Metrology method, apparatus and computer program
Disclosed herein is a metrology method, and an associated metrology apparatus, the metrology method includes measuring a target formed in at least two layers on a substrate by a lithographic process and capturing at least one corresponding pair of non-zeroth diffraction orders, for example in an image field, to obtain measurement data. A simulation of a measurement of the target as defined in terms of geometric parameters of the target, the geometric parameters including one or more variable geometric parameters, is performed and a difference between the measurement data and simulation data is minimized, so as to directly reconstruct a value for each of the one or more variable geometric parameters.
1. A metrology method comprising:
obtaining measurement data relating to a measurement of a target formed in at least two layers on a substrate by a lithographic process, the measurement data being derived from a corresponding pair of non-zeroth diffraction orders;
obtaining simulation data relating to a simulation of a measurement of the target as defined in terms of geometric parameters of the target, the geometric parameters comprising a variable geometric parameter; and
minimizing a difference between the measurement data and the simulation data, so as to directly reconstruct a value for the variable geometric parameter.
2. The method as claimed in claim 1 , wherein the measurement data relates to a measurement of the target derived from a dark-field measurement and wherein the corresponding pair of non-zeroth diffraction orders is detected in an image plane.
3. The method as claimed in claim 1 , wherein the minimizing is performed to minimize the difference between the measurement data and the simulation data in terms of an intensity metric of the detected corresponding pair of non-zeroth diffraction orders.
4. The method as claimed in claim 3 , wherein the measurement data and the simulation data each comprises a plurality of intensity values, one for each combination of non-zeroth diffraction order, measurement radiation characteristic, and/or imposed target bias comprised in the target, and the intensity metric is derived from the plurality of intensity values.
5. The method as claimed in claim 3 , wherein the intensity metric comprises a scaled image intensity asymmetry metric, the scaled image intensity asymmetry metric comprising an intensity difference between corresponding pairs of non-zeroth diffraction orders scaled by their average intensity.
6. The method as claimed in claim 3 , wherein the intensity metric comprises a stack sensitivity metric, wherein the stack sensitivity metric is a ratio of an overlay proportionality constant to an average of intensities of regions of interest within a measurement image.
7. The method as claimed in claim 3 , wherein the intensity metric comprises an average intensity metric, the average intensity metric being an average of intensities of regions of interest within a measurement image.
8. The method as claimed in claim 1 , wherein the measurement data relates to a measurement of the target derived from a measurement with respect to a pupil plane and wherein the corresponding pair of non-zeroth diffraction orders is detected in the pupil plane.
9. The method as claimed in claim 1 , wherein the variable geometric parameter comprises a layer height of a layer comprised in the target.
10. The method as claimed in claim 9 , wherein the variable geometric parameter comprises a layer height of each layer of a plurality of different layers comprised in the target, and wherein the minimizing comprises simultaneously reconstructing values for the layer heights of the plurality of different layers.
11. The method as claimed in claim 9 , further comprising:
calculating a spectral sequence describing a variation of an intensity parameter with wavelength of measurement radiation, for the target with the layer height as determined in the minimizing; and
optimizing the measurement radiation based on the spectral sequence.
12. The method as claimed in claim 1 , wherein the variable geometric parameter comprises a geometric asymmetry parameter of a structure comprised in the target.
13. The method as claimed in claim 12 , wherein the geometric asymmetry parameter comprises one or more selected from: floor tilt, top tilt, side wall angle, and/or critical dimension.
14. The method as claimed in claim 1 , wherein the variable geometric parameter comprises:
a layer height of a layer comprised in the target; and
another variable geometric parameter,
wherein the minimizing is performed in two stages, a first stage for reconstructing a value for the layer height and a second stage for reconstructing a value for the other variable geometric parameter, the layer height determined in the first stage being fed forward as a fixed layer height parameter in the second stage.
15. The method as claimed in claim 14 , wherein the minimizing is performed to minimize the difference between the measurement data and the simulation data in terms of an intensity metric,
wherein the intensity metric comprises a stack sensitivity metric in the first stage, wherein the stack sensitivity metric is a ratio of an overlay proportionality constant to an average of intensities of regions of interest within a measurement image, and
wherein the intensity metric comprises a scaled image intensity asymmetry metric in the second stage, the scaled image intensity asymmetry metric comprising an intensity difference between corresponding pairs of non-zeroth diffraction orders scaled by an average intensity of the corresponding pairs of non-zeroth diffraction orders.
16. The method as claimed in claim 14 , wherein the first stage is used to predict a stack height variation map which describes variation of the layer height across the substrate, the stack height variation map being used to determine the fixed layer height parameter in the second stage.
17. The method as claimed in claim 14 , further comprising:
calculating a spectral sequence describing a variation of an intensity parameter with wavelength of measurement radiation, for the target with the layer height as determined in the minimizing; and
optimizing the measurement radiation based on the spectral sequence.
18. The method as claimed in claim 1 , wherein the minimizing comprises devising an objective function for minimizing the difference between the measurement data and the simulation data.
19. A metrology apparatus comprising a processor configured to at least:
obtain measurement data relating to a measurement of a target formed in at least two layers on a substrate by a lithographic process, the measurement data being derived from a corresponding pair of non-zeroth diffraction orders;
obtain simulation data relating to a simulation of a measurement of the target as defined in terms of geometric parameters of the target, the geometric parameters comprising a variable geometric parameter; and
minimize a difference between the measurement data and the simulation data, so as to directly reconstruct a value for the variable geometric parameter.
20. A non-transitory computer-readable medium comprising instructions therein, the instructions, upon execution by a computer system, configured to cause the computer system to at least:
obtain measurement data relating to a measurement of a target formed in at least two layers on a substrate by a lithographic process, the measurement data being derived from a corresponding pair of non-zeroth diffraction orders;
obtain simulation data relating to a simulation of a measurement of the target as defined in terms of geometric parameters of the target, the geometric parameters comprising a variable geometric parameter; and
minimize a difference between the measurement data and the simulation data, so as to directly reconstruct a value for the variable geometric parameter.