Abstract:
A three-dimensional micro-coil situated in a planar substrate. Two wafers have metal strips formed in them, and the wafers are bonded together. The metal strips are connected in such a fashion to form a coil and are encompassed within the wafers. Also, metal sheets are formed on the facing surfaces of the wafers to result in a capacitor. The coil may be a single or multi-turn configuration. It also may have a toroidal design with a core volume created by etching a trench in one of the wafers before the metal strips for the coil are formed on the wafer. The capacitor can be interconnected with the coil to form a resonant circuit An external circuit for impedance measurement, among other things, and a processor may be connected to the micro-coil chip.

Description:
[0001]    This application is a continuation-in-part of a co-pending U.S. patent application, Ser. No. 09/342,087, filed Jun. 29, 1999, which in turn claims priority from U.S. Provisional Application No. 60/136,471, filed on May 28, 1999, Attorney Docket No. H16-25542. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention pertains to inductive coils. In particular, it pertains to micro-coils in planar substrates, and more particularly, to three-dimensional micro-coils in such substrates operating with an external circuit.  
         BACKGROUND OF THE INVENTION  
         [0003]    Micro-coils on planar substrates in the art are two-dimensional wherein the operation of them results in eddy current losses in the substrate. Other micro-coils are three-dimensional plated metal structures whose height is limited and are difficult to fabricate uniformly. Three-dimensional micro-coils also are fabricated on small rods and ceramic blocks; however, it is difficult to fabricate large numbers of such devices and integrate them with electronics on planar substrates.  
           [0004]    There are micro-coils that consist of spiral inductors fabricated on planar substrates, three-dimensional coils fabricated on the surfaces of tubes, ceramic blocks, or other substrates with cylindrical symmetry, and inductors formed by plating metal structures with high aspect ratios onto substrates.  
           [0005]    There are spiral inductors on planar substrates. This is a type of inductor that is fabricated by deposition and photolithographic processes. One example of its use is to increase the magnetic flux coupled into a magnetometer. Its most serious disadvantage is that a substantial fraction of the stored magnetic energy is contained in the substrate. Thus, if the substrate has a finite conductivity, as is usually the case for silicon, eddy current losses can be substantial  
           [0006]    There are three-dimensional coils on cylindrical objects. Helical inductors have been fabricated by patterning metal deposited onto a tube, and inductors with a square cross-section have been fabricated by laser patterning of metal deposited onto an aluminum oxide rod having a square cross-section The fabrication processes for these devices are not conducive to batch fabrication, and cannot be easily integrated with the fabrication processes for integrated circuits.  
           [0007]    Also, there are high-aspect-ratio plated metal inductors. These devices consist of air bridges of thick metal formed on a patterned metal layer on the surface of a wafer. Many air bridges can be connected electrically to form multi-turn air-core inductors whose stored magnetic energy lies mostly outside the substrate. The air bridges are formed by electroplating metal into molds formed from thick photoresist These inductors can have low eddy current losses, and the fabrication process can be integrated with silicon integrated circuit fabrication. However, the height of the plated structures is limited to the thickness of the photoresist, typically a maximum of 50 to 100 microns, thus limiting the height of the inductor. Also, the thickness of the electroplated metal is non-uniform over the surface of a wafer. This reduces the fabrication yield, and causes the dimensions of the structures, and therefore the electrical characteristics, to vary over the surface of a wafer.  
           [0008]    Inductors are often used in magnetic resonance spectrometer circuits where a pair of coils cooperatively provide a change in reflected RF power in response to a change in impedance. However, prior art inductors used in these circuits still have the above-noted disadvantages, and the present invention depicting three-dimensional micro-coils in a substrate avoids the above-noted disadvantages.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention has applications to portable magnetic resonance sensors and analyzers. Applications to other areas of high frequency electronics include low-loss tuned resonant circuits and filters in radio frequency (RF) wireless communication electronics. The preferred fabrication method for the present invention starts by etching a trench in a wafer substrate to define the air core of the inductor. Metal is then deposited onto the trench and patterned, followed by soldering a second wafer to the first wafer to complete the electrical connections for the inductor windings. With this fabrication methods one-turn inductors having a tubular topology may be fabricated. The magnetic field produced by such an inductor is confined mostly to the interior of the inductor. Thus, eddy current losses in the substrate can be minimized, resulting in the fabrication of high Q resonators.  
           [0010]    Micro-coils may be components of micro-resonators. The invention covers various types of three-dimensional micro-coils as well as electrical resonators formed from these kinds of micro-coils in planar substrates. The resonators typically operate at VHF, UHF or microwave frequencies.  
           [0011]    Micro-resonators having three-dimensional, single-turn tubular micro-coils have been successfully fabricated on silicon wafers. The Q of these resonators is typically about 30 at a resonant frequency of 680 MHz. The inductance of one of these micro-coils is about 0.2 nano-henry (nH). Micro-resonators having micro-coils with two turns and three turns have also been successfully fabricated in silicon wafers. The wafers may be planar substrates made of various materials such as GaAs besides silicon. These multi-turn devices have higher inductance than the one turn devices, but have a substantially lower Q (Q of about 7 at 432 MHz and Q of about 9 at 545 MHz). The lower Q is caused by the RF magnetic field between the windings penetrating into the silicon substrate, producing eddy current losses in the silicon substrate. A wide range of micro-coil inductances (and hence, resonant frequencies) can be obtained by changing the dimensions of the micro-coils. The advantages of the present invention are noted. The micro-coils are batch fabricated by processes compatible with integrated circuit fabrication techniques. Thus, the micro-coils can be fabricated in large quantities at low cost and integrated with active electronic circuitry. The coils have low eddy current losses because they have an air core. The three-dimensional geometry confines the magnetic field to the inside of the coil, thus minimizing eddy current losses in the substrate or other surrounding conductive materials. The height of the air core, determined by the depth of the etch trench, can be as large as the thickness of the substrate wafer, which is typically 500 microns for a 4-inch silicon wafer, and much thicker for the larger wafer diameters typically used in integrated circuit fabrication  
           [0012]    The inductance depends on the dimensions of the etched trench, and the shape of the patterned metal in the etch trench The dimensions of the etched trench can be uniform for many devices over the surface of a wafer, and the shape of the patterned metal is determined by well-defined photolithographic processes.  
           [0013]    Three-dimensional micro-coils have applications in miniature magnetic resonance spectrometers used as sensors and analyzers. Nuclear magnetic resonance (NMR), electron spin resonance (ESR), or nuclear quadropole resonance (NQR) can be measured with such a device. Magnetic resonance spectroscopy is a powerful tool for detection and identification of chemical species. An electron spin resonance (ESR) signal is typically caused by a free radical, and hence is sensitive to the chemical environment. An NMR signal is typically affected by small frequency shifts due to neighboring nuclei and electrons. Thus, each nucleus in a molecule will have a slightly different magnetic resonance frequency. As a result, a complex molecule can have a unique NMR spectrum.  
           [0014]    The greatest obstacle to miniaturization of magnetic resonance spectrometers is the size of the magnet providing the DC field needed to polarize the specimen being measured. A large, uniform polarizing magnetic field is desirable in order to achieve high signal to noise and narrow magnetic resonance linewidth A typical laboratory ESR spectrometer uses a magnet weighing over 1000 kilograms, which provides a uniform field of approximately 0.3 Tesla over a pole-piece diameter of several inches. A typical laboratory NMR spectrometer uses a superconducting magnet providing a field of order 10 Tesla or more. If the size of the pick-up coil can be reduced, then the diameter of the magnet&#39;s pole pieces and the gap between the pole pieces can be reduced, thus allowing the volume of the entire magnet to be dramatically reduced The gap between the pole pieces is important because the number of amp-turns required to achieve a given magnetic field is approximately proportional to the gap spacing. Thus, a small gap reduces the size of the magnet windings and the power supply requirements. The present invention permits the micro-coil thickness, and hence the gap between the pole pieces, to be about one millimeter. The diameter of the pole pieces would be about two centimeters, which is a few times larger than the typical length of the micro-coil Such a magnet is small enough to allow construction of a handheld magnetic resonance analyzer.  
           [0015]    There are further advantages of the present invention for use in miniature magnetic resonance spectrometers. The signal to noise ratio per magnetic resonant spin is higher for small pickup coils than for large pickup coils. Thus, for analyzing very small samples, small coils provide the optimum signal to noise. Also, micro-coils on planar substrates permit inexpensive integration of the pickup coil with the signal processing electronics.  
           [0016]    Analyzers with multiple pickup coils are more cost effective with all the coils integrated onto a single substrate, as made possible by the present invention Integration of the pickup coils with micro-fluidic gas and liquid sampling systems and other microanalysis systems is facilitated.  
           [0017]    The invention has applications for miniaturized wireless communications circuitry. On-chip integrated inductors allow more design flexibility and easier fabrication of filters and tuned resonant circuits at UHF, VHF and microwave frequencies. Such inductors also have applications in microprocessors, especially as clock speeds increase toward one GHz and beyond.  
           [0018]    This invention makes possible the fabrication of arrays of resonant circuits. The resonant circuits can be fabricated by batch fabrication processes. Many of these circuits can be fabricated on a single planar substrate simultaneously. Photolithographic patterning allows the dimensions of each resonant circuit to be precisely defined, therefore providing accurate control of each resonant frequency as well as the properties of circuits that couple energy between them. One application of such an array of resonant circuits would be to form a resonator with flat frequency response over a specified frequency range. Several resonant circuits, each with a slightly different resonant frequency, would be electrically coupled to each other to provide the desired flat frequency response. The coupling would be performed by transmission lines consisting of patterned dielectric and metal layers on one or both of the planar substrates. A transmission line could be connected directly to the capacitor of each resonant circuit, or to a secondary inductor formed near the primary inductor of each resonant circuit so that the mutual inductance between the secondary and primary inductors provides coupling of energy between the transmission line and the resonant circuit.  
           [0019]    A resonator formed from an array of several coupled resonant circuits can be used as an electrical filter having a flat band-pass response. The flat frequency response would also be advantageous for use as the pick-up coil in an NMR or ESR spectrometer. Precise dimensional control is essential for fabrication of such a device, in order to control the resonant frequencies of the individual resonant circuits and the characteristics of the coupling circuitry connecting them together. Batch fabrication using photolithography allows such devices to be built at relatively low cost. Other batch fabrication processes on planar substrates, such as screen-printing, can be used when the device dimensions are large enough to allow such processes. The invention may be fabricated on flexible or rigid planar substrates. Flexible substrates can include polyimide, such as KAPTON, or other polymers.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    For a more complete understanding of the invention, reference is hereby made to the drawings, in which:  
         [0021]    [0021]FIGS. 1 a  and  1   b  show an integrated circuit having a three-dimensional coil and a capacitor.  
         [0022]    [0022]FIGS. 2 a,    2   b  and  2   c  reveal a multi-turn coil within two wafers sandwiched together.  
         [0023]    [0023]FIG. 3 shows a wafer coil having a toroidal configuration.  
         [0024]    [0024]FIGS. 4 a,    4   b  and  4   c  illustrate the interrelationship of the two wafers that encompass the coil and the capacitor.  
         [0025]    [0025]FIG. 5 is a system layout for a device incorporating a micro-resonant circuit used for detecting and identifying electrons and nuclei.  
         [0026]    [0026]FIG. 6 is a circuit diagram of a prior art spectrometer circuit into which the present invention has been inserted to provide an improved performance.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    [0027]FIGS. 1 a  and  1   b  show a resonant circuit device formed from a micro-coil inductor  12  and a capacitor  21  connected to the inductor. FIG. 1 a  shows a “bottom” wafer or substrate  11  of an integrated circuit  10  having a micro-coil  12 . Micro-coil  12  has one turn. An “upper” wafer or substrate  18  is placed on top of wafer  11 . Metal  17  on wafer  11 , solder  14 , metal  16  on wafer  13 , and solder  15  form coil  12 . Item  18  may be a capacitor  21  or be a connection of capacitor  21  to the coil  12  circuit Capacitor  21  is present for completing the basic structure of a micro-resonator on chip  10 . Capacitor  21  may be connected in series or parallel with coil  12 . Trench  19 , etched in wafer  11 , helps establish an inductor cavity  20  for coil  12 . Trench  19  may extend out to the edge of substrate  11 , to allow magnetic resonance specimens to be inserted into trench  19  linearly along its axis, from the trench opening on the edges of substrates  11  and  13 . The magnetic field can be almost entirely confined to the inside of inductor or coil  12  if trench  19  has a toroidal geometry. Plate  25  is an electrode for capacitor  21 . Another plate  25  formed on wafer  13  is another electrode of capacitor  21  in conjunction with electrode  25  on wafer  11 . Also, wafer  13  has conductive interconnect paths for appropriately connecting capacitor  21  and coil  12  with each other, or to item  18 . Solder  14  provides electrical connection between a conductor on wafer  13  and a conductor on wafer  11 , such as pad  22  or metal  17 . Wafer  13  has a metal  16  that is another portion of coil  12 . Wafer  13  has a hole  23  for access to pad  22  and metal  17 . A hole  24  is etched in wafer  13  for access to inductor cavity  20 . Hole  24  in FIG. 1 b  allows insertion of a material to be sensed with ESR or NMR, as well as allowing the magnetic flux to exit the inductor without passing through the substrate  13  or  11  material. Wafers  11  and  13  may have additional pads  22 , coil elements  16  and  17  and capacitor elements  25  for other micro-coils  12  and capacitors  21 . These components may be variously interconnected to form micro-resonators or other devices.  
         [0028]    Three-dimensional coil  12 , formed in planar substrates  11  and  13 , may have a thickness dimension on the order of one millimeter. Substrates  11  and  13  may be wafers of silicon, GaAs, GeSi, silicon-on-insulator (SOI), printed circuit board, plastic flexible circuit substrate, or other like material. Substrates  11  and  13  are bonded together by soldering at, for example, places  14  and  15 .  
         [0029]    Lower substrate  11  is silicon or other material with etched trench  19  that has patterned metal  17 ,  22  and  25  deposited on its surfaces, such that metalized trench  19  forms the core of inductor  12 , and patterned metal  17  partially forms the winding of inductor  12 . Etched trench  19  is typically about 0.5 to 2 millimeters wide, and has a depth that can be comparable to the substrate  11  thickness. Other dimensions are possible, constrained only by the substrate  11  thickness and the minimum size permitted by photolithography. If substrate  11  is silicon, then the preferred method for etching trench  19  is anisotropic wet chemical etching on a (100) oriented silicon wafer  11 . Upper substrate  13  has a patterned layer  16  that completes the electrical current paths for the windings of inductor  12 . (“(100)” describes the crystallographic orientation with respect to the wafer surface, in standard crystallographic terminology). Solder provides electrical connections  14  and  15  between metal layers on upper substrate  13  and lower substrate  11 , as well as providing a mechanical bond between substrates  11  and  13 . The solder is deposited and patterned onto at least one of the substrates  11  and  13  before the wafers are bonded together.  
         [0030]    A resonant circuit can be provided by fabricating a capacitor  21  having a patterned dielectric layer  27  sandwiched between two layers  25  of patterned metal. With certain micromachining techniques, the dielectric may be just a space between electrodes  25 . Capacitor  21  can be fabricated on either of substrates  11  and  13  or both. Capacitor  21  is electrically connected to inductor  12  by patterned metal layers  22  and  26  on the substrates. For connections to external circuitry such as a power source, inductor  12  or capacitor  21  can be connected to wirebond pads  22 . Alternatively, pads  22  can be connected to a second inductor  12  patterned onto etch trench  19  just beyond the end of the first micro-coil  12 , so that the mutual inductance between the two micro-coils provides electrical coupling between a first micro-coil  12  and the external circuitry. Pads  22  are accessed externally for some of the connections through etched holes  23  in substrate  13 . Additional etched holes  23  could reside on substrate  11  with corresponding pads  22  residing on substrate  13 .  
         [0031]    Access to inductive cavity  20  can be attained through etched holes  24 . Etched holes  24  allow measurement specimens to be introduced to inductor cavity  20 . Etched holes  24  also allow magnetic flux to escape inductor cavity  20  without penetrating the substrate material of  11  or  13 . To further prevent penetration of magnetic flux into the substrate material of  11  or  13 , metal  17  can cover the entire trench  19 , and the sidewalls of access holes  24  can be coated with metal. Access holes  24  could be located on substrate  11  and/or substrate  13 .  
         [0032]    Metal layers  16 ,  17 ,  22 ,  25  and  26  are composed of gold, copper, silver or any other material having high conductivity at the operating frequency of device  10 . Metal layers  16 ,  17 ,  22 ,  25  and  26  should be at least as thick as the electrical skin depth of the metal to minimize the electrical resistance of the device and to confine radio frequency (RF) fields to the inside of inductor  12  and capacitor  21 , so as to minimize power dissipation in substrates  11  and  13 . If the substrate material has substantial electrical conductivity, then an insulator layer is required between metal layers  16 ,  17 ,  22 ,  25  and  26 , and the substrate  11 ,  13  material.  
         [0033]    To reduce eddy current losses in substrates  11  and  13 , designing micro-coil  12  to be a tube, or any other shape with cylindrical symmetry, is advantageous because this kind of configuration confines the RF magnetic field mostly to an air core region  20  of inductor  12 . The winding of such an inductor has only one turn as shown by metal layers  16  and  17  in FIGS. 1 a  and  1   b.    
         [0034]    The resonance device  30 , shown in FIGS. 2 a,    2   b  and  2   c,  is a multi-turn micro-coil  12  device. FIG. 2 a  shows a top view of substrate  11 . FIG. 2 b  shows a top view of the substrate  13  that is bonded to the top surface of substrate  11  shown in FIG. 2 a  FIG. 2 c  shows an alternative embodiment of substrate  13  that has an etched trench  29 . Multi-turn inductor  12  of FIGS. 2 a  and  2   b  has been fabricated. However, the RF field of such an inductor can penetrate into substrates  11  and  13  between coil windings  16  and  17 , causing eddy current losses if substrate  11  or  13  is formed from a lossy material such as silicon. Eddy current losses at the ends of micro-coil  12  can be prevented by etching a trench  19  or  29  that forms a closed path on the surface of substrate wafer  11  or  13 , respectively, so that a toroidal inductor is formed when the second wafer  13  or  11 , respectively, is bonded to the first wafer. The magnetic field is then confined almost entirely to the inside of the toroid, thus avoiding the problem of eddy current losses at the ends of inductor  12  (FIGS. 2 a,    2   b  and  2   c ) formed from linear trench  19  or  29  in substrate  11  or  13 .  
         [0035]    A low loss resonant circuit can be fabricated from a one-turn tubular inductor  12  and a capacitor  21 , as shown in FIGS. 1 a  and  1   b . FIG. 3 a  further illustrates this circuit with a cross section of device  40  having a toroidal inductor  12  attached to a capacitor  21 . A top view of inductor  12  would appear circular. On the other hand, the path of the etched trench  29  of device  40  does not need to be circular; it could be any closed path on the surface of substrate  13 . This circuit is a split ring resonator  40  because it has a one-turn inductor  12  formed from a conducting tube (or other shape with cylindrical symmetry) having a slit along its length and a capacitor  21  which is connected to the edges of the slit in inductor tube  20 . A toroidal split-ring resonator  40  can be constructed by joining the ends of tubular inductor  12  to each other. The topology of device  40  is implemented in a planar substrate using micro-machining techniques such as thin-film deposition, wet chemical etching, and photolithographic patterning.  
         [0036]    To produce an inductor  12  having higher inductance and reduced volume, a high-permeability low-loss magnetic material can be deposited into inductor core  20  of micro-coil  12 . This device has application as a compact inductor in integrated circuits, such as filters and resonant circuits in wireless communications, or in high speed digital electronics.  
         [0037]    [0037]FIGS. 4 a,    4   b  and  4   c  are diagrams of a resonator device  50  having coil  12  and capacitor  21 . FIG. 4 a  shows the top side of bottom wafer  11  and FIG. 4 b  shows the bottom side of wafer  13 . One can regard wafers  11  and  13  as two pages of an open book. When the book is closed (i.e., device  50  is assembled), the wafers are put together, and assembled device  50  is shown in FIG. 4 c.  The substrate is assumed to be transparent so that one can see through top wafer  13  in FIG. 4 c.    
         [0038]    A single-turn inductor may have slits perpendicular to the axis of the inductor. Such slits reduce eddy currents caused by an externally applied time-varying magnetic field, thus allowing the external time-varying magnetic field to penetrate into the central region of the inductor. This is useful for performing double magnetic resonance using techniques such as ENDOR (electron-nuclear double resonance), where the specimen must be exposed to two RF magnetic fields having two different frequencies, to excite two different magnetic resonant components within the specimen. The two RF fields would be provided by two resonators, each tuned to a different frequency.  
         [0039]    A single-turn inductor may also have a plurality of longitudinal slits for connection to a plurality of capacitors. The resonant frequency of a resonator fabricated in this way will be proportional to the square root of the number of capacitors, if all the capacitors are identical There are various configurations that can incorporate the invention. The micro-coil can be fabricated within a silicon (or an insulator such as glass or sapphire) wafer, where the diameter of the coil is comparable or less than the thickness of the wafer. The coil may be electrically connected to a capacitor on the same wafer, and be such that the resulting circuit of the coil and the capacitor is resonant. This coil and capacitor may be electrically coupled to an external circuit inductively with a loop of conducting material residing in the same wafer as the coil and having dimensions comparable to those of the coil. Or the coil and the capacitor may be electrically connected to the external circuit by a connection of wires to the electrodes of the capacitor. The micro-coil may be used to excite magnetic resonance of electrons or nuclei in a magnetic field which is constant with time or is slowly varying with time in comparison to the magnetic field generated by the coil, thereby causing a change in electrical impedance of the coil which can be detected by the external circuit.  
         [0040]    [0040]FIG. 5 shows a circuit  31  for identifying matter by exciting the magnetic resonance of electrons  35  or nuclei  36 . Magnet  37  provides the field across micro-coil circuit  31 . An external circuit  32  detects and measures the change of impedance of the micro-coil circuit  31 . This impedance information is fed to processor and indicator  33  so that identification of the detected matter can be achieved.  
         [0041]    A more detailed understanding of the present invention in use with an external circuit,  60  generally, can be seen in FIG. 6. The spectrometer circuit shown in FIG. 6 shows micro coils  61  and  62  connected to a homodyne spectrometer whose basic design is well known in the art. The spectrometer is designed for detection of electron spin resonance (ESR) in low magnetic fields (resonance frequency approximately 1 GHz). The ESR sample is contained in micro coil  62 , which together with an on-chip capacitor, forms a micro resonator  63 . RF power from the signal generator is coupled into coil  62  from coil  61  by inductive coupling. The RF magnetic field in coil  62  can cause the electron spins to flip their orientation in the external field produced by the permanent magnet  64  and the modulation/ramp coil  65 . Significant flipping of the electron spins occurs only when the frequency of the RF magnetic field in micro coil  62  and the slowly varying magnitude of the magnetic field produced by the permanent magnet  64  and the modulation/ramp coil  65  satisfy the electron spin resonance condition. When the electron spin resonance condition is satisfied, the electrons absorb energy from micro coil  62 , changing its impedance, and hence changing the impedance seen looking into coil  61  from the external circuit. This results in a change in reflected RF power from coil  61 , and a change in voltage at the input of the low noise pre-amplifier  66 . The shorted coax  67 , variable attenuator  68  and phase shifter  69  connected to the magic tee  70  are tuned to null out the reflected voltage from micro coil  61  when the ESR resonance condition is not satisfied, so that the RF voltage present at the low noise preamplifier  66  input is very small. The mixer  71  down-converts the output of the low noise preamplifier  66  to produce an intermediate frequency (IF) signal at the AC modulation frequency of the slowly varying magnetic field This audio frequency signal is down-converted to DC by a lock-in amplifier  72 , for display and storage of the ESR signal amplitude and phase.  
         [0042]    The spectrometer circuit can be modified for nuclear magnetic resonance (NMR) detection by using lower frequency RF components suitable for the typically lower NMR resonant frequency. Pulsed magnetic resonance detection is also possible by applying one or more RF pulses to micro coil  61 , rather than applying continuous RF power. The RF pulses produce a rotating magnetization of the sample in micro coil  62  which induces a signal voltage in micro coil  61  and the low noise preamplifier  66 . The RF signal voltage is down-converted by the mixer  71  and observed at the IF output of the mixer  71 , after appropriate filtering and amplification.  
         [0043]    Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.