Method of measuring a parameter of a patterning process, metrology apparatus, target
A technique of measuring a parameter of a patterning process is disclosed. In one arrangement, a target, formed by the patterning process, is illuminated. A sub-order diffraction component of radiation scattered from the target is detected and used to determine the parameter of the patterning process.
1. A method of measuring a parameter of a patterning process, the method comprising:
illuminating a target formed by the patterning process;
detecting a sub-order diffraction component of radiation scattered from the target; and
using the detected sub-order diffraction component to determine a value of the parameter of the patterning process.
2. The method of claim 1 , wherein the sub-order diffraction component is formed predominantly from radiation corresponding to a portion of a theoretical diffraction pattern from the target between and outside of principal first order peaks and outside of a principal zeroth order peak.
3. The method of claim 2 , wherein the detecting of the sub-order diffraction component comprises separating radiation corresponding to the sub-order diffraction component from radiation corresponding to the principal zeroth order peak.
4. The method of claim 3 , wherein the separation comprises directing the radiation corresponding to the principal zeroth order peak and the radiation corresponding to the sub-order diffraction component through separate respective regions in a pupil plane of an optical system between the target and a detector.
5. The method of claim 4 , wherein the parameter of the patterning process is determined by comparing a first sub-order diffraction component with a second sub-order diffraction component.
6. The method of claim 5 , wherein:
the first sub-order diffraction component is formed predominantly from radiation corresponding to a portion of the theoretical diffraction pattern nearer to a+1 principal first order peak than to a−1 principal first order peak; and
the second sub-order diffraction component is formed predominantly from radiation corresponding to a portion of the theoretical diffraction pattern nearer to the −1 principal first order peak than to the +1 principal first order peak.
7. The method of claim 5 , wherein:
the parameter of the patterning process comprises an error in the patterning process; and
the comparing of the first sub-order diffraction component with the second sub-order diffraction component comprises determining a deviation from a nominal relationship between the first sub-order diffraction component and the second sub-order diffraction component expected when the error is zero, the deviation thereby providing information about the parameter of the patterning process.
8. The method of claim 5 , wherein the detecting of the sub-order diffraction component comprises detecting an intensity or an intensity distribution in the pupil plane.
9. The method of claim 8 , wherein:
the first sub-order diffraction component is detected by measuring an intensity or an intensity distribution in a first selected region of the pupil plane; and
the second sub-order diffraction component is detected by measuring an intensity or an intensity distribution in a second selected region of the pupil plane.
10. The method of claim 9 , wherein the first selected region and the second selected region are point symmetric or mirror symmetric with respect to each other.
11. The method of claim 5 , wherein the detecting of the sub-order diffraction component comprises forming an image in an image plane using the sub-order diffraction component.
12. The method of claim 11 , wherein:
the forming of an image comprises forming a first image and a second image;
the first sub-order diffraction component is detected by measuring a property of the first image; and
the second sub-order diffraction component is detected by measuring a property of the second image.
13. The method of claim 1 , wherein:
the detecting of the sub-order diffraction component comprises forming an image in an image plane using the sub-order diffraction component; and
the parameter of the patterning process is determined by comparing a first portion of the image with a second portion of the image.
14. The method of claim 1 , wherein the detected sub-order diffraction component is used to determine a location of an image of the target.
15. The method of claim 14 , further comprising using the determined location of the image of the target to analyze the intensity of the image of the target to determine the parameter of the patterning process.
16. The method of claim 1 , wherein:
the periodic structure comprises a principal periodicity equal to a size of a repeating unit cell in a direction of periodicity of the periodic structure;
the principal zeroth order peak and the principal first order peaks are principal peaks of the theoretical diffraction pattern corresponding to the principal periodicity;
the principal zeroth order peak extends from a maximum of the principal zeroth order peak to minima of the principal zeroth order peak; and
each principal first order peak extends from a maximum of the principal first order peak to minima of the principal first order peak.
17. The method of claim 1 , wherein the illumination and periodic structure are such that radiation corresponding to principal first order peaks of the theoretical diffraction pattern from the target either falls outside of a numerical aperture of an optical system between the target and a detector or is evanescent.
18. The method of claim 1 , wherein:
the target comprises multiple sub-targets, each sub-target comprising a periodic structure with a different nominal applied bias in the parameter of the patterning process or a different orientation; and
the sub-targets are spaced apart from each other by at least 25% of a maximum lateral dimension of each sub-target.
19. A method of measuring a parameter of a patterning process, the method comprising:
illuminating a target formed by the patterning process;
detecting a sub-order diffraction component of radiation scattered from the target; and
using the detected sub-order diffraction component to determine the parameter of the patterning process, the using the detected sub-order diffraction component comparing a first sub-order diffraction component with a second sub-order diffraction component.
20. The method of claim 19 , wherein the comparing comprises determining a deviation from a nominal relationship between the first sub-order diffraction component and the second sub-order diffraction component expected when the error is zero, the deviation providing information about the parameter of the patterning process.
21. A method of measuring a parameter of a patterning process, the method comprising:
illuminating a target formed by the patterning process;
detecting a sub-order diffraction component of radiation scattered from the target, the detecting comprising separating radiation corresponding to the sub-order diffraction component in the scattered radiation from radiation corresponding to a principal zeroth order peak in the scattered radiation; and
using the detected sub-order diffraction component to determine the parameter of the patterning process.