Abstract:
In one example embodiment, a coplanar waveguide signal transition element transitions high-speed signals between vertically stacked coplanar waveguide transmission lines. The signal transition element comprises one or more dielectric layers and a plurality of electrically conductive vias extending through at least a portion of the one or more dielectric layers. The vias include one or more signal vias and one or more ground vias that are configured to transition signals between the vertically stacked coplanar waveguide transmission lines. The signal transition element also comprises a ground plane disposed within the one or more dielectric layers and electrically coupled to the one or more ground vias. The ground plane has one or more openings through which the one or more signal vias respectively pass.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to electrical interconnects in high-speed circuits. In particular, some example embodiments relate to vertical via interconnects between coplanar waveguide (CPWG) transmission lines in high-speed transponders. 
         [0003]    2. Related Technology 
         [0004]    Due to process technology limits and other design challenges, cheap and efficient packaging of components in high-speed circuits, such as high-speed transponders, is difficult. Bulky, expensive, interconnections are instead frequently relied on. Such interconnections include coaxial cable and microwave/radio frequency (RF) connectors, such as GPPO or V-connectors. In addition to their high cost and space consumption, such cables and connectors introduce complexity in component packaging. 
         [0005]    Coaxial cables and their associated connectors can be eliminated by using vertical high-speed interconnects, but not without introducing other design challenges. For example, typical vertical high-speed interconnects critically degrade performance by introducing transmission losses, reflection losses, electromagnetic interference, and reduced bandwidth, among other things. Relatively large pad pitches (e.g., 0.8 mm or more) is a typical design constraint for vertical high-speed interconnects in multi-layer surface-mounted packages, whereby correspondingly large losses in signal quality are introduced. Thus, no satisfactory technology exists for replacing coaxial cables and RF connectors with surface-mountable vertical interconnects in high-speed circuits. 
         [0006]    The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    In general, example embodiments of the invention relate to vertical high-speed interconnects for conveying electrical signals between coplanar waveguide transmission lines. The coplanar waveguide transmission lines may transmit signals between, for example, integrated circuits (ICs) and/or optoelectric circuits (OCs) and packages that include ICs and/or OCs. 
         [0008]    In one example embodiment, a coplanar waveguide signal transition element transitions high-speed signals between vertically stacked coplanar waveguide transmission lines. The signal transition element comprises one or more dielectric layers and a plurality of electrically conductive vias extending through at least a portion of the one or more dielectric layers. The vias include one or more signal vias and one or more ground vias that are configured to transition signals between the vertically stacked coplanar waveguide transmission lines. The signal transition element also comprises a ground plane disposed within the one or more dielectric layers and electrically coupled to the one or more ground vias. The ground plane has one or more openings through which the one or more signal vias respectively pass. 
         [0009]    The signal transition element configured with a ground plane having one or more openings overcomes many of the shortcomings of prior art vertical interconnects by mimicking conventional grounded CPWG transmission lines. Conventional grounded CPWG transmission lines are suitable for routing signals only in a planar surface. However, the proposed signal transition element is suitable for vertical transitions among different layers of CPWG transmission lines. For example, the proposed signal transition element can be employed in connecting one set of planar CPWG transmission lines in one layer of a package to another set of planar CPWG transmission lines in a different layer of the same or a different package. The one or more openings in the ground plane through which the one or more signal vias respectively pass provide smooth electromagnetic mode transitions from a set of planar CPWG transmission lines to the vertical signal vias. 
         [0010]    In another example embodiment, a circuit comprises a printed circuit board (PCB), a first set of coplanar waveguide transmission lines disposed on the PCB, a vertical transition component mounted on the PCB, a ground plane disposed within the vertical transition component, and an integrated circuit mounted on the vertical transition component so as to be in electrical contact with a second set of coplanar waveguide transmission lines. The vertical transition component has electrically conductive vias extending through at least a portion of the vertical transition component, the vias being configured to transition signals between the first set of coplanar waveguide transmission lines and the second set of coplanar waveguide transmission lines arranged in a plane separate from that of the first set of coplanar waveguide transmission lines. In addition, the ground plane is electrically coupled to a first set of one or more of the vias and has one or more openings through which a second set of one or more of the vias pass. 
         [0011]    Additional features of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    To further clarify the above and other features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0013]      FIG. 1  is a simplified block diagram of a high-speed transponder in which an embodiment of the invention may be used; 
           [0014]      FIG. 2A  is a perspective view of CPWG transmission lines for differential signals; 
           [0015]      FIG. 2B  is a perspective view of quasi-CPWG transmission lines for differential signals consistent with an embodiment of the invention; 
           [0016]      FIG. 3  is a top view of quasi-CPWG transmission lines for single-ended signals consistent with an embodiment of the invention; 
           [0017]      FIG. 4  is a perspective view of quasi-CPWG transmission lines with ground openings on an intermediate ground plane; 
           [0018]      FIG. 5  is a perspective view of the quasi-CPWG transmission lines of  FIG. 4  employed in a high-speed multi-layer IC package; and 
           [0019]      FIG. 6  is a plot of forward transmission (insertion loss) and reflection (return loss) characteristics of the quasi-CPWG transmission lines in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Reference will now be made to the figures wherein like structures will be provided with like reference designations. It is understood that the figures are diagrammatic and schematic representations of presently preferred embodiments of the invention, and are not limiting of the present invention, nor are they necessarily drawn to scale. 
         [0021]      FIGS. 1-6  disclose various aspects of some example embodiments of the invention. The embodiments described herein may provide, among other things, a space-efficient and inexpensive way to connect high-speed electrical signals between integrated circuits (ICs) and/or optoelectric circuits (OCs). The term “high-speed” as used herein refers to data rates of about 15 GHz or above. For example, the term “high-speed” as used herein encompasses a data rate of between about 40 GHz and 100 GHz. The high-speed electrical signals may be transferred between packages that include ICs and/or OCs via horizontal transmission lines on a printed circuit board (PCB) and via vertical interconnects and other connections disposed between packages and the horizontal PCB transmission lines. Vertical interconnects consistent with embodiments of the invention are also referred to herein as vertical vias or as quasi-CPWG transmission lines or vertical transition interconnects in a CPWG signal transition component or element because they mimic the function of horizontal CPWG transmission lines. 
         [0022]    Example embodiments of vertical interconnects disclosed herein are configured such that standard package configurations can be employed, obviating the need for specialized IC and OC packages commonly used in high-speed transponders, such as GPPO equipped packages. Additionally, example high-speed vertical interconnects disclosed herein are scalable such that high-speed data rates, such as 40 GHz, 100 GHz, or higher, can be accommodated. Thus, the example high-speed vertical interconnects disclosed herein can be employed to simplify the complexity of transponder design while enabling transfer of high-speed signals between the transponder&#39;s constituent packages. The example vertical interconnects disclosed herein are less expensive, and therefore have better market potential, than interconnects that employ relatively more expensive coaxial cable and GPPO or V-connectors. Some example vertical interconnects disclosed herein can also improve space efficiency within a high-speed transponder. 
         [0023]    With reference to  FIG. 1 , an example application in which vertical interconnects can be used to transfer high-speed signals between packages in a high-speed transponder  100  is disclosed. An OC package  102  interfaces with an IC package  104  via RF traces  106  in a PCB  108  and various intermediate connections. OC package  102  transmits and/or receives optical signals to/from an external circuit or device through a fiber  110  and transmits and/or receives high-speed electrical signals through intermediate connections  112 , which may be conductors in a flex circuit or leads designed for routing high-speed electrical signals to and from RF traces  106 . OC package  102  may integrate various optoelectronic components such as a laser, a photodiode, a transimpedance amplifier, a laser driver, etc. 
         [0024]    IC package  104  transmits and/or receives high-speed electrical signals to and/or from RF traces  106  through vertical interconnects  114  and a surface mount interface  116 . Surface mount interface  116  may be, for example, an array of solder joints such as a ball grid array (BGA), a pin grid array (PGA), a land grid array (LGA), or the like. IC package  104  may integrate various components such as a multiplexer/demultiplexer, a serializer/deserializer, and a clock and data recovery circuit, among other things. The vertical interconnects  114  can be implemented using aspects of quasi-CPWG transmission line technology, which mimics transmissions over horizontal CPWG transmission lines and is disclosed in more detail with reference to  FIGS. 2   b  and  3 - 6  below. 
         [0025]    With reference now to  FIG. 2A , an example set of CPWG transmission lines  200   a  for transmission of differential signals is disclosed. The set of CPWG transmission lines for differential signals  200   a  includes two signal traces  204   a  and  206   a , two side-ground traces  202   a  and  208   a , a ground plane  210   a , and a substrate  212   a . Signal traces  204   a ,  206   a , side-ground traces  202   a ,  208   a , and ground plane  210   a  may be composed of electrically conductive materials, while substrate  212  may be composed of a dielectric material. CPWG transmission lines  200   a  may be used to implement RF traces  106  in  FIG. 1  to route signals between OC package  102  and IC package  104 . 
         [0026]    With reference now to  FIG. 2   b , an example CPWG signal transition component or element  200   b  includes a set of quasi-CPWG transmission lines or vertical vias (or vertical interconnects) for transmission of differential signals. The vertical vias in CPWG signal transition component  200   b  include two signal vias  204   b  and  206   b , two side-ground vias  202   b  and  208   b , two back-ground vias  210   b  and  212   b , and a substrate  214   b . The vertical vias can be employed in a high-speed application as a vertical transition connecting a first set of transmission lines to a second set of transmission lines, for example, on first and second layers of a multi-layer package. Comparing the transmission lines  200   a  in  FIG. 2A  with the vertical vias of  FIG. 2   b , it can be seen that signal traces  204   a  and  206   a  in CPWG transmission lines  200   a  functionally correspond to signal vias  204   b  and  206   b  in CPWG signal transition component  200   b ; side-ground traces  202   a  and  208   a  functionally correspond to side-ground vias  202   b  and  206   b ; ground plane  210   a  functionally corresponds to back-ground vias  210   b  and  212   b ; and substrate  212   a  functionally corresponds to substrate  214   b . Therefore, the vertical vias of CPWG signal transition component  200   b  may be said to mimic the transmission function of transmission lines  200   a.    
         [0027]    The signal vias  204   b ,  206   b , and side-ground vias  202   b  and  206   b  are substantially aligned in a first y-z plane, while back-ground vias  210   b ,  212   b  are arranged in a second y-z plane offset from but parallel to the first y-z plane. Moreover, back-ground vias  210   b ,  212   b  may be disposed in the second y-z plane such that a distance between the ground via  210   b  and signal via  204   b  is minimized and a distance between ground via  212   b  and signal via  206   b  is minimized. Because the second plane is parallel to the first plane, the distance from back-ground via  210   b  to signal via  204   b  is equal to the distance from back-ground via  212   b  to signal via  206   b . In addition, these via to via distances may be equal to the distance between side-ground via  202   b  and signal via  204   b  and the distance between side-ground via  208   b  and signal via  206   b . The distance between signal vias  204   b  and  206   b  and the distance between back-ground vias  210   b  and  212   b  may also be equal to the other neighboring via distances. Thus, the distance between any two neighboring vias may be equal and may be minimized, within pad pitch design constraints, to preserve signal energy. 
         [0028]    Although the example embodiments shown in  FIGS. 2A and 2B  function to transmit differential signals, a single-ended version is also contemplated in which a single signal transmission line and a corresponding single signal via are implemented. For example, with reference now to  FIG. 3 , an example CPWG signal transition component for single-ended transmissions  300  includes quasi-CPWG transmission lines, i.e. vertical vias, for transmission of a single-ended signal. The single-ended vertical vias include a single signal via  304  and two side-ground vias  302  and  306 , arranged in a first plane, and a back-ground via  308  arranged in a second plane offset from the first plane. As with the neighboring via distances in  FIG. 2B , the distance between each neighboring pair of vertical vias in the single-ended embodiment of  FIG. 3  may also be equal. Moreover, although the diameters of all vias in  FIG. 3  are depicted as being equal, the diameters may vary. For example, each of side-ground vias  302 ,  306  and back-ground via  308  may have a first diameter while signal via  304  may have a second diameter. Similarly, with respect to the vias in  FIG. 2B , each of the side-ground vias  202   b ,  208   b , and back-ground vias  210   b ,  212   b  may have a first diameter, while differential signal vias  204   b ,  206   b  may have a second diameter. Each of the via diameters may be selected so as to optimize efficiency of signal transmission using, e.g., standard optimization techniques. 
         [0029]    The vertical via for single-ended signals mimics a partially grounded conventional planar CPWG transmission line for single-ended signals. The vertical vias for single-ended signals can be employed in a high-speed application as a vertical transition connecting a first set of singled-ended CPWG transmission lines to a second set of single-ended CPWG transmission lines. The first set and second set of single-ended CPWG transmission lines can be arranged, for example, on first and second layers of a multi-layer package. 
         [0030]    With reference now to  FIG. 4 , a perspective view depicts a CPWG signal transition component  400  with an intermediate ground plane  406  disposed within a dielectric substrate material and ground cutouts or openings  408  and  410  on intermediate ground plane  406 . CPWG signal transition component  400  has vertical vias corresponding to those of CPWG signal transition component  200   b  in  FIG. 2B . Ground openings  408 ,  410  are formed around vertical vias corresponding to signal vias  204   b ,  206   b  of CPWG signal transition component  200   b  in  FIG. 2B . Signal vias  204   b ,  206   b  extend through ground openings  408 ,  410  to CPWG transmission lines  402  and  404  disposed on a top surface of CPWG signal transition component  400 . The other vias (ground vias  202   b ,  208   b ,  210   b , and  212   b ), on the other hand, do not extend through intermediate ground plane  406 , but instead are electrically coupled to intermediate ground plane  406 . Moreover, intermediate ground plane  406  is parallel to the top surface of CPWG signal transition component  400  and serves as a ground plane for transmission of signals along CPWG transmission lines  402  and  404 . According to one embodiment, intermediate ground plan  406  is separated from the top surface of CPWG signal transition component  400  by a dielectric layer that is six mils thick. 
         [0031]    Although ground openings  408 ,  410  are depicted as half-circles, the shape of one or both may vary. For example, the shape of ground openings  408 ,  410  may be ovoid or polygonal (e.g., having multiple sides corresponding to half of a regular polygon, such as a rectangle, hexagon, octagon, etc., or corresponding to irregular polygonal shapes having, e.g., jagged sides of equal or unequal lengths). The shape and dimensions of ground openings  408 ,  410  may be selected so as to optimize smoothness of mode transition from horizontal planar transmission to vertical transmission using, e.g., standard optimization techniques. 
         [0032]    The dielectric material in CPWG signal transition component  400  may be a substantially monolithic dielectric element or, as in one example embodiment, may comprise one or more high temperature co-fired ceramic (HTCC) layers. For example, a first HTCC layer may be disposed between intermediate ground plane  406  and CPWG transmission lines  402  and  404 . One or more additional HTCC layers may be disposed below intermediate ground plane  406 . The HTCC layers may incorporate other vertical vias (not shown), as well as horizontally disposed signal traces (not shown) to provide interconnections with other components and terminals in integrated circuit package  104 . 
         [0033]      FIG. 5  is a perspective view of the example CPWG signal transition component  400  of  FIG. 4  integrated with other components in an example high-speed multi-layer integrated circuit package  500 . As disclosed in  FIG. 5 , one end of CPWG signal transition component  400  is connected to a first set of CPWG transmission lines  402  and  404  disposed on the top surface of CPWG signal transition component  400 . The other end of CPWG signal transition component  400  is connected to a second set of CPWG transmission lines  502  on another layer (e.g., PCB layer  504 ) via BGA joints  506  (or another surface mount interface, such as PGA or LGA joints). A distance between the first set of CPWG transmission lines  402  and  404  may be tapered (as shown) or widened to interface with other surface-mountable components mounted thereto, which may have a narrower (as shown) or wider pad pitch. Alternatively or in addition, the first set of CPWG transmission lines  402  and  404  may interface with a third set of CPWG transmission lines (not shown) through another CPWG signal transition component (not shown) stacked above CPWG signal transition component  400 . In addition, multi-layer package  500  may have multiple layers. In one embodiment, package  500  has six HTCC layers, for example. However, it is contemplated that the example vertical transition interconnects disclosed herein may also be implemented in multi-layer packages having less than or more than six layers. 
         [0034]      FIG. 6  is a plot  600  showing the forward transmission (insertion loss S 21 ) and reflection (return loss S 11 ) characteristics of the quasi-CPWG transmission lines for differential signals in  FIG. 5 . As disclosed in  FIG. 6 , the example quasi-CPWG transmission line has a bandwidth up to 45 GHz. 
         [0035]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.