Method of Measuring a Structure, Inspection Apparatus, Lithographic System and Device Manufacturing Method

Abstract

An overlay metrology target (600, 900, 1000) contains a plurality of overlay gratings (932-935) formed by lithography. First diffraction signals (740(1)) are obtained from the target, and first asymmetry values (As) for the target structures are derived. Second diffraction signals (740(2)) are obtained from the target, and second asymmetry values (As′) are derived. The first and second diffraction signals are obtained using different capture conditions and/or different designs of target structures and/or bias values. The first asymmetry signals and the second asymmetry signals are used to solve equations and obtain a measurement of overlay error. The calculation of overlay error makes no assumption whether asymmetry in a given target structure results from overlay in the first direction, in a second direction or in both directions. With a suitable bias scheme the method allows overlay and other asymmetry-related properties to be measured accurately, even in the presence of two-dimensional overlay structure.

1 . A metrology target for use in overlay metrology, the metrology target comprising:
 a plurality of target structures, each target structure having first features periodic in both a first direction and a second direction and second features periodic in both the first direction and the second direction,   wherein the first and second directions are non-parallel,   wherein different ones of the target structures have different programmed offsets in placement of the second features relative to the first features in both the first direction and the second direction, and   wherein the target structures are arranged into the metrology target such that any target structure bordering two neighboring target structures has a programmed offset intermediate between the programmed offsets of those two neighboring target structures.   
     
     
         2 . The metrology target of  claim 1 , wherein a bias vector representing the programmed offset rotates less than ninety degrees between neighboring target structures. 
     
     
         3 . The metrology target of  claim 2 , wherein a bias vector representing the programmed offset rotates 45 degrees between neighboring target structures. 
     
     
         4 . The metrology target of  claim 1 , wherein five or more of the target structures are arranged in a closed ring. The metrology target of  claim 1 , wherein eight or more of the target structures are arranged in the closed ring. 
     
     
         6 . The metrology target of  claim 1 , wherein five or more of the target structures are arranged in a line. 
     
     
         7 . The metrology target of  claim 1 , wherein seven or more of the target structures are arranged in a line. 
     
     
         8 . A set of patterning devices for use in a lithographic process, the patterning devices including at least a first patterning device configured to define the first features of a metrology target of  claim 1 , and a second patterning device configured to define the second features of the metrology target. 
     
     
         9 . The set of patterning devices of  claim 8 , wherein the first features of the target structures are formed in a first continuous array and the second features of the first subset of target structures are formed in a second continuous array of features, the different target structures being defined by variation of the positional offsets over one or other of the continuous arrays. 
     
     
         10 . The metrology target of  claim 2 , wherein five or more of the target structures are arranged in a closed ring. 
     
     
         11 . The metrology target of  claim 3 , wherein five or more of the target structures are arranged in a closed ring. 
     
     
         12 . The metrology target of  claim 2 , wherein eight or more of the target structures are arranged in the closed ring. 
     
     
         13 . The metrology target of  claim 3 , wherein eight or more of the target structures are arranged in the closed ring. 
     
     
         14 . The metrology target of  claim 4 , wherein eight or more of the target structures are arranged in the closed ring. 
     
     
         15 . The metrology target of  claim 2 , wherein five or more of the target structures are arranged in a line. 
     
     
         16 . The metrology target of  claim 3 , wherein five or more of the target structures are arranged in a line. 
     
     
         17 . The metrology target of  claim 4 , wherein five or more of the target structures are arranged in a line. 
     
     
         18 . The metrology target of claim  5 , wherein five or more of the target structures are arranged in a line. 
     
     
         19 . The metrology target of  claim 6 , wherein seven or more of the target structures are arranged in a line. 
     
     
         20 . A set of patterning devices for use in a lithographic process, the patterning devices including at least a first patterning device configured to define the first features of a metrology target of  claim 8 , and a second patterning device configured for to define the second features of the metrology target.