Patent Publication Number: US-7711504-B2

Title: Method and system for performing optical proximity correction with process variations considerations

Description:
RELATED APPLICATION 
   The present application claims priority under 35 U.S.C. §119(e)(1) to provisional application No. 60/951,377 filed on Jul. 23, 2007, the contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to photolithographic printing in general, and, in particular, to a method and system for performing optical proximity correction with process variations considerations. 
   2. Description of Related Art 
   When transferring an integrated circuit design pattern onto a semiconductor substrate, the most common technique is to produce a photomask with a photomask layout of the integrated circuit design, and then a lithography process is utilized to expose the patterns of the photomask layout to a semiconductor substrate in a predetermined ratio. 
   As integrated circuit designs become more complex, the number of circuit elements to be produced on a wafer become increasingly large and each object becomes correspondingly smaller. As the size of the objects to be produced become similar in size or smaller than the wavelength of light used to illuminate the wafer, distortions occur whereby the pattern of objects formed on the wafer do not correspond to the pattern of objects defined by the mask. One objective criterion for defining how well an image is formed or an object is produced is the edge placement error (EPE) that indicates how far an edge of an object is shifted from its desired position. Another objective criterion is the edge contrast or slope that describes how sharply the image intensity changes from exposed to not exposed, or vice versa. 
   In order to improve the manufacturability of target layout designs, optical process correction (OPC) techniques have been developed that alter a mask layout pattern in order to correctly create the desired pattern of objects on a wafer. The conventional OPC method of improving the fidelity of a layout is to simulate how a pattern of polygon fragments fabricated on a mask will be lithographically reproduced as corresponding edges on the wafer, and then moves the fragment such that the edge on the wafer will be created at the proper location. 
   In a typical OPC procedure, a target layout includes several polygons that represents the objects desired on the wafer. Referring now to the drawings and in particular to  FIG. 1A , there is illustrated a fragmented polygon in a target layout. As shown, a polygon  10  is divided up into edges  12   a - 12   f . For each of edges  12   a - 12   f , at least one control point (or simulation site) is designated. For example, a control point  13   c  is designated to edge  12   c , and a control point  13   d  is designated to edge  12   d . Although there is typically one site per edge, some edges may have more than one simulation site. 
   Simulations are generated at each of the sites of the edges, usually along a cut line perpendicular to the edge, and measurements of the predicted image slope, maximum and minimum intensities are calculated as shown in  FIG. 1B . From these image parameters, the actual placement of the edge is predicted using techniques such as the variable threshold resist model. The edge location as predicted and the location of the ideal edge in the target layout are then compared in order to calculate the difference as an edge placement error (EPE). 
   Changes are then made in the mask layout to minimize the calculated EPE. For each edge, a fragment in the mask layout is designated, and each mask fragment is moved in an attempt to reduce the calculated EPE. New simulations at the sites are generated again from the revised mask layout, and new EPEs are calculated. This procedure is repeated iteratively until EPE falls within an acceptable tolerance value. 
   Conventional model-based OPC assumes nominal process conditions without considering any process variations due to the current lack of variational lithography models. However, disregarding process variations may lead to erroneous timing, power and yield characterization analysis. For example, post-OPC silicon image based timing analysis is found to be substantially different from that based on the drawn layout. 
   Consequently, it would be desirable to provide an improved method and system for performing OPC with process variations considerations. 
   SUMMARY OF THE INVENTION 
   In accordance with a preferred embodiment of the present invention, the maximum aerial gradient direction for a control point associated with an edge is initially determined. Then, a variational edge placement error  E  along the maximum aerial image intensity gradient direction of the control point is calculated. A determination is made whether or not |C E ·n| is equal to or greater than a manufacturing grid, where n is the direction perpendicular to a segment pointing outward, and C is a constant. If |C E ·n| is equal to or greater than a manufacturing grid, the edge is moved by −C E ·n. 
   All features and advantages of the present invention will become apparent in the following detailed written description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIGS. 1A-1B  illustrate a fragmented polygon in a target layout and a measurement of an edge placement error computed for a simulation site of an edge, respectively, according to the prior art; 
       FIG. 2  is a block diagram of a computer system in which a preferred embodiment of the present invention is incorporated; 
       FIG. 3  is a high-level logic flow diagram of a method for performing optical process correction with process variations considerations, in accordance with a preferred embodiment of the present invention; 
       FIGS. 4A-4B  are rectilinear polygon representations; 
       FIG. 5  shows that the polygon in  FIG. 4B  is segmented into many small segments; and 
       FIGS. 6A-6B  illustrate the concept of edge placement error, in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   With reference now to  FIG. 2 , there is depicted a block diagram of a computer system in which a preferred embodiment of the present invention is incorporated. As shown, a computer system  20  includes a processing unit  21  connected to a display  25 , a keyboard  26  and a mouse  28 . Instructions used by processing unit  21  to implement the optical proximity correction technique of the present invention are received on a computer readable media  23  or can be received as a communication signal from a remote computer over a wired or wireless data link. Processing unit  21  receives a target layer description that defines a number of circuit elements to be manufactured on a semiconductor wafer. The target layer description is received on a computer readable media  22  or can be received from a remote computer over a wired or wireless data communication link. Processing unit  21  generates a mask layout description of one or more lithographic masks that will be used to expose desired portions of a wafer in order to manufacture circuit elements on the semiconductor wafer. The mask layout is exported to a mask writer  24  that produces the physical mask(s) to be used in a wafer processing facility. 
   Referring now to  FIG. 3 , there is depicted a high-level logic flow diagram of a method for performing optical process correction (OPC) with process variations considerations, in accordance with a preferred embodiment of the present invention. Starting at block  30 , polygon segmentation and tagging are performed, as shown in block  31 . Polygon segmentation and tagging can be performed by conventional OPC segmentation and tagging strategies that are well-known in the art. 
   Polygons are parametrized such that they can be efficiently changed.  FIGS. 4A-4B  are examples of rectilinear polygon representations. Each of the rectilinear polygons shown in  FIGS. 4A-4B  is composed of a series of directed edges, and each edge is represented as a triplet e i ={l i ,h i ,p i }. A rectilinear polygon can be represented as {O,c,{e i |0≦i&lt;N}}, and for the present example, N=6. The head of each edge e i  is connected to the tail of a next edge (e mod(i+1,N) ), where N is the number of edges and mod denotes the modulo operation. Each edge can be specified by its length l and two Boolean variables h and p, where h indicates whether it is horizontal or vertical and p indicates whether it points to the positive direction (x or y) or the negative direction (−x or −y). The per polygon Boolean variable indicates whether the interior of the region is to the left or to the right of the edges of the rectilinear polygon. O denotes the starting point of the first edge (e 0 ). 
     FIG. 5  shows that the rectilinear polygon in  FIG. 4B  is segmented into many small segments, denoted as dashed arrows. In addition to parameters {l i ,h i ,p i }, one more parameter d i  should be used to describe the displacement of the segment from its original location. By applying d i &#39;s to each segment, the segments are shifted to the locations denoted as solid arrows. The vertices needed in lithography simulators can be computed based on such representation. Although only rectilinear polygons are described, the above-mentioned technique can be extended to polygons having 45° edges. 
   Referring back to  FIG. 3 , the maximum aerial gradient direction is then computed for each control point on one layer, as depicted in block  32 . A variation edge placement error (EPE)  E  along the maximum aerial image intensity gradient direction of a corresponding control point is then calculated, as shown in block  33 . 
   For any given point on the target contour, its variational EPE is defined as the placement between that point and its nearest printed contour point. For example, a shape  60  is defined by a target contour in solid lines, as shown in  FIG. 6A . The printed contour in illustrated in dashed lines. A portion of the rightmost edge of shape  60  is enlarged to depict a point A on the target contour, as depicted in  FIG. 6B . A point A′ is point A&#39;s closest point on the printed contour. Thus, point A&#39;s variational EPE is defined as the placement between point A and point A′ on the printed contour point. The variational EPE for point A on the target contour is denoted as E. The printed contour is uniquely determined if the variational EPEs of all the target contour points are given. At a certain intensity threshold, the printed contour is the least sensitive to the focus variations, and such intensity threshold is called the iso-focal threshold denoted as I th     iso   , defined as 
                 ∂   E     ⁢     |     I   =     I     th   iso               ∂   z       =   0         
Assuming small I th  variations and small z variations, E can be written as where
 
   
     
       
         
           
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   As an example, when the joint distribution of measured exposure dose E and focus error Z for an actual manufacturing process are available, the joint distribution of I th  and z can be computed as follows 
             I   th     =       β   E       E   -     α   E                     z   =       β   z     ⁡     (     Z   -     α   z       )             
If I th  and z are assumed to be independent and normally distributed, i.e., z˜N(0,σ z   2 ) and I th ˜N(μ Ith ,σ th   2 ), the variational EPE  E  can be calculated as follows:
 
                   〈   E   〉     =       E   iso     +         a   0     ⁡     (     1   +       a   1     ⁢     σ   z   2         )       ⁢     (       μ     I   th       -     I     th   iso         )                     =       E   iso     +       a   0     ⁡     (       μ     I   th       -     I     th   iso         )       +       a   0     ⁢     a   1     ⁢       σ   z   2     ⁡     (       μ     I   th       -     I     th   iso         )                       =       E   norm     +     μ   E                   
where E norm  is the nominal EPE.
 
   After all the control points have been completed, a determination is made, for each edge, as to whether or not |C E ·n| is equal to or greater than a manufacturing grid, where n is the direction perpendicular to the segment pointing outward, and C is a constant preferably chosen by a user based on accuracy and convergent speed, as shown in block  35 . If |C E ·n| is equal to or greater than a manufacturing grid, then the edge is moved by −C E ·n, as depicted in block  36 . The edge movement is preferably rounded to a multiple of manufacturing grid. Otherwise, if |C E ·n| is less than a manufacturing grid, the process proceeds to block  37  for a new edge. The process is stop after all segments within a layer have been completed, as shown in block  38 . The process may start again for a new layer. 
   As has been described, the present invention provides a method and system for performing OPC with process variations considerations. 
   It is also important to note that although the present invention has been described in the context of a fully functional computer system, those skilled in the art will appreciate that the mechanisms of the present invention are capable of being distributed as a program product in recordable type media such as compact disks. 
   While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.