Abstract:
An improved system and method is disclosed for performing a design rule check on a proposed integrated circuit (IC) layout, and for creating customized design rule check command files. The individual layers of the IC (a system on chip—SOC) are separated into different regions having different kinds of features (i.e., memory or logic). Each different type of region is then analyzed in accordance with the customized design rule command file so that so-called “false errors” are eliminated. The invention thus improves, among other things, a development time for getting a design implemented in silicon.

Description:
RELATED APPLICATION DATA 
     The present application claims priority from provisional application Ser. No. 60/278,672 filed Mar. 20, 2001, which is also incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor design technology, and in particular, to systems and techniques used for implementing circuit designs into silicon based integrated circuits (ICs). 
     BACKGROUND 
     In the field of semiconductor design technology, a design rule check (DRC) is a well-known process for inspecting whether mask pattern data of a semiconductor integrated circuit is correctly designed in compliance with fabricators&#39; topological layout rules (TLR). The TLR are unique to each fabrication facility, or semiconductor wafer plant, based on available process technologies, equipment limitations, etc. An example of prior art DRC system is illustrated in U.S. Pat. No. 6,063,132 incorporated by reference herein. 
     Integrated circuit designers transform circuit schematics to mask data by drawing polygons that represent the physical masks to be fabricated on silicon. For example, a transistor symbol on a circuit schematic could be represented by simply drawing a POLY layer polygon (gate region) crossing a DIFFUSION layer polygon (source and drain regions) and both polygons are laid within a WELL layer polygon. These mask pattern data are usually in well-known GDS format (binary) and are used by a design rule checker to check against design rules embodied in coded form in a design rule check command file. 
     FIG. 1 illustrates the basic flow of design rule checking on mask pattern data of an integrated circuit in accordance with a prior art routine  100 . A design rule check (DRC) command file  120  is coded in accordance with a topological layout rule document  101 . DRC command file  120  and the mask pattern data  130  (representing the physical layout of the IC) are used as inputs by a design rule checker  140  (typically a software routine) for design error detection and to generate a results list. Any subsequent design errors detected must be corrected in the layout proposed by the IC designer as shown in block  150 . For some types of ICs, this process of checking the design errors can be automated, but for many others, it cannot. 
     A recent development in the IC industry has been the incorporation of memory and logic within the same IC, as found for example in embedded memory systems, and in so-called system-on-chip (SOC) designs. These systems present a unique challenge to the design process, because memory and logic circuits have different sizing, performance and scaling issues when embodied in silicon. Thus, logic and memory layout regions they must be treated differently during the verification process. 
     A first conventional method of checking design rules for an SOC design is described now with reference to the system  200  shown in FIG.  2 . In the case of a semiconductor foundry (i.e., those plants that specialize in rendering third party designs into silicon) a design rule check command file  220  is written by referring to a Foundry&#39;s Topological Layout Rule document (TLR)  201 . Therefore, this design rule command file typically only consists of so-called “Logic Rules”  210  applicable to a logic section of a chip under consideration. As before the Mask Pattern Data  230  for the chip is fed into a design rule checker  240  to check against these logic rules. The output of this process is a design rule check result file  250 . 
     At this point, the results consist of real logic error(s)  251 , false logic rule errors on memory blocks  252  and possibly real memory error(s)  253 . The false logic rule errors  252  are caused by the fact that logic circuits implemented in silicon typically require greater spacings, sizings and margins than memory circuits. An extra step  260  thus has to be carried out to filter false error(s) from the other real errors reported in the result database. The typical method is by manual effort—i.e., human eye review and filtering. This can be extremely time consuming and cumbersome since the number of false design rule errors will increase relative to the number of memory blocks used and the size of each memory instance. 
     Thus, this approach flags many false DRC errors because each memory bit cells and associated leaf cells can cause many DRC violations that are not “true” because they do not actually violate a memory design rule that is applicable to the memory block. An operator has to manually check all the violations against a memory design rule check at step  270  to see if they are real errors. They then iteratively repeat this process to arrive at a set of real logic errors  271 , and real memory errors  272 . Nonetheless, this process can often cause real errors to be overlooked. Furthermore, this process slows down the design development cycle because each error must be discussed with the IC design supplier, and this interaction can be time consuming as it requires cooperation between the IC designer, the foundry field support engineers, and the foundry itself. 
     A second conventional method of checking design rules is described below with system  300  referenced in FIG. 3, where like numbers are intended to denote like structures and operations unless otherwise noted. In this approach, a Cell Delete or Masking technique  310  masks out the whole memory instance including memory bit cell arrays and other associated logic support circuitry such as wordline decoder, sense amplifier, etc. from design rule checks performed by checker  340  on GDS file  330 . The design rule check (DRC) results  350  will thus consist only of real logic errors  351 . However, this method assumes the memory blocks used are DRC clean and that the interfaces (or intermediate regions) between the memory and logic parts satisfy logic rules. Consequently, it will not detect any real errors in such features. 
     Accordingly, a substantial need exists in this field of art for an improved design verification tool that eliminates the aforementioned problems. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, therefore, is to provide a design rule checking system and method that accurately reports appropriate errors for appropriate regions in a system that includes two different kinds of circuits, i.e., such as memory and logic. 
     A related object is to eliminate false errors caused by design rule checking tools examining regions that are not subject to the same design rules supported by the design rule-checking tool. 
     These objects are achieved by the present invention, which provides a system and method of checking design rules to determine whether or not a logic part (all non-memory devices that have to satisfy TLR) of a mask pattern data obeys logic rules (as specified in a TLR document) and the memory part of a mask pattern data obeys memory rules (in this case, logic rules that are modified appropriately for a memory area to accommodate the more liberal values available for such regions). This method helps users filter out false errors due to logic rule checks in a memory block, and further helps pinpoint real design rule errors in the mask pattern data. 
     Another aspect of the invention covets the creation of customized rules appropriate for different types of memory regions that might be included on a chip, where such customized rules are based on modifying a standard logic rule by “pushing” more liberal memory based parameter on to a stricter logic based parameter. 
     Yet another aspect of the invention pertains to a program that can be executed on any number of conventional computing systems for creating such customized rules, and/or for performing a design rule check on mask pattern data based on such customized rule sets. 
     Still another aspect of the invention concerns a system configured with the above mentioned customized design rules and programs executing on a conventional computer system. 
     In a preferred embodiment, the results are categorized in different categories in accordance with where they occur, such as in Logic, Bordered Single Port (BDSP) SRAM, Borderless Single Port (BLSP) SRAM, Dual Port (DP) SRAM and ROM groups. Furthermore it will more accurately detect and classify mistakes or any modifications made to standardized foundry bit cell designs as may be made by memory compiler vendors or layout engineers at the IC designer. 
     The present invention should find significant usage in the semiconductor industry and similar industries (for example, LCD) where different design rules must be applied to different types of regions on a substrate. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a general prior art process flow sequence for performing design rule checking on mask pattern data of an integrated circuit. 
     FIG. 2 illustrates a flow diagram of steps performed by a 1 st  prior art method for real Logic and Memory errors detection. 
     FIG. 3 illustrates a flow diagram of steps performed by a 2 nd  prior art method for real Logic errors detection. 
     FIG. 4 illustrates a flow diagram of steps performed by a preferred embodiment of the present invention for Real Logic and Memory errors detection. 
     FIG. 5 illustrates a flow diagram of steps performed by [this] the preferred embodiment of the invention for creating customized design rule check command files based on modifying a standard logic rule file to take into consideration minimum dimensional values extracted from memory bit cell design parameters. 
     FIG. 6A illustrates the application of logic and memory rules within or between the logic and memory regions of a mask pattern data; 
     FIG. 6B illustrates the relationship of various circuitry regions in a layout of an IC, and how such are treated by the present invention; 
     FIGS. 7A to  7 C are examples of memory cell violations triggered by a standard logic rule against a variety of types of memory cells, and which violations are used to push appropriate minimum dimensional data onto a modified form of the logic rule to be used for designs utilizing such type of memory cell. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a solution to the disadvantages of the first and second conventional methods of checking design rules as explained above. From a broad perspective, the method generally applies the right set of rules to the right regions of the mask pattern data. To simplify the process (i.e., to avoid having to create an entire set of design rule checks from scratch, or to harmonize several different types of design rules from different memory cell vendors) and ensure its accuracy with respect to any particular set of foundry rules, the customized design rules are based on modifying a standard set of Logic Rules as needed to reflect needs of particular regions in the chip. Thus, a customized design rule is created for each different type of region that may be present on the chip, and this customized design rule is in fact simply based on pushing more liberal parameters onto stricter parameters contained in the standard Logic rules, and only in circumstances where it is necessary to do so. Accordingly, because different types of circuitry (logic, memory) may require different processing steps, lithographic constraints, etc., they can be treated independently by the present invention to ensure that design rules are accurately resolved for a system on chip integrated circuit design which uses a mix or blend of such circuitry. For instance, since memory circuits tend to be more aggressively sized and manufacturable than comparably sized and spaced logic designs, the former are subject to fewer layout constraints. These constraints include, among other things, minimum feature size, allowed feature shapes (i.e., avoiding notches and similar undesirable shapes), minimum distances between different types of feature shapes, etc. For example, a gate width might be smaller in a memory design than a logic design, and the minimum spacing between two signal lines may be smaller as well. Allowable contact sizes and feature shapes may vary from region to region. Other examples will be apparent to those skilled in the art. 
     This system and method is described below with reference to FIG. 4. A system  400  includes a conventional computer system and various software routines and libraries for performing a design rule check as now explained. In particular, system  400  includes a standard logic design rule (for logic areas)  410  that is supplemented by additional customized logic rules  411 - 414  (for other types of areas such as specialized memory areas). One or more rules from this set are used to check a design in GDS form  430 , depending on the types of regions presented in the IC. For example, if logic and (BDSP) SRAM were included in a design, both of these design rules would be used by design rule checker  440  (a software routine operating on the computer system) to check different areas of an IC layout as explained below. As seen in FIG. 4, each type of memory has its own set of customized rules to check against with. Note that in FIG. 4, “BDSP SRAM” stands for Bordered Single Port Static Random Access Memory; “BLSP SRAM” stands for Borderless Single Port Static Random Access Memory; “DP SRAM” stands for Dual Port Static Random Access Memory and “ROM” stands for Read Only Memory. The result is that a design rule check result  450  includes a number of separate error reports for layout violations detected in a layer (or layers) of an IC, including  451  (for real logic errors)  452  (for real BDSP SRAM errors)  453  (for real BLSP SRAM errors)  454  (for teal DP SRAM errors) and  455  (for real ROM errors). Similar customized design rules could be created, of course, for embedded DRAM, flash, etc. The necessity for manual checking, and the possibility of so-called “false” errors, is substantially eliminated. This principle could be extended beyond just memories, of course, to include other design rules for other areas that have differing design rule requirements. 
     A system  500  which derives the customized memory rules from standard logic design rules is shown in FIG.  5 . Note that the system  500  also can be any conventional computing system appropriately configured with the libraries, files and routines explained herein, and in fact, in a preferred embodiment, is the same system as system  400  noted earlier. The first step performed by system  500  is to run a design rule check with checker  540  on a memory bit cell mask pattern data  530  (from the appropriate memory type) against a design rule command file  520  that consists of only standard foundry logic rules  510 . From this report  550 —an example of which is shown in FIG. 7A for a BDSP SRAM—a list of violations is created at  551  as presented by the bit cells. In other words, the various features of the memory cell are checked against standard logic rules to determine where they will fail, and to generate a comprehensive list of all possible errors. These errors are analyzed to determine how the standard logic rules  510  should be modified for a customized design rule set for the particular memory cell for this vendor. Thus, an analysis of the actual memory design rules of such memory cell is made at step  560 , and then the appropriate parameter (minimum dimension) is then “pushed” onto a modified form of the standard logic design rules to create a set of distinct and separate design rules  571 - 575  at step  570 . Further examples are illustrated in FIGS. 7B and 7C for BLSP and DP SRAM cells in such memories for a 0.18 micron design as tested against the present assignee&#39;s own generic design rules as published as of the current date (version 2.2 p0). It is apparent that different violations would be presented by different logic and memory design rules, so that different types of parameters would be pushed as needed onto standard logic rules when creating customized design rules. 
     All these extracted values are used to derive customized memory rules ( 571 - 575 ) for each type of memories. Thus, this invention can be applied to any mask pattern database, including one having no memory blocks, or even multiple types of memory blocks. The only modification required to implement the present invention using conventional GDS formatted data is that different types of memory should be identified in some way, such as with different memory ID layers to defined core bit cell regions. This can be done in advance, by modifying the GDS data file directly, by adding a distinct memory ID layer on top of each type of memory to identify such different respective memory region types. 
     Other techniques for identifying such layers will be apparent to those skilled in the art, and the present invention is by no means limited to any particular embodiment in this respect. The main goal is simply to ensure that design rule checker  540  is able to correlate a particular region in a layer with a particular set of design rules, and this can be accomplished in any number of ways either explicitly or implicitly. 
     FIGS. 6A and 6B illustrate the relationships of different polygons on a mask pattern data, and shows how different design rules are effectuated on a layer  600  within the chip layout. For polygons  610 ,  615  within a logic area  605 , logic rules  620  should be applied. For polygons  630 ,  635  within a memory area  625 , memory rules  640  should be applied. For a polygon  660  that is an intermediate area, i.e., extending from a logic area  605  to a polygon within a memory area  625 , logic rules  620  are also applied in a preferred embodiment. This is because memory rules can only apply to polygons within the memory area due to different process impact. As suggested earlier, the conventional prior art methods do not and cannot distinguish between logic polygons and memory polygons within a layer. Therefore, the same set of rules is used to check against all polygons in a mask pattern data regardless of logic and memory regions, and this leads to improper results. 
     The manner in which the invention checks different regions with different design rules is shown in FIG. 6B as follows. First, in a particular layer A  600  of a layout, a polygon  605  in a logic area is derived as A_logic whereas a polygon  625  of layer A in a memory area is derived as A_memory. To satisfy a foundry&#39;s design rules for implementing a design into silicon, some minimum geometric constraints or dimensions must be observed; these include: a) Minimum A_logic to A_logic spacing defined as logic_value; b) Minimum A_memory to A_memory spacing defined as memory_value; and c) Minimum spacing between A_logic and A_memory is also defined as logic_value. 
     Accordingly, an appropriate standard logic design rule check is executed on region A_logic  605  using logic rules  571 , and not on any other region. A_logic is derived as (layer A NOT MEMORY). This yields any appropriate errors for this logic region of this layer, and is accurate for such region. Next, any memory regions  625  are treated (by examining their ID) in accordance with an appropriate memory region design rule ( 572 - 575 ). The A_memory layer is derived as (layer A AND layer MEMORY). This yields any appropriate errors for this memory region of this layer, and is accurate for such memory region. Any other memory regions are examined in the same way, with a design rule selected based on a particular memory ID. 
     It is apparent, of course, that the sequence is not critical, and that the steps could be reversed. It is only important that the appropriate region receive proper treatment in accordance with an appropriate design rule. All of the above processes can be performed in software with a conventional computer system as noted earlier that is adapted to execute the types of code described herein. Moreover, the aforementioned software routines/programs may be implemented using any number of well-known computer languages known to those skilled in the art in this area, and thus the invention is not limited in this regard. 
     Accordingly, the invention ensures that all types of memory regions have to fulfill all memory rules of their group. Correspondingly, all logic regions have to fulfill logic and memory rules (all logic regions that passed logic rules should have also passed memory rules since memory rules are looser compare to logic rules). The process is superior to prior art techniques in that it avoids false errors, and is more reliable, more efficient, etc. 
     Thus, as noted FIG. 4, Mask Pattern Data  430  is fed into design rule checker  440  to check against both the logic and different memory rules as such may be needed. It is understood, of course, that in the case where an IC does not require mixed types of circuit types (i.e., logic and memory) that it may not be necessary to run both types of checks on each layer. The output of this process is a design rule check result file  450 . The results consist of only real logic  451  and real memory errors  452 . Thus, the present method divides layers of a mask pattern data into LOGIC, BDSP, BLSP; DP and ROM regions (or as many regions as there are different circuit types) so that the right sets of rules will only apply to the right regions. In this manner, false design rules are eliminated, and the implementation of circuit designs into silicon form is expedited as well. 
     Although the present invention has been described in terms of a preferred embodiment, it will be apparent to those skilled in the art that many alterations and modifications may be made to such embodiments without departing from the teachings of the present invention. In addition, many other industries, including liquid crystal display manufacturing and similar micro-patterned technologies, may benefit from the teachings herein. Accordingly, it is intended that the all such alterations and modifications be included within the scope and spirit of the invention as defined by the following claims.