Polarization tuning in scatterometry

Abstract

A method includes projecting an illumination beam of radiation onto a metrology target on a substrate, detecting radiation reflected from the metrology target on the substrate, and determining a characteristic of a feature on the substrate based on the detected radiation, wherein a polarization state of the detected radiation is controllably selected to optimize a quality of the detected radiation.

1 . A method comprising:
 projecting an illumination beam of radiation onto a metrology target on a substrate;   detecting radiation redirected by the metrology target on the substrate; and   determining a characteristic of a feature on the substrate based on the detected radiation, wherein a polarization state of the detected radiation is controllably selected to optimize a quality of the detected radiation.   
     
     
         2 . The method according to  claim 1 , wherein the controllably selecting comprises selecting a ratio of TE to TM polarization in the illumination beam. 
     
     
         3 . The method according to  claim 2 , wherein the optimization comprises determining a performance characteristic for each of a plurality of polarization states and selecting a polarization state for which the performance characteristic is the best or crosses a threshold. 
     
     
         4 . The method according to  claim 3 , wherein the performance characteristic is selected from stack sensitivity, overlay error, signal to noise ratio, and/or stability versus wavelength. 
     
     
         5 . The method according to  claim 1 , wherein the controllably selecting comprises performing a plurality of measurements using a plurality of polarization states and combining the measurements to obtain a combined measurement having characteristics related to a superposition of the plurality of polarization states. 
     
     
         6 . The method according to  claim 5 , wherein each polarization state is selected from TE or TM, and a respective number of measurements using each polarization state defines a ratio of TE to TM polarization in the illumination beam. 
     
     
         7 . The method according to  claim 2 , wherein the selection of a ratio of TE to TM polarization is performed by controllably attenuating one or both of a TE illumination and a TM illumination. 
     
     
         8 . The method according to  claim 2 , wherein the selection of a ratio of TE to TM polarization is performed by rotation of a half-wave plate configured and arranged to change a polarization state of the illumination. 
     
     
         9 . The method according to  claim 1 , wherein the controllably selecting a polarization state of the detected radiation comprises filtering the radiation using an addressable array of polarizing elements to polarize selected portions of the radiation at a pupil plane of a metrology apparatus used to perform the method. 
     
     
         10 . The method according to  claim 9 , wherein the addressable array of polarizing elements comprises a liquid crystal panel. 
     
     
         11 . A non-transitory computer program product comprising machine-readable instructions that, when executed by a processor system, are configured to cause the processor system to at least:
 cause projection of an illumination beam of radiation onto a metrology target on a substrate;   cause detection of radiation redirected by the metrology target on the substrate; and   determine a characteristic of a feature on the substrate based on the detected radiation, wherein a polarization state of the detected radiation is controllably selected to optimize a quality of the detected radiation.   
     
     
         12 . A method of manufacturing devices wherein a device pattern is applied to a series of substrates using a lithographic process, the method including adapting an aspect of the method of manufacturing devices using a determined characteristic obtained using the method according to  claim 1 , and exposing a substrate using the adapted method of manufacturing devices. 
     
     
         13 . A scatterometer comprising:
 a radiation source configured and arranged to project an illumination beam of radiation onto a metrology target on a substrate;   a detector, configured and arranged to detect radiation redirected by the metrology target on the substrate; and   a polarization controller, configured and arranged to controllably select a polarization state of the detected radiation to optimize a quality of the detected radiation based on a characteristic of a feature on the substrate determined based on the detected radiation.   
     
     
         14 . The scatterometer according to  claim 13 , wherein:
 the radiation source comprises:   a pair of sources, one of the pair of sources being configured and arranged to produce TE polarized radiation in the illumination beam of radiation, the other of the pair of sources configured and arranged to produce TM polarized radiation in the illumination beam of radiation, and   an optical element configured and arranged to controllably select a ratio of TE to TM polarization in the illumination beam.   
     
     
         15 . The scatterometer according to  claim 14 , wherein the optical element comprises a rotatable half-wave plate configured and arranged to change a polarization state of the illumination. 
     
     
         16 . The computer program product of  claim 11 , wherein the instructions configured to controllably select the polarization state of the detected radiation are further configured to select a ratio of TE to TM polarization in the illumination beam. 
     
     
         17 . The computer program product of  claim 11 , wherein the instructions configured to optimize the quality of the detected radiation are further configured to determine a performance characteristic for each of a plurality of polarization states and select a polarization state for which the performance characteristic is the best or crosses a threshold. 
     
     
         18 . The computer program product of  claim 17 , wherein the performance characteristic is selected from stack sensitivity, overlay error, signal to noise ratio, and/or stability versus wavelength. 
     
     
         19 . The computer program product of  claim 11 , wherein the instructions configured to controllably select the polarization state of the detected radiation are further configured to cause performance of a plurality of measurements using a plurality of polarization states and combine the measurements to obtain a combined measurement having characteristics related to a superposition of the plurality of polarization states. 
     
     
         20 . The computer program product of  claim 19 , wherein each polarization state is selected from TE or TM, and a respective number of measurements using each polarization state defines a ratio of TE to TM polarization in the illumination beam.