Abstract:
Signal line conductors passing through vertical vias in an insulative substrate for supporting and interconnecting integrated circuit chips are provided with shielding conductors in adjacent vias that link respective power and ground planes. The shielding conductors&#39; presence in positions around a signal via is made possible through the employment of power plane and ground plane conductive grids that are laid out in rhomboid patterns. The power plane and ground plane grids possess a left-right mirror relation to one another and are displaced to place the rhomboid&#39;s corners to avoid overlapping any of the grid lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 11/610,082, filed Dec. 13, 2006, entitled “Ceramic Substrate Grid Structure for the Creation of Virtual Coax Arrangement”, by Becker et al., the entirety of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates in general to substrates to which integrated circuit chips are affixed so as to provide interconnectivity between the chips and with other circuits and systems external to the substrate. More particularly, the present invention is directed to a to a configuration of ground plane and power plane conductors which reduce cross coupling, lowers losses and permits higher frequency operation. Even more particularly, the present invention is directed to a configuration of conductive via paths which act essentially as a coaxial shielding arrangement. (A “via” is a vertical path in the substrate through which conductive material therein carries electrical power and signals.) It is also noted that the present invention is particularly useful with glass ceramic substrates; however, the conductive configurations of the present invention are employable in conjunction with any insulative substrate material including polymeric materials. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is well known that with the continuing shrinkage of electronic circuit components, there is a concomitant need for operation at higher frequencies. At these higher frequencies, cross coupling between physically adjacent conductive paths becomes a greater problem. It is expected that chip-to-chip interconnections will require a one gigabit per year increase in the data rate. To achieve this goal it is desirable to further improve packaging structures in the first and second levels of packaging in order to support advanced circuit designs. This means that the losses and coupled noise attributes of the interconnect systems should be reduced relative to the current design. For glass ceramic MCMs (MultiChip Modules) the signal line losses are virtually zero. However, this makes the contribution of the coupled noise, to support the higher data rates, even more pronounced. The severity of this problem is highlighted in the Apr. 11, 2005 issue of EE Times. 
       SUMMARY OF THE INVENTION 
       [0004]    In the present invention, the coupled noise for the x, y and z interconnects is controlled by reducing signal density by adding EM (Electromagnetic) shielding to the traces (that is, to the conductive paths). In the present invention a manufacturable geodetic approach is employed as a solution of the problem. The structure of the present invention reduces the coupled noise interaction for both the vertical and the x-y plane interconnects by a factor of from four to six, while at the same time minimizing wirability problems. 
         [0005]    The solution proposed herein creates a virtual coax (that is, coaxial conductor) arrangement for the vertical signal interconnections and by doing so it allows their operation at a data rate that is two to three times higher than existing technology. 
         [0006]    A central aspect of the present invention is the replacement of the usual orthogonal grid array currently used for the power supply planes of glass ceramic substrate modules by a geodetic structure of equilateral triangles implemented trough the use of rhomboid shapes so that the manufacturing complexity is not increased. Specific spacing of these shapes allows their construction within the current ground rules of ceramic technology. The displacement of the rhombus shapes among the different planes minimizes the loss of wiring density, while at the same time it reduces the coupled noise by a factor of four. In addition, calculations indicate that the proposed structure reduces the average interconnect latency by 16.6%. 
         [0007]    In accordance with one aspect of the present invention a structure for providing electrical interconnection for integrated circuit chips comprises an insulative substrate wherein at least one conductive layer within the substrate (say a ground layer) has two sets of parallel conductors crossing each other in a substantially rhomboid shaped pattern. A second conductive layer within the substrate is patterned in substantially the same way but in a mirror image patter. Nonetheless, it too possesses a substantially rhomboid shaped pattern. The second layer is displaced horizontally from the first layer. One or two signal layers are disposed between the ground plane and power plane layers. 
         [0008]    Furthermore, it is noted that while the description herein focuses upon a situation in which there are only three or four layers, in practice such substrates include many tens of layers, with 30 layers being typical for the ones contemplated herein. When reference is made herein to a “vertical” direction, it should be understood that this is a relative term referring to a direction from one conductive plane in the substrate to another. It is also understood that while the present invention is best used with thicker glass or glass ceramic substrates, the advantages obtained apply also to polymeric substrates. 
         [0009]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. 
         [0010]    The recitation herein of a list of desirable objects which are met by various embodiments of the present invention is not meant to imply or suggest that any or all of these objects are present as essential features, either individually or collectively, in the most general embodiment of the present invention or in any of its more specific embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which: 
           [0012]      FIG. 1  is a top view of the desired grid line structure for one of the conductive power planes in a substrate to which integrated circuit chips are affixed; 
           [0013]      FIG. 2  is a view similar to  FIG. 1  but now showing the relation between the desired grid structure as between the ground plane and the power plane. 
           [0014]      FIG. 3  is a view similar to  FIG. 2  except that signal lines are now included and are located in two planes between the power planes shown in  FIG. 2 ; 
           [0015]      FIG. 4  is a view similar to  FIG. 3  but now illustrating the presence of conductive structures that provide a virtual coaxial shielding configuration for the signal path coming from the bottom or top of the packaging structure; 
           [0016]      FIG. 5  is a view which shows, in an enlarged fashion, a portion of a desired coaxial shielding structure to demonstrate the placement of the vertical power connections for the ground and supply voltage planes; 
           [0017]      FIG. 6  is a view similar to  FIG. 2  but which more particularly illustrates the grid line dimensioning parameters as a function of the technology parameter dimension, a; 
           [0018]      FIG. 7 , in contrast to the top views above, is a side elevation view illustrating the relations amongst a representative sample of the ground, signal and power planes wherein one or two planes carrying signal lines are sandwiched between two power planes but it is note that, for higher signal frequencies, only one signal plane is used; 
           [0019]      FIG. 8  is an isometric three dimensional view illustrating the coaxial shielding structure; and 
           [0020]      FIG. 9  is a side, projection view of a portion of the substrate illustrated in  FIG. 8  which provides a slightly different view and which seeks to emphasize, as much as possible, the coaxial shielding aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  is a top view of the patterns of conductive lines in one of the layers of a substrate used for providing the ground voltage to the interconnected circuit chips. For illustrative purposes only, this particular layer is referred to a ground plane or ground plane layer. Typically, one of these substrates includes a plurality of layers having various patterns of conductors. In conventional substrate designs these conductors are arranged in orthogonal patterns. In such substrates there are typically three different kinds of layers: ground plane layers, power plane layers and signal layers. The signal layers are typically found between a ground plane layer and a power plane layer. However, there is no hard and fast rule specifying the number of signal layers present. 
         [0022]    For purposes of illustration, it is assumed that the layer shown in  FIG. 1  is a ground plane layer and it is designated by reference numeral  100 . It is a significant feature of the present invention is that the conductive patterns present in  FIG. 1  are configured to produce rhomboid shaped regions between the conductors. At the intersections of the conductive lines shown in  FIG. 1 , the circular dots represent the presence of a via opening to a conductive pattern in another layer of the substrate. It is not a requirement of the present invention that all of these vias are in fact occupied by conductive material, which provides an electrically conductive path between layers. It should be appreciated that in any given substrate there may be present a plurality of ground plane layers and a plurality of power plane layers. The vias are employed to electrically connect the ground planes in each different layer. The same is true for power plane layers, as is seen in the discussion below. 
         [0023]      FIG. 2  illustrates the presence of a second layer of conductive patterns. For explanatory purposes only, it is assumed that this second layer ( 200 ) is a power plane layer. Just as with ground plane layers, typical substrates include a plurality of power plane layers as a mechanism for distributing power to various ones of the integrated circuit chips connected to an upper or lower surface of the substrate. Likewise, ground level voltage potentials are provided throughout the substrate by means of vias which connect ground planes in different layers at the bottom of the substrate in question. 
         [0024]    It is particularly noted that the conductive patterns shown in  FIG. 2  for a power plane layer are also disposed in a fashion in which the conductors form rhomboid shaped areas, as in  FIG. 1 . However, it is important to note that, for the second layer the conductive patterns forming the power plane include conductors which are essentially disposed in a mirror image fashion as compared to the patterns shown in  FIG. 1 . It is also important to note that, as between the patterns in the two layers there is a displacement that exists. For example, it is noted that the corner vertices of each of the rhombus patterns in  FIG. 2  lies at a point which is not directly above the vertex of the conductive patterns for the ground plane below it. Furthermore, just for the sake of clarity, it is worthy to note that the references to “up” and “down” are merely relative and are employed herein only for the sake of convenience. 
         [0025]    Throughout the discussion herein the ground plane is indicated by a plurality of conductors shown as dashed lines. In contrast power plane conductors are shown as solid lines. In typical conditions, the voltage V DD  is found to be present on the power plane conductors. 
         [0026]    It is also noted that the conductive patterns present in the power plane layer also include vias at the vertices of the rhombus patterns. As with the ground plane structure, these vias are employed to provide power to different layers within the substrate. As indicated above, substrates typically employ a plurality of such layers. However it is noted that in order to appreciate and understand the structure and operation of the present invention, it is sufficient to describe the structure present in only three or four layers. 
         [0027]      FIG. 3  illustrates the placement of signal paths  301 ,  302  and  302  in the substrate structures of the present invention. Though not evident from the illustration in  FIG. 3 , the signal lines shown are present in two signal layers that exists between power plane layer  200  and a ground plane layer  100 . For example, see  FIG. 7 . Furthermore, it is noted that the advantages provided by the present invention are in fact best illustrated by assuming that the signal lines, S 1 , S 2  and S 3  ( 301 ,  302  and  302 ) shown in  FIG. 3  lie in two or three different layers. For example, it is not uncommon for several signal layers to be present between a ground plane layer and a power plane layer. 
         [0028]    When one employs the pattern of conductive lines, as shown in  FIGS. 2 and 3 , certain advantages are gained. In particular, it is noted that the routing of signal conductors within the signal planes can be carried out using more direct routes, thus shortening the signal path. A shortened signal path has two significant advantages: lower losses and the ability to operate at a higher circuit speed. 
         [0029]    In addition to the advantages provided solely by the use of the rhomboid patterns shown in  FIGS. 1 through 4 , there is an additional advantage that accrues with respect to the vertical via connections. Conductors through the vias connecting respective ones of the power and ground planes together provide a virtual coaxial shielding arrangement for the vertical part of the signal paths. This is illustrated in  FIG. 4  and is shown in even more detail in  FIG. 5 . Before discussing  FIG. 5 , however, it is particularly noted that vertical signal paths  310  and  315  are shown as open circles in  FIG. 4 . In addition, conductive pattern  400 , which is substantially hexagonal in shape (although any repeatable shape is employable), is employed in the power plane and in the ground plane layers to provide additional structure to produce a the coaxial shielding configuration. It should be noted that since  FIG. 4  is a top view the pattern of  400 ,  FIG. 4  is meant to suggest the pattern that is visible in the upper layer which in this case is power plane  200 . 
         [0030]    In order to better understand the virtual coaxial structure provided by the present invention,  FIG. 5  is presented as an enlargement of a portion of the structure shown in  FIG. 4 . In particular is noted that a single signal line is shown as being present. There are vias connecting the ground planes and there are separate vias through which the power planes are connected. The ground plane vias are shown as open circles in  FIG. 5  and are designated by reference numerals  110 ,  112  and  114 . Likewise, power plane  200  is connected to other power plane levels by means of vias  210 ,  212  and  214 . As in  FIG. 4  conductive structure  400 , as shown, represents a structure that is present in any one of the ground or power planes (or at least in the ones through which an effectively shielded signal line passes). In preferred embodiments of the present invention, (hexagonal) conductive structure  400 , visible in  FIGS. 4 and 5  only in power plane  200 , is present in other ones of the ground or power planes, as needed or desired. This conductive pattern surrounds signal line via  315  to whatever extent necessary with respect to its vertical passage through the insulative substrate. 
         [0031]    One of the advantages of the present invention, is that it is scalable. In particular, the dimensions that may be assigned to the grid are a function of a single parameter. This is illustrated in  FIG. 6 . There are four dimensions illustrated for the grid shown. However, the most important one is the distance a which represents a fundamental grid spacing. The other dimensions shown are selectable as a function of the single parameter a. The other parameters are D, d and v. The parameter D is the altitude of the rhomboid areas shown. The parameter d is the (vertically projected) distance between signal line vias and ground plane conductors, as shown. The parameter v is the (vertically projected) distance between rhomboid vertices in the ground plane and in the power plane. To be slightly more precise this distance is the distance between via openings for the ground plane and power planes. Again in  FIG. 6 , open circle  310  represents a signal line via. 
         [0032]    In preferred embodiments of the present invention, the following represents the relationship between the distance parameters shown in  FIG. 6  and the so-called technology parameter a:
       D=0.8667 a;   d=0.2887 a;   v=0.5774 a.       
 
         [0036]    In order to provide a better appreciation of the fact that the various conductive layers present in the substrate exist in different planes,  FIG. 7  is shown.  FIG. 7  provides a side elevation view illustrating conductors  200  in the power plane, conductors  300  and  400  in two signal planes, and conductors  100  in the ground plane. As indicated above, these are merely representative layers, and in fact, any given a substrate typically employs tens of layers with power planes and ground planes being connected to one another, respectively through via openings in the substrate material. While  FIG. 7  shows the presence of only a single signal plane  300 , it is not all unusual to have several signal planes present between a ground plane and a signal plane. 
         [0037]      FIG. 8  is a isometric three-dimensional view, which particularly illustrates the use of power plane and ground plane vias as a mechanism for providing a coaxial shielding configuration for vertical signal paths. Because of the complexity of  FIG. 8 ,  FIG. 9 , as discussed below, should also be considered at the same time when attempting to construe the structure shown in  FIG. 8 . This three-dimensional figure includes a lower ground plane grid  100 . Through vias  110 ,  112  and  114  (and the conductors therein), the conductive grid in this layer is electrically connected to correspondingly laid out conductors  100 ′ in a higher layer (see  FIG. 9 ). Likewise, power conductors  200 , lying in a plane above the ground plane, but insulated therefrom by the substrate material, are connected to corresponding power conductors  200 ′ in a superior layer. This connection is made through vias  210 ,  212 , and  214  as seen in  FIGS. 5 and 9 . Particularly relevant to the present invention,  FIG. 9  also illustrates the vertical connection through via  310  made between signal conductors  300  and  400  lying in different planes within the insulative substrate. In this regard it is especially useful to observe the coaxial effect with conductive material in via  310  being surrounded by conductive material in vias  110 ,  210 ,  112 ,  212 ,  114  and  214 . It is noted that the horizontal and vertical scales used in  FIG. 9  are not necessarily intended to be an accurate rendition of the dimensions indicated elsewhere but is merely intended to show the desired coaxial structure. 
         [0038]    While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.