Abstract:
A method, computer program product, and data processing system for performing an improved optical proximity correction are disclosed, which better respect the electrical properties of the device being manufactured. A preferred embodiment of the present invention performs OPC by first dividing the perimeter of a mask region into a plurality of segments, then grouping the segments into at least two distinct groups, wherein segments in the first of these groups are adjusted in position so as to minimize edge placement error (EPE) when the photolithography using the mask is simulated. Segments in the second group are adjusted in position so as to minimize cumulative error in a dimension spanning the region, wherein the span of such dimension extends from segments in the first group to segments in the second group. Correction so obtained by this process more readily preserves the intended electrical behavior of the original device design.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates generally to computer-aided design of photolithographic masks used in the manufacturing of integrated circuits. More specifically, the present invention is directed to an improved method, computer program product, and apparatus for performing optical proximity correction in such masks. 
         [0003]    2. Description of the Related Art 
         [0004]    Integrated circuits (ICs) are commonly manufactured through a photolithographic process. In photolithography, a layer of light-sensitive material (photoresist) is applied atop a layer of material to be etched by the process. A geometric pattern is applied to the photoresist by shining light of a prescribed wavelength through a photolithographic mask (also referred to as a “photomask” or, simply, “mask”) containing the pattern. A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. (Henceforth herein, the term “region of transparency” shall be used to denote either a hole in the plate or an actual transparent portion of the plate, so that a single term may be used to refer to either possibility. A “region of opacity” shall be used to denote an opaque feature in the photomask. The term “region” shall be used to denote either a region of transparency or a region of opacity.) In practice, lithographic photomasks are typically transparent fused silica blanks covered with a pattern defined with a chrome metal absorbing film. They are typically manufactured to a size that is substantially larger than that of the circuit itself, and reduction optics are used to project the mask pattern onto the photoresist at the correct size. The light passing through the mask causes changes to occur in the photoresist such that a subsequent etching process leaves selected portions of the photoresist and immediately underlying material layer intact, while removing undesired portions of such layers. For example, photolithography can be used to define the geometric pattern of the polysilicon layer in a MOS (metal-oxide semiconductor) integrated circuit technology (the layer in which the gates of MOSFETs [metal-oxide semiconductor field-effect transistors] are defined). 
         [0005]      FIG. 1 , for example, depicts the IC layout of a simple MOSFET  100 . MOSFET  100  is made up of a region of doped silicon  102  (either N-type or P-type) overlapped by a region of polysilicon material  104 . The gate of the transistor is defined by the rectangular portion ( 106 ) of polysilicon region  104  that overlaps doped silicon region  102 . In the standard Shichman-Hodges MOSFET model, the electrical behavior of the MOSFET thus constructed is modeled by a parameter “K,” which is directly proportional to the ratio of the width ( 108 ) of gate  106  to its length ( 110 ). Thus, the relative dimensions of transistor features are critical to the proper electrical behavior of a given device. 
         [0006]    The minimum feature sizes available in a given technology are limited, at least in part, by the nature of the photolithographic process itself. Specifically, as feature sizes approach the wavelengths of light used in the process (e.g., at sub-micron feature sizes for conventional photolithography using ultraviolet light), diffraction (a result of the wave-nature of electromagnetic radiation) reduces the fidelity of the image projected onto the photoresist. For example, a rectilinear feature  200  in a photomask (existing either as a region of transparency or as a region of opacity), as depicted in  FIG. 2 , might be rendered in a distorted curvilinear shape (such as shape  300 ) in the photoresist, as depicted in  FIG. 3 . 
         [0007]    One potential way of dealing with this problem is to shorten the wavelengths of light used. Thus, the use of extremely short-wavelength ultraviolet radiation or X-ray radiation for photolithography is a topic of current research. Another method of dealing with this problem is to employ what has come to be known as optical proximity correction (OPC). In OPC, diffusion-induced distortions in the pattern applied to the photoresist are reduced in severity by introducing small irregularities into the mask itself. The effect of these irregularities is to “predistort” the mask pattern in such a way that the diffraction-induced distortions are minimized. For example, in  FIG. 4 , an OPC-modified version  400  of mask pattern  200  is depicted, wherein irregularities (e.g., irregularities  402  and  404 ) are introduced into the mask shape so as to counteract the distortions caused by diffraction. OPC, as it is practiced in the art, relies on computerized optimization algorithms to minimize “edge placement error” (EPE). EPE is a metric that represents the distance between a location on the projected image and a corresponding location in the mask pattern. In a typical implementation, modifications to the mask pattern are made using a gradient-descent approach to minimize the overall EPE of a mask pattern. 
         [0008]    Although EPE-based OPC is a useful technique, the results obtained from such technique are not necessarily optimal in the sense of electrical behavior. More specifically, although the projected image may better conform to the desired shape, the electrical behavior of the resulting device may differ substantially from what is desired, thus necessitating complex electrical simulation and further adjustment of the mask pattern in response. 
         [0009]    What is needed, therefore, is a method of performing optical proximity correction which respects the electrical properties of the device being manufactured. The present invention provides a solution to this and other problems, and offers other advantages over previous solutions. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, the present invention provides a method, computer program product, and data processing system for performing an improved optical proximity correction that better respects the electrical properties of the device being manufactured. Specifically, the present invention recognizes that the electrical properties of a given device are a function of its dimensions (size), as opposed to its edge positions (shape). In particular, a preferred embodiment of the present invention performs OPC by first dividing the perimeter of a mask region in an integrated circuit into a plurality of segments, then grouping the segments into at least two distinct groups, wherein segments in the first of these groups are adjusted in position so as to minimize edge placement error (EPE) when the photolithography using the mask is simulated. Segments in the second group are adjusted in position so as to minimize cumulative error in a dimension spanning the region, wherein the span of such dimension extends from segments in the first group to segments in the second group. Optical Proximity Correction so obtained by this process, because such correction is obtained with reference to desired dimensions, as opposed to merely position, more readily preserves the intended electrical behavior of the device as originally designed (before the correction took place). 
         [0011]    The foregoing is a summary and thus contains, by necessity, simplifications, generalizations, and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a diagram illustrating the silicon layout of a simple MOSFET; 
           [0014]      FIG. 2  is a diagram illustrating an exemplary rectilinear region in a photomask; 
           [0015]      FIG. 3  is a diagram illustrating diffraction-induced distortions in the projected image obtained from a photomask at the sub-micron level; 
           [0016]      FIG. 4  is a diagram of an exemplary region in a photomask design that is a possible result of applying optical proximity correction (OPC) to the photomask design in  FIG. 2 ; 
           [0017]      FIG. 5  is a diagram illustrating a step of dividing a perimeter of a region into a plurality of segments and further grouping those segments for electrically-motivated optical proximity correction in accordance with a preferred embodiment of the present invention; 
           [0018]      FIG. 6  is a diagram illustrating a step of preparing various modified versions of a region for the purpose of simulating the effects of various segment adjustments in accordance with a preferred embodiment of the present invention; 
           [0019]      FIG. 7  is a flowchart representation of a process of performing electrically-motivated optical proximity correction in accordance with a preferred embodiment of the present invention; and 
           [0020]      FIG. 8  is a diagram of a data processing system in which a preferred embodiment of the present invention may be implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following is intended to provide a detailed description of an example of the invention and should not be taken to be limiting of the invention itself. Rather, any number of variations may fall within the scope of the invention, which is defined in the claims following the description. 
         [0022]    Operation of a preferred embodiment of the present invention is illustrated with respect to an exemplary feature or “region”  502  in a photomask pattern. For the purpose of this example, a purely rectangular region is selected, which would be representative of a simple polysilicon layer utilized to form the gate of a single MOSFET, such as MOSFET  100  in  FIG. 1 . The process of performing optical proximity correction in this preferred embodiment of the invention begins with the division of the perimeter of region  502  into a plurality of segments, such as segment  504 . These segments are then grouped into two groups, shown here as group  506  and group  508 . One of these groups (we will choose group  506  in this example) will have the positions of its segments adjusted as to minimize edge placement error (EPE), whereas the other group (group  508 ) will have the positions of its segments adjusted so as to minimize dimensional error. 
         [0023]    The actual process of determining which corrections to make to region  502  is illustrated in  FIG. 6 . Each segment is considered individually in this process. In this example, we will start with segment  602 , which is a segment that is to be adjusted for minimal edge placement error (EPE) because it belongs to group  506  (as shown in the previous figure)—we will refer to this type of adjustment as “edge-optimization.” To perform the adjustment, a photolithography simulation is first performed on region  502  to obtain a simulated printed contour  612 . If the printed contour generated from the simulation (contour  612 ) is within the boundary of the desired shape (as shown in  FIG. 6 ), segment  602  is displaced outward, thus replacing segment  602  with an outwardly extending serif  604 . In the opposite case, where the simulation contour extends outside of the boundary of the desired shape, segment  602  would instead be replaced with a concave notch  608 . The size of the notch or serif generated, in either case, is computed as a function of the magnitude of the edge placement error of segment  602  (so that greater errors result in greater displacements, for instance). 
         [0024]    For the segments in group  508  (for example, segment  610 ), the same simulation data is used. However, for these segments, the choice of positional adjustment is made not for the purpose of minimizing EPE, but for the purpose of minimizing the dimensional error for each segment. In this example, the dimensional error at a single point would, in a preferred embodiment, be the difference between width  618  (the width of the desired shape at that point) and width  616  (the width of the simulated shape at the same point). If the dimensional error for segment  610 , for example, indicates that the printed contour is too narrow between segment  602  and segment  610 , segment  610  can be replaced with a convex serif. If the contour is too wide, on the other hand, segment  610  can be replaced with a concave notch. As with the EPE-optimized segments, the size of each notch or serif created is a function of the magnitude of the error associated with that segment. 
         [0025]    This overall process is depicted in more complete detail in  FIG. 7 , where it is presented in flowchart form. Given a region (feature) for which OPC is to be performed, the perimeter of that region is divided into a plurality of segments (block  700 ). These segments are then grouped into “edge-optimized” and “dimension-optimized” groups (block  702 ). Then, the photolithographic process is simulated on the current mask (block  704 ) and an EPE score and dimensional error score are computed from this simulation (block  706 ). If these error scores are acceptable (i.e., there is no more than an acceptable level of overall error), the process terminated (block  708 : No), but if the error scores exceed acceptable levels, the process continues to consider each segment in the region individually. 
         [0026]    Thus, the next segment in the region is examined (block  710 ). A determination is then made as to whether the segment is to be “edge-optimized.” If so (block  714 : Yes), then the “error” score for each displacement is calculated as the EPE score for that displacement (block  716 ). If not, however, (block  714 : No), the “error” score for each displacement is calculated as the dimensional error score for that displacement (block  718 ). 
         [0027]    A displacement that reduces the “error” score (either EPE or displacement, depending on context) for that segment is then computed on the basis of the determined error and the photomask design adjusted according to that choice (block  720 ). If there are more segments in the region to be considered (block  722 : Yes), the process loops back to block  710  to consider the next segment. If not (block  722 : No), the process returns to block  704  to determine the over all acceptability of the design as modified. 
         [0028]    In an alternative embodiment of the present invention, a gradient-descent search method may be used to optimize the mask according to global objective functions. In this alternative embodiment, for each segment, simulations are run for different displacements (inward or outward) and the displacement that, according to the simulation, minimizes the overall EPE or dimensional error (depending on the segment in question) is chosen. 
         [0029]      FIG. 8  illustrates information handling system  801  which is a simplified example of a computer system/data processing system capable of performing the computing operations described herein with respect to a preferred embodiment of the present invention. Computer system  801  includes processor  800  which is coupled to host bus  802 . A level two (L2) cache memory  804  is also coupled to host bus  802 . Host-to-PCI bridge  806  is coupled to main memory  808 , includes cache memory and main memory control functions, and provides bus control to handle transfers among PCI bus  810 , processor  800 , L2 cache  804 , main memory  808 , and host bus  802 . Main memory  808  is coupled to Host-to-PCI bridge  806  as well as host bus  802 . Devices used solely by host processor(s)  800 , such as LAN card  830 , are coupled to PCI bus  810 . Service Processor Interface and ISA Access Pass-through  812  provides an interface between PCI bus  810  and PCI bus  814 . In this manner, PCI bus  814  is insulated from PCI bus  810 . Devices, such as flash memory  818 , are coupled to PCI bus  814 . In one implementation, flash memory  818  includes BIOS code that incorporates the necessary processor executable code for a variety of low-level system functions and system boot functions. 
         [0030]    PCI bus  814  provides an interface for a variety of devices that are shared by host processor(s)  800  and Service Processor  816  including, for example, flash memory  818 . PCI-to-ISA bridge  835  provides bus control to handle transfers between PCI bus  814  and ISA bus  840 , universal serial bus (USB) functionality  845 , power management functionality  855 , and can include other functional elements not shown, such as a real-time clock (RTC), DMA control, interrupt support, and system management bus support. Nonvolatile RAM  820  is attached to ISA Bus  840 . Service Processor  816  includes JTAG and I2C buses  822  for communication with processor(s)  800  during initialization steps. JTAG/I2C buses  822  are also coupled to L2 cache  804 , Host-to-PCI bridge  806 , and main memory  808  providing a communications path between the processor, the Service Processor, the L2 cache, the Host-to-PCI bridge, and the main memory. Service Processor  816  also has access to system power resources for powering down information handling device  801 . 
         [0031]    Peripheral devices and input/output (I/O) devices can be attached to various interfaces (e.g., parallel interface  862 , serial interface  864 , keyboard interface  868 , and mouse interface  870  coupled to ISA bus  840 . USB hub  845 , for instance, is shown connected to a media reader  846  for the purpose of reading machine-readable tangible data storage media, such as memory cards, optical discs, and the like. Alternatively, many I/O devices can be accommodated by a super I/O controller (not shown) attached to ISA bus  840 . 
         [0032]    In order to attach computer system  801  to another computer system to copy files over a network, LAN card  830  is coupled to PCI bus  810 . Similarly, to connect computer system  801  to an ISP to connect to the Internet using a telephone line connection, modem  875  is connected to serial port  864  and PCI-to-ISA Bridge  835 . 
         [0033]    While the computer system described in  FIG. 8  is capable of executing the processes described herein, this computer system is simply one example of a computer system. Those skilled in the art will appreciate that many other computer system designs are capable of performing the processes described herein. 
         [0034]    One of the preferred implementations of the invention is a computer program, namely, a set of instructions (program code) or other functional descriptive material in a code module that may, for example, be resident in the random access memory of the computer. Until required by the computer, the set of instructions may be stored in another computer memory, for example, in a hard disk drive, or in a removable memory such as an optical disk (for eventual use in a CD ROM) or floppy disk (for eventual use in a floppy disk drive), or downloaded via the Internet or other computer network. Thus, the present invention may be implemented as a computer program product for use in a computer. In addition, although the various methods described are conveniently implemented in a general purpose computer selectively activated or reconfigured by software, one of ordinary skill in the art would also recognize that such methods may be carried out in hardware, in firmware, or in more specialized apparatus constructed to perform the required method steps. Functional descriptive material is information that imparts functionality to a machine, namely executable computer code. 
         [0035]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from this invention and its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of this invention. Furthermore, it is to be understood that the invention is solely defined by the appended claims. It will be understood by those with skill in the art that if a specific number of an introduced claim element is intended, such intent will be explicitly recited in the claim, and in the absence of such recitation no such limitation is present. For non-limiting example, as an aid to understanding, the following appended claims contain usage of the introductory phrases “at least one” and “one or more” to introduce claim elements. However, the use of such phrases should not be construed to imply that the introduction of a claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an;” the same holds true for the use in the claims of definite articles. Where the word “or” is used in the claims, it is used in an inclusive sense (i.e., “A and/or B,” as opposed to “either A or B”).