Abstract:
Methods of and systems providing high reliability relative humidity sensors having integrated signal conditioning are described, providing a series capacitive circuit including a humidity-sensitive dielectric, a common top plate and first and second bottom plates associated with a first and second capacitor, respectively. Changes in humidity affect the humidity-sensitive dielectric thereby causing changes in the capacitive value of said series capacitive circuit. The system includes a common top plate; a common humidity-sensitive dielectric formed under said common top plate; a first series capacitor Cx1 including a first bottom plate formed under said common humidity-sensitive dielectric opposite said common top plate; a second series capacitor Cx2 including a second bottom plate formed under said common humidity-sensitive dielectric opposite said common top plate; wherein said first series capacitor Cx1 and said second series capacitor Cx2 commonly share said dielectric and commonly share said common top plate and are arranged in a series capacitor circuit configuration within a semiconductor system. A sampling voltage source can provide a fixed voltage through said series capacitive circuit.

Description:
TECHNICAL FIELD 
     This invention relates to semiconductor wafer-based devices. More particularly the present invention relates to semiconductor-based sensors. The present invention is also related to humidity sensors. More particularly, the present invention is related to a semiconductor-based relative humidity sensor including integrated signal conditioning. 
     BACKGROUND OF THE INVENTION 
     Solid-state semiconductor devices are found in most electronic components today. Semiconductor-based sensors, for example, are fabricated using semiconductor processes. Humidity sensors are but one class of semiconductor-based sensors finding many industrial applications. Modern manufacturing processes, for example, generally require measurement of moisture contents corresponding to dew points between −40° C. and 180° C., or a relative humidity between 1% and 100%. There is also a need for a durable, compact, efficient moisture detector that can be used effectively in these processes to measure very small moisture content in gaseous atmospheres. 
     Humidity can be measured by a number of techniques. In a semiconductor-based system, humidity can be measured based upon the reversible water absorption characteristics of polymeric materials. The absorption of water into a sensor structure causes a number of physical changes in the active polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer. Two of the most common physical changes are the change in resistance and the change in dielectric constant, which can be respectively translated into a resistance change and a capacitance change. It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result is erroneous readings, among other problems. 
     Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. It is important in the construction of capacitive elements, however, to avoid the problems that can arise with certain constructions for such elements. In addition, there can also be inaccuracy incurred at high relative humidity values where high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element. By making the component parts of the element thin, it has been found that the above-mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables. 
     Humidity sensing elements of the capacitance sensing type usually include a moisture-insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure along with a layer or coating of dielectric, highly moisture-sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and reaching equilibrium in a short period of time. Capacitive humidity sensors are typically made by depositing several layers of material on a substrate material. 
     Semiconductor-based humidity sensors are well known in the art. For example, U.S. Pat. No. 4,564,882, entitled “Humidity sensing element”, issued Jan. 14, 1986, discloses a capacitance humidity sensing element. The sensing element structure includes a substrate and a set of interdigitated electrodes deposited on the substrate surface. A first water permeable polymer film is deposited over the electrodes and a conductive mesh is formed over the first polymer film. A second polymer film is deposited over the mesh, burying the mesh between the two polymer films. The spacing between openings of the mesh is less than the thickness of the second polymer film. The square of the sum of the two polymer film thicknesses is minimized so that the response time is minimized. The mesh conductivity is made greater than a minimum value so that the resistive component of the impedance is small compared with the capacitive impedance, and thus the device impedance is independent of any instabilities in the conductive mesh impedance. 
     U.S. Pat. No. 6,222,376 entitled “Capacitive moisture detector and method of making the same,” issued Apr. 24, 2001, to Tenney, discloses a multi-layer polymer RH sensor. The Tenney patent describes an improved capacitive moisture detector that includes a ceramic substrate with a plurality of layers of interdigitated electrodes and a plurality of interleaved moisture sensitive dielectric layers. Alternate electrodes are electrically coupled to provide two electrical contacts and a circuit representing multiple capacitors connected in parallel. According to an exemplary embodiment, six layers are provided, resulting in a structure presenting five parallel connected capacitors, thereby providing a total capacitance of the detector which is ten times that of previous detectors having the same footprint. According to a preferred embodiment of the Tenney invention, each dielectric layer is made relatively thin to effectively decrease the distance between the plates of each of the five capacitors. Since capacitance is inversely proportional to the distance between the dielectric layers, the thinness of the electrodes in the present invention also serves to increase capacitance. According to another preferred embodiment of the Tenney patent, a floating porous conductive film is provided over the sixth electrode to contain the field of the top capacitor and render the detector immune from the effects of surface contaminants. 
     U.S. Pat. No. 4,831,325, entitled “Capacitance measuring circuit”, issued May 16, 1989, to Watson, discloses yet another semiconductor-based sensor for measuring humidity. In the Watson patent, a variable capacitor, which may be a humidity-sensitive capacitor, and a fixed reference capacitor are connected at a node. The node is clamped at a reference potential during a first phase of a two-phase measuring cycle as the variable capacitor is charged to a fixed voltage and the fixed capacitor is charged to a feedback voltage. The node is unclamped during the second phase and the capacitors are connected in a series loop to allow a redistribution of the charge in the capacitors or force a reversal of that charge with a voltage source. The deviation of the node from its reference potential after charge redistribution occurs is used as input to a feedback circuit that integrates that deviation over a number of cycles until it provides a feedback voltage of magnitude sufficient to cause the node deviation to be reduced to zero. A second reference capacitor can be supplied to provide an offset. The capacitors are constructed by simultaneous deposition on a substrate of a first plate followed by a dielectric film and a second plate. The second plate of the variable capacitor is porous to admit water molecules and the second plate of the fixed capacitor is impervious to water. According to Watson, simultaneous deposition provides similar characteristics for the capacitors. 
     An electrical connection problem has been found to reside in some semiconductor humidity sensor designs as described in, but not limited to, the Watson patent. Failed electrical connection can result where a large step height exists between the conductive electrical plates and the electrical interconnects, particularly during thermal/humidity cycling due to the relatively high Temperature Coefficient of Expansion of the sensing medium (e.g., dielectric film, polyimide insulator) and the swelling and shrinking of the sensing medium. By this mechanism, changing humidity or temperature can disconnect the sensing capacitor system from the chip electronics, or at least change the capacitive value of the sensor arrangement. 
     The present inventors have found that there is a need for improved humidity sensor designs. In view of the above-identified shortcomings of previous devices, the present inventors have invented an improved relative humidity sensor. Accordingly, the present invention is described and presented as a means to address the shortcomings currently found in present humidity sensor devices. 
     SUMMARY OF THE INVENTION 
     The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole. Additional objects and advantages of the current invention will become apparent to one of ordinary skill in the art upon reading the specification. 
     It is an object of the invention to provide an improved capacitive moisture detector that is not adversely affected by surface contamination. 
     It is another object of the invention to provide an improved capacitive moisture detector that is sensitive and capable of detecting very small to very large quantities of moisture. 
     It is still another object of the invention to provide an improved capacitive moisture detector that is compact. 
     In accordance with another aspect of the present invention, an interlayer dielectric of silicon nitride is added in a sensor to protect aluminum metal interconnects. 
     In accordance with another aspect of the present invention, aluminum or metal interconnects can also be covered with gold to virtually eliminate the corrosion failures. 
     In accordance with another aspect of the present invention, a new capacitor configuration is incorporated into the humidity sensor that eliminates the weak mechanical connection between the thin platinum and the thick aluminum or metal interconnects. 
     In accordance with another feature of the present invention, the capacitive sensor structure of the present invention is incorporated into an integrated sensor system on silicon. 
     In accordance with another aspect of the present invention, a charge balance circuit can be incorporated into the integrated silicon sensor. 
     In accordance with another aspect of the present invention, the sensing capacitor can be constructed for minimization of stray capacitance, which adversely affects the charge balance circuit. 
     In accordance with yet another aspect of the present invention, the calibration method employs an area reduction technique for balancing capacitance in a sensor structure, which minimizes error due to unbalanced capacitance. 
     In accordance with another aspect of the present invention, an integrated relative humidity sensor (IRHS) can include a planar humidity sensitive capacitor structure that includes: a thin porous platinum top plate, a humidity sensitive polyimide dielectric, and a metal (e.g., titanium-tungsten) bottom plate. Two capacitors can be effectively wired in series such that the metal bottom plates form the independent, electrically driven connections, and the thin platinum top layer is used to form the top plates and the floating series interconnection. The floating series interconnection thus eliminates problems associated with interconnection between the prior art thin platinum plate to thick aluminum or metal interconnections. 
     In accordance with another aspect of the present invention, the charge balance circuit can generally reject stray capacitance from the bottom plate of each capacitor to the underlying substrate. 
     Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
     The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form part of the specification, further illustrate the present invention and together with the detailed description of the invention, serve to explain the principles of the present invention. 
     FIG. 1 is an illustration of a prior art humidity sensor; 
     FIG. 2 is a cut-away side view of a prior art humidity sensor; 
     FIG. 3 is a top plan view of the prior art humidity sensor of FIG. 2; 
     FIG. 4 is a cut-away side view of a humidity sensor in accordance with a preferred embodiment of the present invention wherein; 
     FIG. 5 is a top plan view of the humidity sensor of FIG. 4; 
     FIG. 6 is an illustration of another embodiment of the present invention wherein conductivity of the top plate is modified through area reduction in order to balance capacitance between Cx1 and Cx2; 
     FIG. 7 illustrates relative humidity-to-voltage transfer function components; 
     FIG. 8 illustrates circuit connectivity for operation during a Phase 1 operation; 
     FIG. 9 illustrates circuit connectivity for operation during a Phase 2 operation; and 
     FIG. 10 illustrates a parasitic capacitance included in an analysis of humidity sensor circuitry to understand the parasitic capacitances influence on a transfer function during circuit operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow. 
     Referring to FIG. 1 (labeled prior art), a perspective view of the basic components found in a semiconductor-based humidity sensor  100  is shown. A semiconductor humidity sensor  100  is generally grown on a silicon substrate  110 . The active sensor components include a lower  120  and an upper  140  electrically conductive plates sandwiching a humidity sensing medium  130 , such as a polymer. The polymer material is sensitive to humidity, and its electrically conductive properties (resistance and/or capacitance) change as it absorbs moisture or as it dries. Lower  120  and upper  140  plates are electrically connected to sensor circuitry (not shown). A protective layer  150  can be used to protect the active components of the sensor (e.g., top plate  120  and sensing medium  130 ) from debris  160 . Upper plate  140  is designed to be porous in order to enable humidity to enter into the sensing medium from an external environment of interest  170  (the monitored environment of interest). 
     Referring to FIG. 2 (labeled prior art), an Integrated Relative Humidity Sensor (IRHS) that has been manufactured and sold by Honeywell, the present assignee, is illustrated. The illustrated semiconductor sensing system uses a planar humidity sensitive capacitor structure  200  that includes a thin porous platinum metal top plate  240  (top porous plate) covered by a protective polymer layer  250 , and a humidity sensitive polyimide dielectric  230  sandwiched between the top porous plate  240  and a silicon bottom plate  220  (or substrate). The top plate  240 , the polyimide dielectric  230  and the bottom plate  220  are at the heart of the capacitor function. Capacitance (Cx) formed with these three components is symbolized schematically in the drawing as  280 . Variations in humidity affecting the dielectric  230  create a variable capacitive value between the top porous plate  240  and bottom plate  245 . 
     The top porous plate  240  makes an electrical connection to chip electronics by contacting the thick aluminum interconnects  220 . It has been found that the large step height between what consists of a very thin and porous conductive layer as the top porous plate  240  and the thick aluminum interconnects  220  tend to result in the formation of a weak mechanical connection. A common sensor failure mechanism is found when this weak mechanical connection breaks down during thermal/humidity cycling of the sensing device  200  due to the relatively high Temperature Coefficient of Expansion (TCE) of the sensing medium  230  (polyimide dielectric) and its swelling and shrinking due to changing humidity, which can physically keep the sensing capacitor from effective operation. 
     FIG. 3 (labeled prior art) illustrates a top plan view of the top porous plate  240  layer of the prior art humidity sensor shown in FIG.  2 . As seen in FIG. 3, top porous plate  240  is electrically connected with an electrical contact  310 . Bottom plate  220  (P silicon) is also electrically connected to an electrical contact  320 . The border area represents the interconnects  245  to which the top porous layer  240  becomes electrically connected during fabrication. It is along the interconnects  245  where the top porous plate  240  can become electrically detached due to expansion of the sensing medium (not shown). The top porous plate  240  is shown as one large conductive layer operating as a first plate of the humidity sensing capacitor, whereas the bottom plate operates as the second plate of the humidity sensing capacitor. 
     Referring to FIG. 4, a new capacitor configuration in accordance with the present invention will now be described for an improved semiconductor-based humidity sensor  400 . The humidity sensor of the present invention includes a substrate  410 . Insulating materials  405  can function as a buffer between the substrate  410  and first  420  and second  425  lower contact plates. First lower contact plate  420  is electrically connected to a first connector  417 . Second lower contact plate  425  is electrically connected to a second connector  427 . A sensing medium  430  is disposed on top of the first  420  and second  425  lower contact plates. A porous platinum top plate  440  is then disposed on top of the sensing medium  430 . A breathable or porous protective layer  450  can also be disposed on top of the top plate for protection of the top plate  440  and sensing medium layer  430 . Two capacitors Cx1 and Cx2 are schematically illustrated in respective positions within the sensing medium  430  between the first lower contact plate  420  and top plate  440  and the second lower contact plate  425  and the top plate  440 . The gap/barrier  460  formed as an electrical disconnect between first  420  and second  425  lower contact plates creates the series capacitor configuration for Cx1 and Cx2. 
     Referring to FIG. 5, a representative top view of the capacitive plates that provide electrical contact for generating capacitance through a sensing medium (not shown) for the present invention is shown. As shown in FIG. 5, capacitor Cx1 includes common top plate  440  (common to both Cx1 and Cx2) and a first lower contact plate  420  in further electrical contact with a first electrical contact  417 . Capacitor Cx2 includes common top plate  440  as its first contact and second lower contact plate  425 , which is in further electrical contact with second electrical contact  427 . As shown in FIG. 5, a conductive barrier or insulating gap  460  is provided between the lower contact plates  420  and  425 . This gap controls stray capacitance in parallel with Cx1 and Cx2, and controls parasitic capacitance between common top plate  440  and the underlying silicon substrate  410  (shown in FIG.  4 ). In the preferred embodiment, the gap  460 , which can also be seen in FIG. 4 between the lower contact plates, near the center of the device, is filled with the humidity-sensitive dielectric (although other dielectrics could be utilized). 
     The capacitive values of Cx1 and Cx2 must not only be physically realized using the gap/barrier shown in FIGS. 4 and 5, but also adjusted for the device to operate properly (i.e., calibrated). It is known in the art to control or adjust capacitance through variable capacitors. In the present invention, however, it is not feasible to provide for ongoing capacitive control. Therefore, the present inventors have devised a method of adjusting the capacitive values of Cx1 and Cx2 while keeping their values substantially equal prior to applying a protective layer  450  as shown in FIG.  4  and/or packaging the device. 
     Referring to FIG. 6, a representative top view of the capacitive plates that provide electrical contact for generating capacitance through a sensing medium (not shown) for the present invention is illustrated. As shown in FIG. 6, the common top plate  440  (common to both Cx1 and Cx2) has been etched at three locations. A first location relative to Cx1 shows a box-like etching  610  on common top plate  440 , and a second location relative to Cx2 shows box-like etching  620 . An etching procedure will cut through the entire common top plate  440  layer, effectively removing capacitive value associated with box-like areas  615  and  625  of the common top plate  440 . The resulting area available for electron build-up on the plate  440  is thereby lessened, thus lowering the capacitive value of Cx1 and Cx2. Care is taken to make areas  615  and  625  approximately equal. 
     As seen on the top common plate  440  area associated with Cx2, a slit  630  and/or  640  has also been etched into the top common plate  440 . The purpose of etching slits  630  and/or  640  is similar in reason to that of creating the box-like areas  625  and  615 , which was to reduce the amount of common top plate  440  area that will contribute to the overall capacitance of either or both Cx1 or Cx2. 
     The initial adjustments made to Cx1 ( 615 ) and Cx2 ( 625 ) can be seen as coarse adjustments to the net value of the sensing capacitance, whereas slits  640  and  630  can be seen as fine adjustments. The etching procedure can be used anywhere on the top plate deemed necessary in order to adjust the net capacitance of Cx1 and Cx2, while keeping the values of Cx1 and Cx2 substantially equal. It should be appreciated to those skilled in the art from the foregoing discussion that adjustments can be carried out in a series of etchings and with etchings of various shapes and sizes. Any adjustments will generally be carried out while trying to maintain equality in value between Cx1 and Cx2. 
     The charge balancing method used to transduce relative humidity to a linear voltage will now be described. Most linear control circuits are utilized to regulate voltages or currents; however, the high impedance capacitive nature of the humidity sensor is more readily handled by control of charge. FIG. 7 shows the essential components that comprise the relative humidity-to-voltage transfer function in terms of a circuit diagram  700 . C 0 , C 1 , C 2 , C 3 , and Cref are all capacitors that are designed to be insensitive to humidity and that are fabricated at the same time and from the same materials. Thus, while their absolute values of capacitance will vary, the ratios will track very closely. Cx is designed to be sensitive to humidity and is fabricated at a different time and from different materials than the aforementioned capacitors. The switching matrix  710  varies the wiring scheme for capacitors: Cx, Co, and Cref using two-phase, non-overlapping, dual polarity clocks, as can be provided by clock generator  705 . Note that one end of all three capacitors is always connected in common, thus providing a charge summing node, Qs. Inverters A 1 , A 2 , and A 3 , and capacitor C 1 , and the pair of associated transmission gates  715  and  720  form a high gain comparator. C 2  and its pair of associated transmission gates  725  and  730  are the switched capacitor equivalent of a resistor which coupled with amplifier A 4  and feedback capacitor C 3  form an integrator. 
     FIGS. 8 and 9 show the circuit connectivity for operation during “Phase 1” and “Phase 2” respectively. Neglecting Cref for the moment and concentrating on Co and Cx, note that they effectively form a voltage divider. In Phase 1 Co is pulled up to Vcc and Cx is pulled down to GND and vice versa during Phase 2. Thus a periodic differential voltage is created which is a function of the difference in capacitance values. Those skilled in the art will recognize this as a half bridge sensor configuration. During Phase 1, inverters A 1  and A 2  short their input node to their output node, which, when implemented with complementary FETs, forms a voltage divider. 
     FIG. 8 indicates that all three of these FETs are designed to produce a half supply transfer function in this configuration, thus driving both the charge summing node and the output of A 3  to Vcc/2 during Phase 1. These transmission gate shorts are opened up during Phase 2, creating a high gain inverting comparator, which allows small movement in the charge summing node voltage relative to Vcc/2 to drive the output of A 3  to Vcc or GND. Thus A 3 , the output of the comparator, controls the integrator. During Phase 1, the output of A 3  and the non-inverting input of A 4  are both at Vcc/2, which puts the integrator into a “Hold” state. So Phase 1 can be thought of as a measurement or sampling phase during which Cref is charged. During Phase 2, Cref is disconnected from the integrator output and reconnected to GND and the comparator responds to the charge-summing node. If the comparator output goes to GND, then the output of the integrator increases linearly. If the comparator output goes to Vcc, then the output of the integrator decreases linearly. If the charge-summing node effectively remains at Vcc/2 during Phase 2, then the integrator remains in the “Hold” state, though this is rarely the case. So Phase 2 can be thought of as the negative feedback adjustment phase. 
     The following equations mathematically describe the operation of the aforementioned circuit. Equations 1 and 2 calculate the charge at the summing node during Phase 1 and 2, respectively. The negative feedback results in Qs1 and Vs1 being substantially equal to Qs2 and Vs2, equation 3 mathematically describes the resulting transfer function for the complete circuit operation. 
     
       
           Qs 1= Cx·Vs 1+ Co· ( Vs 1− Vcc )+ Cref· ( Vs 1− Vout )  EQ.1  
       
     
     
       
           Qs 2= Cx· ( Vs 2− Vcc )+ Co·Vs 2+ Cref·Vs 2  EQ.2 
       
       
         
           
             
               
                 EQ 
                 . 
                 
                     
                 
                  
                 3 
               
                
               
                   
               
                
               Vout 
             
             = 
             
               
                 
                   ( 
                   
                     Cx 
                     Cref 
                   
                   ) 
                 
                 · 
                 Vcc 
               
               - 
               
                 
                   ( 
                   
                     Co 
                     Cref 
                   
                   ) 
                 
                 · 
                 Vcc 
               
             
           
         
                 
         
             
         
      
     
     As described previously, the new construction method for the humidity sensitive capacitor, Cx, minimizes the problems due to mechanical weakness found in previous construction methods. As shown in FIG. 10, however, this present invention creates a parasitic capacitance, Cct, which is included in the analysis to understand its influence on the transfer function for the complete circuit operation. Equation 4 mathematically describes the new transfer function, including the effect of the parasitic capacitor, Cct, and mismatch (mm) between Cx1 and Cx2.            EQ   .              4                   Vout     =         (       4   ·     Cx   2     ·     (     1   -     m                   m   2         )           (       4   ·   Cx     +   Cct     )     ·   Cref       )     ·   Vcc     -       (     Co   Cref     )     ·   Vcc                              
     Note that if the parasitic capacitance, Cct, is much less than four (4) times Cx, and the squared mismatch, mm 2 , between Cx1 and Cx2 is much less than one, then Equation 4 simplifies back to Equation 3. Therefore, the parasitic capacitance from the floating interconnection to the underlying silicon should be minimized. Also, since the interconnection, Vx, is electrically floating, charge build-up on the top surface of the chip is minimized to avoid destructive breakdown of the sensing capacitors. And, as shown in FIG.  6  and discussed previously, the adjustments made to Cx should be divided substantially equally between Cx1 and Cx2 to minimize the sensitivity reduction due to mismatch error. 
     The present device is used to sense the relative humidity in the ambient environment around the sensor. During operation, a relative humidity level is sensed and then the sensor generates a voltage output proportional to the relative humidity. This voltage can then be used by other circuits to implement functions such as relative humidity control, enthalpy control for building HVAC, weather sensing instruments, process controls for drying, process controls for batch or continuous production where relative humidity is a parameter that controls the output of a process or is related to some process variable to be controlled, length or end of cycle in drying applications, and other applications. 
     The embodiment and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. The description as set forth is not intended to be exhaustive nor to limit the scope of the invention. Many modifications and variations are possible in light of the above teaching without departing from the scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto, giving full cognizance to equivalents in all respects.