Abstract:
A combination edge- and broadside-coupled transmission line element formed in an integrated circuit chip, using semiconductor processes, in a stack of metal layers separated by dielectric layers. Each of the metal layers includes a number of transmission lines. Interconnects between the transmission lines are formed at predetermined locations, each interconnect electrically connecting together a group of the transmission lines to form a conductor. The efficiency of the coupling between the lines in the different conductor is increased by positioning the lines such that both edge and broadside-coupling is realized. For example, at least one of the transmission lines in one of the conductors is positioned either above or below a transmission line in the other conductor to achieve broadside-coupling and laterally adjacent to another transmission line in the other conductor to achieve edge-coupling. In a preferred embodiment each of the lines in one of the conductors is edge- and broadside-coupled to lines in the other conductor. The transmission line element may contain two, three or more conductors, and each conductor may contain two, three or more transmission lines. The transmission line element can be used, for example, to fabricate various types of balanced and unbalanced transformers.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is related to commonly owned application Ser. No. 09/768,865, filed Jan. 23, 2001, and application Ser. No. 09/863,779, filed May 22, 2001, each of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to impedance transforming elements, and in particular to segmented and interdigitated integrated coupled transmission line elements. 
     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 balanced and unbalanced transmission lines, balanced-unbalanced (balun) transmission lines, 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 that operate at higher than UHF frequencies is desirable. However, the incorporation of these conventional transmission line elements into RF devices such as cellular telephones is not competitively feasible due to size and cost. Moreover, conventional coupled transmission line elements are not suitable for use in the desired frequency range. 
     Therefore, a need has arisen for a coupled transmission line element that addresses the disadvantages and deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     A transmission line element in accordance with this invention comprises a plurality of metal layers that are formed in an integrated circuit chip. Each of the metal layers is separated from an adjacent metal layer by a dielectric layer. In a bifilar embodiment, a first conductor comprises at least two transmission lines in different metal layers; and a second conductor comprises at least two transmission lines also in different metal layers. The transmission lines in the first and second conductors run parallel to each other. A plurality of interconnects are located at predetermined positions along the conductors, each of the interconnects containing an electrical connection between the transmission lines in the first conductor and an electrical connection between the transmission lines in the second conductor. At least one transmission line in the first conductor is edge-coupled to at least one transmission line in the second conductor and broadside-coupled to at least one other transmission line in the second conductor. 
     In addition, at least a second transmission line in said first conductor may be edge-coupled to at least one transmission line in said second conductor and broadside-coupled to at least one other transmission line in said second conductor. 
     The first and second conductors may be formed in the shape of a spiral or a variety of other shapes. 
     At least one of the interconnects may comprise a via through the dielectric layer, a first tongue extending to the via from one of said transmission lines and a second tongue extending to the via from another one of said transmission lines. 
     In one bifilar embodiment the first conductor comprises two transmission lines and the second conductor comprises two transmission lines. Alternatively, the first conductor comprises three transmission lines and said second conductor comprises three transmission lines. 
     In a trifilar embodiment the transmission line element comprises a third conductor, and each of said first, second and third conductors may comprise three transmission lines. 
    
    
     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 an bifilar broadside- and edge-coupled transmission line element according to the invention. 
     FIG. 2A is a view of the top metal layer in the crossover region of the transmission line element of FIG.  1 . 
     FIG. 2B is a view of the bottom metal layer in the crossover region of the transmission line element of FIG.  1 . 
     FIG. 3 is a composite view of the top and bottom metal layers in the crossover region of the transmission line element of FIG.  1 . 
     FIG. 4 is a cross-sectional view of the broadside- and edge-coupled transmission line element of FIG.  1 . 
     FIG. 5A is a view of the transmission line element of FIG. 1 taken at cross-section  5 A— 5 A of FIG. 3, showing the via that connects one pair of associated transmission lines. 
     FIG. 5B is a view of the transmission lines of FIG. 1 taken at cross-section  5 B— 5 B of FIG. 3, showing the via that connects the other pair of associated transmission lines. 
     FIGS. 6A and 6B are cross-sectional views of alternative bifilar transmission line elements in accordance with the invention. 
     FIGS. 7A-7G are schematic circuit diagrams of various transformers that may be fabricated using the transmission line element of this invention. 
     FIG. 8 is a top view of a trifilar broadside- and edge-coupled transmission line element according to the invention. 
     FIG. 9 is a cross-sectional view of the trifilar broadside- and edge-coupled transmission line element of FIG.  8 . 
     FIGS. 10A-10C are top views of the bottom, middle and top metal layers of an interconnect in the transmission line element of FIG.  8 . 
     FIG. 11 is a top composite view of the interconnect in the transmission line element of FIG.  8 . 
     FIGS. 12A-12C are cross-sectional views of the interconnect in the transmission line element of FIG.  8 . 
     FIGS. 13 and 14 are top views of the crossover region in the transmission line element of FIG.  8 . 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a general view of a bifilar transmission line element  10  in accordance with the invention. Bifilar element  10  includes two pairs of broadside- and edge-coupled transmission lines formed in two metal layers separated by a dielectric layer. FIG. 4 illustrates the relative positions of the transmission line pairs. Lines  104 A and  102 A are formed in a top metal layer TM, and lines  102 B and  104 B are formed in a bottom metal layer BM. The top and bottom metal layers TM and BM are separated by a dielectric layer  103 . This structure is fabricated using conventional semiconductor processes that are well-known to those skilled in the art and will not be detailed here. Metal layers TM and BM may be formed, for example, of aluminum, gold, or another conductive material. 
     As shown in FIG. 1, the transmission line pairs extend from a first terminus  106  to a second terminus  108 . While the transmission lines in FIG. 1 are laid out in the pattern of a square spiral, many other geometries may be used. For example, other spiral shapes (circular, rectangular, etc.) can be used, or the transmission lines can be linear or variety of other shapes. This invention is not limited to any particular shape of transmission lines. 
     Transmission lines  102 A,  102 B,  104 A,  104 B are “segmented” in the sense that at predetermined intervals line  102 A is connected to line  102 B, and line  104 A is connected to line  104 B. Lines  102 A,  102 B thus together constitute a first conductor  102  and lines  104 A  104 B together constitute a second conductor  104 . The intervals between such connections are referred to as “segments”. Referring to FIG. 1, the connections are made at interconnects  112 ,  114 ,  116 ,  118 ,  120 ,  122 ,  124 ,  126 ,  128 ,  130 ,  132  and  134 . The length between interconnects  112 ,  114  (terminus  106 ) and interconnects  116 ,  118  constitutes a first segment, the length between interconnects  120 ,  122  and interconnects  124 ,  126  constitutes a second segment, and the length between interconnects  128 ,  130  and interconnects  132 ,  134  (terminus  108 ) constitutes a third segment. Preferably, the interconnects are spaced such that, at the operating frequency of the transformer, the segments are less than 30 degrees long. 
     FIG. 3 is a detailed view of the area  110  shown by the dashed lines in FIG.  1 . Included are interconnects  116 ,  118 ,  124  and  126 . The views at cross-sections  5 A— 5 A and  5 B— 5 B are shown in FIGS. 5A and 5B, respectively. Referring first to FIG. 5A, the top metal layer TM, which forms lines  104 A and  102 A, and the bottom metal layer BM, which forms lines  102 B and  104 B, are separated by a dielectric layer  103 . Dielectric layer  103  may be made of bisbenzocyclobutene (BCB), a nitride or oxide of silicon, or some other insulating material. Dielectric layer  103  is deposited using conventional techniques. 
     FIG. 2A shows the top metal layer TM in area  110 , and FIG. 2B shows the bottom metal layer BM in area  110 . Referring to FIG. 2A, at interconnect  116  a tongue  102 X extends from line  102 A to a via  103 A, and at interconnect  118  a tongue  104 X extends from line  104 A to a via  103 B. Referring to FIG. 2B, at interconnect  116  a tongue  102 Y extends from line  102 B to via  103 A, and at interconnect  118  a tongue  104 Y extends from line  104 B to via  103 B. Thus, as shown in FIG. 5A, at interconnect  116  an electrical connection is formed between lines  102 A and  102 B by means of tongue  102 X, the metal in via  103 A, and tongue  102 Y. As shown in FIG. 5B, at interconnect  118  an electrical connection is formed between lines  104 A and  104 B by means of tongue  104 X, the metal in via  103 B, and tongue  104 Y. Using tongues and vias, similar connections between lines  102 A and  102 B and lines  104 A and  104 B are formed at interconnects  112 ,  114 ,  120 ,  122 ,  124 ,  126 ,  128 ,  130 ,  132  and  134 . 
     As shown in FIGS. 2A and 2B, area  110  includes a crossover region  136 , where the transmission lines cross. In crossover region  136  the top metal layer TM terminates between interconnects  118  and  120 , and the bottom metal layer BM terminates between interconnect  134  and terminus  108 , thereby allowing the transmission lines to pass from the inside of the spiral to terminus  108 . 
     By reference to FIG. 4 it will be understood that broadside coupling occurs between lines  102 A and  104 B and between lines  104 A and  102 B; and edge coupling occurs between lines  102 A and  104 A and between lines  102 B and  104 B. As compared with the broadside-coupled arrangement described in the above-referenced application Ser. No. 09/768,865, the addition of segmented edge-coupling between the lines and phasing the alternate interdigitated segments increases the surface area for the RF currents and enhances the coupling coefficient. Transformer losses are significantly reduced. For example, simulated tests show a reduction of losses from −0.3 dB to −0.15 dB. 
     The embodiment described above is bifilar in the sense that in essence there are two conductors  102 ,  104  running adjacent to each other. Each conductor consists of two lines:  102 A,  102 B and  104 A,  104 B. The invention is not limited to this embodiment, however. In other bifilar embodiments, each of the two conductors may include three or more transmission lines formed in two, three or more metal layers. Two alternative embodiments are shown in FIGS. 6A and 6B. FIG. 6A shows a two-layer embodiment wherein each conductor includes three transmission lines. Transmission lines  202 A,  204 B and  202 C are formed in the top metal layer, and transmission lines  204 A,  202 B and  204 C are formed in the bottom metal layer. Lines  202 A,  202 B and  202 C are connected together to form a conductor  202 , and lines  204 A,  204 B and  204 C are connected together to form a conductor  204 . The connections between lines  202 A- 202 C and  204 A- 204 C are preferably made at interconnects similar to those shown in FIGS. 2A,  2 B,  5 A and  5 B, the interconnects being spaced such that transmission line segments of an appropriate length are formed. FIG. 6B shows a three-layer embodiment wherein each conductor includes three transmission lines. Transmission lines  302 A and  304 A are formed in the top metal layer, transmission lines  302 B and  304 B are formed in the middle metal layer, and transmission lines  302 C and  304 C are formed in the bottom metal layer. Lines  302 A,  302 B and  302 C are connected together to form a conductor  302 , and lines  304 A,  304 B and  304 C are connected together to form a conductor  304 . The connections between lines  302 A- 302 C and  304 A- 304 C are preferably made at interconnects similar to those shown in FIGS. 2A,  2 B,  5 A and  5 B, the interconnects being spaced such that transmission line segments of an appropriate length are formed. 
     In both of the embodiments shown in FIGS. 6A and 6A, it will be noted that any line that is located above, below or laterally adjacent to a given line in one of the conductors is a part of the other conductor. Taking line  202 B in FIG. 6A as an example, line  204 B lines directly above line  202 B and lines  204 A and  204 C lie on opposite sides of line  202 B. Line  202 B is a part of conductor  202 , and lines  204 A,  204 B and  204 C are parts of conductor  204 . This maximizes the extent of broadside- and edge-coupling between the signal in line  202 B and the signal in lines  204 A,  204 B and  204 C. In some embodiments, however, broadside- and edge-coupling may not be required with respect to all of the transmission lines. 
     The positions and locations of the termini and connecting ends shown in FIG.  1  and the accompanying diagrams are meant to be illustrative and not limiting. Other embodiments of the invention readily apparent to those skilled in the art will have such ends located in a variety of positions. Furthermore, it is to be understood that reference to the metal layers as “top” and “bottom” is purely arbitrary and that the position of the layers with respect to each other when looking downward on them could be reversed. 
     As noted above, the interconnects preferably are in the form shown in FIGS. 2A,  2 B,  5 A and  5 B, with vias being formed in the dielectric layer laterally in between adjacent transmission lines. It will be understood, however, that other techniques and structures may be used to connect the transmission lines at the interconnects. 
     Referring again to FIG. 1, each of the conductors  102 ,  104  has a separate terminal at each of the termini  106  and  108 . FIGS. 7A-7F illustrate how these terminals can be connected to form different types of transformers. In FIGS. 7A-7F, the terminal of conductor  102  at terminus  106  is designated  106 A; the terminal of conductor  104  at terminus  106  is designated  106 B; the terminal of conductor  102  at terminus  108  is designated  108 A; and the terminal of conductor  104  at terminus  108  is designated  108 B. Typically an input signal is applied at terminals  106 A and  108 A, and an output signal is generated at terminals  106 B and  108 B. FIG. 7A shows a balanced transformer. The version shown in FIG. 7B is similar but it is unbalanced because output terminal  106 B is grounded. The embodiment of FIG. 7D is also unbalanced because both input terminal  108 A and output terminal  106 A are grounded. The embodiment of FIG. 7C is similar to the embodiment of FIG. 7D except that terminals  108 A and  106 B are tied together. Note that in the embodiments of FIGS. 7C,  7 E and  7 F, a connections is made between conductors  102  and  104  using vias at the specified locations in the transmission line. 
     The embodiments described above are bifilar, meaning that, regardless of how many transmission lines are present, they are connected together to form two conductors. Other embodiments according to this invention may include three or more separate conductors. 
     FIGS. 8-14 illustrate a trifilar transmission line element  401 , in which there are three conductors  402 ,  404  and  406 . Conductor  402  contains transmission lines  402 A,  402 B and  402 C; conductor  404  contains transmission lines  404 A,  404 B and  404 C; conductor  406  contains transmission lines  406 A,  406 B and  406 C. Transmission line element  10  is in the form of a rectangular spiral, although any other shape could also be used, and interconnects  420 ,  422 ,  424 ,  426 ,  428 ,  430 ,  432  and  434  between the transmission lines in each conductor are formed at periodic intervals around the spiral. The spiral runs from a first terminus  412  to a second terminus  414 , both of which are on the outside of the spiral, and the transmission lines run from the inside to the outside of the spiral in a crossover area  436 . 
     The arrangement of transmission lines  402 A- 402 C,  404 A- 404 C, and  406 A- 406 C is shown in FIG. 9, which is taken at cross-section  9 — 9  shown in FIG.  8 . Lines  402 C,  404 B and  406 A are formed in a bottom metal layer B, lines  402 B,  404 A and  406 C are formed in a middle metal layer M, and lines  402 A,  404 C and  406 B are formed in a top metal layer T. Bottom metal layer B and middle metal layer M are separated by a dielectric layer  405 , and middle metal layer M and top metal layer T are separated by a dielectric layer  403 . This stacked structure of metal lines and dielectric layers is fabricated using semiconductor processes well-known to those skilled in the art. 
     As FIG. 9 indicates, to maximize the broadside- and edge-coupling between the conductors, each transmission line is bounded above and/or below and laterally by transmission lines that are part of a different conductor. For example, transmission line  402 B is bounded above and below by transmission lines  406 B and  404 B, respectively, and on opposite sides by transmission lines  404 A and  406 C. This configuration also provides a more uniform distribution of the capacitance between the transmission lines and ground. 
     Interconnect  424  is shown in detail in FIGS. 10A-10C,  11  and  12 A- 12 C. FIGS. 12A-12C are cross-sectional views taken at sections  12 A— 12 A,  12 B— 12 B and  12 C— 12 C, respectively, shown in FIG.  11 . As shown in FIG. 12A, transmission lines  402 A,  402 B and  402 C are joined together by means of a via  408 E through dielectric layer  403  and a via  408 F through dielectric layer  405 . Tongues  402 W and  402 X extend laterally from lines  402 A and  402 B, respectively, to make the connection between lines  402 A and  402 B through via  408 E. Tongues  402 Y and  402 Z extend laterally from lines  402 B and  402 C, respectively, to make the connection between lines  402 B and  402 C through via  408 E. In this manner lines  402 A,  402 B and  402 C are joined together. 
     Similarly, transmission lines  404 A- 404 C and transmission lines  406 A- 406 C, respectively, are joined together as follows. As shown in FIG. 12B, lines  404 A,  404 B and  404 C are joined together by means of a via  408 A through dielectric layer  405  and a via  408 B through dielectric layers  403  and  405 . Tongues  404 W and  404 X extend laterally from lines  404 A and  404 B, respectively, to make the connection between lines  404 A and  404 B through via  408 A. Tongues  404 Y and  404 Z extend laterally from lines  404 B and  404 C, respectively, to make the connection between lines  404 B and  404 C through via  4084 . As shown in FIG. 12C, lines  406 A,  406 B and  406 C are joined together by means of a via  408 C through dielectric layer  403  and  405  and a via  408 D through dielectric layer  403 . Tongues  406 W and  406 X extend laterally from lines  406 A and  406 B, respectively, to make the connection between lines  406 A and  406 B through via  408 C. Tongues  406 Y and  406 Z extend laterally from lines  406 B and  406 C, respectively, to make the connection between lines  406 B and  406 C through via  408 D. 
     A top view of each metal layer in interconnect  424  is shown in FIGS. 10A,  10 B and  10 C. A composite top view of metal layers T, M and B is shown in FIG.  11 . 
     Referring again to FIG. 8, transmission line element  401  includes a crossover region  436  where the conductor pass from the inside of the spiral to terminus  414  on the outside of the spiral. Detailed views of crossover region  436  are shown in FIGS. 13 and 14. As indicated, the top metal layer T terminates at the dashed lines  438  and  440  and the middle and bottom metal layers M, B; The middle and bottom metal layers M, B terminate at the dashed line  442 , and thus metal layer T passes over metal layers M, B in the crossover region  426 . 
     FIG. 7G is a schematic circuit diagram of a trifilar balanced-unbalanced (balun) transmission line that may be constructed using the structure illustrated in FIGS. 8-14. Conductors  402 ,  404  and  406  are shown, along with the terminal  412  and  414 . The conductors  402 ,  404 ,  406  are connected together by vias at the locations indicated. 
     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.