Method of designing metrology targets, substrates having metrology targets, method of measuring overlay, and device manufacturing method
Metrology targets are formed by a lithographic process, each target comprising a bottom grating and a top grating. Overlay performance of the lithographic process can be measured by illuminating each target with radiation and observing asymmetry in diffracted radiation. Parameters of metrology recipe and target design are selected so as to maximize accuracy of measurement of overlay, rather than reproducibility. The method includes calculating at least one of a relative amplitude and a relative phase between (i) a first radiation component representing radiation diffracted by the top grating and (ii) a second radiation component representing radiation diffracted by the bottom grating after traveling through the top grating and intervening layers. The top grating design may be modified to bring the relative amplitude close to unity. The wavelength of illuminating radiation in the metrology recipe can be adjusted to bring the relative phase close to π/2 or 3π/2.
1. A method for use in a lithographic process using a pair of patterning devices comprising:
measuring, using an inspection apparatus, a first amplitude and/or phase of light of a first radiation component of illuminating radiation, the first radiation component having been diffracted by a second periodic structure of a metrology target formed on an upper layer of a substrate;
measuring, using the inspection apparatus, a second amplitude and/or phase of light of a second radiation component of the illuminating radiation after the second radiation component has traveled through the upper layer to a lower layer of the substrate and returning to the upper layer, the second radiation component having been diffracted by a first periodic structure of the metrology target formed on the lower layer;
determining, using the inspection apparatus, a relative amplitude and/or phase difference between the first and second radiation components;
generating, using the inspection apparatus, a metrology recipe parameter for the illuminating radiation and a design parameter for the metrology target based on the relative amplitude and/or phase difference,
wherein the metrology recipe parameter and the design parameter maximize accuracy of measurement of overlay,
wherein the metrology target has periodic structures designed according to the design parameter values generated, and whereby the metrology target with the periodic structures is utilized in conjunction with the pair of patterning devices when subsequently producing product features on the substrate.
2. The method of claim 1 , wherein the target design parameters are selected so that amplitudes of the first and second components are of the same order.
3. The method of claim 1 , wherein the second amplitude of the second radiation component is no more than ten times the first amplitude of the first radiation component, optionally no more than five times or two times.
4. The method of claim 1 , wherein the second periodic structure comprises primary features and secondary features within each period, and the method further comprises adjusting relative dimensions of the primary and secondary features so as to divert a proportion of the illuminating radiation into unobserved portions of a diffraction spectrum of the diffraction of the illumination radiation from the target.
5. The method of claim 1 , wherein:
the first and second radiation components represent substantially radiation that has been diffracted at first order by the periodic structures, and
unobserved portions of a diffraction spectrum of the diffraction of the illumination radiation from the metrology target includes orders higher than first order.
6. The method of claim 1 , wherein the values of metrology recipe parameters and design parameters are selected so as to make the relative phase between the first and second radiation components close to π/2 or 3π/2 radians rather than to 0 or π.
7. The method of claim 6 , wherein a wavelength of the illuminating radiation is one of the metrology recipe parameters and is adjusted to make the relative phase between the first and second radiation components have a desired value.
8. The method of claim 1 , wherein the desired value of the relative phase is π/2 or 3π/2 radians.
9. The method of claim 1 , further comprising selecting the values for the metrology recipe parameters and design parameters, wherein the selecting comprises:
identifying at least one candidate design and a plurality of candidate recipes based on the relative amplitude and/or phase difference,
for each recipe, determining simulated overlay measurements while applying simulated process variations in a mathematical model of the metrology target, and
selecting a final combination of design and recipe based on measurement error in the simulated overlay measurements.