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. Most likely, the Y-shift will grow linearly with the distance from the center, so small samples will be | 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. Most likely, the Y-shift will grow linearly with the distance from the center, so small samples will be less affected, while full wafers will experience shifts in Y that far exceed the ±1µm specification. It might be a good idea to include an alignment mark at 0;0 as the first mark when aligning to a pattern exposed on a different tool. | ||
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The alignment test with 4 alignment marks shows a +40ppm scaling on the X-axis, as well as a 0.1mRad shearing of the axes. The result is a decent alignment in X, but a shift in Y as well as a relatively large deviation. The raw data shows the deviation in Y is due to a -40ppm scaling along the Y-axis, as seen in the MLA1-MLA1 test with 4 marks, suggesting that the scaling in Y is consistently overestimated. | |||
<br>Aligning using only 2 marks yields acceptable shifts in the center of the wafer, but very large shifts in X towards the edges, as evidenced by the 7.4µm deviation in X. The raw data suggests that this deviation is mainly due to a 0.2mRad tilt in the Y-axis, which corresponds well with the 0.1mRad shearing measured using 4 marks. There is also a (-)40ppm scaling along the X-axis, again similar to what was measured during 4 mark alignment. Even a 5mm chip would be affected by the 0.2mRad tilt, so clearly 4 mark alignment is needed when aligning to a pattern that was not exposed using MLA1. | |||
=Optimal use of the maskless aligner= | =Optimal use of the maskless aligner= | ||