Abstract:
A method for improving signal levels between capacitively-coupled chips in proximity communication (PxC) includes depositing a high permittivity dielectric material layer over a signal pad of a first chip, and placing a second chip in close proximity to the first chip such that faces of the signal pads align to enable for capacitive signal coupling. The high permittivity dielectric material layer that fills at least a portion of a gap between the first chip and the second chip, and improves capacitive coupling between signal pads of the first chip and the second chip by providing for an increased permittivity in the gap between the first chip and the second chip. The increased permittivity ensures that electric fields are substantially confined to a space between the signal pad of the first chip and the signal pad of the second chip.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a method for increasing signal coupling between pads of chips engaged in capacitive proximity communication (PxC). More specifically, the present invention relates to a method for increasing signal coupling, where a high permittivity dielectric material is directly deposited atop the pads to enable confinement of electric fields within pad gaps of the chips. 
         [0003]    2. Background Art 
         [0004]    With scaling of processor technology to future generational requirements, the pressing need for increased serial data rate over many communication channels to off-chip memory is not eased by the inability of electrical signaling to drive off-chip structures efficiently due to mismatch between on-chip and off-chip bandwidths. Thus, the ability of processors to exploit advancements in computational power, clock frequency, transistor count, multi-core architectures and the like is limited. One way to circumvent the aforementioned limitation is to employ PxC as a means to signal between processor and memory. 
         [0005]    PxC is a form of wireless communication between chips, with the possible chip-to-chip coupling being capacitive, inductive or optical. Capacitive coupling may be employed to provide a high-bandwidth and high density channel between two chips placed face-to-face and extremely close to each other such that the transmitter circuits of one chip align with the receiver circuits of the other. An input/output (I/O) technology using the capacitive coupling of PxC can scale with on-chip features unlike traditional area ball bonds, leading to a largely improved I/O density. Thus, PxC with capacitive coupling offers the advantages of a hundred-fold increase in bandwidth density with a simultaneous three-fold reduction of on-chip power dissipation over traditional off-chip I/O signaling, thereby aiding utility in applications with power hungry, I/O limited multi-chip systems. 
         [0006]      FIG. 1  shows two chips ( 110 ,  120 ) placed close to each other in a capacitively coupled PxC mode. Each chip includes transmitter ( 114 ,  122 ) and receiver circuits ( 112 ,  124 ), where a transmitter circuit of one chip ( 114 ,  122 ) communicates with the receiver circuit ( 112 ,  124 ) of the other chip by way of a metal plate on one chip and the metal plate on the other chip forming a capacitor. The metal plates of the transmitter circuit ( 114 ,  122 ) are depicted as shaded rectangles and the metal plates of the receiver circuit ( 112 ,  124 ) are depicted as non-shaded rectangles. A metal plate on the transmitter side is driven by the transmitter circuit ( 114 ,  122 ) and a metal plate on the receiver side drives the receiver circuit ( 112 ,  124 ). 
         [0007]    When the two chips ( 110 ,  120 ) are moved close to each other, the surfaces of the dielectric and passivation layer touch each other, thereby allowing for excellent capacitive coupling between the metal plates due to their proximity. Also, the sizes of the transmitter and receiver structures may be reduced, allowing for reduced parasitic capacitances and huge power savings. The need for electrostatic discharge (ESD) protection devices is dispensed with as the metal plates lie under the passivation layer. This contributes to an additional reduction in capacitance and power consumption. 
         [0008]      FIG. 2  shows a typical cross-section of transmitter  212  and receiver  222  pads of two chips ( 210 ,  220 ) involved in capacitive PxC. Communication between the transmitter  212  and receiver  222  pads is affected by three capacitances in positive and negative ways. The desired capacitance, i.e., the signal capacitance C s , is the capacitance between the transmitter  212  and the receiver  222  pads. The unwanted capacitance due to coupling between adjacent transmitter  212  and receiver  222  pads is the noise capacitance C n . As shown in  FIG. 2 , guard rings  224  may be provided between receiver  222  pads to suppress crosstalk between individual pads. However, an unwanted parasitic capacitance C p  may have to be factored into received signal calculations. This parasitic capacitance C p  may arise due to coupling between receiver  222  pads and guard rings  224 , coupling between receiver  222 /transmitter  212  pads and the ground plane, and/or intrinsic capacitance. Thus, these capacitances depend on inter-pad distance and inter-chip distance. Conservation of charge deems that the received signal, V r , is dependent on the capacitances, and may be obtained from example Equation (1) as follows. 
         [0000]      ( C   s   +C   n   +C   p ) V   r   =V   s   C   s ,   (1) 
         [0009]    where V s  is the source signal. The received signal, V r , as a function of capacitances, may, therefore, be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     r 
                   
                   = 
                   
                     
                       
                         C 
                         s 
                       
                        
                       
                         V 
                         s 
                       
                     
                     
                       ( 
                       
                         
                           C 
                           s 
                         
                         + 
                         
                           C 
                           n 
                         
                         + 
                         
                           C 
                           p 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0010]    Assuming equal dimensions w×l of two plates, equal plate thickness of t, and a separation between plates d, the capacitance C between two plates may be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     = 
                     
                       
                         
                           ɛ 
                           d 
                         
                          
                         
                           ( 
                           
                             w 
                             - 
                             
                               t 
                               2 
                             
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             l 
                             - 
                             
                               t 
                               2 
                             
                           
                           ) 
                         
                       
                       + 
                       
                         
                           πɛ 
                            
                           
                             ( 
                             
                               l 
                               + 
                               w 
                             
                             ) 
                           
                         
                          
                         
                           f 
                            
                           
                             ( 
                             
                               
                                 d 
                                 2 
                               
                               , 
                               t 
                             
                             ) 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0011]    where ε=ε 0 ε r  is the permittivity of the medium between the two plates, ε 0  being the permittivity of free space and ε r  being the dielectric constant of the medium between the two plates, and f(d,t) is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                      
                     
                       ( 
                       
                         d 
                         , 
                         t 
                       
                       ) 
                     
                   
                   = 
                   
                     1 
                     
                       [ 
                       
                         ln 
                          
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 2 
                                  
                                 d 
                               
                               t 
                             
                             + 
                             
                               
                                 
                                   
                                     2 
                                      
                                     d 
                                   
                                   t 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       
                                         2 
                                          
                                         d 
                                       
                                       t 
                                     
                                     + 
                                     2 
                                   
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0012]    Using the method of images, Equation (3) is obtained by modeling the fringing fields of a rectangular micro-strip line over a ground plane as field lines due to a ground plane between two plates. Equation (3) does not account for the capacitance due to the four corners of the plates, which however is very small, but accounts for both the fringing fields and the parallel plate component. Equation (3) may be used to model the capacitance between transmitter  212  and receiver  222  pads, or any two finite pads separated by a small distance. 
         [0013]    The crosstalk problem that arises in PxC may be due to fringing fields emanating from the length and width of the transmitter  212  pad not being coupled to the receiver  222  pad above and below the transmitter  212  pad (a receiver  222  pad above is not shown in  FIG. 2 ). The crosstalk capacitance C ct1  may be modeled by subtracting from the capacitance when the receiver  222  pad is infinite the capacitance when the receiver  222  pad is of the same dimensions as the transmitter  212  pad, and maybe expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     ct 
                   
                   = 
                   
                     
                       πɛ 
                        
                       
                         ( 
                         
                           l 
                           + 
                           w 
                         
                         ) 
                       
                     
                      
                     
                       ( 
                       
                         
                           f 
                            
                           
                             ( 
                             
                               d 
                               , 
                               t 
                             
                             ) 
                           
                         
                         - 
                         
                           0.5 
                            
                           
                             f 
                              
                             
                               ( 
                               
                                 
                                   d 
                                   2 
                                 
                                 , 
                                 t 
                               
                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0014]    The crosstalk problem may also have a contribution from two adjacent receiver  222  pads forming a parallel plate capacitor between each other, where the thickness of the pad contributes mainly to the capacitance C ct2 , which may be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       C 
                       
                         ct 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     = 
                     
                       ɛ 
                        
                       
                         ( 
                         
                           
                             tl 
                             
                               ( 
                               
                                 p 
                                 - 
                                 w 
                               
                               ) 
                             
                           
                           + 
                           
                             Af 
                              
                             
                               ( 
                               
                                 
                                   
                                     ( 
                                     
                                       p 
                                       - 
                                       w 
                                     
                                     ) 
                                   
                                   2 
                                 
                                 , 
                                 w 
                               
                               ) 
                             
                           
                           + 
                           
                             1.47 
                              
                             l 
                           
                         
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0015]    where p is the center to center distance between adjacent pads, and A may be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   A 
                   = 
                   
                     0.5 
                      
                     
                         
                     
                      
                     
                       π 
                        
                       
                         ( 
                         
                           t 
                           + 
                           l 
                         
                         ) 
                       
                     
                      
                     
                       ( 
                       
                         1 
                         - 
                         
                           0.0543 
                            
                           
                             w 
                             
                               ( 
                               
                                 p 
                                 - 
                                 w 
                               
                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0016]    Typically, field lines from receiver  222  pads are not only coupled to adjacent receiver  222  pads but also coupled to the transmitter  212  plates below (not shown in  FIG. 2 ). Therefore, the capacitance C ct2  reduces by a fraction, and may be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     
                       ct 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       ɛ 
                        
                       
                         ( 
                         
                           d 
                           
                             p 
                             - 
                             w 
                             + 
                             d 
                           
                         
                         ) 
                       
                     
                      
                     
                       ( 
                       
                         
                           tl 
                           
                             ( 
                             
                               p 
                               - 
                               w 
                             
                             ) 
                           
                         
                         + 
                         
                           Af 
                            
                           
                             ( 
                             
                               
                                 
                                   ( 
                                   
                                     p 
                                     - 
                                     w 
                                   
                                   ) 
                                 
                                 2 
                               
                               , 
                               w 
                             
                             ) 
                           
                         
                         + 
                         
                           1.47 
                            
                           l 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0017]    Misalignment is another problem in PxC.  FIG. 3  shows a one-dimensional misalignment between two plates  310  and  320  in the direction of the width w. w 1  is the width of receiver  310  plate that overlaps with a transmitter plate  320 , whose width of non-overlap is w 2 . Thus, w can be expressed as: 
         [0000]        w=w   1   +w   2    (9) 
         [0018]    The capacitance C ma  between two misaligned parallel plates  310  and  320  may be approximately expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     ma 
                   
                   = 
                   
                     ɛ 
                      
                     
                       ( 
                       
                         
                           
                             
                               w 
                               1 
                             
                              
                             l 
                           
                           d 
                         
                         + 
                         
                           π 
                            
                           
                               
                           
                            
                           
                             w 
                             1 
                           
                            
                           
                             f 
                              
                             
                               ( 
                               
                                 
                                   d 
                                   2 
                                 
                                 , 
                                 t 
                               
                               ) 
                             
                           
                         
                         + 
                         
                           π 
                            
                           
                               
                           
                            
                           
                             lf 
                              
                             
                               ( 
                               
                                 
                                   
                                     0.25 
                                      
                                     
                                       ( 
                                       
                                         
                                           d 
                                           2 
                                         
                                         + 
                                         
                                           w 
                                           2 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                                 , 
                                 t 
                               
                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0019]    With the rapid changing status quo of Silicon technology, PxC requires chip separation on the order of 10 microns, which would shrink to smaller levels with future generation technology. In order to ensure high fidelity transmission of data streams with very low power and ultra-high density, gap requirements of a few microns and even sub-micron gaps may be the order of the day. Typical low-cost chip manufacturing processes achieve tolerances that are no better than a few mils (1 mil=25.4 microns). 
         [0020]      FIG. 4  shows the plot of signal levels in flinging fields that are potential contributors to crosstalk in a dense PxC array as a function of chip separation for different pad dimensions. The x-axis denotes chip separation  410  in microns, and the y-axis signal levels  420  expressed as a percentage. Three different pad dimensions are chosen, viz. 1 micron pads  430 , 10 micron pads  440 , and 50 micron pads  450 .  FIG. 4  clearly demonstrates that when pad sizes are reduced to convenience high-density packaging, the signal levels coupled from pad to pad decrease with increase in chip separation  410 . Alternatively, signal levels  420  in fringe fields increase with increase in chip separation  410 . 
       SUMMARY OF INVENTION 
       [0021]    In general, in one aspect, the present invention relates to a method for improving signal levels between capacitively-coupled chips in PxC. The method includes the steps of depositing a high permittivity dielectric material layer over a signal pad of a first chip, and placing a second chip in close proximity to the first chip such that faces of the signal pads align to enable for capacitive signal coupling. The high permittivity dielectric material layer that fills at least a portion of a gap between the first chip and the second chip, and improves capacitive coupling between signal pads of the first chip and the second chip by providing for an increased permittivity in the gap between the first chip and the second chip. The increased permittivity ensures that electric fields are substantially confined to a space between the signal pad of the first chip and the signal pad of the second chip. 
         [0022]    In general, in one aspect, the present invention relates to a processing unit that provides for increased signal coupling between capacitively-coupled chips. The processing unit includes a first semiconductor chip having a signal pad atop which a high permittivity dielectric material layer is deposited, and a second semiconductor chip in proximity to the first semiconductor chip such that a face of a signal pad of the second chip is aligned with a face of the signal pad of the first chip to enable for capacitive signal coupling. The face of the signal pad of the first chip is an area of deposition of the high permittivity dielectric material layer that fills at least a portion of a gap between the first chip and the second chip. The high permittivity dielectric material layer improves capacitive coupling between signal pads of the first chip and the second chip by providing for an increased permittivity in the gap between the first chip and the second chip. The increased permittivity ensures that electric fields are substantially confined to a space between the signal pad of the first chip and the signal pad of the second chip. 
         [0023]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  shows two chips placed close to each other in a capacitively-coupled PxC mode. 
           [0025]      FIG. 2  shows a cross-section of transmitter and receiver pads of two chips involved in capacitive PxC. 
           [0026]      FIG. 3  shows a one-dimensional misalignment between two plates engaged in capacitive PxC in the direction of the width. 
           [0027]      FIG. 4  shows the plot of signal levels in fringing fields that are potential contributors to crosstalk in a dense PxC array as a function of chip separation for different pad dimensions. 
           [0028]      FIG. 5  shows a PxC chip with a patterned high permittivity ferroelectric ceramic material atop the pads in accordance with one or more embodiments of the invention. 
           [0029]      FIG. 6  shows the plot of signal enhancement factor associated with a patterned ScTiO 3  pad (with oil between pad gaps and without oil between pad gaps) as a function of chip separation  610  in accordance with one or more embodiments of the invention. 
           [0030]      FIG. 7  shows the steps involved in an exemplary method to fabricate a ferroelectric material directly atop a PxC signal pad on a semiconductor/semiconductor-on-insulator substrate in accordance with one or more embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Specific embodiments of the invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency. 
         [0032]    In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description. 
         [0033]    In general, embodiments of the present invention describe a specific method for increasing coupling between signal pads of chips in a PxC configuration by depositing a layer of a high permittivity dielectric material directly atop signal pads. In one or more embodiments, such a high permittivity dielectric material may provide for electric field confinement in the gap between signal pads of the chips. 
         [0034]      FIG. 5  shows a PxC chip  500  with a patterned high permittivity ferroelectric ceramic material  520  atop the pads  510  in accordance with one or more embodiments of the invention. In one or more embodiments, the high permittivity ferroelectric ceramic material  520  may be selected from a group consisting of, but not limited to, BaTiO 3 , SrTiO 3 , BaSrTiO 3 , and ScTiO 3 . In one or more embodiments, BaTiO 3 , SrTiO 3 , and BaSrTiO 3  ceramic materials  520  may provide for permittivity enhancement factors of 150, 300, and 1000 respectively. In one or more embodiments, the gap between two PxC chips  500  having signal pads  510  that serve as transmitters and receivers (not shown in  FIG. 5 ) may be filled with oil, in addition to the ceramic material  520  being deposited atop the signal pads  510 . 
         [0035]    In one or more embodiments, the width of the high permittivity ferroelectric ceramic material  520  atop the pads  510  may be less than the width of the pads  510  as shown in  FIG. 5 . In one or more embodiments, the width of the high permittivity ferroelectric ceramic material  520  may also be equal to or greater than the width of the pads  510 , depending on the electric field characteristics in the PxC set-up the chip  500  is a part of and the relative transmitter and receiver chip sizes. In one or more embodiments, capturing fringing fields is made easy by the high permittivity ferroelectric material  520  atop the pads  510 . 
         [0036]    In one or more embodiments, parallel plate models described above may be employed to calculate signal levels associated with bare pads with no patterned ferroelectric material. In one or more embodiments, signal levels may be calculated by accounting for the ferroelectric material  520  patterned atop signal pads  510  in the models. In one or more embodiments, the contribution of fields from neighboring pads may be ignored. 
         [0037]      FIG. 6  shows the plot of signal enhancement factor  620  associated with a patterned ScTiO 3  pad (with oil between pad gaps  630  and without oil between pad gaps  640 ) as a function of chip separation  610  in accordance with one or more embodiments of the invention calculated using the parallel plate model described above. The signal enhancement factor  620  is the ratio of the signal levels associated with pads including the ScTiO 3  material and the signal levels associated with bare pads with a  10  micron chip-to-chip gap. In one or more embodiments, PxC using high permittivity ferroelectric materials may potentially enable chip separations  610  in excess of 10 microns as it provides for high signal enhancement factors  620  even at increased chip separations  610  as shown in  FIG. 6 . 
         [0038]      FIG. 7  is a flowchart showing the steps involved in an exemplary method to fabricate a ferroelectric material directly atop a PxC signal pad on a semiconductor/semiconductor-on-insulator substrate in accordance with one or more embodiments of the invention. In Step  710 , a photolithographic mask may be used to expose the PxC signal pads on the substrate to define areas for high permittivity ferroelectric material deposition. Step  720  then involves sputter deposition of high permittivity ferroelectric material directly atop the PxC signal pads. In one or more embodiments, the sputter deposition may be a physical vapor deposition (PVD) process. In one or more embodiments, the PxC signal pads may be sputter cleaned before the sputter deposition process. In one or more embodiments, the sputter deposition may be plasma-assisted to enable good grain growth of the high permittivity ferroelectric material, along with high densification of the alloy components of the ferroelectric material. In one or more embodiments, the PVD process of sputter deposition may be alternated with RF back-sputtering or chemical etching to form a layer of desired thickness. In one or more embodiments, the aforementioned sputtering, PVD, or plasma assisted sputtering may involve deposition of a ferroelectric doped spin-on glass or a ferroelectric impregnated epoxy as a preliminary step. Step  730  involves deposition of an insulation coating layer to encompass the PxC signal pads, thereby ensuring environmental encapsulation of the entire surface. In one or more embodiments, a deposition of a thin coating layer of conformal Si 3 N 4  may provide for the insulation. In one or more embodiments, Chemical Vapor Deposition (CVD) of SiO 2  may be used instead of conformal Si 3 N 4 . 
         [0039]    In one or more embodiments, step  710  may involve deposition of the high permittivity ferroelectric material over the entire wafer substrate. Step  720  may then involve lithographic patterning over the PxC signal pad areas. In one or more embodiments, the non-patterned high permittivity ferroelectric material may then be etched away in a reactive-ion etching (RIE) or wet etching process. In one or more embodiments, patterning of the ferroelectric material layer may be accomplished by lift-off, silk screening, stenciling or a selective area deposition process. In one or more embodiments, ferroelectric high permittivity materials with thickness approaching 10 microns may be deposited over PxC signal pads. In one or more embodiments, as the ferroelectric high permittivity materials are CMOS compatible, the sputter deposition may be performed at the last level of a CMOS process, with minimal impact to the underlying circuitry. 
         [0040]    It will be obvious to one of ordinary skill in the art that individual PxC pads are not limited to a uniform size but may be varied in accordance with the PxC chip-to-chip set-up, and accordingly, the patterning method may be modified to accommodate the variation. The pad sizes and the insulator layers employed may also be varied. High permittivity dielectric materials instead of the exemplary ferroelectrics may also be employed. All of the aforementioned variations are within the scope of the invention. Advantages of various embodiments of the invention may include one or more of the following. In one or more embodiments, use of high permittivity dielectric materials may ensure that the electric fields are entirely confined to the pad space between chips. In one or more embodiments, high permittivity dielectric materials may allow for significantly larger chip separation when compared to air-gaps, which are smaller than 8 microns to allow for significant pad-to-pad signal coupling. 
         [0041]    In one or more embodiments, a group of chips having varied functionalities, such as a processor and a memory, may be brought in PxC with high permittivity ferroelectric materials atop signal pads. In one or more embodiments, individual chips engaged in such PxC coupling may be of different sizes as the small chip air-gap requirements may be relaxed when high permittivity dielectric materials are used. In one or more embodiments, PxC coupling with high permittivity dielectric materials may potentially enable for large chip separations while providing for signal levels higher than that of an ideally aligned chip with only 1 micron of air-gap between chips. In one or more embodiments, such a large chip separation would ease removal and replacement of flawed chips. 
         [0042]    In one or more embodiments, the sputter deposition of high permittivity dielectric materials atop PxC signal pads may be performed at the last level of a CMOS process, as discussed above. In one or more embodiments, crosstalk between small signal pads may be reduced by increased coupling levels between PxC pads with high permittivity dielectric materials. In one or more embodiments, manufacturing complexity, cost of alignment, and cost of packaging may be reduced in a PxC chip-to-chip configuration with high permittivity dielectric material layers, with excellent signal fidelity between signal pads of opposing chips. 
         [0043]    While the invention has been described with respect to an exemplary embodiment of a method for increasing coupling between signal pads of chips engaged in PxC, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.