Abstract:
Integrated high frequency balanced-to-unbalanced transformers and inductors suitable for operation in high frequencies, such as radio frequencies. Embodiments disclosed give consideration to issues related to the layout of the top and bottom inductors for the minimization of capacitive effects between layers and methods of manufacturing thereof. A displacement between the conductive paths of the top inductor and the bottom inductor is shown that provides for superior performance over prior art solutions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/897,446 filed Oct. 4, 2010, now allowed, which is a continuation of U.S. patent application Ser. No. 11/821,313 filed Jun. 22, 2007, now U.S. Pat. No. 7,808,356 which is a continuation-in-part of U.S. patent application Ser. No. 11/208,979 filed Aug. 22, 2005, now U.S. Pat. No. 7,253,712, entitled “Integrated High Frequency Balanced-to-Unbalanced Transformers”, which claims the benefit of U.S. Provisional Patent Application No. 60/605,511 filed Aug. 31, 2004. 
     
    
     STATEMENT OF SUPPORT 
       [0002]    The work that led to the development of this invention was co-financed by Hellenic Funds and by the European Regional Development Fund (ERDF) under the Hellenic National Strategic Reference Framework (NSRF) 2007-2013, according to Contract no. MIKRO2-34 of the project “Next Generation Millimeter Wave Backhaul Radio-THETA”, within the Program “Hellenic Technology Clusters in Microelectronics-Phase-2 Aid Measure”. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to semiconductor integrated circuits, and more particularly to integrated circuit inductors that are magnetically-coupled for the purpose of creating a high frequency transformer. 
         [0005]    2. Prior Art 
         [0006]    In wireless communications, an antenna is commonly coupled, typically by means of passive components, to a transformer. In many cases, a balanced-to-unbalanced (BALUN) transformer is used. Such a transformer allows the conversion of a single-ended signal into a differential signal and vice versa. In wireless communications, the antenna receives a single-ended radio frequency (RF) signal. The signal is converted to a differential signal using a BALUN transformer. The operation of BALUN transformers is well-known in the art, and such transformers are usually represented by the schematic  10  shown in  FIG. 1A . The unbalanced side of the BALUN transformer has two ends marked  12  and  14  respectively. The balanced side of the BALUN transformer has three connections, two on each end of the inductor marked  22  and  26  respectively, and one at the center of the inductor, marked  24 . The balanced side provides for a differential mode. In some cases the inductors of the transformer are wound around a core, directly impacting the mutual inductance between the primary and secondary inductors and therefore the performance of the transformer. Typically node  14  of the primary inductor and node  24  of the secondary inductor are AC grounded, as shown in  FIG. 1B . 
         [0007]    With the advent of solid state electronics, the ability to integrate components in a single semiconductor device has increased manyfold. This allows the reduction in size, power consumption, and cost, and further provides overall improvement in system performance. It is therefore natural that many attempts have been made to integrate transformers, including BALUN transformers, in order to take advantage of these features. Providing a symmetrical BALUN transformer has been known to be a challenge in the art, as specifically shown in U.S. Pat. No. 6,608,364 by Carpentier (hereinafter “Carpentier”) and U.S. Pat. No. 6,707,367 by Castaneda al. (hereinafter “Castaneda”). Carpentier suggests an implementation of a BALUN transformer that has five metallization layers, therefore requiring a complex manufacturing process having many layers often restricting conductor routing over the BALUN transformer. Castaneda suggests an elaborate scheme to provide a symmetrical BALUN transformer, also requiring several layers of metal and dielectric as shown in the various Figs. of Castaneda. Another example may be found in U.S. Pat. No. 6,882,263 by Yang et al. Symmetrical primary and secondary windings of an on-chip BALUN transformer are shown. However, the issue of capacitive coupling between the primary and secondary windings is not addressed, as the windings are essentially positioned such that a maximum capacitive coupling is achieved, having a disadvantage in operation at high frequencies, for example several GHz, as the capacitive coupling will tend to short-circuit the BALUN at these higher frequencies. 
         [0008]    As the demand for integrated circuit radios increases, many attempts have been made to integrate transformers and/or transformer BALUNs onto radio frequency integrated circuits. However, such integration has been limited due to flux leakage, capacitive coupling limits, and significant series resistance. To reduce these limitations, advances have been made in transformer IC design including coplanar interleaved transformers, toroidal and concentric transformers, overlay transformers and symmetric coplanar transformers. Coplanar interleaved transformers have the primary and secondary windings interleaved on the same integrated circuit layer, where the primary and secondary windings are constructed of planer metal traces. While coplanar interleaved transformers reduce size and are widely used, they suffer from low quality (Q) factor, small coupling coefficients, and, if used as a BALUN, the center tap is often at an undesirable location, resulting in an asymmetric geometry. As is known, asymmetry of a transformer winding causes an imbalance in the resulting differential signal and/or an imbalance in the resulting single ended signal from a differential signal. 
         [0009]    The advent of nm-scale CMOS RFIC design poses new challenges in the design of cost-effective integrated telecommunication transceivers. Despite the fact that the geometry of active devices in such processes is significantly scaled down, passive devices do not follow: integrated resistors, capacitors and inductors, tend to occupy the same silicon area as in more conventional CMOS or BiCMOS processes. From all passive devices, the integrated inductor is obviously the most area hungry. On the other hand, real estate is much more expensive in advanced sub-micron processes such as 90 nm or—even worse—in a 65 nm technology node so the design of area effective integrated inductors becomes imperative. 
         [0010]    Therefore, for the development of large L inductor structures, a multi-layer device is typically proposed. The conventional multi-layer inductor structure however, suffers from low self-resonance frequency mainly due to the increased inter layer parasitic capacitance: metal segments running on different layers form excellent Metal-Insulator-Metal structures that drastically affect the electrical behavior of the integrated inductor. U.S. Pat. Nos. 6,801,114 and 6,759,937 are examples for this class of solutions. While desired values may be calculated based on this solution it suffers from the limitations of the conventional multi-layer inductor structure. In particular, the vertical separation does not sufficiently overcome the parasitic capacitance that limits the high-frequency operation of the BALUN. 
         [0011]    There is therefore a need in the art for a BALUN transformer which is essentially symmetrical, can be implemented in a minimal number of layers of metal, and still provide the electrical characteristics of a BALUN transformer, and especially a reduced capacitive coupling, for the purposes of RF applications, for example in the gigahertz range. Furthermore, there is a need in the art for a design of an area effective inductor that overcomes the deficiencies of prior art solutions. It would be further advantageous if the electrical characteristics of the inductor are of high quality, and especially the reduction of the capacitive coupling, for the purposes of high-frequency RF applications, for example in the gigahertz range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1A  is a schematic drawing of a BALUN transformer. 
           [0013]      FIG. 1B  is a schematic drawing of a BALUN transformer with a grounded node of the primary coil and a grounded center node of secondary coil. 
           [0014]      FIG. 2  is an exemplary layout of a primary inductor of a first BALUN transformer using a single metal layer. 
           [0015]      FIG. 3  is an exemplary layout of a symmetrical secondary inductor of the first BALUN transformer having a displacement with respect to the primary inductor. 
           [0016]      FIG. 4  is an exemplary layout of the shunts of the first BALUN transformer. 
           [0017]      FIG. 5  is a layout of the first BALUN transformer showing the relationship between all three metal layers. 
           [0018]      FIG. 6  is an exemplary layout of a pseudo-differential and symmetrical primary inductor of a second embodiment of BALUN transformer. 
           [0019]      FIG. 7  is an exemplary layout of a symmetrical secondary inductor of the second BALUN transformer having a displacement in respect to the primary inductor. 
           [0020]      FIG. 8  is an exemplary layout of the shunts for the primary inductor of the second BALUN transformer. 
           [0021]      FIG. 9  is an exemplary layout of the shunts for the secondary inductor of the second BALUN transformer. 
           [0022]      FIG. 10  is a layout of the second BALUN transformer showing the relationship between all four metal layers. 
           [0023]      FIG. 11  is a diagram of a portion of primary coil metal layer and the secondary coil metal layer showing the displacement between the respective windings. 
           [0024]      FIG. 12  is a flowchart of the process steps respective of the manufacture of the BALUN transforms in accordance with the disclosed invention. 
           [0025]      FIG. 13A  is a schematic of a full 3D inductor designed in accordance with the principles of the disclosed invention. 
           [0026]      FIG. 13B  is a schematic of the top part of the 3D inductor of  FIG. 12 . 
           [0027]      FIG. 13C  is a schematic of the bottom part of the 3D inductor of  FIG. 12 . 
           [0028]      FIG. 14  shows an enlarged cross section  1400  corresponding to cross section A-A of inductor  1300  of  FIG. 13A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    Deficiencies of the prior art have lead to a need to provide BALUN transformers that are more efficient in their design, particularly in the number of metallization layers used for their implementation without significantly adversely affecting the BALUN transformer performance. The solution of the present invention accomplishes this target by having the windings of the primary inductor in one metal layer and the windings of the secondary inductors in another metal layer not only vertically separated from, but also horizontally displaced from the first metal layer. The displacement reduces the capacitive coupling between the primary and secondary coils. Furthermore, the implementations shown enable the use of only three or four layers of metal for a BALUN transformer. It should be noted that the displacement should be such that a substantial magnetic coupling between the primary and secondary inductors of the BALUN is still achieved to ensure the proper performance of the BALUN. 
         [0030]    Reference is now made to  FIGS. 2 through 4  where each of the three metal layers comprising a BALUN transformer  500 , shown in  FIG. 10 , are implemented in accordance with the disclosed invention are shown. The implementation makes use of three metal layers, metal layer  100 , metal layer  200 , and metal layer  300 . A person skilled-in-the-art will realize that it is not required that the metal layers used are consecutive metal layers, and specific choices may be made for the desired characteristics of the BALUN transformer, such as BALUN transformer  10 , including, but not limited to, the grounding of both one of the nodes, for example node  14 , of the primary inductor and the center node  24  of the secondary inductor. 
         [0031]    In  FIG. 2 , a primary coil is composed of a continuous winding  210  and ends  12  and  14 , implemented on a metal layer  100 , and designed to be pseudo-symmetrical, i.e., essentially symmetrical, with a slight asymmetry when curving to implement an internal winding. In  FIG. 3  a secondary coil, implemented in metal layer  300 , is composed of winding segment  310  having ends  312  and  26 , winding segment  320  having ends  22  and  322 , and winding segment  330  having ends  332  and  334 . The complete coil of the secondary coil is achieved by the use of shunt  410 , connecting ends  322  and  334  of winding segments  320  and  330  respectively, and shunt  420 , connecting ends  312  and  332  of winding segments  310  and  330  respectively. The shunts are shown in  FIG. 4 . 
         [0032]    Winding segments  310 ,  320  and  330  of the secondary coil of  FIG. 3  have a displacement with respect to winding  110  of the primary coil of  FIG. 2 , as explained in more detail below. The displacement reduces the horizontal overlap between the primary and secondary coils and hence reduces the capacitive coupling between them. Preferably the displacement is such that there is less than fifty percent overlap in the conductive paths between the windings of the secondary and the primary windings, excluding the shunts. A non-overlapping implementation is also possible as long as there is sufficient magnetic coupling between the primary and secondary inductors of the BALUN. In some embodiments of the disclosed invention, the input nodes of the primary inductor are physically one-hundred and eighty degrees from the outputs of the secondary inductor, further achieving symmetry.  FIG. 5  shows such an embodiment, with the center tap  24  of the secondary being connected to node  14 , typically both being grounded or coupled to a circuit common by a single connection thereto. 
         [0033]    Referring now specifically to  FIG. 5 , a top view of the three metal layers comprising BALUN transformer  10  are shown. In one preferred embodiment, metal layer  100  is the bottom layer, metal layer  200  is the middle layer and metal layer  300  is the upper layer. In particular, the primary coil metal layer  100  would be deposited over an insulator such as silicon dioxide (SiO 2 ), for example on a substrate, typically a silicon substrate, and then patterned using conventional photolithography techniques. Notably, metal layer  100  may be any one of the metal layers available for use in the device. Then another SiO 2  layer is deposited, followed by the depositing and patterned of another metal layer  200  to form the shunts. A further SiO 2  is deposited and windows opened (etched) therein to expose the ends of the shunts for VIAS, and in the embodiment being described, an opening through the last two SiO 2  layers to expose node  14  of the primary inductor. Then a final metal layer is deposited and patterned, making electrical contact with the shunts the form the complete secondary winding, and providing a common connection to one primary node ( 14 ) and the center node  24  of the secondary winding. It should be further noted that it is not required that the metal layers, used in the BALUNs of the present invention, be consecutive metal layers. Hence if a semiconductor device has available a total of seven metal layers, then if three metal layers are used for the BALUN, any three of the seven metal layers may be of use. 
         [0034]    By using this arrangement, the vertical distance between the primary coil and the secondary coil is further increased and therefore contributes to a reduction in the capacitive coupling between the coils. The primary coil is accessed at nodes  12  and  14  in metal layer  100 . Since node  14  is connected to the center node  24  of the secondary inductor, it is further possible to access node  14  in metal layer  200 . The secondary coil ends  22  and  26  are accessed in metal layer  300 , while center node  24  of the secondary coil is accessed at end  24  in metal layer  300 , as well as through node  14  in metal layer  100  as explained above. 
         [0035]    In one alternate embodiment, the order of the layers may be reversed, namely layer  300 , then layer  200  and finally layer  199 . In another embodiment of the disclosed invention, metal layer  300  follows metal layer  100  in the vertical stack, with the last metal layer being metal layer  200 . Connection between layers is achieved by the use of VIAS or stacked VIA holes which are well-known in the art. The inventors have noted that this implementation provides for minimal losses and has a narrowband balancing. 
         [0036]    Typical external diameter for a BALUN transformer in accordance with the disclosed invention is between 200 and 800 micron. Spacing between turns in the primary coil is typically 5 to 10 microns, and between turns of the secondary coil is typically 5 microns. A conduction path width of the primary inductor is typically between 10 and 20 microns and the secondary inductor path width is typically 5 microns. Therefore, in a preferred embodiment of the invention, with a fifty percent overlap of the secondary with respect to the primary, only 2.5 micron of width, or less, of the secondary inductor will be in overlap with the windings of the primary inductor. The typical numbers provided herein are of course exemplary only, and are not intended to limit the scope of the disclosed invention. 
         [0037]    Reference is now made to  FIGS. 6 through 9  where each of the four metal layers comprising a BALUN transformer  1000 , shown in  FIG. 10  in accordance with another embodiment of the present invention are shown. This embodiment is designed to provide broadband balancing. The implementation makes use of four metal layers, metal layer  100 , metal layer  200 , metal layer  300 , and metal layer  400 . These layers are shown in  FIGS. 6 through 9 . A person skilled-in-the-art will realize that it is not required that the metal layers used be consecutive metal layers, and specific choices may be made to accommodate the specific characteristics of BALUN transformer  1000 . The schematic of BALUN transformer  1000  is identical to the schematic shown for BALUN transformer  10  in  FIG. 1B , and therefore node designation shall again remain the same. 
         [0038]    In  FIG. 6 , a primary coil is composed of a winding segment  610  having ends  12  and  612 , and a winding segment  620  having ends  622  and  14 . Winding segments  610  and  620  are implemented in a patterned metal layer  100 . In  FIG. 8 , there is shown a shunt  810  implemented in patterned metal layer  200 . Shunt  810  connects ends  612  and  622  of windings  610  and  620  respectively. By connecting winding segments  610  and  620 , shunt  810  completes an implementation of a primary coil of BALUN transformer  1000 , creating a pseudo-differential inductor, having only two spirals. In  FIG. 7  a secondary coil is composed of winding segment  710  having ends  22  and  712 , winding segment  720  having ends  26  and  722 , and winding segment  730  having ends  732  and  734 . Segments  710 ,  720  and  730  of the secondary coil of BALUN transformer  1000  are implemented in patterned metal layer  400 . In  FIG. 9  there are shown shunts  910  and  920  implemented in patterned metal layer  300 . Shunt  910  connects ends  722  and  734  of windings  720  and  730 , and shunt  920  connects ends  712  and  732  of windings  710  and  730 . By connecting winding segments  710 ,  720  and  730 , shunts  910  and  920  complete an implantation of a differential secondary coil of BALUN transformer  1000 , where typically center node  24  is grounded, and connected to one of the nodes of the primary coil, for example node  14 . Winding segments  710 ,  720  and  730  have a displacement with respect to winding segments  610  and  620  of the primary coil, as explained in more detail below. The displacement reduces the overlap between the primary and secondary coils and hence the capacitive coupling between them. Preferably the displacement is such that there is less than fifty percent overlap in conductive path width between the windings of the secondary and the primary windings, excluding the shunts. A non-overlapping implementation is also possible as long as there is sufficient magnetic coupling between the primary and secondary inductors of the BALUN. In one embodiment of the disclosed invention, the output nodes of the primary inductor are physically one-hundred and eighty degrees from the outputs of the secondary inductor, further allowing for achieving symmetry. 
         [0039]    Referring now to  FIG. 10 , the four metal layers comprising BALUN transformer  1000  are shown. In one preferred embodiment, metal layer  100  is the bottom layer, metal layer  200  is a first middle layer followed by metal layer  300 , and metal layer  400  is the upper layer. However, a person skilled-in-the-art would easily note that a reverse order could be used, or in fact, any order that would not cause a restriction on the connection between the different metal layers. The primary coil is accessed at ends  12  and  14  in metal layer  100 . End  14  may be further accessed via node  24  of the secondary coil, connected through shunt  24  shown in  FIG. 8 . The secondary coil ends  22  and  26  are accessed in metal layer  400 . Center node  24  of the secondary coil is accessed via metal layer  200  which is also connected, for example by use of a VIA to node  14  in metal layer  100 . Connection between layers is achieved by the use of VIA or stacked VIA holes which are well-known in the art. The fabrication process in general may be similar to that previously described. 
         [0040]    In the BALUNs of the present invention, each layer is separated from adjacent layers by an electrically insulative (dielectric) layer, preferably SiO 2 , though other substrates and other electrically insulative layers could be used if desired. In that regard, silicon and SiO 2  are preferred as being most compatible with integrated circuit fabrication processes. The metal layers may be of various metals, though high electrical conductivity metals are preferred, such as aluminum, gold or silver. It should be further noted that it is not required that the metal layers, used in the BALUNs of the present invention, be consecutive metal layers. Hence if a semiconductor device has available a total of seven metal layers, then if three metal layers are used for the BALUN, any three of the seven metal layers may be of use. 
         [0041]    The inventors have noted that the foregoing implementation provides for minimal losses and has a broadband balancing. Typical external diameter for a BALUN transformer in accordance with the disclosed invention is between 200 and 800 micron. Spacing between winds in the primary coil are typically 5 to 10 microns, and between windings of the secondary coil are typically 5 microns. A path width of the primary inductor is typically between 10 and 20 microns and the secondary inductor is typically 5 microns. Therefore, in a preferred embodiment of the invention, with a fifty percent overlap, only 2.5 micron of width, or less, of the secondary inductor conductive path will be in overlap with the windings of the primary inductor. Again, the typical numbers provided herein are exemplary purposes only and are not intended to limit the scope of the disclosed invention. 
         [0042]    Reference is now made to  FIG. 11  where a diagram of a first portion  1110  of a primary coil metal layer and a second portion  1120 -A and a third portion  1120 -B of a secondary coil metal layer are shown. The layout of the second portion and third portion is in displacement with respect to the first portion. By avoiding full coverage between the primary and secondary coils, the parasitic coupling capacity is reduced and overall performance of the BALUN transformer improved. This separation further allows the use of a wider first portion and therefore reduces the resistance of the primary inductor. 
         [0043]    Reference is now made to  FIG. 12  where an exemplary flowchart  1200  of the process of manufacture of the BALUN transformers disclosed herein is shown. In one embodiment of the manufacturing process, in step S 1210 A there is created in a first metal layer an essentially pseudo-symmetrical winding. Alternatively, step S 1210 B is used where there is created a first winding that is symmetrical, as explained above with respect to  FIG. 6 . In step S 1220  there is deposited a layer of non-conducting material that is an insulator between one layer of metal and another layer of metal, and has further known dielectric characteristics. Therefore, when depositing another metal plate on top of the dielectric, there will be formed a parasitic capacitor, known also as a coupling capacitance, between the two layers of metal, reducing the performance of the BALUN. In accordance with the disclosed invention, in step S 1230  there is created a symmetrical second winding, as may be seen with respect to  FIGS. 3 and 7 , the second winding being concentric with, but horizontally displaced from the turns of the first winding. In one embodiment, the overlap between the second winding and the first winding is no more than fifty percent of the conductive path width of the second winding, excluding shunts. A non-overlapping implementation is also possible as long as there is sufficient magnetic coupling between the primary and secondary inductors of the BALUN. In some embodiments of the disclosed invention, the output nodes of the primary inductor are physically one-hundred and eighty degrees from the outputs of the secondary inductor, further providing symmetry. In step S 1240 , shunts are created to ensure continuous conducting paths through each of the first winding and the second winding. A person skilled in the art would readily recognize that the shunts may be created at multiple metal layers and hence the specific order shown herein should not be viewed as a limitation of the invention. Furthermore, it should be noted that the preferred manufacturing processes in general are well-known in the art, and are not provided herein in great detail in order to maintain clarity of the disclosed invention. Also while certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. As an example, while in embodiments shown herein with respect of  FIGS. 6 ,  7  and  8 , where the primary inductor has two turns and the secondary inductor has three turns, other configurations may be used. For example, and without limitation to the disclosed invention, embodiments of a BALUN having three turns in the primary inductor and five turns in the secondary inductor, or, four turns in the primary inductor and seven turns in the secondary inductor, are also possible. 
         [0044]    The principles discussed hereinabove may be also used to design large L inductors. This way, the overlap capacitance between the different metal layers is reduced and the self-resonance frequency is not affected significantly. Reference is now made to  FIG. 13A through 13C  that show a large L inductor designed in accordance with the principles of the disclosed invention.  FIG. 13A  shows a schematic drawing of the overall “3-D” inductor  1300  structure. The inductor  1300  is comprised of a top inductor  1310 , shown in  FIG. 13B , and a bottom inductor  1320 , shown in  FIG. 13C . The top inductor  1310  generally corresponds to the upper portion discussed above with respect of the BALUN. The bottom inductor  1310  generally corresponds to the lower portion discussed above with respect of the BALUN. In accordance with the principles of the disclosed invention the winding of the top inductor is displaced with respect to the bottom inductor, thereby reducing the overlap between the metals comprising the top inductor and the bottom inductor. The reduced overlap further accounts for the reduction in the parasitic capacitance between the windings and thereby contributing to the overall superior design over prior art solutions. 
         [0045]    The construction of a large L inductor in accordance with the principles of the disclosed invention is as follows: First the top inductor  1310  is followed from the outer winding to the inner winding. Once the inner winding is reached, a pair of metal bridge segments (not shown) transfer the spiral windings to the bottom inductor  1320  which is now deployed from the inner winding to the outer winding, each winding being in displacement to windings of the top inductor  1310 . The bridges connect the edges  1312  and  1314  of the top inductor  1310  to the edges  1322  and  1324  of the bottom inductor  1320  respectively. The center tap is placed at the outer spiral of bottom part. The current flow is always in the same winding sense so the mutual inductance developed is in favor of the overall spiral inductance. The ports of the inductor are ports  1316  and  1318 . The center tap  1326  in the bottom inductor is in fact the center of the large L inductor. In one embodiment of the disclosed invention the overlap between the conductive paths of the top inductor and the bottom inductor does not exceed fifty percent of the width of at least one of the conductive paths. 
         [0046]    Reference is now made to  FIG. 14  where a cross section  1400 , corresponding to cross section A-A from  FIG. 13A , is shown. In the enlarged cross section it can be seen, that in accordance with the principles of the disclosed inventions, the windings of the top inductor  1310  are placed in a displacement to the windings of the bottom inductor  1320 . In one embodiment of the disclosed invention an inductor may be created using a sandwich of two metal layers, the effective thickness of the spiral is increased and, therefore, the quality factor of the device is kept as high as possible. Surface  1410  is the face of the portion of the integrated circuit while surface  1420  is the back side and the substrate of the integrated circuit. 
         [0047]    While a preferred embodiment of the present invention has been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.