Patent Publication Number: US-11038344-B2

Title: Shunt power rail with short line effect

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Patent Application No. 62/712,862, filed on Jul. 31, 2018, and titled “SHUNT POWER RAIL WITH SHORT LINE EFFECT,” the disclosure of which is expressly incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to cell circuits. More specifically, the present disclosure relates to a cell circuit having a shunt power rail with short line effect to mitigate risks associated with electron migration. 
     BACKGROUND 
     The design complexity of mobile radio frequency (RF) transceivers is complicated by added circuit functions and devices to support communication enhancements. For example, advances in technology have resulted in cellular communication standards and protocols, such as fifth generation (5G) new radio (NR) millimeter wave (mmW) using physically small antenna arrays that contain multiple antenna elements. The complexity is further heightened by the desire to support multiple frequency bands in the millimeter wave frequency range, such as frequency bands located near 24 gigahertz (GHz), 26 GHz, 28 GHz, 37 GHz, 39 GHz, 48 GHz, and 56 to 71 GHz. Processor-based computer systems can include a vast array of integrated circuits (ICs). 
     Each IC has a complex layout design comprised of multiple IC devices. Standard cell circuits are often employed to assist in making the design of ICs less complex and more manageable. For example, standard cell circuits provide a designer with pre-designed cells corresponding to commonly used IC devices that conform to specific design rules of a chosen technology. As non-limiting examples, standard cell circuits may include gates, inverters, multiplexers, and adders. Using standard cell circuits enables a designer to create ICs having consistent layout designs, thereby creating a more uniform and less complex layout design across multiple ICs, as compared to custom designing each circuit. 
     Conventional standard cell circuits are fabricated using process technologies that form device elements with a pre-defined technology node size. For example, a process technology may be employed to fabricate a conventional standard cell circuit with device elements fourteen (14) nanometers (nms) or ten (10) nms wide. Improvements in fabrication processes and related technologies are enabling decreases in technology node size, which allows a higher number of device elements, such as transistors, to be disposed in less area within a circuit. As technology node size scales down, gate and metal lines within a conventional standard cell circuit also scale down to reduce the area of a conventional standard cell circuit. The standard cell circuit may scale down in area by reducing a total height. For example, a gate length can be scaled down to reduce a width of a conventional standard cell circuit, and metal line width can be scaled down to reduce the cell height. However, continuous integrated circuit scaling drives a reduction in the standard cell height, and consequently a reduction in widths of metal power rails and metal shunt power rails. The reduction in the widths of metal power rails and the metal shunt power rails leads to an increasing risk of electron migration (EM). 
     SUMMARY 
     A cell circuit includes a first power rail having a first line length. The first power rail is in a first conductive layer. The first power rail is configured to receive a first voltage for the cell circuit. The cell circuit also includes a plurality of conductive lines in a second conductive layer. The cell circuit also includes a first conductive shunt in a third conductive layer. The first conductive shunt is electrically coupled to the first power rail and a first set of conductive lines of the plurality of conductive lines. The first conductive shunt has a second line length that is shorter than the first line length. The cell circuit further includes a second conductive shunt in the third conductive layer. The second conductive shunt is electrically coupled to the first power rail and a second set of conductive lines of the plurality of conductive lines. The second conductive shunt has a third line length that is shorter than the first line length. 
     A method of fabricating a cell circuit includes fabricating a first power rail having a first line length. The first power rail is in a first conductive layer. The first power rail is configured to receive a first voltage for the cell circuit. The method also includes fabricating a plurality of conductive lines in a second conductive layer. The method also includes fabricating a first conductive shunt in a third conductive layer. The first conductive shunt is electrically coupled to the first power rail and a first set of conductive lines of the plurality of conductive lines. The first conductive shunt has a second line length that is shorter than the first line length. The method further includes fabricating a second conductive shunt in the third conductive layer. The second conductive shunt is electrically coupled to the first power rail and a second set of conductive lines of the plurality of conductive lines. The second conductive shunt has a third line length that is shorter than the first line length. 
     A cell circuit includes means for receiving a first voltage for the cell circuit. The first voltage receiving means has a first line length. The first voltage receiving means is in a first conductive layer. The cell circuit also includes a plurality of conductive lines in a second conductive layer. The cell circuit also includes a first conductive shunt in a third conductive layer. The first conductive shunt is electrically coupled to the first voltage receiving means and a first set of conductive lines of the plurality of conductive lines. The first conductive shunt has a second line length that is shorter than the first line length. The cell circuit further includes a second conductive shunt in the third conductive layer. The second conductive shunt is electrically coupled to the first voltage receiving means and a second set of conductive lines of the plurality of conductive lines. The second conductive shunt has a third line length that is shorter than the first line length. 
     This has outlined, rather broadly, the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the present disclosure will be described below. It should be appreciated by those skilled in the art that this present disclosure may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the teachings of the present disclosure as set forth in the appended claims. The novel features, which are believed to be characteristic of the present disclosure, both as to its organization and method of operation, together with further objects and advantages, will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1  shows a wireless device communicating with a wireless communication system. 
         FIG. 2  shows a block diagram of the wireless device in  FIG. 1 , according to an aspect of the present disclosure. 
         FIG. 3  is a top view of a standard cell circuit including a first power rail and a second power rail. 
         FIG. 4  illustrates a top view of one conductive layer of a standard cell circuit juxtaposed with another conductive layer of the standard cell circuit. 
         FIG. 5  illustrates a cross-section of a layout of a cell circuit in a short line configuration, according to aspects of the present disclosure. 
         FIG. 6  illustrates a top view of a layout of a cell circuit in a short line configuration according, to aspects of the present disclosure. 
         FIG. 7  illustrates a top view of another layout of a cell circuit in a short line configuration, according to aspects of the present disclosure. 
         FIG. 8  illustrates a top view of a layout of a cell circuit in a short line configuration having multiple options for interconnecting to various layers of the cell circuit, according to aspects of the present disclosure. 
         FIG. 9  illustrates a cross-section of a first interconnections option to the various layers of the cell circuit of  FIG. 8 , according to aspects of the present disclosure. 
         FIG. 10  illustrates a cross-section of a second interconnection option to the various layers of the cell circuit of  FIG. 8 , according to aspects of the present disclosure. 
         FIG. 11  depicts a simplified flowchart of a method of device fabrication, according to aspects of the present disclosure. 
         FIG. 12  is a block diagram showing an exemplary wireless communication system in which a configuration of the disclosure may be advantageously employed. 
         FIG. 13  is a block diagram illustrating a design workstation used for circuit, layout, and logic design of a cell circuit according to one configuration. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below, in connection with the appended drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts. As described herein, the use of the term “and/or” is intended to represent an “inclusive OR”, and the use of the term “or” is intended to represent an “exclusive OR”. 
     Cell-based designs (e.g., standard cell circuits) have power rails that supply voltage potentials such as Vdd or Vss to the cells. Continuous integrated circuit scaling drives a reduction in a standard cell circuit height, and consequently, a reduction in widths of conductive (e.g., metal) power rails and conductive (e.g., metal) shunt power rails. However, the power rails and the shunt power rails are limited or constrained by direct current specifications. 
     The reduction in the widths leads to an increasing risk of electron migration (EM) associated with, for example, the metal shunt power rail. Electron migration is the transport of material caused by the gradual movement of the ions (e.g., copper ions) in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The migration of the ions of the conductor can result in a physical breakage of the conductor in the regions depleted of the ions. One way to reduce the risk of electron migration is to increase the width of a metal shunt power rail. However, increasing the width of the metal shunt power rail can cause an expensive place and route (P&amp;R) resource (or track) loss. 
     Aspects of the present disclosure are directed to a cell circuit that mitigates shunt power rail electron migration while reducing place and route resource loss. For example, the cell circuit includes a first power rail that has a first line length. The first power rail is in a first conductive layer. The first power rail is configured to receive a first voltage for the cell circuit. The cell circuit includes a plurality of conductive lines in a second conductive layer that are orthogonal to the first power rail. 
     The cell circuit further includes a first conductive shunt (or first conductive shunt power rail) in a third conductive layer. The first conductive shunt is parallel to the first power rail. The first conductive shunt is electrically coupled to the first power rail and a first set of conductive lines of the plurality of conductive lines. The first conductive shunt has a second line length shorter than the first line length. Furthermore, the cell circuit includes a second conductive shunt (or second conductive shunt power rail) in the third conductive layer. The second conductive shunt is also parallel to the first power rail. The second conductive shunt is electrically coupled to the first power rail and a second set of conductive lines of the plurality of conductive lines. The second conductive shunt has a third line length shorter than the first line length. 
     The cell circuit further includes a first set of vias, a second set of vias, and a third set of vias. The first set of vias are between the first conductive layer and the second conductive layer. Each of the first set of vias electrically couples the first power rail to the plurality of conductive lines in the second conductive layer. The second set of vias are between the second conductive layer and the first conductive shunt. Each of the second set of vias electrically couples the first set of conductive lines to the first conductive shunt. The third set of vias are between the second conductive layer and the second conductive shunt. Each of the third set of vias electrically couples the second set of conductive lines to the second conductive shunt. 
     The cell circuit also includes a second power rail, a third conductive shunt, and a fourth conductive shunt. The second power rail includes a fourth line length and is in a second row of the first conductive layer. The second power rail is configured to receive a second voltage for the cell circuit, in which the first voltage is different from the second voltage. For example, the first voltage has a positive polarity and the second voltage has a negative polarity. The third conductive shunt is in a second row of the third conductive layer and electrically coupled to the second power rail and a third set of conductive lines of the plurality of conductive lines. The third conductive shunt includes a fifth line length shorter than the fourth line length. The fourth conductive shunt is in the second row of the third conductive layer and is electrically coupled to the second power rail and a fourth set of conductive lines of the plurality of conductive lines. The fourth conductive shunt has a sixth line length shorter than the fourth line length. 
     A first space or gap between the first conductive shunt and the second conductive shunt is aligned with a second space between the third conductive shunt and the fourth conductive shunt. In some aspects, the first space between the first conductive shunt and the second conductive shunt is staggered relative to the second space between the third conductive shunt and the fourth conductive shunt. Each of the second line length and the third line length is between approximately two (2) micrometer (μm) and ten (10) μm. 
     In some aspects, the third conductive shunt and the fourth conductive shunt may be replaced with a continuous conductive shunt in the second row of the third conductive layer. The continuous conductive shunt is electrically coupled to the second power rail as well as the third set of conductive lines and the fourth set of conductive lines of the plurality of conductive lines. The continuous conductive shunt has a fifth line length substantially equal to the fourth line length. 
       FIG. 1  shows a wireless device  110  that includes the described cell circuit. The wireless device  110  communicates with a wireless communication system  120 . The wireless communication system  120  may be a 5G system, a long term evolution (LTE) system, a code division multiple access (CDMA) system, a global system for mobile communications (GSM) system, a wireless local area network (WLAN) system, millimeter wave (mmW) technology, or some other wireless system. A CDMA system may implement wideband CDMA (WCDMA), time division synchronous CDMA (TD-SCDMA), CDMA2000, or some other version of CDMA. In a millimeter wave (mmW) system, multiple antennas are used for beamforming (e.g., in the range of 30 GHz, 60 GHz, etc.). For simplicity,  FIG. 1  shows the wireless communication system  120  including two base stations  130  and  132  and one system controller  140 . In general, a wireless system may include any number of base stations and any number of network entities. 
     A wireless device  110  may be referred to as a user equipment (UE), a mobile station, a terminal, an access terminal, a subscriber unit, a station, etc. The wireless device  110  may also be a cellular phone, a smartphone, a tablet, a wireless modem, a personal digital assistant (PDA), a handheld device, a laptop computer, a Smartbook, a netbook, a cordless phone, a wireless local loop (WLL) station, a Bluetooth device, etc. The wireless device  110  may be capable of communicating with the wireless communication system  120 . The wireless device  110  may also be capable of receiving signals from broadcast stations (e.g., a broadcast station  134 ), signals from satellites (e.g., a satellite  150 ) in one or more global navigation satellite systems (GNSS), etc. The wireless device  110  may support one or more radio technologies for wireless communication such as 5G, LTE, CDMA2000, WCDMA, TD-SCDMA, GSM, 802.11, etc. 
     The wireless device  110  may support carrier aggregation, which is operation on multiple carriers. Carrier aggregation may also be referred to as multi-carrier operation. According to an aspect of the present disclosure, the wireless device  110  may be able to operate in low-band from 698 to 960 megahertz (MHz), mid-band from 1475 to 2170 MHz, and/or high-band from 2300 to 2690 MHz, ultra-high band from 3400 to 3800 MHz, and long-term evolution (LTE) in LTE unlicensed bands (LTE-U/LAA) from 5150 MHz to 5950 MHz. Low-band, mid-band, high-band, ultra-high band, and LTE-U refer to five groups of bands (or band groups), with each band group including a number of frequency bands (or simply, “bands”). For example, in some systems each band may cover up to 200 MHz and may include one or more carriers. For example, each carrier may cover up to 40 MHz in LTE. Of course, the range for each of the bands is merely exemplary and not limiting, and other frequency ranges may be used. LTE Release 11 supports 35 bands, which are referred to as LTE/UMTS bands and are listed in 3GPP TS 36.101. The wireless device  110  may be configured with up to five carriers in one or two bands in LTE Release 11. 
     Some carrier aggregation implementations in the sub 6 GHz include multiple frequency bands in the millimeter wave frequency range, such as frequency bands located near 24 gigahertz (GHz), 26 GHz, 28 GHz, 37 GHz, 39 GHz, 48 GHz, and 56 to 71 GHz. For example, these bands may include 24.25-24.45 GHz, 24.75-25.25 GHz, 27.5-28.35 GHz, and 37-40 GHz. In these systems, the carriers may be 50 MHz, 100 MHz, 200 MHz, or 400 MHz and the bands may be up to 2.4 GHz and may include one or more carriers. 
       FIG. 2  shows a block diagram of an exemplary design of wireless device  110  in  FIG. 1 . In this exemplary design, the wireless device  110  includes a transceiver  220  coupled to a primary antenna  210 , a transceiver  222  coupled to a secondary antenna  212 , and a data processor/controller  280 . The transceiver  220  includes multiple (K) receivers  230   pa  to  230   pk  and multiple (K) transmitters  250   pa  to  250   pk  to support multiple frequency bands, multiple radio technologies, carrier aggregation, etc. The transceiver  222  includes L receivers  230   sa  to  230   s   1  and L transmitters  250   sa  to  250   s   1  to support multiple frequency bands, multiple radio technologies, carrier aggregation, receive diversity, multiple-input multiple-output (MIMO) transmission from multiple transmit antennas to multiple receive antennas, etc. 
     In the exemplary design shown in  FIG. 2 , each receiver  230  includes an LNA  240  and receive circuits  242 . For data reception, the antenna  210  receives signals from base stations and/or other transmitter stations and provides a received radio frequency (RF) signal, which is routed through an antenna interface circuit  224  and presented as an input RF signal to a selected receiver  230 . An antenna interface circuit  224  may include switches, duplexers, transmit filters, receive filters, matching circuits, etc. The description below assumes that the receiver  230   pa  is the selected receiver. Within the receiver  230   pa , an LNA  240   pa  amplifies the input RF signal and provides an output RF signal. Receive circuits  242   pa  downconvert the output RF signal from RF to baseband, amplify and filter the downconverted signal, and provide an analog input signal to data processor  280 . Receive circuits  242   pa  may include mixers, filters, amplifiers, matching circuits, an oscillator, a local oscillator (LO) generator, a phase locked loop (PLL), etc. Each remaining receiver  230  in the transceivers  220  and  222  may operate in a similar manner as the receiver  230   pa.    
     In the exemplary design shown in  FIG. 2 , each transmitter  250  includes transmit circuits  252  and a power amplifier (PA)  254 . For data transmission, a data processor  280  processes (e.g., encodes and modulates) data to be transmitted and provides an analog output signal to a selected transmitter. The description below assumes that the transmitter  250   pa  is the selected transmitter. Within the transmitter  250   pa , transmit circuits  252   pa  amplify, filter, and upconvert the analog output signal from baseband to RF and provide a modulated RF signal. The transmit circuits  252   pa  may include amplifiers, filters, mixers, matching circuits, an oscillator, an LO generator, a PLL, etc. A power amplifier (PA)  254   pa  receives and amplifies the modulated RF signal and provides a transmit RF signal having the proper output power level. The transmit RF signal is routed through the antenna interface circuit  224  and transmitted via the antenna  210 . Each remaining transmitter  250  in the transceivers  220  and  222  may operate in a similar manner as the transmitter  250   pa.    
       FIG. 2  shows an exemplary design of a receiver  230  and transmitter  250 . The receiver  230  and a transmitter  250  may also include other circuits not shown in  FIG. 2 , such as filters, matching circuits, etc. All or a portion of transceivers  220  and  222  may be implemented on one or more analog integrated circuits (ICs), RF ICs (RFICs), mixed-signal ICs, etc. For example, LNAs  240  and receive circuits  242  within transceivers  220  and  222  may be implemented on multiple ICs, as described below. The circuits in transceivers  220  and  222  may also be implemented in other manners. 
     The data processor/controller  280  may perform various functions for the wireless device  110 . For example, the data processor  280  may perform processing for data being received via the receivers  230  and data being transmitted via the transmitters  250 . The controller  280  may control the operation of the various circuits within the transceivers  220  and  222 . In some aspects, the transceivers  220  and  222  may also comprise a controller to control various circuits within the respective transceiver (e.g., LNAs  240 ). A memory  282  may store program codes and data for the data processor/controller  280 . The data processor/controller  280  may be implemented on one or more application specific integrated circuits (ASICs) and/or other ICs. 
       FIG. 3  is a top view of a standard cell circuit  300  including a first power rail  312   a  and a second power rail  312   b . For example, the first power rail  312   a  corresponds to a terminal that receives a positive supply voltage (Vdd) and the second power rail  312   b  corresponds to a terminal that receives a negative supply voltage (Vss). The negative supply voltage may be a ground voltage. Each of the first power rail  312   a  and the second power rail  312   b  has a rail width W rail . The standard cell circuit  300  includes active devices (not shown) that include corresponding gates  301   a - 301   e . The gates  301   a - 301   e  are perpendicular to the first power rail  312   a  and the second power rail  312   b  and have a gate pitch GP (e.g., fifty (50) to fifty five (55) nanometers). The first power rail  312   a  and the second power rail  312   b  are included in a first conductive layer (e.g., a first back-end-of-line (BEOL) layer (M 0 ) or a second a second BEOL layer (M 1 )) of the integrated circuit. 
     The standard cell circuit  300  also includes routing/signal lines  303   a - 303   f  in the first conductive layer. The signal lines  303   a - 303   f  are between the first power rail  312   a  and the second power rail  312   b  and are orthogonal to the gates  301   a - 301   e . Each of the signal lines  303   a - 303   f  corresponds to a routing track, and has a line width W line . The signal lines  303   a - 303   f  are used, in part, to interconnect elements in the standard cell circuit  300  to form various devices, such as particular logic gates. The signal lines  303   a - 303   f  are also used, in part, to interconnect elements to the first power rail  312   a  and the second power rail  312   b . To further assist in interconnecting elements in the standard cell circuit  300 , as well as to interconnect elements to the first power rail  312   a  and the second power rail  312   b , conductive (metal) lines  305   a - 305   d  in subsequent conductive layers (e.g., a second conductive layer or a third BEOL layer (M 2 )) are included between the respective gates  301   a - 301   e . The conductive lines  305   a - 305   d  are orthogonal to the first power rail  312   a  and the second power rail  312   b.    
       FIG. 4  illustrates a top view of one conductive layer (e.g., a first conductive layer  407 ) of a standard cell circuit  400  juxtaposed with another conductive layer (e.g., a third conductive layer  409 ) of the standard cell circuit  400 . The first conductive layer  407  is juxtaposed with the third conductive layer  409  to illustrate design constraints of the different conductive layers. In standard cell circuit design, conductive layers (e.g., a second conductive layer (not shown)) directly above or below another conductive layer are perpendicular to the conductive layers directly above or below them. For example, signal lines  403   a - 403   d  and power rails  412   a - 412   b  of the first conductive layer  407  are perpendicular to signal lines (not shown) and shunt power rails (not shown) of the second conductive layer (not shown). 
     Conductive layers that are separated by one conductive layer are parallel to each other. For example, the signal lines  403   a - 403   d  and the power rails  412   a - 412   b  of the first conductive layer  407  are parallel to the signal lines  411   a - 411   e  and shunt power rails  413   a - 413   b  of the third conductive layer  409 . This follows because the second conductive layer is between the first conductive layer  407  and the third conductive layer  409 . The signal lines and the power rails are subject to design constraints. For example, the number of signal lines between the power rails  412   a - 412   b  of the first conductive layer  407  and signal lines between the shunt power rails  413   a - 413   b  of the third conductive layer  409  are constrained by a cell height, which is based on a design standard. For example, the total cell height of the standard cell circuit  400  is measured from a center of the power rail  412   a  to a center of the power rail  412   b . The total cell height of the standard cell circuit  400  can also be measured from a center of the shunt power rail  413   a  to a center of the shunt power rail  413   b . In this aspect, the power rails  412   a - 412   b  of the first conductive layer  407  and the shunt power rails  413   a - 413   b  of the third conductive layer  409  have a same continuous length. 
     For example, the first conductive layer  407  is constrained to four signal lines  403   a - 403   d  between the power rails  412   a - 412   b  that are wider than the shunt power rails  413   a - 413   b  of the third conductive layer  409 . The third conductive layer  409  is further constrained because more signal lines (e.g., five signal lines  411   a - 411   e ) are specified between the shunt power rails  413   a - 413   b . This means that a width W rail2  of the shunt power rails  413   a - 413   b  are smaller than the width W rail1  of the power rails  412   a - 412   b . If the width W rail1  of the of the power rails  412   a - 412   b  are not wide enough to support an increased power supply, the extra power can be supported by the shunt power rails  413   a - 413   b . However, if the width W rail2  of the shunt power rails  413   a - 413   b  are limited by space, the standard cell circuit  400  may encounter electron migration. In this case, the shunt power rails  413   a - 413   b  are specified for a reduced direct current Idc. 
     Reducing the rail width W rail2  as a result of scaling decreases the conductive area of both the shunt power rails  413   a - 413   b . Such a reduction in the conductive area results in both the shunt power rails  413   a - 413   b  having an increased resistance, and thus, a decrease in direct current Idc (where Idc=voltage divided by resistance (V/R)). The reduction in the direct current Idc causes the standard cell circuit  400  to be subjected to electron migration that may cause a physical break in the shunt power rails  413   a - 413   b.    
       FIG. 5  illustrates a cross-section of a layout of a cell circuit  500  in a short line configuration, according to aspects of the present disclosure. The cell circuit  500  includes a first power rail  502  that has a first line length (L 1 ). The first power rail  502  is in a first row of a first conductive layer (e.g., layer M 1 ). The first power rail  502  is configured to receive a first voltage for the cell circuit  500 . The cell circuit  500  includes a plurality of conductive lines (e.g.,  504   a ,  504   b ,  504   c ,  504   d ,  504   e , and  504   f ) in a second conductive layer (e.g., layer M 2 ). The process flow for semiconductor fabrication of the cell structure may include front-end-of-line (FEOL) processes, middle-of-line (MOL) processes, and back-end-of-line (BEOL) processes. The MOL process is the set of process steps that enable connection of the transistors to the back-end-of-line or BEOL interconnects (e.g., layer M 1  or metal  1 , layer M 2 , layer M 3 , etc.) using MOL contacts. Although metal  1  (M 1 ), metal  2  (M 2 ) and metal  3  (M 3 ) are referred to throughout this document, the present disclosure is not limited to metal  1  (M 1 ), metal  2  (M 2 ) and metal  3  (M 3 ), as other metal layers may be used. In one aspect, the plurality of conductive lines are perpendicular to the first power rail  502 . The plurality of conductive lines  504   a - 504   f  are electrically coupled to the first power rail  502  by a first set of vias  510  between the plurality of conductive lines and the first power rail  502 . 
     The cell circuit  500  further includes a first conductive shunt  506  (or first conductive shunt power rail) in a first row of a third conductive layer (e.g., layer M 3 ). The first conductive shunt  506  is parallel to and electrically coupled to the first power rail  502  and a first set of conductive lines  504   a ,  504   b , and  504   c  of the plurality of conductive lines. For example, the first conductive shunt  506  is electrically coupled to the first power rail  502  by a second set of vias  512  and some of the first set of vias  510 . The first conductive shunt  506  is electrically coupled to the first set of conductive lines  504   a ,  504   b , and  504   c  of the plurality of conductive lines by the second set of vias  512 . The first conductive shunt  506  has a second line length (L 2 ), which is shorter than the first line length L 1 . 
     Furthermore, the cell circuit  500  includes a second conductive shunt  508  (or second conductive shunt power rail) in the first row of the third conductive layer (e.g., M 3 ). The second conductive shunt  508  is parallel to and electrically coupled to the first power rail  502  and a second set of conductive lines  504   d ,  504   e , and  504   f  of the plurality of conductive lines. For example, the second conductive shunt  508  is electrically coupled to the first power rail  502  by a third set of vias  514  and some of the first set of vias  510 . The second conductive shunt  508  is electrically coupled to the second set of conductive lines  504   d ,  504   e , and  504   f  of the plurality of conductive lines by the third set of vias  514 . In some aspects, a distance from via-to-via (e.g., between two vias  514 ) is 20-40 contacted poly/gate pitch (CPP) which is about one to two micrometers (1-2 μm.) 
     The cell circuit  500  is in the short line configuration to achieve a short line effect with respect to the first conductive shunt  506  and the second conductive shunt  508  of the cell circuit  500 . Short line effect occurs when a stress gradient that builds up during copper migration (e.g., copper ion migration) leads to reflective force that opposes an electron migration force. To achieve the short line effect, the first conductive shunt  506  and the second conductive shunt  508  of the third conductive layer are two separate conductive shunts instead of one continuous conductive shunt that is substantially a same length as the first power rail  502 . For example, the first conductive shunt  506  has a second line length L 2 , which is shorter than the first line length L 1 . Similarly, the second conductive shunt  508  has a third line length L 3 , which is shorter than the first line length L 1 . But each of the second line length L 2  and the third line length L 3  is shorter than a critical length Lc to achieve the short line effect. 
     The short line effect is described as follows. A stress gradient that builds up during copper migration (e.g., copper ion migration) leads to a back or reflective force that opposes the electron migration force. In short lines (e.g., first conductive shunt  506  having a second line length L 2  or the second conductive shunt  508  having a third line length L 3 ), a relatively small transfer of material is specified to produce a gradient sufficient to offset the electron migration force, halt the metal migration, and save the conductive shunt. Consequently, all conductive shunts whose length (e.g., second line length L 2  or third line length L 3 ) is shorter than the critical length Lc (e.g., as large as eight micrometers or more) are less subject to electron migration failure. Thus, the shorter lines are unable to generate sufficient stress to cause the line to rupture and the line is unable to grow a void to a size sufficient for failure to occur. 
     Accordingly, a maximum specified direct current Idc for the conductive shunts that have a length less than the critical length is much higher (e.g., three to four time higher) than the conventional continuous shunt power rail length that is substantially equal to the length of the underlying power rail (e.g., first power rail  502 ). The conductive shunts (e.g., the first conductive shunt  506  or the second conductive shunt  508 ) may have a width of twenty (20) to twenty five (25) nanometers. In some aspects, these ranges are associated with seven nanometer (7 nm) or five nanometer (5 nm) technology nodes. 
       FIG. 6  illustrates a top view of a layout of a cell circuit  600  in a short line configuration, according, to aspects of the present disclosure. The cell circuit  600  includes conductive shunts  606   a - 606   h  in a same conductive layer (e.g., the third conductive layer (e.g., M 3 ) of  FIG. 5 ). 
     Some of the conductive shunts (e.g., the conductive shunt  606   a  and the conductive shunt  606   b ) correspond to terminals that receive a positive supply voltage (Vdd). Other conductive shunts (e.g., the conductive shunt  606   c  and the conductive shunt  606   d ) correspond to terminals that receive a negative supply voltage (Vss). The negative and positive terminals can be laid out in an alternating configuration. For example, a first row of conductive shunts may include positive terminals corresponding to the conductive shunt  606   a  and the conductive shunt  606   b  and a second row immediately adjacent to the first row may include negative terminals corresponding to the conductive shunt  606   c  and the conductive shunt  606   d , and the next row is a positive terminal, and so on. 
     The conductive shunts  606   a - 606   h  are grouped into rows and columns such that some conductive shunts (e.g., the conductive shunt  606   a  and the conductive shunt  606   b ) in a same row are aligned in a direction of the row and conductive shunts (e.g., the conductive shunt  606   b ,  606   d ,  606   f  and  606   h ) in a same column are aligned in a direction of the column. A gap G 1  between the conductive shunt  606   a  and the conductive shunt  606   b  in a first row is the same and aligned with a gap between the conductive shunts in each of the other rows. This configuration mitigates shunt power rail electron migration while reducing place and route resource loss. 
       FIG. 7  illustrates a top view of another layout of a cell circuit  700  in a short line configuration, according to aspects of the present disclosure. The cell circuit  700  includes conductive shunts  706   a - 706   j  in a same conductive layer (e.g., the third conductive layer (e.g., M 3 ) of  FIG. 5 ). The conductive shunts  706   a - 706   j  are grouped into rows and columns such that some conductive shunts (e.g., the conductive shunts  706   a - 706   c ) are staggered relative to other conductive shunts (e.g., the conductive shunt  706   d  and the conductive shunt  706   e ). In the staggered configuration, a gap G 2  between the conductive shunt  706   a  and the conductive shunt  706   b  is staggered relative to a gap G 4  between the conductive shunt  706   d  and the conductive shunt  706   e . Similarly, a gap G 3  between the conductive shunt  706   b  and the conductive shunt  706   c  is staggered relative to the gap G 4  between the conductive shunt  706   d  and the conductive shunt  706   e.    
     Some of the conductive shunts (e.g., the conductive shunt  706   a , the conductive shunt  706   b , and the conductive shunt  706   c ) correspond to terminals that receive a positive supply voltage (Vdd). Other conductive shunts (e.g., the conductive shunt  706   d  and the conductive shunt  706   e ) correspond to terminals that receive a negative supply voltage (Vss). The positive terminals and the negative terminals are laid out in the alternating configuration, as described with respect to  FIG. 6 . In some aspects, conductive shunts corresponding to terminals of a same polarity (e.g., positive or negative) may be aligned while conductive shunts corresponding to terminals of different polarity are not aligned. For example, the conductive shunt  706   a , the conductive shunt  706   b , and the conductive shunt  706   c  are aligned with the conductive shunt  706   f , the conductive shunt  706   g , and the conductive shunt  706   h . The conductive shunt  706   d  and the conductive shunt  706   e  are aligned with the conductive shunt  706   i  and the conductive shunt  706   j.    
     The different configurations incorporate different potential power net styles or patterns. For example, in real power net, there may be two different vertical positive supply voltage (Vdd) and negative supply voltage (Vss) configurations. One configuration includes separate or ungrouped positive and negative supply voltages in an alternative configuration, (e.g., ---Vdd-------Vss------Vdd Vss--) where Vdd represents vertical lines (not shown). Each “-” may illustrate a separation between the supply voltages. Another configuration includes grouped positive and negative supply voltages in a side-by-side configuration (e.g., ---Vdd-Vss------Vdd-Vss-------Vdd-Vss--). In this case a set of Vdd and Vss are grouped side-by-side as one set of vertical column similar to the Vdd and Vss configuration illustrated in  FIG. 8 . A staggered pattern is beneficial because IR drop can be mitigated in undesirable scenarios. For example, an undesirable (e.g., worst) device location for IR drop is between two vertical Vdd or Vss metal lines. With respect to aligned patterns, however, worst locations for Vdd and Vss IR drop are aligned. 
       FIG. 8  illustrates a top view of a layout of a cell circuit  800  in a short line configuration having multiple options for interconnecting to various layers of the cell circuit  800 , according to aspects of the present disclosure.  FIG. 8  illustrates different ways in which the conductive shunts are connected or coupled using vias to layers directly above and directly below the conductive shunts such that the positive voltages Vdd and the negative voltages Vss are provided to the various conductive layers. The cell circuit  800  includes conductive shunts  806   a - 806   f  in a same conductive layer (e.g., the third conductive layer (e.g., M 3 ) of  FIG. 5 ). 
     The conductive shunts  806   a - 806   f  are grouped into rows and columns such that some conductive shunts (e.g., the conductive shunt  806   b , the conductive shunt  806   c , the conductive shunt  806   e , and the conductive shunt  806   f ) are laid out in accordance with a short line configuration, similar to the illustration in  FIG. 6 . The other conductive shunts (e.g., the conductive shunt  806   a  and the conductive shunt  806   d ) are in a continuous configuration like the first power rail  502  of  FIG. 5 . 
     Some of the conductive shunts (e.g., the conductive shunt  806   a  and the conductive shunt  806   d ) correspond to terminals that receive a positive supply voltage (Vdd). Other conductive shunts (e.g., the conductive shunt  806   b , the conductive shunt  806   c , the conductive shunt  806   e , and the conductive shunt  806   f ) correspond to terminals that receive a negative supply voltage (Vss). The positive terminals and the negative terminals of the cell circuit  800  are laid out in an alternating configuration, similar to the illustration in  FIG. 6 . 
     The conductive shunt  806   a  and the conductive shunt  806   d  provide the positive voltages Vdd to the various conductive layers and the conductive shunt  806   b , the conductive shunt  806   c , the conductive shunt  806   e , and the conductive shunt  806   f  provide the negative voltages Vss to the various conductive layers. For example, the conductive shunt  806   a  provides the positive voltage Vdd to a first conductive line  816   a  of a fourth conductive layer (e.g., M 4 ) and to a second conductive line  804   a  (e.g., the conductive line  504   a ) of a second conductive layer (e.g., M 2 ) through at least one via V 1 . The conductive shunt  806   b  provides the negative voltage Vss to a third conductive line  816   b  of the fourth conductive layer (e.g., M 4 ) and to a fourth conductive line  804   b  (e.g., the conductive line  504   b ) of the second conductive layer (e.g., M 2 ) through at least one via V 2 . 
       FIG. 9  illustrates a cross-section  900  of a first interconnection option to the various layers of the cell circuit  800  of  FIG. 8 , according to aspects of the present disclosure. The cross-section  900  is taken across a line AA, as shown in  FIG. 8 .  FIG. 9  is focused on a direct interconnection from the first conductive line  816   a  of the fourth conductive layer (e.g., M 4 ) to the second conductive line  804   a  of the second conductive layer (e.g., M 2 ) through the conductive shunt  806   a  of the third conductive layer (e.g., M 3 ), the via V 1 , and a via V 3  in a substantially straight line path P 1 . Because the interconnection in  FIG. 9  is direct and the signal does not flow along the length of the third conductive layer (e.g., M 3 ) as it would if the first conductive line  816   a  and the second conductive line  804   a  were staggered, the signal path P 1  is not affected or only mildly affected by electron migration on the conductive shunt  806   a . Accordingly, the conductive shunt  806   a  is designed in a continuous configuration, much like the continuous configuration of the first power rail  502  of  FIG. 5 . 
       FIG. 10  illustrates a cross-section  1000  of a second interconnection option to the various layers of the cell circuit  800  of  FIG. 8 , according to aspects of the present disclosure. The cross-section  1000  is taken across a line BB, as shown in  FIG. 8 .  FIG. 10  is focused on an indirect interconnection from the fourth conductive line  804   b  of the second conductive layer (e.g., M 2 ) to the third conductive line  816   b  of the fourth conductive layer (e.g., M 4 ) through the conductive shunt  806   b  of the third conductive layer (e.g., M 3 ), the via V 2 , and the via V 4  in a staggered line path P 2 . Because the interconnection in  FIG. 10  is indirect and the signal flows along the length L 4  of the conductive shunt  806   b , the signal path P 2  (which includes the length L 4 ) is affected by electron migration on the conductive shunt  806   b . Accordingly, the conductive shunt  806   b  is designed in accordance with a short line configuration as illustrated in  FIG. 6  to mitigate the effects of the electron migration. 
       FIG. 11  depicts a simplified flowchart of a method  1100  for fabricating a cell circuit. At block  1102 , a first power rail having a first line length is fabricated. The first power rail is in a first conductive layer (e.g., a first row of the first conductive layer). The first power rail is configured to receive a first voltage for the cell circuit. At block  1104 , a plurality of conductive lines are fabricated in a second conductive layer. At block  1106 , a first conductive shunt is fabricated in a third conductive layer (e.g., a first row of the third conductive layer). The first conductive shunt is electrically coupled to the first power rail and a first set of conductive lines of the plurality of conductive lines. The first conductive shunt has a second line length, which is shorter than the first line length. At block  1108 , a second conductive shunt is fabricated in the third conductive layer (e.g., the first row of the third conductive layer). The first conductive shunt is electrically coupled to the first power rail and a second set of conductive lines of the plurality of conductive lines. The second conductive shunt has a third line length, which is shorter than the first line length. 
     According to a further aspect of the present disclosure, a cell structure is described. The cell structure includes means for receiving a first voltage for the cell circuit. The first voltage receiving means may be the first power rail  312   a  and/or first power rail  502 , shown in  FIGS. 3 and 5 . The cell structure includes means for receiving a second voltage for the cell circuit. The second voltage receiving means may be the second power rail  312   b , shown in  FIG. 3 . In another aspect, the aforementioned means may be any module or any apparatus configured to perform the functions recited by the aforementioned means. 
       FIG. 12  is a block diagram showing an exemplary wireless communication system in which a configuration of the disclosure may be advantageously employed. For purposes of illustration,  FIG. 12  shows three remote units  1220 ,  1230 , and  1250  and two base stations  1240 . It will be recognized that wireless communication systems may have many more remote units and base stations. Remote units  1220 ,  1230 , and  1250  include IC devices  1225 A,  1225 B, and  1225 C that include the disclosed cell circuit. It will be recognized that other devices may also include the disclosed cell circuit, such as the base stations, switching devices, and network equipment.  FIG. 12  shows forward link signals  1280  from the base station  1240  to the remote units  1220 ,  1230 , and  1250  and reverse link signals  1290  from the remote units  1220 ,  1230 , and  1250  to base station  1240 . 
     In  FIG. 12 , remote unit  1220  is shown as a mobile telephone, remote unit  1230  is shown as a portable computer, and remote unit  1250  is shown as a fixed location remote unit in a wireless local loop system. For example, a remote unit may be a mobile phone, a hand-held personal communication systems (PCS) unit, a portable data unit such as a personal digital assistant (PDA), a GPS enabled device, a navigation device, a set top box, a music player, a video player, an entertainment unit, a fixed location data unit such as a meter reading equipment, or other communications device that stores or retrieves data or computer instructions, or combinations thereof. Although  FIG. 12  illustrates remote units according to the aspects of the disclosure, the disclosure is not limited to these exemplary illustrated units. Aspects of the disclosure may be suitably employed in many devices, which include the cell circuit. 
       FIG. 13  is a block diagram illustrating a design workstation used for circuit, layout, and logic design of a cell circuit, such as the transistor array disclosed above. A design workstation  1300  includes a hard disk  1301  containing operating system software, support files, and design software such as Cadence or OrCAD. The design workstation  1300  also includes a display  1302  to facilitate design of a circuit  1310  or a cell circuit  1312 . A storage medium  1304  is provided for tangibly storing the design of the circuit  1310  or the cell circuit  1312 . The design of the circuit  1310  or the cell circuit  1312  may be stored on the storage medium  1304  in a file format such as GDSII or GERBER. The storage medium  1304  may be a CD-ROM, DVD, hard disk, flash memory, or other appropriate device. Furthermore, the design workstation  1300  includes a drive apparatus  1303  for accepting input from or writing output to the storage medium  1304 . 
     Data recorded on the storage medium  1304  may specify logic circuit configurations, pattern data for photolithography masks, or mask pattern data for serial write tools such as electron beam lithography. The data may further include logic verification data such as timing diagrams or net circuits associated with logic simulations. Providing data on the storage medium  1304  facilitates the design of the circuit  1310  or the cell circuit  1312  by decreasing the number of processes for designing semiconductor wafers. 
     For a firmware and/or software implementation, the methodologies may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. A machine-readable medium tangibly embodying instructions may be used in implementing the methodologies described herein. For example, software codes may be stored in a memory and executed by a processor unit. Memory may be implemented within the processor unit or external to the processor unit. As used herein, the term “memory” refers to types of long term, short term, volatile, nonvolatile, or other memory and is not to be limited to a particular type of memory or number of memories, or type of media upon which memory is stored. 
     If implemented in firmware and/or software, the functions may be stored as one or more instructions or code on a computer-readable medium. Examples include computer-readable media encoded with a data structure and computer-readable media encoded with a computer program. Computer-readable media includes physical computer storage media. A storage medium may be an available medium that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer; disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     In addition to storage on computer-readable medium, instructions and/or data may be provided as signals on transmission media included in a communication apparatus. For example, a communication apparatus may include a cell circuit having signals indicative of instructions and data. The instructions and data are configured to cause one or more processors to implement the functions outlined in the claims. 
     The various illustrative logical blocks, modules, and circuits described in connection with the disclosure herein may be implemented or performed with a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the technology of the disclosure as defined by the appended claims. For example, relational terms, such as “above” and “below” are used with respect to a substrate or electronic device. Of course, if the substrate or electronic device is inverted, above becomes below, and vice versa. Additionally, if oriented sideways, above and below may refer to sides of a substrate or electronic device. Moreover, the scope of the present application is not intended to be limited to the particular configurations of the process, machine, manufacture, and composition of matter, means, methods, and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding configurations described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.