Patent Publication Number: US-6335494-B1

Title: Multiple power distribution for delta-I noise reduction

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to electronic circuit packaging and, more particularly, to power distribution for single and multi-chip packages, especially packages including CMOS chips. 
     2. Description of the Prior Art 
     Recent advances in performance, integration density and manufacturing economy have made complementary metal-oxide-semiconductor (CMOS) designs the technology of choice for digital processing circuits in virtually all applications except where extreme switching speed approaching microwave frequencies is required. At the current time, a new generation of CMOS technology is introduced every twelve to eighteen months with a 100% increase in integration density and a 20%-30% increase in performance. 
     Scaling active elements such as transistors to smaller sizes and increased integration density and obtaining higher switching speeds has generally required reduced operating voltages and increased input and output (I/O) connection densities on the package. Each new generation of CMOS technology also represents about a 30% reduction in operating voltage and multiple voltages must be accommodated by I/O connections to allow communication and operation with past (or future) generation devices. 
     These trends lead to application specific integrated circuits (ASICS), memories, microprocessors and other types of integrated circuits which must include multiple drivers to provide voltages for signals communicated to other chips which operate at voltages different from the voltages at which the integrated circuits themselves are intended to principally operate. These multiple voltages, of course, require separate power distribution networks in the circuit package supporting the chip(s) which contain these drivers. At the same time, high clock speeds and switching frequencies must be maintained for increased numbers of devices which switch simultaneously. Current transients which are caused by core logic and I/O switching activity cause voltage fluctuations in the chip and the package power distribution network due to parasitic inductance in the package, resulting in so-called delta-I noise. 
     This delta-I noise not only affects on-chip circuits but also transmits through quiet off-chip drivers, and, combined with signal coupled noise and reflection noise, can propagate to receivers. This combined noise tends to increase while signal levels decrease with succeeding generations of technology; significantly reducing the signal-to-noise ratio (SNR) and leading to performance degradation and data integrity concerns, particularly in regard to memory read operations and dynamic logic circuits which are precharged prior to input signal evaluation. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a power distribution arrangement for single and-or multi-chip packages with significantly reduced parasitic and loop-inductance and consequently reduced delta-I noise. 
     It is another object of the invention to provide an electronic integrated circuit package having an increased signal-to-noise ratio even when CMOS circuits operating at reduced voltages are included therein. 
     In order to accomplish these and other objects of the invention, an integrated circuit device and multi-layer connection structure forming a power distribution network for a plurality of voltages and ground are provided comprising alternating layers of patterned conductive layers and insulating layers, alternating patterned conductive layers being power distribution layers and signal layers, respectively, alternating power distribution layers being ground and voltage distribution layers, respectively, and vias selectively connecting selected conducting regions of the patterned conductive layers, wherein a voltage distribution layer is partitioned into respective areas for respective voltages that may appear in signal connections in an adjacent layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
     FIG. 1A is a schematic, exploded, side cross-sectional view of an exemplary conventional multi-voltage package design, 
     FIG. 1B includes a portion of the view of FIG. 1A illustrating the generation of delta-I noise in the conventional package design, 
     FIG. 2 is a plan view of the partitioned V 2  voltage plane of the conventional package of FIG. 1A, 
     FIG. 3A is a schematic, exploded, side cross-sectional view of an exemplary multi-voltage package design in accordance with the invention, 
     FIG. 3B includes a portion of the view of FIG. 3A illustrating the avoidance of delta-I noise in accordance with the invention, and 
     FIG. 4 is a plan view of the partitioned V 2  voltage plane of the package of FIG.  3 A. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1A, there is shown a schematic representation of a cross-sectional view of the power and signal distribution networks of a conventional electronic integrated circuit package in exploded form, for clarity. While FIGS. 1A,  1 B and  2  represent devices over which the present invention represents a significant improvement, the illustrations are arranged to facilitate explanation of the invention and are highly schematic and generalized for that purpose. Therefore no portion of any of these Figures is admitted to be prior art in regard to the present invention. These Figures are, accordingly, labelled “Related Art”. 
     FIG. 1A shows and exemplary eight layer arrangement  10  in which the layers are formed of insulating material  12  such as alumina with patterned conductive material formed thereon by screening or other techniques. Generally, the patterned conductive material will be formed only on the bottom side of each layer except for the top layer which will also have top surface metallurgy (TSM) mostly in the form of patterned connection pads on the top surface, as well. Portions of respective layers are connected by vias such as  14   a ,  14   b ,  14   c  and  14   d . The order of layers, regardless of the number of layers, seeks to place ground and power plane layers (e.g. V 1 , V 2  and ground (GND)) between signal layers (e.g. S 1 , S 2  and S 3 ) to provide shielding and reduce capacitive coupling between planes that carry high frequency signals. The networks formed by patterning of the layers are not important to the practice of the invention except to the extent that the connections necessary to support operation of the chip must be provided. 
     The conductive pattern on the bottom of layer  8  is generally referred to as bottom surface metallurgy (BSM) and will be constructed to support the attachment of package electrical connections such as connection pins or solder balls. For that reason, the BSM layer will generally be much more coarsely patterned than other layers. Patterned conductive material on the top of layer  1  is generally referred to as top surface metallurgy (TSM) which must make connections to the chip or chips that are mounted thereon in the completed package. Therefore the TSM layer is generally the most finely patterned and includes a so-called flip-chip pad layer. 
     The intermediate layers  2 - 6  may have conductive areas and vias at any desired pitch, as required to support the package function, and connection pad arrangements in the TSM and BSM layers. In this regard, however, it should be noted that in the arrangement of FIG. 1A, the number of connections by vias such as  14   d  to the V 2  conductive pattern on the bottom of layer  1  will be limited to the number of V 2  connections supported by the BSM layer (e.g. the number of BSM pin or ball locations reduced by the number of V 1  and ground connections within a given S/P (signal-to-power connection) ratio; tending to concentrate currents in the limited number of vias available. Both the concentration of current and the length of the vias  14   d  to extend through the full depth of the package increases the self-inductance of the V 2  distribution in the package. 
     If the package is required to support a chip including off-chip drivers which operate at a different voltage than the remainder or another portion of the chip, the V 2  layer is generally used to distribute power at the different, generally higher, voltage and, if more than one additional voltage is needed, the V 2  layer is also generally partitioned into areas in accordance with the respective voltages (collectively referred to as V 2  voltages). An exemplary form of such partitioning is illustrated in plan view in FIG. 2 although it should be understood that such partitioning is substantially arbitrary and not limited to diagonal divisions into quadrants, as shown. Nevertheless, diagonal partitioning is generally desirable since different voltages are generally required principally for off-chip drivers which are often most conveniently placed at the periphery of the chip to ease the package redistribution wiring congestion. 
     Referring now to FIG. 1B, the development of delta-I noise in the structure of FIG. 1A will now be explained. FIG. 1B shows the same multi-layer structure as illustrated in FIG.  1 A and the connection thereto of an exemplary off-chip driver circuit  16 . It should be understood that driver circuit  16  is integrated within a chip (generally with numerous similar circuits) mounted on and connected to TSM layer  1  but is illustrated beside the multi-layer structure for clarity in FIG.  1 B. 
     At the outset, it should be appreciated that, at the high clock frequencies at which current digital circuits operate, conductive region on a signal plane such as S 3  (layer  6  of FIG. 1A) surrounded by ground and/or reference voltage planes operates substantially as a transmission line. Thus for every signal current, there will be return or image current in the opposite direction at some location(s). At high frequencies, most return current for a transmission line travels on the nearest reference plane. 
     The return current must eventually return to the same plane from which it is drawn and the current loop may or may not be complete or closed (e.g. without traversing a power supply at a different voltage). Thus when the driver circuit  16  transitions from a high to low output state connected to signal layer S 3  (layer  6 ), the return current principally travels in the adjacent ground layer  7 . The current path is closed and little delta-I noise results. 
     However, when the driver circuit  16  output transitions from low to high, as illustrated by solid arrows in FIG. 1B, the V 2  voltage on the V 2  plane (layer  1 ) is connected to a conductive region on the S 3  plane (layer  6 ). As the signal propagates in the S 3  plane, the return or image current is reflected principally in the adjacent ground layer  7 , with some in the V 1  plane (layer  5 ), as indicated by dashed arrows. This current path is not closed and substantial delta-I noise results due to the return of current from ground or V 1  to V 2  through decoupling capacitors. Further, this path is physically longer than the path for the high to low transition and has increased resistance and loop-inductance. 
     Referring now to FIGS. 3A and 4 the multi-layer structure in accordance with the invention will now be described and explained. The gross physical structure in regard to patterned conductor layers separated by insulating layers is the same as that of FIG.  1 A. The patterning of the conductive layers is similarly substantially arbitrary but some additional flexibility is provided, as will be discussed below. The principal characteristic of the invention is that voltage distribution in the conductive pattern on the bottom of layer  1  and voltage distribution on the bottom of layer  5  are both partitioned or split to carry both V 1  and V 2  voltages. An exemplary partitioning of a voltage distribution layer is shown in FIG.  4 . Again, the locations of divisions between different regions which are intended to carry different voltages is also substantially arbitrary but it is preferred and generally convenient to provide a V 1  voltage area centrally of the layer (or centrally of the location intended for mounting and connection of a chip). Diagonal separation of V 2  voltage areas, if provided, is also preferred for the reasons discussed above. 
     The partitioning or splitting of the power layers  1  and  5  also allows a significant difference in via structures compared to FIG.  1 A. Specifically, while via  14   a  is an exemplary connection to the S 3  signal layer, via  14   b  is an exemplary connection to ground layer  7 , via  14   c  is an exemplary connection to the V 1  area of layer  5  and via  14   c ′ is an exemplary connection from V 1  to the chip are effectively the same as those of FIG. 1A, via  14   d  need only reach to layer  5  and a plurality of further vias  14   e , possibly of reduced size, complete the connection to layer  1  and/or the chip. This division of the current reduces the effective resistance/voltage drop of each via and reduces the self-inductance of the package. 
     The reduction of delta-I noise in accordance with the invention will now be discussed in connection with FIG.  3 B. The high to low transition causes a return or image current on ground layer  7 , in the same manner as described above in connection with FIG.  1 B. However, as can be seen from FIG. 3B, the return or image current path for the low to high transition is principally carried on the V 2  portion of layer  5  and the current path is similarly complete or closed. Therefore, very little delta-I noise will be generated. Further, the small amount of noise generated is symmetric for both layers  5  and  7  and both output signal transition directions. Accordingly, a substantial degree of noise cancellation can be statistically projected over the large number of switching devices on current and foreseeable chips. Additionally, the current path length is much reduced and distributed (e.g. over plural vias  14   e ), as well; both serving to reduce self-inductance of the multi-layer structure. Thus delta-I noise is minimized through a plurality of mechanisms. 
     These effects of delta-I noise reduction are also derived for all signal planes (e.g. S 1  and S 2  as well as S 3 ) in the multi-layer structure of FIG.  3 A and are referenced to the correct voltage supplies on adjacent power planes. The return or image current paths are all closed or complete and the length of current loops is reduced and made largely symmetrical for both signal transitions. Via length can also be reduced and current distributed to reduce self-inductance. 
     Several other important advantages accrue from the splitting of the power planes to include V 1  and V 2  voltages in accordance with the invention. Specifically, it is preferred that the V 1  area of the power layers be somewhat smaller than the chip or die in order to also include V 2  voltage chip connections. Therefore, it is preferable to place all V 1  BSM connections under the die area to minimize the length thereof and many BSM connections allocated to V 1  in previous designs can simply be eliminated since additional distribution and connections that may be required by the chip or die can be accommodated in the power layers and with vias therefrom. Therefore, BSM connection locations which are not needed for V 1  can be allocated to V 2  to further reduce the self-inductance of the multi-layer structure. The signal to V 2  loop inductance is also reduced, further reducing delta-I noise. 
     At the same time, all BSM connections under the chip or die areas can now be dedicated to V 1  and ground. This lowers the self-inductance of both the V 1  and Ground connections and the V 1 -ground loop inductance. A highly coupled and direct vertical power feed is thus provided to the core logic reducing voltage fluctuations with current and reducing the potential for switching/data errors. 
     In view of the foregoing it is seen that the invention provides substantial reduction in delta-I noise through a plurality of mechanisms and design refinements made possible through partitioning of the power layers to accommodate both V 1  and V 2  voltages which significantly reduce loop inductance and self-inductance to very low levels which are substantially minimized. By the same token, the noise levels attributable to the power distribution network is so significantly reduced that good operating margins and signal-to-noise ratios can be maintained to signal levels well below those for generations of technology currently being developed. 
     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. For example, the principles of the invention are applicable to both single and multi-chip packaging but are not limited to power and signal connections to a chip. The principles of the invention are equally applicable to any power and high frequency signal networks in, for example, multiple wiring layers on the chip itself or circuit boards to which single or multiple chip packages may be attached. 
     Additionally, it is not necessary that the power layers such as the V 1 /V 2  layers or planes of FIG. 3 be similarly partitioned, limited to power distribution (e.g. signal connections may be included in a power layer and vice-versa) or even carry all voltages that are present in the package or on the chip. It is only necessary to provide power distribution in any given plane proximate to portions of signal connections in an adjacent plane at voltages that may be present therein in order to obtain significant reduction in delta-I noise and other advantages of the invention.