Abstract:
A three-level semiconductor balun is disclosed. In one embodiment, the balun includes a first spiral-shaped transmission line overlying a substrate. The first transmission line has first and second ends. A second spiral-shaped transmission line is substantially vertically aligned with the first transmission line. The second transmission line has a first end electrically connected to the second end of the first transmission line. A third spiral-shaped transmission line is substantially vertically aligned with the first and second transmission lines. The third transmission line has a first end electrically connected to a second end of the second transmission line. The balun may be integrated on the same chip with other RF circuit components, and is suitable for use at higher frequencies than most conventional baluns.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to integrated circuits, and in particular to a three-level semiconductor balun and method for creating the same. 
     BACKGROUND OF THE INVENTION 
     The use of twisted pairs of copper wires to form coupled transmission line elements is well known. These transmission line elements may be used to create baluns, balanced and unbalanced transformers and current and voltage inverters. Examples of the use of conventional transmission line elements are presented in C. L. Ruthroff, “Some Broad-Band Transformers,”  Proceedings of the IRE (Institute for Radio Engineers ), vol. 47, pp. 1337-1342 (August 1959), which is incorporated herein by reference. These transmission line elements are typically found in forms that are useful in frequency bands through UHF. 
     The use of such transmission line elements in integrated circuits such as RF power amplifiers and low noise amplifiers is desirable. However, the incorporation of off-chip devices such as these conventional transmission line elements into RF devices such as cellular telephones is not competitive due to size and cost. Moreover, conventional coupled transmission line elements are not suitable for use in the desired frequency range. 
     SUMMARY OF THE INVENTION 
     Therefore, a need has arisen for a coupled transmission line element that addresses the disadvantages and deficiencies of the prior art. In particular, a need has arisen for a low-loss balun suitable for integration in RF integrated circuits. 
     Accordingly, a three-level semiconductor balun is disclosed. In one embodiment, the balun includes a first spiral-shaped transmission line overlying a substrate. The first transmission line has first and second ends. A second spiral-shaped transmission line is substantially vertically aligned with the first transmission line. The second transmission line has a first end electrically connected to the second end of the first transmission line. A third spiral-shaped transmission line is substantially vertically aligned with the first and second transmission lines. The third transmission line has a first end electrically connected to a second end of the second transmission line. In one embodiment, a first balanced-side terminal is electrically connected to the first end of the first transmission line, a second balanced-side is terminal electrically connected to the first end of the third transmission line, and an unbalanced-side terminal is electrically connected to the second end of the third transmission line. 
     In another aspect of the present invention, a method for creating a balun on a semiconductor substrate is disclosed. The method includes forming a first transmission line on the substrate, forming a second transmission line substantially overlying the first transmission line, the second transmission line having a first end electrically connected to the second end of the first transmission line, and forming a third transmission line substantially overlying the first and second transmission lines, the third transmission line having a first end electrically connected to a second end of the second transmission line. 
     An advantage of the present invention is that the balun may be integrated on the same chip with other RF circuit components. Another advantage of the present invention is that the balun is suitable for use at higher frequencies than most conventional (non-integrated) baluns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a top view of a balun constructed in accordance with the present invention; 
     FIG. 2 is a perspective view of a crossover area of the balun; 
     FIGS. 3A through 3E are top views of the balun at various stages of fabrication; and 
     FIG. 4 is an equivalent schematic diagram of the balun. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention and their advantages are best understood by referring to FIGS. 1 through 4 of the drawings. Like numerals are used for like and corresponding parts of the various drawings. 
     Referring to FIG. 1, a top view of a balun  10  constructed in accordance with the present invention is shown. In balun  10 , a first transmission line  12  primarily occupies a top metallization layer. Second and third transmission lines  13  and  14 , respectively, primarily occupy middle and bottom metallization layers, respectively, underneath the top metallization layer. The top and middle metallization layers are separated by a dielectric layer (not shown in FIG.  1 ), as are the middle and bottom metallization layers. Each transmission line  12 ,  13 ,  14  has an outer terminus  12   a,    13   a,    14   a.  From the outer terminus  12   a,    13   a,    14   a,  each transmission line  12 ,  13 ,  14  spirals inward to an inner terminus  12   b,    13   b,    14   b.    
     The transmission lines of balun  10  are referred to as “broadside-coupled” because the transmission lines are substantially vertically aligned, giving rise to transmission line coupling between the conductors. Naturally, other effects such as edge coupling between conductor loops within the same metallization layer are also observed. However, the spiral shape of transmission lines  12 ,  13  and  14  allows the transmission line coupling to predominate over other undesired effects. 
     The dimensions of balun  10  are preferably such that each transmission line  12 ,  13 ,  14  has an overall length that is less than or approximately equal to one-eighth of the signal wavelength. The lower limit of transmission line length will vary depending on device characteristics, but is generally determined by transmission line coupling. In general, it is preferable for the desired “odd mode” or “push-pull” coupling between the transmission lines to predominate over the undesired “even mode” or “common mode” coupling between the transmission lines, as is known to those skilled in the art. 
     In one exemplary embodiment, signals in the frequency range of 1 GHz to 5 GHz are to be conducted by balun  10 . In this embodiment, each transmission line  12 ,  13 ,  14  has a width of 15 microns and an overall length of four millimeters. Transmission line  12  has a thickness of approximately 5.5 microns, while transmission lines  13  and  14  each have a thickness of approximately two microns. Transmission lines  12 ,  13 ,  14  are separated by dielectric layers (transparent in the illustration of FIG. 1) with a thickness of 1.5 microns. 
     At the inner terminus  12   b,    13   b,    14   b,  each transmission line  12 ,  13 ,  14  is electrically connected to a respective connector  16 ,  17 ,  18 . In one embodiment, connectors  16 ,  17  and  18  reside in the middle and bottom metallization layers. Connectors  16 ,  17  and  18  are used to establish electrical contact between the respective inner termini  12   b,    13   b,    14   b  and other electrical terminals, as will be described below. 
     Each loop of the balun  10  requires transmission lines  12 ,  13  and  14  to cross over connectors  16 ,  17  and  18 . To accomplish this without the use of an additional metallization layer, bridge segments  12   c  and  12   d  of transmission line  12  share space in the top metallization layer with transmission line  12  in each crossover area  20 . 
     Referring to FIG. 2, a perspective view of a crossover area  20  is shown. Transmission line  12  and bridge segments  12   c  and  12   d  occupy the top metallization layer while connectors  16 ,  17  and  18  occupy the middle and bottom metallization layers. Dielectric layers (not shown) separate the metallization layers. 
     A process for creating balun  10  is illustrated in FIGS. 3A through 3E, where top views of balun  10  at various stages of fabrication are shown. Referring to FIG. 3A, the pattern of the bottom metallization layer  22  is shown. Metallization layer  22  may be, for example, a layer of copper or another conductive material. Metallization layer  22  is deposited on a substrate  24  and etched to create transmission line  14  using conventional deposition and photolithography techniques. Substrate  24  may be, for example, a semi-insulating substrate such as gallium arsenide. The bottom layer of connectors  16 ,  17 ,  18  are formed with metallization layer  22 . As shown in the figure, the bottom layer of connector  18  is contiguous with transmission line  14  at inner terminus  14   b.    
     Also included in metallization layer  22  are two contact strips  12   e,    13   e.  Strips  12   e  and  13   e  provide electrical contacts in bottom metallization layer  22  to transmission lines  12  and  13 , respectively. The manner in which strips  12   e  and  13   e  are connected to their respective transmission lines is described below. A similar extension strip  14   e  of transmission line  14  is provided in proximity to contact strips  12   e  and  13   e.  Thus, all three transmission lines  12 ,  13 ,  14  may be contacted from bottom metallization layer  22 . All of these strips  12   e,    13   e,    14   e  may be connected to other wiring (not shown) patterned in bottom metallization layer  22 . 
     Referring to FIG. 3B, a dielectric layer  26  is deposited over metallization layer  22 , which is shown in dashed lines in this figure. Dielectric layer  26  may be, for example, bisbenzocyclobutene (BCB), a nitride or oxide of silicon, or some other insulating material. Dielectric layer  26  is deposited using conventional techniques. Dielectric layer  26  is selectively etched to form openings or vias  27  (shown in solid lines), which allow electrical contact to be establish with the middle metallization layer as described below. 
     Referring to FIG. 3C, the middle metallization layer  30  is formed over dielectric layer  26 . Metallization layer  30  may be, for example, a layer of copper or another conductive material. Metallization layer  30  is deposited on dielectric layer  26  and etched to create transmission line  13  and the top layer of connectors  16 ,  17 ,  18  using conventional deposition and photolithography techniques. As shown in the figure, the top layer of connector  17  is contiguous with transmission line  13  at inner terminus  13   b.    
     Vias  27  in dielectric layer  26  beneath metallization layer  30  are shown in dashed lines in FIG.  3 C. These vias provide points of contact between middle metallization layer  30  and bottom metallization layer  22 . Thus, connectors  16 ,  17  and  18  reside in both the bottom and middle metallization layers  22  and  30 . 
     An extension  13   f  contiguous with the outer terminus  13   a  of transmission line  13  is connected with contact strip  13   e  in bottom metallization layer  22  by means of another via  27 . A metal portion  29  is formed over a via  27  in electrical contact with contact strip  12   e  in bottom metallization layer  22 . Metal portion  29  provides electrical contact between contact strip  12   e  and transmission line  12  in the top metallization layer, as described below. 
     Similarly, metal portions  31  are formed separate from transmission line  13 . These metal portions  31  provide electrical contact between transmission line  14  in bottom metallization layer  22  and bridge segments  12   c  in the top metallization layer, as described below. 
     Referring to FIG. 3D, a dielectric layer  32  is deposited over metallization layer  30 , which is shown in dashed lines in this figure. Dielectric layer  32  may be made using the same insulating material as dielectric layer  26  described above. Dielectric layer  32  is deposited using conventional techniques. Vias  34  are formed in dielectric layers  32  and  26  using conventional photolithography techniques. Vias  34  are formed in the locations shown to establish electrical contact between metallization layers, as described below. 
     Referring to FIG. 3E, the top metallization layer  36  is formed over dielectric layer  32 . Metallization layer  36  may be, for example, a layer of copper or another conductive material. Metallization layer  36  is deposited on dielectric layer  32  and etched to create transmission line  12  and bridge segments  12   c,    12   d  using conventional deposition and photolithography techniques. During deposition, metallization layer  36  fills in the vias  34  in dielectric layer  32 , establishing electrical contact to middle metallization layer  30 . 
     Specifically, each bridge segment  12   c  is electrically connected on either end to a metal portion  31  in middle metallization layer  30 , and is thereby electrically connected to transmission line  14  in bottom metallization layer  22 . Bridge segments  12   c  therefore provide a conduction path for transmission line  14  across the gaps necessitated by connectors  16 ,  17  and  18 . 
     Similarly, each bridge segment  12   d  is electrically connected on either end to transmission line  13  in middle metallization layer  30 . Bridge segments  12   d  therefore provide a conduction path for transmission line  13  across the gaps necessitated by connectors  16 ,  17  and  18 . 
     At its outer terminus  12   a,  transmission line  12  is electrically connected to metal portion  29  in middle metallization layer  30 , and is thereby electrically connected to contact strip  12   e  in bottom metallization layer  22 . Contact strip  12   e,  as previously described, provides a means to connect transmission line  12  to other wiring (not shown) patterned in bottom metallization layer  22 . At its inner terminus  12   b,  transmission line  12  is electrically connected to connector  16  by means of a via  34 . 
     Referring to FIG. 4, an equivalent schematic diagram of balun  10  is shown. In FIG. 4, transmission lines  12 ,  13 ,  14  are represented (in no particular order) by three parallel inductors  40 ,  42  and  44 . The balanced side of balun  10  has two terminals  46  and  48 , while the unbalanced side has one terminal  50  and a connection to a common potential (e.g. ground). 
     In the schematic diagram of FIG. 4, the transmission line coupling of the transmission lines  12 ,  13 ,  14  is reflected in the alignment of inductors  40 ,  42  and  44 . Thus, the left side of each inductor may represent the inner terminus of the corresponding transmission line  12 ,  13 ,  14 , while the right side of each inductor represents the outer terminus of the corresponding transmission line, or vice versa. All three inductors  40 ,  42 ,  44  must have the same orientation, so that, for example, the left side of the schematic represents the inner termini of all three transmission lines. 
     There are six possible ways to substitute transmission lines  12 ,  13  and  14  for the three inductors  40 ,  42  and  44  in FIG.  4 . Furthermore, the “handedness” of the schematic may be changed by changing which side (left or right) represents the inner termini of the transmission lines  12 ,  13 ,  14 . This gives a total of 12 possible interconnections of transmission lines  12 ,  13  and  14  to create balun  10 . 
     These 12 possible interconnect cases for forming balun  10  are shown in Table A. Each row of the table represents a separate interconnect case, and provides the reference numeral of the terminal (or common potential) to which each transmission line terminus is connected. 
     Differences in actual circuit performance may be observed among the various interconnect cases listed in Table A. Experimentation may be conducted to determine the optimal interconnect scheme for a given circuit implementation. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE A 
               
             
             
               
                   
               
               
                 Transmission line terminus 
               
             
          
           
               
                 Case 
                 12a 
                 13a 
                 14a 
                 12b 
                 13b 
                 14b 
               
               
                   
               
               
                 1 
                 48 
                 common 
                 46 
                 50 
                 48 
                 common 
               
               
                 2 
                 46 
                 common 
                 48 
                 common 
                 48 
                 50 
               
               
                 3 
                 common 
                 48 
                 46 
                 48 
                 50 
                 common 
               
               
                 4 
                 46 
                 48 
                 common 
                 common 
                 50 
                 48 
               
               
                 5 
                 common 
                 46 
                 48 
                 48 
                 common 
                 50 
               
               
                 6 
                 48 
                 46 
                 common 
                 50 
                 common 
                 48 
               
               
                 7 
                 50 
                 48 
                 common 
                 48 
                 common 
                 46 
               
               
                 8 
                 common 
                 48 
                 50 
                 46 
                 common 
                 48 
               
               
                 9 
                 48 
                 50 
                 common 
                 common 
                 48 
                 46 
               
               
                 10  
                 common 
                 50 
                 48 
                 46 
                 48 
                 common 
               
               
                 11  
                 48 
                 common 
                 50 
                 common 
                 46 
                 48 
               
               
                 12  
                 50 
                 common 
                 48 
                 48 
                 46 
                 common 
               
               
                   
               
             
          
         
       
     
     It will be appreciated that balun  10  provides a transition of balanced to unbalanced conductors in a manner readily apparent to those skilled in the art. Balun  10  may be used, for example, as a high performance balun for an RF push-pull amplifier with integrated matching network. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.