Abstract:
The present invention pertains to a method for analyzing a semiconductor chip design for determining potential voltage drop and electromigration problems. Initially, the semiconductor chip design is divided into a plurality of blocks. A block level verification is then performed based on the assumption that full voltage is being supplied to each of the blocks. Next, the blocks are modeled by an equivalent RC network. This RC network is then reduced into a simpler representation. The voltage drops are determined based on the reduced, equivalent model. The blocks are then reanalyzed with the supply voltage input to the blocks reduced according to the calculated voltage drops. Thereby, a more realistic simulation can be achieved.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a power network simulation and analysis tool for testing the reliability of the physical designs of integrated circuit semiconductor chips. 
     BACKGROUND OF THE INVENTION 
     A highly specialized field, commonly referred to as &#34;electronic design automation&#34; (EDA), has evolved to handle the demanding and complicated task of designing semiconductor chips. In EDA, computers are extensively used to automate the design process. Computers are ideally suited to performing tasks associated with the design process because computers can be programmed to reduce or decompose large, complicated circuits into a multitude of much simpler functions. Thereupon, the computers can be programmed to iteratively solve these much simpler functions. Indeed, it has now come to the point where the design process has become so overwhelming that the next generation of integrated circuit (IC) chips cannot be designed without the help of computer-aided design (CAD) systems. 
     And after the circuit for a new semiconductor chip has been designed and physically laid out, there still remains extensive testing which must be performed to verify that this new design and layout works properly. A multitude of different combinations of test vectors are applied as inputs to the design in order to check that the outputs are correct. In the past, many prior art testing and reliability tools assumed a constant power supply voltage source. This approach was deficient because although the design might be functioning perfectly from a logic standpoint, it might, nevertheless, still not meet specifications due to hidden voltage drop and electromigration problems in interconnect wires. In real life, each of the transistors of a semiconductor circuit consumes a small amount of power (during the logic switching period.). Individually, the voltage drop in the power network attributable to a single transistor is negligible. However, due to rapid advances in semiconductor technology, today&#39;s chips can contain upwards of ten million or more transistors. The cumulative effect of all these voltage drops may lead to serious performance degradation or even critical failures. For example, a transistor might be specified to be a logic &#34;0&#34; from 0.0 to 0.7 volts and to be a logic &#34;1&#34; from 3.3 to 2.1 volts. However, due to the voltage drops in the power network, a transistor output might not switch to those specified ranges and thus results in a logic error. And even if a voltage-tolerant CMOS process is used whereby the transistor has more noise margin, its switching speed is detrimentally impacted. Higher power supply voltages makes transistors switch faster, whereas lower voltages makes them switch slower. Consequently, if the voltage in a power network of a circuit drops below a critical level, the speed of that circuit might be reduced to an unacceptable rate. 
     Another problem which might arise relates to electromigration. It has been established that high current density can cause the metal in the lines distributing the power through the semiconductor chip to migrate along the path of the current flow. Eventually, over a period of time (e.g., several years), this electromigration can result in an open circuit so that power is cut off from parts of the IC, thereby causing the IC to fail. The electromigration may even result in a short circuit which also causes the IC to fail. 
     Thus, it would be prudent to test for any potential power distribution problems as part of the overall testing and simulation process. However, testing a circuit with millions of transistors is an extremely complex and time-consuming process. It requires expert knowledge and highly skilled EDA specialists. Furthermore, it requires the dedication of a powerful and expensive mainframe computer with gigabytes of memory. Indeed, advances in semiconductor technology has led to submicron designs having even greater numbers of transistors being crammed into ever greater densities at higher levels of complexities which threaten the capability of today&#39;s most powerful computers to simulate. 
     Thus, there exists a need in the prior art for some reliability analysis tool to test and simulate gigantic power network of multi-million transistor submicron IC designs. The present invention provides a unique, efficient solution by implementing a hierarchical scheme. Basically, the present invention extracts an accurate; yet reduced RC model of the power network and current characteristics for each circuit block in the design layout file and then simulates the entire power network of the design with those derived models to determine the current flow and voltage drops in each interconnect wire in the entire circuit at each instance of time, that would otherwise be impossible to simulate due to the prohibitively large memory and CPU time requirement when tried with a conventional flat simulation method. Based on this transistor level simulation, circuit designers can pinpoint where voltage drop and electromigration may pose problems. The designers may then take corrective action before chips are fabricated and sold. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to a reliability analysis tool to test and simulate the power network of submicron IC designs. A hierarchical approach incorporating four stages is applied: block level verification, modeling, full chip simulation, and revisiting the blocks. In the block level verification stage, rather than analyzing the entire chip at once, the new chip design is divided into a number of blocks at the top level. The layout of each block matches that of the schematic. The power connection locations of each block are identified in the chip layout. These blocks are then analyzed for voltage drop and electromigration with the assumption that the full voltage levels are being supplied at each power connection point. Any problems detected during the analysis are connected and the blocks are reanalyzed as necessary. Next, in the modeling stage, the power network in each of the blocks is modeled as equivalent RC network. Current characteristics at each power connection point are also modeled as a piece wise linear function. These RC networks are then further reduced into much simpler circuits, which can be analyzed more easily, efficiently and quickly. The actual voltage drops and current waveforms through the top-level interconnect wires are then determined in the full chip RC network simulation stage based on the functionally equivalent, but reduced RC networks and the recorded current models. Once the actual voltage drops and current flows through the interconnect wires have been determined, the last stage is to revisit the blocks with this new information. Instead of assuming that the full supply voltage is being supplied to the blocks, the actual calculated voltage drops are substituted thereto as inputs to the various blocks. The blocks are then re-analyzed with the updated voltage and current values to determine whether there may exist any potential voltage, current, thermal, or electromigration problems with any nodes or wires of the new design. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The operation of this invention can be best visualized by reference to the drawings. 
     FIG. 1 shows a flowchart describing the basic stages of the present invention. 
     FIG. 2 shows a semiconductor chip which may be tested and simulated by the present invention. 
     FIG. 3 is a flowchart describing in detail, the steps for performing the block level verification. 
     FIG. 4 shows an exemplary block having three probe points with power lines supplying power to the block. 
     FIG. 5 shows an exemplary current profile flowing through a dynamic power connection as determined by the test vectors. 
     FIG. 6 shows a flowchart describing in detail, the steps for modeling the blocks. 
     FIG. 7 shows a simple example of how an RC circuit can be reduced. 
     FIG. 8 is a flowchart describing in detail, the steps associated with the full chip simulation stage. 
     FIG. 9 is a flowchart describing in detail the steps for performing the top level simulation. 
     FIG. 10 is a flowchart describing in detail the steps for revisiting the blocks. 
     FIG. 11 shows an exemplary computer system (e.g., personal computer, workstation, mainframe, etc.), upon which the present invention may be practiced. 
    
    
     DETAILED DESCRIPTION 
     A reliability analysis tool for testing and simulating the power network of submicron IC designs is described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the present invention. 
     Referring now to FIG. 1, a flowchart describing the basic stages of the present invention is shown. Rather than simulating and analyzing all of the chip&#39;s transistors and parasitic resistors and capacitors originating from the interconnect wires at the same time, the present invention performs the simulation and analysis in four basic stages 101-104. In the first stage 101, the semiconductor chip is broken into several top level blocks, with each block having four million or less transistors. Since the values of the voltage drops to the power connection points have not been calculated, it is assumed at this point that there are no voltage drops. A block level verification is then performed on each of these blocks. Next, stage 102 models the power RC networks of these blocks with simplified, equivalent circuits. The simplified circuits are electrically the same as actual circuits of the blocks that they represent, except that the total number of components (resistors and capacitors) have been reduced. This reduction makes it easier to analyze the RC networks of the blocks. Thereupon, a full chip RC network simulation can be performed in stage 103. Based on this simulation, the voltage drops corresponding to each of the blocks can now be more easily determined. In the last stage 104, the blocks are then revisited (e.g., simulated and analyzed) with the calculated voltage drops and current in flows. 
     FIG. 2 shows a ten million transistor semiconductor chip 200 as which may be tested and simulated by the present invention. The circuits inside chip 200 are broken into sixteen separate blocks 201-216. The functions of each of the four stages are now described in detail with reference to semiconductor chip 200. 
     In particular, FIG. 3 is a flowchart describing in detail, the steps for performing the block level verification. In the first step 301, all of the blocks 201-216 which are to be modeled are identified. Note that not each and every block has to be modeled in this fashion. Next, the blocks 201-216 are arranged in step 302 into subdirectories of the circuit directory. All the blocks and the top level should be in parallel directories. For each block, steps 303-310 are performed. In step 303, the layout versus schematic (LVS) checking is performed to ensure that the layout of each block matches with that of the schematic. The test vectors for driving the blocks from the top level are then obtained, step 304. The probe points corresponding to each of the power connection points are generated in step 305. A probe point corresponds to the physical location (x, y) at which power is supplied to a block. A block may have one or more probe points. An example of three probe points 401-403 for power lines 404-406 supplying power to block 201 is shown in FIG. 4. 
     The next step 306 involves performing an RC extraction to determine the equivalent RC networks of the interconnect wires. For example, a metal line may be represented with its equivalent resistance, which is a function of its length, width, and intrinsic properties. There are many different tools which are commercially available for RC extraction. In the currently preferred embodiment, the RC extraction is performed by a tool Arcadia sold by Synopsys, Inc. of Mountain View, Calif. This particular tool generates an .espf file for RC networks to VDD and GND. Based on this RC network, the voltages at the nodes and branch currents can be calculated using well-known electrical principles. Once the voltages and currents are determined, an approximate simulation of voltage drop and electromigration analysis can be created at step 307. Based on this simulation, potential problem areas are isolated. For example, the voltage drop at a particular node may be too much or the current density through one path may be too high. The user may choose in step 308 to resolve potential voltage drop and electromigration issues by tweaking the design or layout. 
     Next, the currents through the power connections or &#34;pads&#34; (e.g., probe points 401-403) are captured with a &#34;power&#34; configuration command, step 309. Finally, step 310 generates a file (e.g., piece-wise linear--.pwl), to store all the pad current waveforms for the block as a function of time. At each instant in time, the currents through a pad are recorded and a running average is calculated. After some duration of time, a sample current waveform can be constructed. In the currently preferred embodiment, these steps are accomplished through the use of an &#34;em --  make --  pad --  ipwl&#34; command to generate the .pwl file. During block level simulation, the em --  make --  pad --  ipwl function records a piece-wise linear current waveform for each power connection point in the block. FIG. 5 shows an exemplary current profile flowing through a dynamic power connection as determined by the test vectors. It can be seen that the current fluctuates as a function of the switching states of the transistors coupled to that particular power connection. In step 311, the program stores all the peak voltage drops for each pad and the average current flowing through each branch in the power network. In the currently preferred embodiment, this step is accomplished through an &#34;em --  vector --  compaction&#34; command, which compares all the voltage drops calculated for that node to find the largest voltage drop. For example, one set of test vectors might result in a voltage drop of only 0.2 volts, whereas another set of test vectors might result in a higher voltage drop of 0.7 volts. Similarly, the em --  vector --  compaction command averages all the current values over some period of time. This information is stored in separate files (e.g., VDD.ave, VDD.vec, GND.ave, and GND.vec). Step 312 determines whether all designated blocks have been verified. If so, the block verification stage 101 is completed. Otherwise, steps 303-311 are repeated for subsequent designated blocks. It should be noted that each block can be analyzed whenever the design of the block is finished. 
     FIG. 6 shows a flowchart describing in detail, the steps for modeling the blocks. The goal of this stage 102 is to reduce the extracted RC networks (stored as an .espf file) for the blocks to equivalent connectivity and impedance models. In step 601, an .espf file is created from performing an RC extraction. Next, the power connection points are specified, step 602. The reduction is accomplished in part by first performing serial and/or parallel reductions in step 603. The principles behind the reductions are well known in the art. For example, FIG. 7 shows an equivalent circuit for a simple case of an resistor/capacitor (RC) circuit reduction. By iteratively reducing the resistors, inductors, and capacitors (RLCs), a complex circuit can be reduced into simpler circuits that is electrically equivalent to the original circuit. In the currently preferred embodiment, step 603 is accomplished by running an &#34;em --  list --  espf --  file reduction&#34; command to generate an .spi file containing the equivalent circuit having serial/parallel reductions. Further reductions are possible in step 604 by applying commercially available reduction tools. One such tool is the AWE product, manufactured and sold by Synopsis Inc. of Mountain View, Calif. The &#34;make --  awe&#34; program can reduce a typical circuit by a factor of ten. The resulting reduced, equivalent circuit is stored in an .awl file. The power connection points were preserved during step 310 (see FIG. 3). In the currently preferred embodiment, they are specified as probe 13  points by the em --  add --  pad comment. Step 605 ensures that steps 601-604 are repeated for each of the blocks. 
     FIG. 8 is a flowchart describing in detail, the steps associated with the full chip simulation netlist preparation stage. The first step 801 of the full chip simulation stage 103 involves assigning probe --  points for power connection points to the blocks. In the currently preferred embodiment, a utility known as &#34;top --  rail&#34; has been developed to generate the power connection points of all the blocks for the top level extraction according to an input file. The input file (e.g., block --  instance file) contains one line description of each block instance. An exemplary format might be: instance --  name block --  name x y orientation vdd --  probe file gnd --  probe --  file. An example of such an input file might look like: x1 ram1 5709.60 2469.40 0ram1/probe.vdd ram1/probe.vss. The composite power connection points (probe --  points) of all the specified blocks are stored in an output file. Next, step 802 performs the actual RC extraction for the top level composite mode. The modeled blocks are represented as black boxes with inputs, functions performed on the inputs, and outputs. The VDD and GND lines terminate at the boundaries of the blocks which are modeled as black boxes (with probe --  points assigned). In step 803, the top level power netlist is prepared. As part of step 803, the reduced power network is inserted to the top level power netlist for each block. This can be achieved by using the same &#34;top --  rail&#34; utility which inserts a call from the top level .espf file to the awe file for each block instance. The port orders are matched by name. Next, step 804 prepares the blocks for simulation. In the currently preferred embodiment, this is accomplished by attaching em --  ipwl commands to the probe --  points locations. In other words, the current waveforms (see FIG. 5) are applied to the power connection points of the block instances. The &#34;em --  ipwlfile filename instance --  name&#34; config command is specified to match the .pwl file of a block to multiple instantiations. If necessary, top level vectors will be applied. Finally, step 805 performs the top level simulation. 
     FIG. 9 is a flowchart describing in detail the steps for performing the top level simulation. First, the whole chip netlist files (.espf ), including calls to reduced .awe block files, is accessed in step 901. Next, step 902 drives the transistors contained in a top level netlist which also includes the vectors for driving those transistors. There is no need to include the blocks for step 902. In step 903, the em --  ipwl config commands are applied to the appropriate probe --  points for each block. The simulation vectors are then assigned in step 904 to drive the transistors at the top level. At this point, step 905, the voltage drop and current density analysis can be performed for the top level. Step 906 resolves all top level issues that have been identified. Finally, step 907 records the peak voltage drop and the average current in flow for each power connection points for all of the blocks. It should be noted that steps 902 and 904 are optional. 
     FIG. 10 is a flowchart describing in detail the steps for revisiting the blocks. Once the voltage drops for the supplies to the blocks have been determined (step 907), the power supplies to the blocks are adjusted based on this new information and the block is resimulated. This is accomplished by first applying the recorded voltage drops to the power supply connection points for the block and then simulating the RC network for voltage drops, steps 1001 and 1002. Next, the recorded current to the power supply connection points for the block are applied, and the RC network is simulated for current density, steps 1003 and 1004. A determination is then made in step 1005 as to whether there are any voltage drop violations that will be caused by the voltage drop at the block connections. Similarly, the current density at each wire is resimulated with the current flowing into the pads into consideration (calculated in step 907). This may include new voltage drop highlight and/or violation files as well as new current density highlight and/or violation files, which are placed in the directory of that particular block. If there is a violation, the designer can fix the violation by changing either the netlist or layout geometries to modify the design, step 1006. This may include new voltage drop highlight and/or violation files as well as new current density highlight and/or violation files, which are placed in the directory of that particular block. Optionally, the user can choose to expand a block to a transistor level in step 1007. If a block is expanded, the top level simulation is performed (see process of FIG. 8), step 1008. The user may desire to leave the block modeled at the top level. In either case, the number of transistors needs to be consistent with the capacity associated with the workstation running this tool. This process of steps 1001-1008 can be iteratively repeated for each of the blocks, step 1009. 
     Referring to FIG. 11, an exemplary computer system 1112 (e.g., personal computer, workstation, mainframe, etc.) upon which the present invention may be practiced is shown. The reliability analysis tool to test and simulate the power network of semiconductor chips is operable within computer system 1112. When configured with the simulation and testing procedures of the present invention, system 1112 becomes a computer aided design (CAD) tool 1112, for reliability analysis. The four stages of the present invention described in Figures are implemented within system 1112. 
     In general, computer systems 1112 used by the preferred embodiment of the present invention comprise a bus 1100 for communicating information, one or more central processors 1101 coupled with the bus for processing information and instructions, a computer readable volatile memory 1102 (e.g., random access memory) coupled with the bus 1100 for storing information and instructions for the central processor 1101. A computer readable read only memory (ROM) 1103 is also coupled with the bus 1100 for storing static information and instructions for the processor 1101. A random access memory (RAM) 1102 is used to store temporary data and instructions. A data storage device 1104 such as a magnetic or optical disk and disk drive coupled with the bus 1100 is used for storing information and instructions. A display device 1105 coupled to the bus 1100 is used for displaying information to the computer user. And an alphanumeric input device 1106 including alphanumeric and function keys is coupled to the bus 1100 for communicating information and command selections to the central processor 1101. A cursor control device 1107 is coupled to the bus for communicating user input information and command selections to the central processor 101, and a signal input/output port 1108 is coupled to the bus 1100 for communicating with a network. The display device 1105 of FIG. 11 utilized with the computer system 1112 of the present invention may be a liquid crystal device, cathode ray tube, or other display device suitable for creating graphic images and alphanumeric characters recognizable to the user. The cursor control device 1107 allows the computer user to dynamically signal the two dimensional movement of a visible symbol (pointer) on a display screen of the display device 1105. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.