Abstract:
A structure and process for a copper-containing, wire-bonding pad structure for bonding to gold wires. The structure includes a nickel-containing film to improve metal lurgical characteristics. The structure also has a laminated impurity film within the copper pad, which complexes with the nickel-containing pad to prevent a destructive interaction between nickel and copper at elevated temperatures, or during the lifetime of the device or the wirebond.

Description:
FIELD OF THE INVENTION 
     The present invention is related to a process and structure for providing a copper interconnect film used in the semiconductor packaging industry. More particularly, the present invention is directed to providing a copper film structure that contains impurities and provides superior wire bonding characteristics. 
     BACKGROUND OF THE INVENTION 
     Low-cost, wire-bonding processes for copper interconnect and copper packaging technologies are a critical issue that needs to be resolved for copper interconnection technologies to rapidly penetrate the consumer goods sector of the electronics market. Using the present technology, gold wire may not be directly attached to a final copper metal level because of the poor reliability of the copper-to-gold wire-bonding metallurgy. In addition, wire-bond pads containing copper surfaces that are exposed to the environment are unacceptable because exposed copper surfaces are prone to corrosion. 
     One process, directed to circumventing the issues associated with the copper-to-gold bonding, is the use of aluminum as a final metal film to contact the gold wire bond. The use of an aluminum film requires, however, an additional photomask step and an additional patterning step, which is typically reactive ion etching. These two additional steps, together with the associated pre-cleaning and photoresist removal steps, require additional processing materials, additional time, and additional expenses. 
     The prior art process which uses an aluminum film to provide contact between copper and gold bonding wires is best described as follows. After the final metal cooper structure is formed on the surface to be bonded, a dual dielectric film is deposited over the copper surface. This deposition is followed by a polyimide layer formed over the dielectric film. The polyimide serves as a further insulator. The polyimide and underlying passivating dielectric are patterned and etched using reactive ion etching. In this manner, a section of the final copper metal film is exposed. This exposed section will be used to provide contact to the wire-bonding gold. Exposed surfaces of copper film are undesirable for providing direct contact to gold wire bonds. 
     Therefore, at this point in the conventional process, an aluminum film is added. A dual metal layer is deposited as follows. First, a barrier film, which isolates the copper and aluminum metals from each other, is deposited. The deposition of the barrier material is not a selective deposition process. As such, the barrier material contacts the exposed copper surface and also covers the patterned dielectric and polyimide films. Next, an aluminum film is formed over the entire surface of the barrier layer. This dual layer (barrier layer and aluminum) material must then be patterned and subsequently etched. A photolithographically sensitive film is used to form a pattern of this dual layer metal film. After the films are removed by etching, and the photolithographically sensitive film is removed, the gold wire bond may be connected to the aluminum film which is contacted to the copper film through the barrier material. 
     Another, less expensive, alternative to aluminum wire bonding over copper may be the use of an electroless deposition process to form compounds such as CoP and NiP as contact layers for gold wire bonding. In this process, after a pattern has been created in the insulating layers to expose the portion of the final copper film which is to be bound (as above), a layer of CoP is selectively deposited over the exposed film region. This CoP film acts a copper barrier. The CoP layer is then passivated with electroless NiP to prevent the oxidation of the CoP. The NiP is plated selectively over the CoP barrier film. The NiP is then selectively covered with gold using immersion or electroless plating techniques. After the immersion or electroless gold film has been selectively deposited, the structure is ready for wire bonding to the gold wire-bonding line. 
     A simpler structure could be formed by eliminating the CoP barrier layer and depositing NiP directly over the exposed copper surface. A major shortcoming of this approach is that, above 300° C., the NiP film interacts with the exposed copper to produce a bond pad metallurgy with a resistance which is too high for wire-bonding applications. Therefore, what is needed in the art is a procedure which uses the selectively deposited NiP film but does not require the CoP barrier layer, yet produces a bond pad metallurgy with acceptably low resistance values. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the shortcomings of current processing approaches. The present invention provides for the introduction of the NiP film directly onto the copper metal pad, and also prohibits the interaction between the NiP film and the copper which has resulted in unacceptably high resistance values as in the prior art. The present invention discloses a process for laminating impurities such as oxygen, copper, nitrogen, and sulfur close to the exposed upper surface of the copper metal pad. After this laminated impurity film is created and additional copper is added, a transition film such as NiP is deposited directly over the pad which contains the buried, laminated impurities. The NiP layer is then capped with immersion or electroless gold. 
     To stabilize the structure, the entire structure is heated in an inert ambient. During this heat-treatment stabilization, the buried, laminated impurities (such as oxygen, carbon, nitrogen, and sulfur) segregate to the Cu-NiP interface. Here, the laminated impurities interact with NiP to form complexes such as NIP(O) or other complexes including carbon, nitrogen, and sulfur at the interface between the NiP and the exposed copper surface. This NiP(O) or other complex impedes the undesired high-temperature a reaction of NiP and copper thereby enhancing the reliability of the Cu-NiP interface. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures. 
     FIGS. 1 through 6 are cross-sections showing the process sequence used to form an exemplary embodiment of the present invention. More specifically, FIG. 1 shows a plated metal copper film formed on a damascene structure; 
     FIG. 2 shows the structure of FIG. 1 after the exposed copper surface has been modified; 
     FIG. 3 shows an impurity film laminated within the structure of FIG. 2; 
     FIG. 4 shows a photoresist pattern formed on the structure of FIG. 3; 
     FIG. 5 shows the metal film structure of FIG. 4 after it has been patterned; 
     FIG. 6 shows a completed bond pad of an exemplary embodiment of the present invention; 
     FIGS. 7 and 8 are cross-sectional views showing the process sequence used to form a second exemplary embodiment of the present invention. More specifically, FIG. 7 is a cross-section showing a structure of the present invention using the plate-through process; 
     FIG. 8 is a cross-section showing a completed bond pad of the second exemplary embodiment of the present invention; 
     FIGS. 9 through 13 are cross-sectional views showing the process sequence used to form a third exemplary embodiment of the present invention using damascene techniques. More specifically, FIG. 9 is a cross-section showing an impurity-laminated film within an electroplated film structure; 
     FIG. 10 is a cross-section showing the damascene structure of FIG. 9 after it has been polished; 
     FIG. 11 is a cross-section showing the structure of FIG. 10 after insulating films have been added and patterned; 
     FIG. 12 shows the structure of FIG. 11 after a nickel phosphorus film has been added; and 
     FIG. 13 is a cross-section showing a completed structure of the third exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     An important feature of the present invention is the formation of an impurity film which is laminated onto the surface of a copper film which has been roughened. The function of this laminated impurity is to complex with a subsequently deposited NiP film. Such a complex prevents the undesirable interaction between the NiP film and the copper film which would otherwise cause high resistance and produce a bond pad unsuitable for wire-bonding applications. Because of this feature, the CoP film commonly required in the prior art, to serve as a barrier between the exposed copper surface and the NiP film, is not required. 
     When damascene processing techniques are used, the impurity film is laminated onto the surface of the copper film in a region close to the top of the trench formed within an insulator. In this manner, after additional copper is added and the completed damascene pad is formed, the laminated impurity layer is near the top surface of the pad which will contact the subsequently added NiP. 
     FIG. 1 is a cross-section showing an insulating film  3  formed on a semiconductor substrate  1 . A trench  7  has been formed within the insulating film  3 . Via  5  is used to connect a metal structure, such as a wire-bond pad formed within the trench, to subjacent circuitry. The damascene structure includes an exposed upper surface  9  as well as exposed surfaces  9 A (side walls) and  9 B (the bottom of trench  7 ). A barrier layer  14  is formed covering all of the exposed surfaces  9 ,  9 A, and  9 B. The barrier layer  14  may be any suitable barrier material common in the art and may be formed by any suitable process. 
     On top of the barrier layer  14 , a bulk copper film  10  is formed. Copper film  10  has an upper surface  12 . In the preferred embodiment, copper film  10  may be formed by electrodeposition using a plating solution. The plating solution is also subsequently used to form an impurity film which is laminated onto the surface of the electroplated bulk copper film  10 . The plating bath formulation may consist of copper sulfate pentahydrate with a concentration of 20 to 150 grams per liter, but preferably within the range of 30 to 150 grams per liter. The copper sulfate pentahydrate is dissolved in a solution containing deionized water and containing approximately 3 to 25% sulfuric acid by volume. In the preferred embodiment, the sulfuric acid content may range from 5 to 20% by volume. 
     The plating formulation also includes additives. For example, chlorine may be added to the solution within a range of 20 to 180 parts per million and, in the preferred embodiment, may range between 30 to 150 ppm. The plating bath may also contain other commercially available additives such as MD and ML o . With respect to additive MD, the concentration may range from 2 milliliters per liter (ml/l) to 25 ml/l and, in the preferred embodiment, is within the range of 3 ml/l to 20 ml/l. With respect to the concentration of additive ML o  within the bath, the concentration range of the preferred embodiment is between 0.3 to 3.5 ml/l, but may range from 0.2 ml/l to 5 ml/l. 
     After a brief dwell period within the plating solution, the plating process is initiated. During the plating process, the electroplating current density ranges from 7 mA/cm 2  to 35 mA/cm 2  in the preferred embodiment, but may range from 5 mA/cm 2  to 70 mA/cm 2 . The anode material consists of a CuP alloy, as typically used in copper electrodeposition. The electroplating time is determined by the plating current density and the desired thickness of the electroplated film being formed. The desired thickness is chosen so that, after the copper film is formed, at least 85% of the cross-sectional area of original trench  7  is filled with the copper film. As deposited, copper film  10  is formed over the barrier layer  14  formed on surfaces  9 ,  9 A, and  9 B. Electroplated copper film  10  includes impurities electrodeposited along with the copper from the additives included in the plating solution. 
     Now turning to FIG. 2, upper surface  12  shown in FIG. 1 is treated to form roughened surface  12 ′. Roughened surface  12 ′ is formed by moving the substrate  1  from the plating solution into an oxygen-containing environment. In alternate embodiments, in which the copper film  10  is formed by processes other than electroplating, the upper surface  12  is contacted with a plating solution as described above. The plating solution is then mechanically removed from upper surface  12  in the oxygen-containing environment. In the preferred embodiment, the oxygen-containing environment may simply be air, and the preferred process for mechanical removal may be spin-drying the substrate  1 . The spin speed used to remove the plating solution from the substrate  1  may range from 10 to 2,500 RPM, but preferably will lie within the range of 200 to 800 RPM. The spin time necessary to completely remove the plating solution from the upper surface  12  depends on the spin speed used, and will typically range from 10 to 45 seconds. This “spin-off” step increases the surface area of the exposed surface by forming a micro-surface within the originally formed upper surface  12  of copper film  10 . The step exposes high crystallographic index planes to form exposed roughened surface  12 ′ which, as modified, provides for increased adsorption of impurities laminated onto the roughened surface  12 ′ during subsequent processing steps. 
     The following examples are included to more clearly demonstrate the overall nature of the invention. These examples are exemplary, not restrictive, of the invention. 
     EXAMPLE 1 
     FIG. 3 shows an laminated impurity film  15  formed on top of roughened surface  12 ′. This laminated impurity film  14  is formed by placing the substrate  1  into the same plating solution, used to electrodeposit bulk copper film  10 , and allowing the substrate  1  to dwell in the plating solution before the application of the electroplating current. Typical dwell times in the solution may range from 0 to 30 seconds and, in the preferred first embodiment, may lie within the range of 2 to 10 seconds. During this time, a minimal current of 3 to 5 mA/cm 2 , which is much less than the electroplating current, may be applied, or the solution may be maintained in an electrically neutral state. During this dwell period before the electroplating current is applied, additives from the bath adsorb onto the large micro-surface area and the high crystallographic index planes created by the prior processing step. 
     In this manner, an impurity film  15  is laminated onto bulk copper film  10  and, more directly, onto roughened surface  12 ′. The additives contained in the plating solution are preferentially absorbed from the solution and onto the copper microstructure during this lamination process. Thus, a thin film with a high impurity content is laminated onto the microstructure to provide a discrete region of impurity concentration within the composite film structure. The impurity concentration in this discrete region is much higher than a corresponding impurity concentration within the bulk copper film  10  produced by electroplating from the same electroplating solution. In the preferred first embodiment, oxygen may be laminated onto roughened surface  12 ′, but carbon, nitrogen, and sulfur may be added alternatively or additionally. The laminated impurity film  15  has a top surface  16  and may be as thin as an atomic mono-layer in thickness. Impurity film  15  is shown enlarged, in FIG. 3, for clarity. 
     Following this process step, a second copper film  17  is formed by electrodeposition over the laminated impurity film  15  on modified copper roughened surface  12 ′. The thickness of the second copper film  17  is determined by subsequent processing requirements but will typically be much thinner than the first electrodeposited copper film  10 . If chemical mechanical polishing steps will be subsequently used to form a structure within the damascene trench, a minimal thickness of electroplated metal must be added in order to fill the original trench  7 . The process used for electrodeposition may be as described above, but any electrodeposition process suitable in the art may be used. In the preferred first embodiment, the thin electrodeposited second copper film  17  with an upper surface  26  is added to the structure. In this manner, the buried laminated impurity film  15  is close to upper surface  26  and can more efficiently interact with a film such as NiP (not shown) which will subsequently be formed over upper surface  26 . 
     After the metal plating step used to form electroplated second copper film  17 , the deposited metal may be stabilized by an initial annealing process performed in an inert ambient, such as nitrogen or argon, or in a vacuum. The preferred annealing temperature may range from 200° C. to 450° C., but can range from 100° C. to 550° C. in alternate embodiments. The preferred annealing time will range from 15 minutes to 90 minutes, but in alternate embodiments may range from 10 minutes to 120 minutes. After the initial annealing process is completed, a pattern may be formed in the metal film structure to form a bond pad which will be used for wire bonding. 
     In the exemplary first embodiment as shown in FIG. 4, a photoresist film may be used to form the pattern. In the exemplary first embodiment, a photoresist film  18  is applied to upper surface  26 , then patterned. Any process suitable in the art for forming a pattern of the photoresist film  18  may be used. Once patterned, the photoresist film  18  creates exposed field regions  20  and  21  where the composite film will be subsequently removed, and protected region  22  which will remain intact and form the wire-bond pad structure. 
     FIG. 5 shows the structure of the present invention after the portions of the composite film exposed in the field regions  20  and  21  have been removed. The composite film (including second copper film  17 , impurity film  15 , bulk copper film  10 , and barrier layer  14 ) may be removed using any etching process suitable in the art. In the preferred first embodiment, reactive ion etching may be used. The composite film is removed down to exposed surface  9  in field regions  20  and  21 . As shown, side walls  30  are exposed. 
     In protected region  22 , the bond pad structure is formed. After the composite film is removed, the photoresist film  18  is removed to expose upper surface  26 . Any suitable process, which removes a photoresist film and does not attack the underlying metal structure, may be used. 
     Now turning to FIG. 6, the completed structure which is ready for wire bonding is shown. A NiP or other transition film  25  is selectively formed over exposed upper surface  26  and covers side walls  30 . Thus, transition film  25  covers all exposed copper surfaces to preclude copper corrosion resulting from exposure to the environment. The transition film  25  may be formed using any suitable electroless deposition process which provides for selective electroless deposition. Although an NiP film is used in the preferred embodiment, another film such as NiB (nickel boron) or CoWP (cobalt-tungsten-phosphorus) may be substituted. 
     The metal transition film  25  is deposited directly over the laminated pad which includes buried impurities. The laminated impurities such as oxygen, carbon, nitrogen, or sulfur may be close to the pad upper surface  26 , and thus to the transition metal film  25 . In the preferred first embodiment, NiP film  25  is then coated with a gold film  27 , using an immersion gold or electroless gold deposition process. A combination of both immersion plating and electroless plating may also be used. The addition of the gold film  27  is to enhance the integrity of the wire bonding to the NiP film  25  interface. 
     The wire-bonding pad structure, including the immersion or electroless gold film  27 , is then annealed. The anneal stabilizes the microstructure of the deposited gold and urges the interaction between the laminated impurities and the transition film  25  before wire bonding. This final anneal may take place in an inert environment, such as air or nitrogen, at a temperature ranging from 80° C. to 450° C., but most preferably within the temperature range of 100° C. to 400° C. The final anneal time can range from 15 minutes to 3 hours, but most preferably is within the time range of 30 minutes to 2 hours. As the final anneal temperature is increased, the annealing time will be reduced accordingly. In the preferred first embodiment, the anneal conditions are a temperature of 200° C. to 400° C. for about 30 minutes. 
     During this final anneal heat treatment process, the buried impurities laminated below the copper upper surface  26  interact with NiP film  25  in the preferred embodiment. Buried impurities such as oxygen, which originate in laminated impurity film  15  (as in FIG.  5 ), travel to the interface formed between the second copper film  17  and the NiP film  25  at upper surface  26 . Here, the laminated impurities interact with NiP to form complexes such as NiP(O). The NiP (O) complex impedes the undesired reaction of NiP and copper at high temperatures, thereby enhancing the reliability of the NiP-copper interface. In alternate embodiments, impurities which form may include NiB(O) or CoWP(O), or complexes with carbon, sulfur, oxygen, and nitrogen. The structure shown in FIG. 6 is now ready for wire bonding. 
     EXAMPLE 2 
     FIG. 7 shows an alternate process for producing a completed metal bond pad structure from the composite film as shown in FIG.  3 . After the structure is annealed according to the initial annealing process described in conjunction with FIG. 3 of Example 1, a “plate-through” pattern is formed on upper surface  26  of the composite film instead of the photoresist pattern as shown in FIG.  4 . 
     In the preferred second embodiment, a photoresist film  51  is used to form the plate-through pattern. In an alternate embodiment, a low-temperature polyimide material may be used. After a pattern has been formed, exposing region  53  which will form a bond pad, an NiP or other transition film  25  is selectively deposited onto region  53 . After the NiP film  25  is formed in the preferred embodiment, a gold film  27  is selectively formed over the NiP film  25 . The gold film  27  may be formed using immersion or electroless plating. 
     Now turning to FIG. 8, the patterned photoresist film  51  is removed. Any process available in the art, which removes the photoresist or low-temperature polyimide film and does not significantly attack the metal structure, may be used. Portions of bulk copper  10 , impurity film  15 , and second copper film  17  which lie outside of region  53 , and which will not form part of the wire-bond pad  55 , are next removed using gold film  27  as a photomask. In the preferred second embodiment, a 1 to 5 percent by volume acidified ammonium persulfate solution may be used to selectively remove the copper. This etch procedure exposes side walls  57  of the bulk copper film  10 . After the composite copper film is removed, the unmasked sections of the barrier film  14  may be removed with a reactive ion etch process using CF 4 . 
     In this manner, final bond pad structure  55  is formed. The final bonding pad structure  55  is then annealed using the final annealing process as described in conjunction with FIG. 6 in Example 1. This annealing process stabilizes the structure and causes the interaction, between the laminated impurities from laminated impurity film  15  shown in FIG.  7  and the NiP film  25  at upper surface  26 , which forms the NiP-copper interface. 
     Final bond pad structure  55  is formed within trench  7  of a damascene structure. Final bond pad structure  55  includes barrier film  14 , first bulk copper film  10  having roughened surface  12 ′, second bulk copper film  17 , and NiP film  25  (in the preferred second embodiment) and is covered by gold film  27 . At the upper surface  26  which forms the interface between the second bulk copper film  17  and the NiP film  25 , the impurities from laminated impurity film  15 , shown in FIG. 7, form complexes with the NiP film  25 . 
     As formed, the final bond pad structure  55  includes exposed copper side walls  57 . In the preferred second embodiment, after the structure is wire bonded to a gold wire, an epoxy is used to cover the entire bond area and to encapsulate the exposed copper side walls  57 . Such encapsulation prevents exposure of the copper to the environment and, possibly, corrosion. In an alternate embodiment, an NiP cover may be deposited onto side walls  57  before removal of the barrier layer  14  using any suitable electroless deposition process which provides for selective deposition. 
     EXAMPLE 3 
     FIG. 9 shows the first step in the formation of the third exemplary embodiment of the present invention. The structure that appears in FIG. 9 is formed in the same manner as the structure described in conjunction with FIG. 3, including the initial annealing operation. In this third exemplary embodiment, chemical mechanical polishing (CMP) is used to form the structure of the present invention. 
     FIG. 9 shows the damascene trench  7  formed within the insulating film  3  of substrate  1 . The structure includes via  5 , and barrier layer  14  formed over the exposed surfaces: upper surface  9 , side wall surfaces  9 A, and bottom surface  9 B. Electrodeposited bulk copper film  10  covers the barrier layer  14  and fills the greater part of the cross-sectional area of the trench  7 . The copper film  10  has a top roughened surface  12 ′ which has been roughened by the spin-etch process as described above. Roughened surface  12 ′ is recessed by a distance  33  below upper exposed surface  9  in the center area of trench  7 . Laminated impurity film  15  covers roughened surface  12 ′. The second electrodeposited bulk copper film  17  covers the structure and fills the trench  7 , as previously described. 
     In the preferred third embodiment, the thickness of the bulk copper film  10  is chosen so that approximately 85% of the cross-sectional area of the trench  7  is filled with the first bulk copper film  10 . The trench  7  must not be completely filled by copper film  10 , to insure that the laminated impurity film  15  and the second deposited bulk copper film  17  have components that remain within the trench  7  after a subsequent polishing step planarizes the structure as will be seen in FIG.  10 . In contrast, in Example 1 the structure is not subsequently planarized and the location of the laminated impurity film  15  with respect to upper exposed surface  9  is not quite as critical. 
     After the structure is annealed according to the initial annealing process described in conjunction with FIG. 3 of Example 1, the overburden of the copper film structure is removed as shown in FIG.  10 . The structure in FIG. 10 is produced after chemical mechanical polishing steps have been used to planarize the structure. The device so planarized includes a copper film structure having an upper surface  31  which is substantially continuous and co-planar with upper exposed surface  9  of the dielectric insulating film  3  formed on the substrate  1 . The metal film structure includes portions of the first electrodeposited bulk copper film  10  and the second electrodeposited bulk copper film  17 . In between these two films, laminated impurity film  15  is interposed. 
     Now-turning to FIG. 11, an insulating film is formed and patterned over surfaces  9  and  31  of the planarized structure. The insulating film may consist of dual interlevel dielectric films  39 , formed directly over the surfaces, and a polyimide film  37  disposed over the dual interlevel dielectric films  39 . The polyimide film  37  and dual interlevel dielectric films  39  are patterned and etched using reactive ion etching chemistry to form a pattern which exposes a region  41  of the exposed upper surface  31  of the composite metal film structure. This region  41  is the region to which a wire-bonding connection will ultimately be made. 
     Now turning to FIG. 12, in the preferred third embodiment, an NiP film  43  is formed by electroless or other suitable selective deposition techniques to cover the exposed region  41  of upper surface  31 . Although an NiP film  43  is used in the preferred third embodiment, other transition films such as NiB or CoWP may be used in alternate embodiments. The NiP film  43  has a top surface  45 . Laminated impurity film  15  and second electrodeposited bulk copper film  17  are included in the structure. 
     Now turning to FIG. 13, a gold film  50  is selectively deposited over NiP top surface  45 . The gold film  50  is preferably formed by immersion plating or electroless plating processes, or the combination of both. This gold layer  50  is then stabilized by using the final annealing process as described in conjunction with FIG. 6 of Example 1. 
     This final annealing process urges the combination of elements from the impurity laminated film  15  (of FIG. 12) with the NiP (or other) film  43  to form chemical mixtures or complexes which prevent the interaction of the NiP (or other) film  43  with copper. The complexes and mixtures formed may include NiP(O), NiB(O), and CoWP(O) and are formed at the upper surface  31  which represents the interface between the NiP film  25  and the bulk copper films  17  and  10 . 
     As described above, Examples 1, 2, and 3 represent exemplary embodiments of the present invention and are not intended to limit the scope of the present invention. The details of the specific processes used to form various components of the structure may be varied and still lie within the scope of the present invention. In addition, the thickness of the various films may be varied. Specifically, the thickness of the first and second deposited bulk copper films  10  and  17  formed within the trench  7  may be varied depending on the subsequent processing sequence anticipated. 
     The semiconductor structures detailed above are intended to be exemplary only. Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.