper-containing material as a metal source to be plated. The above apparatus and process anticipate that one skilled in the art can easily replace the plating solution composition to deposit a variety of metals by applying routine experimentation to the basic scope of the present invention. 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.