Abstract:
The invention relates to a method for optical measurement of an OPC structure ( 306 ), having a pre-determined structure ( 302 ) on a photo-mask, in order to determine a measurement of the structure in at least one direction, whereby, firstly, a region ( 300 ) is determined on the photo-mask, which comprises the OPC structure ( 306 ) to be measured. The intensity of the determined region ( 300 ) is then scanned in a first direction and the region in which the intensity passes a threshold is determined for each scan. The maximum separation between an edge ( 308 ) of the structure ( 302 ) and an edge ( 312 ) of the corresponding OPC structure ( 306 ) is determined, based on the difference of the determined regions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of co-pending International Application No. PCT/EP02/10545, filed Sep. 19, 2002, which designated the United States and was not published in English. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for an optical measuring of an OPC structure, in particular for measuring an OPC structure associated with a predetermined structure on a photo mask. 
   2. Description of the Related Art 
   The trend in semiconductor manufacturing goes more and more toward smaller and smallest structures. The common method here is the illumination of wavers using photomasks by means of light (e.g. using visual wavelengths or wavelengths in the UV range), ion beams, electron beams, x-rays, or other methods to be mapped (lithography). At that, the structures to be mapped, like e.g. thin conductive traces or small contacts, are often in the range of or even smaller than the used wavelengths, which inevitably leads to mapping errors. The limits for mapping for lithography methods which use visible light range from a structure size of about 350 nm to 400 nm, and the mapping limit for lithography methods which use UV light range from about 250 nm to 300 nm. In particular, due to the limited resolution, corners of structures or line ends are mapped strongly rounded off onto the waver. 
   In order to achieve a better pattern fidelity compared to the original design or layout, the above-mentioned critical locations (corners, line ends) on the photomask are provided with OPC structures or OPC-similar structures (OPC=Optical Proximity Correction). 
   In connection with the present description, the term OPC structure refers to any structure or any element which is added to a photomask in order to guarantee or support, respectively, the true mapping of the mask onto a substrate. 
   The OPC structures serve to change the structures actually to be generated on the photomask in a deliberate way in order to achieve a better mapping on the waver, i.e. for example less rounded at the corners. 
   With reference to  FIG. 1 , in the following two examples for the layout of OPC structures for the generation of photomasks are described. In  FIG. 1A  a section  100  of a layout is shown including a portion of a first structure  102 . In order to prevent a roundoff in the area of the corner  104  due to the mapping of the layout  100  onto the photomask, an OPC structure  106  is provided there protruding beyond the horizontal edge  108  and the vertical edge  110  of the structure  102  in the area of the corner  104 . In the example shown in  FIG. 1A  for a layout, the OPC structure  106  is substantially square. The OPC structure at the corner  104  shown in  FIG. 1A  is also referred to as corner serif. 
   In  FIG. 1B  a section  200  of a layout is shown together with a portion of a second structure  202 . The second structure  202  is a line, and in the section  200  of the layout the line end of the structure  202  is illustrated. The structure  202  includes two parallel vertical edges  204  and  206  and a horizontal edge  208  connected to the vertical edges  204  and  206  in the area of a first corner  210  and in the area of a second corner. When transmitting the layout  200  onto a photomask, a similar problem results as in the transmission of the layouts described with reference to  FIG. 1A , i.e. that the structure generated on the photomask in the area of the corners  210  and  212  is rounded off, so that also here, similar to  FIG. 1A , an OPC structure needs to be provided in the area of the corners  210  and  212 . In  FIG. 1B  two OPC structures  214  and  216  are arranged in the area of the corners  210  and  212 , respectively, wherein the OPC structures respectively protrude beyond corners  204  and  208  and  206  and  208 , respectively. As in  FIG. 1A , also here the OPC structures are basically of a square nature. The structure shown in  FIG. 1B  is also referred to as line end serifs. A special case of theses OPC structures in which the serifs are adjacent to each other at the line end is also referred to as a hammerhead. 
   Conditional on the function, the OPC structures  106 ,  214 , and  216  illustrated in  FIGS. 1A and 1B  are very small (approx. 200 nm and smaller). 
   Instead of the structures described in  FIG. 1  also other structures and elements are possible, e.g. so-called jogs or scatterbars, in order to improve the edge quality. 
   In the conventional quality testing and quality assurance of photomasks for example generated using the layouts as they were described with reference to  FIGS. 1A and 1B  using optical microscopy, these small dimensions of the OPC structures represent a special challenge. Further, due to the high number of OPC structures in different spatial orientations on only one photomask a demand exists for an automatic method for the recognition and measuring of these structures. 
   SUMMARY OF THE INVENTION 
   Based on this prior art it is the object of the present invention to provide a method enabling the optical measuring of an OPC structure associated with a predetermined structure on a photomask with a minimum effort. 
   The present invention provides a method for an optical measuring of an OPC structure ( 306 ;  406 ) associated with a predetermined structure ( 302 ;  402 ) on a photo mask with the following steps:
         (a) specifying an area ( 300 ;  400 ) on the photomask including the OPC structure ( 306 ;  406 ) to be measured and a first edge ( 310 ;  404 ,  406 ) of the predetermined structure ( 302 ;  402 ).   (b) sampling of the intensity image of the specified area ( 300 ;  400 ) row-wise in a first direction perpendicular to the first edge ( 310 ;  404 ,  406 ) of the predetermined structure ( 302 ;  402 ), and for each row:
           (b.1) determining the location in which the intensity passes a threshold, and   
           (c) based on the locations specified in step (b.1), determining a location lying farthest out with reference to the predetermined structure ( 302 ;  402 ), and a location lying farthest in with relation to the predetermined structure ( 302 ;  402 ); and   (d) determining the maximum distance between the first edge ( 310 ;  402 ,  404 ) of the predetermined structure ( 302 ;  402 ) and a first edge ( 312 ;  414 ,  416 ) of the associated OPC structure ( 306 ;  406 ) based on the difference of the location lying farthest out and the location lying farthest in.       

   According to a first embodiment, the structure is measured in two directions. In this case, first of all a sampling of the intensity of the specified area in a second direction is performed, and for each sampling in the second direction a location is determined in which the intensity passes a threshold and the maximum distance between an edge of the structure and an edge of the associated OPC structure is determined based on the difference of the specified locations. Alternatively, instead of the additional sampling in the second direction, the photomask may be rotated and the sampling is repeated in the first direction. 
   According to a further embodiment of the present invention, the threshold is determined based on the intensities associated with the structure and a background of the structure. 
   According to a further preferred embodiment, after specifying an area on the photomask first of all the type of the structure and/or the orientation of the structure with regard to a reference position is identified. The type of the structure is preferably identified by sampling the determined area along the edges of the area. For identifying the corner serif, the sampling of the corners of the area is sufficient. In other cases, the intensity course or the number of intensity transitions between light and dark along all edges is used, respectively, to determine the type of the structure. 
   The present invention enables an automatic method, which firstly enables the recognition of a “corner serif”, “line end serifs”, or other OPC or OPC-similar structures on a photomask with a minimum effort on the operator side and to measure the same with a sufficient accuracy. 
   The inventive method reduces errors that may be caused by the operator, as it operates objectively and thus eliminates errors caused by subjective assessments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become clear from the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1A  shows a schematical illustration of a layout for a portion of a photomask, wherein the layout shows a corner with corner serif; 
       FIG. 1B  shows a schematical illustration of a layout for a portion of a photomask, wherein the layout shows a line end with line end serifs; 
       FIG. 2A  shows a section of a photomask generated using the layout section of  FIG. 1A ; 
       FIG. 2B  shows a section of a photomask generated using the layout of  FIG. 1B ; 
       FIG. 3A  shows an example for the edge probing in the x direction with line end serifs; and 
       FIG. 3B  shows an example for the edge probing in y direction with line end serifs. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 2A  an illustration of an intensity image (section  300 ) of a photomask is shown generated using conventional imaging methods, e.g. by a microscope with an associated CCD camera. Alternatively, it may also be an SEM image or another image, which was generated by other imaging methods. 
   In  FIG. 2A  a section  300  of a photomask is shown, wherein the section  300  was generated by transmitting the layouts of  FIG. 1A  onto a photomask. The section  300  includes a portion of a photomask structure  302  including a corner  304 . In the area of the corner  304  of the structure  302  on the photomask the OPC structure  306  was generated which, compared to the structure in the layout (see  FIG. 1A ), was generated with rounded-off edges on the photomask. As it may be seen, the OPC structure  306  is implemented such that the same protrudes beyond a horizontal edge (in the x direction) of the structure  302  and beyond a vertical edge  310  (in y direction) of the structure  302 . 
   In order to test the generated photomask with regard to its quality, it is required now to test the generated OPC structure  306  with regard to its dimensions, in particular with regard to the distance of the same from the edges  308  and  310 , in order to guarantee that the thus generated OPC structure  306  causes the desired correction in the corner area of the structure generated on the substrate in an application of the photomask for generating a structure on a substrate. According to the present invention, a method is provided which firstly enables to measure the distances dx and dy of the OPC structure  306  on the photomask from the structure  302  with a sufficient accuracy with a minimum effort from the operator side. 
   Using an optical microscope, which is provided with a CCD camera, an intensity image of the photomask is generated containing the structure to be examined, i.e. the structure  302  with an associated OPC structure  306 . Around the structure to be examined an image section is defined, section  300 , which is the so-called ROI (ROI=Region of Interest). The selection of this area may either be performed manually by a user or, depending on whether the layout information is known, in an automatically controlled way. The size of the ROI  300  is not critical here, the only important thing is that the ROI  300  does not contain any other structures but only the structure  302  with the associated OPC structure  306 , which is to be measured. As soon as the area  300  is determined, the measuring is performed automatically by the inventive method. 
   Due to the limited spatial resolution of the microscope, the ROI  300  contains a somewhat blurred mapping of the overall structure  302 ,  306  to be examined. Otherwise, the structure  302 ,  306  is mapped with an approximately constant brightness, and via the evaluation of the intensity distribution within the ROI  300  using a suitable method (e.g. histogram), a brightness of the structure  302 ,  306  and a brightness of the background is established. Here, it is not important whether it is a bright structure in front of a dark background or a dark structure in front of a light background, as it is illustrated in  FIG. 2A . 
   After the ROI  300  has been determined, all edges of the overall structure  302 ,  306  are probed in horizontal direction (x direction) and vertical direction (y direction). 
   To this end, e.g. first in the x direction, the intensity in the area  300  is sampled, wherein for each sampling in the x direction the location is specified in which the determined intensity passes a threshold, i.e. for example an intensity signal of a value representing a light background changes to a value representing a dark background (See  FIG. 2A ), whereby the presence of an edge within the photomask may be determined. The edge sampling is performed, as mentioned above, using the threshold value method, which uses a constant threshold which is preferably calculated from the mean intensities of the structure to be measured and the background. At that, the threshold value is selected so that the measuring error resulting from the limited resolution of the microscope and the blurring of the small OPC structure resulting from it is minimized. 
   The sampling in the x direction is performed such that in each row a plurality of sampling points are selected for which the intensity is determined. The location in which an intensity change from light to dark takes place is determined for each row and based on the difference of the locations of the edge  310  of the overall structure  302 ,  306  for each row, the maximum distance between an edge  312  of the OPC structure  306  and the edge  310  of the structure  302  to dx is determined. Analogue to this, a maximum distance between a horizontal edge  314  of the OPC structure  306  and the edge  308  of the structure  302  to dy is determined. 
   In the area of the edge of the structure, the edge sampling with the smallest possible spatial resolution along the edge to be measured is performed. In case of a bad signal-to-noise ratio also a coarser spatial resolution may be selected, wherein then two or more rows or columns, respectively, perpendicular to the edge are combined. 
   Due to the thus performed determination of the locations of the edges it is now possible to obtain all relevant dimensions of the OPC structure  306  overlaid over the structure  302  from the thus obtained edge profiles in horizontal direction and in vertical direction, i.e. the dimensions dx and dy for the corner serif, as it is shown in  FIG. 2A . The final result consists of the dimensions of the OPC structure  306  both in horizontal direction and vertical direction, or the distance, respectively, by which the OPC structure is taller than the structure to be measured, wherein for the example shown in  FIG. 2A  a measurement value pair dx, dy is generated. Optionally, also the edge positions with regard to a predetermined reference position, the overall edge course resulting from individual samplings and the type of the found structure (type, orientation, light/dark) are output. 
   In an alternative approach, instead of the sampling of individual points in the rows, one row or one column, respectively, is completely sampled in order to generate the sum of the intensity values of this sampling. The thus generated overall intensity values for each row or column, respectively, are compared to a first and a second threshold. For the embodiment illustrated in  FIG. 2A , an overall intensity value below the first threshold indicates, that the row comprises no component of the structure  302  or the OPC structure  306 . Such a row is shown as an example in  FIG. 2A  at  316 . The second threshold defines the boundary between the OPC structure  306  and the structure  302  where an intensity exceeding the second threshold is regarded as a combination of the intensities resulting from the background and the structure  302 . Such a row is shown as an example at  318 . If an overall intensity of a completely sampled row lies between the first threshold and the second threshold, as it is indicated as an example in row  320  in  FIG. 2A , this sampled overall row  320  only includes the OPC structure  306 . Thus it is possible to detect the edge  314  of the OPC structure  306  when passing the first threshold and to detect the edge  308  of structure  302  when passing the second threshold, and thus the distance of the edges  314  and  308  to each other or an absolute position of these edges, respectively, with regard to a predetermined reference point. 
   Analogue to that, a corresponding approach is possible when sampling column by column. These approaches are only possible, however, when structures similar to those in  FIG. 2A  are to be sampled, i.e. when only one dimension is to be determined in one sampling direction. If several dimensions are to be detected in one sampling direction, then the row-wise or column-wise approach, respectively, provides no unique result, so that here again the sampling of individual sample points along one row is to be used. 
   In  FIG. 2B  a section of a photomask is illustrated containing a structure which is obtained after mapping the layout of  FIG. 1B  onto the photomask. The section  400  shows a portion of a line  402  including two vertical edges  404  and  406  (in y direction) and a horizontal edge  408  (in x direction) connected to the vertical edges in the area of the corners  410  and  412 . In the area of the corners  410  and  412  the OPC structures  414  and  416  are formed which were generated in a rounded way compared to the layout in  FIG. 1B  on the photomask  400  due to the mapping technology. 
   Similar to the method described as a first alternative with reference to  FIG. 2A , here a distance of the vertical edge  404  of the structure  402  to the vertical edge  418  of the OPC structure  414  is determined to be the distance dx 1 . Further, a distance dy 1  of the horizontal edge  408  of the structure  402  to the horizontal edge  420  of the OPC structure  414  is determined. In the line segment  402 , subsequently further a distance dx 2  between the vertical edge  406  of the structure  402  and the vertical edge  422  of the OPC structure  416  is determined, as well as the distance dy 2  between the horizontal edge  408  of the structure  402  and the horizontal edge  424  of the OPC structure  416 . The proceedings are similar to the embodiment described with reference to  FIG. 2A , it is to be noted, however, that two measurement values each are to be generated for every sampling direction. Thus, first in the x direction for each sampling a location of the edge  404  is determined and in the further sampling the location of the edge  406  and analogue to that the location of the edge  418  or the edge  422 , respectively, is determined, wherein from the difference of the thus determined locations a maximum distance dx 1  or dx 2 , respectively, between the edges  404  and  418  and  406  and  422 , respectively, is determined. Analogue to that, the locations for the edges  420  and  408  or  422  and  408 , respectively, are determined by sampling in the y direction, and from the difference of the locations detected for the edges a maximum distance of the edges dy 1  or dy 2 , respectively, is determined. 
   The proceedings of detecting an overall intensity for one row or one column, respectively, described above as a second alternative with reference to  FIG. 2A , is not possible in the embodiment shown in  FIG. 2B , as by this no unique specification of the distances dy 1  or dy 2 , respectively, would be possible. 
   Analogue to the method in  FIG. 2A , for the line end serifs illustrated in  FIG. 2B  two measurement value pairs dx 1 , dy 1 , and dx 2 , dy 2 , are obtained, indicating the distance of the edges of the OPC structures to the edges of the structure  402 . Optionally, the edge positions with reference to a predetermined reference position, the overall edge course resulting from the individual samplings and the type of the found structure (type, orientation, light/dark) are output. 
   Alternatively, it is also possible to respectively indicate the absolute positions of the edges with reference to a predetermined reference position. 
   According to a preferred embodiment of the present invention, after specifying of the area of the photomask  300  or  400  to be examined it is determined what type of structure is arranged within the selected area  300  or  400 , respectively, in order to thus perform a case differentiation with regard to the steps to be performed for edge detection. If it is determined, for example, that a structure is contained in the area, as it is shown in  FIG. 2A , then here, after reaching an edge in the x direction or the y direction, respectively, the search for a further edge may be terminated. Alternatively, as described above, the overall intensity of a row/column may be used. If it is determined, however, that a structure similar to the one in  FIG. 2B  is present in the area, then it is required to further detect the other edge after detecting one edge in one of the directions, in order to be able to perform the corresponding measurements. 
   After the area  300  or  400 , respectively, was specified on the photomask, the type of structure contained within the same is identified by comparing a brightness course along all four borders or edges, respectively, of the portion  300  or  400 , whereby each structure may uniquely be identified due to the number of intensity transmissions from light to dark determined along each edge. At that, the type of structure (corner or line end), the intensity of the structure (light or dark) and the orientation of the structure with regard to the x or y direction are distinguished. The latter differentiation is facilitated by the fact that on typical photomasks all structures are either oriented horizontally or vertically. If this is not the case, however, the CCD camera itself may be rotated correspondingly and be automatically oriented to the structure. 
   In the following, the determination of the intensity distribution in the intensity image, the corresponding determination of the threshold value and the identification of a structure according to a preferred embodiment of the present invention are described. 
   First of all, the ROI is specified again and the brightness distribution is determined. Further, a threshold is specified, as it is described below. Using a histogram, the brightness distribution in the overall ROI is analysed. Maxima of the histogram distribution are searched for. The condition for this is that the maxima are clearly separated, i.e. that they are different by a certain minimum amount in brightness. A suitable function (Gaussian curve) is adjusted to the two highest maxima in order to determine the brightnesses (I 1  and I 2 ) corresponding to the maxima more accurately. I 1  and I 2  correspond to the mean brightnesses for “dark” and “light”. The absolute brightness threshold value S is calculated from I 1  and I 2  using
 
 S=s/ 100*( I 2 −I 1)+ I 1
 
wherein s is the relative threshold value (in %, commonly 50%) to be set by the user. This threshold value S is used both for the identification of the structure type and also for the later edge probing.
 
   Subsequently, the type of structure is determined. In the above-described embodiments only corners (corner serifs) and line ends (line end serifs) are identified. The expansion to other simple structure types is easily possible, however. 
   The brightnesses in the four corners of the ROI are used in order to enable a first identification of the structure to be measured. For this, the four brightness values are compared to the threshold value S and identified as “light” or “dark” using the same. 
   With a ratio of light/dark=1/3 and 3/1, the identification is clear; it can only be a corner serif. Simultaneously, by this the orientation and the differentiation “dark corner” or “bright corner” is determined and the identification may be ended. 
   What is left is the line ends to be identified. With a corner ratio of light/dark=2/2 it can be no line ending; the identification is terminated with an error message. Only with a ratio of 0/4 or 4/0 can the identification be continued. Now, the ROI is searched along all four edges, and using the threshold value S transitions between light and dark are searched for. In case of a line end, only exactly two such transitions along exactly one edge may be present which then specify the type (light or dark) and the orientation of the line end. In any other cases, the identification is terminated with an error message. 
   After type and orientation of the structure have been specified, now the measuring of the same is started. The measuring is subsequently described with reference to the line end serif OPC structure shown in  FIGS. 3A and 3B . 
   The strategy of edge probing depends on the preceding identification. In the following, the measuring of a “dark upper line end” with line end serifs  500  is described. The generalization to other structure types and orientations is trivial. 
   The first sampling is performed row-wise in the x direction, as shown in  FIG. 3A , wherein in  FIG. 3A  one starts at the bottom and proceeds row by row to the top to the line end (see arrow  502 ). In  FIG. 3A  a row  504  is shown as an example. If required, also two or several rows each may be combined into one. For each row the brightness profile  506  is extracted and from that, using the threshold value S, the positions of the two transitions light/dark are determined with a greatest possible accuracy. In the area of the line end serifs, four transitions are present; here, only the outer two transitions are measured. The sampling is terminated when no transition is visible any more in the profile, i.e. at the upper end of the line end serifs. 
   Thus, the two edge courses left and right are obtained as a series of value pairs x left  and x right . Firstly, the maxima (points of the structure lying farthest out) are determined left and right. Then, the minima (point of the structure lying farthest in) of the edge courses from the bottom boundary of the ROI to the height of the respective maxima are determined. 
   From these four extreme values of the two edge courses left and right, the OPC dimensions dx 1  and dx 2  ( FIG. 2B ) are determined. 
   The determination of dy 1  and dy 2  is performed similarly and is illustrated with reference to  FIG. 3B . Here, the sampling is performed column-wise, wherein in  FIG. 3B  as an example a column  508  is shown. Starting from the middle  510  of the structure  500  (determined using the extreme values of the x edge courses in the last step) movements to the left and right are performed (see errors  512 ,  514 ). For each column the brightness profile  516  is extracted and from this, using the threshold value S, the position of a transition light/dark is determined with a highest possible accuracy. At that, always only the topmost transition is measured and used if several transitions are found. The column-wise sampling is terminated as soon as no more transition are found left and right. 
   For each column the position y of the brightness transition is obtained. The y values obtained for all measured columns determine the upper course of the edge. 
   From the upper course of the edge first of all the maximum values (the topmost points) are determined left and right from the middle  510 , and the minimum value (bottommost point) from the part of the edge course between the two maximum values. From these three values the OPC dimensions dy 1  and dy 2  are obtained. 
   Instead of the above-described structures, the inventive method may also be used for measuring other structures or elements, e.g. so-called jogs or scatterbars. The inventive method may also be used for the determination of an edge roughness of photomask structures. 
   The present invention is not limited to the measuring of the structures and OPC structures described in the preferred embodiment, but is generally directed to the identification and measuring of OPC structures using optical microscopy or other mapping methods in an automatic run. Preferably, an identification of the type of structure and the overlaid OPC structure is performed based on an analysis of the brightness distribution in the intensity image or a section of the same, respectively. The actual measuring of the OPC structure is then performed by the above-described spatial high resolution edge sampling using a threshold value method adjusted to the microscope resolution. The inventive proceedings are used on all types of OPC structures and are not limited to those described above. 
   While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.