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=Alignment=
=Alignment=
The alignment accuracy of the Aligner: Maskless 01 is a combination of the position accuracy of the stage, the accuracy of the alignment mark detection, and the accuracy of the pattern already on the wafer (first print).
<br/> By measuring the stitching accuracy between two layers printed on the same substrate (without unloading the substrate), we can assess the stage accuracy. By aligning to a pattern previously exposed by the Aligner: Maskless 01, we can assess the mark detection accuracy. And finally, by aligning to a pattern exposed on a mask aligner, we can assess the mask-less aligner's ability to compensate for any scaling and orthogonality errors between the two machines.


The results reported here use printed verniers to assess the misalignment along the two axes at different points on the wafer using an optical microscope. Two different designs were used; a ±5µm vernier and a ±1µm vernier. Both consist of a scale of 4µm lines with 10µm pitch, and a vernier scale to enable subdivision of the 5µm or 1µm scale into tenths, i.e. 0.5µm or 0.1µm. During inspection, observation of the symmetry of neighboring lines enables the observer to read the shifts with ±0.25µm or ±0.05µm accuracy.
[[Image:section under construction.jpg|70px]]
<br/>The measurements are used to calculate the misalignment of the second layer with respect to the first print: The Misalignment [µm] is the median of all measurement points in X or Y; the Translation [µm] is the amount by which the image is shifted; the Rotation [ppm] is the angle by which the image is rotated; and the Run-out [ppm] is the amount of gain in the image. The unit of ppm (parts per million) is used as the rotation and run-out are generally small. A rotation of 1ppm corresponds to an angle of 0.2" (arcseconds) or a shift of 100nm across an entire 4" wafer, while a run-out of 1ppm corresponds to a shift of 50nm at the edge of a 4" wafer compared to the center. For comparison, the pixel size at the wafer surface is 500nm X 500nm, and the address grid size is 50nm.
<br/>The deviations (±) given for the results here are calculated as half the range of measurements. If the range is small, the measurement uncertainty is used in stead.
<br/>The samples used for these tests are 100mm Si wafers coated with a 1.5µm layer of the positive tone resist AZ 5214E.


The conclusion to the tests are that the stitching accuracy of the Aligner: Maskless 01 is ±0.1µm. The machine-to-self overlay accuracy is ±0.5µm. The machine-to-machine overlay accuracy could not be determined.  
The alignment accuracy of the Aligner: Maskless 01 is a combination of the position accuracy of the stage, the accuracy of the alignment mark detection (mostly determined by the offset between camera and stage), and the accuracy of the pattern already on the wafer (first print).
<br/> By measuring the stitching accuracy between two layers printed on the same substrate (without unloading the substrate), we can assess the stage accuracy. By aligning to a pattern previously exposed by the Aligner: Maskless 01, we can assess the mark detection accuracy. And finally, by aligning to a pattern exposed on a different aligner, we can assess the mask-less aligner's ability to compensate for any scaling and orthogonality errors between the two machines.  


==Important note about correction options==
==Important note about correction options==
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<span style="color:red">If four marks are used, but scaling and shearing is not applied, ''significant'' misalignment will be observed, even on chips. On a 4" wafer the shift in Y can be several hundred µm.</span>
<span style="color:red">If four marks are used, but scaling and shearing is not applied, ''significant'' misalignment will be observed, even on chips. On a 4" wafer the shift in Y can be several hundred µm.</span>


==Stitching==
==Alignment tests==
In the stitching test, the design consists of ±5µm and ±1µm verniers along the X and Y axis placed in a 3 by 3 matrix covering a 60mm by 60mm area centered on the wafer. The sample is loaded, and the first layer (the linear scales) is printed. Without unloading, the second layer (the vernier scales) is printed on top of the first, and then the sample is developed.
In the stitching test, the design consists of ±5µm and ±1µm verniers along the X and Y axis placed in a 3 by 3 matrix covering a 60mm by 60mm area centered on the wafer. The sample is loaded, and the first layer (the linear scales) is printed. Without unloading, the second layer (the vernier scales) is printed on top of the first, and then the sample is developed.


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==Overlay==
 
In the overlay test, two alignment accuracies are assessed: The machine-to-self overlay accuracy (MLA-MLA), and the machine-to-machine (MA6-MLA) overlay accuracy. Because alignment is possible using two marks or four marks, both are tested in each case. Exposing the first print with or without flat alignment was also tested, but no significant effect was observed.
In the overlay test, two alignment accuracies are assessed: The machine-to-self overlay accuracy (MLA-MLA), and the machine-to-machine (MA6-MLA) overlay accuracy. Because alignment is possible using two marks or four marks, both are tested in each case. Exposing the first print with or without flat alignment was also tested, but no significant effect was observed.


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