Patent Publication Number: US-2018048044-A1

Title: High-density stacked grounded coplanar waveguides

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 14/864,679, filed Sep. 24, 2015. 
    
    
     TECHNICAL FIELD 
     This application relates to waveguides, and more particularly to a two-layer stacked grounded coplanar waveguides. 
     BACKGROUND 
     It is conventional to use grounded coplanar waveguides (GCPWs) for signal routing in a millimeter wave circuit board for signal frequencies of 28 GHz or higher. An example GCPW  100  is shown in  FIG. 1 . An upper-most metal layer M 1  is patterned to include a signal trace  105  and a surrounding upper ground plane  110 . An adjacent metal layer M 2  forms a lower ground plane  120 . The electrical properties for GCPW  100  depends on a number of factors including the separation between the metal layers M 1  and M 2 , the gaps between signal trace  105  and upper ground plane  110 , and the width of signal trace  105  as known in the GCPW arts. Metal layer M 1  can support additional signal traces for additional GCPWs (not illustrated) so long as there is no intersection of the resulting signal traces. 
     As the number of signal traces increases, it becomes increasingly difficult to route all the signal traces onto metal layer M 1  such that a stacked GCPW architecture is used, which requires additional metal layers. The metal layers are formed in a substrate such as an organic circuit package substrate that uses a central pre-impregnated (prepreg) layer to provide sufficient rigidity. The inclusion of the prepreg layer complicate the resulting stacking of GCPWs. For example, a conventional substrate  200  is shown in  FIG. 2  that includes a prepreg layer  230 . An upper core (dielectric layer)  226  lies between an upper-most metal layer M 1  and a lower metal layer M 2 . A lower core (dielectric layer)  227  lies between an lower-most metal layer M 4  and an adjacent metal layer M 3 . Each core and its corresponding metal layers are separately patterned to form a corresponding GCPW. For example, metal layer M 1  on upper core  226  may be patterned into a signal trace  210  and an upper ground plane  215  for an upper GCPW  211 . Metal layer M 2  forms a lower ground plane  220  for GCPW  211 . Similarly, metal layer M 4  may be patterned into a signal trace  235  and an upper ground plane  240  for a GCPW  205 . Metal layer M 3  forms a lower ground plane  245  for GCPW  205 . 
     After formation of cores  226  and  227  and their corresponding metal layers M 1  through M 4 , the completed cores may then be laminated onto either side of prepeg layer  230 . A ground source (not illustrated) may then be coupled to ground plane  215  to provide the desired ground to GCPW  211 . Core  226  may include a plurality of vias  225  to couple ground to lower ground plane  220 . It would be convenient to use a plurality of vias  250  to couple the same ground source to ground planes  245  and  240  for GCPW  205 . But vias  250  are not allowed through prepreg layer  230  due to the lamination of cores  226  and  227  as discussed above. 
     An realizable construction of a conventional GCPW stack may be better appreciated through a consideration of GCPW stack  300  shown in  FIG. 3 . An upper core  301  is configured with a metal layer M 1  and a second metal layer M 2 . Metal layer M 1  is patterned into a signal trace  315  and an upper ground plane  320  for a first GCPW  305 . Metal layer M 2  forms a lower ground plane  325  for first GCPW  305 . Vias  340  through upper core  301  couple ground planes  320  and  325  together. Similarly, a lower core  302  and its metal layers M 3  and M 4  are configured to form a second GCPW  301 . In particular, metal layer M 4  is patterned to form a signal trace  330  and an upper ground plane  335  for second GCPW  310 . Metal layer M 3  forms a lower ground plane  350  for second GCPW  310 . A set of vias  345  extending through lower core  302  couple ground planes  335  and  350  together. The completed cores  302  and  301  may then be laminated onto prepreg layer  230 . But note that a ground source (not illustrated) would then be needed to couple to ground plane  320  to provide ground to first GCPW  305  while a second ground source (not illustrated) would be needed to couple to ground plane  335  to provide ground to second GCPW  310 . Such a coupling to ground from both sides of GCPW stack  300  is awkward. Since vias from M 2  to M 4  or from M 3  to M 1  are not allowed or very impractical due to the lamination onto prepreg layer  230 , a laser or mechanical drill may thus be used to form a through-hole via (not illustrated) through ground planes  320 ,  325 ,  350 , and  335  that may then be plated to couple ground planes  320 ,  325 ,  350 , and  335  to a common ground. Since this ground via must penetrate through all four metal layers, it must be relatively thick, which lowers density. In addition, note that all four metal layers are used to form GCPW stack  300 . The routing of additional signals besides those propagated by GCPWs  305  and  310  is thus hindered by the occupation of all four metal layers by GCPW stack  300 . 
     Accordingly, there is a need in the art for stacked GCPWs with improved density and enhanced signal routing. 
     SUMMARY 
     A pair of stacked ground coplanar waveguides (GCPWs) is provided in two consecutive metal layers that are deposited on opposing surfaces of a dielectric layer. A first metal layer on a first side of the dielectric layer forms a first signal trace and an upper ground plane for a first GCPW in the pair. Similarly, a second metal layer on a second surface of the dielectric layer forms a second signal trace and an upper ground plane for a second GCPW in the pair. The upper ground plane for the first GCPW also functions as the lower ground plane for the second GCPW. Similarly, the upper ground plane for the second GCPW also functions as the lower ground plane for the first GCPW. 
     The resulting combination of the dielectric layer and the patterned first and second metal layers is readily laminated onto, for example, a pre-impregnated layer to form a millimeter wave circuit board for millimeter wave applications. The resulting millimeter wave circuit board advantageously offers enhanced signal routing in that just two consecutive metal layers are used to form the pair of stacked GCPWs. Additional metal layers in the millimeter wave circuit board may thus be dedicated to other purposes. Moreover, a ground connection to the upper ground plane for the first GCPW may be readily coupled through a plurality of vias extending through the dielectric layer to also ground the upper ground plane for the second GCPW. In this fashion, the grounding of the stacked GCPWs does not require any through-hole vias through the pre-impregnated layer, which enhances density. 
     These and other advantageous features may be better appreciated through the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is cross-sectional view of a conventional grounded coplanar waveguide (GCPW). 
         FIG. 2  is a cross-sectional view of a conventional pair of stacked GCPWs in a four-metal-layer substrate with a central pre-impregnated layer highlighted to show a forbidden via formation through the pre-impregnated layer. 
         FIG. 3  is a cross-sectional view of a conventional pair of stacked GPCWs in a four-metal-layer substrate with a central pre-impregnated layer without any forbidden vias. 
         FIG. 4  is a cross-sectional view of a pair of stacked GCPWs formed using two consecutive metal layers in a substrate including a central pre-impregnated layer, wherein the GCPWs in the stack are configured such that their corresponding signals are substantially de-coupled in accordance with an aspect of the disclosure. 
         FIG. 5  is a cross-sectional and perspective view of a pair of stacked GCPWs formed using two metal layers in a substrate having a central pre-impregnated layer, wherein the GCPWs in the stack are configured such that their corresponding signals are substantially coupled in accordance with an aspect of the disclosure. 
         FIG. 6  is a partially cutaway plan view of a pair of stacked GCPWs formed using two consecutive metal layers in which the signal trace for a first GCPW in the stack longitudinally extends at a right angle to a longitudinal axis for a signal trace in a second GCPW in the stack. 
         FIG. 7  is a perspective view of a circuit board including a pair of stacked GCPWs formed using two consecutive metal layers coupled to a radio frequency integrated circuit (RFIC) and a patch antenna in accordance with an aspect of the disclosure. 
         FIG. 8  is a flowchart for a method of coupling a first signal propagating in a first GCPW formed in consecutive two-metal-layer stack with a second signal propagating in a second GCPW formed in the consecutive two-metal-layer stack in accordance with an aspect of the disclosure. 
     
    
    
     Implementations of the present disclosure and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION 
     Two consecutive metal layers are configured to form two or more stacked grounded coplanar waveguides (GCPWs) to increase density and provide improved signal routing. As used herein, two metal layers are deemed to be consecutive if no other metal layers intervene between the two metal layers. A first one of the metal layers is patterned to form a signal trace and an upper ground plane for a first GCPW. The upper ground plane for the first GCPW also functions as a lower ground plane for a second GCPW. The remaining second metal layer is patterned to form a signal trace for the second GCPW and an upper ground plane for the second GCPW. The upper ground plane for the second GCPW also functions as the lower ground plane for the first GCPW. In that regard, note that “upper” and “lower” with respect to ground planes are defined herein with regard to a particular GCPW. What is an upper ground plane from one GCPW in a stack formed in two consecutive metal layers is the lower ground plane for the remaining GCPW in the stack. 
     An example GCPW stack  400  is shown in  FIG. 4 . The two consecutive metal layers are an upper-most metal layer M 1  and an adjacent metal layer M 2  that sandwich an upper core dielectric layer  401 . Metal layer M 1  is patterned such as through photolithography or other suitable techniques to form a signal trace  415  and to form an upper ground plane  420  for a first GCPW  405 . Upper ground plane  420  also forms the lower ground plane for a second GCPW  410 . Metal layer M 2  is patterned such as through photolithography or other suitable techniques to form a signal trace  430  for second GCPW  410  and to form an upper ground plane  435  for second GCPW  410 . Upper ground plane  435  also forms a lower ground plane for first GCPW  405 . A plurality of vias  436  couple from ground plane  420  to ground plane  435  on either side of signal trace  415  in first GCPW  405 . Similarly, a plurality of vias  436  couple from ground plane  420  to ground plane  435  on either side of signal trace  430 . Although  FIG. 4  is a cross-sectional view, note that signal traces  415  and  430  are extending longitudinally in the same direction. Signal trace  415  thus does not cross over signal trace  430 . Similarly, signal trace  430  does not cross under signal trace  415 . Vias  436  on a first side of signal trace  415  in GCPW  405  are arranged in a series that extends longitudinally with signal trace  415  to form a “via wall” as will be further explained herein. Similarly, vias  436  on a remaining second side of signal trace  415  in GCPW  405  are arranged in a similar via wall. Signal vias  436  on either side of signal trace  430  in GCPW  410  are arranged into a similar pair of via walls that sandwich signal trace  430 . The resulting grounded via walls form a very strong isolation between a signal propagated through GCPW  405  and any signal propagated (or not) through GCPW  410  since signal trace  415  does not cross over signal trace  430 . This isolation is reciprocal in that should there be a signal propagated through GCPW  410 , it too will be strongly isolated from coupling into GCPW  405 . In one implementation, vias  436  may be deemed to comprise means for coupling upper ground plane  420  for the first GCPW  405  to an upper ground plane  436  for the second GCPW  410 . 
     The resulting patterned core layer  401  and its GCPWs  405  and  410  may be laminated onto a first surface of prepreg layer  403 . Metal layer M 2  is thus fused or adhered onto the first surface of prepreg layer  403 . At the same time or in a separate manufacturing step, another dielectric core layer  402  and its metal layers M 3  and M 4  may be similarly laminated onto an opposing second surface of prepreg layer  403  such that metal layer M 3  fuses or adheres to the second surface of prepreg layer  403 . Note that metal layers M 3  and M 4  may be patterned (not illustrated) to support other signals independently from the routing of signals through GCPWs  405  and  410 . In this fashion, signal routing flexibility is enhanced. In addition, no through-hole via is necessary to ground metal layers M 1 , M 2 , M 3 , and M 4  together since one or more ground contacts (not illustrated) coupled to ground plane  420  is sufficient to provide ground to both GCPWs  405  and  410 . 
     In an alternative implementation, a GCPW stack  500  as shown in  FIG. 5  is configured such that a signal propagating through a first GCPW  501  will strongly couple into a second GCPW  505 . This coupling may be reciprocal such that a signal propagating through GCPW  505  will also strongly couple into GCPW  501 . GCPWs  501  and  505  are formed in a first metal layer M 1  and a consecutive metal layer M 2  that sandwich a core dielectric layer  503 . Metal layer M 1  is patterned to form a signal trace  510  and an upper ground plane  515  for GCPW  501 . Upper ground plane  515  also functions as a lower ground plane for GCPW  505 . Metal layer M 2  is patterned to form a signal trace  530  and an upper ground plane  520  for GCPW  505 . Upper ground plane  520  for GCPW  505  also functions as the lower ground plane for GCPW  501 . 
     In contrast to GCPW stack  400  of  FIG. 4 , signal trace  510  of GCPW  501  overlays signal trace  530 . Both signal traces  510  and  530  extend longitudinally in the same direction such that signal trace  510  completely overlays signal trace  530  along its entire longitudinal extent. Given this complete overlay of signal trace  510  onto signal trace  530 , a plurality of vias  525  extending through core layer  503  from ground plane  515  to ground plane  520  form a pair of vias walls that are shared by both GCPWs  501  and  505 . In particular, a first set of vias  525  form a first via wall  540  on a first side of signal traces  510  and  530 . A second set of vias  525  form a second via wall  545  on an opposing second side of signal traces  510  and  530 . There are thus no via walls in GCPW stack  500  that isolate GCPW  501  from GCPW  505 . This lack of isolation and the overlay of signal trace  510  over signal trace  530  causes a signal propagated through GCPW  501  to couple relatively strongly into GCPW  505 . Similarly, a signal propagated through GCPW  505  will strongly couple into GCPW  501 . 
     Core  503  with its vias  525  and its patterned metal layers M 1  and M 2  may then be laminated onto a first surface of a prepreg layer  550 . Another core layer  504  sandwiched by metal layers M 3  and M 4  may also be laminated onto an opposing second surface of prepreg layer  550 . Prior to this lamination, metal layers M 3  and M 4  may be patterned as desired to carry signals besides those propagated through GCPWs  501  and  505 . In addition, a ground contact (not illustrated) may supply ground to GCPWs  501  and  505  through a contact to first upper ground plane  515  without the need for any through-hole vias through prepreg layer  550 . 
     GCPW stacks  400  and  500  of  FIGS. 4 and 5  represent two extremes: relatively strong isolation between GCPWs  405  and  410  in stack  400  versus relatively little isolation between GCPWs  501  and  505  in stack  500 . In stack  400 , signal trace  415  never overlays signal trace  430  so that the resulting via walls formed by vias  436  provide strong isolation between GCPWs  405  and  410 . Conversely, signal trace  510  completely overlays signal trace  530  so that vias walls  540  and  545  are shared and provide relatively little isolation. Given these two extremes, a moderate amount of coupling from one GCPW to another in a stack may be accomplished by varying the degree of overlay. For example, a signal trace  605  for an upper GCPW shown in  FIG. 6  crosses a signal trace  610  for an underlying GCPW at a 90 degree angle. In contrast, the overlay for signal trace  510  onto signal trace  530  in stack  500  may be deemed to be a zero degree overlay. The 90 degree crossing for signal trace  605  over signal trace  610  thus presents a reduced cross-over area  615  in which signal trace  605  overlays signal trace  610 . By varying the angle at which one signal trace overlays another in a pair of stacked GCPWs, a circuit designer may vary the coupling between the upper and lower GCPWs in the stack accordingly. With regard to signal trace  605 , the 90 degree crossing over signal trace  610  produces a moderate amount of coupling that would have a magnitude in between the extremes of GCPW stacks  400  and  500 . If the longitudinal axis of signal trace  605  were made to be more and more parallel to the longitudinal axis of signal trace  610  while signal trace  605  continues to overlay signal trace  610 , cross-over area  615  would continue to grow so as to produce more and more signal coupling. At the extreme of a zero degree crossing angle, cross-over area  615  becomes identical to the surface area of either signal trace  610  and  605  (assuming they have the same widths). By thus varying the cross-over area of one signal trace over another in a GCPW stack, a circuit designer may provide a desired amount of signal coupling between the corresponding GCPWs. For example, a bandpass filter may require a certain amount of coupling between GCPWs whereas a built-in-self test (BIST) may require another amount of coupling. In that regard, the formation of a pair of stacked GCPWs into two consecutive metal layers as disclosed herein provides a compact and convenient structure for BIST operation. During a BIST mode, a BIST signal may be driven into one of the GPCPWs in the stack. Depending upon the cross-over area, the BIST signal will then couple into the remaining GCPW in the stack so that it may be used to confirm desired functionality of the tested system. 
     The GCPW stacks in two consecutive metal layers as disclosed herein may be advantageously applied in a millimeter-wave circuit board including an RFIC. For example, a millimeter-wave circuit board  700  shown in  FIG. 7  includes an RFIC  705  mounted on an upper-most metal layer M 1 . Metal layer M 1  may be patterned into a plurality of conventional traces  710  through which RFIC  705  may drive a corresponding plurality of digital signals. In addition, metal layer M 1  may be patterned to form a signal trace  725  and an upper ground plane for an upper GCPW in a stack that includes a signal trace  765  patterned into metal layer M 2  for a lower GCPW. A lower ground plane  745  formed in metal layer M 2  for the upper GCPW having signal trace  765  also functions as the upper ground plane for the lower GCPW including signal trace  765 . In this configuration, signal trace  725  crosses signal trace  765  at a right angle to introduce a limited amount of coupling between signal traces  725  and  765 . Signal trace  725  couples to a through-hole via  735  that extends through metal layer M 2  to a patch antenna  740  formed in a bottom-most metal layer M 3 . A prepreg layer (not illustrated) may intervene between metal layers M 2  and M 3  such that circuit board  700  includes three metal layers. Rather than use a via  735  to drive patch antenna  740 , signal trace  725  could also indirectly couple to patch antenna  740  through an aperture (not illustrated) in metal layer M 2 . A fourth metal layer (or even additional metal layers) may be included in circuit board  700  in an alternative implementations. Another GCPW signal trace  715  in metal layer M 1  may cross over another GCPW signal trace  760  in metal layer M 2  at right angles to again introduce a limited amount of coupling between the signals propagated in traces  715  and  760 . 
     A method of operating a GCPW stack formed in two consecutive metal layers in accordance with an aspect of the disclosure will now be discussed with regard to the flowchart of  FIG. 8 . The method includes an act  800  of driving a first signal through a first signal trace in a first metal layer for a grounded coplanar waveguide (GCPW) having a first ground plane formed in a consecutive second metal layer. An example of act  800  comprises driving a signal through signal trace  510  of GCPW stack  500  in  FIG. 5  or through signal trace  605  of  FIG. 6 . The method also includes an act  805  of driving a second signal through a second signal trace in the second metal layer for a second GCPW having a second ground plane formed in the first metal layer, wherein the first signal trace crosses over the second signal trace in a cross-over area for the first signal trace and the second signal trace. An example of act  805  comprises driving a signal into signal trace  530  of  FIG. 5  or into signal trace  610  of  FIG. 6 . Finally, the method includes an act  810  of coupling the first signal into the second signal responsive to a size for the cross-over area. The large cross-over area for GCPW stack  500  that leads to a large signal coupling as well as the reduced cross-over area  615  of  FIG. 6  that leads to a reduced signal coupling are examples of act  810 . 
     As those of some skill in this art will by now appreciate and depending on the particular application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of use of the devices of the present disclosure without departing from the scope thereof. In light of this, the scope of the present disclosure should not be limited to that of the particular embodiments illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.