Accuracy improvements in optical metrology

Abstract

Methods, metrology modules and target designs are provided, which improve the accuracy of metrology measurements. Methods provide flexible handling of multiple measurement recipes and setups and enable relating them to landscape features that indicate their relation to resonance regions and to flat regions. Clustering of recipes, self-consistency tests, common processing of aggregated measurements, noise reduction, cluster analysis, detailed analysis of the landscape and targets with skewed cells are employed separately or in combination to provide cumulative improvements of measurement accuracy.

1 . A method comprising:
 identifying measurement setups which correspond to each other in measurement values of at least one metric by applying a self-consistency test to the measurement values, to yield a plurality of clusters of setups,   determining a most reliable cluster according to statistical characteristics of the clusters, and   deriving a measurement result from at least one setup in the determined most reliable cluster.   
     
     
         2 . The method of  claim 1 , wherein the identifying is carried out using a distance metric in a space of the measurement values. 
     
     
         3 . The method of  claim 1 , wherein the statistical characteristics comprise at least a size and a diversity of each cluster. 
     
     
         4 . The method of  claim 1 , further comprising relating the identified measurement setups to corresponding at least one flat region in a measurement landscape that comprises an at least partially continuous dependency of at least one metrology metric on at least one parameter. 
     
     
         5 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 1 . 
     
     
         6 . A metrology module configured to carry out the method of  claim 1 . 
     
     
         7 . A method comprising:
 selecting a plurality of measurement setups for a corresponding plurality of wafer regions, wherein the wafer regions are pre-determined or determined on-the-fly and wherein the measurement setups are selected according to at least one accuracy metric, and   carrying out metrology measurements with the selected measurement setup for each corresponding wafer region.   
     
     
         8 . The method of  claim 7 , wherein the measurement setups are selected according to a clustering of measurements or simulation values of at least one metric by applying a self-consistency test thereto, to yield a plurality of clusters of setups from which a most reliable cluster is determined for each of the wafer regions according to statistical characteristics of the clusters. 
     
     
         9 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 7 . 
     
     
         10 . A metrology module configured to carry out the method of  claim 7 . 
     
     
         11 . A method comprising modifying a pixel value in a metrology pupil image according to an average of neighboring pixel values to reduce random noise in the pupil image. 
     
     
         12 . The method of  claim 11 , further comprising symmetrizing pixel values in the pupil images prior to the modifying, wherein the symmetrizing is in a direction that is perpendicular measurement direction of a measured periodic structure target. 
     
     
         13 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 11 . 
     
     
         14 . A metrology module configured to carry out the method of  claim 11 . 
     
     
         15 . A pupil image derived from the method of  claim 11 . 
     
     
         16 . A method comprising deriving a value of a metrology metric from multiple measurements of the metric using different measurement setups and/or different targets, wherein the value is derived from a concatenation of pixels from the multiple measurements. 
     
     
         17 . The method of  claim 16 , further comprising weighting the pixels according to a weighting function and optimizing the weighting function to minimize pupil variation. 
     
     
         18 . The method of  claim 16 , further comprising estimating an algorithmic stability by comparing results from multiple concatenations of different sub-sets of the measurements. 
     
     
         19 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 17 . 
     
     
         20 . A metrology module configured to carry out the method of  claim 17 . 
     
     
         21 . (canceled) 
     
     
         22 . A method comprising deriving a measurement setup by applying a Principal component analysis (PCA) to a plurality of metrology measurements from using a plurality of setups, and selecting a measurement setup which exhibits a smallest ratio of small to large eigenvalues from the PCA. 
     
     
         23 . The method of  claim 22 , further comprising applying the PCA to a plurality of pixels from the metrology measurements. 
     
     
         24 . The method of  claim 23 , wherein the metrology measurements comprise both pupil- and field-scatterometry measurements. 
     
     
         25 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 22 . 
     
     
         26 . A metrology module configured to carry out the method of  claim 22 . 
     
     
         27 . A method comprising indicating inaccuracy in metrology measurements by calculating a difference between at least two values of at least one metrology metric, the at least two values derived from at least two metrology algorithms, and setting a threshold for the calculated difference. 
     
     
         28 . The method of  claim 27 , further comprising reporting the difference as a flag indicator for measurement inaccuracy. 
     
     
         29 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 27 . 
     
     
         30 . A metrology module configured to carry out the method of  claim 27 . 
     
     
         31 . (canceled) 
     
     
         32 . A method comprising indicating a proximate region in a landscape which is proximate to a resonance region, wherein the landscape comprises an at least partially continuous dependency of at least one metrology metric on at least one parameter and the resonance region in the landscape corresponds to a resonance of optical illumination in a measured location, the indicating being carried out by calculating a variability of pupil signal or at least one function thereof, and comparing the calculated variability with a predefined threshold, wherein surpassing the predefined threshold indicates the proximate region. 
     
     
         33 . The method of  claim 32 , further comprising reporting the calculated variability as a flag indicator for measurement inaccuracy. 
     
     
         34 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 32 . 
     
     
         35 . A metrology module configured to carry out the method of  claim 32 . 
     
     
         36 . (canceled) 
     
     
         37 . A method comprising determining resonant regions in a landscape, wherein the landscape comprises an at least partially continuous dependency of at least one metrology metric on at least one parameter and the resonance region in the landscape corresponds to a resonance of optical illumination in a measured location, the determining being carried out by detecting non-analytic behavior of at least one metrology metric comprising: spanning a space of the analytic functions over pupil coordinates by a basis of analytic functions, expressing the metrology metric in terms of the basis of analytic functions and estimating a residual of the expressing, wherein a residual larger than a predetermined threshold indicates the non-analytic behavior. 
     
     
         38 . The method of  claim 37 , further comprising reporting the residual as a flag indicator for resonance. 
     
     
         39 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 37 . 
     
     
         40 . A metrology module configured to carry out the method of  claim 37 . 
     
     
         41 . (canceled) 
     
     
         42 . A metrology target comprising a plurality of target cells having periodic structures along at least two different and non-perpendicular measurement directions. 
     
     
         43 . The metrology target of  claim 42 , wherein at least one of the measurement directions is at an angle φ to another measurement direction, wherein the angle φ is selected to reduce an effect of target cell asymmetry. 
     
     
         44 . The metrology target of  claim 42 , wherein the measurement directions comprise at least three measurement directions at angles different from 0° and 90° with respect to each other. 
     
     
         45 . (canceled) 
     
     
         46 . (canceled) 
     
     
         47 . A target design method comprising adding to target cells having periodic structures along a measurement direction, at least one skewed target cell having a different and non-perpendicular direction with respect to the measurement direction. 
     
     
         48 . The target design method of  claim 47 , further comprising designing the at least one skewed target cell to have a direction at an angle  4  to the measurement direction, wherein the angle  4  is selected to reduce an effect of target cell asymmetry. 
     
     
         49 . A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of  claim 47 .