Optimization of target arrangement and associated target
A method of devising a target arrangement, and associated target and reticle. The target includes a plurality of gratings, each grating having a plurality of substructures. The method includes: defining a target area; locating the substructures within the target area so as to form the gratings; and locating assist features at the periphery of the gratings, the assist features being configured to reduce measured intensity peaks at the periphery of the gratings. The method may include an optimization process including modelling a resultant image obtained by inspection of the target using a metrology process; and evaluating whether the target arrangement is optimized for detection using a metrology process.
1. A method of devising a target arrangement, the target comprising a plurality of gratings, each grating comprising a plurality of substructures, the method comprising:
defining a target area;
locating the substructures within the target area so as to form the gratings;
locating assist features at the periphery of the gratings, the assist features being configured to reduce measured intensity peaks at the periphery of the gratings; and
before locating the substructures forming a particular grating, adapting the length of the grating substructures for that particular grating to a remaining target area such that the length of the substructures forming that particular grating is different than that of the substructures of another grating in the target area.
2. A method as claimed in claim 1 , wherein the locating the substructures comprises forming each individual grating sequentially.
3. A method as claimed in claim 1 , wherein the locating the substructures comprises, for each grating, locating the substructures sequentially beginning from the target area periphery towards the target area center.
4. A method as claimed in claim 1 , wherein the assist features which are adjacent to and orientated with a particular grating are positioned in phase with that grating.
5. A method as claimed in claim 1 , wherein each grating is substantially surrounded by the assist features so as to isolate each grating from its surrounding environment.
6. A method as claimed in claim 1 , wherein a pitch of the assist features is such that the assist features are not detected during inspection of the target using a metrology process.
7. A method as claimed in claim 1 , wherein assist features are located immediately adjacent each outermost substructure of each grating.
8. A method as claimed in claim 1 , wherein each grating has approximately equal area.
9. A method as claimed in claim 1 , wherein the target comprises two or more overlaid target layers, with the top target layer comprising an overlay bias, and wherein the bias is not applied to assist features comprised in the top layer.
10. A method as claimed in claim 1 , wherein the target comprises two or more overlaid target layers, with the top target layer comprising an overlay bias, and wherein the top layer does not comprise any assist features.
11. A method as claimed in claim 1 , comprising modelling a resultant image obtained by inspection of the target using a metrology process, and evaluating whether the target arrangement is optimized for detection using a metrology process.
12. A method as claimed in claim 11 , wherein the method is repeated iteratively in order to optimize the target arrangement.
13. A method as claimed in claim 11 , wherein criteria for considering whether a particular target arrangement is considered optimized include one or more selected from:
determining whether intensities at a grating periphery are of the same order of magnitude as those at a grating center, when inspected using a diffraction based metrology process;
determining whether there is minimum intensity variation at a grating periphery in the presence of overlay, defocus and/or aberrations when inspected using a diffraction based metrology process;
determining whether there is sufficient spacing between the gratings for optimum target-recognition for a relevant inspection wavelength range; and/or
determining whether a total grating area is maximized.
14. A target comprising:
a plurality of gratings, each grating comprising a plurality of substructures;
assist features comprising lines having a pitch substantially smaller than the pitch of the gratings and separated from the substructures,
wherein the target comprises assist features at the periphery of the gratings, the assist features configured to reduce measured intensity peaks at the periphery of the gratings, and
wherein the assist features which are adjacent to and orientated with a particular grating are positioned in phase with the substructures of that grating.
15. A reticle comprising features configured to form the target as claimed in claim 14 .
16. A target as claimed in claim 14 , wherein a length of the substructures for one of the plurality of gratings is different than that of the substructures of another grating of the plurality of gratings.
17. A target as claimed in claim 16 , wherein each grating has approximately equal area.
18. A method of devising a target arrangement, the target comprising a plurality of gratings, each grating comprising a plurality of substructures, the method comprising:
defining a target area;
locating the substructures within the target area so as to form the gratings;
locating assist features within the target area, the assist features configured to reduce measured intensity peaks at the periphery of the gratings;
modelling a resultant image obtained by inspection of the target using a metrology process; and
evaluating whether the target arrangement is optimized for detection using a metrology process,
wherein criteria for considering whether a particular target arrangement is considered optimized include one or more selected from:
determining whether intensities at a grating peripheral portion are of the same order of magnitude as those at a grating central portion, when inspected using a diffraction based metrology process,
determining whether there is minimum intensity variation at a grating peripheral portion in the presence of overlay, defocus and/or aberrations when inspected using a diffraction based metrology process,
determining whether there is sufficient spacing between the gratings for optimum target-recognition for a relevant inspection wavelength range, and/or
determining whether a total grating area is maximized.
19. A method as claimed in claim 18 , wherein the assist features which are adjacent to and orientated with a particular grating are positioned in phase with that grating.
20. A method as claimed in claim 18 , further comprising adapting the length of the grating substructures for, or an overall length of, a particular grating to a remaining target area such that that length of the grating substructures or overall length is different than that of another grating in the target area.