Diffraction-based focus metrology
AssigneeKLA-Tencor Corporation
InventorVladimir Levinski
Life of this patent
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Abstract
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 +1st and −1st 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.
Claims13
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 +1st and −1st 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.
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 +1st and −1st 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.
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 +1st and −1st 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.
Dependencies are inferred from the claim text.