Abstract:
A method for analyzing a layout for a semiconductor integrated circuit, which includes a plurality of physical devices, to generate physical parameter distribution enabling accurate recognition of changes in transistor characteristics caused by systematic variations. The method includes holding systematic variation tables for physical parameters dependent on the layout of the semiconductor integrated circuit among physical parameters related to characteristics of the semiconductor integrated circuit, analyzing a design layout pattern of the semiconductor integrated circuit and selecting tables corresponding to the plurality of physical devices, and generating a physical parameter distribution based on the selected tables.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-323807, filed on Nov. 8, 2005, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a layout analysis method and apparatus for analyzing systematic variations in a semiconductor integrated circuit and generating a physical parameter distribution that depends on the layout. 
     Large-scale integrated circuits (LSIs) have increasingly been miniaturized in recent years. For such miniaturized LSIs, variations in layout patterns and arrangements of circuit elements or variations in manufacturing processes greatly affect circuit performance. Systematic variations (variations for which cause can be identified using design data) cannot be distinguished from random variations (variations for which cause cannot be identified using design data) in present LSI design environments. Thus, circuits employ worst-case designs, which take into consideration all possible variations and include excessive margins for overcoming the worst conditions. In recent years, LSIs are required to operate at a lower voltage to reduce power consumption and operate at higher speeds. However, circuits with excessive margins hinder reduction in power consumption and increase in operation speed. Moreover, it is difficult to provide sufficient margins. To enable circuit designing with reduced margins, it is necessary to analyze systematic variations for a semiconductor integrated circuit and generate a physical parameter distribution that depends on the layout. 
       FIG. 1  shows a transistor layout on a chip  1 . The transistor is formed with poly gates  3  and diffusion regions  2 . In a miniaturized LSI, transistor characteristics change greatly in accordance with differences in transistor pattern shapes and density (interval) and positions of poly gates. 
     For example, the intervals of the poly gates  3  differ between transistors formed in areas a, b, and c. This is the same in areas d, e, and f. The transistors formed in areas a and d have the same pattern. However, since the transistors in areas a and d are located at different positions and arranged in different orientations, the transistor characteristics in area a and area d are different. 
     Under present designing environments, there are no analyzing means or processes for locating causes of processing variations. Thus, characteristics, such as delay time, power consumption, and leakage current, are analyzed using transistor characteristics under the worst condition (worst point) and best condition (best point) as parameters for characteristic analysis. 
     SUMMARY OF THE INVENTION 
     In the above example of the prior art, only the transistor characteristics at the worst point and the best point are used as characteristic analysis parameters when designing a semiconductor integrated circuit. Accordingly, designing must be performed with sufficient margins based on the transistor characteristics at the worst point and the best point. However, recent LSIs operate at higher speeds and lower voltages to reduce power consumption. Thus, designing that provides excessive margins hinder reduction in further reduction in power consumption and further increase in operation speeds. 
     Japanese Laid-Open Patent Publication No. 2002-318829 describes a method for easily conducting a circuit simulation, which takes into consideration variations, with high accuracy. In this circuit simulation, variations in layout patterns and arrangements are represented by expressions. Parameters for the expressions are stored as a device parameter group associated with devices. The parameters in the device parameter group are varied to conduct a simulation. 
     However, since the parameters for the expressions are stored as a device parameter group associated with devices, the simulation does not completely conform to the actual variations. 
     The present invention provides a layout analysis method and layout analysis apparatus for generating physical parameter distribution enabling accurate recognition of changes in transistor characteristics caused by systematic variations through simulations by improving the analysis accuracy of transistor characteristics. 
     One aspect of the present invention is a method for analyzing a layout for a semiconductor integrated circuit including a plurality of physical devices. The method includes holding systematic variation tables for physical parameters dependent on the layout of the semiconductor integrated circuit among physical parameters related to characteristics of the semiconductor integrated circuit, analyzing a design layout pattern for the semiconductor integrated circuit and selecting tables corresponding to the plurality of physical devices, and generating a physical parameter distribution based on the selected tables. 
     Another aspect of the present invention relates to a method for analyzing a layout for a semiconductor integrated circuit including a plurality of physical devices. The method includes holding systematic variation tables for physical parameters dependent on the layout and assembly stress of the semiconductor integrated circuit among physical parameters related to characteristics of the semiconductor integrated circuit, analyzing a design layout pattern for the semiconductor integrated circuit and selecting tables corresponding to the plurality of physical devices, and generating a physical parameter distribution based on the selected tables. 
     A further aspect of the present invention is a method for analyzing a layout for a semiconductor integrated circuit including a plurality of physical devices. The method includes holding systematic variation tables for physical parameters dependent on the layout and process stress of the semiconductor integrated circuit among physical parameters related to characteristics of the semiconductor integrated circuit, analyzing a design layout pattern for the semiconductor integrated circuit and selecting tables corresponding to the plurality of physical devices, and generating a physical parameter distribution based on the selected tables. 
     Another aspect of the present invention is a layout analysis apparatus for analyzing a layout for a semiconductor integrated circuit including a plurality of physical devices. The apparatus includes a library for holding systematic variation tables for physical parameters dependent on the layout of the semiconductor integrated circuit among physical parameters related to characteristics of the semiconductor integrated circuit. An analyzer analyzes a design layout pattern for the semiconductor integrated circuit, selects and extracts tables corresponding to the plurality of physical devices, and generates and extracts a physical parameter distribution based on the extracted tables. 
     A further aspect of the present invention is a layout analysis apparatus for analyzing a layout for a semiconductor integrated circuit including a plurality of physical devices. The apparatus includes a library for holding systematic variation tables for physical parameters dependent on the layout and assembly stress of the semiconductor integrated circuit among physical parameters related to characteristics of the semiconductor integrated circuit. An analyzer analyzes a design layout pattern for the semiconductor integrated circuit, selects and extracts tables corresponding to the plurality of physical devices, and generates a physical parameter distribution based on the extracted tables. 
     Another aspect of the present invention is a layout analysis apparatus for analyzing a layout for a semiconductor integrated circuit including a plurality of physical devices. The apparatus includes a library for holding systematic variation tables for physical parameters dependent on the layout and process stress of the semiconductor integrated circuit among physical parameters related with characteristics of the semiconductor integrated circuit. An analyzer analyzes a design layout pattern for the semiconductor integrated circuit, selects and extracts tables corresponding to the plurality of physical devices, and generates a physical parameter distribution based on the extracted tables. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing a layout on a chip; 
         FIG. 2  is a block diagram of a layout analysis apparatus according to a preferred embodiment of the present invention; 
         FIG. 3  is a diagram showing one example of a graph; 
         FIG. 4  is a schematic diagram showing a layout on a chip; 
         FIGS. 5A to 5D  are distribution models that are stored as graphs; 
         FIG. 6  is a front view showing a chip in a state in which assembly stress is produced; 
         FIG. 7  is a cross-sectional view of a package in a state in which assembly stress is produced; 
         FIG. 8  is a front view showing a chip and illustrating assembly stress; 
         FIG. 9  is a schematic diagram showing analysis of a physical parameter distribution with assembly stress; and 
         FIG. 10  is a cross-sectional diagram of a chip illustrating process stress. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A layout analysis method and apparatus for a semiconductor integrated circuit according to a preferred embodiment of the present invention will now be discussed.  FIG. 2  shows a layout analysis apparatus  100 . The layout analysis apparatus  100  includes a first library  11 , a second library  12 , a third library  13 , and an analyzer  14 . 
     The first library  11  stores layout pattern data, which is generated beforehand. In the preferred embodiment, the first library  11  stores layout patterns of cells or macros laid out for each chip or macro. 
     The second library  12  stores various types of process sensitivity parameters. The third library  13  stores various types of assembly stress sensitivity parameters. 
     The process sensitivity parameters stored in the second library  12  are tables associated with systematic factors that are dependent on the layout of the semiconductor integrated circuit, such as the pattern shapes of physical devices like transistors, the density (intervals) of patterns, and the location of patterns on the chip. These tables are distribution tables for various types of parameters that vary transistor characteristics, such as the delay time, noise characteristic, leakage current, and power consumption. As for transistors, the parameters includes the gate length, gate width, gate oxide film thickness, threshold, diffusion resistance, and wire resistance. As for wires, the parameters include wire thickness, wire width, interlayer film thickness, dielectric constant, and resistance. 
     The assembly stress sensitivity parameters stored in the third library  13  are tables associated with systematic factors that are dependent on stress produced in an assembly process. These tables are distribution tables for various types of parameters that vary transistor characteristics, such as the delay time, noise characteristic, leakage current, and power consumption. 
       FIG. 3  shows an example of a graph representing a table stored in the second library  12 . The table illustrates gate length variations that are dependent on the density (intervals) of the poly gates.  FIG. 3  shows variations in the gate length that are dependent on the intervals between poly gates. The variations in the gate length were measured through experiments conducted under a plurality of exposure conditions, and the measured values were each stored as a table in second library. 
     As for the other various types of tables, measured values obtained in the same manner through experiments are stored as tables in the second library  12  or the third library  13 . 
     The analyzer  14  is formed by a computer-aided design (CAD) device and analyzes the layout patterns stored in the first library  11 . The analyzer  14  selects a table from the second library  12  and the third library  13 . For example, the analyzer  14  selects a table for a sensitivity parameter associated with transistors. Based on the selected table, the analyzer  14  generates a physical parameter distribution that is dependent on the layout or a physical parameter distribution that is dependent on assembly stress and stores the physical parameter distribution in a fourth library  15 . 
       FIG. 4  shows an example of a layout pattern stored in the first library  11 . A plurality of transistors are laid out on a chip. When generating a poly gate physical parameter distribution that is dependent on the layout pattern, the analyzer  14  analyzes the layout pattern. For example, the analyzer  14  analyzes the pattern shapes, the density, and locations of the poly gates G 1  to G 4 . 
     Then, the analyzer  14  selects from the second library  12  a sensitivity parameter table associated with the pattern shapes, poly gate density, and poly gate location (area) of the poly gates G 1  to G 4 . For example, tables 1 to 4 (represented in a graphical format as shown in  FIGS. 5A to 5D ) respectively corresponding to poly gates G 1  to G 4  are selected. 
     Tables  1  to  4  include variation distribution models (distribution curves) stored in the fourth library  15  as a physical parameter distribution that is dependent on the layout. The physical parameter distribution stored in the fourth library  15  is used for statistical analysis of the timing characteristic (delay time), statistical analysis of the power consumption characteristic, statistical analysis of the leak characteristic, and statistical analysis of the noise characteristic. 
       FIGS. 5A to 5D  show physical parameter distributions of transistors. Physical parameter distributions for wire resistance and wire capacitance of wire patterns are also generated in the same manner and stored in the fourth library  15 . 
     In this case, the tables stored in the second library  12  are for variation information of the shapes of wire patterns, wire thickness, wire width, interlayer film thickness, dielectric constant, resistance, contact resistance, and wire thickness that is dependent on the density and location of the wires. 
     The analyzer  14  analyzes the layout pattern data stored in the first library  11  and extracts the pattern shape, density, and location of the wires for each segment. Then, the analyzer  14  selects a table corresponding to the extraction result from the second library  12  and generates variation distributions of the wire resistance and wire capacitance from the table. 
     Subsequently, the analyzer  14  synthesizes the variation distributions of the wire resistance and wire capacitance in the segments to generate the variation distribution of the wire resistance and wire capacitance for each net. That is, the analyzer  14  generates the physical parameter distribution. Then, the analyzer  14  stores the physical parameter distribution in the fourth library  15 . 
     Referring to  FIG. 6 , assembly stress acting on a chip, which is installed in a package, will now be described. When connecting pads  17  formed in the rims of the chip  16  to lead frames  18  with bonding wires  19 , bonding stress (arrow A) acts on the pads  17 . This changes the physical shape of the chip  16  at areas a 1  and a 3 . Further, tensile stress (arrow B) produced by the bonding wires  19  acts on the pads  17 . This changes the physical shape of the chip  16  at areas a 2  and a 4 . 
     The tensile stress produced by the bonding wire  19  differs depending on the shape of the lead frames  18 . Further, the stress acting in the directions of arrows A and B change the shape of the chip  16 . This causes changes in the physical shape, such as the gate width and gate length of the transistors in the chip  16 , and changes the transistor characteristics. 
       FIG. 7  shows an IC package mounted on a substrate. In this state, thermal stress acts on the chip rims and causes systematic variations in the transistor characteristics. More specifically, when soldering a package  20  to a substrate  21 , thermal stress acts on a chip  22  in the package  20 . As shown in  FIG. 8 , such stress F acts on the four corners of the chip  22  and increases at locations closer to each corner. This deforms the four corners of the chip  22  and changes the physical shape of the transistors, such as the gate width and gate length, at the corresponding positions thereby changing the transistor characteristics. 
     In such a manner, stress produced by the package shape, the substrate, and the mounting conditions changes the chip shape. To generate tables when the chip shape changes, referring to  FIG. 9 , the chip  22  is divided into a plurality of areas (segments)  24 . The physical shape change distribution caused by the stress F during the mounting is modeled for each area  24  and stored as a table in the third library  13 . 
     More specifically, with regard to the physical parameters for the gate length, gate width, and gate oxide film thickness, change distribution models of the physical parameters caused by stress, which is dependent on the package type and chip size, are generated for each area  24  and stored in the third library  13 . 
     The analyzer  14  analyzes the layout pattern, selects from the third library  13  a table corresponding to a transistor in a cell or macro arranged in the corresponding area  24 , and stores the table as the physical parameter distribution in the fourth library  15 . 
       FIG. 10  shows an example in which the transistor characteristic changes in a systematic manner due to the difference in thermal expansion coefficients between a transistor formation region and an insulator, which is located near the transistor formation region. 
     To insulate an N-channel MOS transistor formation region  25  and a P-channel MOS transistor formation region  26 , a shallow trench isolation (STI)  27  is formed between the two transistor formation regions  25  and  26 . 
     The difference in the thermal expansion coefficients between the insulator, which forms the STI  27 , and the transistor formation region produces strain caused by stress acting on the transistor formation region in the direction of arrow C. This changes the transistor characteristics, which are dependent on the pattern shape, density, and locations of the transistors. (Materials and manufacturing conditions differ depending on the process technology thereby changing the source and direction of stress.) 
     Tables for physical parameter distributions of factors causing characteristic changes, which are dependent on the process technology, pattern shape, density, and location, are generated and stored in the second library  12 . 
     The analyzer  14  analyzes the layout patterns, selects corresponding variation distribution models from the second library  12 , and stores the models as physical parameter distributions in the fourth library  15 . 
     The preferred embodiment has the advantages described below. 
     (1) Variation tables for physical parameters dependent on layouts are generated beforehand as variation distribution models. The layout patterns are analyzed and corresponding distribution models are selected. This enables the generation of physical parameter distributions for characteristic analysis. Accordingly, realistic characteristic analysis that is dependent on the layouts may be performed. Further, by designing semiconductor integrated circuits with such physical parameter distributions, margins are reduced during designing, and power consumption of semiconductor integrated circuits is lowered. 
     (2) Physical parameter distributions for characteristic analysis may be generated for variations in physical parameters dependent on the layout and assembly stress. 
     (3) Physical parameter distributions for characteristic analysis may be generated for variations in physical parameters dependent on the layout and process stress. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following form. 
     In the preferred embodiment, physical parameter distributions for variations in physical parameters dependent on the layout, assembly stress, and process stress may be generated. 
     The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.