Abstract:
A method of inspecting a mask, where a structure on the mask is optically imaged at a resolution specified by a criteria set including at least one of a method by which the structure was formed, a tolerance, and a structure type, to produce an optical image. The optical image is compared to a baseline image, and the differences between the optical image and the baseline image are identified. The differences are compared to a threshold value, and any differences that exceed the threshold value are flagged as defects.

Description:
FIELD 
     This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to methods for inspecting masks used in the fabrication of integrated circuits. 
     BACKGROUND 
     The fabrication of integrated circuits typically involves transferring circuit patterns from a photolithographic mask or reticle onto a photosensitive resist layer on a substrate, such as a semiconductor substrate. As the terms are typically used, a mask may be somewhat different than a reticle. A mask, for example, typically includes a pattern that covers an entire substrate, while a reticle includes a pattern that covers only a few devices, and which is stepped across the surface of the substrate to pattern the entire substrate. In addition, a mask is typically a one to one representation of the images to be formed, while a reticle may be a larger representation of the images to be formed, such as a four to one representation, which is then optically reduced in size during the exposure process. However, as used herein, the term “mask” is intended to include all such imaging structures, whether they be masks or reticles. 
     After a pattern from the mask has been defined in the resist layer, a process is performed on the substrate, such as etching or doping an underlying layer, or depositing a new layer. Once this has been accomplished, the layer of resist is removed. Multiple different patterns are repeatedly transferred onto the substrate in this general manner, with each step typically using a different mask or mask to form the separately patterned layers. At the end of the fabrication process the substrate is singulated into a plurality of dice for subsequent packaging as separate integrated circuits. 
     There are a few different methods by which a mask can be formed. The fabrication process generally begins with data corresponding to the circuit pattern, which is typically a representational layout of the physical layers of the integrated circuit. However, there are different methods by which the data is transferred to a physical representation of an image on the mask substrate. One general method is by directly writing the image onto the mask substrate, and another general method is by using an intermediate imaging technique, such as a photo repeater. Each method has its benefits and drawbacks. 
     A mask writer typically uses an imaging system such as a laser scanner or an electron beam writer. With this method, the image data is written directly onto the mask substrate, such as burning away an opaque layer on the transparent mask substrate, depositing an opaque layer in a pattern on the mask substrate, or directly exposing a pattern into a photoresist layer on the mask substrate. Regardless of the specific method used, direct writing of the desired pattern is accomplished without any type of intermediate structure. Direct writing of the mask is a good method to be used when, for example, the pattern to be formed on the mask is highly customized. By direct writing such a customized pattern, other intermediate imaging structures do not need to be formed as a part of the mask fabrication process. Typically, such intermediate structures cost a lot to produce. Since the intermediate structures for a highly customized pattern would not be widely used thereafter, their cost would have to be entirely absorbed by the few masks in which they were used. Thus, direct writing of a mask may tend to reduce the cost of the mask in certain circumstances. 
     The other general method of mask fabrication uses a master mask, which is an intermediate structure used to pattern the mask being fabricated. Typically, patterns on the master mask are formed at a larger size, and are then reduced during the exposure process onto the mask being formed. Thus, the patterns on the master mask are repeated as many times as desired onto the mask being fabricated. Mask repeaters are typically used to fabricate masks that include standard device patterns, such as patterns for central processing units, random access memory, read only memory, digital signal processors, digital to analog converters, and other standard designs that are shared by devices such as system on chip and memory. Because the patterns for such functional units are typically used again and again in various integrated circuit designs, the cost of the intermediate structures is spread across many different mask sets. Thus, photo repeating tends to reduce the cost of certain masks formed with this method. 
     In some instances, both direct writing and photo repeating are used to form a mask. For example, some portions of the integrated circuit pattern of a given layer may be formed by direct writing of the pattern, and other portions of the integrated circuit pattern of the given layer may be formed by photo repeating from a master mask. Specifically, the patterns for standard functional units may be formed on the mask using photo repeating, while more customized functional units on the mask may be formed using direct writing. 
     Once fabricated, the mask is inspected for manufacturing defects. Inspection of the mask is typically accomplished using an automated inspection apparatus. Typically optical images of the mask structures are compared to baseline images. The baseline image may be generated from the circuit pattern data, from another mask, or from an adjacent corresponding image on the mask being inspected. During comparison of the images, any difference between the inspected structure and the baseline structure is compared to a threshold value, with any difference in excess of the threshold indicating a defect. 
     The magnification at which the inspection is conducted influences how many discrepancies are flagged between the inspected structure and the baseline structure. At a higher magnification, or in other words at a high resolution, a greater number of discrepancies are typically found, while at a lower magnification, or in other words at a lower resolution, a fewer number of discrepancies are typically found. Thus, it is desirable at the onset of inspection to specify the resolution at which the inspection will be conducted. 
     However, mask patterns formed by different methods tend to have different optimal inspection resolutions. For example, patterns imaged using mask repeaters generally have improved pattern uniformity and relatively smooth line edges, which generally tolerate a relatively higher inspection resolution. On the other hand, patterns imaged using direct writing generally have relatively rougher line edges, which generally require a relatively lower inspection resolution. In addition, different functional units on a mask layer may have different optimal critical dimension criteria, regardless of the method by which they are imaged on the mask, and thus would most preferably be inspected at different resolutions. Unfortunately, inspection equipment does not have the flexibility to make inspections at variable resolutions based upon such criteria. 
     What is needed, therefore, is a system for inspecting a mask using different resolutions that are tailored to the properties of the patterns being inspected. 
     SUMMARY 
     The above and other needs are met by a method of inspecting a mask, where a structure on the mask is optically imaged at a resolution specified by a criteria set including at least one of a method by which the structure was formed, a tolerance, and a structure type, to produce an optical image. The optical image is compared to a baseline image, and the differences between the optical image and the baseline image are identified. The differences are compared to a threshold value, and any differences that exceed the threshold value are flagged as defects. 
     In this manner, various structures on the mask can be inspected at different resolutions, based on the criteria set. Thus, for example, structures that require tighter tolerances on a critical dimension can be inspected at a higher resolution, and any differences that are detected can be compared to a tighter threshold. On the other hand, structures that do not require such a tight tolerance on a critical dimension can be inspected at a lower resolution, and any differences that are detected can be compared to a looser tolerance. Further, portions of the mask that are formed with a photo repeater, and which tend to have very clean edges, can also be inspected with a higher resolution or a tighter tolerance, while other portions of the mask that are formed with direct writing, and which tend to have rougher edges, can be inspected with a lower resolution or a looser tolerance. Thus, the inspection of the various portions of the mask are tailored to the characteristics of that portion of the mask, and the inspection parameters dynamically change as appropriate during the inspection process. 
     In various preferred embodiments, a higher resolution is used when the method by which the structure was formed is a photo repeater method. On the other hand, a lower resolution is preferably used when the method by which the structure was formed is a direct writing method. A higher resolution is preferably used when the structure type requires a more exact critical dimension, and a lower resolution is preferably used when the structure type requires a less exact critical dimension. In one embodiment the threshold value is zero. Preferably, the threshold value is dependant on at least one of the method by which the structure was formed, a tolerance, and the structure type. In one embodiment, at least one of the method by which the structure was formed, the tolerance, and the structure type are read from a database by an instrument on which the method is performed. In another embodiment at least one of the method by which the structure was formed, the tolerance, and the structure type are input to an instrument on which the method is performed. The threshold value is read from a database by an instrument on which the method is performed in one embodiment, and in an alternate embodiment the threshold value is input to an instrument on which the method is performed. 
     According to another aspect of the invention there is described a mask inspection device adapted to inspect a mask. The mask inspection device includes an optical system to image a structure on the mask at a variable resolution, and produce an optical image. A controller specifies the resolution at which the structure is imaged based on a criteria set including at least one of a method by which the structure was formed, a tolerance, and a structure type. The controller compares the optical image to a baseline image, identifies differences between the optical image and the baseline image, compares the differences to a threshold value, and flags as defects any differences that exceed the threshold value. 
     In various preferred embodiments, a higher resolution is used when the method by which the structure was formed is a photo repeater method, and a lower resolution is used when the method by which the structure was formed is a direct writing method. Preferably, a higher resolution is used when the structure type requires a more exact critical dimension, and a lower resolution is preferably used when the structure type requires a less exact critical dimension. In one embodiment the threshold value is zero. The threshold value is preferably dependant on at least one of the method by which the structure was formed, the tolerance, and the structure type. At least one of the method by which the structure was formed, the tolerance, and the structure type are preferably read from a database by the inspection device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1  is a representational view of an integrated circuit. 
         FIG. 2  is representational view of one of the masks used to fabricate the integrated circuit of  FIG. 1 . 
         FIG. 3  is a functional embodiment of a mask inspection device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to  FIG. 1 , there is depicted a representation of an integrated circuit  10  formed on a substrate  12  and having defined thereon a plurality of functional blocks  14 - 28 . Each functional block  14 - 28  represents a component of the integrated circuit  10  that has been formed using a mask to pattern the substrate  12  with the circuit pattern corresponding to the functional block. It is appreciated that within each functional block there are defined many structures, also referred to variously as elements or features, which structures have not been depicted so as to not overly burden the figures with excessive detail that does not significantly increase an understanding of the present invention. 
     As depicted in  FIG. 1 , functional blocks  14 - 22  are arranged in what is commonly called a Manhattan configuration, and functional blocks  24 - 28  are arranged in what is commonly called a non Manhattan configuration. In a Manhattan configuration, the functional blocks  14 - 22  are disposed in an orthogonal arrangement, with the spaces between the functional blocks  14 - 22  running parallel and perpendicular to the peripheral edges of the substrate  12 . In a non Manhattan configuration, the functional blocks  24 - 28  are disposed in a skewed arrangement, with the spaces between the functional blocks  24 - 28  running at angles other than ninety degrees to the peripheral edges of the substrate  12 . 
     It is appreciated that the integrated circuit  10  of  FIG. 1  is formed using a variety of masks, such as mask  30  depicted in  FIG. 2 . Mask  30  contains the patterns for a single layer of the integrated circuit  10  of  FIG. 1 . Thus, Mask  30  also contains functional blocks  34 - 48  disposed on a substrate  32 . Similar to that as described above in regard to the integrated circuit  10 , the mask  30  has functional blocks  34 - 42  disposed in a Manhattan configuration, and blocks  44 - 48  disposed in a non Manhattan configuration. 
     As depicted in  FIGS. 1 and 2 , functional blocks  24 ,  26 , and  28  of the integrated circuit  10  and functional blocks  44 ,  46 , and  48  or the mask  30  are depicted as having non Manhattan shapes. Typically, an integrated circuit  10  and mask  30  at a large scale block and die level as depicted would have strictly Manhattan shapes. It is the actual circuit features (substantially undetectable at the level depicted in the figures) within the block or die that may have non Manhattan geometry. So, at the level of detail as provided in  FIGS. 1 and 2  as depicted, one would expect to see rectangular geometries only. However, the blocks  24 ,  26 ,  28 ,  44 ,  46 , and  48  have been depicted as having non Manhattan geometries for the sake of simplifying the figures, and so as to not become overly burdensome in detail. Thus, it is appreciated that the blocks  24 ,  26 ,  28 ,  44 ,  46 , and  48  are representative of much smaller non Manhattan pattern features that have diagonal, horizontal, and vertical lines and dimensions. 
     When non Manhattan functional blocks, such as functional blocks  44 - 48 , are directly written with a pattern generator, the edges of the various features tend to be especially rough. Thus, non Manhattan functional blocks  44 - 48  are especially good candidates for being formed on the mask using a photo repeater method, which tends to form very smooth line edges in the features of the non Manhattan functional blocks  44 - 48  so formed. Of course, it is possible to form any of the functional blocks  34 - 48  using either pattern generation or photo repetition, as desired. However, as indicated herein, there may be reasons in a particular case for a given functional block  34 - 48  to be formed with one or the other of the two methods. 
     Some of the functional blocks  34 - 48  may be formed from standardized functional blocks that are maintained in a pattern library, and which perform functions that are commonly used in a variety of different integrated circuit  10  designs. Others of the functional blocks  34 - 48  may be formed in a highly customized manner, and thus are not used in many different integrated circuit designs. However, the patterns for such customized functional blocks are preferably fully specified, and can be placed in a library, even though they may not ever be used for another integrated circuit pattern. Thus, the patterns for each of the functional blocks  34 - 48  are all preferably available in an electronic format that is readable by a properly programmed computing device. These electronically readable patterns are preferably used to create the functional blocks on the mask. 
     As introduced elsewhere herein, the mask is preferably inspected after it has been fabricated, to ensure that the various elements within the functional blocks  34 - 48  have been formed correctly. Such inspection is preferably accomplished on a mask inspection device  50 , such as depicted in  FIG. 3 , which images a portion of the mask at a time, and compares the imaged portion to some type of baseline reference. For example, the baseline reference may be as simple as comparing two similar portions of the mask to each other. If there is any difference between the two imaged portions of the mask, the difference is interpreted as some type of flaw. If the flaw is of a great enough severity, then the difference is interpreted as a mask defect. 
     The mask inspection device  50  according to the present invention is preferably able to make such comparisons, but has additional capabilities. For example, the mask inspection device  50  according to the present invention is preferably able to receive input through an input  56  in regard to different sections of the mask that are inspected, and using this input is able to make such inspections at different optical resolutions, with an optical system  52 . This information can be input manually, such as by an operator, but is most preferably retrieved in an automated fashion, such as from an online database. 
     In a most preferred embodiment, the inspection device  50  is adapted to read the pattern files from which the mask was generated through the input  56 . In this embodiment, the inspection device  50  can determine the methods used to create the various portions of the mask from the pattern file, and can adjust the resolution of the optical system  52  accordingly during the inspection process for those portions. In other embodiments, the input information is in regard to the desired critical dimensions for a given portion of the mask, and again, the resolution of the optical system  52  used during the inspection process for those portions is adjusted accordingly. 
     In one embodiment, the controller  54  for the inspection device  50  reads the information from the mask library, which was used to create the mask, through the input  56  as the mask is inspected. Thus, as the optical system  52  is optically inspecting the mask and sending optical information to the controller  54 , the controller  54  is comparing that optical information to the pattern information that was used to create the mask. Thus, the baseline reference in this embodiment comes from the data used to create the mask in the first place. 
     Additionally, the controller  54  can determine, from the pattern information read through the input  56  from the pattern library, the method by which a given portion of the mask was fabricated, and can adjust the resolution of the optical system  52  accordingly. Further, the controller  54  can also receive or determine other information on which it can adjust the resolution of the optical system  52  for a given portion of the mask. For example, the controller  54  may access a recipe which instructs the controller  54  to inspect some portions of the mask with a higher resolution, and other portions of the mask with a lower resolution. As another example, the controller  52  in one embodiment determines from the input the type of structure on the mask that is being inspected, and adjusts the resolution of the optical system  52  based on the structure type. Thus, structures that require a tighter tolerance on a critical dimension can be inspected at a higher resolution, and structures that can allow a looser tolerance on a critical dimension can be inspected at a higher resolution. 
     In this manner, the resolution of the optical system  52  of the inspection device  50  is dynamic and changeable under the control of the controller  54 , as it receives input through the input  56 . By increasing the resolution of the optical system  52  for certain portions of the mask inspection, a more thorough inspection can be performed in those portions which require such. However, by being able to reduce the resolution of the optical system  52  for other portions of the mask inspection, inspection speed can be increased because of the reduced amount of data that is delivered per unit time for a lower resolution inspection. Thus, the inspection system  50  according to the present invention provides benefits and versatility that are not found in prior art systems. 
     The controller  54  determines a difference between the optical information received from the optical system  52  and the baseline reference, whether that baseline reference is another portion of the same mask, a similar portion of a different mask, or the pattern information received through the input  56 . The difference between the optical information and the baseline is compared to a threshold, and if the threshold is exceeded, then the difference is flagged as a mask defect. 
     Most preferably, the threshold value is also variable for different portions of the mask, as determined by the controller  54 . As mentioned above in regard to the resolution adjustment, the value for the threshold can also be determined based at least in part on information received through the input  56 . For example, the method by which a given portion of the mask was fabricated can determine the threshold value by which a defect is determined. Additionally, the structure type that is being inspected can also be used to determine the threshold value. Further, the threshold can also be determined based on a tolerance value that is read by the controller  54 , such as from a recipe. 
     Thus, one or both of a resolution of the inspection and a threshold by which a difference is flagged as a defect can be dynamically configured based on one or more of a variety of different inputs to the inspection device  50 . 
     The method and apparatus of the invention is preferably implemented on any suitable inspection tool. For example, a KLA TeraStar and TeraScan Mask Inspection Systems, commercially available from KLA-Tencor Corporation of San Jose, Calif., may be adapted for such use. 
     The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.