Mitigation of inaccuracies related to grating asymmetries in scatterometry measurements
Scatterometry overlay targets as well as target design and measurement methods are provided, which mitigate the effects of grating asymmetries in diffraction based overlay measurements. Targets comprise additional cells with sub-resolved structures replacing resolved coarse pitch gratings and/or comprise alternating sub-resolved structures with coarse pitch periodicity—to isolate and remove inaccuracies that result from grating asymmetries. Measurement methods utilize orthogonally polarized illumination to isolate the grating asymmetry effects in different measurement directions, with respect to the designed target structures.
1. A scatterometry overlay (SCOL) target comprising, along at least one measurement direction:
two cells having periodic structures with a specified coarse pitch at least at a first wafer layer and a second wafer layer, wherein the first wafer layer is disposed over the second wafer layer, wherein the two cells have opposite offsets of a top periodic structure with respect to a bottom periodic structure in the respective layers,
wherein a first of the two cells has a periodic structure with the specified coarse pitch at the first wafer layer and a sub-resolved periodic structure at the second wafer layer, and
wherein a second of the two cells has a periodic structure with the specified coarse pitch at the second wafer layer and a sub-resolved periodic structure at the first wafer layer,
wherein the specified coarse pitch is at least 500 nm and the sub-resolved periodic structure has a sub-resolved pitch smaller than 300 nm, wherein the sub-resolved periodic structure in the first of the two cells is configured to have a same filling factor as the periodic structure with the specified coarse pitch in the first of the two cells, and wherein the sub-resolved periodic structure in the second of the two cells is configured to have a same filling factor as the periodic structure with the specified coarse pitch in the second of the two cells,
wherein the periodic structures have cells along each of two, X and Y, measurement directions, wherein eight of the cells are arranged in two rows each with four of the cells having a same spatial arrangement with respect to the measurement directions, wherein each of the rows comprises two X direction cells and two Y direction cells that individually alternate along the row such that one of the X direction cells in the row is adjacent two of the Y direction cells in the row and such that one of the Y direction cells in the row is adjacent two of the X direction cells in the row.
2. The SCOL target of claim 1 , wherein the sub-resolved pitch is smaller than 100 nm.
3. A target design file of targets designed according to claim 1 .
4. A method comprising adding, to a scatterometry overlay (SCOL) target comprising, along at least one measurement direction, two cells having periodic structures with a specified coarse pitch at least at a first wafer layer and a second wafer layer, wherein the first wafer layer is disposed over the second wafer layer, wherein the two cells have opposite offsets of a top periodic structure with respect to a bottom periodic structure in the first wafer layer and the second wafer layer, wherein one of the two cells has a periodic structure with the specified coarse pitch at the first wafer layer and a sub-resolved periodic structure at the second wafer layer and another of the two cells has a periodic structure with the specified coarse pitch at the second wafer layer and the sub-resolved periodic structure at the first wafer layer, wherein the specified coarse pitch is at least 500 nm and the sub-resolved periodic structure has a sub-resolved pitch smaller than 300 nm, wherein the sub-resolved periodic structure in the two cells is configured to have a same filling factor as the periodic structure with the specified coarse pitch in the two cells, wherein the periodic structures have cells along each of two, X and Y, measurement directions, wherein eight of the cells are arranged in two rows each with four of the cells having a same spatial arrangement with respect to the measurement directions, wherein each of the rows comprises two X direction cells and two Y direction cells that individually alternate along the row such that one of the X direction cells in the row is adjacent two of the Y direction cells in the row and such that one of the Y direction cells in the row is adjacent two of the X direction cells in the row.
5. The method of claim 4 , further comprising measuring the cells along the two measurement directions with orthogonally polarized illumination radiation.
6. The method of claim 5 , wherein the measuring is carried out simultaneously for pairs of cells with pairs of orthogonally polarized illumination spots.
7. The method of claim 4 , wherein the sub-resolved pitch is smaller than 100 nm.