Specific Process Knowledge/Lithography/Aligners/Aligner: Maskless 01 processing: Difference between revisions
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After installation, multiple tests were conducted in order to assess the overlay accuracy of Aligner: Maskless 01. The conclusion to the early tests were that the stage accuracy is ±0.1µm, and the machine-to-self overlay accuracy is ±0.5µm. The machine-to-machine overlay accuracy was not determined (due to the lack of a suitable mask for the mask aligners). In 2019, efforts to establish regular QC of the equipment were started, and the accuracy of the alignment mark detection has been measured regularly since 2020. While both the average and the spread of the alignment errors for the x-axis (measured in 3x3 positions covering a 60x60mm<sup>2</sup> area) has consistently been within the ±1µm specification of the machine, the spread of the alignment errors for the y-axis is typically 3±1µm, despite the average error being in spec, due to negative offsets on the upper half of the wafer and positive offsets on the lower. In 2025, it was decided to investigate this problem further, in order to determine whether a specific alignment protocol could remedy the alignment error, or whether the acceptance limits for the QC would have to be changed. | After installation, multiple tests were conducted in order to assess the overlay accuracy of Aligner: Maskless 01. The conclusion to the early tests were that the stage accuracy is ±0.1µm, and the machine-to-self overlay accuracy is ±0.5µm. The machine-to-machine overlay accuracy was not determined (due to the lack of a suitable mask for the mask aligners). In 2019, efforts to establish regular QC of the equipment were started, and the accuracy of the alignment mark detection has been measured regularly since 2020. While both the average and the spread of the alignment errors for the x-axis (measured in 3x3 positions covering a 60x60mm<sup>2</sup> area) has consistently been within the ±1µm specification of the machine, the spread of the alignment errors for the y-axis is typically 3±1µm, despite the average error being in spec, due to negative offsets on the upper half of the wafer and positive offsets on the lower. In 2025, it was decided to investigate this problem further, in order to determine whether a specific alignment protocol could remedy the alignment error, or whether the acceptance limits for the QC would have to be changed. | ||
The result of these tests suggest that when aligning to a pattern exposed using MLA1, only 2 alignment marks on the X-axis should be used. If the first pattern was exposed using a different tool, 4 alignment marks must be used (with all corrections applied), but the alignment accuracy in Y-direction suffers. The Y-shift grows linearly with the size of the pattern/sample, so small samples will be less affected, while full wafers will experience shifts in Y that far exceed the ±1µm specification. In general, larger alignment error in Y must be accepted when aligning to a pattern exposed on a different tool. By scaling the design 40ppm in Y, however, it is possible to compensate for the Y-shift and achieve alignment within the machine specification across an entire wafer. | The result of these tests suggest that when aligning to a pattern exposed using MLA1, only 2 alignment marks on the X-axis should be used. If the first pattern was exposed using a different tool, 4 alignment marks must be used (with all corrections applied), but the alignment accuracy in Y-direction suffers. The Y-shift grows linearly with the size of the pattern/sample, so small samples will be less affected, while full wafers will experience shifts in Y that far exceed the ±1µm specification. In general, larger alignment error in Y must be accepted when aligning to a pattern exposed on a different tool. By scaling the design 40ppm in Y, however, it is possible to compensate for the Y-shift and achieve alignment within the machine specification across an entire wafer. Based on these results, it was decided to keep the QC procedure as it is, but to adjust the QC limits to ±2µm, in order to match the tolerance needed when using the machine. | ||