Abstract:
A method ( 300 ) for optimising transistor performance in semiconductor integrated circuits built from standard cells ( 12 ), or custom transistor level layout, is disclosed. An active area of NMOS diffusion is extended with a joining area ( 102 ) between two adjacent cells ( 112 ) having the same net on diffusion at the adjacent edges of each cell. The diffusion area is extended to limit the occurrence of active and nonactive interface to minimise lattice strain effects and improve transistor performance.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to integrated circuits, and more particularly to methods for optimising performance of logic cells within a standard cell library for large scale integration of semiconductor integrated circuits. 
     BACKGROUND 
     Conventional standard cell libraries in semiconductor integrated circuits (IC) primarily contain a logic cell layout based in a metal oxide semiconductor (MOS) environment, in particular a complimentary metal oxide semiconductor (CMOS) environment. A standard cell is a pre-designed layout of transistors or non-specific collection of logic gates that are typically designed with computer assisted design (CAD) applications. The cells are usually interconnected or wired together in a particular manner with means of a placement and routing tool to perform a specific type of logical operation in an application specific IC (ASIC). A conventional ASIC layout is typically defined by an array of logic cells arranged in adjacent rows. Such a row  10  is shown in  FIG. 1 . The row of cells is depicted for illustrative purposes as a layout representation of abutting logic cells  12 , 31 , 32 , 33 , 35  bound by power and ground rails  14 , 16 . Each logic cell defines a specific logic circuit. The active areas or components of the logic cell include negative-channel diffusion  24 , positive-channel diffusion  26 , and gate  34  layers. The components of the logic cells are wired internally with vias  28  and metal layer  18 , 20 , 22  to form simple logic (NMOS and PMOS) gates to perform Boolean and logic functions, for example INVERTER (or NOT)  12 , 35 , AND, OR, NAND  31 , NOR  32 ,  33  XOR, XNOR, ADDERS, FLIP-FLOP, and the like. In the design of the interconnection layout, integrated circuit design rules must be observed, for example, minimum width of transistor width, minimum width of metal tracks, and the like. 
     Recent advances made in semiconductor technology have enabled cell library layout designers to work on the nanometer scale. However, as a result of this technology scaling, additional problems have surfaced concerning the physical properties of the ASIC. Such a problem includes stress occurring in materials near an interface of different materials with different crystallographic structures or thermal expansion coefficients. The stress creates strain in the active and shallow trench isolation (STI) regions  36  within the cell. The strain related with these physical interactions substantially effects the characteristics of the component transistors in the integrated circuit. For example, in the NMOS and PMOS devices, such as field effect transistors (FET), the impact of the stress may be severe and result in 10% or more variation of output performance. Noticeably, in the NMOS devices, lattice strain is responsible for a reduction in the current between source and drain (I D ). A similar variation is reflected in the PMOS device, however, the fluctuation is a positive increase which actually is a beneficial variation improving the PMOS device performance. 
     Thus, there is a need for a method to avoid the reduction in drain current which is effected adversely by lattice stresses between the active regions and STI regions of the transistor devices of semiconductor integrated circuits. 
     STATEMENT OF THE INVENTION 
     An aspect of the invention provides a method for optimising transistor performance in an integrated circuit, comprises providing an integrated circuit having at least two cells, each cell having a logic function defined by the interconnection between active regions, non-active regions and power rails, each cell having an edge defined by a non-active region each cell having a connection between an active diffusion region and a power rail; identifying at least two adjacent cells each having a corresponding connection between an active region to the same power rail; and joining the respective active regions associated with each corresponding connection of each cell with an additional diffusion area across the boundary between the cells forming the connected diffusion of the two adjacent cells. 
     An aspect of the invention provides a method for building an integrated circuit having a connected diffusion cell architecture, comprises: providing a logic cell library having at least one cell, each cell having a logic function defined by the interconnection between active regions, non-active regions and power rails, each cell having an edge defined by a non-active region, each cell having a connection between an active region and a power rail; placing cells from the logic cell library into the integrated circuit; identifying at least two adjacent cells in the integrated circuit each having a corresponding connection between an active region to the same power rail; and joining the respective active regions associated with each corresponding connection of each cell with an additional diffusion area across the boundary between the cells forming the connected diffusion of the two adjacent cells. 
     In an embodiment the active regions comprise a positive diffusion area, a negative diffusion area and a gate area. The connected diffusion may be the negative diffusion area or wherein each negative diffusion area of each adjacent cell are connected and each positive diffusion area of each adjacent cell are connected. The corresponding connection may be positioned along the edge of the cell forming a boundary between the two cells and the two corresponding connections. Additionally, an embodiment may further comprise reorientating a cell having a corresponding connection positioned along an edge, the cell being reoriented with the edge having the corresponding connection adjacent to and forming the boundary between the two cells for joining the diffusion areas. 
     An aspect of the invention provides an integrated circuit having a connected diffusion cell architecture, comprising: at least two adjacent cells, each cell having a logic function defined by the interconnection between active regions, non-active regions and power rails, each cell having an edge defined by a non-active region, each cell having a connection between an active region and a power rail; additional diffusion area across the boundary between the cells joining the respective active diffusion region associated with the corresponding connection of each cell forming the connected diffusion of the two adjacent cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A method for incorporating the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a physical layout design of a row of cell structures within a CMOS environment; 
         FIG. 2  shows a physical layout design of cell structures of  FIG. 1  reconfigured in accordance with an embodiment of the invention; 
         FIG. 3  Shows a physical layout design of cell structures of  FIG. 2  having continuous active areas in accordance with an embodiment of the invention; 
         FIG. 4A-B  shows a schematic diagram of a side view of NMOS regions of two adjacent cells in accordance with an embodiment of the invention; and 
         FIG. 5A-B  are flow charts of methods in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a physical layout of a row of logic cells is shown arranged in a conventional manner. A conventional cell includes non-active areas  36 , for example STI regions, surrounding active areas or regions  24 , 26 , 34  within each cell. There may be STI regions within a cell, especially when the cell has more than two stages, however the STI regions  36  also act to divide and isolate active areas from one another and form cell boundaries between the cells at the block level. The STI regions forming cell boundaries straddle the border of two adjacent cells. The active areas include the diffusion  24 , 26  and gate areas  34 . The supply nets  20 , 18  are supply connections between positive-channel  26  and negative-channel diffusion  24  and respective power rails  14 , 16  (V DD ,V SS ). The supply nets may be configured along a boundary or an edge  39  of the cell. A boundary or edge  39  of the cell is defined by a continuous STI region  36  that straddles across two adjacent cells, at the block level, between power and ground rails  14 , 16 . With this arrangement each cell may have at least one positive net and/or at least one negative net along an edge. Therefore adjacent cells may or may not have corresponding supply nets along bordering edges. 
       FIG. 2  shows a layout symbolic representation  70  of the row of logic cells of  FIG. 1 , however, cells with corresponding nets along an edge of bordering cells may be flipped about a y-axis (not shown) of the cell. The flipped cells  71 , 73 , 75  have edges with corresponding positive nets and negative nets of adjacent cells orientated to lie adjacent across the cell boundary of the adjacent cell  78 . It will be appreciated that it may not be necessary to flip any cells, for example, adjacent cells may already have corresponding nets along the boundary  78 . Additionally,  FIG. 2  shows respective positive and negative net orientation, however, advantages of embodiments of the invention may be achieved with orientating the cells such that only the same net type (i.e. positive nets or alternatively the negative nets) of each adjacent cell lie along the edge of the boundary between the two cells. 
       FIG. 3  shows the cell row arrangement of  FIG. 2  in accordance with an embodiment of the invention the area bound between the corresponding supply nets of the two adjacent cells are filled at the block level above the cells with additional diffusion  102 . This filling of diffusion alters the overall diffusion profile of the adjacent cells. The two adjacent cells having an edge with corresponding NMOS (V SS ) negative nets that lie on either side of the cell boundary form a continuous negative-channel diffusion  24 . Nets  18 , 20  accordingly continue to interconnect diffusions  24 , 26  for each cell. It will be appreciated that the same process may be applied to the positive-channel  26  diffusion. In CMOS technology the advantages of filling the negative-channel diffusion of NMOS improves performance, however, the invention is not limited to filling only the negative-channel diffusion. 
     The additional diffusion  102  may be filled with various techniques known in the field, such as a dummy layer or abutment algorithm method, grow/shrink method, and the like. For example in the dummy layer approach, each cell is built, or modified such that a dummy mask layer is added in the region between the cell border, and the diffusion which connects to a V SS  net. This will have a minimum width of “W”, by virtue of manufacturing design rules. At the block level, when a dummy layer width is equal to “2W”, then the dummy layer is replaced with diffusion fill. In the grow/shrink approach, at the block level all NMOS diffusion is grown in the horizontal direction by an amount “W”. All regions where a short circuit between different nets has occurred on diffusion, the area “shrinks” back to “W”. All other areas that do not shrink to “W” are diffusion fills between the same nets (such as V SS ) across a cell boundary. It will be appreciated by a skilled reader that other techniques exist, and the dummy layer and grow/shrink approaches are provided as examples. 
       FIG. 4A-B  shows schematic diagrams of a side view of NMOS regions of two adjacent cells in accordance with an embodiment of the invention.  FIG. 4A  shows the STI and active regions and the stress resulting from STI being a harder material than the diffusion regions (NMOS or PMOS). The pushing force between the two materials creates stress which impacts the performance. By filling the STI region with diffusion, the stress is removed, as shown in  FIG. 4B . Diffusion may be added where adjacent supply nets connected to the same power or ground. The diffusion fill is added between cell  1  and cell  2  at the block level of the IC, which is the next level up in the design hierarchy, above the cell level, and consists of multiple rows of such cells. 
       FIG. 5A  shows a flow chart of a method  300  in accordance with an embodiment of the invention. As discussed, a standard cell library is provided  310 . The ASIC  315  is built from a cell, or more typically a collection of cells in the library. In building an integrated circuit such as ASIC (or block thereof) cells are interconnected to effect a specific function. Those cells may be arranged in an array of rows. When an ASIC block is built, using automated or custom place and route tools two adjacent cells are identified  320  along the row that have the same net, for example V SS , on diffusion on either side of a boundary between the two cells. The diffusion areas associated with the supply nets of the two cells are joined  330  such that the two adjacent cells that are identified with corresponding nets form a single continuous diffusion area. An additional step of flipping  352  a cell is optional (as indicated by dashed box) if the cell needs to be flipped so that the same net is on diffusion on either side of a boundary.  FIG. 5B  shows a flow chart of a method  350  in accordance with another embodiment that shares steps  320 , 330  and may also have the optional flipping step  352  as set out in the method of  FIG. 5A . However, the method of  FIG. 5B  begins with having the ASIC provided  305 . 
     With a shared diffusion configuration, the active areas, i.e. negative and positive diffusion channels  24 , 26  have a longer continuous length than the arrangement of  FIG. 1 . A longer continuous diffusion channel length reduces the number interfaces occurring between active diffusion areas and STI regions along a row of cells. For example, for cells  33 , 35  in  FIG. 1 , the number of active diffusion area and STI region interactions is halved as shown in comparing  FIGS. 1 and 3 . The reduction in the occurrences of stress between the STI regions and the active diffusion areas correlates with an improvement in performance. 
     An embodiment of the invention may be implemented with computer aided design (CAD) systems that are well known to a skilled person. Well known hardware description languages (HDL), such as Very High Speed Integrated Circuit Hardware Description Language (VHDL) and VERILOG, both international standard languages of Institute of Electrical and Electronic Engineers, Inc. (IEEE), may be used to implement the invention to describe an ASIC, which is then synthesized into detailed logic functions comprising standard cells. An example of a tool to perform the synthesis is DESIGN COMPILER (DESIGN COMPILER is a trademark in certain countries of Synopsys, Inc. of Mountain View, Calif., United States of America). The cell library may also be designed with modelling tools or logic schematic programs on a CAD system to create symbolic representations such as a layout of logic functions, for example, VIRTUOSO (VIRTUOSO is a trademark in certain countries of Cadence Design Systems, Inc., of San Jose, Calif., United States of America). Of course, the ASIC may be built in rows of standard cells as mentioned above, or by other techniques known such as custom transistor level layout or the like. ASIC developers may use “place and route” (P&amp;R) tools to flip the cells as required for increasing the applicability of the technique. The placement and routing tools generate the associated mask patterns to physically wire the standard cells in the manner required to implement the ASIC function. While placement tools provide initial placement of cells in a block or IC when the routing needs are estimated, routing tools are capable of moving cells from their initial placement once the routing needs are known. Examples of “place and route” tools that may be used are PHYSICAL COMPILER and ASTRO, respectively (PHYSICAL COMPILER and ASTRO are trademarks in certain countries of Synopsys, Inc.). The hardware and software required to implement the invention, and indicated for explaining the preferred embodiment should not be limiting. Similarly, the software processes running on them may be arranged, configured or distributed in any manner suitable for performing the invention defined by the claims. 
     It will be understood that the block level diffusion fill method as described above provides advantages such as limiting the occurrence of stress between active areas and STI regions of the logic cells and improving overall performance. Additionally, embodiments may be applied to all types of ASICS, whether the ASIC is built from a collection of logic cells of a standard cell library (as discussed above) or from other techniques such as from full custom layout and the like. It will be appreciated that specific embodiments of the invention are discussed for illustrative purposes, and various modifications may be made without departing from the scope of the invention as defined by the appended claims.