CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a divisional patent application of U.S. patent application Ser. No. 09/912,588, filed Jul. 24, 2001, now allowed. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of semiconductor integrated circuits. The invention particularly relates to methods for designing and manufacturing integrated circuits, and to integrated circuits so produced.  
         BACKGROUND OF THE INVENTION  
         [0003]    The creation of a new semiconductor integrated circuit includes the following activities. Integrated circuit designers start with some description of the circuit&#39;s function. From this description or specification, the designers generate a design netlist. The design netlist contains a list of the components of the circuit and also contains a list of the interconnections between those components. The design netlist also contains a list of I/O buffers with their associated bonding pads and their connections to the other components. The designers then apply electronic design automation (EDA) tools to the netlist to create the physical layout of the components as they will appear in the integrated circuit die. With present day semiconductor technology, and using 0.25 micron feature size, designers can put up to 2 million logic gates and up to 250 input/output (I/O) ports on a 7 mm by 7 mm die. The majority of designs at this date, however, contain only about 50K to 80K gates of logic and about 200 I/O&#39;s.  
           [0004]    The physical layout of a 0.25 μm design consists of a set of about 18 to 26 unique mask levels. This information is supplied to a mask-making facility where the information is used to generate a set of glass masks for that particular design.  
           [0005]    The mask set is then sent to a wafer fabrication facility where the mask set is applied to process a batch of about 20 semiconductor wafers. After the wafers have been completely processed, they are sent to wafer electrical test where dice that do not meet specification are inked or otherwise marked reject. The tested wafers are then sawn into individual dice, each die containing one integrated circuit. The dice are packaged and then electrically tested in a procedure known generally as final test. Based on the wafer electrical probe test results and the final test results, it may be necessary to correct the design and generate a second generation design with a second generation mask set. The costs associated with the design and prototyping activities just described are quite substantial.  
           [0006]    There are several methods in use currently that aim to share some of the costs of the activities mentioned above.  
           [0007]    So-called Gate Arrays provide one method of sharing costs. At this date, a mask set may have from 18 to 26 levels in a typical 0.25 micron CMOS process. In the gate array method, the first two thirds of these mask levels are common to all designs. Only the final one third of the mask levels are specific to each new design. The benefit here is that the first two thirds of the masks do not have to be designed and made anew for each new integrated circuit design and also that wafer fabrication time for a new design can be reduced by processing wafers in advance through the common masks and holding the partially processed wafers in stock ready to continue prototype processing through the last one third of the mask layers.  
           [0008]    There are disadvantages to the gate array approach. Since the basic transistors have to be fabricated ahead of time, they all have to be of standard fixed sizes. Without the flexibility of being able to use varying transistor sizes in different parts of the new circuit design and of adding user-specific circuit components, the integrated circuit size increases and the performance degrades. The engineering costs to generate each design layout and to personalize the last one third of the mask layers are incurred for each design and are not sharable. Another disadvantage is the inability to incorporate custom-designed blocks such as analog cells or true static random access memory (SRAM) blocks. With the increasing levels of integration that are now available in deep submicron process technologies, almost all designs made today in 0.25 micron or better technologies incorporate some amount of SRAM.  
           [0009]    Another approach to sharing prototyping costs is known as the multi-project wafer approach. In the multi-project approach, several projects, or integrated circuit designs, are combined in one mask set. Each design has its own bonding pads and input/output buffers located around the periphery of that design. In this approach, certain procurement costs are shared between the different designs that are included in the one mask set. If different users own the different designs on a given mask set, then the mask cost and wafer fabrication cost is reduced for each user. There are several organizations now running multi-project wafer programs. Among these is MOSIS, a not-for-profit organization located in southern California.  
           [0010]    The multi-project wafer approach has some disadvantages. For example, since the several different designs in a multi-project die have their own unique bonding pads, probe testing at the wafer level is difficult in a production environment. Assembly of these designs into integrated circuit packages is difficult because of complications in the wafer sawing process. When a semiconductor wafer is sawed into dice, the entire wafer has to be sawed at a fixed indexing increment. In the multi-project approach, if there are 10 designs of different die sizes on the wafer, the saw is indexed to yield dice containing 1 of the designs, and thus the saw destroys other designs. Also, if one of the designs is to go into production, a new mask set will need to be made, containing only that design. Cost sharing is possible only at the prototyping stage and not at the production stage.  
           [0011]    Therefore, there is a need for an integrated circuit architecture that provides multiple integrated circuit designs in a single die, wherein all of the designs on a die are testable at wafer probe test, and wherein all of the designs survive the wafer sawing operation and the package assembly operation, thus providing complete cost sharing from design through final test. There is also a need for an integrated circuit architecture that, during assembly, allows any number of the designs to be made accessible or inaccessible in the packaged device. There is also a need for a design method that produces this integrated circuit architecture.  
         OBJECTS AND ADVANTAGES  
         [0012]    It is an object of the present invention to provide a multi-design integrated circuit die that contains multiple integrated circuit designs and a set of bonding pads that is shared by the different designs on the die.  
           [0013]    It is an object of the present invention to provide a multi-design integrated circuit, in which the multiple circuit designs in the integrated circuit can be probe tested via one set of shared bonding pads.  
           [0014]    It is a further object of the present invention to provide a multi-design integrated circuit die that has bonding pads located, not around each design, but rather in an area of the die near the periphery of the multi-design die.  
           [0015]    It is a further object of the present invention to provide a multi-design integrated circuit die, in which, after the die has been assembled into an integrated circuit package, any of the circuit designs can be accessed.  
           [0016]    It is a further object of the present invention to provide a design method for designing multi-design integrated circuits.  
           [0017]    It is a further object of the present invention to provide an automated design method for designing multi-design integrated circuits.  
           [0018]    It is a further object of the present invention to provide an automated design method for designing a multi-design integrated circuit die that not only provides cost sharing during prototyping but that is also production-ready.  
           [0019]    It is a further object of the present invention to provide an automated design method for designing a multi-design integrated circuit die that allows the designer freedom to customize the individual components in each design within the integrated circuit die.  
         SUMMARY  
         [0020]    The objects and advantages of the present invention are obtained by generating a multi-design netlist that contains and is a combination of individual design netlists and that shares I/O buffers among designs. The objects and advantages of the present invention are further obtained by using the multi-design netlist and masks derived from it to process semiconductor wafers to obtain multi-design integrated circuit dice. The semiconductor wafers would most commonly be silicon wafers but may also be any other semiconductor such as for example gallium arsenide, and may also be semiconductor on insulator wafers such as for example silicon on sapphire.  
           [0021]    In one aspect, the method of the present invention is a method of design and layout of a set of integrated circuit designs, the set of designs being combined on a single integrated circuit die. The set of designs may include only two designs or may include ten or more designs. In the method of the present invention, netlists are prepared for each of the designs in the set of designs that is to be incorporated in a single die (also called a chip). Each design netlist includes a pin-pad assignment table. The pin-pad assignment table for a design assigns numbered package pins to numbered bonding pads on a one to one basis. This pin-pad assignment would apply if the die contained only that one design, in which case that one design would have its own bonding pads and I/O buffers.  
           [0022]    In the multi-design integrated circuit die, the individual designs do not each have their own I/O buffers and bonding pads, but they do have corresponding input and output connection points, which, for convenience, will be referred to herein as ports.  
           [0023]    The netlists for the individual designs are combined by a computer program into one multi-design netlist. The multi-design netlist includes a pin-pad assignment table that assigns numbered ports of each design to numbered I/O buffers, to numbered bonding pads, and to numbered package pins. The multi-design netlist includes, in addition to the netlists for the individual designs, provision for accessing the designs via shared bi-directional input/output buffers, and also includes logic for enabling only the outputs of one design at a time. EDA tools are then applied to the multi-design netlist to generate mask layouts. The mask layouts are used in a mask making facility to produce a mask set. The mask set is used to process silicon wafers. After fabrication is completed, the wafers are electrically probe tested. In this procedure, probes are placed on bonding pads on a die and each of the designs can be tested. The testing of all the designs on the die can be completed in one probing operation, each of the designs being operated in turn. The wafer is then cut into individual dice, each die containing the set of designs. Each die is then assembled into a semiconductor package with connections between bonding pads and package pins being made in accordance with the pin-pad assignment table. During assembly, access to the various designs in the multi-design integrated circuit can be restricted or can be left unrestricted. In either case, access to any design is made by applying appropriate signals to design active pins.  
           [0024]    In another aspect, the integrated circuit die of the present invention is a multi-design integrated circuit die that contains at least two integrated circuit designs, wherein each of the designs may be operated independent of the others. The design that is to be operated is designated the active design. The selection of an active design is made by supplying design active signals to the die and the selection may be easily changed.  
           [0025]    Shared bonding pads and shared bi-directional I/O buffers provide access for input and output signals to whichever design is active.  
           [0026]    Input signals are applied to bonding pads and pass through I/O buffers to the designs. Each input signal is distributed to the number of designs that require that signal in the overall design. Thus, at least some of the I/O buffers pass input signals to two or more designs. An I/O buffer may be connected to send an input signal to all the designs.  
           [0027]    Output control logic is connected between the designs and the I/O buffers so that the output signals reaching the bonding pads are from the active design only.  
           [0028]    The output control logic includes output enable control logic wherein an output enable signal from a design is passed to an I/O buffer only if that design is active.  
           [0029]    The output control logic also includes, when more than one design is served by a given I/O buffer, output selection logic which selects the output of the active design for transmission to the I/O buffer.  
           [0030]    In the layout of the integrated circuit die of the present invention, the shared bonding pads and I/O buffers are located near the periphery of the die and the designs are located in the central or core area of the die. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 shows the general layout of a multi-design integrated circuit die in accordance with the present invention.  
         [0032]    [0032]FIG. 2 shows a bi-directional I/O buffer as used for the “input” function.  
         [0033]    [0033]FIG. 3 shows a bi-directional I/O buffer as used for the “output” function.  
         [0034]    [0034]FIG. 4 shows a bi-directional I/O buffer as used for the bi-directional function.  
         [0035]    [0035]FIG. 5 shows one example of connections to a bi-directional I/O buffer in a multi-design integrated circuit die that contains two designs, in accordance with the present invention.  
         [0036]    [0036]FIG. 6 shows another example of connections to a bi-directional I/O buffer in a multi-design integrated circuit die that contains two designs in accordance with the present invention.  
         [0037]    [0037]FIG. 7 is a flow diagram showing the overall function of design consolidation software in accordance with the present invention.  
         [0038]    [0038]FIG. 8 is a flow diagram showing the overall function of test consolidation software, in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    [0039]FIG. 1 shows a simplified layout for a multi-design integrated circuit die in accordance with the present invention. In FIG. 1 only the top surface of the die is shown. The multi-design die  10  contains designs  12 . In FIG. 1, by way of example, the die contains a set of four designs. The number of designs could be as small as two or as large as 20 or more. These designs  12  can be of different levels of complexity and of different shapes. In FIG. 1 the designs  12  are located in the central or core portion of the chip. FIG. 1 also shows shared I/O buffers  14  located near the periphery  16  of the die  10 . Each I/O buffer  14  is connected to its assigned bonding pad  18 . The bonding pads may be replaced by any other suitable type of mechanical connection, such as solder bumps. There are connections between the I/O buffers  14  and the designs  12  but these connections are not shown.  
         [0040]    The design process for the multi-design die  10  of FIG. 1 includes generating, for each design  12 , a separate netlist. The netlist for a single design  12  may be referred to as a sub-netlist or a lower level netlist. These netlists are combined and augmented to produce a super-netlist, which may also be called an upper level netlist, that includes shared I/O buffers and the connections between I/O buffers and the designs.  
         [0041]    The bi-directional I/O buffers  14  used in the multi-design die  10  of the present invention may each be operated either as an input buffer as shown in FIG. 2, as an output buffer as shown in FIG. 3 or as a bi-directional buffer as shown in FIG. 4. Furthermore a bi-directional I/O buffer may be left unconnected if so required by the multi-design netlist.  
         [0042]    In FIG. 2, bi-directional I/O buffer  24  has an input signal port  26 , an output signal port  32 , an output enable port  34  and an off-chip interface port  36 . The off-chip interface port  36  is connected to a bonding pad, not shown. The bi-directional I/O buffer  24  operates, in this case, as an input buffer because a signal applied to the output enable terminal  34  disables the output portion of the I/O buffer shown in dotted lines.  
         [0043]    In FIG. 3 the bi-directional I/O buffer  24  operates as an output buffer because an output enable signal is applied to the output enable signal port  34  and because the input signal port  26  is not connected.  
         [0044]    In FIG. 4 the bi-directional I/O buffer functions as a bi-directional buffer for transmitting input signals and output signals. In this case, the output function is enabled by the output enable signal applied to the output enable signal port  34 , and the input signal port  26  is connected to at least one input port on a design, not shown in FIG. 4.  
         [0045]    The multi-design netlist includes the individual netlist for each design and also includes an additional netlist that defines additional logic for multiplexors and control signals such that for each bi-directional I/O buffer, the four functions of “input”, “output”, bi-directional” and “no-connect” functions are implemented as required to operate any one of the designs.  
         [0046]    The function required of a given bi-directional I/O buffer will change depending on which of the designs is active. Any one design may require any one of the four functions enumerated above. By way of illustration, if there are only two designs there are ten possible combinations of first design function and second design function. Table 1 shows these ten combinations, designated case #1 to case #10.  
                                                                   TABLE 1                                       Design #1                Input   Output   Bi-directional   No Connect                        Design #2                       Input   Case #1   Case #2   Case #4   Case #7       Output       Case #3   Case #5   Case #8       Bi-directional           Case #6   Case #9       No Connect               Case #10                  
 
         [0047]    The implementation of cases 4 and 6 will now be discussed in detail.  
         [0048]    [0048]FIG. 5 shows a schematic of buffer connections and output control logic that can be used for case number 4. In case number 4 the bi-directional I/O buffer is required to perform the “bi-directional” function when design  12 - 1  is active and to perform “input” function when design  12 - 2  is active. The bi-directional I/O buffer  24  has an input signal port  26  that is electrically connected to an input port  28  in design  12 - 1  and to an input port  28  in design  12 - 2 , so that any input signal applied to a bonding pad (not shown) is transmitted by the I/O buffer  24  to the two input ports  28 . The output port  38  in design  12 - 1  is connected to the output signal port  32  of the I/O buffer  24 . The output enable port  40  of design  12 - 1  is inputted to AND gate  42 . A design #1 active signal is applied via a design active signal line  50  to AND gate  42 . The output control logic in this case is the output enable logic consisting of AND gate  42 . The output of AND gate  42  is the output enable signal that is applied to the output enable signal port  34  of buffer  24 . In this case, when the design # 1 active signal makes design  12 - 1  active, the output function of the bi-directional buffer  24  is enabled, and the output signal from output port  38  of design  12 - 1  appears at off-chip interface  36 .  
         [0049]    If design  12 - 2  is active (made active by a design # 2 signal, not shown in FIG. 5), then the output enable signal from the output enable port  40  of design  12 - 1  is blocked at the AND gate  42  and the bi-directional I/O buffer  24  performs the “input” function only.  
         [0050]    [0050]FIG. 6 shows a schematic of the connections and output control logic that can be used to implement case  6 . In case  6 , the bi-directional I/O buffer  24  is required to perform the “bi-directional” function for design  12 - 1  and also for design  12 - 2 .  
         [0051]    The output control logic in this case includes output enable control logic  44  and output selection logic  46 . When the design #1 active signal on line  50  is high and design #2 active signal on line  52  is low, then the output enable signal from output enable port  40  of design  12 - 1  is sent by the output enable logic  44  to the output enable signal port  34  of the buffer  24  and buffer  24  is output enabled. Again, when design # 1 active signal is high on line  50  and design # 2 active signal is low on line  52 , then design  12 - 1  is selected and active, the output signal from output port  38  of design  12 - 1  is passed via output selection logic  46  to the output signal port  32  of the buffer  24  and appears at port  36  of the buffer  24 . At the same time, any output signal from output port  38  of design  12 - 2  is blocked by output selection logic  46 .  
         [0052]    When design # 2 signal is high on line  52 , the operation is similar except that the output enable signal from output enable port  40  of design  12 - 2  output enables buffer  24 , and the output signal from output port  38  of design  12 - 2  is transmitted by logic  46  while the output signal from output port  38  of design  12 - 1  is blocked by logic  46 .  
         [0053]    The design of a multi-design die in accordance with the present invention includes, as stated above, generating a multi-design netlist. The design software requires that, for each design, the pin-pad-buffer assignments be defined. The user of each design can supply this information. The pin-pad-buffer assignments for the individual designs are combined by software to provide a pin-pad assignment table for the multi-design integrated circuit die. Table 2 is an example of such a pin-pad assignment table for an oversimplified situation.  
         [0054]    Pin-Pad Assignment table example:  
                                                                             TABLE 2                               Design   Design   Design   Design               Pin #   Pad #   #1   #2   #3   #4       Top Design                                1   1   Input   Output   Input   Bi-Dir       Bi-Dir       2   2   NC   Bi-dir   Output   Bi-Dir       Bi-Dir                                                 3   3   Bi-Dir   Input   Bi-Dir   NC       Bi-Dir       4   4   V dd     V dd     V dd     V dd         V dd         5   5   V ss     V ss     V ss     V ss         V ss                    
 
         [0055]    Table 2 shows pin-pad assignments for 4 designs and five pins, including V dd  and V ss  pins. The body of the table shows the buffer functions required between each pin and the pads in each design. In the first column of table 2, “pin #” refers to package pin number. In the second column of the table “pad #” refers to pad numbers in each design, for example “pad #” 1 refers to input pad 1 or output pad 1 or both, as required. Pad number 1 refers to input pad 1 in design 1, to output pad 1 in design 2, to input pad 1 in design 3, and to both input pad 1 and output pad 1 in design 4.  
         [0056]    Taking pin 1 for example, the input function is required between pin 1 and pad 1 of design 1, thus pin 1 is assigned to input port 1 of design 1. The output function is required between pin 1 and pad 1 of design 2, thus pin 1 is also assigned to output port 1 of design 2. The input function is required between pin 1 and pad 1 of design 3. The bi-directional function is required between pin 1 and pad 1 of design 4, thus pin 1 is also assigned to input port 1 and to output port 1 of design 4. In the rightmost column of table 2, the top-level design requires a bi-directional buffer for pin 1.  
         [0057]    Note that pin 3 is not assigned to any pad in design 4 as is indicated by the notation NC, meaning not connected. Though not shown in Table 2, a pin and its associated I/O buffer may not be assigned to a pad in any design, in which case it is assigned to zero input ports and to zero output ports.  
         [0058]    From table 2 it can be said that if there are N designs then an input pin may be assigned an input port in each of M designs and to an output port in each of P designs, where M and P are integers in the set of integers zero through N.  
         [0059]    The term “pads” at the head of column 2 in table 2 derives from the fact that if a design is fabricated as a single design integrated circuit then its input ports and output ports would each require a bonding pad. These bonding pads are eliminated in the multi-design structure, though the design input ports and Is output ports remain. The term “pad” is, however, retained in table 2.  
         [0060]    The column headed “top design” shows that bi-directional buffers are required for certain of the pins. In generating the multi-design netlist, design automation techniques can be used to remove buffers from the design netlists and insert bi-directional I/O buffers in the multi-design netlist where they are shared by the lower level netlists. The software will also read the pin-pad assignment for each design so that the overall pin-pad assignments are correctly generated.  
         [0061]    [0061]FIG. 7 shows a flow diagram for the operation of generating a top-level consolidated design for a multi-design integrated circuit. In FIG. 7 design consolidation software  82  receives data  84  for all of the designs that are to be built in a single multi-design die. The data  84  includes, for each design, pin-pad-buffer assignment  86  and a netlist  88 . The design consolidation software may also receive design selection inputs  90 . The design selection inputs are configuration information externally applied to the consolidated design. This configuration information can be implemented by tying input pins to either the high or low states. Alternatively, the high or low state termination can be achieved by wire-bonding techniques during the assembly process. The design consolidation software outputs a top-level consolidated design for a multi-design integrated circuit die containing pin-pad-buffer assignment and top-level netlist or multi-design netlist.  
         [0062]    Each netlist represents a complete design. The designer generates simulation vectors for each design. The simulation vectors from each design are combined into one set of simulation vectors for the top-level design. Design automation techniques can be used to automate this process. The combined vector set will then be used for generating a semiconductor production test program to test the multi-design device.  
         [0063]    [0063]FIG. 8 is a flow diagram illustrating the process of test vector consolidation. In FIG. 8 test pattern consolidation software  92  receives data  94  for all of the designs that are to be included in a multi-design chip. The data  94  includes stimulus vectors and response vectors for each design. The test pattern consolidation software may also receive design selection inputs  96 . The test pattern consolidation software outputs data  98  including stimulus vectors and response vectors, both for the top-level consolidated design. From the stimulus vectors and response vectors  98  a semiconductor test program  100  is generated for the multi-design integrated wafers and devices.  
         [0064]    Once the design consolidation of FIG. 7 and the test pattern consolidation of FIG. 8 have been completed, the physical layout of the multi-design chip is generated and a mask set is created. The masks are used in a wafer fabrication facility to process a batch of semiconductor wafers. The processed wafers are probe tested using the semiconductor production test program and the good multi-design dice are identified. The wafers are cut into individual multi-design dice. Each die is then attached to an integrated circuit package. The bonding pads are connected to the package external leads and the package is sealed. The packaged multi-design integrated circuits are again tested.  
         [0065]    Exemplary embodiments of the present invention have been described. These are intended to be illustrative and not restrictive. Thus, the present invention is capable of many variations in detailed implementation, which variations may be derived from the description contained herein by a person of ordinary skill in the art. All such variations are considered to be within the scope of the appended claims.
A method for designing an integrated circuit die having bi-directional I/O buffers shared by multiple designs therein. The multi-design integrated circuit die is designed by combining netlists, each of which represents a complete design, and pin-pad assignment lists for the individual designs into a top-level consolidated netlist or multi-design netlist with its own consolidated pin-pad assignment. Data for the top-level consolidated design, including stimulus vectors and response vectors, are used to generate a semiconductor test program to test the integrated circuit die. Consolidation of netlist, pin-pad assignments and vectors are accomplished using design automation software and techniques. The top-level consolidated design data may include configuration information externally applied to the consolidated design.