Reduction or elimination of pattern placement error in metrology measurements

Abstract

Metrology methods and targets are provided for reducing or eliminating a difference between a device pattern position and a target pattern position while maintaining target printability, process compatibility and optical contrast—in both imaging and scatterometry metrology. Pattern placement discrepancies may be reduced by using sub-resolved assist features in the mask design which have a same periodicity (fine pitch) as the periodic structure and/or by calibrating the measurement results using PPE (pattern placement error) correction factors derived by applying learning procedures to specific calibration terms, in measurements and/or simulations. Metrology targets are disclosed with multiple periodic structures at the same layer (in addition to regular target structures), e.g., in one or two layers, which are used to calibrate and remove PPE, especially when related to asymmetric effects such as scanner aberrations, off-axis illumination and other error sources.

What is claimed is: 
     
         1 . A method of reducing a difference between a device pattern position and a target pattern position of at least one periodic structure in a corresponding metrology target, the method comprising adding, to a mask design of the at least one periodic structure, sub-resolved assist features at a same periodicity as, and in continuation of, the at least one periodic structure, wherein the sub-resolved assist features have a critical dimension (CD) smaller than a corresponding printability threshold. 
     
     
         2 . The method of  claim 1 , further comprising using optimized phase shift masks to minimize target asymmetry. 
     
     
         3 . The method of  claim 2 , wherein the phase shift masks are optimized using simulation results of optical proximity corrections for the metrology target. 
     
     
         4 . The method of  claim 1 , further comprising optimizing the sub-resolved assist features to minimize target asymmetry by deriving a maximal pitch therefor which is below a printability threshold. 
     
     
         5 . The method of  claim 4 , wherein the optimizing is carried out using simulations. 
     
     
         6 . The method of  claim 1 , further comprising configuring the at least one periodic structure to have a Line:Space (L:S) ratio between 0.9 and 1.1, and measuring from the metrology target second harmonics comprising an interference of second and zeroth diffraction order signals. 
     
     
         7 . The method of  claim 1 , applied to imaging metrology and corresponding targets. 
     
     
         8 . The method of  claim 1 , applied to scatterometry metrology and corresponding targets. 
     
     
         9 . The method of  claim 7 , wherein elements of the at least one periodic structure are further segmented at a fine pitch. 
     
     
         10 . The metrology target designed according to the method of  claim 1 . 
     
     
         11 . A method of reducing pattern placement error (PPE) discrepancy between a semiconductor device and corresponding metrology targets, the method comprising:
 deriving, from multiple target structures and metrology signals measured therefrom, a PPE correction related to asymmetric aberrations, by applying a learning procedure to a calibration term, and   adjusting corresponding measurements by the PPE correction.   
     
     
         12 . The method of  claim 11 , wherein the learning procedure is carried out according to
   DiffOverlay i     th     harmonics   wafer1,wafer2 =RealOffset+ PPE   i =RealOffset( A   1   −A   2 )+ F   i ( A   1   −A   2 )  Equation 2,
       D=OVL   i   −OVL   j   =PPE   i   −PPE   j =( F   i   −F   j ) A,   Equation 3, and
       OVL=OVL   i   −PPE   i   =OVL   i   −R·D,   Equation 4,
   wherein PPE i  is a pattern placement error of an i th  harmonic, PPE j  is a pattern placement error of a j th  harmonic, A is an aberration of a wafer, A 1  and A 2  are the aberrations of wafers 1 and 2, F i  is a slope of the measurement error of the i th  harmonic, F j  is a slope of the measurement error of the j th  harmonic, OVL is a corrected overlay, OVL i  is the overlay of the i th  harmonic, OVL j  is the overlay of the j th  harmonic, and R is a calibration constant.   
     
     
         13 . The method of  claim 11 , further comprising calibrating metrology measurements by reiterating the derivation of the PPE correction 
     
     
         14 . The method of  claim 11 , further comprising configuring the multiple target structures in at least one layer having two or more target structures to provide the PPE correction upon measurement. 
     
     
         15 . The method of  claim 14 , wherein the two or more target structures in each layer have identical fine pitches. 
     
     
         16 . The method of  claim 14 , wherein the two or more target structures differ in pitch and/or critical dimension (CM. 
     
     
         17 . The method of  claim 11 , further comprising producing the multiple target structures in one layer by applying a cutting procedure to a periodic structure at a minimal design rule pitch to yield periodic structures of different pitches and CDs. 
     
     
         18 . The method of  claim 11 , applied to imaging metrology, corresponding targets and harmonics measured therefrom. 
     
     
         19 . The method of  claim 11 , applied to scatterometry metrology, corresponding targets and diffraction signals measured therefrom. 
     
     
         20 . Metrology measurements derived from the method of  claim 11 .