Abstract:
One embodiment of the invention provides a system that simulates effects of a manufacturing process on an integrated circuit to enhance process latitude and/or reduce layout size. During operation, the system receives a representation of a target layout for the integrated circuit, wherein the representation defines a plurality of shapes that comprise the target layout. Next, the system simulates effects of the manufacturing process on the target layout to produce a simulated printed image for the target layout. The system then identifies problem areas in the simulated printed image that do not meet a specification. Next, the system moves corresponding shapes in the target layout to produce a new target layout for the integrated circuit, so that a simulated printed image of the new target layout meets the specification.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates to the process of designing an integrated circuit. More specifically, the invention relates to a method and an apparatus for simulating effects of a manufacturing process on an integrated circuit to enhance process latitude and/or reduce layout size. 
     2. Related Art 
     Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise as a consequence of the optical lithography process that is typically used to manufacture the integrated circuits. This optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photoresist layer coated wafer. (Note that the term “mask” as used in this specification is meant to include the term “reticle.”) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some other type of electromagnetic radiation together with suitably adapted masks and lithography equipment. 
     This light is reduced and focused through an optical system that contains a number of lenses, filters and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, the light is blocked by opaque regions of the mask, leaving underlying portions of the photoresist layer unexposed. 
     The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer. 
     As feature sizes continue to decrease, optical effects, resist effects, mask writer beam effects, and/or other effects can degrade the quality of the printed image obtained through the optical lithography process. The upper portion of FIG. 1 illustrates how an optical lithography process  102  converts a layout  510  into a printed image  113  on a semiconductor wafer. As mentioned above, this optical lithography process  102  involves: a mask fabrication process  104 , an exposure of the mask through stepper optics  106 , a photoresist development process  108  and a development or etching process  112 . Note that each of these processes can degrade the resulting printed image  113 . 
     A layout of an integrated circuit is typically created in accordance with a set of design rules that specify a number of constraints, such as minimum spacings or minimum line widths, to increase the likelihood that the finished integrated circuit functions properly in spite of different manufacturing effects. These design rules can be thought of as guidelines for a layout to circumvent process limitations. 
     It is advantageous to use such design rules because they simplify the layout process by hiding the complexity of the photolithography process. Design rules can be thought of as transforming a continuous problem into a discrete problem. Moreover, design rules can be easily verified by checking dimensions in the layout, such as minimum spacing between shapes. 
     However, the use of design rules can lead to sub-optimal layouts. For example, a design rule may specify a minimum spacing between specific shapes. However, a circuit designed using this minimum spacing may only function properly for a narrow range of variations in the manufacturing process. It may be preferable to use a larger spacing between shapes whenever possible to improve “process latitude”. 
     For this reason, some foundries have “recommended rules” to improve process latitude. The layout designer/tool uses these recommended rules in addition to standard design rules during the cell generation process to improve process latitude. The layout designer/tool attempts to satisfy these recommended rules. However, unlike standard design rules, they are not required to be satisfied. The layout designer/tool must relax minimum spacing, width and/or size between layout shapes to implement recommended rules. However, if recommended rules applied everywhere, they can lead to unnecessary expansion of the layout. The designer also has to make tradeoffs between recommended rules. 
     Hence, what is needed is a method and an apparatus for optimizing the spacing, width and/or size of layout shapes in order to enhance process latitude. 
     Note that in addition to the process latitude for standard design rules, there can be “fighter design rules” that decrease process latitude compared to standard design rules. There can also be “relaxed rules”, which increase process latitude compared to the standard design rules. Note that a recommended rule is a special case of a relaxed rule with a fixed value. Relaxed rules can allow for multiple values. 
     SUMMARY 
     One embodiment of the invention provides a system that simulates effects of a manufacturing process on an integrated circuit to enhance process latitude and/or reduce layout size. During operation, the system receives a representation of a target layout for the integrated circuit, wherein the representation defines a plurality of shapes that comprise the target layout. Next, the system simulates effects of the manufacturing process on the target layout to produce a simulated printed image for the target layout. The system then identifies problem areas in the specification that do not meet a specification. Next, the system moves corresponding shapes in the target layout to produce a new target layout for the integrated circuit, so that a simulated printed image of the new target layout meets the specification. 
     Note that the above-described process differs from optical proximity correction. Optical proximity correction modifies a layout to compensate for optical effects so that the actual printed layout matches a target layout. In contrast, the above-described process moves shapes within the target layout to produce a new target layout that has better process latitude. 
     In a variation on this embodiment, moving the corresponding shapes in the target layout involves optimizing process latitude for the target layout. 
     In a variation on this embodiment, moving the corresponding shapes in the target layout involves performing a compaction process to minimize layout size. 
     In a variation on this embodiment; moving the corresponding shapes in the target layout involves providing objectives and/or constraints to the compaction process. 
     In a variation on this embodiment, moving the corresponding shapes in the target layout involves applying relaxed rules to the problem areas of the target layout to improve process latitude. In a further variation, the relaxed rules include priority values for resolving conflicts between relaxed rules. 
     In a variation on this embodiment, moving the corresponding shapes in the target layout involves applying relaxed rules to the problem areas of the target layout. 
     In a variation on this embodiment, the effects of the manufacturing process are simulated over a range of manufacturing parameters. 
     In a variation on this embodiment, the system additionally uses the simulated printed image to estimate a yield for the target layout. 
     In a variation on this embodiment, the system performs optical proximity correction (OPC) on the new target layout to produce a modified layout, wherein a simulated printed image of modified layout more closely matches the new target layout than the simulated printed image of the new target layout. 
     In a variation on this embodiment, the system uses information obtained from examining the simulated printed image to formulate new design rules for the target layout. These new design rules may be more aggressive tightened design rules. Alternatively, these new design rules can merely be different rules that apply to specific cases. For example, a certain pitch may not print well with off-axis illumination, and a new design rule could prohibit this case. 
     In a variation on this embodiment, the target layout defines a standard cell that is used as a building block for the integrated circuit. 
     In a variation on this embodiment, the system performs a design rule checking operation on the target layout prior to simulating the effects of the manufacturing process. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee. 
     FIG. 1 illustrates how the manufacturing process is simulated in accordance with an embodiment of the invention. 
     FIG. 2 is a flow chart illustrating the wafer fabrication process in accordance with an embodiment of the invention. 
     FIG. 3 is a flow chart illustrating the design process and the manufacturing process for an integrated circuit in accordance with an embodiment of the invention. 
     FIG. 4 is a flow chart illustrating how a standard cell is designed in accordance with an embodiment of the invention. 
     FIG. 5 is a flow chart illustrating how a layout is generated and enhanced in accordance with an embodiment of the invention. 
     FIG. 6 illustrates the difference between optical proximity correction and process-complaint layout optimization in accordance with an embodiment of the invention. 
     FIG. 7 illustrates an exemplary printed image including problem areas in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Wafer Fabrication Process 
     FIG. 2 is a flow chart illustrating the wafer fabrication process in accordance with an embodiment of the invention. The system starts by applying a photoresist layer to the top surface of a wafer (step  202 ). Next, the system bakes the photoresist layer (step  204 ). The system then positions a mask over the photoresist layer (step  206 ), and exposes the photoresist layer through the mask (step  208 ). Next the system optionally bakes the wafer again (step  214 ) before developing the photoresist layer (step  216 ). Next, either a chemical etching or ion implantation step takes place (step  218 ) before the photoresist layer is removed (step  220 ). (Note that in the case of a lift-off process, a deposition can take place.) Finally, a new layer of material can be added and the process can be repeated for the new layer (step  222 ). 
     Design and Manufacturing Process 
     FIG. 3 is a flow chart illustrating the design process and the manufacturing process for an integrated circuit in accordance with an embodiment of the invention. The design process begins with a library design process (step  302 ). During the library design process, basic building blocks are created, such as standard cells, memory blocks (or compilers), I/O cells, and data path cells (or compilers). 
     Next, the logical design process (step  304 ) takes place. During the logical design process, functionality for the integrated circuit is defined, partitioned and verified. The output of the logical design process is a gate-level netlist. 
     This gate-level netlist is fed through a physical design process that converts the logical design into a physical implementation (step  306 ). Note that this physical implementation can include cells from cell libraries or other cells. 
     Next, a tapeout process takes place (step  308 ) in which a number of operations occur. Layout finishing is performed to merge abstracts with the routed layout, and to insert dummy structures and manufacturing patterns into the layout. The system also extracts devices, checks design rules at the device level and performs mask corrections as necessary. 
     Finally, the mask(s) resulting from the layout are fed through a manufacturing process (e.g. the process of FIG.  1 ), which manufactures the integrated circuit from the layout (step  310 ). 
     Standard Cell 
     FIG. 4 is a flow chart illustrating how a standard cell is created during the library design process (step  302  of FIG. 3) in accordance with an embodiment of the invention. 
     First, a schematic for the circuit is manually created with a schematic tool, such as CADENCE COMPOSER (step  402 ). During this process, netlist connectivity and transistor size are specified. A circuit simulation is also performed to verify functionality and performance. Next, a layout is generated from the netlist (step  404 ). This involves translating the netlist into a layout. Note that this layout must meet a set of design rules. 
     The system then performs an extraction operation (step  406 ) to extract transistor size, as well as capacitance and resistance of wires and devices. The output of this extraction process is a netlist with resistance and capacitance parameters. 
     Next, a physical verification operation takes place (step  408 ) to check design rules and to ensure that the transistor network is consistent with the layout. 
     The system then performs a characterization operation (step  410 ), which simulates the cell then extracts propagation delays and power dissipation for the circuit. 
     Next, a modeling process translates timing and power data into formats needed by design tools (step  412 ). 
     Finally, a quality assurance (QA) operation checks consistency of the simulation models (step  414 ). 
     Note that some of the above-described steps are optional depending on specific details of a given manufacturing process. Moreover, some of the above-described operations can be performed in parallel. 
     Layout Generation and Enhancement 
     FIG. 5 is a flow chart illustrating how a layout is generated and enhanced in accordance with an embodiment of the invention. Note that this process can be applied to the design of a standard cell for a library as well as to the design of an entire layout. Layout creation process  503  takes as input a design  502  and ensures that the resulting layout  510  satisfies a set of design rules  505 . The design  502  may be expressed in a number of suitable input formats ranging from SPICE netlists to VHDL or RTL descriptions of the design. 
     Next, layout  510  feeds through process simulator  512 . This process simulator  512  uses a process model  513  to generate a simulated printed image  514  for the layout. Note that this simulated printed image  514  may include a number of printed images generated using different process parameters. In this way, process simulator  512  can determine how the printed image will be affected by changes in process parameters. For example, the simulation could be performed for a range of defocus conditions. Note that FIG. 1 illustrates how process simulator  512  can make use of an optical model  114  as well as other models including a resist/etch model  116  to produce a simulated printed image  118 . 
     Next, an image analyzer  516  uses the simulated printed image  514  to generate local layout requirements  518  to optimize the process latitude and/or layout characteristics, e.g. area. These additional constraints  518  feed into a layout optimizer  520 , which further optimizes the layout. Note that this further optimization can involve identifying problem areas in the layout as is illustrated in FIG.  7 . 
     In one case, layout optimizer  520  attempts to update the layout to produce a layout  522  with enhanced process latitude. In this case, the goal in producing enhanced layout  522  could be to achieve a pre-determined target process latitude  517  regardless of the area impact. Hence, the layout optimizer  520  relaxes the layout to achieve the target process latitude. 
     In another case, the layout area is a fixed constraint and the layout optimizer  520  increases the process latitude as much as possible without changing the area. In this case, optimizing the global process latitude while maintaining the target area may result in decreased process latitude in some regions of the layout. 
     In yet another case, layout optimizer  520  additionally performs a compaction operation on the layout. 
     At a later time, enhanced layout  522  can be further refined through optical proximity correction  524  (as well as phase shifting, if desired) to produce a corrected layout  526 . 
     Furthermore, layout  522  can additionally feed into yield estimator  523  to produce an estimated yield  527  for the integrated circuit. 
     Note that the above-described simulation process can be applied to the enhanced layout in an iterative fashion to further improve process latitude for the layout (as is indicated by the arrow feeding enhanced layout  522  back into process simulator  512 ). 
     Comparing OPC and Process-Compliant Layout Optimization 
     FIG. 6 compares OPC with process-compliant layout optimization in accordance with the invention. OPC starts with a target layout T and produces a modified layout M that includes modifications to edges of shapes to compensate for optical effects during the manufacturing process. This results in a printed image P(M) which is closer to the target layout T than the printed image, P(T), of the unmodified target layout T. 
     In contrast, the process compliant layout optimization uses simulation results to produce a new target layout T′. Within this new target layout T′ shapes have been moved to improve process latitude, specifically in the original layout T, the two features were a distance d 1  apart, but in the new target layout the features are a spaced further apart. As such, the new target layout T′ is expected to have greater process latitude than the original layout T, this can be seen somewhat by the improved printed image P(T′) that shows less likelihood that the two features will bridge together than shown in P(T). 
     OPC seeks to produce a modified image M that results in a printed image P(M) that closely matches the target image T, whereas process-compliant layout optimization produces a new and different target image T′ so that the resulting printed image P(T′) has better process latitude when compared against P(T) (and P(M)). 
     Also note that OPC can be subsequently applied to the new target image T′ to produce a new layout M′ (not shown) that in turn produces a printed image P(M′) (not shown) that more closely matches the new target layout T′ than P(T′). 
     Example Printed Image 
     FIG. 7 illustrates an exemplary simulated printed image in accordance with an embodiment of the invention. This simulated printed image has problem areas that are highlighted by white boxes. Note that relaxed rules are only applied to shapes within these problem areas, and not to other areas in the layout. 
     In each of these problem areas, there is potential bridging between the printed lines. Hence, process latitude can be improved by moving the edges of the features so that a larger space is created between the features. 
     Alternative Embodiments 
     The foregoing descriptions of embodiments of the invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the invention. The scope of the invention is defined by the appended claims. 
     For example, a number of additional variations on the above-described process for selectively applying to problem areas are possible. In one embodiment, the process applies standard design rules to the entire layout and only applies relaxed rules as needed to local areas. Alternatively, the process can apply standard design rules (or relaxed rules) to the entire layout, and can then apply new rules, which can be tighter or looser than the original rules, to local areas. 
     Moreover, the data structures and code described in this detailed description can be stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet and the computer instruction signals may include the programs being accessed across the network. 
     Conclusion 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.