Abstract:
A method is provided for determining pitch of lithographic features of a mask. The method includes determining a bias based on an interaction between a plurality of reference features positioned according to a lithographic parameter of the mask, applying the bias to a plurality of lithographic features of the mask, and determining pitch of the plurality of lithographic features based on interactions between the biased plurality of lithographic features of the mask.

Description:
FIELD 
     The invention relates generally to systems and methods for semiconductor device manufacture. 
     BACKGROUND 
     In the semiconductor industry, intricate designs or patterns of electronic chips are generally made using lithographic techniques, such as photolithography, X-ray lithography, or extreme ultraviolet (EUV) lithography. These techniques utilize a patterned photomask or reticle in combination with certain systems to transfer patterns onto objects such as semiconductor wafers and electronic chips. For example, in a photolithographic process, a patterned photomask is used in combination with laser exposure systems to transfer patterns. Processing situations, however, may distort the resulting pattern defined on a semiconductor wafer. For example, optical diffraction may cause the pattern defined on the wafer to differ from the pattern of the photomask. 
     A photomask may include assist or auxiliary features that compensate for distortions in a resulting pattern transferred onto a wafer. The auxiliary features aid in the transfer of primary features of the photomask. In one technique for compensating distortions, a photomask may include sub-resolution assist features (SRAFs). An SRAF is designed to improve the process margin of a resulting wafer pattern, but not to be printed on the wafer. Typically, the SRAF is small enough and properly located on the mask so that that the SRAF is not transferred onto the wafer because the wafer features are below the dimensional resolution of the lithography system. The SRAF, however, is large enough to affect the passage of light and impacts a nearby lithographic feature. 
     In certain situations, however, the SRAFs may be unsatisfactory. For example, the SRAFs may print on a wafer or may violate mask rules. The unsatisfactory SRAF may be caused by interactions between SRAFs of neighboring printed photomask features or neighboring SRAFs. Accordingly, the position of the SRAFs must be accurately determined in order to prevent unsatisfactory effects in the photomask. Typically, the SRAFs positions are determining by simulating the entire mask layout including all primary and secondary features and looking for any interactions. 
       FIGS. 1A and 1B  are diagrams illustrating a conventional method for determining the interaction of mask features for positioning SRAFs. As illustrated in  FIG. 1A , a mask design  100  includes several wafer features, such as contact holes  102 . Contact holes  102  are positioned in the design according to the requirements of the design. 
       FIG. 1B  illustrates a conventional method for determining the interaction of contact holes  102  in mask layout  100 . As illustrated in  FIG. 1B , projections  104  are simulated for each contact hole  102 . If projections  104  overlap, the overlapping edge is considered to interact with the corresponding overlapping edge. As such, the SRAFs&#39; position and number must be determined considering the interaction. The SRAFs&#39; position and number is determined by the pitch between the interacting contact holes. If projections  104  do not overlap, the non-overlapping edges are considered to be isolated (ISO) edges  106 . As such, the conventional method will not consider the interaction between ISO edges  108  even though these contact holes are in close proximity. 
     According to the conventional method, the entire design must be simulated first to determine if the lithographic features and SRAFs will interact. Further, the conventional method does not recognize all possible interactions between lithographic features. In  FIG. 1B , according to the conventional method, edges  108  of contact  102  do not interact. Due to their proximity, however, contacts  102  with edges  108  are not independent, but are strongly coupled. As such, if the conventional method was utilized, the SRAFs of these contacts  102  would interact and may print on the wafer. Alternatively, if the conventional method was utilized, the process margin of the main feature may be insufficient to meet the requirements of the process 
     SUMMARY 
     An embodiment is directed to a method of determining pitch of lithographic features of a mask. The method comprises determining a bias based on an interaction between a plurality of reference features positioned according to a lithographic parameter of the mask, applying the bias to a plurality of lithographic features of the mask, and determining pitch of the plurality of lithographic features based on interactions between the biased plurality of lithographic features of the mask. 
     Another embodiment is directed to a method of preventing photolithographic mask problem sites. The method comprises generating a plurality of reference auxiliary features for a plurality of random contacts, determining a minimum separation distance between the plurality of random contacts based on lithographic parameters, and determining a bias of the plurality of random contacts, separated by the minimum separation distance, by enlarging the area of the plurality of random contacts until the plurality of random contacts have a projection, applying the bias to a plurality of contacts of a mask, determining an interaction of the biased plurality of contacts by determining portions of an edge of one of the plurality of biased contact that interacts with projections of adjacent biased plurality of contacts, and determining pitch of the plurality of biased contacts based on interaction of edges of the plurality of biased contacts. 
     Another embodiment is directed to a system for determining pitch of lithographic features of a mask. The system comprising a processor and an application configured for execution by the processor comprising program instructions for determining a bias based on an interaction between a plurality of reference features positioned according to a lithographic parameter of the mask, applying the bias to a plurality of lithographic features of the mask, and determining pitch of the plurality of lithographic features based on interactions of the biased lithographic features of the mask. 
     Additional embodiments of the disclosure will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present disclosure. The embodiments of the disclosure will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description, serve to explain the principles of the embodiments. 
         FIGS. 1A and 1B  are diagrams illustrating a conventional method for positioning SRAFs. 
         FIG. 2  is a block diagram illustrating a computing platform for determining the pitch of lithographic features consistent with embodiments of the present disclosure. 
         FIG. 3  is a flow diagram illustrating a method for determining pitch of lithographic features consistent with embodiments of the present disclosure. 
         FIG. 4  is a flow diagram illustrating a method for determining bias of reference features consistent with embodiments of the present disclosure. 
         FIG. 5  is a block diagram illustrating a method for determining bias of reference features consistent with embodiments of the present disclosure. 
         FIG. 6  is a flow diagram illustrating a method for determining pitch of biased lithographic features consistent with embodiments of the present disclosure. 
         FIGS. 7A and 7B  are block diagrams illustrating a method for determining pitch of biased lithographic features consistent with embodiment of the present disclosure. 
         FIG. 8  is a block diagram illustrating a method for generating auxiliary features consistent with embodiments of the present disclosure. 
         FIG. 9  is a table illustrating SRAF position and size corresponding to various pitch values. 
     
    
    
     DETAILED DESCRIPTION 
     According to conventional methods, an entire mask layout for a semiconductor wafer design must be simulated first in order to determine if lithographic features and auxiliary features will interact and possibly produce unwanted printing. Further, the conventional method does not recognize all possible interactions between lithographic and auxiliary features. 
     According to embodiments of the present disclosure, an application, such as a semiconductor device design application, determines pitch by determining a bias that is based on an interaction between reference features. The references features may be exemplary lithographic features of a mask layout, such as a contact hole. The references features may be features which ultimately appear in the mask layout. Likewise, the reference features may be simulated features used only for determining bias and may not be actual features which appear in the mask layout. 
     Once the bias is determined, the bias may be applied to the lithographic features of a mask layout. Once the features are biased, the application determines the pitch based on the interaction of the biased features. Instead of determining the pitch for the entire edge of the biased lithographic feature, the application determines the pitch for portions of an edge of the biased lithographic feature. As such, a single lithographic feature may have multiple pitch values for a single edge of the feature. By having the multiple pitch values, a single lithographic feature may have multiple configurations for auxiliary features, such as SRAFs. 
     According to embodiments, by utilizing reference features, the application is not required to simulate the entire mask layout in order to determine feature interaction. Further, by biasing the lithographic features and dividing the edges, the application may determine interactions not typically determined by the conventional method. 
     Reference will now be made in detail to the exemplary embodiments of the present disclosure, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration specific exemplary embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the invention. The following description is, therefore, merely exemplary. 
     Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5. 
       FIG. 2  is a block diagram of an exemplary computing platform  200  capable of performing embodiments of the present disclosure. Computing platform  200  may be utilized to perform the methods of the present disclosure. For, example, methods for determining pitch of lithographic features may be implemented in an application, such as a photomask design application, written in program code and executed by the computing platform  200 . The application may be implemented in computer languages such as PASCAL, C, C++, JAVA, HTML and the like. One skilled in the art will realize that the methods described above may be implemented in any computer language and any application capable of designing semiconductor devices. 
     As shown in  FIG. 2 , computing platform  200  may include components such as a processor  202 , a cache  204 , a memory  206 , a secondary memory  208 , a display  210 , a keyboard  212 , a mouse  214 , a display adaptor  216 , a network adaptor  218 , and input/output (I/O) interface  220 . Each of these components may be coupled and communicate via a bus  203 . 
     Processor  202  may be embodied in one or more processors. Processor  202  provides an execution platform for applications performing methods of the present disclosure. Commands and data from the processor  202  may be communicated over communication bus  203  to and from main memory  206 . Main memory  206  may be, for example, a Random Access Memory (RAM). Main memory  206  may store the operating system and applications implementing the methods of the present disclosure, which may be executed during runtime. 
     Likewise, applications implementing the method of the present disclosure may be stored on secondary memory  208 . Secondary memory  208  may include, for example, one or more of a hard disk drive and/or a removable storage drive, a floppy diskette drive, a magnetic tape drive, a compact disk drive, and the like. Computer platform  200  may read from and/or write to secondary memory  208 . Likewise, secondary memories  208  may read from and/or write between themselves in a well-known manner. 
     Users of computing platform  200  may interface with and control computing platform  200  utilizing keyboard  212  and mouse  214 . Computing platform may utilize display  210  and display adapter  216  to output data, such as the mask layout, for viewing by the users. For example, display adapter  216  may receive display data from the processor  202  and convert the display data into display commands for display  210 . 
     Network adapter  218  may allow computing platform  200  to send and receive data via a network. Additionally, I/O interface  220  may allow computing platform  220  to input data from and output data to other electronic devices. For example, I/O interface  220  may be coupled to a lithographic apparatus for receiving semiconductor design data and transmitting the results of the methods of the present disclosure. 
     As mentioned above, any of the methods may be performed by an application executed on computing platform  200 . The application may be embodied on a computer readable storage medium as instruction for causing computer platform  200  to perform the instructions. The computer readable storage medium may include storage devices and signals, in compressed or uncompressed form 
     Exemplary computer readable signals, whether modulated using a carrier or not, are signals that a computer system hosting or running the present invention can be configured to access, including signals downloaded through the Internet or other networks. Concrete examples of the foregoing include distribution of executable software programs of the computer program on a CD-ROM or via Internet download. In a sense, the Internet itself, as an abstract entity, is a computer readable medium. The same is true of computer networks in general. 
     According to embodiments of the present disclosure, an application, such as a photomask design application, may determine pitch of lithographic features.  FIG. 3  is a flow diagram illustrating an exemplary method  300  for determining pitch of lithographic features in a semiconductor device consistent with embodiments of the present disclosure. Method  300  may be implemented in any type of application for designing, editing, and creating lithographic masks executed on a computing platform. For example, method  300  may be performed on any computing platform or computing platform in a network system, such as computer platform  200  described above. One skilled in the art will realize that method  300  may be performed on any computing platform in which semiconductor devices are designed. 
     An application, which implements method  300 , determines pitch by determining a bias that is based on an interaction between reference features. Once the bias is determined, the bias may be applied to the lithographic features of a mask layout. Once the features are biased, the application determines the pitch based on the interaction of the biased features. 
     Method  300  begins with the application determining a bias for reference features (stage  302 ). The reference features may be exemplary lithographic features of a semiconductor device, such as a contact hole. The reference features may be features which ultimately appear in the mask layout. Likewise, the reference features may be simulated features used only for determining bias and may not be actual features which appear in the mask layout. 
     The bias represents an amount the reference features may be enlarged to cause an interaction between the reference features. The application may determine the bias by determining the interaction between the reference features after an enlargement. To determine the enlargement amount, the application may first position the reference features based on a lithographic parameter. For example, the lithographic parameter may be reticle parameters or lithographic apparatus parameters such as exposure wavelength, numerical aperture of the lens, and/or geometry of the entrance pupil. 
     For example, the reference features may include auxiliary features. The application may position the reference features and auxiliary features relative to one another such that any closer positioning may violate a rule of mask design. 
     After the bias is determined, the application applies the bias to the lithographic features of the mask layout (stage  304 ). In order to apply the bias, the application creates the mask layout. Then, the application applies the bias by enlarging the lithographic features of the mask layout by the bias. 
     Then, the application determines pitch of the biased lithographic features (stage  306 ). The application determines the pitch by determining the interaction of the biased lithographic features. For example, the application may determine interaction by utilizing a projection method. 
     Instead of determining the pitch for the entire edge of the biased lithographic feature, the application determines the pitch for portions of an edge of the biased lithographic feature. As such, a single lithographic feature may have multiple pitch values for a single edge. By having the multiple pitch values, a single lithographic feature may have multiple auxiliary feature configurations. 
     For example, if only half of one edge of a first biased lithographic feature interacts with another lithographic feature, only the interacting half may be considered to interact with the other lithographic feature. The other half of the edge may be considered isolated. As such, the application may determine a pitch value for the isolated half and a separate pitch value for the interacting half. By dividing the edge, the lithographic feature will have two different auxiliary feature configurations. 
     As mentioned above, in method  300 , the application determines a bias for the lithographic.  FIGS. 4 and 5  are a flow diagram and block diagram, respectively, illustrating an exemplary method  400  for determining a bias of the reference features. Method  400  may be performed at stage  302  of method  300  in order to determine the bias. 
     Method  400  begins with the application generating reference features (stage  402 ). The references features represent exemplary lithographic features of a semiconductor device which may appear in the mask layout.  FIG. 5  illustrates exemplary reference contact holes  502  which may be utilized during method  400 . 
     As shown in  FIG. 5 , reference contact holes  502  may be placed in close proximity. Reference contact holes  502  may be created to be any shape and size which is representative of the mask layout. Contact holes  502  may be features which ultimately appear in the mask layout. Likewise, contact holes  502  may be simulated features used only for determining bias and may not be actual features which appear in the mask layout. 
     Returning to method  400 , after generating the reference features, the application generates auxiliary features for the reference features (stage  404 ). The auxiliary features may be generated according to standard lithographic techniques. The auxiliary features may be generated by assuming that the reference features are isolated. That is, the auxiliary features may be generated without considering interaction with other lithographic features. 
     For example, the application may generate SRAFs  504  for reference contact holes  502  as illustrated in  FIG. 5 . SRAFs  504  may be generated for each contact holes  502  as if contact holes  502  were isolated. 
     After generating the auxiliary features, the application determines a minimum spacing distance for the reference features (stage  406 ). The application may determine the minimum spacing distance by decreasing the distance between the reference features until the reference features violate a lithographic parameter, such as a rule of the mask design. 
     For example, as illustrated in  FIG. 5 , the application may decrease the distance between contact holes  502  until a spacing  508  between SRAFs  504  violate a mask rule. Spacing  508  may be the minimum distance at which a lithographic device may resolve the features or spacing  508  may exceed the manufacturing capability of the reticle manufacture. The distance between contact holes  502  at which the rule is violated may be the minimum spacing distance  506 . 
     Next, the application positions the reference features at the minimum spacing distance (stage  408 ). For example, as illustrated in  FIG. 5 , the application may position contact holes  502  at minimum spacing distance  506 . 
     Then, the application calculates the bias for the reference features (stage  220 ). The application may determine the bias by enlarging the reference features until the reference features are no longer considered isolated. For example, the application may enlarge the reference feature until a projection of the features overlap. The bias would be the difference between the original size of the reference feature and the enlarged size. 
     For example, as illustrated in  FIG. 5 , contact holes  502  may be enlarged until contact holes  502  have projections that overlap. The difference between the original size of contact holes  502  and the enlarged contact holes  502  would be bias  510 . 
     As mentioned above, during method  300 , the application determines pitch of the biased lithographic features. FIGS.  6  and  7 A-B are a flow diagram and a block diagrams, respectively, illustrating an exemplary method  600  for determining the pitch of lithographic features. Method  600  may be utilized at stage  306  of method  300 . 
     Prior to beginning method  600 , the application may have previously generated the mask layout and biased the lithographic features with a determined bias. Method  600  begins with the application generating a projection of each biased feature (stage  602 ). A projection may be an extension projected out from one edge of a lithographic feature. The projection may be the width of the projected edge. 
       FIG. 7A  illustrates an exemplary mask layout with three contact holes  701 ,  702 , and  703  that are biased to an amount  704 . One skilled in the art will realize that the mask layout in  FIG. 7A  is exemplary and that the mask layout may include any type and number of lithographic features. 
     As illustrated in  FIG. 7A , the application generates a projection  706  from one edge of each contact hole  701 ,  702 , and  703 . The projection consists of a simulated rectangle that has a width equal to the edge from which the rectangle is projected. 
     Next, the application determines an interaction of the projection and the biased features (stage  604 ). The application may determine the interaction by utilizing any method to determine if the lithographic features will interact during printing. For example, as illustrated in  FIG. 7A , the application may determine an interaction for biased contact holes  701 ,  702 , and  703  if projections  706  overlap any other biased contact hole. 
     Next, the application divides edges of the biased features into bins (stage  606 ). The application divides the edges based on the interaction between the biased lithographic features. For example, as illustrated in  FIG. 7B , the application divides the edges into portions that may have an interaction and portions with no interaction. In other words, the application divides the edges based on the overlap of projection  706 . 
     In this example, the edge of contact hole  701  may be divided into two portions  708  and  710 . Portion  708  may have no interaction and may be considered isolated. Based on projection overlap, portion  710  may interact with portion  712  of contact hole  702 . As such, portion  710  of the edge of contact hole  701  and portion  712  of the edge of contact hole  702  may be classified as Bin a  714 . 
     The edge of contact hole  702  may be divided into three portions. Contact hole  702  may include portion  712  which interacts with portion  710  of contact hole  701 . Contact hole  702  may include an isolated portion  716 . Further, based on the projection overlap, contact hole  702  may include a portion  718  which interacts with portion  720  of contact hole  703 . As such, portion  718  of the edge of contact hole  702  and portion  720  of the edge of contact hole  703  may be classified as Bin b  722 . The edge of contact hole  703  may also have a portion  724  with no interaction and may be considered isolated. 
     Then, the application determines a pitch for each bin (stage  608 ). The application may determine pitch for each bin in which interaction was determined. Likewise, the application may determine pitch for portions with no interaction. The application may determine the pitch by measuring the distance between portions of the edges of the lithographic features included in the bin. 
     As illustrated in  FIG. 7B , the application may determine the pitch for Bin a  714  by measuring the distance between portion  710  and  712 . Likewise, the application may determine the pitch for Bin b  722  by measuring the distance between portion  718  and  720 . The application may determine the pitch for isolated portions  708 ,  716 , and  724  by considering these portions to be isolated. 
     Once the application determines the pitch for all the lithographic features, the application may generate auxiliary features for the lithographic features. The auxiliary features may be generated according to any known method for generating auxiliary features. Once these auxiliary features are generated, the auxiliary features may be used without modification, or the auxiliary features may be subsequently modified prior to generation of the mask or reticle. 
       FIG. 8  is a diagram illustrating exemplary SRAFs  802  for the mask layout described in  FIGS. 7A and 7B . As illustrated in  FIG. 8 , contact holes  701 ,  702 , and  703  may include different SRAF configuration for a single edge. This is due to the multiple pitch values determined for an edge of a contact hole. For example, the application may generate three separate SRAFs for contact hole  702 . That is, the application generates an SRAF for bin a  714 , an SRAF for isolated portion  716 , and an SRAF for bin b  722 . The application may determine the position and size of the SRAFs by utilizing a pitch table illustrated in  FIG. 9 . 
     Other embodiments of the present teaching will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.