Abstract:
A novel routing rule definition for standard cells placement is disclosed. The method comprises following steps. At beginning, a statistical analysis is carried out to analyze the frequency of standard cells used in a design plane. The most frequency used types in standard cells is then used as bases for a greatest common divisor (GCD) calculation. The GCD value acquired is then as a routing pitch criterion, which is a distance of contact hole-to-contact hole, or says the standard cell width.

Description:
FIELD OF THE INVENTION 
     The present invention relations to a method of layout design of integrated circuit, specifically, to about doing a routing defined modification for standard cells, so as to make the layout optimization. 
     BACKGROUND OF THE Invention 
     In the semiconductor manufacturing process, the design of integrated circuit, especially, to application-specific integrated circuit (abbreviated as ASIC), generally, the steps are as follows: firstly, producing a netlist, and then system partitioning and prelayout simulation are successively followed. Next, a floorplanning to arrange the blocks of the netlist on the chip is performed . Thereafter, placing the standard cell in a block. Then perform routing to make connection between cells and blocks. Finally, extracting the resistance and capacitance of the connection, and postlayout simulation to check the design works are carried out. The standard cells includes logic gate, such as NAND, OR, NAND, NOR, AND, XOR and NOT or sequential device, flip-flop, latches, register and so on. After performing simulation for qualified approval, the manufacturing department is then implemented the physical processes in accordance with the layout design. 
     Generally, to place a standard cell, for example for an inverter, three via pitch distance is demanded. A width of two via pitches is not enough to accommodate such an inverter. A NAND gate with two inputs and one output requires about four via pitches. A NOR gate requires a space almost equivalent to a NAND gate. It is because the pitch width of connection line for standard cells is based on the definition of via-to-via between connection level according to the current design rule. 
     Please refer to FIG. 1, a schematic diagram shows a design of via-to-via about 0.15 μm feature length process. The metal via width w1 is 0.22 μm. The surround area to via hole with board width w 2 , w 3  are about 0.05 μm and 0.01 μm, respectively. A space s1 between metal board is about 0.24 μm. And thus a pitch of via-to-via is about 0.56 μm. However, a width needed for a inverter is about 1.18 μm according to the design rule. Consequently, if an inverter is placed into a space of two via itches for saving the planar area of wafer. The poly-gate has to form by having about 45° turning angle so as to accommodate the spacing issue. Whereas, for deep sub-micron technology with device feature length of about 0.15 μm, 0.13 μm or beyond, the poly-gate with turning angle is not being allowed. 
     A similar problem is encountered for NAND gate layout. A two-inputs NAND gate, needs a width of about 1.77 μm. As a consequent, the routing rule according to prior art, of about four via pitches i.e., 2.24 μm is necessary to fit such a NAND gate. An extra planar area cost cannot be saved. 
     The layout for standard cells in accordance with the aforementioned prior art is failed to save some extra planar area. However, as is known skilled in the art, the NAND gate, NOR gate, and inverter together are occupied about 70 to 80% of the components for a typical logic circuit using basic standard cells. Thus, the better of the layout design will decide what a degree of the integrity of IC may be, or the chip size while the same amount of devices are placed. 
     An object of the invention is thus to provide a method which utilize the planar area of a chip fully. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a novel routing definition so as to improve the non-compact layout issues. 
     The present invention is to provide a routing rule method. The method comprises following steps. At beginning, a statistical analysis is carried out to analyze the standard cells used times in a design plane. The most frequency used types in standard cells is then used as bases for a greatest common divisor (GCD) calculation. The GCD value acquired is then as a routing pitch criterion, which is a distance of contact hole-to-contact hole, or says the standard cell width. 
     According to the present invention the cross-point between the margins of standard cells along the cell height and the connection lines can be function as substrate contact to increase the cell reliability and 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a conventional routing pitch according via to via. 
     FIG. 2 shows a synoptic layout diagram of an inverter according to the present invention. 
     FIG. 3 shows a synoptic layout diagram of a NAND gate according to the present invention. 
     FIG. 4 shows a contact-hole pitch according to the present invention. 
     FIG. 5 shows a substrate contact array provides the standard cells below and above its position for common contact-hole use. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As forgoing design rule for standard cells according to prior art depicted is restricted, currently, by the design rule bases on the metal via pitch, which will result in wasting more planar areas of a chip. The present invention disclosed a method for routing pitch definition, which can solve aforementioned issue. 
     For very deep sub-micron design, the feature length of device of 0.15 μm, 0.13 μm or beyond, the inventor found that the core cells are almost abutment with the routing metal layers of five or six, or more. However, dominant factor to the chip size determine should be the size of the core cell but not the connection line pitch. 
     FIG. 2 shows an inverter gate is composed of a PMOS transistor  100  and a NMOS transistor  110  has a width W 1  of about 1.18 μm and a cell high H 1  of about 4.32 μm for 0.15 μm process technology. The width W 1  is measured from a contact hole  115  through a polygate to another contact hole  115 . For NAND gate and the same process technology, as shown in FIG. 3, includes two PMOS transistors  120  in parallel (contact holes in between two polygates  122 ) and two NMOS transistors  130  (without contact holes in between two polygates. The core cell size of a NAND gate is thus determined by the total size of two PMOS transistors. The size of a NAND gate is about 1.77 μm wider than the inverter. However, the height of NAND cell, however, is about 4.32 μm, the same as the inverter. 
     A NOR gate occupied area is of about the same as that of a NAND gate. The NOR gate has two PMOS transistors in series, and two NMOS transistors in parallel. The core cell size of a NOR gate is thus determined by the size of two NMOS transistors which have about 1.77 μm in width and 4.32 μm in height. As is in aforementioned prior art, the NOR gate and the NAND gate together with inverter gate are usually the most portion of the standard cells. 
     The more compact the layout to the NOR gate, the NAND gate and the inverter gate, as a consequent, will better the layout design. The inventor proposes thus a newly routing definition by using a greatest common divisor (GCD) of about 0.59 μm among them as a base unit. Using the GCD not only provides the close packed to the NOR, NAND and inverter, but also provides the D-flip-flop with most compact design, wherein a D-flip-flop is composed of NAND gates and NOR gates which is about 17 times the GCD in width, that is 0.59 μm×17=10.03 um. 
     Thus, using the contact-hole pitch as a routing criterion would be the most appropriate. 
     According to the preferred embodiment, the contact-hole pitch is 0.59 μm, as shown in FIG. 4, includes ½×contact hole width 50+a board width which enclosures the contact hole+spacing 70+another board width+½×contact hole width 50=½×0.18+0.085+0.24+0.085+½×0.18=0.59 (μm), The spacing  70  is a distance between active regions (please refer to FIG.  2  and FIG.  3 . 
     Above data information is using a NAND gate of two-inputs and one out as an exemplification. The routing rule can be used in three-inputs, or four inputs NAND gate. For example, the former requires 4×0.59 μm spacing, and the latter requires 5×0.59 μm=2.95 μm. 
     The concept of the present invention can be summary as follows: 
     For an inverter of one input terminal, it requires two contact-hole pitches, and an NAND gate of three inputs, requires three contact-hole pitches. Similarly, for a NAND gate with three inputs, a width of four pitches is generally demanded. 
     Using the value of a GCD of the standard cells, in addition to compact the standard cells during layout, furthermore, the contact holes in the layout can be designed as a symmetry substrate contact array. Please see FIG. 5, the synoptic layout diagram, the substrate contacts are located at each cross point of connection line and a margin  147  of the standard cells along the cell height direction. Noted that in the figure, only the positions of standard cells instead of standard cells thereof are shown. The substrate contacts provide common contacts for the standard cells below and above thereof. 
     Still referring to FIG. 5, the substrate contacts provide the standard cells  150  (above substrate contact  170 B) and the standard cells  160  (below substrate contact  170 B) for common power connection terminal or common ground terminal use. The designer can then connect the substrate contacts depending on the routing requirement. Similarly, the substrate contacts  170 A are provided for standard cells  150  and the standard cells above to use (not shown). The substrate contacts  170 C are provided for standard cells  160  and the standard cells below (not shown) use. No extra area is required. The substrate contact concept thus saves the planar areas and increases the reliability. 
     The benefits of this invention are: 
     (1) The pitch of contact hole to contact hole as routing pitch unit can provide compact layout requirement. 
     (2) The routing pitch is based on the GCD of contact holes of the standard cells used. As a result, while altering the feature sizes of standard cells, it can thus very ease to modify the routing pitch according to the method of the present invention. 
     (3) Since the substrate contacts provided by both substrate cells about and below thereof, and thus the reliability is significantly increased. 
     As is understood by a person skilled in the art, the foregoing preferred embodiment of the present o invention is an illustration of the present invention rather than limiting thereon. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.