Test structure design for metrology measurements in patterned samples

Abstract

A test structure is presented for use in metrology measurements of a sample pattern formed by periodicity of unit cells, each formed of pattern features arranged in a spaced-apart relationship along a pattern axis. The test structure comprises a test pattern, which is formed by a main pattern which includes main pattern features of one or more of the unit cells and has a symmetry plane, and a predetermined auxiliary pattern including at least two spaced apart auxiliary features located within at least some features of the main pattern, parameters of which are to be controlled during metrology measurements.

2 . The test structure according to claim  1 , wherein the auxiliary features of the auxiliary pattern are arranged such that the auxiliary pattern is asymmetric with respect to the symmetry plane of the main pattern. 
     
     
         3 . The test structure according to claim  1 , wherein the main pattern is symmetric with respect to the symmetry plane irrespective of whether parameters of similar features of the main pattern are equal or not. 
     
     
         4 . The test structure according to claim  1 , wherein the auxiliary features of the auxiliary pattern are configured and arranged with respect to the main pattern such that data indicative of optical response from the test structure satisfies a condition of symmetry around a measurement plane being parallel to the symmetry plane of the main pattern, if and when at least one parameter of the main pattern being controlled is maintained within the test structure, and does not satisfy said condition of symmetry around said measurement plane if and when said at least one parameter varies within the test structure. 
     
     
         5 . The test structure according to  claim 4 , wherein said data indicative of the optical response comprises a relation between first and second measured data pieces corresponding to first and second measurements taken at different measurement conditions. 
     
     
         6 . The test structure according to  claim 5 , wherein said relation is a difference between the first and second measured data pieces. 
     
     
         7 . The test structure according to  claim 5 , wherein said different measurement conditions are characterized by different polarizations of light collected from the structure. 
     
     
         8 . The test structure according to claim  1 , wherein the test pattern comprises the main pattern comprising at least one unit cell, the unit cell comprising three lines with two respective spacings between them, and the auxiliary pattern associated with the at least one unit cell and comprising two vias located in and extending across said two spacings, respectively, thereby enabling control of widths of the spacings by optical measurements using a measurement plane parallel to the symmetry plane of the main pattern which is parallel to the pattern axis. 
     
     
         9 . The test structure according to  claim 8 , wherein said two vias are located at opposite sides of the line between said two spacings, and at opposite ends of said line. 
     
     
         10 . The test structure according to claim  1 , wherein the test pattern comprises the main pattern comprising at least one pair of locally adjacent unit cell, the unit cell comprising two spaced lines, and the auxiliary pattern associated with one unit cell of the pair and comprising two vias located in and extending across said two spacings, respectively, thereby enabling control of widths of the spacings, by optical measurements using a measurement plane parallel to the symmetry plane of the main pattern which is perpendicular to the pattern axis. 
     
     
         11 . A patterned structure comprising a test region and at least one test structure located in said test region, the test structure being configured according to any one of the preceding claims. 
     
     
         12 . The patterned structure according to  claim 11 , being a semiconductor wafer progressing on a production line. 
     
     
         13 . A system for use in metrology measurements of a sample having a main pattern having symmetry planes, the system comprising: an optical unit defining an illumination channel for illuminating a test region in a sample with an illuminating beam and a collection channel for detecting collected light reflected from the test region, and a control unit for receiving and processing measured data indicative of detected light, wherein
 the optical unit is configured and operable such that the illumination and collection channels are located in a plane perpendicular to a sample plane and parallel to the symmetry plane of the main pattern, the optical unit comprises a polarization unit comprising at least one polarization filter located in the collection channel and configured and operable to collect and allow detection of light of a polarization state different from polarization of the illuminating light, the optical unit being operable to perform at least one measurement session, each measurement session including first and second measurements with different polarizations of the detected light each being different to polarization of the illuminating light, and generating first and second measured data pieces respectively;   the control unit is configured and operable to process the said first and second measured data pieces, and determine a relation between the first and second measured data pieces being indicative of a condition of symmetry of the main pattern in said test region.   
     
     
         14 . The system of  claim 13 , wherein said at least one polarization filter is shiftable between its different operational states corresponding to different polarization states of the collected light. 
     
     
         15 . The system of  claim 13 , wherein said at least one polarization filter is a linear polarization filter, and the control unit is configured to controllably rotate the polarization filter to ±45° with respect to p linear polarization state. 
     
     
         16 . The system of  claim 13 , wherein the polarization filter is a circular polarization filter configured to selectively allow detection of light of clockwise or counterclockwise polarization states; the control unit being configured to selectively switch between said clockwise and counterclockwise states. 
     
     
         17 . The system of  claim 13 , wherein the illuminating light is unpolarized light. 
     
     
         18 . The system of  claim 17 , wherein said at least one polarization filter is shiftable between its different operational states corresponding to different polarization states of the collected light. 
     
     
         19 . The system of  claim 17 , wherein the polarization unit is a static unit, allowing for concurrently collecting light of the first and second polarization states. 
     
     
         20 . The system of  claim 13 , wherein the polarization unit comprises at least one polarization filter in the illumination channel operable to sequentially provide at least first and second illuminating light beams having at least first and second different polarization states. 
     
     
         21 . The system of  claim 20 , wherein the control unit is configured to controllably operate the polarization unit to illuminate the test region with the first and second illuminating beams of the first and second polarization states for the detection of light of respectively the second and first polarization states. 
     
     
         22 . A method for use in metrology measurements of patterned samples, the method comprising:
 associating the patterned sample being measured with a corresponding test structure configured according to claim  1 ;   performing at least one measurement session on said test structure using a measurement plane parallel to the symmetry plane of the main pattern in the test structure, the measurement session comprising at least first and second measurements, each directing illuminating light onto said test structure along an illumination channel and collecting light reflected from said test structure propagating along a collection channel to be detected, such that detected light has a polarization state different from polarization of the illuminating light, and generating a measured data piece indicative of the light detected in the measurement, thereby generating at least first and second measured data pieces for said at least first and second measurements on the same test structure corresponding to the detected light having different first and second polarization states; and   determining a relation between said at least first and second measured data pieces indicative of parameters of said main features in the main pattern.