Abstract:
A power distribution system for a semiconductor die includes bonding pads located adjacent to and connected to power busses with connections between the bonding pads providing a parallel path for current. Connections may be provided by stitch bonds, by conductors within a substrate or by other means.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to gate arrays, more particularly to an electrical distribution system for gate arrays exposed to radiation environments, and specifically to gate arrays that may experience short term exposure to a high level of ionizing radiation. This type of exposure is referred to as dose rate or prompt dose. 
     It is well known to use standardized gate arrays to construct semi-custom integrated circuits. A gate array formed in a semiconductor die or chip requires an electrical power distribution or power bussing system to provide the necessary voltage levels to circuits integrated in the die. This electrical distribution system typically takes the form of a conductive paths for providing a positive voltage, for example 3 volts and a conductive path for providing a reference voltage, for example ground. For gate array applications not subject to radiation environments the electrical distribution system is designed to meet various requirements and to provide the proper voltage level throughout the entire chip. To meet this requirement considerations is given to factors such a resistive voltage drops along the bus and electromigration. For non radiation applications calculations for power bussing systems that consider the maximum current requirements and include appropriate safety factors can be made and these calculations along with knowledge of the integrated circuit layout details can be used to determine the conductor sizing and spacing in a relatively straight forward fashion. 
     The design of a power bussing system for a gate array that will be subjected to a high level of ionizing radiation requires that additional factors be considered. The photocurrents generated in PN junctions of semiconductors subjected to ionizing radiation such as gamma or X-rays cause soft error rates in SRAM circuits, storage devices and failures in ROM. The photocurrent effects are global, i.e., every device on a chip served by the same power bussing is simultaneously affected by the photocurrents. One of the most common problems that results from the dose rate event is the reduction in the differential power supply voltage, that is a reduction in the difference between the positive voltage, for example VDD and the reference voltage, for example VSS. This reduction of course occurs across individual devices, for example, transistors, across circuit paths in the chip and in the package and across bonding wires connecting the chip to the package. The cause of this reduction in the differential power supply voltage is the droop of the local VDD voltage and the rise of the local VSS voltage due to the photocurrent contributions of all circuit components generated through the finite resistance and inductance of the power bussing and chip package. The result of this reduction is to lower the transient radiation upset threshold voltage. In addition the reduction can permit latch-up and burn out a device at sufficiently high dose rates. 
     The details of the power supply bussing of the prior art can be explained with reference to FIG. 1 which shows a greatly simplified top view drawing of a power bussing arrangement for a portion of a radiation resistant chip as found in the prior art. Chip 2 includes a Vdd buss 4 and a Vss buss 6, each of which extend around the perimeter 9 of chip 2 and have chip bond pad 5 and chip bond pad 7 for connection to power and ground, respectively. Chip 2 is typically housed in a package 8 which includes a method for bringing power and ground to chip 2, for example, package Vdd bond pad 10 and package Vss bond pad 12 which receive power and ground through conductors (not shown) located within package 8. Individual pairs of power and ground busses are located within the chip. For example, second metal Vdd bus 16 and second metal Vss buss 18 comprise one pair and second metal Vdd Buss 20 and second metal Vss buss 22 comprise another pair. Several additional Vdd and Vss pairs would exist if a more complete top view of chip 2 were shown in FIG. 1. Within chip 2, first metal Vdd busses 24 are connected to Vdd buss 16 by vias 26 and extend horizontally. Transistors 28, for example, p-channel transistors, would be connected to first metal buss 24. First metal Vss busses 30 are connected to Vss buss 18 by vias 32 and extend horizontally. Transistors 34, for example, n-channel transistors, would be connected to first metal buss 30. 
     FIG. 1a shows a cross-sectional view of a first metal buss 24, via 26 and second metal buss 16. 
     When a dose rate event occurs electron hole pairs are formed at PN junctions and photocurrents result. Electrons are attracted to the Vdd voltage and flow, for example from chip areas to first metal busses 24 through vias 26 and Vdd busses 16 to Vdd pad 5. A voltage due to the IR drop and the Ldi/dt drop causes the voltage at first metal buss 24 to be reduced. Holes are attracted to the Vss voltage and flow, for example from chip areas to first metal busses 30 through vias 32 and Vdd busses 18 to Vss pad 7. A voltage due to the IR drop and the Ldi/dt drop causes the voltage at first metal buss 30 to rise. Thus the differential voltage between first metal Vdd buss 24 and first metal Vss buss 30 is reduced. 
     Using the approach of the prior art, an increased dose rate requirement means that the increased size or quantity of power buss pairs requires more chip space, which in turn means loss in gate count and/or routing resources which, in turn, can result in the original circuit design no longer fitting on the gate array. 
     Thus a need exists for an electrical power distribution system that will meet the requirement of higher dose rates without significantly increasing the power bussing network. 
     SUMMARY OF THE INVENTION 
     The present invention meets these and other needs by providing a power distribution system for a semiconductor die comprising: 
     a first plurality of bond pads distributed along a first path on said die and having underlying connections to integrated circuits requiring a voltage at a first level; 
     means for connection to a first source for said voltage at a first level; 
     first conductive means connecting said means for connection to a first source to said first plurality of bond pads with said first conductive means spaced from a surface of said die except at said first plurality of bond pads; 
     a second plurality of bond pads distributed along a second path on said die and having underlying connections to integrated circuits requiring a second voltage; 
     means for connection to a second source for said voltage at a second level; and 
     second conductive means connecting said means for connection to a second source to said second plurality of bond pads with said second conductive means spaced from a surface of said die except at said second plurality of bond pads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a portion of a gate array chip according to the prior art. 
     FIG. 1a is a cross-sectional view of a portion of FIG. 1. 
     FIG. 2 is a plan view of a portion of a gate array chip according to the principles of the present invention. 
     FIG. 2a is a cross-sectional view of a portion of FIG. 2. 
     FIG. 2b is a cross-sectional drawing illustrating additional details of the present invention. 
     FIG. 3 is a cross-sectional view of an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     An electrical power distribution system for a semiconductor die according to the principles of the present invention is shown in the drawings and generally designated 50. 
     Referring to FIG. 2 where reference numbers having the suffix &#34;a&#34; are used to designate items similar to those shown in FIG. 1. Power distribution system 50 includes, in a preferred embodiment, third metal Vdd bond pads 52 which are located adjacent second metal Vdd bus 16a. Bond pads 52 have a via connection to second metal connected to second metal Vdd buss 16a. 
     A simplified cross-sectional drawing in FIG. 2a illustrates, for example, first metal Vdd buss 24a, via 26a, second metal Vdd buss 16a, and , bond pad 52 connected to second metal by via 54. 
     Thin wires 56 connect bond pads 52, bond pad 5a and bond pad 10a. Power distribution system 50 also includes Vss bond pads 62 which are connected by a via arrangement to second metal Vss buss 18a in a manner similar to that described for Vdd bond pads 52. Vss buss 18a is connected to first metal buss 32a. Thin wires 64 connect bond pads 62, bond pad 7a and bond pad 12a. Only one pair of Vdd and Vss busses are shown in FIG. 2 for simplicity, but it is to be understood that there are multiple pairs in the portion of the chip that is not shown. 
     Now that the construction of power distribution system 50 has been described, certain features may be set forth and appreciated. When a dose rate event occurs. electron hole pairs are generated. Electrons flow to Vdd buss 16a and holes flow to Vss buss 18a. As an example, electron flow into first metal Vdd buss 24a and then through via 26a to second metal buss 16a. At this juncture, the electron may flow along buss 16a or may flow through via 54 to bond pad 52 and through thin wires 56 to bond pad 5a and to bond pad 10a. 
     In a similar way, hole current will flow into first metal Vss buss 30a and then through via 32a to second metal Vss buss 18a. At this junction, hole current may flow along buss 18a or may flow through via connecting buss 18a to wire bond pad 62 and through thin wires 64 to bond pad 7a and bond pad 12a. 
     The embodiment of FIG. 2 may be implemented through the use of stitched bonding. Thin wires 56 and 64 are attached by stitched bonds. FIG. 2b shows a cross-sectional view of a portion of a package 70, with a power pad 72 and semiconductor die 74, with bond pads 76. Stitched bonds 78 extend from pad 72 to pads 76 and, according to the present invention, would act to shunt current from a power buss (not shown) within chip 74. Wire bonds are spaced from chip 74 except at bond pads located on the chip. 
     A simulation of the effect of the present invention shows dramatic results. The details of the simulation and the results are shown in Table 1. 
     
                       TABLE 1______________________________________                          Dose Rate  Vdd/Vss     Stitch    Upset  Bus Width   Bonds     rad(Si)/Sec______________________________________Standard power bussing of gate array             42.55 μm                      No      2E10  Stitch bond approach                 42.55 μm Yes 1.3E11  To obtain the same dose rate upset     300 μm No 1.3E11  without stitch bonds,  Vdd/Vss bus width needs  to increase to:______________________________________ 
    
     For the simulation of Table 1, the following conditions apply: 
     Bond wire diameter=1.25 mils 
     Aluminum pads for stitched bonds with passivation opening 88 μm×125 μm 
     Typical wire bond length between pads 75 mils to 80 mils 
     Chip active area 12,395 microns by 11,582 microns. 
     Photocurrent calculated at 1E11 rad(Si)/Sec=12.6A. 
     Thus Table 1 shows that to obtain the same dose rate upset achieved with stitched bonds, by simply increasing the buss width, it would be necessary to expand the buss width to 300 microns from 42.55 microns. 
     The present invention permits gate array applications that require higher dose rate upset. The present invention is easily implemented and the number of stitched bonds can vary depending on the dose rate requirements. Furthermore, stitch bonding may be implemented using a readily available standard manufacturing process. 
     While the use of stitched bonds has been described, other embodiments of the present invention may be utilized. For example an alternate embodiment of power distribution system 50 is shown in FIG. 3 which is intended for a flip chip arrangement. In FIG. 3, chip 80 includes pads 82 connected by vias 84 to buss 86. Substrate 90 includes pads 92 connected by vias 94 to shunt buss 96. Pads 82 and pads 92 would be round rather than rectangular. Solder balls 98 separate chip 80 from substrate 90 prior to solder reflow. In operation, when a dose rate event occurs, electron hole pairs are generated. Current flows from chip 80 to substrate 90 through the solder balls 98 with a very low resistance and inductance path. This would result in a dramatic improvement on the dose rate upset. 
     The scope of the invention is to be indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.