Diffraction-based focus metrology
Diffraction-based focus target cells, targets and design and measurement methods are provided, which enable sensitive focus measurements to be carried out by overlay measurement tools. Cells comprise a periodic structure having a coarse pitch and multiple elements arranged at a fine pitch. The coarse pitch is an integer multiple of the fine pitch, with the fine pitch being between one and two design rule pitches and smaller than a measurement resolution and the coarse pitch being larger than the measurement resolution. The elements are asymmetric to provide different amplitudes in +1 st and −1 st diffraction orders of scattered illumination, and a subset of the elements has a CD (critical dimension) larger than a printability threshold and the other elements have a CD smaller than the printability threshold.
1. A diffraction-based focus target cell, printable using a lithographic printing tool, comprising a periodic structure having a coarse pitch and a plurality of elements arranged at a fine pitch, wherein the coarse pitch is an integer multiple of the fine pitch, with the fine pitch being between one and two integer multiples of a design-rule pitch and smaller than a measurement resolution and the coarse pitch being larger than the measurement resolution, wherein the measurement resolution is a resolution of a measurement tool,
wherein the elements are asymmetric to provide different amplitudes in +1 st and −1 st diffraction orders of scattered illumination,
wherein the design-rule pitch is a smallest pitch printable by the lithographic printing tool, and
wherein a subset of the elements has a CD (critical dimension) larger than the design-rule pitch and the other elements have a CD smaller than the design-rule pitch.
2. The diffraction-based focus target cell of claim 1 , further comprising lines having a CD smaller than the design-rule pitch, the lines delimiting the elements arranged at the fine pitch.
3. A diffraction-based focus target comprising a plurality of the diffraction-based focus target cells of claim 1 , having at least two pairs of cells with opposite asymmetry of the elements, and wherein the target has a 180° rotational symmetry.
4. A target design file of the target of claim 3 .
5. A method comprising measuring a focus position of a printing tool using at least one diffraction-based focus target cell of claim 1 .
6. A method comprising:
measuring a focus position of a printing tool using at least one diffraction-based focus target cell, printable using a lithographic printing tool, having asymmetric elements arranged at a fine pitch between one and two integer multiples of a design-rule pitch and smaller than a measurement resolution, the at least one cell having a coarse pitch which is an integer multiple of the fine pitch and is larger than the measurement resolution, wherein the measurement resolution is a resolution of a measurement tool,
wherein the design-rule pitch is a smallest pitch printable by the lithographic printing tool, and
wherein the asymmetric elements are designed to provide different amplitudes in +1 st and −1 st diffraction orders of scattered illumination and wherein a subset of the elements has a CD larger than the design-rule pitch and the other elements have a CD smaller than the design-rule pitch.
7. The method of claim 6 , wherein the cells are arranged in a focus target having a 180° rotational symmetry.
8. A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to implement the method of claim 6 .
9. A scatterometry overlay measurement tool comprising a focus module configured to implement the method of claim 6 .
10. A method comprising designing a diffraction-based focus target by:
designing diffraction-based focus target cells, printable using a lithographic printing tool, to have asymmetric elements arranged at a fine pitch between one and two integer multiples of a design-rule pitch, wherein the design-rule pitch is a smallest pitch printable by the lithographic printing tool, and smaller than a measurement resolution, the at least one cell having a coarse pitch which is an integer multiple of the fine pitch and is larger than the measurement resolution, wherein the measurement resolution is a resolution of a measurement tool,
configuring the asymmetric elements to provide different amplitudes in +1 st and −1 st diffraction orders of scattered illumination, wherein a subset of the elements has a CD larger than the design-rule pitch and the other elements have a CD smaller than the design-rule pitch, and
arranging the designed cells in a focus target having a 180° rotational symmetry.
11. A diffraction-based focus target comprising a plurality of the diffraction-based focus target cells of claim 2 , having at least two pairs of cells with opposite asymmetry of the elements, and wherein the target has a 180° rotational symmetry.
12. A target design file of the target of claim 11 .
13. A method comprising measuring a focus position of a printing tool using at least one diffraction-based focus target cell of claim 2 .