Method for designing overlay targets and method and system for measuring overlay error using the same

Abstract

A method for designing an overlay target comprises selecting a plurality of overlay target pairs having different overlay errors or offsets, calculating a deviation of the simulated diffraction spectrum for each overlay target pair, selecting a plurality of sensitive overlay target pairs by taking the deviation of the simulated diffraction spectrum into consideration, selecting an objective overlay target pair from the sensitive overlay target pairs by taking the influence of the structural parameters to the simulated diffraction spectrum into consideration, and designing the overlay target pair based on the structural parameter of the objective overlay target pair.

1 . A method for designing an overlay target, comprising the steps of:
 selecting a plurality of overlay target pairs having different overlay errors or offsets;   calculating a deviation of the simulated diffraction spectrum for each overlay target pair;   selecting a plurality of sensitive overlay target pairs by taking the deviation of the simulated diffraction spectrum into consideration;   selecting an objective overlay target pair from the sensitive overlay target pairs by taking the influence of the structural parameters to the simulated diffraction spectrum into consideration; and   designing the overlay target pair based on the structural parameter of the objective overlay target pair.   
   
   
       2 . The method of  claim 1 , wherein each of the overlay target pairs comprises a first overlay target and a second overlay target, and the step of calculating a deviation of the simulated diffraction spectrum for each overlay target pair comprises the steps of:
 generating simulated spectra of the first overlay target and the second overlay target by using a spectrum simulation algorithm; and   calculating the deviation between the simulated spectra of the first and second overlay targets.   
   
   
       3 . The method of  claim 2 , wherein the deviation is a difference value. 
   
   
       4 . The method of  claim 3 , wherein the difference value is a root mean square error. 
   
   
       5 . The method of  claim 2 , wherein the spectrum simulation algorithm is based on a rigorous coupled wave theory. 
   
   
       6 . The method of  claim 1 , wherein the step of selecting a plurality of sensitive overlay target pairs by taking the deviation of the simulated diffraction spectrum into consideration further comprises the steps of:
 generating a graph having a plurality of spectrum difference curves from the deviation of the simulated diffraction spectra of the overlay target pairs; and   selecting a plurality of sensitive overlay target pairs by analyzing slopes of spectrum difference curves in the graph.   
   
   
       7 . The method of  claim 6 , wherein the step of selecting an objective overlay target pair further comprises the steps of:
 changing a predetermined structural parameter of the sensitive overlay target pairs;   calculating the deviation of simulated spectra of the sensitive overlay target pairs with the changed predetermined structural parameter; and   selecting the objective overlay target pair by taking a correlation data into consideration, wherein the correlation data correlates the deviation of the sensitive overlay target pairs and the predetermined structural parameter.   
   
   
       8 . The method of  claim 7 , wherein the objective overlay target pair has a minimum correlation data. 
   
   
       9 . The method of  claim 1 , wherein the overlay target pair comprises a first overlay target and a second overlay target, the first overlay target has a predetermined overlay error, and the second overlay target has an error equal to the sum of the predetermined overlay error and the offset. 
   
   
       10 . A method for measuring an overlay error, comprising the steps of:
 measuring a diffraction spectrum of an overlay target pair;   calculating a deviation of the measured diffraction spectrum of the overlay target pair;   performing a matching process to search a matched deviation in a library, wherein the library stores a plurality of the deviations of the simulated diffraction spectra and corresponding overlay errors thereof; and   determining the overlay error of the overlay target pair by taking the matched deviation into consideration.   
   
   
       11 . The method of  claim 10 , wherein the overlay target pair comprises a first overlay target and a second overlay target, and the calculating step further comprises the steps of:
 generating measured spectra of the first overlay target and the second overlay target; and   calculating the deviations of the measured spectra of the first and second overlay targets.   
   
   
       12 . The method of  claim 10 , wherein the deviation is a difference value. 
   
   
       13 . The method of  claim 12 , wherein the difference value is a root mean square error. 
   
   
       14 . The method of  claim 10 , wherein the overlay target pair comprises a first overlay target and a second overlay target, the first overlay target has a predetermined overlay error, and the second overlay target has an error equal to the sum of the predetermined overlay error and the offset. 
   
   
       15 . The method of  claim 10 , further comprising the step of defining the library, including:
 selecting a plurality of the overlay target pairs having different overlay errors or offsets; and   calculating the deviation of the simulated diffraction spectrum for each overlay target pair.   
   
   
       16 . The method of  claim 15 , wherein the overlay target pair comprises a first overlay target and a second overlay target, and the step of calculating the deviation further comprises the steps of:
 generating simulated spectra of the first and second overlay targets; and   calculating the deviation between the simulated spectra of the first and second overlay targets.   
   
   
       17 . The method of  claim 16 , wherein the deviation is a difference value. 
   
   
       18 . The method of  claim 17 , wherein the difference value is a root mean square error. 
   
   
       19 . The method of  claim 16 , wherein the step of generating simulated spectra of the first overlay target and the second overlay target is based on a rigorous coupled wave theory. 
   
   
       20 . A system for measuring an overlay error, comprising:
 an angular scatterometer configured to acquire a measured spectrum of an overlay target pair;   a data processing module configured to calculate a deviation of the measured spectrum of each overlay target pair;   a library unit configured to store the deviations of a plurality of simulated spectra and corresponding overlay errors thereof; and   a match unit configured to compare the deviations of simulated diffraction spectra with the deviation of the measured diffraction spectrum of the overlay target pair so as to select an overlay error of the overlay target pair.   
   
   
       21 . The system of  claim 20 , wherein the overlay target pair comprises a first overlay target and a second overlay target, the first overlay target has a predetermined overlay error, and the second overlay target has an error equal to the sum of the predetermined overlay error and an offset. 
   
   
       22 . The system of  claim 21 , wherein the library unit is further configured to perform the steps of:
 selecting a plurality of overlay target pairs having different overlay errors or offsets; and   calculating deviations of simulated diffraction spectra of the individual overlay target pairs.   
   
   
       23 . The system of  claim 22 , wherein the overlay target pair comprises a first overlay target and a second overlay target and the library unit is further configured to perform the steps of:
 generating simulated spectra of the first and second overlay targets by using a spectrum simulation algorithm; and   calculating deviations of the simulated spectra of the first and second overlay targets.   
   
   
       24 . The system of  claim 23 , wherein the deviation is a difference value. 
   
   
       25 . The system of  claim 23 , wherein the difference value is a root mean square error. 
   
   
       26 . The system of  claim 23 , wherein the spectrum simulation algorithm is based on a rigorous coupled wave theory.