Patent Publication Number: US-8122418-B2

Title: Capacitor arrangement method and layout apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a technique of arranging, in LSI layout design, capacitors used for reducing noise. 
     BACKGROUND ART 
     As the operating frequency of an LSI is increased, power-supply noise inside the LSI is increased. In order to reduce the power-supply noise, a capacitor element called on-chip capacitor or decoupling capacitor is formed on a power-supply line of the LSI. PTL 1 discloses a technique concerning the layout of such capacitor elements. 
     The technique disclosed in PTL 1 is a technique in which a target area in which a decoupling capacitance is formed is divided into a plurality of areas, and unit cells of the decoupling capacitance are arranged in each area. At this time, the unit cells are laid out such that the power consumption in each area does not exceed an average value that has previously been calculated. 
     CITATION LIST 
     Patent Literature 
     
         
         {PTL 1} JP-A-2006-040962 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the technique of PTL 1, each unit cell to be laid out is a cell of a predetermined single-size. The smaller the size of each unit cell, the more convenient it is for the unit cells to be arranged in a small area. However, in the case where small cells are arranged in a large area, a large number of cells are required, complicating the arrangement pattern and subsequent manufacturing process. On the contrary, a larger cell size is convenient for the unit cells to be arranged in a larger area but inconvenient in a smaller area. Thus, achievement in efficient layout is likely to be difficult depending on the size setting of the unit cell. 
     Further, in order to achieve noise reduction, measures based on the frequency of the noise are required. However, PTL1 does not consider the frequency of noise. 
     Thus, the present invention relates to the layout of capacitor cells, and an object thereof is to provide a capacitor arrangement method and a layout apparatus capable of achieving both efficiency in layout design and frequency-based noise reduction. 
     Solution to Problem 
     According to a first aspect of the present invention, there is provided a capacitor arrangement method, including: storing a plurality of capacitor cells which are classifiable into a first classification for identifying capacitor cells having different sizes by frequency characteristic correlating with gate width of a capacitor and a second classification for identifying capacitor cells having different frequency characteristics by cell size; recognizing a plurality of combinations of a directed frequency characteristic and arrangement area; selecting, for each of the combinations, capacitor cells corresponding to the directed frequency characteristic based on the first classification, reading out the capacitor cells in the descending order of cell size from the selected capacitor cells, and arranging the read out capacitor cells to fill the directed arrangement area; checking a violation of capacitor density for all the directed arrangement areas of the plurality of combinations; replacing, when detecting the violation, a capacitor cell having larger gate width out of the arranged capacitor cells with a capacitor cell having smaller gate width besides the same cell size as the capacitor cell having larger gate width in accordance with the second classification; and retrying checking the violation of capacitor density after finishing the replacement. 
     According to a second aspect of the present invention, there is provided a layout apparatus including: a storage section storing a plurality of capacitor cells which are classifiable into a first classification for identifying capacitor cells having different sizes by frequency characteristic correlating with gate width of a capacitor and a second classification for identifying capacitor cells having different frequency characteristics by cell size; and a layout section: recognizing a plurality of combinations of a directed frequency characteristic and arrangement area; selecting, for each of the combinations, capacitor cells corresponding to the directed frequency characteristic based on the first classification; reading out the capacitor cells in the descending order of cell size from the selected capacitor cells; arranging the read out capacitor cells to fill the directed arrangement area; checking a violation of capacitor density for all the directed arrangement areas of the plurality of combinations; replacing, when detecting the violation, a capacitor cell having larger gate width out of the arranged capacitor cells with a capacitor cell having smaller gate width besides the same cell size as the capacitor cell having larger gate width in accordance with the second classification; and retrying checking the violation of capacitor density after finishing the replacement. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to achieve both efficiency in layout design and frequency-based noise reduction in the layout of capacitor cells. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory view concerning a capacitor cell in an embodiment of the present invention. 
         FIG. 2  is an explanatory view concerning a sell list in the embodiment. 
         FIG. 3  is a flowchart showing an operation procedure in the embodiment. 
         FIG. 4  is a configuration of a layout apparatus in the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. The present embodiment assumes two types of frequencies: high frequency and low frequency as a frequency band of noise to be reduced by means of an on-chip capacitor arranged in LSI. For example, a frequency band of 1 GHz or more may be defined as high frequency, and frequency band of less than 1 GHz may be defined as low frequency. 
       FIG. 4  shows a configuration of a layout apparatus  100  according to the present invention. A storage section  101  stores a cell list indicating a plurality of types of capacitor cells used as capacitor models, classification of the capacitor cells, and the like. A layout section  102  executes layout of the capacitor cells read out from the storage section  101  according to a procedure to be described later. 
       FIG. 1  shows a configuration of a capacitor cell. In this example, with the size of a cell CH 11  for high frequency set as a reference (1×1), a cell (CH--) for high frequency and a cell (CL--) for low frequency are prepared. Further, a fill cell F 11  serving as a blank cell used at the time of density violation is prepared. The size type includes 2×1 type obtained by doubling the cell width of the reference size, 4×1 type obtained by quadrupling the cell width of the reference size, and 2×2 type obtained by doubling both the cell width and cell height of the reference size. 
     The frequency characteristic of each capacitor cell is defined by a width “W” of a transistor in the cell and a gate width “L” of the transistor. An on-chip capacitor having a smaller gate width L is suitable for higher-frequency noise reduction. Thus, in the present embodiment, the gate width L of the capacitor cell for high frequency is designed smaller than that of the capacitor cell for low frequency. In the cell for high frequency, the capacitor density in the cell, that is, a ratio of the area occupied by a capacitor pattern including a transistor and a wiring relative to the total area of the cell is set lower than in the cell for low frequency. 
     Further, in the case where two capacitor cells have the same frequency characteristic and same aspect ratio of the transistor width W and gate width L, a cell having a larger size has less dead space in the cell and higher capacitor density. Specifically, comparing cells CH 11  and CH 22  for high frequency, cell CH 22  having a cell size larger than cell CH 11  has a higher capacitor density. 
       FIG. 2  shows an example of the cell list for identifying the capacitor cells. A cell list  10  includes a first classification  11  classifying the capacitor cells by frequency characteristic and a second classification  12  classifying the capacitor cells by cell size. In the first classification  11 , the capacitor cells of respective categories are listed in descending order of cell size. In the case of “CELL FOR HIGH FREQUENCY”, the capacitor cells are listed starting from cell CH 22  having the largest size, followed by cell CH 41 , cell CH 21 , and cell CH 11  in the order mentioned. According to this listing order, the cells are read out in layout operation to be described later. In the second classification  12 , the cells are classified by cell size. 
     A layout procedure for arranging the capacitor cells in an LSI will be described with reference to a flowchart of  FIG. 3 . When recognizing that the frequency characteristic and arrangement area have been directed (step S 1 ), the layout section  102  of the layout apparatus  100  refers to the cell list  10  ( FIG. 2 ) stored in the storage section  101  and selects, from the first classification  11 , the category corresponding to the directed frequency characteristic (step S 2 ). For example, in the case where high frequency is directed as the frequency characteristic, the layout section  102  selects the category of “for high frequency” from the first classification  11 . 
     The layout section  102  uses the capacitor cells of the selected category to sequentially arrange the capacitor cells so as to fill the directed area. The capacitor cells to be arranged are read out from the storage section  101  in the order corresponding to the cell list  10  (step S 3 ). If the “CELL FOR HIGH FREQUENCY” is directed, cell CH 22  is arranged first in the arrangement area. If there remains a space smaller than the cell size of CH 22 , the layout section  102  tries to arrange, in the space, the subsequent cells CH 41 , CH 21 , and CH 11  in the same category in the order mentioned. In this manner, the layout section  102  fills the arrangement area with capacitor cells for high frequency (Step S 4 : No→S 3 ). 
     After finishing the arrangement of the capacitor cells in the directed area (step S 4 : Yes), the layout section  102  checks whether frequency characteristic and arrangement area are newly directed. In the case where the layout section  102  recognizes a new direction (step S 1 : Yes), the layout section  102  arranges the capacitor cells of the directed frequency characteristic in the directed arrangement area in the descending order of cell size (steps S 2  to S 4 ). 
     In the case where there is no further direction as to frequency characteristic and arrangement area (step S 1 : No), the layout section  102  checks, based on a predetermined design rule, whether there is a density violation in the current layout. To this end, the layout section  102  calculates the capacitor density in all the arrangement areas (step S 5 ). In the case where the calculated density does not exceed a specified value, the layout section  102  determines that there is no violation (step S 6 : No), that is, determines that the layout has been made properly and ends the processing. 
     On the other hand, in the case where the calculated capacitor density exceeds the specified value, the layout section  102  determines that there is a density violation in the current layout (step S 6 : Yes). When detecting the violation, the layout section  102  replaces cells having a high capacitor density with cells having a low capacitor density, and retries the density calculation. As described above, a cell for high frequency has a smaller gate width besides a lower capacitor density as compared to a cell for low frequency. Thus, in the present embodiment, when there occurs a density violation, the cells for low frequency are replaced by the cells for high frequency or fill cells so as to reduce the density. 
     Then, the layout section  102  checks whether the number of violations has exceeded the upper limit. In the case where the number of violations at the current time point is less than the upper limit (step S 7 : No), the layout section  102  replaces cells for low frequency with cells for high frequency having the same cell size as that of the cells for low frequency (step S 8 ). The cells having the same cell size are listed on the second classification  12  of the cell list  10  ( FIG. 2 ). For example, cell CL 21  of 2×1 type for low frequency is replaced by the same 2×1 type cell CH 21  for high frequency. After finishing the replacement, the layout section  102  retries the density calculation in the current layout (step S 5 ) and determines presence/absence of a violation. 
     Although the above replacement of the cell may be performed for all the cells for low frequency, it may be performed for some cells for low frequency. This allows the density to be reduced gradually. 
     On the other hand, in the case where the number of violations has exceeded the upper limit (step S 7 : Yes), a given cell for low frequency is replaced by fill cells having the same size as that of the given cell for low frequency (step S 9 ). Also in this case, all the cells for low frequency may be replaced by the fill cells or only some cells for low frequency may be replaced by the fill cells. In the case where the density violation occurs even when all the cells for low frequency have been replaced by the fill cells, some cells for high frequency may be replaced by the fill cells, for example. After completion of the replacement, the layout section  102  performs the density calculation once again (step S 5 ). 
     According to the present embodiment, capacitors corresponding to the assumed frequency band of noise are arranged, thereby increasing noise reduction accuracy. Further, cells having the same frequency characteristic and different sizes are prepared, and the capacitors are arranged in the descending order of cell size, whereby layout operation can be made efficient. Therefore, both efficiency in layout design and frequency-based noise reduction can be achieved. 
     The present invention is not limited to the above embodiment, but various modifications may be made within the spirit of the present invention. For example, although two types of frequencies: high frequency and low frequency are assumed as the frequency characteristic of the capacitor, the number of types of frequencies may be three or more. Also in that case, the gate width of the capacitor in the cell having a higher frequency is made smaller. 
     The present invention can be implemented also as a computer program corresponding to the operation ( FIG. 3 ) of the layout section  102  in the above embodiment or a recording medium storing the program. When the above implementation is realized, the program is stored in the recording medium, and computer reads out the program from the recording medium and executes it. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2008-262713, filed on Oct. 9, 2008, the disclosure of which is incorporated herein its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
         CH 11 , CL 21 , CH 21 , CH 22 , CL 41 , CH 41 : Capacitor cell 
         F 11 : Fill cell (blank cell) 
         L: Gate width 
         W: Transistor width 
           10 : Cell list 
           11 : First classification 
           12 : Second classification