Abstract:
The present invention discloses a system for improving power management for a class of spatial power combiners, called active loop probes, or, active loops. One aspect of the invention includes the provision of a patterned conductor on the surface the semiconductor chip that opposes the surface upon which the active devices of the loop are disposed. This metal material can be used to both enhance heat removal from the chip and to provide a new and more efficient DC biasing path (with the use of vias) for the active components on the other (front) surface of the chip. Another aspect of the invention is the introduction of a dielectric superstrate that attaches to the front surface of the chip to provide an alternative or complementary heat removal and/or biasing structure to the conventional substrate that is typically attached to the back side of the chip. Various combinations of the above features are disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a particular class of spatial power combiners called active-loop, spatially-combined, microwave and millimeter wave amplifiers, and in particular to architectures for efficiently removing heat generated by, and supplying bias to, active loops.  
           [0003]    2. Description of Related Art  
           [0004]    Broadband communications, radar and other imaging systems require the generation and transmission of radio frequency (“RF”) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called “power combining” has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.  
           [0005]    Several years ago, “spatial power-combining” was proposed as a potential solution to these problems. In spatial power-combining, the individual outputs of microwave and millimeter wave solid-state devices combine in free space to provide a single combined power output more effectively than can ostensibly be achieved using the above-described conventional power-combining techniques.  
           [0006]    As used herein, the term “spatial power-combining” broadly applies to all structures that couple component outputs in free space. One subset of spatial power-combining that has recently received significant attention is called “quasi-optical power combining.” The theory underlying “quasi-optics” is that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, thus minimizing conductor losses. Furthermore, monolithic fabrication enables potentially thousands of microwave or millimeter wave active devices to be incorporated on a single wafer.  
           [0007]    Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the field has been explosive.  
           [0008]    A quasi-optical array amplifier includes a two-dimensional sheet of active devices, interconnected with patterned conductors, that accepts a polarized electromagnetic wave as an input and radiates an amplified output wave with a polarization that is orthogonal to the input polarization. Two array amplifier configurations that have been previously reported are transmission-mode arrays and reflection-mode arrays. FIG. 1 shows a typical transmission-mode grid amplifier  10 , wherein an array of closely-spaced interconnected differential pairs of transistors  14  are formed in a grid-like pattern on the front surface of a semiconductor chip  12 . The chip is sandwiched between an input polarizer  18  and an output polarizer  24 . An input signal  16  passes through the horizontally polarized input polarizer  18  and creates an input beam incident from the left that excites rf currents on the horizontally-patterned, polarized, metal input antennas  20  of the grid on the chip  12 . These currents drive the inputs of the transistor pairs  14  in the differential mode. The output currents are redirected along the grid&#39;s vertically-patterned polarized antennas  22 , producing, out the right side of the array, a vertically polarized output beam  30  via an output polarizer  24 .  
           [0009]    Reflection-mode arrays also use two-dimensional active grids similar to the active grids in transmission mode arrays. However, reflection-mode arrays require that the EM radiation have access to only the front (active grid) surface of the chip. The back surface of the chip is mounted on a dielectric substrate atop a metallic mirror.  
           [0010]    Unfortunately, drawing heat away from quasi-optical arrays and, more generally, from any type of spatial power combiners remains a problem, especially for high-power, high-frequency applications. In microwave and millimeter wave semiconductor devices, heat is generated in the circuitry fabricated on the front (or top) surface of the semiconductor. In conventional microwave and millimeter wave circuits, in order to enhance heat removal, the semiconductor wafer is typically thinned to a final thickness of 50 to 150 microns. Further, the back (or bottom) surface of the semiconductor chip is completely covered with a metal “ground plane,” which acts as an equipotential shield and return conductor to the power supply, that is electrically connected to the devices on the front surface through metallic “vias.” Additionally, this metal layer provides both a mechanical support as well as a surface for soldering devices to a metalized carrier, thus enabling a high thermal conductivity interface between the device and its package.  
           [0011]    In quasi-optical grid arrays, the microwave/millimeter wave signals radiate into or are absorbed from fields above and below the semiconductor. Covering the back surface of the semiconductor with metal would impair the coupling of radiation fields to the patterned conductors on the front surface of the semiconductor, and would completely prevent the transmission of the radiated signals through the semiconductor wafer.  
           [0012]    Previous implementations of monolithic grid arrays have been fabricated on semiconductor chips without metal ground planes disposed on their back surfaces. As seen in FIG. 1, in order to draw the substantial amount of heat generated in the circuitry away from the grid array, the back surface of the chip is generally mounted on a dielectric heat spreading slab  13 . This dielectric slab is typically attached to the bottom, or back surface of the semiconductor chip  12  using an electrically non-conductive adhesive. Further, DC power supply to the active devices (biasing) has been carried out using the patterned metal lines placed exclusively on the front surface of the semiconductor chip. Unfortunately, having all of the conductors on the front side of the chip limits the efficiency of the biasing, and the absence of backside metal precludes the use of high thermal conductivity attachments using a solder or traditional eutectic die-attach.  
           [0013]    Another subset of spatial power-combining, described below, is called the “active loop” power combiner. Many of the drawbacks described above with respect to quasi-optic devices apply to active loop devices.  
           [0014]    Thus, there is a definite need for an improved means for removing heat generated by active devices in spatial power combiners, such as quasi-optical grid arrays and active loops, especially where higher output powers are required. There is also a need for improved methods and structures that allow for more flexibility in the placement of bias and tuning elements in these devices.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention, which addresses these needs, resides in novel structures and techniques for monolithic, active loop spatial power-combining devices and systems that improve power management in these systems and devices.  
           [0016]    In one aspect of the present invention, a planar, active loop, spatial power-combining chip having a front surface and a back surface is disclosed. The chip includes a plurality of active devices disposed on the front surface of the chip in an interconnected loop, and patterned, electrically-conductive material disposed on the back surface of the chip. This pattern either complements or supplants the patterned metal that is typically disposed on the front surface of the chip. The patterned metal on the back surface (a) provides an alternative (and improved) location from which to supply DC biasing to the components on the front of the chip; and/or (b) enables this heat-conducting material to be joined to other metallic, heat-conducting structures for efficient heat removal from the chip. Thus, the chip may further include patterned, electrically-conductive material disposed on the front surface of the chip and vias disposed within the chip and between the front and back surfaces for providing a DC power-supplying path to the front surface of the chip.  
           [0017]    In another aspect, an integrated, active loop, spatial power-combining system is disclosed. The system includes an active loop, spatial power-combining chip having a front surface with a plurality of active devices disposed thereon in an interconnected loop, a back surface with patterned, electrically-conductive material disposed thereon; and a thermally-conductive, dielectric substrate having a front surface attached to the back surface of the chip. Unlike the continuous metal ground plane used on the back surface of conventional semiconductor chips, the patterned metal on the backside of the active loop spatial power-combining semiconductor chip and on the dielectric slabs can be designed so as not to impair the coupling between, for example, the active loop and the incident radiation fields, but to actually improve it.  
           [0018]    It should be understood that the active devices disposed onto the front surface of the active loop refers broadly to any conventional technique for fabricating or integrating active devices onto, into or with semiconductor wafers or chips. Further, the active devices may be any appropriate three-terminal active devices that can be fabricated with a semiconductor chip, such as, for example, pHEMT or BJT transistors. These active devices are interconnected and comprise the active loop circuit that is improved upon by the present invention.  
           [0019]    The chip in this system may further include vias disposed between the front surface and patterned, electrically-conductive material on the back surface.  
           [0020]    Moreover, the front surface of the dielectric substrate includes patterned, electrically-conductive material disposed thereon that is connected to the patterned material on the back surface of the chip. This metal-to-metal connection can provide the dual benefits of electrical conduction for supplying bias to the chip and excellent thermal conduction for drawing heat away from the chip. It should be understood that this metal-to-metal connection may be accomplished using any appropriate electrically-conducting joint that may or may not use additional material for making the connection. Thus, the patterned, electrically-conductive material on the back surface of the chip may be connected to the patterned, electrically-conductive material on the front surface of the substrate with one of a solder bond, a thermo-compression bond, an ultrasonic bond and an electrically-conductive adhesive.  
           [0021]    In a more detail aspect of the present invention, the system may further includea thermally-conductive, dielectric superstrate having a back surface joined to the front surface of the spatial power-combining chip. The back surface of the dielectric superstrate may be joined to the front surface of the power-combining structure via a thermally-conductive dielectric joint disposed therebetween. The back surface of the dielectric superstrate may also include patterned, electrically-conductive material disposed thereon that is joined to the front surface of the power-combining structure. The front surface of the chip may also include patterned, electrically-conductive material. One advantage that a superstrate structure may have over a substrate is that it is physically located closer to the front surface of the chip—the source of the heat generation—and thus can provide very efficient heat removal.  
           [0022]    In yet a more detailed aspect of the present invention the system may further include vias disposed within the spatial power-combining structure and between the patterns on the front and back surfaces of the structure that provides an electrical path for DC power to the front surface of the chip. In this embodiment, the patterned, electrically-conductive material disposed on the front surface of the substrate may be joined to the patterned, electrically-conductive material disposed on the back surface of the power-combining structure via an electrically-conducting joint, and the patterned, electrically-conductive material disposed on the back surface of the superstrate may joined to the patterned, electrically-conductive material disposed on the front surface of the power-combining structure via an electrically-conducting joint.  
           [0023]    In an alternative aspect of the present invention, the integrated active loop, spatial power-combining system has a thermally-conductive, dielectric superstrate having a back surface attached to the front surface of the active loop spatial power-combining chip. In this embodiment, the back surface of the dielectric superstrate may include patterned, electrically-conductive material disposed thereon that is connected to the front surface of the power-combining chip. The front surface of the chip may further include patterned, electrically-conductive material that is connected to the patterned, electrically-conductive material on the back surface of the superstrate with one of a solder bond, a thermo-compression bond, an ultrasonic bond and an electrically-conductive adhesive. This system may further include a first thermally-conductive joint disposed between the front surface of the power-combining chip and the back surface of the superstrate, a thermally-conductive, dielectric substrate having a front surface attached to the back surface of the chip, and/or a thermally-conductive joint disposed between the back surface of the power-combining chip the front surface of the substrate.  
           [0024]    In yet a further aspect of the present invention, a method of drawing heat away from an active loop spatial power-combining chip having a front surface with heat-generating, active components disposed thereon and a back surface that is connected to a thermally-conducting substrate is disclosed. The method includes providing a metal pattern on the back surface of the chip, providing a metal pattern on the front surface of the substrate; and joining the metal pattern on the back surface of the chip to the metal pattern on the front surface of the substrate in order to draw heat generated by the active devices away from the chip.  
           [0025]    An alternative method of drawing heat away from an active loop spatial power-combining chip is also disclosed. This method includes providing a thermally-conducting dielectric superstrate having front and back surfaces, and attaching the front surface of the power-combining chip to the back surface of the superstrate via a thermally-conducting joint. In this method, the back surface of the thermally-conducting dielectric superstrate may include a metal pattern disposed thereon, and the attaching may include joining the metal pattern disposed on the back surface of the superstrate with the front surface of the chip. A metal pattern disposed on the front surface of the chip and the metal pattern on the back surface of the superstrate is joined to the metal pattern on the front surface of the chip.  
           [0026]    A method of providing DC power to active components on an active loop power-combining chip having front and back surfaces, wherein the components are disposed on the front surface of the chip is also disclosed. The method includes providing a metal pattern on the back surface of the power-combining chip, selectively connecting one or more of the active components to the metal pattern on the back surface of the chip, and providing DC power to the metal pattern on the back surface of the chip. In this method, one or more of the active components may be selectively connected to the metal pattern on the back surface of the chip with vias disposed between the front and back surfaces of the power-combining chip. The method may also include the further steps of providing a thermally-conducting, dielectric substrate having a front surface with a metal pattern disposed thereon, electrically connecting the metal pattern on the front surface of the dielectric substrate to the metal pattern on the back surface of the power-combining chip, and supplying DC power to the metal pattern on the front surface of the dielectric substrate.  
           [0027]    An alternative method of providing DC power to active components on an active loop power-combining chip having front and back surfaces, with the components being disposed on the front surface of the chip, is also disclosed. This method includes providing a thermally-conducting dielectric superstrate having a back surface with a metal pattern disposed thereon, electrically connecting the metal pattern on the back surface of the superstrate to selected components on the front surface of the chip, and applying DC power to the metal pattern on the back surface of the superstrate. 
       
    
    
       [0028]    Other features and advantages of the present invention will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is an exploded view of a conventional transmission mode quasi-optical grid array with one of the differential pair unit cells in the array magnified;  
         [0030]    [0030]FIG. 2 is an exploded view of a quasi-optical grid array that implements several features of the present invention;  
         [0031]    [0031]FIG. 3 is a cross-sectional side view of a quasi-optical grid array according to one preferred embodiment of the present invention;  
         [0032]    [0032]FIG. 4 is a cross-sectional side view of the improved heat-dissipating, quasi-optical grid array system according to another preferred embodiment of the present invention;  
         [0033]    [0033]FIG. 5 is a cross-sectional side view of a the improved heat-dissipating, quasi-optical grid array system according to yet another preferred embodiment of the present invention;  
         [0034]    [0034]FIG. 6 is a cross-sectional side view of the improved heat-dissipating, quasi-optical grid array system according to still another preferred embodiment of the present invention;  
         [0035]    [0035]FIG. 7 is a simplified schematic of an active loop power combiner;  
         [0036]    [0036]FIG. 8 is a top view of an active loop power combiner designed with front side and back side metallic patterns according to one preferred embodiment of the present invention; and  
         [0037]    FIGS.  9 ( a ) and  9 ( b ) are perspective views of the improved active loop assembly in a waveguide. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    The present invention relates to structures and techniques that improve the performance of spatial power combiners by providing efficient heat removal from the semiconductor chips that carry the combiners. Various embodiments of the present invention are first described in the context of quasi-optical grid arrays. Then, the present invention is described as applied to active loop probes.  
         [0039]    [0039]FIG. 2 shows an exploded view of the primary components of one illustrative embodiment of an improved quasi-optical grid array system  100  according to the present invention. The system includes a “substrate”  120 , a “spatial power-combining semiconductor chip structure” (and more specifically, in this embodiment, a quasi-optical grid array)  110  and a “superstrate”  130 . The semiconductor chip structure  110  has a grid array with active devices and patterned electrical conductors on the front surface  114  (not shown, but is similar to the patterns on the front surface of the grid array shown in FIG. 1) and patterned electrical conductors  116  on its back surface  112 . The chip may also include ‘vias’ connecting the conductors on its two faces, or surfaces,  114  and  116 . The vias and the back surface conductors tend to improve heat removal from the array, and to improve flexibility in providing biasing and rf matching for the grid array. Alternatively, the patterned conductors on the back surface of the chip may actually replace the patterned conductors that are conventionally found on the front surface of grid arrays. In this variation, the back surface patterned metal would preferably be connected directly to the active components on the front side of the array through vias. One advantage of placing the patterned conductors on the back surface is that the patterned metal does not compete for real estate on the back surface with components as it must on the front surface of the chip. Thus, it is possible to provide thicker metalization patterns on the back surface than is possible on the front side, thereby providing inherently lower voltage drops (lower resistance/higher efficiencies), simplified layout and enhanced manufacturability.  
         [0040]    The substrate  120  is a slab made of any appropriate thermally-conductive material, such as ceramic, and, in this embodiment, has patterned, electrically-conductive material (e.g. a metal pattern)  124 ,  126  disposed on its front surface  122 . This material may also optionally be disposed on the back surface of the substrate (not shown). When mated with the patterned metal on back surface of the chip, this patterned material can improve delivery of DC power to the active components in the array on the chip  110  (biasing), can enhance the removal of waste heat from the array, and can improve radio frequency (rf) matching.  
         [0041]    The superstrate  130  is a thermally conductive dielectric material such as a ceramic slab or thermally conductive encapsulant. The superstrate may also have patterned electrically-conducting material  132  on its back surface in order to enhance heat removal from, and biasing and tuning for the grid array. Each section is attached to the adjacent section through a process and/or means that provides mechanical support, heat removal, and (in some embodiments) electrical connection. Adjacent sections may be attached, or joined, with thermally-conductive adhesives or epoxies and for regions in which there is metal-to-metal contact, any appropriate bonding technique and material, collectively called joint, may be used, such as soldering, thermo-compression, ultrasonic bonding, electrically-conductive adhesives or any appropriate combination of these techniques and/or materials.  
         [0042]    Various permutations of these features that incorporate the principles of the invention and result in novel spatial combining devices, such as quasi-optical grid array amplifiers or oscillators, with enhanced thermal properties and potentially improved biasing options are now described. One embodiment includes a spatial-power combining chip  110  (with or without patterned metal on its back surface) attached to the superstrate  130  and without a substrate  120 . FIG. 3 shows in cross-sectional view, another alternative embodiment of an improved spatial power-combining system  300  according to the present invention. In particular, the system includes two primary structures, namely, a quasi-optical grid array chip  310  and a thermally-conductive, dielectric substrate  330 . The chip has a front surface  312  and a back surface  314  with patterned, electrically-conductive material  320  disposed on the back surface. The substrate also has patterned, electrically-conductive material  340  disposed on it front surface  332 . The chip is attached to the substrate by joining the two sets of patterned materials together via solder joint  350 . DC bias is supplied to the devices on the chip&#39;s front surface  312  (not shown) either from the patterns  320  on the back surface by means of vias through the chip or by the conventional means of connecting the DC source(s) to the patterns on the front surface  312 .  
         [0043]    [0043]FIG. 4 shows in cross-sectional view, a second alternative embodiment of an improved spatial power-combining system  400  according to the present invention. In particular, the system includes three primary structures, namely, a quasi-optical grid array chip  410  and a substrate  430  and a superstrate  420 . The back surface  414  of the chip is attached to the front surface  432  of the substrate via a dielectric thermally conductive adhesive  450 . The front surface  412  of the chip is attached to the back surface  422  of the superstrate via another dielectric thermally-conductive adhesive  440 .  
         [0044]    [0044]FIG. 5 shows in cross-sectional view, a third alternative embodiment of an improved spatial power-combining system  500  according to the present invention. In particular, the system includes three primary structures, namely, a quasi-optical grid array chip  510  and a substrate  530  and a superstrate  520 . The chip has a front surface  512  and a back surface  514  with patterned, electrically-conductive material  516  disposed on the back surface. The substrate also has patterned, electrically-conductive material  534  disposed on its front surface  532 . The chip is attached to the substrate by matching and joining the patterned materials  516  on the chip to the patterned material  534  on the substrate via solder joint  550 . The back surface  522  of the superstrate  520  is attached to the front surface  512  of the chip via a dielectric thermally-conductive adhesive  540 .  
         [0045]    DC bias is supplied to the devices on the chip&#39;s front surface  512  (not shown) either from the patterns  516  on its back surface by means of vias through the chip or by the conventional means of connecting the DC source(s) to the patterns on the front surface  512 .  
         [0046]    [0046]FIG. 6 shows in cross-sectional view, a fourth alternative embodiment of an improved spatial power-combining system  600  according to the present invention. In particular, the system includes three primary structures, namely, a quasi-optical grid array chip  610  and a substrate  630  and a superstrate  620 . The chip has a front surface  612  and a back surface  614 , each with patterned, electrically-conductive material  616  and  618  disposed thereon, respectively. The substrate also has patterned, electrically-conductive material  634  disposed on its front surface  632  that partly or substantially matches the metal pattern on the back surface  614  of the chip. The chip is attached to the substrate by joining the two sets of patterned materials  616 ,  634  together via solder joint  650 . The back surface  622  of the superstrate  620  includes patterned, electrically-conductive material  624  disposed thereon. The chip is attached to the superstrate by matching and joining the patterned material  616  on the chip to the patterned material  624  on the superstrate via thermo-compression bonding at  640 . DC bias may be supplied via the conventional means to the front of the chip itself, via the metal patterns on the substrate, or via the metal pattern on the superstrate, or via a combination of the above.  
         [0047]    As noted, the above discussion discloses various inventive aspects of the present heat dissipation and improved biasing features and functions as applied to quasi-optical grid arrays. Another class of spatial power combiners to which the present invention is equally applicable is the active loop architecture, which will now be reviewed. The active probe is a periodically-loaded loop that acts as a free-space radiating element that couples to electromagnetic (EM) fields. The probe is related to the active-transformer architecture. In the active transformer, a single-loop primary winding is periodically loaded with active devices, and the magnetic field generated by this loop is coupled to an interior, concentric secondary winding, from which a microstrip output is taken. The primary advantages of the active transformer are providing impedance matching from the very low-voltage CMOS devices to a 50 ohm load, and reducing losses associated with capacitive coupling to a conductive silicon substrate.  
         [0048]    The probe is also a periodically loaded loop, but in this case, rather than comprising the primary winding of a transformer, the loop is a radiating element that couples to propagating electromagnetic fields either in a waveguide or quasi-optical environment. The coupling of the active loop can take place either directly or via a passive “parasitic element,” in close analogy to parasitic elements used for impedance matching in ordinary loop antennas. The advantages of this probe for microwave/millimeter-wave amplification are the combination of the output power of several devices in free-space using a broadband, non-resonant radiating structure along with the option for providing a microstrip to waveguide transition within the same structure.  
         [0049]    [0049]FIG. 7 shows a representative embodiment. FIG. 7 shows a schematic circuit representation of an active-loop amplifier chip  700 , showing a loop loaded with four differential-pairs  702 ,  704 ,  706  and  708 , fed from a common differential input  710 . FIG. 8 illustrates an exemplary embodiment of an active loop circuit combined with the heat removal structures of the present invention. In particular, FIG. 8 is a schematic layout of the active-loop amplifier chip with a metal pattern  802  applied to the back side of the chip  800  in accordance with one aspect of the present invention and a metal pattern  804  applied to portions of the front side of the chip according to another aspect of the invention, both metals designed to improve heat dissipation and potentially provide one or more paths for DC power to be supplied to the active components of the loop. FIG. 9( a ) shows the improved chip  910  shown in FIG. 8 having its backside with patterned metal mounted to a ceramic heat spreader  920 , with a printed wiring board  930  carrying the input (in microstrip format), DC power, and an optional “parasitic element.” FIG. 9( b ) shows the subassembly of FIG. 9( a ) assembled into waveguide (both a cutaway front view as well as a rear view illustrating the microstrip feed). Thus, it is readily observed that each embodiment described with respect to the quasi-optical grid array equally applies to the active loop probe.  
         [0050]    Having thus described exemplary embodiments of the invention, it will be apparent that further alterations, modifications, and improvements will also occur to those skilled in the art. Further, it will be apparent that the present technique and system is not limited for use with quasi-optical array structures (e.g. grid amplifiers, oscillators, mixers, etc.) or active loops, but is applicable to any spatial power combiner that can benefit from being integrated with heat-removing and bias-supplying structures. In fact, the present invention is applicable to any type of integrated semiconductor chip that cannot properly function with a conventional conducting solid ground plane that covers substantially the entire back surface of the chip, but can benefit from the selective conducting pattern structures and methods of the present invention. Accordingly, the invention is defined only by the following claims.