Abstract:
Inductors and methods for integrated circuits that result in inductors of a size compatible with integrated circuits, allowing the fabrication of inductors, with or without additional circuitry on a first wafer and the bonding of that wafer to a second wafer without wasting of wafer area. The inductors in the first wafer are comprised of coils formed by conductors at each surface of the first wafer coupled to conductors in holes passing through the first wafer. Various embodiments are disclosed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the field of integrated circuits, and more specifically, integrated circuits with passive elements which include inductors. 
         [0003]    2. Prior Art 
         [0004]    Integrated circuits normally comprise not just a combination of active devices (transistors), but also the interconnection of the active devices with passive devices such as resistors, capacitors and inductors. Resistors are relatively easily formed as part of the integrated circuit, with physical sizes being generally comparable to the physical sizes of the active devices. Similarly, techniques are known for forming relatively small capacitors of relatively low capacitance as part of an integrated circuit. Historically capacitors of a larger capacitance and inductors have generally not been part of the integrated circuit, but instead have been incorporated in passive circuitry off the integrated circuit chip and coupled to the chip as necessary. In many such circuits, the integrated circuit is by far the smallest part of the overall circuit, and is relatively dwarfed by the size of the off-chip passive devices. Further, the required connections to the passive devices usually require a substantial increase in the number of input and output pins on the integrated circuit, which in turn increases the size of the required chip. 
         [0005]    More recently some inductors have been formed on chip as part of the integrated circuit, though at the expense of substantial chip area. In particular, whether formed on the integrated circuit or as part of separate passive circuitry, inductors are normally formed in what will be referred to herein as a two-dimensional structure, namely, as spiral windings insulated from and in a plane parallel to the face of the chip. In at least some instances second and third layers of the windings are also provided, each insulated from the other and interconnected by vias through the insulative layers. 
         [0006]      FIG. 1  is a face view of an RF transceiver circuit comprising a flip chip assembly of an integrated circuit  20  on a passive circuit  22 , each of which includes such two-dimensional inductors. In particular, the integrated circuit  20  includes two two-dimensional inductors  24  and the passive circuit  22  also includes two two-dimensional inductors  26 . It is apparent that the two-dimensional inductors  24  on the integrated circuit occupy a significant fraction of the chip area, not only because of their size but because the magnetic fields generated thereby can adversely affect linear circuitry that is too close to the inductors. Similarly, inductors  26  utilize an area as large as, or perhaps even larger than, the entire integrated circuit itself. The net result of this assembly is that an integrated circuit chip of 1.91 millimeters by 1.91 millimeters is mounted on a substrate with passive circuitry having dimensions of 4.99 millimeters by 4.99 millimeters, or approximately 6.8 times the area of the integrated circuit itself, with the final package having dimensions of 6 by 6 by 0.85 millimeters, over 9 times the area of the integrated circuit itself. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a view of a prior art integrated circuit, which circuit includes inductors on the integrated circuit, all mounted on a passive device circuit packaged therewith. 
           [0008]      FIGS. 2 through 31  are local cross sections of a wafer illustrating the partial fabrication of an inductor in accordance with an embodiment of the present invention. 
           [0009]      FIGS. 32 and 33  are local cross sections illustrating the bonding of two wafers in accordance with an embodiment of the present invention. 
           [0010]      FIG. 34  illustrates the coupling of an inductor to circuitry on a second wafer. 
           [0011]      FIG. 35  illustrates the coupling of circuitry on a bottom wafer through an upper wafer for connection to external circuitry, and the coupling of circuitry on a bottom wafer to circuitry on the upper wafer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    In the Figures referred to in the description to follow, such Figures are far from being scaled Figures, but instead are drawn with certain dimensions relatively exaggerated and others relatively compressed so as to better illustrate the fabrication process. In most cases, suitable dimensions will be obvious to those skilled in the art, and in other cases where important or unique to the present invention, representative dimensions will be given. 
         [0013]    Now referring to  FIG. 2 , a schematic illustration of an inductor formed by the methods of the present invention may be seen. The inductor is formed by the interconnection of vertical members  28  and horizontal members  30  and  32  to form a continuous coil-like structure between contacts  34 . Obviously the number of turns may be increased or decreased as desired. Also, one of the contacts  34  may be brought out at the lower level of the coil by adding or subtracting half a turn, or both contacts might be brought out at the lower level by simply turning over the structure illustrated. The process for fabricating the coil as described below is illustrated by representative cross sections taken along the view plane of  FIG. 2  for specificity in the description. That specificity however is not a limitation of the invention. 
         [0014]    Now referring to  FIG. 3 , a silicon substrate  36  may be seen. The substrate has a backside oxide layer  38  and integrated circuit devices formed on the top surface thereof with interconnect metal layers schematically illustrated as interconnect metal layer  40 , all within various oxide layers  42 . This structure would be formed by typical integrated circuit fabrication techniques and may comprise any of a wide variety of circuits, depending on the application. Preferably the substrate is a wafer size substrate, i.e., with which multiple devices will be formed and later diced to separate the multiple devices. 
         [0015]    The structure of  FIG. 3  is then coated with a hard mask layer  44  and patterned as shown in  FIG. 4  using a conventional photomask and etching process. Thereafter, a silicon trench type etch using a standard commercial process is made, as shown in  FIG. 5 . Then the photoresist is stripped as shown in  FIG. 6  and an oxide layer  46  is deposited as shown in  FIG. 7 . That layer is then coated with a barrier seed layer  48  as shown in  FIG. 8  and a layer of copper  50  is electroplated to fill the holes in the silicon substrate  36 , at least to a level above the top of the oxide layers  40 , as shown in  FIG. 9 . Then a chemical mechanical polishing (CMP) process is used to remove the copper layer  50 , the oxide layer  48  and barrier seed layer  46  between the holes in the substrate  36  that are now filled with copper, as shown in  FIG. 10 . 
         [0016]    The next step in the exemplary process is to deposit a stop layer  52  as shown in  FIG. 11 , then apply and pattern a photoresist layer  54  as shown in  FIG. 12  and etch down to the interconnect layer  40  as shown in  FIG. 13 . Then the photoresist layer  54  is stripped as shown in  FIG. 14 , a metal layer is deposited to fill the opening created by the etch, and a further CMP is used to remove stop layer  52  and the excess metal, leaving metal  56  contacting interconnect  40  as shown in  FIG. 15 . Then an oxide layer  58  is deposited as shown in  FIG. 16  and a photoresist layer  60  is then spun on the wafer in a standard manner and patterned as shown in  FIG. 17 . The oxide layer  58  is then etched through the photoresist ( FIG. 18 ) and the photoresist removed as shown in  FIG. 19 . Thereafter a metal barrier seed layer  62  is deposited as shown in  FIG. 20 , followed by a copper layer  64  sufficiently thick to fill the etched regions in the oxide layer  58 , as shown in  FIG. 21 . This is followed by another CMP to remove the copper and the metal barrier seed layer between filled regions  64  as shown in  FIG. 22 . This forms regions  34  and  32  in the coil of  FIG. 2  (as can be seen in  FIG. 2 , the region  32  of  FIG. 22  angles out of the view plane of this cross section). 
         [0017]    Thereafter a passivation oxide layer  66  is deposited as shown in  FIG. 23 , a photoresist layer  68  is applied and patterned as shown in  FIG. 24 , openings are etched to allow contact to one or both regions  34  and other integrated circuit contacts as needed ( FIG. 25 ) and the photoresist layer is removed ( FIG. 26 ). Note that in  FIG. 24 , region  34  is electrically accessible from the top of the wafer and is also electrically connected to the IC metal interconnect layer  40 . Depending on the circuit design, either one of these connections may not be present. By way of example, if the coil is in series with an output terminal and this end of the coil is to form the output terminal, connection of region  34  to the metal interconnect layer  40  would not be present, and if the coil is connected entirely to internal circuitry, the access through the passivation layer would not be provided. 
         [0018]    Now a temporary glue layer  70  is deposited ( FIG. 27 ) and the wafer is temporarily bonded to a carrier  72  as shown in  FIG. 28 . Then the opposite side of the substrate of wafer  36  is thinned by a coarse grind ( FIG. 29 ) and then given a fine polish using CMP ( FIG. 30 ). A silicon plasma etch is then used to expose the ends of copper  50  (vertical members  28  in  FIG. 2 ) as shown in  FIG. 31 , and then the lower end of copper vertical members  50  are thermo-compression bonded to copper horizontal members  30  (see also  FIG. 2 ) accessible through a passivation oxide layer  74  on another integrated circuit wafer  76  ( FIG. 32 ). The copper horizontal members  30  are separated by a photo-defined polymer, layer  77  in  FIG. 32 . This layer  77  serves two main purposes. Primarily, it serves as a strong adhesive layer between the top wafer and the bottom wafer. It also serves as a stress-distribution level during thermo-compression bonding. The left copper layer  64  is a region  34  of  FIG. 2  and the right copper layer  64  is a region  32  of  FIG. 2 . Thereafter the temporary carrier  72  and the glue layer  70  are removed to provide the structure of  FIG. 33  wherein the two silicon wafers are physically and electrically interconnected, both of which wafers may include integrated circuits with an inductor coil being formed by the combination of conductors extending entirely through the upper silicon wafer (as thinned) and interconnected at the top and bottom of the upper wafer to form the inductor coil, in the embodiment described being interconnected at the bottom by the pattern of copper regions on the lower substrate. Alternatively the lower interconnection of the copper vertical members  28  could be made by depositing and patterning a copper layer on the bottom of the first wafer by a photoresist process or CMP, though it is preferred to interconnect the copper vertical members  28  using a patterned layer of copper on the second wafer, as a patterned copper layer is needed on the second wafer anyway for thermo-compression bonding of the two wafers together. 
         [0019]    Now referring to  FIG. 34 , an alternate embodiment of the inductor coil of the present invention may be seen. In the embodiment previously described one (or both) coil leads is accessible through the top of the upper wafer. In the embodiment of  FIG. 34 , the inductor coil is not externally accessible but rather is flipped so that potentially both inductor leads  34  (see also  FIG. 2 ) are internally connected to the integrated circuit  76 . Thus one, both or none of the inductor leads may be made externally accessible, depending on the circuit being fabricated. 
         [0020]      FIG. 35  illustrates, at the left side thereof, how connections to the integrated circuit on the lower wafer are made accessible through the top of the upper wafer, and on the right thereof, how interconnections are made to the integrated circuits on the two wafers. In both cases, copper members  78  form vias through the upper substrate to connect copper member  80  and  82  to interconnect copper member  80  with the integrated circuit metal interconnect  84 , and at the right, to interconnect copper members  86  and  88  to interconnect integrated circuit metal interconnects  90  and  92 . Thus using the methods of the present invention, all required externally accessible connections to the integrated circuits on both wafers are accessible through the top of the upper wafer, and are ready for solder bumping or wire bonding and dicing. Simultaneously, all required interconnection between wafers and connections to the inductor leads are made through the same process. 
         [0021]    In a preferred embodiment, the final thickness of the upper wafer is approximately 100 microns, with the vertical members  28  ( FIG. 2 ) having a diameter of approximately 5 microns, thus providing an aspect ratio of approximately 20 to 1. However such dimensions and aspect ratio are not limitations of the invention. Also the upper wafer, if silicon, should be substantially pure silicon which has a very high resistivity at ordinary operating temperatures. Of course doped regions may be formed in other parts of the upper wafer for providing other integrated circuit components therein. 
         [0022]    As a further alternative, substrate  36  in  FIGS. 3 through 28  may be silicon with a thick oxide layer thereon, with the silicon subsequently being removed to leave the substrate in  FIG. 31  and subsequent Figures as a silicon oxide substrate. Other starting substrates might also potentially be used, such as by way of example, glass or ceramic. In any case, the resulting inductor coil, having an axis parallel to the plane of the substrate and coils extending all the way through the substrate, can have a substantial length in comparison to the prior art, yet still occupy a very small substrate area, allowing the realization of one or more inductors along with other passive or active elements on the upper substrate within an area consistent with the area of a typical integrated circuit in the lower substrate, allowing wafer to wafer bonding without significant wafer area waste as described, followed by solder bumping at the top of the upper wafer for making all connections to circuitry on both wafers, after which the pair of wafers may be diced to separate the multiple devices or integrated circuits on the wafers, and packaged. 
         [0023]    Thus the present invention has a number of aspects, which aspects may be practiced alone or in various combinations or sub-combinations, as desired. While 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 defined by the full breadth of the following claims.