Abstract:
A method for establishing distortion properties of an optical system in a microlithographic measurement system is provided. The optical system has at least one pupil plane, in which the distortion properties of the optical system are established on the basis of measuring at least one distortion pattern, which the optical system generates when imaging a predetermined structure in an image field. The distortion properties of the optical system are established on the basis of a plurality of measurements of distortion patterns, in which these measurements differ from one another in respect of the intensity distribution present in each case in the pupil plane.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to German patent application 10 2013 106 320.9, filed on Jun. 18, 2013. The above application is incorporated by reference. 
     TECHNICAL FIELD 
     The invention relates to a method for establishing distortion properties of an optical system in a microlithographic measurement system. 
     BACKGROUND 
     Microlithography is used for producing microstructured components, such as, for example, integrated circuits or liquid crystal displays (LCDs). The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. In this case, the image of a mask (also referred to as a reticle) illuminated by the illumination device is projected, by the projection lens, onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate. 
     A characterization of the structures on the mask is performed both in respect of present deviations of the respective structure on the mask from the intended position predefined by the design (so-called positioning error or “registration error”) and in respect of the line width of the structures (“critical dimension” (CD)). 
     For determining the positioning error, various methods are known in the prior art. By way of example, a “threshold-based” image evaluation can be applied to the structures of the aerial image, as is known from US 2012/0063666 A1. Alternatively, by use of a position measurement system, a first aerial image of a segment of the mask can be recorded and compared with a simulated second aerial image, whereupon the positioning error is then equated with the distance between the structures to be measured in the measured first aerial image and the simulated second aerial image. 
     One problem that occurs in practice, however, is that the measurement image is deformed or distorted on account of the properties of the optical system (that is to say that a coordinate grid is not exactly at right angles on the measurement image), whereas the simulated image as an ideal simulated grid does not have this property. 
     One known approach for taking account of the distortion consists in the latter being calibrated or “extracted computationally,” i.e., the distortion being determined metrologically by a targeted measurement with test structures in the image field. In this case, however, the further problem occurs that the distortion taken as a basis in such a calibration is dependent both on the pupil illumination specifically used within the imaging optical unit of the position measurement system and on the type of structure used for calibration. In so doing, here and in the following text, “pupil illumination” is understood to mean the intensity distribution obtained in a pupil plane within the imaging optical unit of the position measurement system, in which the imaging optical unit images light coming from the mask onto a detector unit. 
     The distortion underlying the above-described calibration is no longer exactly valid for any other possible structures, in which the resulting structure-dependent and illumination-dependent differences in the distortion on which the calibration is based are measurable in the sub-nanometer range and may be significant. 
     With regard to the prior art, reference is made for example to WO 2001/012265 A1, DE 10 2007 033 815 A1 and DE 10 2006 059 431 A1, US 2010/0104128 A1, DE 10 2007 033 815 A1 and also the publication M. Längle et al.: “Pattern placement metrology using PROVE high precision optics combined with advanced correction algorithms,” Proc. SPIE 8082, 80820J (2011). 
     SUMMARY 
     In a general aspect, a method for establishing distortion properties of an optical system in a microlithographic measurement system is provided. The system enables a more accurate specification of the distortion properties. The optical system has at least one pupil plane, in which the distortion properties of the optical system are established on the basis of measuring at least one distortion pattern, which the optical system generates when imaging a predetermined structure in an image field. The distortion properties of the optical system are established on the basis of a plurality of measurements of distortion patterns, in which these measurements differ from one another in respect of the intensity distribution present in each case in the pupil plane. 
     Initially, the invention proceeds from the consideration that the distortion occurring when measuring at least one structure in the generated image field depends on the pupil illumination specifically used in the imaging optical unit of the position measurement system such that the measurement images recorded with the position measurement system are also to be processed taking into account the distortion respectively emerging for this specific pupil illumination. 
     Proceeding from this consideration, the invention is based upon the concept of, in particular, establishing the distortion properties of the optical system on the basis of a plurality of measurements of distortion patterns having different pupil illuminations (i.e., different intensity distributions in the pupil plane). In particular, the invention contains the concept of carrying out a distortion correction using a suitable distortion function when processing the measurement images, which distortion function describes the dependence of the distortion on the pupil illumination such that the specific pupil illumination used in the specific case when recording the relevant measurement image can also be taken into account in respect of the influence thereof on the distortion. 
     In accordance with one embodiment, during the plurality of measurements of distortion patterns, only one segment from a plurality of segments is illuminated in each case in the pupil plane. 
     In accordance with one embodiment, a measurement image or one or more relevant portions thereof, recorded by the optical system is corrected on the basis of the established distortion properties of the optical system. 
     In accordance with one embodiment, the corrected measurement image is used for establishing registration errors and/or structure widths on a mask. 
     The corrected measurement image can be used in different ways and by means of methods respectively known per se for, e.g., establishing registration errors. In accordance with one application example, the corrected measurement image can be used in an image comparison with a simulated image (such that, in other words, in an image comparison between a measurement image and a simulated image, these images are aligned with respect to distortion effects on the basis of the established distortion properties of the optical system). 
     In further applications of the invention, the evaluation of the corrected measurement images for establishing registration errors can also be brought about by other methods, in which the actual position of the structure on the mask is established in each case. By way of example, a symmetry correlation as described in DE 10 2010 047 051 A1 can be carried out to this end, in which at least one symmetry operation (e.g., a point reflection or mirroring in a reference mirror plane) is carried out for a provided image comprising the structure to be established. Alternatively, it is also possible for edge detection to be carried out in the corrected measurement image, in which the position of the structure is established from the detected edge positions. In the two methods mentioned last, the registration error then emerges as difference between the actually established position and the intended position of the structure. 
     In particular, the distortion function ultimately to be used when processing or correcting the recorded measurement image can be established by virtue of a plurality of individual distortion functions being established initially, of which each is assigned to an (as it were “elementary”) pupil illumination. Then, depending on the extent to which this specific pupil illumination corresponds to the elementary pupil illuminations, the distortion function to be used for the respectively current or specific pupil illumination when processing the measurement images recorded with the position measurement system can be calculated as a weighted sum of the individual distortion functions. 
     Therefore, in accordance with one embodiment, the method in particular includes the following steps:
         establishing a distortion function (V i (x, y), i=1, . . . , n)) in each case for a plurality (n) of segments in the pupil plane, which distortion function specifies the distortion generated by the optical system in an image plane when illuminating the respective segment; and   calculating an overall distortion function (V tot  (x, y)) for a given intensity distribution in the pupil plane as       

                 V   tot     ⁡     (     x   ,   y     )       =       ∑     i   =   1     n     ⁢       w   i     ·       V   i     ⁡     (     x   ,   y     )                 
where
 
               w   i     =       I   i       I   tot             
denotes a weighting factor assigned to the i-th segment in the pupil plane and I i  denotes the intensity in the i-th segment for the given intensity distribution.
 
     The plurality of individual distortion functions can be established in such a way that a portion of the pupil plane within the imaging optical unit, i.e., in each case a “pupil segment,” is assigned in each case to each of these individual distortion functions. When establishing the distortion function ultimately to be used when processing the recorded measurement images, the individual distortion functions are thereupon taken into account to the extent to which the relevant “pupil segment” contributes to the actually employed pupil illumination or the specific illumination setting. 
     The optical system according to the invention can, in particular, be equipment for determining the position of structures on a microlithographic mask, an inspection measurement system for measuring defects of photomasks, equipment for determining the line width in photomasks, a phase measurement system for photomasks or inspection equipment for localizing defects of photomasks. 
     Further embodiments of the invention can be gathered from the description and the dependent claims. 
     The invention is explained in more detail below on the basis of exemplary embodiments depicted in the attached drawings. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a schematic illustration for explaining an exemplary design of a position measurement system which can be used in the method according to the invention; 
         FIG. 2  shows a schematic illustration for explaining the concept underlying the present invention; 
         FIG. 3  shows a flowchart for explaining an embodiment of the method according to the invention; and 
         FIG. 4  shows a flowchart for explaining a conventional method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a possible design of a position measurement system  100 , in which the present invention can be realized. 
     In accordance with  FIG. 1 , a mask  102  is mounted on a platform  101  displaceable in three spatial directions in a position measurement system  100 . The structures to be measured on the mask  102  are illuminated by illumination light, in which in the depicted exemplary embodiment, provision is made both for an illumination device  103  for transmitted illumination of the mask  102  and for an illumination device  104  for reflected illumination of the mask  102 . Light coming from the mask  102  is imaged by an imaging optical unit  105  onto a detector unit  107  via a semitransparent mirror  106  and detected. 
     A control device  108  serves to control both the movement of the platform  101  supporting the mask  102  and the recording of the image data by the detector unit  107 , and it is connected to an evaluation unit  109  in which the image data recorded by the detector unit  107  are evaluated for determining the position of the structures. To this end, the image data of the generated recordings are fed to the control device  108 , from where the data are transmitted to the evaluation unit  109 . The measurement image (in the form of a first aerial image) of a section of the mask  102 , established by the position measurement system  100 , can be compared to, e.g., a simulated second (aerial) image, whereupon the positioning error is then equated to the distance between the measurement image and the simulated image. In  FIG. 1 , “PP1” merely schematically indicates a pupil plane within the illumination device  103  and “PP2” indicates a pupil plane within the imaging optical unit  105 . 
     In the following text, a method according to the invention is now described with reference to  FIGS. 2 and 3 . What a suitable calibration achieves in this method is that, taking into account the specifically measured structures in each case and the pupil illumination specifically used in the imaging optical unit of the position measurement system, the images to be compared (namely the measurement image and the simulated image) in the carried out image comparison (e.g., for establishing registration errors) also correspond in view of distortion effects, i.e., in other words, that structure-dependent and pupil illumination-dependent distortion effects or image aberrations can be correctly taken into account in each case. 
     By way of illustration,  FIG. 2  shows an exemplary decomposition or segmentation of an illumination pupil or of the pupil plane PP2 within the imaging optical unit  105  of the position measurement system  100  from  FIG. 1 , in which the number of segments (which is merely exemplary in  FIG. 2  and, in principle, arbitrary) is denoted by “n” and in which “i” specifies the index of the respective segment. I i  specifies the intensity obtained for the respective pupil illumination in the i-th segment. 
     Proceeding from the decomposition indicated in  FIG. 2 ,  FIG. 3  now shows a flowchart for explaining an embodiment of the method according to the invention. 
     In accordance with  FIG. 3 , a plurality (n) of distortion measurements are carried out in a first (calibration) step S 10  in order to establish a plurality (n) of individual distortion functions V i  (x, y), in which each of these individual distortion functions V i (x, y) in each case is assigned to a portion of the pupil plane PP2 within the imaging optical unit  105  from  FIG. 1 , that is to say, for example, to in each case one “pupil segment” in accordance with the exemplary decomposition of  FIG. 2 . The field coordinates, i.e., the coordinates in the image field of the imaging optical unit  105 , are denoted by “x” and “y”. 
     The method known per se from U.S. Pat. No. 8,416,412 B2 can be used for the calibration or for the carrying out of the aforementioned individual distortion measurements in step S 10  (i.e., determining the individual distortion functions V i (x, y)). Here, a test mask including a multiplicity of adjustment marks is arranged in different rotational or displacement positions and a measurement image is generated in each case, in which the positions for the respective adjustment marks obtained in the measurement image emerge from the position of the platform  101  supporting the mask, the position of the adjustment marks on the mask and the distortion (from which the respective distortion can be calculated). 
     Thereupon, in step S 20 , a pupil illumination, specifically used in the current measurement, within the imaging optical unit  105  of the position measurement system  100 , i.e., a specific intensity distribution in the pupil plane PP2 in accordance with  FIG. 1 , is measured. Here, the associated intensity I i  is determined for each “pupil segment” (e.g., in the exemplary decomposition of  FIG. 2 ). The overall intensity in the pupil plane PP2 is 
     
       
         
           
             
               
                 
                   
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     The overall distortion function V tot  (x, y) now emerges from the individual distortion functions V i (x, y) (i.e., the distortion functions for the i segments) as 
     
       
         
           
             
               
                 
                   
                     
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     In step S 40 , the measurement images are actually recorded by the detector unit  107  from  FIG. 1 . A distortion correction is then applied to these measurement images in step S 50  by virtue of the image data recorded by the detector unit  107  being processed taking into account the weighted distortion function calculated in step S 30 . 
     When using the measurement images, corrected according to the invention, it is now possible, for example, to obtain increased accuracy in a subsequently carried out image comparison between a measurement image and a simulated image for establishing registration errors since, according to the invention, the pupil illumination specifically used in the imaging optical unit of the position measurement device is also taken into account, i.e., in other words, pupil illumination-dependent distortion effects or image aberrations were correctly taken into account. 
     Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments are evident to a person skilled in the art, e.g., by combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for a person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is defined by the accompanying patent claims and the equivalents thereof.