Abstract:
A method for writing a design to a material using an electron beam includes assigning a first dosage to a first polygonal shape. The first polygonal shape occupies a first virtual layer and includes a first set of pixels. The method also includes simulating a first write operation using the first polygonal shape to create the design, discerning an error in the simulated first write operation, and assigning a second dosage to a second polygonal shape to reduce the error. The second polygonal shape occupies a second virtual layer. The method further includes creating a data structure that includes the first and second polygonal shapes and saving the data structure to a non-transitory computer-readable medium.

Description:
BACKGROUND 
       [0001]    The present disclosure relates generally to semiconductor manufacturing. Specifically, the present disclosure relates to systems and methods that provide proximity effect correction for electron beam writing. 
         [0002]    Electron-beam (or “e-beam”) writing relates to a process for creating changes in a medium using e-beams. Specifically, some e-beam processes use e-beams to write designs onto mediums. Examples of mediums that can be written on with e-beams include semiconductor wafers and photomasks (e.g., fused silica and chrome masks). E-beam writing provides a way to create features on a medium where the features are smaller than a resolution limit for light. 
         [0003]    However, as feature sizes shrink, behaviors of e-beams, such as forward scatter and back scatter phenomena, become more significant. As a result of scattering phenomena, a shape actually written to a medium may differ somewhat from the pattern of the pixels written by the e-beam. This is also referred to as the proximity effect. Conventional methods correct for the proximity effect by adding alterations such as serifs or hammerheads to the pixel pattern so that the resulting shape is closer to the desired design feature. Other conventional methods use a pixel-by-pixel dosage modulation process, where dosage of a pixel is equal to beam intensity multiplied by exposure time. The dosage at each pixel can be set to make the resulting shape closer to the desired shape. In an example of a conventional process, a pixel map is provided to a mathematical modeling system that predicts how the resulting shape would appear upon inspection of the medium. The model is then compared to the desired shape to produce an error. The error is minimized by use of shape correction or pixel-by-pixel dosage modulation. 
         [0004]    The above-described conventional techniques have some disadvantages. For instance, shape correction by itself may not provide sufficient correction for very fine features. Also, pixel-by-pixel dosage modulation requires much processing power because of the large number of pixels that may be present in a given design. More efficient and effective proximity effect correction is called for. 
       SUMMARY 
       [0005]    The present disclosure provides for many different embodiments. In a first embodiment, a method for writing a design to a material using an electron beam includes assigning a first dosage to a first polygonal shape. The first polygonal shape occupies a first virtual layer and includes a first set of pixels. The method also includes simulating a first write operation using the first polygonal shape to create the design, discerning an error in the simulated first write operation, and assigning a second dosage to a second polygonal shape to reduce the error. The second polygonal shape occupies a second virtual layer. The method further includes creating a data structure that includes the first and second polygonal shapes and saving the data structure to a non-transitory computer-readable medium. 
         [0006]    In another embodiment, a computer-based system for facilitating electron beam writing includes a dosage modulation module for proximity effect correction. The dosage modulation module is operable to: create a plurality of polygons, each polygon having a respective dose applied to a respective set of pixels, and further each polygon occupying a respective virtual layer; iteratively apply ones of the layers to correct a simulation writing error; and save the applied ones of the layers to a data structure. The system also includes a non-transitory computer-readable medium storing the data structure. 
         [0007]    In another embodiment a program product having a non-transitory computer readable medium tangibly recording computer program logic for performing proximity effect correction includes code to calculate an error for a first shape using a first write simulation operation. The computer program product also has code to apply a first polygon in a first virtual layer to the first shape. The polygon defines a first set of pixels with a first dosage and is generated to address the error. The computer program product also includes code to create a data structure that includes the first shape and the first virtual layer and code to output the data structure in a computer-readable file. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
           [0009]      FIG. 1  is a simplified diagram of an exemplary network system in which proximity effect correction embodiments may be implemented. 
           [0010]      FIG. 2  is a simplified block diagram of an exemplary client computer system that may be configured to implement embodiments of proximity effect correction systems. 
           [0011]      FIG. 3  is an illustration of an exemplary process for performing proximity effect correction. 
           [0012]      FIG. 4  is an illustration of an exemplary shape correction operation. 
           [0013]      FIG. 5  is an illustration of an exemplary polygon-based correction operation. 
           [0014]      FIG. 6  shows an exemplary dosage/shot number map, which corresponds to the scenario of  FIG. 5 . 
           [0015]      FIG. 7  illustrates exemplary polygons, where in each layer there is a freedom of shape, position, and number of polygons. 
           [0016]      FIG. 8  illustrates freedom of dosage in the context of the polygons of  FIG. 7 . 
           [0017]      FIG. 9  is an illustration of an exemplary layer-by-layer writing process. 
           [0018]      FIG. 10  is an illustration of an exemplary design pattern and a close-up of an exemplary dosage correction applied to part of the design pattern by one or more processes and systems described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present disclosure relates generally to semiconductor manufacturing. Specifically, the present disclosure relates to proximity effect error correction techniques that use polygons to apply dosage modulation. While the examples herein discuss applying the techniques to write to photolithographic masks and semiconductor wafers, it is understood that the scope of embodiments can include any system for writing to any appropriate medium using e-beam technology. 
         [0020]    The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
         [0021]    With reference now to the figures,  FIG. 1  is a simplified diagram of a network system  100  in which embodiments may be implemented. Network system  100  includes a network  102  that provides a medium for communications between various devices and computers communicatively coupled by network system  100 . Network  102  may be implemented as one or more of various networks, such as the Internet, an intranet, a local area network, a wide area network (WAN), or another network architecture that facilitates communications between network connected devices. Network  102  may include any one or more of various communication connections, such as wire, wireless, fiber optic, satellite links, or other communication media. 
         [0022]    In the present example, various servers  110 - 112  are connected to network  102 . In addition, a client  120  is connected to network  102 . Servers  110 - 112  may be implemented as hypertext transfer protocol (HTTP) servers, file transfer protocol (FTP) servers, application servers, or other devices that provide data sources such as web pages or other content to client  120  connected therewith. Client  120  may be implemented as a personal computer, a portable computer, a network computer, a super computer, or another computational device. Client  120  is interconnected with a local printer  121  and/or a network printer for producing hardcopy output. 
         [0023]    Additionally, client  100  is connected to e-beam writing hardware  130 , which receives files from client  120  and writes to a physical medium according to the data in the file. The data in the files includes layout patterns that are stored and transmitted in a layout description language such as GDS-II or Oasis. Techniques for proximity effect correction discussed further below may be performed by one or more computers, such as client  120  and/or servers  110 - 112 . For instance, one or more computers perform the simulation, error calculation, and polygon generation that is used in the error correction. 
         [0024]      FIG. 2  is a simplified block diagram of a computer system  200 , such as client  120  or any of servers  110 - 112  shown in  FIG. 1 , that may be configured to implement embodiments of a proximity effect correction system. Computer system  200  includes a processor  202  interconnected with a system bus  204 . System bus  204  provides couplings to subsystems and components of computer system  200 . A memory controller  206  interconnected with a system memory  208  provides a communicative coupling between memory  208  and processor  202 . Memory  208  may store executable instructions that provide proximity effect correction functionality as described more fully below. An input/output bridge  210  may be connected with system bus  204 , and one or more input/output devices may be connected with an I/O bus  212 . For example, a hard disk  216  (or other memory, such as a flash drive) may provide non-transitory, non-volatile storage, and a modem or network adapter  214  may provide a communication interface that facilitates communication exchanges between computer system  200  and one or more data resources on a network. Additionally, user input devices, such as a mouse/keyboard  218 , may be coupled with I/O bus  212  and facilitate user input to computer system  200 . The configuration of computer system  200  is illustrative and is chosen only to facilitate an understanding of embodiments described herein. 
         [0025]      FIG. 3  is an illustration of exemplary process  300  for performing proximity effect correction. Process  300  may be performed, for example, by one or more computers  110 - 112 ,  120  of  FIG. 1 . In block  305 , a file is received. In this example, the file includes data for writing a design to a medium. 
         [0026]    In block  310 , shape correction is performed on the data in the file. In this example, a shape in the file is simulated, and an error is calculated. Changes to the shape are generated to reduce the error, and the shape-corrected file is analyzed at block  320 . Shape correction is explained in more detail with respect to  FIG. 4 . 
         [0027]    Moving to  FIG. 4 , an illustration of an exemplary shape correction operation is given. Shape  405  is the shape of the pixels that are written. Shape  415  is the target shape for the design. Target shape  415  includes target points  420 , which in this example include a multitude of points around the perimeter of target shape  415 . Contour  410  is the result of simulation, showing what can be expected when shape  405  is written to a physical medium. 
         [0028]    After simulation, a cost function is run to discern an error between contour  410  and target shape  415 . The cost function can be any function able to identify an error between contour  410  and target shape  415 . Further, the cost function may be a function of any one or more of the following parameters: Edge Placement Error (EPE, the distance between an edge of a contour and a given target point), Process Window (PW), Normalized Image Slope (NILS), Wafer Error Enhancement Factor (WEEF), Image Log Slope (ILS), Depth of Focus (DoF), Exposure Latitude (EL). One example of a cost function is given by Equation (1) below. It is understood that the scope of embodiments includes any other cost functions capable of identifying an error. 
         [0000]    
       
         
           
             
               
                 
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         [0029]    Of note in  FIG. 4  is the number of target points  415 .  FIG. 4  shows eight target points  415 , though other embodiments may have more or fewer. Generally, when more target points are used, the shape correction process requires more time and more processing power but may provide more precise results. Such considerations can be matched to the particular application of a given embodiment, and in some applications the number of target points can be defined by a user. 
         [0030]    If the size of the error as given by the cost function is within an acceptable range, then the proximity effect correction may skip to block  345  ( FIG. 3 ). On the other hand, if the size of the error is not within an acceptable range, then shape correction is performed on shape  405 . Examples of shape correction include adding serifs, hammerheads, and other shape modifications to shape  405  to reduce the error. In  FIG. 4 , contour  410  matches target points  420  closely, so error in the particular case of  FIG. 4  is expected to be low, though it may or may not be low enough to be within an acceptable range. In some embodiments, acceptable error ranges can be set by a user. 
         [0031]    The embodiment of  FIG. 3  uses shape correction as a preliminary or coarse correction function and uses polygon-based correction as a finer correction operation. In other embodiments, shape correction is omitted, such that the first correction applied to the file is polygon-based correction. Different applications may include or omit the shape correction of block  310  depending on various factors, such as time constraints and available processing power. 
         [0032]    Returning to  FIG. 3 , in block  320 , the shape-corrected file is simulated again to calculate an error. The error can be calculated using the same or a different cost function from the shape correction example above. If the calculated error is within an acceptable range, then process  300  may omit polygon-based correction. However, if the calculated error is not within the acceptable range, then polygon-based correction is performed to reduce the error. Polygon-based correction is explained in more detail below with respect to  FIGS. 5-10 . 
         [0033]    Moving to  FIG. 5 , an example of polygon-based correction is shown. Target shape  415  and target points  420  are the same as in  FIG. 4 . Shape  405  is overlaid with polygons  520 ,  530  to create contour  510  during simulation, which has an incrementally smaller error than does contour  410  of  FIG. 4 . 
         [0034]    Polygons  520  and  530  each define a set of pixels that have a dosage associated therewith. Moreover, each of polygons  520 ,  530  is associated with a virtual layer. For example, the layer that includes polygon  520  overlays shape  405 , and the layer that includes polygon  530  overlays shape  405  as well as the layer that includes polygon  520 . The layers are virtual in that they do not represent physical layers, but rather, provide a way to conceptually separate and/or stack different polygons. 
         [0035]    Also, each one of polygons  520 ,  530  can be additive or subtractive. In one example, polygon  520  is additive so that the pixels therein have a dosage of +3 shots (where shots is a unit of dosage, explained further below). The dosage +3 adds to the pixels of the shape  405  that are directly below polygon  520 . In the same example, polygon  530  is subtractive so that the pixels therein have a dosage of −1. The dosage −1 subtracts one shot from each of the pixels of shape  405  and polygon  520  that lie directly below polygon  530 . Such concept is described further with respect to  FIG. 8 . 
         [0036]      FIG. 6  shows exemplary dosage/shot number map  600 , which corresponds to the scenario of  FIG. 5 . The set of pixels in area  605  has a uniform dosage. Similarly, the pixels in area  610  have a different uniform dosage that includes the dosage of shape  405  modified by the dosage associated with polygon  610 . The pixels in area  615  have a third uniform dosage that is cumulative of the dosages of polygons  520 ,  530  and shape  405 . 
         [0037]    Polygon-based correction is iterative in this example. For instance, when a shape with an error is submitted for polygon-based correction, the system generates a layer with a polygon that has a shape, position, and dosage that is calculated to reduce the error. In the example of  FIG. 5 , the system generates a layer that has polygon  520 . The system then runs a simulation and calculates an error again. The system determines that the new error is still not in an acceptable range and generates a layer with polygon  530  to further reduce the error. The system runs another simulation and error analysis and can end polygon-based correction or can perform further iterations by generating more polygons in more layers. Various embodiments can perform any number of iterations to reach a desired error range. 
         [0038]    In each layer, there is a freedom of shape, position, and number of polygons. This principle is illustrated by  FIG. 7 , which shows polygons  710 ,  720 ,  730 . In case  1 , there are three layers—one that includes polygon  710 , one that includes polygon  720 , and one that includes polygon  730 . In case  2 , there may be three layers, as in case  1 , but with a different spatial arrangement of polygon  730  in one of the layers. Alternatively, there could be two layers—one for polygon  710  and one that has polygons  720  and  730 . Case  3  shows that any of polygons  710 ,  720 ,  730  may be included in more than one layer and/or may be repeated in any single layer. 
         [0039]    Furthermore, in each layer, there is freedom of dosage.  FIG. 8  illustrates freedom of dosage in the context of three polygons  710 ,  720 ,  730 . In this example, polygon  710  is assigned one shot, where one shot can be an arbitrarily defined unit of dosage for the pixels in a polygon. For instance, a shot can indicate a specific beam intensity applied for a specific time. Shot equivalents can be produced by modulating the intensity of a beam and/or the speed of writing at a particular intensity. 
         [0040]    Further in this example, polygon  720  is assigned a two-shot dose, and polygon  730  is assigned a three-shot dose. Any arbitrary dosage can be assigned to a given polygon. Although not illustrated in  FIG. 8 , dosage sign is an aspect of dosage freedom. For instance, polygons  710 ,  720 ,  730  are shown as having positive dosages, though any one or all three could have a negative dosage. A negative dosage indicates that, for the multi-layered shape as a whole, some pixels may have dosage subtracted by one or more layers. Polygons  710 ,  720 ,  730  are arranged to give particular sets of pixels desired dosages, and some layers may include both negative- and positive-signed polygons. 
         [0041]    Referring back to  FIG. 3 , block  320  includes iteratively applying layers with one or more polygons until the cost function shows that an error is within an acceptable range. As explained above, for each of the layers, there is freedom of number of polygons, shape of polygons, position of polygons, and dosage of polygons. The layers, polygons, shapes, positions, and dosages to be applied in block  320  can be determined in any of a number of ways. In one example, shape correction is determined by a human user using knowledge or skill to apply an appropriate number of layers and polygons and to position, shape, and apply dosage to the polygons. In another example, a computer program uses models and/or rules to determine the specific details of the correction. The scope of embodiments is not limited to any particular method for deciding the number of layers and polygons and the positions, shapes, and dosages of the polygons. 
         [0042]    Once the system discerns that the error is within an acceptable range, the system prepares to select a mode at block  340 . Process  300  provides for two modes. The first mode is a merged mode, as shown at block  330 . In the merged mode, the multiple layers and the shape are added to give a map of pixels with cumulative dosage values. After block  330 , the file is effectively a single-layer pixel map where the dosages of the pixels have been summed from the different virtual layers. 
         [0043]    The second mode is a layer-by-layer mode. Each of the virtual layers are kept separate so that they are not added before the file is output. A mode can be selected at block  340  by a human user or by computer logic, depending on the application. 
         [0044]    The file is output at block  345 . The file can be saved to a non-transitory medium, such as a hard drive, optical disc, flash drive, or the like. The file can be loaded to an e-beam writing machine over a network or by physical media. In any event, the e-beam writing machine (e.g., machine  130  of  FIG. 1 ) uses the data and instructions in the file to write the specified design to a medium. 
         [0045]    If the merged mode was selected at block  340 , the e-beam writing machine may perform a single pass, modulating dosage of the individual pixels during the pass. On the other hand, if the layer-by-layer mode was selected at block  340 , then the e-beam writing machine may make as many passes as there are layers.  FIG. 9  is an illustration of an exemplary layer-by-layer writing process. Pattern  900  is written in a three-step process. In pass one, a first dosage is applied according to polygon  930  in a first layer. In pass two, a second dosage is applied according to second polygon  920  in a second layer. The pixels that are within polygon  920  thereby receive a cumulative dose defined by the first two layers. In pass three, a third dosage is applied according to polygon  910  in a third layer. The pixels that are within polygon  910  receive a cumulative dose defined by the three layers. 
         [0046]      FIG. 10  is an illustration of an exemplary design pattern  1010  and a close-up of an exemplary dosage correction  1020  applied to part of the design pattern by the processes and systems described herein. The dosage correction includes an underlying shape that is modified by polygons  1021 - 1026 . The different shading represents different dosages. Polygons  1021 - 1026  can be applied using one or more virtual layers and can be merged or written layer-by-layer, as described above. 
         [0047]    The e-beam writing process includes writing to a physical medium. In one example, the e-beam writing process is used to etch material from a semiconductor wafer in a design that facilitates the manufacture of structures on the wafer. In an other example, the e-beam writing process is used to remove portions of a photomask that is made of, e.g., chrome and fused silica. The photomask can then be used in the manufacturing process of semiconductor devices. E-beam writing processes provide an effective way to create designs on a physical medium where some of the features of the design may be too small to be made by other conventional processes, such as photolithography. 
         [0048]    Various embodiments may include advantages over other techniques. For instance, as explained above, pixel-by-pixel dosage modulation may use more processing power than is desirable because designs that vary dosage pixel-by-pixel can be quite complex. By contrast, some embodiments using a polygon-based technique for dosage modulation may experience increased efficiency because regimes employing large multi-pixel polygons may be less complex that regimes that employ pixel-by-pixel techniques. Additionally, currently-available tools to write e-beam designs can readily adopt the polygon-based processes described above with software and/or firmware changes, thereby allowing various embodiments to be implemented quickly and cost-effectively. 
         [0049]    The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.